Rajapalot PEA — 2023 Technical Report

A proposed processing plant designed for two feed types will produce gold doré and cobalt concentrate from the Rajapalot project.

Report context

This technical report describes the recovery methods for the Rajapalot project based on the 19 December, 2023 preliminary economic assessment. The process design is based on metallurgical test work and typical engineering principles deployed widely in Finland and globally in the gold and base metals industry.

Processing route

Feed classification and campaign operation

Run-of-mine plant feed will be classified as "gold" and "gold-cobalt" types and processed on a campaign basis. Both feed types share the same process with the exception of an added flotation step for the gold-cobalt feed.

Crushing

The plant feed is crushed in a three-stage crushing process to 100% passing 12 mm. Primary crushing uses an open circuit jaw crusher with a feed opening of 1,200 x 830 mm and closed side setting (CSS) of 100 mm. The assumed blasted top size is 600 mm. Oversized material will be broken down with a hydraulic rock breaker. The primary crushed material is conveyed to a triple deck screen with 60 mm, 20 mm, and 10 mm apertures. Secondary crushing uses a cone crusher with CSS of 20 mm, and tertiary crushing uses a cone crusher with CSS of 9 mm. The crushed product P80 is approximately 8 mm. The minus 10 mm particles are conveyed to the fine ore bin, which has a 2.8 day residence time.

Grinding

The crushed plant feed is ground in a single stage conventional overflow ball mill to a P80 of 70–75 µm. The ball mill installed power is 2,750 kW and size 4.7 m diameter x 7.05 m effective grinding length. Lime powder is fed to the mill to maintain slurry pH of 10.5 before cyanide addition. The ball mill discharge is pumped to a hydrocyclone cluster.

Gravity separation and intensive leaching

A split of the cyclone classification underflow reports to a gravity and intensive leach circuit. One third of the hydrocyclone underflow is passed through a 2 mm wet screen, with undersize reporting to a centrifugal gravity separator. Gravity concentrate is upgraded by shaking table, and approximately 1.2 tpd of gravity concentrate is produced. Shaking table concentrate is treated in batches in an intensive leach reactor using concentrated sodium cyanide solution with oxidant and pH adjustment.

Carbon-in-leach (CIL)

The grinding circuit cyclone overflow is transferred to a pre-CIL thickener to increase solids content from 40 w/w% to greater than 50 w/w%. Cyanide leaching is undertaken in a CIL process with one leach-only tank and six CIL tanks. Tanks are sized at 1,100 m³ each at 11 m diameter x 11.05 m height. Residence time is 30 hours. Slurry flows from the first tank to the last, with activated carbon moving in the opposite direction.

Acid wash, stripping and electrowinning

The loaded carbon is transferred to acid wash and stripping before electrowinning. Acid wash uses a 3% HCl solution. Elution follows a pressurized Zadra process. Electrowinning for the intensive leach solution and stripped carbon eluate is completed in separate electrowinning circuits using stainless steel wool mesh cathodes. After electrowinning, the gold sludge is filtered, dried and smelted into gold doré bars. The stripped carbon is regenerated in a reactivation kiln for return to the CIL circuit.

Cyanide detoxification

The CIL circuit tailings are detoxified by cyanide destruction using the INCO process, with oxygen and sulphur dioxide introduced via sodium metabisulphite at pH 8.5–9.0. Design allows for two consecutive tanks with a volume of 220 m³ each and 5.9 m diameter, providing 1 hour total residence time.

Flotation for cobalt recovery

For gold-cobalt feed, the detoxified CIL residues are processed by flotation to produce a marketable cobalt concentrate. Cobalt is dominantly contained in cobaltite and fine-grained linnaeite intimately associated with pyrrhotite. Cobalt recovery is maximised by bulk sulphide flotation. The flotation circuit consists of a rougher circuit (five 30 m³ mechanical flotation cells) and two cleaner stages (four 5 m³ cells each). Reagents include sodium ethyl xanthate as collector, copper sulphate as activator, and methyl isobutyl carbinol as frother. The rougher flotation residues are final plant residues. The second cleaner concentrate is pumped to cobalt concentrate thickening and filtration. Concentrate is thickened in an 11 m diameter high-rate thickener and filtered using a pressure filter.

Water management

Water circulation is maximised in the process design, making up to 80% of the process water requirement. Input water is majority sourced from underground dewatering, with approximately 112 m³/h pumped from the mine and rock storage runoff areas. Raw water is required at a nominal rate of 7 m³/h from a ground water source. Water treatment includes a metals precipitation facility and a moving bed biofilm reactor for nitrogen removal with a capacity of 150 m³/h.

Key reported parameters

Technical qualifications

The process described is based on metallurgical test work and typical engineering principles. Multiple design criteria values have been estimated without laboratory or pilot scale testing and have been marked as "assumption", "assuming", or "typical" in the preliminary process design criteria. All such values require further confirmatory test work. The design criteria will be updated accordingly in later project stages as more information becomes available.

Source: Rajapalot PEA – Main Report, 19 December, 2023, Section 17 Recovery Methods.