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The hydrogen precipitation of vanadium from aqueous solutions O'Brien, Robert Neville 1952

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TIB HYDROGEN PRECIPITATION OP VANADIUM FROM AQJMXiJS SOLUTIONS by ROBERT NEVILLE O'BRIEN A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of Mining and Metallurgy at the University of Bri t i s h Columbia We accept this thesis as conforming to the standard required from candidates for the degree of "Master of Applied Science" Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER, 1952 Acknowledgment The author i s grateful for the financial assistance of the National Research Council of Canada which made this work possible. The author i s likewise grateful for the assistance and encouragement of the staff of the Department of Mining and Metallurgy especially DroJ.HalpeBa whose direction was invaluable to this investigation'. Table of Contents Page ABSTRACT 1 INTRODUCTION 3 A* Chemistry of Vanadium 3 B« Extraction of Vanadium from i t s Ores 6 Co Scope of Present Research 7 EXPERIMENTAL 10 A* Materials 10 B» Equipment ' 11 C, Procedures 12 (i) Analytical 12 ( i i ) Precipitation Experiments 12 RESULTS AND DISCUSSION 14 A* Nature of the Reaction 14 B. Precipitation Rate Curves 18 Ct Reproduceability of Results 21 D. Effect of Vanadium Concentration 23 E« Effect of Catalyst 24 P. Effect of Hydrogen Pressure 30 G« Effect of Temperature 35 H. Effect of Solution Volume 35 I. Effect of Solution Composition 40 CONCLUSIONS A. Summary of Kinetic Data B. Mechanism of Reaction C. Suggestions for Further Research APPENDIX A. Summary of Kinetic Measurements APPENDIX B> Details of Typical Precipitation Experiments REFERENCES LIST OF ILLUSTRATIONS Page Figure 1. Typical Bate Curves for the Precipitation of 19 Vanadium from Carbonate Solutions., Figure 2. Effect of I n i t i a l Vanadium Concentration on 20 the Rate of Precipitation* Figure 3. Bate Curves for the precipitation of Vanadium 25 i n the Presence of Different Amounts of Nickel Figure 4. Effect of Nickel Catalyst Concentration on the 27 Bate of Precipitation of Vanadium. Figure 5* Effect of Recycling Catalyst on the Bate of 29 Precipitation. Figure 6. Bate Curves for the Precipitation of Vanadium 31 under Different Pressures of Hydrogen Figure 7» Effect of Hydrogen Pressure on the Bateof 32 Precipitation of Vanadium. Bate versus Pressure. Figure 8. Effect of Hydrogen Pressure on the Bate of 33 Precipitation of Vanadium. Bate versus Square Boot of Pressure. Figure 9« Bate Curves for the Precipitation of Vanadium 36 at Different Temperatures. Page Figure 10. Arrhenius Plot Showing the Effect of 38 Temperature on the Bate of Precipitation of Vanadium. Figure 11. Effect of Solution Volume on the Bate of Precipit- 39 ation of Vanadium. Figure 12. Hate Curves for the Precipitation of 41 Vanadium from Different Solutions. ABSTRACTo It has been found that sodium vanadate i n aqueous solutions, can he reduced by gaseous hydrogen, at temperatures of 3 0 0 «. l+0Q°Ft and i n the presence of a nickel catalyst, and precipitated as an oxide of lower valence. The following reaction has been proposed: 2 TO3- -t 2H2 —» V 20 3 •+- 20BT +- HgO The kinetics of this reaction have been examined, and the following effects observed. (l) Two distinct stages of reduction are involved.In the overall reaction, V03~is f i r s t reduced to a 1. soluble tetravalent ion, believed to be followed by further reduction to the trivalent oxide which i s precipitated. The * latter stage is rate controlling* (2) The kinetics of the reaction are i n i t i a l l y of zero order, the rate being independent of the concentration of vanadium down to a value of one gram per l i t r e . Below this * value the kinetics become f i r s t order. (3) The rate is directly proportional to the amount of nickel catalyst present. (4) The rate is proportional to the square root „ of the pressure of hydrogen. (5) The activation energy for the reaction is 7850 calories per mole. The mechanism of the reaction i s discussed i n the light of these kinetic results and a rate controlling step i s proposed consisting of a reaction between VgOip ions.and H atoms, both adsorbed on the surface of the nickel. Such a mechanism i s consistent with a l l the kinetic observations. INTRODUCTION A Chemistry of Vanadium Vanadium which finds extensive use today, p a r t i c u l a r l y as an a l l o y i n g constituent of s t e e l s , i s one of the' more recent metals to have been discovered and examined. Although references to vanadium can lie found i n the l i t e r a t u r e as early as 1801. ( l ) , there i s good reason to doubt that the materials referred to were a c t u a l l y the metal i t s e l f . Like uranium, vanadium i s highly e l e c t r o p o s i t i v e , and i t s oxides are reduced to the metal only with great d i f f i c u l t y . It thus appears that the material which Eers e l i u s examined i n 1831 and / identified with the element vanadium, was actually an oxide of the metal. In 1 8 6 7 , Rosiae ( 2 ) , f i n a l l y isolated and character-ised the pure metal and established i t s place i n Period IV,Group 5A of the periodic table. The atomic weight of the element is 5 0 . 9 5 . i t s atomic number is 23 , and the electron configuration is 1 § 2 , 2 § 2 , 2p6, 3 B 2 , 3 ? 6 , 4s 2, 3 a 3 , ( 3 ) . It appears that in addition to the three 3d electrons, the ks electrons i n the ground state can be excited (probably to a 3d level) and function as valence electrons. The element thus forms compounds in which i t exhibits valencies ranging from 2 to 5« As expected from the electron configuration the valencies of 3 and 5 are,the most important. Because of this wide range of valencies, and the fact that compounds of vanadium undergo substantial hydrolysis and polymerization* in solution, the chemistry of vanadium i s probably more complex than that of any other element. An attempt is made below to review the most important aspects of this chemistry insofar as i t has been established and applies to the present investigation ( 4 , 5 ) . (i) Pentavalent Vanadium Vanadium is readily oxidised to the pentavalent state, and i t s pentavalent compounds are thermodynamically more stable than:those of anyoother valency. These compounds derive from the well known oxide ^0^. This oxide is amphoteric and is soluble i n both acid and alkaline solutions, but insoluble i n neutral solutions. In alkaline solutions, the oxide is con-verted to vanadates, of which several are known, the principal ones "being meta-, ortho- and pyrovanadates having the forms MVO3, M 3 V U J ; and M2V2O7, respectively, and corresponding to different pH ranges. The orthovanadate ion 70^-, appears to be stable at high pH values (>12). In the pH range 12 to 10.6 i t is converted to the pyrovanadate ion, V20"p=, which predominates down to the pH of 9»0. Below pH a 9. metavanadates ions of the form VO3"", and polymerized ions of the form V^0^ =, etc., are obtained, the degree of polymerization increasing as the pH i s lowered. It is likely that a l l these ions co-exist in any given solution, and there i s practically no information about the thermodynamics or the chemistry of the more complex forms. ( i i ) Tetravalent Vanadium The compounds of tetravalent vanadium derives from the oxide VOj, or as i t is more generally regarded, V20^. Very l i t t l e is known about this oxide or i t s solutions as these compounds are unstable and readily oxidized to the pentavalent state by atmospheric oxygen. V2O4 is also reported as being amphoteric, dissolving in acid and alkaline solutions to form vanadites of the form M2Vi|0c;. These salts are very soluble in water giving rise to brown solutions. ( i i i ) Trivalent Vanadium The compounds of trivalent vanadium derive', from the oxide, V2O3, a black powder of d i s t i n c t l y basic character, insoluble i n a l k a l i s but re a d i l y soluble i n acids,, Very l i t t l e 1 j : i s known of the s o l u t i o n chemistry of th i s compound, (i v ) Divalent Vanadium The oxide VO and other compounds of the vandous series are so unstable that they cannot be i s o l a t e d i n the presence of water or oxygen. It should also be noted that i n a d d i t i o n to the simple stoichiometric oxides, v'.jO-j, V 2 0 ^ and V 2 0 ^ r e f e r r e d to above vandium and oxygen farm a continuous series of compounds of intermediate composition. These are sometimes considered to be solutions of the elementary oxides, although such a i" c l a s s i f i c a t i o n appears to be a r b i t r a r y . B. E x t r a c t i o n of Vanadium from i t s Ores. Vanadium generally occurs i n nature i n the form of pentavalent compounds, the most common minerals being vana-d i n i t e , Pb^PbClJVO^; patronite, V2S5> r o s c o e l i t e , HgK 2(M 8,Ee) (A.%V)4(3103)2 and carnotite K 2° * W Q 3 • V 2 ° 5 * $2$' C O M M O N procedures f o r extracting vanadium from these ores generally involve roasting or fusing the ore with an a l k a l i such as sodium carbonate, followed by leaching. With some ores, notably carnotite, leaching alone with a carbonate s o l u t i o n can sbe used.(6) When eit h e r of these procedures are used' the vanadium i s obtained as a sod'ium vanadate s a l t , generally i n a sodium carbonate sol u t i o n . Since most vanadates are soluble i n a l k a l i n e solutions, i t has always been found necessary to neu t r a l i z e the solutions by a d d i t i o n of a c i d to a pH of about 6 i n order to p r e c i p i t a t e the vanadium as hydrated V 2 O 5 , and recover i t from the leach s o l u t i o n . Such a procedure i s c obviously wasteful of reagents (ie EfaoCC^ &nd B^IQii.) and i s therefore not r e a d i l y applicable to ores of low grade. C. Scope of the Present Research. An 'improvement over t h i s method of p r e c i p i t a t i o n appeared to be afforded by a xreantion discovered i n t h i s l a b -oratory, whereby vanadium was p r e c i p i t a t e d by reducing i t with gaseous hydrogen from the pentavalent state to a lower valence state, apparently insoluble i n a l k a l i n e solutions. •This procedure i s a modification of the method which has been used f o r p r e c i p i t a t i n g nickel,, copper and cobalt from ammonia leach solutions. However whereas with n i c k e l , copper and cobalt the metal i t s e l f i s produced and p r e c i p i t a t e d on hydrogen reduction, i n the case of vanadium which i s more e l e c t r o p o s i t i v e , reduction w i l l not proceed as f a r as the metal, and instead a lower oxide i s p r e c i p i t a t e d . In a d d i t i o n . i t was found that the presence of a c a t a l y s t such as "flffettallic n i c k e l was necessary f o r the reduction of vanadium to proceed with measureable rates under moderate conditions of 8. temperature and pressure. Because this reaction makes use of the principle of altering only the metal which i t is desired to precipitate, rather than the whole solution, during the course of precipita» tion, i t offers obvious advantages over the existing methods of precipitating vanadium from alkaline leach solutions by neutralizations The essential character of the leach solutions is not altered during precipitation and the solutions can be recycled for further leaching* Such a cyclic process, permitting re-use of leach solutions with low reagent consumption is essential i n treating low grade ores. The present research, i n which the kinetics of the reduction and precipitation of vanadium by hydrogen were investi-gated, was undertaken with a view to obtaining information about the chemistry and mechanism of the reaction, to determine quantitatively the variables upon which the rate depends, and to evaluate i t s scope and possible applications. Experiments were carried out i n a high pressure autoclave i n which the temperature and pressure of hydrogen could be controlled* Precipitation rate curves were determined by periodic sampling and analysis of the solution. Variables investigated included solution composition, vanadium concentration, catalyst con-centration, temperature, and hydrogen pressure. The results of the research are presented and discussed i n the present thesis* EXPERIMENTAL A.Materials. (i) Ammonium Me ta vanadate, CP. Grade, was supplied by Brickman and Co., Montreal, Que. and used in making up the solutions. ( i i ) Nickel Powder, CP. Grade was supplied by Eimer and Amend. This powder was used as catalyst i n the precipitation experiments. Surface area measurements by the B.E.T. method, kindly carried out by the National Research Council, Ottawa, gave an apparent surface area of 0,72 square metres per gram of nickel. ( i i i ) Hydrogen and Nitrogen Gas, were of commercial grade and supplied i n cylinders "by Canadian L i q u i d A i r Co. ( i v ) Sodium Carbonate, = "bicarbonate, sulphate, hydroxide, hydrogen peroxide and other chemicals used i n preparing solutions and i n the a n a l y t i c a l procedures were of C P . Grade* (v) D i s t i l l e d Water was used i n the preparation of a l l sol u t i o n s . B. Equipment* ( i ) Autoclave P r e c i p i t a t i o n experiments were c a r r i e d out i n a high pressure autoclave manufactured by Autoclave Engineers Incorporated, and designed for pressures up to 2000 p s i . The inside dimensions of the v e s s e l were 5 inches, diameter by 14" height, corresponding to a volume of about one g a l l o n . The autoclave^ was equipped with an externally driven s t i r r e r , and thermocouple well, coToling c o i l and sampling tube, a l l connected through the l i d . Pressure was measured by a standard gauge. The autoclave was heated e l e c t r i c a l l y by external s t r i p heaters. The temperature was recorded and co n t r o l l e d by a Leeds and Northrop Micromax unit to within± 5°E. ( i i ) Spectrophotometer: A Beckman Model DU Spectrophotometer was used i n measuring absorption spectra and i n carrying out vanadium determinations. ( i i i ) pH Meter 12 pH measurements and potentiometric t i t r a t i o n s were made with a Beckman Model H-2 pH meter. C. Procedures ( i ) A n a l y t i c a l Vanadium was determined spectophotometrically using the H2SO^ - H2O2 method, as given by Sandell (7). To a 5 cc a l i q u o t of s o l u t i o n being analyzed were added 1 cc of jf> HgOjpand 30 cc of 10$ HgSOij,, The resultant mixture was made up to 100 cc and the i n t e n s i t y of the colour measured at a wave length of 460 m^vwith a Beckman spectrophotometer. The vanadium concentration was calculated from a suitable c a l i - ' b r a t i o n graph. . Carbonate.,., bicarbonate and hydroxyl ion concen-t r a t i o n s were determined by the usual potentiometric t i t r a t i o n prodedures with standard HG1 (8)» ( i i ) P r e c i p i t a t i o n Experiments A standard s o l u t i o n of sodium vanadate was prepared by d i s o l v i n g a weighed amount of ffi^VO^in 5$ NagCO^ and' b o i l i n g to drive o f f ammonia i n accordance with the following equation: 2NB4VO3 + Na2C03 —»> 2 KaV03-+ M3 + H2O + CP2"U)* Aliquots of t h i s s o l u t i o n were d i l u t e d to give the desired concentration of vanadium and used i n the p r e c i p i t a t i o n 13 experiments. The charge co n s i s t i n g , generally, of 2 l i t r e s of s o l u t i o n and a desired quantity of n i c k e l c a t a l y s t was placed i n the autoclave, which was sealed, flushed with nitrogen and heated to the desired temperature. A desired pressure of hydrogen v/as then introduced and maintained while the charge was s t i r r e d c ontinually. Samples of the mixture were withdrawn at measured time i n t e r v a l s , generally of 10 minutes.centrifuged to remove the n i c k e l and p r e c i p i t a t e d vanadium, and the solutions were analyzed,for vanadium, and i n some instances f o r CO = and OEP. The r e a c t i o n was generally continued u n t i l s o l u t i o n a n a l y s i s showed that p r e c i p i t a t i o n was complete, or u n t i l a s u f f i c i e n t number of points had been obtained from which to determine the rate of p r e c i p i t a t i o n . A f t e r each experiment the autoclave was cleaned with hydrogen peroxide to remove any vanadium p r e c i p i t a t e d on the walls or i n t e r n a l parts of. the v e s s e l . 14 RESULTS AMD DISCUSSION" A. Nature of the Reaction Most of the experiments were made using a s o l u t i o n containing 50 gm/l of NagCO-jand 2 gm/l of Vanadium as Na VOg. Sodium carbonate was selected as the standard medium, since i t dissolves vanadium re a d i l y and i s convenient to work with, being less corrosive then most a l k a l i n e media* In addition, since most leaching processes f o r vanadium involve the use of sodium carbonate solutions, the r e s u l t s obtained by using this medium, could be more r e a d i l y evaluated i n terms of the proposed a p p l i c a t i o n s . 15 The f i r s t effect observed i n reacting such a solution with hydrogen in the presence of nickel powder at temperatures of about 300°F, was a marked colour change. The vanadate solutions, i n i t i a l l y colourless, always turned a deep red brown before precipitation commenced. Following this colour change vanadium was precipitated as a solid ranging from brown to black in colour. As the precipitation proceeded the intensity of the solution colour decreased, remaining proportional to the vanadium concentration. The changes i n the appearance of the mixture during the course of precipitation are shown i n Appendix B. These observations already suggest that the precipitation of vanadium involves two consecutive reactions, as follows: (1) The reduction of vanadate to a soluble ion of lower valence, (2) the further reduction of this ion to an insoluble compound of s t i l l lower valence, General considerations of the chemistry of vanadium would suggest that the intermediate soluble compound trivalento The following equations are proporsed as being i n conformity with this scheme, 2V03^rH2 —=> 7 2G 5- -j H20 (2) followed by 72°5 % ^> 2 0 H t V2°3 W The o v e r a l l r e a c t i o n obtained by combining these equations, i s 27O3" + 2H2 —» V2O3 + 20H" *+ H2O (4) An attempt to i d e n t i f y the intermediate coloured compound by i t s absorption spectrum, which was determined and found to have a peak at 3150 Angstroms, was unsuccessful as there was no record of a s i m i l a r spectrum i n the l i t e r a t u r e . S i m i l a r l y , attempts to i d e n t i f y the p r e c i p i t a t e by x^ray d i f f r a c t i o n measurements were also unsuccessfulo The observed patterns did not correspond with those recorded f o r any van-adium compound, but i t should be pointed out that such i n -formation about vanadium oxides, whose complexity has already b been r e f e r r e d to i s very meagre. • •No information about the reaction was'56btained by attempts to measure the oxidation p o t e n t i a l of samples of the s o l u t i o n , withdrawn at d i f f e r e n t times during the course of p r e c i p i t a t i o n * The measured p o t e n t i a l s were found to be irreproducible and to vary considerably with time and with atmospheric exposure of the solutions* To obtain s i g n i f i c a n t information along these l i n e s i t would appear to be necessary to incorporate an electrode into the auto-clave assembly and measure the a c t u a l p o t e n t i a l changes occuning i n the s o l u t i o n i n the autoclave during p r e c i p i t a t i o n . The r e s u l t s of such measurement would be of great i n t e r e s t and should throw considerable l i g h t on the nature and thermodynamics of the reactions accompanying the reduction and p r e c i p a t i o n of vanadium* However appreciable experimental d i f f i c u l t i e s are involved, the s o l u t i o n of which was beyond the scope of the present i n v e s t i g a t i o n * Some support f o r the suggested r e a c t i o n was obtained from measurements of the amount of OH" formed during p r e c i p i t a t i o n * It was found that one 0H~* ion was produced f o r every atom of V p r e c i p i t a t e d * This i s i n agreement with equation 4, and rules out the p o s s i b i l i t y of the p r e c i p i t a t e being a vanadite s a l t rather than an oxide» Further support f o r equation 4 i s furnished by a v a i l a b l e thermodynamic information* The standard heats of r e a c t i o n f o r the reduction of V0 3 by hydrogen to various lower oxides are l i s t e d below* The c a l c u l a t i o n s are based on data from Bichowsky and R o s s i n i (9)* 2703"+ H2 ~* 7204 +-20H**(5);AH°* 23 Keal/mole 2VO3"" - 2H2 v 2 ° 3 - H 2 ° - 20lT (4); AH°*7 Keal/mole 2YO3- ~ H V 2 ° 2 w 2 H 2° " 2 0 K * (7); A H 0 * 74 Zeal/mole Of the three possible reactions l i s t e d , equation 4, corres^ ponding to the formation of ^2^3appears to be thermodyna-* mically the most l i k e l y one having the lowest heat of reaction* The other reactions i n v o l v i n g the formation of YgO^pT Y2O2, would require excessive energies and are therefore thermoe dynamically u n l i k e l y , even at the temperatures and pressures at which the p r e c i p i t a t i o n experiments were c a r r i e d out*. The reactions represented "by equations 2, 3 and k are thus consistent with the changes observed during p r e c i p i t a t i o n , and with the general features of the chemistry of vanadium, outlined e a r l i e r . However, i t should be empha-sized that the equations are not considered to be accurate i n d e t a i l , but only to represent the general changes i n valence and i n s o l u b i l i t y involved. The a c t u a l r e a c t i o n i s l i k e l y to be more complex inv o l v i n g simultaneously several forms of each ion, d i f f e r i n g i n degree of hydrolysis and polymerization, as well as a mixture of several oxide modi-f i e a t i o n s and hydrates ahs- theinfinal product, B. P r e c i p i t a t i o n Bate Curves. T y p i c a l p r e c i p i t a t i o n rate curves depicting the course of p r e c i p i t a t i o n of vanadium from a sodium car.* bonate s o l u t i o n as a function of time, are shown i n Figure 2. The i n i t i a l part of the rate curves, sometimes preceded by a short lindanetion period during which the colour change i n the s o l u t i o n occurs, i s always l i n e a r doxvn to a concen-t r a t i o n of vanadium of about 1 gm/l. This i s referred to as the zero order region. .As the concentration of vanadium i n s o l u t i o n , i s reduced further by p r e c i p i t a t i o n , below 1 gnv v / l , the rate of p r e c i p i t a t i o n f a l l s o f f progressively, • m f i g u r e 1. T y p i c a l Bate Curves f o r the P r e c i p i t a t i o n of vanadium from Carbonate solutions* 0 ioo T I M E — M I N U T E S VANADIUM-GRAMS PER LITRE 21 following an exponential or f i r s t order r e l a t i o n i i . e * the rate of p r e c i p i t a t i o n i n t h i s region i s approximately proportional to the concentration of vanadium remaining i n solution* A l l the rate curves obtained with sodium carbonate solutions were e s s e n t i a l l y of t h i s form. C. R e p r o d u c i b i l i t y An example of the r e p r o d u c i b i l i t y of the p r e c i p i t a t i o n r e s u l t s obtained i n d i f f e r e n t experiments under the same conditions i s shown i n Figure 1. The two rate curves are s h i f t e d s l i g h t l y , due to differences i n the i n i t i a l vanadium concentration and i n the p r e c i p i t a t i o n s t a r t i n g time, but are e s s e n t i a l l y s i m i l a r i n shape and i n slope* P r e c i p i t a t i o n rates, calculated from the slopes of the zero order regions of the rate curves f o r a group of experiments made under i d e n t i c a l conditions, are l i s t e d i n Table I« The mean deviation from the average rate i s £ 3-7$ and the maximum d e v i a t i o n ^ 11»7$* On 'the whole the measured rates are considered reproducible to v/i t h i n 10$* The r e p r o d u c i b i l i t y i s believed to be l i m i t e d by the p r e c i s i o n with which the temperature and pressure i n the autoclave could be controlled, by the sampling and a n a l y t i c a l procedures, by v a r i a t i o n s i n the d i s p e r s a l of c a t a l y s t caused by changing stlming rate and by the presence of variable impurities i n the system. The rates of c a t a l y t i c 22: TABLE I. Reproducibility of Bate Measurements. I n i t i a l 7 Concentration - 2.0 gm/l. ^2 C 0 3 Concentration - 5° gm/l Hickel Catalyst - 10 gm/l Temperature - 300°E. Experiment Bate of Precipitation " 7 Concentration No. Deviation from at Break i n Curve (gm7/l/Min) Mean Rate ( gm/l) 7-60 o.oi35 - 0.7 $ 1.05 7-35 0.0140 2.9 0.85 7-59 0.0149 9.6 7-47 0.0120 « 11.7 Average 0.0136 3*7$ reactions are known to be se n s i t i v e to such impurities.. D« E f f e c t of Yanadium Concentration. Rate curves depicting the p r e c i p i t a t i o n of vanadium from a ser i e s of solutions, d i f f e r i n g i n i n i t i a l vanadium concentration, are shown i n Figure 2* The rates calculated from the slopes of the zero order regions are summarized i n Table I I , The following e f f e c t s should be noted* 1„ The rate curves are a l l essentialy p a r a l l e l . The i n i t i a l rates range from.0,0135 to 0,0153 gm V/l/min. but the v a r i a t i o n s i s considered to be wi'thinn the experimental error and no systematic trend i s r e f l e c t e d * Fromthis i t i s concluded that the k i n e t i c s i n t h i s region are t r u l y zero order, the rate being independent of the i n i t i a l vanadium concentration, as well as of the changing concentration of vanadium during p r e c i p i t a t i o n . Z.t The t r a n s i t i o n from zero order to f i r s t order k i n e t i c s always occurs at approximately the same vanadium concentration of about one gram per l i t r e * This i s important as i t shows that the change i s not r e l a t e d to the time of p r e c i p i t a t i o n , to the amount of vanadium p r e c i -p i t a t e d , or to the amount of 013" produced i n accordance with equation 4, a l l of which vary with the i n i t i a l vanadium concentration. 2k E . E f f e c t of Catalyst. Hate curves de p i c t i n g the p r e c i p i t a t i o n of vanadium with d i f f e r e n t amounts of n i c k e l c a t a l y s t present are shown i n Figure 3» The r e s u l t s are summarized i n Figure k and Table II» The rate of p r e c i p i t a t i o n i s seen to be d i r e c t l y proportional to the amount of c a t a l y s t present showing that the r e a c t i o n i s e n t i r e l y c a t a l y t i c * No r e a c t i o n was observed i n the absence of n i c k e l . An experiment was made to determine whether the a c t i v i t y of the c a t a l y s t or the properties of the s o l u t i o n . were a l t e r e d during the course of p r e c i p i t a t i o n * A f t e r completion of a p r e c i p i t a t i o n experiment the autoclave was opened, and without removing the catalyst or p r e c i p i t a t e , more vanadium was dissolved i n the s o l u t i o n and the p r e c i p i t a -t i o n repeated. The r e s u l t i n g rate curve i s compared to the o r i g i n a l one i n Figure 5» The two curves are seen to be e s s e n t i a l l y s i m i l a r i n shape, except f o r a f l u c t u a t i o n i n the second rate curve which appears to have been caused by a f l u c t u a t i o n i n the s t i r r i n g temperature or pressure during the experiments The i n i t i a l rates, are 0,0135 and 0,0132 grm v/l/min respectively f o r the two experiments. The s i m i l a r i t y of the two rate curves shows that the f a l l o f f i n rate of p r e c i p i t a t i o n toward the end of each experiment, i s not caused by a change i n the a c t i v i t y of the c a t a l y s t or by a change i n the properties 255 Figure 3* Bate Curves for the Precipitation of Vana&itaia i n the Presence of D i f f -erent Amounts of Nickel. 0 TIME- MINUTES 26 TABLE II. Effect of I n i t i a l Vanadium Concentration on the Hate of Precipitation NagCO-j Concentration - 50 gm/l Nickel Catalyst - 10 gm/l Hydrogen Pressure - 300 psi Temperature - 300°P. I n i t i a l V Concentration - as given "below. Eperiment I n i t i a l Vanadium Rate of Precipitation No. gm/l Gm. V/l/min V-57 1*1 Continually decreasing V-60 2,02 0.0135 V-65 3.9 0.0153 V-72 5.0 0.0148 figure 4. Effect of Kickel Catalyst 0 oncentration on the Eat© ©f Precipitat-ion of Yanadium. 28 TABLE IIIo Effect of Catalyst on the Bate of Precipitation  of Vanadium I n i t i a l V Concentration - 2.0 gm/l NagCO^Concentration - 50 gm/l Nickel Cat alyst - as given "below Hydrogen Pressure - 300 psi ' Temperature - 300°F Eperlment Ho. V-60 V-69 V-63 Nickel Catalyst Bate of Precipitation Bate gm/l gm/v/l/min Catalyst 5 10 15 20 0.0067 0.0135 0.0205 0.0265 0.00134 0.00135 0.00137 0.00133 Average 0.00135 30 of the s o l u t i o n other than the concentration of vanadium, to which i t has already been r e l a t e d . Jm. E f f e c t of Evdroaen Pressure The e f f e c t of hydrogen pressure on the p r e c i p i t a t i o n of vanadium was investigated i n a series of experiments i n which the pressure was v a r i e d from 100 to 400 pounds per square inch. The rate curves are shown i n Figure 6, and the v a r i a t i o n of the rate with hydrogen pressure i s summarized i n Figures 7 and 8 and i n Table IV. The rate i s seen to be d i r e c t l y proportional to the square root of the pressure of hydrogen, suggesting that the hydrogen p a r t i c i p a t i n g intthe rate c o n t r o l l i n g step i s d i s s o c i a t e d , i n accordance with the following equilibrium, H2 — > 2 H 0 (7) 00 ± K 0 * 2 ) ( 9 ) This d i s s o c i a t i o n probably occurs on the n i c k e l surface, and i s generally considered to be responsible f o r the c a t a l y t i c a c t i v i t y of n i c k e l i n hydrogenation reactions. Varying the hydrogen pressure had no e f f e c t on the shape of the rate curves or on the concentration of vanadium which the t r a n s i t i o n from zero to f i r s t order k i n e t i c s was observed to occur* 31 -figure 6. Effect of P resuuxej Bate Curves for the Precipitation of Vanadium tinder Different Pressures of Hydrogen, i I ' 0 100 TIME- MINUTES Figure 7. Effect of Hydrogen Pressure on the Bate of Precipitation of Yanadium 1 ate versus Pressure I B Figure 8. Effect of Hydrogen Pressure on the Bate of Precipitation of Vanadium Bate versus Square Boot of Pressure. 34:-X&BLE IVo Effect of Rvdrogen Pressure on the Rate of Precipitation of Vanadium. I n i t i a l V Concentration - 2.0 gm/l NagCO^Concentration - 50 gm/l Nickel Catalyst - 10 gm/l Hydrogen Pressure — as given "below Temperature - 300°P. Experiment Hydrogen Pressure Rate of Precipitation Rate Rate 2. Hbe psi gm V/l/min Pressure (Pressure) V-44 100 0.00750 0.000075 0.00075 V-62 200 0.0110 0.000055 0.00078 V«60 300 **0.0135 0.000045 0.00078 V«6l 400 *>Qpl56 0.000039 0.00078 Average 0.00077 Gr«-Effect of Temperature. The e f f e c t of temperature on the p r e c i p i t a t i o n of vanadium was investigated i n a seri e s of experiments' i n which the temperature was va r i e d from 300 to 400° P. The rate curves are shown in'Figure 9 and the r e s u l t s are summarised i n Table V and Figure 10. A good ArrheuiuB p l o t was obtained; the a c t i v a t i o n energy, E, calculated from the slope of t h i s p l o t i s 7850, cal/mole. Many heterogeneous reactions have a c t i v a t i o n energies of this order. As the temperature was increased, a progressive tendency was noted f o r the t r a n s i t i o n from zero to f i r s t order k i n e t i c s to occur at higher concentra^ tions of vanadium but the r e l a t i o n could not be determined accurately* HsEffect of Volume* The e f f e c t of changing the volume of the s o l u t i o n on the rate of p r e c i p i t a t i o n i s shown i n Figure 11* \ When the volume was increased from 2 to 3 l i t r e s (with a f corresponding increase i n the amount of catalyst),the rate curve remained essentially unchanged, except that a small decrease in the rate from 0*0135.to 0*0114 gm v/l/min was noted. This i s believed to be due to the tendency of the heavy nickel catalyst to settle,thus f a i l i n g to remain uniformly dispersed throughout the larger volume of solution. • 36 Figure 9. Bate Curves for the Precipitation of Vanadium at Different Temperatures. TIME- MINUTES 37 TABLE V, Effect of Temperature on the Bate of  Precipitation of Vanadium^ I n i t i a l V Concentration - 2*0 gm/l NagCO-j Concentration - 50 gm/l Nickel Catalyst - 10 gm/l Hydrogen Pressure - 300. psi. Temperature - as given below* Experiment No. Temperature^ Bate of Precipitation (gm v/l/min) V-60 V-53 V-79 T-80 V-77 300 325 350 375 400 0.0135 0.0196 0.0250 d.0315 0.0395 38 39 Figure 11. Effect of Solution volume on the Bate of Precipitation of Vanadium. I. Effect of Solution Composition. A series of experiments were made to examine i the effect of varying the composition of the solution on the precipitation of vanadium. Sodium vanadate was dissolved i n water as well as i n solutions of sodium carbonate,sodium hydroxide, sodium sulphate and sodium bicarbonate. Comparative rate curves are shown i n Figure 12 and the results are summarized i n Table VI. Although the rate curves for the various solu-tions showed significant differences both i n shape and slope, the results are d i f f i c u l t to interpret and the variations bear no definite relation to either the pH or the electrolyte content of the solution. Thus the rates were lowest i n BaOH solutions ( a 5$ BaOH solution failed to give precipita-tion) and highest i n KaHCO^ solution. Water and Na 2S©^ solu-tions,with s t i l l lower pH values, gave intermediate rates. Ha2 c o 3 and Na2 SO4 with widely differing pH values, hado nearly identical rate curves. The shapes of the rate curves also varied significantly, the curves for NaH CO-j and water being linear down to very low vanadium concentrations, while that for NaOH showing a very marked curvature from the start. There i s some suggestion that the reaction i t s e l f may not be identical i n a l l the solutions as evidenced by the fact til Figure 12. Bate Curvee for the Precipitation of VamAtua from. Different Sol* utlone* 0 IOO TIME-MINUTES TABLE VI. Effect of Solution Composition on the Precipitation of Vanadium. I n i t i a l V Concentration - 2.0 gm/l Nickel Catalyst Hydrogen Pressure Temperature Solution Composition - 10 gm/l - 300 psi - 300 °7. - as given below. Experiment No. Solution Salt Composition Concentration Bate of Precipitation gm V/l/min V-73 None - 0,0220 V-60 NagCOj 50 gm/l 0.0135 V-91 BaHC03 50 gm/l 0.0520 V-75 Na2S0^ 50 gm/l 8.0160 V-93 NaOH 20 gm/l Continually decreasing V-92 NaOH 50 gm/l No Precipitation that the intermediate colours of the solutions d i f f e r . This is shown i n Appendix B. A further factor which i s known to "be affected by changes i n the nature and concentration of salts, is the solubility of hydrogen. It i s li k e l y that this accounts i n part for the observed variations i n the rate. However i t appears that other factors are also involved and i t would clearly be of interest to investigate .theergactton i n solutions other than carbonate. CONCLUSIONS At SW»mrY Qf Kinetic Pate* It has teen found that sodium vanadate In carbonate solutions Is reduced by hydrogen gas at tempera-tures of the order of 300°F, and precipititated as an oxide of lower valence, i n accordance with a reaction which i s believed to proceed as follows: 2 2 Hg —* *2°3 + 2 0 f f * + ^jP She kinetics of this precipitation reaction have been examined, and the results described and discussed In the preceding 1'. sections* She following effects have been observed, (1) She precipitation reaction appears to involve two distinct stages of reduction, YO3* is f i r s t reduced to a soluble tetravalent vanadium lon, followed by the further reduction of this lon to the trivalent oxide which is precipitated. The first reaction is fast and the second or slow stage appears to control the rate of the precipita-tion process. (2) She kinetics of the reaction are initially of zero order, the rate being independent of the concentration of vanadium In the solution, down to a concentration of about one gram per l i t r e . Below this value the rate decreases following approximately a f i r s t order relation. (3) She rate of precipitation is directly pro-portional to the amount of nickel catalyst present, confirming the catalytic nature of the reaction. She activity of the catalyst apparently remains unchanged during the course of the reaction. (4) She rate of precipitation is proportional to the square root of the pressure of hydrogen gas and conse-quently to the square root of the concentration of hydrogen In the solution. Shis indicates that the hydrogen partici-pating In the .rate controlling step is in a dissociated state. (5) She reaction was found to have an activation energy of 7850 calories per mole. She rate doubles with every 46 increase in temperature of about 60°F in the range 300 «• 400°F, (6) In accordance with these kinetic results, the rate of precipitation of vanadium in the zero order region is expressed by the following relation: - dCvJ m E Org Ql2$ (IO) where (J) is the concentration of vanadium in the solution CJiJ is the surface area of nickel catalyst per unit volume of solution (kg) is the pressure of hydrogen and E is the rate constant of the reaction and is given by the following expressions I t A e - 7850/ET ( u ) where A is the frequency factor* At 300°F, 300 psl H ^ pressure, and in the presence of 10 gm/l nickel catalyst (corresponding to a sur-4 9* face area of 7*2 x 10 cur) the rate of precipitation is 1*35 x lO^gm 7 / 1 / min or 4>4 x 10"^moles 7 / l / sec* The rate constant, k, has the value9 E m U35 x 10* 1 1 mole 7 cm*2 atm*^ " sec" 1 and A « 1*4 x IO"7 mole 7 cm**2 atm"£" sec" 1 Be Mechanism of the Reaction, Since the first stage in the reaction is fast, the kinetic results can provide no information about its 4? mechanism* However some conclusions may he drawn regarding the mechanism of the second or rate controlling step i n the precipitation process, which Aas been postulated to proceed as follows: V 2 0 5 % E, ^  72G3-t-2QH-It has been shown that the reaction i s heterogenous, occur— ing on the surface of the nickel catalyst* General consid-erations of the kinetics of heterogenous reactions, show that the following stages must be Involved ( l l ) * ( i ) Diffusion of the reactants i n solution to the catalyst surface* ( i i ) Adsorption of the reactants on the surface* (111) Reaction on the surface* (iv) Desorptlon of the products into the solu-tion and diffusion away from the surface* The kinetic results indicate that step(l) above i s not rate-controlling, at least i n the i n i t i a l zero order region of the reaction* The rate of diffusion of a dissolved reactant to the catalyst surface i s proportional to Its concentration i n the solution (by Pick's Law), and a diffusion controlled reaction rate would therefore be of f i r s t order with respect to one of the reactants* The observed kinetics of precipitation, however, were of zero order with respect 48 to the concentration of vanadium and of one half order with respect to the concentration of hydrogen* This would appear to rule out the possibility of the diffusion step being rate-controlling* The observed activation energy of 7850 calories per mole i s also considerably i n excess of that to be expected for a diffusion process In aqueous solution* Similarly i t Is unlikely that step (iv) i n the above sequence, involving the removal of the reaction products from the catalyst surface i s rate controlling, as the kinetics of this step would be Independent of the pressure or concentration of hydrogen* Either of the remaining stages i n the reaction sequence, the adsosptlon of the reactants or the reaction at the surface may be rate-controlling* Both appear to be consistent with the observed reaction kinetics* The f i r s t p ossibility i s that the adsorption of hydrogen i s rate controlling* It i s known that the rate of adsorption of hydrogen on nickel i s proportional to the square root of i t s pressure* This agrees with the observed kinetics* At high concentration of vanadium, the adsorption of vanadium i s more |»pid than that of hydrogen and the rate of reaction i s independent of the vanadium concentration* This corresponds to the zero order region* At lower concen-trations of vanadium i t s adsorption may become slower than 49 that of hydrogen and control the rate of precipitation* $M<* would account for the observed transition from zero to f i r s t order kinetics at low vanadium concentrations* However It would he expected that the vanadium concentration at which the transition occurs, would Increase with the pressure of hydrogen* She results f a i l e d to confirm this* The most likely rate-controlling process thus appears to he step ( i l l ) i n the reaction sequence i s the surface reaction i t s e l f * This Implies that the adsorption of both vanadium and hydrogen on the nickel surface i s rapid , the adsorption equilibria being main-tained throughout the reaction* The rate of reaction i n this case, would be proportional to the concentrations of hydrogen atoms and vanadium ions adsorbed on the surface* She observed reaction order suggests that the surface i s only sparsely covered with hydrogen, so that the concentration of adsorbed hydrogen is proportional to the square root of the gas pre** ssure* At the same time, the adsorption of vanadium Ions appears to increase with the vanadium i n solution up to a concentration of about one gram of vanadium per l i t r e , when the surface becomes saturated, and the concentration of adsorbed vanadium remains constant* Shis would account for the zero order reaction kinetics and the transition to f i r s t order kinetics at a given value of the vanadium concentration* It i s also consistent with the 50 observation that this value tends to inc rease with temperature, since i t i s known that adsorption generally decreases with r i s i n g temperatures, and higher concentrations of vanadium i n solution would be required to saturate the surface. While this explanation of the observed kinetics, and the assumptions on which It i s based are reaeeinable, they do not exclude the possibility of other mechamsasconsistent with the experimental data* Further verification of the nature of the rate controlling step by application of the absolute reaction rate theory(ll), which permits values of the frequency factor to be calculated and compared with the experimental value, was considered impossible i n this case, i n view of the uncer-tainties surrounding the nature of the reaction and the identi-f i c a t i o n of the reactants and products involved* ff* Suflfiqstftons faff Further ffesear^h. She research described and discussed i n this thesis, has shown that vanadium can be reduced by gaseous hydrogen i n the presence of a suitable catalyst, and thereby precipitated from a variety of alkaline solutions* She general kinetic features of this precipitation process have been elucidated and i t has been shown how the rate depends on a number of Important variables* However the detailed course of the reaction, and the nature of the reactants, intermediates and products Involved have not been f u l l y determined; nor has the mechanism or rate controlling step been definitely established* She study must therefore be 53. considered to be of a preliminary nature only and It is sug-gested that i t would be of interest to continue It along the following lines* (l) To attempt, by means of more refined physical and chemical measurements (i»e* pH, potentials, absorption spectra, x-ray diffraction ;k.) on the solution and precipitates, to identify the reacting ions and the products in order to establish the exact nature of the reaction* (2) To Investigate the adsorption characteristics of each of the species involved in the reaction, in order to obtain information which would be useful In interpreting the kinetic results* (3) To investigate quantitatively the effect of further variables on the kinetics of the reaction* The following' would be of particular Interest: pH(using buffered solution); ionic strength, potentials; different catalysts* (4) Using more precise analytical methods, to investigate more fully the kinetics of the reaction at lower vanadium concentrations i.e» in the first order region below one gram of vanadium per litre* The kinetic measurement in the present study were largely concerned with the zero order region at higher vanadium concentrations* (5) So extend the investigations to solutions other than carbonate in order to establish the nature and kinetics of the reactions occuring in different solutions* 5& (6) fo carry out studies on the chemistry of vanadium and i t s compounds p a r t i c u l a r l y those of lower valence* E x i s t i n g knowledge i n t h i s f i e l d i s very meagre* 53 APPENDIX A Summary of Elastic Measurements* Experiment Solution Init ial Nickel No* 7 Cone Catalyst (gm/l) (gm/l) 7-35 5$ BagCO^ 2*0 10 7-44 5$ NagCO-j 1.98 10 7-47 5# Ba2C03 1*8? 10 7-53 5% Na2C03 1.96 10 v-57 5$ Ba2C03 1.1 10 V-59 555 BagCOj 2.13 10 7-60 5$ BagCO-j 2*02 10 7-61 5£ Na2C03 1.97 10 7-62 5$ Hs^ CC^ 2*0 10 7-63 5# EkgCOj 1*98 20 7-64 5^  lagCOj 1.97 5 7-65 5$ Na2C03 3.9 10 7-69 5$ NagCO-j 2.04 15 7-72 5# SagC03 5.0 10 V-73 S 0 2 2*04 10 7-75 5$ Ba2S0^ 2.07 10 V-77 5# Na2C03 2*08 10 7-79 Na2C0-j 2.08 10 7-80 5$ Na2C03 2.02 10 Hydrogen Temp Init ial Pressure Precipitation (psl) 9f (gm7/l/min) 300 300 •0140 100 300 .0075 300 300 •0120 300 325 .0196 300 300 continually decreasing 300 300 .0149 300 300 •0135 400 300 .0156 200 300 •0110 300 300 •0265 300 300 .0067 300 300 .0153 300 300 •0205 300 300 •0143 300 300 •0220 300 300 •0160 300 400 .0395 300 350 .0250 300 375 .0315 5* Experiment Solution Initial Hieke1 Hydrogen Temp Init ial Ho* V Cone Catalyst Pressure Precipitation (gm/l) (gm/l) (psi) °F (gm V/l/min) 7-81 5$ JfagCo^  2*0 10 300 300 •0135 7-82 2*0 10 300 300 •0220 7-87 5i BagCOj 1*94 10 300 300 •0114 7-88 5$ Ba2C03 2*11 10 300 300 .0132 7-91 5# BaHCOj 2*0 10 300 300 •0520 7-92 5$ BaOH 1*94 10 300 300 0 V-93 2$ HaOH 2.21 10 300 300 continual1/ decreasing APPENDIX £ Details of Typical Precipitation Experiments. P or the purpose of indentif ying the centrifuge tubes in the above photograph a numbering system from l e f t to right w i l l he used for both the upper and lower rows. The tubes and the samples i n them w i l l be referred to as U-l to U-14 for the upper row and L - l to 1-14 for the lower row. The straight-walled test tubes w i l l be identified only by position. 56 She color picture was taken of samples (after centrlfuging) of Bun 7-81; a typical standard run In 5$ Ba200jsolution, 300° F, 300 psi hydrogen pressure vith 10 gms/lit re 51 catalyst, and Bun 7-82 per-formed under the same conditions except in water solution to show the difference In color change during the reaction due to solution compos-ion japle No. Time Taken Minutes Concentration O m s . y / l l t r e U - l 0 2.6 U-2 16 1.90 U-3 30 1.6? U-4 47i 1.45 U-5 60 1.30 U-6 72^ 1.15 U-7 80 1.04 U-8 90 .95 U-9 100 .85 U-10 116 • 72 U - l l 136 .53 U-12 140 .50 u-13 150 .43 U-14 160 .38 57 Sample No. Time Taken. Minutes Concentration Gms. V/ l i tre L ~ l 0 2.0 L~2 15 2.0 L-3 31 2.0 50 2.0 1-5 60 1.95 L-6 70 1.70 L-7 80 1.49 L-8 92£ 1.15 1-9 102^ .82 L-10 n e .37 L - l l 132£ .08 L~3£ 140 •05 L-13 150 .05 L»14 160 .©5 The f irst test tube in the top row from the left is sample U-4 as diluted and treated for analysis. The second test tube in the upper row is sample TJ-14 similarly prepared. The firsttest tube from the left in the bottom row is a 15 ml. aliquot of L - l plus 2 ml. of 3$ HgOg. The second is the solution that was diluted to make up the charge used in these runs. Its concentration is 50 gms. V/l itre at pH 7»1» BIBLIOGHAPHY li) von Huobolt, A . , Gehlen*s Journal, 2.695, (1804) 2) Ho8Coe, H.E. , Philosophical Transactions of the loyal Society 1, (1868) 3) Bice, O.K., Electronic Structure and Chemical Bonding. McGrav H i l l , (1940) 4) Sidgvick, XT. 7., The Chemical Elements and Their Compounds Oxford, (1950) 5) Mellor, J.W., A Somprehenslve Treatise on Inorganic and Theoretical Chemistry, 7ol IX Longmans Green, (1922) 6) Llddell, D.M., Handbook of Honferrous Metallurgy, 7ol. 2 McGrav H i l l , (1945) 7) Sandell, S.B., Colorimetrie Determinations of Traces of Metals, Interscience (1950) 8) Eblthoff, I .M., and Sandell, E . B . , Quantitative Inorganic Analysis, MacMillan, (1943) 9) Bichovsky, F.B. and Rossini, P.D., Thermochemistry of Thermochemical Substances, Beinhold Publishing Co. (1940) 10) Seidell, A., Solubilities of Inorganic compounds, Van Uostrand (1940) -11) Glasstone, g., Laidler, E . J . and Byring, H . , Theory of Bate Processes, McGrav H i l l (1943) 

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