The Copperstone process plant design adopts whole ore cyanide leaching based on metallurgical testwork that indicated 97% gold recovery, using existing crushing and grinding equipment supplemented by new leaching and solution recovery circuits.
Report context
This technical report, dated August 2, 2023, documents the process design for the Copperstone Project. The report presents design criteria for a processing rate of 592 tonnes per day (652 short tons per day) based on mine delivery of 198,560 tonnes of mineralized material per year (219,000 short tons per year). Plant availability is set at 92%, with operations 24 hours per day, 7 days per week.
Processing route
Flowsheet selection
Metallurgical testing evaluated three process options: grinding and flotation to produce a gold concentrate; grinding, flotation, and cyanide leaching of the gold flotation concentrate; and grinding and whole ore cyanide leaching. The final two options produce doré bars. Testwork conducted by RDI for screening these flowsheet options indicated 97% recovery of gold for whole ore leaching, while the other processes achieved 88% to 90% recovery. Based on economic and technical analysis, whole ore leaching was selected as the best processing option. The report notes that although operating costs for whole ore leaching are the highest of the three options, the increase in recoveries and elimination of smelter charges make this option economically superior.
Crushing and grinding
The flowsheet takes advantage of existing equipment including the primary crusher (jaw), secondary crusher (cone), screen, ore bin, conveyors, and ball mill. This equipment will be refurbished prior to commissioning. The target grind size is 104 µm (cyclone overflow P80). The existing ball mill is larger than required for the target 600 short tons per day throughput and will operate with reduced steel load and lower power. The existing Hardinge ball mill has a motor power of 700 hp, operates at 75% of critical speed, uses 3-inch diameter balls, and has a ball charge of 25% to 35%. A single new 20-inch cyclone is specified, with a design circulating load of 250%.
Whole ore leaching
The new whole ore leaching circuit includes a pre-leach thickener followed by four leaching tanks. The pre-leach thickener is a new high-rate unit with a diameter of 6.1 m (20 ft) and a settling rate of 295 ft²/tons per day, producing an underflow density of 45 wt%. The four leach tanks each have an operating volume of 258 m³ (68,150 gallons), with a height of 30 ft and diameter of 20 ft. Total residence time with four tanks is 27.2 hours, and with three tanks is 20.4 hours. Free board is 10%. Design sodium cyanide concentration is 1,750 ppm, with design consumption of 4.3 lbs per short ton (2.15 kg per tonne).
Solution recovery and Merrill Crowe
Pregnant solution is recovered through four CCD thickeners operating in countercurrent decantation. The CCD thickeners have a diameter of 6 m (20 ft), a settling rate of 0.04625 m²/tonnes per day (0.452 ft²/tons per day), and produce underflow density of 50 wt%. The wash ratio is 2.3 based on barren to feed solids and 2.7 based on barren to feed aqueous.
Pregnant solution is processed in a Merrill Crowe circuit to precipitate gold, silver, and copper. Design feed to Merrill Crowe is 383 gpm (87 m³/hr), with a design maximum of 459 gpm (104 m³/hr). Pregnant solution grades are: Au 1.82 g/m³, Ag 0.12 g/m³, Cu 262 g/m³, and CN 383 g/m³. Zinc consumption is 36 kg/hr. The pregnant solution tank has 0.67 hours residence time with a volume of 58 m³. The barren solution tank has 2 hours residence time with a volume of 174 m³.
Following precipitation, the product will be batch leached with acid in the gold room to extract residual copper, which can be recovered using a cementation process, prior to smelting and pouring doré bars.
Detoxification
A detox tank is specified with a feed of 50 wt% solids and 4 hours residence time. The tank volume is 128 m³. Sodium metabisulfite (SMBS) consumption is 0.248 lb/ton (0.497 kg/tonne).
Key reported parameters
| Parameter | Unit | Design Value | Basis |
|---|---|---|---|
| Plant throughput | tonnes per day | 544 | Plant tonnage |
| Plant throughput | short tons per day | 600 | Plant tonnage |
| Plant availability | % | 92 | Operating basis |
| Design capacity | tonnes per day | 592 | Design |
| Design capacity | short tons per day | 652 | Design |
| ROM F80 | mm | 102 | Design |
| ROM specific gravity | , | 3.18 | Design |
| Milling feed F80 | µm | 12,500 | Design |
| Bond ball mill work index (average) | kWh/t | 13.94 | Testwork |
| Bond ball mill work index (maximum) | kWh/t | 18.03 | Testwork |
| Bond ball mill work index (minimum) | kWh/t | 10.04 | Testwork |
| Cyclone overflow P80 | µm | 104 | Design |
| Circulating load | % | 250 | Design |
| Pre-leach thickener diameter | m (ft) | 6.1 (20) | Design |
| Pre-leach thickener underflow density | wt% | 45 | Design |
| Leach tanks, quantity | , | 4 | Design |
| Leach tanks, residence time (4 tanks) | hr | 27.2 | Design |
| Leach tanks, residence time (3 tanks) | hr | 20.4 | Design |
| Leach tank operating volume (each) | m³ (gal) | 258 (68,150) | Design |
| Sodium cyanide concentration | ppm | 1,750 | Design |
| Sodium cyanide consumption | lb/st (kg/t) | 4.3 (2.15) | Design |
| CCD thickeners, quantity | , | 4 | Design |
| CCD thickener diameter | m (ft) | 6 (20) | Design |
| CCD thickener underflow density | wt% | 50 | Design |
| Merrill Crowe feed | gpm (m³/hr) | 383 (87) | Design |
| Merrill Crowe feed (design maximum) | gpm (m³/hr) | 459 (104) | Design |
| Merrill Crowe pregnant solution Au | g/m³ | 1.82 | Design |
| Merrill Crowe pregnant solution Ag | g/m³ | 0.12 | Design |
| Merrill Crowe pregnant solution Cu | g/m³ | 262 | Design |
| Merrill Crowe zinc consumption | kg/hr | 36 | Design |
| Detox tank residence time | hr | 4 | Design |
| Detox tank SMBS consumption | lb/ton (kg/tonne) | 0.248 (0.497) | Design |
Project website: https://www.sabre.gold/projects/copperstone/
Technical qualifications
The report states that testwork results indicated 97% recovery of gold for whole ore leaching, while the other processes achieved 88% to 90% recovery. Testing was conducted by RDI for screening these flowsheet options. The present testwork indicated that the deposit does not have free coarse gold. Samples representing softer and harder material were tested for Bond's Ball Mill Work Index at a closed size of 100 mesh, yielding BWi values of 10.74 kWh/ton.
The existing ball mill is larger than required for the target throughput and will operate with reduced steel load and draw lower power. The crushing and grinding circuit is currently in place and will require only minor modification, with equipment to be refurbished prior to commissioning.
Dr. Deepak Malhotra visited the Copperstone property in February 2018 and reviewed the plant flowsheet and equipment in the plant.
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*Source: Copperstone Project , 2023 Technical Report, August 2, 2023, Sections 17.1–17.3*
