Almas Gold Project — 2021 Technical Report

This technical report presents a proposed gold processing plant design for the Almas Gold Project, based on feasibility-study testwork and historical testing.

Report context

The technical report is dated March 10, 2021, and describes a process design developed for a feasibility study. The design is based on results from several testwork programs, including testwork completed for the feasibility study and historical testing that evaluated different flowsheet options. The selected flowsheet is based on typical industry unit operations for gold processing plants.

Processing route

Overall process design

The flowsheet includes primary crushing followed by grinding to achieve a particle size distribution of 80% passing 75 µm. Part of the cyclone underflow will be processed in a gravity circuit and the cyclone product (overflow) will feed a pre-leach thickener, with the underflow processed through a leach/carbon in leach (CIL) circuit. CIL tailings will be treated for cyanide destruction. The carbon from CIL will go to elution, regeneration and the final solution will go to electrowinning and the gold room.

Primary crushing and stockpiling

The crushing circuit is designed for an annual operating time of 6,130 h or 70% availability at the capacity of 3,560 t/d. Material is hauled from the mine or stockpiles and fed by front-end loader into the mobile crushing system, which is composed of a run-of-mine (ROM) hopper, a vibrating grizzly feeder, a primary crusher and a discharge conveyor, along with auxiliary equipment. Provision for dumping on the ROM pad for blending and re-handling into the ROM hopper is provided. Material from the ROM hopper is crushed by a primary jaw crusher. ROM hopper material is reclaimed by a vibrating grizzly at 212 t/h to feed the jaw crusher.

A mobile rock breaker is utilized to break oversize rocks at the feed to the jaw crusher. The crushed material is conveyed to a surge bin that provides approximately 3 hours of live storage at the nominal processing rate. The bin has an overflow system, which forms an emergency stockpile next to the bin. Given the milling operation is designed for an annual operating time of 8,059 h or 92% availability, this will result in excess crushed material production when the crusher is operational. The mill feed surge bin is equipped with two vibrating feeders to regulate feed at 161 t/h into the SAG mill.

Grinding and classification

The grinding circuit includes a low-aspect SAG mill with trommel screen and cyclone classification. Pebbles from the SAG mill are fed to a recycle circuit via conveyor and discharged on the SAG mill feed conveyor to recycle to the SAG mill. The transfer point of the pebble recycle conveyor has a chute that allows purging of the pebbles as required.

Gravity recovery

Gravity recovery of the cyclone underflow slurry is performed by one semi-batch centrifugal gravity concentrator. The gravity circuit feed source is cyclone underflow slurry, with 25% of the cyclone underflow directed to the gravity circuit. Scalping screen undersize is fed to the centrifugal concentrator. The gravity concentrate is collected in the concentrate storage cone and subsequently leached by the intensive cyanidation reactor circuit. The tails from the gravity concentrator report to the gravity tails pumpbox.

Intensive leach reactor

Concentrate from the gravity circuit reports to the intensive leach reactor (ILR) to extract the contained gold by intensive cyanidation. The concentrate from the gravity concentrator is directed to the ILR gravity concentrate storage cone and de-slimed before transfer to the ILR. ILR leach solution (mixture of NaCN, NaOH and LeachAid – an oxidant) is made up within the heated ILR reactor vessel feed tank. The leached residue within the reaction vessel is washed, with wash water recovered to the reaction vessel feed tank, and then the solid gravity leach tailings are pumped to the CIL circuit. The ILR pregnant leach solution is pumped from the reaction vessel feed tank to the ILR pregnant solution tank located in the gold room. ILR pregnant solution is treated in the gold room for gold recovery as gold sludge using a dedicated electrowinning cell.

Pre-leach thickening

Trash screen undersize feeds the pre-leach thickener, which increases the solids concentration to 50% (w/w) prior to the leach-CIL circuit. Flocculant is added to the thickener feed to improve solids settling. The thickener overflow is reused as process water throughout the plant, mainly at the cyclone feed pumpbox.

Leach and adsorption circuit

The leach-adsorption circuit consists of one leach tank and six carbon-in-leach (CIL) tanks. The circuit is fed by the pre-leach thickener. The leach and CIL tanks are identical in size, with a total circuit residence time of 24 hours at 50% w/w density. Air is sparged to each tank to maintain adequate dissolved oxygen levels for leaching at 5-8 mg/L. Hydrated lime is added to adjust the operating pH to the desired set point of 10.5-11. Cyanide solution is added to the first leach tank. Fresh/regenerated carbon from the carbon regeneration circuit is returned to the last tank of the CIL circuit and is advanced counter-currently to the slurry flow by pumping slurry and carbon. Slurry from the last CIL tank gravitates to the cyanide detoxification tanks.

Cyanide destruction

CIL tails at approximately 50% w/w solids flow by gravity to the two cyanide destruction tanks. Each tank operates with a total residence time of approximately 60 mins to reduce weak acid dissociable cyanide (CNWAD) concentration from 150 mg/L to less than 2.0 mg/L to comply with environmental requirements prior to deposition in the TSF. Cyanide destruction is undertaken using the SO2/air method. The reagents required are air, lime, copper sulphate, and sodium metabisulphite (SMBS). From the detoxification tank, the tailings report to the carbon safety screen.

Carbon acid wash, elution and regeneration

Prior to gold stripping stage, loaded carbon is treated with a weak hydrochloric acid solution to remove calcium, magnesium, and other salt deposits. The acid-washed carbon is then hydraulically transferred to the elution column for gold stripping.

The gold stripping (elution) circuit uses the Anglo-American Research Laboratory (AARL) process. The elution sequence commences with the injection of a set volume of water into the bottom of the elution column, along with the simultaneous injection of cyanide and sodium hydroxide solution. During the pre-soak, the caustic/cyanide solution is circulated through the column and the elution heater until a temperature of 95°C is achieved. Upon completion of the pre-soak period, additional water is pumped through the trim heat exchanger and elution heater, then through the elution column to the pregnant eluate tank at a rate of 2.0 Bed Volumes (BV)/h. At this stage, the temperature of the strip solution passing through the column is increased to 120°C.

Carbon is reactivated in a gas-fired rotary kiln. Dewatered barren carbon from the stripping circuit is held in a 3-t kiln feed hopper. A screw feeder meters the carbon into the reactivation kiln, where it is heated to 650° to 750°C in an atmosphere of superheated steam to restore the activity of the carbon.

Electrowinning and gold room

Gold is recovered from the pregnant solution by electrowinning and smelted to produce doré bars. The pregnant solution from both elution and the intensive cyanidation circuit is pumped through one electrowinning cell with stainless steel mesh cathodes. The gold-rich sludge is washed off the steel cathodes in the electrowinning cell using high-pressure spray water and gravitates to the sludge hopper. The sludge is filtered, dried, mixed with fluxes, and smelted in an electrical induction furnace to produce gold doré.

Key reported parameters

Technical qualifications

The process design is based on the results of several testwork programs, including testwork completed for the feasibility study and historical testing. Historical testing evaluated different flowsheet options. The flowsheet selected for the feasibility study is based on typical industry unit operations for gold processing plants. The report does not provide actual operating data from a built plant, as the design is proposed. Key process design criteria are presented as design values, not confirmed operating results. The report states that a source for the flowsheet and layout drawings is Ausenco, 2020.

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*Source: NI 43-101 – Almas Gold Project – March 10, 2021, Section 17 Recovery Methods*