The Tarkwa CIL process plant is a 14 Mt per annum conventional crush-grind-leach circuit for gold extraction using cyanide, with a dual crushing and stockpile system feeding a SAG and ball mill grinding circuit.
Report context
The Tarkwa 2021 Technical Report describes the processing facilities for the Tarkwa operation as of the report date. The process plant has a design capacity of 14 Mt per annum and operates using a conventional carbon-in-leach (CIL) flowsheet. During 2018, a gravity recovery circuit was added, and additional gravity concentrators were installed during 2020. Debottlenecking projects were implemented during the period 2019 to 2021, and the plant achieved treatment rates of approximately 14 Mtpa in both 2020 and 2021 based on actual performance.
Processing route
Crushing and stockpiling
Ore is transported from the mine pits to the crusher pad. Material is tipped directly or reloaded by front-end loaders into an Allis Gyratory Superior 54″ x 75″ gyratory crusher. The crushed ore, at 80% passing 150 mm, passes into a bin feeding an apron feeder. The apron feeder supplies ore via a dual stockpile system consisting of the original coarse ore stockpile and a new auxiliary stockpile with a combined live storage capacity of 45,000 t, equivalent to 30 hours of operation.
Additional ore from the pits is tipped into the North Heap Leach (NHL) 50″ x 65″ M Π Sandvik Superior primary gyratory crusher. The crushed product, also at 80% passing 150 mm, gravitates to a 1,829 m x 6,815 m D4 Jacques A Terex apron feeder, which supplies two secondary screens via conveyor. Oversize from two 2,440 mm x 7,320 mm Nordberg double deck scalping screens, with aperture sizes of 50 mm and 32 mm for the upper and lower decks respectively, feeds two Superior gyratory secondary crushers (S6000). Undersize joins the final product to the new CIL auxiliary stockpile via a series of overland conveyors.
The secondary crushing product is fed to a tertiary hopper via conveyors. Ore from this hopper is fed by six 750 mm x 1,800 mm Nordberg vibratory feeders to six 2.4 m x 7.3 m Nordberg double deck scalping screens operating in closed circuit with six tertiary H4000 hydrocone crushers. Oversize from the upper and lower decks (32 mm and 25 mm respectively) feeds the tertiary crushers, and the crusher discharge is conveyed back to the tertiary bin. Undersize from the tertiary screens joins the final crushing product at 80% passing 25 mm to the new CIL auxiliary stockpile.
Grinding and classification
Under each stockpile, a reclaim tunnel with apron feeders feeds onto a conveyor belt supplying the milling circuit. Mill feed comprises 30% fine material from the NHL crushing circuit and 70% coarse material from the CIL crusher. The milling circuit consists of a SAG mill and a ball mill with recycle crushing in closed circuit with the SAG mill. The SAG mill has an effective grinding length (EGL) of 42 feet with an internal diameter of 27 feet and 14 MW of installed power (2 x 7,000 kW twin drive motors). The ball mill has an EGL of 36 feet with an internal diameter of 26 feet and 14 MW of installed power (2 x 7,000 kW twin drive motors).
The milling circuit operates at a capacity of 1,600 t/hr (1 Mt/month). Crushed ore from the dual stockpile system is fed into the SAG mill for primary milling. The SAG mill discharge is screened at 12 mm, with the oversize fraction fed to a recycle crusher. The crushed product is recycled to the SAG mill feed. The SAG mill discharge screen undersize is classified in a cluster of twelve 26-inch Krebs cyclones, with underflow reporting to the ball mill feed. The ball mill discharge is classified in a cluster of twenty-six 20-inch Krebs cyclones, with underflow reporting back to the ball mill feed. Overflow from both cyclone clusters reports to three 32 m Outotec thickeners via linear trash removal screens.
Gravity recovery circuit
During 2018, Gold Fields added a gravity recovery circuit to increase plant recoveries. The gravity circuit consisted of three 48″ Knelson concentrators and an Acacia Reactor. During 2020, an additional two Knelson concentrators were added.
Thickening and leaching
The cyclone overflow slurry is thickened from 25% solids to 55% solids in the thickeners, then pumped to the CIL tanks. Clear water from the thickener overflow is reused in the plant. The CIL circuit consists of two trains of eight tanks in series fed from a common leach tank. Seven of these tanks in each train contain activated carbon for gold adsorption. Cyanide is added to ensure stability of the dissolved gold complex, which is then adsorbed onto the carbon. The residue slurry overflowing the last CIL tank forms the tailing, which is analysed for weak acid dissociable (WAD) cyanide with an online analyser and then pumped to any of the three TSFs (TSF 1, 2 and 5).
Carbon elution and gold recovery
Carbon loaded with gold is recovered over the loaded carbon screen, where it is cleaned of slurry. The loaded carbon passes into a 15 t acid wash column, where it is washed with hydrochloric acid solution to remove calcium. Gold is recovered from the loaded carbon in two 15 t elution circuits using the AARL stripping method. The eluted carbon is passed through a regeneration kiln operating at 700 °C to remove organic foulants. The gold-bearing solution from the elution circuits is pumped through electrowinning cells, where gold plates onto stainless steel cathodes as gold particles. Gold is removed from the cathodes with high pressure sprays and dried before being smelted in an induction furnace at 1,400 °C. Fluxes are added before smelting to assist in separation of impurities. The molten gold is poured into moulds and allowed to cool. Once cooled, the doré bar is cleaned, stamped and transported to a refinery.
Debottlenecking
During the period 2019 to 2021, Gold Fields implemented debottlenecking projects that included upgrading the SAG and ball mill discharge pump sets, the tailings pumps, and the CIL intertank screens. The debottlenecked CIL plant can treat approximately 14 Mtpa, as demonstrated by actual performance achieved in both 2020 and 2021.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Plant design capacity | 14 Mt per annum | Design |
| Crushed ore size, primary crusher | 80% passing 150 mm | Design |
| Crushed ore size, tertiary product | 80% passing 25 mm | Design |
| Combined stockpile live capacity | 45,000 t (30 hours) | Design |
| Mill feed composition | 30% fine from NHL, 70% coarse from CIL | Design |
| Milling circuit operating capacity | 1,600 t/hr (1 Mt/month) | Operating data |
| SAG mill dimensions | 42 ft EGL x 27 ft internal diameter | Design |
| SAG mill installed power | 14 MW (2 x 7,000 kW) | Design |
| Ball mill dimensions | 36 ft EGL x 26 ft internal diameter | Design |
| Ball mill installed power | 14 MW (2 x 7,000 kW) | Design |
| Thickener feed solids | 25% | Design |
| Thickener underflow solids | 55% | Design |
| CIL tanks | Two trains of eight tanks each | Design |
| Carbon tanks per train | 7 | Design |
| Gravity concentrators (post-2020) | 5 x 48″ Knelson concentrators | Design/historical |
| Elution and acid wash columns | 2 x 15 t elution, 1 x 15 t acid wash | Design |
| Carbon regeneration temperature | 700 °C | Design |
| Smelting furnace temperature | 1,400 °C | Design |
| Actual plant throughput (2020 and 2021) | Approximately 14 Mtpa | Actual performance |
Project website: https://www.goldfields.com/reports/rr_2009/tech_tarkwa.php
Technical qualifications
The report notes that all ore sources are amenable to gold extraction using cyanide. The schematic process flow sheet is provided as Figure 17.1 in the original technical report. The debottlenecked plant throughput of approximately 14 Mtpa is based on actual performance demonstrated in 2020 and 2021, not on a new design basis. The gravity circuit addition in 2018 and the expansion in 2020 are described as additions by Gold Fields, with results described as successful.
Source: Tarkwa 2021 Technical Report, Section 17 Recovery methods, Section 17.1 Flow sheet and design, pages 89-91 (original pagination).
