Bilboes Gold Project: BIOX® Processing for Refractory Gold Recovery

Source: Caledonia Mining Corporation Plc (2026)
Website: https://www.caledoniamining.com/operations/bilboes/

Critical Data

Overview

Caledonia Mining Corporation Plc, through its feasibility study for the Bilboes Gold Project in Zimbabwe, has designed an advanced processing plant to recover gold from refractory sulphide ores. Located at the Isabella McCays complex, with ore trucked 23 km from the Bubi deposit, the facility will process up to 2.88 million tonnes per annum in Phase 1 (Isabella McCays) and 2.16 million tpa in Phase 2 (Bubi). The project, with an effective date of 31 October 2025, addresses the challenge of ultrafine gold particles encapsulated in sulphide minerals. The selected recovery method integrates comminution, flotation, Metso’s proprietary BIOX® biological oxidation, carbon-in-leach (CIL), carbon treatment, and electrowinning. This multi-stage approach ensures maximum liberation of gold from the refractory matrix. The plant design incorporates a two-stage crushing circuit, ball milling to a P80 of 75 microns, flotation to concentrate sulphides, and a 6.5-day BIOX® residence time for bacterial sulphide destruction. Gold dissolution in CIL achieves approximately 99% recovery onto activated carbon, followed by elution and electrowinning to produce gold doré. The process is designed to handle varying ore characteristics, with average head grades of 2.54 g/t Au at Isabella McCays and 2.43 g/t Au at Bubi, and sulphide contents of 0.77% and 2.64% respectively. The project’s significance lies in its use of proven biological oxidation technology to treat refractory gold ores in Zimbabwe, supported by a robust water supply from Bafana dam and a 132 kV overhead power line.

Key Process Stages

• Stage 1: Comminution – Crushing & Milling: Run-of-mine ore (650 mm top size) is fed to a primary jaw crusher via a vibrating grizzly feeder (150 mm aperture). The secondary cone crushers operate in closed circuit with a double-deck screen (30 mm aperture) to produce a crushed ore stockpile with a live capacity of 24 hours (26,000 tonnes). Milling is performed by a grate-discharge ball mill with steel liners and forged Cr-Mo steel balls (80 mm diameter). Cyclone classification targets a product P80 of 75 microns; the circuit processes 240,000 tpm for Isabella McCays and 180,000 tpm for Bubi due to harder ore (Bond ball work index 16.82 vs 21.67 kWh/t).
• Stage 2: Flotation – Sulphide & Gold Concentration: Cyclone overflow (P80 75 µm) enters a conditioning tank where process water, collector (Sodium Ethyl Xanthate), activator (copper sulphate), and depressants (starch, sodium carbonate) are added. The circuit operates at natural pH in a rougher-cleaner-recleaner-cleaner scavenger configuration. Mass pull is 5% for Isabella McCays and 10% for Bubi. Rougher tails report to neutralisation, while recleaner concentrate is thickened (thickener) and fed to the BIOX® circuit. Water recovery is recycled to process water pond.
• Stage 3: BIOX® Biological Oxidation: Thickened flotation concentrate (≈18% solids) enters two parallel primary reactors followed by three series secondary reactors (total residence time 6.5 days). Mesophilic bacteria (40–45°C) oxidize sulphides at pH 1.2–1.6, controlled by sulfuric acid addition. Oxygen and CO₂ are supplied via medium-pressure blowers (250 kPa) with 15% air hold-up. Temperature is maintained at ~42°C using cooling coils. Oxidized slurry exits to a 3-stage counter-current decantation (CCD) circuit where acidic solution is separated from solids (washed with makeup water). Acidic overflow is neutralized with limestone (Phase 2) and lime to pH 7, precipitating arsenic as basic ferric arsenate. Neutralized slurry is thickened and sent to the flotation tailings storage facility.
• Stage 4: Carbon-in-Leach (CIL): Washed BIOX® product (CCD underflow) is first pH-adjusted with lime and aerated in CIL tank 1 to oxidize cyanide consumers. From tank 2 onward, sodium cyanide is added and activated carbon (retained by interstage screens) adsorbs dissolved gold. Total slurry residence time is 36 hours, achieving ~99% gold loading onto carbon. Carbon is moved counter-currently upstream; loaded carbon from tank 2 is screened and sent to acid wash. Barren slurry is detoxified to WAD cyanide <50 ppm using the INCO SO₂/air process (SMBS, copper sulphate, lime) before disposal to the BIOX® tailings storage facility.
• Stage 5: Carbon Treatment, Electrowinning & Smelting: Loaded carbon (5-tonne batches) undergoes acid wash (3% HCl, 1 hour) to remove calcium, then is eluted using the split AARL method (1% NaCN, 2% NaOH at 110–125°C, 4–5 hours). The gold-bearing eluate feeds two electrowinning cells (each with 16 cathodes of stainless steel mesh, 18 anodes) operating at up to 2,000 A DC over 12 hours. Gold is stripped from cathodes by high-pressure water jets, filtered, dried, and smelted at 1,100°C with fluxes (borax, silica, sodium carbonate) in an electric furnace. Barren carbon is regenerated at 750°C in an electrically heated rotary kiln before returning to CIL.

Additional Interesting Data and Summary

The Bilboes Gold Project’s processing plant is designed to treat refractory gold ores from two deposits: Isabella McCays (Phase 1, years 1–6) and Bubi (Phase 2, years 6–11). The ore bodies exhibit distinct metallurgical characteristics; Bubi ore has higher sulphide content (2.64% vs 0.77%) and greater hardness (Bond work index 21.67 vs 16.82 kWh/t), necessitating phased throughput reduction from 240,000 tpm to 180,000 tpm. The flotation circuit achieves a mass pull of 5–10%, concentrating sulphides for BIOX® treatment. The biological oxidation stage uses mesophilic bacteria at 42°C and pH 1.2–1.6, with a 6.5-day residence time, and includes a CCD washing circuit to separate acidic solution. Neutralisation of acidic effluents is a key environmental control: Phase 1 uses lime only, while Phase 2 requires a dedicated limestone milling circuit to handle the higher acid load from Bubi ore. Arsenic is fixed as basic ferric arsenate, a stable precipitate. Water management is critical: raw water from Bafana dam (5,300 m³/day in Phase 1) supports the process, with a 10,400 m³ process water dam for recycle. The water balance is negative, requiring 3,300 m³/day make-up in Phase 2. Reagents are imported except locally sourced limestone; the plant uses sodium ethyl xanthate, copper sulphate, sulphuric acid, sodium cyanide, caustic soda, hydrochloric acid, and sodium metabisulphite. The gold recovery circuit achieves over 99% gold dissolution in CIL, with electrowinning and smelting producing doré bars. Tailings are detoxified to <50 ppm WAD cyanide using the INCO SO₂/air process before disposal in a dedicated BIOX® tailings storage facility. Sustainability initiatives include recycling process water, fixing arsenic in stable form, and using a 132 kV overhead line for reliable power. The feasibility study, effective 31 October 2025, confirms the technical and economic viability of applying BIOX® technology to Zimbabwean refractory gold ores, positioning Caledonia Mining for long-term production from a complex, high-grade resource.

Key Processes: Flotation, CIP/CIL, Cyanidation, Gravity Separation, Ball Mill, Crushing

Target Commodities: Gold, Copper