LIMITATIONS, CHALLENGES, AND ENVIRONMENTAL IMPACTS OF THIOUREA LEACHING
Although the toxicity of thiourea is far less than that of cyanide (a lethal dosage of thiourea is 10 g/kg), it is not an entirely environmentally friendly reagent. This is a subject of debate because thiourea is used in human thyroid treatments as a non- carcinogenic reagent (IARC, 1974; Shubik 1975). The discharge of various degradation products of thiourea, including urea, ammonia, formamidine disulphide, sulphur, cyanamide, carbon dioxide, sulphate and nitrate ions, is problematic and proper effluent treatment is needed (Gupta 1963; Preisler and Berger 1947; Hiskey and DeVries 1992). Cyanamide and formamidine disulphide are unstable and short-lived. The half-life of thiourea decomposition in surface water and soil is up to 168 h under aqueous aerobic biodegradation, and up to 336 h in groundwater without biodegradation (Howard 1991). There is some evidence of heavy metals dissolution in thiourea from minerals/soils causing soil and water pollution.
It must be noted that in the absence of any oxidizing agent, thiourea alone in lix- iviant is less effective. Fe2(S04), is preferred as a cheaper reagent than H202, but it also increases thiourea consumption. An excess of Fe2(S04), suppresses the leaching of gold. An initial concentration of thiourea and Fe,(S04)3 along with the rate of oxidation of thiourea are the factors that most affect the leaching of gold. The formation of degradation products in successive oxidation stages of thiourea makes it difficult to control the leaching system. High consumption of reagents including SC(NH2)2, Fe2(S04)3 and H2S04 makes the process more expensive than using cyani- dation. Therefore, unless these complexities are handled carefully, thiourea leaching in gold extraction is quite difficult.
Commercial applications of thiourea in gold recovery processing have been hindered by the higher consumption of reagents and the complexity of the leaching system when treating various ores/concentrates and different recycled materials. At 154 pm. 90% Au and 50% Ag could be achieved. Further size reduction achieves full precious metal recovery. However, thiourea consumption is usually very high and it is more expensive than cyanide and thiosulfate lixiviants. There are still many challenges in the recovery of precious metal from electronic wastes, including low total recovery. In practice, <20% of the PMs from electronic wastes have been recovered and recycled. Precious metal loss during pre-treatment and multistep leaching should be minimized in electronic waste treatment.
ACKNOWLEDGEMENTS
This work was supported by the Brain Pool Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (Grant No. 2019H1D3A2A02101993) and the Basic Science Research Programme through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Project no. 2020R111 A1 AO 1074249).
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