The 2022 Prefeasibility Study proposes a 195,000 t/d conventional flotation and cyanidation processing plant fed from the Mitchell, Sulphurets, and East Mitchell deposits.
Report context
The proposed KSM plant for the 2022 Prefeasibility Study (PFS) is designed to start at a nominal process rate of 130,000 tpd and expand to 195,000 t/d by Year 3 of operations. The process plant will receive ore from Mitchell, Sulphurets, and East Mitchell deposits. The planned mill life is approximately 33 years, excluding the production development stage and closure stage. The Mitchell deposit will be the dominant resource and will supply mill feed throughout the projected life of mine.
Processing route
General approach
A combination of conventional flotation and cyanidation processes are proposed for the PFS. The processing flowsheet was developed based on test results. The mineralization from Mitchell, Sulphurets, and East Mitchell deposits are amenable to the combined flotation and cyanidation process.
Facility arrangement
The process plant will consist of three separate facilities: an ore primary crushing and handling facility at the mine site; an ore transportation system by trains through the MTT; and a main process facility in the Treaty OPC area at the plant site.
Crushing and grinding
At the Mitchell OPC site, primary crushing will consist of three 60 in by 89 in gyratory crushers, three apron feeders, one train load surge bin feed conveyor, and associated transfer conveyors. The ROM will be approximately 80% passing 1,200 mm. The gyratory crushers will reduce the ROM material to a particle size of 80% passing 150 mm or less.
The crushed ore will be reclaimed by two automatic train loading systems from two coarse ore surge bins at the Mitchell site and transported to the Treaty site by a train transport system through the MTT. Each train will consist of two 140-t electric locomotives and eighteen 42 m³ belly dump ore cars. On average, 11 trains will deliver approximately 195,000 t/d of ore. An additional two trains will be on standby.
At the Treaty site, the crushed ore is unloaded into a bin, reclaimed by two apron feeders, and conveyed to two coarse ore stockpiles, each with a live capacity of 90,000 t. The ore is then reclaimed via apron feeders and conveyed to the secondary crushing circuit.
Secondary crushing consists of four cone crushers in closed circuit with four double-deck vibrating screens. Screen undersize finer than 50 mm is delivered to three HPGR feed surge bins, each with a 3,250-t live capacity. Tertiary crushing uses six HPGR crushers in closed circuit with double-deck vibrating screens. Discharge is conveyed to three 3,250-t ball mill feed surge bins. Material is wet screened at a cut size of approximately 6 mm.
The grinding circuit employs conventional ball mills to grind the HPGR product to a particle size of 80% passing 125 to 150 µm. The primary grinding circuit includes six milling circuits (four circuits for the initial 130,000 t/d at Year 1 and six circuits for 195,000 t/d at Year 3), each with a 7.9 m diameter by 14.0 m long ball mill driven by two 7.5 MW synchronous motors, in closed circuit with hydrocyclone clusters.
Flotation
There are three copper-gold-molybdenum bulk rougher flotation circuits, each consisting of five 500 m³ flotation cells. A bulk copper-gold/molybdenum rougher flotation concentrate, approximately 6% to 7% of the flotation feed by weight, is reground. The flotation tailing is sent to the pyrite flotation circuit.
The bulk concentrates are combined and reground to a particle size of 80% passing 15 to 20 µm in a two-stage regrind circuit. The first stage uses three tower mills each with 2,240 kW, the second stage uses stirred mills.
The reground concentrate is cleaned in three stages: six 160 m³ tank cells for first stage, three 100 m³ tank cells for second stage, and one 100 m³ tank cell for third stage. First cleaner tailing is further floated in two cleaner scavenger flotation cells (160 m³ each). The tailing from cleaner scavenger reports to the gold leaching circuit.
Depending on molybdenum content, the final copper-gold/molybdenum concentrate may be further processed to produce a copper-gold concentrate and a molybdenum concentrate. The separation employs copper suppression by sodium hydrosulphide and four stages of molybdenum cleaner flotation and regrinding, with molybdenum flotation cells aerated by nitrogen gas.
Gold recovery
Gold is recovered by cyanide leaching of bulk copper-gold scavenger cleaner tails and copper-removed pyrite rougher concentrate, followed by carbon elution and production of gold doré. A cyanide recovery circuit and cyanide destruction of washed leach residue occurs prior to disposal in the lined pond within the TMF.
Concentrate dewatering
The upgraded copper-gold concentrate, together with copper sulphide precipitate generated from the cyanide recovery circuit by SART treatment, will be thickened.
Key reported parameters
| Criteria | Unit | Value |
|---|---|---|
| Average Daily Process Rate | t/d | 195,000 |
| Operating Year | d | 365 |
| Availability – Primary Crushing | % | 70 |
| Availability – Secondary Crushing | % | 85 |
| Primary Crushing Product Particle Size, P80 | mm | 150 |
| Secondary Crushing Product Particle Size, P80 | mm | <45 |
| Availability – HPGR/Grind/Flotation/Leach | % | 94 |
| Milling and Flotation Process Rate | t/h | 8,644 |
| Mill Feed Size, P80 | mm | 2.0 |
| Primary Grind Size, P80 | µm | 125-150 |
| Bond Ball Mill Work Index – Design | kWh/t | 16 |
| Bond Abrasion Index | g | 0.293 |
| Cu/Au Rougher/Scavenger Concentrate Regrind Size, 80% Passing | µm | 15-20 |
| Au-Pyrite Concentrate Regrind Size, 80% Passing | µm | 20 |
| Gold-bearing Materials Leach Method | Cyanidation | |
| Feed Mass to CIL Circuit | t/h | 1,117 |
| Initial Mill Feed Rate (Year 1-2) | t/d | 130,000 |
Project website: https://www.seabridgegold.com/projects/kerr-sulphurets-mitchell
Technical qualifications
The process description and design criteria presented in this article are based on the 2022 Prefeasibility Study level of engineering. The proposed flowsheet was developed from test results discussed in Section 13 of the technical report. The design uses a Bond Ball Mill Work Index – Design of 16 kWh/t and a Bond Abrasion Index of 0.293 g. The concentrator is designed to process an average of 195,000 t/d, with initial two years at 130,000 t/d. The process plant will consist of three separate process lines excluding bulk cleaner flotation, copper molybdenum separation, and gold and silver extraction circuits which will be operated in one line.
Source: KSM (Kerr-Sulphurets-Mitchell) Prefeasibility Study and Preliminary Economic Assessment, NI 43-101 Technical Report, 2022, Sections 17.0-17.3.
