Bayan Khundii Gold Project — Feasibility Study Update

The Feasibility Study Update for the Bayan Khundii Gold Project details a 650,000 tpa conventional cyanide leaching plant with carbon-in-pulp recovery, designed to produce an average of 78,200 ounces of gold per year within a doré at 92.7% recovery over the life of mine.

The Bayan Khundii Gold Project Feasibility Study Update presents a Class 2 estimate based on a complete engineered design, with only detailed design and incorporation of specific vendor information remaining. The process plant design was developed from the Prefeasibility design and additional metallurgical testing undertaken during the feasibility study. The Eurocode was used as an overarching internationally recognized code, as it addresses gaps in the Mongolian guidelines and is comprehensive and prescriptive as to the application of design loads.

A significant body of metallurgical and process test work was completed during the Prefeasibility Study for the Bayan Khundii deposit. The body of test work has been sufficient to fully develop the process design, which in turn has allowed the development of the design to the level required. The feasibility study determined that the process plant will have an annual capacity of 650,000 tpa throughput and is expected to produce an average 78,200 oz per year of gold within a doré, over the planned mine life, at an average 92.7% recovery.

The process design criteria were used to size and cost the processing plant for this phase of the project. The plant design basis has an overall plant operating time of 92% or 8,060 hours per annum. The process plant has been designed to produce up to approximately 88 Koz/a gold as doré bar as achieved in Year 5, but averages approximately 77 Koz per year over the life of mine, consistent with the 78,200 oz per year statement in the feasibility summary.

Critical Data

ROM Ore

Carbon in Pulp (CIP)

Overview

The Bayan Khundii processing facility has been designed to treat oxide and transition ore through a conventional crushing, grinding, and cyanide leaching circuit. The plant design incorporates duplicated crushing circuits, duty/standby pumps, and bypass launders to ensure high reliability and maintenance flexibility. All selected circuits were challenged during the feasibility study update to determine if they defined the most appropriate circuit, with sizing further developed using initial test-work from the PFS and supplementary test-work during the FS.

The process plant uses a single-stage jaw crusher in each of the two permanently installed crushing circuits, followed by a SAG mill and ball mill grinding circuit. A gravity circuit has not been included in the design, as gravity recovery has not indicated any additional overall recovery beyond the leach recoverable gold at the design grind size. However, it is noted that there is potential ability to reduce the load to the leach and CIP circuits by removing some gold and silver from high grade feeds prior to leaching.

The design prioritizes water management and cyanide conservation. The tailings circuit includes a thickener to wash residual cyanide from the CIP tail, detoxification on a high solid phase tailing pulp using the sulphur dioxide/air method, and pressure filtration to produce a dry stack tail material to be encapsulated in the in-pit tailings facility.

Key Process Stages

The crushing circuit consists of two permanently installed crusher circuits, each including a ROM bin, apron feeder, vibrating grizzly to allow the passing of smaller material, and a jaw crusher. A grizzly will be placed over the feed bin to control oversize material and minimize crusher blockages, with any material that pegs the grizzly managed by a rock breaking hammer fixed to an excavator. The SAG feed surge bin has an overflow chute to allow material to be stockpiled as an emergency crushed stockpile, which is fed back to the SAG mill feed belt when required via a hopper and belt feeder. The reclaim feeders are controlled via a belt weigh scale on the SAG mill feed conveyor to ensure a consistent feed rate to the grinding circuit, which is critical to ensure consistency in the grind size and control over the mill circuit.

The grinding circuit consists of a SAG mill and ball mill. After additional modelling, the solid percentage will be increased to the optimum level as determined by the test work program, targeted at 42% solids by mass. The leach circuit design was reviewed to determine the appropriateness of CIP versus CIL. A single train of leach tanks was selected, which provides a reduced leach time and simplification of process understanding, in that there are not two separate dynamics occurring at the same time. A higher amount of gold is retained in the circuit as a result.

The CIP circuit operates with carbon concentration of 30 g/l and includes four tanks with bypass launders, so any one tank can be taken out of circuit for maintenance while the remainder still operates. Carbon control is by intertank rotary screen. Duty/standby for the carbon recovery pump has not been included as the carbon recovery cycle is estimated to be approximately 6 hours every 24, giving sufficient time for maintenance between cycles. Pumps from the thickener underflow are installed as duty/standby sets to ensure operations continue during maintenance.

The elution circuit uses a packaged modular Pressure Zadra system, selected based on water quality and mineralogy simplicity. The packaged modular solution is complete with all interconnecting pipework, a self-contained Process Logic Controller (PLC automated control system), and a built in gold room containing the filters, driers, furnace and gold safe so the high value material does not leave the high security area during the process. Carbon reactivation and handling are also incorporated into the same facility. All water used in the elution process, including flushing water, is filtered water to minimize contamination of the carbon.

The tailings circuit is designed to manage cyanide and water effectively. The circuit allows additional cyanide to be flushed from the CIP tail before the detoxification circuit by routing the transfer of raw water to the process water tank by way of the tails thickener feed. This additional feed dilution allows for the displacement of cyanide containing fluids back to the process water tank for further utilization and assists in the densification of the thickener underflow by reducing the settling distance. Detoxification is conducted using the sulphur dioxide/air method on a high solid phase tailing pulp, followed by plate and frame pressure filtration to produce a dry stack tail material with approximately 12% moisture.

The plant water supply is managed through five water circuits: Raw Water, Process Water, Fire Water, Filtered Water, and Potable Water. The raw water transfer to the process water tank is achieved via the tails thickener feed dilution, with the thickener overflow pumped to the process water tank. Filtered water from a reverse osmosis plant is further treated by a UV sterilization unit for potable water requirements. A dedicated fire water volume has been allocated within the raw water tank. A gland water circuit supplies gland water to all slurry pumps within the process.

Reagent requirements were firmed up with additional test-work, with reagent make-up systems sized according to expected utilization. A mix is required per day for major reagents including cyanide, sodium hydroxide, SMBS, lime, and every 3 or 4 days for copper sulphate. Hydrochloric acid, used in the elution circuit, will be supplied as a 33% strength in 1,000-liter bulky cubes. All reagent areas using bulk bags for mixing are designed accordingly.

Additional Interesting Data and Summary

The Bayan Khundii project has deliberately excluded a gravity circuit from the design based on metallurgical test results indicating no additional overall recovery beyond leach-recoverable gold at the design grind size. However, the study acknowledges potential future ability to reduce load to the leach and CIP circuits by removing gold and silver from high grade feeds prior to those stages.

Water management is central to the plant design. In operation, it is likely that there will be a build-up of copper within the process water circuit due to the high copper content in the ore, which is managed through the water circuits. Air accumulation tanks are used to maintain steady air pressure within the cycle operation of the compressors.

The process plant has been designed with maintainability as a key consideration, with duty/standby pumps specified for continuous use primary process pumps and bypass launders in multi-tank processes to enable temporary bypass of individual tanks for maintenance.

Key Processes

• Two-stage crushing with dual permanently installed jaw crusher circuits, each with ROM bin, apron feeder, vibrating grizzly, and rock breaking hammer
• SAG mill and ball mill grinding circuit with surge bin and emergency crushed stockpile reclaim
• Single train leach circuit followed by Carbon-in-Pulp (CIP) recovery with four tanks and bypass launders
• Packaged modular Pressure Zadra elution circuit with integrated gold room, carbon reactivation, and PLC automation
• Tailings thickening with cyanide flush, sulphur dioxide/air detoxification, and plate and frame pressure filtration for dry stack tailings
• Five-circuit water management system (Raw, Process, Fire, Filtered, Potable) with reverse osmosis and UV sterilization
• Reagent make-up systems sized for daily major reagent mixing (cyanide, sodium hydroxide, SMBS, lime) and periodic copper sulphate

Source: Bayan Khundii Gold Project , Feasibility Study Update, NI 43-101 Technical Report, 2023.

Project website: Bayan Khundii Gold Project, Feasibility Study Update