Ind. Eng. Chem. Res. 1997, 36, 2757-2759
2757
Reduction of Thiourea Consumption in Gold Extraction by Acid Thiourea Solutions Takami Kai,* Tamaki Hagiwara, Hiroko Haseba, and Takeshige Takahashi Department of Applied Chemistry and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890, Japan
The thiourea leaching of gold and silver from the gold-bearing silicate ore of the Hishikari mine (Kagoshima, Japan) was studied. The apparent activation energy of the extraction of gold and silver was smaller than that of the reaction rate of thiourea consumption by ferric iron. We carried out the leaching after the leaching solution was preserved for several hours. The gold recovery at 288 K was greater than that at 308 K when the pretreatment period was over 3 h. This is because the concentration of thiourea was decreased by the oxidation at 308 K, while the thiourea was not consumed by ferric ion at 288 K. It is considered that the lower temperature operation is preferable when the leaching solution is recycled. Introduction
2CS(NH2)2 + 2Fe3+ S
Gold and silver are mainly extracted from their ores by the cyanidation process in view of its relative economy and technological simplicity. However, the cyanidation process has some disadvantages, such as the low rate of extraction of gold and the high toxicity of cyanide solutions. The development of an alternative process to leach gold from its ores has been interesting. Since Plaksin and Kozhukhova (1941) have proposed the leaching method using thiourea, many studies have been carried out, and the effect of thiourea concentration, the concentration and type of oxidant, pulp density and temperatures, and the concentration and type of acid on the performance were investigated (Groenewald, 1976; Chen et al., 1980; Goto et al., 1985; Descheˆnes and Ghali,1988; Farinha et al., 1992; Ac¸ ma et al., 1993; Shengming et al., 1996). Some literature surveys on thiourea leaching have been also published (Descheˆnes, 1986). According to the results in the previously published papers, ferric ion is most effective as the oxidant, and sulfuric acid is more effective than nitric or hydrochloric acids. Some pilot plants and industrial applications that use thiourea have already been built (Raudsepp and Allgood, 1987). However, very few plants are in leaching operation and the process has not been widely understood. The overall reaction describing the extraction of gold in acidic solutions of thiourea (Kazakov et al., 1964) is
Au + 2CS(NH2)2 S Au(CS(NH2)2)2+ + e-
(1)
If ferric ion is used as an oxidant, gold can be leached by the following equation:
Au + 2CS(NH2)2 + Fe3+ S Au(CS(NH2)2)2+ + Fe2+ (2) However, thiourea is easily oxidized by ferric ion in acidic solutions, and formamidine disulfide is produced (Gupta, 1963): * To whom correspondence should be addressed. Telephone: +81-99-285-8361. FAX: +81-99-257-5895. E-mail:
[email protected]. S0888-5885(97)00064-X CCC: $14.00
(CS(NH2)(NH))2 + 2H+ + 2Fe2+ (3) Formamidine disulfide is not stable in acidic solutions and decomposes irreversibly producing elemental sulfur and cyanamide:
(CS(NH2)(NH))2 w CS(NH2)2 + NH2CN + S (4) Thiourea is also lost by the formation of a stable ferric sulfate complex (Hiskey, 1984; Pyper, 1981; Groenewald, 1976):
Fe3+ + SO42- + CS(NH2)2 S [FeSO4(CS(NH2)2)]+ (5) When the temperature is over 333 K, thiourea thermally decomposes (Groenewald, 1976; Pyper, 1981). Although some researchers concluded that the acidothioureation method is promising to replace the traditional cyanidation method (Chen, 1980), this method has some problems which remain to be solved. As pointed out by many researchers, the high consumption of the reagent and the lack of technical information on the gold recovery from thiourea solutions are the main disadvantages in the utilization of thiourea. It was reported that the decomposition of thiourea was suppressed by the addition of a reducing agent, such as SO2. It is known that other suitable reductants are hydrogen sulfide and its salts such as sodium sulfide, zinc blende, pyrrhotite, as well as dithionites, dithinates, and thiosulfates (Schulze, 1986). Wakamatsu et al. (1987) have reported that thiourea consumption can be reduced to half and the leaching time can be shortened by increasing the leaching temperature to 333 K. Descheˆnes and Ghali (1988) reported an improvement in gold extraction with an increase in temperature up to 333 K. On the other hand, Farinha et al. (1992) have reported that gold extraction decreased by 44.2%, when the temperature was changed from 303 to 323 K. Groenewald (1976) attributed this decrease to rapid thiourea decomposition when the temperature is over 318 K. Pyper (1981) considered it to be caused by the thermal degradation of the gold thiourea complex. The objective of this work is to reduce thiourea consumption in the thiourea leaching of a gold concentrate. Because the temperature dependence of the © 1997 American Chemical Society
2758 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
Figure 1. Arrhenius plot for thiourea consumption by ferric iron.
Figure 2. Effect of leaching temperature on the extraction rate of Au and Ag from Hishikari ore.
leaching rate was lower than that of thiourea consumption by the unfavorable reaction, we considered that the lower temperature was preferable to reducing the thiourea consumption without a large decrease in the leaching rate. We investigated the effects of pretreatment time on the recovery of gold and silver. Experimental Section Material. The ore sample used in the leaching was mined from Kagoshima Prefecture in Japan and was supplied by Sumitomo Metal Mining Co., Japan. The sample contained 56.1 g/ton gold and 43.6 g/ton silver. The ore used in this study was silicate and contained more than 90 wt % of insoluble components. The ore sample was crushed and sized under 53 µm for the leaching. Leaching. The extraction of gold and silver was carried out in 300 mL Erlenmeyer flasks containing 20 g of the crushed ores and 100 mL of leaching solution. The solution contained thiourea (1.0 wt %), ferric ion, and sulfuric acid. The fraction of thiourea used for the extraction of gold and silver is lower than 0.3% according to the stoichiometry. The flasks were shaken in a constant temperature bath at 303 K for a prescribed period. Samples were taken for analysis at predetermined intervals. The concentration of gold and silver in the leaching solution was determined using atomic absorption spectrophotometry. The concentration of ferrous iron was analyzed by the o-phenanthroline method. Ferric iron was also analyzed after reduction by hydroxylamine hydrochloride. The thiourea concentration was determined by the spectrophotometric method. Results and Discussion Oxidation of Thiourea. The oxidation rate of thiourea by ferric ion was determined in the solution without the ores. The concentration change in thiourea was measured for various initial concentrations of thiourea and ferric ion. The concentration of sulfuric acid was fixed to 0.5 N. The consumption rate of thiourea was determined by differential method using the concentration change in thiourea with time. It was found that the consumption rate of thiourea was firstorder dependent on thiourea and ferric ion concentrations. The reaction rate constant of this second-order reaction at 303 K was determined to be 1.55 × 10-6 m3 mol-1 s-1. We obtained the values of the rate constant at various temperatures. Figure 1 shows the Arrhenius plot for the thiourea oxidation by ferric ion. The activation energy was calculated from the slope of the
Figure 3. Effect of pretreatment time on the recovery of Au.
line in this figure to be 79 kJ mol-1. This result agreed with the value of 73 kJ mol-1 which was obtained by Zhu (1992). Temperature Dependence of Leaching Rate. We measured the extraction rate of gold and silver under various conditions and obtained the initial rate of the extraction. The apparent activation energy for the extraction of gold and silver was lower than that for the thiourea consumption. Figure 2 shows the dependence of the initial extraction rate of gold and silver on the leaching temperature. In this case, the thiourea concentration was 1.0 wt %, and the concentration of sulfuric acid was 0.5 N. The apparent activation energy obtained from this figure was 8.5 and 19 kJ mol-1 for gold and silver, respectively. Especially in the case of gold extraction, the apparent activation energy was small, about 11% of that for thiourea consumption. These small values do not correspond to the activation energy of the reaction itself presented by eq 1, because the dissolution of the metals was controlled by the physical processes such as diffusion and adsorption. Effect of Pretreatment of Leaching Solution. From the results described above, it is thought that the consumption of thiourea by the side reaction might be reduced by carrying the leaching at lower temperature. The leaching solution was prepared and preserved for a prescribed time at a constant temperature. The thiourea in this solution was consumed by ferric ion in this pretreatment. After this pretreatment, this solution was used for the leaching. The temperatures of the pretreatment and the leaching were the same. The leaching time was 2 h, and the concentrations of thiourea and ferric iron were 1.0 and 0.2 wt %, respectively. Figure 3 shows the effect of the duration of the pretreatment on the recovery of gold. In the case of the
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2759
ent on the material and the composition of leaching solution, but in this study we pointed out the possibility that the consumption of thiourea could be reduced in the leaching operation at lower temperature. Acknowledgment The authors wish to thank Sumitomo Metal Mining Co., Ltd., for supplying the gold ores. Nomenclature
Figure 4. Effect of pretreatment time on the recovery of Ag.
extraction of gold, because the effect of leaching temperature on the extraction of gold was small as shown in Figure 2, the recovery of gold without the pretreatment was not influenced by the leaching temperature. The recovery decreased with increasing pretreatment time. The degree of the decrease was large when the temperature was higher. This occurs because thiourea was oxidized by ferric ion in the pretreatment. The gold extraction was not so influenced by the pretreatment when the temperature was 288 K, whereas the gold recovery decreased by 30% at 308 K. Figure 4 shows the effect of the duration of the pretreatment on the recovery of silver. In this case, the extraction without the pretreatment was affected by temperature. As shown in Figure 2, the apparent activation energy of silver extraction was 19 kJ mol-1, and this value was larger than that for the gold extraction. The silver recovery was 97% at 308 K, while the recovery was 71% at 288 K. However, the consumption of thiourea by the side reaction was promoted when the temperature was 308 K. After a 4 h pretreatment at 308 K, the silver recovery decreased by 40%. On the other hand, the decrease was about 10% when the temperature was 288 K. The silver recovery was somewhat higher for the 288 K leaching than for the 308 K leaching after a 4 h pretreatment. Although the thiourea oxidation was promoted at higher temperature, the higher temperature was selected in the previous studies. This is because the leaching could be operated at lower thiourea concentration and the consumption of thiourea was reduced in a batch operation. When the leaching solution is recycled, however, the consumption by the oxidation should be avoided. The thiourea consumption can be reduced without a large decrease in the recovery by setting the leaching temperature at a lower level as shown in the present study. The oxidation of thiourea by ferric iron is a homogeneous reaction which occurs in the bulk liquid phase, and the oxidation rate was affected by temperature. On the other hand, the dissolution rate of gold and silver was dominated by the physical processes such as diffusion of thiourea into the pore of ores and adsorption on the solid surface. Therefore, the dissolution rate of the metals was insensitive to temperature. Although the leaching rate slightly decreased at a lower temperature, this decrease is not a significant problem because the extraction of both gold and silver in a suitable thiourea medium is over 10 times faster than that in the cyanide solution. There are many parameters to be adjusted for the most economical operation. The optimum operating condition is depend-
Ea ) activation energy, kJ mol-1 k ) rate constant of thiourea oxidation, m3 s-1 mol-1 re ) initial rate of metal extraction, mol s-1 kg-1 T ) temperature, K
Literature Cited Ac¸ ma, E.; Arslan, F.; Wuth, W. Silver extraction from a refractory type ore by thiourea leaching. Hydrometallurgy 1993, 34, 263. Chen, C. K.; Lung, T. N.; Wan, C. C. A study of the leaching of gold and silver by acidothioureation. Hydrometallurgy 1980, 5, 207. Descheˆnes, G. The recovery of gold from thiourea solutions and the comparison with cyanidation. CIM Bull. 1986, 79, 76. Descheˆnes, G.; Ghali, E. Leaching of gold from a chalcopyrite concentrate by thiourea. Hydrometallurgy 1988, 20, 179. Farinha, P. A.; Correia, M J. N.; Carvalho, J. R. Leaching of gold from a Portuguese concentrate with thiourea. Miner. Eng. 1992, 5, 953. Goto, S.; Ogawa, O.; Asakura, I.; Nakamura, S. The leaching of gold and silver from ore with thiourea in sulfuric acid solutions. Nippon Kogyou Kaishi (in Japanese), 1985, 101, 75. Groenewald, T. The dissolution of gold in acidic solutions of thiourea. Hydrometallurgy 1976, 1, 277. Gupta, P. C. Analytical chemistry of thiocarbamides. I. Quantitative determination of thiourea. Z. Anal. Chem. 1963, 196, 412. Hiskey, J. B. Thiourea leaching of gold and silversTechnology update and additional applications. Miner. Metall. Process. 1984, 11, 173. Kazakov, V. P.; Lapshin, A. I.; Peshchevitskii, B. I. Redox potential of the gold(I) thiourea complex. Russ. J. Inorg. Chem. 1964, 9, 708. Plaksin, I. N.; Kozhukhova, M. A. The solubility of gold and silver in thiourea, C. R. Acad. Sci. USSR 1941, 31, 671. Pyper A.; Hendrix, J. L. Extraction of gold from finely disseminated gold ores by use of acidic thiourea solution. Ext. Metall. ’81, Pap. Symp. 1981, 57. Raudsepp, R.; Allgood, R. Thiourea leaching of gold in a continuous pilot plant. Proc. Int. Symp. Gold Metall. 1987, 87. Schulze, R. G. Thiourea leaching of precious metals Erzmetall 1986, 39, 57. Shengming, X.; Chuanfu, Z.; Tiancong, Z.; Yanjun, W. Thiourea Leaching of gold from a calcine of gold-bearing arsenical pyrite concentrate Trans. Nonferrous Met. Soc. China 1996, 6, 21. Wakamatsu, T.; Nakahiro, Y.; Kawaguchi, T.; Ardiwilaga, S.; Shibata, J.; Sano, M.; Nishimura, S. A study on the leaching of gold ore from Cimanggue mine, Indonesia by acidothioureation. Sigensyori-Gijyutu 1987, 34, 113. Zhu, T. The Redox Reaction between Thiourea and ferric iron and catalysis of sulphide ores. Hydrometallurgy 1992, 28, 381.
Received for review January 22, 1997 Revised manuscript received April 17, 1997 Accepted April 21, 1997X IE970064R
Abstract published in Advance ACS Abstracts, June 1, 1997. X
