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.

Highlights

  • Bio-beneficiation is the concentration of mineral species by employing microorganisms that interact with either the gangue or the valuable mineral species

  • This paper has demonstrated the immense potential of the biobeneficiation of Platinum Group of Metals (PGMs)

  • Biobeneficiation of the sulphide minerals closely associated with PGMs has been carried out and the conditions are well known

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Summary

Introduction

Bio-beneficiation is the concentration of mineral species by employing microorganisms that interact with either the gangue or the valuable mineral species. Bio-beneficiation can be described as the use of microorganisms to interact with minerals to subsequently induce processes such as magnetic separation [1], flotation, and flocculation [2]. It has been reported that oxidation reactions are responsible for bio-modification [7] Another proposed mechanism for biomodification is the adsorption of bacterial proteins and exopolysaccharides on mineral surfaces and/or the chemical reaction of mineral surfaces with metabolite products [8]. As in conventional beneficiation, knowledge of the flotation and flocculation behavior of various PGM-associated sulphide species in the presence of microorganisms is useful in the postulation of PGM biobeneficiation. We provide a background on the typical chemical and mineralogical compositions of PGM ores as well as their mineral associations This information is important, as mineralogy plays a critical role in mineral beneficiation. The challenges and opportunities of PGM bioprocessing are highlighted

Chemical and Mineralogical Composition of PGMs and Their Mineral Associations
Biobeneficiation of Base Metal Sulphides Associated with PGMs
Bacillus polymyxa
Paenibacillus polymyxa
Mycobacterium phlei
Acidithiobacillus ferrooxidans
Acidithiobacillus thiooxidans
Leptospirillum ferrooxidans
Bacillus subtilis
Bacillus pumilus and Alicyclobacillus ferrooxidans
Halophilic Bacteria
4.10. Sulphate-Reducing Bacteria
4.11. Mixed Cultures
4.12. Biobeneficiation of Pentlandite
Removal of Silicates and Chromites during Biobeneficiation
Potential Challenges and Opportunities of PGM Biobeneficiation
Findings
Conclusions
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