Successful MesoTherm Demonstration Trial at Fairview
The continuous 2006, 2015, and 2016 Fairview MesoTherm BIOX® pilot trials firmly established MesoTherm technology as an effective bio-oxidation process that realizes lower cyanide consumption of mesophile residues ultimately oxidized through a thermophile bio-oxidation stage. The trials began with pilot-plant primary vessel sizes of 240 liters in 2006 and progressed to a primary vessel size of 1,000 liters in the 2015 and 2016 campaigns. The premise of the two-stage bio-oxidation process was that the final thermophile treatment stage would yield a more “sterile” residue by way of limiting the residual reactive polysulfide levels and hence limit the reaction of these active sulfur species with cyanide. A continuous demonstration and scale-up trial in the 21 m3 reactor at Fairview was therefore the next logical step in terms of investigating the process operational parameters to further develop the MesoTherm technology as a process that can be effectively integrated in BIOX plants. The objectives of this large-scale trial were therefore to:
- Demonstrate the culture robustness in the 21 m3 reactor relying on blends of partially oxidized material and fresh concentrate slurry;
- Evaluate the suitability of delivering the oxygen mass transfer requirements only using air at thermophilic temperatures (~65°C) when feeding a blend of concentrate and partially oxidized slurry;
- Confirm that the low cyanide consumptions determined in the 1 m3 continuous trial are obtainable and reproducible from the thermophile residue.
Figure 1. Fairview 21 m3 test reactor and photomicrographs of the bacteria
Supporting technical investigations on the more fundamental aspects were being conducted in parallel between 2015 and 2018 at the universities of Cape Town and Pretoria, who were tasked with looking at the bio-hydrometallurgy in this new operating regime.
The 21 m3 reactor was thus operated as a demonstration reactor capable of receiving various blends of flotation concentrate and/or partially oxidized primary BIOX® mesophile product (from the commercial Fairview reactors). Design variables were primary retention time, feed density, feed blending ratios, data capture-targeted oxygen off-gas and aqueous dissolved oxygen concentrations, oxidation extents, and reactor species levels.
A successful engineering scale-up and demonstration tollgate was achieved in the trial at the Fairview gold mine in South Africa during 2018. This large engineering demonstration stage not only validated the metallurgical performances achieved earlier – namely lower cyanide consumptions, comparative gold dissolutions, and sulfide oxidations – but also allowed the evaluation of important design scale-up parameters such as mechanical shear tolerances and oxygen mass transfer rates.
Figure 2. Gold dissolution results from the 21 m3 demonstration trail
Figure 3. Sulphide oxidation results from the 21 m3 demonstration trail
Figure 4. Cyanide consumption results from the 21 m3 demonstration trail
The successful demonstration trial in the 2018 campaign set the focus for a larger commercial demonstration in 2019 that realized the thermophile culture introduction into one of the commercial reactors at Fairview. This reactor is currently receiving direct bleed streams of the plant feed material at much higher solid concentrations and higher sulfide sulfur levels with varying sulfide mineralogies than can typically be realized in a commercial MesoTherm plant; its robustness is being stretch-tested and benchmarked against the commercial mesophile primary reactors. This benchmark is ongoing and will mark the final validation step before formally rolling out the technology to industry with the launch of a slightly modified and improved bio-oxidation refractory technology.
Figure 5. Fairview S10 Reactor (middle) and 21 m3 test reactor (right)
Figure 6. Typical MesoTherm plant process flow sheet