Abstract
High reagent consumption and low efficiencies are the main obstacles against common use of thiourea for the leaching of precious metals. A new technique of universal application has been developed by which reagent consumption is depressed to about 0.5 kg/t while high metal yields are achieved and passivation of feed material is avoided. The basic principle is the selective reduction of formamidine disulfide to thiourea during leaching, maintaining the oxidized part of the initial thiourea to about 50%. This prevents the generation of passivating sulfur and enables high redox potentials of above 400 mV for rapid metal dissolution at low thiourea concentration. Pulp processes with activated carbon or resins as adsorbents can be applied. Stripping of precious metals is achieved with hot thiourea or acid solutions.
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