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Manufacture of sodium dithiouite from sodium-mercury amalgam and aqueous solution of sulfur dioxide Nayar, Raman 1972

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|5"IV) MANUFACTURE OF SODIUM DITHIONITE FROM SODIUM-MERCURY AMALGAM AND AQUEOUS SOLUTION OF SULFUR DIOXIDE by RAMAN NAYAR B. Tech. (Hon.), I.I.T., Kharagpur, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMICAL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA November, 1972 In presenting this thesis in partial fulfullment of the requirements for an advanced degree at the University of British Columbia/ I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that publication, in part or in whole, or the copying of this thesis for financial gain shall not be allowed without my written permission. RAMAN NAYAR Department of Chemical Engineering The University of British Columbia Vancouver 8, Canada Date K^~ot Xo' \^ 7 3 ii ABSTRACT A relatively dilute (approximately 1 to 2%) water solution of sodium dithionite was produced from sodium-mercury amalgam and aqueous solution of sulfur dioxide in a simple "once through" reactor [proposed process]. The reactor could be run in conjunction with the Castner-Kellner type cell. The manufactured solution could then be used directly for the brightening of groundwood pulp. The bench scale experiments were carried out in a continuous-flow-stirred-tank reactor where the aqueous and amalgam phases formed an interface. The effects of important process variables on the steady-state concentration of sodium dithionite in the reactor and yields of sodium dithionite on sulfur dioxide in the aqueous feed and on sodium consumed in a single pass were determined. The above-mentioned yields are important in assessing the economic feasibility of the pro posed process. The steady-state yield of sodium dithionite on sodium in the amalgam entering the reactor and'conversion of sodium to different products in the reactor were also determined. The present investigation showed that the process variables can be controlled to give approximately 2.3% sodium dithionite solution with steady-state Na2S20^ yields of about 21% on sulfur dioxide in the aqueous feed and about 67% on sodium consumed. The yields obtained depend on the levels of process variables such as: 1. the concentration of sodium in the amalgam enter ing the reactor, 2. the concentration of total sulfur dioxide in the aqueous feed solution, 3. the agitation in the aqueous phase, 4. the agitation in the amalgam phase, 5. the residence time in the aqueous phase, 6. the residence time in the amalgam phase, 7. the interfacial-area/aqueous-volume ratio, 8. the temperature of the aqueous phase, and 9. the pH of the aqueous phase. This experimental study indicates that it may be economically feasible for a pulp mill to change .from zinc dithionite produced in situ to sodium dithionite produced in situ by the proposed; process. Further, the proposed process compared to the manufacture of zinc dithionite in situ avoids the discharge of zinc ions which act as biocidal agents when discharged into the effluent receiving waters. The models suggested by Ketelaar (44) and Gerritsen (30) were found inadequate to explain the processes occurring iv in the reacting system sodium-mercury amalgam and aqueous sulfur dioxide. A qualitative model has been suggested on the basis of the experimental work and the information available in the literature. This work also sheds some light on the type of reactor which would be suitable for the proposed process. V ACKNOWLEDGEMENT The author wishes to express his thanks to the faculty and staff of the Chemical Engineering Department, The University of British Columbia. Special thanks are extended to Dr. F.E. Murray, who suggested the project and under whose guidance this work was undertaken. The author is indebted to the Chemical Engineering Workshop personnel for their assistance in assembling the experimental equipment. The author wishes to thank Mr. E. Rudischer, in particular, for his assistance and cooper ation . Financial support for this research was most gratefully received from the National Research Council of Canada and from the British Columbia Research Council. The author is also indebted to his wife Jane for her invaluable help throughout this work. vi TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION 1 II. REVIEW OF PERTINENT PRIOR WORK 7 A. Manufacturing Processes for Sodium Dithionite 7 1. Zinc dust: sodium carbonate process ... 7 2. Electrolytic or cathodic reduction process 9 3. Sodium formate process 9 4. Sodium borohydride process 10 5. Sodium amalgam process 1 (a) Advantages of the sodium amalgam process 1(b) Types of the sodium amalgam process ... 12 (i) Sodium amalgam: S02-organic solvent process(ii) Sodium amalgam: gaseous SO2 process and sodium amalgam: liquid SO2 process 13 (iii) Sodium amalgam: S02-NaHS03/Na2S03 buffer process 13 B. Recommended Conditions for Improving the Yield of Sodium Dithionite in the Sodium Amalgam: S02-NaHS03/Na2S03 Buffer Process . . 15 C. Mercury Contamination of Sodium Dithionite Produced by the Sodium Amalgam: SOj-NaHSO,/ Na„S0o Buffer Process ...... 17 CHAPTER vii PAGE D. Lignin Preserving Bleaching of Ground-wood Pulp by Sodium Dithionite 19 1. Definition of the terms "brightening" and "bleaching" . . . . . 19 2. Characteristics of the groundwood bleach ing process . 20 3. Effects of groundwood brightening . . . 21 4. Conditions for groundwood brightening by sodium dithionite 21 E. Sodium-mercury Amalgam 4 1. Molecular structure of sodium-mercury amalgam .......... 24 2. Surface tension of sodium-mercury amalgam 25 3. Sensitivity to oxidation of sodium-mercury amalgam ...... 25 4. Density of sodium-mercury amalgam ... 26 F. Sulfur Dioxide Solution in Water ..... 26 1. Principal equilibria ......... 26 2. Diffusion of sulfur dioxide in water ................. 32 G. Important Reactions in the Proposed Process ..... 33 1. The Sodium dithionite formation reaction . ..... 35 2. The water reaction 39 3. The sodium dithionite decomposition reactions ....... 43 (a) Homogeneous decomposition of sodium dithionite 43 (b) Heterogeneous decomposition of sodium dithionite 4 6 viii CHAPTER PAGE 4. The sodium dithionite oxidation reaction ..... 48 III. THEORETICAL MODELS 50 IV. EXPERIMENTAL 4 A. Experimental Materials .... 54 1. Sodium-mercury amalgam ..... 54 2. Aqueous sulfur dioxide solution 54 B. Experimental Apparatus . 55 1. Reactor 52. pH measurement of the aqueous phase ... 64 3. Temperature measurement of different streams 65 4. Insulation of the equipment ....... 67 5. Electrical wiring diagram ........ 67 C. Calibration Curves ..... 67 D. An Experimental Run ........ 69 E. Analytical Procedures and Errors ...... 71 1. Sodium-mercury amalgam .... 71 (a) Analysis of sodium-mercury amalgam 7(b) Accuracy and precision of the analytical procedure 72 2. Aqueous sulfur dioxide solution ..... 75 (a) Analysis of aqueous sulfur dioxide solution 7(b) Accuracy and precision of the analytical procedure 76 3. Aqueous sodium dithionite solution .... 77 ix CHAPTER PAGE (a) Analysis of sodium dithionite in the product stream 77 (b) Accuracy and precision of the analytical procedures 79 V. EXPERIMENTAL RESULTS 8 2 A. Batch Experiments 2 B. Introduction to Experiments in the CFSTR .......... 87 C. Definitions of Some Important Quantities which are used for the Interpretation of Data ... 90 D. Reproducibility of Experimental Runs in the CFSTR 94 E. Data from CFSTR Experiments ......... 101 1. Concentration of sodium in fresh amalgam 101 2. Concentration of "total" sulfur dioxide in the aqueous feed solution 121 3. Agitation in the aqueous phase 133 4. Flow rate of aqueous sulfur dioxide solution, i.e. residence time in the aqueous phase ...... 144 5. Interfacial-area/aqueous-volume ratio . . 153 6. Temperature of the aqueous phase 164 7. pH of the aqueous phase 168 8. Flow rate of fresh amalgam, i.e. residence time in the amalgam phase . . . 168 VI. DISCUSSION I70 A. Model for the Reacting System in the Proposed Process 170 X CHAPTER PAGE 1. Development of the model 170 2. The model 185 B. Conditions for Improving the Yields of Sodium Dithionite in the Proposed Process 188 C. Economic Feasibility of the Proposed Process ....... 194 D. Reactor for the Proposed Process ...... 198 VII. CONCLUSIONS 199 VIII. RECOMMENDATIONS FOR FURTHER WORK 203 IX. NOMENCLATURE 205 BIBLIOGRAPHY ........ 208 APPENDIX A. EQUIPMENT SPECIFICATION A-l 1. pH measurement A-2. Digital temperature recording ........ A-l 3. Calibration curves ....... A-2 B. STATISTICAL EVALUATION OF ACCURACY AND PRECISION B-l 1. Error of a measurement process ....... B-l 2. Evaluation of accuracy B-3 3. Evaluation of precision (or imprecision). . . B-4 4. Propagation of random error B-8 C. SODIUM-MERCURY AMALGAM C-l 1. Purity of the chemicals in preparing amalgam C-2. Problems encountered in preparation of amalgam ..... C-2 xi APPENDIX PAGE 3. Calculation of sodium content in an amalgam sample C-3 4. Estimation of the precision of the analytical procedure C-4 D. AQUEOUS SOLUTION OF SULFUR DIOXIDE D-l 1. Purity of the chemicals in preparing aqueous sulfur dioxide solution D-l 2. Calculation of the total sulfur dioxide con centration in an aqueous solution sample . . D-2 3. Estimation of the precision of the analytical procedure ..... D-3 E. SODIUM DITHIONITE IN THE PRODUCT STREAM ..... E-l 1. Details of the analytical procedures and sample calculations E-l (a) The iodine-formaldehyde method E-l (b) The Rubine-R method E-8 2. Estimation of the precision of the Rubine-R E-16 method F. DATA PROCESSING F-l 1. Mathematical expressions for calculating CS204' YS02, YNa' C0NNA' XNa and Na/S°2< rate of sodium consumption, and sample calculations 2. The 95 per cent confidence limits of the steady-state C8 Y , CONNA, X^, Na/S02 and rate of sodium consumption _ for an experimental run 3. Experimental data and results F-12 xii LIST OF FIGURES Figure Page 1. Concentrations of free sulfur dioxide, bisulfite ions and sulfite ions versus pH in a 0.399 molar sulfur dioxide solution in water at 25°C 2. Effective diffusion coefficients for sulfur dioxide in aqueous (^4)2603. 'Total* sulfur dioxide concentration = 1 gm mole/ 1 i ter 3. Schematic flow-sheet of the experimental apparatus 4. Reactor assembly 61 5. Digital temperature measurement (schematic). . 66 6. Electrical wiring diagram (schematic) 68 7. Reproducibility of the experimental runs in the CFSTR 97 8. Reproducibility of the experimental runs in the CFSTR 100 9. Steady-state values of sodium dithionite concentration and various yields versus Na/S02 ratios entering the CFSTR for the experimental set: 65-77 . 103 10. Steady-state values of sodium dithionite con centration and various yields versus concen trations of sodium in amalgam entering the CFSTR for the experimental set: 65-77 .... 11. Rates of sodium consumption versus Na/S02 ratios entering the CFSTR for the experimental set: 65-77 107 12. Rates of sodium consumption versus concentra tions of sodium in amalgam entering the CFSTR for the experimental set: 65-77 .... 108 xiii Figure Page 13. Rates of sodium consumption versus concen trations of sodium in amalgam entering the CFSTR for the experimental sets: 95-105, 23-28, 65-77 and 42-46 ..... Ill 14. Concentrations of sodium dithionite in the product stream versus time for runs in the experimental set: 4 2-46 . 115 15. Concentrations of sodium in amalgam leaving the CFSTR versus time for runs in the experimental set: 42-4 6 16. Concentrations of sodium dithionite in the product stream versus time for runs in the experimental set: 23-28 117 17. Concentrations of sodium in amalgam leaving the CFSTR versus time for runs in the experimental set: 23-28 118 18. Concentrations of sodium dithionite in the product stream versus time for runs in the experimental set: 65-77 119 19. Concentrations of sodium in amalgam leaving the CFSTR versus time for runs in the experi mental set: 65-77 120 20. Steady-state values of sodium dithionite concentration and various yields versus Na/SC>2 ratio entering the CFSTR for the experimental set: 23-28 123 21. Steady-state values of sodium dithionite concentration and various yields versus Na/SC>2 ratio entering the CFSTR for the experimental set: 4 2-46 124 22. Steady-state sodium dithionite concentrations versus Na/S02 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed ^25 23. Steady-state sodium dithionite concentrations versus concentrations of sodium in amalgam entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed . . . 127 xiv Figure Page 24. Critical concentrations of sodium in amalgam entering the CFSTR versus molar ity of sulfur dioxide in the aqueous solutions entering the CFSTR 128 25. Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/SC>2 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed 129 26. Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/S02 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed 130 27. Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/S02 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed . 131 28. Steady-state conversions of sodium to different products in the CFSTR versus Na/SC«2 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed 1329. Steady-state values of sodium dithionite concentration and various yields versus Na/S02 ratios entering the CFSTR for the experimental set: 86-90 137 30. Steady-state sodium dithionite concentrations versus Na/SC^ ratios entering the CFSTR at different levels of agitation in the aqueous feed , 139 31. Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/SC>2 ratios entering the CFSTR at different levels of agitation in the aqueous feed . . 140 32. Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/SC>2 ratios entering the CFSTR at different levels of agitation in the aqueous feed 141 XV Figure Page 33. Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/SC»2 ratios entering the CFSTR at different levels of agitation in the aqueous feed 142 34. Steady-state conversions of sodium to different products in the CFSTR versus Na/S02 ratios entering the CFSTR at different levels of agitation in the aqueous feed . 143 35. Steady-state values of sodium dithionite concentration and various yields versus Na/S02 ratios entering the CFSTR for the experimental set: 94-104 145 36. Steady-state values of sodium dithionite concentration and various yields versus Na/SC>2 ratios entering the CFSTR for the experimental set: 95-105 . 146 37. Steady-state sodium dithionite concentrations versus Na/S02 ratios entering the CFSTR at different flow rates of the aqueous feed . . 147 38. Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/S02 ratios entering the CFSTR at differ ent flow rates of the aqueous feed 14 8 39. Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR ver sus Na/S02 ratios entering the CFSTR at different flow rates of the aqueous feed . . 149 40. Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/SC>2 ratios entering the CFSTR at differ ent flow rates of the aqueous feed 150 41. Steady-state conversions of sodium to different products in the CFSTR versus Na/S02 ratios entering the CFSTR at different flow rates of the aqueous feed . . 151 XVI Figure Page 42. Steady-state values of sodium dithionite concentration and various yields versus Na/S02 ratios entering the CFSTR for the experimental set: 122-134 .......... 156 43. Steady-state values of sodium dithionite concentration and various yields versus Na/S02 ratios entering the CFSTR for the experimental set: 123-135 157 44. Steady-state sodium dithionite concentra tions versus Na/S02 ratios entering the CFSTR at different values of interfacial-area/aqueous-volume ratio 158 45. Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/S02 ratios entering the CFSTR at different values of interfacial-area/aqueous volume ratio 159 46. Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/S02 ratios entering the CFSTR at different values of interfacial-area/aqueous-volume ratio 160 47. Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/S02 ratios entering the CFSTR at differ ent values of interfacial-area/aqueous-volume ratio 161 48. Steady-state conversions of sodium to different products in the CFSTR versus Na/S02 ratios entering the CFSTR at differ ent values of interfacial-area/aqueous-volume ratio 162 49. Steady-state values of sodium dithionite concentration and yield of sodium dithionite on sulfur dioxide in the aqueous feed versus interfacial-area/aqueous-volume ratios at different Na/S02 ratios entering the CFSTR 163 50. Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/S02 ratios entering the CFSTR at different steady-state temperatures of the aqueous phase 166 xvii Figure Page 51. Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/SG^ ratios entering the CFSTR at different steady-state temperatures of the aqueous phase . 167 A-I. Flow rate of mercury pumped by Moyno pump versus micrometer setting on Graham transmission A~3 A-II. Flow rate of aqueous sulfur dioxide versus reading on the rotameter scale A-4 A-III. Flow rate of cooling water versus reading on the rotameter scale . Ar-5 A-IV. Millivolt output of iron-constantan thermocouples versus temperature °C . . . . A-6 A-V. RPM of the propeller versus micrometer setting on the thyratron controller for the variable speed drive (Heller motor) . . xviii LIST OF TABLES TABLE PAGE 1. Conditions for Brightening Groundwood by Na2S2°4 23 2. Precision of the Amalgam Analytical Procedure ..... 74 3. Precision of the Sulfur Dioxide Analytical Procedure 76 4. Process Variables and Their Units ....... 93 5. Levels of the Process Variables in Set: 47-57 96 6. Levels of the Process Variables in Set: 65-777. Levels of the Process Variables in Set: 66-76 99 8. Levels of the Process Variables in Set: 62-63 ..... ...... 99 9. Levels of the Process Variables in Set: 87-91 ........ 99 10. Levels of the Process Variables in Set: 95-105 ....... 110 11. Levels of the Process Variables in Set: 23-28 1112. Levels of the Process Variables in Set: 42-46 110 13. Levels of the Process Variables in Set: 86-90 13g 14. Levels of the Process Variables in Set: 94-104 1315. Levels of the Process Variables in Set: 122-134 . 155 xix TABLE PAGE 16. Levels of the Process Variables in Set: 123-135 155 17. Levels of the Process Variables in Set: 106-118 1618. Levels of the Process Variables in Set: 110-113 165 19. Levels of the Process Variables for the Run 44 19C-I. Measured Values, Range of Random Errors and Variances of different measured quanties (amalgam analytical procedure) C-7 F-I. Steady-State Data and Results for Experimental Set: 47-57 F-14 F-II. Steady-State Data and Results for Experimental Set: 65-77 F-15 F-III. Steady-State Data and Results for Experimental Set: 66-76 F-16 F-IV. Steady-State Data and Results for Experimental Set: 62-63 ..... F-17 F-V. Steady-State Data and Results for Experimental Set: 87-91 F~l8 F-VI. Steady-State Data and Results for Experimental Set: 95-105 F~l9 F-VII. Steady-State Data and Results for Experimental Set: 23-28 F~20 F-VIII. Steady-State Data and Results for Experimental Set: 42-46 F-21 F-IX. Steady-State Data and Results for Experimental Set: 86-90 F-22 F-X. Steady-State Data and Results for Experimental Set: 94-104 F-23 XX TABLE PAGE F-XI. Steady-State Data and Results for Experimental Set: 122-134 F-24 F-XII. Steady-State Data and Results for Experimental Set: 123-135 F-25 F-XIII. Steady-State Data and Results for Experimental Set: 106-118 F-26 F-XIV. Steady-State Data and Results for Experimental Set: 110-113 ....... F-27 F-XV. Unsteady-State Results for Experimental Run 42 F-28 F-XVI. Unsteady-State Results for Experimental Run 43 F-2F-XVII. Unsteady-State Results for Experimental Run 44 F-29 F-XVIII. Unsteady-State Results for Experimental Run 4 5 F-2F-XIX. Unsteady-State Results for Experimental Run 46 F-30 F-XX. Unsteady-State Results for Experimental Run 23 F-3F-XXI. Unsteady-State Results for Experimental Run 24 F-31 F-XXII. Unsteady-State Results for Experimental Run 25 F-3F-XXIII. Unsteady-State Results for Experimental Run 26 F-32 F-XXIV. Unsteady-State Results for Experimental Run 27 F-3F-XXV. Unsteady-State Results for Experimental Run 28 F-33 F-XXVI. Unsteady-State Results for Experimental Run 65 F-3XXI TABLE PAGE F-XXVII. Unsteady-State Results for Experimental Run 67 . F-34 F-XXVIII. Unsteady-State Results for Experimental Run 69 F-3F-XXIX. Unsteady-State Results for Experimental Run 71 F-35 F-XXX. Unsteady-State Results for Experimental Run 73 . F-35 F-XXXI. Unsteady-State Results for Experimental Run 75 F-36 F-XXXII. Unsteady-State Results for Experimental Run 77 F-31 CHAPTER I INTRODUCTION Groundwood pulps, especially those obtained from western softwood species, are dark in colour and must be brightened for use in newsprint manufacture. Brightening is done usually with a solution of zinc dithionite (ZnS20^), manufactured on the mill site from zinc dust and aqueous sulfur dioxide solution, according to the reaction: Zn + 2S02 ZnS204 This process is economical and efficient but suffers from one serious drawback. That is, the zinc ion which remains in the spent bleach solution must be discharged to the available effluent receiving waters. Since zinc ion is an active biocidal agent (46, 37), this is very object ionable from the standpoint of water pollution control. Already the use of zinc dithionite is prohibited in some areas and in others, pressure is developing to prohibit its use. For a mill which can not use zinc dithionite, the available alternative is to purchase manufactured sodium 2 dithionite (^2820^). Sodium dithionite is available in the market as the anhydrous crystalline salt. It is quite unstable in aqueous solution or when it carries water of crystallization (Na2S204' 2H20). The cost of ZnS204 produced in pulp mills (62) is approximately 16<:/lb of $2®^ Ion. The cost of crystalline ^2820^ bought in the market (65) is approximately 39<:/lb of $2°^ i°n« Preferred application of sodium dithionite for the brightening of ground-wood pulp is 0.2 - 1.5% of the fibre weight (62, 106). For the following example, it was assumed that in the bleaching plant, the concentration of sodium dithionite is 1% of the fibre weight. For a mill producing 500 tons per day of groundwood pulp, forced to change from ZnS20^ produced in situ to crystalline ^2^2^^ bought in the market, the extra cost involved would be about one million dollars per year. A good part of the cost in purchased crystalline sodium dithionite is incurred in the crystallization and subsequent drying steps. It would be possible to reduce substantially the cost of sodium dithionite for mill use if it could be manufactured in situ in a simple "once through" reactor and the solution fed directly to the groundwood bleaching plant. One possible means of doing this would be by the sodium-mercury amalgam route (Chapter II). In this process, a solution of sulfur dioxide (or sodium bisulfite) in water is allowed to react with sodium dissolved in mercury to produce a solution of sodium dithionite. This approach may be of a special interest to pulp mills where chlorine and caustic soda are manufactured in mercury cells. Considering the fact that ^2820^ (~39<Vlb of 4 ^on ^n tne annY^rous crystalline salt) is a more valuable product than NaOH [-2.75C/lb(65)], sodium-mercury amalgam produced in the brine electrolysis unit could be used as a relatively inexpensive source of sodium to produce sodium dithionite. In other words, it might be advantageous to divert a part of the sodium-mercury amalgam to the proposed sodium dithionite manufacturing unit. The rest of the amalgam could go to the conventional "decomposer" to produce caustic soda. The sodium-depleted amalgam from the decomposer and the sodium dithionite manufacturing unit could be sent back to the brine electrolysis unit. The sodium amalgam process for making sodium dithionite has been investigated quite extensively. A summary of the literature search is given in Chapter II. Although a good deal of information has been developed on this process, it is directed primarily at producing concentrated solutions of sodium dithionite for easy recovery of the solid salt. The reason behind this was probably to compete with the existing zinc dust: sodium carbonate method (Chapter II), which has been used industrially to manufacture anhydrous sodium dithionite crystals. For the proposed sodium amalgam process, solutions obtained could be as dilute as 1 to 2% Na^S-,0. . It can be 4 shown from a mass-balance that if an approximately 1% solution of sodium dithionite is manufactured, it would not change appreciably the pulp consistency which is employed in the normal groundwood brightening process (62, 106). Based on the above discussion the problem for the present research was defined. It was decided to investigate the effect of different process variables on the yields of sodium dithionite produced as a relatively dilute (approxi mately 1-2%) water solution from sodium-mercury amalgam and sulfur dioxide in a simple "once through" reactor. The manufactured aqueous solution would be used directly for the brightening of groundwood pulp. It was also hoped that the investigation would lead to a better understanding of the sodium amalgam process and might yield sufficient information for the design of a semi-commercial or a commercial plant. Since it was decided to have no recycle of sulfur dioxide in this investigation, yields of sodium dithionite on * sulfur dioxide entering t and on sodium consumed in a single pass must be economical. Yield of sodium dithionite on total sulfur dioxide entering the reactor (%), (gm molar cone, of Na2S20^ in product) x 2 x 100 Y — 2 (gm molar cone, of total SO- in aqueous feed) 5 A rough economic assessment of a sodium amalgam process to produce sodium dithionite solution in situ can be made. In the manufacture of chlorine and caustic soda using mercury cells, the cost of production may be apportioned to these two products on a weight basis (57). Thus, the cost of chlorine gas and caustic soda is almost equal. The market value of caustic soda can be taken as approximately 2.75<Vlb (65). An evaluation of the sodium in the amalgam would be approximately 5$ (40/23 x 2.75 = 5) per pound. The cost of crude sulfur varies depending on the market conditions but may be taken as 1.8£/lb (65). The cost of sulfur dioxide gas produced by oxidation of this sulfur would be about 0.9$ plus processing cost for a total of about 1.5C/lb. The reaction between sodium and sulfur dioxide is given stoichiometrically by the equation: 2 Na + 2S02 -*• 2 Na+ + * Yield of sodium dithionite on sodium consumed in the reactor (%) , (gm moles of Na^SpO. in product/min) x 2 x 100 CONNA = gm moles of Na entering with fresh amalgam/min -gm moles of Na leaving with spent amalgam/min 6 If it is assumed that a 100% yield of sodium dithionite on sodium consumed is obtained and the yield of sodium dithionite on sulfur dioxide entering is 100%, then the cost of sodium dithionite (chemical cost only) would be about 3.3<?/lb of S20~ ion. This cost can be compared with the cost of anhydrous sodium dithionite crystals bought in the market (=39<:/lb of S2O4 ion) as well as the cost of zinc dithionite produced in situ (~16<:/lb of ^>2Q~i ion) . If reasonably high yields of sodium dithionite on sulfur dioxide entering and on sodium consumed could be obtained by the proposed sodium amalgam process, it seems possible to decrease the cost of sodium dithionite substantially for mill use. It is also possible that the cost of dithionite ions produced by the proposed sodium amalgam process may not be very different from the cost of dithionite ions in the zinc dithionite produced in situ. 7 CHAPTER II REVIEW OF PERTINENT PRIOR WORK A. Manufacturing Processes for Sodium Dithionite A number of methods are available in the literature describing preparation of sodium dithionite. All of them are based on the reduction of sulfur dioxide (or sodium bisulfite). The ones which could be used commercially are: 1. Zinc dust: sodium carbonate process 2. Electrolytic or cathodic reduction process 3. Sodium formate process 4. Sodium borohydride process 5. Sodium amalgam process. Brief outlines of these processes are given below. Special emphasis has been put on the sodium amalgam process because of its similarity to the proposed process. 1. Zinc dust: sodium carbonate process Sodium dithionite is manufactured by reacting an aqueous solution of sulfur dioxide with a stirred suspension of zinc dust in water and then converting the zinc salt into the sodium salt by the addition of sodium carbonate (8-10, 8 17, 19, 30, 73, 76, 78, 94, 98, 148). The chemical reactions involved are: Zn + 2S02 -> ZnS204 ZnS204 + Na2C03 •*• ZnC03 + Na2S204 After the removal of the zinc carbonate by filtration, Na2S204. 2H20 is salted out by the addition of NaCI. Dehydration and drying steps follow to make the anhydrous salt, Na2S204. Badischen Anilin - und Sodafabrik (Germany) used this method industrially (9, 10). A 20% solution of sodium dithionite was crystallized, dehydrated, then dried. This German firm obtained a yield of 65-75% based on sulfur dioxide feed and about 70% on zinc. Virgina Chemical Inc. of the U.S.A. has also been reported to use this method industrially. Some of the disadvantages of the zinc dust:sodium carbonate process to produce sodium dithionite are: (a) The market for zinc oxide, which is a by-product, is contracting. Zinc dust is quite expensive, and the initial cost incurred in the raw material is not compensated by revenue from the sale of zinc oxide. (b) The process is discontinuous and involves the extra steps (compared to the other processes) of pre cipitating zinc carbonate and calcining the precipi tated carbonate to obtain zinc oxide. 9 (c) Zinc is always present as an impurity in the Na2S204 and zinc ions, as mentioned earlier, are a pollution hazard if released with mill effluent. (d) Dehydration and drying steps to make stable anhydrous Na2S204 are complicated and expensive. 2. Electrolytic or cathodic reduction process In this method dithionite ion is produced cathodically (8, 10, 21, 39, 40, 61, 71, 75, 131, 132, 135, 136, 139, 142-144, 147) according to the reaction: 2HSO~ + 2H+ + 2e -»• S20= + 2H20 Although the cathodic reduction of sulfur dioxide (or bisul fite) solution in water has been studied repeatedly since 1904, this method has never been used industrially. The process is reported to give low yields of sodium dithionite on sulfur dioxide and low current efficiencies. However, in a recently published investigation based on laboratory experiments, Oloman (67, 68) claims that ^28204 produced by this method could compete with ZnS20^ produced in situ. 3. Sodium formate process This process was invented by Kinzlberger & Co. (122, 123). In this process sulfur dioxide is dissolved in a solu-10 tion of sodium hydroxide in methanol. The sulfur dioxide is reduced in turn, by a suspension of sodium formate in aqueous methanol at a temperature of about 70°C according to the reaction: HCOONa + NaOH + 2S02 * 2Na+ + S^ + C02 + H20 The product is filtered, washed with methanol and then dried to give anhydrous sodium dithionite. According to Gerritsen (30) this process was commer cially used in the past, but it was dropped because the product was a very finely divided powder and therefore not very stable to oxidation. Recently some investigators (128, 141, 146, 151, 152), on the basis of laboratory experiments, have reported high yields of sodium dithionite on sulfur dioxide (=80%) and high purity of the product (==90%) from the formate process. 4. Sodium borohydride process In this process an aqueous solution of sulfur dioxide (or sodium bisulfite) is reduced by sodium borohydride (140, 150) according to the reaction: NaBH. + 8 NaHSO- -*• NaB0o + 6 H„0 + 4 NaoS„0. So far this method has not been used industrially. 11 5. Sodium amalgam process In this process sodium metal dissolved in mercury is used to reduce sulfur dioxide to give sodium dithionite. The overall reaction can be written as follows: 2 Na + 2S02 •*• Na2S204 The standard oxidation potential of pure sodium metal is greater than that of sodium in sodium-mercury amalgam (35, 36, 57). The overvoltage of hydrogen gas on sodium-mercury amalgam is high (57) . Thus it is possible to bring about a controlled reaction between sodium in mercury solution and various other reactive compounds in aqueous solution, with little loss due to reaction of the sodium with water. (a) Advantages of the sodium amalgam process (i) Compared to the zinc dust: sodium carbonate process, which is used industrially, extra steps are avoided such as separation of zinc carbonate and calcining the precipitated carbonate to obtain zinc oxide. (ii) Sodium dithionite obtained by this process is not contaminated by the zinc ions as is the dithionite obtained by the zinc dust: sodium carbonate method. 12 (iii) The reactor could work as the decomposer in con junction with mercury cells which may be used for chlorine manufacture at a mill site. (iv) As the reducing agent is a solution, continuous reduction can be easily carried out. (v) Sodium-mercury amalgam is an inexpensive source of sodium metal,particularly if the sodium is produced in a mer cury chlor-alkali cell. (b) Types of the sodium amalgam process Sulfur dioxide in a number of forms may be reacted with sodium-mercury amalgam to produce sodium dithionite. Some of these forms are: (i) Sulfur dioxide dissolved in organic solvents (ii) Gaseous or liquid sulfur dioxide (iii) Sulfur dioxide introduced into aqueous buffer containing NaHSO^ and Na2S0.j. (i) Sulfur dioxide dissolved in organic solvents (sodium amalgam: SG^ - organic solvent process) A number of investigators (49, 83, 117, 126) have pre pared sodium dithionite from sodium-mercury amalgam and sulfur dioxide dissolved in non-aqueous media such as hydrocarbons, ethers, amides, alcohol and kerosene. They have claimed high 13 yields of the anhydrous product which is obtained directly. According to the information available, this process has never been used industrially. (ii) Gaseous or liquid sulfur dioxide (sodium amalgam: gaseous process and sodium amalgam: liquid SO2 process) Rougeot (89) reported finding Na2S20^ in the reaction mass when SO2 was bubbled through sodium-mercury amalgam for 7 to 8 hours, or when liquid sulfur dioxide was in contact with sodium amalgam for 2 to 3 hours at room temperature. According to the information available, this method has never been used industrially. (iii) Sulfur dioxide introduced into aqueous buffer containing NaHS03 and Na2S03 (Sodium amalgam: S02-NaHS03/Na2S03 buffer process) This method has been a subject of interest for many investigators (10, 18, 19, 26, 56, 64, 69, 120, 121, 124, 129, 130, 132-134, 137, 145). In this process sodium metal dissolved in mercury reduces the sulfur dioxide added to an aqueous buffer solution of NaHS03 and Na2S03, at a pH of about 5 to 6 and at a temperature of about 15 to 30°C, to give a concentrated solution of sodium dithionite. Hydrated sodium dithionite crystals (Na2S204« 2H20) are salted out by the addition of sodium chloride and the slurry is taken to a 14 filtration unit where the crystals are removed and the mother liquor is sent back to the reactor. These hydrated crystals are then dehydrated by heating them rapidly to a temperature of about 60 to 65°C/ after which they are filtered, washed with alcohol and dried under vacuum. High yields of sodium dithionite and high purity of the product have been reported. Some other investigators (74, 125, 138, 149) have recommended that a certain percentage of water-miscible alcohol should be added to the aqueous buffer solution to improve the recovery of anhydrous sodium dithionite. The steps in the reaction for this process are as follows: Formation reaction 2Na + 2NaHSO_ NaoSo0. + 2NaOH Neutralization reaction NaOH + NaHSO, + Na-SO-, + Ho0 Overall formation and neutralization reaction 2Na + 4NaHSO_ -»• Na^S-O. + 2Na~SO., + 2Ho0 Regeneration reaction 2Na2S03 + 2S02 ->- 4NaHSO 15 Overall reaction 2Na + 2S02 Na2S204 This method of preparation of sodium dithionite was used industrially (30 to 40 tons Na2S204/month) from 1923 to 1930 by Farbenfabriken Bayer in Leverkusen (9) but it was plagued by poor yields and formation of unwanted by-products. As late as the early 1940's they had concluded that the zinc dust: sodium carbonate process was the most economical route to sodium dithionite unless the price of zinc were unusually high and provided that the by-product zinc oxide could be sold (26) . J. Olmestead (66) reported that R.B. MacMullin Associates of Niagara Falls, N.Y. had built a sodium dithionite plant some years ago, using the sodium-mercury amalgam route, for the Marathon Paper Company, but that this operation was discontinued for some unknown reason. B. Recommended Conditions for Improving the Yield of Sodium Dithionite in the Sodium Amalgam: S02-NaHS03/Na2S03 Buffer Process A brief description of this process has been given on pp. 13-15.Although the objective of the proposed investigation (study of the sodium-mercury amalgam process for the manufacture 16 of a relatively dilute water solution of sodium dithionite) is quite different from the aim of most of the previous investigations (manufacture of a solution of sodium dithionite, generally 15-25% solution, for efficient recovery of the solid salt), it was felt important to consider the recommended levels of process variables for the latter case as they might yield some useful information for the proposed investigation. A number of investigators (10, 18, 19, 26, 56, 64, 69) have spelled out conditions for increasing the yield of sodium dithionite by the sodium amalgam: SG^ - NaHS02/Na2SO.j buffer process. Their conclusions are given below. It was generally recognized that conditions of extreme acidity and high temperatures in the reactor increase the rate of decomposition of sodium dithionite. Therefore, to obtain reasonable yields of sodium dithionite, it was found necessary to maintain the reactor pH between 5 and 6 and the reactor temperature in the range 15 to 30°C. It was also found that sodium dithionite is rapidly oxidized by atmospheric oxygen; therefore, an inert atmosphere of nitrogen (or carbon dioxide) was provided in the entire apparatus. The concentration of sodium in the inlet-sodium amalgam was kept in the range .01% to .05% (by weight) and sulfur dioxide was introduced in stoichiometric proportions to the sodium fed from the amalgam. The residence time of the aqueous solution * The word "yield" has not been defined very carefully in the available literature. In most cases it appears that the authors considered the yield of sodium dithionite on sodium consumed. 17 in the reactor was kept as short as possible to avoid the decomposition of sodium dithionite formed. The aqueous solution was well agitated in the reactor in order to disperse the sulfur dioxide and to renew the solution at the interface. Some other investigators (74, 125, 138, 149) modified the process by using a 20-30% ethanol solution in water rather than water alone as the solvent for NaHSO^- Na2S02 buffer. The presence of ethanol helped in the recovery of better crystals of sodium dithionite. Most of the information mentioned above has been obtained from the patent literature and different investigators used varied experimental devices and contacting methods for amalgam and aqueous phases. Unfortunately, very little effort has been made to improve understanding of the sodium amalgam process in general. For the proposed "once through" process, recommen dations such as the addition of 20 to 30% ethanol to the buffer solution were considered uneconomical. Some other recommen dations provided guide lines for the proposed investigation. C. Mercury Contamination of Sodium Dithionite Produced by the Sodium Amalgam: S02 - NaHSO^/NaSO^ buffer process At least two investigators (10, 19) have reported that sodium dithionite obtained by the sodium amalgam: S09 -18 NaHS03/Na2S03 buffer process was contaminated with small amounts of mercury. However, it is not indicated whether the mercury was entrained in the product stream or some chemical compounds of mercury were present. Farbenfabriken Bayer (10) manufactured sodium dithion ite industrially in a reactor where sodium-mercury amalgam was thoroughly mixed with the sulfur dioxide solution in the presence of a buffer solution of NaHS03 and Na2S03(pH 5-7). Sodium dithionite produced by their process was contaminated with mercury, but they reported that it was possible to prepare a product with not more than 20 ppm mercury. At that time the mercury content of their product was not considered objectionable in the bleaching of groundwood pulp. Dijs, Hoogland and Waterman (19) manufactured a 15% solution of sodium dithionite in a packed bed (column packed with glass rods). Sodium-mercury amalgam entered the reactor at the top of the column, divided into fine droplets which, after interaction with sulfur dioxide in the presence of a 5% NaHS03 and Na2S03 buffer (pH 5-5.5), gathered at the bottom of the column. The buffer solution flowed up the column, counter current to the amalgam, and the product stream was recycled to the bottom of the reactor. The operation was made continuous by introducing an aqueous solution of sulfur dioxide to the feed and bleeding off an equal volume of the product stream. According to Dijs et al • tne product stream was sometimes contaminated with mercury. 19 Removal of small amounts of mercury from the product stream is relatively easy. Dijs et al, recommended a solution of Na2S to precipitate small amounts of mercury as HgS, which might be removed by filtration. Recently, Imperial Chemical Industries, England, has patented (127) a process to remove mercury from the waste brine of mercury cells. Generally, the waste brine contains 3 to 4 ppm of Hg and this mercury can be removed by treating the brine with a solution of NaHS and subsequently filtering the suspension to remove HgS. The same principle can also be used to remove mercury from the sodium dithionite solution. D. Lignin Preserving Bleaching of Groundwood Pulp by Sodium Dithionite(62, 106) 1. Definition of the terms "brightening" and "bleaching" When referring to groundwood, the term "brightening11 is generally used to describe the whitening action of reducing agents such as the sulfites and dithionites. The term "bleaching" generally refers to the action of oxidizing agents, such as peroxides, which modify the coloured substances of the wood pulp for a longer period of time. However, the term "bleaching" is sometimes used to describe the action of both dithionites and peroxides. 2. Characteristics of the groundwood bleaching process The essence of the groundwood bleaching process can be summarized as follows: (a) A major requirement in groundwood bleaching, in contrast to chemical pulp bleaching, is that lignin must be retained. (b) Unlike chemical pulp brightness, unbleached groundwood brightness varies widely with wood species and, for each species, varies with the condition of the wood being ground. Therefore, a great deal of care in cleaning and selecting wood must be taken so that all groundwood produced can be raised to the desired brightness by the available processes at an acceptable cost. (c) The brightness of groundwood after bleaching is much more unstable than that of bleached chemical pulps. The bright ness reversion rate is greater after the reducing treatment by dithionites than after the oxidizing action of peroxides. This is understandable, considering that the oxygen of the air will tend to reverse the reducing action of the dithionites. For papers of limited use, like newsprint, groundwood brightening with dithionite gives just adequate brightness gains [10-12 G.E. brightness points with 1-2% (by weight) dithionite on fibre]. Since peroxide bleaching costs are between 5 and 10 times as high as for dithionite brightening, the latter has gained wide acceptance in the production of newsprint. 21 3. Effects of groundwood brightening The action of dithionite on groundwood pulps has two major effects; (a) The sheet reflects more light, and hence it becomes brighter. (b) The colour of the sheet shifts appreciably from yellow-pink towards a bluer shade, and so it becomes whiter. This is because dithionite treatment affects particularly the substances which absorb strongly the blue components of light and are responsible for the yellow-pink appearance of unbleached pulps. 4. Conditions for groundwood brightening by sodium dithionite In the chemical action of dithionite during groundwood brightening, the oxidation state of the sulfur atom changes 3+ 4 + from S to S , that is, from dithionite to sulfxte or bi sulfite (62). Depending on pH, the reaction can be written in two ways. In acid solution, dithionite reacts as follows: 2H20 + HS20~ :^=£ 2HS0~ + 3H+ + 2e The standard oxidation potential of this reaction at 25°C on the hydrogen scale is E° = + .08 volts (51). In strongly alkaline solution the reaction is: 22 S204 + 40H ~ " 2S03 + 2H20 + 2e The standard oxidation potential of this reaction at 25°C on the hydrogen scale is E° = + 1.12 volts. Thus, dithionite is a far stronger reducing agent in alkaline than in acidic solution. However, high alkalinity causes degradation and dis colouration of wood lignin; so, in practice, it is necessary to carry out the dithionite treatment at pH levels 1 to 2 units below neutrality. The instability of dithionite solutions to acids, oxygen and high temperature has been mentioned earlier and will be discussed in detail in section II. G. The mixing of the groundwood pulp with dithionite should be rapid, intimate and uniform; and the pulp should be as free of air as possible. Ranges for permissible and preferred conditions (62) for brightening groundwood from most species are shown in Table 1. 23 TABLE 1 CONDITIONS FOR BRIGHTENING GROUNDWOOD BY NaoSo0 Process Variable Permissible Conditions Preferred Conditions Temperature Reaction Time pH Consistency Dithionite Treatm % (by weight) of Fibre 32 to 82°C 1/2 to 8 hrs 5 to 7.5 3 to 18% tent .2 to 2% 50 to 74°C 1 to 2 hrs 5 to 5.6 3 to 6% .2 to 1.5% 24 E. Sodium-mercury Amalgam For the proposed continuous manufacture of sodium dithionite, it would be necessary to use liquid sodium-mercury amalgam. It has been reported in the literature (35, 36) that the solubility of sodium in mercury, at 25°C, is about 0.6% by weight. If the sodium dithionite manufactur ing unit was to work in conjunction with a mercury cell, sodium in mercury would be available at a certain composition, and that would, without the use of some special equipment, put an upper limit on the concentration that could be used. Normally, the maximum concentration of sodium in the amalgam produced in the mercury cell is about .05 to .15% by weight (2, 28). In the proposed process, the maximum concen tration of sodium in the inlet-amalgam would be about 0. 15% by weight. The following information available in the literature on some of the important properties of liquid sodium-mercury amalgam is pertinent. 1. Molecular structure of sodium-mercury amalgam According to Vanstone (107-109) and Schuller (92), the discontinuities in the freezing point diagram and in the specific volume versus weight per cent curve, as well as the micrographic study of mixtures of sodium and mercury, indicate that there are six compounds of the two elements 25 in solid sodium-mercury amalgam. However, the molecular structure of liquid sodium-mercury amalgam remains uncertain. Vanstone (107, 109) and Bent (7) determined the specific volume, electrical conductivity, oxidation potential, depres sion of freezing point, lowering of vapour pressure and heat of formation of liquid sodium-mercury amalgams. On the basis of their results, it can be assumed that liquid sodium-mercury amalgams are monoatomic, true solutions of sodium in mercury. 2. Surface tension of sodium-mercury amalgam The surface tension of pure mercury has been deter mined against air and vacuum at different temperatures (16). Schmidt (90) determined the surface tension of mercury against air at 20°C and reported a value of 435.5 dynes/cm. According to Hohn (35) the surface tension of mercury is lowered when sodium is dissolved in it. 3. Sensitivity to oxidation of sodium-mercury amalgam Liquid sodium mercury amalgam is highly sensitive to oxidation by atmospheric oxygen. According to Hohn (35), when the liquid sodium amalgam is exposed to air it is immediately coated with a grey film of oxide. A thin layer of paraffin oil has been recommended to avoid oxidation. 26 4. Density of sodium-mercury amalgam Copious data on the specific volume of sodium-mercury amalgam, in liquid as well as solid state, has been reported in the literature (4, 58, 79, 107-109, 112). Maey's (58) data can be used to plot density of the liquid amalgam at a temperature of 17°C against weight per cent of sodium in the amalgam upto a concentration, of 0.3% sodium. A regression fit on his data gives the following expression. Density of sodium amalgam at 17°C = 13.55 - .9986 x weight per cent of sodium in amalgam It can be seen from this expression that the density of liquid sodium-mercury amalgam decreases with increasing concentration of sodium in the above-mentioned range. F. Sulfur Dioxide Solution in Water 1. Principal equilibria On the basis of numerous investigations (5, 24, 25, 29, 31, 42, 96, 99-101, 115) it can be concluded that when sulfur dioxide is dissolved in water, the following equilibria exist. 27 SO- , %+ H-0 -7— S0-» HO -—»- HSO" + H+- SO- + 2H+ 2 (g) 2 2 2 3 3 It is interesting to note that at very low pH's, an aqueous solution of sulfur dioxide does not contain H2S03 molecules; sulfur dioxide exists in the molecular state. Simon and Waldmann (100) , working with Raman spectra and aqueous solu tions of sulfur dioxide of concentration greater than 1 molar, detected lines attributed to S2Og, SU(3<3es^n9 that another ionic equilibrium exists in the system, namely 2HS03 -—^ S205 + H20 Not much is known about this equilibrium, hence, the presence of S2OJT ions has been ignored in the present investigation. Thus, the principal equilibria may be written as follows: Khs S02(g) + H2° — S02 " H2° Kl + S02« H20 , H + HS03 K2 HS0~ , H+ + S03 where, = Thermodynamic equilibrium constant for S02-H20 system 28 aS02'H20 [S02'H20] Khs = = ~Z * fS0o-Ho0 * * * x aTI „ P„„ 2 2 S02 H2° S02 K.^ = Thermodynamic ionization constant for the first dissociation of S02» H20 aH+ X aHSOl [H+][HSOl] fH+ X fHSOl ± = . J ... .(2) aS02«H20 [S02«H20] fS02*H20 K0 Thermodynamic ionization constant for the second dissociation of S02 • H20 aH+ X aS0= [H+][SOl] fH+ X fS0= = ^— • .... (3) aHSO~ [HS03] fHSO-In the equations (1), (2) and (3) 'a' is the activity of the indicated species. [ ] is the concentration in gram ions or gram moles per liter of the enclosed species. 'f is the activity coefficient of the indicated species. P is the pressure of sulfur dioxide gas in atm. bU2 The values of , K2 and K^g have been reported by several investigators (93, 111, 118). According to Scott 29 (93), at 25°C, K± = .0127; K2 = 6.28 x 10~8; and Khg = 1.233 gm moles/liter atm. In the proposed investigation the concentration of total sulfur dioxide (S02 + HSO^ + SO3) in the aqueous solutions used was in the range 0.4 molar to 1.3 molar. No data is available to-date on the molar activity coefficients of the sulfur dioxide molecules dissolved in water and mean molar activity coefficients of hydrogen and sulfite ions. Mean molar activity coefficients of hydrogen and bisulfite ions have been reported (34, 77) for very dilute solutions -3 -3 of sulfur dioxide in water (3.27 x 10 to 12.6 x 10 molar) . In the absence of any reasonable data, for the purposes of this investigation, the activity coefficients were assumed to be unity. Hence, the principal equilibria, at 25°C, could be written as follows: [S09'H,0] - - = 1.233 ... .(4) [H+][HSO~] — = .0127 ... .(5) [S02'H20] [H ][SO"] [HSO3] = 6.28 x 10 -8 . . . .(6) The concentration of total sulfur dioxide in the aqueous solution could be written as follows: [S02] + [HSO~] + [SO3] = [S02]TOTAL . . . .(7 If the concentration of total sulfur dioxide in the aqueous solution and the pH were known, the concentrations of all the ionic species and the dissolved sulfur dioxide in molecular state could be calculated using the equations (5), (6) and (7). A computer program was written using these equations and for a typical [S*-^ Total = molar, the concentrations of different species of sulfur dioxide in the aqueous solution were calculated. These concentrations are plotted against pH (.1 to 9) in Figure 1. It can be observed from this figure that at very low pH (pH - 1) , most of the sulfur dioxide in the aqueous solution is avail able as molecular sulfur dioxide; the remainder is present as bisulfite ions. As the pH increases, the proportion of bisulfite ions increases and reaches a maximum at a pH of about 4.5. With further increase in the pH the concentration of sulfite ions starts building up and above a pH of 9, most of the sulfur dioxide is available as sulfite ions. The conclusions arrived at by the author, agree with the following comments made by Eriksen (22), despite the latters usage of slightly different ionization constants Figure 1 Concentrations of free sulfur dioxide, bisulfite ions and sulfite ions versus pH in a 0.39 9 molar sulfur dioxide solution in water at 25°C (K± = 1.54 x 10~2; K2 = 6.9 x 10 1) . "More than 97 per cent of the sulfur atoms in the solutions were contained as sulfur dioxide molecules in a solution of pH 0.3 and more than 99 per cent as bisulfite ions at pH 4.5 and as sulfite ions at pH 9.0. This selection of different pH values seems to be the closest one could come to achieving pure solutions of these molecules or ions." 2. Diffusion of sulfur dioxide in water Eriksen (22, 23) investigated the self-diffusion of 35 sulfur dioxide in aqueous solutions using S as a tracer. He used Anderson and Saddington's (1) assumption that there was no interaction between the three diffusing species, namely, SC^, HSO^ and SO^". He calculated the composition of a saturated solution of sulfur dioxide in water at 20°C and atmospheric pressure, finding 1.6 molar SG^ and 0.15 molar HSO^", i.e. about 11 per cent was ionized. Therefore, he concluded that the diffusion coefficient of sulfur dioxide in water could not be ascribed to any one diffusing species but might be treated as an "effective diffusion coefficient." He reported the integral concentration weighted (effective) diffusion coefficient of sulfur dioxide -5 in a saturated aqueous solution at 20°C to be 1.45 x 10 2 cm /sec. This value of effective diffusivity is comparable to the values obtained by several other investigators (33, 48, 55, 72, 105, 113). 33 According to Eriksen the diffusivities of the differ ent species of sulfur dioxide in water are unequal, and the relative amounts of molecular SC^, HSO^ ano^ S0~ will vary with pH for a fixed concentration of total sulfur dioxide in the system. Hence, the "effective diffusion coefficient" will be a variable with pH. The effective diffusion coeffic ient for sulfur dioxide, in aqueous (NH^^SO-j, as a function of pH is shown in Figure 2 (22). It can be seen from Figure 2 that the effective diffusivity of sulfur dioxide decreases with increasing pH of the solution. The effect of temperature, total ionic concentration and type of cations on the effective diffusion coefficient of sulfur dioxide in aqueous solutions were also investigated by Eriksen. G. Important Reactions in the Proposed Process After carefully studying different manufacturing processes for sodium dithionite (including the patent literature), it was possible to outline some of the major reactions that may take place if a dilute aqueous solution of sodium dithionite were to be produced by contacting sodium-mercury amalgam and a solution of sulfur dioxide in water (proposed process). These reactions are: Figure 2 Effective diffusion coefficients for sulfur dioxide in aqueous (NH^JoSC^. 1 Total1 sulfur dioxide concentration = I gm mole/liter 35 1. The sodium dithionite formation reaction 2. The water reaction 3. The sodium dithionite decomposition reactions 4. The sodium dithionite oxidation reaction . Unfortunately, very little information has been reported about the kinetics and mechanisms of these reactions. The avail able information is as follows: 1. The sodium dithionite formation reaction Although the mechanism of dithionite formation on sodium-mercury amalgam is not exactly known, it can be postu lated (44) that the atomic sodium in the amalgam reduces bisulfite ion to give sodium dithionite according to the reaction: 2Na + 2HS0~ + 2Na+ + + 20H~ The mechanism of the formation reaction has been in vestigated by several workers (32, 44, 48) using polaro-graphic techniques. They studied the reduction of sulfur dioxide in aqueous solutions at a dropping mercury electrode. None of the investigators found reduction waves at dropping mercury electrodes with neutral or alkaline sulfite solutions, but well-defined waves were observed in acid medium. In other words, aqueous solutions of sulfur dioxide were reduced 36 in strong as well as weakly acidic media. In strongly acidic medium (pH 0 to 2) , the reduction of aqueous sulfur dioxide to dithionite takes place in one step at a half-wave potential, = -0.37 volts (reduction potential, at pH = 1 and temperature = 25°C, measured against a saturated calomel electrode) (32, 48). The reaction could be written as follows: 2S02 + 2e -* S204 In weakly acidic medium (pH 6), the polarograph shows two waves (32, 44, 48), one at E^y2 = -0.67 volt and the other at Ejy2 = -1.23 volts (reduction potentials, at pH = 6 and temperature = 25°C, measured against a saturated calomel electrode). According to Ketelaar (44) , at both of these half-wave potentials, dithionite is formed exclusively. This is contradictory to the explanation of Kolthoff and Miller (48), who reported that the step at -1.23 volt is connected with the formation of thiosulfate according to the empirical equation: 2HSO~ + 4H+ + 4e -»• + 3H2<J or 2HSO~ + H„0 + 4e + S„o!T + 40H~ Ketelaar concluded that thiosulfate is formed at still more negative potentials, and hydrogen is liberated at the same time (E-jy2 = volts). At more negative potentials, further reductions occur, which lead to the formation of sulfide ions and other products. The sulfite ions are not reducible at these potentials. Ketelaar postulated that the reduction reaction at E^^2 = -0.67 volt may be written as: 2HSO~ + 2e S204 + 20H" . . . .(8) However, the reaction based on the step at Ejy2 = -1-23 volt is uncertain. According to Ketelaar, this reaction could be written as follows: HS03 + S03 + 3H20 + 4e •*• 2HS02 + 50H HS02 + HS03 -»• S204 + 2H20 The total reduction at ^^y2 =-1'23 volts could be written: 3HS03 + S03 + H20 + 4e 2S204 + 50H . . . .(9) When the formation reaction (9) is combined with the equilib rium between HS03 and S03 ions in the aqueous medium, HSO~ + OH~ S03 + H20 38 the resultant formation reaction is equivalent to equation (8) . Previous investigators (30, 44) assumed that when sodium-mercury amalgam is brought in contact with an aqueous solution of sulfur dioxide,-the dithionite formation probably takes place at the interface of the two phases. The author has attempted to explain this assumption as follows: According to information available in the literature, sodium cannot exist in aqueous solutions in atomic form. Sodium, from the amalgam gets ionized at the interface to give an electron: Na -*• Na+ + e Free electrons or hydrated electrons (95) have not been known to exist in aqueous solutions for any measurable length of time. They would react with bisulfite ions and hydrogen ions rapidly. This may be taken as a justification of the assumption that the reduction by sodium-mercury amalgam probably does not take place inside the aqueous medium. No data has been reported on the solubility of aqueous sulfur dioxide in sodium-mercury amalgam or pure mercury. It is believed that the aqueous solution of sulfur dioxide is insoluble in the amalgam. Hence, the reduction reactions would not take place inside the amalgam phase. Having accepted the assumption that the dithionite formation takes place at the interface of the two phases, attention was directed to determine if the reaction is reversible or irreversible. The equilibrium constant for the reaction 2Na + 2HSG~ •*• 2Na+ + + 20H~ , at 25 °C, was calculated by using the standard free energy of formation values (51) of different atomic and ionic species for the reaction at 25°C. The value of the equilibrium 67 constant, at that temperature was approximately 10 . Such a high value of the equilibrium constant implies that the reaction can be considered an irreversible reaction at 25°C. No data is available in the literature on the rate of the sodium dithionite formation reaction. However, it has been suggested by previous investigators (30, 44) that the rate of the dithionite formation on the amalgam surface is exclusively dependent on the mass-transfer . rate of bisulfite ions on the aqueous side and the supply of sodium from the amalgam to the interface. This implies that compared to the mass-transfer rate in the two phases, the chemical reaction rate is infinitely fast. 2. The water reaction Avedikian (125), in 1956, was granted a patent for the manufacture of sodium dithionite by the sodium amalgam process. He reported that, in his process when the con centration of sodium in the amalgam was greater than .04% (by weight), some of the sodium was consumed in what he called the water reaction. This is an unproductive reaction compared to dithionite formation, and produces sodium hydroxide and gaseous hydrogen thus lowering the yield of sodium dithionite. According to him the water reaction may be written as follows: 2Na + 2H20 -»• 2 NaOH + H2 The evolution of hydrogen has also been reported by Ketelaar (44) in his investigations. It was felt necessary to investigate this reaction at different pH values, for it would compete with the sodium dithionite formation reaction. The reactions of sodium-mercury amalgam with different acids, buffer solutions, water and sodium hydroxide have been studied by several investi gators (12, 20, 45). Unfortunately, none of the investigators studied the rate of the water reaction from the point of view of the modern approach to mass-transfer with chemical reaction as outlined by Astarita (3). Different investigators experimented under entirely different conditions, hence, it is difficult from the information available in the literature, to formulate a mathematical model which would predict the rate of the water reaction under a given set of conditions. 41 The most reliable work has been done by Dunning and Kilpatrick (20) and their conclusions are as follows: The rate of the water reaction is proportional to the square root of concentration of sodium in the amalgam, -4 independent of hydrogen ion concentration in the range 10 to 10-1^ moles/liter, and proportional to the area of the interface. Thus, the empirical equation could be written as follows: dC Na dt k . A V~Z water C . . . .(10) Na where, 'Na water Na dt Concentration of sodium in the amalgam, gm moles/liter Surface area of the sodium-mercury amalgam 2 in contact with water, cm Reaction rate constant Rate of dissolution of sodium from the amalgam into water, gm moles/liter sec Presence of certain electrolytes in the aqueous phase also affects the rate of the water reaction; however, no quantitative relationship has been proposed. Dunning and Kilpatrick (20) and Fletcher and Kil-patrick (27) also showed that the rate of dissolution of sodium (or lithium) from the amalgam into water increased very sharply when the concentration of hydrogen ions -4 increased above 10 moles/liter. The nature of the empirical rate equation (10) implies that, in the pH range 4 to 10, the rate of the water reaction is not controlled by the mass-transfer rate of sodium to the interface. Dunning and Kilpatrick suggested that the rate is probably controlled by the rate of chemical reaction at the interface. Like the sodium dithionite formation reaction, the water reaction probably also takes place at the interface of the amalgam and aqueous phases. The equilibrium constant for the water reaction, Na + H20 + Na+ + 0H~ + 1/2 H2 , at 25°C, was calculated by using the standard free energy of formation values (51) of the different atomic and ionic species in the reaction. The equilibrium constant, for 3 2 the water reaction, at 25°C is approximately 10 . This high value of the equilibrium constant indicates that the water reaction is essentially irreversible. 43 3, The sodium dithionite decomposition reactions Two types of decomposition reactions should be con sidered. The homogeneous decomposition reaction takes place in all parts of the aqueous phase. Heterogeneous decomposition takes place at the interface of the amalgam and aqueous phases where dithionite is reduced by sodium of the amalgam. (a) Homogeneous decomposition of sodium dithionite Studies (40, 43, 53, 54, 59, 60, 70, 82, 116) of the decomposition of sodium dithionite in various aqueous solu tions, in the absence of oxygen, have shown that at a pH of about 7, the main overall reaction is: 2Na2S204 + H20 -»• 2NaHS03 + Na2S203 Several side reactions occur, however, and there has been little agreement on their importance or products. The kinetics have been in dispute and different mechanisms have been suggested. Spencer (102), in 1967, reviewed most of the previous work and pointed out that the kinetic studies by earlier workers had been in dispute because the rate of decomposition is a function of solution pH and the concentrations of bi sulfite, thiosulfate and dithionite ions; all of which change during the decomposition reaction. By the use of buffered 44 solutions containing high bisulfite concentrations, he avoided the kinetic irregularities of the earlier work. He was also able to elucidate some features of the decomposition reaction mechanism. Sodium dithionite decomposition was followed in standard solutions over a range of concentrations (.015 to .20 moles/liter) and temperature (15 to 40°C). In most of his experiments, the pH of the aqueous buffer containing NaHSO^ and Na2S03 was about 5.2. According to Spencer, two sets of stable products, (Na2S203 + NaHS03) and Na2S30g are obtained. It was also found that about 10% of the overall decomposition reaction gives trithionite ions, even when the concentrations of NaHS03 and Na2S03 are low. Ketelaar (113) also reported the formation of trithionite ions in the homogeneous decomposition reaction. According to Spencer, the decomposition reactions could be written as follows: 2Na2S204 + H20 -»• 2NaHS03 + Na2S2C>3 .... (11) Na2S204 + 2NaHS03 Na^Og + Na2S03+ H20 .... (12) He also postulated that the initial, rate-determining steps are common to both reactions (11) and (12), with the final product distribution being determined by subsequent altern ative routes. The mechanism of the rate determining step was also suggested. It was found that the rate of decomposition is first order in [820^] and, of various solution components, bisulfite ion has the strongest influence on the decomposition rate. The effects of pH, [HSO^] and [S0~] can not be separated rigorously, but it seems probable that the decomposition rate is first order in [HSO3], variable fractional order in [H+] and zero order in [S0~]. The presence of S2Q3 i°ns accelerates the rate of decomposition. In most of the investigations (24, 121-129) on the rate of decomposition of sodium dithionite in aqueous solutions, it was found that an increase in temperature increases the rate of decomposition. Thus, the rate of decomposition of sodium dithionite in aqueous solutions can be given by the following expression. rhomo = kc[S2OI]1[HSOP1[SO3]0[H+]X[S2O3]y ' ' * ' (13) where, k = Reaction rate constant; increases with an in-c crease in the temperature. No data has been reported on its value by Spencer, x = Variable. y = Not determined by Spencer Some idea about the rate of decomposition may be obtained from Spencer's experiments. In concentrated buffer solutions of bisulfite and sulfite (2.38 moles NaHSO.,/liter; 0.25 moles Na2S03/liter) at a pH of 5.2 and a temperature of 20°C, sodium dithionite decomposed from a concentration of 0.135 to .02 moles/liter in approximately 350 minutes. Decomposition of sodium dithionite in the solutions at very low pH levels was investigated by Scholder and Denk (91). According to them, auto-decomposition of sodium dithionite is very rapid in strongly acidic solutions and eventually elemental sulfur is precipitated. The reaction can be written as follows: 2H2S204 -*> 3S02 + S + 2H20 . . . .(4) Decomposition of sodium dithionite in highly alkaline aqueous solutions has also been investigated (10, 60). It was suggested that the reaction can be written as follows: 3Na2S204 + 6 NaOH 5Na2S03 + Na2S + 3H20 Hence, for a sodium dithionite manufacturing process, the conditions of extreme acidity or alkalinity and high temperatures must be avoided to obtain reasonable yields of the desired product. (b) Heterogeneous decomposition of sodium dithionite Chassain and Ostertag (14) reported that when sodium dithionite was prepared by bubbling gaseous sulfur dioxide through sodium-mercury amalgam in the presence of traces of water, some of the dithionite was further reduced by sodium dissolved in mercury according to the following reaction: 3Na2S204 + 2Na -*• 2Na2S203 + 2Na2S03 Kolthoff and Miller (4 8) and Ketelaar (44) , from the polaro-graphic investigation of aqueous sulfur dioxide solutions, also concluded that the dithionite ions can be further reduced according to the equation: S204 + 2H+ + 2e •*• S203 + H20 Ketelaar also observed that at stronger reduction potentials, S204 ion in the aqueous solution was reduced to S~ ion. Thus, when sodium-mercury amalgam is contacted with an aqueous solution of sulfur dioxide,the heterogeneous decomposition of the dithionite ions formed at the interface can be written schematically as follows: Reduction by Reduction by Reduction by 2 4 2 3 n . . . .(15) No data has been reported, in the literature, on the rates or mechanisms of these reactions. Like the sodium dithionite formation and the water reaction, the heterogeneous 48 decomposition reaction probably also take place at the inter face of the amalgam and aqueous phases. 4. The sodium dithionite oxidation reaction Since sodium dithionite is a strong reducing agent, it is rapidly attacked in solution and more slowly as a hydrated solid, by common oxidants such as atmospheric oxygen (13, 30, 38, 83) . Many investigations (6, 43, 54, 63, 80, 81) have been done on the kinetics and mechanism of air oxidation of the dithionite ions in aqueous solutions. According to Jouan (43), the oxidation of sodium dithionite by oxygen in aqueous solutions involves two reactions, and sulfite and sulfate ions are the products exclusively. The first reaction is very rapid and involves 82.4 per cent of the dithionite in solution: HoS„0. + 0o + Ho0 -*• Ho0'S0o + HnSO. 224 2 2 2 2 24 Simultaneously, the remaining 17.6 per cent of the dithionite is converted into an unidentified compound of similar reducing power, presumably sulfoxylic acid, which then oxidizes more slowly to sulfite: • (H2S02) + \ 02 H2O.S02 49 According to Rinker et al. (81), the rate of oxidation of sodium dithionite in aqueous solutions by atmospheric oxygen under the conditions where diffusion of air was not the rate controlling step, could be given by the following expression: = V2 roxidation - ko[S2°4] [02] A correlation of the rate constants with temperature was made by use of the Arrhenius equation. A value of 9.3 Kcal/mole was obtained for the activation energy and the frequency 5 -1/2 -1 factor was found to be 7.2 x 10 (moles/liter) (sec) In view of the information mentioned above, an inert atmosphere should be provided in an apparatus which is used to manufacture sodium dithionite. 50 CHAPTER III THEORETICAL MODELS As outlined in the Section II.G., when sodium-mercury amalgam is contacted with an aqueous solution of sulfur dioxide to produce sodium dithionite, the following reactions should be considered: A. The Sodium Dithionite Formation Reaction B. The Water Reaction C. The Heterogeneous Decomposition of Sodium Dithionite D. The Homogeneous Decomposition of Sodium Dithionite E. The Oxidation of Sodium Dithionite. The literature cited in the Section II.A.5,b.iii. shows that, although several investigators have tried to develop the sodium amalgam: S02 - NaHSO^/Na^O^ buffer process from the point of view of obtaining good yields of the solid salt, none considered all of the reactions mentioned above. In most of the studies only the sodium dithionite formation reaction, the homogeneous decomposition reaction and the oxidation reaction were considered. Even for such a simplified case, in the absence of kinetic data on the reactions considered, no mathematical models have been suggested which would predict the yields of sodium dithionite 51 on sulfur dioxide and on sodium consumed as a function of the process variables. Ketelaar (44) suggested a mathematical model for the rate of sodium dithionite formation reaction on the basis of the diffusion rates of the reactants in the amalgam and the aqueous phases. He also discussed the importance of homogeneous decomposition of the dithionite briefly. Gerritsen (30) attempted to correlate some process variables to the yield of sodium" dithionite on sodium consumed for a theoretically ideal reactor (the amalgam and aqueous phases perfectly mixed separately). He assumed no loss of dithionite due to oxidation and considered, primarily, the sodium dithionite formation reaction and the homogeneous decomposition of dithionite along with the mass-transfer of the reactants in the two phases. In the absence of mass-transfer coefficients and kinetic data, he arrived at the following general conclusions. According to Gerritsen, yields of sodium dithionite on sodium consumed could be improved by using large interfacial-area/aqueous-volume ratios, large mass-transfer rates of sulfur dioxide, high concentrations of sulfur dioxide in the aqueous phase and low rates of homogeneous decomposition of the dithionite. For the present investigation, where dilute aqueous solutions of sodium dithionite were produced, mass-transfer of the different reacting species in the amalgam and aqueous 52 phases along with all of the major reactions mentioned above should be considered to formulate a theoretical model. The oxidation of sodium dithionite in the aqueous solution could be avoided by providing an inert atmosphere of nitrogen in the entire apparatus used to manufacture sodium dithionite and by using fresh, oxygen-free distilled water to prepare the sulfur dioxide solution. Already, a lot of information has been developed on the homogeneous decomposition of sodium dithionite in acidic, aqueous solution, and a quantitative relationship may be obtained by studying this reaction in the samples taken from the experimental reactor. Although some information (II.G.2.) is available on the rate of the water reaction, it is not known how this rate is quantitatively affected by the electrolytes present in the experimental reactor. It would be very difficult to isolate the water reaction, the sodium dithionite formation reaction and the heterogeneous decomposition of dithionite for kinetic investigation separately because they are parallel-series reactions. Other than lack of reliable kinetic data on the major reactions, the formulation of a theoretical model is further complicated by the following: 1. The diffusion of ionic species in the aqueous phase (50) may not be treated as molecular diffusion. 2. The mass-transfer of sodium in the amalgam may increase due to the Marangoni effect. 3. Some products may get adsorbed at the mercury surface. Thus, it was difficult to suggest a theoretical model for the proposed process from the information available. However, a qualitative model has been postulated on the basis of the information in the literature and the experiments carried out by the author. This model will be discussed in Chapter VI. 54 CHAPTER IV EXPERIMENTAL A. Experimental Materials 1. Sodium-mercury amalgam A known quantity of distilled mercury was taken in a stainless-steel container and covered with a thin layer of paraffin oil. Sodium metal was cut under paraffin oil and cleaned to remove the layer of oxide formed on its surface.The desired amount of clean sodium metal was slowly added to the mercury with constant agitation, and was allowed to amalgamate. (The layer of paraffin oil prevents oxidation of the sodium amalgam by the atmospheric oxygen). The purity of the chemicals and some problems involved in the preparation of amalgam are presented in Appendix C. 2. Aqueous sulfur dioxide solution Solutions of sulfur dioxide gas in freshly distilled water were made by using an absorption column (2 1/2" x 30") filled with 1/2" Raschig Rings. This solution was always kept under an atmosphere of nitrogen to avoid oxidation • The absorption column could prepare solutions up to 1.4 molar sulfur dioxide in water. When pH of the SC^ solution required adjustment, a concentrated aqueous solution of sodium hydroxide was added. The purity of chemicals used in the preparation of aqueous sulfur dioxide solutions is given in Appendix D. B. Experimental Apparatus The experimental apparatus used in this study is described schematically in Figure 3. Legend for Figure 3 has been enclosed in the following pages. For the sake of clarity, the types of valves used in the apparatus have not been described in the flow-sheet or the legend. For precise con trol of flow rate (e.g. the flow rate of the aqueous sulfur dioxide solution), a needle valve made of stainless-steel-316 was used. For less accurate control of flow rates, vee valves made of stainless-steel-316 were utilized. When an on-off control of flow rates was required, toggle-operated valves made of stainless-steel-316 were employed. Some important features of the experimental apparatus are pre sented below. 1. Reactor To obtain the desired information, a continuous-flow-stirred-tank reactor (CFSTR) was designed with aqueous and CYCLE Figure 3 Schematic flow-sheet of the experimental apparatus 57 LEGEND FOR FIGURE 3 A. Amalgam and Product Cycle: 1. Fresh amalgam storage, 8 liter polyethylene bottle 2. Moyno pump (Type 3M1, SSQ). Positive displacement pump for pumping amalgam to the reactor (0-730 ml/ min) 3. Graham transmission; variable speed drive for Moyno pump (0-450 rpm) 4. Amalgam cooling heat-exchanger 5. Variable speed stirrer in the amalgam heat-exchanger (200-1400 rpm) 6. Plexiglass connector 7. Fresh amalgam sampling place 8. Stainless-steel-sheathed iron-constantan thermocouple 9. Pyrex CFSTR 10. Propeller mixer (1 1/2" dia) mounted on 5/16" stainless-steel shaft 11. Heller motor; variable speed drive (0-3000 rpm) 12. Thyratron controller for Heller motor 13. Plexiglass level controller for amalgam layer in the reactor 14. Spent amalgam sampling place 15. Stainless-steel-sheathed iron-constantan thermocouple 16. Spent amalgam reservoir (1" x 12" glass) 17. Plexiglass level controller for spent amalgam in the reservoir 58 Legend for Figure 3 (continued) 18. Spent amalgam storage, 8 liter polyethylene bottle 19. Plexiglass connector 20. Product sampling place 21. Stainless-steel-sheathed iron-constantan thermocouple 22. Burette filled with the product 23. Erlenmeyer flask with Rubine-R solution 24. N2 gas cylinder 25. Combination pH electrode 26. Thermocompensator B. SO_2 Cycle: 27. Distilled water storage tank, 10 gallon polyethylene bottle 28. Absorption column (2.1/2" x 30") packed with Raschig Rings 29. S02 solution storage tank, 20 gallon polyethylene bottle 30. Centrifugal pump; Eastern type D-ll 31. Rotameter (0-400 ml/min) 32. Plexiglass connector 33. S02 solution sampling place 34. Stainless-steel"sheathed iron-constantan thermocouple 35. S02 solution cooling heat-exchanger 36. Variable speed stirrer in SC»2 solution heat-exchanger (200-1400 rpm) 37. S09 gas cylinder 59 Legend for Figure 3 (continued) 38. N2 gas cylinder C. Cooling Water Cycle 39. Cooling water storage tank, 10 gallon enamelled container 40. Centrifugal pump, Eastern type E-1 41. Rotameter (0-3920 ml/min) 60 amalgam phases forming an interface. This was selected rather than a packed, spray or a tubular reactor because all of the variables could be investigated independently and conveniently. This reactor might not be the ideal commercial reactor, but it was convenient to provide information on the proposed process. One of the require ments for the present investigation was to design a CFSTR that could be scaled-up, if necessary, for a future semi-commercial or full-scale plant. The reactor consisted of a 4" x 6" Pyrex pipe section. The amalgam/aqueous solution interface was about 4 inches in diameter. The reactor assembly is shown in Figure 4 along with the applicable legend. The major parts of the reactor are described in the following paragraphs. Initially it was planned to stir the two phases in the reactor perfectly but independently. This would have required separate stirrers and baffles for each phase. To simplify the reactor design for the present investigation, no stirrer or baffles were provided in the amalgam phase. Mixing in the amalgam phase was due to flow of the amalgam. The part of the reactor which contained the aqueous phase was designed for perfect mixing using the recommended dimensions given by Sterbacek and Tausk (103) . It was decided to use a centrally-located marine propeller (Part 2), fixed to a rotating vertical shaft (Part 3) and driven by a variable speed drive (Heller Figure 4 Reactor assembly 62 LEGEND FOR FIGURE 4 Part No. 1 4" x 6" Pyrex pipe section 2 1 1/2" diameter 3 blade stainless-steel marine propeller; angle of tilt=42° from verticle 3 5/16" stainless-steel shaft 4 nylon bushing 5 stainless-steel packing gland 6 teflon washer 7 stainless-steel collar for the teflon washer 8 stainless collar 9 four baffles (stainless-steel) 10 stainless-steel slab 11 1/5" thick stainless-steel disc mounted on part 10 12 1/5" thick stainless-steel disc mounted on part 11 13 stainless-steel bottom flange 14 polyethylene weir 15 stainless-steel top flange 16 combination pH electrode 63 motor), in the aqueous phase. The advantages of the propeller mixer were its high speed, axial flow pattern and great pumping effect, which permits short mixing times. The aqueous sulfur dioxide solution was introduced just above the propeller. The flow patterns were such that the solution was forced to the interface primarily by axial flow, and then mixed with the rest of the aqueous medium by radial flow, axial flow and tangential flow. The shaft of the propeller entered the reactor through a nylon bushing (Part 4) in a stainless-steel packing gland (Part 5). The lower end of the bushing was sealed by a teflon washer (Part 6), held in place by a stainless-steel collar (Part 7) which, in turn, rested on a circlip. Another stainless-steel collar (Part 8) was mounted on the propeller shaft above the bushing, such that the distance between its lower face and the top end of the bushing was approximately 1/16". Distilled water was injected into this gap periodically to lubricate the shaft in the bushing. This collar also restricted any movement of the bushing in the packing gland. Four baffles (Part 9) were provided in the aqueous phase to promote perfect mixing conditions. To ensure that stirring in the aqueous phase did not agitate the amalgam phase significantly, the baffles in the aqueous phase extended only to the interface. The sodium-mercury amalgam entered at the bottom of the reactor centrally and then it flowed outward radially over a stainless-steel slab (Part 10). The hold-up of the amalgam in the reactor could be changed, without changing the interfacial area, by using 1/5 inch thick stainless-steel discs (Parts 11 and 12). The surface area of the interface could be changed by opening the bottom flange (Part 13) of the reactor and introducing thin stainless-steel discs with annular holes of various diameters. These discs were pressed against the bottom of the baffles. The product stream containing sodium dithionite flowed over a polyethylene weir (Part 14), thus fixing the height of the aqueous medium. The weir rested on top of the Pyrex pipe section and it was kept in its position by the top flange (Part 15) made of stainless-steel. Holes were provided in the top flange for inserting a pH electrode, the sulfur dioxide inlet tube, and the nitrogen inlet and outlet tubes. Holes were provided in the bottom flange for introducing fresh sodium-mercury amalgam and removing the spent amalgam. 2. pH measurement of the aqueous phase The combination pH electrode was located in the reactor and the thermocompensator placed in the product stream. The pH output was recorded continuously. The specifications of the instruments used are presented in Appendix A. When the combination pH electrode was placed in the reactor, it became contaminated repeatedly. This was prob ably caused by entry of the aqueous solution resulting from the greater pressure exerted on the reference-electrode liquid junction by the aqueous phase in the reactor than the hydro static pressure of KCl-solution inside the reference electrode. However, the combination pH electrode worked satisfactorily when the reference electrode was pressurized by about 15 psig N2 through the refill aperture. 3. Temperature measurement of different streams The temperatures of the inlet sulfur dioxide solution, product stream, fresh amalgam and spent amalgam were measured by stainless-steel-sheathed iron-constantan thermo couples. These temperatures were recorded digitally with the aid of electronic instruments including a digital clock, digital millivolt meter, scanner, multiplexer and a printer system (printer and printer controller). The system used to measure the temperatures digitally is out lined schematically in Figure 5. The specifications of the different temperature measuring instruments used are given in Appendix A. 66 MAIN IRON-CONSTANTAN THERMOCOUPLES 115 V, 60 C/S S02 SOLN. FRESH AMALGAM SPENT AMALGAM DITHIONITE SOLN. DIGITAL CLOCK O U m tt tt SCANNER o o ca 1 DIGITAL m-VOLT METER o u 00 MULTIPLEXER BCD PRINTER SYSTEM T PRINT-OUT Figure 5 Digital temperature measurement (schematic) 67 4. Insulation of the equipment The heat of formation of the sodium dithionite at 25°C was calculated. A value of AH° = -88.29 Kcal/gm mole implies that the dithionite formation reaction is highly exothermic. To maintain low temperatures in the reactor, the inlet streams were cooled and most of the equipment was insulated with glass-wool. 5. Electrical wiring diagram A schematic wiring diagram for the experimental apparatus is shown in Figure 6. C. Calibration Curves The following calibrations were done and the cali bration curves have been attached in Appendix A: 1. Flow rate of mercury pumped by the Moyno pump against micrometer setting on the Graham transmission. 2. Flow rate of aqueous sulfur dioxide against reading on the rotameter scale. 3. Flow rate of cooling water against reading on the rotameter. 208 V,60 C/S EASTE PUMP 2 A THYRATRON CONTROLLER -EXCHAN. "T^^^Y TIRRER-1 \ pr/ 33 3 A Ar-/-HEAT-S HEAT- EXCHAN. STIR RER-2 LIGHT BULB DIGITAL CLOC DIGITAL M-VOLT METER 'J3 SCANNE MULTIPLEXER P CONT RINTER "7^^" ONTROL JLyL nter 33 pH METER 3A , AT-/-1A • 1 A A/-1 A AT 1 A 1 A Ar-1 A Ai-1 A A/-1 A •Ar-N1G 12 A r 3 A Figure 6 Electrical wiring diagram (schematic) 69 4. Millivolt output of iron-constantan thermocouples against temperature (°C). 5. RPM of the propeller against micrometer setting on the thyratron controller for the variable speed drive (Heller motor). D. An Experimental Run Sodium-mercury amalgam, the sodium content of which was known approximately, was prepared and stored in the fresh amalgam storage bottle under a thin layer of paraffin oil. A concentrated aqueous solution of sulfur dioxide was prepared in the absorption column and stored, under N2, in the sulfur dioxide storage bottle. The concentration of total sulfur dioxide in the solution was determined and then it was diluted with distilled water, so that it was slightly above the desired concentration. The pH of this solution was adjusted in the desired range, by the addition of concentrated sodium hydroxide solution. The amalgam and sulfur dioxide solution streams were then circulated through the heat-exchangers (closing the inlets to the reactor) and cooled by water. Samples were taken from the amalgam and sulfur dioxide-solution streams for analysis and their concentrations were determined. 70 The reactor was flushed with N2 and kept under a slight pressure of nitrogen gas. The cooled amalgam was sent through the heat-exchanger to the reactor and the height of the amalgam layer in the reactor (slightly above the bottom of the baffles) was adjusted by the mechanical level controller. The flow rate of the amalgam was adjusted by the micrometer screw oh -the Graham transmission which was driving the Moyno pump. The propeller in the reactor, driven by the variable speed drive (Heller motor), was started and its rpm was fixed at the desired level with the help of a thyratron controller. The combination pH electrode was then introduced into the reactor and the pH meter was kept in the "standby" position. The cooled sulfur dioxide solution entered the reactor at a very high flow rate until the reactor was filled and then the flow rate was reduced and adjusted to the desired level, using a needle valve and a rotameter. Generally, the amalgam level controller had to be re-adjusted so that the amalgam just touched the bottom of the baffles. The pH meter was turned on and pH of the aqueous phase in the reactor was recorded continuously. The desired temperature of the aqueous phase in the reactor was obtained by controlling the temperatures of the reactant streams. The temperatures of the reactant streams were controlled by the flow rate of the cooling water. The temperatures of the fresh amalgam, spent amalgam, inlet sulfur dioxide solution and the product stream were recorded periodically. 71 Outlet amalgam samples were taken periodically during the run from a sample point on the level controller. From time to time, the product stream was taken from the top of the reactor to the burette (under a N2 atmosphere) and • titrated against Rubine-R dye for its dithionite content. The experimental run was continued until the process attained steady-state. A steady-state was attained when successive titrations with Rubine-R dye gave the same concentration of sodium dithionite in the product stream. E. Analytical Procedures and Errors 1. Sodium-mercury amalgam (a) Analysis of sodium-mercury amalgam The analytical procedure used was similar to that employed by Rinker and Lynn (83) . The sodium-mercury amalgam sample was taken using a hypodermic syringe and injected under distilled water (to avoid oxidation by air) in an Erlenmeyer flask. A known volume of standard H^SO^ was pipetted into it and the Erlenmeyer flask was agitated thoroughly. The sodium dissolved in mercury completely reacted with H^SO^ to give H2 provided there was an excess of I^SO^. Excess of H-SO. was back-titrated with a standard solution of NaOH using phenolphthalein as an indicator. A sample calculation to determine the concentration of sodium in amalgam is presented in Appendix C. (b) Accuracy and precision of the analytical procedure The theory behind the estimation of uncertainty in the results obtained by an analytical method is discussed in Appendix B. The most efficient way to test the accuracy and precision of the proposed analytical method in our laboratory, would have been to make an amalgam of known sodium concen tration and check its sodium content several times by the analytical method. Unfortunately, it was found difficult to prepare an amalgam of known sodium concentration starting with known quantities of mercury and sodium metal (Appendix C). Hence, it was not possible to calculate the accuracy of the analytical method by the above-mentioned approach. However, it was evident from the information available in the literature (52, 97, 104, 110) that the chemical reactions involved in the analytical procedure were irreversible and went to completion in a very short time. Precautions were taken to avoid any systematic errors in various steps of the procedure. Based on the above arguments, it was assumed that the analytical procedure, as applied in the present investigation, provided accurate estimates of sodium content in the amalgam. The precision of the analytical procedure was estimated by the method of "propagation of random error" (Appendix B). In short, the precision of measurement of directly measured quantities, in various steps of the ana lytical procedure, was known. From this knowledge, the 95 per cent confidence limits (precision of the analytical procedure) of the weight per cent of sodium in the amalgam were estimated (Appendix C). In the instances where fairly dilute (- 0.1N) standard solutions of H^SO^ and NaOH were used and sufficiently large samples (= 30 gms) of the amalgam were taken, the analytical procedure was precise enough for the present in vestigation. To illustrate this, three amalgams of different sodium content were analysed by the proposed analytical method. The sodium concentrations of these amalgams, covered the range of sodium content in the amalgams used in the present work. The precision of the method, at 95 per cent confidence level, for determining the concentration of sodium in these three amalgams was estimated (Appendix C) and the results are given in Table 2. 74 TABLE 2 PRECISION OF THE AMALGAM ANALYTICAL PROCEDURE Amalgam Sample No. gm of Na/ 100 gm of Amalgam Precision (95% Confi dence limits) Percentage Precision I .0015 +.0005 -± 33 II .0383 +.0005 *± 1.3 III .1533 +.0005 -± .33 It has been shown in Appendix F that the large imprecision in determining very small concentrations of sodium in the amalgam (e.g. amalgam no. I) did not cause appreciable error in the desired yields and conversions. To verify the estimated precision mentioned above, several samples of spent amalgam were taken from the reactor after steady-state had been attained during an experimental run. These samples were analyzed for their sodium content by the analytical method used. The scatter in the data was caused by: (i) random errors involved in various steps of the analytical procedure, and (ii) the change of sodium content in the spent amalgam depending on the reactor dynamics. 75 The variance was estimated from the experimental data using the equation (B-2). In almost all of the experimental runs, it could be said with 95 per cent confidence that there was no significant difference between the experimentally estimated variance and the variance estimated by the method of propagation of random error. The variances were compared by the *F* test. This conclusion also implied that the fluctuations of sodium content in the spent amalgam caused by reactor dynamics were insignificant compared to the imprecision caused by the analytical method. 2. Aqueous sulfur dioxide solution (a) Analysis of aqueous sulfur dioxide solution The total sulfur dioxide (SC»2 + HSO^ + SO^) in aqueous solution was determined by iodometric analysis as outlined by Vogel (110) and Kolthoff and Belcher (47). A liquid sample of the aqueous sulfur dioxide solution was injected, using a hypodermic syringe, into an excess of standard iodine solution. The excess of iodine in the solution was back-titrated with a standard solution of sodium thiosulfate using a starch-solution as an indicator. The method used to calculate the concentration of total sulfur dioxide in water is presented in Appendix D. 76 (b) Accuracy and precision of the analytical procedure The iodometric method to determine total sulfur dioxide in aqueous solution is a standard text-book method (47/ 110) and is considered to give accurate results by several investigators (22, 93). Three different concentrations of sulfur dioxide in aqueous solutions were used in the present investigation; 0.40 molar, 0.655 molar and 1.30 molar. The method used to estimate the precision of the analytical procedure, at 95 per cent confidence level, in determining the concen tration of total sulfur dioxide in these three solutions has been outlined in Appendix D. The results are presented in Table 3. TABLE 3 PRECISION OF THE SULFUR DIOXIDE ANALYTICAL PROCEDURE Aqueous sulfur dioxide solution No. Moles of Total sulfur dioxide/ liter Precision (95% Confidence limits) Percentage Precision I 0.400 ±0.002 *± 0.5 II 0.655 ±0 .004 = ± 0.6 III 1.300 ±0.007 *± 0.5 77 3. Aqueous sodium dithionite solution When air oxidation was avoided, the product stream from the CFSTR contained mainly S.^^, S20~ and HSO~ (or S0~ or S02 depending on pH) ions. (a) Analysis of sodium dithionite in the product stream The assay of sodium dithionite has been thoroughly studied by a committee and their findings have been published (114) . In its attempt to determine the most suitable method for sodium dithionite analysis, the committee reviewed about fourteen different methods. This committee concluded that it is possible to obtain precise and comparable results by faithful application of the following three methods. (i) The iodine-formaldehyde method (ii) The Rubine-R method (iii) The ammonical-copper method. The iodine-formaldehyde (47) and the Rubine-R (114, 119) methods were modified and then used in the present investigation. A detailed description of these analytical procedures along with sample calculations are presented in Appendix E. The general principle of the iodine-formaldehyde method is as follows. The dithionite ions are oxidized quantitatively to sulfate ions by iodine according to the 78 following reaction: + 3I2 + 4H20 -> 2S0J + 6I~ + 8H+ However, the direct titration is of no practical value since sulfite ions (or bisulfite ions), which are always present with the dithionite ions, also react. This interference can be eliminated by the addition of an excess of formaldehyde which reacts with the dithionite to form formaldehyde-bisulfite and formaldehyde-sulfoxylate according to the reaction: S20^ + 2HCH0 + H20 •*• HCHO'HSO" + HCHO»HSO~ Formaldehyde-bisulfite is inert to iodine, whereas the sul-foxylate reacts readily according to: HCHO'HSO" + 2I2 + 2H20 -*• SOj + HCHO + 4l" + 5H+ Rubine-R is a bright red dye. A solution of Rubine-R is reduced instantaneously by the dithionite ions, to an amber coloured liquid. Although, HSO-j and S^^" ions also reduce Rubine-R, the rate of reaction is very slow and it takes them weeks to reduce a small quantity of the dye solution. Thus, Rubine-R can be considered specific for determination of dithionite ions. In the present investigation, the iodine-formaldehyde and the Rubine-R methods gave comparable results. The Rubine-R 79 method was used most of the time because of its simplicity and better precision. The disadvantage of the Rubine-R method was that it gave only S204 ^n t*ie product stream while S^^, S20~ and HSO-j (or S03) could be determined by the iodine-formaldehyde method. (b) Accuracy and precision of the analytical procedures The most efficient way to test the accuracy and precision of the two analytical methods used in the present investigation would have been to make aqueous sodium dithionite solution of a known concentration and check its sodium dithionite content several times by both methods. It was found impractical to prepare a solution of known Na2S204 content as a primary standard because reagent grade sodium dithionite purchased from the market contains an unknown amount of Na2S204. Fresh stock obtained from the market may contain as high as 90% Na2S204 while old stock may have less than 60% Na2S204 due to the decomposition of Na2S204 to NaHS03 and Na2S2C<3. As mentioned in the last section, the iodine-formaldehyde and the Rubine-R methods have been accepted as the most accurate and precise methods for determining the concentration of Na2S204 in a solution. After steady-state conditions had been obtained in an experimental run, samples of the product stream were taken from the reactor, and were analyzed by both methods. The mean concentration 80 > (gm Na2S2O4/100 ml) of Na2S204, determined by the Rubine-R method, was about 5 per cent higher than the mean concen tration determined by the iodine-formaldehyde method. The scatter in the data for an analytical method was caused by: (i) random errors involved in various steps of the analytical procedure, and (ii) the change of Na2S204 content in the product stream depending on the reactor dynamics. From the experimental data, the precision of the two analytical methods at the 95 per cent confidence level, for determining the concentration of Na2S204 in the product stream was estimated. In most of the cases, the precision of the Rubine-R method derived from experimental data was within ± 1% and the precision of the iodine-formaldehyde method was about ± 5%. The precision of the Rubine-R method was also estimated by the method of propagation of random error. Various steps involved in the Rubine-R method for precision calculations have been outlined in Appendix E. Using this approach, the estimated precision of the Rubine-R method was also found to be within approximately ± 1% for the concentrations of sodium dithionite analyzed in the present investigation. The observation that, at 95 per cent confidence interval, there was no significant difference between the experimentally estimated precision and,the precision estimated by the method of propagation of random error, implied that the fluctuations of ^2820^ concentration in the product stream caused by the reactor dynamics were insignificant compared to the imprecision caused by the analytical method. 82 CHAPTER V EXPERIMENTAL RESULTS A. Batch Experiments On the basis of information available in the litera ture, five possible reactions were outlined in Section II. G that may take place in the proposed process. Some batch experiments showed that, under the experimental conditions of the present investigation, almost all of these reactions would take place. 1. Presence of the sodium dithionite formation reaction Four 50 ml samples of an approximately 0.22% sodium-mercury amalgam (0.22 gms Na/100 gms of amalgam) were taken in four separate beakers. An approximately 2.2 molar solution of sodium bisulfite in water was prepared and the solution was divided into four parts. The pH of the'four portions of the NaHSO^ solution was adjusted to 0.8, 1.7, 4.0 and 9.0 respectively by the addition of concentrated ^SO^ or NaOH. Four 50 ml samples of the NaHSO^ solutions at the different pH's were added to the four amalgam samples respectively and the contents in the beaker were stirred with a glass rod. An approximately 1 ml solution of dilute 83 Rubine-R dye was added to each of the beakers. The bright red colour of the Rubine-R dye disappeared almost immediately in the beakers where NaHSO^ solutions at pH's 0.8, 1.7 and 4.0 were added. In all of the three cases, immediately after the Rubine-R was discoloured, the pH of the aqueous phase was below 7. In the fourth beaker, where the pH of the NaHSO^ solution sample was 9, the Rubine-R dye was not discoloured. These experiments demonstrated that when sodium-mercury amalgam is brought in contact with an aqueous solution of sodium bisulfite, sodium dithionite is produced at a rapid rate. The results also agree with the literature information presented in section II.G.l that the sodium dithionite formation reaction can take place only when the pH of the aqueous solution is in the acidic range. This reaction can be written as follows: 2Na + 2HSO~ -»• 2Na+ + + 20H~ . . . . (16) 2. Presence of homogeneous decomposition of sodium dithionite in an acidic solution Approximately 20 gms of the anhydrous sodium dithionite (reagent grade salt) was dissolved in a liter of oxygen-free distilled water (this is the maximum concentration of Na~S904 expected in the present investigation) and the solu-84 tion was stored under an inert atmosphere of N2 gas. The concentration of sodium dithionite in the solution was determined by the Rubine-R method. The solution was then divided into three parts and their pH was adjusted to 0.8/ 1.7 and 5.5 respectively. All these solutions were kept at room temperature (20°C) and under a N2 blanket to avoid oxidation of sodium dithionite. By taking samples from these solutions at different intervals of time and analyzing them for their sodium dithion ite content by the Rubine-R method, it was concluded that the rate of homogeneous decomposition of sodium dithionite increased with decreasing pH. By varying the temperature of a sodium dithionite solution at a fixed pH, it was observed that the rate of homogeneous decomposition of sodium dithionite increased with increasing temperature. These conclusions agree with the information available in the literature (Section II.G.3.a). These experiments were not exhaustive enough to determine a quantitative expression for the rate of decomposition. The following observation of some special significance was obtained from the above-mentioned experiments. No elemen tal sulfur was observed in any of the sodium dithionite solutions kept at 20°C and pH's of 0.8, 1.7 and 5.5 even after 48 hours. This implies that the rate of auto-decomposition of sodium dithionite according to the equation, 85 2H2S204 -»• 3S02 + S + 2H20 , in the pH range 0.8 to 5.5 is negligibly small. It was thought that, in the proposed investigation, the pH of the aqueous medium would not be less than 0.8 and the residence time of the product in the aqueous medium would not be more than an hour, hence, the auto-decomposition of dithionite could be neglected. Thus, only the following homogeneous decomposition reactions, outlined by Spencer (10 2), were considered to occur. 2Na2S204 + H20 2NaHS03 + Na2S203, and (11) NaoSo0. + 2NaHSO, •*• NaoS-0c + NaoS0- + Ho0 (12) 224 3 236 23 2 3 . Presence of the heterogeneous decomposition of sodium dithionite Approximately 50 ml of a 0.22% sodium-mercury amalgam was taken in a beaker. A n approximately 5 ml sample of a 1% sodium dithionite solution in water was added to the amalgam sample and the contents in the beaker were stirred with a glass rod. After a short time H2S gas was detected as one of the products by its distinctive smell. The gas was confirmed to be H2S when it turned lead acetate paper black. The experiment was repeated with approximately 1 ml 86 of the 0.22% amalgam and a large excess (approximately 200 ml) of the 1% Na2S2C>4 solution; no H2S was detected. These results qualitatively agree with the reported (Section II.G.3.b) heterogeneous decomposition of sodium dithionite by sodium-mercury amalgam: Reduced by Reduced by Reduced by S20= *a * S20= £2 S= ^ * S= . . .(15) When the Na/Na2S204 ratio was very high, the reduc tion of S204 took place progressively and eventually S~ ions were obtained which reacted with the H+ ions present in the aqueous medium to give H2S. On the other hand, when the Na/Na2S204 ratio was very small, the reduction probably stopped after the first or the second step and, therefore, no S_ ions were formed to produce H2S. 4. Presence of the water reaction In the batch experiments, at low Na/NaHSO^ ratios and low pH's of the solution (pH 0.8 and 1.7), a gas evolving from the interface could be seen. This gas was not H2S. Most probably it was hydrogen generated due to the water reaction (Section II.G.2) 2Na + 2H20 2 NaOH + H2 ... .(17) 87 At high Na/NaHSO^ ratios, the water reaction probably also takes place. 5. Presence of the sodium dithionite oxidation reaction in an aqueous solution Approximately 10 gms of reagent grade sodium dithion ite salt was dissolved in 1 liter of oxygen-free distilled water and the solution was neutralized by sodium hydroxide to avoid acidic decomposition. This solution was stored under N2 gas at 20°C. The Na2S2C>4 content of this solution was accurately determined by the Rubine-R method. A sample of the standard solution was taken in a beaker; the beaker was exposed to air and its contents were stirred with a glass rod. The concentration of Na2S204 in the sample decreased with increasing time as has been reported in the literature (Section II.G.4). B. Introduction to Experiments in the CFSTR Nine process variables were identified for considera tion. They were; 1. the concentration of sodium in the fresh amalgam, 2. the concentration of total sulfur dioxide in the aqueous feed solution, 88 3. the agitation in the aqueous phase, 4. the agitation in the amalgam phase, 5. the flow rate of the aqueous sulfur dioxide solution (residence time in the aqueous phase), 6. the flow rate of the sodium-mercury amalgam (resi dence time in the amalgam phase), 7. the interfacial-area/aqueous-volume ratio, 8. the temperature of the aqueous phase, and 9. the pH of the aqueous phase. The effect of different process variables and their interactions on yields of sodium dithionite, based on total sulfur dioxide entering the reactor and on sodium consumed in the reactor could possibly be determined if a factorial design of experiments were done at least at five levels of o each variable. This would amount to doing (5) = 1953125 experimental runs. Since considerable time was required for preparing reagents and analyzing products for an experimental run, it was difficult to do more than four to five experi mental runs a week. Doing all possible experiments would have taken an exceptionally long time, and many would not have produced reasonable dithionite yields or dithionite concentrations from the point of view of economics and use respectively. Therefore, the effects of process variables considered of primary importance and their interactions on the yields of sodium dithionite were investigated. For all the experimental runs the depth of the 89 amalgam and the volume of the aqueous layer in the CFSTR were kept fixed. As a result of this arrangement, for the reactor of a specified geometry (Section IV.B.l), the volume of the aqueous layer was approximately 980 ml and the volume of the amalgam layer was about 96 ml. It was mentioned in Section II.G. that probably none of the process reactions take place inside the amalgam phase. The sodium metal dissolved in mercury is transferred to the mercury/aqueous interface where the sodium dithionite formation reaction, the water reaction and the hetrogeneous decomposition of sodium dithionite take place. A critical look at the list of the process variables showed that the effect of variables (4), (6) and (1) should have a similar effect on the rate of mass-transfer of sodium in the amalgam. In other words, an increase in the level of agitation in the amalgam phase by introducing a mechanical stirrer and baffles in that phase, at a fixed concentration of sodium in the fresh amalgam entering the reactor, should increase the rate of mass-transfer of sodium to the interface. Similarly, an increase in the volumetric flow rate of the fresh amalgam at a fixed concentration of sodium in fresh amalgam or an increase in the concentration of sodium in fresh amalgam at a fixed flow rate of fresh amalgam should also increase the rate of mass-transfer of sodium to the interface. Therefore, an increase in the level of any of these three variables should affect the yields of sodium dithionite in the same manner. 90 In the present investigation, the effect of the rate of mass-transfer of sodium to the interface on the yields of sodium dithionite was investigated primarily by changing the concentration of sodium in fresh amalgam and keeping the volumetric flow rate of the amalgam fixed. As mentioned in Section IV.B.l, to simplify the reactor design, no stirrer or baffles were provided in the amalgam phase. Due to a limited supply of mercury available for the experiments, the volumetric flow rate of the amalgam could not be changed significantly. In any case, it was hoped that by a systematic investigation of the other process variables, one could pre dict the effect of a variation in the level of agitation in the amalgam phase or the volumetric flow rate of the amalgam. The effects of the remaining process variables on the yields of sodium dithionite are discussed in the follow-ing sections. C. Definitions of Some Important Quantities which are Used for the Interpretation of Data These quantities have been defined below. The mathe matical expressions to calculate them and sample calculations have been presented in Appendix F. 1. Concentration of sodium dithionite in the product stream, °S204 " (gms of sodium dithionite) (100 ml of the product stream) 2. Yield of sodium dithionite on total sulfur dioxide entering the reactor (%), (gm molar cone, of Na2S20^ in product) x 2 x 100 ^SO ~~ 2 (gm molar cone, of total SG^ in aqueous feed) 3. Yield of sodium dithionite on sodium entering the reactor with the fresh amalgam (%), (gm moles of Na2S20^ in product/min) x 2 x 100 Na (gm moles of Na entering with fresh amalgam/min) 4.Yield of sodium dithionite on sodium consumed in the reactor (%), (gm moles of Na2S20^ in product/min) x 2 x 100 CONNA = gm moles of Na entering with fresh amalgam/min1 -gm moles of Na leaving with spent amalgam/mini 92 5. Conversion of sodium from the amalgam to different products in the reactor (%), gm moles of Na entering with fresh amalgam/min -gm moles of Na leaving with spent amalgam/min x X = — (gm moles of Na entering with fresh amalgam/min) 6. Na/SC>2 ratio entering the reactor, (gm moles of Na entering with fresh amalgam/min) (gm moles of total SC>2 entering with aqueous feed/min) 7. Rate of sodium consumption in the reactor, = (gm moles of Na entering with fresh amalgam/min) -(gm moles of Na leaving with spent amalgam/min) For brevity, symbols were used for the different process variables in the following sections. These symbols along with their units have been described in Table 4. TABLE 4 PROCESS VARIABLES AND THEIR UNITS SYMBOLS DESCRIPTION UNITS CHGF Concentration of sodium in the fresh amalgam gm of Na/100 gm of the fresh amalgam STS02 Concentration of total sulfur dioxide in the aqueous feed solution gm mole/liter (RPM)_ Aq Speed of the marine propeller in aqueous phase revolutions/min FLS02 Flow rate of the aqueous sulfur dioxide solu tion liter/min FLHG Flow rate of the sodium-mercury amalgam ml/min <A/v)Aq The interfacial-area/aqueous-volume ratio 2 3 cm /cm TEMP Temperature of the aqueous phase degree centigrade PH pH of the aqueous phase . , gm ions of H » CO 94 D. Reproducibility of Experimental Runs in the CFSTR Before a detailed experimental investigation could be started, it was decided to check the reproducibility of the experimental runs in the CFSTR. Unfortunately, the following problems made the task somewhat difficult. The sodium-mercury amalgam for the experimental runs was prepared in a batch. It is explained in Appendix C why it was difficult to make an amalgam of a specified sodium-concentration starting with known quantities of mercury and sodium. Generally, it took over an hour to obtain steady-state conditions during an experimental run. The supply of mercury was limited (- 7.5 liters), therefore, it was not convenient to do more than two experimental runs, at the flow rate of the fresh amalgam considered, under identical amalgam concen trations. The results obtained from only two experimental runs under identical levels of different process variables would not offer a very powerful statistical test for re producibility. Hence, the following scheme was used. A set of seven experimental runs (set: 47-57, expts. 47, 50, 52, 53, 54, 55 and 57) was performed under almost identical levels of different process variables except the concentration of sodium in the fresh amalgam which was varied in the range .0403% to .0932%. The experimental conditions for the set: 47-57 have been presented in Table 5. From the observations made, the steady-state Cc n , S2U4 , Y„ , CONNA and X.T and their 95 per cent confidence S02' Na Na r limits were calculated for each experimental run. Sample calculations for these quantities in the experimental run 54 are shown in Appendix F. For all the experimental runs in the set: 47-57, the steady-state Cg Q , Yg0 , YNa, CONNA and XNa were plotted 2 4 2 against the Na/S02 ratio entering the reactor in each run. Thus, five smooth curves were drawn as shown in Figure 7. The 95 per cent confidence limits are not shown on the graphs. It is important to point out that the nature of the curves obtained by plotting steady-state Cg Q , Yg0 , vNa, CONNA 2 4 2 and XNa against the concentrations of sodium in fresh amalgam would be similar because the molarity of the sulfur dioxide solution entering the reactor, STS02, during all of the experimental runs in this set was approximately the same. However, plotting these quantities against Na/S02 ratios offered certain advantages in the interpretation of data as discussed in later sections. At a later date, another set of seven experimental runs (set: 65-77, expts. 65, 67, 69, 71, 73, 75 and 77) was performed using different batches of reagents for the analytical work. The levels of the process variables were nearly the same as for the set: 47-57, except that the concentration of sodium in fresh amalgam was varied in the range .0392 % to .1010 %. The experimental conditions for the set: 65-77 have TABLE 5 LEVELS OF THE PROCESS VARIABLES IN SET: 47 - 57 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM) j^q FLSO2 FLHG (A/V)Aq TEMP PH .0403 - .0932 .639 -.656 673 .096 47.5 .0784 17 5.6 - 6.0 TABLE 6 LEVELS OF THE PROCESS VARIABLES IN SET: 65-77 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLS02 FLHG (A/V)Aq TEMP pH .0392 - .1010 .650 -.658 673 .096 47.5 .0784 1 7 5.4 - 6.0 vo 97 J I I L •2 -3 -4 -5 Na/S02 Figure 7 Reproducibility of the experimental runs in the CFSTR. See Tables 5 and 6 for the levels of the process variables 98 been presented in Table 6. Once again, the steady-state C„ _ , Y„„ , Y„ , CONNA and X.T , calculated from the obser-2 4 2 vations made for each experimental run, were plotted against their respective Na/S02 ratios and are shown in Figure 7. Figure 7 shows that, within the 95 per cent confidence limits there is no significant difference between the steady-state Cg 0 , Yg0 , YNa, CONNA and XNa curves for the two 2 4 2 sets: 47-57 and 65-77. Furthermore, for two experimental runs, run 55 and 73, which were performed under almost identical conditions, it can be said with 95 per cent confidence that there is no significant difference between their respective steady-state C_ _ , Y _ , Y , CONNA and X values. 2 4 2 Further confirmation of the reproducibility of the experimental runs was provided by the results obtained from the set: 66-76 (expts. 66, 68, 70, 72, 74 and 76), set: 62-63 (expts. 62 and 63) and set: 87-91 (expts. 87, 89 and 91). These sets used different batches of the reagents for the analytical work but they basically differed from the sets: 47-57 and 65-77 in that the rpm of the propeller in the aqueous phase was decreased from 673 to 225. The experimental con ditions for the sets: 66-76, 62-63 and 87-91 are presented in the Tables 7, 8 and 9 respectively and the results are shown in Figure 8. TABLE 7 LEVELS OF THE PROCESS VARIABLES IN SET: 66-76 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLSO 2 FLHG (A/V)Aq TEMP pH .0392 - .0892 .650 -.658 225 .096 47.5 .0784 17 5.35-5.95 TABLE 8 LEVELS OF THE PROCESS VARIABLES IN SET: 62-63 Range of the Changed Variable VALUES OF THE FIXED VARIABLES STS02 (RPM) Aq FLS02 FLHG (VV)Aq TEMP PH CHGF .0635 - .1012 .639 -.646 225 .096 47.5 .0784 17 5.7-6.15 TABLE 9 LEVELS OF THE PROCESS VARIABLES IN SET: 87 - 91 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLSO 2 FLHG (A/V)Aq TEMP pH .0372 - .0986 .655 -.658 225 .096 47.5 .0784 17 5.35-5.85 vo vo 100 Figure 8 Reproducibility of the experimental runs in the CFSTR. See Tables 7, 8 and 9 for the levels of the process variables 101 E. Data from CFSTR Experiments 1. Concentration of sodium in fresh amalgam The general plan of the work involved a set of experi mental runs at various concentrations of sodium in fresh amalgam, and at chosen fixed levels of the other process variables. The effects of the concentration of sodium in fresh amalgam on the steady-state values of Cc _ , Y_n , 2 4 2 Yr, , CONNA and X.T were determined at different levels of Na Na each variable. Generally, these calculated quantities were plotted against the Na/S02 ratios entering the reactor. These quantities were also plotted against the concentration of sodium in the fresh amalgam to obtain additional information. The nature of the curves obtained by plotting the steady-state CS 0 ' YS0 ' YNa' C0NNA and XNa a9ainst tne Na/S02 ratios 2 4 2 (or the concentration of sodium in the fresh amalgam) was found to be similar for different sets of the experimental runs. During an experimental run, the unsteady-state concentration profiles of sodium dithionite in the product stream and sodium in the spent amalgam, as well as the pH of the aqueous phase, were also investigated. The effect of varying the concentration of sodium in fresh amalgam on the nature of these unsteady-state curves was also determined. 102 (a) The variation of steady-state values of C S204' YS02/ YNa' CONNA and XNa with change in the Na/S02 ratios entering the reactor The experimental set: 65-77 may be considered a typical set of experimental runs where, as mentioned in Section V.D., the concentration of sodium in fresh amalgam was varied in the range .0392 % to .1010 %. The experimental conditions for this set are presented in Table 6. the concentration of sodium in fresh amalgam in Figure 10. The curves in Figure 9 and 10 were similar because the molarity of sulfur dioxide solution entering the reactor during all of the experimental runs in this set was approximately the same. (i) The variation of steady-state sodium dithionite concen tration in the product stream, C_ n , with change in Na/S02 ratios entering the reactor Steady-state concentrations of sodium dithionite in the product stream are of considerable importance if this product stream is to be used for groundwood brightening. Figure 9 shows the effect on values of steady-state C n caused by changing Na/S02 ratios. The concentration of sodium dithionite in the product stream increases with increasing Na/SO- ratios to a value of Na/S09 equal to 0.29* Steady-state values of Cg Q , Yg0 , YNa, CONNA and X. 2 4 2 were plotted against Na/S0» ratios in Figure 9 and against Na 103 1 I I I I I I L •2 -3 -4 *5 Na/S02 Figure 9 Steady-state values of sodium dithionite concen tration and various yields versus Na/SO~ ratios entering the CFSTR for the experimental set: 65-77. See Table 6 for the levels of the process variables 104 100 90-80 CC * 70 o o CO 60 50 40 30 20 Figure 10 o - 19 - 15 - 11 o o CN CO CM cc E o CM CO o - 1-2 •8 - .4 • \ CONNA 05 •10 •15 20 CHGF (gm Na/100gmAmalgam) Steady-state values of sodium dithionite concen tration and various yields versus concentrations of sodium in amalgam entering the CFSTR for the experimental set: 65-77. See Table 6 for the levels of the process variables At this point, steady-state C reaches its maximum value of 0.9%. At values of the Na/S02 ratio greater than 0.29, the concentration of dithionite drops sharply; then levels ,off to decrease slowly with further increase in the Na/S02 ratio. For the set: 65-77, a Na/SC»2 ratio of about 0.29 corresponds to a sodium concentration in fresh amalgam of .064% (molarity of sulfur dioxide in the aqueous solution was fixed at about 0.65 molar) . (ii) The variation of steady-state yield of sodium dithionite on total sulfur dioxide entering the reactor, Ycn , with change in Na/S02 ratios entering the reactor Figure 9 shows the effect of Na/S02 ratios on the yield of sodium dithionite calculated on sulfur dioxide. The steady-state concentration of sodium dithionite when the Na/S02 ratio is changed. Yield on sulfur dioxide reaches a maximum of 15.6% at the Na/S02 ratio of 0.29. (iii) The variation of steady-state conversion of sodium (from the amalgam) to different products in the reactor, X^a, with change in Na/S02 ratios entering the reactor 2 values of steady-state Y SO respond in a way similar to the As shown in Figure 9, when the steady-state values of XM are plotted against the Na/SO- ratios entering the 106 reactor, a curve is obtained showing a maximum. A maximum . conversion of 94% is obtained at a Na/SC^ ratio of 0.29. For the set: 65-77, the rate of sodium consumption by the aqueous phase (gm moles of sodium entering the reactor with fresh amalgam minus gm moles of sodium leaving the reactor with spent amalgam per minute) is plotted against the Na/SG^ ratios in Figure 11 and against the concentration of sodium in fresh amalgam in Figure 12. These curves demonstrate that when the Na/SC^ ratio entering the reactor or the concentration of sodium in fresh amalgam is increased, keeping the levels of all of the other process variables fixed, the rate of sodium removal from the amalgam phase increases. Figures 11 and 12 also show that at Na/SC^ ratios below 0.29 corresponding to sodium concentration in fresh amalgam of less than .064%, the rate of sodium consumption is linearly related to the Na/SC^ ratio entering the reactor or the concentration of sodium in fresh amalgam. However, at Na/SG^ ratios above 0.29 the rate of sodium consumption is not linearly proportional to the Na/SC^ ratios entering the reactor (Figure 11). A similar result is obtained at CHGF above .064% when the rate of sodium consumption is plotted against the concentration of sodium in fresh amalgam (Figure 12). In Figure 12, when the curve is extrapolated to zero rate of sodium consumption, the line does not go through the origin. For the set: 65-77, it appears that upto 107 "E co Q O CO Ll. o LU 0250h o -0225 E E 3 ^ -0200 O h-Q_ 0175h O O -0150 0125h 0100 Figure 11 •2 -3 -4 Na/S02 Rates of sodium consumption versus Na/S0o ratios entering the CFSTR for the experimental set: 65-77. See Table 6 for the levels of the process variables 108 CHGF (gmNa/100 gm Amalgam) Figure 12 Rates of sodium consumption versus concentrations of sodium in amalgam entering the CFSTR for the experimental set: 65-77. See Table 6 for the levels of the process variables 109 approximately 0.0035% sodium in fresh amalgam, no sodium is consumed by any reaction. A few statistical calculations (95 per cent confidence limits) show that this deviation from the origin can be attributed neither to the errors in the analytical method nor to the calculation technique. To further verify the above-mentioned phenomena, the rate of sodium consumption is plotted against the concentration of sodium in fresh amalgam for different sets of the experi mental runs. For the sets: 95-105 (expts. 95,97,99,103 and 105), 23-28 (expts. 23,24,25,26,27 and 28), 65-77 (expts. 65,67,69,71,73,75 and 77) and 42-46 (expts. 42,43,44,45 and 46), the plots are shown in Figure 13 and the experimental conditions have been presented in Tables 10,11,6 and 12 respectively. Figure 13 shows that for each experimental set at values of CHGF below the steady-state sodium dithionite concentration maximum, the rate of sodium consumption is linearly related to the concentration of sodium in the fresh amalgam (CHGF). But at values of CHGF above the steady-state Na2S204 concentration maximum, the rate of sodium consumption is not linearly proportional to the values of CHGF. Further, for none of the sets does the extrapolated line pass through the origin. (iv) The variation of steady-state yield of sodium dithionite on sodium consumed in the reactor, CONNA, with change in Na/S09 ratios entering the reactor TABLE 10 LEVELS OF THE PROCESS VARIABLES IN SET: 95-105 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLS02 FLHG <Vv)Aq TEMP PH .0439 - .0912 .653 -.660 673 .198 47.5 .07 84 17 5.20 - 5.65 TABLE 11 LEVELS OF THE PROCESS VARIABLES IN SET: 23-28 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLSO 2 FLHG (A/v)Aq TEMP PH ' .0173 - .0544 .397 -.403 673 .096 47.5 .0784 17 5.2 - 5.9 TABLE 12 LEVELS OF THE PROCESS VARIABLES IN SET: 4 2-46 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM) Aq FLSO 2 FLHG (A/V>Aq TEMP PH .0656 - .2127 1.270 -1.310 67 3 .096 47.5 .0784 17 5.0 - 5.75 Ill in) E •048 CD O E E •040 w o lid •032 CO z: o •024 o OD •016 CO LL. O LU i •008 < cr • A O V SET:95-105 SET:23-28 SET-.65-77 SET.-42-46 • 04 -08 -12 -16 -20 CHGF (gmNa/lOOgm Amalgam) Figure 13 Rates of sodium consumption versus concentrations of sodium in amalgam entering the CFSTR for the experimental sets: 95-105, 23-28, 65-77 and 42-46. See Tables 6, 10, 11 and 12 for the levels of the process variables 112 When the steady-state CONNA values were plotted against the Na/S02 ratios for the set: 65-77, the curve shown in Figure 9 was obtained. This curve shows no maximum. The steady-state yield of sodium dithionite on sodium consumed in the reactor is about 71% at a Na/S02 ratio of .176 (small est Na/S02 ratio for the experimental set: 65-77). The steady-state value of CONNA decreases gradually with increas ing Na/S02 ratios to a value of Na/S02 equal to 0.29. At values of the Na/S02 ratio greater than 0.29, the yield of sodium dithionite on sodium consumed in the reactor drops sharply then levels off to decrease slowly with further increase in the Na/S©2 ratio. (v) The variation of steady-state yield of sodium dithionite on sodium entering the reactor with the fresh amalgam, YNa' w^tn change in Na/S02 ratios entering the reactor Figure 9 shows that the form of the curves obtained by plotting steady-state YNa versus Na/S02 ratios and steady-state CONNA versus Na/S02 ratios is similar; the latter having been discussed in the last section. However, at a fixed Na/S02 ratio entering the reactor the steady-state yield of sodium dithionite on sodium consumed in the reactor (CONNA) is always greater than the steady-state yield of sodium dithionite on sodium entering the reactor with fresh amalgam, Y . 113 (b) The variation of unsteady-state concentration profiles with change in Na/SC^ ratios entering the reactor (i) Concentration of H+ions in the aqueous phase For an experimental run, starting with sulfur dioxide in water at pH = 3 to 3.5 and the system in unsteady-state, the pH of the aqueous phase in the CFSTR increases and then attains a steady-state value in the range 5 to 6. Starting with an aqueous sulfur dioxide solution of a fixed concentration and pH, when the Na/SC^ ratio entering the reactor is increased (or the concentration of sodium in fresh amalgam is increased) , the steady-state pH of•< the aqueous phase in the reactor also increases. (ii) Concentration of sodium dithionite in the product stream and concentration of sodium in spent amalgam For the following presentation 'M1 has been defined as the Na/SC^ ratio, for a set of experimental runs, at which maximum steady-state concentration of sodium dithionite is obtained in the product stream. The concentration of sodium dithionite in the product stream versus time curves for two typical sets, sets: 42-46 and 23-28, at various Na/SG^ ratios entering the reactor are shown in Figures 14 and 16 respectively. The concen tration of sodium in spent amalgam versus time curves for 114 these sets are shown in Figures 15 and 17 respectively. The levels of the different process variables, under steady-state conditions, for these sets (42-46, 23-28) have been presented in Tables 12 and 11 respectively. At Na/S02 ratios less than M, for the experimental runs in various sets including the sets: 42-46 and 23-28 (Figures 14 and 16), the concentration of sodium dithionite in the product stream from the reactor increases with time and then attains a steady-state value. Figures 15 and 17 show that the concentration of sodium in the spent amalgam decreases and also attains a steady-state value. At Na/SG^ ratios above M, as can be seen from Figures 14 to 17, transient maxima (for sodium dithionite) and minima (for sodium in spent amalgam) are observed in the unsteady-state concentra tion profiles. However, no such maximum in pH is observed on the pH versus time curves in this region. A better understanding of this phenomenon is obtained by examining the curve for concentration of sodium dithionite in the product stream versus time (Figure 18) and the concen tration of sodium in spent amalgam versus time curve (Figure 19) for the experimental runs in the set: 65-77. The steady-state experimental conditions for this set have been presented in Table 6. As mentioned earlier, for this set, M = 0.29. The experimental run 69 was done at a Na/SC^ ratio <0.29. The pH of the aqueous phase increased from 3.35 to 5.65 and as L_ I I I I I I i I 10 20 30 40 50 60 70 TIME ( min ) Figure 14 Concentrations of sodium dithionite in the product stream versus time for runs in the experimental set: 42-46. See Table 12 for the steady-state levels of the process variables. See Tables F-XV to F-XIX for the unsteady-state results h J I I I I I l_ 10 20 30 40 50 60 70 TIME ( min) Figure 15 Concentrations of sodium in amalgam leaving the CFSTR versus time for runs in the experimental set: 42-46. See Table 12 for the steady-state levels of the process variables. See Tables F-XV to F-XIX for the unsteady-state results i 1 i 1 1 1 1 r TIME (min) Figure 16 Concentrations of sodium dithionite in the product stream versus time for runs in the experimental set: 23-28. See Table 11 for the steady-state levels of the process variables. See Tables F-XX to F-XXV for the unsteady-state results RUN A - 23 0 - 28 • - 27 • - 24 A - 26 • - 25 Na/S02>M -A-A-=•0=0= •tQ= 1 1 -O Cr 30 40 50 60 TIME (min) 70 80 Figure 17 Concentrations of sodium in amalgam leaving the CFSTR versus time for runs in the experimental set: 23-28. See Table 11 for the steady-state levels of the process variables. See Tables F-XX to F-XXV for the unsteady-state results RUN A -75 V -69 O -73 • -67 • -71 • -77 • - 65 1 Na/S02>M l_ 10 20 30 40 50 TIME (min) 60 70 Figure 18 Concentrations of sodium dithionite in the product stream versus time for runs in the experimental set: 65-77. See Table 6 for the steady-state levels of the process variables. See Tables F-XXVI to F-XXXII for the unsteady-state results .A A A. RUN A - 75 ^ " 69 O - 73 • - 67 • - 71 A - 77 • -65 Na /S02>M L) '.Q—rrT^rCr—gfisQ^l rt 9 U V-1 1 10 20 30 40 50 TIME (min) 60 70 Figure 19 Concentrations of sodium in amalgam leaving the CFSTR versus time for runs in the experimental set: 65-77. See Table 6 for the steady-state levels of the process variables. See Tables F-XXVI to F-XXXII for the unsteady-state results 121 shown in Figures 18 and 19 no maximum nor minimum is observed in the unsteady-state curves. The experimental run 73 was also done at a Na/S02 ratio <0.29/ however, the pH of the aqueous phase was fixed at about 5.8 during the unsteady-state as well as the steady-state periods. Again, no maximum nor minimum is observed in the unsteady-state concentration profiles (Figures 18 and 19). The experimental run 65 was done at a Na/S02 ratio >0.29 and the pH of the aqueous phase increased from 3.53 to 6.0 at the steady-state. Figures 18 and 19 show that for this experimental run maximum and minimum are observed in the unsteady-state curves. The experimental runs 71 and 77 were also done at Na/S02 ratios >0.29, however, the pH of the aqueous phase was fixed at about 5.9 under the unsteady-state and the steady-state conditions. In these experimental runs, no maxima nor minima are observed in the unsteady-state concentration profiles (Figures 18 and 19). it >• 2. Concentration of total sulfur dioxide in the aqueous feed solution To investigate the effect of variation in the concen tration of total sulfur dioxide in the aqueous feed solution on the steady-state C„ , Y„ , Y.T , CONNA and XM at different Na/S02 ratios entering the reactor, sets: 23-28, 65-77 and 4 2-46 were performed. The concentration of total sulfur dioxide in these sets was about 0.4 molar, 0.65 molar and 1.30 molar respectively. The levels of the other process variables for these sets, under steady-state conditions, are presented in Tables 11, 6 and 12 respectively. The steady-state Cg Q , YSQ , vNa, CONNA and XNa, for the sets: 2 4 2 23-28, 65-77 and 42-46, are plotted against the Na/S02 ratios entering the reactor, in Figures 20, 9 and 21 respectively. (a) The variation of steady-state sodium dithionite concen tration in the product stream, Cc , with change in b2u4 the concentration of sulfur dioxide in the aqueous feed solution Figure 22 shows that the nature of the curves obtained by plotting the steady-state concentration of sodium dithionite in the product stream against the Na/S02 ratios entering the reactor for sets: 23-28 and 4 2-46 was the same as for the set: 65-77, which has been discussed in Section V.E.l.a.i. Figure 22 also shows that at a fixed Na/S02 ratio, when all of the other process variables are kept constant, an increase in the concentration of total sulfur dioxide in the aqueous feed solution increases the steady-state concentration of sodium dithionite in the product stream. As the concentration of sulfur dioxide in the aqueous feed for all of the experimental runs in each set was fixed, the steady-state Cc _ is plotted against the concentration s2°4 of sodium in fresh amalgam, CHGF, for the sets: 23-28, 65-77 123 •2 -3 -4 Na/S02 Figure 20 Steady-state values of sodium dithionite concen tration and various yields versus Na/SCu ratio entering the CFSTR for the experimental set: 23-28. See Table 11 for the levels of the process variables 124 — ml) — o o >^ d CM CO CM CM o >°° E CM O CM CO u - 19 - 2-4 — - 15 - 2-0 — - 11 - 1-6 — 7 - 1-2 — Figure 21 •2 -3 -4 Na/S02 Steady-state values of sodium dithionite concen tration and various yields versus Na/SO- ratio entering the CFSTR for the experimental set: 42-46. See Table 12 for the levels of the process variables 125 T T 3-0 STS02 O- 1.30 MOLAR A-0-65 « • -0.40 " E 2-5 o o c5 2-o CM CO OJ CO E O) 1-5 c? O 1-0-•5-1 •3 -4 Na/S02 Figure 22 Steady-state sodium dithionite concentrations versus Na/S02 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed. See Tables 6, 11 and 12 for the levels of the process variables 126 and 42-46. These curves are shown in Figure 23. For the sets: 23-28, 65-77 and 42-46, the maximum steady-state concentration of sodium dithionite in the product stream is 0.385%, 0.89% and 2.3% at the CHGF values of 0.0303%, 0.064% and 0.1439% respectively. For the following presentation, the concentration of sodium in fresh amalgam at which the maximum steady-state concentration of sodium dithionite was produced for a set of experimental runs, is called CHGF_ . Therefore, the CHGF ... , for the sets: 23-28, 65-77 and 42-46 are 0.0303%, 0.064% and 0.1439% respectively. The CHGF ... i is found to be linearly related to critical 2 the molarity of sulfur dioxide in the aqueous feed as shown in Figure 24. The correlation shown in Figure 24 could be very useful from the practical point of view. If the con centration of sulfur dioxide in the aqueous feed solution is known, the concentration of sodium in the fresh amalgam at which the maximum concentration of sodium dithionite would be obtained, can be determined. (b) The variation of steady-state values of Yg0 , vNa/ CONNA and XNa with change in the concentration of sulfur dioxide in the aqueous feed solution The trend of the steady-state Y _ versus Na/S0o, YNa versus Na/S02, CONNA versus Na/S02 and XNa versus Na/S02 curves for the sets: 23-28 and 4 2-46 is the same as that of the corresponding curves for the set: 65-77. These 127 1 1 1 1 STS02 O - 1.30 MOLAR I I I I I I •05 -10 -15 -20 CHGF (gmNa/100gm Amalgam) Figure 23 Steady-state sodium dithionite concentrations versus concentrations of sodium in amalgam entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed. See Tables 6, 11 and 12 for the levels of the process variables 128 E cc co £ < £ o o CO £ D) CO (J CD X o •3 -6 -9 1-2 CONC.OF S02 (gmmole/liter) Figure 24 Critical concentrations of sodium in amalgam entering the CFSTR versus molarity of sulfur dioxide in the aqueous solutions entering the CFSTR. See Tables 6, 11 and 12 for the levels of the process variables 129 31 27 23 STS02 O-L30 MOLAR A -0.65 " • -0-40 -3 -4 Na/S02 Figure 25 Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/S02 ratios entering the CFSTR at different concen trations of sulfur dioxide in the aqueous feed. See Tables 6, 11 and 12 for the levels of the process variables 130 •2 -3 -4 -5 Na/S02 Figure 26 Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/S02 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed. See Tables 6, 11 and 12 for the levels of the process variables 131 Na/S02 Figure 27 Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/SC>2 ratios entering the CFSTR at different concen trations of sulfur dioxide in the aqueous feed. See Tables 6, 11 and 12 for the levels of the process variables 132 X 70 60 50 40 1 STS02 O - 1.30 MOLAR A - 0.65 " • — 0-40 // 1 •3 -4 Na/S02 Figure 28 Steady-state conversions of sodium to different products in the CFSTR versus Na/S02 ratios entering the CFSTR at different concentrations of sulfur dioxide in the aqueous feed. See Tables 6, 11 and 12 for the levels of the process variables 133 curves for the sets under consideration are shown in Figures 25 to 28 respectively. Figures 25 to 27 show that at a fixed Na/SC^ ratio entering the reactor, when all of the other process variables are kept constant, an increase in the concentration of sulfur dioxide in the feed solution increased the steady-state yield of sodium dithionite on total sulfur dioxide entering the reactor (vs0 )/ the steady-state yield of sodium dithionite 2 on sodium entering the reactor with fresh amalgam (YNa) an^ the steady-state yield of sodium dithionite on sodium consumed in the reactor (CONNA). Figure 28 shows that at relatively low values of Na/S02 ratio entering the reactor, an increase in the concen tration of sulfur dioxide in the feed solution does not affect the steady-state conversion of sodium to different products in the reactor, XNa, appreciably (considering the 95% error limits). However, at relatively high values of Na/S02 ratio entering the reactor, an increase in the concen tration of sulfur dioxide in the feed solution increases the steady-state values of XNa significantly. 3. Agitation in the aqueous phase Before describing the experimental work in detail it should be pointed out that there were some difficulties which influenced the choice of lower and upper limits on 134 degree of agitation in the aqueous phase. Below a stirrer Reynolds No.= 10,000 (corresponding to the propeller rpm of 376),the results obtained from the laboratory investigation could not be used in making estimations using scale-up methods (84-88, 103). On the other hand when the rpm of the propeller was well above 700, a substantial agitation was imparted to the amalgam phase causing ripples. Some preliminary experiments were performed to assess the effects of agitation but they did not yield any useful information. A short account of these experiments follows. The reactor was filled up to the lower end of the baffles with an amalgam of a known sodium concentration. Then, known volumes of a sulfur dioxide solution of a known concentration and a Rubine-R solution of a known normality were added to the reactor and the agitation in the aqueous phase was started. The rpm of the propeller was varied through the range of 110 to 7 00 for the different experi ments and the times taken for the Rubine-R colour to disappear were compared. These experiments did not provide very useful data, since even for large volumes of Rubine-R (the dye solution available was very dilute), the colour disappeared almost immediately. Informative results were obtained when the effect of agitation in the aqueous phase of the CFSTR, on the steady-state CG n , Y n , Y , CONNA and X. was investigated. During the sets: 65-77 and 47-57, the rpm of the propeller in the aqueous phase was kept fixed at 673. The levels of the other process variables, under steady-state conditions, are presented in Tables 6 and 5 respectively and the steady-state C_ ~ versus Na/S0o, Ycri versus Na/SO~, YT versus ^2 4 2 wa Na/SC^/ CONNA versus Na/S02 and XNa versus Na/S02 curves are shown in Figure 7. The experimental sets: 66-76, 62-63 and 87-91 were performed at an rpm of 225 and the steady-state Cc _ versus Na/SO», Ycr. versus Na/S0o, Y versus "2 4 2 Na/S02f CONNA versus Na/S02 and XNa versus Na/S02 curves are shown in Figure 8. For these sets, the levels of all of the other process variables were the same as for the sets: 65-77 and 45-57 as shown in Table 7 to 9. Similarly, the experimental set: 86-90 (expts. 86, 88 and 90) was performed at an rpm of 110. The steady-state C_ , Yori , Y„. , CONNA •^2 4 2 i^a and X„ were calculated at the three Na/SO,, ratios and the Na ' 2 results are shown in Figure 29. As there were only three points for each calculated quantity, only broken line curves are drawn. The levels of the process variables, under the steady-state conditions for the set: 8 6-9 0 are presented in Table 13. Prior to presenting the results, it should be mentioned that a few experiments were carried out in the experimental TABLE 13 LEVELS OF THE PROCESS VARIABLES IN SET: 86-90 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 . (RPM>Aq FLS02 FLHG (W)Aq TEMP PH .0372 - .0986 .655 -.658 110 .096 47.5 .0784 17 5.35 - 5.8 TABLE 14 LEVELS OF THE PROCESS VARIABLES IN SET: 94-104 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Ag FLSO 2 FLHG (A/v)Aq TEMP pH .0280 - .0912 .652 -.660 673 .066 47.5 .0784 17 5.75 - 6.1 LO 137 I 1 I I I I I L_ •2 -3 -4 -5 Na/S02 Figure 29 Steady-state values of sodium dithionite concen tration and various yields versus Na/SG^ ratios entering the CFSTR for the experimental set: 8 6-90. See Table 13 for the levels of the process variables 138 reactor to ensure that the conditions of perfect mixing existed at the different levels of agitation. Johnson (41) carried out tracer studies in the experimental reactor under the conditions used in the present investigation. Using the Cholette and Cloutier model (15) for the CFSTR his results showed that, under the specified experimental conditions, perfect mixing occurred in the reactor. The curves obtained by plotting the steady-state Cg Q versus Na/S02, Yg0 versus Na/S02, Y^versus Na/S02,CONNA versus Na/S02 and X^versus Na/S02 at three different levels of agitation in the aqueous phase are shown Figures 30 to 34 respectively. Figures 30 to 33 show that at a fixed Na/S02 ratio entering the reactor, when the level of agitation in the aqueous phase was increased, the steady-state concentration of sodium dithionite in the product stream (C_ n ), the yield b2u4 of sodium dithionite on total sulfur dioxide entering the reactor (Yc_ ), the yield of sodium dithionite on sodium entering the reactor with the fresh amalgam (YNa) and tne yield of sodium dithionite on sodium consumed in the reactor (CONNA) also increased. However, the percentage increase in the steady-state Cc _ , Ycr. , YM and CONNA at values of 2 4 2 Na/S02 ratio below the steady-state Na2S204 concentration maximum (i.e. at the Na/S02 ratios <0.29) is less than the percentage increase in these values at Na/S02 ratio above the steady-state sodium dithionite concentration maximum (i.e., at the Na/S02 ratios >0.29). Figure 34 shows that at 139 ^ 2-4 E o ? 2-0 c? 1-6 CvJ CO E C? CN CO O 1-2 •8 RPM) Aq • • - 673 - 225 - 110 1 1 •3 -4 Na/S02 Figure 30 Steady-state sodium dithionite concentrations versus Na/SC^ ratios entering the CFSTR at different levels of agitation in the aqueous feed. See Tables 5-9 and 13 for the levels of the process variables 140 27-23 CN o 19 15 11 (RPM) Aq • • • - 673 - 225 - 110 1 1 •3 -4 Na/S02 Figure 31 Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/SO- ratios entering the CFSTR at different levels of agitation in the aqueous feed. See Tables 5-9 and 13 for the levels of the process variables 141 J I I L •2 -3 -4 -5 Na/S02 Figure 32 Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/S02 ratios entering the CFSTR at different levels of agitation in the aqueous feed. See Tables 5-9 and 13 for the levels of the process variables 142 •2 -3 -4 -5 Na/S02 Figure 33 Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/S02 ratios entering the CFSTR at different levels of agitation in the aqueous feed. See Tables 5-9 and 13 for the levels of the process variables 143 co X Figure 34 Steady-state conversions of sodium to different products in the CFSTR versus Na/SC>2 ratios entering the CFSTR at different levels of agitation in the aqueous feed. See Tables 5-9 and 13 for the levels of the process variables 144 Na/S02ratios <0.29, the steady-state conversion of sodium to different products in the reactor (xNa) does not change appreciably, however, at Na/S02 ratios >0.29, the steady-state XNa values increase significantly with increase in the level of agitation. 4. Flow rate of aqueous sulfur dioxide solution, i.e. residence time in the aqueous phase The effect of variation in the flow rate of the aqueous sulfur dioxide solution on the steady-state C n , Y_rt , 2 4 b 2 YNa' C0NNA anc^ XNa' at var^-ous Na/S02 ratios entering the reactor, was investigated in the experimental sets: 94-104 (expts. 94, 96, 98, 100, 102 and 104), 65-77 and 95-105. The flow rates of the aqueous solution in these sets were 66 ml/min, 96 ml/min and 198 ml/min respectively. The levels of the other process variables, under the steady-state conditions, have been reported in Tables 14, 6 and 10 respectively. The steady-state C_ _ , Ye_ , YM , CONNA and &2^4 2 ™a XNa, for the sets: 94-104, 65-77 and 95-105, are plotted against the Na/S02 ratios entering the reactor in Figures 35, 9 and 36 respectively. The curves obtained by plotting the steady-state Cg 0 versus Na/S02, Yg0 versus Na/S02, Y^a versus Na/S02, CONNA versus Na/S02 and XNa versus Na/S02 at the three different flow rates of the aqueous sulfur dioxide solution are shown in Figures 37 to 41 respectively. 145 90h 3 -4 Na/S02 Figure 35 Steady-state values of sodium dithionite concen tration and various yields versus Na/SC>2 ratios entering the CFSTR for the experimental set: 94-104. See Table 14 for the levels of the process variables 14 6 2 -3 Na/S02 Figure 36 Steady-state values of sodium dithionite concen tration and various yields versus Na/SC>2 ratios entering the CFSTR for the experimental set: 95-105. See Table 10 for the levels of the process variables 147 Figure 37 Steady-state sodium dithionite concentrations versus Na/SC>2 ratios entering the CFSTR at different flow rates of the aqueous feed. See Tables 6, 10 and 14 for the levels of the process variables 148 i ~i 1 1 r FLSG-2 A / _J I I I I L •1 -2 -3 -4 -5 -6 Na/S02 Figure 38 Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/SC»2 ratios entering the CFSTR at different flow rates of the aqueous feed. See Tables 6, 10 and 14 for the levels of the process variables 149 co 70 60 50 40 30 20 FLSO2 • -.066 LIT /MIN O —.096 1* A - .198 // •3 -4 Na/S02 Figure 39 Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/S02 ratios entering the CFSTR at different flow rates of the aqueous feed. See Tables 6, 10 and 14 for the levels of the process variables 150 •3 -4 Na/S02 Figure 40 Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/S02 ratios entering the CFSTR at different flow rates of the aqueous feed. See Tables 6, 10 and 14 for the levels of the process variables 151 § 70 X 60 50 FLS02 • -.066 LIT/MIN O - .096 • - .198 1 1 1 •3 -4 -5 Na/S02 Figure 41 Steady-state conversions of sodium to different products in the CFSTR versus Na/S02 ratios entering the CFSTR at different flow rates of the aqueous feed. See Tables 6, 10 and 14 for the levels of the process variables 152 Figure 37 shows that at all of the values of Na/S02 ratio considered, the steady-state concentration of sodium dithionite in the product stream, Cc n , increases when the h2u4 flow rate of the aqueous sulfur dioxide solution is decreased (i.e., when the residence time in the aqueous phase is increased). Similar behaviour is observed when the steady-state values of Yg0 (yields of sodium dithionite on total 2 sulfur dioxide entering the reactor) are plotted against the Na/SC^ ratios entering the reactor at the three levels of the flow rate (Figure 38). Figure 39 shows that at low as well as high Na/S02 ratios, the percentage of sodium that is converted to sodium dithionite under steady-state conditions, CONNA, increases when the flow rate of the aqueous feed solution is decreased. The trend of the curves obtained by plotting the steady-state yields of sodium dithionite on sodium entering the reactor (YNa) against the Na/S02 ratios at the three levels of the flow rate was similar. Figure 41 shows that at values of the Na/S02 ratio below the steady-state sodium dithionite concentration maximum (i.e., at the Na/S02 ratios <0.29), the steady-state conversion of sodium to different products in the reactor (XNa) decreased when the flow rate of the aqueous sulfur dioxide feed was reduced. But at Na/S02 ratios >0.29 the steady-state values of X^a did not change appreciably with change in flow rate. 153 The geometry of the reactor was such that the aqueous sulfur dioxide solution entered the reactor through a 1/4 inch stainless-steel tube, the mouth of which was situated approximately 2 inches above the amalgam surface. Therefore, it was suspected that an increase in the flow rate of the aqueous solution would impart greater turbulence to the amalgam phase. To investigate this a colour tracer was introduced in the inlet aqueous solution. Despite the fact that the propeller was placed directly below the mouth of the tube which brought the aqueous solution to the reactor, it was observed that, at the rpm under consideration, the stream of the inlet aqueous solution did not break completely by the mixer and it impinged on the amalgam surface. 5. Interfacial-area/aqueous-volume ratio To investigate this process variable, the volume of the aqueous phase was not changed from 980 ml. The inter-facial area was changed, as mentioned in Section IV.B.l, by opening the bottom flange of the reactor and introducing thin stainless-steel discs with variable diameter annular holes. In the experimental sets: 65-77, 122-134 (expts. 122, 125,128,131 and 134) and 123-135 (expts. 123,129,132 and 2 2 135) the area of the interface was 76.8 cm , 24.2 cm and 2 9.35 cm respectively. These areas corresponded to inter-154 2 3 facial-area/aqueous-volume ratios of .0784 cm /cm , •0247 2 3 2 3 cm /cm and .0095 cm /cm respectively. The levels of the other process variables, under the steady-state conditions are presented in Tables 6, 15 and 16. The steady-state Co n ' Ycr> ' YM ' CONNA and X._ , for the sets: 65-7 7, 122-2 4 2 134 and 123-135, are plotted against the Na/S02 ratios entering the reactor in Figures 9, 4 2 and 43 respectively. The curves obtained by plotting the steady-state Cg Q versus Na/S02, Y versus Na/S02, YNa versus Na/S02, 2 4 2 CONNA versus Na/S02 and XNa versus Na/S02 at the three interfacial-area/aqueous-volume ratios are shown in Figures 44 to 48 respectively. Figures 44 to 4 8 show that, at a fixed Na/S02 ratio entering the reactor, when the interfacial-area/aqueous-volume ratio increases, the values of steady-state C n , fa2u4 Y„_ , Y.T , CONNA and XXT also increase. S02 Na Na Figure 49 shows that when the values of steady-state concentration of sodium dithionite in the product stream (C n ) or the yield of sodium dithionite on sulfur dioxide S2°4 entering the reactor (vs0 ) are plotted against the inter-2 facial-area/aqueous-volume ratios, at different Na/S02 ratios, straight line relationships are not obtained. One of the major problems in investigating the process variable under consideration was the following. Every time a stainless-steel disc was forced into position to change the interfacial-area/aqueous-volume ratio, it TABLE 15 LEVELS OF THE PROCESS VARIABLES IN SET: 122-134 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLS02 FLHG (A/V)Aq TEMP pH .0488 - .1238 .658 -.664 673 .096 47.5 .0247 17 5.7 - 5.95 TABLE"16 LEVELS OF THE PROCESS VARIABLES IN SET: 123-135 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLS02 FLHG <W)Aq TEMP pH .0488 - .0879 .658 -.664 673 .096 47.5 .0095 17 5.65 - 5.8 156 •2 -3 -4 -5 Na/S62 Figure 42 Steady-state values of sodium dithionite concen tration and various yields versus Na/SC>2 ratios entering the CFSTR for the experimental set: 122-134. See Table 15 for the levels of the process variables 157 90 80 70 * 60 50 40 30 20 10 — ml) — /inn / lUU cf C\J CO fN o £ <3 CM CO o - 19 - 15 - 1-2 — - 11 - -8 — 7 - -4 — •3 -4 Na/S02 Figure 43 Steady-state values of sodium dithionite concen tration and various yields versus Na/SC-2 ratios entering the CFSTR for the experimental set: 12 3-135. See Table 16 for the levels of the process variables 158 2-1 £ o o 1-8 o 1-5 CM CO CM Na 1-2 E •9 o CM CO •6 o —i 1 lA/VlAq • - .0784 CM2/CM3 O - .0247 // A - .0095 ** 1 1 3 -4 -5 Na/S02 Figure 44 Steady-state sodium dithionite concentrations versus Na/SO- ratios entering the CFSTR at different values of interfacial-area/aqueous-volume ratio. See Tables 6, 15 and 16 for the levels of the process variables 159 I 1 1 1— 27h (A/v)Aq • - .0784 CM2/CM3 O - .0247 " 23| A — .0095 » VP 19 Figure 45 Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/S02 ratios entering the CFSTR at different values or inter-facial-area/aqueous-volume ratio. See Tables 6, 15 and 16 for the levels of the process variables 160 T VP 80 70 60 50 40 30 20 10 I ,A/v)Aq • - .0784 CM2/CM3 O- .0247 » A — .0095 » \ I I 3 -4 Na/S02 Figure 46 Steady-state yields of sodium dithionite on sodium in the amalgam entering the CFSTR versus Na/S02 ratios entering the CFSTR at different values of interfacial-area/aqueous-volume ratio. See Tables 6, 15 and 16 for the levels of the process variables 161 • - .0784 CM2/CM3 O - .0247 " I 1 I I I I •2 -3 -4 '5 Na/S02 Figure 47 Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/SC>2 ratios entering the CFSTR at different values of interfacial-area/aqueous-volume ratio. See Tables 6, 15 and 16 for the levels of the process variables 162 90 80 <3* 70 2 60 X 5 0 40 30 o • U/v)Aq - -0784 CM2/CM3 - .0247 - .0095 » 1 3 -4 Na/S02 Figure 48 Steady-state conversions of sodium to different products in the CFSTR versus Na/S02 ratios entering the CFSTR at different values of inter-facial-area/aqueous-volume ratio. See Tables 6, 15 and 16 for the levels of the process variables 163 (A/V) 'Aq Figure 49 Steady-state values of sodium dithionite concen tration and yield of sodium dithionite on sulfur dioxide in the aqueous feed versus interfacial-area/aqueous-volume ratios at different Na/S02 ratios entering the CFSTR. See Tables 6, 15 and 16 for the levels of the process variables 164 was found difficult to position it exactly horizontal. In other words, the interfacial-area/aqueous-volume ratio reported may have been less than the actual interfacial-area/ aqueous-volume ratio. This problem could be avoided by using separate reactors with different interfacial-area/ aqueous-volume ratios. However, this was not done in the present investigation. 6. Temperature of the aqueous phase The experimental sets: 106-118 (expts. 106, 109, 112, 115 and 118), 65-77 and 110-113 (expts. 110 and 113) were performed to investigate the effect of variation in the temperature of the aqueous phase on the steady-state yields of sodium dithionite based on the sulfur dioxide entering the reactor (Yg0 ) and on the sodium consumed in the reactor (CONNA). The steady-state temperature of the aqueous phase in the reactor, for the sets: 106-118, 65-77 and 110-113, was 13°C, 17°C and 27°C respectively. Lower temperatures could not be obtained due to the exothermic nature of the reactions. The levels of the process variables, under the steady-state conditions are presented in Tables 17, 6 and 18. The steady-state Y and CONNA values are plotted bU2 against the Na/S02 ratios entering the reactor at the three temperatures of the aqueous phase. These curves are shown TABLE 17 LEVELS OF THE PROCESS VARIABLES IN SET: 106-118 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLS02 FLHG <Vv)Aq TEMP pH .0372 - .0951 .653 -.660 673 .096 47 .5 .0784 13 5.55 - 6.0 TABLE 18 LEVELS OF THE PROCESS VARIABLES IN SET: 110-113 Range of the Changed Variable VALUES OF THE FIXED VARIABLES CHGF STS02 (RPM)Aq FLSO 2 FLHG (VV)Aq TEMP pH .0372 - .0646 .653 . -.654 673 .096 47 .5 .0784 27 5.7 - 5.9 166 23-19 CM O 15 11 I I I I TEMP — • - 27 ° C — O - 17 »/ A - 13 » —— • 1 • \ ""~A^^O»«, I I I I •2 •3 •4 -5 Na/S02 Figure 50 Steady-state yields of sodium dithionite on sulfur dioxide in the aqueous feed versus Na/S02 ratios entering the CFSTR at different steady-state temperatures of the aqueous phase. See Tables 6, 17 and 18 for the levels of the process variables 167 80 70 °N°60 <50 z: o O 40 30 20 TEMP • -27° O - 17 * - 13 1 1 •3 -4 Na/S02 Figure 51 Steady-state yields of sodium dithionite on sodium consumed in the CFSTR versus Na/S02 ratios entering the CFSTR at different steady-state temperatures of the aqueous phase. See Tables 6, 17 and 18 for the levels of the process variables 168 in Figures 50 and 51 respectively. No curves are drawn for the set: 110-113 because there are only two experimental runs in this set. Figure 50 shows that for different experimental sets (carried out at different temperatures)/ the maximum in the steady-state Ycn versus Na/S00 curves is obtained at a Na/SC^ ratio of 0.29. This figure also shows that there may be an optimum temperature at every Na/SG^ ratio entering the reactor, above which the steady-state Yon would decrease. &°2 7. pH of the aqueous phase For all experimental runs discussed in the previous sections, the steady-state pH of the aqueous phase was in the recommended range (Section II.B.) of 5 to 6. To investi gate the effect of a variation in the pH, some exploratory experimental runs (expts. 1 to 11) were performed where the pH of the aqueous phase was in the range 0.8 to 2.0. Various combinations of the other process variables were used in these experimental runs. In all these cases, the yields of sodium dithionite on sulfur dioxide entering the reactor were very small. 8. Flow rate of fresh amalgam, i.e. residence time in the amalgam phase This process variable was not investigated systematically for the reasons outlined in the Section V.B. 169 However, a few experimental runs (expts. 5 to 11) were performed which gave interesting results. When the flow rate of the fresh amalgam was increased such that the Na/SC^ ratio entering the reactor was very high (greater than 1), a white precipitate of sulfur was obtained. It was noted that for none of the experiments was the pH of the aqueous phase less than 0.8. 170 CHAPTER VI DISCUSSION A. Model for the Reacting System in the Proposed Process 1. Development of the model It was mentioned in Chapter III that Ketelaar (44) and Gerritsen (30) proposed models for the reacting system sodium-mercury amalgam and aqueous sulfur dioxide. According to Ketelaar's model, the overall rate of sodium dithionite formation is controlled by the rate of sodium mass-transfer to the amalgam-aqueous solution interface. Gerritsen's model does not take into consideration the heterogeneous decomposition of the dithionite formed at the interface and assumes a very simplified expression for the rate of the homogeneous de composition of dithionite (i.e. rate is a function of the concentration of sodium dithionite in the bulk). Both these models were found inadequate to describe the results presented (Chapter V). A model for the reacting system at a steady-state pH of 5 to 6 is developed on the basis of the experimental results obtained in this work and the information available in the literature. In the formulation of this model it was found necessary to consider the mass-transfer of different 171 reacting species in the amalgam and aqueous phases along with the following chemical reactions: - The sodium dithionite formation reaction 2 Na + 2 HSO~ -> 2 Na+ + S20~ + 2 OH." .... (16) - The water reaction 2 Na + 2 H20 -»• 2 NaOH + H2 .... (17) - The heterogeneous decomposition of sodium dithionite Na Na _ Na S2°4 "*" S2°3 Sn "* S .... (15) - The homogeneous decomposition of sodium dithionite 2 Na2S204 + H20 2 NaHS03 + Na2S203 .... (11) Na2S204 + 2 NaHS03 -»• Na^Og + Na2S03 + H20 ....(12) The oxidation of sodium dithionite was eliminated by provid ing an inert atmosphere of N2 in the reactor system. The experimental results indicate that the production of sodium dithionite in the amalgam process depends primarily upon the Na/S02 ratio entering the reactor which is related to the Na/S02 ratio in the reacting system. The former was controlled in the present investigation and will be called •Na/SO- ratio1 henceforth. 172 In all CFSTR experimental sets, where the concen tration of sulfur dioxide in the feed solution was about 0.65 molar, the steady-state concentration of sodium dithionite (Cc n ), yield of sodium dithionite on sulfur b2°4 dioxide entering the reactor (Ycr. ) and conversion of sodium fa°2 to different products in the reactor (XNa) show a maximum at a Na/S02 ratio of about 0.29. However, this ratio changes when the concentration of sulfur dioxide in the feed solution changes as shown in Figures 22, 25 and 28. The shift in maxima is probably due to the fact that these quantities are plotted against the Na/S02 ratios entering the reactor and not against the Na/S02 ratios in the bulk of the two phases. In any event, Figure 24 shows that if the molarity of sulfur dioxide in the aqueous feed solution is known, the concentration of sodium in the amalgam entering the reactor (therefore the Na/S02 ratio entering the reactor) at which maximum steady-state Cc n , Y__ and X„ are obtained, 2 4 2 Na can be estimated. Further, the Na/S02 ratio of about 0.29 is not a magic number and it will change, for instance, if the level of turbulence in the amalgam phase is changed. At values of the Na/S02 ratio below the steady-state C maximum, for all experimental sets, the consumption rate of sodium to produce sodium dithionite and other products is directly proportional to the Na/S02 ratio or to the concentration of sodium in fresh amalgam. For a typical set 173 this is shown in Figures 11 and 12 respectively. This result implies that the overall rate of S204 formation is limited by sodium mass-transfer rate to the interface in the range vof Na/SC>2 ratios under consideration. At values of the Na/S02 ratio above the steady-state sodium dithionite concentration maximum, for all experimental sets, the rate of sodium consumption is no longer a linear function of the Na/S02 ratio (Figure 11) or concentration of sodium in amalgam (Figure 12). This observation indicates that the mass-transfer of sodium to the interface is no longer the limiting step in the dithionite formation process in this Na/S02 ratio range. The conclusions reached above are supported by evidence presented below which, in some cases, also indicates that at values of Na/SO~ ratio above the steady-state C n maximum, - * b2u4 the mass-transfer rate of bisulfite ions in the aqueous phase controls the rate of dithionite formation. (a) Figure 34 shows that the steady-state conversion of sodium to different products in the reactor (XNa) above a Na/S02 ratio of 0.29 is increased significantly by in creased stirring in the aqueous phase while at Na/S02 ratios below 0.29 the change is insignificant. Figure 30 shows that the rate of dithionite production above a Na/S02 ratio of 0.29 increased to a greater extent by increased agitation in the aqueous phase than at values of the Na/SO- ratio 174 below 0.29. These results would be expected if the sodium mass-transfer rate controlled the dithionite formation at values of Na/SC^ ratio below 0.29 and if mass-transfer rate of bisulfite ions controlled at Na/SG^ ratios greater than 0.29. (b) Figures 14 to 17 show that at values of Na/SC^ ratio above the steady-state C_ n maxima, the unsteady-state b2U4 concentration profiles of sodium dithionite in the product stream and sodium in the outlet-amalgam (spent amalgam) show maxima and minima respectively. However such maxima and minima are not observed at Na/SC^ ratios below the steady-state Cc n maxima. &2°4 It was discussed in Section II.F. that when sulfur dioxide gas is dissolved in water, depending on the pH of the solution, it can exist in the form of molecular sulfur dioxide, bisulfite ions or sulfite ions. Assuming unit activity coefficients for the different ionic and molecular species that exist when sulfur dioxide gas is dissolved in water, it was shown that most of the sulfur dioxide is available as bisulfite ions at a pH of about 4.5. Above this pH the concentration of sulfite ions increases and the concentration of bisulfite ions decreases. Sulfite ions, as mentioned in the previous sections, can not be reduced by sodium dissolved in mercury to give dithionite. 175 When an experimental run starts, the rate of sodium dithionite formation increases, thus increasing the con centration of dithionite in the product stream. Starting with the aqueous feed solution at pH = 3 to 3.5 and the system at unsteady-state the pH of the aqueous phase in the CFSTR increases and then attains a steady-state value in the range 5 to 6. At Na/S02 ratios above the steady-state Cc _ maximum, as the pH of the aqueous phase increases b2°4 above 4.5, the bulk concentration of the bisulfite ions in the reactor decreases. This decreases the rate of sodium dithionite formation, thus lowering the concentration of sodium dithionite in the product stream. As the same bulk concentration of the bisulfite ions is not available for dithionite formation, the concentration of sodium in the spent amalgam increases and then attains a steady-state value. These maxima and minima are not seen at Na/S02 ratios below the steady-state C _ maxima because the rate S2°4 of the sodium dithionite formation is controlled by the rate of mass-transfer of sodium from the bulk of the amalgam to the interface and at all times there are sufficient bisulfite ions present at the interface for the reaction. Further confirmation of the explanation given above is obtained from Figures 18 and 19. These figures show that for the experimental run 65 a maximum and a minimum are observed in the curves obtained by plotting the concentration 176 of sodium dithionite in the product stream against time and the concentration of sodium in spent amalgam against time respectively. This experiment was done at a Na/SO^ ratio above 0.29 and the pH of the aqueous phase increased from 3.53 to 6.0. On the other hand the unsteady-state concen tration profiles for the experimental runs 71 and 77 do not show any maxima or minima. These experiments were also done at values of Na/S02 ratio above 0.29, but the pH of the aqueous phase was fixed at about 5.9 under the unsteady-state and steady-state conditions. (c) Figure 22 shows that throughout the range of Na/S02 ratios investigated, when the concentration of sulfur dioxide in the feed solution increases at a fixed Na/S02 ratio, the rate of sodium dithionite formation increases. Figure 28 shows that in the range of relatively low Na/S02 ratios, when the concentration of sulfur dioxide is increased at a fixed Na/S02 ratio, the steady-state conversion of sodium to different products in the reactor (XN&) does not change significantly. However, in the range of high Na/S02 ratios, the steady-state XNa increases with molarity of sulfur dioxide at a fixed Na/S02 ratio. This indicates that at low Na/S02 ratios although the chemical reaction rates of the sodium consuming reactions may increase, the rate of sodium consumption is still limited by the rate of mass-transfer of sodium to the interface. This is not true at high Na/S05 ratios. (d) Figure 41 shows that the steady-state conversion of sodium to different products in the reactor (xNa) above a Na/SC>2 ratio of 0.29 is not changed significantly by increasing the flow rate of the aqueous feed solution (shortening the residence time in the aqueous phase). Similar behaviour is expected at Na/SC^ ratios below 0.29 if the rate of sodium mass-transfer to the interface limits the rate of sodium consumption. However, Figure 41 shows that the steady-state XNa increases with increase in the flow rate of the aqueous solution in this range of Na/SC^ ratios. This observation is consistent with the above-mentioned conclusions as explained below. During the experiments outlined in Section V.E.4., it was observed that an increase in the flow rate of the aqueous solution increases the level of turbulence in the amalgam phase. At low Na/SO^ ratios, where the rate of sodium consumption is limited by the rate of mass-transfer of sodium from the bulk of the amalgam phase to the interface, increased turbulence in the amalgam phase would increase the rate of mass-transfer of sodium. At a fixed Na/S02 ratio (for values of Na/S02 ratio below 0.29), this increased sodium mass-transfer increases the steady-state XNa value. At values of Na/SC^ ratios above 0.29, the steady-state conversion of sodium to different products in the reactor (X. ) does not increase because the rate of 178 sodium consumption is not controlled by the rate of mass-transfer of sodium to the interface. The nature of the response curves obtained during the study and shown in Chapter V is not only affected by the sodium mass-transfer and bisulfite mass-transfer but is also strongly influenced by the chemical reactions taking place. Some of these reactions are listed at the start of this chapter (equations 16, 17, 15, 11 and 12). The relative importance of these reactions at different Na/S02 ratios is discussed below. (i) Na/S00 ratios below the steady-state Cc _ maximum A b2°4 Figures 12 and 13 show that at very low concentrations of sodium in mercury (or the Na/SG^ ratios) no sodium is consumed. However, as the Na/SC^ ratio entering the reactor is increased, the rate of sodium consumption increases linearly. This is understandable because for all reactions of sodium-mercury amalgam with aqueous solutions, the oxidation potential of sodium at the mercury/water interface determines whether the reaction will take place. At very small concentrations of sodium in mercury, the oxidation potential of sodium is not sufficient to initiate a reaction. As the Na/SC^ ratio is increased, the concentration of sodium in amalgam required to initiate different sodium-consuming reactions is reached. 179 It was concluded above that, in the region under con sideration, the rate of sodium consumption is limited by the sodium mass-transfer rate to the interface. If it is assumed that only the sodium dithionite formation reaction takes place in this region then all of the sodium consumed by the aqueous phase would be used to give sodium dithionite. Under those conditions, the rate of sodium dithionite for mation would increase proportionately with an increase in the rate of sodium consumption (therefore, with an increase in the Na/S02 ratio). If the above assumption were true and the values of steady-state yield of sodium dithionite on sodium consumed in the reactor (CONNA) were plotted against the Na/S02 ratios, a straight line with zero slope (CONNA = 100) would be expected. However, this is not the case as shown in Figure 9. This figure shows the steady-state yield of sodium dithionite on sodium consumed (CONNA) for a typical set of experimental runs. It is proposed that the steady-state CONNA is less than 100 per cent due to the homogeneous decomposition of the dithionite in the bulk of the aqueous phase and the heterogeneous decomposition of the dithionite, produced at the interface, by the sodium transferred there. The rate of sodium consumption by the water reaction, under steady-state conditions, is expected to be negligible in the range of Na/S02 ratios below the steady-state Cg Q maximum. The 180 information available in the literature (Section II.G.2.) shows that the water reaction takes place at the interface but its rate, in the pH range 4 to 10, is not limited by the mass-transfer rate of sodium to the interface; this rate is controlled by the rate of chemical reaction at the interface. On the basis of the batch experiments and the infor mation available in the literature (Section II.G.3.a.) homogeneous decomposition of sodium dithionite takes place as given by equations (11) and (12). The empirical expression for the rate of this decomposition has been outlined by Spencer (102). rhomo = kc [S20=]1tHSO-]1[SO=]°[H+]X[S20=]y . . . .(13) From the experimental results it is concluded that, in the range of Na/S02 ratios under consideration, essentially all of the sodium transferred to the interface is consumed by the sodium dithionite formation reaction and the hetero geneous decomposition of dithionite. The results discussed below (a and 8) imply that although the rate of sodium consumption by these two reactions is limited by the sodium mass-transfer rate to the interface, the product distribution depends on their relative reaction rates. 181 (a) Figures 25 and 27 show that in the region under considera tion, when the concentration of sulfur dioxide in the aqueous feed solution is increased at a fixed Na/S02 ratio, the steady-state yields of sodium dithionite on sulfur dioxide (Ycr. ) and on sodium consumed in the reactor (CONNA) increase, b02 but the steady-state conversion of sodium in the reactor (XNa) does not change significantly as shown in Figure 29. These results indicate that at a fixed Na/S02 ratio, an increase in the concentration of sulfur dioxide in the aqueous feed causes a greater increase in the rate of the sodium dithionite formation reaction than in the rate of the hetero geneous decomposition of the dithionite. ($) Figures 31 and 33 show that below a Na/S02 of about 0.29, the steady-state Y n and CONNA increase at a fixed Na/S00 ratio with an increase in the level of agitation in the aqueous phase. However, the increase in the steady-state XNa at that Na/S02 ratio is negligible as shown in Figure 35. These results indicate that in the region under consideration, an increase in the level of agitation in the aqueous phase at a fixed Na/S02 ratio increases the rate of sodium dithionite formation more than the rate of the heterogeneous decomposition of the dithionite. (y) For a typical experimental set the rate of the sodium dithionite formation reaction increases with increasing Na/S09 ratios because in the region under consideration 182 the rate depends on the sodium mass-transfer rate to the interface. The rate of the homogeneous decomposition of dithionite also increases because the concentrations of the $2°^ ions and the S2®~3 xons increase. However, it is doubt ful that the homogeneous decomposition of the dithionite alone would cause such a decrease in the steady-state CONNA with increasing Na/S02 ratio as shown in Figure 9. (ii) Na/SO- ratios above the steady-state Cc n maximum * £>2U4 It was concluded earlier that in this region, the rate of sodium consumption is not limited by the sodium mass-transfer rate to the interface. Further, it was con cluded that the rate of the sodium dithionite formation reaction is limited by bisulfite mass-transfer to the inter face . In other words, for an experimental set where the concentration of sulfur dioxide in the aqueous feed is fixed, the rate of the sodium dithionite formation reaction in creases to a maximum value with increasing Na/S02 ratios. An equation for the rate of homogeneous decomposition of the dithionite was given by Spencer (equation 13). This rate probably increases as the Na/S02 ratio is increased above the value corresponding to a maximum steady-state concen tration of sodium dithionite in the product stream. The increase in the rate would be due to increase in the con centration of S>2®~?, i°ns which are a product of the homogeneous and heterogeneous decomposition reactions. As the Na/SO~ 183 ratio is further increased the rate of homogeneous decom position is expected to attain its maximum value because the concentration of dithionite in the bulk of the aqueous phase decreases. This is indicated by the gradual levelling off of the steady-state Cc _ versus Na/S0o curves at high b2u4 values of the Na/SO^ ratio for all of the experimental sets. The occurrence of the heterogeneous decomposition of the dithionite, at least at very high values of the Na/S02 ratio, is indicated by the results presented in sections V.A.3. and V.E.8. (in conjunction with Section V.A.2.). For none of the experimental runs outlined in Section V.E.8. was the pH of the aqueous phase less than 0.8, so the sulfur could not have formed due to the auto-decomposition of sodium dithionite (see Section V.A.2.). Most probably, the sulfur was formed by the oxidation of the sulfide ions by the sulfur dioxide present in the aqueous phase. These sulfide ions were formed by the heterogeneous decomposition of sodium dithionite at the interface (equation 3). In view of the occurrence of this reaction at Na/SG^ ratios below the steady-state C _ maximum and at very high Na/S0o 2 4 ratios, it is logical to assume its presence in the intermediate range of Na/SG^ ratios. The experimental results tend to support this. For a typical experimental set, Figure 9 shows that the steady-state C decreases b2U4 sharply when the Na/S02 ratio is increased above 0.29 and then tends to level off. It is unlikely that the sharp fall in the steady-state concentration of sodium dithionite in the product stream (C ) is caused by the homogeneous b2U4 decomposition of the dithionite alone. These results also imply that the rate of heterogeneous decomposition of the dithionite, immediately above Na/S02 ratio of about 0.29, increases with an increase in the Na/S02 ratio. This can happen if the rate of this reaction is limited by the rate of chemical reaction at the interface. However, at very high Na/S09 ratios the steady-state C _ versus Na/S0o z s2°4 ratio curve tends to level off implying that the rate of the heterogeneous decomposition also attains its maximum value. This suggests that the rate of this reaction at very high Na/SC"2 ratios is controlled by the removal rate of the S204 ions from the interface. The following discussion indicates that at values of Na/S09 ratios above the steady-state Cc ~ maximum, the z s2o4 interfacial concentration of sodium in mercury required to initiate the water reaction is reached. (a) Figure 9 shows that the steady-state yield of sodium dithionite on sodium consumed in the reactor (CONNA) falls very sharply at Na/S02 ratios above 0.29; the fall is gradual at very high Na/S02 ratios. The sharp fall in the steady-state CONNA is observed because the rate of the sodium dithionite formation reaction does not change with an increase in the Na/SO~ ratio but the rate of the water 185 reaction and the rates of the heterogeneous and homogeneous decomposition of the dithionite increase. Thus, an increas ing proportion of the sodium transferred to the interface is used up by the water reaction and the heterogeneous decomposition of the dithionite. The decrease in the steady-state CONNA is more gradual at very high Na/S02 ratios because the rates of the heterogeneous and homogeneous decomposition reactions depend upon S2Q~4 concentration and hence do not increase indefinitely. (3) The curve obtained by plotting the rate of sodium con sumption against the concentration of sodium in the fresh amalgam (CHGF) for an experimental set shows that at very high values of CHGF (or Na/S02 ratio) the rate of sodium consumption increases gradually with an increase in CHGF. The general nature of this curve, in the region under consideration, is similar to that expected for the water reaction as outlined by Dunning and Kilpatrick (20) [Section II .G. 2 . ] . 2. The model At fixed levels of all the other process variables, when the concentration of sodium in the amalgam entering the reactor increases (the Na/S02 rat^-° increases at a fixed conc.of SO2 in the feed), the steady-state concentration of sodium dithionite in the reactor passes through a 186 maximum. The processes occurring in the reactor, at a steady-state pH of 5 to 6 and in the inert atmosphere of may be described as follows: Ca) Na/SO- ratios below the steady-state Cc _ maximum z fa2 4 At very small concentrations of sodium in mercury (or low Na/S02 ratios), the oxidation potential of sodium is not sufficient to initiate any reaction. As the Na/SC^ ratio entering the reactor is increased, the threshold concentration of sodium in mercury required to initiate the sodium dithionite formation reaction is reached. Once the dithionite ions start forming at the interface, their heterogeneous and homogeneous decomposition also start. In the region under consideration the sodium dithion ite formation reaction and the heterogeneous decomposition of the dithionite are the important sodium consuming reactions. The rate of sodium consumption by these reactions is limited by the sodium mass-transfer rate to the interface, however, the product distribution depends on their relative reaction rates. As the Na/S02 ratio increases towards the value corresponding to maximum steady-state sodium dithionite concentration (C_ _ ), the rate of mass-transfer of sodium S2°4 to the interface increases which in turn increases the rates of the sodium dithionite formation reaction and the hetero-187 geneous decomposition of the dithionite. The rate of homo geneous decomposition of dithionite also increases [see equation 13 by Spencer (102)]. However, the increase in the rate of sodium dithionite formation is greater than the increase in the rate of its decomposition. (b) Na/S0o ratios above the steady-state Cc rt maximum z b2u4 When the Na/SG^ ratio is increased above the value corresponding to the steady-state C_ n maximum, the con-b2°4 centration of sodium in the amalgam interface increases and the threshold concentration of sodium in the mercury required to initiate the water reaction is reached. Therefore, the sodium transferred to the interface is consumed by the sodium dithionite formation reaction, the heterogeneous decomposition of the dithionite and the water reaction. However, in the region under consideration the rate of sodium consumption by these reactions is not limited by the sodium mass-transfer rate to the interface. The rate of the sodium dithionite formation reaction is limited by the rate of mass-transfer of bisulfite ions to the interface. This rate does not change with increasing Na/S02 ratio. The rate of the heterogeneous decomposition of sodium dithionite is controlled by the rate of chemical reaction at the interface and this rate increases with increasing Na/SO~ ratio. At very high values of Na/SO- ratio the rate 188 of this reaction becomes controlled by the removal rate of the S204 ions from the interface and tends to attain its maximum value. The rate of sodium consumption by the water reaction is controlled by the rate of chemical reaction at the interface. This rate can be given by the empirical expression (equation 10) derived by Dunning and Kilpatrick (20). The rate increases with increasing Na/SO^ ratio. The rate of homogeneous decomposition of the dithionite may be given by Spencer's (102) expression shown in equation 13. This rate increases with increasing Na/SG^ ratio. However, at very high values of the Na/SG^ ratio the rate of homogeneous decomposition tends to attain a maximum value. B. Conditions for Improving The Yields of Sodium Dithionite In The Proposed Process It was mentioned in Chapter I that for the proposed process the steady-state yields of sodium dithionite on sulfur dioxide entering the reactor (Ycn ) and on sodium consumed in a single pass (CONNA) must be economical. The experimental results show that the conditions for the highest yield on sulfur dioxide are different from the 189 conditions for the highest yield on sodium consumed. There fore, the relative costs of the reacting chemicals must be considered. Figure 10 shows the variation of steady-state CONNA and Yc~ as a function of the concentration of sodium in fresh amalgam for a typical set of experimental runs where all the other process variables were kept fixed. This figure demonstrates that, for such an experimental set, the highest steady-state CONNA is obtained at a concentration of sodium in the amalgam where the steady-state Ycn is relatively small. Figures 25 and 27 show that at a fixed Na/S02 ratio entering the reactor, the steady-state yields on sulfur dioxide in the feed and on sodium consumed in a single pass increase with an increase in the concentration of sulfur dioxide in the feed. At a fixed Na/S02 ratio, there may, however, be an optimum concentration of sulfur dioxide in the aqueous feed solution above which the steady-state Yen £>u2 would decrease. There may be such an optimum concentration of sulfur dioxide for the steady-state CONNA also. These concentrations could not be determined in the present in vestigation because an aqueous sulfur dioxide solution whose concentration is substantially greater than 1.30 molar would require pressurization of the reactor system. Figure 24 shows that when the concentration of sulfur dioxide in the aqueous feed is increased, the concentration 190 of sodium in the fresh amalgam at which maximum steady-state Ycn is obtained in a set of experimental runs (critical CHGF) also increases. The critical concentration of sodium in fresh amalgam for the experimental set performed at 1.30 molar sulfur dioxide in the feed was about 0.14 39 per cent. The results show that the steady-state Y and CONNA under these conditions are higher than the steady-state values of these quantities, under critical conditions, at 0.65 molar er.0.4 molar sulfur dioxide. The critical CHGF would increase with a further increase in the molarity of sulfur dioxide in the aqueous feed and the steady-state Y and CONNA may also increase. These concentrations of sodium in fresh amalgam are well outside the limits recommended for the sodium amalgam: S02 - NaHS03/Na2S03 buffer process (Section II.B.) by previous investigators. This is understandable, because the previous workers were primarily interested in the yield of sodium dithionite on sodium consumed. Figures 31 and 33 show that an increase in the level of agitation in the aqueous phase increases the desired yields. The increase in the steady-state Y was found to b02 be greater at values of Na/S02 ratio above the steady-state Yg0 maximum. For the reasons given in Section V.E.3. the effect of agitation in the aqueous phase could not be investigated exclusively at propeller speeds above 700 rpm. 191 Figures 38 and 40 show the variation in the values of steady-state Y _ and CONNA as a function of the flow rates of the aqueous solution. The recommendation by pre vious investigators that the residence time of the aqueous solution in the reactor should be as short as possible (Section II.B) does not seem to be correct. The model based on the experimental results suggests that there would be an optimum residence time in the aqueous phase at different Na/S02 ratios below which the steady-state Yg0 and CONNA may decrease. Figures 45 and 4 7 show that at a fixed Na/S02 ratio entering the reactor, the steady-state yields of sodium dithionite on sulfur dioxide in the feed (Ycn ) and on bU2 sodium consumed in the reactor (CONNA) increase with an increase in the interfacial-area/aqueous-volume ratio. Figure 4 9 shows that at different Na/S02 ratios investigated this relationship is nonlinear. The decrease in the steady-state CONNA values with a decrease in the interfacial-area/ aqueous-volume ratio, at a fixed Na/S02 ratio was unexpected. This was probably caused by the manner in which the experi ments were conducted. The discs which were introduced to decrease the interfacial-area/aqueous-volume ratio were of a finite thickness. When they were introduced, the level of turbulence in the aqueous phase at the interface, where the sodium consuming reactions take place, decreased. That probably decreased the rate of sodium dithionite formation/ rate of the heterogeneous decomposition ratio. Figures 50 and 51 shows that at certain Na/SOj ratios entering the reactor an increase in temperature increases the steady-state Y„ and CONNA. The experimental results suggest that there may be optimum temperatures at every Na/S02 ratio above which the steady-state YgQ and CONNA will decrease. For most of the experimental runs done in this in vestigation, the steady-state pH of the aqueous phase was in the range 5 to 6. This range has been recommended by previous investigators (Section II.B). Few experimental runs that were done at very low pH values (0.8 to 2.0) gave very poor yeilds. When the pH is lowered, the rates of the homogeneous decomposition of the dithionite and the water reaction increase. The rate of the sodium dithionite formation reaction is also expected to increase with limited pH decrease because the effective diffusivity of sulfur dioxide solutions in the aqueous phase would increase as would the concentration of bisulfite ions. The model for the reacting system based on the experimental results suggests that at values of the Na/S02 ratio where the rate of sodium consumption is limited by the sodium mass-transfer rate, the steady-state Ycr. and bU2 CONNA would increase with an increase in the agitation in the amalgam phase. This information may be of a special 193 interest from the point of view of selecting the operating conditions for the proposed process. It was mentioned earlier that at relatively low Na/SC^ ratios, in the region under consideration, the steady-state CONNA is high but the steady-state Ycn is low. Considering the fact that both of these yields affect the cost of the product, it may be advantageous to increase the level of agitation in the amalgam phase at relatively low Na/S02 ratios. At fixed levels of all the other process variables if the flow rate of the amalgam is increased, the effect on the yields would be the same as if the Na/S02 ratio and the agitation in the amalgam phase had been increased. C. Economic Feasibility of the Proposed Process Experimental run 44 was chosen for costing. For this run, under the steady-state conditions, the concentra tion of sodium dithionite in the product stream, CS204 =2.3 gms Na2S2O4/100 ml the yield of sodium dithionite on the total sulfur dioxide entering the reactor, YS02 = 20.4% the yield of sodium dithionite on the sodium consumed in the reactor, CONNA = 67% the yield of sodium dithionite on sodium entering the reactor with the fresh amalgam YNa = 63% the conversion of sodium from the amalgam to different products in the reactor, XM = 95.1 % Na The levels of the process variables are presented in the Table 19. TABLE 19 LEVELS OF THE PROCESS VARIABLES FOR THE RUN 44 CHGF .1439 stso2 moles1 lit , 1.297 (RPM) jrpmj Aq 673 FLSO. lit min .096 FLHG ml min, 47.5 cm 3 cm , .0784 TEMP 17 PH 5.75 196 BASIS: 100 lb moles of total sulfur dioxide in the aqueous feed. The overall sodium dithionite formation is: 2 Na + 2 SO„ + NaoS~0, 2 2 2 4 For every 100 lb moles of the total sulfur dioxide in the feed, 20.4 lb moles are converted to give sodium dithionite. By stoichiometry, 20.4 lb moles of the sulfur dioxide give = 1/2 x 20.4 = 10.2 lb moles of Na2S204. Further 10.2 lb moles of Na2S204 contains = 2 x 10.2 = 20.4 lb atoms of sodium. The yield of sodium dithionite on sodium consumed = 67 per cent. * • • Total sodium consumed to give 10.2 lb moles of sodium 20.4 dithionite = = 30.45 lb atoms. 0.67 It was shown in Chapter I that, the cost of sodium in the amalgam - 5<=/lb, and the cost of S02 gas - 1.5$/lb © • • The chemical cost of sodium dithionite 100 x 64 x 1.5 + 30.45 x 23 x 5 10.2 x 174 = 7.4<:/lb of sodium dithionite The chemical cost of the dithionite ions produced by the 174 proposed process = 7.4 x - 10<r/lb S204 197 A small correction could be made for the cost of caustic soda used to adjust the pH of the aqueous sulfur dioxide solution. The actual cost of sodium dithionite produced by the proposed process would be higher because it would also include the capital and operating costs. However, this cost can be reduced by increasing the yields of Na^^O^ on sulfur dioxide entering the reactor and on sodium consumed in the reactor which can be accomplished by changing the levels of certain process variables as discussed in the last section. By comparison, zinc dithionite produced from zinc dust at 19<:/lb and sulfur dioxide at 1.5<r/lb, assuming 100% yields on both zinc and sulfur dioxide, gives dithionite at a cost of about llC/lb of S204 ions for chemicals alone. The actual cost of ZnS204 produced in pulp mills, considering lower yields, capital and operating costs, is approximately 16<Vlb of S2°4 ions' Thus, it seems economically feasible for a pulp mill to change from zinc dithionite produced in situ to sodium dithionite produced by the proposed process without involving extra cost. Additional advantages of the proposed process are that it can run in conjunction with Castner-Kellner type cells and does not discharge zinc ions to the effluent receiving waters. 198 D. Reactor for the Proposed Process The bench-scale experiments provide information on the type of reactor which would be suitable for the pro posed process. The chosen reactor should be such that it would be possible to have large interfacial-area/aqueous-volume ratio, high level of agitation in the aqueous and amalgam phases and high concentration of sulfur dioxide in the aqueous feed solution. The reactor used in the present investigation (CFSTR) has the advantages that it does not allow much entrainment of mercury in the product stream and permits fairly good control of the variables such as concentration of sodium in the feed-amalgam, flow rates of the sulfur dioxide solution and the amalgam, temperature and pH. It can be improved by redesigning it so that it has larger interfacial-area/ aqueous-volume ratio. The level of agitation in the amalgam phase can be increased by providing a stirrer and baffles in that phase and the concentration of sulfur dioxide in the aqueous feed can be increased by pressurizing the reactor system. 199 CHAPTER VII CONCLUSIONS 1. It is possible to produce sodium dithionite as a rel atively dilute (approximately 1-2%) water solution from sodium-mercury amalgam and sulfur dioxide in a simple "once-through" reactor [proposed process]. This solution could be used directly for the brightening of groundwood pulp. 2. The proposed process to produce sodium dithionite can economically compete with the manufacture of zinc dithionite in situ. 3. The reactor chosen for the proposed process should be such that it would be possible to have large interfacial-area/aqueous-volume ratio, high level of agitation in the aqueous and amalgam phases and high concentration of sulfur dioxide in the aqueous feed solution. Further, it should permit good control of the variables such as concentration of sodium in the feed-amalgam, the flow rates of the sulfur dioxide solution and the amalgam, temperature and pH. 4. The models suggested by Ketelaar (44) and Gerritsen (30) are inadequate to explain the experimental results 200 obtained in this investigation. To formulate a model for the reacting system sodium-mercury amalgam and aqueous sulfur dioxide, at a steady-state pH of about 5 to 6 and in the inert atmosphere of N2, it is necessary to consider mass-transfer of the reacting species in the amalgam and aqueous phases along with the chemical reactions such as the sodium dithionite formation reaction, the homogeneous and heterogeneous decomposition of the dithionite and the water reaction. 5. The production of sodium dithionite in the proposed sodium amalgam process depends primarily upon the Na/S02 ratios entering the reactor. At fixed levels of all the other process variables, when the concentration of sodium in the amalgam entering the reactor increases (the Na/S02 ratio entering the reactor increases at a fixed concentration of sulfur dioxide in the aqueous feed solution), the steady-state values of the concen tration of sodium dithionite in the reactor, the yield of sodium dithionite on sulfur dioxide in the aqueous feed and the conversion of sodium to different products in the reactor pass through a maximum. The steady-state values of the yield of sodium dithionite on sodium entering with the mercury and on sodium consumed in the reactor decrease with increasing Na/SO- ratio. 201 6. At Na/SG^ ratios below the steady-state sodium dithionite concentration maximum, the rate of sodium consumption is limited by the sodium mass-transfer rate to the inter face. At Na/S02 ratios above the steady-state sodium dithionite concentration maximum, the rate of sodium consumption is not limited by the sodium mass-transfer rate to the interface. 7. At Na/S02 ratios below the steady-state sodium dithionite concentration maximum, the rate of the sodium dithionite formation reaction is controlled by the rate of sodium mass-transfer to the interface, but at Na/S02 ratios above the steady-state sodium dithionite concentration maximum, the rate of the sodium dithionite formation reaction is controlled by the mass-transfer rate of bi sulfite ions to the interface. 8. When the steady-state pH of the aqueous phase in the reactor is between 5 to 6, the proportion of sodium consumed by the water reaction is small at Na/S02 ratios below the steady-state sodium dithionite concentration maximum. 9. At Na/S02 ratios below the steady-state sodium dithionite concentration maximum, the rate of the heterogeneous decomposition of the dithionite is limited by the sodium mass-transfer rate to the interface. At Na/S00 ratios 202 above the steady-state sodium dithionite concentration maximum, this rate is controlled by the chemical reaction at the interface. However, at very high Na/S02 ratios the rate of this reaction is controlled by the removal rate of the dithionite ions from the interface. The auto-decomposition of the dithionite according to the equation, 2 H2S204 3 S02 + S + 2 H20 , in the pH range 0.8 to 6 is negligibly small under the conditions of the present investigation. 203 CHAPTER VIII RECOMMENDATIONS FOR FURTHER WORK 1. The reactor used in the present investigation (CFSTR) should be redesigned or another type of reactor should be chosen according to the guidelines given in the discussion (Chapter VI). Then the proposed process should be optimized at a pilot-scale. 2. Further studies should be conducted to elucidate the processes in the reactor at lower values of pH (pH below 4). 3. The mercury content of the product stream should be determined to see if the product meets the pollution standards. If not, then the process should be modified along the lines suggested in Chapter 2. 4. If possible, the kinetics of the sodium dithionite formation reaction and the heterogeneous decomposition of the dithionite should be investigated separately. 5. The effect of variation in the concentration of electrolytes used in the proposed process on the rate of the water reaction should be investigated. 204 6. A quantitative expression should be determined for the rate of homogeneous decomposition of the dithionite. It would be of special interest to know how this rate is quantitatively affected by change in the concentration of ^2^3 ions. 7. The mean activity coefficients of the ionic species and activity coefficients of molecular species pre sent in the - H^O system should be determined at high concentrations of SO- gas in water (.5 to 2 molar). 205 CHAPTER IX NOMENCLATURE Symbol Explanation Typical Units A (A/V) Aq CHGF CHGS CNa CONNA 'S2°4 D E° El/2 ECONNA EX. Na EY Na EY SO, Activity of the indicated species in equations (1)/ (2), and (3) Surface area of interface Interfacial-area/aqueous-volume ratio Concentration of sodium in fresh amalgam Concentration of sodium in spent amalgam Concentration of sodium in amalgam Yield of sodium dithionite on sodium consumed Concentration of sodium dithionite in the product stream Diffusion coefficient Standard oxidation potential on hydrogen scale at 25°C Half-wave potential (reduction potential) The 95 per cent confidence limits of CONNA The 95 per cent confidence limits of XM Na The 95 per cent confidence limits of Y™ Na The 95 per cent confidence limits gm moles/liter cm 2 3 cm /cm gm Na 100 gm amalgam gm Na 100 gm amalgam gm moles liter % gm/100 ml cm /sec volts volts 206 Symbol Explanation Typical Units FLHG FLSO. AH' Activity coefficient of the indi cated species in equations (1)/ (2) , and (3) Flow rate of sodium-mercury amalgam Flow rate of aqueous sulfur dioxide solution Heat of formation of sodium dithionite at 25°C Reaction rate constant for the oxidation of sodium dithionite ml/min liter/min Kcal/gm moles ,g moles,-^-(sec)"1  ( liter > 2 water K, K hs M Na/S02 pH P Reaction rate constant for the homogeneous decomposition of sodium dithionite in equation (13) Reaction rate constant for the water reaction in equation (10) Thermodynamic ionization constant for the first dissociation of SO. •H20 in equation (2) SO, Thermodynamic ionization constant for the second dissociation of S02«H20 in equation (3) Thermodynamic equilibrium constant for S02*H20 system in equation (1) The Na/S02 ratio for a set of experimental runs at which maximum steady-state Cc _ is obtained b2°4 Ratio of sodium to sulfur dioxide entering the reactor pH of the aqueous phase Pressure of sulfur dioxide gas gm moles lit atm , (gm ions o -lQ9 ^ liter f H +) atm Symbol Explanation 207 Typical Units homo Rate of homogeneous decompo sition of sodium dithionite gm moles 3 . cm mm oxidation (RPM)Ag STRUB STS02 TEMP V VDITHI Na -Na SO. Rate of oxidation of sodium gm moles dithionite - liter sec Speed of the marine propeller rev/min Weight of sodium dithionite gms which would discolour 5 ml of a standard Rubine-R solution Concentration of total sulfur gm moles dioxide in the aqueous feed liter solution Temperature of the aqueous °C phase 3 Volume of the aqueous phase cm Volume of the product stream ml required to discolour 5 ml of a standard Rubine-R solution % Variable exponent in the rate equation (13) Conversion of sodium from the % amalgam to different products in the reactor Exponent in the rate equation (13) Yield of sodium dithionite on % sodium entering the reactor Yield of sodium dithionite on % sulfur dioxide entering the reactor BIBLIOGRAPHY Anderson, J.S. and Saddington, K., J. Chem. Soc, 381 (1949) . Anon., Chem. Eng., p. 146, Aug. (1952). Astarita, G., "Mass-transfer With Chemical Reaction," Elsevier Publishing Co., New York (1967). Bain, E.C. and Withrow, J.R., J. Phys. Chem., 25, 535 (1921) . Baly, E.C.C. and Bailey, R.A., J. Chem. Soc, 121, 1813 (1922) . Bassett, H. and Durrant, R.G., J. Chem. Soc, 2_, 1401 (1927) . Bent, H.E., J. Phys. Chem., 3J7, 431 (1933). BIOS Report 271 (PB 22049) . BIOS Report 422 (PB 34027). BIOS Final Report 1373 (1947) . Briamacombe, J.K., Graves, A.D. and Inman, D., Chem. Eng. Sci., 25, 1817 (1970). Bronsted, J.N. and Kane, N.L.R., J. Am. Chem. Soc, 53, 3624 (1931) . Chappell, N., Soc. of Dyers and Colourists J., 37, 206 (1921). Chassain, Y. and Ostertag, H., Compt. Rend., 242, 1732 (1956) . Cholette, A. and Cloutier, L., Can. J. Chem. Eng., 37, 105 (1959) . CRC Handbook of Chemistry and Physics, 51st Ed., 1970-71, p. F-23. 209 17. Crespi Gherzi, R.A., de Venturini, N.C. and de Alvarez, J.S., An.Asoc. Quim. Arg., 40, 1, 3A90 (1952). 18. Dijs, T., Dissertation (Delft), 1950. 19. Dijs, T., Hoogland, J.G. and Waterman, H.I., Chemistry and Industry, No. 44, 1073 (1952). 20. Dunning, W.G. and Kilpatrick, M., J. Phys. Chem., 42, 215 (1938). 21. Elbs, K. and Becker, K., Zeit. Electrochemie, No. 21, 361 (1904). 22. Eriksen, T.E., Chem. Eng. Sci., 22, 727 (1967). 23. Ibid., 2_4, 273 (1969). 24. Falk, M. and Giguere, P.A., Can. J. Chem., 35, 1195 (1957). 25. Ibid., 3±, 1121 (1958). 26. FIAT Final Report No. 818 (PB40351), June 25, 1946. 27. Fletcher, F.A. and Kilpatrick, M., J. Phys. Chem., 42/ 113 (1938). 28. Gardiner, W.C., Chem. Eng., 54, 11, 108 (1947). 29. Garrett, C.S., J. Chem. Soc, 107 , 1324 (1915). 30. Gerritsen, D.J., Dechema Monograph, 2_6, 279 (1956). 31. Getman, F.H., J. Phys. Chem., 30_, 266 (1926). 32. Gossman, B., Coll. Czechoslov. Chem. Commun., 2, 185 (1930). 33. Groothius, H. and Kramers, H., Chem. Eng. Sci. 4, 17 (1955) . 34. Hogg, R.D., B.A.Sc. Thesis, The University of British Columbia, Canada, 1965. 35. Hohn, H., Research, 3, No. 1, 16 (1950). 36. Ibid., 3, No. 9, 407 (1950). 37. Hynes, H.B.N., "The Biology of Polluted Waters," Liverpool University Press, 1960. 210 38. Ingruber, O.V. and Kopanidis, J., Pulp and Paper Mag. of Canada, 68^ No. 6, T-258 (1967). 39. Jellinek, K., Zeit. Electrochemie, No. 17, p. 157, p. 245 (1911) . 40. Jellinek, K. and Jellinek, E., Physik. Chem., 93, 325 (1919) . 41. Johnson, G., B.A.Sc. Thesis, The University of British Columbia, Canada, 197 2. 42. Jones, L.H. and McLaren, E., J. Chem. Phys., 28_, 995 (1958) . 43. Jouan, R., J. Chim. Physique, 5£, 327 (1959). 44. Ketelaar, J.A .A., Chemie - Ing. - Techn., 3_5, No. 5, 372 (1963). 45. King, C.V. and Braverman, M.M., J. Am. Chem. Soc, 54, 1744 (1932). 46. Klein, L., "River Pollution," Vol. II - Causes and Effects, Butterworth Inc., Washington, D.C, 1962. 47. Kolthoff, I.M. and Belcher, R., "Volumetric Analysis," Vol. Ill - Titration Methods: oxidation-reduction reactions, Interscience Publishers Inc., New York (1957) . 48. Kolthoff, I.M. and Miller, C.S., J. Am. Chem. Soc, 6_3, 2818 (1941) . 49. Kui-Shih Chia and Ung-Ping Wang, Annual meeting of Chinese Chemical Soc. at Chaiyi, Sept. 27 (1959) . 50. Lakshminarayanaiah, N., "Transport Phenomena in Membranes ," Academic Press, New York (1969). 51. Latimer, W.M., "Oxidation Potentials," 2nd Ed., Prentice-Hall, New York (1952). 52. Laura Creanga, Rev. Chim. (Bucharest), 9_, 219 (1958). 53. Lister, M.W. and Gravie, R.C., Can. J. Chem., 37, 1567 (1959). 54. Lynn, S., Ph.D. Thesis, California Inst, of Tech., 1954 . 211 55. Lynn, S., Straatemeier, J.R. and Kramer, H., Chem. Eng. Sci., 4, 49 (1955) . 56. MacMullin, R.B., Chem. Inds., 61, 41 (1947). 57. MacMullin, R.B., Chem. Eng. Prog., 46_, No. 9, 440 (1950) . 58. Maey, E., Zeit. f. Phys. Chem., 29, 119 (1899). 59. Maros, L., Koros, E., Feher, I. and Schulek, E., Magyar Kern. Folyoirat, 65_, 58 (1959) . 60. Marshak, E.M., Khim. Nauka i Prom., 2, 524 (1957). 61. McGlynn, A. and Brown, O.W., J. Phys. Chem., 33, 1165 (1929). 62. Meret, R.G., "Brightening and Bleaching of Mechanical Pulp," MacMillan, Bloedel and Powell River Ltd., Research Division, Powell River, B.C., Canada, 1966. 63. Meyer, J., z. anorg. chem., 3_4, 43 (1903). 64. Nicolai, H.W., Chimie und Industrie, 73, 1149 (1955). 65. Oil Paint and Drug Reporter, Sept. 29, 1969. 66. Olmstead, J.L., "Sodium Hydrosulfite ," Hooker Chemical Corp., Aug. 10, 1964. 67. Oloman, C. and Austin, R., Pulp and Paper Mag. of Canada, T529 (1969) . 68. Oloman, C, J. Electrochem. Soc, 117, No. 12, 1604 (1970) . 69. Patel, C.C. and Rao, M.R.A., Proc. Natl. Inst. Sci., India, 15, 127 (1949) . 70. Ibid., 19, 231 (1953). 71. Patel, C.C. and Rao, M.R.A., Bull. India. Sect. Electro chemical Soc, 8_, 37 (1959). 72. Peaceman, D.W., Sc. D. Thesis, Chem. Eng., M.I.T., 1951. 212 73. Pratt, L.A., Chem. and Met. Eng., 31^, 11 (1924). 74. Prestat, G., Industrie Chim. belg., 2_0, spec. No. II, 600 (1955). 75. Rabinowitsch, M. and Fokin, A.S., Z. Electrochem., 36_, 839 (1930) . 76. Ranshaw, N., Chemical Age, 41, 359 (1939). 77. Ratkowsky, D.A. and McCarthy, J.L., J. Phys. Chem., 66, 516 (1962) . 78. Riegel, E.R., "Industrial Chemistry," Reinhold Publish ing Corp., New York (1962). 79. Rindich, N.A., Nauk. Povidomlennya, Kiiv Univ., No. 1, 45 (1956) . 80. Rinker, R.G., Gordon, T.P., Mason, D.M., Sakaida, R.R. and Corcoran, W.H., Am. Doc. Inst., Washington 25, D.C, Document 6104 (1959). 81. Rinker, R.G., Gordon, T.P., Mason, D.M., Sakaida, R.R. and Corcoran, W.H., J. Phys. Chem., 64_, 573 (1960). 82. Rinker, R.G., Lynn, S., Mason, D.M. and Corcoran, W.H., Ind. Eng. Chem. (Fundamentals), 4_, 282 (1965). 83. Rinker, R.G. and Lynn, S., I. & E.C Product Research and Development, 8_, No. 4, 338 (1969) . 84. Rushton, J.H., Chem. Eng. Prog., 4_7, No. 9, 485 (1951). 85. Ibid., 48, No. 1, 33 (1952). 86. Ibid., 48, No. 2, 95 (1952). 87. Rushton, J.H. and Oldshue, J.Y., Chem. Eng. Prog., 49, No. 4, 161 (1953). 88. Ibid., 49_, No. 5, 267 (1953). 89. Rougeot, L., Compt. Rend., 222, 1497 (1946). 90. Schmidt, F., Ann. d. Physik, 3_9, 1108 (1912). 91. Scholder, R. and Denk, G., z. anorg. allgem. chem., 222, 48 (1935) . 213 92. Schuller, A., zeit. anorg. chem., 4_0, 385 (1904). 93. Scott, W.D., Ph.D. Thesis, University of Washington, Seattle, U.S.A., 1964. 94. Scribner, A.K., Am. Dyestuff Reporter, p. 185, March (1933). 95. Shaede, E.A. and Walker, D.C, Chem. Soc., London, Sp. Publ. No. 22, 277 (1967) . 96. Shaeffer, K. and Koehler, W., z. anorg. u. allgem. chem., 104, 212 (1918). 97. Shanahan, C.E.A. and Cooke, F., J. Appl. Chem., 7, 645 (1957). 98. Shreve, R.N., "Chemical Process Industries," 3rd Ed., McGraw-Hill Book Co., New York (1967). 99. Simon, A. and Waldmann, K., z. anorg. allg. chem., 281, 113 (1955). 100. Ibid., 283, 359 (1956). 101. Ibid., 28£, 36 (1956). 102. Spencer, M.S., Trans, of the Faraday Soc, 6_3, No. 10, 2510 (1967) . 103. Sterbacek, Z. and Tausk, P., "Mixing in the Chemical Industry," Pergamon Press Ltd., New York (1965). 104. Swift, E.H., "A System of Chemical Analysis (Qualitative and Semiqualitative) for the Common Elements," Prentice-Hall Inc., New York (1946). 105. Toor, H.L. and Chiang, S.H., Chem. Eng. Sci., 11_, 344 (1959). 106. Tyminski, A., Pulp and Paper Mag. of Canada, 6_8_, No. 6, T-258 (1967). 107. Vanstone, E., Trans. Faraday Soc, 7_, 4 2 (1911). 108. Vanstone, E., Chem. News, 103, p. 181, p. 198, p. 207 (1911) . 109. Vanstone, E., J. Chem. Soc, 105, 2617 (1914). 214 110. Vogel, A.I., "A Text-book of Qualitative Inorganic Analysis," 3rd Ed., John Wiley & Sons, New York, (1961) . 111. Wang, Jui-Chung, Ph.D. Thesis, The University of Texas, U.S.A., 1963. 112. Withrow, J.R., J. Phys. Chem., 2_0, 528 (1916). 113. Witney, R.P. and Vivian, J.E., Chem. Eng. Prog., 45, 322 (1949). 114. Wood, P.J., Am. Dyestuff Reporter, p. 443, June 17 (1957). 115. Wright, R., J. Chem. Soc, 105, 669 (1914). 116. Yost, D.M. and Russell, H., "Systematic Inorganic Chemistry," Prentice-Hall, New York (1944). 117. Yuferev, R.F. and Malushin, P.U., J. Chem. Ind. (Moscow), 7, 553 (1930). 118. Yui, N., Tokyo Inst. Phys. Chem. Res. Bull., 19, 1229 (1940) . 119. "Zinc Hydrosulfite," Bulletin 407 J, Virginia Chemicals, U.S.A. 120. Zjiwotinsky, Masjowetz, Fokin, Ukrain. Khem. Zhur, 6_, 205 (1931) . 121. Ibid., 6, 212 (1931) . PATENTS 122. British P. 25,870 (1910). 123. British P. 11,010 (1913). 124. British P. 700,042 (1953). 125. British P. 763,278 (1956). 126. British P. 786,212 (1957). 127. British P. 916,866 (1963). 128. British P. 1,148,248 (1969). 215 129 . D.R.P. 431,254 (1925) . 130 . D.R.P. 671,883 (1933) . 131. D.R.P. 641,673 (1935) . 132. D.R.P. 666,184 (1936) . 133 . D.R.P. 864,935 (1953) . 134 . D.R.P. 900,336 (1953) . 135. French P. 806,126 (1936). 136. French P. 819,947 (1937) . 137 . French P. 1,064 ,998 (1954). 138 . French P. 1,065 ,064 (1954). 139. French P. 1,363 ,064 (1964). 140. Ger. Offen. 1,036,228 (1958) 141. Ger. Offen. 2,000,877 (1970) 142. Holl. P 45,450 (1938) . 143. Holl. P '. 46,832 (1939) . 144 . Holl. P '. 47,030 (1939) . 145. Italian , P. 489, 137 (1954). 146. Japan P . 7,016, 327 (1970) . 147 . U.S.P. 2,193,323 (1940). 148 . U.S .P. 2,226,576 (1940). 149 . U.S.P. 2,938,771 (1960). 150. U.S.P. 3,226,185 (1965). 151. U.S.P. 3,411,875 (1968) . 152. U.S.P. 2,010,651 (1969). A-1 APPENDIX A EQUIPMENT SPECIFICATION 1. pH measurement The specifications of the instruments used for contin uous pH recording were as follows: pH electrode; Sargent-Welch S-30072-15 Thermocompensator; Sargent-Welch S-30115-03 pH meter; Sargent-Welch Model PBL S-30009 pH recorder; Sargent-Welch Model SRG pH recorder chart; Sargent Welch S-72166 2. Digital temperature recording All electronic instruments used for recording the temperature of the different streams were purchased from United Systems Corporation, Dayton, Ohio, U.S.A. Their specifications were as follows: Digital clock - 661 Scanner - 635 Digital millivolt meter - 451 Multiplexer - 64 2 Printer system - 611/620D BCD cable connecting digital clock to multiplexer 4378-6 BCD cable connecting scanner to multiplexer 4837-6 A-2 BCD cable connecting digital millivolt meter to printer system 4378-20 BCD cable connecting multiplexer to printer system 9054-20. 3. Calibration curves The calibration curves mentioned in Section IV.C. are shown in Figures A-I to A-V. A-3 10 20 30 40 SCALE A-I Flow rate of mercury pumped by Moyno pump versus micrometer setting on Graham transmission A-II Flow rate of aqueous sulfur dioxide versus reading on the rotameter scale > A-5 SCALE A-III Flow rate of cooling water versus reading on the rotameter scale A-6 A-IV Millivolt output of iron-constantan thermocouples versus temperature °C A-7 1.0 2-0 3-0 SCALE A-V RPM of the propeller versus micrometer setting on the thyratron controller for the variable speed drive (Heller motor) B-l APPENDIX B STATISTICAL EVALUATION OF ACCURACY AND PRECISION 1. Error of a measurement process 12 3 Eisenhart, Ku and Murphy have described the meaning of certain terms such as "precision" and "accuracy" which 3 specify the error of a measurement process. Strictly speaking, the actual error of a reported value, that is, the magnitude and sign of its deviation from the truth, is usually not ascertainable. Limits to this error, however, can usually be inferred, with some risk of being incorrect, from the precision of the measurement process by which the reported value was obtained, and from reasonable limits to the possible bias (or systematic error) of the measurement process. Precision of a measurement process refers to the closeness of successive independent measurements of a single magnitude. The measurements are generated by repeated applications of the process under specified conditions. The accuracy is determined by the closeness to the true value 1. Eisenhart, C, Science, Vol. 160, p. 1201, June (1968). 2. Ku, H.H., Measurements and Data, p. 72, July-August (1968) . 3. Murphy, R.B., Material Research and Standards, p. 264, April (1961) . B-2 of such measurements. Thus, if the bias, or systematic error, of a measurement process is known, its accuracy can be specified. Precision and accuracy are inherent characteristics of the measurement process employed and not of the particular end result obtained. It is important to note that a measure ment process may be extremely precise and at the same time not very accurate. Precision, in statistical language, is some times called "imprecision.11 Since imprecision and systematic error are distinctly different components of the uncertainty of a reported value, and are subject to different treatments and interpretation in usage, two numerics respectively expressing the imprecision and bounds to the systematic error of the measurement process should be given along with the measured quantity, whenever both of these errors are factors requiring consideration. According to Eisenhart^" four distinct cases need to be recognized, viz., (a) both systematic error and imprecision negligible, in relation to the requirements of the intended and likely use of results; (b) systematic error not negligible, imprecision negligible; (c) neither systematic error nor imprecision negligible; and B-3 (d) systematic error negligible, imprecision not negligible. Eisenhart discussed and outlined the recommended practices on the expression of uncertainties in the above-mentioned cases. 2. Evaluation of accuracy It has been mentioned in the last section that the bias or systematic error of a measurement process is a measure of its accuracy. The systematic error may be due to uncertainty in constants, uncertainty in calibrated values or bias in the method of computation. The following devices may yield some information re garding the presence of systematic error in a measurement process. (a) Measurement of a quantity whose true value is known. (b) Comparison with other measurement processes. (c) Comparison with modifications of the given measure ment process. The evaluation of systematic error of a measurement 4 process has been discussed in great detail by Youden. In short, (i) Systematic error can be estimated experimentally if the true value of the measured quantity is known. 4. Youden, W.J., Material Research and Standards, p. 268, April (1961) . B-4 (ii) Systematic error can be estimated from experience or by judgment. (iii) Systematic error can be estimated by a number of elemental systematic errors in the measurement process if they are known. The mode of specifying limits of systematic error 2 1 (or accuracy) has been well-described by Ku and Eisenhart. 3. Evaluation of precision (or imprecision) In any investigation, it is necessary to know how well the particular measured value is likely to agree with other values that the same measurement process might have provided in this instance, or might yield on remeasurement of the same magnitude on another occasion. Such information is provided by the estimated standard error of the reported value, which measures (or is an index of) the characteristic disagreement of repeated determinations of the same quantity by the same method and, thus, serves to indicate the precision of the reported value (more correctly, precision of the measurement process in reporting the measured value). In the following example the precision of a measure ment process (or test method) "within a laboratory" has been considered and an expression for its statistical estimate, standard error, has been derived. B-5 Suppose that a given measurement were repeated 10 times. It is called set 1, and n^ = 10. From this set of 10 values, the sample mean, x^, and the sample estimate of 2 the population variance, s^, can be calculated by using 5 the following equations n Z x. i=l 3 x = . . . .(B-l) n n 2 I (Xj-X) s2 = J-Zi—; . . . .(B-2) n-1 Therefore, 10 - _ A*1* x 2^ — —————— . . . . (B—• 3) 10 10 - v2 1 (xLi"xl) 2 i =1 J s^ = -2-^ ... . (B-4) (10 - 1) These quantities are estimates of the population mean, u, 2 and population variance a . If a second set of 10 measure ments of the same quantity were made, the values of x2 and 5. Mickley, H.S.; Sherwood, T.K. and Reed, C.E., "Applied Mathematics in Chemical Engineering," 2nd Ed., McGraw-Hill Book Co. Inc. (New York), 1957. B-6 2 s2, calculated from the second set of 10 values, would be expected to differ from the first set and from the population values. If it is supposed that a large number of sets, each of 10 measurements, were obtained, a new set comprising the sample means x^, x2, x^ could be generated. This set of means would exhibit some very important characteristics. If the grand mean, x^, is calculated k _ x_ = .... (B-5) m v (where k represents the number of sets), it will be found that xm is a better estimate of the population mean, u, than individual x^1s. Further, on the average, the deviation of x. from x will be less than the deviation of a single term, x m x^j, from the mean of the ith Set, x^. The sample estimate of the variance of the set of means Z (x.-x ) 2 1 1 m sm = ... .(B-6) m k-1 will be smaller than the sample estimate of the population 2 variance, s^. It is also found that the frequency distribution of the sample means, x^, about the population mean, y, is essentially normal, even though the population frequency distribution may be non-normal 1 When these findings are treated analytically, it is found that the sample variance of 2 the mean, s^, may be estimated from the variance calculated from a single set: 2 s2 2 In equation (B-7), s is the sample estimate of the population variance calculated from a single set of measurements, n is 2 the number of measurements in the single set, and s„ is the 3 m estimated variance of the set of means. The square root of 2 the estimated variance of the set of means, s , is called ' m' standard deviation of the mean or standard error. The standard error is usually denoted by s . u m It is desired to determine the limits within which the true population mean, y, will fall 95 times out of 100 termed the "95 per cent confidence limits on y" (sufficient for most engineering investigations). Student's 't' statistics could be used for small sample sizes. For the case discussed above "n-l,a= .05 _ x-y _ x-y sm s m — /n .(B-8) 95 per cent confidence limits on y are B-8 x - t , nc- s < u < x + t , nK sm ... (B-9) n-±,a=.uo m — — n-l,a=.05 m 4. Propagation of random error Most often an investigator comes across a situation where the precisions (more correctly/ precisions in the measurement processes) of several directly measured quantities are known and it is desired to estimate the precision (pre cision in the measurement process) of any function of these quantities. To illustrate propagation of random error, it is assumed that a quantity z is calculated from directly measured values of several quantities P^ by means of a mathematical relation which can be represented formally by z = f (Px, P2/ P3/ .../ P±) ... .(B-10) In a very general case, each measurement P is made m times under supposedly identical conditions. The action of random errors results in a series of values of P1: P12' P13' **"' Plm wnicn form the P^ set and a corresponding set forming the P2 set, etc. It is then desired to estimate the best value of z, variance of z and standard error of z (standard deviation of mean of z). B-9 If the errors in the P's are normally distributed and the variances of P's are small, then the best value of z 5 6 will be given by the following expression: ' z = fCP^ P2, P3, P±) ... .(B-ll) where P is the mean of a series of values in the set P. If the errors in P's are independent and not too large, 5—8 the variance of z is given by the following expression: 2 32 2 2 82 2 2 82 2 2 ot = (-=r a* + (_ )z of + ... + (—)* a; . .(B-12) 8P-L 1 3P2 *2 3P± i 3z The partial derivative —— is evaluated numerically at the 3P. mean value of each set: P^, P2, etc. It is important to note that validity of equation (B-12) is not dependent upon any 6. Parratt, L.G. "Probability and Experimental Errors in Science," Chapter 3, John Wiley and Sons,N.Y. (1961) . 7. Ratkowsky, D.A. "Notes on Statistical Techniques," ChE 453/81, October 14 , 1965, Dept. of Chem. Eng., U.B.C, Canada. 8. Bannett, CA. and Franklin, N.L. "Statistical Analysis in Chemistry and the Chemical Industry," John Wiley and Sons , N.Y. (1957) . B-10 assumption concerning a normal error frequency distribution. Only two conditions are required. First, that no correlation exist between the AP. terms (P, -P, = AP. P_ -P. = im^ lm^ 1 lm^; 2m2 2 AP2m , etc.), an assumption frequently termed "the assumption of statistical independence." Secondly, the errors in the P's must not be too large. On the other hand, the usual methods of relating variance to probability do involve the assumption of a normal frequency distribution. For the mathematical expression (B-10), the standard deviation of mean of z (standard error of z), a , can be z 6 m written as follows: 9 32 2 0*1 3Z 2 ^2 32 2 ^i a 2 = (__)2 __L_ + (_)2 _L_ + ... + (_)2 m 3P. m, 3P0 m^ 8P. m. 112 2 ii ....(B-13) Knowing the standard error of z, the 95 per cent confidence limits of the population mean of z can be estimated by using equation (B-9). One major problem involved in the use of equation (B-9) is that *t' statistics with known degrees of freedom are used for estimating the confidence limits. It may seem difficult to estimate the degrees of freedom of z in the case in point. However, if P^, P2, P^/ P^ are independent, considering different combinations of measurements in different sets, z will have very large B-ll number of degrees of freedom. Under these conditions 't' distribution would approach the 'N' (normal) distribution. Hence the 95 per cent confidence limits on the population mean of z can be estimated by Z - N AC S < Z < Z + N r\ c s ,n , .» a=.05 z — — a=.05 z ....(B-14) m m where, z = best estimate of population mean, z s = estimated standard deviation of the mean of z z m Na= 05 = [From normal probability table]. Practical application of equation (B-12) and (B-13) for a function like (B-ll) depends upon one's ability to 2 2 2 determine aB , a_ , , aD from experimental data, or to *1 *2 Fi make an a priori estimate of them. If it is experimentally possible to have estimates (from random samples) of the variances (by equation (B-2)) and variances of the mean (by equation(B-7)) of P^, P2, P3, •••,P^ etc., then 95 per cent confidence limits on the population mean of z can be estimated by equations (B-13) and (B-14) In the absence of an experimental random sample, it is possible to get an a priori estimate of the variance (or its square root, the standard deviation) from a con sideration of the maximum range of random error expected B-12 7 using a particular measurement method. It is known from elementary statistics that practically all (actually 99.73 per cent) of the area under the curve of the normal distribution is contained within ±3 standard-deviations of the population mean of a variable . Considering ±3a^^ to be synonymous with the range of the variable P^, then one merely needs to know or to be able to estimate the range to obtain a reasonable estimate of the standard deviation of the variable P. . l Sometimes the value of a particular quantity obtained from an instrument or measuring device remains very constant, but the limiting factor to precision of reading is the smallest scale division available on the instrument. If a temperature measurement made with a thermometer is considered in which the smallest division is 1°C, then it seems reasonable to assume that the maximum error range obtainable (due to human reading error alone) is -about 1°C (or ±.5°C). Thus an a priori estimate of standard deviation of temperature measure ment would be 0.5/3 = .167. Of course, the range may be considerably greater than 1°C, due to large fluctuations or instabilities in the temperature. In that case, one would once again require experimentation in order to obtain an estimate of the variance. The above discussion only applies to steady readings where the smallest scale division imposes a limiting factor on the precision of the reading. Once again B-13 if one considers the problem where z = f (P1# P2, P3 Pi) , the standard deviation of z, sz, can be estimated by equation (B-12) using a priori estimates of a , a • • •, a . Also, the 95 per cent confidence limits on the population mean of 7 z can be estimated. z " Na=.05 Sz 1 z 1 z + Na=.05 Sz ' • ' *(B"15) The '95 per cent confidence limits' estimated by equation (B-15) are broader than those calculated by equation (B-14). C-l APPENDIX C SODIUM-MERCURY AMALGAM 1. Purity of the chemicals in preparing amalgam (a) Mercury Technical grade mercury, obtained from various sources, was thoroughly washed and then distilled in a mercury still available in the laboratory. It was assumed that after distillation the mercury was 100 per cent pure. (b) Sodium metal Reagent grade sodium of the following assay was purchased from Canlab Supplies Ltd. (Toronto). Chloride (Cl) 0.0015% Nitrogen (N) ...... 0.003% Phosphate (P04) 0.0005Sulfate (S04) 0.002% Heavy metals ( as Pb) 0.0005% Iron (Fe) 0.001(c) Paraffin oil White, heavy (Saybolt viscosity 335/350), laboratory grade paraffin oil was purchased from Fisher Scientific Co. (Montreal). C-2 2. Problems encountered in preparation of amalgam (a) Heat of solution When sodium was dissolved in mercury, a large quantity of heat was liberated. Therefore, small pieces of sodium metal were slowly added to the mercury to promote controlled amalgamation. (b) Black precipitate in amalgam preparation When sodium was dissolved in mercury (kept under a thin layer of paraffin oil), a finely divided black precipitate was obtained which could be easily separated from the amalgam by mechanical means. This finely divided black precipitate was identified as mercury (formed by the reduction of HgO by sodium) which did not dissolve in the bulk of the amalgam because of a coat ing of paraffin oil on the droplets. Due to this problem it was difficult to make an amalgam of a known sodium-concentration starting with known quantities of mercury and sodium. (c) Disposal of small pieces of sodium and its oxide The layer of oxide, on the surface of sodium metal, was removed by scraping the surface of the metal with a knife under a layer of paraffin oil. The most effective way to dispose of tiny pieces of sodium and scrapings of the oxide was to dissolve them in a solution of iso-propanol and benzene (2:3 V/V). C-3 3. Calculation of sodium content in an amalgam sample (a) Normality of the standard NaOH solution, N„ _ = 0.1N -1 NaOH (b) Normality of the standard HoS0, solution, N„ = 0.1N (c) Weight of the Erlenmeyer + 100 ml distilled water, W1 = 275.3 gms (d) Weight of the Erlenmeyer + 100 ml distilled water + approximately 2.5 ml amalgam sample, W2 = 305.3 gms (e) Weight of the amalgam sample, W^^ = W2~W^ = 30 gms (f) Volume of N„ e_ sulfuric acid added to the Er lenmeyer, H2S°4 V"H2SO4 = °'025 LITER (g) Volume of NNaQH sodium hydroxide used to titrate the excess of sulfuric acid, V" = 0.020 liter. NaOH Sodium dissolved in mercury reacts with sulfuric acid according to the following reaction 2 Na + H2S04 -> Na2S04 + H2 By stoichiometry, the weight per cent of sodium in the amalgam sample, NAPT, can be given by the following expression. NAPT = lAP-P- (v" N - V" N ) (C-l) WAML H2S04 H2S04 Na0H Na0H * * C-4 For the above-mentioned example, NAPT = •2-|°p (0.025 x 0 .1 - 0 .020 x 0 .1) = 0.03 833 gms of sodium/100 gms of the amalgam. 4. Estimation of the precision of the analytical procedure The following steps were involved in the determination of weight per cent of sodium in amalgam by the proposed ana lytical procedure. (a) Standardization of an aqueous sodium hydroxide solution 1 2 by potassium hydrogen phthalate ' V„ ^„liters of N„ sodium hydroxide solution was NaOH NaOH J titrated with WpHp gms of potassium hydrogen phthalate dissolved in approximately 200 ml of distilled water. W N = PHP , * * NaOH 204 .22 V rtTT ... . ^-zj NaOH (b) Standardization of a sulfuric acid solution by the standardized sodium hydroxide solution 1. Swift, E.H., "A system of chemical analysis (qualitative and semiqualitative) for the common element." Prentice-Hall Inc. , New York, (1946) . 2. Vogel, A.I., "A text-book of qualitative inorganic analysis." 3rd Ed. John Wiley and Sons, New York, 1961. C-5 V I^SO^ liters of NH g0 sulfuric acid was titrated 2 4 with v'NaQH liters °f NNaOH so<^um hydroxide solution. N = NNaOH V'NaOH _ {Q_3) z q v' H2S04 (c) Sodium-mercury amalgam sample weighed WAML ^ms °^ an ama-L9am sample was weighed under a layer of distilled water using a Mettler balance. (d) Addition of standard sulfuric acid solution to the amalgam sample V" liters of the N sulfuric acid was pipetted H2b°4 2 °4 into the flask containing the amalgam sample. (e) Excess sulfuric acid back-titrated with the standard sodium hydroxide solution Sulfuric acid solution that did not react with sodium in the amalgam was titrated with V" liters of NXT rt„ sodium ^ NaOH NaOH hydroxide solution. As mentioned in Appendix C.3, the weight per cent of sodium in the amalgam can be given by the following expression. NAPT = 2300 (V" N - V" N ) WAML H2S04 H2S°4 Na0H Na0H (C-l) The measured values of the different quantities involved in the above mentioned steps ((a) to (e)) and the range of random errors associated with them have been presented in Table C-I • Standard deviations (and hence, the variances) of W^^, V" 0l~ and V" were estimated from the total range of the 2 4 NaOH 3 random errors in these quantities by the method outlined in Appendix B.4. The results are shown in the Table C-I. Standard deviations (and hence, the variances) of the different measured quantities in the equations (C-2) and (C-3), viz. WPHP' VNaOH' V,NaOH and V'H9S0. were also estimated from the knowledge of the total range of the random errors in these quantities (Table C-I). The values and the estimates of variances of N.T _TT and NTT n n . , c ,, NaOH H2S04 were calculated as follows: (i) Value and variance of N„ _„ NaOH Using equation (C-2), M WPHP 0.6127 n lft N = ; = =0.10 iNaun 204 .22 VNaQH 204 .22 x 0.03 Using equation (B-12), 2 _ (%aOH)2 02 + (8NNaOH 2 2 N ' W 3 V V NNaOH dWPHP WPHP °VNaOH NaOH = 31.229 x 10 TABLE C-I MEASURED VALUES, RANGE OF RANDOM ERRORS AND VARIANCES OF DIFFERENT QUANTITIES Variable Explanation Measured Value Measured To grange a=—3 a2 W PHP Weight of potassium hydrogen phthalate taken to standardize N NaOH .6127 gm ±.0001 gm 3.3xl0~5 1.09xl0~9 v NaOH Volume of NXT titrated with r. NaOH p.n.p. .030 lit ±.00005 lit 1.67xl0~5 2.79xl0"10 V NaOH Volume of NXT _„ taken to . , ,. NaOH standardize N„ ori H2so4 .025 lit * ±0 lit (pipetted) 0 0 V* H2S04 Volume of N„ titrated H2S04 With V'NaOH VOlume °f NNaOH .025 lit ±.00005 lit 1.67xl0"5 2.79xl0~10 W AML Weight of amalgam sample taken for analysis 30 gm ±.05 gm 1.67xl0~2 2.79xl0~4 V" H2S04 Volume of N„ er. added to the H2 4 amalgam sample .025 lit - ±0 lit (pipetted) 0 0 V" NaOH —__________ Volume of N„ used in back NaOH titration of the acid Amalgam No I Amalgam No II Amalgam No III .0248 lit .0200 lit .005 lit ±.00005 lit ±.00005 lit ±.00005 lit 1.67x10*";? 1.67x10*";? 1.67x10*° 2.79x10"^ 2.79x10 2.79xlO"-LU o I C-8 (ii) Value and variance of N„ H2S04 By equation (C-3), NNaOH V'NaOH 0.1 x 0.025 A N qn = = = 0.10 *VU4 V' Qn 0.025 H2S°4 Using equation (B-12), 2 ,Vy 2 . (9V42 a2 NH2S04 ~ NaOH NaOH ^a^ *aOH H.SO. 2 4 2 9 + ( ) <C, 9V,H2S04 H2S04 75.869 x 10'10 Knowing the variances of w , V"H , N , V and 2 4 2 4 NNaOH ^n ec3uati°n (c~l) i tne variance of NAPT was estimated by equation (B-12). The 95 per cent confidence limits on population mean of NAPT were estimated by equation (B-15). Sample calculations have been presented below, where the precision of the analytical procedure in determining the con centration of sodium in three different amalgams was estimated, Amalgam No. I (V"Na0H = 0.0248 liter) By equation (C-l), C-9 NAPT = ^|^- (0 .025 x 0.1 - 0.0248 x 0 .1) = 0.00153 Using equation (B-12), 2 _ ,8NAPTv2 .2 , ,3NAPT . 2 2 A , 3NAPT N2 NAPT " (3WTI7) AMT ) °V" ort + } X ^ 3V H2S04 H2S04 9NH2S04 2 + (a^T_) 2 02 + {^T_)2 2 H2S04 aV"NaOH NaOH 9 ^NaOH NaOH = 5.5561 x 10"8 <W =0-0002357 The 95 per cent confidence limits on the population mean, of NAPT (By equation (B-15)) = 0.00153 ± 1.96 (0.0002357) = 0.0015 ± 0.0005 • • Percentage precision - ±33% . Similar calculations were made for Amalgam No. II and No. Ill, Amalgam No II (V"Na0H = 0.0200 liter) NAPT = 0.03833 °2NAPT = 5'2068 x 10~8  aNAPT = 0.0002282 C-10 The 95 per cent confidence limits on the population mean of NAPT = 0.03833 ± 1.96 (0.0002282) = 0.0383 ± 0.0005 .*. Percentage precision - ±1.3% . Amalgam No. Ill (V"N QH = 0.005 liter) NAPT = 0.15333 °2NAPT = 5*2018 X 10"8 °NAPT = °-0002281 The 95 per cent confidence limits on the population mean of NAPT = 0.15333 ± 1.96 (0.0002281) = 0.1533 ± 0.0005 Percentage precision * ±0.3 3% D-l APPENDIX D AQUEOUS SOLUTION OF SULFUR DIOXIDE 1. Purity of the chemicals in preparing aqueous sulfur dioxide solution (a) Sulfur dioxide gas A C.I.L. sulfur dioxide tank was used as a source of S02 gas. (b) Water Freshly distilled water was used for making the aqueous solution of sulfur dioxide. (c) Sodium hydroxide Fisher certified ACS sodium hydroxide pellets were used for pH adjustment. Their composition was as follows: not less than 97% 0.5% 0.005% 0.001% 0.001% 0.003% 0 .02% 0 .002% Assay (NaOH) Sodium carbonate (Na^O^) Chloride (Cl) Nitrogen compounds (as N) Phosphate (PO^) Sulfate (S04) Ammonium hydroxide ppt Heavy metals (as Ag) D-2 Iron (Fe) 0.001% Nickel (Ni) ............ 0.001% Potassium (K) 0.02Copper (Cu) 0.001% 2. Calculation of the total sulfur dioxide concentration in an aqueous solution sample (a) Normality of the standard iodine solution, Nj = 0.1N (b) Normality of the standard sodium thiosulfate solution, N = 0.1N S2U3 (c) Volume of Nj iodine solution taken in the Erlenmeyer flask, Vx = 0.075 liter (d) Volume of the aqueous sulfur dioxide solution sample, injected into the iodine solution, V = 0.005' liter b°2 (e) Volume of the N sodium thiosulfate solution S2°3 required to titrate unreacted iodine in the Erlen meyer flask, V2 = 0.0095 liter. • • Molarity of total sulfur dioxide in the aqueous solution = —-— (NTV, - Nc _ V~) so 2 = - (0 .1 x 0 .075 - 0.1 x 0 .0095) 2x0.005 = 0.655 molar D-3 3. Estimation of the precision of the analytical procedure The calculation method was similar to that used in estimating the precision of the analytical procedure for evaluating the concentration of sodium in the amalgam. The following steps were involved in determining the concen tration of total sulfur dioxide in an aqueous solution by iodometric analysis as performed in our laboratory. (a) Preparation of a standard potassium dichromate solution (primary standard) W. pD(_ gms of pure anhydrous potassium dichromate was dissolved in V . liter of distilled water, water W Nry. n = — .... (D-l) ur2u7 49.035 V . water (b) Standardization of a sodium thiosulfate solution by the standard potassium dichromate solution V' _ liters of a N n sodium thiosulfate solution S2°3 S2°3 was titrated with V"Cr Q liters of the NCr Q potassium 2 7 2 7 dichromate solution. N V Cr-,0- Cr907 NQ n = — —- . . . . (D-2) b2U3 (c) Standardization of an iodine solution with the standard sodium thiosulfate solution liters of iodine solution was titrated with V _ liters of N _ sodium thiosulfate solution. S2°3 S2°3 Ns2°3 Vs2°3 • • N = —— — . . . .(D-3) VI (d) Excess of iodine solution taken in an Erlenmeyer flask liters of the iodine solution was taken in an Erlenmeyer flask and was diluted with distilled water to get a sharp end point. The volume was dependent on the con centration of sulfur dioxide in the fixed-volume sample to be tested. (e) Injection of the sulfur dioxide sample Vg0 liters of the sulfur dioxide solution was taken by a hypodermic syringe, and was injected into the contents in the Erlenmeyer. (f) Back-titration of excess iodine The unreacted iodine was titrated with liters of N S2O.J sodium thiosulfate solution. On the basis of the steps (a) to (f)/ the molarity of total sulfur dioxide could be given by the following expression: D-5 Molarity = —- (NTV, - Nc V0) ... .(D-4) 2 V_ n 11 S2°3 2  S2°3 Substituting the values of N and Nc _ from the equations i b2o3 (D-3) and (D-2) respectively, MI •* 1 , WPDC VCr207 % ,V1 VS2Q3 „ , Molarity = ( ) ( V„) 98.07 V' _ V0 V . VT S203 3 water I .... (D-5) Knowing the variances of the different quantities in the equation (D-5), the 95 per cent confidence limits on the population mean of molarity were estimated. E-1 APPENDIX E SODIUM DITHIONITE IN THE PRODUCT STREAM 1. Details of the analytical procedures and sample calculations (a) The iodine formaldehyde method (i) Detailed description of the iodine-formaldehyde method Two equal-volume (- 5 ml) samples of the product stream that contained primarily ^2^1' S2°3 and HS(^3 were taken using a hypodermic syringe. These samples (Sample I and Sample II) were analysed by the iodine-formaldehyde method in the following three parts. It is obvious that the amount of reagents used in different parts of the analytical procedure would depend on the concentrations of S2®~4' S2^3 and HSO.J in the sample. Hence, the values given below are only approximate. Part A - Determination of dithionite + thiosulfate + bisulfite: [a] A known volume (- 50 ml) of a standard (- 0.1N) iodine solution was taken in an Erlenmeyer flask and diluted to about 200 ml with oxygen-free distilled water. [3] A 5 ml sample (Sample I) of the product stream was in jected under the diluted iodine solution. Iodine oxidizes E-2 S2°4' S2°3 and HS03 Present in tne sample as follows: .(E-l) 2S203 + I2 S.O, + 21 4 6 . (E-2) HSCU + I0 + H„0 * HSO. + 2HI .(E-3) [y] Excess iodine was back-titrated with a standard (= 0.1N) solution of sodium thiosulfate using starch-solution as an indicator. Part B - Determination of dithionite + thiosulfate: [a] Approximately 25 ml of a formaldehyde solution (35 ml of 37 per cent HCHO: 50 ml of H20) at about pH = 9 was taken in an Erlenmeyer flask. It was diluted with oxygen-free distilled water (by bubbling N2 through freshly distilled water). A drop of phenolphthalein was added which turned the colour of the solution pink. [8] A 5 ml sample (Sample II) of the product stream was in jected under the diluted formaldehyde solution. If the pink colour of the solution disappeared, some NaOH solution was immediately added. The sulfoxylate formation took place as follows. The amount of iodine consumed by the sample in Part A gave the amount of (S^"^ + S20~ + HSO~) in Sample I. E-3 S204 + 2HCH0 + H20 -»• HCHO'HS03 + HCHO'HS02 . . . . (E-4) Any bisulfite (or sulfite) present in the sample was also tied down as formaldehyde bisulfite. [y] The flask was stoppered and allowed to stand for about 20 minutes during which time the reaction (E-4) completed. [<S] The solution was acidified (till the pink colour disappeared) by 20 per cent acetic acid solution in water. [e] A known volume of a standard (- 0.1N) iodine solution was added to the contents in the Erlenmeyer. Provided there was an excess of iodine, formaldehyde sulfoxylate and thiosulfate present in the system were oxidized as follows: HCHO'HS02 + 2I2 + 2H20 + S04 + HCHO + 4I~ + 5H+ . .(E-5) 2S2°3 + Z2 * S4°6 + 2I~ • • • .(E-6) Both reactions took place in acidic medium [?] Excess iodine was back-titrated with a standard (- 0.1N) solution of thiosulfate using starch-solution as an indicator. The amount of iodine consumed by E-4 HCHO»HS02 and S20~ gave the amount of (S20~ + S20~) in Sample II. Part C - Determination of thiosulfate: [a] After the end-point had been reached in Part A, approxi mately 30 ml of a 5 per cent sodium sulfite heptahydrate solution was added to the contents of the Erlenmeyer. The following reaction took place. S4Og + SO3 -> S30= + S20= .... (E-7) [83 The optimum pH range, within which the reaction was quantitative in five minutes, was 5.5 to 9.5. So a drop of phenolphthalein was added and a normal solution of sodium hydroxide was added until the solution turned pink. It was . allowed to stand for five minutes. [y] Approximately 5 ml of the formaldehyde solution was added to bind excess sulfite. [6] The contents in the flask were acidified with 10 ml of 20 per cent acetic acid. [e] Thiosulfate generated due to reaction (E-7) was titrated with a standard (- 0.1N) solution of iodine using a starch-solution as an indicator. The iodine consumed in Part C corresponds to 1/2 (thiosulfate present in Sample I + thiosulfate added in Part A). E-5 (ii) Sample calculations for the iodine-formaldehyde method At steady-state, during experimental run 23, two samples of 5 ml were taken by a hypodermic syringe. Normality of sodium thiosulfate solution used for analysis = 0.1N Normality of iodine solution used for analysis = .099N Data obtained in Part A: Volume of 0.099N iodine taken =60.0 ml Volume of sample = 5 ml Volume of 0.1N Na2S203 used in back-titration = 18.9 ml Data obtained in Part B: Volume of sample dissolved in formaldehyde • 5 ml Volume of 0.099N iodine taken = 20.0 ml Volume of 0.1N Na2S203 used in back-titration = 16.55 ml Data obtained in Part C: After adding 30 ml of 5 per cent sodium sulfite hepta-hydrate, Volume of .099N iodine used in the titration = 9.75 ml On the basis of the above mentioned data, S~07 + S~oT + S07 consumed (by oxidation in Part A) = (60.0 - 18,QXg,1)ml of .099N Iodine = 40.91 ml of .099N Iodine S203 in Sample I + S20_, added in Part A = 2 x 9.75 = 19.50 ml of .099N Iodine S2°3 added in Part A 5 18 'ogg'1 = 19.09 ml of .099N Iodine Therefore, S20_, in a 5 ml sample of product stream = 19.50 - 19.09 = 0.41 ml of .099N Iodine Na2S203 in the product stream _ 0.41X.099 _ 0Q004059 gm m°leS Na2S2°3 1000 -00004059 j-— S2°4 + S2°3 :'"n SamPle 11 consumed on 16.55x0.1 " 20 ^99^ = 3.283 ml of .099N Iodine t't Na2S204 in the product stream E-7 (3.283 - 0.41) x .099 _ Q000716 ^ moles Na2S2°4 4 x IOOO " : 5 ml Concentration of Na2S204 in the product stream .249 gms Na2S204 = .0000716 x 174 x 20 = 100 ml NaHSO 2 in the product stream (40.91 - 2.893 x | - 0.41) .099 2 x 1000 gm moles NaHSO_ = .0017899 5 ml sample 9•# Total SO2 in 5 ml of the product stream, S02 from Na2S204 = 2 x .0000716 = .0001432 gm moles S02 from Na2S203 = 2 x .00004059 = .0000812 gm moles S02 from NaHS03 = .0017899 = = .0017899 gm moles Total = .0020143 gm moles The concentration of 'total' SO2 going into the reactor was checked by iodometry. The concentration of sulfur dioxide in aqueous solution entering the reactor was .4 00 molar • • 5 ml of the sulfur dioxide solution entering the reactor contained .002 moles of S02 which compares well with the SO in the product stream. (b) The Rubine-R method (i) Detailed description of the Rubine-R method A standard solution of Rubine-R dye for dithionite analytical purposes was purchased from Virginia Chemicals/ Portsmouth/ Va., U.S.A. This dye solution was re-standar dized before use in the laboratory. Five ml of a standard (N^) solution of Rubine-R was pipetted into an Erlenmeyer flask. The dye solution was diluted with approximately 1.50 ml of oxygen-free distilled water. The Erlenmeyer flask with its contents was flushed with a stream of N2 gas and then it was stoppered with a rubber bung. Samples of product stream were drawn into a burette from the reactor, periodically, during an experi mental run. These samples were titrated with the bright red dye solution, with vigorous agitation, to an amber end-point. Throughout the titration, the burette and the Erlenmeyer flask were kept under N2 atmosphere. During the course of experiments, it was observed that at low pH (about pH = 1.0), the colour changed from bright red to colourless very sharply at the end-point. Hence, approximately 2 ml of concentrated sulfuric acid E-9 was added to the diluted Rubine-R before titrating it with dithionite in the product stream. It was observed that the addition of sulfuric acid to the Rubine-R solution did not have any significant effect on the results. If Vp liters of the product stream were titrated with V_._. liters of N..-, Rubine-R solution. #•m the concentration of ^28204 in the product stream 87 .0526 VRR NRR gmsNa^ 10 Vp 100 ml . . . .(E-8) The use of the Rubine-R method depends on accurate standar dization of the dye solution. Rubine-R solution was standardized with a standard solution of 112(804)-,. Pre paration and standardization of Ti2(S04)_, solution and standardization of Rubine-R with Ti2(S04)-, solution have been discussed below. [a] Preparation of Ti2(S04)-. solution A 20 per cent standardized solution of Ti2(S04)3 was purchased from La Motte Chemical Products CO.,Chestertown, Maryland, U.S.A. According to manufacturer's specification, the solution was prepared for chemical tests and was free of sulfides (commercial titanous sulfate solution sometimes contains sulfides which can be eliminated as hydrogen sulfide on boiling). One hundred ml of the concentrated E-10 solution was diluted to one liter by oxygen-free distilled water. The solution was kept under an atmosphere of oxygen-free carbon dioxide gas (C02 was scrubbed through a solution that contained one part of Na2S204, tWO Parts °^ NaHCO^ and 20 parts H20). [3] Standardization of Ti2(S04)3 Solution Ti2(S04)3 solution is unstable at high temperatures, in presence of oxygen and when exposed to direct sunlight. The following text-book methods were used to standardize diluted Ti2(S04)3 solution before its use in the laboratory. - The ferric ammonium sulfate method"*" - The potassium dichromate method^" 2 - The iron content determination method The results obtained by these three methods were comparable and reproducible. Only potassium dichromate method has been described here, briefly, for it is a very accurate and precise method. Information on the other methods can be obtained from the literature cited. 1. Vogel, A.I., "A text-book of qualitative inorganic analysis," 3rd Ed. John Wiley and Sons, New York, 1961. 2. Pierson, R.H. and Gantz, E. St. Clair., Analytical Chemistry, 26, No. 11, 1809 (1954). E-ll The potassium dichromate method: If very accurate results are desired or if the normality of ferric ammonium sulfate solution is not exactly known, the standardization of Ti^SO^)-, solution can be carried out with 0.1N potassium dichromate solution using the approximately 0.1N ferric ammonium sulfate solution as an intermediary. Into a 250 ml flask, 25.0 ml of primary standard 0.1N potassium dichromate and 20 ml of dilute sulfuric acid (2:5 W/W) were placed, oxygen-free carbon dioxide was passed to displace air and the stream of gas was maintained during the titration. Forty ml of the titanous sulfate solution was added to the contents in the flask. The liquids were mixed by swirling the flask gently and the excess titanous solution was titrated with 0.1N ferric ammonium sulfate solution. When the dark colour due to the titanous salt had nearly disappeared, 10 ml of 10 per cent ammonium thiocyanate solution was added and the addition of ferric ammonium sulfate solution was continued until a red or pink colouration was obtained which was permanent for at least one minute. In the same way 40.0 ml of the titanous sulfate solution was titrated with 0.1N ferric ammonium sulfate solution but the addition of 0.1N potassium dichromate solution was omitted. From the results obtained, the exact normality of the ferric ammonium sulfate solution and the E-12 normality of titanous sulfate solution was calculated. The method of calculation has been described below. If, NT = Normality of Ti2(S04)3 solution Npe = Normality of ferric ammonium sulfate solution N^ n = Normality of potassium dichromate solution or2u7 V"T = Volume of Ti^SO^)^ solution taken in the flask during the first titration where potassium dichromate solution was added V_ _ = Volume of potassium dichromate solution taken in <-r2u7 the first titration V_ = Volume of ferric ammonium sulfate solution used Fe in the first titration V.J, = Volume of Ti2(S04>2 solution taken in the flask during the second titration where no potassium dichromate solution was added V* = Volume of ferric ammonium sulfate solution used Fe in the second titration during first titration, VT NT " VCr207 NCr207 = VFe NFe ' ' * '<E"9) and during second titration, E-13 V1 N = V' N VT T Fe ™ Fe . (E-10) From equations (E-9) and (E-10), N, T .(E-ll) [y] Standardization of Rubine-R solution by standard Ti2(S04) 3 solution into a 300 ml Erlenmeyer flask. To this was added, 25 ml of distilled water, 5 ml of 10 per cent Na^O^ solution, 50 ml of methanol and 25 ml of 25 per cent sodium tartarate solution. The contents in the Erlenmeyer flask were boiled gently for five minutes while sweeping with oxygen-free C02 and were titrated hot with the standard solution of Ti2(SG"4) The colour change at the end-point was from pink to amber and was quite sharp. Twenty five ml of Rubine-R dye solution was pipetted 3. Wood, P.J./ Am. Dyestuff Reporter, 443, June 17 (1957) . E-14 If, N-^ = Normality of Rubine-R solution N T = Normality of Ti^SO^)-. solution V-^ = Volume of the Rubine-R solution taken in the flask V" = Volume of Ti-(SO.)solution used in the titration T 2 4 3 II VT N •'• NRR = ' ' • .(E-12) VRR (ii) Sample calculations for the Rubine-R method [a] Standardization of Ti-,(S04)2 solution by potassium dichromate solution: The following data was obtained for the parameters in equation (E-ll) Npy. = 0.10204 V_ = 0.020 liter VCr207 = °-010 liter V__ = 0.0112 liter Fe V,J, = 0.010 liter V' = 0.1055 liter Fe Using equation (E-ll), E-15 NT V N Cr O Cr O ^r2u7 cr2u7 0.010 x 0.10204 Vm-VT VFe 0.020 - 0.010 * 0.0112 T v, 0.01055 Fe = 0.10874N [8] Standardization of the Rubine-R solution by Ti2(S04)-, solution The following data was obtained for the parameters in equation (E-12) N_ = 0.10874N = 0.010 liter Vj = 0.0106 liter Using equation (V. 12), XT T NT 0.0106 x 0.10874 n 11KoCM NPR = ~v = = 0-11526N RR 0.010 [y] Concentration of sodium dithionite in the product stream During the experimental run 23, under steady-state conditions, the values of the parameters in equation (E-8) were as follows: E-16 VDD = 0.005 liter = 0 .11526N VP = 0.01828 liter • • Using equation (E-8), The concentration of Na2S204 ^n tne product stream 87.0526 VRRNRR 10 VP 87.0526 x 0.005 x 0.11526 10 x 0.01828 gms Na2S204 = 0.2744 100 ml of the product stream 2. Estimation of the precision of the Rubine-R method In this section various steps involved in determining the concentration of Na2S204 in a sample, from the product stream of the reactor, by the Rubine-R method have been outlined. The method of error calculation was indicated in connection with the analysis of an amalgam sample (Appendix C). For brevity, the calculations have not been attached. The different steps were: E-17 (a) Preparation of a standard solution of iodine. (b) Standardization of a sodium thiosulfate solution by the standard iodine solution. (c) Standardization of a potassium dichromate solution by the standard solution of sodium thiosulfate. (d) Standardization of a titanous sulfate solution by the standard solution of the potassium dichromate. (e) Standardization of a Rubine-R solution by the standard solution of titanous sulfate. (f) Titration of a product stream sample, containing sodium dithionite, with a known volume of the standard solution of Rubine-R. F-l APPENDIX F DATA PROCESSING 1. Mathematical expressions for calculating CQ n , Y 2 YNa, CONNA, XNa, Na/SO^y rate of sodium consumption, and sample calculations. (a) Mathematical expressions Mathematical expressions to calculate these quantities were derived from their definitions and the results have been included. The symbols used in these expressions have been explained in Chapter IX. It was assumed that the density of an amalgam did not change significantly in the range of temperatures used for the present investi gation. Therefore, the density of amalgam was calculated by the following expression (Section II.E.4): Density of the amalgam (gm/ml) = 13.55 - 0.9986 x (gms of sodium/100 gms of the amalgam) (i) Concentration of sodium dithionite in the product stream, C (gms of sodium dithionite) (100 ml of the product stream) 100 x STRUB VDITHI . (F-l) (ii) Yield of sodium dithionite on total sulfur dioxide entering the reactor (%), (gm molar cone, of ^2820^ in product) x 2 x 100 2 (gm molar cone, of total SC^ in aqueous feed) 2 x 105 x STRUB .... (F-2) 174.1052 x STS02 x VDITHI (iii) Yield of sodium dithionite on sodium entering the reactor with the fresh amalgam (%) , (gm moles of ^2820-4 in product/min) x 2 x 100 ^Na ~ (gm moles of Na entering with fresh amalgam/min) 46 x 107 x FLSO2 x STRUB 174.1052 x VDITHIx FLHG x CHGF x (13.55-0.9986 x CHGF) (iv) Yield of sodium dithionite on sodium consumed in the reactor (%), (gm moles of Na_S904 in product/min) x 2 x 100 CONNA = * * gm moles of Na entering with fresh amalgam/min\ -gm moles of Na leaving with spent amalgam/min J F-3 46 x 105 x FLSO, x STRUB CONNA =1 174.1052 x VDITHI x FLHG x (CHGF-CHGS) x (100 - CHGS) (F-4) (13.55 - 0.9986 x CHGF) (v) Conversion of sodium from the amalgam to different products in the reactor (%), (gm moles of Na entering with fresh amalgam/min -gm moles of Na leaving with spent amalgam/min x Na Cgm moles of Na entering with fresh amalgam/min) (100/CHGF - 1) = 100 x | 1 J . . . .(F-5) (100/CHGS - 1) (vi) Na/SQ2 ratio entering the reactor, (gm moles of Na entering with fresh amalgam/min) -(gm moles of Na leaving with spent amalgam/min) FLHG x CHGF x (13.55 - 0.9986 x CHGF) 2300 x STS02 x FLS02 ....(F-6) F-4 (vii) Rate of sodium consumption in the reactor, = (gm moles of Na entering with fresh amalgam/min) -(gm moles of Na leaving with spent amalgam/min) FLHG x (13.55 - 0.9986 x CHGF) 2300 x CHGF - (100 - CHGF) (100 - CHGS) x CHGS . .(F-7) (b) Sample Calculations Experimental run 54 was considered for the sample calculations. The process variables were fixed at the following levels: CHGF = .04029% STS02 = .6425 gm moles/liter (RPM)Ag = 673 rpm FLSO-, = .0955 liters/min FLHG = 47.5 ml/min (AA)A„ = .0784 cm2/cm3 TEMP = 17°C pH =5.6 F-5 Under the steady state conditions, CHGS = .00407% VDITHI = 7.40 ml STRUB = .049397 gms of Na2S204 (i) Steady-state concentration of sodium dithionite in the product stream Using equation (F-l) , 100 x STRUB 100 x .049397 gms C_ = = = .66753 &2U4 VDITHI 7.40 100 ml (ii) Steady-state yield of sodium dithionite on total sulfur dioxide entering the reactor (%) Using equation (F-2), 2 x 10 x .049397 SO- =  11.935% ^ 174.1052 x .6425 x 7.40 (iii) Steady-state yield of sodium dithionite on sodium entering the reactor with fresh amalgam (%) F-6 Using equation (F-3), 46 x 10' x .0955 x .049397 "Na 174.1052 x 7.40 x 47.5 x .04029 x (13.55-0.9986x.04029) = 65.145% (iv) Steady-state yield of sodium dithionite on sodium consumed in the reactor (%) Using the equation (F-4) , CONNA 46 x 10J x .0955 x .049397 ^ 174 .1052 x 7 .40 x 47 .5 x (.04029-.00407)/ x (100 - .00407) (13.55 - 0.9986 x .04029) = 72.462% (v) Steady-state conversion of sodium from the amalgam to different products in the reactor (%) Using equation (F-5), xNa - 100 *|1 -(100/.04029) - 1) (100/.00407) - 1) = 89.902% F-7 (vi) Na/S02 ratio entering the reactor Using equation (F-6), Na/S02 = 47.5 x .04029 x (13.55 - 0.9986 x .04029) 2300 x .6425 x .0955 = .18320 (vii) Rate of sodium consumption Using equation (F-7), Rate = 47.5 x (13.55 - 0.9986 x .04029) 2300 .04029 -(100 - .04029) (100 - .00407) x .00407 = .010106 gm moles/min 2. The 95 per cent confidence limits of the steady-state CS 0 ' YSO ' YNa' C0NNA' XNa' Na/S02 and rate of sodium 2 4 2 consumption for an experimental run. To estimate the 95 per cent confidence limits of the steady-state C , Y , Y , CONNA and XM for an experim _>2^^ 2 Wcl Ala run, it was necessary to know the values and the variances of the parameters used in equations (F-l) to (F-7). The data taken during experimental run 54 was used for the sample calculations. (a) Value and variance of STRUB The normality of the Rubine-R solution used for the analysis of sodium dithionite in the product stream was determined by the method outlined in the Section E.l.b. The normality of the Rubine-R solution used in the experimental run 54 was, NRR = .1134885N. The variance of the normality of the Rubine-R solution was estimated by the method of propagation of random error. The steps involved have been outlined in the Appendix E (steps a to e Section E.2). The variance of the normality of the Rubine-R solution was estimated to be, a = .856884 -7 . RR x 10 (• • 95 per cent confidence limits of N__, = ± .0006). STRUB = gms of Na2S204 which would discolour 5 ml of .1134885N Rubine-R solution = 87.0526 x .005 x .1134885 = .0493973 gms of Na2S204 Neglecting any error involved in pipetting the 5 ml of Rubine-R solution, the variance of STRUB was estimated to 2 7 * be, aSTRUB = .16236 x 10 (• • 95 per cent confidence limits of STRUB = ± .0003) . (b) Value and variance of VDITHI Volume of the product stream, at steady-state, required to discolour 5 ml of 0.1134885N Rubine-R solution, VDITHI, was 7.40 ml. The variance of VDITHI was estimated by knowing the maximum range of random error expected in using a 50 ml burette. aVDITHI = <X) 2 = 2,7889 X 10"4 • (• • 95 per cent confidence limits of VDITHI = ±.03) (c) Value and variance of STS02 2 The method to calculate STS02 and °gTgQ has been out lined in Appendix D. Knowing the values and the variances of the parameters in equation (D-5), the value and the variance of STS02 were calculated. STS02 = 0.6425 molar a2 -5 STS02 = .327441 x 10 (• • 95 per cent confidence limits of STS02 = ±.004). (d) Value and Variance of FLS02 The flow rate of the aqueous sulfur dioxide solution, FLS02, was determined by measuring the volume of the aqueous solution that flowed into a graduated cylinder in a fixed length of time. This procedure was repeated several times and the mean flow rate, FLS02, was found to be .0955 liters/ 2 mm. The variance of FLSO-, c?„T , was estimated by £ rLo(J2 equation (B-2). OFLS02 " -11758 * 10"5 (• v 95 per cent confidence limits of FLS02 = ±.002). (e) Value and variance of FLHG The flow rate of pure mercury, FLHG, was determined by measuring the volume of the mercury that collected in a graduated cylinder in a fixed length of time. This pro cedure was repeated several times and the following results were obtained. FLHG = 47.5 ml/min 2 FLHG = .188695 (• • 95 per cent confidence limits of FLHG = ± 0.8). F-ll (f) Values and variances of CHGF and CHGS The values of CHGF and CHGS were calculated by the method indicated in Appendix C. Knowing the variances of different measured quantities in the mathematical expression to calculate CHGF or CHGS (equation C-l/ Appendix C), the variances of CHGF and CHGS were calculated by the method of propagation of random error. The following results were obtained CHGF = .04029% aCHGF = '63426 X 10~7 (• • 95 per cent confidence limits of CHGF = ± .0005) . CHGS = .00407% rr2 7 °CHGS = .82510 x 10 (• • 95 per cent confidence limits of CHGS = ± .0005). Knowing the values and the variances of the parameters mentioned above, the variances of Cc n , Y _ , Y , CONNA, X. , Na/S02 and rate of sodium consumption were estimated by the method of propagation of random error. The 95 per cent confidence limits of these quantities were estimated by using equation (B-15) (Appendix B). The results were as follows: F-12 Cc ._ = .668 ± .005 (• • Percentage error - .75%) b2°4 Ycn = 11.9 ± .1 (• • Percentage error - .84%) bU2 YNa = 65 ± 2 (• • Percentage error = 3.1%) CONNA = 72 ±3 (• • Percentage error = 4.2%) XNa = 90 ± 1 (• • Percentage error = 1.1%) Na/S02= .183 ± .006 (•"• Percentage error = 3.3%) Rate of sodium consumption = .0101 ± .0003 (•*• Percentage error = 3%) It is interesting to note that despite a large error in determining CHGS (- 12%) , the error in CONNA and XNa is relatively small. 3. Experimental data and results Tables F-I to F-XIV show the steady-state values of the process variables and other quantities which were used to calculate the steady-state values of Na/S02, rate of sodium consumption, C , Y _ , Y , CONNA and XM . The 2 4 2 results and the 95 per cent confidence limits of Y„^ , Y„ S02 Na CONNA and are also presented. F-13 Tables F-XV to F-XXXII show the unsteady-state results for runs in the experimental sets: 42-46, 23-28 and 65-77. TABLE F-I STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 47-57 (Expts. 47, 50, 52, 53, 54, 55 and 57) (RPM) = 673; FLSOj = .0955; FLHG = 47.5; (A/V) = .0784; TEMP = 17; STRUB = .049397 RUN CHGF CHGS STS02 pH VDITHI Na/S02 cs2o4 Yso2  ±EYso2 ±EYNa CONNA ±ECONNA XNa ±EXNa 47 .09322 .01347 .65500 5.75 7.700 .414 .642 11.3 27.2 32 85.6 ±.1 ±.8 ±1 ±.5 50 .04573 .00383 .64500 5.80 6.650 .207 .743 13.2 64 70 92 ±.1 ±2 ±2 ±1 52 .04257 .00384 .63950 5.60 7.520 .194 .657 11.8 61 67 91 + .1 ±2 ±2 ±1 53 .07735 .00762 .64775 6.0 7.158 .348 .690 12.2 35 39 90.2 ±.1 ±1 ±1 ±.7 54 .04029 .00407 .64250 5.60 7.400 .183 .668 11.9 65 72 90 ±.1 ±2 ±3 ±1 55 .05776 .00399 .63900 5.80 5.770 .264 .856 15.4 58 63 93.1 . ±.1 ±2 ±2 ±.9 57 .06854 .00391 .65625 5.95 5.60 .304 .882 15.4 51 54 94.3 ±.1 ±2 ±2 ±.7 TABLE F-II STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 65-77 (Expts. 65, 67, 69, 71, 73, 75 and 77) (RPM)Ag = 673; FLSO, > = .0955 ; FLHG = 47.5; (A/V)Ag . = .0784; TEMP - 17; STRUB = .049397 RUN CHGF . CHGS STS02 pH VDITHI RATE OF Na CONSUMPTION* Na/S02 Cs2°4 Yso2 ±EY YNa ±EYNa CONNA +ECONNA XNa ±EXNa 65 .10104 .01932 .65125 6.00 8.250 .0227 .451 .599 10.56 ±.09 23.4 ±.7 28.9 ±.9 80.9 ±.5 67 .06626 .00387 . .65000 5.95 5.600 .0174 .297 .882 15.6 ±.1 52 ±2 56 ±2 94.2 ±.8 69 .04677 .00374 .65050 5.65 6.733 .0120 .210 .734 13.0 ±.1 62 ±2 67 ±2 92 ±1 71 .07542 .00609 .65800 5.95 6.717 .0193 .334 .735 12.8 ±.1 38 ±1 42 ±1 91.9 + .5 73 .05914 .00414 .65300 5.80 5.700 .0153 .264 .867 15.2 ±.1 58 ±2 62 ±2 93.0 ±.9 75 .03921 .00391 .65075 5.40 7.795 .0098 .176 .634 11.2 ±•1 64 ±2 71 ±3 90 ±1 77 .08920 .01104 .65550 5.70 7.770 .0217 .396 .636 11.1 ±.1 28.1 ±.8 32 ±1 87.6 ±.6 * gm moles/min TABLE F-III STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 66-76 (Expts. 66, 68, 70, 72, 74 and 76) (RPM) = 225; FLSO, = .0955; FLHG => 47.5; (A/V) = .0784; TEMP = 17; STRUB = .049397 RUN CHGF CHGS STSO- pH VDITHI Na/S02 Cs2°4 Yso2  ±EYso2 YNa ±EYNa CONNA ±ECONNA XNa ±EXNa 66 .06626 .00421 .6500 5.95 6.120 .297 .807 14.3 48 51 93.7 ±.1 ±1 ±2 ±.7 68 .04677 .00415 .65050 5.65 7.237 .210 .683 12.1 57 63 91 ±.1 ±2 ±2 ±1 70 .07542 .00703 .65800 5.90 7.450 .334 .663 11.6 35 38 90.7 ±.1 ±1 ±1 ±.5 72 .05914 .00471 .65300 5.75 6.237 .264 .792 13.9 53 57 92.0 ±.1 ±2 ±2 ±.9 74 .03921 .00361 .65075 5.35 8.400 .176 .588 10.38 59 65 91 ±.09 ±2 ±2 ±1 76 .08920 .01437 .65550 5.75 9.330 .396 .529 9.28 23.4 27.9 83.9 ±.08 ±.7 ±.8 ±.5 TABLE F-IV STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 62-63 (Expts. 62 and 63) (RPM) - 225; FLSOj = .0955; FLHG - 47.5; (A/V) = .0784; TEMP = 17; STRUB - .049397 RUN CHGF CHGS STS02 pH VDITHI Na/S02 Cs2°4 Yso2  ±EYso2 YNa ±EYNa CONNA ±ECONNA XNa ±EXNa. 62 .10116 .02563 .64550 6.15 9.750 .456 .507 9.02 19.8 26.5 74.7 ±.07 ±.6 ±.8 ±.5 63 .06352 .00413 .6390 5.70 6.210 .290 .795 14.3 49 53 93.5 ±.1 ±1 ±2 ±.7 TABLE F-V STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 87-91 (Expts. 87, 89 and 91) (RPM) = 225; FLS02 = .0955; FLHG = 47.5; (A/V)A = .0784; TEMP =17; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 cs2o4 Yso2  ±EYso2 YNa ±EYNa CONNA +ECONNA XNa ±EXNa 87 .05605 .00472 .65800 5.75 6.172 .249 .770 13.4 54 59 92 ±.1 ±2 ±2 ±1 89 .09863 .02162 .65450 5.85 9.064 .438 .524 9.2 21.0 26.9 78.1 ±.1 ±.6 ±.8 ±.5 91 .03723 .00360 .65800 5.35 8.38 .165 .567 9.9 60 66 90 ±.1 ±2 ±2 ±1 TABLE F-VI STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 95-105 (Expts. 95, 97, 99, 103 and 105) (RPM) = 673; FLS02 = .198; FLHG = 47.5; (A/V)A = .0784; TEMP = 17; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI RATE OF Na CONSUMPTION* Na/S02 cs2o4 Y-2 ±EYso2 YNa ±EYNa CONNA ±ECONNA XNa ±EXNa 95 .04387 .00533 .65700 5.20 14.040 .0108 .094 .338 5.92 63 72 88 ±.06 . ±2 ±2 ±1 97 .09119 .00487 .65800 5.65 8.010 .0240 .195 .593 10.4 53 56 94.7 ±.1 ±1 ±1 ±.6 99 .08184 .00467 .65700 5.60 8.450 .0215 .175 .562 9.8 56 ' 60 94.3 ±.1 ±1 ±1 ±.6 103 .05460 .00455 .66000 5.40 11.267 .0140 .116 .422 7.34 63 69 92 ±.08 ±1 ±2 ±1 105 .07107 .00487 .65300 5.65 9.017 .0184 .153 .527 9.3 61 65 93.2 ±.1 ±1 ±2 ±.8 TABLE F-VII STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 23-28 (Expts. 23, 24, 25, 26, 27 and 28) (RPM)- = 673; FLSC2 = .0955; FLHG = 47.5; (A/V)A = .0784; TEMP = 17; STRUB = .050169 RUN CHGF CHGS STS02 pH VDITHI RATE OF Na CONSUMPTION* Na/S02 cs2o4 Yso2  ±EYso2 YNa ±EYNa CONNA ±ECONNA XNa ±EXNa 23 .01725 .00176 .39996 5.20 18.283 .0043 .126 .274 7.88 62 70 90 ±.08 ±3 ±4 ±4 24 .03765 .00376 .40300 5.70 14.200 .0095 .273 .353 10.1 36.9 41 90 ±.1 ±1 ±1 ±1 25 .05444 .01243 .39898 5.90 19.100 .0117 .398 .263 7.56 19.0 24 .6 77 ±.08 ±.6 ±.9 ±1 26 .04712 .00688 .39924 5.70 17.000 .0112 .345 .295 8.49 24.6 29 85 ±.09 ±.8 ±1 ±1 27 .03027 .00268 .39700 5.40 13.100 .0077 .223 .383 11.1 50 55 91 ±.1 ±2 ±2 ±2 28 .02428 .00239 .39949 5.30 14.850 .0061 .178 .338 9.7 55 61 90 ±.1 ±2 ±3 ±2 gm nles/min TABLE F-VIII STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 42-46 (Expts. 42, 43, 44, 45 and 46) (RPM) = 673; FLS02 = .0955; FLHG = 47.5; (A/V) = .0784; TEMP = 17; STRUB = .098795 RUN CHGF CHGS STS02 pH VDITHI RATE OF Na CONSUMPTION* Na/S02 Cs2°4 Yso2  ±EYso2 YNa ±EYNa CONNA ±ECONNA XNa ±EXNa 42 .06561 .00542 1.3100 5.00 7.119 .0168 .146 1.39 12.2 83 91 91.7 ±.1 ±2 ±3 ±.7 43 .10344 .00678 1.3000 5.50 5.217 .0268 .231 1.89 16.7 72 77 93.5 ±.2 ±2 ±2 .±•5 44 .14386 .00708 1.2970 5.75 4.300 .0379 .322 2.30 20.4 63 67 95.1 ±.2 ±2 ±2 ±.3 45 .21265 .03749 1.2875 5.00 6.400 .0483 .476 1.54 13.8 28.9 35 82.4 ±.1 ±.9 ±1 ±.2 46 .18438 .01515 1.2638 5.70 5.600 .0467 .422 1.76 16.0 38 41 91.8 ±.2 ±1 ±1 ±.2 gm moles/min TABLE F-IX STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 86-90 (Expts. 86, 88 and 90) (RPM)Aq = 110; FLS02 " -0955; FLHG - 47.5; (A/V) = .0784; TEMP = 17; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 %o4 YSO-±EYso2 YNa ±EYNa CONNA ±ECONNA XNa ±EXNa 86 .05605 .00494 .65800 5.75 6.394 .249 .743 13.0 52 57 91 ±.1 ±2 ±2 ±1 88 .09863 .02232 .65450 5.80 9.467 .438 .502 8.81 20.1 26.0 77.4 ±.09 ±.6 ±.8 ±.5 90 .03723 .00353 .65800 5.35 8.590 .165 .553 9.7 58 64 91 ±.1 ±2 ±2 ±1 TABLE F-X STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 94-104 (Expts. 94, 96, 100, 102 and 104) (RPM) = 673; FLS02 = .0656; FLHG = 47.5; (A/V) = .0784; TEMP = 17; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 °S2°4 Yso2 YNa CONNA Na iEYso2 ±EYNa ±ECONNA ±EX„ Na 94 .04387 .00351 .65700 5.95 4.580 .284 1.037 18.1 64 69 92 ±.2 ±2 ±2 ±1 96 .09119 .03275 .65800 6.10 7.850 .587 .605 10.6 18.0 28.1 64 ±.1 ±.5 ±.9 ±.5 98 .08184 .02440 .65700 6.10 7.502 .528 .633 11.1 21.0 29.9 70.2 ±.1 ±.6 ±.9 ±.6 100 .02801 .00324 .65150 5.75 6.945 .183 .684 12.1 66 74 88 ±.1 ±2 ±3 ±2 102 .05460 .00449 .66000 6.10 5.420 .351 .876 15.3 43 47 92 ±.2 ±1 ±2 ±1 104 .07107 .01469 .65300 6.10 7.270 .462 .653 11.5 24.9 31 79.3 ±.1 ±.7 ±1 ±.8 TABLE F-XI STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 122-134 (Expts. 122, 125, 128, 131 and 134) (RPM)^ = 673; FLSO,, = .0955; FLHG = 47.5; (A/V)- = .0247; TEMP = 17; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 Cs2°4 *so2  ±EYso2 YNa ±E*Na CONNA ± ECONNA XNa ±EXNa 122 .06283 .00629 .66000 5.70 8.930 .278 .532 9.3 33 37 90 . ±.1 ±1 ±1 ±1 125 .12382 .05016 .66064 5.90 12.793 .544 .371 6.46 11.9 19.9 59.5 ±.07 ±.4 ±.6 ±.4 128 .04884 .00527 .65802 5.70 10.038 .217 .473 8.26 38 43 89 ±.09 ±1 ±1 ±1 131 .07673 .00942 .65628 5.95 9.900 .341 .480 8.40 24 .7 28.1 87.7 ±.09 ±.8 ±.9 ±.7 134 .08789 .01267 .66400 5.80 10.160 .385 .467 8.09 21.0 24.5 85.6 ±.08 ±.6 ±.7 ±.6 TABLE F-XII STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 123-135 (Expts. 123, 129, 132 and 135) (RPM) = 673; FLS02 = .0955; FLHG = 47.5; (A/V). = .0095; TEMP = 17; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 cs2o4 Yso2 +EY xso2 YNa ±EYNa CONNA ±ECONNA Na ±EX„ Na 123 .06283 .01603 .66000 5.65 15.897 .278 .299 5.20 18.8 25.2 74.5 ±.04 ±.4 ±.7 ±.9 129 .04884 .01494 .65802 5.65 19.350 .217 .245 4.29 19.8 28' 69 ±.04 ±.6 ±1 ±1 132 .07673 .02303 .65628 5.80 17.275 .341 .275 4.81 14.1 20.2 70.0 ±.05 ±.4 ±.7 ±.7 135 .08789 .03064 .66400 5.65 17.100 .385 .278 4.81 12.5 19.1 65.2 ±.05 ±.4 ±.6 ±.6 t TABLE F-XIII STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 106-118 (Expts. 106, 109, 112, 115 and 118) (RPM) = 673; FLSOj = .0955; FLHG = 47.5; (A/V). = .0784; TEMP = 13; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 Cs2°4 vso2  ±E*so2 CONNA ±ECONNA 106 .08996 .00742 .65300 6.00 7.740 .401 .614 10.8 29.3 ±.1 ±.9 109 .06455 .00421 .65350 5.80 5.198 .288 .803 14. T 52 ±.2 ± 2 112 .03715 .00344 .65250 5.55 7.947 .166 .598 10.5 70 ±.1 ± 3 115 .05455 .00431 .65950 5.80 6.073 .241 .782 13.6 61 ±.2 ± 2 118 .09505 .00875 .65471 6.00 7.932 .422 .599 10.5 27.4 ±.1 ±.9 TABLE F-XIV STEADY-STATE DATA AND RESULTS FOR EXPERIMENTAL SET 110-113 (Expts. 110 and 113) (RPM) = 673; FLS02 = .0955; FLHG = 47.5; (A/V)A = .0784; TEMP = 27; STRUB = .047495 RUN CHGF CHGS STS02 pH VDITHI Na/S02 Cs2o4 Yso2  ±EYso2 CONNA +ECONNA 110 .06455 .00354 .65350 5.90 5.037 .288 .943 16.6 61 ±.2 ±2 113 .03715 .00357 .65250 5.70 7.850 .166 .605 10.7 71 ±.1 ±3 TABLE XV UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 4 2 Initial pH = 3.4; Final pH = 5.0; CHGF » .06561 TIME (min) Cs2o4 10.4 .915 14.3 1.040 17.5 1.116 21.5 1.198 24.4 1.235 30.0 1.283 35.0 1.291 39.5 1.300 44.0 1.309 51.6 1.392 54.0 1.372 57.5 1.411 61.1 1.392 63.8 1.382 66.6 1.392 69.6 71.4 1.353 1.372 73.8 1.392 TIME 4.5 7.5 12.0 19.0 29.0 34.0 41.0 68.0 73.0 (nin) CHGS .00560 .00593 .00524 .00545 .00503 .00550 .00558 .00506 .00449 TABLE XVI UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 43 Initial pH - 3.3; Final pH = 5.5; CHGF = .10344 TIME (min) 9.7 13.7 16.9 21.3 27.2 30.8 34.7 37.3 41.8 44.8 49.4 52.0 56.0 58.4 cs2o4 1.176 1.432 1.544 1.689 1.780 1.830 1.864 1.864 1.882 1.918 1.900 1.900 1.882 1.882 TIME (min) 3.1 8.5 11.5 18.5 23.0 29.0 33.0 46.5 54.0 59.5 CHGS .00888 .00649 .00630 .00673 .00685 .00634 .00697 .00666 .00702 .00688 I ro co TABLE XVII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 44 initial pH •= 3.3; Final pH = 5.75; CHGF = .14386 TIME (min) 6.5 9.8 14.3 16.6 22.0 25.0 28.8 32.8 37.5 41.4 65.2 67.6 74.5 %o4 1.326 1.620 1.976 2.058 2.171 2.195 2.245 2.271 2.271 2.298 2.298 2.298 2.298 TIME (nin) 4.5 7.8 11.3 18.0 27.0 52.0 57.0 63.0 71.0 CHGS .00754 .00712 .00707 .00705 .00717 .00720 .00697 .00715 .00689 TABLE XVIII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 45 Initial pH = 3.15; Final pH = 5.0; CHGF = .21265 TIME (min) 8.4 11.6 14.9 20.4 23.3 27.9 31.5 35.4 40.9 45.5 49.5 51.9 55.5 Cs2o4 1.718 1.918 2.016 1.882 1.813 1.689 1.633 1.620 1.556 1.556 1.544 1.544 1.544 TIKE (min) 2.5 7.0 10.5 16.3 22.0 25.3 29.5 33.3 37.5 47.0 61.5 CHGS .04245 .00802 .00777 .01504 .02939 .03519 .03608 .03698 .03735 .03850 .03664 I ro io TABLE XIX UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 46 Initial pH «= 3.0; Final pH = 5.7; CHGF - .18438 T I"E (r.in) 6.2 10.8 15.8 18.3 21.8 25.2 28.7 32.3 36.8 40.1 43.1 46.4 48.9 51.6 54.8 57.3 60.0 62.5 65.3 67.8 70.3 1.259 1.900 2.148 2.195 2.245 2.171 2.102 1.976 1.956 1.882 1.882 1.830 1.830 1.813 1.749 1.7 96 1.780 1.780 1.749 1.764 1.764 TIKE (r.in) 3.0 8.S 14.3 20.2 24.0 27.0 33.8 39.0 4S.0 53.0 63.5 CHGS .02508 .00776 .00717 .00915 .01070 .01166 .01348 .01384 .01468 .01616 .01462 TABLE XX UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 23 Initial pH - 4.0 Final pH 5.2; CHGF - .01725 TIMS (.-in) 4.S 7.4 13.2 23.6 27.2 30.6 32.7 34.5 41.7 46.6 52.5 54.7 57,1 60.9 65.5 68.9 .148 .181 .216 .255 .263 .265 .267 .273 .268 .274 .276 .271 .278 .271 .276 .274 (.-in) 3.8 13.8 30.0 45.0 50.0 73.0 74.0 CHGS .00174 .00176 .00178 .00170 .00200 .00206 .00150 f I co o TABLE XXI UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 24 Initial pH = 4.2; Final pH •» 5.7; CHGF «• .03765 TIME 7.7 11.5 15.7 19.8 32.9 39.5 52.8 57.5 61.1 63.9 66.7 68.4 (min) Cc n .261 .320 .364 .369 .364 .366 .356 .353 .353 .353 .344 .353 b2°4 TIME 10.0 15.4 24.8 35.0 41.0 49.3 60.9 (min) CHGS .00220 .00300 .00370 .00370 .00400 .00400 .00376 TABLE XXII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 25 Initial pH = 4.1; Final pH - 5.9; CHGF - .05444 TIKE 6.8 10.3 14.2 18.9 22.3 29.0 37.0 72.4 77.0 80.0 84.0 (min) C, n .215 .261 .282 .282 .280 .267 .263 .261 .263 .263 .263 i>2°4 TIME 8.4 11.8 20.2 26.8 59.5 70.2 78.0 84.0 (min) CHGS .00487 .00618 .01081 .01157 .01243 .01306 .01243 .01243 TABLE XXIII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 26 Initial pH - 4.1; Final pH » 5.7; CHGF •» .04712 TIME ( min) 9.7 12.7 16.6 19.9 23.3 28.3 35.0 39.3 42.5 47.9 53.0 55.9 60.5 cs2o4 .282 .326 .330 .330 .328 .318 .302 .299 .295 .297 .293 .295 .295 TIME (min) 6.5 16.0 23.0 37.0 45.0 50.0 58.0 63.0 64.0 CHGS .00334 .00400 .00601 .00718 .00737 .00672 .00668 .00677 .00657 TABLE XXIV UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 27 Initial pH = 4.0; Final pH - 5.4; CHGF = .03027 T I ME (min) 8.0 12.9 16.9 19.8 26.0 29.0 33.2 37.3 41.6 46.9 50.6 55.5 Cs2o4 .237 .295 .337 .344 .366 .380 .383 .380 .382 .386 .382 .386 TIME (min) 5.1 10.8 22.0N 35.1 48.4 52.0 56.8 CHGS .00325 .00185 .00234 .00232 .00292 .00298 .00283 TABLE XXV UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 28 Initial pH = 3.9; Final pH = 5.3; CHGF - .02428 TIKE (nir.) 9.7 13.7 16.9 21.7 24.9 33.8 40.6 45.2 47.9 51.3 54.3 61.1 64.7 67.5 S°4 .227 .270 .287 .304 .312 .322 .326 .330 .330 .333 .335 .339 .337 .338 TIME (min) 7.3 19.0 35.5 49.5 55.8 62.9 66.6 CHGS .00248 .00281 .00187 .00248 .00232 .00239 .00238 TABLE XXVI UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 65 Initial pH » 3.53; Final pH - 6.0; CHGF - .10104 TIKE ( min) 9.8 14.0 20.0 27.5 34.7 44.7 54.7 61.3 65.0 68.0 71.0 CS2°4 .602 .650 .642 .602 .599 .595 .606 .606 .599 .599 .595 TIME (min) 6.6 11.6 18.0 33.1 47.0 56.4 68.0 CHGS .00869 .00769 .01456 .01945 .01903 .01922 .01960 TABLE XXVII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 67 Initial pH = 3.38; Final pH = 5.95; CHGF - .06626 TIME 8.3 13.5 19.8 26.3 32.2 41.7 46.7 51.7 55.3 / (m in) C. n .561 .721 .817 .867 .882 .882 .882 .882 .882 s2°4 TIME 6.3 11.8 17.8 24.0 29.0 40.0 44.0 50.2 53.8 (rr. in) CHGS .00436 .00386 .00358 .00343 .00361 .00377 .00393 .00405 .00374 TABLE XXVIII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 69 Initial pH «• 3.35; Final pH = 5.65; CHGF - .04677 TIME 8.6 13.7 20.9 27.1 33.1 39.4 45.3 48.5 52.0 57.3 60.8 63.5 66.3 (min) C_ n .447 .567 .643 .696 .707 .716 .716 .726 .726 .734 .732 .737 .732 t,2u4 TIME 6.7 12.1 18.9 25.1 31.5 36.7 43.3 50.2 55.7 68.4 (min) CHGS .00421 .00383 .00348 .00388 .00388 .00399 .00368 .00358 .00373 .00364 TABLE XXIX UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 71 Initial pH • 5.88; Final pH = 5.95; CHGF = .07542 TIKE (min) 5.3 8.3 14.0 19.3 25.3 31.8 34.6 38.4 46.0 49.8 53.6 S*4 .706 .721 .726 .721 .726 .737 .732 .737 .737 .732 .737 TIME (min) 3.7 10.3 17.1 23.5 30.2 36.8 44.1 51.8 ChGS .00554 .00615 .00614 .00599 .00615 .00610 .00605 .00610 TABLE XXX UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 73 Initial pH = 5.75; Final pH = 5.8; CHGF = .05914 TIME (min) 9.1 15.3 21.3 30.0 35.4 41.0 45.5 53.0 58.5 Cs2o4 .844 .849 .867 .867 .867 .867 .867 .867 .867 TIME (min) 5.3 11.9 18.2 23.8 32.3 37.8 42.3 CHGS .00429 .00454 .00464 .00414 .00422 .00408 .00412 I. co TABLE XXXI UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 75 Initial pH » 5.35; Final pH » 5.4; CHGF - .03921 TIME (min) 4.5 10.3 14.6 20.2 24.7 32.5 37.6 44.0 49.2 51.6 Cs2°4 .587 .613 .618 .625 .633 .633 .632 .633 .637 .633 TIME (ain) 7.8 12.9 17.3 27.1 35.2 40.8 CHGS .00391 .00399 .00394 .00387 .00392 .00386 TABLE XXXII UNSTEADY-STATE RESULTS FOR EXPERIMENTAL RUN 77 Initial pH = 5.76; Final pH - 5.7; CHGF - .08920 TIHE (min) 8.5 14.3 19.7 24.8 32.8 40.8 44.9 52.3 55.0 CS2°4 .608 .629 .633 .629 .625 .633 .635 .642 .633 TIME (min) 11.1 17.7 22.1 30.7 39.0 43.1 48.8 55.0 CHGS .00976 .01074 .01094 .01167 .01113 .01094 .01121 .01087 I GO cn 

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