Abstract
The kinetic study of nonoxidative leaching of cinnabar concentrate ore in aqueous hydrochloric acid-thiourea solutions is presented. The dissolution rates were investigated over a temperature range of 60 to 100 °C, at an acid concentration of 0 to 5 M and thiourea concentration ranging from 0.3 to 1.5 M at atmospheric pressure.The dissolution rate is of the second order for thiourea concentration, first order for HCl in the concentration range of 0 to 2 M, and zero order for HCl concentration above 3 M. An activation energy of 53.6 kJ/mole was found. To obtain mercury, the leaching solution was treated electrolytically, with metallic mercury depositing on the cathode and thiourea oxidizing on the anode to formamidine disulfide. The spent electrolyte may be recycled to the leaching reactor. In the leaching reaction formamidine disulfide is reduced to thiourea by the H2S generated in the cinnabar attack according to: $$\begin{gathered} HgS + 2HCl + 2CS(NH_2 )_2 \to HgCl_2 \cdot 2CS(NH_2 )_2 + H_2 S \hfill \\ H_2 S + \mathop C\limits_{\mathop {||}\limits_{NH} }^{\mathop |\limits^{NH_3^ + } } - S - S - \mathop C\limits_{\mathop {||}\limits_{NH} }^{\mathop |\limits^{NH_3^ + } } \to 2CS(NH_2 )_2 + S^ \circ + 2H^ + \hfill \\ \end{gathered} $$ No change was found in leaching power after four complete cycles in laboratory-scale experiments. The single byproduct is elemental sulfur.
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