From hazardous waste to fertilizer: Recovery of high-value metals from smelter slags

This work proposes a method to valorize lead slag for fertilizer purposes. The research concept was to selectively recover valuable microelements (Cu(II), Fe(II), Zn(II) in an amount of at least 0.2% m/v of each) by chemical leaching while retaining toxic elements in the slag (i.e. As and Pb). Among acids, hydroxides, salts and their mixtures tested for slag treatment, it was potassium hydrogen sulfate and ammonia liquor under strongly oxidizing conditions (in the presence of hydrogen peroxide) that proved to be the most effective leaching agents. Response Surface Methodology applied to optimize the slag leaching conditions set the most favorable process parameters (concentration of leaching agents, slag to reagent weight ratio, and temperature). As a result, the concentration of Cu(II) in the extract was 3751 mg/L (for ammonia liquor) and Fe(II) and Zn(II) concentrations in potassium hydrogen sulfate were 4738 mg/L and 6102 mg/L, respectively. To close the life cycle of the waste, immobilization in polyethylene and binding to cement were indicated as methods to manage the solid waste material after leaching. The mixed extracts rich in Cu(II), Fe(II) and Zn(II) ions were tested in germination tests on cucumber. No phytotoxic effect was observed, which raises the possibility of utilizing the solutions after chemical leaching of slag as an alternative source of micronutrients for the production of multicomponent fertilizers. The results are promising and fit in the assumptions of circular economy.