Nickel ammonium thiosulphate leaching of gold from waste mobile phone printed circuit boards

Abstract

Over the years, the quantity of e-waste, primarily waste mobile phone printed circuit boards (PCBs), has increased worldwide, as a result of technological advancements in the fields of electronics, telecommunications and computing. Therefore, to sustain the protection of the environment and human health, sustainable measures should be implemented. Electronic waste (e-waste) is known for its wider variety of base and precious metal content compared to naturally occurring ores. Waste mobile phone PCBs are the most attractive type of e-waste due to their higher and more structured precious metal content. Ammonium thiosulphate leaching is considered as one of the promising alternatives to cyanidation in hydrometallurgy. However, its industrial application is limited due to the high reagent consumption which has a direct impact on its cost-effectiveness. This research aimed to investigate the ammonium thiosulphate leaching of gold from waste mobile phone PCBs, with an emphasis on the thiosulphate consumption and interference of the extractable copper in the leaching process. The objectives were to study the effect of acid pre-treatment and copper replacement with nickel as the metal oxidant on gold extraction and thiosulphate consumption. The experiments in this study were all conducted in a batch setup. The PCBs were reduced to less than 3 mm particle size by cutting and crushing. In the acid pre-treatment, sulphuric acid and hydrogen peroxide concentrations were varied between two levels, namely 2 M and 3 M, to identify the optimum conditions that maximised copper extraction and minimised gold extraction. In the ammonium thiosulphate leaching, PCB pre-treatment and metal oxidant were investigated as categorical factors to establish the optimum conditions that maximised gold extraction and minimised thiosulphate consumption. The PCB pre-treatment was varied between two levels: acid-pretreated PCBs and untreated PCBs. The metal oxidant factor was varied between copper and nickel. The other leaching conditions were fixed, as prescribed in the existing literature. The mobile phone PCBs used in this study were found to contain 524 g Au/ton-PCB and 461.8 kg Cu/ton-PCB, as determined by aqua regia leaching. Copper thus contributed to more than 40% of the total PCB mass. In the acid pre-treatment, the optimum reagent combination was 2 M H2SO4 and 3 M H2O2 and resulted in 93.72% copper extraction and 8.83% gold loss in 150 minutes at a pulp density of 50 g/L, 25°C and stirring speed of 350 rpm. Furthermore, it was determined, through material balance, that the PCB mass reduction induced by the acid pre-treatment was mostly attributed to the copper extraction, evidence that the other base metals such as iron and aluminium that could have dissolved in the acid pre-treatment stage were in small quantities in the PCBs to impact the overall mass reduction significantly. The acid pre-treatment was a PCB beneficiation process with an increase in the extractable gold content from 524 to 842 g/ton-PCB and a decrease in the copper content from 461.8 to 51.1 kg/ton-PCB. The variation in H2SO4 concentration had more statistical impact on gold extraction, whereas varying H2O2 concentration had a more statistical influence on copper extraction. The analysis of the goodness of fit of the shrinking-core model to the experimental results indicated that the acid pre-treatment was chemically controlled, with moderate control due to the turbulence. In ammonium thiosulphate leaching, the optimum conditions consisted in using nickel as the oxidant in the thiosulphate leaching of acid-pretreated PCBs at a pulp density of 50g/L, 0.1 M thiosulphate, 0.2 M NH3, 0.03 M Ni2+, pH 10.5, 25°C and stirring speed of 350 rpm. The gold extraction and thiosulphate consumption were found to be 65.41% and 61.03 kg/ton-PCB in 5 hours of leaching time, a significant improvement from the conventional copper-thiosulphate leaching of untreated PCBs which resulted in gold extraction and thiosulphate consumption of 18.61% and 90.9 kg/ton-PCB, respectively. The statistical analysis of experimental results indicated that the variation of the metal oxidant had a higher significance level than the PCB pre-treatment with respect to gold extraction. The assessment of the goodness of fit of the shrinking-core model to the experimental data indicated that the copper-thiosulphate and nickel-thiosulphate leaching processes were both chemically controlled. To assess the degree of comparison between the copper-thiosulphate and nickel-thiosulphate leaching processes for gold extraction from waste PCBs, a preliminary economics assessment was performed, with emphasis on production costs. Considering a basis of 1 metric ton of processed PCB, replacing copper with nickel as the metal oxidant reduced the raw materials costs from 3,768 USD to 2,868 USD, approaching previously reported cyanidation costs closely. The revenue and gross margin were increased from 8,969 and 5,201 USD in copper-thiosulphate leaching to 16,287 and 13,419 USD in nickel-thiosulphate leaching, respectively. Therefore, the use of ammonium thiosulphate as an environment-friendly alternative lixiviant shows potential by virtue of the improved process economics.