Exploring metal extraction potential from historical metallurgical sites and waste deposits using solvometallurgical and hydrometallurgical processes.

Metalliferous waste from past metallurgical sites present significant environmental risks yet provide opportunities for metal extraction and recovery to supplement global demands in the shift towards renewable technologies. Characterising site conditions, metal specificity of green solvents and chelators, and an initial assessment of operating condition are required to develop these approaches into a commercial setting. This study evaluates the efficacy of solvo- and hydro-metallurgical approaches in extracting metals from real-world metalliferous waste. Specifically, it examines the performance of green deep eutectic solvents (DES) and chelating agents (CA) on blast oxide slag dust (BOS-D) and goethite, by-products of iron smelting and zinc refinery processes, respectively. Through batch tests and column studies, the impact of varying operational conditions, including molar ratio for DES and pH for CA were assessed. The results were used to inform sequential bioleaching to solvo- or hydroleaching tests to investigate the potential of hybrid extraction processes. The extraction efficiencies across batch, column and the sequential study from BOS-D were compared against a commercial chelating agent ethylenediaminetetraacetic acid (EDTA). DES extracted 60% more of the total metal load compared to CA and EDTA from BOS-D in batch tests. When applied to goethite, CA extracted 70% more of the total metal load than DES in batch tests. There was a 1% difference in metal extraction between DES and CA from BOS-D and 2 % from goethite in the column study. EDTA extracted the same or up to 15% more of the total metal load than DES and up to 18% than CA in the column study. The sequential study demonstrated that DES and CA have minimal influence in metal extraction under the operational conditions used in earlier batch tests, when applied to the residue after bioleaching extraction using Acidithiobacillus ferrooxidans. The extraction of the total metal load was improved by 4% using CA. The findings of this study demonstrate that CA and DES can extract a robust suite of metals from real metalliferous wastes. In BOS-D batch and column tests, analysis of metal group selectivity showed that DES had greater extraction than EDTA across all metal groups except rare earth lanthanides. In BOS-D batch studies, EDTA had greater extraction than CA across all metal groups except alkaline metals, but column studies showed that CA had greater extraction than EDTA across all metal groups except alkaline metals. This provides evidence for a robust metal extraction process and could improve the suite of extractable metals, providing sustainable alternatives in metal supply chains. The understanding of these extraction processes will be improved through, optimisation of more physical controls e.g., temperature, agitation, and further optimisation of the initial chemical conditions to improve metal extraction efficiencies, metal selection and potential revenue generation.