The valorisation of mine wastes: Metals dissolution through the action of acid-producing bacteria

Over 3 billion tonnes of metal are produced annually to be used in structural elements, as alloys, andas dopants in electronics; all of which affect every facet of our modern lives.As ore grades worsened with exploitation, mining technology improved to access the gradually scarcermetals. However, the last 50 years have experienced an inflexion point where rare earth metals havecome into greater focus as a ‘green’ surge toward global electrification has taken place. Rare earthmetals are, by definition, present in ores at low concentrations and result in greater volumes ofprocessing waste.Mine waste includes tailings, metallurgical slag, process residues, and waste effluents. These wastesare an environmental hazard, but also present an opportunity as they are often (semi)selectivelyenriched with metals other than what initially mined for. Several wastes were investigated as a sourceof critical raw materials and rare earth elements through dissolution of their bulk by organic andmineral acids produced by bacteria.The wastes investigated were a bauxite residue from Greece, several magnesium-rich wastes fromSpain, platinum group metal-containing wastes from the UK, and vanadium-containing magnetitefrom Norway.The bauxite residue showed maximal dissolution (71.22 %) in spent media from a Gluconobacteroxydans culture that contained gluconic acid at significantly lower normality than the other acidstested. This indicated that while availability of hydronium ions affect dissolution, an acid’s conjugatebase is as an important consideration. In addition, a combination of organic acids in these spentmedia may affect leaching. Although contact leaching of bauxite residue in a live G. oxydans culturemay promote leaching of some metals, other metals appeared to be minimally soluble and leavesolution after three days of exposure.The magnesium containing waste was shown to rapidly undergo dissolution in aFervidacidithiobacillus caldus culture, which produced sulfuric acid from elemental sulfur – itself awaste product. Dissolution was up to 74 % within 5 minutes of exposure, and up to 99 % after 57 days.In addition, F. caldus acid production with exposure to the ore was enhanced over that on elementalsulfur alone, accumulating up to 16 g/L magnesium in solution. Dissolution with sulfuric acid alsoappeared to be selective toward magnesium over iron, calcium, and manganese.Finally, the vanadium-containing magnetite, when cultured with Shewanella loihica, did not yieldsignific