Selinsing Gold Mine and Buffalo Reef Project — 2016 Technical Report

This report details the existing oxide ore processing plant at Selinsing and the proposed Phase IV expansion for treating sulphide ore from both Selinsing and Buffalo Reef using flotation and bioleaching.

Report context

The 2016 technical report for the Selinsing Gold Mine and Buffalo Reef Project presents processing information for the existing oxide ore treatment plant, historical production data from fiscal years 2010 to 2016, and a proposed Phase IV expansion design for processing sulphide ore. The report includes process design criteria developed by Lycopodium based on testwork carried out by Inspectorate from 2010 to 2013, along with capital and operating cost estimates for the bioleach facility.

Processing route

Existing Selinsing oxide ore process treatment plant

The existing processing plant is designed to treat oxidised ore using conventional crushing and grinding to approximately 80% passing 74 µm to the cyclone overflow. The ball mill operates in closed circuit with a hydro-cyclone. A split of the cyclone underflow is subjected to gravity recovery using a Knelson centrifugal concentrator, with the concentrate then subjected to intense cyanidation. The cyclone overflow is forwarded to a 36-hour CIL cyanidation leach process.

The current unit operations at the Selinsing processing plant comprise crushing, grinding and classification, gravity concentration (Knelson centrifugal concentrator), intense leaching (Acacia reactor) of gravity concentrate, CIL with cyanidation and carbon adsorption, carbon desorption, electrowinning, smelting, tailings disposal and effluent reclaim, and cyanide detoxification.

The loaded carbon from the CIL is stripped and forwarded with the precious metal pregnant solution from intense cyanidation to an electrowinning station, from which the final precious metal is recovered by smelting to Dore. CIL slurry is disposed into the TSF, from which effluent is recycled back to the process plant. Excess water from the TSF is detoxified with hydrogen peroxide and ferric sulphate to destroy cyanide and precipitate arsenic prior to discharge.

The capital cost for Phases I and II was in the region of US$18.2 million. The mill operates 24 hours per day, seven days per week with an expected 92% availability.

Phase III expansion

The Phase III plant expansion, designed by Metallurgical Design and constructed by SGM, incorporated the installation of an additional crusher, an additional ball mill, three additional leach tanks, and improvements to the gold room, detox circuit, tailings pipelines, and pumping system. In conjunction with this expansion, an additional US$1.7 million was invested to enlarge the TSF capacity. The current design has a dam top elevation of 530 mRL with a capacity of 6.7 Mt of tailings, with final design covering 45 ha and capable of holding a maximum of 11 Mt of tailings with the highest dam top planned at an elevation of 540 mRL.

Proposed Phase IV expansion

An initial conceptual flowsheet for processing sulphide ores from Selinsing and Buffalo Reef was developed based on tests from 2010 to 2013 carried out by Inspectorate. The concept involves producing gold-bearing sulphide concentrate using a froth flotation circuit that could be adapted into the existing plant, followed by regrinding and bacterial leaching. The resulting slurry would be neutralised and forwarded to the CIL tanks.

Lycopodium was engaged in 2013 to prepare a PFS level process design with capital and operating cost estimates for a bioleach plant to service a 1 Mt/a ore milling facility. Based on testwork, bio-oxidation has shown to be the best pre-treatment method for processing sulphide ore material at both Selinsing and Buffalo Reef.

The Phase IV expansion involves the addition of four distinct sections: the flotation circuit, the bioleach circuit, reagents/nutrients, and utilities/services.

The flotation circuit comprises flash flotation on the secondary mill cyclone underflow, rougher flotation of the cyclone overflow, scavenging flotation of rougher tails, and one stage of cleaning of rougher and scavenger concentrate, including a regrind mill circuit. Slurry from the grinding circuit, at 30% solids, reports to a conditioning tank where flotation reagents are added.

The bioleach circuit will consist of a single train with three primary reactors in parallel and three secondary reactors in series, followed by a three-stage counter-current decantation wash circuit and liquor neutralisation circuit. The bioleach circuit will operate 24 hours per day with a concentrate throughput of 12.1 t/hr at design availability of 95%. Maintaining high availability is essential as frequent interruptions can significantly reduce bacterial activity and overall oxidation performance.

The oxidation reactions are highly exothermic and the reactors will be equipped with cooling coil baffles to control slurry temperature at 42°C in each reactor. Medium pressure air will be injected by sparge rings, and slurry pH will be controlled between 1.0 and 1.6 by limestone slurry addition.

During bioleaching, iron, sulphur and arsenic are solubilised. These will be washed from the oxidised residue in a series of three 15 m diameter CCD thickeners with a wash ratio of 7.8 to reduce soluble cyanicides and acid to acceptable levels. The neutralisation circuit will consist of six aerated and agitated 289 m³ tanks in series, with two-stage neutralisation using flotation tails, limestone, and lime to precipitate sulphuric acid as gypsum and soluble arsenic and iron as stable basic ferric arsenate.

The estimated capital cost for the bioleach plant is US$39.2 million. The estimated operating cost for the bioleach process is US$12.46/t, which is additional to the existing operating cost, bringing the total process plant operating cost to US$23.50/t.

The Qualified Person’s opinion is that the proposed flowsheet is applicable and correctly designed for the proposed sulphide material. The operating cost is considered somewhat optimistic but achievable at steady state, and the capital cost is within the PFS range and achievable. Production schedule going forward will stockpile oxide material and treat oxide and sulphide material separately in campaign mode, not as a blend.

Key reported parameters

Technical qualifications

The process design criteria developed by Lycopodium are considered sufficiently detailed for PFS purposes, but additional detail on limestone and lime requirements will be required for final process design as limited data is available from testwork and it is a major contributor to operating costs. The Qualified Person stated that the operating cost is somewhat optimistic but achievable when the plant is at steady state, and that during commissioning and ramp-up this operating cost will not be achieved and will be substantially higher than proposed. The biggest risk with the capital cost estimation is exchange rate fluctuations. Lycopodium recommends thickening and rheological testwork be conducted during the next phase to identify the limiting thickener underflow percent solids. The capital cost estimate excludes duties and taxes, and owners’ and EPCM cost estimates were provided by Monument Mining Limited and have not been vetted by Lycopodium.

Source: Selinsing Gold Mine and Buffalo Reef Project , 2016 Technical Report, Section 17 Recovery Methods