The Las Chispas Project technical report describes a conventional gold-silver processing flowsheet using SAG milling, flotation, separate cyanide leaching for concentrate and tailings streams, and a Merrill Crowe circuit for doré production.
Article Body
The Las Chispas Project process plant is designed to treat mineralized material through a flowsheet comprising semi-autogenous grinding (SAG), flotation, independent cyanide leaching circuits for both flotation concentrate and tailings streams, a Merrill Crowe circuit, and tailings handling facilities. According to the feasibility study, the flowsheet is based on well-proven unit operations and there are no unique or novel processing methods required for gold and silver recovery. The plant design incorporates a conventional comminution circuit including a primary jaw crusher and a SAG mill to reduce feed material to the target particle size distribution for processing.
The process design shows metallurgical testing results, Ausenco's design expertise, and experience from local operations treating similar types of mineralized material. The addition of the flotation circuit and separate cyanide leach circuits for the concentrate and tailings streams provides flexibility and robustness in processing different grades of material. Process control and data management will be handled through a distributed control system (DCS) and operator interface stations, performing equipment and process interlocking, control, alarming, trending, event logging, and report generation.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Plant Throughput | 1,250 | t/d | Process design basis |
| Head Grade – nominal Au | 8 | gpt | Nominal feed grade |
| Head Grade – nominal Ag | 800 | gpt | Nominal feed grade |
| Head Grade – design Au | 11 | gpt | Design feed grade |
| Head Grade – design Ag | 1,100 | gpt | Design feed grade |
| Head Grade – peak Au | 13 | gpt | Peak feed grade |
| Head Grade – peak Ag | 1,300 | gpt | Peak feed grade |
| Overall Recovery Au | 97.6 | % | Mineralized material to doré |
| Overall Recovery Ag | 94.3 | % | Mineralized material to doré |
| Plant Availability | 91.3 | % | Design basis |
| SMC Axb – design (75th percentile) | 41 | kWh/t | Breakage characterization |
| Bond Ball Mill Work Index – design (75th percentile) | 19.4 | kWh/t | Breakage characterization |
| Flotation Concentrate Mass Pull | 2 | % | Design basis |
| Flotation Slurry Density | 48 | % solids (w/w) | Design basis |
| Total Sodium Cyanide Consumption Rate (design) | 3.5 | kg/t | Design basis |
| Total Quick Lime Consumption Rate (design) | 1.76 | kg/t | Design basis |
| Number of CCD Wash Stages | 5 | , | Counter-current decantation |
| CCD Wash Efficiency | 99.7 | % | Design basis |
| Metal Recovery Method | Merrill Crowe | , | Zinc precipitation |
| Total Peak Operating Load | 7.3 | MW | Process facility |
| Total Normal Operating Load | 7.1 | MW | Process facility |
| Lime Storage | 30 | t | Storage silo capacity |
| Lime Annual Consumption | 800 | t | At design production rate |
| Sodium Cyanide Annual Consumption | 1,600 | t | At design production rate |
| Sodium Cyanide Stock Solution | 200 | g/L | 20 vol% NaCN |
| Oxygen Plant Units | 5 | , | Each 954 kg/day capacity |
| Oxygen Unit Installed Power | 38 | kW | Each unit |
| Oxygen Utilization | 80 | % | Sizing basis |
| Instrument Air Pressure | 700 | kPa(g) | Process plant air |
| High Pressure Air for Tailings Filters | 1,000 | kPa(g) | Lead-lag configuration |
| Diesel Storage Vessels | 2 | , | Each at least 22,000 L |
| Diesel Total Storage | 44,000 | L | Combined capacity |
| Process Plant Air Compressors | 2 | , | Lead-lag configuration |
Overview
The Las Chispas Project process plant is designed to treat gold and silver bearing mineralized material at a nominal throughput of 1,250 t/d. The flowsheet integrates semi-autogenous grinding with flotation concentration and separate cyanide leaching circuits for both concentrate and tailings streams. The design incorporates a Merrill Crowe circuit for precious metal recovery through zinc precipitation, followed by smelting to produce doré bars. The process design criteria include overall recoveries of 97.6% gold and 94.3% silver from mineralized material to doré, based on life-of-mine average grades.
The mineralized material is characterized as moderate competence with very abrasive properties. SAG Mill Comminution index design values include an SMC Axb of 41 kWh/t and a Bond Ball Mill Work Index of 19.4 kWh/t at the 75th percentile. The very abrasive nature of the material indicates above average wear rates and media consumption can be anticipated.
Key Process Stages
The crushing area is the first stage of the comminution circuit, with a primary jaw crusher reducing feed material for processing. The SAG mill further reduces material, with cyclone underflow returning to the SAG mill feed. Cyclone overflow gravitates over a vibrating trash screen to remove foreign material prior to flotation.
The flotation circuit includes four 5 m³ forced-air tank cells arranged in series. Flotation concentrate mass pull is designed at 2% of feed. The flotation concentrate leaching circuit includes three 6.8 m diameter by 6.8 m high leach tanks and one 5 m diameter high-rate thickener. The tailings stream reports to a separate cyanide leaching circuit, allowing independent treatment of both process streams.
The Merrill Crowe circuit incorporates zinc mixing and precipitation equipment, including one zinc mixing cone with hopper and feeder, two precipitation filter press units each having 185 m² filtration area, one pre-coat preparation tank, and one body feed preparation tank. The filters operate in a duty/standby configuration and operate until pressure reaches a predetermined value. Filtrate reports to the barren solution tank for reuse mainly as CCD wash solution. Excess barren solution reports to the cyanide detoxification circuit for recycling as process water or is bled from the plant as required.
Mercury is collected in a mercury trap and decanted into a mercury flask. Once mercury-free material is available, the doré room includes a melting furnace with fume and dust exposure controlled through a ventilation system including hoods, enclosures, and fans to follow local regulations. Spillage within this area is returned to the cyanide detoxification feed box.
The tailings circuit is designed for a nominal throughput of approximately 57 t/hr and includes one 14.0 m diameter high-rate thickener and one 8.6 m diameter by 8.6 m high agitated tails filter feed tank. The cyanide detoxification circuit configuration and reagent consumptions were designed based on Ausenco's design expertise and experience from successful installations in similar operations, combined with metallurgical testwork completed at SGS Lakefield in 2019–2020.
A counter-current decantation (CCD) circuit with five wash stages provides a wash efficiency of 99.7%. The facility design includes the ability to add process water as a washing solution. Overflow solution from the final CCD thickener flows in counter-current mode to the preceding thickener.
Additional Interesting Data and Summary
The process facility includes a dedicated oxygen plant with five units, each with oxygen production capacity of 954 kg/day and 38 kW installed power per unit, sized using an 80% oxygen utilization factor. Quick lime is delivered in regular 20 t bulk shipments and received in a 30 t storage silo, providing about 13 days of storage at design operating rates. At the design production rate, the process facility consumes about 800 t of lime and about 1,600 t of sodium cyanide annually.
Sodium cyanide is supplied in solid briquette form in 1 t bulk bags, with a stock solution of 20 vol% (200 g/L NaCN) generated on site using a solid to liquid system. High pressure air at 1,000 kPa(g) is provided by two compressors in a lead-lag configuration for tailings filters. Plant and instrument air at 700 kPa(g) is provided by two process plant air compressors in lead-lag configuration, with dried air distributed through the process plant air receiver, plus additional receivers for the precipitate filter in the Merrill Crowe circuit and dust collector in the refinery. Low pressure air to flotation cells is supplied by one dedicated blower.
Diesel is supplied to trucks and light vehicles from two vessels each containing at least 22,000 L, for a total of 44,000 L. The metallurgical laboratory will conduct daily assays for quality control and optimize process performance, undertaking all basic testwork to monitor metallurgical performance and improve process flowsheet and efficiencies.
Metso Outotec completed a simulation using testwork data generated by SGS in 2019–2020 for flotation cell sizing and downstream equipment specification. Trade-off studies examined bulk leach residence times of 48, 72, and 96 hours, three, four, and five CCD stages, CCD thickener sizes of 12 m versus 14 m, and CCD circuit versus filters. A combination of a 96-hour leach with five CCD stages provided the highest net present value, while reducing thickener size from 14 m to 12 m resulted in minor capital savings with the 14 m option retained.
Key Processes
- Semi-autogenous grinding (SAG) with primary jaw crushing
- Flotation concentration with forced-air tank cells
- Independent cyanide leaching for flotation concentrate and tailings streams
- Counter-current decantation (CCD) washing with five stages
- Merrill Crowe zinc precipitation
- Smelting to doré bars
- Cyanide detoxification
- Tailings thickening and filtration with dry-stack filter cake handling
Source: Las Chispas Project , 2021 Technical Report, 2021.
Project website: Las Chispas Project, 2021 Technical Report
