Towards the Biobeneficiation of PGMs: Reviewing the Opportunities

Abstract

Conventional beneficiation of the Platinum Group of Metals (PGMs) relies on the use of inorganic chemicals. With the depreciation of high grade deposits, these conventional processes are becoming less economically viable. Furthermore, the use of chemicals has serious negative impacts on the environment. To address the challenges of conventional PGM beneficiation, biobeneficiation has been proposed. In conventional flotation, the flotation behavior of the associated sulphides determines overall PGM recovery. The same principle may also be applied for the bio-beneficiation of PGMs. Therefore, this paper discusses the biobeneficiation behavior of sulphides closely associated with PGMs with the aim of postulating the bio-beneficiation behavior of PGMs associated with the same base metal sulphides. Conventional PGM processes are briefly discussed, as bio-beneficiation of PGMs is governed by similar underlying principles. Potential microorganisms for the biobeneficiation of PGMs are highlighted, as well as the corresponding conditions for their effectiveness. The use of both single cultures and mixed cultures is discussed. Depending on conditions, PGMs associated with pyrite and/or chalcopyrite were projected to be biofloatable with B. polymyxa, P. polymyxa, A. ferrooxidans, L. ferrooxidans, B. pumilus, B. subtilis, halophilic bacteria, Alicyclobacillus ferrooxidans, sulphate reducing bacteria, and mixed cultures of A. ferrooxidans, A. thiooxidans and L. ferrooxidans. Pyrite-associated PGMsare expected to be generally prone to biodepression, whereas chalcopyrite-associated PGMs are expected to be generally recovered as the floatable phase. Sulphate-reducing bacteria were reported to have a dual role on the bioflotation of sulphide ores (flotation and depression), depending on the conditions. Therefore, this type of microorganism may serve as both a depressant or a collector in the recovery of PGMs. Based on the bioflotation response of pyrrhotite to L. ferrooxidans, it is anticipated that pyrrhotite-associated PGMS can be biodepressed using L. ferrooxidans. In terms of bioflocculation, PGMs associated with chalcopyrite may be recovered using L. ferrooxidans, whereas A. ferrooxidans, A. thiooxidans, B. polyxyma and B. subtilis can be used in the bioflocculation of pyrite-associated PGMs. M. phlei can be employed in the reverse bioflocculation of pyrite-associated PGMs. Although no information was found on the biobeneficiation of pentlandite, postulations were made based on other sulphide minerals. It was postulated that biobeneficiation (biodepression and bioflotation) with pentlandite-associated PGMs should be possible using A. ferrooxidans. It is also projected that sulphate-reducing bacteria will be suitable for the bioflotation of PGMs associated with pentlandite. The removal of gangue species such as silicates and chromites associated with PGM concentrates was also discussed. A. ferrooxidans, P. polymyxa and B. mucilaginous are candidates for the removal of gangue species. Furthermore, the need to control process conditions was highlighted. The most suitable conditions for biobeneficiation of the various base metal sulphide minerals associated with PGMs are presented in the paper. Most of the challenges associated with biobeneficiation of PGMs are already common to conventional methods, and the means of circumventing them are already well established. Developments in genetic engineering and the advent of new data science techniques are tools that could make the biobeneficiation of PGMs a possibility.

Acidithiobacillus Ferrooxidans Applied for the Beneficiation of Platinum Group Metals-Bearing Base Metal Sulphides

“…The capacity and contribution of micro-organisms and their metabolites towards mineral processing and metallurgical practices is well established. Some of the topics includes [3,4,17,32] Several mechanisms that explain the interaction between the micro-organism and metal-bearing minerals draw down to biomodification and bioremediation. Biomodification mechanism can be imputable to [3] a) The adhesion of microbial cells to the mineral surface, b) Oxidation reactions brought by the micro-organisms and their metabolites onto the minerals, c) The adhesion of bacterial proteins and exopolysaccharides to mineral surfaces and/or the chemical reaction between mineral surfaces and metabolite products.…”

“…The microorganisms break down the coating around the fines and/or gangue minerals on the mineral surfaces by using them as an energy source or co-metabolizing them with an energy source. In both biomodification and bioremediation, the adhesion of the micro-organism depends on the mineral's chemical properties, toxicity of chemical composition as well as dissolution towards the micro-organism [3].…”

“…Current reagent suites also has drawback with high dosages, cost implication, toxicity and in ability to achieve high recoveries. The conventional methods of recovering PGMs have become less economical and less effective [3] . As such, bio-beneficiation of PGMs-containing ores is receiving interest since it has a potential of being economical, effective, and environmentally friendly.…”

“…These include Acidithiobacillus ferrooxidans (A. ferrooxidans) previously known as Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans, Leptospirillum, Ferroplasma acidiphilum, members of the genera Acidimicrobium, Ferromicrobium, Sulfobacillus, Sulfolobus, Acidianus, Metallosphaera, Sulphurisphaera, and mesophili [8,[17][18][19][20][21][22][23]. Their various applications are presented in a recently published review paper by Chipise et al [3]. In the current review, the capacity of A. ferrooxidans to beneficiate PGMs directly as metals is not reported, no relevant data was available in the open literature.…”

“…It was found that mesophile and thermophile selectively modified the compounds of the UG2 ore such that during the flotation of the conditioned ore, the chromite was suppressed for the ore exposed to mesophile, while the sulphide flotation was enhanced in the ore conditioned with thermophile. Since PGMs are primarily associated with sulphides, the current work investigates the potential of using A. ferrooxidans as a modifier, collector or a depressant in common sulphide processes as per in the existing literature review [3,40]. The sulphides of interest include FeXS, (Fe, Ni)9 S8, (CuFeS2), (NiS), (FeS2), due to their association with PGMs.…”