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

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L (5% ft 7 Cxf Is Cop • i THE INFLUENCE OF ROASTING TEMPERATURE UPON GOLD RECOVERY FROM A REFRACTORY GOLD ORE. A thesis submitted as partial requirement for a Master of Applied Science Degree i n the Department of Mining and Metallurgy at the University of British Columbia Ralph Carter University of British Columbia Apr i l 1949 i ABSTRACT SUMMARY The results of this investigation indicate that the gold i n the cyanide tailings of refractory gold calcines i s occluded i n fused or recrystallized iron oxide particles formed during roasting. These high temperature particles cannot be prevented by controlling 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 decom- posing samples of completely sulphated concentrate under such conditions that the maximum particle temperature varied between 650 C and 1100 C. This comparison showed that the particle temperature during a normal roast is several hundred degrees higher than the furnace temperature. An approximate relationship between the two is established for roasting the "refractory" arsenical gold concentrate examined during this investigation. The unavoidable high particle temperatures occurring i n furnace roasts are not upheld by theory since calculated radiation and convection losses exceed the heat generated. Rate of reaction and rate of oxygen diffusion calculations, however, do explain the failure of controlled atmospheres to give low particle temperature roasts at moderate furnace temperatures where the reaction is not diffusion controlled. Because high particle temperatures could not be prevented during any furnace roast, and because low particle temperatures are essential for complete gold extraction by cyanidation, aqueous medium oxidation under high oxygen pressures was studied. Gold recoveries from calcines produced by this means was over 98% compared with 80% from furnace calcines. It i i was shown that a medium with a high hydroxyl concentration gives adequate and rapid oxidation of the sulphides by increasing the chemical driving force by the oxygen-hydroxyl half c e l l potential. ACKNOWLEDGEMENT The author wishes to express his appreciation for the financial 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 investigation was continued this winter. In addition he wishes to thank Mr. F. A. Forward, Head of the Department of Mining and Metallurgy, and a l l members of the Metallurgical Department for their helpful co- operation and suggestions. INDEX OBJECT OF INVESTIGATION 1 INTRODUCTION 2 EXPERIMENTAL WORK 4 I Description of Ore 4 I I Apparatus . 5 I I I Experimental Work and Results........ 6 (A) Sulphide Oxidation 6 (a) Furnace Oxidation 6 ( i ) A i r Atmosphere Roasts............. 8 ( i i ) Controlled Atmosphere Roasts 11 (b) Aqueous Media Oxidation.. 12 ( i ) D i s t i l l e d Con Concentrate 13 ( i i ) Raw Con Concentrate 15 ( i i i ) Combined Aqueous Media Oxidation and Pressure Cyanidation.. 16 (B) Study of Calcines 16 (a) Thermal Decomposition of Sulphated Con Conoentiate.. 16 (b) Gold Nucleolation Experiments 17 (c) Microscopic Examination of Calcines 19 DISCUSSION OF RESULTS i •.. 24 I Furnace Oxidation • 24 (A) Experimental Results 24 (B) Calculation of Heat Flow From Oxidizing Sulphide P a r t i c l e s 26 (C) Calculation of Sulphide Oxidation Reaction Rates 28 I I Aqueous Media Oxidation 31 CONCLUSIONS 34 APPENDIX 35 I Calculation of Heat Flow From Oxidizing Sulphide Particles.... 35 (A) Heat Flow by Radiation J 35 (B) Heat Flow by Convection 36 II Calculation of Sulphide Oxidation Rates 37 (A) Oxidation Rate When Limited by Rate of Oxygen Diffusion.. 37 (B) Oxidation Rate When Chemical Rate Controlled 41 III Calculation of The Oxidation Potentials For The Aqueous Media Oxidation 42 (A) Oxidation In The Alka 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 Pyritic and arsenical ores containing sub- microscopic gold have long been regarded as refractory requiring special treatment to free the gold for solution by cyanide. The sulphides are usually roasted to free the gold and the calcines cyanided. This investigation was under- taken to determine why this treatment seldom yields gold recoveries of over 90%. -1- -2- INTRODUCTION This work i s intended to explore some of the suggestions made 1 2 by Mr. Haszard and Mr. Norwood i n t h e i r investigations of the roasting of auriferous f l o t a t i o n concentrates. To date general agreement on refractory gold concentrates and t h e i r treatment stresses the following points: 1. Gold occurs i n elemental form i n a very fine d i v i s i o n , as 3 o l i d 3 4 solution i n the pyrite or arsenopyrite l a t t i c e ' . 2. A low p a r t i c l e temperature during roasting i s e s s e n t i a l i f the res u l t i n g calcine i s to give high gold extraction by cyanidation. While agreement exists on the above points, 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 oxidation and measuring the temperature, therefore, formed the p r i n c i p a l objectives of t h i s investigation. From a comparison of roasting and cyanidation tests on many refractory gold ores the following points appeart 1. The more f i n e l y divided the gold the more refractory the ore. 2. The more refractory the ore the greater the dependence of gold recovery upon p a r t i c l e temperature during the roast. 1 Haszard, N. I . , The Condition of Refractory Gold i n Lake View and Star (Kalgoorlie) Ore. Australasian Institute of Mining and Metallurgy, No. 108, 1937. 2 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. 3 Edwards, A., Texture of the Ore Minerals. Australasian I n s t i t u t e of Mining and Metallurgy (Inc.), 1947, P.93. 4 Ginziro Kuranti, Synthetic Study of Gold-Bearing P y r i t e . Sueyokwai- S i , 10, 419-424, 1941. Chemical Abstracts, 35, 3563, 1941. To date strong evidence indicates that the gold is locked up by recrystallized or fused iron oxide during the roast. According to Mellor 5 two crystalline forms of fe r r i c oxide exist, an <* f e r r i c oxide stable at low temperatures, and a/?ferric oxide stable at high temperatures. The transformation temperature for this change has been reported as occurring between 678 C and 1320 C. Mellor also reports ferromagnetic ferric oxide with the cubic lattice 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 this transformation being i n the hundreds of atmospheres^. Opposed to these chemical changes is the mechanical locking of the gold by incipient fusion of the iron oxide. Giving weight to the suggestion that the gold is occluded by FegOg decomposing to dense FejO^ is the change of ochres during roasting, where the color darkens with increasing roasting temperature from yellow through red to black, while the specific gravity gradually increases. 5 Mellor, J . M., A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 13, Longmans Green and Co., 1928, p. 787. 6 White, J., The Physical Chemistry of Open Hearth Slags, The Iron and Steel Institute, 1944, p. 614. - 4 - EXPERIMENTAL WORK To reduce the number of variables during this investigation the following standard cyanidation procedure was adopted: 1 . One hundred grams of the calcine was washed for one hour with 500 ml of water in an open f i v e - l i t r e wide-mouth jar 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 f i l t e r . 2 . The washed calcine was returned to the original jar and cyanided on the above mentioned r o l l s for 24 hr with 500 ml of water, 0 . 5 g of NaCN, and 1 .0 g of CaO. I. Description of Ore The material used as a basis for the tests throughout this investigation was a flotation concentrate produced by Con Gold Mines Limited having the following analysis: Sb s 0.60% The uniform structure of the concentrate, principally arsenopyrite, is 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 refractive character of this concentrate. Au o 1 .63 oz per ton Pb s 0 . Fe s 3 4 . 3 / £ S at 33.0/o As = 17.1% Cu = 0 . 3 0 ^ Zn = 3.00% Insoluble = Q.30% Plate I 375X Con Concentrate. Large, Uniform, Dense Arsenopyrite Grains with no Free Gold. TABLE I No. Treatment Gold Assay of Tailing Gold (oz per ton) Extraction C-l None 1.28 21.5% C-2 1.0 hr milling 1.18 31.1% i n laboratory pebble m i l l # #Note: M i l l Size e 7 i n . by 7 i n . Pebble Charge = 2765 g M i l l Speed s 74 rpm Ore Charge s 1000 g Water = 400 ml II. 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 in Figure I. The whole assembly was placed on two inch-high fire-clay blocks i n a 13 kw Walker electric furnace. Thus i t was possible to shield the surface of the charge from radiation - 6 - Pipe to Back of fur/joce to Ac/mir tt'XIZ*' Silica Tray Steel Door1 /5"/f £> /v/-e C / c / Mvff/e 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 tests were made i n the 1400 ml mechanically-agitated autoclavte shown i n Figure I I . I I I . 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 for a calcine produced in a furnace held at 450 C for 11 hr, to 16$ for 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 off 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 by increasing i t s specific surface area. This d i s t i l l e d concentrate was used for a l l futur furnace oxidation tests. The porous sulphide, F e S i # i , containing 0.6$ to 0.8$ arsenic and 31.8$ to 33.1$ sulphur is shown i n Plate II. Plate II 375X Di 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 Particle of Undistilled Arsenopyrite. (i) Air 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 air 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 results of these tests are summarized on Graph I, and a few typical results are given i n detail i n Table II. IO0 \ o I 1 1 1 1 I L _ I 400 300 SOO 700 I Furnace Temperature C I : Graph I Effect 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 II No. Max. Time at Calcine Assay Gold Assay Gold Temp. Max.Temp. Sulphide Sulphate of Tailings Extraction Sulphur Sulphur (oz per ton) C-44 32.3 % 1.08 45.0 % C-53 450 C 3Hr 5.48 % 4.19 % 0.48 79.8 % C-54 500 C 1.5Hr 4.68 % 1.30 % 0.54 76.6 % C-47 550 C 0.5Hr 1.23 % 1.31 % 0.84 60.9 % C-37 600 C 0.5Hr 1.98 % 1.86 % 0.98 56.2 % C-34 750 C 0.5Hr 0.07 % 0.15 % 1.58 16.3 % Note; No. C-44 is the Con concentrate after heating under nitrogen for one hour at 600 C. No. C-53 and C-54 showed no glowing particles during roasting. No. C-47 and C-37 glowed slightly during roasting. No. C-34 glowed strongly during roasting. From Graph I the complete dependence of gold recovery upon roasting temperature is obvious. Because of the exponential dependence -10- of roasting rate on temperature, the true particle temperature is much higher than shown on the abscissa of Graph I . To study the influence of roasting time upon gold recovery a series of roasts was made in the sealed muffle under an a i r atmosphere at the optimum temperature of 400 C to 450 C found from the previous series. A plot of recovery against roasting time is shown in Graph I I and a few typical results are given i n detail-in Table I I I . 6 a IO 12. Roosting Time in Hours Graph I I Effect 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 I I I No. Time Calcine Assay Sulphide Sulphate Sulphur Sulphur Gold Assay of Tailings (oz per ton) Gold Extraction C-61 31.8 % 1.08 44.8 % C-66-1 3 Hr 14.6 % 2.5 % 0.54 76.5 % C-66-2 6 Hr 5.1 f. 4.8 % 0.52 78.8 % C-66-3 9 Hr 6.2 % 4.8 % 0.50 79.8 % C-66-4 12 Hr 6.1 % 5.1 % 0.46 84.0 % C-70 15 Hr 3.6 % 5.7 % 0.48 81.5 % Mote: During these roasts no glowing of the sulphide particles occurred, ( i i ) Controlled Atmosphere Roasts Controlled atmosphere roasts were investigated to determine i f high particle temperature by flashing could be prevented by reducing the oxygen partial pressure over the sulphides. These low oxygen atmospheres were divided into two series, 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 results of this study are given i n Table IV and Graph III. TABLE IV No. Max. Time Atmosphere Calcine Assay Gold Assay Gold Temp. ©2 SOg Sulphide Sulphate, of Tailing Extraction Sulphur Sulphur (02 per ton) C-50 450 C 2 Hr 14 $ C-52 450 C 4 Hr 14 $ C-49 450 C 7 Hr 14 $ C-63 450 C 4 Hr 14 $ 7.4 $ C-62 450 C 7 Hr 14 $ 7.4 $ C-64 450 c 9 Hr 14 $ 7.4 $ 17.6 $ 4.4 $ 3.89$ 2.0 $ 1.10$ 0.80$- 2.75 $ 0.52 77.0 $ 3.97 $ 0.50 78.7 $ 5.03 $ 0.46 82.6 $ 4.8 $ 0.54 76.8 $ 5.52 $ 0.54 76.0 $ 5.15 $ 0.52 78.8 $ Note: During these roasts no glowing of the sulphide particles occurred. These low oxygen roasts, as shown by Table IV, did not raise gold recovery above that given by a i r atmosphere roasts at the same furnace temperatures. The sulphating roasts, similarly, did not improve recovery. The small amount of iron sulphate formed during these roasts implies that either sulphation at 450 C is very slow and incomplete, or that the particle temperature during the roast was above the decomposition temperature of -12- "1 I 3 <0 fa s / /a /a ay 70 SO Gold Recovery X J Graph I I I Influence of Sulphate Sulphur Content Upon Gold Recovery from Calcines Produced by Furnace Oxidation of Di s t i l l e d Con Concentrate. iron sulphate. This latter supposition finds support i n nature where both ferric and ferrous sulphates are formed in atmospheres containing negligible amounts of sulphur dioxide. Thus the f i n a l sulphate content of a calcine under the conditions maintained during this series of experiments appears, to be independent of the percent sulphur dioxide in the atmosphere but to depend upon the maximum particle temperature during the oxidation. If the sulphate sulphur is a record of the maximum particle temperature, the extreme sensitivity of gold recovery to particle temperature i s shown by the large positive slope of Graph III. (b) Aqueous Media Oxidation Because of the impossibility of preventing high particle temperatures during a furnace roast, aqueous medium oxidation under high oxygen pressures was investigated. By such an oxidation the combination of low particle -13"- temperature and high reaction rate is possible, because of the high oxygen pressure or driving force present and the high specific heat and intimate contact of the liquid medium with the sulphide particles. Thus local overheating of the sulphide particles i s impossible. Aqueous media oxidation of sulphides under high oxygen pressures 7 8 9 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 to 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 resulting pulps were then f i l t e r e d , washed, and cyanided as usual. The results of this work are given in Table V. TABLE V Aqueous Media Oxidation of D i s t i l l e d Con Concentrate No. Treatment i n Autoclave Sulphide Gold Assay Gold Temp. Time Oxygen Liquid Media Sulphur of Tailings Extraction •esaae . I n i t i a l Final In CaMnas (oz per ton) absolu' C-92 33.2 % 1.08 44.5 % C-101 33.2 % .96 49.9 % C-97 90C 4Hr 47psi 13N NB4OH NH4§H-Low 29.1 % .28 83.8 % C-98 90C 4Hr 47psi 13N NH4OH NH4OH-L0W 24.5 % .08 95.5 % C-105 150C 4Hr 115psi 13N NH4OH NH4OH-O.3N 27.8 % .90 53.3 % C-106 150C <lHr 115psi 12N NaOH NaOH-2.5N 7.7 % .16 92.4 % C-10715X <lHr 115psi 12N NaOH NaOH-1.7N 5.5 % .08 96.2 % C-108 150C lHr 115psi 12N NaOH NaOH-0.2N 0.4 % .04 98.0 % C-109150C 4Hr 115psi .35N NaOH CaO-e.04N 27.6 % .90 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 a, 1947. 8 Tronev, V., and Bondin, S., Oxidation of Zinc Sulphide and Transference of Zinc into Aqueous or Alkali Solution Under Air Pressure. Compt.Rend,Acad. Sci., 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 Air Pressure. Compt.Rend.Acad. Sci. , 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 lis 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 psi to 170 psi, as the vapor pressure of water decreased on cooling. 3. Vapor pressure of 13 N aqua ammonia at 80 C = 20 p s i . 4. Vapor pressure of 13 N aqua ammonia at 150 C - HO p s i . 5. Vapor pressure of water at 150 C - 70 psi. 6. To C-109 was added 140 g of CaO. As seen by the high gold recoveries l i s t e d i n Table V a low particle temperature oxidation has been obtained. Surprisingly, test3 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 far off this graph has not been determined. O 3 IO IS 20 23 JO SS Sulphide S u/phur /£ Graph IV Dependence of Gold Recovery upon Sulphide Sulphur Content for D i s t i l l e d Con Concentrate decomposed by Aqueous Media Oxidation. -15r_ Apparently test C-105 did not oxidize because of the low oxygen partial 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 in table VI. TABLE VT Aqueous Media Oxidation of Ground Con Concentrate C-99-A No. Treatment in Autoclave Sulphide Gold Assay Gold Temp. Time Oxygen Media Sulphuran of Tailings Extraction aDsoiute I n i t i a l Final Calcine (oz per ton) C-99-A 33.0 % 1.16 31.2 % C-99 80C 4Hr 47 psi 13N NH4OH Nt^OH-Low 29.8 % 1.06 35.1 % C-100 80C 4Hr 47 psi 13N NR̂ OH NI^OH-Low 29.1 % 0.96 40.1 % C-110 150C 4Hr 115 psi 12N NaOH NaOH-ION 27.6 % 0.38 76.5 % These tests indicate that aqueous media oxidation of raw Con concentrate is much slower than for the d i s t i l l e d concentrate. This drop i n reaction rate results from the small specific surface area of the raw concentrate compared to that of the d i s t i l l e d concentrate. In this series of tests as for 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 elimination. In this series the irregularity occurs i n the sodium hydroxide rather thanthe aqua ammonia medium. Hence the medium cannot be responsible for the phenomenon. ( i i i ) Combined Aqueous Media Oxidation and Pressure Cyanidation Pressure cyanidation tests of a Con calcine, C-79, at 21 psi gauge oxygen pressure showed the cyanidation rate to be increased by the oxygen pressure, with no improvement in overall gold extraction. The result of this work is shown in graph V. A few tests were made, therefore, on 8O1 i 1 1 1 1 1 1 70 40 o s 10 /s zo zs JO JS Cyan/ding Time in Hours Graph V Effect 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 rate. Thus the gold would be dissolved i n the autoclave as soon as i t was freed from the sulphide. Such treatment, however, dissolved only about half the gold that a standard 24 hr cyanidation dissolved from the same calcines. Hence the cyanidation rate under 115 psi of oxygen can only be approximately half the oxidation rate of the sulphides. (B) Study of Calcines (a) Thermal Decomposition of Sulphated Con Concentrate To substantiate the supposition that the gold is occluded by iron oxide and to determine the particle temperature during a normal roast, -17- completely 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 acid, the excess acid being fumed off. This sulphated material was then decomposed by heating i n a tube furnace under both oxygen and nitrogen atmospheres and the resulting calcine cyanided. In this way local overheating of individual particles, which always occurs during an ordinary roast, was prevented, the decomposition of iron sulphate being an endothermic reaction. The results of this work are given i n table VII. TABLE VII No. Max. Time at Calcine Assay Gold Assay Gold Calcine Temp. Max.Temp. Fe*^ Fe+* S as SO4. of Tailing Extraction Appear- (oz per ton) ance Sulphated C-69ConQ> 21.8$ 22.8$ C-7I 650C 6 Hr:: 42.0$ 8.0$ 0.10 96.2$ Fluffy,Brick Red C-76 875C 0S5Hr : 51.5$ 0.5$ 0.9$ 0.10 95.8$ Darker than C-71 C-1023C00C 0.5Hr 51.0$ 1.1$ 0.2$ 0.48 75.6$ Dark Red C-73 11QDC 0.5Hr 50.8$ 1.8$ 0.2$ 0.96 50.7$ Very Dense, Black In these results the atmosphere i s not recorded since i t did not influence the gold recovery. Once again the particle temperature appears as the factor governing gold extraction from calcines. (b) Gold Nucleolation Experiments Because the state of division of the gold is believed responsible for the refractory qualities of gold ores, the refractory degree of the ore should be reduced by nucleolation of the gold particles. Following the procedure of Burg 1 0, such attempts were made by heating 125 g portions of raw Con concentrate under nitrogen i n a sealed steel bomb for several hours 10 Burg, G., Nutur des i n den Pvriten nicht Sichlbar enthaltenen Goldes, Zestschrift fur praktische Geologie, February 1935. -18- at 600 C. The resulting calcine3ware then washed and cyanided. The results of this work are l i s t e d i n Table VIII where they are compared with extractions from (a) raw concentrate, (b) concentrate ground for one hour in the laboratory pebble m i l l , and (c) d i s t i l l e d concentrate. TABLE VIII No. Treatment Gold Assay of Tailing Gold (oz per ton) Extraction C-l None C-2 1.0 hr grind i n laboratory pebble mil l C-92 D i s t i l l e d Concentrate C-87 16 hr at 600 C under ^ i n bomb C-88 24 hr at 600 C under N 2 i n bomb C-89 32 hr at 600 C under Ng i n bomb C-90 47 hr at 600 C under Ng i n bomb 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 is evident that the described treatment has only slightly increased gold extraction. Hence, the gold could not have been nucleolated, since nucleolated gold particles could be expected to cyanide readily. The slight increase i n gold recovery after heating must result from the decrease in particle size shown by Plate III. Plate III Tailing 1.38 oz per ton 375 X Calcine Cyanidation Residue C-90. Con Concentrate Heated 47 Hr in 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 yield 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 in Plates IV to VI and the thermally-decomposed samples i n Plates VII to X. The combination of low gold extraction and large iron oxide particles at high roasting temperatures i s clearly shown by these plates. Plate IV Tailing 0.44 oz per ton 1000X Calcine Cyanidation Residue C-49. Di s t i l l e d Concentrate Roasted at 400 C. The pores of much of the iron oxide are extremely small and the walls thin, although partial fusion has occurred to a considerable extent. Plate V Tailing 0.84 oz per ton 1000X Calcine Cyanidation Residue C-47. Di s t i l l e d Concentrate Roasted at 500 C. The pore walls are thicker than for C-49 and some grains show almost complete fusion around their boundaries. Plate VI Tailing 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 for C-49 or C-47 while the walls are thick and continuous. -21- Plate VII Tailing 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 Tailing 0.10 oz per ton 1800X Calcine Cyanidation Residue C-76. Sulphated Concentrate decomposed at 875 C. Particles are very fine as i n C-71 Plate IX Tailing 0.48 oz per ton 1000X Calcine Cyanidation Residue C-102, Sulphated Concentrate decomposed at 1000 C. Calcine particles have grown probably due to partial fusion of iron oxide. Plate X Tailing 0.96 oz per ton 675X Calcine Cyanidation Residue C-73 . Sulphated Concentrate decomposed at 1100 C. Calcine particles have grown and increased i n density with increasing temperature. Once the gold has been coated by the partial fusion shown i n the previous plates i t cannot be freed by fine grinding nor by hydrochloric acid washes as shown i n table IX. Throughout this work the influence of fine TABLE IX No. Treatment Gold Assay of t a i l i n g Gold (oz per ton) Extraction C-56 None 0.58 75.0 % C-56-B 0.5 hr grind i n pebble m i l l 0.56 75.8 % C-56-C 1.0 hr wash i n 1% HC1 0.56 77.4 % C-56-D 1.0 hr wash i n Z% HC1 0.56 77.4 % C-56-E 1.0 hr wash i n 5% HQ. 0.58 76.6 % Note; C-56 was produced by roasting d i s t i l l e d concentrate at 450 C for 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 yielding a high gold recovery the unextracted gold i s locked i n particles smaller than can be obtained by grinding. Table III.Shows Relationship Between Particle and Furnace Temperature During A Roast of D i s t i l l e d Con Concentrate Series I Furnace Calcines Produced by Direct Roasting of Temp. Dist i l l e d Con Concentrate Particle Series II Calcines Produced by Temp. Decomposing Sulphated Dis t i l l e d Con Concentrate 750C C-34 1000X Tailings = 1.58 oz./T Evidence of greater fusion than i n C-73 1100C 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 1000C 400C C-49 1000X Tailings = 0.44 oz./T Evidence of fusion similar to C-102 875C IP C-73 675X Tailiness 0.96 oz./T Evidence of considerable fusion and particle growth. C-102 1000X Tailings a 0.48 oz./T Evidence of fusion similar to C-49 1 C-76 1800X Tailings : 0.10 oz./T No evidence of fusion of particles -24. DISCUSSION OF RESULTS I Furnace Oxidation (A) Experimental Results This investigation has-clearly shown how gold extraction from the Con concentrate, as for most refractory sulphide gold ores, depends upon the maximum particle temperature during the roast. In Figure III, opposite, the comparison of calcines produced by furnace oxidation of d i s t i l l e d concentrate and produced by thermal decomposition of sulphated concentrate shows that high particle temperatures during furnace oxidation cannot be prevented. The arrangement of the calcines i n order of decreasing particle temperatures, and the known particle temperatures during decomposition of the sulphated concentrate gives an estimate of the maximum particle temperature during a furnace roast. This relationship i s found by plotting i n Graph VI. temperature against gold extraction for both the furnace calcines and the decomposed sulphated-concentrate calcines. 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 is drawn relating the maximum particle temperature to the furnace temperature. These graphs are approximate but they provide an estimate of the particle temperature at any furnace temperature for the d i s t i l l e d Con concentrate roasted under the conditions previously described. The failure 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 is the apparent explanation for this failure. Graph VI Effect of Temperature upon Tailing Assay for both Furnace Roasts of Dis t i l l e d Con Concentrate and Thermally Decomposed Sulphated Concentrate. Graph VII A Plot of Particle 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 particle temperatures than has been possible by roasting. When oxidation begins during a furnace roast, the particle temperature cannot be controlled because the heat generated by the reaction cannot be dissipated from the sulphide particles. Radiation calculated with the surroundings at 450 C, an emissivity of one, a particle diameter of 0 . 0073 cm (200 mesh) and an area to diameter ratio of 10 is given by the following equation: Qr = 9 . 4 x 1 0 6 ^ g U J 4 - 0 .274J cal per mole of FeS per sec. (a) From calculations which w i l l be shown later the maximum li k e l y roasting rate is given by the equation: 25 5 . , N _ 2 00 r =/T 2(2.84 x lO^e TZ moles of FeS per,mole per sec The heat evolved by the reaction: 7 2FeS + Z 0 2 = Fe 20 3 + 2S0 2 is given by: -3 2 AH * - 146,229 + 6.84T - 15.212 x 10 T cal per mole of FeS The combination of these expressions gives the following equation for the rate at which heat is generated by the oxidation of FeS: QR - f% (l46,229 - 6.84T + 15.212 T 2)/ 2.835 x 10 4) e" cal per V A mole 0/FeS per sec (b) These results, compiled i n Table X, indicate that at a l l particle temperatures radiation w i l l dissipate the heat generated. TABLE X . ' T 2 QR (Heat Generated) Q R (Heat loss by Radiation) Q C (Heat loss by Conyestkn) Particle Temp, (cal per mole FeS per sec) (cal per mole FeS per sec) (cal per mole FeSpersec) 500 C 3.6 0.078 x 10 7 0.019 x 107 700 C 12.4 0.584 x l O 7 0.085 x 10 7 1000 C 3.3 x 10* 2.21 x 10? Q < 2 2 x 1 q 7 1500 C 6.6 x lCg 2l:§Vl09 0.501 x 107 2000 C 1.4 x 10 7 0.763 x 10 7 -27- Unfortunately 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 practically zero, the heat flow from the roasting bed by radiation and subsequent conduction through the muffle walls w i l l be practically zero. Therefore, most of the radiated heat w i l l be reflected back to the roasting bed. It might be argued that the assumption that a l l the sulphide particles are free to radiate heat is invalid. This d i f f i c u l t y i s overcome by considering that those particles 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 is less than that by radiation since air i s a good insulator because of the large mean free path of it s molecules. Hence each sulphide particle may be assumed to be surrounded by an insulating air blanket. Heat loss by convection from spheres is given by Langmuirs' equation: Qc = S(^2 " ̂  watts per sphere Assuming, as before, a particle diameter of 0.0073 cm and taking Langmuirs' thermal conduction f a c t o r s , ^ , for s t i l l a i r , the heat flow by convection i s given bys Qc r (0.123 x 10$ - 0.2o) cal per mole^FeS per sec Correcting the above equation for an extreme a i r velocity of 1000 cm per second increases the convection loss by 5.45 to give the results 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 particle temperatures occurring during furnace oxidation, since heat flow by either mechanism exceeds the heat generated. Either the roasting rate equation or the heat transfer equations must be i n error. -28- (C) Calculation,, of Sulphide Oxidation Reaction Rates Diev and Karyakin3-! studied the roasting rates of FeS i n oxygen- enriched air with the following conclusions: 1. At 500 C the rate of oxidation is independent of the oxygen pressure of the atmosphere. 2. Above 700 C the rate of oxidation is dependent upon the oxygen pressure of the atmosphere. These measurements indicate that the oxidation of the FeS used by Diev and Karyakin is reaction rate controlled below 500 C and diffusion 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 this investigation. The roasting rate of d i s t i l l e d Con concentrate near 500 C is therefore independent of the atmosphere but depends exponentially upon the absolute temperature. Thus the heat liberated by the reaction and the particle temperature are practically impossible to control during a furnace roast, because of temperature gradients i n the roasting bed. When the particle temperature reaches approximately 700 C, the reaction rate becomes equal to the oxygen diffusion rate to the sulphide interface and depends upon the oxygen pressure of the atmosphere. High particle temperatures and the accompaning low gold extractions are inevitable by such roasts, because of the combination of high reaction rate, high heat of reaction, and low specific 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 Air Enriched with Oxygen, Journal of Applied Chemistry, U.S.S.R., 11. 1112-22, 1938. -29- 1. At 500 C the roasting rate i s reaction controlled and equals 0.0000238 moles per mole per second. 2. At 700 C the roasting rate i n a i r i s diffusion controlled and equals 0.000525 moles per mole per second. JS'/RT From this data and the Arrhenius equation, K s Ae l.-v , and assumed activation energies of 25, 35, and 45 kilocalories per gram mole of FeS the oxidation rate when reaction rate controlled is estimated as shown i n Graph VIII. This range of activation energies for FeS oxidation was selected for the following reasons: 1. The range 20 to 50 kilocalories per gram mole i s given by Eyring^ 2 for the majority of chemical reactions. 2. The relation between activation energy and the temperature at which a reaction attains a measurable rate^as given by Hinshelwood^ yields a value of 45 kilocalories per gram mole. Consider the reaction rate when diffusion controlled. The average diff u s i v i t y of oxygen through the stagnant nitrogen f i l m and against the counter-diffusing sulphur dioxide may 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 individual gases. The oxygen concentration at the sulphide interface is zero, because the reaction is diffusion controlled. The sulphur dioxide concentration i s assumed 12$ by volume at the interface and zero i n the gas stream. This gradient yields a sulphur dioxide diffusion rate 0.571 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., The Theory of Rate Processes. McGraw-Hill Book Company, 1941. 13 Hinshelwood, C , The Kinetics of Chemical Change in Gaseous Systems. Clarendon Press, 1933, p. 121. 14 G i l l i l a n d , E. R., Industrial Engineering Chemistry, 26, 1934, p. 681. -30- As for the previous convection calculations a gas film 0.1 cm.thick i s considered to surround each sulphide particle. The measured roasting rate for FeS of 0.000525 moles per mole per second at 700 C, gives the following oxidation rate equation when diffusion controlled: 6.38 x 10"6 ff, (1 - X ) 2 / 3 r s 1.379 - (l - X ) V ^ moles of FeS per mole per second X - fraction of the sulphide particle oxidized. This equation is plotted on Graph V I I I for various values of X. 300 GOO 700 GOO QOO A~ Temperature C Graph V I I I Effect of Temperature upon Oxidation Rate of FeS when Chemical Rate Controlled and when Diffusion Controlled. Graph V I I I shows clearly that at low temperatures the oxidation rate i s lower than the oxygen diffusion rate. At these temperatures the oxidation rate is independent of atmosphere composition. Similiarly the graph shows that at high temperatures the diffusion rate or atmosphere composition controls the oxidation rate. -31- Graph VIII also indicates that as roasting proceeds diffusion controls the oxidation rate to lower temperatures. This mechanism explains the calcine structure shown by Plate V, where the surfaces of several particles are fused indicating a high i n i t i a l roasting rate and the centres are unresolved at 1000 magnification indicating a low f i n a l roasting rate. II Aqueous Media Oxidation A roast with low particle temperatures obtained by an aqueous medium oxidation gives complete extraction of the gold by cyanide solution which confirms the supposition that gold i n cyanide tailings results from high particle temperatures. The oxidation i n a liquid medium is increased to measurable rates at 150 C by the increased solubility of oxygen at high partial pressures. The oxidation rate i s further increased by the restrictions placed upon the vibrations of the oxygen molecules by the medium. An oxygen molecule i n solution whenever i t approaches a particle collides 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 liquid i s much slower than in a gas, the c o l l i s i o n frequency i s therefore approximately the same as for an equal oxygen concentration i n a gas. Because of i t s high specific heat and intimate contact with the sulphide particles, the liq u i d medium prevents overheating or flashing of the particles during the oxidation. A medium having a high hydroxyl ion concentration i s essential for rapid andcomplete oxidation as shown by a comparison of the sulphide content of C-108 and C-109 and by the following calculations. At 25 C and 115 psi, oxygen solubility i n the average 4N NaOH medium of C-108 and i n the neutral medium of C-109 are 1.0 x IO""3 and 9.6 x 10~ 3 moles per l i t r e respectively. At 150 C these concentrations are reduced by a factor of 0.61. The oxidation potential 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 ion concentrations -32- are as follows: 1. For the al k a l i medium reaction: FeS ( 3) f 2 0 2 ^ f 3(0H") = Fe(0H) 3 * S0 4 = (c) The driving potential - 1.2 volta. 2. For the neutral medium reaction: FeS( S) *• 2 0 2 ( L ) * F e * + + + S 0 4 ( d) The driving potential = 0.9 volts. The above 0.3 volt difference is 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 slight oxidation occurring i n the neutral medium either of two reactions is suggested: 1. The driving potential of 0.9 volts compared to 1.2 volts for the a l k a l i medium gives a lower rate of reaction. This i s unlikely, 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 lik e l y since the FeS has a metal type lattice involving no electron exchange. The iron on oxidizing to the ionic state, in common with a l l metals, would evolve hydrogen and develop a hydrogen overvoltage. The free energy calculations definitely show that the concentration difference i n hydroxyl and fe r r i c ions between the two media cannot account for the difference i n oxidation potential. The free energy difference resulting from the hydroxyl and fe r r i c ion concentrations being only 3700 calories compared to a total free energy change of -219,000 calories for reaction (c) and -164,000 calories for reaction (d). The caustic used by the a l k a l i medium reaction can be regenerated with lime by the reaction: H20 f Na 2(S0 4) • CaO = CaSO^ + 2NaOH after f i l t e r i n g off the calcine. -33- The above calculations were made at 25 C rather than 150 C, because of the li m i t e d high temperature free energy data av 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 for both reactions, the difference i n oxidation potential between the two media at 150 C should not vary greatly from 0.3 v o l t s . 34- CONCLUSIONS The following conclusions are drawn from this investigation: 1. Gold recovery by cyanidation from refractory ores i s determined by the maximum particle temperature during a roast. The starting temperature, f i n a l temperature, and roasting speed do not affect gold recovery except as they affect the maximum temperature of the individual roasting particles. 2. Gold recovery from each calcine particle i s determined by the degree to which some compound, probably iron oxide, imprisons the gold by incipient fusion or recrystallization. 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 solution. In the liquid medium,natural weathering or oxidation is 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 is required to obtain a driving force during aqueous medium oxidation sufficient 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 in a furnace calcine, due probably to the near atomic size of the gold particles. APPENDIX -35 I Calculation of Heat Flow From Oxidizing Sulphide Particles The following assumptions were held constant throughout this report: An average particle diameter of .0073 cm,(200 mesh, Tyler Screen Series). An area to diameter ratio of 10. A temperature of 450 C for the atmosphere and surrounds of the oxidizing particles. An emissivity of one for a l l radiation calculations. A specific gravity of 4.6 for the iron sulphide. A composition of FeS, molecular weight u 87.8, for the sulphide. (A) Heat Flow by Radiation The heat flow from the roasting sulphide particles was calculated by the Stefan-Boltzmann formula: 'ftjotjo/ -viooo/ ( w a t t s p e r sq c m where: Tg and T^ are the temperatures of the surface and the surroundings i n degrees Kelvin and ^ is a fraction, the emissivity, which depends on the radiation body and which approaches one for rough black surfaces. When the above mentioned assumptions are substituted into this formua the heat loss by radiation i s given by: Qr = 5.76E Qr = 9.4 x lO^DT^)" 4 1 - .274> cal per mole of FeS per sec. It is obvious that the above formula assumes the area of the particles to be constant during roasting. Although the diameter of the particles increases slightly because of the formation of porous iron oxide from the dense sulphide, i n view of the other approximations this increase is not serious. -36- (B) Heat Flow by Convection Heat loss by convection from spheres is given by Langmuirs'1^ equation: Qc = S ((jl, - $2.) watts per sphere, where: S includes the space factor and the thickness of the a i r film 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 • particle temperature. For spheres, 3 is given by: • = d (i * *f^S?) » where: a is the sphere diameter. B is the thickness of the air film 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. For a bulk a i r temperature of 450 C, B therefore equals 1.0 cm. To ensure that the calculated convection losses were not low, B was reduced to 0.1 cm. Langmuirs* thermal conduction factors {0) for s t i l l a i r are as follows Temperature 0 450 C 0.20 watts per cm 500 C 0.23 watts per cm 700 C 0.36 watts per cm 1000 C 0.55 watts per cm 1500 C 1.00 watts per cm 2000 C 1.60 watts per cm 15 Langmuir, M., loc. c i t . -37- From the previously l i s t e d assumptions, S equals, 0.0555 cm and the heat flow by convection is given by: Q„ = (0.123 x 10 7) Ôp - 0.20) cal per mole of FeS per sec II Calculation of Sulphide Oxidation Rates. (A) Oxidation Rate When Limited By Rate Of Oxygen Diffusion According to the classical kinetic theory of gases the diffusion of component A i n a gaseous mixture of A and B results from a driving force equal to the partial pressure gradient - *f fA which exists i n the direction d i i of diffusion. This driving force is used to overcome a resistance to diffusion proportional to the product of the concentration of the gases. Mathematically* - ! - r - • ° S B ° A °B < VA " » B > u> where: P^ s partial pressure of A. L = distance i n the direction of diffusion of A. = a proportionality factor. C^,Cg s molal concentrations of A and B, respectively. V^,Vg = linear velocities of diffusion of A and B i n the direction of diffusion of A. For the roasting of iron sulphide: 2FeS + 7/ 2 02 - Fe 20 3 + 2 S0 2 ' (A) there exists counter diffusion of oxygen and sulphur dioxide through an inert nitrogen gas film. G i l l i l a n d ^ has developed a rigorous treatment of the simultaneous diffusion of two gases in the presence of a stagnant gas film which leads to complex equations for even this relatively simple case. However, useful approximations are obtained by simple relationships based"on the following assumption. In a complex system of diffusing gases, the diffusional gradient for any component A is equal to the sum of the gradients 16 G i l l i l a n d , E.R., loc. c i t . -38- which result from the separate diffusion of A with each of the other components in separate binary systems in which the concentrations and rate are the same as for the complex system. Thus for the diffusion of oxygen to the sulphide interface, equation (1) develops into d Po 2 - d — * ° o 2 s o 2 G 0 2 C S 0 2 ( V 0 2 - v S 0 2 ) + - 0 2 N 2 c 0 2 c N 2 v 0 2 (2) Assuming the ideal gas law to hold, the coefficient of diffusion DAB °^ components A and B is defined ast (RT) 2 DAB where : TT is the tot a l pressure i n atmospheres. T - temperature, degrees Kelvin. & n d * v. - r U - rAR.T  VA - rA UA = - j r - where: r A z "tbe molal rate of diffusion of A i n moles per unit time per unit of cross - sectional area of diffusional path. 2 molal volume of component A. Making these substitutions, equation (2) yields: Because of the uncertainty of the proportionality f a c t o r s , ^ , and the corresponding dif f u s i v i t i e s i n mixtures of several components, an average dif 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 diffusion coefficient taken as the weighted mean for each pair is given by: & - NA> DAm = NB DAB * NC DAC + * " ( 4) where: NA* NB»*'* a r e " t h e average mole fractions of components, A,B,-«" i n the diffusional film. From the stoichiometry of the roasting reaction, equation A: - 3 9 - r S 0 2 = " 7 R 0 2 a n d rN 2 = 0 ( 5 ) ( 6 ) Substituting equations ( 4 ) and ( 5 ) into ( 3 ) yields: 'M T T 2 2 = Do~" R 0 2 ( P S 0 2 + P N 2 + 7 P 0 2 ) or, since 7T = • P ^ «• V d L z - ^ J C D P 0 P - ^ ( 7 ) ° 2 R T | P 0 2) Integrating between the film boundaries, L and L i , corresponding to an effective f i l m thickness Bg giy,es: r n = l %XZL-Z- in ^ " I fe1 ( 8 ) The diffusion coefficient D „ for the interdiffusion of two gases, AB A and B, may be estimated from the following emperical relationship developed by G i l l i l a n d : DAB = °-0043 Au Al / 3 „ 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 = 0 * 5 0 6 sq cm per sec ° 2 S 0 2 and D Q ^ ^ S 0 . 6 4 4 sq cm per sec 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 S g ; " 7 r f l , and assuming the oxygen gradient across Bg to 2 2 equal 0 . 2 1 atmospheres. Noting that the diffusion coefficient varies as the 3 / 2 power of the absolute temperature, at any temperature T : and D n - 0.00002947 T 3/ 2 sq cm per sec U2m T^r- (0.00000336) , r f t ;/T — L moles per sq cm per sec °2 B G Thus, since one mole of oxygen oxidizes 0.571 moles FeS, the oxidation rate of FeS when oxygen diffusion controlled equals: r*> a s TT 0.00000192 moles FeS per sq cm per sec BG From the i n i t i a l roasting rate measurements for FeS by Diev and 17 Karyakin at 700 C, 0.000525 moles per mole per second, diffusion rate controlled, the effective area,A, per mole of FeS is found to equal: .000525 B Q A = ( .00000192)^73 " 8 ' 7 6 B G 8<* c m Thus the roasting rate at the start of oxidation i s given by r_ 0.00000168) moles FeS per mole per sec *eS t> G : Consider now the sulphide particles, assuming spheres, and let X equal the fraction of each particle oxidized at any time, r the effective i n i t i a l radius of the particles and the effective instantaneous radius per mole. The momentary thickness of the oxide layer equals r - r ^ = X) cm per mole and the momentary area of the particles = 4#~r 2 (1 - X ) 2 / 3 sq cm per mole. When B Q is 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 diffusion controlled is given by: rF 8S -fY (0.00000168) 4?-(.264) 2(l-X) 2/ 3 (.876)(0.1 + .264 - .264 ^1-X ) moles per mole per sec 17 Diev, N.P., and Karyakin, Yu.V., loc. cit, -41- and simplifing: _ 6.38 x 10~6 (1-X) 2/ 3 rFeS " 1.379 - (l-X)V* moles per mole per sec (10) This roasting rate is shown plotted against temperature for different values of X i n Graph VTII. For a l l values of X, the energy of activation for this diffusion, FeS. While this value is considerably lower than the 3000 to 5000 cal per mole usually found for diffusion controlled reactions i t nevertheless is 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 Law: an expression for the rate of oxygen diffusion to the iron sulphide interface may be derived: dm a D 47-r S R dt R - r where: m = weight of oxygen absorbed. r = instantaneous radius of the sulphide particle. R 8 original radius of the sulphide particle. 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 activation energy of zero. For these reasons i t i s not considered further i n this report. (B) Oxidation Rate when Chemical Rate Controlled. found by plotting In r F eS VS ""» is approximately 1000 cal per gram mole of dC _ n ,d2C 2 dCx dt " D W * X dx; The use of the Arrhenius equation, K • A e .E/RT , is too well -42- known to require further explanation at this 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 solubility data for the aqueous media oxidation are as follows* 18 1. Free energy data: i FeS ( s ); AF°25 Q = - 22,900 cal per mole. i i 0H7Tx ; AF° n -'(!•) 25 C - 37,470 cal per mole. i i i Fe(0H) 3;2lF° 5 Q = - 164,030 cal per mole. i v s o 4 ; 4 F ° 5 C = - 176,100 cal per mole. v F e * + * 5 4 F ° 5 C » - 2,530 cal per mole. 2. Experimental results: i Average normality of a l k a l i medium = 4 N NaOH. i i Final concentration of Fa"1"** and SO* - 4.13 N. i i i Oxygen partial pressure = 115 psi s 7.83 atmospheres. 3. Solubility data: 1 9 i Oxygen solubility i n water at 14.7 psi and 25 C = 28.22 ml per l i t r e . i i Oxygen solubility in 0.5 N NaOH at 14.7 psi and 25 C = 22.9 ml per l i t re . i i i Oxygen solubility i n 2.0 N NaOH at 14.7 psi and 25 C • 12.2 ml per l i t r e . iv The solubility of slightly soluble gases i n aqueous salt 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 Solubility of Gases i n Liquids. Chemical Reviews, 28, 1941. -43- is given by: S s S Q (A m + T^TE m) where: m molarity of the salt . S «- gas sol u b i l i t y . S Q * gas solubility in pure water. A,B, are arbitrary constants. On solving, A = - 0.078, B = 0.356, and at 14.7 psi, 25 C, and i n 4 N NaOH, S « 2.82 ml per l i t r e . v When a slightly soluble gas does not form a chemical compound with the solvent, Henry's Law i s obeyed within the limits allowed i n engineering calculations. Thus the solubility of oxygen at 115 psi and 25 C i n water = 220. ml per l i t r e » 9.6 x 10~3 moles per l i t r e . Similarly the solubility of oxygen at 115 psi and 25 C i n 4 N NaOH = 22. ml per l i t r e = 1.0 x 10~ 3 moles per l i t r e . (A) Oxidation In The Alka l i Medium. The oxidation reaction i n the a l k a l i medium is given by the equation: (B) This equation may be considered i n two steps: A F, 25C - 204,820 - 4,050 A F. 25C -208,870 cal per mole FeS (Ixl0"dmoles/1) (7.83 atmos.) fn_ .12 10,560 cal -44- Therefore the free energy change for equation (B) « - 219,430 cal per mole FeS Hence The oxidation potential: E,- p = -£L- = 1.2 volta 25C NF (B) Oxidation In The Neutral Medium. The oxidation reaction i n the neutral medium is given by the equation: FeS / ex + 20- = Fe*** + SO^ (C) As before this equation may be s p l i t into two steps: I FeS/.v + 20, = Fe +* + + SO," IS) 2 ( g ) 4 (7.83 atmos.) 4F • AF° + R T In -Fe***Jfg°4*J AF25Q - - 155,730 - 753 A F 2 5 C « - 156,480 cal per mole FeS II 20 9 = 20 9 2(L) 2 ( g ) (9.6x10"-"moles/1) (7.83 atmos.) AF° « -RT In [ Q2( J [°2(L)P Apo - - 7900 cal Therefore the free energy change for equation (C) s-164,000 cal per mole FeS. Hence the oxidation potential: AF E25C = " TTT 8 0.9 volts. 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, 7340a, 1947. (3) Burg, G., Nutur des i n den Pyriten nicht Sichlbar enthaltenen Goldes. Zestschrift fur 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 Air Enriched with Oxygen, Journal of Applied Chemistry, U. S. S. R., 11, 1112-22, 1938. (6) Edwards, A., Texture of the Ore Minerals. Australasian Institute of Mining and Metallurgy (Inc.), 1947. (7) G i l l i l a n d , E. R., Industrial Engineering Chemistry. 26, 681, 1934. (8) Ginziro, K., Synthetic Study of Gold-Bearing Pyrite. Sueyokwai-Si, 10, 419-424, 1941. Chemical Abstracts, 35, 3563, 1941. (9) Glasstone, S., Thermodynamics For Chemists, D. Van Nostrand Company, 1947. (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 Principles. Part III, 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, Bulletin 406, United States Government Printing Office, 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 Solubility 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 Flotation Concentrates, Australasian Institute of Mining and Metallurgy, No. 116, 1939. (24) Peretti, 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 Extraction i n the Study of Balei 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 Air 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 of Open Hearth Slags. The Iron and Steel Institute, 1944. (33) Woods, S., The Reduction of Oxides of Iron as a Diffusion 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 Air Pressure, Compt. Rend. Acad. Sci., U. R. S. S., 23, 537-40, 1939.

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