Public defence in ecology: Nathan van Wyk

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Abstract

Over 3 billion tonnes of metal are produced annually to be used in structural elements, as alloys, and as 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 scarcer metals. However, the last 50 years have experienced an inflexion point where rare earth metals have come into greater focus as a ‘green’ surge toward global electrification has taken place. Rare earth metals are, by definition, present in ores at low concentrations and result in greater volumes of processing waste.

Mine waste includes tailings, metallurgical slag, process residues, and waste effluents. These wastes are an environmental hazard, but also present an opportunity as they are often (semi)selectively enriched with metals other than what initially mined for. Several wastes were investigated as a source of critical raw materials and rare earth elements through dissolution of their bulk by organic and mineral acids produced by bacteria.

The wastes investigated were a bauxite residue from Greece, several magnesium-rich wastes from Spain, platinum group metal-containing wastes from the UK, and vanadium-containing magnetite from Norway.

The bauxite residue showed maximal dissolution (71.22 %) in spent media from a Gluconobacter oxydans culture that contained gluconic acid at significantly lower normality than the other acids tested. This indicated that while availability of hydronium ions affect dissolution, an acid’s conjugate base is as an important consideration. In addition, a combination of organic acids in these spent media may affect leaching. Although contact leaching of bauxite residue in a live G. oxydans culture may promote leaching of some metals, other metals appeared to be minimally soluble and leave solution after three days of exposure.

The magnesium containing waste was shown to rapidly undergo dissolution in a Fervidacidithiobacillus caldus culture, which produced sulfuric acid from elemental sulfur – itself a waste 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 elemental sulfur alone, accumulating up to 16 g/L magnesium in solution. Dissolution with sulfuric acid also appeared to be selective toward magnesium over iron, calcium, and manganese.

Finally, the vanadium-containing magnetite, when cultured with Shewanella loihica, did not yield significant metal release. Leaching with gluconic acid produced by Gluconobacter oxydans resulted in a maximum yield of 3.3 % of the available vanadium. Microorganisms endogenous to the mineral may also explain the elevated levels of vanadium in effluent and could potentially be utilised for the bioleaching of magnetite in future work.

Acid dissolution of mine waste presents a viable path to the valorisation of these wastes; however, mineralogy of the wastes is pivotal to yield and requires further investigation.

Keywords: Magnesium, scandium, PGM, vanadium, elemental sulfur, bioleaching, Gluconobacter oxydans, Fervidacidithiobacillus caldus