Critical raw materials - Advanced recycling technologies and processes: Recycling of rare earth metals out of end of life magnets by bioleaching with various bacteria as an example of an intelligent recycling strategy
The current ongoing transition in the energy and mobility sector is partially based on elements which can be classified as critical, like rare earths and cobalt. For these elements recycling can be one option to lower the demand on primary raw materials. Nevertheless the complexity of devices is high making recycling for technology metals complicated. Therefore the generation of concentrated fractions is an important step in the process chain of treating secondary raw materials. In addition to the established classification and sorting processes based on physical principles there is an increasing demand for intelligent solutions to meet the growing complexity and heterogeneity of material flows. Conventional recycling strategies based on pyrometallurgical or hydrometallurgical processes can quickly become costly due to a high energy requirement and usage of chemicals. Bioleaching offers a green recycling strategy, where leaching of waste material is performed by microorganisms. In this study the recycling potential of end-of life magnets was investigated by means of bioleaching with various bacteria. The experiments were done in shaking batches, whereby in parallel the chemical leaching represented by abiotic controls was followed. As sample material magnets with different alloy composition and particle sizes were examined. The experiments were quantified using ICP-OES, µ-XRF, XRD and SEM with EDX. All samples could be successfully bioleached. The highest leaching efficiencies were determined in approaches with Acidithiobacillus and Leptospirillum ferrooxidans. Leaching efficiencies up to 100% (Dy, Pr) were achieved. According to leaching efficiencies of the abiotic controls were in the same range as in biotic batches a chemical leaching by acids was detected as dominant. However, batches with an iron addition showed higher leaching due to the catalytic effect of Fe3+ ions. On the other hand bioleaching was exposed as more efficient due to lower costs, a less use of chemicals and lower pollution due to emissions and contaminated residues. An up-scaling of the investigated process was done in a bioreactor, wherein no loss in efficiency was recorded. The obtained solution was further purified by processes to extract the rare earth elements. The precipitation with oxalic acid and a two-step extraction was identified as the most efficient methods. Extraction rates of the REE up to 100% with a purity of 98% were achieved.