Turnagain PEA Swaps SAG Milling for HPGR in a 90,000 t/d Crush-Grind-Flotation Nickel Concentrator

Giga Metals' 2020 preliminary economic assessment for the Turnagain nickel project in northern British Columbia designs a phased concentrator that starts at 45,000 t/d and doubles after five years.

The N.I. 43-101 Technical Report and Preliminary Economic Assessment for the Turnagain Project, British Columbia, was prepared by Hatch Ltd. and other qualified persons for Giga Metals Corporation, with an effective date of October 28, 2020 and a report date of November 18, 2020. It builds on the 2011 AMC PEA and Hatch's 2018 conceptual study. The plant is designed around the project's highly competent nickel ore, and the report says that comminution equipment selection is vital to the project's economics.

Critical Data

Overview

Earlier Hatch designs for Turnagain used a semi-autogenous grinding (SAG) mill for coarse grinding. The 2020 study instead uses high-pressure grinding rolls (HPGR) as a tertiary crushing stage, followed by two ball mill circuits in series. The report says HPGR was chosen mainly for its higher energy efficiency than SAG milling, and cites Freeport-McMoRan's Metcalf concentrator as a recent example of large HPGR systems used as the main comminution equipment.

The concentrator is built in two phases. The first phase, at 50% of full capacity, has one crushing train, one grinding train, two rougher flotation trains and one cleaner and cleaner scavenger train. The full plant has two grinding trains, four rougher trains and two cleaner trains. The primary crusher, overland conveyor and secondary stockpile are sized for both phases.

Key Process Stages

Primary crushing. Haul trucks deliver ore to a dump pocket above a gyratory crusher. Crushed ore is reclaimed by an apron feeder and conveyed to the primary stockpile.

Secondary crushing. Ore is reclaimed from the primary stockpile by apron feeders, with a self-cleaning magnet removing tramp metal. Two parallel secondary circuits each have a 3.0 m by 7.3 m double-deck, multi-slope vibrating screen ahead of a 933 kW cone crusher operating in open circuit. Variable speed conveyors keep the crushers choke fed. Crushed product joins the screen undersize on its way to the HPGR feed bin.

HPGR. Two parallel tertiary circuits each use a 2.6 m by 2.4 m HPGR with 9,000 kW installed. Edge material from the rolls is recycled to the HPGR feed, and the centre product travels by a 4 km overland conveyor to the secondary stockpile at the concentrator.

Grinding. Each grinding circuit has two 7.9 m by 13.7 m (26 by 45 ft) ball mills of 18,000 kW in series. The primary mill runs in direct closed circuit with a 14-place cluster of 0.84 m cyclones. Its cyclone overflow goes to a 10-place cluster of 0.66 m cyclones, which feeds the secondary mill in an indirect configuration, so that material already at final size is scalped to the overflow first. Secondary cyclone overflow goes to rougher feed conditioning.

Flotation. A conventional sequential flowsheet of rougher, cleaner and cleaner scavenger flotation makes the nickel concentrate, with three cleaning stages in mechanical cells. Regrinding is not included. The report notes that limited testwork has shown a potential recovery increase from regrinding and that further testwork will support a trade-off study.

Concentrate and tailings. High-rate thickeners and a pressure filter dewater the concentrate to 9% moisture, and it is discharged to a shed for truck load-out. Rougher tailings and cleaner scavenger tailings flow by gravity to the tailings management facility (TMF), which also supplies reclaim process water.

Additional Interesting Data and Summary

The reagents are MIBC as frother, SIPX as collector, Calgon as depressant, and flocculant. Fresh water is used for pump gland seals, reagent mixing and similar needs. All buildings and conveyors are enclosed with dust suppression and collection, to address exposure to silicate or fibrous material.

The report notes that the primary crusher is larger than needed in the first phase, which leaves room to optimize its operating schedule. It also says that building the plant at full scale at once, or shortening the gap between the two phases, could improve the project economics.

In summary, the 2020 Turnagain design trades the SAG mill of earlier studies for an HPGR and two-stage ball mill circuit to handle hard ore more efficiently, producing an 18% nickel concentrate by sequential flotation in a plant that doubles from 45,000 to 90,000 t/d after five years.

Key Processes

• Gyratory primary crushing
• Screened secondary cone crushing
• HPGR tertiary crushing with edge recycle
• Two-stage ball milling with cyclone classification
• Rougher, cleaner and cleaner scavenger flotation
• Concentrate thickening and pressure filtration
• Tailings deposition in the TMF with water reclaim

Source: N.I. 43-101 Technical Report & Preliminary Economic Assessment for the Turnagain Project, British Columbia, Canada, prepared for Giga Metals Corporation, effective date October 28, 2020.