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The influence of roasting temperature upon gold recovery from a refractory gold ore Carter, Ralph 1949

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L (5%ft7 Cxf  Cop  Is  •i  THE INFLUENCE OF ROASTING TEMPERATURE UPON GOLD RECOVERY FROM A REFRACTORY GOLD ORE.  A thesis submitted as p a r t i a l requirement f o r a Master of Applied Science Degree i n the Department of Mining and Metallurgy at the University of B r i t i s h  Columbia  Ralph Carter  University of B r i t i s h  A p r i l 1949  Columbia  i ABSTRACT SUMMARY  The r e s u l t s of t h i s i n v e s t i g a t i o n indicate that the gold i n the cyanide t a i l i n g s of refractory gold calcines i s occluded i n fused or r e c r y s t a l l i z e d i r o n oxide p a r t i c l e s formed during roasting.  These high  temperature p a r t i c l e s cannot be prevented by c o n t r o l l i n g the roasting atmosphere or by maintaining a low furnace temperature. The above conclusions have been reached following a microscopic examination of calcines produced at various furnace temperatures with different atmospheres, and a comparison with calcines produced by decomposing samples of completely sulphated concentrate under such conditions that the maximum p a r t i c l e temperature varied between 650 C and 1100 C. This comparison showed that the p a r t i c l e temperature during a normal roast i s several hundred degrees higher than the furnace temperature.  An  approximate r e l a t i o n s h i p between the two i s established f o r roasting the "refractory" a r s e n i c a l gold concentrate examined during t h i s i n v e s t i g a t i o n . The unavoidable high p a r t i c l e temperatures occurring i n furnace roasts are not upheld by theory since calculated r a d i a t i o n and convection losses exceed the heat generated.  Rate of reaction and rate of oxygen  d i f f u s i o n calculations, however, do explain the f a i l u r e of controlled atmospheres to give low p a r t i c l e temperature roasts at moderate furnace temperatures where the reaction i s not d i f f u s i o n controlled. Because high p a r t i c l e temperatures could not be prevented during any furnace roast, and because low p a r t i c l e temperatures are e s s e n t i a l f o r complete gold extraction by cyanidation, aqueous medium oxidation under high oxygen pressures was studied.  Gold recoveries from calcines produced  by t h i s means was over 98% compared with 80% from furnace c a l c i n e s .  It  ii was shown that a medium with a high hydroxyl concentration gives adequate and rapid oxidation of the sulphides by increasing the chemical d r i v i n g force by the oxygen-hydroxyl half c e l l p o t e n t i a l .  ACKNOWLEDGEMENT  The author wishes to express h i s appreciation for the f i n a n c i a l assistance received during the past summer from the Consolidated Mining and Smelting Company's Research Fund and for the National Research Council Bursary under which the i n v e s t i g a t i o n was continued t h i s winter.  In addition he wishes t o  thank Mr. F. A. Forward, Head of the Department of Mining and Metallurgy, and a l l members of the M e t a l l u r g i c a l Department f o r t h e i r h e l p f u l cooperation and suggestions.  INDEX  OBJECT OF INVESTIGATION  1  INTRODUCTION  2  EXPERIMENTAL WORK  4  I II III  D e s c r i p t i o n of Ore Apparatus  4 .  5  Experimental Work and R e s u l t s . . . . . . . .  6  (A)  Sulphide Oxidation  6  (a)  Furnace Oxidation  6  (i)  8  (ii) (b)  C o n t r o l l e d Atmosphere Roasts  11  Aqueous Media Oxidation..  12  (i)  D i s t i l l e d Con Concentrate  13  Raw Con Concentrate  15  Combined Aqueous Media Oxidation and Pressure Cyanidation..  16  (ii) (iii) (B)  A i r Atmosphere Roasts.............  Study of Calcines  16  (a)  Thermal Decomposition of Sulphated Con Conoentiate..  16  (b)  Gold N u c l e o l a t i o n Experiments  17  (c)  Microscopic Examination of Calcines  19 i •..  DISCUSSION OF RESULTS I  Furnace Oxidation  •  (A)  Experimental Results  (B)  C a l c u l a t i o n of Heat Flow From O x i d i z i n g Sulphide  II  C a l c u l a t i o n of Sulphide Oxidation Reaction Rates  Aqueous Media Oxidation  CONCLUSIONS  24 24  Particles (C)  24  26 28 31 34  APPENDIX I  II  III  35 Calculation of Heat Flow From Oxidizing Sulphide P a r t i c l e s . . . . (A)  Heat Flow by Radiation  (B)  Heat Flow by Convection  J  35 35 36  C a l c u l a t i o n of Sulphide Oxidation Rates  37  (A)  Oxidation Rate When Limited by Rate of Oxygen D i f f u s i o n . .  37  (B)  Oxidation Rate When Chemical Rate Controlled  41  Calculation of The Oxidation Potentials For The Aqueous Media Oxidation  42  (A)  Oxidation In The A l k a l i Medium  43  (B)  Oxidation In The Neutral Medium  44  THE  INFLUENCE OF ROASTING TEMPERATURE  UPON GOLD RECOVERY FROM A REFRACTORY GOLD ORE.  OBJECT OF INVESTIGATION  P y r i t i c and arsenical ores containing submicroscopic gold have long been regarded as refractory requiring s p e c i a l treatment to free the gold for s o l u t i o n by cyanide.  The sulphides  are usually roasted to free the gold and the calcines cyanided.  This investigation was under-  taken to determine why t h i s treatment seldom yields gold recoveries  of over 90%.  -1-  -2INTRODUCTION  This work i s intended t o explore some of the suggestions made 1 by Mr. Haszard  2 and Mr. Norwood  i n t h e i r i n v e s t i g a t i o n s of the r o a s t i n g  of a u r i f e r o u s f l o t a t i o n concentrates. To date general agreement on r e f r a c t o r y gold concentrates  and  t h e i r treatment stresses the f o l l o w i n g p o i n t s : 1.  Gold occurs i n elemental form i n a very f i n e d i v i s i o n , as 3 o l i d 3 4 s o l u t i o n i n the p y r i t e or arsenopyrite l a t t i c e ' .  2.  A low p a r t i c l e temperature during r o a s t i n g i s e s s e n t i a l i f the r e s u l t i n g c a l c i n e i s t o give high gold e x t r a c t i o n by c y a n i d a t i o n . While agreement e x i s t s on the above p o i n t s , no one has  maintained  a low p a r t i c l e temperature during a roast or measured the i n d i v i d u a l p a r t i c l e temperature.  Obtaining a low p a r t i c l e temperature o x i d a t i o n and  measuring the temperature, t h e r e f o r e , formed the p r i n c i p a l o b j e c t i v e s of this investigation. From a comparison of r o a s t i n g and cyanidation t e s t s on many r e f r a c t o r y gold ores the f o l l o w i n g points appeart 1.  The more f i n e l y d i v i d e d the gold the more r e f r a c t o r y the ore.  2.  The more r e f r a c t o r y the ore the greater the dependence of gold recovery upon p a r t i c l e temperature during the r o a s t .  1 Haszard, N. I . , The Condition of Refractory Gold i n Lake View and Star ( K a l g o o r l i e ) Ore. A u s t r a l a s i a n I n s t i t u t e of Mining and M e t a l l u r g y , No. 108, 1937. 2 Norwood, A., Roasting and Treatment of Auriferous F l o t a t i o n A u s t r a l a s i a n I n s t i t u t e of Mining and Metallurgy, No. 116, 1939.  Concentrates.  3 Edwards, A., Texture of the Ore Minerals. A u s t r a l a s i a n I n s t i t u t e of Mining and Metallurgy ( I n c . ) , 1947, P.93. 4 G i n z i r o K u r a n t i , Synthetic Study of Gold-Bearing P y r i t e . SueyokwaiS i , 10, 419-424, 1941. Chemical A b s t r a c t s , 35, 3563, 1941.  To date strong evidence indicates that the gold i s locked up by r e c r y s t a l l i z e d or fused i r o n oxide during the roast.  According to M e l l o r  5  two c r y s t a l l i n e forms of f e r r i c oxide e x i s t , an <* f e r r i c oxide stable at low temperatures,  and a / ? f e r r i c oxide stable at high temperatures.  The  transformation temperature f o r t h i s change has been reported as occurring between 678 C and 1320 C.  Mellor also reports ferromagnetic f e r r i c oxide  with the cubic l a t t i c e of magnetite transforming at 700 C into paramagnetic halmatite with a rhombohedral l a t t i c e .  In addition the  iron-oxygen  diagram shows FegOg transforming to magnetite and oxygen at 1450 C, the oxygen pressure required to prevent t h i s transformation being i n the hundreds of atmospheres^.  Opposed to these chemical changes i s the mechanical  locking of the gold by i n c i p i e n t fusion of the i r o n oxide.  Giving weight  to the suggestion that the gold i s occluded by FegOg decomposing to dense FejO^ i s the change of ochres during r o a s t i n g , where the color darkens with increasing roasting temperature from yellow through red to black, while the s p e c i f i c gravity gradually increases.  5 Mellor, J . M., A Comprehensive Treatise on Inorganic and Theoretical Chemistry, V o l . 13, Longmans Green and Co., 1928, p. 787. 6 White, J . , The Physical Chemistry and Steel I n s t i t u t e , 1944, p. 614.  of Open Hearth Slags, The Iron  -4-  EXPERIMENTAL WORK  To reduce the number of variables during t h i s i n v e s t i g a t i o n the following standard cyanidation procedure was 1.  adopted:  One hundred grams of the calcine was washed for one hour with 500  ml  of water i n an open f i v e - l i t r e wide-mouth j a r with ten small pebbles and revolving on r o l l s at 10 rpm.  The calcine was then washed  twice by decantation from two l i t r e s or by f i l t r a t i o n on a buchner filter. 2.  The washed calcine was returned to the o r i g i n a l j a r and cyanided on the above mentioned r o l l s f o r 24 hr with 500 ml of water, 0 . 5 of NaCN, and 1 . 0  I.  g of  g  CaO.  Description of Ore The material used as a basis f o r the tests throughout  this  investigation was a f l o t a t i o n concentrate produced by Con Gold Mines Limited having the following a n a l y s i s : Au  o  1.63  Fe  s  34.3/£  S  at  33.0/o  As Sb  =  17.1%  s  oz per ton  0.60%  Pb  s  Cu = Zn = Insoluble =  0. 0.30^  3.00%  Q.30%  The uniform structure of the concentrate, p r i n c i p a l l y arsenopyrite, i s shown i n Plate I .  A microscopic examination at 1800 magnification showed  no free gold. The low gold extraction by cyanidation of the raw  concentrate,  shown i n Table I, i ndicates the r e f r a c t i v e character of t h i s concentrate.  Plate I 375X Con Concentrate. Large, Uniform, Dense Arsenopyrite Grains with no Free Gold.  TABLE I No.  Treatment  C-l C-2  None 1.0 hr m i l l i n g i n laboratory pebble m i l l #  Gold Assay of T a i l i n g (oz per ton)  Gold Extraction  1.28 1.18  21.5% 31.1%  #Note: M i l l Size Pebble Charge M i l l Speed Ore Charge Water II.  e =  s s  =  7 i n . by 7 i n . 2765 g 74 rpm 1000 g 400 ml  Apparatus A l l furnace oxidation tests were made i n s i l i c a trays sealed i n  a f i r e clay muffle, as shown i n Figure I .  The whole assembly was placed  on two inch-high f i r e - c l a y blocks i n a 13 kw Walker e l e c t r i c furnace. Thus i t was possible to s h i e l d the surface of the charge from r a d i a t i o n  -6-  Steel Door  1  Pipe  to  Back  tt'XIZ*' Tray  of fur/joce to Ac/mir /5"/f  £>  /v/-e  Silica  Mvff/e  C/c/  l_<43l?estvs PacAiny  Spring  C/omp  Thermocouple  F I G U R E  T.  from the furnace elements, and to maintain any desired atmosphere around the charge. The aqueous media oxidation t e s t s were made i n the 1400 mechanically-agitated autoclavte shown i n Figure III.  ml  II.  Experimental Work and Results (A) Sulphide Oxidation (a) Furnace Oxidation Preliminary roasts showed that below 450 C Con concentrate  roasted very slowly and above 450 C, despite constant rabbling, the charge glowed and the calcine bed temperature  rose over 550 C.  Gold extraction  FIGURE IT  from these calcines dropped from a maximum of  f o r a calcine produced  i n a furnace held at 450 C f o r 11 hr, t o 16$ f o r a calcine produced i n a furnace where the temperature,  i n i t i a l l y 450 C, was raised 50 C every  30 min to a maximum of 750 C. Because of these high i n i t i a l roasting temperatures, the Con concentrate was heated under nitrogen at 600 C for one hour to d i s t i l l o f f the arsenic as a mixture of AsS and AS2S3.  This treatment removed  the arsenic as a possible cause of low gold extraction and increased the roasting rate of the concentrate at low temperatures s p e c i f i c surface area. furnace oxidation t e s t s .  by increasing i t s  This d i s t i l l e d concentrate was used f o r a l l futur The porous sulphide, F e S i i , containing 0.6$ #  to 0.8$ arsenic and 31.8$ to 33.1$ sulphur i s shown i n Plate II.  Plate II 375X D i s t i l l e d Con Concentrate. Shows Porous Structure of D i s t i l l e d Concentrate and One Dense P a r t i c l e of U n d i s t i l l e d Arsenopyrite. (i)  A i r Atmosphere Roasts  To determine the influence of temperature upon gold extraction the porous pyrrhotite from the d i s t i l l a t i o n was roasted i n the sealed muffle of Figure I with constant rabbling under an a i r atmosphere.  During  this series the i n i t i a l furnace temperature was fixed at 400 C, the lowest temperature at which roasting would begin, and the f i n a l temperature was varied from 450 C to 750 C.  During the roast the temperature was raised  50 C every 15 min to the maximum temperature, which was held u n t i l the evolution of sulphur dioxide ceased.  The r e s u l t s of these tests are  summarized on Graph I, and a few t y p i c a l results are given i n d e t a i l i n Table II.  IO0  \ I  I I  oI 400  1  1 300  1  1  Furnace  I  SOO  Temperature  C  L _ 700  :  Graph I E f f e c t of Furnace Temperature on Gold Recovery from Calcines Produced by Furnace Oxidation of D i s t i l l e d Con Concentrate. TABLE I I No.  C-44 C-53 C-54 C-47 C-37 C-34  Max. Temp.  450 500 550 600 750  C C C C C  Time at Max.Temp.  3Hr 1.5Hr 0.5Hr 0.5Hr 0.5Hr  Calcine Assay Sulphide Sulphate Sulphur Sulphur 32.3 5.48 4.68 1.23 1.98 0.07  % % % % % %  4.19 1.30 1.31 1.86 0.15  Gold Assay Gold of T a i l i n g s Extraction (oz per ton)  % % % % %  1.08 0.48 0.54 0.84 0.98 1.58  45.0 79.8 76.6 60.9 56.2 16.3  % % % % % %  Note; No. C-44 i s the Con concentrate a f t e r heating under nitrogen f o r one hour at 600 C. No. C-53 and C-54 showed no glowing p a r t i c l e s during r o a s t i n g . No. C-47 and C-37 glowed s l i g h t l y during roasting. No. C-34 glowed strongly during roasting. From Graph I the complete dependence of gold recovery upon roasting temperature i s obvious.  Because of the exponential dependence  -10of roasting rate on temperature, the true p a r t i c l e temperature i s much I.  higher than shown on the abscissa of Graph  To study the influence of roasting time upon gold recovery a series of roasts was made i n the sealed muffle under an a i r atmosphere at the optimum temperature of 400 C to 450 C found from the previous s e r i e s . A plot of recovery against roasting time i s shown i n Graph I I and a few III.  t y p i c a l results are given i n d e t a i l - i n Table  6  Roosting  a  12.  IO  Time  in  Hours  Graph I I E f f e c t of Roasting Time at Optimum Temperature upon Gold Recovery from Calcine Produced by Furnace Oxidation of D i s t i l l e d Con Concentrate. TABLE No.  Time  C-61 C-66-1 C-66-2 C-66-3 C-66-4 C-70  3 6 9 12 15  Hr Hr Hr Hr Hr  Calcine Assay Sulphide Sulphate Sulphur Sulphur  31.8 14.6 5.1 6.2 6.1 3.6  % % f. % % %  2.5 4.8 4.8 5.1 5.7  % % % % %  III Gold Assay of Tailings (oz per ton)  1.08 0.54 0.52 0.50 0.46 0.48  Gold Extraction  44.8 76.5 78.8 79.8 84.0 81.5  % % % % % %  Mote: During these roasts no glowing of the sulphide p a r t i c l e s occurred, ( i i ) Controlled Atmosphere Roasts Controlled atmosphere roasts were investigated to determine i f high p a r t i c l e temperature  by f l a s h i n g could be prevented by reducing the  oxygen p a r t i a l pressure over the sulphides.  These low oxygen atmospheres  were divided into two s e r i e s , one having a high and the other a low sulphur dioxide content, to determine the influence of the sulphating roast, which has been claimed by many metallurgists to increase gold recovery from refractory ores.  The r e s u l t s of t h i s study are given i n Table IV and  Graph I I I . TABLE IV No.  Max. Temp.  Time  Atmosphere ©2 SOg  C-50 C-52 C-49 C-63 C-62 C-64  450 450 450 450 450 450  2 4 7 4 7 9  14 14 14 14 14 14  C C C C C  c  Hr Hr Hr Hr Hr Hr  $ $ $ $ 7.4 $ 7.4 $ 7.4  $ $ $  Calcine Assay Gold Assay Gold Sulphide Sulphate, of T a i l i n g Extraction Sulphur Sulphur (02 per ton)  17.6 $ 4.4 $ 3.89$ 2.0 $ 1.10$ 0.80$-  2.75 3.97 5.03 4.8 5.52 5.15  $ $ $ $ $ $  0.52 0.50 0.46 0.54 0.54 0.52  77.0 $ 78.7 $ 82.6 $ 76.8 $ 76.0 $ 78.8 $  Note: During these roasts no glowing of the sulphide p a r t i c l e s occurred. These low oxygen roasts, as shown by Table IV, d i d not raise gold recovery above that given by a i r atmosphere roasts at the same furnace temperatures.  The sulphating roasts, s i m i l a r l y , did not improve recovery.  The small amount of i r o n sulphate formed during these roasts implies that either sulphation at 450 C i s very slow and incomplete, or that the p a r t i c l e temperature  during the roast was above the decomposition temperature  of  -12-  "1  fa  I  s /  /a  3  ay  <0  /a  SO  70  Gold  Recovery  X  J  Graph I I I Influence of Sulphate Sulphur Content Upon Gold Recovery from Calcines Produced by Furnace Oxidation of D i s t i l l e d Con Concentrate. i r o n sulphate.  This l a t t e r supposition finds support i n nature where both  f e r r i c and ferrous sulphates are formed i n atmospheres containing negligible amounts of sulphur dioxide.  Thus the f i n a l sulphate content of a calcine  under the conditions maintained during t h i s series of experiments appears, to be independent of the percent sulphur dioxide i n the atmosphere but to depend upon the maximum p a r t i c l e temperature during the oxidation.  I f the  sulphate sulphur i s a record of the maximum p a r t i c l e temperature, the extreme s e n s i t i v i t y of gold recovery to p a r t i c l e temperature i s shown by the large positive slope of Graph I I I . (b) Aqueous Media Oxidation Because of the i m p o s s i b i l i t y of preventing high p a r t i c l e  temperatures  during a furnace roast, aqueous medium oxidation under high oxygen pressures was  investigated. By such an oxidation the combination of low p a r t i c l e  -13"temperature and high reaction rate i s possible, because of the high oxygen pressure or driving force present and the high s p e c i f i c heat and intimate contact of the l i q u i d medium with the sulphide p a r t i c l e s .  Thus l o c a l  overheating of the sulphide p a r t i c l e s i s impossible. Aqueous media oxidation of sulphides under high oxygen pressures 7 89 has been reported by very few investigators ' ' . (i)  D i s t i l l e d Con Concentrate  Two hundred gram samples of d i s t i l l e d Con concentrate, C-92, which had been ground for one hour i n the laboratory pebble m i l l t o give C-101, were agitated i n 500 ml"of either a l k a l i or ammoniacal solution under high oxygen pressures i n the autoclave shown i n Figure I I . The r e s u l t i n g pulps were then f i l t e r e d , washed, and cyanided as usual.  The results of  t h i s work are given i n Table V. TABLE V No.  Aqueous Media Oxidation of D i s t i l l e d Con Concentrate Treatment i n Autoclave Sulphide Gold Assay Gold Temp. Time Oxygen Liquid Media Sulphur of T a i l i n g s Extraction •esaae . I n i t i a l Final In CaMnas (oz per ton) absolu'  C-92 C-101 C-97 90C 4Hr C-98 90C 4Hr C-105 150C 4Hr C-106 150C <lHr C-10715X <lHr C-108 150C lHr C-109150C 4Hr  47psi 47psi 115psi 115psi 115psi 115psi 115psi  13N NB4OH 13N NH4OH 13N NH4OH 12N NaOH 12N NaOH 12N NaOH .35N NaOH  NH4§H-Low  NH4OH-L0W  NH4OH-O.3N NaOH-2.5N NaOH-1.7N NaOH-0.2N CaO-e.04N  33.2 33.2 29.1 24.5 27.8 7.7 5.5 0.4 27.6  % % % % % % % % %  1.08 .96 .28 .08 .90 .16 .08 .04 .90  44.5 49.9 83.8 95.5 53.3 92.4 96.2 98.0 52.9  % % % % % % % % %  7 Bognar, A., Humid Oxidation of Sulphide Ores Under High Pressure, Magyar Memok Esiteszegylet Kozl'dnye, 78, p.200-4, 1944. Chemical Abstracts,7340 , 1947. a  8 Tronev, V., and Bondin, S., Oxidation of Zinc Sulphide and Transference of Zinc into Aqueous or A l k a l i Solution Under A i r Pressure. Compt.Rend,Acad. S c i . , U.R.S.S., 23,541-3,1939. 9 Zvyagentsev, 0., and Tronev , V . , Oxidation of Copper Sulphide and Solution of Copper i n Aqueous Solution Under A i r Pressure. Compt.Rend.Acad. S c i . , U.R.S.S., 23,537-40,1939.  Note: 1.  To a l l times l i s t e d i n Tables V and VI, except tests C-97 and C-98, must be added 0.5 hr to heat the autoclave, oxygen pressure 80 p s i .  2.  To a l l times l i s t e d i n Tables V and VI, except tests C-97 and C-98, must be added 0.5 hr to cool the autoclave, oxygen pressure increasing from 115 p s i to 170 p s i , as the vapor pressure of water decreased on cooling.  3.  Vapor pressure of 13 N aqua ammonia at 80 C  4.  Vapor pressure of 13 N aqua ammonia at 150 C - HO p s i .  5.  Vapor pressure of water at 150 C - 70 p s i .  6.  To C-109  was added 140 g of  =  20 p s i .  CaO.  As seen by the high gold recoveries l i s t e d i n Table V a low p a r t i c l e temperature  oxidation has been obtained.  Surprisingly, t e s t 3 C-97 and  C-98  gave high gold recoveries with low sulphur elimination, although the other calcines gave recoveries which depended upon the sulphur elimination as shown i n G"raph IV.  Why  tests C-97 and C-98 should l i e so f a r o f f t h i s graph  has not been determined.  O  3  IO  Sulphide  IS  20  S u/phur  23  JO  SS  /£  Graph IV Dependence of Gold Recovery upon Sulphide Sulphur Content f o r D i s t i l l e d Con Concentrate decomposed by Aqueous Media Oxidation.  -15 _ r  Apparently test C-105  d i d not oxidize because of the low oxygen  p a r t i a l pressure i n the autoclave resulting from the high vapor pressure of aqua  ammonia at 150  C.  The f i n a l ammonia concentrations have no significance because of vapor losses while emptying the hot autoclave. ( i i ) Raw  Con  Concentrate  To determine i f the dense arsenopyrite of the raw  concentrate  could be decomposed by aqueous medium oxidation, 200 gram samples of concentrate, after grinding two hours to give C-99-A, were treated as outlined i n table VI. TABLE VT Aqueous Media Oxidation of Ground Con Concentrate C-99-A No.  Treatment i n Autoclave Temp. Time Oxygen Media aDsoiute I n i t i a l  C-99-A C-99 80C C-100 80C C-110 150C  4Hr 4Hr 4Hr  47 p s i 47 p s i 115 p s i  13N NH4OH 13N NR^OH 12N NaOH  Sulphide Sulphuran Calcine  Final  Nt^OH-Low NI^OH-Low NaOH-ION  33.0 29.8 29.1 27.6  Gold Assay Gold of T a i l i n g s E x t r a c t i o n (oz per ton)  % % % %  1.16 1.06 0.96 0.38  These t e s t s indicate that aqueous media oxidation of raw concentrate i s much slower than f o r the d i s t i l l e d concentrate.  31.2 35.1 40.1 76.5  Con  This drop  i n reaction rate r e s u l t s from the small s p e c i f i c surface area of the raw concentrate compared to that of the d i s t i l l e d  concentrate.  In t h i s series of tests as f o r the aqueous media oxidation of d i s t i l l e d concentrate, one sample, C-110, gave high gold recovery with low sulphur e l i m i n a t i o n . In t h i s series the i r r e g u l a r i t y occurs i n the sodium hydroxide rather thanthe aqua  ammonia medium.  be responsible for the phenomenon.  Hence the medium cannot  % % % %  ( i i i ) Combined Aqueous Media Oxidation and Pressure Pressure cyanidation t e s t s of a Con c a l c i n e , C-79,  Cyanidation  at 21 p s i gauge  oxygen pressure showed the cyanidation rate to be increased by the oxygen pressure, with no improvement i n o v e r a l l gold extraction. t h i s work i s shown i n graph V.  8O1  The r e s u l t of  A few t e s t s were made, therefore, on  i  1  s  10 /s Cyan/ding  1  1  1  1  1  70  40  o  zo Time in  zs Hours  JO  JS  Graph V E f f e c t of Oxygen Pressure upon Cyanidation Rate for Con Calcine C-79 d i s t i l l e d and raw concentrate to determine i f the cyanidation rate could not be increased to equal the sulphide aqueous medium oxidation r a t e . the gold would be dissolved i n the autoclave as soon as i t was the sulphide.  Thus  freed from  Such treatment, however, dissolved only about h a l f the gold  that a standard 24 hr cyanidation dissolved from the same c a l c i n e s . Hence the cyanidation rate under 115 p s i of oxygen can only be approximately h a l f the oxidation rate of the sulphides. (B) Study of Calcines (a) Thermal Decomposition of Sulphated Con Concentrate To substantiate the supposition that the gold i s occluded  by  i r o n oxide and to determine the p a r t i c l e temperature during a normal roast,  -17completely sulphated d i s t i l l e d Con concentrate was decomposed at various temperatures.  Several kilograms of d i s t i l l e d concentrate was completely  sulphated with excess sulphuric a c i d , the excess acid being fumed o f f . This sulphated material was then decomposed by heating i n a tube furnace under both oxygen and nitrogen atmospheres and the r e s u l t i n g calcine cyanided.  In  t h i s way l o c a l overheating of i n d i v i d u a l p a r t i c l e s , which always occurs during an ordinary roast, was prevented, the decomposition of i r o n sulphate being an endothermic  reaction.  The r e s u l t s of t h i s work are given i n table V I I . TABLE VII  No.  Max. Time at Temp. Max.Temp.  Sulphated C-69ConQ> C-7I 650C 6 Hr: C-76 875C 0S5Hr : C-1023C00C 0.5Hr C-73 11QDC 0.5Hr  :  Calcine Assay Fe*^ Fe * S as SO4. +  21.8$ 42.0$ 51.5$ 0.5$ 51.0$ 1.1$ 50.8$ 1.8$  Gold Assay Gold of T a i l i n g E x t r a c t i o n (oz per ton)  22.8$ 8.0$ 0.9$ 0.2$ 0.2$  0.10 0.10 0.48 0.96  96.2$ 95.8$ 75.6$ 50.7$  Calcine Appearance  F l u f f y , B r i c k Red Darker than C-71 Dark Red Very Dense, Black  In these results the atmosphere i s not recorded since i t d i d not influence the gold recovery.  Once again the p a r t i c l e temperature  appears  as the factor governing gold extraction from c a l c i n e s . (b) Gold Nucleolation Experiments Because the state of d i v i s i o n of the gold i s believed responsible for the refractory q u a l i t i e s of gold ores, the refractory degree of the ore should be reduced by nucleolation of the gold p a r t i c l e s .  Following the  procedure of B u r g , such attempts were made by heating 125 g portions of 10  raw Con concentrate under nitrogen i n a sealed s t e e l bomb f o r several hours  10 Burg, G., Nutur des i n den Pvriten nicht Sichlbar enthaltenen Goldes, Z e s t s c h r i f t f u r praktische Geologie, February 1935.  -18at 600 C. The r e s u l t i n g calcine3ware then washed and cyanided.  The results  of t h i s work are l i s t e d i n Table VIII where they are compared with extractions from (a) raw concentrate,  (b) concentrate  laboratory pebble m i l l , and (c) d i s t i l l e d  ground for one hour i n the concentrate.  TABLE VIII No.  Treatment  Gold Assay of T a i l i n g Gold (oz per ton) Extraction  C-l C-2 C-92 C-87 C-88 C-89 C-90  None 1.0 hr grind i n laboratory pebble m i l l D i s t i l l e d Concentrate 16 hr at 600 C under ^ i n bomb 24 hr at 600 C under N i n bomb 32 hr at 600 C under Ng i n bomb 47 hr at 600 C under Ng i n bomb 2  1.28 1.18 1.08 1.34 1.36 1.36 1.38  21.5 31.1 45.0 35.1 34.6 34.2 33.3  % % % % % % %  From the above results i t i s evident that the described treatment has only s l i g h t l y increased gold extraction.  Hence, the gold could not have  been nucleolated, since nucleolated gold p a r t i c l e s could be expected to cyanide r e a d i l y . The s l i g h t increase i n gold recovery a f t e r heating must result from the decrease i n p a r t i c l e size shown by Plate I I I .  Plate III T a i l i n g 1.38 oz per ton 375 X Calcine Cyanidation Residue C-90. Con Concentrate Heated 47 Hr i n Sealed Bomb. Shows small grains of sulphide having l i t t l e porosity.  (c) Microscopic Examination of Calcines Three calcines produced by furnace oxidation of d i s t i l l e d  concentrate  to y i e l d high, intermediate, and low gold extractions were microscopically compared with samples of sulphated d i s t i l l e d concentrate thermally decomposed at 650 C, 875 C, 1000 C and 1100 C.  These three furnace calcines are shown  i n Plates IV to VI and the thermally-decomposed samples i n Plates VII to X. The combination  of low gold extraction and large i r o n oxide p a r t i c l e s at  high roasting temperatures i s c l e a r l y shown by these plates.  Plate IV T a i l i n g 0.44 oz per ton 1000X Calcine Cyanidation Residue C-49. D i s t i l l e d Concentrate Roasted at 400 C. The pores of much of the i r o n oxide are extremely small and the walls t h i n , although p a r t i a l fusion has occurred to a considerable extent.  Plate V T a i l i n g 0.84 oz per ton 1000X Calcine Cyanidation Residue C-47. D i s t i l l e d Concentrate Roasted at 500 C. The pore walls are thicker than f o r C-49 and some grains show almost complete fusion around t h e i r boundaries.  Plate VI T a i l i n g 1.58 oz per ton 1000X Calcine Residue C-34. D i s t i l l e d Concentrate Roasted at 750 C. The pores are much larger than f o r C-49 or C-47 while the walls are thick and continuous.  -21-  Plate VII T a i l i n g 0.10 oz per ton 1800X Calcine Cyanidation Residue C-71. Sulphated Concentrate decomposed at 650 C. Particles are so fine that they cannot be resolved at 1800 magnification.  Plate VIII T a i l i n g 0.10 oz per ton 1800X Calcine Cyanidation Residue C-76. Sulphated Concentrate decomposed at 875 C. P a r t i c l e s are very fine as i n C-71  Plate IX T a i l i n g 0.48 oz per ton 1000X Calcine Cyanidation Residue C-102, Sulphated Concentrate decomposed at 1000 C. Calcine p a r t i c l e s have grown probably due t o p a r t i a l fusion of i r o n oxide.  Plate X T a i l i n g 0.96 oz per t o n 675X Calcine Cyanidation Residue C-73 . Sulphated Concentrate decomposed at 1100 C. Calcine p a r t i c l e s have grown and increased i n density with increasing temperature.  Once the gold has been coated by the p a r t i a l fusion shown i n the previous plates i t cannot be freed by f i n e grinding nor by hydrochloric acid washes as shown i n table IX.  Throughout t h i s work the influence of f i n e TABLE IX  No.  Treatment  C-56 C-56-B C-56-C C-56-D C-56-E  None 0.5 hr 1.0 hr 1.0 hr 1.0 hr  grind i n pebble m i l l wash i n 1% HC1 wash i n Z% HC1 wash i n 5% HQ.  Gold Assay of t a i l i n g (oz per ton)  0.58 0.56 0.56 0.56 0.58  Gold Extraction  75.0 75.8 77.4 77.4 76.6  % % % % %  Note; C-56 was produced by roasting d i s t i l l e d concentrate at 450 C f o r three hours, to 1.29 % sulphide sulphur. grinding upon gold extraction decreased as the quality of the calcine improved. This suggests that i n a calcine y i e l d i n g a high gold recovery the unextracted gold i s locked i n p a r t i c l e s smaller than can be obtained by grinding.  Table III.Shows Relationship Between P a r t i c l e and Furnace Temperature During A Roast of D i s t i l l e d Con Concentrate Series I Calcines Produced by Direct Roasting of D i s t i l l e d Con Concentrate  Furnace  Particle  Temp.  Temp.  Series I I Calcines Produced by Decomposing Sulphated D i s t i l l e d Con Concentrate  750C  1000X C-34 Tailings 1.58 oz./T Evidence of greater fusion than i n C-73 =  1100C  675X C-73 T a i l i n e s s 0.96 oz./T Evidence of considerable fusion and p a r t i c l e growth.  500C  C-47 1000X Tailings 0.84 oz./T Evidence of greater fusion than i n C-49 but less than i n C-73  IP  =  1000C  C-102 1000X T a i l i n g s a 0.48 oz./T Evidence of fusion similar to C-49  400C  C-49 1000X Tailings = 0.44 oz./T Evidence of fusion similar to C-102  1  875C  C-76 1800X Tailings 0.10 oz./T No evidence of fusion of p a r t i c l e s :  -24. DISCUSSION OF RESULTS  I  Furnace Oxidation (A) Experimental Results This i n v e s t i g a t i o n has-clearly shown how gold extraction from the  Con concentrate, as for most r e f r a c t o r y sulphide gold ores, depends upon the maximum p a r t i c l e temperature during the roast. In Figure I I I , opposite, the comparison of calcines produced by furnace oxidation of d i s t i l l e d concentrate and produced by thermal of sulphated concentrate shows that high p a r t i c l e temperatures oxidation cannot be prevented.  during furnace  The arrangement of the calcines i n order of  decreasing p a r t i c l e temperatures, decomposition  decomposition  and the known p a r t i c l e temperatures  during  of the sulphated concentrate gives an estimate of the maximum  p a r t i c l e temperature during a furnace roast.  This r e l a t i o n s h i p i s found by  p l o t t i n g i n Graph VI. temperature against gold extraction f o r both the furnace calcines and the decomposed sulphated-concentrate c a l c i n e s .  The  temperature scale of the furnace calcines i s then adjusted to coincide with the plot for the decomposed sulphated concentrate.  From the two temperature  scales Graph VII i s drawn r e l a t i n g the maximum p a r t i c l e temperature to the furnace temperature.  These graphs are approximate but they provide an  estimate of the p a r t i c l e temperature at any furnace temperature f o r the d i s t i l l e d Con concentrate roasted under the conditions previously described. The f a i l u r e of atmosphere controlled roasts to increase the gold recovery are surprising i n view of the favorable results such roasts have given during other investigations.  The low oxidation temperature required  for high gold recovery from the Con concentrate i s the apparent for t h i s f a i l u r e .  explanation  Graph VI E f f e c t of Temperature upon T a i l i n g Assay f o r both Furnace Roasts of D i s t i l l e d Con Concentrate and Thermally Decomposed Sulphated Concentrate.  Graph VII A Plot of P a r t i c l e Temperature during a Furnace Roast against Furnace Temperature for the D i s t i l l e d Con Concentrate.  -26(B) Calculation of Heat Flow From Oxidizing Sulphide Particles The experimental results of this investigation indicate that increased gold extraction from the Con concentrate can be obtained only by oxidizing the sulphides at lower p a r t i c l e temperatures than has been possible by roasting.  When oxidation begins during a furnace roast, the p a r t i c l e  temperature cannot be controlled because the heat generated by the reaction cannot be dissipated from the sulphide p a r t i c l e s . Radiation calculated with the surroundings at 450 C, an emissivity of one, a p a r t i c l e diameter of  0.0073  cm  mesh) and an area to diameter  (200  r a t i o of 10 i s given by the following equation: Q  r  x  9.4  =  10  6  ^g J U  -  4  c a l per mole of FeS per sec.  0.274J  (a)  From calculations which w i l l be shown l a t e r the maximum l i k e l y roasting rate i s given by the equation: . , N_ r =/T (2.84 x l O ^ e  22500 25 TZ5  2  moles of FeS per,mole per sec  The heat evolved by the reaction: 7  2FeS + Z 0  2  =  Fe 0 2  3  + 2S0  2  i s given by: -3 2 AH  * - 146,229 + 6.84T - 15.212 x 10  T  c a l per mole of FeS  The combination of these expressions gives the following equation f o r the rate at which heat i s generated by the oxidation of FeS: QR -  f%  (l46,229 - 6.84T + 15.212 T ) / 2.835 x 10 ) e" c a l per mole 0/FeS per sec (b) These r e s u l t s , compiled i n Table X, indicate that at a l l p a r t i c l e temperatures 2  V  4  A  radiation w i l l dissipate the heat generated. TABLE X . ' QR (Heat Generated) Particle Temp, ( c a l per mole FeS per sec) T  2  500 700 1000 1500 2000  C C C C C  3.6 12.4 3.3 x 10* 6.6 x lCg 1.4 x 10 7  Q (Heat loss by Radiation) Q (Heat loss by Conyestkn) ( c a l per mole FeS per sec) ( c a l per mole FeSpersec) R  C  0.078 x 10 0.584 x l O 2.21 x 10? 2l:§Vl09  0.019 x 107 0.085 x 10  7 7  7  Q  <  2  2  x  1  q  7  0.501 x 107 0.763 x 1 0 7  -27Unfortunately the above results are invalidated by enclosing the roast i n a f i r e clay muffle.  Because the temperature gradient across the  muffle walls i s p r a c t i c a l l y zero, the heat flow from the roasting bed by radiation and subsequent conduction through the muffle walls w i l l be p r a c t i c a l l y zero.  Therefore, most of the radiated heat w i l l be r e f l e c t e d  back to the roasting bed. It might be argued that the assumption that a l l the sulphide p a r t i c l e s are free t o radiate heat i s i n v a l i d .  This d i f f i c u l t y i s overcome  by considering that those p a r t i c l e s which are not free to dissipate heat are not free to oxidize or generate heat since they are not receiving oxygen. Heat flow by convection to the atmosphere i s less than that by r a d i a t i o n since a i r i s a good insulator because of the large mean free path o f i t s molecules.  Hence each sulphide p a r t i c l e may be assumed to be  surrounded by an i n s u l a t i n g a i r blanket. Heat loss by convection from spheres i s given by Langmuirs' equation: Q  = (^2 S  c  "^  watts per sphere  Assuming, as before, a p a r t i c l e diameter of  0.0073 cm and taking Langmuirs'  thermal conduction f a c t o r s , ^ , f o r s t i l l a i r , the heat flow by convection i s given bys Q  c  r  (0.123 x 10$  - 0.2o) c a l per mole^FeS per sec  Correcting the above equation f o r an extreme a i r v e l o c i t y of 1000 cm per second increases the convection loss by 5.45 t o give the r e s u l t s l i s t e d i n Table X. The calculations i n Table X show that neither radiation nor convection w i l l explain the high p a r t i c l e temperatures  occurring during furnace oxidation,  since heat flow by either mechanism exceeds the heat generated.  E i t h e r the  roasting rate equation or the heat transfer equations must be i n e r r o r .  -28(C)  Calculation,, of Sulphide Oxidation Reaction Rates Diev and Karyakin -! studied the roasting rates of FeS i n oxygen3  enriched a i r with the following  conclusions:  1.  At 500 C the rate of oxidation i s independent of the oxygen pressure of the atmosphere.  2.  Above 700 C the rate of oxidation i s dependent upon the oxygen pressure of the atmosphere.  These measurements indicate that the oxidation of the FeS used by Diev and Karyakin i s reaction rate controlled below 500 C and d i f f u s i o n rate controlled above 700  C. Because the d i s t i l l e d Con concentrate has the composition FeS^^i,  the above findings should be approximately true for t h i s i n v e s t i g a t i o n . The roasting rate of d i s t i l l e d Con concentrate near 500 C i s therefore independent of the atmosphere but depends exponentially upon the temperature.  absolute  Thus the heat l i b e r a t e d by the reaction and the p a r t i c l e  temperature are p r a c t i c a l l y impossible  to control during a furnace roast,  because of temperature gradients i n the roasting bed.  When the p a r t i c l e  temperature reaches approximately 700 C, the reaction rate becomes equal to the oxygen d i f f u s i o n rate to the sulphide interface and depends upon the oxygen pressure of the atmosphere.  High p a r t i c l e temperatures and  the  accompaning low gold extractions are inevitable by such roasts, because of the combination of high reaction r a t e , high heat of reaction, and  low  s p e c i f i c heat of a i r . This condition i s shown by the following calculations based on two roasting rate measurements for FeS by Disv and Karyakin:  11 Diev, N. P., and Karyakin, Yu. V., Roasting of Sulfides with A i r Enriched with Oxygen, Journal of Applied Chemistry, U.S.S.R., 11. 1112-22, 1938.  -291.  At 500 C the roasting rate i s reaction controlled and 0.0000238 moles per mole per second.  2.  At 700 C the roasting rate i n a i r i s d i f f u s i o n controlled and equals 0.000525 moles per mole per second. JS'/RT  From t h i s data and the Arrhenius equation, K energies  Ae l.-v  s  equals  , and assumed a c t i v a t i o n  of 25, 35, and 45 k i l o c a l o r i e s per gram mole of FeS the  oxidation  rate when reaction rate controlled i s estimated as shown i n Graph V I I I . This range of a c t i v a t i o n energies  f o r FeS oxidation was  selected for the  following reasons: 1.  The range 20 to 50 k i l o c a l o r i e s per gram mole i s given by E y r i n g ^ for the majority of chemical reactions.  2.  The r e l a t i o n between a c t i v a t i o n energy and the temperature at which a reaction attains a measurable rate^as given by Hinshelwood^ yields a value of 45 k i l o c a l o r i e s per gram mole. Consider the reaction rate when d i f f u s i o n c o n t r o l l e d .  The average  d i f f u s i v i t y of oxygen through the stagnant nitrogen f i l m and against counter-diffusing sulphur dioxide may  2  the  be estimated from the relationships  developed by G i l l i l a n d ^ * knowing only the molecular weights, molecular volumes, and concentrations  of the i n d i v i d u a l gases.  The oxygen concentration at the  sulphide interface i s zero, because the reaction i s d i f f u s i o n c o n t r o l l e d . The  sulphur dioxide concentration i s assumed 12$ by volume at the interface  and  zero i n the gas stream.  rate 0.571  This gradient yields a sulphur dioxide d i f f u s i o n  times that of the oxygen as demanded by the stoichiometric  equation: 2 FeS -f 7/2  Og  =  ^2°3  *  2  s o  2  12 Glasstone, S., Lardler, K., Eyring, H., McGraw-Hill Book Company, 1941.  The Theory of Rate Processes.  13 Hinshelwood, C , The Kinetics of Chemical Change i n Gaseous Systems. Clarendon Press, 1933, p. 121. 14 G i l l i l a n d , E . R.,  I n d u s t r i a l Engineering  Chemistry, 26, 1934,  p.  681.  -30As f o r the previous  convection calculations a gas f i l m 0.1 cm.thick i s  considered to surround each sulphide p a r t i c l e .  The measured roasting rate  for FeS of 0.000525 moles per mole per second at 700 C, gives the following oxidation rate equation when d i f f u s i o n c o n t r o l l e d :  6.38 x 10" ff, ( 1 - X ) / 2  6  3  r s 1.379 - ( l - X ) V ^  moles of FeS per mole per second  X - f r a c t i o n of the sulphide p a r t i c l e oxidized. This equation i s plotted on Graph  300  VIII  f o r various values of X.  GOO  700  Temperature  GOO  QOO  A~  C  Graph V I I I E f f e c t of Temperature upon Oxidation Rate of FeS when Chemical Rate Controlled and when D i f f u s i o n Controlled. Graph  VIII  shows c l e a r l y that at low temperatures the oxidation  rate i s lower than the oxygen d i f f u s i o n r a t e .  At these temperatures the  oxidation rate i s independent of atmosphere composition.  S i m i l i a r l y the  graph shows that at high temperatures the d i f f u s i o n rate or atmosphere composition controls the oxidation r a t e .  -31Graph VIII also indicates that as roasting proceeds d i f f u s i o n controls the oxidation rate to lower temperatures.  This mechanism explains  the calcine structure shown by Plate V, where the surfaces of several p a r t i c l e s are fused i n d i c a t i n g a high i n i t i a l roasting rate and the centres are unresolved at 1000 magnification i n d i c a t i n g a low f i n a l roasting rate. II Aqueous Media Oxidation A roast with low p a r t i c l e temperatures obtained by an aqueous medium oxidation gives complete extraction of the gold by cyanide s o l u t i o n which confirms the supposition that gold i n cyanide t a i l i n g s results from high p a r t i c l e temperatures.  The oxidation i n a l i q u i d medium i s increased t o  measurable rates at 150 C by the increased s o l u b i l i t y of oxygen at high p a r t i a l pressures.  The oxidation rate i s further increased by the r e s t r i c t i o n s  placed upon the vibrations of the oxygen molecules by the medium.  An oxygen  molecule i n s o l u t i o n whenever i t approaches a p a r t i c l e c o l l i d e s many times with the sulphide surface rather than just once as would a gaseous oxygen molecule.  Although the net movement of a molecule i n a l i q u i d i s much  slower than i n a gas, the c o l l i s i o n frequency i s therefore the same as f o r an equal oxygen concentration i n a gas.  approximately  Because of i t s high  s p e c i f i c heat and intimate contact with the sulphide p a r t i c l e s , the l i q u i d medium prevents overheating or f l a s h i n g of the p a r t i c l e s during the oxidation. A medium having a high hydroxyl i o n concentration i s e s s e n t i a l f o r rapid andcomplete oxidation as shown by a comparison of the sulphide content of C-108 and C-109 and by the following c a l c u l a t i o n s . At 25 C and 115 p s i , oxygen s o l u b i l i t y i n the average 4N NaOH medium of C-108 and i n the neutral medium of C-109 are 1.0 x IO"" and 9.6 x 10~ moles per l i t r e 3  respectively. 0.61.  3  At 150 C these concentrations are reduced by a f a c t o r of  The oxidation p o t e n t i a l calculated at 25 C from these oxygen s o l u b i l i t i e s ,  free energy data, and the f i n a l f e r r i c , sulphate, and hydroxyl i o n concentrations  -32are as follows: 1.  For the a l k a l i medium reaction: FeS ) f 2 0 (3  The  2  ^ f 3(0H") =  3  * S0  (c)  = 4  d r i v i n g potential - 1.2 v o l t a . 2.  For the neutral medium reaction: FeS( ) *• 2 ( ) * 2 0  S  The  Fe(0H)  F e  L  driving p o t e n t i a l  =  *  + +  +  S 0  4  ( ) d  0.9 v o l t s .  The above 0.3 volt difference i s equivalent  to the standard oxygen-  hydroxyl half c e l l of 0.401 volts corrected for the concentrations of the reactants and products.  Part of the difference between the 0.3 and 0.401  volts probably resulted from errors i n the free energy values used. Because of the s l i g h t oxidation occurring i n the neutral medium either of two reactions i s suggested: 1.  The d r i v i n g potential of 0.9 volts compared to 1.2 v o l t s for the a l k a l i medium gives a lower rate of reaction. This i s u n l i k e l y , however, since the rate appears to be reaction controlled and should therefore depend upon temperature.  2.  The true driving potential i n the neutral medium may approach zero because of an overvoltage developed on the surface of the sulphide. This supposition i s more l i k e l y since the FeS has a metal type l a t t i c e involving no electron exchange. The i r o n on oxidizing to the i o n i c state, i n common with a l l metals, would evolve hydrogen and develop a hydrogen overvoltage. The  free energy calculations d e f i n i t e l y show that the concentration  difference i n hydroxyl and f e r r i c ions between the two media cannot account for the difference i n oxidation p o t e n t i a l .  The free energy difference  r e s u l t i n g from the hydroxyl and f e r r i c i o n concentrations being only 3700 calories compared to a t o t a l free energy change of -219,000 calories for reaction (c) and -164,000 calories f o r reaction (d). The  caustic used by the a l k a l i medium reaction can be regenerated  with lime by the reaction: H 0 f Na (S0 ) • CaO = CaSO^ 2  2  4  a f t e r f i l t e r i n g o f f the c a l c i n e .  + 2NaOH  -33The above c a l c u l a t i o n s were made at 25 C rather than 150 C, because of the l i m i t e d high temperature free energy data a v a i l a b l e . Since the change i n free energy with temperature i s l i k e l y to be of the same order f o r both r e a c t i o n s , the difference i n o x i d a t i o n p o t e n t i a l between the two media at 150 C should not vary g r e a t l y from 0.3 v o l t s .  34CONCLUSIONS  The following conclusions are drawn from t h i s i n v e s t i g a t i o n : 1.  Gold recovery by cyanidation from r e f r a c t o r y ores i s determined by the maximum p a r t i c l e temperature during a roast.  The s t a r t i n g  temperature, f i n a l temperature, and roasting speed do not a f f e c t gold recovery except as they a f f e c t the maximum temperature of the i n d i v i d u a l roasting p a r t i c l e s . 2.  Gold recovery from each calcine p a r t i c l e i s determined by the degree to which some compound, probably i r o n oxide, imprisons the gold by i n c i p i e n t f u s i o n or r e c r y s t a l l i z a t i o n .  3.  A low temperature roast i s obtained by oxidation of the sulphides i n an a l k a l i medium under high oxygen pressures.  This oxidation  gives a calcine i n which a l l the gold i s free and 100$ soluble i n cyanide s o l u t i o n .  In the l i q u i d medium,natural weathering or  oxidation i s thus obtained i n as l i t t l e as one hour for a porous sulphide. 4.  A concentrated a l k a l i solution i s required to obtain a d r i v i n g force during aqueous medium oxidation s u f f i c i e n t to give a measurable rate of reaction at low temperatures.  5.  Neither hydrochloric acid washes nor extreme fine grinding w i l l free locked gold i n a furnace c a l c i n e , due probably to the near atomic size of the gold p a r t i c l e s .  -35 APPENDIX  I  Calculation of Heat Flow From Oxidizing Sulphide P a r t i c l e s The following assumptions were held constant throughout t h i s  report: An average p a r t i c l e diameter of .0073 cm,(200 mesh, T y l e r Screen S e r i e s ) . An area to diameter r a t i o of 10. A temperature of 450 C f o r the atmosphere and surrounds of the oxidizing particles. An emissivity of one for a l l r a d i a t i o n c a l c u l a t i o n s . A s p e c i f i c gravity of 4.6 for the i r o n sulphide. A composition of FeS, molecular weight u 87.8, f o r the sulphide. (A) Heat Flow by Radiation The heat flow from the roasting sulphide p a r t i c l e s was  calculated  by the Stefan-Boltzmann formula: Q  = 5.76E'ftjotjo/  r  where:  -viooo/ (  w a t t s  p e r  s  q  c m  Tg and T^ are the temperatures of the surface and the surroundings  i n degrees Kelvin and ^ i s a f r a c t i o n , the emissivity, which depends on the r a d i a t i o n body and which approaches one f o r rough black surfaces.  When the  above mentioned assumptions are substituted into t h i s formua the heat loss by radiation i s given by: 4  Q  r  =  9.4 x l O ^ D T ^ ) "  1  - .274> c a l per mole of FeS per sec.  It i s obvious that the above formula assumes the area of the p a r t i c l e s to be constant during roasting.  Although the diameter of the  p a r t i c l e s increases s l i g h t l y because of the formation of porous i r o n oxide from the dense sulphide, i n view of the other approximations t h i s increase is not serious.  -36(B) Heat Flow by Convection Heat loss by convection from spheres i s given by Langmuirs' ^ 1  equation: Q  c  = S ((jl, - $2.) watts per sphere,  where: S  includes the space factor and the thickness of the a i r f i l m around each sphere.  0\  i s the thermal conduction of a i r at t ;p the bulk a i r temperature.  02  I  s  the thermal conduction of a i r at t g, the interface •  p a r t i c l e temperature. For spheres, 3 i s given by:  • =  d (i *  *f^S?)  »  where: a  i s the sphere diameter.  B  i s the thickness of the a i r f i l m around each sphere.  For s t i l l a i r at atmospheric pressure, experiments have shown B to equal 0,41 cm at 20 C, to be independent of the sphere temperature, and to be inversely proportional to the a i r density. of 450 C, B therefore equals 1.0 cm.  For a bulk a i r temperature  To ensure that the calculated convection  losses were not low, B was reduced to 0.1  cm.  Langmuirs* thermal conduction factors {0) f o r s t i l l a i r are as follows Temperature 450 500 700 1000 1500 2000  C C C C C C  15 Langmuir, M., l o c . c i t .  0 0.20 0.23 0.36 0.55 1.00 1.60  watts watts watts watts watts watts  per per per per per per  cm cm cm cm cm cm  -37From the previously l i s t e d assumptions,  S equals, 0.0555 cm and  the heat flow by convection i s given by: Q„ II  = (0.123 x 10 ) O^p - 0.20) c a l per mole of FeS per sec 7  Calculation of Sulphide Oxidation Rates. (A) Oxidation Rate When Limited By Rate Of Oxygen D i f f u s i o n According to the c l a s s i c a l k i n e t i c theory of gases  the d i f f u s i o n  of component A i n a gaseous mixture of A and B results from a d r i v i n g force equal to the p a r t i a l pressure gradient - *f f A  which exists i n the d i r e c t i o n  d i i  of d i f f u s i o n .  This d r i v i n g force i s used to overcome a resistance to d i f f u s i o n  proportional to the product of the concentration of the gases.  - !-r-•  ° S B °A °B  Mathematically*  u>  < A " »B> V  where: P^ s  p a r t i a l pressure of A.  L  =  distance i n the d i r e c t i o n of d i f f u s i o n of A.  =  a proportionality f a c t o r .  s  molal concentrations of A and B, r e s p e c t i v e l y .  C^,Cg V^,Vg  =  l i n e a r v e l o c i t i e s of d i f f u s i o n of A and B i n the d i r e c t i o n of d i f f u s i o n of A.  For the roasting of i r o n sulphide: 2FeS + / 2 7  2 0  -  Fe 0 2  3  + 2 S0  2  '  (A)  there exists counter d i f f u s i o n of oxygen and sulphur dioxide through an i n e r t nitrogen gas f i l m .  G i l l i l a n d ^ has developed a rigorous treatment of  the simultaneous d i f f u s i o n of two gases i n the presence of a stagnant gas f i l m which leads to complex equations f o r even t h i s r e l a t i v e l y simple case. However, useful approximations are obtained by simple relationships based"on the following assumption.  In a complex system of d i f f u s i n g gases, the  d i f f u s i o n a l gradient f o r any component A i s equal t o the sum of the gradients 16 G i l l i l a n d , E.R., l o c . c i t .  -38which result from the separate d i f f u s i o n of A with each of the other components i n separate binary systems i n which the concentrations and rate are the same as f o r the complex system.  Thus f o r the d i f f u s i o n of oxygen to the sulphide  interface, equation (1) develops into d Po - d — * °o so G C ( V - v 2  2  2  0  2  S  0  2  0  2  S 0 2  )  +  -  0  2  N  2  c  0 2  c  N 2  v  (2)  0 2  Assuming the i d e a l gas law t o hold, the c o e f f i c i e n t of d i f f u s i o n D  AB °^ components A and B i s defined ast (RT) D  2  AB  where : TT  i s the t o t a l pressure i n atmospheres.  T - temperature, degrees * v . - r U - AR.T A - A A = -jrwhere: & n d  r  V  r  r  Kelvin.  U  z "tbe molal rate of d i f f u s i o n of A i n moles per unit time  A  per unit of cross - sectional area of d i f f u s i o n a l path. 2 molal volume of component A. Making these substitutions,  equation (2) y i e l d s :  Because of the uncertainty of the proportionality  factors,^,  and the corresponding d i f f u s i v i t i e s i n mixtures of several components, an average d i f f u s i v i t y  i s used to replace the separate values i n equation (3).  The average value of the d i f f u s i o n c o e f f i c i e n t taken as the weighted mean for each pair i s given by: &  - A> Am = B AB * C AC + * " N  D  N  D  N  ( )  D  4  where: N  A* B»*'* N  a  r  e  "  t h e  average mole fractions of components, A,B,-«"  i n the d i f f u s i o n a l f i l m . From the stoichiometry of the roasting  reaction,  equation A:  -39-  S  r  " 7  = 0 2  R  0  2  (5) a n d  r  N  =  2  0  Substituting equations ( 4 ) and ( 5 ) into ( 3 ) y i e l d s :  TT  'M  = Do~"  2 2  or, since 7 T  0  R  (  2  P  • P^  =  V d L z - ^ J C R  °2  S 0  + 2  P  N  +  7  2  (6)  P ) 0 2  «• D  P  0P- ^ |  T  (7)  P ) 0 2  Integrating between the f i l m boundaries, L and L i , corresponding to an e f f e c t i v e f i l m thickness Bg giy,es:  r  n  l %XZL-Z- i n ^ " I  =  fe  1  (8)  The d i f f u s i o n c o e f f i c i e n t D „ f o r the i n t e r d i f f u s i o n of two AB A and B, may be estimated from the following emperical relationship  gases,  developed  by G i l l i l a n d : D  AB  =  °-  Au l/3  0043  A  „  U  B  1 / 3  }  2  f  I  A  * ^  0)  where: and Mg are the molecular weights of A and B. A l l other symbols are as previously l i s t e d . Therefore at 5 0 0 C: D  A  ° 2  S  0  =  0*506  sq  cm  per  sec  S  0.644  sq  cm  per  sec  2  and DQ^^  The required sulphur dioxide gradient across BQ, i s now found by t r i a l and error to equal 0 . 1 2 atmospheres from equations ( 4 ) , ( 8 ) , the rate 4  relationship  r g  ; "7r  S  2  , and assuming the oxygen gradient across Bg to  f l  2  equal 0 . 2 1 atmospheres. Noting that the d i f f u s i o n c o e f f i c i e n t varies as the 3 / 2 power of the absolute temperature, at any temperature T :  D  - 0.00002947 T / 3  n  Um  2  sq cm per sec  2  and r  T^r- (0.00000336) , ;/T — moles per sq cm per sec L  ft  °2  BG  Thus, since one mole of oxygen oxidizes 0.571 moles FeS, the oxidation rate of FeS when oxygen d i f f u s i o n controlled equals: r  *> a s TT B  0.00000192 moles FeS per sq cm per sec  G  From the i n i t i a l roasting rate measurements f o r FeS by Diev and Karyakin  17  at 700 C, 0.000525 moles per mole per second, d i f f u s i o n rate  controlled, the effective area,A, per mole of FeS i s found to equal: .000525 B A  =  Q  ( .00000192)^73 "  8  '  7 6 B  G <* 8  c  m  Thus the roasting rate at the s t a r t of oxidation i s given by r_ *eS  t>  0.00000168)  moles FeS per mole per sec  :  G  Consider now the sulphide p a r t i c l e s , assuming spheres, and l e t X equal the f r a c t i o n of each p a r t i c l e oxidized at any time, r the e f f e c t i v e i n i t i a l radius of the p a r t i c l e s and per mole.  the e f f e c t i v e instantaneous radius  The momentary thickness of the oxide layer equals r - r ^  =  X) cm per mole and the momentary area of the p a r t i c l e s = 4#~r  (1 - X ) / sq cm per mole.  2  2  3  When B Q i s assumed equal to 0.1 cm, as  during the convection heat flow calculations,the above measured roasting rate gives an effective i n i t i a l radius per mole: r = 0.264 cm With these values the instantaneous oxidation rate of FeS when d i f f u s i o n controlled i s given by: -fY (0.00000168) 4 ? - ( . 2 6 4 ) ( l - X ) / (.876)(0.1 + .264 - .264 ^1-X ) moles per mole per sec 2  r  F S 8  2  3  17 Diev, N.P., and Karyakin, Yu.V., l o c . c i t ,  -41and s i m p l i f i n g : _ 6.38 x 10~ (1-X) / FeS " 1.379 - (l-X)V* moles per mole per sec 6  r  2  3  (10)  This roasting rate i s shown plotted against temperature for d i f f e r e n t values of X i n Graph VTII. For a l l values of X, the energy of a c t i v a t i o n f o r t h i s d i f f u s i o n , found by p l o t t i n g In rF S VS ""» i s approximately 1000 c a l per gram mole of e  FeS.  While t h i s value i s considerably lower than the 3000 to 5000 c a l per  mole usually found for d i f f u s i o n controlled reactions i t nevertheless i s of the right order and equation (10) may be considered to give the rate of the reaction with reasonable  accuracy.  By using the spherical analogy of Ficks Second dC _ dt "  ,d C 2  n  W  D  Law:  2 dCx ; * X dx  an expression f o r the rate of oxygen d i f f u s i o n to the i r o n sulphide interface may be derived: dm dt  a  47-r  D  S  R  R - r  where: m = weight of oxygen absorbed. r  =  instantaneous radius of the sulphide p a r t i c l e .  R  8  o r i g i n a l radius of the sulphide p a r t i c l e .  D = diffusion coefficient. S - concentration of oxygen i n atmospheres. t = time. This expression, however, does not check measured reaction rates and i s independent of temperature and thus yields an a c t i v a t i o n energy of zero. For these reasons i t i s not considered further i n t h i s report. (B) Oxidation Rate when Chemical Rate Controlled. The use of the Arrhenius equation, K • A e  .E/RT  , i s too well  -42known to require further explanation at t h i s point and w i l l therefore not be considered further, except to state that A i s assumed to vary as the square root of the absolute temperature. I l l Calculation Of The Oxidation Potentials For The Aqueous Media Oxidation. The free energy, experimental, and s o l u b i l i t y data for the aqueous media 1.  oxidation are as follows*  18 Free energy data:  i FeS ii  ( s )  ;  0H7 x  ;  T  '(!•)  AF° AF°  25 C  i i i Fe(0H) ;2lF° 3  ivso  vFe* * +  2.  5  5  4 F °  5  =  Q  5  -  =  C  »  C  - 22,900 c a l per mole. - 37,470 c a l per mole. - 164,030 c a l per mole. - 176,100 c a l per mole. - 2,530 c a l per mole.  Experimental r e s u l t s : i  Average normality of a l k a l i medium = 4 N NaOH.  ii  F i n a l concentration of Fa" "** and SO* - 4.13 N.  iii 3.  n  ; 4 F °  4  =  25 Q  1  Oxygen p a r t i a l pressure = 115 p s i  s  Solubility data: i ii  7.83 atmospheres.  1 9  Oxygen s o l u b i l i t y i n water at 14.7 p s i and 25 C = 28.22 ml per l i t r e . Oxygen s o l u b i l i t y i n 0.5 N NaOH at 14.7 p s i and 25 C = 22.9 ml per  litre. iii iv  Oxygen s o l u b i l i t y i n 2.0 N NaOH at 14.7 p s i and 25 C • 12.2 ml per litre. The s o l u b i l i t y of s l i g h t l y soluble gases i n aqueous s a l t solutions  18 Lewis, G., and Randell, M., Thermodyamics and The Free Energy of Chemical Substances, McGraw-Hill Book Company, New York, 1923. 19 Markhan, A., and Kobe, K., The S o l u b i l i t y of Gases i n Liquids. Chemical Reviews, 28, 1941.  -43i s given by: S  s  SQ  (A m + T^TE  m  )  where:  m  molarity of the s a l t .  S «- gas s o l u b i l i t y . S Q * gas s o l u b i l i t y i n pure water. A,B, are a r b i t r a r y constants. On s o l v i n g , A = - 0.078, B = 0.356, and at 14.7 p s i , 25 C, and i n 4 N NaOH, S « 2.82 ml per l i t r e . v  When a s l i g h t l y soluble gas does not form a chemical compound with the solvent, Henry's Law i s obeyed within the l i m i t s allowed i n engineering c a l c u l a t i o n s . Thus the s o l u b i l i t y of oxygen at 115 p s i and 25 C i n water = 220. ml per l i t r e » 9.6 x 10~ moles per l i t r e . 3  S i m i l a r l y the s o l u b i l i t y of oxygen at 115 p s i and 25 C i n 4 N NaOH = 22. ml per l i t r e = 1.0 x 10~ moles per l i t r e . 3  (A)  Oxidation In The A l k a l i Medium. The oxidation reaction i n the a l k a l i medium i s given by the  equation:  (B) This equation may be considered i n two steps:  A F,  - 204,820 - 4,050  A F.25C  -208,870 c a l per mole FeS  25C  (Ixl0" moles/1) (7.83 atmos.) d  fn_  .12  10,560 c a l  -44Therefore the free energy change f o r equation (B) « - 219,430 c a l per mole FeS Hence The oxidation p o t e n t i a l : E , - = -£L25C NF  = 1.2 volta  p  (B) Oxidation In The Neutral Medium. The oxidation reaction i n the neutral medium i s given by the equation: FeS x + 20-  =  /e  Fe*** + S O ^  (C)  As before t h i s equation may be s p l i t i n t o two steps: I  FeS/.v + 20, IS) 2  = Fe * +  ( g )  +  + SO," 4  (7.83 atmos.)  4F  • AF° + RT In - ***Jfg°4*J  AF  - - 155,730 - 753  Fe  25Q  AF II  2 5 C  20 2  « - 156,480 c a l per mole FeS =  9  20  (L)  (9.6x10"-"moles/1) AF°  «  9  2  (  g )  (7.83 atmos.)  -RT In [ 2( Q  J  [° (L)P 2  Apo - - 7900 c a l Therefore the free energy change f o r equation (C) s-164,000 c a l per mole FeS. Hence the oxidation p o t e n t i a l : AF E  25C  =  " TTT  8  0.9 v o l t s .  BIBLIOGRAPHY  (1)  Anderson, J . S., The Primary Reactions i n Roasting and Reduction Processes. Trans, of the Faraday Society, June 1948.  (2)  Bognar, A., Humid Oxidation of Sulphide Ores Under High Pressure. Magyar Mernb'k Epiteszegylet Kozhonye, 78, 200-4, 1944. Chemical Abstracts, 7340 , 1947. a  (3)  Burg, G., Nutur des i n den Pyriten nicht Sichlbar enthaltenen Goldes. Z e s t s c h r i f t f u r praktische Geologie, February 1935.  (4)  Butts, A., Metallurgical Problems, McGraw-Hill Book Company, Inc., New York, 1943.  (5)  Diev, N. P., and Karyakin, Yu. V., Roasting of Sulfides with A i r Enriched with Oxygen, Journal of Applied Chemistry, U. S. S. 11, 1112-22, 1938.  (6)  R.,  Edwards, A., Texture of the Ore Minerals. Australasian I n s t i t u t e of Mining and Metallurgy (Inc.),  1947.  (7)  G i l l i l a n d , E. R., I n d u s t r i a l Engineering Chemistry. 26, 681,  (8)  Ginziro, K., Synthetic Study of Gold-Bearing P y r i t e . Sueyokwai-Si, 10, 419-424, 1941. Chemical Abstracts, 35, 3563, 1941. Glasstone, S., Thermodynamics For Chemists, D. Van Nostrand Company, 1947.  (9)  1934.  (10) Glasstone, S., Lardler, K., and Eyring, H., The Theory of Rate Processes, McGraw-Hill Book Company, 1941. (11) Goodeve, C , Phvsico-Chemical Principles i n Process Metallurgy. Trans. of the Faraday Society, July 1948. (12) Haszard, N., The Condition of Refractory Gold i n Lake View and Star (Kalgoorlie) Ore. Australasian Institute of Mining and Metallurgy, No. 108, 1937. (13) Hinshelwood, C , The Kinetics of Chemical Change i n Gaseous Systems, Clarendon Press, 1933. (14) Hougen, 0., and Watson, K., Chemical Process P r i n c i p l e s . Part I I I , John Wiley and Sons, 1947. (15) International C r i t i c a l Tables of Numerical Data, Physics, Chemistry and Technology. National Research Council of the United States of America, McGraw-Hill Book Company, Inc., 1926.  (16) Kelley, K., Contributions to the Data on Theoretical Metallurgy, The Thermodynamic Properties of Sulphur and Its Inorganic Compounds, United States Bureau of Mines, B u l l e t i n 406, United States Government Printing O f f i c e , Washington, 1937. (17) Khundkar, M., Thermal Decomposition of Iron Pyrite , Journal of the Indian Chemical Society, 24, 407-8, 1947. (18) Langmuir, M., Convection and Radiation of Heat. Transactions of the American Electrochemical Society, 23, 299, 1913. (19) Lewis, C , and Randall, M., Thermodynamics and The Free Energy of Chemical Substances. McGraw-Hill Book Company, New York, 1923. (20) Markham, A., and Kobe, K., The S o l u b i l i t y of Gases i n Liquids. Chemical Reviews, 28, 1941. (21) Markham, A., and Kobe, K., The Journal of the American Chemical Society, 63, 1165, 1941. (22) Mellor, J . , A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol 13, Longmans Green and Co., 1928. (23) Norwood, A., Roasting and Treatment of Auriferous F l o t a t i o n Concentrates, Australasian I n s t i t u t e of Mining and Metallurgy, No. 116, 1939. (24) P e r e t t i , E., A New Method For Studying The Mechanism of Roasting Reactions, Trans, of the Faraday Society, June 1948. (25) Perry, J . , Chemical Engineers' Handbook, McGraw-Hill Book Company, Inc., New York, 1941. (26) Plaksin, I., Ways of Increasing Gold E x t r a c t i o n i n the Study of B a l e i Ores, Sovet. Zolotoprom, 3, 28-36, 1938. (27) Schwab, G., Catalysis, Macmillan and Co. Limited, London,  1937.  (28) Thompson, M., The Total and Free Energies of E'ormation of The Oxides of Thirty-Two Metals. The Electrochemical Society, Inc., New York, 1942. (29) Tronev, V., and Bondin, S., Oxidation of Zinc Sulphide and Transference of Zinc into Agueous or-"Alkali Solution Under A i r Pressure, Compt. Rend. Acad. S c i . , U. R. S. S., 23, 541-3, 1939. (30) Truesdale, E., and Waring, R., Relative rates of Reactions Involved i n Reduction of Zinc Ores, Metals Technology, T. P. 1295, A p r i l 1941. (31) Vogel, A., A Text Book of Quantitative Inorganic Analysis, Longmans Green and Co., London, 1946. (32) White, J . , The Physical Chemistry S t e e l I n s t i t u t e , 1944.  of Open Hearth Slags. The Iron and  (33) Woods, S., The Reduction of Oxides of Iron as a D i f f u s i o n Controlled Reaction, Trans, of the Faraday Society, September 1948. (34) Zvyagentsev, 0., and Tronev, V., Oxidation of Copper Sulphide and Solution of Copper i n Agueous Solution Under A i r Pressure, Compt. Rend. Acad. S c i . , U. R. S. S., 23, 537-40, 1939.  

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