CEReS Deliverable 4.3: Extractive Metallurgy Unit Development Report

WP4 Char Leaching aims to demonstrate the feasibility for utilisation of the acidic leachates (biolixiviant) generated from coal wastes bioleaching (WP3) as leaching agent for extraction of the base and high-value metals contained in the char originating from the pyrolysis of PCB’s (WP2). The main tasks of this WP consist of three components: 1) investigation on the suitable reactor design and its working parameters for treating such material, which is unique compared to conventional materials treated in (bio)hydrometallurgy reactors, e.g. mineral ores, PCBs, etc., 2) exploring the option to extract metal(s) of interest and recover it/them to obtain a high-purity product, and 3) exploring the option to valorise the residue and excess iron in the char treatment system.
Stirred-tank reactor (STR) is the most commonly employed since the early days of biohydrometallurgy campaigns, ranging from lab-scale to industries, treating from mineral ores to electronic wastes and residues. The main advantage of this type of reactor is achieving the desired levels of mass transfer, mixing, and agitation, in which STR outperforms other types of reactors. Due to that, STR has been given priority during WP4 of CEReS project.
Despite the advantages, STR faces the challenge linked to the char’s special physical characteristics. The disintegration of fibres followed by their suspension inside reactor needs a careful hydrodynamic design to maintain sustained mixing and mass transfer. Moreover, the high shear stress of impeller rotation may harm the presence of biofilm, risking the elimination of microorganisms from the biolixiviant. As an alternative, leaching performance of rotary-drum reactor (RDR) has also been studied in this WP4. Step-leaching has also been studied in order to outline the possibility to increase the kinetic rate of copper dissolution by doing the leaching in several successive steps.
Different from mineral ores, char (or pyrolyzed PCBs) contains various metals and elements, oftentimes in high grades. More often than not, the leaching of the char is non-selective, meaning that other than metals of interest, the un-desired ones should also be dissolved at high concentration. This is proven to be problematic for recovery stage, thus highlight the importance of extraction stage as intermediate between leaching and recovery.
The solvent extraction stage consists of two steps; extraction step to transfer metal ions from solution (aqueous) to extractant (organic), and stripping step to transfer metal ions back from organic to aqueous. Concentrated metal ions in the aqueous phase are then transferred to recovery stage to produce metal product with high purity. This report will subsequently summarize the result on investigation of solvent extraction to produce concentrated electrolyte solution, followed by the results on recovery stage by electrowinning to produce metal product.
The treatment of residues came after the char leaching and solvent extraction-electrowinning occurred. The residue from char leaching still contains fairly high grade of precious metals (gold, silver) as well as platinum group metals (PGMs). Although the processing of such by-products is not in the scope of CEReS, the possibility of such treatment has been explored in relation to CEReS applying the concept of biohydrometallurgy. The bleeding of iron from the system is important to maintain balanced level of accumulative iron from overall process, and has been explored by performing jarosite precipitation.