Lac Knife Graphite Project — 2023 Feasibility Study Update

The 2023 feasibility study update for the Lac Knife graphite project describes a concentrator designed to produce 50,000 dry t/y of graphite concentrate through crushing, grinding, flotation, polishing, magnetic separation, and dewatering circuits.

Report context

The 2023 feasibility study update (FSU) for the Lac Knife graphite project was prepared in April 2023. This update increased the plant throughput from the 2014 feasibility study target of 44,300 dry t/y of high-grade salable graphite concentrate to 50,000 t/y of graphite, containing 47,781 t/y of high-grade salable concentrate. The concentrator is located near the open pit mine and consists of a crushing area, beneficiation, dewatering, and bagging areas.

Processing route

Crushing

Run-of-mine ore with an estimated moisture content of 5% is dumped by mine haul trucks onto run-of-mine ore stockpiles, with accommodations for two 7-day stockpiles at the crusher. Material is rehandled by front-end loaders and transferred to a material feeder. A static grizzly at the feed point provides top-size protection, and a mobile rock breaker handles large rocks. The feeder delivers ore into a jaw crusher (1230 mm x 920 mm opening, 160 kW installed power) designed to produce a product with a particle size distribution 80% less than (P80) 124 mm. The primary crushed ore is transported via conveyor to a coarse ore bin with 1-day (1,000-tonne) live capacity.

Grinding and flotation

Crushed ore is withdrawn from the coarse ore bin using two redundant apron feeders and transferred via conveyor to the SAG mill (4.70 m diameter x 2.35 m EGL, 800 kW installed power). The SAG mill operates in closed circuit with a double-deck vibrating screen (1.22 m x 3.05 m) having a top deck with 4.8 mm openings and a bottom deck with 1.7 mm openings. Proper density control with the appropriate ball charge produces a continuous coarse grinding product with a P80 of 0.68 mm. Screen undersize flows by gravity to the ball mill circuit.

The ball mill (2.40 m diameter x 3.60 m EGL, 250 kW installed power) operates in closed circuit with a coarse flotation cell and a set of four hydrocyclones (350 mm, 3 operating plus 1 standby). Ball mill discharge is pumped to the hydrocyclones for size classification. Hydrocyclone underflow flows by gravity to the coarse flotation cell (25.4 m³, 55 kW), while overflow is sent to rougher flotation. The coarse flotation cell is in closed circuit with the ball mill, allowing removal of large graphite flakes as soon as they are liberated. Fuel oil and methyl isobutyl carbinol (MIBC) are added as collector and frother, respectively. No pH modifier is required. Coarse graphite concentrate is expected to contain 66% C(t). Coarse flotation tailings are returned to the ball mill.

The hydrocyclones overflow is designed to have a P80 of 0.274 mm. Rougher flotation uses five 10 m³ flotation cells (15 kW each) providing 18 minutes of retention time. Rougher concentrate is expected to contain 37% C(t) and is combined with coarse concentrate and directed to the polishing circuit. Rougher tails containing 0.60% C(t) are directed to the tailings thickener. The combined concentrate from coarse and rougher circuits is expected to achieve 97% graphite recovery.

Primary cleaner circuit

Cleaning of graphite concentrate is done in two distinct phases. The primary cleaning phase consists of polishing, magnetic separation, and column flotation. The primary polishing mill (3.6 m diameter, 7.3 m EGL, 550 kW) uses ceramic media to scrub gangue minerals from the surface of graphite flakes. The polishing mill discharge is transferred to a low-intensity magnetic separator (LIMS, 36 inch diameter x 24 inch wide drum, 0.7 kW). The non-magnetic material continues to the primary flotation cleaner column (2.44 m diameter x 6.0 m height). This column selectively floats graphite flakes and upgrades combined concentrate from an estimated 57% C(t) to 88% C(t). Column tailings are sent to scavenger flotation (five 1.5 m³ cells, 3.7 kW each) to recover remaining liberated or partially liberated graphite. Primary cleaner scavenger concentrate is returned to polishing mill #1; tailings are sent to the tailings thickener. Cleaner scavenger tailings containing 6.4% C(t) and LIMS magnetics containing 6.8% C(t) report to the tailings thickener. Primary cleaning circuit stage recovery is 95%.

Secondary cleaner circuit

Primary cleaner concentrate is screened with a rotary screen (2.2 m L x 1.8 m W x 1.8 m H frame, 5.5 kW) having panel openings of 0.30 mm. Screen oversize with a P80 of 0.37 mm goes to polishing mill #2 (1.8 m diameter, 3.7 m EGL, 50 kW) for coarse polishing using ceramic media. Polishing mill discharge goes to the secondary coarse cleaner flotation column (1.22 m diameter x 6.0 m height), producing concentrate above 99% C(t) that is sent to concentrate dewatering. Coarse column tailings are sent to coarse cleaner scavenger flotation (one 1.5 m³ cell, 3.7 kW); concentrate is returned to polishing mill #2 and tailings go to the tailings thickener.

Rotary screen undersize goes to polishing mill #3 (3.0 m diameter, 6.1 m EGL, 300 kW) for slightly harsher polishing using ceramic media. This discharge goes to the fine cleaner flotation column (1.83 m diameter x 6.0 m height), producing concentrate above 97% C(t) that is sent to concentrate dewatering. Fine column tailings are sent to fine cleaner scavenger flotation (two 1.5 m³ cells, 3.7 kW each); concentrate is returned to polishing mill #3 and tailings go to the tailings thickener. Secondary cleaning circuit stage recovery is 96%.

Concentrate dewatering

Concentrates from both coarse and fine cleaner flotation columns are dewatered in this circuit. The combined concentrate is first deslimed with a desliming cyclone (250 mm ID), removing -400 mesh graphite. There is currently no market for this product and it is sent to tailings.

Deslimed graphite concentrate is sent to a graphite thickener (7 m diameter high-capacity thickener, 1.9 kW) where it is dewatered to 37% solids. Thickener overflow is pumped to the process water tank for recirculation. Concentrate thickener underflow is sent to a concentrate holding tank prior to being filtered to 15% moisture using a horizontal pressure filter (28 m², 24.3 kW). Filtered concentrate is conveyed to a dryer feed hopper. The dryer is an electric continuous tray dryer (7.6 m diameter x 5.5 m high, 1,500 kW) that dries graphite concentrate to 0.1% moisture. Dried product is transferred using dense phase conveyance to a bulk graphite holding bin (45 t capacity, 4 m diameter x 10 m high).

Graphite dry screening and bagging

Dried graphite is transferred via dense phase conveyance to the bulk graphite bin and then to the dry sifting system. The dry sifting system comprises eight sifting cabinets with 27 sizing screens per cabinet. The 27 screen decks are arranged in a series/parallel arrangement to split a single feed stream into four products. The four screened fractions discharge into appropriate holding bins: graphite flake bin (18 t capacity), coarse graphite bin (26 t capacity), intermediate graphite bin (46 t capacity), and fine graphite bin (46 t capacity). Below each bin is a semi-automatic bagging system with an automated product sampler. Each bag can contain up to 1,000 kg of graphite. Super sack filling is automated; super sack positioning is manual. Filled bags are transferred to a bag storage area prior to loading into trucks via forklift.

Tailings dewatering

Combined process plant tailings are sent to a tailings thickener feed tank where they are mixed with flocculant. Material is thickened to produce an underflow of 65% w/w solids in a tailings thickener (9 m diameter high-capacity thickener, 4.8 kW). Thickener overflow is pumped to the process water tank for recirculation. Tailings thickener underflow is sent to a tailings holding tank prior to being filtered to 15% moisture using a plate-and-frame pressure filter (2.5 m x 2.5 m plates, 96.7 kW). Filtered tails are conveyed to the tailings stockpile, then rehandled via front-end loader and trucked to the filtered tailings storage facility.

Reagents

Fuel oil #2 is used as collector for graphite flotation, delivered by mine fuel truck and stored in a 14 m³ double-walled tank. Expected usage is 66 litres per day. MIBC is used as frother, delivered by tanker truck to a 14 m³ storage tank, with expected consumption of 147 litres per day. Flocculant is used in both graphite concentrate and tailings thickeners, supplied in 25 kg bags, with expected consumption of 20 kg per day. Lime is not used in the process; an allowance for a lime system has been included in case pH or alkalinity adjustment is required.

Utilities

Fresh water from Lac Knife and underground water wells is pumped to a 10.0 m diameter x 12.0 m high freshwater tank at a nominal flow rate of 13.5 m³/h. Potable water is used at 0.9 m³/h; gland water is the remainder at 12.6 m³/h. Reclaim water is recycled from tailings and concentrate thickeners at a nominal rate of 247.1 m³/h, with additional water from concentrate thickener overflow at 62.5 m³/h. The process water tank is 8 m diameter x 8 m high with 325 m³ capacity. Fire water comes from the freshwater system with pumping capacity up to 440 m³/h, stored in a 10.0 m diameter x 12.0 m high tank with 880 m³ capacity.

The concentrator has two sets of high-pressure air compressors: Set #1 for plant air and pressure filter air including an air dryer and instrumentation air receiver; Set #2 comprises two air compressors dedicated to flotation columns (one variable speed, one fixed speed standby). Low-pressure air is supplied by two air blowers for mechanical flotation cells.

Processing power requirements

Installed power for mechanical equipment in the process plant is estimated at 7.5 MW. Mechanical operational power is estimated at 5.8 MW.

Key reported parameters

Technical qualifications

Total graphite recovery of 90.7% and salable graphite concentrate recovery of 86.7% (excluding ultrafine) are average figures based on pilot plant test work results and may change depending on ore composition. All throughput rates are based on the production of 50,000 dry t/y of graphite concentrate containing 47,781 dry t/y of high-grade 97.8% C(t) salable graphite concentrate from a feed grade of 14.8% C(t). There is no current market for ultrafine graphite, and the balance (-400-mesh) is assumed sent to tailings; if upon further market reviews the ultrafine graphite can be utilized, especially in the region, then the ultrafines would be sold as product. Equipment sizing and design were based on metallurgical test results, DRA's experience, and supplier input where applicable. Flotation residence times were based on pilot plant test work. The crusher operating availability of 33.3% differs from the concentrator operating availability of 93%.

Source: NI 43-101 Technical Report, Feasibility Study Update, Lac Knife Graphite Project, April 2023, Sections 17.1 through 17.6.