NI 43-101 Technical Report on the Blanket Gold Mine, Zimbabwe

This profile is based on the NI 43-101 Technical Report on the Blanket Gold Mine, Zimbabwe, with an effective date of 1 September 2022. The processing route is a conventional gold recovery circuit comprising crushing, milling, gravity concentration, carbon-in-leach (CIL) leaching, batch elution and electrowinning, and smelting.

Report context

The NI 43-101 technical report, prepared by Caledonia Mining Corporation Plc in conjunction with Minxcon (Pty) Ltd, describes the Blanket Gold Mine processing plant as a conventional operation treating run-of-mine (RoM) ore at a rate of 70.1 kilotonnes per month (ktpm). The crushing and milling circuits were designed for RoM material, while the CIL and downstream circuits were originally designed to treat tailings dam material at approximately 110 ktpm (3,800 tpd). As of the report date, the CIL circuit was used exclusively for RoM treatment at up to 70.1 ktpm. Historic production data from 2012 to 2022 show milled tonnes ranging from approximately 364,000 tonnes per annum (tpa) to 752,000 tpa. The report states the plant is expected to maintain a similar recovery when treating current Blanket RoM material, but notes that RoM from other sources may be more refractory and would require testing before treatment.

Processing route

The plant comprises a multi-stage comminution circuit followed by gravity concentration and cyanide leaching. The process flow is detailed below.

Comminution

RoM ore is fed over 14-inch by 24-inch jaw crushers to reduce the top size from minus 300 millimetres to less than 80 millimetres. Ore from No. 4 Shaft is crushed to minus 135 millimetres underground. At the new central shaft, a 25 by 40 jaw crusher crushes RoM ore to minus 150 millimetres, after which material is trucked to the plant. Tramp iron magnets ahead of the crushers remove scrap iron, and metal detectors are installed to detect any residual metal.

Secondary crushing uses two 38-inch hydraulically adjusted Nordberg cone crushers, which operate independently and feed Osborn vibrating screens. Screened product smaller than 13 millimetres is delivered to a 2,000-tonne fine-ore bin with a live capacity of approximately 1,500 tonnes, providing about 12 to 14 hours of feed to the primary mills.

Four 6.5-foot by 12-foot rod mills operate in parallel. Each feed belt has a mill feed mass meter for control and measurement. The BM3 regrind mill, which had been running in parallel with BM6, was converted to a primary rod mill, enabling an increase in RoM milling capacity from 100 tonnes per hour (tph) to 115 tph.

Regrind duty is performed by two ball mills operating in parallel: BM6 (12 ft x 16 ft) and BM10 (12 ft x 18 ft, commissioned in August 2022), both in closed circuit with a set of eight 250-millimetre-diameter cyclones.

Gravity concentration

Knelson concentrators are fed from the primary mills. The report states that approximately 48% to 55% of gold production is recovered as gravity gold using these concentrators. Concentrate is stored and processed every 24 hours on a Gemini shaking table. Gemini table concentrates go directly to smelting, while the table tailings are sent to an intensive leach (IL) circuit. Tailings from the IL are returned to milling. The tails of the Knelson concentrators are pumped to the cyclone cluster, with underflow going to regrind and overflow going to the CIL plant.

Carbon-in-leach and downstream recovery

The CIL circuit consists of one pre-aeration tank and eight leach tanks, where alkaline-cyanide leaching and simultaneous adsorption of dissolved gold onto granular activated carbon occur. Oxygen from a Pressure Swing Absorption (PSA) oxygen plant is added into the first CIL tank; liquid oxygen is available as backup. A TAC 1000 cyanide online analyser measures and controls cyanide addition. This process control system, combined with oxygen injection, has reduced cyanide consumption. The report notes that oxygen use improved CIL recovery from an average of 82.6% to 85.4%.

Elution and electrowinning are conducted on site in two 2.5-tonne elution columns operating in parallel. The design is described as unique in that elution and electrowinning occur in the same pressurised vessel, eliminating the need for external solution circulation and heat exchangers, resulting in energy efficiency. During electrowinning, gold is deposited on steel wool cathodes within the elution column; loaded cathodes are removed on a planned cycle and acid digested.

Gold solids from acid digestion and gravity concentrate are smelted into doré bars. Granular activated carbon is regenerated in a kiln before recirculation to the CIL section. Loaded carbon is not acid treated to reduce reagent costs. The report states that carbon reactivation has remained acceptable, though acid treatment can be re-introduced if required. Doré bars are delivered, as required by Zimbabwean gold-mining law, to the Government-operated Fidelity Printers and Refiners for sampling and refining.

Tailings, water, and power

Plant tailings from CIL are deposited to a tailings storage facility (TSF) located close to the plant. The maximum amount of tailings water is pumped back to the metallurgical plant for re-use. Daily management and operation of the tailing deposition area is contracted to the Zimbabwean subsidiary of Fraser Alexander. The current TSF was expected to support deposition until 2023, after which a new TSF must be commissioned. The new TSF was designed by Knight Piesold (Pty) Ltd, with estimated design work and construction of USD 16 million.

Power is supplied from the national grid, with a fully automated 10-megawatt diesel-driven power plant available for power outages. A 12.5-megawatt solar plant was erected and commissioned to act as an alternative power source for both plant and mining operations.

Reagents and consumables

Key reagent and consumable consumptions reported for the average past year include:

• Grinding media (rods and balls): 1.57 kg/t total steel media
• Lime: 1.68 kg/t
• Sodium cyanide: 0.85 kg/t
• PSA oxygen: 3.42 kg/t
• Carbon: 0.07 kg/t
• Steel wool: 0.244 kg/t of carbon
• Caustic soda: 42.91 kg/t of carbon

The report notes that higher reagent consumptions were partly due to the high retention time of approximately 72 hours in the CIL circuit, which affects carbon consumption from prolonged agitation and abrasion, as well as cyanide consumption.

Key reported parameters

The report distinguishes gravity contribution, CIL-stage recovery, and overall recovery. Gravity recovery and CIL-stage recovery are not additive; overall recovery incorporates both the gravity circuit and the CIL circuit. The historic production table shows that for 2022, gravity recovery was 48.21%, CIL recovery was 87.02%, and overall recovery was 93.80%.

Technical qualifications

This profile is derived solely from the specified technical report section (Item 17, Recovery Methods) and does not show any subsequent operational changes. The report is dated effective 1 September 2022, with an issue date of 13 March 2023. All process descriptions, metrics, and design parameters are presented as reported in that document.

The report notes that the plant was designed and constructed by Kinross Mining Company to treat RoM ore from the Blanket mine. The crushing and milling circuits were designed for RoM, while the CIL and downstream circuits were designed for tailings dam material but were used exclusively for RoM at the time of reporting. The new BM10 regrind mill was commissioned in August 2022, prior to the effective date.

Historic production data (2012-2022) are reported as actual measured results. The report states the plant is expected to maintain similar recovery when treating current Blanket RoM material, but this is a forward-looking statement. RoM material from other sources may be more refractory and would require testing.

The assay laboratory, located 200 metres from plant offices, is not accredited but has necessary equipment for sample preparation and analysis. External verification is performed through sample submission to Met Solution and Performance Laboratories. The report notes that the laboratory does not have an electronic tracking system and that implementation of a Laboratory Information Management System (LIMS) would reduce human error.

The plant does not have a central process control system; there are local controls in areas such as mill feed control and cyanide addition. The report notes a higher labour complement (114 total plant employees as per Table 52) is in part due to the manual control nature of the plant. The Electrical Engineer is shared between the plant and the mine.

Source: NI 43-101 Technical Report on the Blanket Gold Mine, Zimbabwe, effective date 1 September 2022, Item 17 (Recovery Methods), sections 17(a), 17(b), and 17(c). of the original technical report.