The use of thiourea, CS(NH2)2 as extracting reagent for gold and silver has shown promise for implementation in the gold mining industry. Laboratory results have shown that a thiourea process for gold extraction has several advantages over the classical cyanidation process. Thiourea is not restricted by the same toxicity factors as cyanide. Data shows high threshold limit values for mammals and humans. On this consideration, thiourea can be considered safer than cyanide. Other factor very important is the kinetics of thiourea dissolution of gold. Under proper conditions, a thiourea system has been shown to leach gold as much as twelve times faster than cyanide. Other important advantage is referred to cyanicides. Test results have probed that classical cyanicides affect partially the leaching action of thiourea. This reagent has been tested on different materials and ores with good results. For example pre-roasted materials such as pyrite and arsenopyrite were leached with gold recoveries as high as 95% in the first 35 minutes. Recovery of gold from pyrite concentrate has given around 95% and extraction of 89% on carbonaceous material. Most the work performed on thiourea was focused on leaching conditions and not much investigation was practiced on recovery of gold from pregnant solution. Research reports show the potential application of the same process employed for pregnant solutions obtained from cyanidation processes (i.e. activated carbon, ion exchange resins, cementation and solvent extraction). Obviously, some processes are better than other and the ore type has influence on the process to be used.
Probably, the major disadvantage of the thiourea process is the higher cost due to high thiourea and acid consumption. Excessive consumption is created by the oxidation-degradation by ferric ion. Other aspects that favor the degradation of this reagent are temperature and complex formation with other metals. Thiourea in an acid environment forms single cationic species with gold according to the following reaction,
Au + 2CS(NH2)2 = Au(CS(NH2)2)2+ + e-
The reaction is reversible. In the presence of an oxidizing agent such as hydrogen peroxide or ferric ions, thiourea oxidizes in successive stages to a number of products. The first step is the formation of formamidine disulfide.
2CS(NH2)2 = C2S2(NH)2(NH2)2 + 2H+ + 2e-
The reaction is fast with an electrode potential of +420mV. It has been noted that formamidine disufide is an active oxidant and is required for the gold dissolution. the presence of ferric ions is important for the process. Combining the previous equations,
2Au + 2CS(NH2)2 + C2S2(NH)2(NH2)2 +2H+ = 2Au(CS(NH2)2)2+
Au + Fe+3 + 2CS(NH2)2 = Au(CS(NH2)2)2+ + Fe+2
Continue oxidation of thiourea results in the slow formation of products in which sulphur has the higher oxidation state. At alkaline pH values, oxidation products are urea and sulphate ion. Formamidine disulfide breaks down with time to cyanamide, hydrogen sulfide and elemental sulphur. These secondary reactions promote the passivation of the gold surface reducing the rate of leaching. The addition of ferric ion as an oxidant reduces the rate of these secondary reactions. In this way, leaching rate is improved with the presence of ferric ion as oxidant. It is believed that thiourea reacts slowly with ferric ions in a sulphate media forming stable compounds,
SO4-2 + Fe+3 + CS(NH2)2 = FeSO4(CS(NH2)2)2+
About the pH, it has been reported that the formamidine disulfide reaction is reversible and the variation of reduction potential is independent of pH from 0 to 4.5. Nevertheless, the leaching rate is better at lower pH values. In the absence of surface films, the overall rate of dissolution of gold is controlled by diffusion of either reactants to the surface or products from the surface. Leaching conditions are approximately 140-160 g/L sulphuric acid, 40-50 g/L thiourea, 20-25% solids, 200-250 mV potential, 20-25oC temperature, pH 1.4-1.6. The addition of ferric salts is influenced by the ore type. It has been reported that gold leaching is improved by adding sulphur dioxide (SO2). This reagent prevents the passivation created by sulphur on gold particle surface. However the addition must be practiced is small dosages, otherwise the efficiency of the process is affected.