Bacterial leaching of critical metal values from Polish copper metallurgical slags using Acidithiobacillus thiooxidans

Global economy faces an increasing problem of the supply risk of critical raw materials, therefore the search for secondary resources has become an urgent issue. Copper orebodies in Poland contain substantial amounts of metals deemed critical (e.g. Co, Mo, rare earth elements (REE) or V), which are not recovered during processing. The metals of interest are concentrated in metallurgical waste residues that should be classified as a secondary resource rather than as a waste. Bioleaching is a frontier technology promising for environment-friendly treatment of slags. Therefore, the objective of this work was to study the feasibility of metal (Co, Mo, REE, V) bioleaching from copper metallurgical wastes employing Acidithiobacillus thiooxidans bacterial strain as the leaching agent. The effect of particle size (fractions < 0.25 mm and 0.25-0.5 mm) and pulp density (1%, 2%) was studied using three different slag samples (lead slag - LS, shaft furnace slag - SFS and granulated slag - GS). The bioleaching experiment was set up for 28 days under acidic conditions (pH t(0) = 2.5). The results revealed that the microorganisms can catalyze metal extraction from slags as compared to leaching achieved under abiotic conditions. The optimal bioleaching yield was achieved under conditions at 0.25-0.5 mm particle size and 1% pulp density, regardless of used type of slag. After 28 days, the extracted amounts of metals were: 88% Co, 40% Mo, 83% REE and 55% V from LS, 100% Co, 44% Mo, 70% REE and 70% V from SFS and 95% Co, 70% Mo, 99% REE and 93% V from GS. All examined slags showed good potential for bioleaching of valuable elements. However, optimization of initial parameters is still needed for further efficiency improvement, especially in terms of the process duration.