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Geometallurgical Approach for Implications of Ore Blending on Cyanide Leaching and Adsorption Behavior of Witwatersrand Gold Ores, South Africa

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Abstract

Gold production in South Africa is projected to continue its decline in future, and prospects for discovery of new high-grade deposits are limited. Many of the mining companies have resorted to mining and processing low-grade and complex gold ores. Such ores are technically challenging to process, which results in low recovery rates, excessive reagent consumption and high operating costs when compared to free-milling gold ores. In the Witwatersrand mines, options of blending low-grade gold ores with high-grade ores exist. Although it is well known that most of the Witwatersrand gold ores are highly amenable to gold cyanidation, not much is known on the leachability of blended ores, especially the effects of mineralogical and metallurgical variability between different gold ores. In this study, we apply a geometallurgical approach to investigate mineralogical and metallurgical factors that influence the leaching of blended ores in a set of bottle shaker and reactor column tests. Three gold-bearing conglomerate units, so-called reefs, i.e., Carbon Leader Reef, Ventersdorp Contact Reef and the Black Reef, all in the Carletonville goldfield, were sampled. The ores were prepared using a terminator jaw crusher followed by vertical spindle pulverizer (20 kg aliquot) and high-pressure grinding rolls (80 kg aliquot). Mineralogical analysis was conducted using a range of complementary tools such as optical microscopy, QEMSCAN and micro–XCT. The results show that Witwatersrand gold ores are amenable to the process of ore blending. Some of the ores, however, contain impervious inert gangue and reactive ore minerals. Leach solution can only access gold locked in impervious gangue minerals through HPGR-induced pores and/or cracks. The optimum ore blending ratio of the bottle shaker experiments (p80 = − 75 μm) comprises 60% Carbon Leader Reef, 20% Ventersdorp Contact Reef and 20% Black Reef and yields 92% recovered Au over a leach period of 40 h. Blended ores with high carbonaceous material (> 1 wt% carbonaceous material, (Black Reef = 36–60%) yield lower recoveries of 60–69% Au). Ore leaching at the mixed-bed reactor column (− 75 μm and − 5.6/+ 4 mm) yields about 70% over a leach period of two weeks. We therefore suggest that the feasibility of ore blending is strongly controlled by the mineralogy of the constituent ores and that a mixed-bed reactor may be a viable alternative method for leaching of the low-grade Witwatersrand gold ores. Material from certain reefs, such as the Black Reef, has synergistic/antagonistic (nonadditive) blending effects. The overall implication of this study is that ore blending ratios, effects of comminution on mineral liberation, an association of gold with other minerals, and gold adsorption behavior will greatly inform future technology choices in the area of geometallurgy.

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Acknowledgments

Glen Nwaila thanks CIMERA (Centre of Excellence for Integrated Mineral and Energy Resource Analysis) for funding this research. Various mining companies (Sibanye-Stillwater, Harmony Gold and AngloGold Ashanti) are acknowledged for providing samples. Sibanye-Stillwater is further acknowledged for providing facilities for running experimental work. We thank the University of Cape Town’s Department of Chemical Engineering for providing access to the QEMSCAN, leaching columns schematics and for assisting with analyses. Dr Frikkie De Beer and Mr Lunga Bam of Necsa are acknowledged for providing access to the X-ray tomography facilities.

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Authors and Affiliations

  1. School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa

    Glen T. Nwaila & Steven Zhang

  2. Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, 97187, Luleå, Sweden

    Yousef Ghorbani

  3. Minerals to Metals Initiative, Department of Chemical Engineering, University of Cape Town, Private Bag X6, Rondebosch, 7701, South Africa

    Megan Becker & Jochen Petersen

  4. Bavarian Georesources Centre (BGC), Institute of Geography and Geology, University of Würzburg, Am Hubland, 97074, Würzburg, Germany

    Hartwig E. Frimmel

  5. Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa

    Hartwig E. Frimmel

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  1. Glen T. Nwaila
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Nwaila, G.T., Ghorbani, Y., Becker, M. et al. Geometallurgical Approach for Implications of Ore Blending on Cyanide Leaching and Adsorption Behavior of Witwatersrand Gold Ores, South Africa. Nat Resour Res 29, 1007–1030 (2020). https://doi.org/10.1007/s11053-019-09522-4

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