The Candelones Oxide PEA outlines a 5,000 t/d heap leach operation that will process oxide and transition mineralization through crushing, agglomeration, and a carbon-in-columns recovery circuit to produce doré bars.
The Candelones Oxide Pre-Feasibility Study (PEA) describes a 5,000 t/d heap leach operation for processing oxide and transition mineralization from the Candelones project. The conceptual process design, developed by Halyard, is based on metallurgical testwork and design criteria prepared by Micon's Qualified Person (QP). The proposed flowsheet integrates crushing, agglomeration, heap leaching, and a carbon adsorption-desorption-recovery (ADR) plant. This article details the process design criteria, key stages of the flowsheet, and the metallurgical basis for the design, as described in the technical report.
Gold recovery estimates for the oxide and transition mineralization are based on metallurgical testwork performed by Bureau Veritas Commodities Canada Ltd. in Vancouver. The process design criteria were derived from a series of bottle roll leach tests and column leach testwork programs. Notably, phase 1 column leach testwork was completed in 2020, while phase 2 column leach testwork was still ongoing at the time of the report.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Nominal throughput | 5,000 | t/d | Design value |
| Annual throughput | 1,825,000 | t/y | Derived from daily rate |
| Average hourly throughput | 417 | t/h | Crushing circuit design |
| Crushed bulk density | 1.30 | t/m³ | Estimate from phase 1 2020 column test |
| Moisture in material | 4.0 | wt.% | Estimate |
| Feed size (F100) | 500 | mm | Static grizzly opening |
| Final product size (P80) | 100 – 150 | mm | Estimated |
| Crushing circuit utilization | 75.0 | % | Assumed |
| Operating days per year | 365 | days | From client |
| Shifts per day | 2.0 | # | From client |
| Hours per shift | 8.0 | h | From client |
| Days per week | 7.0 | days | From client |
| Cyanide concentration | 0.05 | % | Dosing solution |
| Cyanide make-up | 15 | g/t | Assumed for design |
| Hydrated lime consumption | 4,000 | g/t | Estimate from testwork |
| Pregnant pond operating volume | 100,000 | m³ | Derived |
| Barren pond operating volume | 60,000 | m³ | Derived |
| Carbon column capacity | 6 | t | Per column |
| Carbon loading rate | 3 | t/d | Maximum |
| Carbon cycles | 2-3 | cycles/week | Average |
| Number of leach pads | 1 | # | Assumed |
| Gold recovery | Not stated | % | Testwork-based |
Overview
The Candelones project is designed as a conventional open pit mining operation feeding a heap leach processing facility. The operation is planned to treat a total of 5,000 t/d of mineralization, which is hauled approximately 3 km from the open pit to the heap leach pad. The process design is based on crushed feed material, with the ore being prepared through a combination of primary crushing and lime addition before leaching.
The metallurgical testwork program, including bottle roll and column leach tests, provides the basis for the gold recovery estimates. The process design criteria table (Table 17.1) summarizes the key parameters used for plant design, drawing on data from the testwork programs and assumptions made by the study team.
Key Process Stages
The crushing circuit is designed to handle 417 t/h, operating 16 hours per day, seven days per week, with a 75% equipment availability. Run-of-mine material is dumped onto a coarse leach feed stockpile, from which it is fed to a hopper equipped with a 500 mm opening static grizzly. Oversized material is crushed in a mineral sizer to less than 150 mm. The crushed material is then conveyed to a stockpile where loaders transfer it to the heap leach pad.
Hydrated lime is added to the crushed material at a rate of 4,000 g/t. The lime is delivered to the site and stored in a silo before being screw-conveyed to the haul truck feed conveyor. The material is stacked on the heap leach pad and mixed with the lime before irrigation begins.
The heap leach pad consists of a single pad, with the design based on a single-stage leach, meaning no intermediate ponds were considered. Irrigation is applied using a 0.05% sodium cyanide solution. Barren solution is distributed onto the heap using one vertical centrifugal pump located at the barren pond and a second horizontal centrifugal booster pump operating in series. Pregnant solution is collected in a 100,000 m³ pond and pumped to the carbon-in-columns (CIC) circuit using a vertical centrifugal pump.
The CIC adsorption circuit consists of columns arranged in a cascade counter-current configuration. Each column holds 6 t of activated carbon and operates at atmospheric pressure. Carbon is advanced through the circuit, with a maximum of 3 t of carbon loaded per day and an average of 2 to 3 cycles processed per week. Carbon in the columns is advanced one at a time, with fresh or regenerated carbon transferred into the final empty column.
The desorption circuit uses a 3-t modular Zadra ADR plant operating on a batch basis. The ADR plant comprises acid wash, elution, carbon handling, carbon regeneration, electrowinning cells, and a refinery. Carbon contaminated with organic species is regenerated to restore its adsorption capacity. The elution process recovers gold and silver from the loaded carbon, and the resulting precious metal sludge is filtered, dried in an oven, and smelted to produce doré bars.
Cyanide is delivered as briquettes in 1,000 kg bulk bags and stored in a covered area. The briquettes are mixed with water to produce a 20% w/w cyanide solution at the mixing tank. This concentrated solution is injected into the barren solution pipe feeding the heap leach pad irrigation system to achieve a final dosing concentration of 0.05%. Sodium hydroxide is delivered as a 25% solution in 1,000-litre totes and transferred to the ADR plant via a metering pump. Hydrochloric acid is delivered as a 35% solution in similar totes.
Additional Interesting Data and Summary
The design criteria for the Candelones heap leach process were developed from a combination of metallurgical testwork and design assumptions. The testwork program included bottle roll leach tests and column leach tests, with the phase 1 column testwork completed in 2020 providing data for the crushed ore bulk density estimate of 1.30 t/m³.
The process design emphasizes a relatively simple flowsheet with no tailings facility required, as the process is a heap leach operation. The pregnant pond operating volume is designed at 100,000 m³, while the barren pond is designed at 60,000 m³. The plant is designed to operate year-round at 365 days per year.
The conceptual design by Halyard is based on process design criteria prepared by Micon's QP. This collaborative approach ensures the process design is aligned with industry best practices while being grounded in project-specific testwork data.
Key Processes
- Crushing and agglomeration: Mineral sizer crushing to less than 150 mm with static grizzly scalping at 500 mm
- Heap leaching: Single-stage, single-pad irrigation with 0.05% sodium cyanide solution
- Carbon adsorption: Cascading counter-current carbon-in-columns circuit with 6 t carbon capacity per column
- Desorption and recovery: Modular 3-t batch Zadra ADR plant with acid wash, elution, and carbon regeneration
- Electrowinning and refining: Sludge filtration, oven drying, smelting, and doré bar production
- Reagent preparation: Cyanide briquette dissolution to 20% w/w solution and hydrated lime addition at 4,000 g/t
Source: Candelones Oxide PEA Technical Report, 2021.
Project website: Candelones Oxide PEA Technical Report
