Caledonia Mining's May 2024 technical report on the Blanket gold mine in Zimbabwe, effective December 31, 2023, credits oxygen addition with raising CIL recovery from 85.0% to 87.6%, and records a new tailings facility commissioned in February 2024.
The NI 43-101 Technical Report on the Blanket Gold Mine, Zimbabwe, was prepared by Caledonia Mining Plc with an effective date of December 31, 2023 and an issue date of May 15, 2024. Kinross Mining Company originally designed and built the Blanket plant to treat run-of-mine ore from the mine. It is a conventional crushing, milling, carbon-in-leach (CIL), batch elution and smelting plant, currently running at 66 kt a month. Gravity recovery produces about half of the gold.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Report effective date | December 31, 2023 | Not applicable | Caledonia Mining Plc; issued May 15, 2024 |
| Current throughput | About 66 | kt/month | Not applicable |
| Tonnes milled, 2023 | 770,421 | t | Up from 390,734 t in 2014 |
| Peak gold production | 80.77 | koz | 2022 |
| Overall recovery | 93.8 | % | Up from 93.0% after oxygen plant commissioned (Q4 2019) |
| CIL recovery | 85.0 to 87.6 | % | Before and after oxygen addition |
| Gravity recovery | 46.5 to 58.1 | % | Over 2014 to 2023 |
| Primary crushing | Jaw crushers, to below 80 mm | Not applicable | No. 4 Shaft ore crushed to −135 mm underground |
| Secondary crushing | Two 38 inch Nordberg cone crushers | Not applicable | Product below 10 mm |
| Primary milling | Three 6.5 x 12 ft rod mills in parallel | Not applicable | Open circuit |
| CIL | One pre-aeration tank and seven leach tanks | Not applicable | PSA oxygen, TAC 1000 cyanide analyser |
| Elution | Two 2.5 t combined elution and electrowinning columns | Not applicable | No acid wash of loaded carbon |
| Backup power | 10 | MW | Automated diesel plant |
Overview
Milled tonnes rose by about 42,200 t a year between 2014 and 2023, reaching 770,421 t in 2023. Gold production rose by about 3,738 oz a year and peaked at 80.77 koz in 2022. Oxygen from a PSA plant, commissioned in the fourth quarter of 2019, raised CIL recovery from an average of 85.0% to 87.6% and overall recovery from 93.0% to 93.8%. Gravity recovery varied between 46.5% and 58.1% over the decade with no clear trend. The report expects similar recoveries on current Blanket ore. It cautions that ore from other sources may be more refractory and should be tested before treatment.
Key Process Stages
Crushing. Ore passes 14 by 24 inch jaw crushers, reducing it from up to 300 mm to less than 80 mm. Tramp iron magnets protect the cone crushers. Crushed ore goes to a 900 t open stockpile and on to two recently installed 38 inch hydraulic Nordberg cone crushers. These run independently, each with an Osborn vibrating screen, and send product below 10 mm to the mill feed bin. Because the rod mill feed bin is small, the crushers run three shifts to keep the mills fed.
Milling. Three 6.5 by 12 ft rod mills run in parallel, each with a feed mass meter. Regrinding is done by a 12 by 16 ft ball mill (BM6) and a 6.5 by 12 ft ball mill (BM3) in parallel and closed circuit. BM3 was converted from primary to regrind duty in 2019.
Gravity and intensive leach. Knelson concentrators fed from the primary mills collect gravity concentrate, which is upgraded on a Gemini shaking table every 24 hours. Table concentrate is smelted directly. Table tails go to an intensive leach circuit, whose tails return to milling. Gravity recovers about 50% to 60% of the gold produced.
Classification and CIL. Knelson tails are cycloned, with the underflow going to regrind and the overflow to CIL. The CIL has one pre-aeration tank and seven leach tanks. Oxygen from the PSA plant, with liquid oxygen as backup, is added to the first tank, and a TAC 1000 online analyser controls cyanide addition, which has reduced cyanide consumption.
Elution and electrowinning. Two 2.5 t columns run in parallel. Elution and electrowinning take place in the same pressurized vessel, so no solution circulates outside it and no heat exchangers are needed, which the report calls very energy efficient and cost effective. Steel wool cathodes are removed on a planned cycle and acid digested. Loaded carbon is not acid washed, to save reagent costs.
Smelting and regeneration. Digested gold and gravity concentrate are smelted to doré, which Zimbabwean law requires to be delivered to the government's Fidelity Printers and Refiners. Carbon is reactivated in a kiln.
Tailings. CIL tails, reduced in cyanide, go to licensed impoundments close to the plant, and as much water as possible is returned for reuse.
Additional Interesting Data and Summary
A new tailings storage facility designed to the Global Industry Standard on Tailings Management began receiving tailings in December 2023 (Phase 1A) and was fully commissioned in February 2024. Phase 1B was due by the fourth quarter of 2024 and Phase 2 in 2025. Together they give about 20 more years of deposition, to 2044, enough for the life of mine. The decommissioned facility has live-monitored electronic piezometers. Power is from the national grid, backed by a 10 MW automated diesel plant.
In summary, Blanket is a mature rod mill, gravity and CIL plant whose output has roughly doubled in a decade. Gravity recovers about half of the gold, and oxygen-assisted CIL with combined elution-electrowinning cells recovers most of the rest, for 93.8% overall.
Key Processes
- Jaw and cone crushing to below 10 mm
- Rod milling in open circuit and ball mill regrind with cyclones
- Knelson gravity concentration, Gemini tabling and intensive leaching
- Oxygen-assisted CIL with online cyanide control
- Combined pressure elution and electrowinning
- Kiln carbon regeneration
- Smelting to doré
- Lined tailings storage with water return
Source: NI 43-101 Technical Report on the Blanket Gold Mine, Zimbabwe, Caledonia Mining Plc, effective date December 31, 2023.
Earlier report summaries
Earlier versions of this profile, based on older technical reports, are retained below for source traceability. Dates and figures reflect each report’s own effective date and are superseded by the current information above.
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
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Plant throughput (design, RoM) | 70.1 | ktpm | Design |
| Milled tonnes (2022 actual) | 752,033 | t/yr | Actual |
| Plant head grade (2022 actual) | 3.56 | g/t | Actual |
| Gravity gold recovery (2012-2022 range) | 48.51 – 56.71 | % | Actual |
| Gravity gold recovery (11-year average) | 50.87 | % | Actual |
| CIL circuit recovery (11-year average) | 86.14 | % | Actual |
| Overall gold recovery (2012-2022 range) | 92.91 – 93.79 | % | Actual |
| Overall gold recovery (11-year average) | 93.41 | % | Actual |
| RoM milling capacity (after BM3 conversion) | 115 | tph | Actual |
| CIL retention time | ~72 | hours | Design/actual |
| Fine-ore bin live capacity | ~1,500 | tonnes | Design |
| CIL tanks (pre-aeration + leach) | 1 + 8 | tanks | Design |
| Elution columns | 2 x 2.5 | tonnes | Design |
Project website: https://www.caledoniamining.com/operations/blanket-gold-mine/
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.
