Preliminary Economic Assessment of the Valentine Lake Gold Project — 2018

This report describes a proposed processing route comprising separate heap leach and mill facilities for the Valentine Lake Gold Project, based on a 13-year mine life and a combined throughput of 6.0 Mtpa.

Report context

The Preliminary Economic Assessment of the Valentine Lake Gold Project, dated October 2018, presents a process plant design based on two gold processing operations: a heap leach facility and a mill facility. The report was prepared by Lycopodium Minerals Canada Ltd. The design is based on nominal throughput of 9,000 tonnes per day for each facility, equivalent to 6.0 Mtpa combined. Both plant designs are based on a 13-year mine life. The mill will process 3.0 Mtpa of high-grade material from open pit mines, while the heap leach plant will process 3.0 Mtpa of low-grade material from open pit mines.

Processing route

Overall process design

The process plant design is based on a metallurgical flowsheet incorporating unit operations well proven in the industry. The mill will consist of crushing, milling, gravity recovery, flotation of gravity tails, flotation concentrate regrind, cyanidation of both flotation concentrate and flotation tailings via a CIL circuit, carbon elution and gold recovery circuit. CIL tails will be treated for cyanide destruction and disposed of as tails in the tailings storage facility. The heap leach plant will consist of crushing, heap leaching and carbon-in-column gold adsorption. The loaded carbon from the heap leach facility will be sent in carbon transport vessels to the milling facility for further processing in the elution circuit.

Heap leach process

The proposed heap leach process design comprises primary, secondary and tertiary crushing; conveyor, lime addition, transport to the leach pad; heap leach conveying and stacking; barren solution heater, pregnant solution pond, barren solution pond, emergency/storm water pond; and an adsorption plant (CIC). A three-stage crushing circuit is proposed to reduce run-of-mine material from 800 mm to a P80 of 9.0 mm and a P100 of 13.0 mm. Feed to the crushing plant will be accomplished via a front-end loader dumping into a feed bin, with ROM material drawn from the feed bin at a controlled rate of 460 t/h via a variable-speed vibrating grizzly feeder to feed the primary jaw crusher. Primary crushing product, together with secondary and tertiary crusher products, will be sent to a double deck screen.

A total of three lifts, each 10 metres in height, will be used for heap leaching of the material. The design bulk density for the stacked material is 1.68 kg/m³. The pad will be constructed to approximately 942 m x 942 m in size. An emergency/storm water pond will be constructed to capture precipitation and run-off based on a 100-year rainfall event. Cyanide solution will be injected directly into the barren solution line. The cyanide and anti-scalant bearing solution will be applied onto the leach pad at a rate of 5 L/m²/h through a piping network consisting of drip irrigation systems. Based on preliminary column leach test results, a leach period of 130 days is required for optimal gold recovery.

The adsorption circuit will consist of a single train of six open, up-flow columns, each with a 2.5 tonne carbon capacity, operating as an expanded bed contactor. Carbon advancement between the columns will be manually controlled by the operator using carbon advance pumps on each column.

Mill process

The proposed mill process design comprises primary crushing of ROM material; a covered crushed material stockpile; a grinding circuit consisting of a SAG mill with trommel screen, ball mill and cyclones; pebble recycle via FEL reclaim; gravity recovery from ball mill discharge by two semi-batch centrifugal gravity concentrators, followed by intensive cyanidation of the gravity concentrate and electrowinning; trash screen and rougher flotation; thickening of flotation concentrate and flotation tails prior to leaching; regrind mill for flotation concentrate; flotation concentrate leach and CIL, and flotation tails CIL; acid washing of loaded carbon and AARL type elution followed by electrowinning and smelting to produce doré; carbon regeneration by rotary kiln; cyanide destruction of tailings using the Air/SO₂ process; and a polishing pond for water treatment.

Material will be hauled from the mine and dumped on the ROM pad. A ROM hopper apron feeder will regulate feed at 457 t/h into a vibrating grizzly and the jaw crusher. The SAG mill specific energy requirement is 6.1 kWh/t based on an F80 of 75 mm and a P80 of 1,000 to 3,000 µm, without the use of a pebble crusher. The ball mill specific energy requirement of 12.3 kWh/t was determined based on a F80 of 1,000 to 3,000 µm and a P80 of 75 µm.

The gravity circuit comprises two parallel centrifugal concentrators complete with feed scalping screens. Operation of the gravity concentrator will be semi-batch and the gravity concentrate will be collected in the concentrate storage cone and subsequently leached by the intensive cyanidation reactor circuit.

Key reported parameters

Technical qualifications

This is a preliminary economic assessment and the process design is proposed. The basis includes design criteria derived from testwork, with specific limitations noted. A 25% scale-up factor was applied to testwork consumption for sodium cyanide addition in the heap leach process. The leach period of 130 days is based on preliminary column leach test results. The report does not present actual operating data from the Valentine Lake Gold Project, as the facilities are proposed. The head grades provided are based on the mine plan and are presented as a range, blended average, and design value. The report contains figures referenced as Figure 17.1, 17.2, 17.3, and 17.4 depicting the overall process flow diagram and 3D images of the proposed plants, but these figures are not included in the text provided.

Source: Preliminary Economic Assessment of the Valentine Lake Gold Project, October 2018, Section 17.0 Recovery Methods.