Prestea West Reef — 2018 Technical Report

This report details the proposed processing of Prestea West Reef underground ore through the existing Bogoso oxide plant, including batch milling scenarios, CIL circuit design, and associated metallurgical testwork findings.

Report context

The March 2018 NI 43-101 Technical Report on Resources and Reserves for Golden Star Resources' Bogoso/Prestea Gold Mine presents processing plans for ore from the Prestea West Reef deposit. Ore is to be trucked approximately 15 km from Prestea Central Shaft to the Bogoso process plant for treatment through a modified Oxide circuit employing conventional carbon-in-leach (CIL) gold recovery.

Processing route

Comminution study background

Metallurgical testwork programs have been conducted on samples from the PUG West Reef deposit, with an initial program in 2008 and subsequent testwork supporting feasibility studies for mechanised cut and fill (2013) and shrinkage mining (2015). A comminution study by MDM/AMEC Foster Wheeler/KEMIX evaluated both continuous and batch operation modes for the existing circuit.

Proposed feed scenarios

Two operating scenarios were examined. Initially, oxide and PUG ore would be processed together at a RoM rate of 1.154 Mt/a dry basis for oxide and 0.237 Mt/a dry basis for PUG ore, over 7,076 operating hours per annum at 81% availability. Once the oxide ore supply is exhausted, 0.237 Mt/a of PUG ore would be processed through the same circuit but on a batch basis, decoupling comminution from CIL via medium-term slurry storage.

Crushing and stockpiling

Run-of-mine ore at approximately -350 mm will be delivered by truck to the Bogoso ROM stockpile and stored under a newly constructed ore shed with 24-hour security surveillance. A front end loader reclaims ore from the stockpile; material of -150 mm bypasses the crusher via a vibrating grizzly, while oversize reports to a KEMCO C160 Jaw crusher producing P80 150 mm product. Bath crushing will be undertaken, with a 6,000 t capacity covered stockpile installed to manage security risks and rainwater ingress.

Milling and classification

Crushed ore is delivered at a nominal 120 tph to a 1.5 MW Morgardshammer SAG mill operating in open circuit with variable speed capability from 60% to 80% of critical speed. Mill discharge slurry at P80 700 micron reports to a bank of GMAX 15-3117 Krebs cyclones. Cyclone overflow at P80 -75 micron is thickened and fed to the CIL circuit, while underflow reports to a gravity circuit comprising two operating 30" Knelson concentrators and one standby unit. The 1.5 MW Ball mill operates at 75% critical speed via an air clutch.

CIL circuit design

Feed thickened to 45% solids is stored in six 1,200 m³ tanks providing 3,600-4,200 t storage. The CIL circuit consists of five 800 m³ leach and adsorption tanks in series with interconnecting launders. Sodium cyanide, oxygen/hydrogen peroxide, and activated carbon are added to enhance leaching. The TAC 1000 Automatic Cyanide analyser/controller optimises cyanide addition based on CIL feed rate.

Each CIL tank is 1,200 m³ in size with a live volume of approximately 1,140 m³. Loading at 25-40 tph of solids would occur from these tanks at 45% w/w solids.

Elution and gold recovery

Loaded carbon undergoes acid washing with dilute hydrochloric acid followed by pressurised Zadra elution, with flexibility to use either 5 t or 16 t elution columns. The 5 t column is recommended, giving one elution every 5 days and ensuring steady cash flow. Electrowinning cell cathodes are removed weekly, calcined at 750°C for a minimum of 12 hours, fluxed, smelted, and cast into bullion bars.

Barren carbon is regenerated in a 275 kg/hr Electric Kiln at 650-750°C in an inert atmosphere. Concentrate from the Knelsons is processed through the Consep Acacia CS2000 within the gravity fence, using a heated leach system to maximise leach rate and achieve high gold solutions.

Proposed carbon management

Simulations indicate that CIL tanks are oversized for lower throughput by a factor of 2-4 times, with no economic justification for a new CIL circuit. A low carbon upgrade ratio of 900:1 is proposed, with staged carbon concentrations of 10 g/L in the first stage and 5 g/L in four subsequent stages. While six stages are indicated by simulations, a reduction to five or fewer might be achievable, requiring confirmation under real operating conditions.

Key reported parameters

Technical qualifications

The report notes that the number of CIL stages required will only be established under real operating conditions. The metal recovery estimate of exceeding 94% applies to processing PUG ore alone on an annualised basis, based on testwork. The leach is expected to be complete within 15-20 hours, theoretically achievable in the first two tanks, but gold adsorption onto carbon depends on number of stages, carbon concentration, activity, and gold on regenerated carbon rather than residence time alone.

The comminution section as currently configured is stated as suitable for both continuous and batch modes, with greater operational flexibility and cost savings realised in batch mode at lower throughputs. Recommendations to minimise carbon inventory include using a relatively low carbon upgrade ratio of 900:1, with the objective of minimising CIL tanks subject to not incurring gold losses in excess of CIL stage operating costs.

Source: Prestea West Reef , 2018 Technical Report, NI 43-101 Technical Report on Resources and Reserves, Golden Star Resources, Bogoso/Prestea Gold Mine, March 2018, Sections 17.1, 17.2, and 17.3.