Pretium's 2019 technical report for the Brucejack gold mine in northwest British Columbia reviews operation since the June 2017 first pour. It sets out how the existing flowsheet can reach 3,800 t/d with modest changes.
The Technical Report on the Brucejack Gold Mine, Northwest British Columbia, was prepared by Tetra Tech and others for Pretium Resources Inc., with an effective date of April 4, 2019. Brucejack mineralization is in quartz-carbonate-adularia veins, stockwork and breccia zones, and much of the gold and silver is in nugget or metallic form. The concentrator was designed for 2,700 t/d at 92% availability, using gravity concentration and bulk sulphide flotation. It was commissioned from March to May 2017, poured first gold on June 20, 2017 and declared commercial production on July 1, 2017. It reached full operation in the fourth quarter of 2017. In 2018 the mill processed 1,005,603 t at 2,755 t/d with 97.3% gold and 87.0% silver recovery.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Report effective date | April 4, 2019 | Not applicable | Tetra Tech and others for Pretium Resources Inc. |
| Original design | 2,700 t/d | Not applicable | 92% availability |
| Upgraded design | 3,800 t/d (172 t/h) | Not applicable | Same flowsheet |
| 2018 performance | 2,755 t/d, Au 97.3%, Ag 87.0% | Not applicable | Head grade 11.9 g/t Au |
| Primary crusher | Underground jaw, 150 kW | Not applicable | 264 t/h at 60% availability, P80 120 mm or finer |
| Surge bin | 2,500 t live | Not applicable | Two 1,067 mm x 15,000 mm apron feeders |
| SAG mill | 6.1 m x 3.05 m EGL, 2,013 kW VFD | Not applicable | 40 mm pebble ports, HP100 pebble crusher |
| Ball mill | 3.96 m x 7.26 m EGL, 2,013 kW VFD | Not applicable | About 300% circulating load |
| Grind size | P80 90 to 100 | µm | Simulated limit about 4,240 t/d at 90 µm |
| Gravity | Two QS40 concentrators, 250 t/h each | Not applicable | Treat 100% of ball mill discharge |
| Rougher and scavenger | Four and two 100 m³ tank cells | Not applicable | PAX and D250 at natural pH |
| Cleaners | Three stages | Not applicable | About 44 t/h feed at 3,800 t/d |
| Hardness | BWi 14.0 average, 16.6 design | kWh/t | Axb 41.4 |
Overview
Run-of-mine ore is crushed underground and conveyed to a surge bin on surface. It is ground in a SABC circuit (SAG mill, ball mill and pebble crusher) in which the whole ball mill discharge passes through centrifugal gravity concentrators. Gravity concentrate is upgraded on shaking tables and a secondary Knelson concentrator and smelted on site to doré, which is flown to refineries. Cyclone overflow goes through rougher, scavenger and three-stage cleaner flotation. The flotation concentrate is dewatered, loaded into bulk containers and trucked to Stewart for shipment to smelters or traders. Part of the flotation tailings is made into paste backfill, and the rest is stored in Brucejack Lake.
Key Process Stages
Primary crushing. The underground station has a rock breaker, a stationary grizzly, a vibrating grizzly feeder and a 150 kW jaw crusher at a closed side setting of about four inches. Feed is typically under 700 mm.
Grinding and gravity. SAG mill discharge passes an 8 mm vibrating screen, and the oversize goes to the HP100 pebble crusher. The whole ball mill discharge is split between two safety screens and QS40 concentrators. The gravity tails join the SAG screen undersize at the cyclone feed. Six 381 mm cyclones (five operating) produce overflow at about 33% solids. Simulations show the 3,800 t/d target can be met by raising SAG speed and charge and bringing a standby cyclone into service.
Flotation. The rougher concentrate is about 15% of feed mass. Rougher tails and first cleaner tails are scavenged, and the scavenger concentrate returns to the rougher head or the first cleaner. Metso testwork and simulations found that the existing rougher and scavenger cells give enough retention time at 3,800 t/d. The tailings pumps are being upgraded.
Tailings and water. Tailings are thickened and sent either to the paste backfill surge tank or to the disposal tank for deposition in Brucejack Lake. Thickener overflows are recycled as make-up water. Treated water from the water treatment plant is used for cooling, gland seal, reagent preparation and make-up.
Additional Interesting Data and Summary
In 2018, plant metallurgists, engineers, equipment suppliers and independent consultants carried out throughput reviews and testwork. The gold room runs on day shift only. The underground crusher's 60% availability allows for higher crushing rates, maintenance and weather interruptions.
In summary, the 2019 Brucejack report covers a high-grade underground gold operation whose SABC mill recovers much of the gold by gravity ahead of bulk flotation. It shows that the plant can reach 3,800 t/d through operating changes and a few equipment upgrades rather than a new flowsheet.
Key Processes
- Underground jaw crushing and surface surge bin
- SABC grinding to P80 90 to 100 µm
- Centrifugal gravity concentration of the full ball mill discharge
- Gravity concentrate tabling and on-site smelting to doré
- Rougher, scavenger and three-stage cleaner flotation
- Concentrate dewatering and bulk container shipment
- Paste backfill and subaqueous tailings storage in Brucejack Lake
Source: Technical Report on the Brucejack Gold Mine, Northwest British Columbia, presented to Pretium Resources Inc., effective date April 4, 2019.
Earlier report summaries
Earlier versions of this profile, based on older technical reports, are retained below for source traceability. Dates and figures reflect each report’s own effective date and are superseded by the current information above.
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
| Parameter | Unit | Design Value |
|---|---|---|
| Daily Processing Rate | t/d | 2,700 |
| Operating Days per Year | d/a | 365 |
| Operating Schedule | – | Two shifts/day; 12 hours/shift |
| Mill Feed Grade – Gold (Average) | g/t Au | 10 to 15 |
| Mill Feed Grade – Silver (Average) | g/t Ag | 40 to 100 |
| Mill Feed Grade – Sulphur (Average) | % S | 3 |
| Primary Crushing Availability | % | 65 |
| Crushing Product Particle Size (80% passing) | mm | 120 to 150 |
| Grinding/Flotation/Gravity Availability | % | 92 |
| Milling and Flotation Process Rate | t/h | 122 |
| SAG Mill Feed Size (80% passing) | mm | 120 to 150 |
| SAG Mill Grinding Particle Size (80% passing) | µm | 1,070 |
| Drop Weight Breakage Parameter (Axb) | – | 41.4 |
| Ball Mill Grinding Particle Size (80% passing) | µm | 90 |
| Ball Mill Circulating Load | % | 300 |
| Bond Ball Mill Work Index | kWh/t | 16.0 |
| Regrind Particle Size (80% passing) | µm | 35 to 40 |
| LOM Annual Average Gold in Doré | kg | 5,600 |
| LOM Annual Average Silver in Doré | kg | 1,900 |
| LOM Annual Average Flotation Concentrate Produced | t | 44,000 |
| Flotation Concentrate Gold Grade (LOM Average) | g/t Au | 157 |
| Flotation Concentrate Silver Grade (LOM Average) | g/t Ag | 1,000 |
| Gold Recovery to Doré (LOM Average) | % | 43.3 |
| Gold Recovery to Flotation Concentrate (LOM Average) | % | 53.4 |
| Total Gold Recovery (LOM Average) | % | 96.7 |
| Silver Recovery to Doré (LOM Average) | % | 3.5 |
| Silver Recovery to Flotation Concentrate (LOM Average) | % | 86.5 |
| Total Silver Recovery (LOM Average) | % | 90.0 |
| Mill Feed Grade – Gold (LOM Average) | g/t Au | 14.1 |
| Mill Feed Grade – Silver (LOM Average) | g/t Ag | 57.7 |
| Arsenic Content of Feed | ppm | 281 |
| Sulphur Content of Feed | % | 2.4 |
Project website: https://projects.eao.gov.bc.ca/p/588511caaaecd9001b825bcb/project-details
Project website: https://www.mining-technology.com/marketdata/five-largest-gold-mines-canada-2020/
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
