Nathan van Wyk, Mark Dopson
Mining waste and the residues of metals extractions present both a toxic hazard and waste storage problem, but also a potential secondary source of rare earth elements (REEs) and critical raw materials (CRMs). Our BIORECOVER partners within the third work stream “Treatment to isolate interesting CRMs” have investigated the matter.
We investigated the leaching and solubilisation of several waste ores supplied by our industry partners (MAGNA, Spain, MYTILINEOS, Greece, and Johnson-Matthey, UK) using biologically produced mineral and organic acids. Treatment of waste ores with these biogenic acids present myriad benefits: a reduction of the energy requirements for metals extraction with a greener environmental profile, reductions in the need for, and reliance on, hazardous chemicals for extractions, and lowering of the pH and mineralogy of these often toxic and extremely alkaline wastes. In addition, the post-leach waste residues may be more amenable as an aggregate for construction materials, or as a soil bulking agent, reducing the space requirement when dumping.
Another significant benefit our work has found is that the dissolution systems used for some ores required the addition of wastes from unrelated processes, thus consuming multiple waste streams. Finally, we found that the dissolution process can be modulated through its components: alkaline ore can be used to raise the pH of an acid-generating culture to a more habitable range for the microbes used, while the acid-producing microbes drive the pH down.
Dissolution of waste ores result in the increase of solubilised metals, potentially poisoning the metabolism of the microbes of interest, and especially those isolated from non-soil environments. Our work shows that the toxic effect of a high-metals environment can be mitigated by switching from a batch to a semi-continuous process, and by implementing a custom settling system, we were able to retain the less acid-soluble ore component in the bioreactors while washing out fully solubilised and potentially toxic metals, enabling better leaching kinetics with a lower loss of solids as a result of wash-out.
Our work also highlighted how different acids, and combinations thereof, affected leaching rates in the tested ores. Mixed acid solutions of biogenic origin frequently outperformed isolated, pure acids as well as synthetic ‘mimics’ of the mixed biogenic acid solutions.
The research combines established knowledge with new processes and systems, updating the current state and scope of leaching and solubilisation technology. Using these data, we will move on to molecular work that focusses more on stress reactions of the microbes used in the processes. We have also made discoveries that centre on organism survival and stress responses that may have a wider impact on the understanding of these responses in microorganisms.
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