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Factors affecting the leaching process as applied to nickel sulphide ores and concentrates Kudryk, V. 1948

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UNIVERSITY OF BRITISH COLU.V.&IA DEPARTMENT OF M I N I N G AND M E T A L L U R G Y cop. / FACTORS AFFECTING THE LEACHING. STAGE OF THE AMMONIA LEACHING PROCESS AS APPLIED TO NICKEL SULFIDE ORES AND CONCENTRATES by V. Kudryk A t h e s i s submitted as p a r t i a l requirement f o r a Master of A p p l i e d Science Degree i n the Department of Mining and M e t a l l u r g y at the U n i v e r s i t y o f B r i t i s h Columbia. June 1 9 4 S . UNIVERSITY OF BRITISH COLU.v.BiA DEPARTMENT O F M I N I N G AND M E T A L L U R G Y INDEX INTRODUCTION .....1 SUMMARY OF PROPOSED PROCESS '.. 3 OBJECT OF INVESTIGATION . 5 DISCUSSION 6 Reduction • ... • .....6 D i g e s t i o n •«..«•• ••• Leaching • . . . . 1 7 SUMMARY • «c .23 ACKNOWLEDGEMENT The author wishes to express h i s s i n c e r e a p p r e c i a t i o n f o r t h e guidance and encouragement r e c e i v e d from P r o f e s s o r s F.A. Forward and C,S. Samis throughout the work. The a s s i s t a n c e of Messrs. A. A f f l e c k and J . Montgomery i n c a r r y i n g out the a n a l y t i c a l work i s g r a t e f u l l y acknowledged. INTRODUCTION The p r i n c i p l e u n d e r l y i n g the process f o r n i c k e l r e c o v e r y by ammonia l e a c h i n g , i s the high s o l u b i l i t y of f i n e l y d i v i d e d m e t a l l i c n i c k e l i n an oxygenated ammonia s o l u t i o n . Caron^"^recognized the i n d u s t r i a l p o s s i b i l i t i e s of such a process and obtained a patent i n 1924 c o v e r i n g t h i s p r i n c i p l e f o r commercial purposes. A p p l i c a t i o n of t h i s process was p r i m a r i l y d i r e c t e d towards n i c k e l ores of an o x i d i z e d nature, such as l i m o n i t e and s e r p e n t i n e , i n which the n i c k e l i s present as a hydrated nickel-magnesium s i l i c a t e known as g a r n i e r i t e . Due to v a r i o u s d i f f i c u l t i e s encountered, v e r y l i t t l e commercial development f o l l o w e d u n t i l World War I I , when the Free p o r t Sulphur Company ( 2 ) c o n s t r u c t e d a p l a n t 'to t r e a t l a t e r i t i c n i c k e l ores o c c u r r i n g i n Cuba, u t i l i z i n g ammonia l e a c h i n g . The b a s i c o p e r a t i o n s were as f o l l o w s 1 1. Decompose the hydrated s i l i c a t e ore at e l e v a t e d temperatures. 2 . S e l e c t i v e l y reduce the n i c k e l oxide to metal l e a v i n g most of the i r o n i n the o x i d i z e d form. 3. Leach out the reduced n i c k e l with an oxygenated s o l u t i o n of ammonium hydroxide and ammonium carbonate. 4* F i l t e r o f f the r e s i d u e and p r e c i p i t a t e the n i c k e l as a b a s i c carbonate by b o i l i n g o f f the f r e e ammonia. 5. C a l c i n e the b a s i c n i c k e l carbonate to produce n i c k e l o x i d e . 6. Recover NH^ and CO2, to regenerate the l e a c h i n g s o l u t i o n . 2 -Copper, copper oxide and m e t a l l i c c o b a l t behave s i m i l i a r i l y when subjected to an ammonia s o l u t i o n , so i f present they w i l l be leached out along with the n i c k e l . In 1946, F.A. F o r w a r d w / a n d C.S. Samis of the Department of Mining and M e t a l l u r g y i n v e s t i g a t e d the p o s s i -b i l i t i e s o f a p p l y i n g an ammonia l e a c h i n g process to a n i c k e l s u l f i d e ore, i n p a r t i c u l a r to a 16% n i c k e l concen-t r a t e from the S h e r r i t t Gordon Lynn Lake p r o p e r t i e s i n Northern Manitoba. R e s u l t s appeared to be promising, but r e c o v e r i e s were too low f o r an economic p r o c e s s , t h e r e f o r e a comprehensive i n v e s t i g a t i o n ^ ^ w a s undertaken the f o l l o w i n g year, to evolve a process t h a t would produce high r e -c o v e r i e s and would be economical f o r l a r g e s c a l e o p e r a t i o n . The r e s u l t s f r o m . t h i s i n v e s t i g a t i o n i n d i c a t e d a p o s s i b i l i t y of o b t a i n i n g a 96$ n i c k e l and an 85% copper r e c o v e r y . G e n e r a l l y , i t i s d i f f i c u l t to p r e d i c t a c c u r a t e l y the economics of a process from r e s u l t s on a l a b o r a t o r y s c a l e , however,there was good i n d i c a t i o n of an eco n o m i c a l l y f e a s i b l e p r o c e s s . A t e n t a t i v e flow sheet r e s u l t i n g from t h i s i n v e s t i g a t i o n i s reproduced i n F i g u r e 1, and f o l l o w i n g i s a summary of the proposed p r o c e s s . F i g . l - Proposed Flowsheet f o r Ammonia L e a c h i n g of N i c k e l S u l f i d e Concentrates A i r o,— CONCENTRATE ROASTING-REDUCTION DIGESTION LEACHING FILTER Residue ( T a i l s ) N i c k e l Powder F i l t r a t e I CEMENTATION F i l t r a t e J NICKEL RECOVERY S o l i d s ( N i , C u ) N i c k e l Leach S o l u t i o n - 3 -SUMMARY OF PROPOSED PROCESS Ro a s t i n g No p a r t i c u l a r d i f f i c u l t y was encountered i n r o a s t i n g the ore, however a f i n i s h i n g temperature around 800°C i s r e q u i r e d to decompose s u l f a t e s t h a t tend to form at lower temperatures. The s u l f a t e s are s o l u b l e and thus the s u l f a t e c o n c e n t r a t i o n s would tend to b u i l d up i n the l e a c h -i n g s o l u t i o n , i f a c y c l i c process were used. Higher temperature; i n c r e a s e s f e r r a t e f o r m a t i o n and s i n c e copper f e r r a t e i s not r e a d i l y r e d u c i b l e and i s i n s o l u b l e i n the ammoniacal s o l u t i o n , copper recovery w i l l decrease with i n c r e a s e d f i n i s h i n g temperature. Reduction The n i c k e l , which appears as NiO i n the c a l c i n e , must be converted to the m e t a l l i c form b e f o r e d i s s o l u t i o n can o c c u r . H 2 was used as t h e reducing agent, however CO or water gas are p o s s i b l e s u b s t i t u t e s . Optimum r e -d u c t i o n temperature was found to be i n the range of 480 -500°C when H 2 was used f o r r e d u c t i o n . D i g e s t i o n It was found necessary to i n t r o d u c e t h i s stage to r e o x i d i z e the s m a l l q u a n t i t y of m e t a l l i c i r o n formed d u r i n g the r e d u c t i o n stage, which otherwise, would s e r i o u s l y e f f e c t the l e a c h a b i l i t y of the n i c k e l . T h i s r e o x i d a t i o n was c a r r i e d out i n the l e a c h s o l u t i o n p r i o r - 4 -to l e a c h i n g . The optimum c o n d i t i o n s f o r d i g e s t i o n were found to be a temperature of 35°- 40°C with m i l d a g i t a t i o n of the m i x t u re. Leaching A maximum s o l u b i l i t y of 14-16 gm. of n i c k e l per l i t e r was obtained using an &% NH^OH / 8%(^iii¥) 2 CO^ (by weight) aqueous s o l u t i o n . The n i c k e l must be present i n the m e t a l l i c s t a t e before d i s s o l u t i o n can occur, as NiO i s i n s o l u b l e . Copper behaves d i f f e r e n t l y as both the metal and oxide (CuO) are s o l u b l e . Oxygen i s a prime requirement f o r the l e a c h i n g process, and the r a t e of l e a c h i n g depends upon the c o n c e n t r a t i o n of oxygen i n s o l u t i o n . S o l u t i o n P u r i f i c a t i o n Copper i s present with the n i c k e l i n s o l u t i o n and must be removed. Copper can be cemented out completely by n i c k e l powder, p r o v i d i n g f r e e ammonia i s present.and the temperature i s maintained at 6B° - 75°C. N i c k e l Recovery From S o l u t i o n The r e l a t i v e l y pure n i c k e l ammonia s o l u t i o n r e s u l t i n g from the f o r e g o i n g o p e r a t i o n i s heated to . b o i l o f f NH^, CO2 and water vapor. As the ammonia c o n c e n t r a t i o n i s d e p l e t e d , the n i c k e l p r e c i p i t a t e s as a b a s i c n i c k e l carbonate. The l a t t e r i s r e a d i l y c a l c i n e d to NiO and may be marketed i n t h i s form. OBJECT OF INVESTIGATION The i n v e s t i g a t i o n as c a r r i e d out i n the l a b o r a t o r i e s of the Department of Mining and M e t a l l u r g y was p r i m a r i l y concerned with the f e a s i b i l i t y of a p p l y i n g ammonia l e a c h i n g to a n i c k e l s u l f i d e ore, however the importance of the t h e o r e t i c a l a s p e c t s of the v a r i o u s stages evolved were r e a l i z e d , t h e r e f o r e f u r t h e r s t u d i e s were undertaken with t h i s p o i n t i n view. T h i s paper i n c l u d e s the r e s u l t s of an i n v e s t i g a t i o n of t h e f a c t o r s t h a t a f f e c t the l e a c h i n g stage of the proposed p r o c e s s . As the d e s c r i p t i o n of most of the equipment used and mode of o p e r a t i o n were d e s c r i b e d i n the p u b l i s h e d (4) paper, they w i l l be d e l e t e d here, as w e l l as r e s u l t s t h a t have no b e a r i n g on t h i s paper. - 6 -DISCUSSION Reduction The temperature at,which the c a l c i n e s were reduced, had a v e r y important e f f e c t on the n i c k e l r e c o v e r y from the l e a c h i n g stage. T h i s e f f e c t i s q u i t e evident i n F i g u r e 2, where r e s u l t s of l e a c h i n g t e s t s , u s i n g c a l c i n e s reduced at v a r i o u s temperatures, are p l o t t e d . As can be seen from t h i s curve, the optimum r e d u c t i o n temperature appears to be i n the neighborhood of 480~500°C. too 400 450 5 0 0 550 6 0 0 ; Reduction Temp.'C F i g . 2 - E f f e c t of Reduction Temperature on N i c k e l L e a c h a b i l i t y . Using hydrogen, NiO begins to reduce to m e t a l l i c n i c k e l around 2 3 0 ° C ^ ^ , but t h e r a t e of r e d u c t i o n i s q u i t e slow. T h i s r a t e i n c r e a s e s w i t h r i s i n g temperature, but i s not r a p i d enough u n t i l 4^0° C i s reached, to get complete r e d u c t i o n of the NiO i n the allowed r e a c t i o n time. Therefore, below 480°C, complete r e d u c t i o n does not occur and i n d i c a t e s the reason f o r the decreased n i c k e l r e c o v e r y . - 7 -On the other hand, apparently magnetite begins to reduce to iron around 475°C and the quantity of reduced iron formed, increases rapidly as the temperature i s raised above 500°C. As indicated earlier, this reduced iron seriously interferes with the nickel leachability and explains the sharp decrease in nickel recovery when temperatures above 500°C are used in reduction. R. J o h n s o n ^ a p p l y i n g a similar scheme to a Cobalt sulfide ore, obtained similar results. Results of the re-duction temperature on Cobalt recovery are reproduced in Figure 3> with Mr. Johnson's kind permission. Reduction of L i t t l e Gen Oro with Hvdropen Graph Bho»ing relutlonahip between cofcalt recovery and troporatu re of reduction tflth rx) preliminary dicentine period b) otraig'r.t leaching 100 GO A 60 Recovery = ^ ^ \ \ per cent 40 \ 20 4J!J , ADO iiOO Teriocrature of Reduction I decrees Centigr'-de ) i Fig. 3-Effect of Reduction Temperature on Cobalt Leachability. Various reducing gasesisuch as H2, CO, or mixtures such as water gas, can be used for this reduction, however H 2 is probably the best suited for low temperature reduction as i t apparently produces the most active form^-^of nickel. - 8 -Hydrogen being a h i g h l y p e n e t r a t i n g gas would a l s o tend to give a more r a p i d and complete r e d u c t i o n . D i g e s t i o n With the i n t r o d u c t i o n of d i g e s t i o n p r e c e d i n g the l e a c h i n g stage, r e c o v e r i e s o f 96$ of the n i c k e l were r e a l i z e d and c o n s i s t e n t r e s u l t s were obta i n e d , p r o v i d i n g proper c o n d i -t i o n s were maintained d u r i n g the r o a s t i n g and r e d u c t i o n s t a g e s . Upon f i r s t c o n s i d e r a t i o n s , no apparent reason was evident t h a t could s a t i s f a c t o r i l y e x p l a i n the n e c e s s i t y f o r t h i s p r e l i m i n a r y treatment, t h e r e f o r e a study of the d i g e s t i o n stage was undertaken i n an attempt to determine; the cause of the i n h i b i t i o n of the n i c k e l l e a c h i n g without d i g e s t i o n and how t h i s i n h i b i t i o n was overcome when d i g e s t i o n was employed. The o r i g i n a l method u t i l i z e d f o r d i g e s t i o n was simply to add 10 grams of the reduced c a l c i n e to 500 ml. of oxygen-ated ammonia l e a c h s o l u t i o n i n a 2 l i t e r Winchester b o t t l e and a l l o w the mixture to stand 24 hours without a g i t a t i o n . I f the mixture was s t i r r e d at room temperature d u r i n g t h i s p e r i o d , no b e n e f i t was d e r i v e d from the d i g e s t i o n . However, at temperatures above 35°C, a g i t a t i o n had no adverse e f f e c t on the d i g e s t i o n r e a c t i o n . ^ In o b s e r v i n g the appearance of the mixture as the d i g e s t i o n r e a c t i o n proceeded, red Fe(OH)^ c o u l d be seen to form i n the s o l u t i o n above the reduced c a l c i n e and would s e t t l e as a reddish-brown l a y e r on the b l a c k p u l p . The - 9 -r e s i d u e s from the l e a c h s o l u t i o n s that had been subjected to d i g e s t i o n and produced high n i c k e l r e c o v e r i w s , were r e d d i s h -brown, while those that were not d i g e s t e d and consequently produced low r e c o v e r i e s , remained b l a c k i n c o l o r as the o r i g i n a l reduced c a l c i n e . From the f o r e g o i n g , t h e r e was strong i n d i c a t i o n t h a t the i r o n present was r e s p o n s i b l e f o r p r e v e n t i n g the n i c k e l from being l e a c h e d . During the r e d u c t i o n of the c a l c i n e , temperatures (480 - 500°C) are maintained to s e l e c t i v e l y reduce the NiO to m e t a l l i c n i c k e l and the F©2°3 t o ^ e 3 ^ ^ » however, some m e t a l l i c i r o n i s formed which a p p a r e n t l y must be converted t o Fe(OH)^ during the d i g e s t i o n , before s a t i s -f a c t o r y l e a c h i n g can be a t t a i n e d . The f i r s t s e r i e s of t e s t s were c a r r i e d out to o b t a i n evidence that the i r o n d i d i n h i b i t the n i c k e l l e a c h i n g and to determine whether the manner i n which the i r o n and n i c k e l were a s s o c i a t e d had any e f f e c t on the l e a c h i n g r e a c t i o n . In these t e s t s , pure n i c k e l powder, mechanical mixtures of i r o n and n i c k e l powder, and s y n t h e t i c i r o n - n i c k e i mattes t h a t had been r o a s t e d and reduced, were su b j e c t e d to ammonia l e a c h i n g with and without d i g e s t i o n and the r e s u l t i n g n i c k e l r e c o v e r i e s compared. T y p i c a l r e s u l t s f o r these t e s t s appear i n Table I. - 10 -Table I - E f f e c t of I r o n - N i c k e l A s s o c i a t i o n on N i c k e l r e a c h a b i l i t y . No. M a t e r i a l D i g e s t e d Leach Time % Ni Recovered T - l Ni Powder No T-2 Ni & Fe Powder No T-3 Syn. Matte No T-4 " Yes The r e s u l t s i n d i c a t e t h a t n i c k e l i s r e a d i l y s o l u b l e i f present alone and i s not a f f e c t e d by the i r o n , p r o v i d i n g the l a t t e r i s present as a mechanical mixture. In c o n s i d e r -i n g the m a t e r i a l r e s u l t i n g from the r o a s t e d and reduced matte, i t i s h i g h l y probable t h a t the n i c k e l and i r o n are a s s o c i a t e d as an a l l o y or are i n d i r e c t contact with one another. T h e r e f o r e , i t i s apparent t h a t i f the i r o n i s a s s o c i a t e d i n t h i s manner i t must be r e o x i d i z e d , t h a t i s , i t must be sub^cted to d i g e s t i o n b e f o r e the n i c k e l can be leach e d s u c c e s s f u l l y . From the f o r e g o i n g i t can be concluded t h a t the i r o n present i n the reduced c a l c i n e i s mutually a s s o c i a t e d with the n i c k e l , n e c e s s i t a t i n g the p r e l i m i n a r y treatment termed d i g e s t i o n , i f high n i c k e l r e c o v e r i e s are to be a t t a i n e d . E v i d e n t l y the d i g e s t i o n stage pro v i d e s the necessary mechanism f o r the i r o n removal t o occur, t h e r e f o r e a t t e n t i o n was d i r e c t e d to determining how t h i s mechanism takes plac e and the c o n d i t i o n s necessary f o r i t to o c c u r . 20 h r s . lie 99.8 100.0 53.0 99.5 - 11 -The l e a c h s o l u t i o n was s a t u r a t e d with oxygen be f o r e d i g e s t i o n , t h e r e f o r e i t was necessary to determine whether the oxygen was u t i l i z e d i n the r e a c t i o n . D i g e s t i o n was c a r r i e d out u s i n g s o l u t i o n s s a t u r a t e d with N 2 > a i r and 0 2 . Use of n i t r o g e n r e s u l t e d i n v e r y low r e c o v e r i e s , but improved when a i r was used, p a r t i c u l a r l y i f s u f f i c i e n t time was allowed f o r d i g e s t i o n . High n i c k e l r e c o v e r i e s r e s u l t e d when oxygen was employed with a d i g e s t i o n time of 4 - 6 hours at 23°C, and \ - 1 hour at a temperature of 35°C or h i g h e r . C o n s i d e r a t i o n of the mechanism of d i g e s t i o n can now be undertaken. With the preceding p o i n t s i n mind, i t becomes apparent t h a t i n t h e d i g e s t i o n r e a c t i o n , a system e x i s t s i n v o l v i n g two c o n t a c t i n g metals present i n an e l e c t r o -l y t e and r e q u i r i n g oxygen f o r the r e a c t i o n t o proceed. T h i s a l l p o i n t s to the p o s s i b i l i t y t h a t a s h o r t c i r c u i t e d g a l v a n i c c e l l i s set up and t h a t the i r o n i s removed by a process s i m i l a r to e l e c t r o c h e m i c a l c o r r o s i o n . The i r o n must a c t as the anode, as i t goes i n t o s o l u t i o n and t h e r e f o r e the n i c k e l must become the cathode. The r e a c t i o n s o c c u r r i n g at each e l e c t r o d e can be r e p r e s e n t e d as f o l l o w s : Anode: i i i Fe • F e ^ ^ / 3 e Cathode 0 2 / 2H 2 0 f 4e - ~ 4 OH" the o v e r a l l r e a c t i o n being represented by the f o l l o w i n g equation: 4 Fe / 3 0 2 /r,6H 20 ^ 4 Fe(OH) - 12 -I n v e s t i g a t i o n o f the f i n e l y reduced c a l c i n e by micro-s c o p i c methods r e v e a l e d v e r y l i t t l e to s u b s t a n t i a t e t h i s t h e o r y , t h e r e f o r e a macroscopic method was used. Since i t was b e l i e v e d t h a t the r e a c t i o n was s i m i l a r to e l e c t r o c h e m i c a l . c o r r o s i o n , the method adopted f o r s t u d y i n g t h i s r e a c t i o n was s i m i l a r to those f r e q u e n t l y used f o r g a l v a n i c c o r r o s i o n t e s t s w ' i n the l a b o r a t o r y . The c o r r o s i o n c e l l used i n the study of the d i g e s t i o n , c o n s i s t e d of an i r o n e l e c t r o d e and a n i c k e l e l e c t r o d e , immersed i n the ammonia l e a c h s o l u t i o n and shorted through a milliammeter. A m i l l i v o l t -meter measured the d i f f e r e n c e i n p o t e n t i a l e x i s t i n g between the two e l e c t r o d e s . An i n d i c a t i o n of the r a t e of c o r r o s i o n and the p o l a r i t y could thus be obtained from the cu r r e n t and vo l t a g e measurements. In the f i r s t c o r r o s i o n t e s t , an attempt was made to determine the reason why the i r o n i n h i b i t e d n i c k e l l e a c h i n g i f d i g e s t i o n was not used, A n i c k e l and an i r o n e l e c t r o d e were immersed i n an oxygen s a t u r a t e d s o l u t i o n , and the solution a g i t a t e d v i g o r o u s l y to simulate c o n d i t i o n s encountered i n l e a c h i n g . Upon immersion of the e l e c t r o d e s , the i r o n became anodic with a p o t e n t i a l d i f f e r e n c e of 60 m i l l i v o l t s and a c u r r e n t of 3 m i l l i a m p s . W i t h i n a h a l f hour, a t h i n , l u s t r o u s , brown f i l m formed on the i r o n e l e c t r o d e and the cur r e n t decreased t o 0.4 m i l l i a m p s , i n d i c a t i n g t h a t the c o r r o s i o n had p r a c t i c a l l y ceased. When the i r o n e l e c t r o d e was removed, the f i l m was observed to be ve r y tough and - 13 -continuous as i l l u s t r a t e d on the l e f t e l e c t r o d e of F i g u r e 4* The l i g h t area surrounding the f i l m , was the p o r t i o n of the e l e c t r o d e covered with wax to prevent a t t a c k and thus have some method f o r comparison. The n i c k e l e l e c t r o d e appeared t o be unattacked, however upon re-immersing the e l e c t r o d e f o r 24 hours, without the presence o f the i r o n e l e c t r o d e , some of the n i c k e l d i s s o l v e d as i n d i c a t e d on the r i g h t e l e c t r o d e of Fi g u r e 4* T h i s would be expected, as i t was i n d i c a t e d e a r l i e r that i f no i r o n was present i n contact with the n i c k e l , the n i c k e l was r e a d i l y s o l u b l e . F i g . 4-Corrosion E f f e c t s on Iron and N i c k e l E l e c t r o d e s when Leaching s o l u t i o n i s A g i t a t e d . V.R. Evans s t a t e s t h a t , "In g e n e r a l , where anodic r e a c t i o n leads to the formation of a s o l u b l e s a l t of the anode metal, nothing w i l l occur which can i n t e r f e r e s e r i o u s l y with - 14 -the c o r r o s i o n of the anode. When, however, cases are c o n s i d -ered i n which the discharge o f the anion can l e a d to the p r o d u c t i o n of an i n s o l u b l e s a l t at or near the anode s u r f a c e , o ther e f f e c t s may come i n . In c o n s i d e r i n g the p r o t e c t i v e e f f e c t of the anodic c o r r o s i o n product, we have to determine whether that c o r r o s i o n product i s adherent and continuous or whether l o o s e and porous. F u r t h e r a t t a c k w i l l be prevented i f the c o r r o s i o n product i s o r becomes c l o s e l y adherent t o the metal". A p p a r e n t l y a case where a t i g h t l y adherent, continuous-^ f i l m on the i r o n s u r f a c e r e s u l t s when the reduced c a l c i n e i s j leached without d i g e s t i o n , as i n d i c a t e d by the r e s u l t s from the c o r r o s i o n c e l l t e s t . The i r o n becomes anodic p r e v e n t i n g d i s s o l u t i o n of the n i c k e l , but due to the t i g h t l y adhering f i l m t h a t forms on the i r o n , f u r t h e r removal of the i r o n i s prevented. However, the p o t e n t i a l s t i l l e x i s t s , t h e r e f o r e the n i c k e l does not d i s s o l v e . In a c t u a l p r a c t i c e some n i c k e l d i s s o l u t i o n does occur, even though no d i g e s t i o n i s c a r r i e d out. T h i s may be exp l a i n e d by the f a c t t h a t at the r e d u c t i o n temperature employed, on l y s m a l l q u a n t i t i e s of m e t a l l i c i r o n are produced, t h e r e f o r e there probably i s some n i c k e l not mutually a s s o c i a t e d with the i r o n and thus^oula^bT)soluble without d i g e s t i o n . To determine the e f f e c t of the d i g e s t i o n stage, e l e c t r o d e s of n i c k e l and i r o n were immersed i n an oxygen s a t u r a t e d l e a c h s o l u t i o n but no a g i t a t i o n was employed and - 1 5 - -the temperature maintained a t 2 3°C. When the e l e c t r o d e s were f i r s t immersed the n i c k e l was anodic, but i n a few minutes the p o l a r i t y was r e v e r s e d and the i r o n became anodi c . The p o t e n t i a l d i f f e r e n c e i n c r e a s e d to 60 m i l l i v o l t s and the c u r r e n t i n c r e a s e d to 3 milliamps and both remained constant f o r the d u r a t i o n of the t e s t , i n d i c a t i n g t h a t i n t h i s case no i n t e r f e r e n c e with the c o r r o s i o n r a t e was encountered. As the t e s t progressed, a f l o c c u l e n t c o a t i n g of Fe(OH)^ was observed forming on the i r o n e l e c t r o d e . T h i s c o r r o s i o n product d i d not adhere t i g h t l y , t e n d i n g to f a l l o f f , exposing a f r e s h s u r f a c e f o r c o r r o s i o n . A f t e r 48 hours, the i r o n e l e c t r o d e was c o n s i d e r a b l y corroded as can be seen on the l e f t e l e c t r o d e of F i g u r e 5« F i g . 5-Corrosion E f f e c t s on Iron and N i c k e l E l e c t r o d e s when Leaching s o l u t i o n i s not a g i t a t e d . - 16 -During t h i s p e r i o d no a t t a c k of the n i c k e l occurred,' however, upon removing the i r o n e l e c t r o d e and a g i t a t i n g the s o l u t i o n c o n s i d e r a b l e d i s s o l u t i o n o c c u r r e d , as i n d i c a t e d on the r i g h t , e l e c t r o d e of F i g u r e 5» E v i d e n t l y , due to the quiescent s t a t e e x i s t i n g i n the d i g e s t i o n stage, the oxygen c o n c e n t r a t i o n i n the immediate v i c i n i t y of the i r o n e l e c t r o d e i s l e s s than when vigo r o u s a g i t a t i o n i s employed and under these c o n d i t i o n s a porous c o r r o s i o n product i s formed. Without a g i t a t i o n , the oxygen must reach the s u r f a c e by slow d i f f u s i o n a l p r ocesses, r e s u l t i n g i n the decrease oxygen c o n c e n t r a t i o n . The e f f e c t of temperature on the d i g e s t i o n was s t u d i e d by use of the c o r r o s i o n c e l l maintained at 3 5°C. At the h i g h e r temperature, the e l e c t r o d e s were e f f e c t e d i n a s i m i l a r manner ^ s ^ the c o r r o s i o n c e l l maintained at 23°C without a g i t a t i o n . However, when g e n t l e a g i t a t i o n was employed a porous f i l m was formed whereas the use of a g i t a t i o n at 23°C r e s u l t e d i n a continuous f i l m . Here a g a i n the reason may be to the decrease i n oxygen s o l u b i l i t y a t higher tempera-tu r e . The c u r r e n t f l o w i n g through the m i l l i a m e t e r from the c e l l maintained at 35°C i n c r e a s e d to 30 m i l l i a m p s , thus i n d i c a t i n g t h a t the c o r r o s i o n was proceeding about 10 times as f a s t as a t 23°C, which e x p l a i n s the s h o r t e r time r e q u i r e d f o r d i g e s t i o n when c a r r i e d out at 35°C or h i g h e r . - 17 -LEACHING The n i c k e l must be present i n the m e t a l l i c s t a t e }/ before d i s s o l u t i o n can occur, as NiO i s i n s o l u b l e ^ copper ^ behaves d i f f e r e n t l y , as both the metal and the oxide CuO are s o l u b l e i n the l e a c h s o l u t i o n . The reason f o r t h i s may be t i e d i n with the r e l a t i v e s t r e n g t h s of the bonding between the oxygen and metal. The copper oxide i s r e l a t i v e l y e a s i e r to reduce than NiO, t h e r e f o r e the oxygen i s not h e l d as t e n a c i o u s l y as i n the case of NiO. In the reduced c a l c i n e the n i c k e l i s present i n the m e t a l l i c form but i s s o l u b l e i n the l e a c h s o l u t i o n o n l y i f oxygen i s a v a i l a b l e . The oxygen i s necessary f o r o x i d a t i o n of the n i c k e l from m e t a l l i c to the i o n i c form t h a t appears i n s o l u t i o n . Oxygen i s q u i t e s a t i s f a c t o r y f o r t h i s necessary o x i d a t i o n j however i f stronger o x i d i z i n g agents such as H2O2 are used no l e a c h i n g o c c u r s . This may be due to the p o s s i b i -l i t y of forming NiO, which i s insoluble,, Tests i n d i c a t e d t h a t not o n l y i s oxygen necessary f o r the l e a c h i n g r e a c t i o n , but. t h a t the r a t e of l e a c h i n g i s dependent upon the oxygen c o n c e n t r a t i o n i n the l e a c h s o l u t i o n . R e s u l t s of l e a c h i n g time, when v a r i o u s oxygen pres s u r e s are used, appear i n F i g u r e 6. The decrease i n l e a c h i n g time as oxygen pressure i s i n c r e a s e d i s q u i t e e v i d e n t . - 18 -The curves of F i g u r e 7 were obtained from r e s u l t s i n F i g u r e 6, thus g i v i n g a d i r e c t r e l a t i o n s h i p between l e a c h i n g r a t e and oxygen p r e s s u r e . The l o g a r i t h m i c p l o t of the curves of F i g u r e 7 are i l l u s t r a t e d i n F i g u r e 8. - 19 -0.2 0.5 1.0 1.5 2.0 3.0 0XY6CN PRESSURE i» ATM OS. F i g . 8-Logarithmic P l o t of Oxygen Pressure Versus Leaching Rate; 'The l e a c h i n g tank was. not designed to operate below atmosphere p r e s s u r e , t h e r e f o r e a i r at v a r i o u s pressures was s u b s t i t u t e d f o r the oxygen atmosphere, to a t t a i n oxygen p a r t i a l p r e s s u r e s of l e s s than one atmosphere. However, as can be seen from F i g u r e 7 and F i g u r e 8, a d i s t i n c t change occurs when oxygen i s r e p l a c e d by a i r , t h e r e f o r e the a i r atmosphere can not be used to represent the l e a c h i n g r a t e of an oxygen atmosphere[wfi pressure equal to the corresponding p a r t i a l p r e s s u r e of the 0 2 i n the a i r atmosphere. From the curves of F i g u r e 7 and F i g u r e 8, an equation approximating the curve o f F i g u r e 7 was determined r .=' 0.85 P 3 where r a ^ n i c k e l leached per minute P 8 O2 pressure i n atmospheres. For the a i r atmosphere, the f o l l o w i n g equation was - 20 -obtain e d , r a 0.8 P i n d i c a t i n g the r a t e was d i r e c t l y p r o p o r t i o n a l to the parti a l p r e s s u r e of the 0 2 i n the a i r atmosphere. It should be emphasized t h a t these equations do not repre s e n t r i g o r o u s mathematical d e r i v a t i o n s , but are simply e m p i r i c a l r e l a t i o n s h i p s t h a t appear to s a t i s f y the data over the v e r y l i m i t e d range o f pressures i n v e s t i g a t e d . In c o n s i d e r i n g the system t h a t e x i s t s i n the l e a c h i n g r e a c t i o n , i t i s apparent t h a t three phases are i n v o l v e d -gaseous, l i q u i d and s o l i d , which giv e s r i s e to a very complex s i t u a t i o n when r e a c t i o n k i n e t i c s are c o n s i d e r e d . In a case of t h i s nature, g e n e r a l l y s e v e r a l mechanisms.are i n v o l v e d and a l l must be con s i d e r e d i f a mathematical d e r i v a t i o n , or even a q u a l i t a t i v e e x p l a n a t i o n of the r e a c t i o n r a t e i s to be attempted. In g e n e r a l , where these r e a c t i o n mechanisms are co n s e c u t i v e , i f one step i s slow i n comparison t o the o t h e r s , i t w i l l a c t u a l l y be the c o n t r o l l i n g mechanism and the o v e r a l l r a t e i s a measure of t h i s p a r t i c u l a r mechanism. However, i f the r a t e s of a l l steps are about equal, complexity i n c r e a s e s as It i s d i f f i c u l t to a s s i g n the measured r a t e to any p a r t i -c u l a r mechanism. D i f f i c u l t y may a l s o a r i s e i f the c o n t r o l l i n g step i s not evident and attempts/of) a s s i g n i n g the measured ^ r a t e to the obvious mechanisms w i l l prove f r u i t l e s s , t h e r e -f o r e i n a complex case of t h i s type i t i s v e r y d i f f i c u l t t o make a complete a n a l y s i s of the system without a v e r y - 21 -d e t a i l e d i n v e s t i g a t i o n . However, we may c o n s i d e r some of the obvious mechan-isms t h a t occur i n the system and i f any one of them ge n e r a l r e a c t i o n r a t e s s i m i l a r to the measured r a t e , i t i s q u i t e probable that i t i s the c o n t r o l l i n g mechanism. The obvious mechanisms t h a t do occur are as f o l l o w s : (1) Mass t r a n s f e r of 0 2 from the gaseous phase i n t o the l i q u i d phase. (2) T r a n s f e r of 0 2 from s o l u t i o n t o the surface of the n i c k e l w i t h the p o s s i b i l i t y of the 0 2 being adsorbed on the n i c k e l s u r f a c e , (3) R e a c t i o n on the s u r f a c e to form the complex, (4) Removal of products from the n i c k e l s u r f a c e . The measured r a t e was found t o f o l l o w the equation r • b p x t h e r e f o r e ( l ) and (4) can probably be excluded as they are d i f f u s i o n a l processes and g e n e r a l l y are d i r e c t l y p r o p o r t i o n a l to the c o n c e n t r a t i o n or p r e s s u r e . However, i f a i r i s used as the l e a c h i n g atmosphere, the r a t e i s d i r e c t l y p r o p o r t i o n a l to the p r e s s u r e . In t h i s case ( l ) may be c o n t r o l l i n g due to the presence of the N 2, The l a y e r of a i r i n immediate contact with the s o l u t i o n becomes d e p l e t e d i n 0 2 forming a blanket of W£ through which the 0 2 must d i f f u s e from the bulk of the gas. Thus the measured r a t e becomes the r a t e of 0 2 d i f f u s i o n through t h i s N 2 l a y e r . In c o n s i d e r i n g the p o s s i b i l i t y of oxygen a d s o r p t i o n , i t can be seen t h a t the measured r a t e u s i n g an oxygen atmos-- 22 phere i s s i m i l a r t o F r e u n d l i c h 1 s a d s o r p t i o n isotherm m = a P n t h e r e f o r e , there i s some i n d i c a t i o n t h a t a d s o r p t i o n of oxygen on the n i c k e l s u r f a c e does occur and t h a t t h i s mechanism l i s the c o n t r o l l i n g p r o c e s s . The f o r e g o i n g has been presented to s t i m u l a t e i n t e r e s t i n the t h e o r e t i c a l aspects of r e a c t i o n k i n e t i c s of t h i s and a* s i m i l a r l e a c h i n g processes r a t h e r t h a n A a b s o l u t e e x p l a n a t i o n of what occurs and f u r t h e r work i s contemplated to o b t a i n more c o n c l u s i v e data to uphold or d i s c a r d t h i s theory of a d s o r p t i o n 0 - 23 -SUMMARY The s i g n i f i c a n t f a c t o r s a f f e c t i n g t h e l e a c h i n g stage were found to be: ( 1 ) temperature at which the c a l c i n e was reduced, (2) presence of m e t a l l i c i r o n i n the reduced c a l c i n e , ( 3 ) n e c e s s i t y f o r oxygen d u r i n g l e a c h i n g * The optimum temperature range f o r r e d u c t i o n was found to be 480 - 500°C. In t h i s range a minimum q u a n t i t y of reduced i r o n i s formed, while the n i c k e l oxide i s p r a c t i c a l l y completely reduced w i t h i n a t h r e e hour p e r i o d r e q u i r e d f o r r e d u c t i o n . Reduced i r o n formed during r e d u c t i o n a l l o y s with the n i c k e l and prevents the l a t t e r from d i s s o l v i n g i n the l e a c h s o l u t i o n , t h e r e f o r e the i r o n must f i r s t be r e o x i d i z e d . The d i g e s t i o n stage was i n t r o d u c e d f o r t h i s purpose and under c o n t r o l l e d c o n d i t i o n s the s m a l l q u a n t i t y of reduced i r o n i s r e o x i d i z e d i n the l e a c h s o l u t i o n , l e a v i n g the n i c k e l f r e e to d i s s o l v e d u r i n g the l e a c h i n g stage. I f the d i g e s t i o n , which i s merely a soaking o p e r a t i o n , i s c a r r i e d out at room temperature, no a g i t a t i o n can be t o l e r a t e d ; however i f the temperature i s r a i s e d above 35°C, m i l d a g i t a t i o n can be used and r e a c t i o n r a t e i n c r e a s e s about f i v e f o l d . I f higher r e d u c t i o n temperatures are used, r e p x i d a t i o n cannot be c a r r i e d out completely d u r i n g the d i g e s t i o n , r e s u l t i n g i n lower n i c k e l r e c o v e r i e s . - 24 -D i s s o l v e d oxygen i n the l e a c h s o l u t i o n i s necessary f o r the l e a c h i n g r e a c t i o n t o occur and the r a t e of l e a c h i n g i s dependent upon the oxygen c o n c e n t r a t i o n . I f pure oxygen i s used f o r l e a c h i n g , the l e a c h i n g r a t e i s p r o p o r t i o n e d t o the oxygen pressure cubed, i n d i c a t i n g the c o n s i d e r a b l e i n -crease i n r a t e when l e a c h i n g i s c a r r i e d out under p r e s s u r e . Leaching at room temperature produced the best r e s u l t s , higher temperatures were f a v o r a b l e f o r the f e r r o u s complex formation which would remain i n the s o l u t i o n along with the n i c k e l . BIBLIOGRAPHY (1) "Recovering Values From N i c k e l and Cobalt Ores? M.H. Caron, U.S. Patent #1,487,145 - 1924. (2) " N i c k e l From Cuba" - M.F. Dufour, R.C. H i l l s . Chem. I n d u s t r i e s (N.Y.) 57, 621-7(1945). (3) "Experimental E x t r a c t i o n of N i c k e l From S h e r r i t t - G o r d o n Concentrates by Ammonia Leaching" - Report to S h e r r i t t -Gordon Mines, S h e r r i d o n , Man. - F.A. Forward, C.S. Samis (June 1946). (4) "A Method For Adapting the Ammonia Leaching Process to the Recovery of Copper and N i c k e l From S u l f i d e Ores and Concentrates". - F.A. Forward, C.S. Samis, and V. Kudryk - Can. Min. & Met. B u l l . , June, 1948. (5) "Reduction of N i c k e l Oxides with Hydrogen" - D.P. B o g a t s k i i , M e t a l l u r g y 12, No. 4, 58-65(1937). (6) "Ammonia Leaching of Cobalt Ores" - R. Johnson. Undergraduate T h e s i s , Mining Dept., U.B.C. 1948. (7) " C o r r o s i o n T e s t i n g Methods" - I n t e r n a t i o n a l N i c k e l Co., Inc. T e c h n i c a l B u l l e t i n T-10(0ct. 1946). (8) " C o r r o s i o n of M e t a l s " - U.R. Evans (1924). P u b l i s h e r - Edward Ar n o l d Co. ABSTRACT An I n v e s t i g a t i o n of the f a c t o r s a f f e c t i n g the l e a c h i n g stage of the ammonia l e a c h i n g of n i c k e l s u l f i d e concentra tes t h a t were f i r s t dead r o a s t e d and t h e n s u b j e c t e d t o r e d u c t i o n w i t h hydrogen a t e l e v a t e d temperatures , r e v e a l e d t h a t : (a) The temperature a t which the c a l c i n e was reduced, was c r i t i c a l and had to be a m i n t a i n e d w i t h i n a f a i r l y narrow temperature range o f ^80 - 5 ° ° G C , i f s a t i s f a c t o r y n i c k e l r e -c o v e r i e s were t o be r e a l i z e d , (b) The presence of reduced i r o n produced d u r i n g the r e d u c t i o n s tage , s e r i o u s l y hampered the l e a c h i n g of the n i c k e l , but c o u l d be overcome by a p r e l i m i n a r y t reatment termed " d i g e s t i o n " . Dur ing t h i s stage the mix ture o f reduced c a l c i n e and l e a c h s o l u t i o n were g e n t l y a g i t a t e d a t s l i g h t l y h i g h e r than room temperature i n the presence of an oxygen atmosphere. Under these c o n d i t i o n s the i r o n was p r e f e r e n t i a l l y a t t a c k e d and o x i d i z e d to f e r r i c h y d r o x i d e , thus p r e v e n t i n g any i n t e r -ference d u r i n g the l e a c h i n g s tage . (c) O x i d i z i n g c o n d i t i o n s were necessary d u r i n g the l e a c h i n g s tage , which c o u l d be s a t i s f i e d by a i r o r an atmos-phere o f oxygen. The l e a c h i n g r a t e i n c r e a s e d f i v e f o l d when the a i r was r e p l a c e d w i t h a pure oxygen atmosphere at normal p r e s s u r e . P r e s s u r e had a pronounced e f f e c t on the l e a c h i n g r a t e ; the r a t e b e i n g d i r e c t l y . p r o p o r t i o n a l to the p a r t i a l p r e s s u r e o f t h e oxygen when a i r was.used and a c u b i c a l f u n c t i o n of the pressure when pure oxygen was u s e d . Strong o x i d i z i n g agents such as H 2 0 2 were u n s a t i s f a c t o r y due to t h e i r v i g o r o u s o x i d i z i n g nature r e s u l t i n g i n very low r e c o v e r i e s . 

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