Prairie Creek — 2021 Technical Report

This report presents a process design for the Prairie Creek mineral processing facility based on dense media separation and sequential flotation to produce lead and zinc concentrates.

Report context

The technical report titled "NI 43-101 Technical Report on Preliminary Economic Assessment October 15, 2021" describes a proposed process plant design for the Prairie Creek Mine, prepared by Ausenco. The design relies on metallurgical data and analysis prepared for Section 13 of the report, with the process design based mainly on results from the 2017 metallurgical test programs, including heavy liquid separation, flotation, mineralized material hardness, and dewatering tests. The design also incorporates some existing equipment that was moved from another mine and installed at Prairie Creek in 1981/1982. With an increase in throughput to 2,400 tonnes per day, the crushing plant is the only area retained without modification.

Processing route

Crushing

The existing refurbished crushing circuits consist of a primary crushing unit and a secondary crushing unit in closed circuit with a vibrating screen, reducing ROM mill feed to a particle size of 80% passing 12 mm. Major equipment includes a ROM mill feed dump pocket (40 tonnes live capacity) with a fixed grizzly and vibrating feeder, a coarse mill feed surge bin (136 tonnes) with an apron feeder fitted with grizzly bars, a Kue-Ken 36" x 24" (914 mm by 610 mm) jaw crusher, a secondary crushing feed surge bin (45 tonnes) with a belt feeder, a double deck screen with apertures of 25 mm and 15 mm, a Symons Nordberg 5.5' (1.7 m) shorthead cone crusher, conveyors including a metal detector and magnetic separator, a fine mill feed bin (1,800 tonnes) with a reversible belt feeder, and dust collection systems.

Dense media separation plant

The DMS plant is new equipment designed to reject gangue material, reducing effective feed tonnage and increasing feed grades to the downstream grinding and flotation circuits. Fines are removed from the crushed ore, with material passing a 1.4 mm screen bypassed to grinding. Screen oversize is fed to dense media (ferrosilicon) cyclone separation at a proposed separation specific gravity of 2.8. DMS rejects (the light, float fraction) will be conveyed to a temporary 200 tonnes stockpile (uncovered) and loaded onto haul trucks for transport to the waste rock storage facility. The coarse DMS sink fraction will be conveyed to the ball mills in the grinding circuit. The DMS circuit is designed to be by-passed, whereby feed will be directed to the grinding circuit during times when the DMS circuit is off-line for maintenance.

Grinding and classification

The grinding circuit will consist of two ball mills (one existing, one new) in closed circuit with classifying hydrocyclones. Major equipment includes an existing refurbished 10' (3.05 m) diameter x 14' (4.27 m) long ball mill with a 700 horsepower (522 kW) motor, a new 10' (3.05 m) diameter x 14' (4.27 m) or similar ball mill with a 700 horsepower (522 kW) motor, a new classifying hydrocyclone pack, an existing refurbished ball mill discharge pump box, new hydrocyclone feed pumps, existing and new ball mill feed conveyors, and ancillary equipment including a steel ball storage bin and ball bucket.

Flotation

Polymetallic, sequential flotation will be employed to separate lead and zinc sulphide minerals into concentrates. All flotation cells in both circuits will be new.

The lead flotation circuit consists of rougher, regrind and three stages of cleaner flotation. Major equipment includes one new rougher conditioning tank with a mechanical agitator, six new rougher flotation cells (14.2 m³), one new regrind ball mill (6.5' or 2 m diameter x 10' or 3.05 m long, or similar) with 200 HP (150 kW) motor and cyclones, two existing refurbished cleaner conditioning tanks with mechanical agitators, five new primary cleaner flotation cells (5.1 m³), two new scavenger flotation cells (5.1 m³), five new secondary cleaner flotation cells (2.8 m³), four new tertiary cleaner flotation cells (2.8 m³), and ancillary equipment.

The zinc flotation circuit consists of rougher flotation followed by three stages of cleaner flotation. Major equipment includes two existing rougher conditioning tanks with mechanical agitators, five new rougher flotation cells (14.2 m³), one new cleaner conditioning tank with mechanical agitator, four new primary cleaner flotation cells (5.1 m³), two new scavenger flotation cells (5.1 m³), four new secondary cleaner flotation cells (2.8 m³), four new tertiary cleaner flotation cells (2.8 m³), and ancillary equipment.

Concentrate dewatering and load-out systems

Lead concentrate will be thickened in a new 8.9 m diameter high-capacity thickener. Underflow will be pumped to the existing lead concentrate surge tank at approximately 65% solids. The thickened concentrate will be further dewatered to a moisture level of 8% using existing refurbished Larox pressure filters. Filter cake will be conveyed to a lead concentrate stockpile with one day of storage capacity, then loaded into 20 t concentrate containers.

Zinc concentrate will be thickened in a new 6.8 m diameter high-capacity thickener. Underflow will be pumped to the existing zinc concentrate surge tank at approximately 65% solids. The thickened concentrate will be further dewatered to a moisture level of 8% using a new Larox (or similar) pressure filter. Filter cake will be conveyed to a dedicated zinc concentrate stockpile with temporary storage capacity of one day's production.

Tailings handling and paste plant

Final tailings from the zinc flotation circuit will be pumped to the tailings thickener and then to the backfill plant to produce paste for backfilling underground slopes. A solids underflow concentration of approximately 60 wt% will be achieved. There will be a new paste plant and paste delivery system.

Reagent preparation and delivery

Various chemical reagents will be added to the flotation circuits. The existing reagent preparation area will be refurbished and utilized. Storage tanks will be equipped with level indicators and instrumentation. Appropriate ventilation, fire and safety protection, and MSDS stations will be provided. Storage of bulk reagents will be located inside the mill building.

Assay and metallurgical laboratory

The metallurgical laboratory will be located in existing refurbished office rooms in the mill building, equipped with laboratory crusher, ball mills, sample pulveriser, splitter, Ro-tap sieve size analyser, flotation test cells, vacuum filters, pH meters, weighing scale, hot plate, work bench, and drying oven.

A new stand-alone assay and water treatment laboratory will be housed in a free-standing pre-engineered building, equipped with laboratory crusher, sample pulveriser, splitter, microwave plasma-atomic emission spectrometer (MP-AES), graphite atomic absorption spectrophotometers (AAS), X-ray fluorescence spectrometer (XRF), UV/VIS spectrophotometer, drying oven, pressed pellet, chloride ISE kit, laboratory pressure filter, fusion furnace, cupelling furnace, hot plate, weighing scale, work bench, and pH meters.

Mill water supply and distribution

Fresh water will be supplied from Cell B of the Water Storage Pond (WSP), supplied with mine dewatering non-contact water. Process water will come from concentrate thickener overflows returned to flotation circuits, and from tailings thickener overflow and excess water from the paste plant pumped to Cell A of the WSP, where flotation reagents are allowed to degrade for approximately two months.

Key reported parameters

Technical qualifications

The process design relies on information, data and analysis prepared by the Qualified Person for Section 13 of this report. Metallurgical tests indicate that the Prairie Creek mineralization is amenable to a combined process of pre-concentration by dense media separation and sequential flotation. The process design is based mainly on results from the 2017 metallurgical test programs, including heavy liquid separation, flotation, mineralized material hardness, and dewatering tests. The mineralized material hardness or Bond Ball Mill Work Index has considered the 75th percentile (100 micron) of the 1992 to 2017 BWI tests, which included a total of 12 tests with three tests conducted in 2017.

Source: NI 43-101 Technical Report on Preliminary Economic Assessment, Prairie Creek, October 15, 2021, Sections 17, 17.1, 17.2