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Aqueous oxidation of galena Andersen, John Enevold 1951

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AQUEOUS OXIDATION OF GALENA hy JOHN ENEVOLD ANDERSEN A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of Mining and Metallurgy We accept t h i s t h e s i s as conforming to the standard r e q u i r e d from candidates f o r the degree of MASTER OF APPLIED SCIENCE • •,_»-< .1 i M c a r . w w i . . . . . . Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1951 Abstract To complement the data, on the aqueous o x i d a t i o n of sulphide minerals, that i s , treatment of sulphides i n aqueous s o l u t i o n at elevated temperatures under oxygen pressure, a study was undertaken of the aqueous oxidation of galena i n sodium hydroxide s o l u t i o n . I t wa-s hoped by the study of the k i n e t i c s of the r e a c t i o n to l e a r n something of the mechanism i n v o l v e d . The r e a c t i o n was followed by means of a cathode- ray polarograph. A c r y s t a l of galena was mounted i n a small autoclave equipped with s u i t a b l e e l e c t r o d e s , and during the course of the r e a c t i o n photographs were taken at i n t e r  v a l s of the current-voltage curves. From the height of the lead wave r e l a t i v e concentrations could be recorded, and a f t e r a s u i t a b l e time the run was stopped and the s o l u  t i o n assayed to give absolute values to the wave h e i g h t s . The v a r i a b l e s of oxygen p a r t i a l pressure a g i t a t i o n , sodium hydroxide concentration, temperature, s i l i c a t e - i o n , and e l e c t r i c a l p o t e n t i a l were i n v e s t i g a t e d . I t was found that the polarograph under these conditions gave r e p r o  ducible r e s u l t s . On the basis of the experimental r e s u l t s three a l t e r n a t i v e mechanisms for the reaction were proposed. One of these was too i n d e f i n i t e to t r e a t q u a n t i t a t i v e l y , but the other two were examined by c a l c u l a t i o n s using the theory of absolute r e a c t i o n r a t e s . One was found to give agree ment i n rate with experiment, a model i n which desorption accompanied by hydration and i o n i z a t i o n was the r a t e - c o n t r o l l i n g step. The experimental r e s u l t s were reviewed' i n the l i g h t of t h i s mechanism and appeared to show no serious c o n t r a d i c t i o n s , so t h i s model i s put forward as a postulate of the r e a c t i o n of galena with oxygen i n sodium hydroxide s o l u t i o n . Acknowledgement The author Is g r a t e f u l f o r f i n a n c i a l a i d i n the summer of 195° from the Consolidated Mining and Smelting Company of Canada, Limited, and f o r the Cominco Fellowship from the same company f o r the r e g u l a r session of 1950-51* Without such a i d t h i s work could not have been c a r r i e d out. Profound g r a t i t u d e i s a l s o expressed to the s t a f f of the Department of Mining and Metallurgy f o r t h e i r co operation, encouragement, and advice. The author i s i n  debted p a r t i c u l a r l y to A s s o c i a t e P r o f e s s o r W. M. Armstrong who was most h e l p f u l i n d i r e c t i n g the beginning of t h i s research, and t o Dr. J . Halpe-rn who guided the c a l c u l a t i o n s of r e a c t i o n r a t e s . Table of Contents Page I n t r o d u c t i o n 1 Equipment 3 The Cathode-Ray Polarograph 3 The Autoclave 5 The E l e c t r o d e s 9 Theory of Operation of the Polarograph 9 Experimental 11 M a t e r i a l s 11 Experimental Procedure 12 1 Rates on C r y s t a l s 12 2 Rates on Pulps lk 3 Q u a l i t a t i v e I n v e s t i g a t i o n of the E f f e c t of App l i e d F i e l d s lb R e s u l t s 15 Products of the Pressure Oxidation of Galena i n Sodium Hydroxide S o l u t i o n 15 The Wave Forms 15 The Rate Curves 17 Appearance of the C r y s t a l a f t e r Oxidation 21 I n v e s t i g a t i o n of V a r i a b l e s 23 1 Oxygen P a r t i a l Pressure 23 2 A g i t a t i o n (Rate of Flow of Medium Past the P a r t i c l e ) 2k 3 Concentration of Sodium Hydroxide 25 k Temperature 26 5 E f f e c t of S i l i c a t e - I o n 28 Page 6 E f f e c t of S u b s t i t u t i n g Sodium Acetate f o r Sodium Hydroxide 30 7 E f f e c t of E l e c t r i c a l P o t e n t i a l 3O D i s c u s s i o n of R e s u l t s 32 Numerical Values 32 Mechanism of the Reaction i n 0.5 Normal Sodium Hydroxide 33 Mechanism I 35 Mechanism I I 36 Mechanism I I I 39 C o n s i d e r a t i o n of Mechanism I I with Respect to the Remaining V a r i a b l e s 39 1 A g i t a t i o n and Concentration of Sodium Hydroxide 39 2 E f f e c t of S i l i c a t e - I o n 4-0 3 E f f e c t of S u b s t i t u t i n g Sodium Acetate f o r Sodium Hydroxide 4-0' 4 E f f e c t of Change i n Surface Area 4-1 Conclusions 4 1 Appendix A 4-4- Appendix B 4-5 Appendix C 4-7 B i b l i o g r a p h y 50 I l l u s t r a t i o n s Figure 1 Power Supply 4- Figure 2 Other C i r c u i t s 4- Figure 3 E l e c t r o n i c Equipment 6 Figure 4- Autoclave and A c c e s s o r i e s 6 Figure 5 Photograph showing Autoclave Assembly, Heating and S t i r r i n g Mechanisms 8 Figure 6 Polarogram 16 Page Figure 7 Rate Curve f o r a C r y s t a l 18 Figure 8 Rate Curve f o r a C r y s t a l 18 Figure 9 Rate Curve f o r a Pulp 20 Figure 10 Photomicrograph of Galena C r y s t a l a f t e r A t t a c h i n 0.5N Cau s t i c at 1.5Ooc f o r 200 minutes 22 Figure 11 Photomicrograph of Galena C r y s t a l a f t e r Attack i n 0.5N C a u s t i c at 175°C f o r 132 minutes 22 Figure 12 Rate vs. Normality KaOH 2? Figure 13 Arrhenius P l o t 29 I AQUEOUS OXIDATION OF GALENA I n t r o d u c t i o n The f i r s t mention of pressure o x i d a t i o n of s u l  phides i n aqueous s o l u t i o n i n t e c h n i c a l l i t e r a t u r e i s German Patent D.R.P. Nr. 524, 353, A p r i l 16, 1931, awarded to I. G-. Far b e n i n d u s t r i e Akt.-Ges. i n F r a n k f u r t . T h i s was f o r a pro cess f o r the production of metal sulphates from metal s u l  phides by o x i d a t i o n i n aqueous s o l u t i o n by the use of oxygen or an oxygen-containing gas. The r e a c t i o n was known previous l y , however, and was used i n a c y c l i c process f o r the sep a r a t i o n of H 2S from i l l u m i n a t i n g . g a s . In 1939 Dr. A Bognar was awarded Hungarian Patent 122479 f o r the treatment of ores and f l u e dusts at high temperatures and pressures In the 1 2 presence of some l i q u i d water, with or without the a d d i t i o n of sulphur or sulphur-hearing material, to form water-soluble sulphates of metals. In the same year Tronev, Bondin, and 1 7 Zviagincev ' p u b l i s h e d two papers i n which they describe the o x i d a t i o n of z i n c and copper sulphides i n a l k a l i n e s o l u  t i o n s and i n water, u s i n g h i g h pressure a i r at temperatures up to 250°C. At the U n i v e r s i t y of B r i t i s h Columbia work on pressure o x i d a t i o n has been done by R. Carter3, w. K. A. Congreve , R. B. Mclnto ah5, and J. F. Stenhouse^. Of these, the paper by J. F. Stenhouse i s the only one which deals ex c l u s i v e l y with the mechanism and fundamental r e a c t i o n s of the process. Mr. Stenhouse o x i d i z e d p y r i t e i n sodium hydroxide s o l u t i o n and measured the r a t e of the r e a c t i o n by the r a t e 1 Tronev, V., and Bondin, S., Osidation of Zinc Sulphide and Transference of Zinc i n t o Aqueous or A l k a l i n e S o l u t i o n under A i r Pressure, Comptes Rendus Acad, of Science U.R.S.S. 23, pp. 5^1-3, Moscow, 1939. 2 Zviagincev and Tronev, V., Oxidation of Cupper Sulphide and Transference of Copper i n t o Aqueous S o l u t i o n under A i r Pressure, Comptes Rendus Acad, of Science U.R.S.S. 23, pp. 54-3-4, Moscow, 1939. 3 C a r t e r , R., Influence of Roasting Temperature on Gold Recovery from a R e f r a c t o r y Gold Ore, M. A. Sc. Thesis, U n i v e r s i t y of B r i t i s h Columbia, 194-9. 4- Congreve, W. K. A., Use of High Pressure Oxygen i n Ex t r a c t i o n Metallurgy, Report to the Research Committee, The U n i v e r s i t y of B r i t i s h Columbia, 194-9. 5 Mcintosh, R. B., Recovery of Cobalt from Taylor Gem Ore by Aqueous Oxidation, M. A. Sc. Thesis, U n i v e r s i t y of B r i t i s h Columbia, 1950. 6 Stenhouse, J. F., Humid Oxidation of P y r i t e , M. A. Sc. Thesis, U n i v e r s i t y of B r i t i s h Columbia, 1950. 3 of consumption of oxygen. This r e a c t i o n involved the forma t i o n of a coating of the oxides of i r o n on the f i n e p a r t i c l e s of p y r i t e . A f t e r studying the previous work i n t h i s f i e l d , i t was decided that the mechanism of oxidation where a l l prod ucts were soluble i n the medium had not been adequately i n  v e s t i g a t e d . The pressure o x i d a t i o n of galena i n sodium hydroxide s o l u t i o n appeared to o f f e r a s u i t a b l e r e a c t i o n f o r study. A method of f o l l o w i n g the r e a c t i o n without removing m a t e r i a l from the autoclave was f e l t to be d e s i r a b l e . For t h i s purpose the polarograph seemed to o f f e r good hope of success. A f t e r considering the d i f f e r e n t modifications of t h i s device, the model b u i l t by J . E. B. Randies? was thought to be most s u i t a b l e . A copy of h i s instrument was b u i l t and a f t e r some i n i t i a l d i f f i c u l t i e s was made to give repro ducible r e s u l t s . Equipment The Cathode-Ray Polarograph The c i r c u i t of the cathode-ray polarograph used i n t h i s research i s given i n Figures 1 and 2, page 4. I t c o n s i s t s of s i x u n i t s : a low voltage e l e c t r o n i c a l l y smoothed power supply, a source of increasing p o t e n t i a l , which also includes the current-measuring c i r c u i t ; a synchronizing unit 7 Randies, J . E. B., A Cathode-Ray Polarograph, Trans actions of the Faraday Society, #44,1948, pp. 322-327. 4 1 P O W E R P A C K Figure 1 Power Supply „ . A POTENTIAL SWEEP UNITl H .„„ V AMPLIFIERS Figure 2 Other Circuits 5 which s t a r t s and stops the sweep a f t e r the appropriate time i n t e r v a l s ; a d i r e c t current a m p l i f i e r to amplify the h o r i z o n t a l voltages to give the h o r i z o n t a l sweep; a d i r e c t current a m p l i f i e r to amplify the v e r t i c a l (current) s i g n a l s ; and a cathode-ray tube with i t s high-voltage power supply f o r the v i s u a l p r e s e n t a t i o n . The cathode-ray tube used was incorporated i n an. o x c l l l o s c o p e , T r i p l e t Model 34-40 ( f i v e - i n c h ) , whose chassis was grounded to the chassis housing the r e s t of the e l e c t r o n i c equipment. The output of the D. C. a m p l i f i e r s was connected d i r e c t l y to the d e f l e c t i o n p l a t e s of the cathode-ray tube. The whole i s shown i n F i g . 3, page The Autoclave The autoclave i s i l l u s t r a t e d i n F i g . 4-, page 6. A l l surfaces exposed to the s o l u t i o n are s t a i n l e s s s t e e l , with the exception of the s t i r r i n g mechanism and the thermo meter w e l l . Magnetic material was necessary f o r the s t i r r e r so m i l d s t e e l was used i n the absence of conveniently a v a i l  able magnetic s t a i n l e s s . The thermometer w e l l was an emer gency r e p a i r a f t e r a previous arrangement f a i l e d , and as i t served the purpose i t was not replaced. I t consisted of a piece of ordinary i r o n pipe welded over at the end. Various materials were t r i e d f o r use as a l i n e r — g l a s s , Inconel, cop per, and type 304- s t a i n l e s s s t e e l . On a short-term basis the s t a i n l e s s s t e e l was the only s a t i s f a c t o r y m a t e r i a l . The glass d i s s o l v e d at an excessive r a t e , the Inconel apparently disp l a c e d lead from the s o l u t i o n , and the copper dissolved and r e p r e c i p i t a t e d as a cupric hydroxide suspension. The heating and s t i r r i n g devices are shown i n Figure 3 E l e c t r o n i c Equipment Heavy Pt Wir» *0D Class Tube Triermomet.r Sparkplug Vffl Silver So/eer Joins Short Brass forateit Pt 0isc Gasket o.eoy Pt Wire. Thermomtttr WetJ Wooas Metal Stainless 5tee) Strap Ga/ena Crystal Aoueous Sa/uttoij Stainless Steel imer 'liindum Cement S»«et Steel Figure 4 Autoclave and Accessories 7 F i g . 5, page 8. Heat i s s u p p l i e d from an element made by shaping c o i l e d nichrome wire about a form of the same diameter as the body of the autoclave, p l a s t e r i n g with alundum cement, baking, and s e t t i n g the r e s u l t i n g c y l i n d e r , a f t e r removal of the form, i n a metal can. The space between can and c y l i n d e r was then chinked xtfith more alundum. The element was designed to g i v e 500 watts at 110 v o l t s . The a c t u a l heat s u p p l i e d was c o n t r o l l e d by a v a r i a c and approximated 100 watts. The temperature of the contents of the autoclave was read o f f the thermometer which was i n s e r t e d i n i t s w e l l and connected thermally with molten Wood's metal. The contents of the autoclave were s t i r r e d by r o t a t  i n g a 2.5-inch A l n i c o magnet j u s t below the bomb, which moved the c y l i n d r i c a l r o t o r on the i n s i d e . A rim was turned onto the r o t o r to reduce i t s bearing surface. The speed of r o t a t i o n of the magnet could be v a r i e d from 80 to 250 RPM by changing p u l l e y s on the b e l t d r i v e , and by varying the s e r i e s r e  s i s t a n c e c o n t r o l l i n g the v a r i a b l e speed motor. Oxygen and other gases were supplied from commercial b o t t l e s , r e g u l a t e d by the u s u a l diaphragm valves, and the pressures read o f f the Bourdon gauges incorporated i n t o the r e g u l a t o r assemblies. A piece of high-pressure rubber hose gave f l e x i b i l i t y to the gas connections, and a shut-off valve allowed the autoclave to be disconnected while at elevated temperatures. The autoclave had been t e s t e d to 200 p . s . i . This r e s t r i c t e d the temperature and pressure ranges i n v e s t i g a t e d . 9 The E l e c t r o d e s The e l e c t r o d e s were made as f o l l o w s ; The platinum micro-electrode was made by fu s i n g the end of a piece of f i n e wire and welding the other end to a coarser platinum wire. The heavy wire was fused i n t o a piece of pyrex c a p i l l a r y tubing, which was h e l d i n a rubber packing under a s t a i n  l e s s s t e e l pipe plug. T h i s i s a l l shown i n F i g . 4. The un p o l a r i z e d e l e c t r o d e was a piece of p e r f o r a t e d platinum f o i l , which was brazed to a short rod connected to the centre e l e c t r o d e of a Champion VR-1 model a i r c r a f t spark plug. Theory of Operation of the Polarograph The voltage of the p o t e n t i a l sweep of the cathode- ray polarograph i n c r e a s e s at a r a t e of approximately one v o l t per second. This i s about one hundred times as f a s t as i n the normal polarograph. The r e s u l t of t h i s high r a t e i s to change the law of operation from dependency on the o laws of s p h e r i c a l d i f f u s i o n 0 to dependency on the rate of d e p l e t i o n from very t h i n l a y e r s about the electrode. The theory of operation i s explained by Randies^, who worked out the s o l u t i o n of the d i f f e r e n t i a l equation f o r plane d i f f u s i o n , which t h i s process approximates, by numerical methods. Mathematical d i f f i c u l t i e s here are caused by the complicated equation f o r the boundary c o n d i t i o n s . The 8 K o l t h o f f and Lingane, Polarography^ I n t e r s c i e n c e P u b l i s h e r s Inc., New York, 1941. 9 Randies, J . E. B., The Current Voltage Curves Trans a c t i o n s of the Faraday Society, #44, 1948, pp. 327-338. 10 r e s u l t s of the use of t h i s r a p i d sweep are; 1. The usual sharp r i s e s a s s o c i a t e d with d i f f u s i o n current regions are re p l a c e d by current maxima. 2. I f the point f o r the beginning of the sweep i s taken at a voltage where the preceding r e a c t i o n i s c o n t r o l l e d by steady s t a t e d i f f u s i o n , the current r i s e f o r the succeeding r e a c t i o n i s independent of the preceding current value. 3. Peak height, i n a d d i t i o n to being a f u n c t i o n of a l l the q u a n t i t i e s l i s t e d f o r d i f f u s i o n current i n con v e n t i o n a l polarography, i s a l s o d i r e c t l y p r o p o r t i o n a l to the rate of change of volt a g e . 4. The half-wave p o t e n t i a l , b a s i s of i d e n t i f i c a t i o n by polarography, corresponds to the current maximum. 5. The peak he i g h t i s p r o p o r t i o n a l to the rate of an ele c t r o d e process. For a very slow process there w i l l be a slow r i s e and no maximum i n the mathematical sense. There are no p a r t i c u l a r disadvantages f o r a n a l y t i c a l work i n such a device, and f o r following r a p i d r e a c t i o n s the advantages are obvious. They were r e a l i z e d by Snowdon and Page"1-0, who b u i l t a more r e f i n e d machine than Randies' and used i t to f o l l o w very r a p i d r e a c t i o n s i n synthesis and decomposition of organic compounds. Because of the nature of the process being 10 Snowdon, F. 0., and Page, H. T., A Cathode-Ray P o l a r o - graph, A n a l y t i c a l Chemistry, V o l . 22, No. 8, August, 1950, pp.969-80. 11 i n v e s t i g a t e d , which only takes place at high temperatures and pressures, the usual dropping mercury cathode and calomel h a l f c e l l f o r the anode and reference, p o t e n t i a l could not he used, or, at l e a s t , not e a s i l y . I t has been stated i n Randies' papers and others that reactions at s o l i d e l e c  trodes w i l l give consistent a n a l y t i c a l r e s u l t s , although some of the s i m p l i f y i n g assumptions used i n the mathematical d e r i v a t i o n s of the polarographic current curves are no longer t r u e . Consistent r e s u l t s were obtained i n t h i s a p p l i c a t i o n . The requirement f o r the unpolarized electrode i s that i t have a large area compared to the micro-electrode, and that i t have a steady, known h a l f - c e l l p o t e n t i a l , vary ing l i t t l e with current. The second c r i t e r i o n i s probably not met by a sheet of smooth platinum, but i f the p o t e n t i a l i s reasonably steady, a given ion can be i d e n t i f i e d by the running of s u i t a b l e standards under the same conditions. At elevated temperatures and pressures the half-wave voltages are not known anyway. There was l i t t l e f l u c t u a t i o n of the p o t e n t i a l s of lead and. oxygen waves under l i k e c o n d i t i o n s . Experimental Materials "Bakers Analyzed" chemicals and d i s t i l l e d water were used throughout f o r making up medium. Two types of galena were used, a ground 12 concentrate from the Consolidated Mining and Smelting Company of Canada, and handpicked c r y s t a l s from Violamac Mines (B.C.) Limi t e d at Sandon, B. C. The analyses are given below; C. M. & S. Concentrate OG-S 740 Ag Pb Zn Fe 94.7 oz/ton 82.7$ 1.6$ 1.0$ Spectrographs A n a l y s i s Low Trace Ca A l S i Mg Sb Mn Cu Cd As B Sn Ge V T i Ba Sr B i The a n a l y s i s of three c r y s t a l s of the same type as those used f o r the research, from Violamac Mines, gave the f o l l o w i n g a n a l y s i s ; Ag 126 oz/ton Au 0.02 oz/ton Spectrographic A n a l y s i s Sn 0.2$ S i ) Sb 0.06 Fe ) l e s s than 0.05$ Zn O.O3 Mg ) As ) The r e s u l t s of the spectrographic a n a l y s i s are from the P r o v i n c i a l Assay O f f i c e , V i c t o r i a , B. C., who k i n d l y d i d t h i s work. Experimental Procedure 1_ Rates on C r y s t a l s A slab of galena c r y s t a l measuring approximately 0.2 by 0.3 by 0.5 inches was broken from a l a r g e r piece. The surfaces were ground f l a t and p a r a l l e l to the 100 axes on No. 2 emery, the c r y s t a l was measured by a micrometer, washed i n c o l d , running water, and placed i n a s t a i n l e s s 13 s t e e l wire h o l d e r as I l l u s t r a t e d i n F i g . 5. The hol d e r was placed i n i t s p o s i t i o n i n the l i n e r , 155 ml. of medium added to b r i n g the l i q u i d up to the r e q u i r e d l e v e l , the r o t o r drop ped i n , and, i n l a t e r experiments, a few ml. of water were introduced between the l i n e r and the body of the autoclave. A f t e r washing the l i d and el e c t r o d e s the l i d was b o l t e d i n t o p o s i t i o n , the autoclave placed i n the heating jacket, and the gas l i n e connected. The s h u t - o f f valve was then c l o s e d and gas pressure turned on against i t . (This step was a pre c a u t i o n against steam g e t t i n g i n t o the l i n e i f the shut-off valve d i d not c l o s e p r o p e r l y . ) The heat was then switched on, the a g i t a t o r s t a r t e d , and the polargraph switched on. When the autoclave was up to temperature, the top valve was opened to allow oxygen to reach the s o l u t i o n . T h i s was taken as time zero. Photographs were taken of the cathode-ray tube face at 15-minute i n t e r v a l s , each time shutting o f f the a g i t a t i o n and allowing the s o l u t i o n to come to r e s t . The electrodes were only connected to the polarograph while t a k i n g a reading. A f t e r about 180 minutes the top valve was closed, the heat and oxygen turned o f f and the autoclave quenched under a tap. This was taken as t f i n £ ) - L , and f i v e minutes were added f o r r e a c t i o n o c c u r r i n g a f t e r the apparatus had been shut o f f . The autoclave was opened when cool and the s o l u t i o n with washings put aside f o r assay. The assay procedure i s given i n Appendix A. The c r y s t a l was then examined under the microscope and sometimes photographed. The photographic p l a t e s showing the current-voltage curves were then developed and the peak heights measured, 14 corrected, and p l o t t e d . The rate curve was extrapolated to t f l n a l plus f i v e minutes, and the ordinate at t h i s point corresponded to the t o t a l lead content of the s o l u t i o n . The polarograph was set while taking the f i r s t reading and the adjustments were not changed during the run. The r e s u l t s were consistent w i t h i n 5$, except f o r a few anomalously high r a t e s . These were considered to be due to s p a l l i n g of small fragments of galena, which would be ground under the r o t o r and give an increase i n surface area. 2_ Rates on Pulps When a rate determination was made on ground galena, the procedure was exactly the same except that a weighed amount of pulp was placed i n the l i n e r instead of a c r y s t a l . 3 Q u a l i t a t i v e I n v e s t i g a t i o n of the E f f e c t of Applied F i e l d s When i t was desired to in v e s t i g a t e the ef f e c t of an a p p l i e d f i e l d , a small galena c r y s t a l was set In solder i n a cup d r i l l e d i n t o the end of a brass rod. The other end of the brass rod was welded to the centre electrode of a model a i r c r a f t sparkplug, and a l l metal and c r y s t a l except fo r a small window to a 100 plane was painted with a p l a s t i c - base v a r n i s h . The closed autoclave containing medium was then brought up to b o i l i n g - p o i n t , the sparkplug screwed i n t o p o s i t i o n so that the c r y s t a l was below the l e v e l of the 15 liquid,'oxygen pressure a p p l i e d , and EMF's measured by a vacuum-tube voltmeter. Sometimes the electrodes were con nected to the p e r i o d i c voltages from the polarographic apparatus and the wave forms observed. Results Products of the Pressure Oxidation of Galena i n Sodium Hydroxide S o l u t i o n A n a l y s i s showed the products of the rea c t i o n could be considered to be sodium plurnbite and sodium sulphate, or, more c o r r e c t l y , sulphate-ion and plumbite-ion. No higher valence forms of lead and no lower valence forms of sulphur, f o r example, polythionates, were found. Ap parently the sulphur atom was oxidized completely before i t had t r a v e l l e d f a r i n t o the s o l u t i o n . The Wave Forms The wave forms f o r the platinum micro-electrode were a l l s i m i l a r to the example shown i n F i g . 6, page 16. These were the r e s u l t s of employing the microelectrode as a cathode. I f the microelectrode was made anodic no cur  rent maxima were observed except f o r the evolution of oxygen, which occurred at a very low p o t e n t i a l . Quantitative r e s u l t s were a l l based on the waves f o r the reduction of oxygen and of lead-ion to m e t a l l i c l e a d . There was no s h i f t i n the half-wave voltages f o r these reactions as might have been expected, due to poison ing of the platinum anode, or because of the build-up of ] Figure 6 Polarogram Photograph showing, from left to right, a lead oxidation wave, oxygen reduction wave, and lead reduction wave. 17 lead on the cathode. Wave heights were measured by drawing tangents as described i n K o l t h o f f and Lingane, f o r although the maximum was easy to read, the beginning of the r i s e was not so d e f i n i t e . The v e r t i c a l distance from the lower tangent to the maximum was c a l l e d the wave height. As the l i n e s on the photographic p l a t e were of appreciable t h i c k  ness, the centre of t h i s l i n e was taken as the true value. Because of the r a p i d attack of sodium hydroxide on g l a s s , i t was necessary to use a wire dipping i n t o the s o l u t i o n as the micro-electrode. As current i s a f u n c t i o n of surface area, t h i s made current readings very s e n s i t i v e to changes i n l e v e l of the s o l u t i o n . For t h i s reason, and because no other s u i t a b l e substance was found, oxygen was used as a reference standard. I t was considered i n the case of measuring r a t e s of oxidation of galena c r y s t a l s that the r e a c t i o n was slow enough that the surface l a y e r s of s o l u t i o n would always be saturated with oxygen, a f t e r the f i r s t few minutes. A f t e r water was introduced between l i n e r and autoclave, the changes i n l e v e l became almost i n s i g n i f i c a n t . The wave forms of the r e a c t i o n at a galena, electrode were much more complica.ted and no attempt was made to get q u a n t i t a t i v e information from these. The Rate Curves The r a t e curves f o r c r y s t a l s were a l l of the form shown i n Figures 7 and 8, page 18. The i n i t i a l toe of the curve i s caused by d i f f u s i o n of oxygen into the l i q u i d gradually reaching e q u i l i b r i u m , at which time some I 0 0 3 0 0 5 N N » _ O H 3 0 0 * F 136 P S . G 50 100 TIME MINUTES Figure 7 Rate Curve for a C r y s t a l Figure 8 Rate Curve f o r a C r y s t a l 19 other step controls the o v e r - a l l r a t e . I t i s considered that the medium was approaching saturation with oxygen. I t may he noted i n the two examples given that s a t u r a t i o n was much more r a p i d with more concentrated sodium hydroxide. This can be explained by two c o n t r i b u t i n g factors*. oxygen i s l e s s soluble i n concentrated c a u s t i c , and the r e a c t i o n rate i s slower. I t w i l l be noted that the curves are essen t i a l l y s t r a i g h t a f t e r the i n i t i a l toe. This would seem to mean that the products, at l e a s t i n these low concentrations, have no e f f e c t on the r e a c t i o n , and that the e f f e c t i v e sur face area remains constant, or that these e f f e c t s exactly c a n c e l . The r e a c t i o n was always slow enough and the time short enough that the o v e r a l l s i z e of the c r y s t a l was not much changed. There was an increase i n area, however, due to p i t t i n g e f f e c t s . The straightness of the l i n e a f t e r the i n i t i a l period was checked by stopping the reaction a f t e r shorter periods than usual and seeing i f the concen t r a t i o n - t i m e r e l a t i o n s h i p was l i n e a r . This was found to be so w i t h i n the experimental e r r o r . The rate curve f o r ground pulps was found to have the shape shown i n F i g . 9, page 20. I t appears reasonable considering the reduction i n area a.s the pulp i s consumed. There appears to be no i n i t i a t i o n period here, no doubt due to the f i n e p a r t i c l e s approaching the oxygen-rich g a s - l i q u i d i n t e r f a c e i n the early stages. With incomplete reactions the residue consisted e n t i r e l y of coarser p a r t i c l e s , no doubt due to the e f f e c t mentioned above, and perhaps due i n part to the higher surface energies of very fine p a r t i c l e s . 20 21 A l l r a t e r e l a t i o n s h i p s were worked out from data on s i n g l e c r y s t a l s . There are two dangers i n t h i s : a d i f f i c u l t sampling problem i s created and l a c k of homogeneity i n a l a r g e slab might produce e f f e c t s which would not be met i n a ground pulp. Nevertheless, as i t was d e s i r e d to measure the r a t e s of flow of medium past the surface and to a v o i d the e f f e c t s of a t t r i t i o n , almost a l l the work was done on s i n g l e c r y s t a l s . No d i f f e r e n c e i n r a t e between i n d i v i d u a l slabs of galena, was found. Appearance.of the C r y s t a l a f t e r Oxidation A l l c r y s t a l s , r e g a r d l e s s of r a t e of r e a c t i o n or sodium hydroxide concentration, had three a t t r i b u t e s i n com mon when viewed under the microscope: the surface was crossed by etched l i n e s , apparently some sort of block boundaries, i t was covered with f i n e p i t s of varying s i z e , formed by the i n t e r s e c t i o n of 111 planes, and there was no evidence of change of phase i n the surfa.ce. As agita;tion was p a r t i c u l a r l y poor during the c o o l i n g period, while r e a c t i o n was s t i l l t a k i n g place, there was some accumulation of product, p a r t i c u l a r l y i n h i g h c a u s t i c concentrations. In one case, i n 0,5 N NaOH at 93°C, the surface showed a web of the yellow product (hydrated PbO)along the ri d g e s between these p i t s . Un f o r t u n a t e l y t h i s was not photographed. The coatings were only l o o s e l y adherent. Two photomicrographs are given on page 22. F i g . 10 shows the boundary l i n e s or grooves and F i g . 11 shows the p i t s . Figure 10 35x Photomicrograph of Galena C r y s t a l a f t e r Attack i n 0.5 N Caustic at 1500c f o r 200 minutes mmr • • *r '•«». Figure 11 500x Photomicrograph of Galena C r y s t a l a f t e r Attack i n 0.5N Caustic at 175°C f o r 132 minutes 23 I n v e s t i g a t i o n of V a r i a b l e s The f o l l o w i n g v a r i a b l e s were inv e s t i g a t e d with respect to rea.ction r ates of s i n g l e c r y s t a l s of galena, measured as ra t e of production of lead-ion: 1. Oxygen p a r t i a l pressure. 2. A g i t a t i o n , i . e. rate of flow of medium past the par t i c l e . 3. Concentration of sodium hydroxide. 4. Temperature. 5. E f f e c t of s i l i c a t e - i o n . 6. E f f e c t of s u b s t i t u t i n g sodium acetate f o r sodium hydroxide. ?. E f f e c t of e l e c t r i c a l p o t e n t i a l ( q u a l i t a t i v e l y o n l y ) . 1 Oxygen P a r t i a l Pressure The e f f e c t of oxygen p a r t i a l pressure was i n v e s t i  gated i n 0.5 N NaOH at 123°C i n the range zero to 11 a t  mospheres. The r e s u l t s are tabulated below: Rate, ^ Rate p 1 atm. p2 0 0 1.97 1.41 5.64 2.38 IO.96 3.31 moles cm ~sec x £2 0 5 1 -1 moles cn1, sec - xatm 2 5.77 x 10~9 4.1 x 10-° 10.08 x 10~9 4.2 x 10-9 13.14 x 10-9 4.0 x 10-9 — where P i s the p a r t i a l pressure of oxygen i n atmospheres. I t was noted that at zero p a r t i a l pressure of o x y g e n ( a c t u a l l y not quite zero because commercial nitrogen was used) not only could no le a d or sulphur be detected In the s o l u t i o n , but the surface showed no trace of any kin d of attack a f t e r 3 hours at 123°C. An attempt was also made at t h i s time to i n  v e s t i g a t e the roast r e a c t i o n . The c r y s t a l was heated under nitrogen pressure with sodium plumbite s o l u t i o n at 123°C f o r 3 hours. Again no r e a c t i o n appeared to take place, the surface was unchanged, and there was no increase i n lead content of the s o l u t i o n . 2 A g i t a t i o n (Rate of Flow of Medium Past the P a r t i c l e ) Rate of flow past the p a r t i c l e were ca l c u l a t e d from the speed of r o t a t i o n of the magnet. No allowance was made f o r drag of the walls of the v e s s e l , and the rate c a l  culated f o r the middle of the "leading-edge" of the c r y s t a l was taken as the average. No d i f f e r e n c e i n s p e c i f i c rate was d e t e c t i b l e i n 0.5N NaOH s o l u t i o n at 150°C under 5«64 atmospheres p a r t i a l pressure of oxygen when the "average" rate of flow of s o l u t i o n past the c r y s t a l was varied from 9 cm/sec to 48 cm/sec. Under the same conditions of temperature and pressure, the rates i n 3N and 6N NaOH were dependent on a g i t a t i o n , as shown i n the table below: Rate (moles cm-2sec~:Latm~2') x l O 4 ^ V e l o c i t y of Concentration of NaOH Medium 0.5N 3N .. 6N 9 cm sec" 1 6.17 2.44 0.53 48 cm sec" 1 6.17 3-^ 7 2.01 25 To r e l a t e the v e l o c i t i e s to something more f a m i l i a r , the f r e e s e t t l i n g v e l o c i t i e s of galena i n c o l d w a t e r 1 1 are given below f o r a few screen s i z e s , with approximate Reynold's numbers: Screen Size S e t t l i n g V e l o c i t y Openings/inch cm/sec Reynold's Number 100 6.72 9.9 !50 5.27 5.5 200 2.86 2.1 325 .85 0.37 The Reynold's numbers f o r the flow v e l o c i t i e s associated with the r e a c t i o n r a t e s above va.ry from 1480 to 85. I t would appear from these numbers that the influence of a g i t a t i o n becomes appreciable as soon as flow, by the Reynold's c r i t e r i o n , becomes viscous. From the Reynold's numbers f o r ground pulps i t would appear that t h i s s i t u a t i o n would always obtain, and that increased a g i t a t i o n would give Increases i n rates of r e a c t i o n . This would be inde pendent of any aeration e f f e c t s . _2 Concentration of Sodium Hydroxide The e f f e c t of concentration of sodium hydroxide on the s o l u b i l i t y of oxygen i n aqueous solutions at 25°C i s given by G e f f c k e n 1 2 . The rate of the r e a c t i o n i s known 11 Richards, R. H. and Locke, C. E., Textbook of Ore Dress i n g , Third E d i t i o n , 4th impression, McGraw-Hill Book Co. Inc., New'York and London, 1940. 12 Geffcken G., Beltrage zur Kenntnis der L o s l i c h k e i t - beeinflussung, Z. f . Phys. Chemie 49, 257-302 1904. from experiment to vary with the square root of the p a r t i a l pressure of oxygen, and therefore, assuming oxygen obeys Henry's Law, the r e a c t i o n rate should vary with the square root of the oxygen concentration i n s o l u t i o n . Therefore, at constant p a r t i a l pressure of oxygen the rate should vary as the square root of L, the Ostwald s o l u b i l i t y , which i s defined as concentration of gas i n the l i q u i d divided by concentration i n the gas phase. J- In F i g . 12, page 27, and K, the s p e c i f i c r e  a c t i o n r a t e , are p l o t t e d i n a r b i t r a r y u n i t s against normality of sodium hydroxide. Isk and K are so adjusted that they have the same ordinate at 0.5 N NaOH. Curves are p l o t t e d f o r d i f f e r e n t f l u i d v e l o c i t i e s . These curves would seem to i n d i c a t e that the e f f e c t s of sodium hydroxide are, f i r s t , to reduce the s o l u b i l i t y of oxygen i n the s o l u t i o n , and, second, to increase the v i s c o s i t y of the l i q u i d to cause d i f f u s i o n through the f i l m s around the p a r t i c l e to c o n t r o l the r a t e . I t i s r e a l i z e d there i s an e x t r a p o l a t i o n here from 25°C to 150°C. The assumption implied i s that the v a r i a t i o n of oxygen s o l u b i l i t y with sodium hydroxide con c e n t r a t i o n follows the same shaped curve at 150°C as i t does at 25°C. I t might be expected that the curve would be somewhat f l a t t e r at the high temperatures, thus showing an even greater f i l m e f f e c t . 4 Temperature The e f f e c t s of temperature were in v e s t i g a t e d i n N O R M A L I T Y N t O H Figure 12 Rate vs. Normality NaOH Plot of L"2 and Reaction Rate i n arbitrary units against normality of sodium hydroxide. Temperature 150°C, pressure 5.64 Atmospheres. 28 0.5 N NaOH s o l u t i o n under an oxygen p a r t i a l pressure of 5.64 atmospheres, i n the range 93°C to 175°C. The Arrhenius p l o t i s shown i n F i g . 13, page 29. The slope corresponds to an apparent a c t i v a t i o n energy of 6820 c a l o r i e s per mole. I t i s known that the s o l u b i l i t i e s of gases i n l i q u i d s decrease with temperature, so i t was wondered- i f there should be a c o r r e c t i o n f o r t h i s e f f e c t . However, when lo g L was p l o t t e d against r e c i p r o c r l tempera t u r e , using values from I n t e r n a t i o n a l C r i t i c a l Tables"^, and from the Handbook of Chemistry and P h y s i c s 1 ^ f o r s o l u b i  l i t y i n water, i t was seen that the slope of the curve was very f l a t i n the beginning of the range i n v e s t i g a t e d by the author, with decreasing slope apparently the tre n d . The c o r r e c t i o n at the low temperature end of the r e a c t i o n rate p l o t only amounts to 60 c a l o r i e s per mole. Again, i t i s not known what e f f e c t sodium hydroxide has on the s o l u b i l i t y - temperature curve. J5 E f f e c t of S i l i c a t e - i o n Stenhouse 1^ found that s i l i c a had a pronounced e f f e c t on the o x i d a t i o n of p y r i t e . This was a t t r i b u t e d to some sort of poisoning of the surface with s i l i c a t e - i o n . In f l u i d v e l o c i t i e s of 48cm/sec, 5.64 atm. p a r t i a l pressure 13 I n t e r n a t i o n a l C r i t i c a l Tables of Numerical Data, McG-raw- H i l l Book Co. Inc., New York, 1 9 2 6 . 14 Handbook of Chemistry and Physics 30th E d i t i o n , Chemical Rubber P u b l i s h i n g Co., Cleveland, Ohio, 1946. 15 Stenhouse, J . F., i b i d . -86 ,-d.e 2J2 24 2.8 T 1000 Figure 13 Arrhenlus Plot Log K (in moles cm ^ sec - atm2) versus 1 T of oxygen, at a temperature of 122°C, i t was found that in 0.5 N NaOH, 0.015 Molar s i l i c a t e - i o n had very l i t t l e e f f e c t , and O.O3 Molar s i l i c a t e - i o n gave a reduction i n rate of approximately 10$. It was noticed, however, that at points of low turbulence where a l i t t l e oxide coating might be expected to accumulate, a fine-grained black, adherent coating formed. It was therefore deduced that s i l i c a t e - ion has l i t t l e a f f i n i t y for sulphide surfaces, but i s readily chemisorbed to oxide surfaces, probably i n the case of pyrite sealing them against diffusion of ions. This would be similar to surface reactions occurring i n f l o t a t i o n . _6 Effect of Substituting Sodium Acetate for Sodium Hydroxide When sodium acetate was substituted for sodium hydroxide, the medium at 122°C attacked the mild steel i n the autoclave and even removed the adherent oxide coating from the stainless s t e e l . The result was such a reduction i n oxygen concentration of the solution that there was no reaction with the galena. Shortage of time prevented treat ment i n glass containers. 2 Effect of E l e c t r i c a l Potential Various measurements of the potential of galena under pressure were made, but unfortunately no solution was found for the problem of a reference electrode. A l l measure ments were against bright platinum in the same solution. Potential differences at 100°C in 1 N NaOH solution approxi mated 0.5 volts, the galena being negative. The emf 31 appeared to be s u b s t a n t i a l l y independent of oxygen pressure, as voltages of the same order were measured i n so l u t i o n s from which the oxygen had been removed by b o i l i n g . Using a vacuum tube voltmeter and platinum probes, and reversing probes frequently, an apparent emf i n the order of 10 m i l l i v o l t s was found between a 111 plane and a 100 plane of a galena c r y s t a l i n a s o l u t i o n exposed to the a i r at 50°C. The 111 plane was r e l a t i v e l y p o s i t i v e . When p o s i t i v e p o t e n t i a l s from the polarographic apparatus were applied to galena c r y s t a l s , a coating, apparently PbO, was formed on the exposed face and the cur rent tended to drop. When negative p o t e n t i a l s were appl i e d , there was increased etching of the surface and on the p l a s t i c v a r n i s h around the c r y s t a l a deposit of PbO was l e f t i n a c i r c u l a r pattern with the c r y s t a l the centre. I f a coating e x i s t e d on the c r y s t a l , i t was very r a p i d l y removed. The hydrogen overvoltage on galena appeared to be high. In 3 N II a OH s o l u t i o n , at room temperature, e l e c t r o  l y s i s of a galena c r y s t a l at 1.5 v o l t s with galena the cathode and with an i n e r t anode produced H2S and a coating of spongy lead on the cathode. In view of the increased r a t e of r e a c t i o n with moderate negative p o t e n t i a l s applied to the c r y s t a l , there was some worry that the contact between s t a i n l e s s s t e e l and galena would produce a c e l l which would produce f i e l d s i n the s o l u t i o n and d i s t o r t the r e s u l t s . Two experiments were done to q u a l i t a t i v e l y i n v e s t i g a t e t h i s p o s s i b i l i t y . In 32 each, fragments of the same c r y s t a l were oxidized side by side on a s t a i n l e s s s t e e l rack. I n the f i r s t experiment, one c r y s t a l was i n s u l a t e d with a t h i c k coating of Garbocoat, a p l a s t i c v a r n i s h , and the other made f a i r contact with the s t a i n l e s s s t e e l surface. I n the second experiment, one c r y s t a l was i n s u l a t e d by being placed i n an alundum boat, and the other i n contact with clean s t a i n l e s s s t e e l . In both cases oxida t i o n was f o r a r e a l t i v e l y short time, so that d i f f e r e n c e s i n rate should be more e a s i l y observable under the microscope. In neither case was there any apparent d i f f e r e n c e i n r a t e . A f t e r the f i r s t few experiments the s t a i n l e s s s t e e l was covered with a t h i c k adherent coating of oxide, which should have behaved as an i n s u l a t i n g l a y e r . For these reasons i t i s not believed that the p h y s i c a l s i t u a  t i o n of the c r y s t a l had an appreciable e f f e c t on the r a t e . Discussion of Results Numerical Values The numerical values of such q u a n t i t i e s as a c t i v a  t i o n energy are considered to be correct w i t h i n a few percent under the conditions of the experiment. I f co n t r o l i s by some other mechanism i n another experiment, then of course there w i l l be no r e l a t i o n s h i p between the two values. P a r t i c l e size w i l l have e f f e c t s a l s o . Probably the greatest discrepancies i n extrapolating t h i s data to reactions with 33 suspended pulps would be caused by the increased a c t i v i t y of very f i n e p a r t i c l e s . S u r p r i s i n g l y enough, i n these experiments sampling di d not seem to introduce any appreciable e r r o r . Apparently the half-dozen c r y s t a l s used, which were a l l from Violamac, and a l l selected by t h e i r regular shape, absence of twins, and straightness of axes, were very s i m i l a r i n chemical p r o p e r t i e s . From a l l c r y s t a l s used there were a few very high r e s u l t s which were considered to be due to fragments f a l l i n g under the r o t o r and being ground up to give a large increase i n surface area. In an i n d u s t r i a l a p p l i c a t i o n , c o n t r o l by d i f f u s i o n through the g a s - l i q u i d i n t e r f a c e would be e a s i l y possible with f i n e p a r t i c l e s and inadequate a e r a t i o n . Even more l i k e l y , i n f a i r l y strong ca u s t i c (which would be necessary i n a batch process) would be c o n t r o l by d i f f u s i o n through the f i l m s at the s o l i d - l i q u i d i n t e r f a c e . Motion of the p a r t i c l e would, be c o n t r o l l e d by the laws of viscous flow, and i t would not be possible to get out of t h i s range. As was seen on page 24 t h i s would mean c o n t r o l by d i f f u  sion through the f i l m about the p a r t i c l e . Mechanism of the Reaction i n 0.5 Normal Sodium Hydroxide According to E y r i n g 1 ^ a reaction of a gas at a surface may be separated into f i v e steps, the slowest of 16 Glasstone, S., L a i d l e r , K., and E y r i n g , H., The Theory of Rate Processes, McC-raw-Hill Book Go. Inc., New York and London, 1941. 34 which w i l l " determine the rate of the o v e r - a l l provess. The steps are: 1. Transport of the reactants to the surface. 2. Adsorption of the reactants. 3. Reaction on the surface 4. Desorptlon of the products. 5. Transport of the products from the surface. Using t h i s . c l a s s i f i c a t i o n for the pressure oxida t i o n of galena, the experiments with a g i t a t i o n i n d i c a t e that c o n t r o l must be by steps 2, 3, or 4. I t i s also known that the rate i s proportiona.1 to the square root of the oxygen pa.r t i a l pressure, that a negative p o t e n t i a l on the galena increases the r a t e , and that the a c t i v a t i o n energy i s low. Stenhouse 1? found i n the oxidation of p y r i t e that the rate of oxidation of i r o n depended on the square root of the p a r t i a l pressure of oxygen, but that at low pressures the r a t e of oxidation of sulphur depended on the f i r s t power of the p a r t i a l pressure of oxygen. In other work of the department on aqueous o x i d a t i o n , polythionates formed could be explained by the i n i t i a l formation of S0 2, which would give dependency on the f i r s t power of the oxygen p a r t i a l pressure, i f t h i s were the r a t e - c o n t r o l l i n g step. At low pH, the oxidation of sulphur i s slow or incomplete and apparently sulphur i s l i b e r a t e d to form globules of the element 1^. In Mr. Stenhouse's work, and generally observed 17 Stenhouse, J . F., i b i d . 18 Mcintosh, R. B., i b i d . i n aqueous ox i d a t i o n , i t i s found that the a c t i v a t i o n energy f o r the oxidat i o n of metals i s lower than that f o r sulphur. For these reasons the r a t e - c o n t r o l l i n g step i n t h i s r e a c t i o n , where no new phase appeared at the "boundary between galena a.nd s o l u t i o n , w i l l be considered to involve only the oxida t i o n of l e a d . For the rate to be prop o r t i o n a l to the square root of the p a r t i a l pressure of oxygen there are two p o s s i  b i l i t i e s for the a c t u a l mechanism: 1. Adsorption i s r a p i d but d i s s o c i a t i o n i s slow. 2. The r e a c t i o n i n v o l v i n g d i s s o c i a t i o n i s ra p i d , but the f r a c t i o n of the s i t e s involved i s small. This f o l  lows from the Langmuir isotherm; i x = a p 2 1 - x where x i s the f r a c t i o n of the s i t e s Involved a i s a constant p i s the oxygen p a r t i a l pressure when x i s small and 1 - x approaches 1. Mechanism I For the f i r s t mechanism, consider the reaction: i 0 2 + Pb ? gives (OPb2)# where Pb 2 represents two adjacent s i t e s and (0Pb 2)^ the ac t i v a t e d complex. The rate equation f o r t h i s r e a c t i o n i s : rate ( i n molecules cm"2 sec" 1) =,. c | c _kT t# exp.-H#/RT 36 where" c Q i s the concentration of oxygen i n s o l u t i o n C p k i s the concentration of s i t e s k i s Boltzmann's constant h i s Planck's constant T i s the absolute temperature f7^, fo2> a n d fpt) are the p a r t i t i o n functions of the a c t i v a t e d complex, of oxygen i n s o l u  t i o n , and of the s i t e s , r e s p e c t i v e l y , f o r a standard state of one molecule per cm3 or cm2 R i s the gas constant H# i s the enthalpy of a c t i v a t i o n The rate on the basis of t h i s mechanism has been worked out, using a v a i l a b l e data, and gives a value which i s 23,50° times as f a s t as the experimental value. The c a l c u l a t i o n s are given i n Appendix B. Mechanism I I In the second case above, the rate c o n t r o l l i n g step i s one which follows chemisorption, and i t was postulated that i t was desorption accompanied by hydration of PbO, to give a f t e r i o n i z a t i o n HPbO" i o n . This would explain the e f f e c t of p o t e n t i a l as aiding i n desorption, by r e p e l l i n g the anion. The rate c o n t r o l l i n g step would be: PbO + H 9 0 — ^ ( P b 0 . H 0-...H +)# s £ and the rate equation would be: rate = c P b 0 c H 0 kT f# exp. -H#/RT h f P b 0 fH2° 37 where" cH 2Q i s the concentration of water cPb0 i s t 5 i e concentration of PbO s i t e s fti o i s the concentration p a r t i t i o n function of 2 l i q u i d water and the other symbols have the same s i g n i f i c a n c e as previously By s u b s t i t u t i o n of q u a n t i t i e s from the e q u i l i b r i a : °2 = °? gas s o l u t i o n and Pb + -|02 = P^O the rate equation may be w r i t t e n Rate = c f c „ . c , _ M f# exp. - ( H - H - H^)/RT ^ -rr rarjj ? =f where Hj_ i s the enthalpy of s o l u t i o n of oxygen i n the aqueous medium Hg i s the enthalpy of the chemisorption of atomic oxygen i s the enthalpy of a c t i v a t i o n The term (Hj. + Ez + H"^ ) + RT i s equal to the experimental value of a c t i v a t i o n energy, E. Ej_ i s negative and i s of the order of 100 c a l o r i e s per mole, by c a l c u l a t i o n from data i n I n t e r n a t i o n a l C r i t i c a l Tables. H 2 corresponds to d i s s o c i a t i o n and chemisorption of oxygen to a metal surface. The re a c t i o n i s probably s l i g h t l y exothermic. & - *r This means that W equals E + 100_- H ?, and 38 therefore that H# may be considerably l a r g e r than E. On c a l c u l a t i n g the r a t e from the above expression, s u r p r i s i n g l y good agreement with experiment was achieved. The c a l c u l a t i o n s are given i n Appendix C. By t h i s model, the a c t i v a t i o n energy f o r the d i s s o c i a t i o n and adsorption of oxygen to the surface must be low. I t has been found i n the adsorption of hydrogen on n i c k e l or platinum, a c t i v a t i o n energies of the order of 5 k i l o c a l o r - i e s were observed. P o l a n y i 1 ^ explains t h i s by considering the metal surface to be h i g h l y degenerate, thus rounding o f f the peak corresponding to the a c t i v a t i o n energy and reducing i t s value. There i s a p o s s i b i l i t y that part of the e f f e c t of p o t e n t i a l may be i n increasing the hydrogen-ion concentration. I t Is known from G-effcken's data that hydroxyl has a, depress ing e f f e c t on s o l u b i l i t y of oxygen i n water, but that hydrogen- ion has l i t t l e e f f e c t . These e f f e c t s could be evaluated experimentally by measuring the a c t i v a t i o n energy with applied p o t e n t i a l . I f there were no change i n a c t i v a t i o n energy, then the e f f e c t would only be on the temperature independent term, through increase i n oxygen concentration; whereas i f the e f f e c t i s on desorption there would be a decrease i n the experimental a c t i v a t i o n energy. 19 P o l a n g i , M., C a t a l y t i c A c t i v a t i o n of Hydrogen, The S c i e n t i f i c Journal of the Royal College of Science, V o l . V I I , 193?-39 Mechanism III A t h i r d , p o s s i b i l i t y i s a r e a c t i o n analagous to the corrosion of a metal or a l l o y , with the metal tending to react with hydrogen-Ion and being depolarized by the d i s s o l v e d oxygen. That i s , In terms of c e l l s , the presence of the oxygen-hydroxyl h a l f - c e l l i s necessary to r a i s e the emf s u f f i c i e n t l y to cause r e a c t i o n . The k i n e t i c s of such a p i c t u r e i s not c l e a r enough to attempt evaluation of sulphide oxidation on these terms. I t i s r e a l i z e d that these three mechanisms are by no means exhaustive, that there are other p o s s i b i l i t i e s . However, Mechanism I I does explain the observed e f f e c t s , and the rate deduced from i t by the theory of absolute r e  a c t i o n rates checks wi th the observed r a t e . I t remains to consider Mechanism I I with regard to v a r i a b l e s not included i n the r e s t r i c t e d s i t u a t i o n treated above. Consideration of Mechanism I I with Respect to the Remaining V a r i a b l e s 1 A g i t a t i o n and Concentration of Sodium Hydroxide These v a r i a b l e s appear inseparable and w i l l be treated together. I t i s observed that the rate f a l l s with caustic concentration i n a manner that appears to roughly conform to the square root of the f a l l i n g oxygen s o l u b i l i t y . The general e f f e c t of caustic concentration on the rate of the r e a c t i o n would be by displacement of the equilibrium: ° ? = ° 2 "gas s o l u t i o n 40 to the l e f t . Although the a c t i v i t y of oxygen can be said to remain the same when the temperature and pressure of the gas are the same, but caustic concentration d i f f e r e n t , nevertheless the change i n the a c t i v i t y c o e f f i c i e n t of the oxygen would be r e f l e c t e d by a change i n the a c t i v i t y co e f f i c i e n t of PbO or ac t i v a t e d complex i n the same d i r e c t i o n . This would have the r e s u l t of making the rate proportional to concentration rather than a c t i v i t y . There i s a f u r t h e r complication of v i s c o s i t y here, however, and r e a c t i o n rates are seen to vary with a g i t a t i o n . This i s explained by the mechanical p i c t u r e of stagnant f i l m s about p a r t i c l e s moving r e l a t i v e to a f l u i d . The f i l m r e s i s t  ance i s considered to be high enough that i t takes c o n t r o l of the r e a c t i o n , by v i r t u e of being the slowest of the f i v e steps l i s t e d above. I t i s not known conclusively whether c o n t r o l i s by d i f f u s i o n of reactants i n or of products out from the i n t e r f a c e . 2 E f f e c t of S i l i c a t e - I o n S i l i c a t e - i o n apparently rea.cts with macroscopic coatings of oxide but not with the oxygen chemisorbed to the surface. I t i s not understood why t h i s should be so, unless reaction with s i l i c a t e - i o n i s dependent on hydration. _3_ E f f e c t of S u b s t i t u t i n g Sodium Acetate for Sodium Hydroxide The r e s u l t s of t h i s s u b s t i t u t i o n were abortive, as stated p r e v i o u s l y . 4 E f f e c t of Change i n Surface Area Although not considered as a v a r i a b l e , the s u r  face area was noted to change, mainly by formation of etch- marks c o n s i s t i n g of i n t e r s e c t i n g 111 planes. Apparently t h i s has no e f f e c t on the r a t e . This may be f o r two reasons 1. The surface becomes covered with p i t s i n the i n i t i a l stages of the r e a c t i o n , but t h i s i s concealed by the build.-up of oxygen concentration, marked by the toe of the rate curve. 2. The rates on 111 planes are slower by a f a c t o r which compensates exactly f o r the increase i n area. P i t s may be caused by differences i n a c t i v i t y between d i f f e r e n t parts of the c r y s t a l . Etchmarks are a well-known phenomenon, used i n geology for symmetry deter minations on mineral c r y s t a l s . Conclusions The use of a cathode-ray polarograph method of measuring the rate of oxidation of galena immersed i n a s o l u t i o n , under oxygen pressure, i s s a t i s f a c t o r y f o r measur ing the rate of s o l u t i o n of lead. I t gives no i n d i c a t i o n of the rate of oxidation of sulphur, but as no separate phase appeared and no polythionates were detected, undoubted l y the rate i n terms of formation of sulphate would have been i d e n t i c a l with that i n terms of solu t i o n of l e a d . 42 This technique could be applied p r o f i t a b l y to the measurement of rates of corrosion under extreme conditions of temperature and pressure of a l l metals, a l l o y s , or minerals where a s o l u b l e , reducible or oxidizable product i s formed. I t i s r e s t r i c t e d i n that i t cannot be used i n very concentrated solutions of the ion or molecule being measured, and interference by i m p u r i t i e s may also render i t i n o p e r a t i v e . In any commercial a p p l i c a t i o n of aqueous oxida t i o n , a cathode-ray polarograph would be a very useful con t r o l instrument f o r measuring concentration of oxygen i n s o l u t i o n . Such information might be v i t a l to e f f i c i e n t operation of the process. Even i n fused-salt e l e c t r o l y s i s one can see possible uses f o r a device patterned on t h i s , i n e i t h e r research or plant operation. I t i s not possible from the experimental r e s u l t s here obtained t o decide whether rate of oxidation of lead or of sulphur controls the o v e r - a l l r a t e . From other con s i d e r a t i o n s i t i s thought that lead i s involved i n the rate- c o n t r o l l i n g step. In high c a u s t i c concentrations, rate i s found to be c o n t r o l l e d by d i f f u s i o n through stagnant f i l m s about the p a r t i c l e . Under reaction c o n t r o l , rate of s o l u t i o n of lead i s p r o p o r t i o n a l to the square root" of the oxygen p a r t i a l pres sure. The a c t i v a t i o n energy i s 6820 c a l o r i e s per mole. A negative p o t e n t i a l applied t o the c r y s t a l profoundly increased the r a t e . I t i s not known whether t h i s increase i n rate i s due to r e p u l s i o n of HPbCf or to the increased s o l u b i l i t y of oxygen i n the absence of hydroxyl-ion. The former appears to be most probable. Three possible mechanisms f o r the r e a c t i o n were suggested. Mechanism I , c o n t r o l by the d i s s o c i a t i o n cf oxygen at the surface was found to give too high a c a l c u l a t e d r a t e . Mechanism I I I , modelled on a corrosion p i c t u r e , was not v e r i f i e d as no method of c a l c u l a t i o n from fundamental data appeared to be a v a i l a b l e . In Mechanism I I the r a t e - c o n t r o l l i n g step c o n s i s t  ed of desorption with hydration. The rate c a l c u l a t e d on t h i s basis from fundamental data using the experimental value of a c t i v a t i o n energy was k x lO1-^ molecules cm - 2 sec""! Under c e r t a i n c o n d i t i o n s , compared to an experimental value of 2 .55 x lO-^. This was considered an excellent check. I t i s r e a l i z e d the u n c e r t a i n t i e s i n the p a r t i t i o n f u n c t i o n of the a c t i v a t e d s t a t e , and i n the number of s i t e s a v a i l a b l e f o r reaction, are such that so close an agreement must be considered a coincidence. No experimental data c o n f l i c t e d with the p i c t u r e from Mechanism I I , although i t i s of course possible that there i s an equally p l a u s i b l e , a l t e r n a t i v e model which would also f i t the observations. 44 Appendix A Assay of S o l u t i o n by the Molybdate Method (Gravimetric) The b a s i c s o l u t i o n was made just a c i d to litmus with n i t r i c a c i d , then s l i g h t l y basic by the add i t i o n of ammonium acetat e—usually about f i v e grams. The s o l u t i o n was then heated to b o i l i n g and f i l t e r e d . To the b o i l i n g f i l t r a t e was added 40 ml. of ammonium molybdate s o l u t i o n f o r each 0.1 g of le a d present. A f t e r b o i l i n g a few minutes the s o l u t i o n was allowed to stand, then f i l t e r e d and' washed with hot water containing 2% ammonium n i t r a t e . The residue was i g n i t e d at d u l l red heat, cooled, and weighed as PbMoO^. PbMoO^ x 0.5643 = Pb Ammonium Molybdate Solution Dissolve 4 g of the s a l t per l i t r e of water plus 10 ml a c e t i c a c i d . 4-5 Appendix B C a l c u l a t i o n of Rate from Mechanism I From the re a c t i o n : Pb 2 + -f02 gives (OPb2 the r a t e equation i s given by: rate (molecules cm"2 sec" 1) = c^ Ct31_ kT f# exo. -H# /RT ( l ) The s o l u t i o n of oxygen i s considered to reach e q u i l i b r i u m . Then, Cone, of oxygen i n l i q u i d = u2 ( l i q u i d ) exp. -H-./RT Cone, of oxygen i n gas phase f n (gas) u2 where f represents the designated p a r t i t i o n functions H-^  i s the enthalpy of s o l u t i o n S u b s t i t u t i n g f o r CQ i n ( l ) , and c a n c e l l i n g f Q l i q u i d , rate = c£ £ *_r _ £ ! L w . -Ctl> + H~)/*T Experimental r e s u l t s are conveniently a v a i l a b l e f o r 4-00°K. In t h i s analysis concentration p a r t i t i o n functions based on a standard state of one molecule per square c e n t i  metre or per cubic centimetre were used. Choice of t h i s standard state was purely a matter of convenience. The r e s u l t i n g p a r t i t i o n functions are very large numbers be cause of t h i s choice. The concentration p a r t i t i o n function of gaseous oxygen i s : 46 3/2 T r a n s l a t i o n . . . (2 7r mkT) = 2.73 x 10 Rotation (8 ^ I k T ) = 190 h 2 V i b r a t i o n approximates 1 where I i s the moment of i n e r t i a of the oxygen molecule m i s the mass of the oxygen molecule the- other symbols have the usual s i g n i f i - Then f Q = 5.2 x 10 2 gas cance, 28 c n = 6.02 x 1023 x 273 - I.83 x 1 0 1 9 molecules cm"3 u2 gas 22400 4~00 C P b = 7 x ' t f'* t"2 k, Boltzmann 1 s constant = I.38 x l O - ^ erg deg" 1 h, Planck's constant = 6.62 x 10~2< erg se c e i s the base f o r natural logarithms, 2.72 -1 E i s the experimental a c t i v a t i o n energy, 6820 c a l mol frf = f p ^ and both approximate 1 On s u b s t i t u t i n g these numerical values, rate = z.?z * x * 7 x /o x /.za x-/<? * / y & = 6.0 x l O 1 ^ molecules cm""2 The observed rate i s 4.1 x 10"9 x 6. 02 x 1023 = 2.55 x 101-5 molecules cm - 2 I t i s seen that the calculated rate i s 23,500 times too l a r g e . 47 Appendix G C a l c u l a t i o n of Rate from Mechanism I I I f the r a t e - c o n t r o l l i n g step i s desorption of the PbO, with accompanying hydration: PbO + H20 gives (PbO.HO-.. .H+ )# The f o l l o w i n g reactions w i l l have reached equilibrium: gas s o l u t i o n ( l ) and i - k ) o + Pb = PbO As i n Appendix B c0 = e f ^ -H,/RT- W** 3 " :> - As regards (2) cph£> = ^ ^ -^a-^ *^ ~ "3-/*r~ ff* »U fa where H£ i s the enthalpy of r e a c t i o n Then c„e = c„ /*r And rate = c£ c„t0 e„6 kT -4~T £ **s>.v**)/«T or, expressing enthalpies as an experimental a c t i v a t i o n energy, I t i s seen that t h i s expression d i f f e r s from equation (2) of Appendix B by a f a c t o r . Ctr A = 6 x 10 23 molecules H2° IS7 cm~3 C a l c u l a t i o n of Concentration P a r t i t i o n Function of L i q u i d Water S H a =16.75 E.U. mole - 1 (Hougan and Watson)2^1 Change i n = 2.^ 8 E.U. mole- 1 ( K e l l e y ) 2 1 Then S „ „ = 19-1 E.U. mole" 1 To change from the standard state of l i q u i d water to a standard state of one molecule per cm3, d i l u t e the water I d e a l l y to one molecule per cm3. S = R In 6x10 23 = IO3 E.U. mole- 1 IB Then the entropy i n a state of one molecule per cm3 = 122.1 E.U. mole" 1 122/ f = e * - = 5 x 10 2 26 -/ fee. Rate from Appendix B = 6 x 10 1? molecules cnrd- Rate f o r t h i s model = 6 x l O 1 ^ x 6 x 10 23 x 1 . = IB 5 x 1026 4 x 1015 molcules c i i W which approximately checks with the experimental value of 2.55 x l O 1 ^ . There are two u n c e r t a i n t i e s i n t h i s c a l c u l a t i o n of the r a t e . Cpt), "the concentration of Pb s i t e s a v a i l a b l e f o r r e a c t i o n , may be increased by roughening of the surfa.ce, or reduced by poisoning, the p a r t i t i o n function of the 20 Hougan, 0. A., and Watson, K. M., Chemical Process P r i n c i p l e s , Part I I , John Wiley & Sons Inc., New York, 1947- 21 K e l l e y , K. K., Contributions to the Data on T h e o r e t i c a l Metallurgy X, B u l l e t i n 476, United States Covernment P r i n t  i ng O f f i c e , Washington, 1949. a c t i v a t e d complex , may have a v i b r a t i o n p a r t i t i o n f u n c t i o n o f some m a g n i t u d e , but p r o b a b l y not g r e a t e r t h a n 10. Bibliography 50 Garret, A. B>, Vellenga, S., and Fontana, C M., The S o l u b i  l i t y of Red, Yellow and Black Lead Oxides and Hydrated Lead Oxide i n A l k a l i n e S o l u t i o n s , The Character of the Lead-Bearing Ion, Journal of the American Chemical Soc i e t j 61, 1939, P. 36?. G i l b e r t , P. T., The Corrosion of Zinc and Zinc-Coated S t e e l i n Hot Waters, P i t t s b u r g I n t e r n a t i o n a l Conference on Surface Reactions, Corrosion P u b l i s h i n g Company, Pittsbure Pa., 1948. Glasstone, S., L a i d l e r , K. J . , and Eyr i n g , H., A p p l i c a t i o n of the Theory of Absolute Reaction Rates to Overvoltage, Journal of Chemical Physics, 7, pp.IO53-IO65. Latimer, M. L., The Oxidation States of the Elements and t h e i r P o t e n t i a l s i n Aqueous Solutions, Prentice-Ha.il Inc., New York, I938. L e i d h e i s e r , H., and Gwathmey, A. T., The Influence of C r y s t a l Face on the Electrochemical Properties of a Single C r y s t a l of Copper, Transactions Electrochemical Society, 91, 1947, P 95. Markhan, A., and Kobe, K., S o l u b i l i t y of Gases i n L i q u i d s , Chemical Reviews, 28, 1941, p. 519. M e l l o r , J . ¥., A Comprehensive Treatise on Inorganic and The o r e t i c a l Chemistry, V o l . V I I , Longmans Green & Co. . L t d . , London, 1927. P a u l i n g , L., The Nature of the Chemical Bond, Corn e l l U n i v e r s i t y P r e s s , Ithaca, New York, 194-8. Scott, W. W., Standard Methods of Chemical A n a l y s i s , V o l . I , D.'Van Nostrand Co. Inc., New York, 1939. U h l i g H. H., The Corrosion Handbook, Sponsored by the Electrochemical Society Inc., John Wiley & Sons Inc., New York, 1948. 

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