UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Pressure leaching of copper sulphides in perchloric acid solutions Loewen, Fred 1967

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1967_A7 L64.pdf [ 4.1MB ]
Metadata
JSON: 831-1.0093576.json
JSON-LD: 831-1.0093576-ld.json
RDF/XML (Pretty): 831-1.0093576-rdf.xml
RDF/JSON: 831-1.0093576-rdf.json
Turtle: 831-1.0093576-turtle.txt
N-Triples: 831-1.0093576-rdf-ntriples.txt
Original Record: 831-1.0093576-source.json
Full Text
831-1.0093576-fulltext.txt
Citation
831-1.0093576.ris

Full Text

THE PRESSURE LEACHING OF COPPER SULPHIDES IN PERCHLORIC ACID SOLUTIONS BY FRED LOEWEN B.A.Sc, University of B r i t i s h Golumbia, 1 9 6 4 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of METALLURGY I We accept t h i s thesis as conforming to the standard required from candidates for the degree of MASTER OF APPLIED SCIENCE THE UNIVERSITY OF BRITISH COLUMBIA May, 1967 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t the: L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y , I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f M ^ A l l n r g y  The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada ' D a t e Sppfpnvhpr T i , 1%7 - i i -ABSTRACT The leaching of c o v e l l i t e (CuS), c h a l c o c i t e (Cu^S), chalcopyrite (CuFeS2) and bornite (Cu^FeS^) was c a r r i e d out in,a shaking autoclave i n p e r c h l o r i c acid solutions and using moderate pressures of oxygen.. The temperature range of i n v e s t i g a t i o n was 105-140°C. I t was found that c o v e l l i t e , c h a l c o c i t e and bornite leach at approximately s i m i l a r rates, with chalcopyrite being an order of magnitude slower. I t was found that c h a l c o c i t e leaching can be divided into two stages, f i r s t , the rapid transformation to c o v e l l i t e with an a c t i v a t i o n energy 1.8 Kcal/mole, followed by a slower oxidation stage i d e n t i f i e d as c o v e l l i t e - d i s s o l u t i o n with an a c t i v a t i o n energy of 11.4 Kcal/ mole. These two stages of leaching were also observed i n bornite with c h a l c o c i t e (or digenite) appearing as an intermediate step. No such, transformations were observed i n c o v e l l i t e or chalcopyrite. Two separate reactions were recognized as occuring simultaneously for a l l four minerals during the oxidation process: an electrochemical reaction y i e l d i n g elemental sulphur and creating p i t s on the mineral surface, and a chemical reaction producing sulphate. The f i r s t reaction dominates i n strongly a c i d i c conditions, being responsible for about. 85% of the sulphur released from the mineral, but the sulphate to elemental sulphur r a t i o i n s o l u t i o n increases with decreasing a c i d i t y . Above 120°C the general oxidation process i s i n h i b i t e d by molten sulphur coating the mineral p a r t i c l e s ; the sulphate producing reaction, however, i s not reduced above t h i s temperature. - i i i -For chalcopyrite the a c t i v a t i o n energies for the,sulphate producing reaction and mineral s o l u b i l i z e d were found to be 16.0 Kcal/mole and 11.0 Kcal/mole, re s p e c t i v e l y . It i s suggested that chalcopyrite may leach p a r t i a l l y v i a the .formation of transient c o v e l l i t e on the surface but since i t s leaching rate i s f a s t e r than that for chalcopyrite, tio c o v e l l i t e i s detected a f t e r leaching. - i v -ACKNOWLEDGEMENT Sincere gratitude i s extended to Professor Ernest Peters who provided the i n s p i r a t i o n , information and insig h t necessary i n bringing t h i s work to i t s f i n a l form. Thanks are also extended to Dr. I.H. Warren and Dr. H. Majima for h e l p f u l comments offered during the w r i t i n g of th i s t h e s i s . F i n a n c i a l support v i a a research a s s i s t a n t s h i p from the National Research Council of Canada i s g r a t e f u l l y acknowledged. TABLE OF CONTENTS Page INTRODUCTION 1 Scope of th i s work 3 EXPERIMENTAL 4 1. Materials 4 2. Experimental Apparatus 7 3. Experimental Procedure 9 4. A n a l y t i c a l Methods 10 RESULTS . 13 1. Phase changes during leaching 13 2. Leaching massive specimens 14 3. Formation of sulphate from elemental sulphur 15 4. Evolution of hydrogen sulphide and sulphur dioxide .. 18 5. Comparison of leaching rates 18 6. V a r i a t i o n of a c i d i t y 24 7. V a r i a t i o n of oxygen pressure 24 8. Temperature v a r i a t i o n •••• 31 9. The c a t a l y t i c e f f e c t of cupric ions 35 10. Leaching chalcopyrite 35 DISCUSSION . ... 41 1. Reprodu c i b i l i t y of r e s u l t s 41 2. Phase changes during leaching 41 3. Leaching massive specimens 45 4. The formation df sulphate from elemental sulphur .... 47 5. Evolution of hydrogen sulphide and sulphur dioxide .. 48 6. Comparison of leaching rates .. 48 7. V a r i a t i o n of a c i d i t y 50 8. V a r i a t i o n of oxygen pressure 53 9. Temperature v a r i a t i o n . 54 10. The c a t a l y t i c e f f e c t of cupric ions 56 11. Leaching chalcopyrite 57 CONCLUSIONS ... 62 Suggestions for future work. 63 REFERENCES 65 APPENDIX - X-Ray D i f f r a c t i o n Analysis 67 LIST OF TABLES Table Page 1 Chemical composition of Copper-Sulphide minerals .... 6 2 S p e c i f i c g r a v i t i e s and weights for equal areas 8 3 Temperatures and corresponding oxygen pressures used - . . > 10 4 Leaching C o v e l l i t e , Chalcocite and Chalcopyrite i n 4M acid and Bornite i n 1M acid . 23 5 Leaching of Chalcocite at various a c i d i t i e s at 125°G. 26 6 Leaching of Chalcopyrite at three a c i d i t i e s at 125°C. 28 7 Chalcocite leaching at 110°C i n 1M acid.at three oxygen pressures 28 8 Oxidation rates for leaching Chalcocite i n 1M p e r c h l o r i c acid ........ 31 9 Leaching Chalcopyrite i n 1M acid for 2 hours ........ 38 LIST OF FIGURES Figure Page 1 Surface of massive specimen of C o v e l l i t e before and a f t e r leaching 16 2 Surface of massive specimen.of Chalcopyrite before and a f t e r leaching 17 3 Oxidation curves for leaching C o v e l l i t e i n 4M per- r c h l o r i c acid 19 4 Oxidation curves for leaching Chalcocite i n 4M acid . . ..... 20 5 Oxidation curves for leaching Chalcopyrite i n 4M acid 21 6 Oxidation curves for leaching Bornite i n 1M acid .... 22 7 Leaching Chalcocite at 125°C at various a c i d i t i e s ... 25 8 Leaching Chalcopyrite at 125°C at three a c i d i t i e s ... 27 9 Oxidation curves for the leaching of Chalcocite i n . 1M acid at 110°C at various oxygen pressures .. 29 10 The e f f e c t of oxygen pressure on the leaching rate of Chalcocite i n 1M acid at 110°C . 30 11. Chalcocite leached in.lM p e r c h l o r i c acid,at three temperaturee • • • • • • 32 12 Arrhenius p l o t for primary stage of leaching of Chalcocite i n 1M acid 33 13 Arrhenius plot for secondary stage of leaching of. Chalcocite i n 1M acid 34 14 Chalcopyrite leached i n 1M acid at three temperatures • • • •. 36 15 The e f f e c t of i n i t i a l cupric additions,on the leaching rates of Chalcocite and Chalcopyrite ....... 37 16 Arrhenius p l o t for Chalcopyrite d i s s o l u t i o n in. 1M acid 39 17 Arrhenius p l o t for sulphate formation from Chalcopyrite leaching i n 1M acid ......40 18 Two pairs of experiments: A guide to reproducability of oxygen consumption 42 The Pressure Oxidation of Copper Sulphides i n P e r c h l o r i c Acid Solutions INTRODUCTION Most of the copper i n the earth's crust appears as copper-sulphur or copper-iron-sulphur compounds. The copper bearing sulphide ore i s e a s i l y concentrated to the nearly pure mineral with l i t t l e gangue material present. The c l a s s i c a l method of the recovery of copper by pyrometallurgy produces sulphur dioxide and impure copper. One of the oldest hydrometallurgical processes for the recovery of copper, in v o l v i n g the leaching of roasted concentrates with sulphuric a c i d , also produces sulphur d i o x i d e ^ \ More recently with the introduction of pressure oxidation leaching sulphides could be leached without roasting. Sulphur i s oxidized to sulphate i n ammoniacal leaching and may be recovered as ammonium sulphate When sulphides are subjected to oxidation leaching i n acid (4) media elemental sulphur i s frequently obtained i n good y i e l d . For example almost 100% of the mineral sulphur i s obtained i n the elemental state i n the oxidation of p y r r h o t i t e ^ , g a l e n a ^ ' ^ and s p h a l e r i t e ^ . Elemental sulphur i s also obtained i n acid leaching but i n poor y i e l d (less than 50%) for p y r i t e ^ ' 1 ^ . With copper sulphide minerals the production of elemental sulphur has also been reported but i t s y i e l d does not appear to be r e l i a b l y high. The leaching of copper sulphides i n acid solutions have been studied i n various o x i d i z i n g media. For example, Jackson and Strickland (12) used ch l o r i d e solutions with chlorine as an oxidant. S u l l i v a n . - 2 -studied the chemistry and k i n e t i c s of copper sulphides leaching using acid f e r r i c sulphate as an oxidant and found much slower rates, e s p e c i a l l y for chalcopyrite. F e r r i c solutions were also used, as the o x i d i z i n g (13) 'agent for the leaching of c o v e l l i t e by Thomas and Ingraham . The leaching of c h a l c o c i t e , chalcopyrite and c o v e l l i t e using oxygen as the oxidant was studied by Warren^ 1^ and recently by o t h e r s ' . From these studies the rate of leaching was found to vary with o x i d i z i n g 3+ agent according to the order •^=> Fe > C^; at room temperature and atmospheric pressure, chlorine w i l l oxidize most sulphides i n a few 3+ hours, Fe i n several days, while oxygen takes years. At temperatures (17) over 100°C oxygen w i l l oxidize sulphides i n a few hours . In a l l of the three o x i d i z i n g media the f i n a l products of leaching are s i m i l a r , i . e . , sulphate and elemental sulphur. This i s consistent with the proposition that the reduction of the oxidant i s rate c o n t r o l l i n g , and therefore that the oxidation of the mineral proceeds along a path that i s predictable from thermodynamic considerations. However, the coexistance of cupric solutions with elemental sulphur i s not an equilibrium condition, i n view of the fa c t that the equation 3CuH~ + 4H20 + 4S° 3CuS + S0= + 8H + [1] i s thermodynamically favourable at pH values above -4, at 25°C. The production of elemental sulphur during the d i s s o l u t i o n of copper sulphide minerals, therefore, r e f l e c t s a thermodynamically unstable condition; yet a l l previous investigators have observed that elemental sulphur i s formed when copper i s leached by acid solutions from a l l copper sulphide minerals. However, the conditions of Reaction [1] are s u f f i c i e n t l y compelling to suppose that both copper recovery and elemental sulphur formation w i l l not - 3 -occur i n good y i e l d simultaneously. The deportment of sulphur to elemental sulphur and sulphate during acid leaching of copper sulphide . minerals, therefore, deserves c a r e f u l i n v e s t i g a t i o n . Scope of t h i s Work In t h i s work i t i s proposed to carry the study of the leaching of copper sulphides further, using p e r c h l o r i c acid,and oxygen as the oxidant. The four minerals, c o v e l l i t e , c h a l c o c i t e , chalcopyrite and bornite are investigated. Answers as to the r e l a t i v e rates of leaching among these four minerals and the deportment of sulphur as the product w i l l be attempted on the basis of th i s work and previous knowledge. K i n e t i c studies of these lead hopefully to the proposal of rea c t i o n steps by which the k i n e t i c mechanism of the whole process takes place. Identity of the acid i s i r r e l e v a n t i f the acid anions are chemically i n e r t i n the system. P e r c h l o r i c acid i s known to be completely ionized, strongly r e s i s t a n t to reduction i n d i l u t e solutions and has the lowest tendency to form metal complexes of a l l known anions. - 4 -EXPERIMENTAL Materials Reagent grade chemicals were used e x c l u s i v e l y . A l l solutions were made with d i s t i l l e d w,ater. The p e r c h l o r i c acid used was Baker and Adamson, 60%. The c o v e l l i t e , c h a l c o c i t e and bornite came from Butte, Montana and were supplied by Ward's Natural Science Establishment, Inc. The chalcopyrite used i n the ground form originated from Miyatamata, Japan. The s o l i d specimen of chalcopyrite used for one run came from Rouyn, Quebec and was also supplied by Ward's. The chalcopyrite from Japan was obtained i n the ground form, coarser than 100 mesh. The other minerals were supplied i n massive chunks associated with minor amounts of gangue material. The material fragments were broken o f f with p l i e r s or hammer, crushed i n a mortar and pestle and screened to s i z e f r a c t i o n s . Enough of the mineral was ground i n i t i a l l y for a l l the runs. Only -150 + 200 mesh* and -200 mesh s i z e f r a c t i o n s were used. When examining the ground material microscopically very f i n e p a r t i c l e s , much smaller than the nominal p a r t i c l e s i z e were seen. This * Mesh sizes r e f e r to Tyler standard screens. - 5 -could be considered as dust c l i n g i n g to the larger p a r t i c l e s . When a weighed sample of -150 + 200 mesh material was washed repeatedly i n water and dried, to f i n d the percentage of such dust i n the sample, a weight loss of 0.6% was obtained. Samples of the ground minerals were analyzed for copper, i r o n and sulphur by Coast Eldridge Engineers and Chemists, Limited, and for copper, i r o n and " i n s o l u b l e s " by the author. The'insolubles" were the material that would not dis s o l v e i n hot, concentrated n i t r i c a c id. The r e s u l t of the analyses as well as the formula compositions are presented i n Table 1. X-ray d i f f r a c t i o n patterns* were made from -200 mesh material. The analyses show conclusive i d e n t i f i c a t i o n of c o v e l l i t e and chalcopyrite: every l i n e on the pattern i s matched by a prominent l i n e on the index cards. A good i d e n t i f i c a t i o n i s also obtained from the X-ray patterns of c h a l c o c i t e ; the only extraneous l i n e appearing i s the dominant l i n e of bornite, as the chemical analysis would ind i c a t e . A wide range i n . (18) stoichiometry i s reported for chalcocite thus precluding large amounts of a second phase. The pattern for the bornite sample ihcludes the three strongest l i n e s of c o v e l l i t e ; however, the weakness of these l i n e s indicates that c o v e l l i t e i s a minor constituent. * D-spacings with i n t e n s i t i e s from x-ray d i f f r a c t i o n patterns together with these values obtained from the index cards are presented i n the Appendix. J a b l e 1 - Chemical:: Cpmpp:5±t±^^ Miriera1 Coast'Eldridge. Cu Fe S Sum* Cqawfc "-Efl d--,Jforroalized* Cu Fe S Author Cu Fe- Insol. Formula Comp. Cu Fe S C o v e l l i t e (CuS) Cahlcocite (Cu 2S) Chalcopyrite ,(CuFeS2) Bornite (Cu 5FeS 4) 62.8 63.8; 33.2 58.2 2.49 5.6? 30.4 11-7 31.7 25TQ 34.6 26.; 2 97.0 -94.5 98". 2 96.1 64.7 67 .-6 33; 8 60.6 2.6 6.0: 31.0 12.2. 32.7 25.4 35.2 27.2 61.9 62.7 2.84 6.25. 57.7 12.6. 1.67 2.60. 1.90, 3.15 66.4 79.8 34.5 63.3 30.5 11.1 33.6 20.2 35.0 25.6 * Coast- Eldridge. assays, adjusted to make. Cu. :k Fe +. S... = 100 * Sum of Cu.+ Fe + S i - 7 -To f a c i l i t a t e comparison of r e s u l t s among the experiments approximately equal surface areas of minerals were used. S p e c i f i c g r a v i t i e s of the mineral samples were determined and the values obtained are recorded i n Table 2." Experimental Apparatus The reaction vessel was a shaking, titanium autoclave, manufactured by Pressure Products Industries, Inc., of 117 ml capacity and able to withstand 4500 p s i working pressure. The shaking was a r e c i p r o c a l motion with a stroke of 1.4 inches and 266 cycles/minute. A F l e x i t a l l i c s t a i n l e s s s t e e l gasket sealed the autoclave. Heat was provided by an e l e c t r i c a l resistance heater wound around the aiitoclave. The temperature was sensed by a thermistor located i n a titanium well i n the l i d projecting to the centre of the autoclave. A Thermistemp temperature c o n t r o l l e r (Model 71, Yellow Springs Instruments Co., Inc.) i n series with a voltage regulator c o n t r o l l e d the average temperature within 1/2°C with ±1° cycle s . The t o t a l pressure i n the autoclave was kept constant by a pressure regulator (0-100 p s i range) which l e t oxygen gas,from a res e r v o i r into the autoclave as i t was consumed. A transducer (Consolidated Electrodynamics Corp., Model 4-311, 0-1000 p s i range) relayed a s i g n a l proportional to the pressure of oxygen i n the r e s e r v o i r to a Sargent s t r i p chart recorder. Thus the r i e of reaction was measured by the oxygen, consumption as registered by the pressure drop i n the r e s e r v o i r . - 8 -Table 2 S p e c i f i c G r a v i t i e s and Weights for Equal Areas Mineral S p e c i f i c Gravity Weights for equal areas* D a n a ( 1 9 ) S m i t h ( 2 0 ) Measured and used C o v e l l i t e 4.6 4.6-4.76 4.61 3.25 gm Chalcocite 5.5-5.8 5.5-5.8 5.55 3.91 Chalcopyrite 4.1-4.3 4.1-4.3 4.26 3.00 Bornite 4.9-5.4 5.06-5.08 5.07 357 * Assuming cubic shape f a c t o r , Area = 475 cm for -150 + 200 mesh material. - 9 -The pressure regulator was c a l i b r a t e d and found to read 100 when the pressure i n the autoclave was 95 psig. A l l p r e s s u r e s mentioned hereafter are the corrected values. Experimental Procedure A l l runs were conducted with 70 ml of so l u t i o n . A f t e r the autoclave was sealed i n preparation for a run, shaking and heating were started. When the working temperature was reached,in 10 to 15 minutes, the oxygen was l e t into the autoclave to i n i t i a t e the run. At.the end of the run the oxygen valve to the autoclave was closed, the heat was turned o f f , the i n s u l a t i n g cap was removed and wet towels were wrapped around the sealing nut to promote rapid cooling. Shaking was intermittant during t h i s time. When the temperature f e l l below 85°C, i n 10 to 15 minutes, the autoclave was opened and the contents removed by a suction f l a s k . Between runs, the autoclave was washed with 50% n i t r i c a c i d . A tea-spoonful of powdered s i l i c a was added i f a p r e c i p i t a t e was suspected of coating the walls of the autoclave. The autoclave was sealed and with s h a k i n g j i t was allowed to reach some temperature above 70°C. The n i t r i c acid remained i n the autoclave for no less than 15 minutes. Unless otherwise stated a l l experiments were carried out at 95 p s i gauge pressure regardless of the temperature of the run. Therefore because of s o l u t i o n vapour pressure and nitrogen entrained i n the - 10 -autoclave at the time of closure the pressure of-oxygen i n the, autoclave varied with temperature as l i s t e d i n Table 3. Table 3 Temperatures and Corresponding and Oxygen Pressures Used Temperature Oxygen Press 105°C . 7.7.3 p s i . 110 73.8 115 69.9 120 65.4 125 60.3 130 54.6 140 41.0 Unless otherwise stateclj the mineral charges of a l l experiments were of.the weights l i s t e d i n Table 2.and of -150 + 200 mesh p a r t i c l e s i z e . A n a l y t i c a l Methods 1. The amount of sulphate i n s o l u t i o n was estimated by the (21) standard barium chloride p r e c i p i t a t i o n and gravimetric determination From repeated determination of the same solutions a reproducability of ±1.7% was found. 2. The concentration of copper i n s o l u t i o n was determined by (21) a standard e l e c t r o p l a t i n g procedure using platinum cathodes - 11 -3. The amount of mineral leached, was calculated from the weight loss of the .autoclave charge corrected for elemental sulphur content. The elemental sulphur i n the residue was assumed-to be ..equal to the t o t a l sulphur i n the consumed mineral minus that determined as sulphate i n the s o l u t i o n . The residue was allowed to dry overnight i n the . f i l t e r paper before weighing. This estimate of the amount of mineral leached was used only for the leaching of chalcopyrite i n which no composition change i n the mineral occurs during leaching. Several t r i a l s . i n which mineral samples were shaken with ; pure water i n the autoclave showed that the r e p r o d u c i b i l i t y of the residue recovery method was about ±0.5%. 4. The amount of i r o n i n the o r i g i n a l mineral samples was estimated by the standard double p r e c i p i t a t i o n and gravimetric (21) determination 5. The amount-of oxygen.consumed was estimated from the trace produced on the recorded chart. The r e l a t i o n s h i p between ml of oxygen used and chart reading were established by a gas burette c a l i b r a t i o n . 6. I t was o r i g i n a l l y intended to analyze for elemental sulphur by washing'.the dry residue i n carbon disulphide to d i s s o l v e the sulphur, f i l t e r i n g to remove t h e , s o l i d residue and l e t t i n g the carbon disulphide evaporate to dryness to leave behind a crust of sulphur i n a pre-weighed beaker. The method consisted of immersing the residue for about an hour i n 30 ml carbon disulphide, and then decanting i t through a q u a l i t a t i v e f i l t e r paper. The residue was again immersed and the carbon disulphide decanted. The residue was f i n a l l y placed i n the f i l t e r paper and washed with about 10 ml of carbon disulphide. A warmed watch-glass was - 12 -placed over the funnel to minimize evaporation. The f i l t r a t e was allowed to evaporate overnight. The most elemental sulphur accounted for was about 90%. The amount of oxygen consumption and the reproducability of sulphate determinations between runs indicated that the missing sulphur was not sulphate. A check with potassium permanganate indicated the absence of transient species of sulphur oxide i n s o l u t i o n . Therefore i t was thought that the above procedure was inadequate for extracting a l l the elemental sulphur from the residue. No r e s u l t s from these "analyses" are reported here. (22) Other investigators have encountered a s i m i l a r problem - 13 -RESULTS 1. Phase Changes During Leaching Since c o v e l l i t e i s more stable towards oxidation than other, copper sulphide minerals i t s formation as an intermediate during oxidation of chalcopyrite, c h a l c o c i t e and bornite i s thermodynamically possible. To investigate transformations of c h a l c o c i t e and bornite during leaching these minerals were each leached in.1 M acid at_125°C for 30 minutes. X-ray d i f f r a c t i o n patterns were taken of the residues a f t e r washing with carbon disulphide. Both patterns consisted of dominant l i n e s of both c h a l c o c i t e and c o v e l l i t e ; the pattern for bornite showed no evidence of r e s i d u a l bornite. To. determine s i m i l a r changes i n chalcopyrite, 3 gm of -200 mesh material, was leached i n . l M acid atl20°C for 8 1/2 hours. The residue, a f t e r the run, was washed with carbon disulphide to remove elemental sulphur and an x-ray d i f f r a c t i o n pattern was made of the.unleached material. A s i m i l a r sample of residue was sent to Coast Eldridge for chemical analysis. The x-ray pattern revealed no evidence of c o v e l l i t e or c h a l c o c i t e and the chemical analysis showed the residue had a composition within the accuracy range of the i n i t i a l material assay. Further evidence for no mineral transformation i s that the residues of a l l runs of chalcopyrite a f t e r being washed with carbon disulphide were i d e n t i c a l i n appearance with the i n i t i a l material. - 14 -To determine whether c o v e l l i t e i s p r e c i p i t a t e d from a cupric s o l u t i o n by elemental sulphur, chalcopyrite, 1 gm of -200 mesh material, was shaken i n the .autoclave with 0.3 gm sulphur i n 0.3 M cupric perchlorate s o l u t i o n at 120°C for 3 hours. Another run was performed with 3 gm of the material plus 0.3 gm sulphur i n 2M cupric perchlorate—2M p e r c h l o r i c acid s o l u t i o n at 120°C for 5 hours. The chalcopyrite was added i n the event that the formation of c o v e l l i t e should be catalyzed on the surface of the mineral, but to prevent.any leaching a nitrogen.atmosphere was maintained. Aft e r the runs the chalcopyrite was removed by f i l t r a t i o n and washed with carbon disulphide. Both the chalcopyrite and the,solutions were unaltered. Barium chloride added to the solutions v e r i f i e d the absence of sulphate., 2. Leaching Massive Specimens Large s i n g l e specimens of c o v e l l i t e , c h a l c o c i t e and chalcopyrite were polished smooth on one side, using for f i n a l p o l i s h 1 micron diamond paste for c o v e l l i t e and c h a l c o c i t e and 1 micron alumina s l u r r y on a m e t a l l u r g i c a l p o l i s h i n g wheel for chalcopyrite. Scratches could be observed microscopically on the polished surface. The specimens were mounted i n a f l a t c ylinder of cast sulphur reinforced with titanium wire to mount the specimens firm l y during the run. The three specimens were leached at 110°C i n 1M p e r c h l o r i c acid for 4.8, 3 and 6.5 hours r e s p e c t i v e l y . - 15 -For the c o v e l l i t e specimen, photomicrographs taken of the surface before and a f t e r leaching are presented i n Figure 1. The c h a l c o c i t e turned from grey to black upon leaching. Microscopic examination at low magnification revealed cracks running throughout the surface of the specimen. At low magnification the unleached surface of chalcopyrite showed inclusions of two kinds: grey, s o f t i n c l u s i o n s , probably c o v e l l i t e or c h a l c o c i t e , and hard, yellow i n c l u s i o n s , probably p y r i t e . The hardness of the inclusions r e l a t i v e to the matrix i s estimated by the d i f f i c u l t y of removing scratches from them. Aft e r leaching,the soft i n c l u s i o n s were completely leached out but the hard in c l u s i o n s were unchanged. Photomicrographs taken at 260X magnification are presented i n Figure 2. At lower magnification no change of the matrix was observed with leaching. 3. Formation of Sulphate from Elemental Sulphur. To determine whether elemental sulphur oxidizes to sulphate under the conditions used for leaching a test was made with 0.5 gm of sulphur plus one teaspoonful of s i l i c a i n 0.36M cupric perchlorate - 0.25M per c h l o r i c acid s o l u t i o n at 125°C. A f t e r 2.3 hours the oxygen consumption was n e g l i g i b l e and a barium chloride test revealed that no sulphate was present i n the solut i o n . - 16 -After Leaching Figure 1 Surface of Massive Specimen of C o v e l l i t e before and a f t e r leaching 135X - 17 -Before Leaching Af t e r Leaching gure 2 Surface of Massive specimen of chalcopyrite before and a f t e r leaching 260X - 18 -4. Evolution of Hydrogen Sulphide and Sulphur Dioxide To determine whether hydrogen sulphide or sulphur dioxide are products of the oxidation reactions chalcopyrite, 6gm of -200 mesh, was leached i n 2M p e r c h l o r i c acid at 120°C. After 2 1/2 hours, when 0.0182 moles of oxygen had been consumed corresponding to the oxidation of about 30% of the mineral, the heat was turned o f f to l e t the autoclave cool below 100°C. With the oxygen l i n e closed the gas i n the autoclave was allowed to bubble, for about a minute each, through columns of s i l v e r n i t r a t e , 10 "*M potassium permanganate and d i s t i l l e d water. No p r e c i p i t a t e was formed i n the s i l v e r n i t r a t e which indicated the absence of hydrogen sulphide. The permanganate s o l u t i o n retained i t s f a i n t pink colour and the pH of the d i s t i l l e d water remained constant i n d i c a t i n g the absence of sulphur dioxide. 5. Comparison of Leaching Rates C o v e l l i t e , c h a l c o c i t e and chalcopyrite were leached i n 4M p e r c h l o r i c acid at 110°C and 125°C and two runs were conducted with bornite i n 1M acid at the same temperatures. The oxygen consumption curves for these runs are presented i n Figures 3, 4, 5 and 6. Analyses of copper and sulphate i n s o l u t i o n a f t e r the runs are given i n Table 4. 600 T F i g u r e 3; L e a c h i n g - : c o v e l l i t e i n r 4 M * - p e r c h l o r i c a c i d 600 Fxgnre 4 . 1 2 3 Time-Hours Lnea-dr±ng--chaicQC-ite--ln--.:4M%-acid. Time-Hours Figure 5. Leaching chalcopyrite in-. 4M-acid. 600 Time-Hours Figure 6. Leaching.bornite i n 1M acid - 23 -Table 4 Leaching C o v e l l i t e , Chalcocite and  Chalcopyrite i n 4M Acid and Bornite i n 1M Acid f o r 6 Hours. Mineral Temp. moles O2 consumed moles'^Cu i n soln. moles SO4 i n soln. % S as S O f * % " C u i n soln. C o v e l l i t e 110°C .0079 .0111 .00145 13.0 34.7 125 .0112 .0165 .00172 10.4 51.5 Chalcocite 110 .0170 .0261 .00206 12.0 66.5 125 .0189 .0262 .00309 17.9 66.6 Chalcopyrite 110 .0015 .0007 N.D.* N.D. 4.5 125 .0040 .0017 N.D. N.D. 10.8 Bornite 110 .0277 .0277 .00248 10.3 84.5 125 .0160 .0178 :00340 24.2 54.1 * The residues of Chalcocite and Bornite runs are assumed to be e n t i r e l y transformed to C o v e l l i t e . ^ N.D. = not determined. - 24 -6. V a r i a t i o n of A c i d i t y A se r i e s of runs was done in.which c h a l c o c i t e was leached at 125°C i n pe r c h l o r i c acid ranging i n concentration from 0.1M to 4M. The lengths of these runs were 5, 6 or 9 hours. The oxygen consumption curves for. these runs are given in.Figure 7. The amounts of sulphate and copper i n s o l u t i o n and the f i n a l pH values are presented i n Table 5. After the runs with 0.1M and 0.25M acid a yellow p r e c i p i t a t e coated the insi d e of the autoclave and a light-brown p r e c i p i t a t e was present with the unleached mineral. The amount of p r e c i p i t a t e was much more extensive at the.O.lM tihan at the 0..25M acid run. Chalcopyrite, 9 gm of -150 + 200 mesh, was leached at 125°C i n 0.25M, 1.0M and 4M acid f o r 6 hours each. The oxygen consumption curves for these runs are given i n Figure 8. The amount of sulphate and copper i n s o l u t i o n and the f i n a l pH values are presented i n Table 6. 7. V a r i a t i o n of Oxygen Pressure Three experiments were performed using ch a l c o c i t e i n 1M acid at 110°C and at oxygen p a r t i a l pressures i n the autoclave of 25, 42 and 74 p s i . The lengths of the runs were 5, 6 and 9 hours r e s p e c t i v e l y . The oxygen consumption curves f o r these runs are presented i n Figure 9. . These curves can b e d i v i d e d into two parts, representing an i n i t i a l , rapid oxidation.step followed by a period of steady, slow oxidation. The slopes of these curves are presented in.Table 7 and plotted against pressure of oxygen i n Figure 10. / -Time-Hours Figure 7. Leaching of Chalcocite 125°C at various a c i d i t i e s . - 26 -T a b l e 5 L e a c h i n g C h a l c o c i t e i n P e r c h l o r i c A c i d a t 1 2 5 ° C A c i d i t y Duration of run moles 0 2 consumed moles Cu i n soln. moles SO4 i n soln. % Cu i n soln. %S as S O 5 * f i n a l pH p r e c i p i t a t e 4M 6 hr. . 0189 .0262 .00309 66.7 17.9 0 n i l 6 . 0202 .0332 .00259 84.5 10.7 0 n i l 2 5 .0210 .0261 .00625 66.5 36.3 0.5 n i l 1 9 • 9327 .0327 .0113 83.2 47.5 1.1 n i l 0.5 6 .0356 .0280 .0137 71.2 71.7 1.0 n i l 5 .0363 .0284 .0140 72.3 71.8 1.35 n i l 0.25 6 .0465 .0272 N.D. ppt N.D. 2.3 ppt. 0.10 6 .0228 .00715 N.D. ppt N.D. 3.0 ppt. N.D; = Not determined * Residue i s assumed to be . e n t i r e l y transformed to c o v e l l i t e . - 28 -Table 6 Leaching Chalcopyrite (9gm) at-125°C f or 6 Hours A c i d i t y Moles- • 0'2-consumed molee-Cu i n soln. -moles- SO-^--. i n soln. •%-Cu i n soln. X-S-as SQ^ • f i n a l . pH pr ec i-p i t ate -4M .0217 .01475 .00220 31.3 7.2 0 n i l • 1 .0265 .0116 .00488 24.6 20.4 0.5 ppt 0.25 .0178 .00839 .00261 17.9 15.1 1.2 ppt Table 7 Chalcocite Leaching at 110 PC i n 1M acid at Three Oxygen Pressures Pressure I n i t i a l Secondary oxygen rate rate 25 p s i 270 ml0 2/hr 53.8 ml0 2/hr 42 300 57.4 74 545-830 87.5 3 0 -9 0 0 8 0 0 7 0 0 6 0 0 5 0 0 ^ 4 0 0 o ^ 3 0 0 Hi w 2 0 0 CO u 1 0 0 L i 1 r J L i r I n i t i a l , s t a g e r a t e s o J I L iA e 3' c0 t-i 9 0 8 0 1 0 o S e c o n d a r y s t a g e r a t e s J L _L J L 7 0 8 0 9 0 2 0 3 0 4 0 5 0 6 0 O x y g e n p r e s s u r e ( p s i ) F i g u r e 1 0 T h e e f f e c t o f o x y g e n p r e s s u r e o n t h e l e a c h i n g r a t e o f c h a l c o c i t e i n 1 M a c i d a t 1 1 0 ° C . - 31. -8. Temperature V a r i a t i o n Three runs were done on c h a l c o c i t e i n 1M p e r c h l o r i c acid at temperatures of 110°C, 125°C and 140°C. The lengths of these runs were . 9, 9 and 3 hours r e s p e c t i v e l y . The residue from the run at 140°C was dried and washed with carbon disulphide a f t e r which 1.924 gm of i t were returned to the autoclave to be leached i n 1M acid at 140°C for 3 hours. The oxygen consumption curves for these runs are.given i n Figure 11. The rates were measured for each of the two stages of oxidation and are presented i n Table 8_. The rates divided by pressure of oxygen f o r the two stages of leaching are shown on Arrhenius plots i n Figures 12 and 13. Table 8 Oxidation Rates f o r Leaching Chalcocite i n 1M P e r c h l o r i c Acid Temperature Pressure 0 2 p s i Primary Stage Leaching Rate Secondary stage leaching rate 110°C 74 490-540 ml0 2/hr. 90 ml0 2/hr. 125 9C 60 425-475 140 1$.0 °C 41 340 150 - 32 -i r Time-Hours Figure 11. Chalcocite leached i n 1M p e r c h l o r i c acid at three temperatures. F i g u r e 13 A r r h e n i u s p l o t f o r s e c o n d a r y s t a g e o f l e a c h i n g o f c h a l c o c i t e i n 1M a c i d . - 35 -Three runs were done on chalcopyrite, using 9 gm of -150 +200 mesh, i n 1M acid at temperatures of 110°C, 125°C and 140°C. The lengths of these runs were 11, 6 and 3 hours re s p e c t i v e l y . The oxygen consumption curves for these runs are given i n Figure 14. 9. The C a t a l y t i c E f f e c t of. Cupric Ions In order to test for a c a t a l y t i c e f f e c t of cupric ions on the leaching of copper sulphides, ch a l c o c i t e was leached i n 0.5M copper perchlorate - 1M p e r c h l o r i c acid s o l u t i o n at 125°C for 6 hours. Another run employed chalcopyrite, 9 gm of -150+200 mesh, i n 0.5M copper perchlorate—0.5M p e r c h l o r i c acid s o l u t i o n at the same temperature for also 6 hours. The oxygen consumption curves for these runs together with co n t r o l experiments lacking i n i t i a l cupric s a l t additions are presented i n Figure 15. 10. Leaching Chalcopyrite A series of 2 hour runs was conducted with chalcopyrite, 3 gm of -200 mesh, i n 1M acid i n the temperature range of 105°C to 130°C . The solutions were analyzed for sulphate and the amount of material leached was calculated from the weight of the residue removed from the autoclave. One of the solutions (from a run at 120°C) was analyzed for ferrous content one hour a f t e r removal from the autoclave. In a l l runs, except one at 130°C, the oxygen consumption curves were l i n e a r , i n d i c a t i n g constant reaction rates. The sulphate analyses at varying amounts of mineral leached are given i n Table 9 and shown i n Arrhenius plots i n Figures _16_ and 17. The ferrous t i t r a t i o n showed that 68% of the i r o n present i n s o l u t i o n was i n the ferrous.state. - 36 -1 1 I r 1 — r 0 1 2 3 4 5 6 Time-Hours Figure 14. Chalcopyrite leached i n 1M acid at three temperatures, (9gm of mineral used). Time - Hours Figure 15 The e f f e c t of I n i t i a l cupric additions on the leaching rates of Chalcocite and Chalcopyrite. - 38 -Table 9 Leaching Chalcopyrite i n 1M Acid for 2 Hours (3 gm of -200 mesh) Temperature gm Mineral leached Moles SO^ % S...aa...SQ.4_ 105°C .476 .000638 12.3% 105 .504 .000603 11.0 110 .510 .00078 14.0 110 .545 .00111 18.7 115 .615 .00100 14.9 120 .750 .001105 13.5 120 .606 .00115 17.4 125 .648 .00143 20.2 130 .706 .001635 21.2 2.48 2.52 2.56 2.60 2.64 l / l x 10 3 Figure 16 Arrhenius p l o t for Chacbpyrite d i s s o l u t i o n i n 1M acid - 41 -DISCUSSION 1. R e p r o d u c i b i l i t y of Results Figure 18, showing the oxidation curves of two pairs of experiments, indicates the degree of r e p r o d u c i b i l i t y that can be expected i n t h i s work. Some of the f a c t o r s , i n f l u e n c i n g oxygen consumption are: copper entering the s o l u t i o n , the formation of sulphate, transient species of sulphur oxide being formed and oxidized, ferrous ions o x i d i z i n g to f e r r i c , and impurities i n the o r i g i n a l mineral reacting. Some.of these may be influenced by minute v a r i a t i o n s i n variables that may not have been c o n t r o l l e d . Those experiments c a r r i e d out above the melting point of sulphur are expected to be le s s reproducible than the others because of the wetting and spreading expected of the l i q u i d sulphur and the r e s u l t i n g v a r i a t i o n i n area of anodic regions. 2. Phase Changes During Leaching In the leaching of ch a l c o c i t e the formation of c o v e l l i t e occurs r a p i d l y according to the reaction: Cu„S + 2H +10 2 •>- CuS + Cu ++ [2] ^F°393 - 30.37 Kcal/mole* The rapid s e l f - d i f f u s i o n of the cuprous ion i n ch a l c o c i t e (23,24) i s necessary for the k i n e t i c s to be as fast as observed. * A l l thermodynamic data are taken from Latimer where noted. (25) with few exceptions F i g u r e 1 8 . T i m e - H o u r s Two p a i r s o f e x p e r i m e n t s : a g u i d e to r e p r o d u c i b i l i t y o f o x y g e n c o n s u m p t i o n . - 43 -From the materials analysis (Table 1) i t i s calculated that i n the c h a l c o c i t e sample used the average formal valence of copper i s +1.28. For 3.91 gm of c h a l c o c i t e to react to completion according to Reaction [2] 224 ml of oxygen w i l l be consumed. Examination of the curve for the oxidation of c h a l c o c i t e at 110°C i n Figure 4 shows that t h i s i s near the value for the t r a n s i t i o n between the fast i n i t i a l rate, i d e n t i f i e d as c h a l c o c i t e - c o v e l l i t e transformation, and the subsequent slower oxidation rate, i d e n t i f i e d as c o v e l l i t e d i s s o l u t i o n . Also, comparison of Figures 3 and 4 shows that the c h a l c o c i t e curves have about the same shape as the c o v e l l i t e curves but are displaced upwards by about 200 ml. This indicates that the c h a l c o c i t e - c o v e l l i t e transformation i s p r a c t i c a l l y completed i n 30 minutes and that during t h i s time c o v e l l i t e oxidation i s minor. Because cha l c o c i t e i s a good e l e c t r i c a l conductor and therefore one p a r t i c l e can be at only one e l e c t r i c a l potential} both c o v e l l i t e and elemental sulphur w i l l not coexist on one p a r t i c l e of incompletely reacted c h a l c o c i t e . Therefore the transformation i s expected to go to completion before the next oxidation step begins. However, because a l l the p a r t i c l e s of the charge w i l l not be i n e l e c t r i c a l contact a sharp d i s t i n c t i o n between the two oxidation steps should not be expected with powdered material. The transformation of c h a l c o c i t e to c o v e l l i t e produces cracks i n the mineral p a r t i c l e s as a r e s u l t of shrinkage i n volume, since the molar volumes of chalcocite and c o v e l l i t e are 28.7 cc/mole and 20.7 cc/mole re s p e c t i v e l y . This would prevent the rate of transformation from decreasing due to a d i f f u s i o n b a r r i e r being erected by the product forming on the - 44 -p a r t i c l e s . These cracks also increase surface area and may be responsible for the leaching rate of c h a l c o c i t e at 110°C (Figure 4) being greater than the corresponding rate for c o v e l l i t e (Figure 3). The appearance of both c o v e l l i t e and c h a l c o c i t e from the leaching of bornite indicates, that two t r a n s i t i o n reactions are operative, Cu 5FeS 4 + 4H + + 0 2 » 4CuS + Cu* + Fe"*"1" + 2H20 ' [3] Cu 5FeS 4 + 2H + + 10 2 »-Cu 2S + 3CuS + Fe** + H 20 [4] with A F ° 2 9 g of -80.05 and -47.79 Kcal/mole r e s p e c t i v e l y . * The f i r s t reaction i s the one expected from a k i n e t i c point of view because cuprous ions i n bornite are able to d i f f u s e (28) much f a s t e r than the ferrous ions and therefore are expected to a r r i v e i n s o l u t i o n at l e a s t as f a s t as the ferrous ions. However, thermodynamic c a l c u l a t i o n s predict that i r o n should leach out of bornite at a lower oxidation p o t e n t i a l than that required for copper d i s s o l u t i o n and therefore Reaction [4] probably occurs and may even predominate.. Chalcocite could be formed according to the l a t t e r r eaction only i f the reduction of oxygen i s rate-determining. The presence of c h a l c o c i t e i n the p a r t i a l l y leached bornite i s consistent with a r e v e r s i b l e (27) oxidation step for the mineral , and i s therefore evidence for a r a t e - c o n t r o l l i n g step involving oxygen reduction rather.than mineral oxidation. The reaction i s excluded because cha l c o c i t e and elemental sulphur i n intimate contact react to form c o v e l l i t e , the end r e s u l t being equivalent to Reaction [3]. ( 2 ft) * Thermodynamic data for bornite comes form Majima and Peters CutFeS, + 4H + 0 [5] - 45 -From previous work done with bornite i t i s reported that digenite i s r a p i d l y formed when b o r n i t e - i s heated i n the temperature range of 80° to 1 5 0 ° C ^ 8 \ Digenite i s stable above 105°C and ( 2 9 ) decomposes to c h a l c o c i t e and c o v e l l i t e below t h i s temperature , . This work i s consistant with the r e s u l t s obtained here, i . e . , bornite transforming to c h a l c o c i t e . The r e s u l t s show that chalcopyrite does not form c o v e l l i t e during leaching although the reaction CuFeS 9 + 2H-+ + 10- ». CuS + F e ^ + S° + H 20 [6] 2 has a A F ° of -10.95 Kcal/mole at 120°C* This does not exclude the possibi^ i t y that the formation of c o v e l l i t e may be an intermediate step i n the mechanism of leaching, but because the leaching rate of c o v e l l i t e i s f a s t e r than that of chalcopyrite (comparing Figures 3 and 5) the former would not be present i n observable amounts. 3. Leaching Massive Specimens Examination of the photomicrographs of Figure 1 for the leaching of c o v e l l i t e reveals that before leaching the c o v e l l i t e contained veins of l i g h t grey material, probably c h a l c o c i t e or bornite; the chemical analysis suggests the presence of bornite. After leaching the presence of p i t s and channels i n d i c a t e nonuniform attack on the surface of the c o v e l l i t e . The scratch marks s t i l l v i s i b l e show that l i t t l e uniform attack * Thermodynamic data for chalcopyrite i s taken from Golomzik^*^ - 46 -has occurred. The photographs show that the p i t s were i n i t i a t e d at locations of the o r i g i n a l l i g h t grey phase. If t h i s phase i s c h a l c o c i t e or bornite, t h i s i s consistent with the findings discussed i n the above, section that i f i n e l e c t r i c a l contact then c o v e l l i t e should not leach u n t i l c h a l c o c i t e and bornite have transformed completely to c o v e l l i t e . In the transformation process the grey phase w i l l open up cracks i n the mineral. The p i t s and channels extend over.a larger area than the o r i g i n a l l i g h t grey phase and t h i s indicates that the region adjacent to t h i s phase becomes anodic. Nonuniform attack on e l e c t r i c a l conductors i s i n d i c a t i v e of the occurrence of electrochemical p r o c e s s e s ^ ^ . In the leaching of c o v e l l i t e with acid and oxygen the anodic and cathodic reactions may be written as follows: CuS >-Cu + + + S° + 2e [7] 2 H + + 10 + 2e > H2O [8] 2 On some corroding materials the anodic and cathodic areas may migrate, giving the appearance of general corrosion. However the photographs show the areas were reasonably fixed during the length of this run, with Reaction [7] i n i t i a t i n g and propagating p i t s , and Reaction [8] occuring on the v i r t u a l l y unattacked surface. An explanation for t h i s is that i n p i t s the concentration of oxygen i s expected to be le s s than on the f l a t surface. Therefore Reaction [7] would continue and propagate the p i t s , whereas Reaction [8] i s u n l i k e l y i n p i t s since i t involves the, d i f f u s i o n of oxygen and protons to the surface of the mineral. - 47 -Since c h a l c o c i t e and bornite transform to c o v e l l i t e during the i n i t i a l step of leaching, the d i s s o l u t i o n c h a r a c t e r i s t i c s of c o v e l l i t e discussed here should also apply to the f i n a l leaching of chalcocite and bornite. During leaching the s o l i d specimen of chal c o c i t e i t became black and developed cracks on the surface showing i t s transformation to c o v e l l i t e and a.simultaneous shrinkage i n volume. The photomicrographs from leaching the specimen of chalcopyrite, Figure 2, show traces of l o c a l attack at 260X magnification, however, considering the slowness of the leaching of chalcopyrite the a v a i l a b l e evidence i s i n s u f f i c i e n t to exclude the p o s s i b i l i t y of general attack. 4. Formation of Sulphate from Elemental Sulphur The r e s u l t s i n d i c a t e that with oxygen and i n 0.25M perc h l o r i c acid sulphur does not oxidize to sulphate although the reaction S° + 30. + H00 ^S07 + 2H + [9] — 2 I 4 has a /^F° of -119.39 Kcal/mole at 125°C. Sulphur that oxidizes to sulphate through cataysis by the surface of the mineral would not.be distinguishable from sulphate formation with sulphur o r i g i n a t i n g from the mineral i t s e l f . - 48 -5. The Evolution of Hydrogen Sulphide and Sulphur Dioxide Hydrogen sulphide i s not a product of the leaching reaction even i n solutions of 2M acid as predicted by thermodynamic c a l c u l a t i o n s , since the reaction .. H2S + Cu** >. CuS + 2H + [10] has a ^ F ° 2 9 8 °f -19.34 Kcal/mole and i s rapid i n the d i r e c t i o n as written. Sulphur dioxide also cannot be expected i n these solutions because i t s oxidation by oxygen i s rapid, although the reaction CuS + 30 9 + 2H + > Cut* + H~S0„ [11] 2 1 '(' -* has a AF°29g °f -101.36 Kcal/mole. Sulphate formation through S0 2 or H 2S0^ as an intermediate cannot be excluded. 6. Comparison of Leaching Rates From the oxidation curves of c o v e l l i t e , c h a l c o c i t e and bornite of Figures 3, 4, and 6, i t can be seen that the curves for leaching at 125°C have a higher i n i t i a l slope than the 110°C curves, as expected, but have continuously decreasing slopes over t h e i r range, whereas, the 110°C curves, a f t e r an i n i t i a l steep section, assume constant slopes. The decrease i n the reaction rates at 125°C can be at t r i b u t e d to l i q u i d sulphur covering the mineral and thereby s t i f l i n g the reaction, since the melting point of s u l f u r i s 119 PC. This conclusion was also reached by many previous i n v e s t i g a t o r s ' ^ ' . If the anodic reaction occurs i n p i t s on the mineral, s t i f l i n g of the reaction i s e s p e c i a l l y l i k e l y . - 4 9 -In the leaching of c o v e l l i t e , Figure 3, i t would be expected for the steady rate section of the 110°C oxidation curve to pass through the o r i g i n , but bornite and/or chalcocite impurities i n the c o v e l l i t e react r a p i d l y at f i r s t and a l i n e a r rate i s reached when only c o v e l l i t e remains. The chalcocite curves are the same shape as the c o v e l l i t e curves but displaced upwards by an amount necessary to produce the c h a l c o c i t e - c o v e l l i t e transformation, as discussed e a r l i e r . The f a c t that the 110°C steady leaching rate for chalcocite i s steeper than for c o v e l l i t e , even though both can be considered to be the leaching of c o v e l l i t e , may be explained by the increase i n surface area of c h a l c o c i t e by the cracks formed during the c h a l c o c i t e - c o v e l l i t e transformation. It i s i n c o r r e c t to compare the bornite oxidation curves rigorously with the others since the a c i d i t i e s are d i f f e r e n t . It i s noticed from Figure 7 that there i s a s i g n i f i c a n t difference between leaching chalcocite i n 1M and 4M acid. The fact that a varying amount of s u l f a t e i s produced, and that the i r o n from bornite oxidizes almost completely to f e r r i c tend to make the oxygen consumption curves less meaningful. These factors permit only q u a l i t a t i v e interpretations of the curves. The 110°C oxidation curve i s steeper for bornite than for c h a l c o c i t e and c o v e l l i t e probably because the greater volume change during transformation amplifies the cracks and probably because the transformation of c h a l c o c i t e (or digenite) to c o v e l l i t e , since i t i s a secondary reaction, would be spread over a longer time period than the equivalent transformation - 50 -i n o r i g i n a l c h a l c o c i t e . Thus the f i n a l steady rate f o r 110°C leaching of bornite i s probably not only the leaching of c o v e l l i t e but a simultaneous transformation to some extent. The fac t that the 125°C curve a f t e r 1 hour i s steeper than the chalcocite curve can be explained i n the same way. That i t i s les s subject to s t i f l i n g by molten s u l f u r may again be evidence that the transformation i s spread out over a longer. period. The leaching of chalcopyrite (Figure 5) i s an order of magnitude slower than that of other copper minerals. A f t e r s i x hours the oxygen consumption w a s . s t i l l too small to detect s t i f l i n g by elemental sulphur at 125°C. Apparently fast i n i t i a l rates may be due to dust or traces of other minerals and are therefore not necessarily c h a r a c t e r i s t i c of chalcopyrite. 7. V a r i a t i o n of A c i d i t y An examination of the oxidation curves for leaching chalcocite i n p e r c h l o r i c acid of varying strengths (Figure 7) shows that t h e . t o t a l oxygen consumption increases from the 4M run to a maximum at the 0.25M run. The sharp decrease i n reaction rate a f t e r about one hour for the 4M and 2M cases may be att r i b u t e d to l i q u i d sulphur coating the mineral and i n h i b i t i n g further leaching. This phenomenon would normally be expected to be operative, i n a l l runs i n this s e r i e s . From Table 5 i t i s seen that the decrease of a c i d i t y from 4M to 0.5M i s accompanied by an increase i n the y i e l d of sulphate. This i s not su r p r i s i n g considering that sulphur i s never produced when sulphides - 51 -(2 31) are leached i n basic solutions ' , and that with increasing pH values elemental sulphur becomes les s stable. The observation that the oxidation rate during the l a t t e r h a l f of each run increases with decreasing a c i d i t y leads to the p o s s i b i l i t y that either the s t i f l i n g of the reaction has been eliminated, e.g., sulphur i s being removed, or other reactions that consume oxygen become increasingly dominant. It has already been shown that sulphur i s not removed by oxidation to sulphate i n an acid, cupric s o l u t i o n . This experiment/ was conducted i n 0.25M acid and i s not more favourable at higher a c i d i t i e s . An a l t e r n a t i v e i s . f o r the reaction producing sulphate d i r e c t l y to become dominant, and/or i s not,subject-to l i q u i d sulphur s t i f l i n g . Assuming c h a l c o c i t e i s completely converted to c o v e l l i t e , the sulphate producing reaction i s CuS + 20 2 >-Cu + + + SO* [12] a rea c t i o n which not only consumes four times as much oxygen as that y i e l d i n g elemental sulphur, but also does not consume acid. The suggestion that Reaction [12] may be occurring at locations other than the anodic s i t e s a r i s e s from the p r o b a b i l i t y that i t i s not subject to sulphur s t i f l i n g . If this i s the case the sulphating reaction may be chemical i n nature and occur somewhat uniformly on.the surface of the mineral. It i s not l i k e l y for sulphate to be formed as the counter-part of the cathodic Reaction [8], i e , to proceed 20 »- Cu* + SO^ CuS + 4H? . + 8H + + 8e [13] A F 2 9 8 = - 7 6 - 6 5 K c a l / m o l e - 52 -because of the unfavourable entropy changes involved when t h i s many water molecules are taken up to y i e l d a sing l e sulphate ion. It i s not necessary for a heterogeneous chemical reaction to exhibit uniform attack on the surface of a p a r t i c l e since prefermential adsorption and s e l e c t i v e a c t i v i t i e s , and hence varying reaction rates among c r y s t a l l a t t i c e planes of d i f f e r e n t o r i e n t a t i o n haye been observed i n some (38) systems . However the attack i s expected to vary no more than the or i e n t a t i o n of the exposed faces and therefore , pronounced prefer,rential attack leading to the creation of p i t s i s not expected for chemical processes. A chemical reaction such as Equation [12] involves molecular oxygen rather than water molecules and i s completely chemical, i . e . , i t i s not associated with separate.anodic or cathodic processes. However, i t may, i n f a c t , takei.place i n several steps on the mineral surface, i . e . CuS + 0 2 Cu"1"1" + S 0 2 = (ads) [14] S 0 2 = (ads) + 0 2 »^ S 0 4 = [15] The amount of copper i n so l u t i o n for the runs i n which no p r e c i p i t a t e was formed remained approximately constant as seen i n Table 5. This indicates that as Reaction [12] increases Reaction [7] must decrease correspondingly. I t i s also seen that with decreasing a c i d i t y the slopes of the i n i t i a l sections of the oxidation curves are lowered (Figure 7). Thus i t seems that both the c h a l c o c i t e - c o v e l l i t e transformation and the subsequent c o v e l l i t e leaching without sulphate formation (reactions that consume acid) diminish i n rate with decreasing a c i d i t y . The slopes of the curves f o r leaching at 0.25M and'O.lM acid indicate that sulphate formation - 53 -becomes important a f t e r some time has elapsed, perhaps when the sulphur producing reaction i s nearly stopped because of r i s i n g pH. o r l i q u i d sulphur s t i f l i n g . The oxygen consumption curves for 6 hours of leaching chalcopyrite i n 4M, 1M and 0.25M acid (Figure 8) are comparable .to the . curves of Figure 7 for the f i r s t 2 1/2 hours of leaching c h a l c o c i t e . The 4M curve shows the lowest f i n a l oxidation rate i n d i c a t i n g the highest degree of s t i f l i n g by s u l f u r . The curve for leaching i n 1M acid has the greatest f i n a l slope probably because i t has the highest sulphate formation (Table 3). It may be that as a r e s u l t of the slow leaching rate of chalcopyrite the curve at 0.25M acid may be the lowest of the three curves and show a moderate amount of sulphate i n s o l u t i o n a f t e r s i x hours but a longer leaching time would lead to more rapid oxygen consumption and a much higher percentage production of sulphate. See, for example, the c h a l c o c i t e curve (Figure 7) for the same a c i d i t y . 8. Oxygen Pressure V a r i a t i o n From the oxygen consumption curves of Figure 12 and the oxidation rates plotted against oxygen pressure on Figure 13, i t i s suggested that the rate of both the c h a l c o c i t e - c o v e l l i t e transformation and the c o v e l l i t e leaching reaction are dependent on the pressures of oxygen i n the r e a c t i o n v e s s e l . The experiments are inadaquate for any r e l a t i o n s h i p between oxygen pressure and reaction rate to be postulated; however, i f a Langmuir adsorption isotherm applies, i t seems that at below the pressure of 80 p s i the adsorption of oxygen on the mineral i s incomplete and that the reaction rates would be of f i r s t order i n oxygen pressure. The dependence of leaching rates on the pressure of oxygen as found here j u s t i f i e s the p l o t t i n g of log (rate/press 1 O2) versus 1/T for Arrhenius p l o t s . - 54 -9. Temperature V a r i a t i o n The a c t i v a t i o n energy obtained by p l o t t i n g log (Rate/PC^) versus 1/T i s approximately 1.8 Kcal/mole for the primary stage of leaching chalcocite (Figure 12). This low a c t i v a t i o n energy indicates that the reaction rate i s probably c o n t r o l l e d by d i f f u s i o n , either as part.of the cathodic reaction, i e . , d i f f u s i o n of oxygen or protons through a l i q u i d boundary layer, or the d i f f u s i o n of cuprous ions through the chalcocite l a t t i c e . Jost reports an a c t i v a t i o n energy for the d i f f u s i o n c a l / i (24) (32) of cuprous ions i n chalcocite as approximately 1 Kcal/mole ; another source assigns a value of about 5 Kcal/mole to t h i s The cathodic reaction i s s i m i l a r for both stages of c h a l c o c i t e leaching but may exhibit an e n t i r e l y d i f f e r e n t rate for each stage. I t i s reasonable to expect that the cathodic reaction rate i s proportional to and greatly enhanced by the a r r i v a l of cuprous ions at the surface of the mineral since the cuprous ions i n s o l u t i o n oxidize r a p i d l y to cupric and w i l l reduce adsorbed oxygen i n doing so. There i s evidence for the occurance of t h i s for the leaching of c h a l c o c i t e i n (12) f e r r i c solutions . When the c h a l c o c i t e - c o v e l l i t e transformation i s complete the cathodic reaction must take place on the c o v e l l i t e surface without the aid of cuprous ions and may occur at a rate slow enough to control the rate of the whole process. If the c h a l c o c i t e - c o v e l l i t e transformation i s rate c o n t r o l l e d by d i f f u s i o n through a l i q u i d boundary layer, the transformation rate should increase with a g i t a t i o n . - 55 -The a c t i v a t i o n energy of 11.4 Kcal/mole for the secondary stage of leaching (Figure 13) can be considered to be that for the d i s s o l u t i o n of c o v e l l i t e . This value i s comparable with that of 11.7 Kcal/mole reported elsewhere for the a c t i v a t i o n energy for the leaching (14) of c o v e l l i t e . These values are greater than can be expected for a l i q u i d phase d i f f u s i o n c o n t r o l l e d process. When consideration of the dependence of rate on oxygen pressure.is taken into account, t h i s suggests that the rates of d i s s o l u t i o n of transformed c o v e l l i t e are con-t r o l l e d by heterogeneous processes at the surface of the mineral. Such a process could be a part of the cathodic reaction; for example, the adsorption of oxygen on the surface of the mineral, the transfer of electrons to the oxygen, the combination with protons or the desorption of reaction products. A rate c o n t r o l l i n g step at the' surface would i n f e r that the rate of mineral d i s s o c i a t i o n would be proportional to the i n i t i a l surface area of the mineral, a conclusion reached by other i n v e s t i g a t i o n s ^ ^ It could be that both stages of the leaching of c h a l c o c i t e are rate c o n t r o l l e d by the cathodic Reaction [8] although the apparent a c t i v a t i o n energies found are widely d i f f e r e n t . In the i n i t i a l stage i t may be that the c h a l c o c i t e - c o v e l l i t e transformation i s rapid enough to indue the cathodic reaction to speed up to the point were t h i s r e a c t i o n could be co n t r o l l e d by d i f f u s i o n , (Both reactions must occur to the same degree). but the subsequent c o v e l l i t e d i s s o l u t i o n may be slow enough for d i f f u s i o n to keep up, making a chemical step the slowest i n the whole process. - 56 -When part of the residue of the run at 140°C was washed i n carbon disulphide and returned to the autoclave for further leaching, i t s i n i t i a l oxidation rate was greater than the f i n a l rate of the run from which i t came. This i s further evidence that molten sulphur coating the mineral surface i s responsible f o r i n h i b i t i n g leaching at temperatures above the melting point of sulphur. Figure 14 shows no evidence for more than one step i n the oxidation of chalcopyrite. The runs at 125°C and 140°C show s t i f l i n g of the reaction a f t e r a time of leaching. 10. The C a t a l y t i c E f f e c t of Cupric Ions Figure 15 shows that there i s probably no c a t a l y t i c , e f f e c t on the leaching rate o f c k a l c o c i t e by the addition of cupric ions, but a large increase i n the oxidation rate of chalcopyrite e s p e c i a l l y i n the intermediate time period of leaching i s observed. The increased oxygen consumption rate of chalcopyrite due to the i n i t i a l a ddition of cupric s a l t s , exceeds the amount required f o r complete oxidation of i r o n (9 33 34) i n s o l u t i o n to f e r r i c an oxidation catalyzed by cupric ions ' ' It i s tempting to r e l a t e the c a t a l y t i c e f f e c t of cupric ions on chalcopyrite oxidation to the well established f e r r o u s - f e r r i c oxidation i n s o l u t i o n . However i t i s d i f f i c u l t to associate the depletion of ferrous ions from s o l u t i o n with increased mineral oxidation, and the f e r r i c content of the s o l u t i o n i s kept constant by hy d r o l y s i s , thus preventing i t also from a f f e c t i n g the leaching rate. A better explanation i s probably surface s u b s t i t u t i o n of i r o n by copper i n the mineral l a t t i c e , - 57 -with or without s u p e r f i c i a l transformation to c o v e l l i t e , and the mineral surface may then develop enhanced leaching rates more comparable to c o v e l l i t e . Substitution of ir o n by copper on ir o n sulphide minerals M r o £ \ i s w e ll known i n f l o t a t i o n p r a c t i c e ' The dif f e r e n c e i n leaching rates i s expected to decrease a f t e r some period of leaching because copper from the mineral i s a v a i l a b l e for c a t a l y s i s as leaching progresses. 11. Leaching Chalcopyrite The a c t i v a t i o n energies for chalcopyrite consumed and sulphate produced are 11.0 Kcal/mole and 16.0 Kcal/mole r e s p e c t i v e l y as found from the Arrhenius plots of Figures 16 and 17. The points on the graph for chalcopyrite leached at 120°C, 125°C and 130°C are ignored because these are believed to be too low as a r e s u l t of l i q u i d sulphur s t i f l i n g . Evidence for the supposition that the rate c o n t r o l l i n g mechanism i s the same for the leaching of ch a l c o c i t e and chalcopyrite may be construed from the s i m i l a r i t y of t h e i r a c t i v a t i o n energies (11.4 and 11.0 Kcal/mole). Their v a s t l y d i f f e r e n t leaching rates i n t h i s case i n d i c a t e d i f f e r e n t cathodic area f r a c t i o n s for these two minerals, and therefore, the suggestion that they possess l i k e rate c o n t r o l l i n g steps leads to the proposition that t h e i r cathodic reactions are s i m i l a r and the slowest step i n the process. - 58 -It was pointed out before that i t i s thermodynamically favourable for chalcopyrite to convert of c o v e l l i t e during leaching, according to the reaction CuFeS 2 =>. CuS + F e ^ + S° + 2e. [16] but because of the fast e r leaching rate of c o v e l l i t e i t does not appear i n observable amounts. However microareas of c o v e l l i t e may be formed temporarily and cover a c e r t a i n f r a c t i o n of the mineral surface i n dynamic equilibrium. This c o v e l l i t e may not be massive enough to form a separate phase. Three reactions may occur on the c o v e l l i t e , i e . , CuS + 20 2 ^ C u * * + S 0 4 = [12] CuS ^Cu"1"*" + S° + 2e [7] 4H + + 0 2 + 4e >.2H20 [8] The chemical Reaction [12] and the electrochemical Reaction [7] destroy c o v e l l i t e , whereas, Reaction [8] i s the cathodic complement to Reactions [16] and [7] as well as to the reaction CuFeS 2 . ^ Cut* + Fe"*^ + 2S° + 4e [17] which breaks chalcopyrite into i t s elements. If the cathodic Reaction [8] i s greatly enhanced on the c o v e l l i t e surface, the c o v e l l i t e may constitute a small f r a c t i o n of the t o t a l mineral surface but be responsible for the bulk of the oxygen reduction. When t h i s reaction i s rate determining, t h i s theory, therefore, leads to equal a c t i v a t i o n energies for chalcopyrite and the other three minerals, since the reaction occurs mainly on a c o v e l l i t e surface i n a l l cases. - 59 -The temporary c o v e l l i t e areas may be cathodic during t h e i r l i f e , and when they have disappeared the chalcopyrite surface may again be anodic. The diffe r e n c e i n a c t i v a t i o n energies for the leaching of chalcopyrite between 11.0 Kcal/mole found here and 23 Kcal/mole found by (14) Warren may p a r t i a l l y be due to the following f a c t o r s : A d i f f e r e n t temperature range was employed for the two studies—Warren's work being done i n a generally higher temperature range. Also, the copper i n s o l u t i o n was measured as the rate of rea c t i o n by Warren; i n t h i s work the weight loss of the mineral was used. The d i f f e r e n c e i n a c t i v a t i o n energies may be explained by the c h a l c o p y r i t e - c o v e l l i t e proposition by reasoning that i f the cathodic reaction has an a c t i v a t i o n energy of 23 Kcal/mole on chalcopyrite and 11 on c o v e l l i t e , and i f the net area of c o v e l l i t e on the chalcopyrite shrinks or disappears with increasing temperature, then a r i s e i n temperature would force the main f r a c t i o n of the cathodic reaction onto the chalcopyrite and an a c t i v a t i o n energy of 23 Kcal/mole would then be observed at higher temperatures. At 120°C the wide separation between the two points of chalcopyrite leached (Figure 16) may indic a t e that s t i f l i n g by sulphur may or may not occur at t h i s temperature. This i s reasonable because the melting point of sulphur i s li9°C and the temperature c o n t r o l l e r caused temperature cycles of ±1°C during i t s operation. - 60 -Because the a c t i v a t i o n energy for sulphate production i s somewhat larger than that for chalcopyrite leaching an increase i n the percentage of sulphate formed would be expected and approximate c o r r e l a t i o n with t h i s can be seen from Table 9. That the production of sulphate i s not i n h i b i t e d above the melting point of sulphur may be deduced from the fac t that the points above that temperature do not f a l l below the l i n e on the Arrhenius plot (Figure 17) and also that the scatter for these points i s less than that for chalcopyrite leached. Thus the percentages of sulphate produced for temperatures above 120°C increases because the reaction producing sulphate goes on uninhibited whereas the o v e r a l l r eaction producing sulphur i s hindered. This i s evidence for the occurrence of the two reactions at dif f e r e n t , surface regions for chalcopyrite leaching, as was also stated for chalc o c i t e leaching. If the reaction y i e l d i n g sulphate i s chemical i n nature i t i s then expected to occur,more or less uniformly on the surface of the mineral. Such a reaction may be the one already described, i e . , CuS" + 20 2 ^ Cu"1"1" + S 0 4 = [12] which would lead to the same sulphate producing reaction for a l l four minerals. A chemical reaction for sulphate formation d i r e c t l y on chalcopyrite i s the following: CuFeS 2 + 40 2 ^.Cu4"4" + Fe4"*" + 2S0 4~ [18] A l t e r n a t i v e l y a chemical reaction may occur which produces both sulphur and sulphate, i e . , CuFeS 2 +5/2" 0 2 + 2H + - H > C U + ++ F e ^ + S0 4 + S° + H 2 0 [19] - 61 -Because the rate of sulphur production i s reasonably f a s t , the reaction may be p a r t l y chemical and p a r t l y electrochemical. This would require less stringent atomic configurations :during the formation of the sulphate r a d i c a l since a l l steps of the reaction mechanism need not occur at one l o c a t i o n . If sulphate i s formed by an electrochemical process, the t o t a l reaction may be s p l i t into i t s cathodic and anodic parts, i e . , In t h i s process the sulphur i s required to break up the water molecules i n o x i d i z i n g to sulphate. Because of the s t a b i l i t y of the water molecule and the high c o n f i g u r a t i o n a l entropy required for reaction withw.ater, i t i s more l i k e l y for the sulphur to oxidize with the oxygen dissolved i n the so l u t i o n . This makes the chemical Reactions [12] or [18] more l i k e l y . and chalcopyrite i t i s suggested that the r o l e of ir o n i n these two minerals i s not merely chemical but involves the structure of the mineral so as to produce two widely d i f f e r e n t leaching c h a r a c t e r i s t i c s . In t h i s respect the part played by i r o n i n th i s case i s p a r a l l e l to that i n p y r i t e [8] [20] From the v a s t l y d i f f e r e n t leaching rates observed for bornite and p y r r h o t i t e (37) - minerals which display two d i f f e r e n t c h a r a c t e r i s t i c s i n ^ p i t e of being composed of the same elements. - 62 -1. The order of leaching rates for copper minerals from the slowest to the f a s t e s t was found to be i n the order, chalcopyrite, c o v e l l i t e , c h a l c o c i t e , and bornite. 2. The leaching of c h a l c o c i t e and bornite can be divided into two steps: f i r s t , the transformation to c o v e l l i t e , and second, the d i s s o l u t i o n as c o v e l l i t e . No transformations occur i n the leaching of c o v e l l i t e or chalcopyrite. 3. The major portion of the leaching of c o v e l l i t e i s by non-uniform attack on i t s surface r e s u l t i n g i n the creation of p i t s . 4. The production of sulphate and elemental sulphur as products of leaching occur by two d i s t i n c t reactions. Evidence indicates that the reaction y i e l d i n g elemental sulphur i s electrochemical i n nature occuring at l o c a l i z e d areas on the mineral and that the reaction y i e l d i n g sulphate i s chemical i n nature and therefore may occur uniformly on the surface of the mineral. 5. The a c t i v a t i o n energy for the f i r s t stage of the leaching of c h a l c o c i t e was found to be 1.8 Kcal/mole; the rate was believed to be th^ d i f f u s i o n controlled,either in the l i q u i d phase, or by cuprous d i f f u s i o n i n the mineral. *The second stage of chal c o c i t e leaching was thought to be cont r o l l e d by a step i n the cathodic reaction mechanism, with an a c t i v a t i o n energy of 11.4 Kcal/mole. For the leaching of chalcopyrite and the accompanying production of sulphate the a c t i v a t i o n energies were found to be 11.0 and 16 Kcal/mole. \ CONCLUSIONS - 63 -6. For the leaching of c h a l c o c i t e , the percentage of sulphur appearing as sulphate increased with decreasing a c i d i t y , being about 14% i n 4M and 71% i n 0.5M a c i d . A s i m i l a r e f f e c t i s expected for the other three minerals. 7. The presence of cupric ions i n s o l u t i o n catalyzes the leaching of chalcopyrite but t h e i r e f f e c t i s not detected i n c h a l c o c i t e . 8. The reaction seems to be about f i r s t order i n oxygen pressure at lower, pressures, although an adsorption isotherm form of dependence i s not,excluded. Suggestions for Future Work -This study has uncovered several areas of work which may be u s e f u l l y followed. 1.- It should be determined whether sulphate i s formed by an electrochemical or a chemical process. This could be done by using oxygen-18 as the reactant gas. For example for c o v e l l i t e the chemical reaction CuS + 20 2 »- S 0 4 = + Cu"1"*" would include only oxygen-18 i n the sulphate, whereas, the electrochemical reactions 8H + + 202.+ 8e" 4H20 CuS + 4H 20—Cu"!" 4" + S 0 4 = + 8H + + 8e~ would produce sulphate containing l i t t l e oxygen-18. - 64 -2. To d e t e r m i n e w h e t h e r s u l p h a t e i s formed f r o m e l e m e n t a l s u l p h u r o r f r o m s u l p h u r o r i g i n a t i n g f r o m t h e m i n e r a l r a d i o a c t i v e s u l p h u r -35 c o u l d be added i n i t i a l l y w i t h t h e m i n e r a l and t h e e x t e n t o f i t s o x i d a t i o n to s u l p h a t e d u r i n g t h e r u n may be n o t e d . 3 . T e s t s c a n be c a r r i e d o u t i n w h i c h an e l e c t r i c a l p o t e n t i a l i s imposed on p o l i s h e d , m a s s i v e s p e c i m e n s o f t h e m i n e r a l d u r i n g l e a c h i n g . The e f f e c t o f t h i s on t h e l e a c h i n g r a t e may e l u c i d a t e t h e r e a c t i o n m e c h a n i s m . 4 . E x p e r i m e n t s s h o u l d be c a r r i e d o u t i n w h i c h t h e e f f e c t o f a r i s i n g pH v a l u e i s d e t e r m i n e d . The e x p e r i m e n t s d o n e , w i t h low a c i d i t y and e x t e n s i v e l e a c h i n g underwent a pH change o f s e v e r a l u n i t s d u r i n g t h e l e n g t h o f t h e r u n . The s u l p h u r i c a c i d - s u l p h a t e s a l t s y s t e m c a n be used t o b u f f e r t h e s o l u t i o n a t low pH v a l u e s . 5. Any e f f e c t o f s u l p h a t e i n the s o l u t i o n on t h e l e a c h i n g r a t e o f m i n e r a l s o r on t h e r e s u l t i n g d e p o r t m e n t o f s u l p h u r c o u l d be c h e c k e d by e x p e r i m e n t s i n w h i c h s u l p h a t e i s added t o t h e s o l u t i o n i n i t i a l l y o r removed f r o m s o l u t i o n as i t forms by an i n i t i a l b a r i u m c h l o r i d e a d d i t i o n . 6. The s u g g e s t i o n t h a t c h a l c o p y r i t e forms c o v e l l i t e d u r i n g l e a c h i n g and t h e e f f e c t o f t h i s on t h e s u l p h a t e and c a t h o d i c r e a c t i o n s m e r i t s i n v e s t i g a t i o n . I t may be p o s s i b l e to d e t e c t s u c h c o v e l l i t e on c h a l c o p y r i t e by low e n e r g y e l e c t r o n d i f f r a c t i o n . By k n o w i n g t h e r a t e e q u a t i o n s f o r c o v e l l i t e f o r m a t i o n on c h a l c o p y r i t e and f o r i t s d i s s o l u t i o n , a m a t h e m a t i c a l e x p r e s s i o n f o r t h e f r a c t i o n o f s u r f a c e c o v e r e d by c o v e l l i t e c o u l d be c a l c u l a t e d . - 6 5 -R E F E R E N C E S 1 . L i d d e l l D . M . , " H a n d b o o k o f N o n f e r r o u s M e t a l l u r g y - R e c o v e r y o f t h e M e t a l s " M c G r a w - H i l l ( 1 9 4 5 ) . 2 . F o r w a r d F . A . , T r a n s . C a n . I n s t . M i n . M e t . 5 £ , 3 6 3 ( 1 9 5 3 ) . 3 . V i z s o l y i A . , V e l t m a n H . , a n d F o r w a r d F . A . , M e t . S o c . C o n f . 24, 326 (1963). 4 . T u r c h a n i n o v V . V . , a n d S i n a k e v i c h A . S . , N a u c h n . T r . , I r k u t s k u G o s . N a u c h n . - I s s l e d . I n s t . R e d k i k h M e t a l . 1 1 , 3 0 1 ( 1 9 6 3 ) . 5 . D o w n e s K . W . , a n d B r u c e R . W . , T r a n s . C a n . I n s t . M i n . M e t . 5 8 , 7 7 ( 1 9 5 5 ) 6 . V i z s o l y i A . , V e l t m a n , H . , a n d F o r w a r d F . A . , T r a n s ; A I M E 2 2 7 , 2 1 5 ( 1 9 6 3 ) . 7 . S h e r m a n M . I . , a n d S t r i c k l a n d J . D . H . , J . M e t a l s _ 9 , 7 9 5 ( 1 9 5 7 ) . 8 . F o r w a r d F . A . , a n d V e l t m a n . H . , J . M e t a l s 1 1 , 8 3 6 ( 1 9 5 9 ) . 9 . M c K a y D . R . , a n d H a l p e r n . J . , T r a n s . A I M E 2 1 2 , 3 0 1 ( 1 9 5 8 ) . 1 0 . W a r r e n I . H . , A u s t . J . o f A p p l . S c i . _ 7 , 3 4 6 ( 1 9 5 6 ) . 1 1 . J a c k s o n K . J . , a n d S t r i c k l a n d J . D . H . , T r a n s . A I M E 2 1 2 , 3 7 3 (1958). 1 2 . S u l l i v a n J . D . , T r a n s . A I M E 1 0 6 , 5 1 5 ( 1 9 3 3 ) . 1 3 . T h o m a s G . , a n d I n g r a h a m T . R . ( 1 9 6 7 ) u n p u b l i s h e d . 1 4 . W a r r e n I . H . , A u s t . J . A p p l . S c i . 9_, 3 6 ( 1 9 5 8 ) . 1 5 . V e l t m a n H . , P e l l e g r i n i S . , a n d M a c k i w V . N . , " D i r e c t A c i d P r e s s u r e L e a c h i n g o f C h a l c o c i t e C o n c e n t r a t e s " , p a p e r p r e s e n t e d a t 9 5 t h A n n u a l M e e t i n g o f A I M E , 1 9 6 6 . 1 6 . V i z s o H y i A . , V e l t m a n H . , W a r r e n I . H . , a n d M a c k i w V . N . , " C o p p e r a n d E l e m e n t a l S u l p h u r f r o m C h a l c o p y r i t e b y P r e s s u r e L e a c h i n g " , p a p e r p r e s e n t e d a t 9 6 t h A n n u a l M e e t i n g o f A I M E , 1 9 6 7 . 1 7 . W o o d c o c k J . T . , r e v i e w a r t i c l e , A u s t . I n s t . M i n . M e t . P r o c . N o . 1 9 8 , 47 (1961). 1 8 . G a u d i n A . M . , a n d D i c k e G . , E c o n . G e o l . 3 4 , 4 9 ( 1 9 3 9 ) . 19. D a n a E . S . , " A T e x t b o o k o f M i n e r o l o g y " , 4 t h E d . , J . W i l e y . 2 0 . S m i t h O . C . , " I d e n t i f i c a t i o n a n d Q u a l i t a t i v e C h e m i c a l A n a l y s i s o f M i n e r a l s " , 2 n d E d . , D . V a n N o s t r a n d C o . - 66 -21. Vogel I., "A Textbook of Quantitative Inorganic Analysis - Theory and Prac t i c e " , 2nd Ed., Longmans, Green and Co., pp. 401, 407, 518. 22. Ingraham T.R., private communications 23. Wagner J.B., and Wagner C , J . Chem. Phys. .26, 1602 (1957). 24. Pavlyuchenko M.M., Pokrovskii I.I., and Tikhonov A.S. ,j Dbkl. Akad. Nauk. Belorussk. SSR .9 (4) 235 (1965). 25. Latimer W.M., "The Oxidation States of the Elements and Their Potentials i n Aqueous Solutions", 2nd Ed., Pr e n t i c e - H a l l , Inc. (1952). 26. Majima H., and Peters E., private communication. 27. Garrels R.M., and Ch r i s t C.L., "Solutions, Minerals and Equilibrium", Harper and Row (1965). 28. Takeuchi T., and Nambu.M., Ganseki-kobutsu-koisho Gakkai-shi 3^ 6, 33 (1952). 29. Edwards A.B., "Textures of the Ore Minerals", 1st Ed., Aust. Inst. Min. Met.: Melbourne (1947). 30. Golomzik A.I., Izv. Vysshikh Uchebn Zavedenii Tsvetn. Met. 7 (2) 47 (1964). 31. Majima H., and Peters E., "Oxidation Rates of S u l f i d e Minerals by Aqueous Oxidation at Elevated Temperatures", paper presented at 95th Annual Meeting of AIME, 1966. 32. Jost W., " D i f f u s i o n " , p. 168, Academic Press: New York (1952). 33. Huffman R.E., Davidson N., J . Amer. Chem. Soc. 7_8, 4836 (1956). 34. George P., J . Chem. Soc. (London) 280, 4349 (1954). 35. Bushell C.H.G., Krauss C.J., and Brown G., Trans. Can. Inst. Min. Met. 65, 185 (1962). 36. Bushell C.H.G., Krauss C.J., and Brown G., Trans. Can. Inst. Min. Met. 64, 177 (1961). 37. Hahne H., Doctoral D i s s e r t a t i o n , Technishen U n i v e r s i t a t B e r l i n (1964). 38. L a i d l e r K.J., "Chemical K i n e t i c s " , 2nd Ed., p. 304, McGraw H i l l (1965). - 67 -APPENDIX X-Ray D i f f r a c t i o n Patterns Butte C o v e l l i t e CuS Index d A" I / I m d I / I m 3 .21 30 3 .220 28 3 .05 70 3 .048 67 2 .80 90 2 .813 100 2 .72 100 2 .724 56 2 .32 20 2 ,317 10 2 .04 20 2 .043 7 1 .96 10 1 .902 25 1 .90 70 1 .896 75 1 .73 50 1 .735 34 1 .55 40 1 .556 37 Chalcopyrite, Japan CuFeS^ Index d I/Ln d I / I m 3.02 100 3 .03 100 2 .63 5 2 .63 5 1 .87 20 1 .865 40 1 .86 40 1 .854 80 1 .59 20 1 .591 60 - 68 -CuS Index d ^ I t a 3.30 30 3.32 30 3.16 50 3.16 30 3.02 10 3.048 67 2.94 20 2.81 20 2.813 100 2.74 50 2.75 30 2.724 56 2.51 30 2.52 30 1.94 100 1.94 100 1.65 5 1.65 10 Butte Bornite Cu^eS^ Index d d I/lm Butte Chalcocite Cu 2S Index d ^ i m d ^Im 3.40 10 3.412 25 3.335 40 3.330 . 50 3.16 10 3.181 75 3.07 10 3.051 75 2.95 100 2.950 75 2.85 10 2.864 50 2.65 10 2.665 75 2.55 10 2.560 50 2.51 10 2.528 75 2.39 50 2.401 100 2.398 100 2.32 10 2.328 50 2.02 10 2.206 50 1.96 60 1.972 100 1.94 30 1.94 100 1.87 70 1.879 100 1.80 30 1.795 50 - 69 -B o r n i t e l e a c h e d 30 m i n u t e s C ^ S Index CuS Index d ^ I m d d 4 . 2 3 40 4 .27 50 3 .32 10 3 .33 50 3 .20 20 2 .181 75 3 .220 28 3 .08 20 3 .04 100 3 .048 67 2 .93 50 2 .930 75 2 .80 50 2 .813 100 2 .72 40 2 .724 75 2.724 56 1 .89 100 1 .879 100 1 .896 75 - 1 . 8 5 1 .875 50 1 .79 40 1 .795 50 B o r n i t e l e a c h e d 6 h o u r s CuS Index Cu2S Index d d d ^ I m 4 . 2 5 10 4 .27 50 3 .32 80 3 .330 3 .311 . 50 75 3 .21 20 3 .220 28 3 .181 75 3 .04 100 3 .048 67 2 .93 60 2 .930 75 2 .81 30 2 .813 100 2 .72 30 2 .724 56 1 .90 30 2 .724 75 1 .908 75 1 .87 80 1 .879 1 .875 100 50 1 .73 10 1 .735 34 - 70 -C h a l c o c i t e l e a c h e d 30 m i n u t e s 1/ Im C u 2 S Index 1/ Im CuS Index d 1 / Im 3 .33 3 .20 3 .04 2 .93 2 .80 2 .73 2 .42 1 .89 1 .86 1 . 8 0 1 .74 1 .57 1 .55 1 .53 10 10 50 100 60 40 10 60 10 40 20 10 10 20 3 .330 3 .181 3 .051 2 .930 2 .822 2 .731 2 .401 1 .895 1 .879 1 .862 1 .795 50 75 75 75 50 75 100 50 100 25 50 3 .220 3 .048 2 .813 2 .734 1 .896 1 .735 1 .572 1 .556 28 67 100 56 75 34 15 37 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0093576/manifest

Comment

Related Items