Technical report details a proposed 3.5 Mt/a gold processing facility to treat mineralization from four deposits in the Lynn Lake Gold Camp, Manitoba.
Report context
The Preliminary Economic Assessment (PEA) for the MacLellan, Farley Lake, Burnt Timber, and Linkwood Properties was prepared for Carlisle Goldfields Ltd. The report covers a gold processing facility designed to process nominal 3.5 Mt/a or 10.64 kt/d of gold-silver mineralization from these four deposits. The facility would produce between 196,900 troy oz/a and 375,200 troy oz/a gold-silver doré. The processing route described is a proposed design, supported by test work conducted by SGS in Vancouver, BC, and by Witteck in Mississauga, ON. No historical operating data from these properties is presented in the report sections examined.
Processing route
Crushing and grinding
The proposed flowsheet begins with a covered ROM stockpile with a recovery trench and apron feeder. Material less than 400 mm is fed to a 127 cm by 152 cm primary jaw crusher operating at 458 t/h, reducing material to 180 mm passing. Primary crushing operates 24 h/d with dust collection systems at the crusher and conveyor transfer points.
Secondary crushing uses three 2.13 m diameter short-head cone crushers in closed circuit with a double deck vibrating screen. Oversize from the screen is returned to the cone crusher via a bucket elevator, while undersize at 16 mm passing advances to the fine mineralized material bin. The recirculating load to the cone crusher is 200% of the advancing flow. A fine ore bin with 13,300 t capacity uses a live bottom feeder with load-cell monitoring.
Tertiary grinding employs a wet ball mill, 6.7 m diameter by 9.5 m length, with 7,300 kW drive, in closed circuit with cyclones. Fresh feed is added at 458 t/h with process water to maintain 70% solids in the mill. The ball mill is equipped with a trunnion magnet to remove steel chips. Mill discharge is diluted to 40% solids and pumped to a cyclopack with cut-size of P80 84 µm. The recirculating load is approximately 300%.
Gravity gold recovery
Approximately 30% of cyclone underflow reports to a sieve-bend screen protecting two centrifugal concentrators (KC-QS48, 48-inch diameter or equivalent). Sieve-bend undersize advances to the gravity concentration circuit. Centrifugal concentrators operate continuously, producing a gold concentrate collected in a holding tank sized for approximately 3 t. Concentrator tails are returned to the ball mill.
Gold concentrate undergoes intense cyanide leaching in a ConSep Acacia CS2000 or equivalent system. The leach operates at 50°C with about 1,000 g/L cyanide and pH adjusted to approximately 11 using sodium hydroxide. LeachAid™ may be added. Leached gravity concentrate is returned to the ball mill to liberate any unexposed gold. Pregnant leach solution advances to a dedicated basketless electrowinning (EW) cell.
Leach thickening and pre-aeration
Cyclone overflow from ball milling reports to a 45.7 m diameter conventional thickener to increase density to 40% solids. Flocculent or lime may be added to improve settling. Thickener underflow advances to pre-aeration.
The pre-aeration circuit consists of seven tanks, each 16.5 m diameter by 16.5 m height, providing 24 hours residence time. Pre-aeration is conducted at relatively low pH (8.0 to 11.0) with lime addition. Up to 300 g/t lead nitrate may be added to form an insoluble lead hydroxide layer on pyrrhotite, reducing cyanide consumption. Air or oxygen sparging maintains high dissolved oxygen. During pre-aeration, an insoluble iron hydroxide layer forms on pyrrhotite, preventing its dissolution.
Carbon-in-pulp leach
Four stages of leaching follow pre-aeration, using 14 tanks each 16.5 m diameter by 16.5 m height, providing 48 hours leach residence time. Lime adjusts pH to 10.5 to 11, though the report notes that when processing feed high in arsenopyrite, operating pH should be reduced to prevent an insoluble arsenic oxide layer forming on gold particles. Sodium cyanide is added to the first leach tank at 1.0 to 1.5 g/L NaCN. Leach residue is expected to have approximately 1,500 mg/L CN T and 400 mg/L CN WAD. All leach tanks are sparged with air; the first two may require oxygen sparging based on additional test work. Tanks have feed standpipes to prevent short-circuiting, and flow between tanks is by gravity.
Carbon-in-pulp adsorption
The CIP adsorption circuit uses six tanks, each 11.6 m diameter by 11.6 m height, arranged in a carousel pattern using Kemix pump cells on the same level. The carousel arrangement keeps carbon in one tank, preventing back-mixing and reducing carbon breakage. Slurry flows continuously through the circuit with 1 hour residence time per stage. Carbon is removed from the circuit once every 38 hours. Barren carbon is added to the sixth stage once that tank is at least half-full of slurry, at approximately 25 g/L granular activated carbon. Slurry density may be adjusted between 35 to 50% solids.
When carbon in the first tank is fully loaded, that tank is taken offline and all carbon is pumped to the elution circuit. The offline tank is then refilled with slurry and barren carbon, returning to service as CIP 6. Slurry flow is redirected accordingly through the remaining tanks.
Elution and electrowinning
Gold is recovered from loaded carbon using the AARL elution process. Up to 100% of loaded carbon may undergo acid wash with hydrochloric acid (HCl) to remove calcium scale and adsorbed base metals. Gold is eluted in the elution column with a solution of sodium cyanide and sodium hydroxide made from high quality water at high temperature. Some or all carbon may be regenerated in an electric kiln.
Gold is removed from the pregnant eluate using basketless EW cells. Gold sludge is pressure washed from electrodes to cell bottoms, dewatered in a filter press, and dried. Dried sludge is mixed with fluxes and smelted in an induction furnace to produce gold-silver doré. Gold-rich sludge from the gravity circuit EW is combined with CIP EW sludge for filtration, drying, and smelting.
Cyanide destruction
Barren CIP tailings are pumped to the cyanide detoxification tank. Cyanide is treated using the INCO sulphur dioxide and air process. Sodium metabisulphite (SMBS) is added in the presence of copper from copper sulphate. Slurries thus treated are expected to have less than 5 ppm CN WAD. Treated slurry is pumped to the tailings storage facility.
Key reported parameters
| Parameter | Unit | Design Value | Basis |
|---|---|---|---|
| Overall plant feed | t/d | 10,640 | Plant design |
| Operation rate | d/a | 350 | Plant design |
| Primary crushing availability | % | 94 | Design estimate |
| Secondary crushing availability | % | 94 | Design estimate |
| Plant remainder availability | % | 94 | Design estimate |
| Bond ball mill work index (design) | kWh/t | 12.4 | Testwork |
| Bond abrasion index | , | 0.158 | Testwork |
| Specific gravity plant feed | g/mL | 2.7 | Assumed |
| Moisture content plant feed | % | 3 | Assumed |
| Mill feed size, 80% passing | mm | 400 | Design |
| Primary crushing product size, 80% passing | mm | 180 | Design |
| Secondary crushing product size, 80% passing | mm | 16 | Design |
| Secondary crushing recirculating load | % | 200 | Design |
| Ball mill product size, 80% passing | µm | 80 | Design |
| Ball mill recirculating load | % | 300 | Design |
| Preaeration time | h | 24 | Design |
| Leach circuit retention time | h | 48 | Design |
| CIP circuit retention time (pulp) | h | 6 | Design |
| CIP circuit retention time (carbon) | h | 38 | Design |
| Head grade (design), Au | g/t | 1.94 | Design basis |
| Head grade (design), Ag | g/t | TBD | Design basis |
| Design recovery, Au | % | 90.7 | Based on testwork |
| Design recovery, Ag | % | 60.1 | Based on testwork |
Project website: https://www.canadianminingjournal.com/news/gold-resource-initial-burnt-timber-estimate-includes-780-500-oz/
The design criteria listed are based on test work conducted by SGS and Witteck. The report states the facility will produce between 196,900 troy oz/a and 375,200 troy oz/a gold-silver doré based on 90% recovery of gold and 60% recovery of silver. Silver is only accounted for in the MacLellan deposit; higher production rates only occur when MacLellan is in production. Mineralogical investigations indicated the presence of silver in other mineralizations, but the mining plan does not show silver reporting to mill feed from those deposits.
Technical qualifications
This technical report presents a Preliminary Economic Assessment only. The processing flowsheet and all figures are based on testwork and design assumptions; they do not represent historical operating data from the MacLellan, Farley Lake, Burnt Timber, or Linkwood properties. The design criteria are derived from test work conducted by SGS and Witteck. Several operational parameters carry uncertainty: provision should be made to sparge the pre-aeration tank with air or oxygen depending on the results of additional test work; the first two leach tanks may need oxygen sparging based on future test work; and up to 300 g/t lead nitrate addition depends on results of additional test work on a representative sample of mill feed. The head grade for silver is listed as "TBD". The report contains a simplified process flow diagram (Figure 17.1) that is not reproduced in this article. No actual plant performance, production history, or operating cost data is presented.
Source: Preliminary Economic Assessment for the MacLellan, Farley Lake, Burnt Timber, and Linkwood Properties, Lynn Lake Gold Camp, Manitoba, Carlisle Goldfields Ltd., Section 17.0 Recovery Methods, pages 17-1 through 17-10.
