The 2010 feasibility study examined expanding the existing 5,000 t/d Choco 10 plant to a combined throughput of 20,000 t/d by adding a new 15,000 t/d processing facility.
Report context
The Choco 10 gold recovery plant was installed and commissioned in 2005, with the majority of equipment sourced from a previously closed operation in Canada. This report presents the feasibility study for expanding the facility. Under Rusoro’s management the plant is reported to have achieved a throughput of close to 7,000 t/d while treating saprolitic material. For the purpose of this study, however, a throughput of only 5,000 t/d has been assumed, well within its rated capacity of 5,400 t/d when treating harder rock.
Processing route
Existing plant configuration
The existing process flowsheet is largely conventional, consisting of primary crushing, two-stage milling, cyanide leaching, carbon adsorption and elution, electro-winning and gold smelting. Tailings disposal is undertaken in a valley dam, with decant solution returned as process water to the plant.
Crushing and grinding
Run-of-mine ore is dumped directly by truck or front-end loader into a bin with a grizzly and the undersize fed onto an apron feeder. Material from the apron feeder is fed onto a screen where undersized material bypasses the primary jaw crusher. A separate feed bin is also in place to allow for bypassing the crusher when the feed material is predominantly oxide (saprolitic material). The crusher product, at approximately 80% minus 6 in, and undersize material are fed directly onto the SAG mill conveyor feed-belt. Lime is added onto the SAG mill feed-belt via a bin and screw conveyor.
Milling is undertaken in a 20 ft (6.1 m) diameter by 17 ft (5.2 m) long SAG mill (2,750 HP: 2,052 kW) operated in either open or closed circuit with the SAG mill product feeding a 17 ft (5.2 m) diameter by 23 ft (7.0 m) long ball mill (2,750 HP: 2,052 kW). A facility is in place to return pebbles back to the SAG mill feed-belt. The ball mill product feeds a cyclone pack and the cyclone overflow is screened on a 12 m² Delkor linear screen for removal of trash. The ball mill operates in closed circuit, with cyclones.
CIP leaching and gold recovery
Cyanide leaching of the milled product is undertaken in five large leach vessels each of 3,450 m³. The leach vessels are mechanically agitated with air injection to maintain dissolved oxygen at an acceptable level. The installed leaching capacity allows for theoretical leach residence time in excess of 48 h.
The slurry is transferred from the leach tanks to a conventional carbon-in-pulp circuit, comprising seven CIP tanks, operating in a conventional counter-current flow of slurry and carbon. Carbon is transferred upstream by recessed impellor pumps with inter-stage screens, recently upgraded from Kambalda screens to two 12 m² Kemix screens. Loaded carbon is pumped from the head tank to the elution and regeneration section for stripping of contained gold. The regenerated carbon is returned to the tail adsorption tank.
Elution of loaded carbon is carried out in a 4 t batch through a conventional Pressure Zadra vertical column. Acid washing of the loaded carbon is undertaken predominantly to reduce calcium loadings on the carbon. Carbon regeneration is carried out in a vertical kiln with typical temperatures of 700°C. Tails from the adsorption circuit are screened for removal of carbon, and pumped directly to the tailings dam.
Proposed expansion design
The intent of the increase in plant capacity is for the Choco 10 plant to be able to produce up to 500,000 oz/y of gold utilizing a flowsheet that is essentially similar to the existing one. The economic analysis presented in the 2009 Preliminary Assessment demonstrated that keeping the existing mill at 5,000 t/d and adding a new, 15,000 t/d plant provides the optimum return on capital. Thus, the feasibility study envisages the existing plant continuing to operate “as is”, with the crushing, grinding and leach circuits as previously described.
The processing strategy is designed to maximize the total facility throughput by incorporating existing plant unit processes where practical. For example, the new gyratory crusher and coarse ore stockpile (COS) will feed both the existing and new milling circuits. The existing crushing circuit will be retained as feed source for the existing plant, allowing the two plants to be fed separately and the existing crushing plant to operate when the gyratory crusher is down for maintenance.
The two milling circuits will operate independently through to the classification cyclones. The cyclone overflow from each circuit will be combined in the new pre-leach thickener. Underflow from the thickener will be divided and pumped separately to the new and existing leach tanks. CIL/CIP tailings from the respective circuits will be combined again in the tailings thickener. Loaded carbon from each circuit will be stripped in separate (new and existing) elution circuits. The acid wash and elution columns for the existing plant will remain in their current locations, while all electrowinning and gold handling will be consolidated into a new and common gold room.
Process plant design basis
The Feasibility Study (FS) design criteria were based on the available test work as well as data from similar operations. The timeline of the study meant that the scope of the metallurgical testing typically recommended for an FS could not be completed in time for incorporation in the study. Benchmarking against similar operations was therefore required to confirm some of the design criteria. Ausenco considers this approach to be adequate in terms of managing risk for this project at the FS level of accuracy.
Unit process selection for new plant
The process plant design is based on a flowsheet incorporating the following unit process operations:
- Crushing ore from the open pits in a primary gyratory to a product size of nominally 80% passing (P80) 120 mm for feed to a coarse ore stockpile (COS)
- COS with a live capacity of 20,000 t (24 h) to provide crusher product surge capacity ahead of SAG milling
- Two coarse ore reclaim systems to feed the existing and new plants
- 8 MW SAG mill in closed circuit with pebble crushing
- 11 MW ball mill in closed circuit with cyclones
- Pre-leach thickening in a high-rate thickener to an underflow density of 55% solids
- 4 x 5,000 m³ live capacity cyanidation leach tanks in series
- 6 x 5,000 m³ live capacity carbon-in-leach (CIL) tanks in series
- 10 tonne carbon acid wash and two 10 tonne carbon elution columns
- 20 tonne carbon regeneration kiln
- Electrowinning via six sludging cells with stainless-steel wool cathodes
- Electrowinning sludge drying oven and diesel-fired crucible smelting furnace
- Tailings thickening in a high-rate thickener to an underflow density of 55% solids
- Centrifugal pumping of tailings to a conventional tailings storage facility (TSF)
Historical operating data
The plant is designed to operate as a SAG/ball mill operation; however, the current feed is predominantly saprolite/oxidized material with very low work indices and requires little energy for grinding. At the time of Micon’s site visit (see Leader et al., 2007) milling was set up to operate with only the SAG mill. When operating in this mode, the plant is capable of treating the equivalent of nearly 7,000 t/d of saprolitic ore.
A review of the various reports relating to operating costs for the plant indicates that these are in reasonable agreement with those to be expected for a similar plant treating this type of ore. Additionally, the reagent consumptions also agree well with those on which the feasibility study is based. Barring external factors causing disruption to normal operations, as operating time improves unit operating costs should also reduce. However, these costs will increase somewhat when harder, fresh ore is treated.
Testwork basis
The major comminution design parameters were based on results from the PFS metallurgical test work program. Additional test work was undertaken during the FS, but the results were not available in time to incorporate into the FS design. A preliminary evaluation of the additional test work results against the FS design identified minor discrepancies in the installed powers and mill sizings, but was considered too minor to warrant any changes.
The major leaching design parameters used for this study were based on the 2007 metallurgical test work program. No carbon adsorption or equilibrium test work was completed for the FS. No thickening test work was available for the FS, and the design criteria for the thickeners were therefore based on standard industry values. Subsequent testing indicated that the thickener diameter could be reduced and the target underflow density increased.
Key reported parameters
| Parameter | Unit | Design value | Basis |
|---|---|---|---|
| Plant throughput (new plant) | t/d | 15,000 | Design |
| Plant availability | % | 90.0 | Design, benchmarked |
| Head grade (gold) | g/t | 2.5 | Design |
| Crushing work index | kWh/t | 23.9 | Design, based on testwork |
| Bond ball mill work index | kWh/t | 14.5 | Design, based on testwork |
| SAG mill installed power | kW | 8,000 | Design |
| Ball mill installed power | kW | 11,000 | Design |
| Grind size P80 | μm | 75 | Design |
| CIL gold dissolution | % | 91.0 | Design, based on 2007 testwork |
| CIL carbon adsorption | % | 99.0 | Design |
| Overall recovery | % | 90.1 | Design |
| Leach residence time | h | 48 | Design |
| Existing plant throughput (assumed for study) | t/d | 5,000 | Assumed (design basis) |
| Existing plant throughput (historical, saprolite) | t/d | ~7,000 | Historical operating data |
| Existing plant rated capacity (hard rock) | t/d | 5,400 | Rated capacity |
Project website: https://rusoro.com/
Technical qualifications
The Feasibility Study (FS) design criteria were based on the available test work as well as data from similar operations. The timeline of the study meant that the scope of the metallurgical testing typically recommended for an FS could not be completed in time for incorporation in the study. Benchmarking against similar operations was therefore required to confirm some of the design criteria.
Additional test work was undertaken during the FS, but the results were not available in time to incorporate into the FS design. No carbon adsorption or equilibrium test work was completed for the FS. No thickening test work was available for the FS design, and the design criteria for the thickeners were therefore based on standard industry values.
Ausenco used a power approach for comminution design, based on empirically derived models developed from a database of actual plant operating data and associated bench-scale test work. The installed ball mill power of 11,000 kW incorporates a 10% design contingency to account for the accuracy of the models, calculations, and test work used to determine the expected average pinion power.
Source: Recovery Methods section, Choco 10 Gold Recovery Plant – 2010 Feasibility Study
