Leaching and recovery of precious metals from scrap

Abstract

Electronic waste (E-waste) contains precious metals including gold along with base metals. Pyrometallurgical and hydrometallurgical processes are being used for recovery of these metals. Pyrometallurgical process requires high energy and is hazardous due to toxic emissions during incineration, smelting and roasting of waste. Therefore, hydrometallurgical process is gaining attentions which includes leaching, purification and recovery of metals. Different substances like cyanide, thiosulfate, thiourea, nitric acid and aqua regia are known to be used for leaching agents. Most of them are either less efficient or highly corrosive. Nitric acid dissolves all base metals leaving behind gold particles. In this method, gold can directly be recovered by leaching all base metals except gold. At ambient conditions, its leaching rate is slow with high consumption of nitric acid. The XRD report of recovered particles implies that gold is separated in its pure form. The aim of this study was to maximize the concentration of copper in leachate with minimum use of nitric acid. In order to achieve this objective, Design expert ® software was used to analyze the effect of ultrasound, temperature and nitric acid concentration on copper leaching and to build a mathematical model. A central composite method in response surface methodology (RMS) was applied for the mathematical modeling of leaching concentration of copper from AMD athlon processors. The HNO3 solution was used to leach copper and other base metals, leaving behind the gold particles. Fifteen experimental runs was performed at different concentrations (25-50 %), temperatures (25-50 oC) and ultrasound power (0-300 W) simultaneously. The purposed model was tested by ANOVA and optimization of leaching was carried out by maximizing the concentration of leached copper (mg/l) and by minimizing the concentration of HNO3 (%). The R2 value was found to be 0.902 which implies that model is statistically accurate Quadratic model proposed that at maximum 68.882 mg/l of copper concentration in leachate, minimum 24.393 % HNO3, 37.571oC and 256.066 W ultrasonic power is required. The optimization of leaching suggested that at same nitric acid concentration, the leaching of copper (mg/l) can be increased from 3.30 mg/l to 68.88 mg/l by applying ultrasound waves. This implies that leaching can be enhanced by applying ultrasound waves.