Bio-flotation: Bacterial cell hydrophobicity and adhesion to mineral surfaces

Author: Belinda Bleeze

Bleeze, Belinda, 2023 Bio-flotation: Bacterial cell hydrophobicity and adhesion to mineral surfaces, Flinders University, College of Science and Engineering

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Precious and core metals, such as copper, are essential for the development and improvement of modern-day technologies. As a high proportion of precious metals are found in sulfide minerals, the efficiency of sulfide mineral beneficiation processes is significant. Current beneficiation processes, namely froth flotation, are optimised for high-grade ore using harmful inorganic chemicals to produce a separation efficiency of no more than 80%. This inefficient separation leads to high metal content in mine tailings, which has been linked to environmental issues such as acid mine drainage. As the depletion of high-grade ore continues, beneficiation processes need to evolve to produce more efficient and ecofriendly refinement processes for low-grade ore.

This thesis investigates the optimisation of chalcopyrite recovery through bio-flotation from a mixed mineral system using bioleach microbes, Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans, and their metabolic products. To identify the conditions for optimal recovery, the impact of growth conditions on the behaviour of Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans, and composition of their Extracellular Polymeric Substances (EPS), is required. Investigating this will increase the understanding around what parameters in the bio-flotation system influences the separation and recovery of chalcopyrite.

A combination of microscopic and spectroscopic techniques were used to investigate the interactions between both L. ferrooxidans and A. ferrooxidans with pyrite and chalcopyrite mineral surfaces. These interactions are key in understanding microbial, and bio-reagent, implications on minerals processing of sulfide minerals. Scanning electron microscopy was used to determine preferential attachment and mechanisms of both L. ferrooxidans and A. ferrooxidans. Preferential attachment was observed for L. ferrooxidans but not A. ferrooxidans. This indicates A. ferrooxidans has greater resilience to changing growth conditions and toxic metals, stemming from their ability to utilize both iron and sulfur oxidative pathways to produce energy. This study indicates that the mechanism of attachment is dependent on the growth condition and not the bacterial strain.

Evidence of EPS production and biofilm formation was identified in both bacterial strains with discernible differences observed in the presence and attachment. The A. ferrooxidans were identified to produce both EPS and biofilm before the L. ferrooxidans. Investigating the link between metabolic products and flotation behaviour was made through micro-flotation tests. The presence of bacterial cells, either L. ferrooxidans or A. ferrooxidans, directly impacts the recovery of mineral by increasing the suppression of all minerals within the system. Removal of cells from the system resulted in selective suppression of pyrite, with the degree of suppression dependent on the strain and growth condition. Contact angle measurements support this, with an increase in heterogenicity and wettability of the pyrite surface when exposed to EPS supernatant and minimal change observe on the chalcopyrite surface.

Photoemission spectroscopy and X-ray photoelectron spectroscopy was used to analyse the surface states of pyrite and chalcopyrite exposed to bacterial cells and extracellular polymeric substances. Investigation of the surface chemistry changes can help the flotation behaviour observed be further understood. Analysis confirms the EPS can interact with mineral surfaces with and without the presence bacterial cells. PEEM analysis indicates that the EPS components show preferential oxidation of pyrite in a mixed mineral system, with preferential attachment of EPS to pyrite up to 72 h. The increase in complex organic molecules, identified to contain amides and carboxylic acids, correlates with the changes in flotation behaviour, with the primary EPS component identified as polysaccharides. This finding supports the notion that EPS and its composition is a key factor in the successful separation of chalcopyrite from pyrite in bio-flotation.

Keywords: Bio-flotation, pyrite, chalcopyrite, Leptospirillum ferrooxidans, Acidithiobacillus ferrooxidans

Subject: Science, Technology and Enterprise thesis

Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Professor Sarah Harmer