Candelones Oxide Project — 2022 Technical Report

The proposed processing facilities are designed for a 5,000 t/d run-of-mine heap leach operation treating oxide mineralization over a 3–4 year mine life.

Report context

The December 20, 2022, technical report for the Candelones Oxide Project describes the proposed process facilities for treating oxide mineralization from the Candelones Main and Connector open pit areas. The process design is based on interpretation of metallurgical testwork results and process design criteria. The facilities have been sized to process a resource of 5.6 Mt in a run-of-mine heap leach operation.

Processing route

Screening, Agglomeration and Stacking

Run-of-mine material will be trucked to a plant stockpile area designed to contain a live ROM stockpile of up to 95,500 t. A front-end loader will feed material onto a mobile screening plant that separates coarse leach material feed (above 150 mm) from fines smaller than 12–15 mm. The coarse material reports to a stockpile.

The fine particles will be agglomerated with cement and sodium cyanide to improve permeability and avoid potential percolation issues identified in column testing. The fine material will be transported by conveyor belt into an agglomerating drum. Cement will be delivered to site via truck and stored in a 50 t silo, fed to the agglomerator at a ratio of 5 kg cement per tonne of ore. A sodium cyanide tote will be used for cement curing and fed directly into the agglomerator with additional barren solution as required for moisture control.

The agglomerator nominal throughput is designed as 90 t/h based on 25% of the feed being fines and operating two 12-hour shifts per day. This equipment is expected to be used during the first year of operation when more weathered material is processed, with reduced use once more competent rock is encountered in following years.

The agglomerated material and coarse mineral will be loaded into CAT 740 haul trucks and transported to the stacking area. A front-end loader will feed material into a hopper that supplies the grasshopper conveying and stacking conveyor system. The stacking and conveying system is designed for a nominal throughput of 226 t/h based on operating seven days per week, 24 hours per day, with 92% equipment availability.

Heap Leaching, Pregnant and Barren Solution Management

The ROM heap leach pad will consist of eleven lifts of approximately 4.5 to 5 metres in height, with the stacking system used to move along the pad area to prepare different lifts. After stacking and leveling, the irrigation system will be assembled using drip-tube piping as solution emitters.

Barren solution for irrigation will be controlled to contain cyanide at a concentration of 0.05% NaCN, with milk of lime added to achieve a solution pH of 10.5. The irrigation rate selected for design purposes is 10 L/h/m². The total irrigation volume is selected at 400 m³/h, which will allow 40,000 m² of leaching area to be irrigated.

Solution that has percolated through the heap will drain by gravity to a Pregnant Leach Solution (PLS) pond located at the bottom of the heap leach. The PLS pond has a capacity of 26,530 m³ to contain up to 72 hours of a no-power event. Collected pregnant solution will be pumped to the process plant using one of two vertical turbine pumps, with the second as standby.

An emergency pond located next to the PLS pond will be used in emergency events to collect excess rainwater falling on the heap leach pad. This pond will only be used when contained precious metals and residual cyanide are below threshold levels. The emergency pond is equipped with two submersible pumps that can send water to either the PLS pond or the barren leach solution pond.

Barren solution from the discharge of the carbon-in-columns (CIC) circuit will be pumped to a 140 m³ Barren Leach Solution (BLS) tank, where sodium cyanide and lime are added to achieve desired concentration and pH prior to return to the heap leach pad. Two centrifugal pumps will deliver BLS to the heap leach pads for irrigation. A ring main for barren solution will allow equal distribution to all parts of the heap leach pad as required. Solution losses via evaporation will be made up through fresh-water addition to the barren circuit. A 5,000 m³ barren solution pond will be constructed adjacent to the process facility, with excess barren solution flowing by gravity to this pond and returned to the barren solution circuit as required via one of two submersible pumps.

A DETOX circuit using the SO₂ process will be included to destroy cyanide in solution if required. Evaporation rates at the heap leach are expected to result in net water consumption, with no discharge of solution envisaged. The DETOX circuit may be used as required for inventory management and to ensure no free cyanide bearing solutions leave the plant. An agitated detox tank will be installed and can treat up to 100 m³/h of solution using Sodium Metabisulphite and Copper Sulphate prior to reporting to a polishing pond.

Carbon in Column Circuit

Pregnant leach solution from the heap leach process will be pumped from the PLS pond to a 420 m³ CIC feed tank. From here, one of two centrifugal pumps will send solution through one of two strainers to ensure no coarse material enters the CIC circuit. The discharge from strainers will report to the first of six CIC tanks, consisting of a single train of 3.0 m diameter by 2.1 m high carbon absorption tank columns operating in a cascade counter-current configuration. Each column will have a capacity of 6 t of activated carbon.

Columns will operate in counter current mode with fresh carbon added to the last column and pregnant solution to the first column. Carbon will be moved as required on a daily basis but adjusted to operating conditions.

Barren solution from the last CIC column will be pumped to a safety screen to recover any possible carbon and then transferred to the BLS tank.

Desorption (ADR)

The ADR plant will operate on a daily batch basis. Carbon will first pass through a desliming screen to wash with fresh water, with wash water returned to the CIC process. Carbon will then feed by gravity to an acid washing column. A dilute hydrochloric acid solution (3.0% by mass) will dissolve and remove scale, carbonates, or other acid soluble forms adsorbed in the carbon.

Dilute acid will be transferred to a neutralization tank at the end of the acid wash cycle, with pH raised to 10.0 by caustic soda addition. Once neutralized, solution will be pumped to the BLS tank.

After the acid wash cycle, loaded carbon will be transferred into a pressurized strip vessel for gold desorption. A strip solution with elevated cyanide and high pH will strip gold and silver from the carbon. The strip solution will be heated in a series of heat exchangers to 140°C and passed through the elution column until 10–12 bed volumes have been achieved. Gold and silver desorbed from carbon will create a loaded strip solution, which will pass through heat exchangers before being sent to the electrowinning area.

The elution column is designed in an up-flow manner to improve distribution of the stripping solution inside the column. Loaded strip solution will pass through external dual filters before the cooling heat exchanger to reduce temperature and prevent boiling.

After desorption, stripped carbon will be pumped to a kiln dewatering screen to remove water and carbon fines, then transferred to the carbon regeneration kiln.

Electrowinning and Refining

Loaded strip solution will be continuously pumped to an electrowinning (EW) distribution box feeding two electrowinning cells where gold and silver will be recovered from solution as soft precious metal sludge.

Stripped solution from EW cells will report to an EW barren solution tank, where sodium cyanide and caustic soda are added to achieve a pH of 12 prior to return to the heat exchanger for reheating and elution.

Gold/silver sludge will be washed from cell cathodes and pumped into an EW sludge filter using a diaphragm pump to generate a wet filter cake for subsequent calcining. After filtration, the EW sludge will be calcined in an oven to remove additional moisture before smelting. Dried sludge will be placed on pans, mixed with appropriate flux, and added to a propane fired crucible melting furnace. Gold and silver will be separated from slag material and recovered as a doré bar product.

Carbon Preparation, Regeneration and Handling

Fresh carbon is added into an agitated carbon attrition tank, blended with process water recovered from the carbon regeneration area, to reject near-sized or fragile particles from transport. Carbon is pumped onto a carbon sizing screen, washed with fresh water, with the coarse fraction sent to a regenerated carbon holding tank for transfer to the CIC circuit. Fine carbon goes to a carbon fines tank.

Carbon recovered after the stripping process will be regenerated as required in a carbon regeneration kiln. Recovered carbon is washed in a kiln dewatering screen, then fed into a horizontal rotary diesel fired carbon regeneration kiln at approximately 750°C to remove organic contaminants and regenerate adsorption properties. Hot carbon from the kiln is captured in a carbon quench tank filled with fresh water to drop temperature and store it. This carbon is transferred onto the carbon sizing screen to join fresh carbon when needed for the CIC process.

Carbon fines from fresh and regeneration processes are collected into a carbon fines tank, pumped and dewatered using a filter press. Dewatered carbon fines will be stored in bulk bags for later disposal or sale depending on gold assay.

Reagents

Hydrochloric acid will arrive as 50% solution in trucks of 16.8 m³ and stored in a 20 m³ stainless steel storage tank. Diluted acid preparation will be done in a diluted acid tank, mixing pure acid with fresh water.

Hydrated lime will be delivered in trucks and stored in a 50-ton capacity lime silo with approximately 3.5 days of storage. Lime will be screw-conveyed into an agitated milk of lime mix tank with fresh water to prepare milk of lime at 15% w/w. The milk of lime will be transferred to a distribution tank and injected into the distribution ring, mainly to the BLS tank for pH control and occasionally for the CIC circuit feed tank.

Sodium hydroxide will arrive in 1,500 kg pallets of 25 kg bags of solid caustic soda flake. Bags will be dropped into a hopper bag breaker and mixed with fresh water to produce a 50% w/w concentration. Caustic soda will be transferred to a 10 m³ distribution tank and pumped using metering pumps to the ADR plant, cyanide preparation area, and EW barren solution tank for pH control.

Sodium cyanide will be delivered in 1,500 kg pallets of 25 kg bags of solid sodium cyanide. Bags will be dropped into a hopper bag breaker and mixed with fresh water to produce a 20% w/w concentration cyanide solution. This solution will be transferred to a distribution tank and distributed into the barren solution pipe ring using two metering pumps in duty/standby configuration. The 20% w/w cyanide solution will be pumped into the BLS tank to achieve a final dosing concentration of 0.05% in the leaching irrigation circuit. Cyanide solution will also be added into the EW barren solution tank to achieve a concentration of 0.5%.

Sodium Metabisulphite will be delivered in 1,500 kg pallets of 25 kg bags of solid SMBS. An SMBS solution at 25% concentration will be made up as required in a tote set next to the detox tank for cyanide destruction.

Copper sulphate will be delivered in 1,500 kg pallets of 25 kg bags of copper sulphate crystals. A copper sulphate solution at 10% concentration by mass will be made up as required to support the DETOX circuit operation.

Hard-granular activated carbon sized from 6 to 20 mesh will be required for the adsorption circuit, with a make-up rate of 15 g/t assumed for design.

Key reported parameters

Technical qualifications

The source reports estimated reagent consumption and a proposed recovery circuit. Actual operating results are not stated.