This report details the processing methods at the CMC concentrator, a base metal polymetallic sulfide facility with integrated BIOX and cyanidation circuits for refractory gold recovery.
Report context
The CMC concentrator was designed and commissioned in 1999 to process 600 tpd of ore primarily from the Wellington and Constancia Veins. Operations have been intermittent since then and the plant is currently under care and maintenance status. The report notes that there are no major modifications or additions required to put the plant back into production.
Processing route
Crushing and Grinding Circuits
Run of mine ore is processed underground using a sizing grizzly. Oversize material is stored in the coarse ore stockpile. Grizzly undersize reports to a vibratory screen, with undersize material going to a pump box where it is slurried with water and pumped to the grinding mill. Screen oversize is transferred to the coarse ore storage area, then loaded into trucks and hauled to the crushing plant coarse ore bin.
ROM ore from the coarse ore bin is discharged via a vibratory feeder onto a stationary grizzly. Grizzly oversize reports to the primary jaw crusher, with undersize reporting to the cone crusher feed conveyor. The jaw crusher product also reports to the cone crusher feed conveyor. The cone crusher is operated in closed circuit with the secondary crusher vibratory screen. Undersize from the secondary screen is conveyed to the fine ore storage bin adjacent to the grinding circuit. Oversize is recycled back to the cone crusher.
Fine ore from the fine ore storage bin reports to the mill feed conveyor system via two belt feeders. The mill feed conveyors feed the primary 8-foot x 12-foot ball mill, which operates in closed circuit with D-15 hydrocyclones to achieve a final grind P80 of 75 microns. Hydrocyclone overflow reports to the mill product pump box and is then pumped to the flotation plant. Hydrocyclone underflow reports back to the primary ball mill.
Flotation Circuits
The CMC concentrator produces a Pb concentrate, a Zn concentrate, a Cu concentrate, and an Ag-rich AsPy concentrate. Due to a high percentage of the Au being present in the AsPy matrix, a bulk AsPy/Py flotation circuit is used to recover the AsPy and Py as an intermediate concentrate. The AsPy/Py concentrate is separated using differential flotation, with the AsPy reporting to the concentrate and Py reporting to the tailings. The separation is facilitated using a hot caustic conditioning stage (above 60°C) before flotation, which depresses Py. This minimizes the sulfide sulfur load on the BIOX circuit and reduces the mass load by approximately 60% to the BIOX circuit.
The process plant has five flotation circuits: Pb/Cu, Pb/Cu separation, Zn, bulk AsPy/Py, and AsPy/Py separation.
The mill product slurry is pumped downhill to the concentrator via a surface pipeline. Conditioning tanks are used to activate the appropriate minerals.
The AsPy concentrate, containing the majority of the Au and minor Ag values, is pretreated in the BIOX circuit before being processed in the CIL circuit. Non-oxidized AsPy concentrate using standard cyanide leaching methods resulted in only 8% Au extraction. The Au recovery is accomplished by first producing a bulk AsPy/Py concentrate.
The Py-rich stream, containing a few grams per tonne of Au and approximately 150 g/t of Ag, has no commercial value and is blended with final tailings for disposal.
BIOX Circuit
Several different bacteria are used to oxidize the AsPy concentrate. The primary three are Thiobacillus Ferrooxidans, Thiobacillus Thiooxidans, and Leptospirillum Ferrooxidans. These bacteria metabolize sulfide minerals, generating sulfuric acid and ferric iron. Oxidation of the sulfide matrix exposes entrained Au, which becomes amenable to cyanide leaching in the CIL circuit. The original bacterium was isolated and cultured from acid mine water samples from the CMC underground.
The BIOX tank reactors are mechanically agitated with water cooling and air injectors under the impellers for good air distribution. The BIOX circuit was designed to oxidize 60 tpd of AsPy concentrate. Of the total precious metals in the mill feed, the AsPy concentrate contains 78% of the Au and about 11% of the Ag.
Production data shows that CIL cyanidation of biooxidized AsPy concentrate results in approximately 90% and 65% extraction for Au and Ag respectively. The current configuration of the BIOX process is designed to oxidize 90% of the AsPy at the nameplate production rate of 60 tpd. Based on laboratory bench data, oxidation levels greater than 90% of the AsPy do not significantly improve overall Au and Ag extraction.
The ground AsPy concentrate (at 95% minus 75 microns) is thickened to 50% solids then pumped to a surge tank. The concentrate is then pumped to a splitter box where the slurry is distributed to three bio-reactor tanks and diluted with acidic mine water to a pH of 2.0-1.3, and a slurry density of 20% solids. Limestone is used to control the pH of the bio-reactor tanks.
Primary bio-reactors (306 m³ capacity each) are operated in parallel to oxidize 60% of the AsPy grains. The discharge from the primary bio-reactors is combined and passed through three secondary bio-reactors (also 306 m³ capacity each) operated in series to oxidize an additional 35% of the AsPy grains.
Gold/Silver Leach and Recovery
The BIOX product overflows from the secondary bio-reactors to a three-stage Counter-Current Decantation (CCD) circuit. The CCD thickeners are used to wash a significant amount of BIOX liquor from the BIOX residue, reducing the amount of residual acid and decreasing the amount of limestone and lime required to adjust the residue slurry pH before cyanidation.
Concentrate Dewatering
The Pb and Zn concentrates are dewatered in separate but identical dewatering circuits. These circuits consist of thickeners to produce concentrate densities of 60% solids, with thickener overflow water recycled back into the process water system. The thickened concentrates are pumped to holding tanks, then to banks of vacuum disc filters for final dewatering to achieve a moisture content of less than 10%. Concentrates are shipped by tractor-trailer units or by railway cars to the concentrate buyer’s location at the Port of Callao near Lima.
Deposition of Final Concentrator Tailings
The final tailings consist of AsPy/Py flotation tailings, pyrite concentrate, neutralized BIOX liquor sludge, CIL residue, and mine dewatering neutralization sludge. These are thickened using tailings thickeners with the addition of flocculant before final dewatering in tailings plate and frame filters. The tailings filter cake is shipped to the Chinchan tailings storage facility.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Original plant capacity | 600 tpd | Designed and commissioned in 1999 |
| Final grind P80 | 75 microns | Design |
| Hot caustic conditioning temperature | Above 60°C | Design |
| Mass load reduction to BIOX | Approximately 60% | Design |
| BIOX design capacity (AsPy concentrate) | 60 tpd | Nameplate |
| BIOX design oxidation level | 90% of AsPy | Design |
| AsPy grind for BIOX feed (target) | 95% minus 75 microns | Design |
| AsPy concentrate solids density (to BIOX) | 50% solids | Design |
| BIOX slurry density | 20% solids | Design |
| BIOX reactor pH range | 2.0-1.3 | Design |
| Primary bio-reactor capacity (each, three in parallel) | 306 m³ | Design |
| Secondary bio-reactor capacity (each, three in series) | 306 m³ | Design |
| BIOX liquor wash stages (CCD) | 3 stages | Design |
| Concentrate dewatering moisture target | Less than 10% | Design |
| Concentrate density after thickening | 60% solids | Design |
| Au in AsPy concentrate (as % of mill feed) | 78% | Design |
| Ag in AsPy concentrate (as % of mill feed) | Approximately 11% | Design |
| Au extraction on non-oxidized AsPy concentrate | 8% | Testwork |
| Au extraction on biooxidized AsPy concentrate | Approximately 90% | Historical operating data |
| Ag extraction on biooxidized AsPy concentrate | Approximately 65% | Historical operating data |
| AsPy oxidation in primary reactors | 60% | Design |
| Additional AsPy oxidation in secondary reactors | 35% | Design |
Project website: https://www.pascoresources.com/
Technical qualifications
The report describes a proposed design based on the original 1999 plant configuration. Historical operational data indicates that the plant is capable of meeting or exceeding projected metal recoveries. Operations have been intermittent and the facility is currently under care and maintenance status. The report states that there are no major modifications or additions required to put the plant back into production. The process design relies on bacteria isolated and cultured from acid mine water samples from the CMC underground. The BIOX design parameters are based on nameplate capacity and laboratory bench data.
Source: CMC Project Technical Report, Sections 17.0 and 17.1
