Gold Recovery from Waste Printed Circuit Boards by Advanced Hydrometallurgical Processing

Abstract

The scope of this study was to improve the hydrometallurgical processes involving iodine-iodide leaching and precipitation for recovery of gold from waste printed circuit boards. Firstly, the influence of different precipitating agents, namely ascorbic acid, trisodium citrate and sodium hydroxide on the recovery of gold from gold-iodide leach liquor were investigated in order to define the most effective precipitating agent. The leach liquor was prepared by dissolving pure gold chips in 1:6 molar ratio of iodine-iodide solution at 40°C, 550 rpm for 12 h. The variables, which affect the efficiency of gold precipitation from the leach liquor, were the molar ratio of precipitating agents to gold, pH and redox potential of the solutions. The attained high gold precipitation efficiency from the leach liquor was more than 99% under the highly acidic (pH < 1.6) and alkaline conditions (pH > 13) induced by 0.1 M ascorbic acid and 0.1 M sodium hydroxide respectively, but 64.5% of gold at a weak alkaline condition (pH 8) with 0.1 M trisodium citrate. Secondly, physico-chemical properties of resultant colloidal solutions and prepared gold particles were examined. Finally, recycling of waste printed circuit boards (WPCBs) via iodine-iodide leaching followed by the ascorbic acid reduction was discussed. Results indicate that over 95% of gold extracted from WPCBs by two-step iodine-iodide leaching under the defined conditions, while the dissolution efficiencies of other precious metals (Ag, Pd) and metal impurities (Cu, Al, Fe, Ni, Pb and Zn) were less than 1% and 3%, respectively. The vast majority of Au (99.8%), Cu (95.6%) and Ag (76.8%) were precipitated from the pregnant leach solution by ascorbic acid reduction at ambient conditions.

High-Pressure Oxidative Leaching and Iodide Leaching Followed by Selective Precipitation for Recovery of Base and Precious Metals from Waste Printed Circuit Boards Ash

“…3.5.1. Removal of Metal Impurities from the PLS Based on the preliminary experimental results [25,40], the pH of the PLS was adjusted up to 9.0 from 1.9 using 0.1 M NaOH at ambient temperature and stirring speed of 500 rpm. The efficiency of metals precipitation is shown in Figure 10.…”

“…Metals remained in the alkaline solution that resulted from the precipitation of base metals with NaOH were recovered by reductive precipitation using a 0.1 M, L-AA solution under conditions that were fixed as the molar ratio between Au and L-AA of 1:1, temperature of 25 °C, stirring speed of 500 rpm and precipitation time for 10 min [25,40]. Figure 11 shows the recovery of metals like Au, Cu, Ni, and Pb from the solution via precipitation with L-AA.…”

“…It was observed that the addition of L-AA results in a reduction of pH of the solution from 9 to 1.8 and an increase of Eh from 0.21 V to 0.64 V. The Eh-pH diagram for gold iodide and ascorbic acid species in the solution was constructed at 25 °C within the pH range from 0 to 14 using STABCAL software (N NBS, Helgeson thermodynamic data source, W32-Stabcal Version 1.0, Montana Technological University (Montana Tech), Butte, MT, United States) as shown in Figure 12. The thermodynamic data for gold iodide and L-AA related species were imported from the references [25,[41][42][43]. Table S1 summarizes the thermodynamic data for individual species in the The thermodynamic data for gold iodide and L-AA related species were imported from the references [25,[41][42][43].…”

“…The thermodynamic data for gold iodide and L-AA related species were imported from the references [25,[41][42][43]. Table S1 summarizes the thermodynamic data for individual species in the The thermodynamic data for gold iodide and L-AA related species were imported from the references [25,[41][42][43]. Table S1 summarizes the thermodynamic data for individual species in the solution used to construct the Eh-pH diagram.…”

“…These findings suggest that colloidal gold can be obtained from gold-iodide solutions by reductive precipitation with L-AA. The mechanism of the reduction of gold iodide complexes via L-AA is generally explained by the Equations (14)- (16) [25,46]…”