Sadiola Gold Project — 2022 Technical Report

This report describes the existing oxide processing plant and the proposed new primary ore processing plant at the Sadiola Gold Project in Mali.

Report context

This technical report is dated 12 June 2022 and addresses the Sadiola Gold Project, prepared for Allied Gold Corp and Mondavi Ventures Ltd (to be renamed Allied Gold Corporation). The report presents the recovery methods for both the existing oxide plant, commissioned in 1997, and the proposed new process plant designed to treat primary ore.

Processing route

Existing oxide plant

The oxide plant was commissioned in 1997 with a gravity circuit installed in 2007. The plant flowsheet is typical of free-milling gold ore and consists of two parallel processing streams, sharing a common second stage grinding mill with a nominal capacity of 5 Mt/a. The plant was designed to treat oxidized and soft surface materials with minimal harder rock being presented and as such mineral sizers were incorporated into the original flowsheet. Due to an increasing proportion of harder rock being encountered during the last decade, mobile crushing facilities were incorporated to pre-crush the hard rock for blending with the oxide ore, prior to milling.

Oxide ore (which is soft) is received at the mineral sizer feed bin either directly tipped from Cat 777 trucks or by loaders through a set of grizzly bars. Oversize and harder transitional ores are redirected to a secondary crush circuit for size reduction before being blended into the feed bin using belt feeder hoppers which controls the amount of harder material being treated.

The two primary SAG mills are 4.85 m diameter x 4.88 m in length with 2.5 MW motors, treating the nominal <300 mm ore at a rate of between 275 t/h and 325 t/h each. The one secondary mill is identical to the primary mills. The product from the closed-circuit hydro-cyclone cluster is 80% <75 µm.

A portion of the coarse hydro-cyclone underflow stream is directed to the gravity recovery circuit for the recovery of coarse free gold through an intensive cyanide leach reactor and an associated electrowinning cell. The historical gravity recovery is reported by plant management to be approximately 10% and has been turned off at times over the past 18 months to control milling water balance issues.

The hydro-cyclone overflow stream is screened to remove unwanted wood and plastic prior to being directed to the twin stream pre-leach oxidation tanks and cyanide leach vessels. The pre-oxidation tanks are injected with oxygen to assist with oxidation of some sulphide minerals ahead of the leach circuit. Each leach circuit consists of 10 x 1,470 m³ leach tanks into which cyanide is added to dissolve the gold with a nominal residence of 23 hours. The leached slurry is directed to the twin stream CIP plant with each consisting of seven x 570 m³ vessels operated in counter-current configuration with activated carbon to adsorb the dissolved gold. Each CIP vessel contains ~25 g/L activated carbon with an interstage screen and carbon transfer pump. The loaded carbon is removed from the first CIP vessel and transferred to the twin stream elution circuit.

The elution circuit consists of 2 x 6-tonne acid wash and elution columns utilising the AARL elution process with carbon re-activation in two diesel-fired regeneration kilns. Regenerated carbon is returned to the last CIP vessel in each stream. The solution recovered from the elution process is directed to the electrowinning cells in the gold room for recovery and smelting into doré.

The CIP tailings are treated to remove contained cyanide with the addition of sodium metabisulphite, copper sulphate and oxygen to reduce the contained cyanide to <50 ppm WAD. The treated tailings are pumped to the fully contained TSF.

Proposed new process plant

Previous owners of Sadiola investigated the Expansion Project to treat 6.7 Mt/a of primary ore and procured some of the equipment for the gold process plant, but the project was not implemented. Allied developed production scenarios for up to 10 Mt/a for the Sadiola Expansion Project with a trade-off study including financial analysis and concluding that the optimum size would be a nominal 8 Mt/a process plant for primary material and 10 Mt/a incorporating a blend with up to 30% oxide material.

To realise cost and schedule savings, the design is based on re-using as much of the pre-purchased equipment as feasible with additional machinery and fabrication as required. No portions of the existing plant will be incorporated into the new facility.

The selected process for the recovery of gold from the primary ore at Sadiola is a conventional CIL circuit consisting of crushing, grinding to 80% passing (P80) 75 µm, pre-oxidation, cyanidation and carbon adsorption, followed by electrowinning and smelting to produce gold doré at an average of 73% recovery from the hard primary ore. Plant tailings are thickened, detoxified and arsenic and antimony precipitated prior to disposal to a TSF.

The new plant flowsheet includes: primary and secondary crushing; crushed ore stockpile and reclaim; grinding and classification; pre-leach thickening; pre-oxidation, leaching and adsorption; acid wash and elution; electrowinning and gold room; tailings thickening; cyanide detoxification and arsenic/antimony precipitation; reagents; air and water services.

ROM ore is typically direct tipped into the primary crusher and conveyed to the open circuit secondary crusher and discharged onto the crushed ore stockpile. Apron feeders remove the crushed ore at a controlled rate to feed the open circuit 10 MW SAG mill with the closed-circuit pebble crusher. The SAG mill discharge is delivered to the separate hydro-cyclone clusters with the underflow in closed circuit with the new 18 MW ball mill; there is currently no gravity circuit incorporated in the new plant flowsheet. The hydro-cyclone product is screened and thickened to control the water balance and to maximise the re-use of residual cyanide within the cyanide leach circuit by back-dilution with tailings thickener overflow.

The leach circuit will consist of one pre-oxidation tank (each 3,500 m³) for the addition of oxygen and lead nitrate into the first tank. Primary pH control is achieved by the addition of powdered lime to the mill feed conveyor with milk of lime added to the second tank for secondary pH adjustment. The leach circuit will consist of two leach tanks (each 3,500 m³) in series with seven CIL tanks, also 3,500 m³ capacity. The leach tanks can be fitted with interstage screens and carbon transfer pumps, if justified during operations or if preg-robbing ores are encountered. Fresh and regenerated carbon is charged into the last CIL tank and pumped counter-current with the flow to the first CIL tank, where carbon will be pumped batch-wise to a new existing acid wash and elution circuit.

The CIL tailings will flow to the tailings thickener with the recovered solution being recycled to the pre-oxidation tank feed as dilution. The thickener underflow will be diluted with accumulated and contaminated open pit water prior to cyanide detoxification with arsenic and antimony precipitation in six new 670 m³ tanks. Cyanide content will be reduced with the introduction of SMBS, copper sulphate and oxygen in the first two tanks whilst arsenic will be precipitated with ferric sulphate and antimony with lime to generate chemically stable precipitates. The final tailings will be pumped to the TSF.

The loaded carbon will be acid washed to remove calcium and base metals prior to the high temperature and pressure elution process to desorb gold and silver. There are to be two x 10-tonne elution columns for acid wash and elution duty fabricated from duplex stainless steel. The desorbed gold and silver are electrowon onto cathodes and smelted to doré, prior to shipment to the refinery. Carbon regeneration will be conducted in a diesel fired rotary kiln with a 1,000 kg/h capacity.

Key reported parameters

Technical qualifications

The report states that the historical gravity recovery of approximately 10% is reported by plant management and has been turned off at times over the past 18 months to control milling water balance issues.

The report notes that 2021 production performance was impacted by treating mineralised waste and the ramp-up of mining operations.

The report indicates that structural maintenance is being undertaken with leach tank steelwork being repaired and corrosion protected, refurbishment of the elution plant, gold room facilities and other maintenance items. It has also been stated that one of the mills is showing signs of distress due to internal washaways and the regeneration kiln needs replacement.

The Qualified Person is of the opinion that with adequate and appropriate refurbishment and ongoing maintenance, the existing plant will be able to perform as forecast for the required schedule period and beyond.

The new plant design basis and key inputs for flowsheet development and process design are based on testwork completed in the 2022 FS and are considered representative of future plant feed, based on the testwork carried out at one sample per million tonnes.

The oxide recovery forecast is based on historic results which, in the Qualified Person’s opinion, is fair and reasonable.

The report notes that the primary ore contains some fine gold which is refractory, and a portion is not amenable to cyanide leaching.

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Source: NI 43-101 Technical Report for the Sadiola Gold Project, Mali, 12 June 2022, Section 17 Recovery Methods.