The 2021 NI 43-101 Technical Report for the Ollachea Gold Project describes a two-stage processing plant development that uses gravity concentration and carbon-in-leach technology to recover gold and silver.
The Ollachea Gold Project, operated by Minera IRL Limited, will use a staged construction approach for its treatment plant. Stage 1 consists of a 1,500 tonnes per day processing facility, with Stage 2 doubling throughput to 3,000 tpd. The process flowsheet integrates gravity concentration with carbon-in-leach technology to recover gold and silver. John Thomas serves as the Qualified Person responsible for Section 17 of the report covering recovery methods.
Mineralized material is crushed in three stages to a P80 of 8 mm before grinding to a P80 of -75 microns in a ball mill operating in closed circuit. The plant layout uses a terraced design with distinct process areas. The crushing plant and run-of-mine stockpile occupy the upper platform, the mill and gravity concentration circuits sit on the middle platform, and the leach and elution circuits are located on the lower platform. The tailings filtration plant is positioned separately.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Stage 1 plant throughput | 1,500 | tpd | Gravity concentration design |
| Stage 2 plant throughput | 3,000 | tpd | Full production rate |
| Crushed product size | P80 8 | mm | Three-stage crushing |
| Grind size | P80 -75 | microns | Ball mill closed circuit |
| High-grade concentrate mass | 0.5 | % | Of feed mass |
| Low-grade concentrate mass | 15 | % | Of feed mass |
| Leach plant capacity | 500 | t/d | Concentrate feed |
| Ball mill dimensions | 4.26 x 7.3 | m | 14' by 24' |
| Ball mill motor | 2,000 | kW | 2,700 HP |
| Mill design availability | 92 | % | – |
| CIL design feed rate | 3,000 | tpd | Concentrate from full production |
| Paste production flow | 60 | m³/h | At full 3,000 tpd production |
| Thickener diameter | 10 | m | Tailings feed |
| Filter cake moisture | 16 | % | Expected |
| Cyanide destruction pH | 8.5 | – | Iron cyanide complex destruction |
| Carbon regeneration temperature | 650 – 700 | degrees | Rotary furnace |
| Elution column pressure | 0.4 | MPa | Operating pressure |
| Crushed material feed bin | 10 | m³ | Wheel loader fed |
Project website: https://en.wikipedia.org/wiki/Ollachea_District
Overview
The Ollachea processing facility is designed around a two-stage development strategy. Stage 1 starts at 1,500 tpd, and Stage 2 doubles the treatment rate. The plant uses gravity concentration to produce two concentrate grades, with materials handling and equipment layouts designed to support both stages with minimal additional equipment. The process flowsheet is based on metallurgical test work carried out by Met-Solve and Sepro in 2017 and confirmed in 2021.
The crushing circuit operates in three stages to achieve a P80 of 8 mm. Crushed material reports to a silo that feeds the mill or to a stockpile for reclaim by wheel loader. Water addition in the mill feed chute achieves 65 to 68 percent solids concentration in the mill.
Key Process Stages
The crushing unit is located on the upper platform with the run-of-mine stockpile. For Stage 1, a 900 mm wide conveyor feeds crushed material onto a 1.5 by 4.8 m vibrating screen fitted with 22 mm and 12.7 mm screen decks. The -12.7 mm material becomes the crusher product. Oversize material from the +12.7 to 22 mm fraction is conveyed to a Sepro Blackhawk cone crusher with a 100 kW motor, while the +22 mm fraction goes to an FL Smidth XL350 Raptor cone crusher with a 220 kW drive. A stacker conveyor delivers the -12.7 mm product to the crushed mineralized material stockpile on the middle platform. The crushing unit is supplied with conveyors, steel supports, electrical switch gear, and controls, with crusher and screening modules skid mounted for simplified installation. Stage 2 requires only a larger screen and a second Blackhawk cone crusher.
The grinding circuit consists of a 4.26 m by 7.3 m ball mill fitted with a 2,000 kW motor, designed to process 1,500 tpd with 92 percent availability and producing a product with a P80 of 75 microns. The mill is equipped with rubber liners and supplied as a complete package.
Gravity concentration uses three concentrators operating on cyclone underflow to recover gold to a high-grade gold concentrate. The concentrators discharge on a fixed cycle of 30 minutes, with feed diverted to the other two machines during discharge via an automatic valve. The estimated mass of the two gravity products is 0.5 percent and 15 percent of feed mass for the high-grade and lower-grade concentrates respectively. Concentrate is diluted to 45 percent solids and fed to the leach plant.
The leach plant has capacity to treat 500 tonnes of concentrate per day. High-grade concentrate first passes over a shaking table where material containing greater than 20 percent gold is separated. The carbon-in-leach circuit uses pump-type interstage screens, allowing all tanks to be on the same level, with the screen lifting slurry to flow into the next tank. Carbon moves counter-current to the slurry. Loaded carbon collects in a 2 m³ tank containing 1 tonne of carbon and is pumped to the elution circuit using a hose pump. Three spargers in the tank walls produce fine air bubbles to promote oxygen mass transfer. Sodium cyanide is added to the first CIL tank, with lime added to maintain pH above 10.5.
Carbon regeneration occurs in a rotary furnace heated by propane at 650 to 700 degrees. Regenerated carbon is quenched in water, passed over a sizing screen to remove fines, and transferred back to the carbon adsorption unit. Makeup carbon is agitated in water for one hour before being screened. Elution solution flows to two electrowinning cells operating in series.
Cyanide destruction uses sodium metabisulfite as the sulfur dioxide source with copper sulfate solution added by metering pump. The pH in the destruction tanks is controlled at 8.5 by lime slurry addition to ensure destruction of iron cyanide complexes. Sulfuric acid formed during the process lowers pH, which is corrected by lime addition.
Tailings from the CIL unit flow to a 10 m diameter thickener producing feed slurry at approximately 55 percent solids. Two automated filter presses produce filter cake with an expected 16 percent moisture content. The thickener and filters are supplied as a package with controls, piping, a control room, flocculant makeup system, and filtrate tank. Filter cake is deposited in a dry stacked tailings management unit, with paste backfill at full production requiring 60 m³ per hour.
Additional Interesting Data and Summary
The CIL circuit, cyanide destruction unit, and carbon treatment unit are all sized with sufficient capacity for the concentrate produced from 3,000 tpd of ore. The filtration plant will be duplicated for Stage 2, while the water treatment plant, water supply system, and paste plant require no further additions at the higher throughput. Tailings filtration equipment is installed in a steel framed structure with a roof and partial sides for weather protection.
The reagent building houses sodium cyanide and sodium metabisulfite delivered in 1 tonne big bags, lifted into a dust tight, vented bag cutting unit. Dissolution is rapid and after one hour the solution is pumped to a 10 m³ stock tank. All reagents and materials are delivered by road. The building also contains instrument air compressors, leach circuit blowers, water pumps, and fire pumps. An extraction hood collects fumes in a bag filter unit. Compressors supply instrument and general use air.
Process water recovered from filtration is collected through automatic bomb doors that open when cake is ready for discharge. All contact water, including process water, is treated prior to discharge.
Key Processes
- Three-stage crushing to P80 of 8 mm using screen, Blackhawk cone crusher, and Raptor cone crusher
- Closed circuit ball mill grinding to P80 of -75 microns at 65 to 68 percent solids
- Gravity concentration using three concentrators on cyclone underflow producing high-grade and lower-grade concentrates
- Carbon-in-leach recovery with interstage pump screens and counter-current carbon movement
- Elution circuit with electrowinning cells operating in series
- Carbon regeneration in propane-fired rotary furnace at 650 to 700 degrees
- Cyanide destruction using sodium metabisulfite and copper sulfate catalyst with pH control at 8.5
- Tailings thickening in 10 m diameter thickener followed by automated filter presses to 16 percent moisture
- Dry stacked tailings management with paste backfill at 60 m³ per hour at full production
Source: Ollachea Gold Project , 2021 Technical Report, 2021. Project website: Ollachea Gold Project
Project website: Ollachea Gold Project, 2021 Technical Report
