Technospheric mining of critical and strategic metals from nickel slag – Leaching with citric acid and hydrogen peroxide

Abstract

Critical and strategic metals (CSMs), such as cobalt (Co) and nickel (Ni), are fundamental components of the high technology industry. Securing the supply of CSMs is fast becoming a priority for nations. Thus, secondary sources such as mine wastes, slags, tailings, and residues are being exploited to extract CSMs since they contain notable amounts of those metals. Recovering valuable metals from secondary sources can be categorised as technospheric mining. In this study, the extraction of cobalt and nickel from nickel slag was examined using a combination of an organic acid, citric acid, and hydrogen peroxide as the oxidant. Optimum conditions consisted of 1 M citric acid, 0.5% (v/v) hydrogen peroxide, at a temperature of 60 °C, a pulp density of 2.5%, a particle size of −100 + 75 μm, and an agitation speed of 400 rpm, for a period of 6 h. At the optimum conditions, 84.0% Ni and 91.7% Co were extracted from flash furnace (FF) slag, and 65.0% Ni and 99.1% Co from converter (CV) slag. It is expected that a combination of citric acid and hydrogen peroxide created peroxycitric acid in the solution, which is a strong oxidant and a key factor in the dissolution of the metals. The leach residue from FF slag showed a porous surface resulting from preferential dissolution within the slag structure. The residue from the converter (CV) slag mainly consisted of magnetite as a result of the dissolution of fayalite (Fe2SiO4). The kinetic studies demonstrated that the dissolution of Ni and Co from FF slag is controlled by diffusion. On the other hand, a rate controlling factor for Ni from CV slag is mixed control and for Co is a chemical reaction. Based on the temperature varied experiments, the calculated activation energy for Ni and Co from FF slag were 34.4 kJ/mol and 27.6 kJ/mol, respectively. The activation energies for the leaching of Ni and Co from the CV slag were 16.1 kJ/mol and 23.7 kJ/mol, respectively.