Caribou Mine — 2018 Technical Report

This report presents the process plant flowsheet, design parameters, and historical operating data for the Caribou Mine concentrator.

Report context

Trevali Mining Corporation – Caribou Mine, Project #2929, Technical Report NI 43-101, dated May 31, 2018. The report describes a rehabilitated concentrator that was operating as a stand-alone plant at the time of reporting.

Processing route

Crushing and Grinding

The plant feed is delivered as primary crushed ore at nominally 119 mm (4.5 in.) top size by inclined conveyor from the mine to two 2,000 tonne storage bins at the concentrator. Belt feeders, controlled by variable frequency drives, discharge each bin individually. Plant feed is delivered to the 6.7 m (22 ft) diameter by 2.1 m (7 ft) long Hardinge primary SAG mill, equipped with a 1,491 kW (2,000 hp) drive motor. A maximum ball charge of 15% is allowed in the mill to reduce the feed material to nominal 650 µm as feed to the secondary ball mill. SAG mill discharge is pumped to a Derrick vibratory screen with 3.2 mm (0.13 in.) apertures, and oversize particles are recycled to the SAG mill feed. Screen undersize gravitates to the secondary ball mill cyclone feed pump for classification in a battery of 508 mm (20 in.) diameter Krebs cyclones. Overflow, at 80% passing nominal 30 µm to 35 µm, reports directly to flotation. Cyclone underflow recycles to the 4.3 m (14 ft) diameter by 6.7 m (22 ft) long Nordberg secondary ball mill, equipped with a 1,864 kW (2,500 hp) drive motor. Secondary ball mill discharge combines with the SAG mill product in the cyclone feed pump. Soda ash and sodium cyanide are added to the grinding circuit as pyrite and sphalerite depressants.

Lead Circuit

Product from the grinding circuit, at nominal 35% solids by weight and pH 8.2, gravitates to the pre-aeration circuit. The first two cells in the lead rougher bank are DR500 units used to pre-aerate the slurry and tarnish the iron minerals to aid in their depression. There is no flotation in this circuit. Discharge from the pre-aerators passes to the lead rougher bank, which consists of five Outokumpu 16 m³ (565 ft³) units and three additional DR500 units. The collector 3418A is used with Methyl Isobutyl Carbinol (MIBC) as the frother. Rougher tailings flow by gravity to the scavenger bank, which consists of two banks, three DR500 units and two Outokumpu 16 m³ units. Combined rougher and scavenger concentrate is sent for regrinding, while scavenger tailings form the majority of the feed to the zinc circuit.

Combined rougher-scavenger concentrate passes to the lead regrind circuit, where it is classified in a battery of 152 mm (6 in.) cyclones to divert the plus 12 µm to 15 µm mineralization to an M1000 Isa regrind mill rated at 500 kW (670 hp). Previous operations required two M1000s in parallel owing to higher head grades. Current conditions allowed utilization of the additional ISA mill as a spare. Isa mill discharge combines with cyclone overflow at nominal 12 µm to 15 µm as feed to primary cleaning. Primary cleaner concentrate undergoes secondary regrind in an additional M1000 Isa mill using 100 mm (4 in.) diameter cyclones as classifiers. Product, at nominal 8 µm to -10 µm, passes to secondary cleaning.

The primary cleaner consists of an eight cell rougher and a six cell scavenger, with all cells being DR300 units. Rougher concentrate passes to secondary regrind, while scavenger concentrate is recycled back to the head of the primary cleaner. Primary cleaner tailings report to the primary cleaner scavenger via gravity. The secondary cleaner consists of six DR300 cells, with total secondary concentrate passing to tertiary cleaning. Secondary cleaner tailings combine with rougher concentrate as feed to the regrind cyclone ahead of the primary cleaner. Second cleaner concentrate passes to the combined third and fourth cleaner, consisting of 24 DR100 cells, for final cleaning of the lead. Quaternary cleaner concentrate forms the final lead concentrate and is pumped to dewatering. Tertiary cleaner tailings report to the cyclone feed sump prior to the secondary cleaner. Quaternary cleaner tailings report to the tertiary cleaner. Cleaner scavenger tailings join with rougher scavenger tailings to make up the feed to the zinc circuit.

Final lead concentrate from the fourth cleaner is pumped to a 4.6 m (15 ft) diameter conventional thickener for recovery of excess water. Underflow, at approximately 60% solids, is pumped to a holding stock tank ahead of the Vertical Plate Pressure (VPA) 1515-33 plate and frame filter. Filtrate is recycled to the lead thickener. Thickener overflow is pumped to the tailings pond.

Zinc Circuit

Combined lead rougher scavenger and cleaner scavenger tailings combine as feed to the zinc circuit. A single conditioner is used for reagent addition to allow activation and promotion of zinc mineralization, and to adjust pH for pyrite depression. Conditioner overflow gravitates to a combined zinc rougher-scavenger circuit of eighteen cells, with the rougher being a bank of twelve DR300 cells, and the scavenger a bank of six DR500 units.

Combined rougher and scavenger concentrate is pumped to a single-stage, 3.0 m (10 ft) diameter by 6.7 m (22 ft) long regrind ball mill, equipped with a 746 kW (1,000 hp) drive motor. All mill feed reports to the cyclone feed box for classification in a battery of 254 mm (10 in.) diameter Krebs cyclones, with a product of nominal 80% passing 14 µm. Cyclone underflow recycles to the regrind mill. Cyclone overflow passes by gravity to the cleaner circuit.

The cleaner circuit consists of a four-stage cleaning plant, with the final three stages of cleaning operating in closed circuit. Primary cleaner operates in open circuit, with tailings passing to the cleaner scavenger feed. The primary cleaner consists of a single bank of eleven DR500 cells, with total primary cleaner concentrate passing to the secondary cleaner bank of seven DR300 cells. Tailings from the secondary cleaner recycle to the regrind mill, while secondary concentrate passes to the tertiary cleaner, a bank of five DR300 cells operating in open circuit. Tailings report to the second cleaner feed pump, while third cleaner concentrate is delivered to the fourth cleaner of four DR300 cells. Tailings recycle to the third cleaner feed pump, while the product as final concentrate is delivered to the dewatering section.

Final zinc concentrate is pumped to a 12 m (40 ft) diameter conventional thickener for recovery of excess water. Underflow, at approximately 60% solids, is pumped to a holding stock tank ahead of the VPA 1515-33 plate and frame filter. Filtrate is recycled to the zinc thickener. Thickener overflow is pumped to the tailings pond.

Tailings Disposal

Tailings are pumped from the mill to the STTP and deposited subaqueously via floating pipeline. Reclaim water is pumped back from the STTP to feed the mill from an area close to the dam.

Life of Mine Production Schedule (Proposed Design)

The report presents a life of mine (LOM) process plant production schedule based on Proven and Probable Mineral Reserves. Milled tonnes total 5,106,000 t over the period 2018 to 2024, with annual milled tonnes ranging from 72,000 t (2024) to 1,096,000 t (2019 and 2020). Design zinc grade averages 5.8% Zn, lead grade averages 2.2% Pb, and silver grade averages 63.2 g/t. Design zinc recovery averages 81.4%, with zinc concentrate grade averaging 47.6% Zn. Design lead recovery averages 64.4%, with lead concentrate grade averaging 37.6% Pb. Design silver recovery in lead concentrate averages 37.3%.

Life of Mine Major Consumables (Proposed Design)

Mill consumables include soda ash at 2.19 kg/t, copper sulphate at 1.41 kg/t, Aerophine 3418A promoter at 0.09 kg/t, ceramic beads (2 mm) at 0.20 kg/t, carbon grinding balls (4 in.) at 0.75 kg/t, carbon grinding balls (0.75 in.) at 1.20 kg/t, and hydrated lime at 1.70 kg/t. Energy requirements total 85.8 kW/t annually, with mill energy at 63.3 kW/t.

Historical Plant Performance (Actual Operating Data)

Since recommencement of milling operations in August 2015, the process plant produced a zinc concentrate in the range of 46.0% to 49.7% from head grades ranging from 5.3% Zn to 6.0% Zn. Zinc recovery ranged from 62.9% in August 2015 to a maximum of 81.5% in September 2017. Lead concentrates ranged from 32.5% in August 2015 to 42.9% in June 2017 from head grades ranging from 2.0% Pb to 2.6% Pb. Lead recovery ranged from 44.6% in August 2015 to a maximum of 66.9% in October 2017.

Key reported parameters

Technical qualifications

The report is a technical report prepared under NI 43-101 standards. The process plant was described as a rehabilitated concentrator operating as a stand-alone plant at the time of reporting. Historical operating data was provided for the period August 2015 through December 2017. The life of mine production schedule and consumables estimates were based on Proven and Probable Mineral Reserves.

Source: Trevali Mining Corporation – Caribou Mine, Project #2929, Technical Report NI 43-101, May 31, 2018, Section 17 Recovery Methods.