5 - Improvements to the cyanidation process for precious metal recovery from WPCBs

Abstract

Cyanidation is the main technique for precious metals recovery in the mining industry. This hydrometallurgy technique consists in an electrochemical process that oxidizes gold and reduces oxygen with cyanide solutions at pH values above 10.5. During cyanidation, many factors are critical for precious metals dissolution from Waste Printed Circuit Boards (WPCBs), especially pretreatment techniques and leaching conditions. WPCBs have high concentrations of plastics and copper which can interfere in precious metals dissolution since copper can react to cyanide decreasing the necessary amount of cyanide required for gold dissolution. Therefore the usual methods employed consist in shredding and roasting the WPCBs to decrease particle size and eliminate plastics, respectively. In addition, acidic leaching is used in order to dissolve as much copper as possible from the WPCBs; therefore several acids have been assessed and nitric acid was demonstrated to be the most effective leaching agent for copper removal. Nevertheless, due to large metal particle sizes (>100μm) and the presence of reagents that can react to cyanide (copper), it is essential to increase the cyanide salt concentration from 2 to 4g/L, as well as leaching residence times from hours to days. It is important to take account that despite the effectiveness of nitric acid in copper dissolution, the best results in gold and silver recovery were obtained when ammonia leaching was employed as pretreatment for copper removal. The explanation lies in NH3 ability to displace cyanide in order to generate more free cyanide available for gold dissolution. To sum up, precious metals recovery from WPCBs is conditioned by pretreatment techniques and cyanidation conditions which are essential for the process efficiency.