Fritz, Ruby1, Parbhakar-Fox, Anita2, Jones, Thomas3, Southam, G3
1School of Mechanical & Mining Engineering, The University of Queensland, 4072 Australia, 2W.H.Bryan Mining and Geology Research Centre, Sustainable Minerals Institute, 40 Isles Road, Indooroopilly, Queensland, 4068 Australia, 3School of Environment and Earth Sciences, The University of Queensland, 4072 Australia
Reprocessing mine waste as a potential secondary resource of critical metals can reduce Australia’s reliance on primary sources affected by geopolitical issues and unethical mining, and will support the world’s transition to low-carbon economies. Economic rehabilitation can provide additional environmental opportunities in managing acid mine and metalliferous drainage (AMD) by removing the pollutant source, mitigating geotechnical issues and repurposing mined land.
The project aim was to geometallurgically characterise mine waste collected from the Capricorn Copper mine to inform the feasibility of bioleaching cobalt, specifically from tailings, using indigenous bacteria. Characterisation involved the integrated use of geochemical, mineralogical and mineral chemistry analysis techniques with triplicate bench-scale bioleaching column experiments performed on a composite tailings sample over a 4-month period. During these experiments the pH, solution chemistry, bacterial DNA and enumeration were measured.
The tailings are highly acid-forming (8.8 wt. % pyrite) presenting immediate water quality risks (i.e., low pH and high dissolved heavy metals); highlighting rehabilitation, or an AMD control strategy, needs to be initiated in the short-term. Mineral chemistry analysis confirmed pyrite in tailings as Co-bearing (max. 1,154 ppm). Bioleaching test work proved successful for one of the columns, with a fungal infection inhibiting bioleaching in the other two columns. An 8-week lag/adaptation phase was observed in the indigenous bacteria, showing a drop in leachate concentrations, followed by an exponential growth phase which saw a spike in leached metals (37 % Co recovery after 4 months). The lower than expected recovery rate is attributed to the limited timeframe of the experiment.
Based on these results, it is recommended that: i) sampling at greater depth in the tailings storage facility (i.e., well into the sulphidic zone >2 m depth) will return a higher Co head grade, therefore increasing the quantity of recoverable Co; ii) in order to scale up this work, pyrite flotation and bacterial adaptation to the concentrate is recommended to increase effectiveness prior to bioleaching in a highly-aerated, continuous-flow stirred tank reactor at optimised geochemical conditions; and iii) deleterious elements including As, Sb, and Pb will also leach on bacterial oxidation and therefore need to be managed in the new tailings streams, for example through downstream precipitation methods.
Ultimately, this study has shown the experimental first steps required to develop a business case for reprocessing waste using bioleaching, along with a proposal of integrating circular economy principles into short- and long-term mine planning. If action is not taken immediately, the cost of passively rehabilitating will be unprecedented for communities, ecosystems and the longevity of the industry itself.
Ruby Fritz is a recent graduate of the University of Queensland where she studied a dual degree in mechanical engineering and geology. A core value of Ruby’s is sustainability, and she is interested in pursuing a career that explores ways of making mining methods more efficient and minimising impacts through sustainable long-term planning. She has undertaken her honours project with the Sustainable Minerals Institute on the geometallurgical characterisation and bioprocessing potential of mine waste for the recovery of cobalt. Ruby’s experience in the mining industry ranges from modelling conveyor systems, to underground geotech and open-cut mine geology. In 2021 she will begin a graduate program in exploration geology with Evolution Mining, starting in Kalgoorlie.