Dumont Nickel Project — 2012 Technical Report

The proposed two-phase process plant will treat run-of-mine ore at a nominal 50 kt/d base rate, expandable to 100 kt/d, to produce nickel concentrate via crushing, grinding, desliming, flotation and magnetic separation.

Report context

This 22 June 2012 technical report describes the recovery methods proposed for the Dumont Nickel Project. The process plant and associated service facilities are designed to process run-of-mine (ROM) ore delivered to primary crushers to produce nickel concentrate and tailings. The report presents design criteria, plant design basis, throughput and availability assumptions, processing strategy, head grade selection, flowsheet development and unit process selection.

Processing route

General process description

The proposed process encompasses crushing and grinding of the ROM ore, desliming via hydrocyclone circuit, slimes flotation, nickel sulphide rougher flotation, nickel sulphide cleaning flotation, magnetic recovery of sulphide rougher tailings, regrind of magnetic concentration and sulphide 1st cleaner tailings, and nickel alloy rougher and cleaner flotation.

Concentrate will be thickened, filtered and stored on site prior to being loaded onto railcars or trucks for transport to third-party smelters. The slimes flotation tailings, magnetic separation tailings, alloy rougher tailings and alloy 1st cleaner scavenger tailings will be combined and thickened before placement in the tailings storage facility (TSF).

Phased plant design

The process plant will be built in two phases. Initially, the plant will be designed to process 50 kt/d with allowances for a duplicate process expansion to increase plant capacity to 100 kt/d. Common facilities will include concentrate thickening and handling and reagent storage and preparation.

Plant design basis

The key criteria selected for the base plant (50 kt/d) and expansion plant (100 kt/d) designs are: a nominal base plant treatment rate of 50 kt/d and a nominal expansion plant treatment rate of 50 kt/d for a combined 100 kt/d treatment rate; design availability of 92% (after ramp-up), which equates to 8059 operating hours per year, with standby equipment in critical areas; and sufficient plant design flexibility for treatment of all ore types at design throughput.

Comminution circuit

For each 50 kt/d plant, Ausenco selected one 11.6 m (38 ft) diameter SAG mill and two 7.9 m (26 ft) diameter ball mills. The SAG mill is rated at 22 MW, 11.6 m diameter with 6.7 m EGL, utilizing a trommel screen for classification and oversize recirculation. The two ball mills are each rated at 12 MW, 7.9 m diameter with 11.0 m EGL, in closed circuit with hydrocyclones, grinding to a product size of nominally 80% passing (P80) 150 µm. The selection and sizing of the crushing and grinding circuits was determined through evaluation of variability comminution testwork performed at SGS-Lakefield. Testwork provided a crusher work index, Bond ball and rod mill indices, Bond abrasion index, SMC and JK Axb values for the selected samples. Ausenco elected to use the 75th percentile of each of these values in the design.

Primary crushing and stockpile

Ore from the open pit is crushed using a primary gyratory crusher (assisted with a rock breaker) to a crushed product size of nominally 80% passing (P80) 101 mm. Crushed ore is fed onto the covered stockpile feed conveyor. A covered conical stockpile of crushed ore with a live capacity of 18 hours is served by three apron feeders, each capable of feeding 60% of the full mill throughput.

Desliming circuit

A two-stage desliming circuit via hydrocyclones is proposed. The first stage splits mass with a cut size (D50c) of 40 µm. The second stage splits mass with a cut size (D50c) of 10 µm. Hydrocyclone sizes for each stage are 400 mm and 100 mm, respectively.

Flotation circuit design

Slimes flotation consists of two trains of six 300 m³ forced air tank flotation cells per train to provide 30 minutes of retention time.

Nickel sulphide rougher flotation consists of two trains of seven 300 m³ forced air tank flotation cells per train to provide 60 minutes of retention time.

Nickel sulphide cleaning flotation comprises 1st cleaner, 2nd cleaner and 3rd cleaner stages consisting of five 110 m³ cells each.

Flotation circuit residence times as designed are: slimes 30 minutes, sulphide roughers 60 minutes, sulphide 1st cleaners 15 minutes, sulphide 2nd cleaners 12 minutes, sulphide 3rd cleaners 12 minutes, magnetic sulphide 36 minutes, awaruite roughers 60 minutes, alloy 1st cleaners 18 minutes, alloy 1st cleaner scavenger 18 minutes, and alloy 2nd cleaners 15 minutes.

Magnetic separation

Magnetic recovery of sulphide rougher tailings is included in the flowsheet, followed by regrind of magnetic concentration and sulphide 1st cleaner tailings. Magnetic separation tailings are combined with other tailings streams for thickening and disposal.

Nickel concentrate handling

Concentrate is thickened at a design flux of 0.25 t/m²/h and filtered at a design filtration rate of 450 kg/m²/h. Tailings thickening flux is designed at 0.75 kg/m²/h with tailings thickener underflow density of 40% w/w.

Reagents

Reagent consumptions as designed include: PAX at 140 g/t, MIBC at 19 g/t, Cytec 65 (frothing agent) at 6 g/t, Calgon at 50 g/t, CMC at 25 g/t, sulphuric acid at 8 kg/t, copper sulphate at 30 g/t, and flocculant at 30 g/t.

Media consumption

SAG mill media consumption is designed at 1087 t/a for the 50 kt/d plant and 2175 t/a for the 100 kt/d plant. Ball mill media consumption is designed at 1831 t/a for the 50 kt/d plant and 3662 t/a for the 100 kt/d plant. Regrind mill media consumption is designed at 283 t/a for the 50 kt/d plant and 565 t/a for the 100 kt/d plant.

Throughput and availability

Ausenco nominated an overall plant availability of 92% or 8059 hours per annum. This is described as an industry standard for a large, multi-train flotation plant with moderately abrasive ore. Benchmarking indicates that operating plants have consistently achieved this level. Given the low abrasion index of the Dumont ore, this availability is likely conservative.

Head grade

Each plant is designed to treat ROM ore with a head grade of 0.35% Ni. The design head grade was selected prior to final completion of the mine plan; however, it represents roughly 95% of the peak quarterly head grade. Additionally, 0.35% Ni falls in the 95th percentile of all quarterly maximum head grades. The design head grade value was noted to be revisited in the next phase of study.

Flowsheet development basis

The process plant flowsheet design for the Dumont circuit was conceptually based on those of comparable large flotation plants and then confirmed or altered based on trade-off studies and metallurgical testwork. The process plant design is based on a flowsheet with unit process operations that are well proven in the minerals processing industry.

Key reported parameters

Technical qualifications

The overall engineering approach described in the report was to design robust process plants that could handle a wide range of ore variability and operating conditions. The design criteria and plant design basis are stated as proposed design values. The comminution circuit selection and sizing were based on metallurgical testwork performed at SGS-Lakefield, using the 75th percentile of variability testwork values. The flowsheet was conceptually based on comparable large flotation plants and confirmed or altered based on trade-off studies and metallurgical testwork. The design head grade of 0.35% Ni was selected prior to final completion of the mine plan, and the report notes this value will be revisited in the next phase of study. The report contains no historical operating data from a Dumont plant, as this is a proposed design. Testwork data are limited to the comminution variability study, reagent consumption trials and metallurgical testwork used to develop and confirm the flowsheet.

Source: Dumont Nickel Project , 2012 Technical Report, 2139-RPT-003, Rev 0, 22 June 2012, Section 17 Recovery Methods.