Brucejack Project — 2015 Technical Report

This report presents the proposed processing flowsheet, design criteria, and historical testwork for the Brucejack gold-silver project.

Report context

This technical report, dated 2015, describes the feasibility study update for the Brucejack Project located near Stewart, British Columbia. The report details the proposed mineral processing facilities designed to treat 2,700 t/d of gold-silver ore from an underground mine, producing both gold-silver doré and a gold-silver bearing flotation concentrate. The process plant design is based on test work conducted from 2009 to 2014, including the 2013 and 2014 bulk sample processing programs.

Processing route

Proposed flowsheet overview

The proposed concentrator is a conventional plant combining bulk sulphide flotation with gravity concentration. The flowsheet was developed based on test work conducted mainly from 2009 to 2014, the 2013 and 2014 bulk sample processing programs, and engineering experience. The comminution circuit design is based on the topography of the proposed plant site and operability of the system. The size selection of the grinding mills was based on amenability of the ore to grinding as determined through test programs performed by different laboratories. Grindability tests were performed to determine the following hardness parameters: BWi, RWi, CWi, Ai, and SAG mill comminution breakage.

The gravity circuit was selected based on laboratory gravity concentration tests and GRG test results and related simulations. Flotation cell sizing was based on optimum flotation times determined by test work and using scale-up factors from similar operations.

Various test programs evaluated the cyanidation process to extract gold and silver from test head samples and flotation concentrates. Although the mineralization responds reasonably well to the process, a few samples showed poor cyanidation responses, which may result from the presence of preg-robbing constituents (graphite), potential refractory components (arsenopyrite), and slow leaching kinetics from electrum or other silver bearing minerals. The cyanidation process has not been used for this study, including direct cyanide leach and carbon-in-leach procedures.

Proposed plant description

Primary Crushing (Underground)

The primary crushing facility with an average process rate of 173 t/h will be located underground. A jaw crusher (150 kW) is proposed for primary crushing. Run-of-mine material will be trucked from the underground mine to the underground facility. Feed particle size will be less than 500 mm and the crusher will reduce material to 80% passing 120 to 150 mm. A rock breaker will be installed for oversize rocks. A dust collection and suppression system will control fugitive dust.

Mill Feed Surge Bin

The SAG mill feed surge bin is designed with a live capacity of 2,500 t. Ore will be reclaimed by three belt feeders onto the SAG mill feed conveyor at a nominal rate of 122 t/h.

Primary Grinding, Classification, and Primary Gravity Concentration

A SAG mill/ball mill (SAB) circuit is proposed for primary grinding. The circuit will be equipped with two centrifugal gravity concentrators to recover gold/silver nugget grains. The SAG mill is 5,790 mm diameter by 2,590 mm long (19 ft by 8.5 ft) (EGL) driven by a 1,300 kW variable frequency drive. The ball mill is 3,960 mm diameter by 7,260 mm long (13 ft by 23.8 ft) (EGL) powered by a 1,600 kW fixed speed drive.

The SAG mill will be equipped with 40 mm pebble ports and a trommel screen with 9.5 mm openings. Trommel screen oversize will be returned to the SAG mill feed conveyor. Screen undersize will discharge to the hydrocyclone feed pump box. Provisions have been made for a pebble crushing circuit if required later.

The ball mill operates in closed circuit with hydrocyclones and two centrifugal gravity concentrators. The entire ball mill discharge reports to the gravity concentration circuit. Hydrocyclone overflow particle size will be 80% passing 90 µm at approximately 33% solids. The circulating load to the ball mill will be approximately 300%.

Rougher and Scavenger Flotation

The pulp from primary grinding will undergo conventional flotation at 122 t/h. Flotation reagents include PAX as collector and MIBC as frother. Mass recovery of rougher concentrate is approximately 20% of flotation feed. Rougher/scavenger flotation will be carried out at natural pH. The circuit consists of four 100 m³ rougher flotation tank cells and two 100 m³ scavenger flotation tank cells.

Slimes Flotation

An additional circuit of two 3 m³ flotation cells will handle slime material generated underground. Reagents PAX and MIBC will be added. Concentrate will be pumped to the first cleaner flotation head.

Concentrate Regrinding

Rougher flotation concentrate will be reground to 80% passing 35 to 40 µm in a ball mill 2,710 mm diameter by 4,120 mm long (EGL) driven by a 375 kW motor, in closed circuit with hydrocyclones and one centrifugal gravity concentrator. Approximately 50% of hydrocyclone underflow returns to the ball mill while balance reports to gravity concentration. Circulating load will be approximately 150%. Pulp density of hydrocyclone overflow will be approximately 22% solids.

Cleaner Flotation

Reground concentrates will undergo three stages of cleaning. The circuit includes four 30 m³ tank cells for first cleaner flotation, two 30 m³ tank cells for first cleaner/scavenger, one 30 m³ tank cell for second cleaner, and one 30 m³ tank cell for third cleaner flotation.

Gravity Concentrate Upgrading

Gravity concentrates from both primary grinding and regrind circuits will be upgraded by conventional tabling followed by smelting to produce gold-silver doré. Equipment includes three gravity concentration tables, one centrifugal gravity concentrator with a 12-inch bowl, one table concentrate dryer, one 175 kW induction melting furnace, and associated systems.

Concentrate Handling

Final flotation concentrate will be thickened in a 5 m diameter high-rate thickener, filtered by a tower-type pressure filter up to 40 m², and bagged in 2 t bags. The filter press will reduce moisture to approximately 10 to 12%. Bagged concentrate will be transported to Knipple Transfer Station, then to Terrace, BC, and shipped by rail to a smelter in eastern Canada. On-site storage capacity for up to 10 days of production will be provided.

Tailings Disposal

Final tailings from bulk rougher/scavenger flotation will be thickened in a 20 m diameter high-rate thickener prior to being pumped either to the backfill plant for underground backfilling or to Brucejack Lake for storage.

Testwork basis

The process flowsheet was developed based on test work conducted mainly from 2009 to 2014, the 2013 and 2014 bulk sample processing programs, as well as engineering experience. Grindability tests were performed to determine hardness parameters. Laboratory gravity concentration tests and GRG test results and related simulations were used for gravity circuit selection. Flotation times were determined by test work using scale-up factors from similar operations.

Key reported parameters

Technical qualifications

The design criteria presented in this report are proposed design values based on test work and engineering experience. Gold and silver recovery ranges are reported as ranges (e.g., gold recovery to doré 30 to 60%, gold recovery to flotation concentrate 30 to 70%) showing variability in feed grades and metallurgical performance. The arsenic content of flotation concentrates to be shipped to smelters is expected to be marginally higher than penalty thresholds outlined by most smelters and will require further review. Cyanidation test work showed that a few samples exhibited poor cyanidation responses, possibly due to preg-robbing constituents (graphite), potential refractory components (arsenopyrite), or slow leaching kinetics from electrum or other silver bearing minerals; however, the cyanidation process has not been used for this study.

Source: Brucejack Project , 2015 Technical Report, Sections 17.0 and 17.1