This updated prefeasibility study proposes processing Croinor ore at the recently acquired Beacon Mill at a rate of approximately 575 tpd using gravity concentration, cyanide leaching and the Merrill-Crowe process.
Report context
This NI 43-101 updated prefeasibility study for the Croinor Gold Property is dated March 22, 2018. Since the previous prefeasibility study was issued, the issuer acquired the Beacon Mill facility. This updated version is therefore based on processing ore at the Beacon Mill at an average production rate of around 575 tpd. The Beacon gold mill is situated 15 km east of Val-d'Or. The plant started operating in 1987. The flowsheet uses gravity concentration, cyanide leaching and the Merrill-Crowe process to recover the gold.
Processing route
Crushing circuit
The plant feed is first dumped on a heavy-duty grizzly. An impact hammer breaks the oversize rock directly on it. The crushing circuit consists of a 32" by 42" jaw crusher that can be fed with ore of about 20" at a rate of 175 m³/h to 200 m³/h. The ore crushed to about 4" is sent to the screen that sends the -¾ fraction to the fine ore silos. The +¾ fraction is sent to the conical crushers. A standard 4 ft crusher and a 4 ft short head crusher are used. The plant has two fine ore silos with capacity of about 1,000 t each. The silos feed the grinding circuit with a conveyor at a rate of 32 t/h.
Grinding and gravity circuits
The grinding circuit is composed of a primary rod mill of 8 ft by 12 ft and one ball mill of 8 ft by 12 ft. The rod mill runs in open circuit. The ball mill is currently set up to run with both mill discharges going to the same common cyclone. The first ball mill will operate in closed circuit with the existing cyclone cluster which is composed of two cyclones, one unit of which would be in operation and one in standby. A portion of the underflow is sent to the gravity recovery circuit and the remainder is returned to the first ball mill. The cyclone cluster overflow is sent to thickener.
The gravity circuit is composed of one Knelson concentrator operating to recover free gold. One unit is considered sufficient for operation at 750 tpd. The Knelson concentrate is treated on a shaking table. The gold concentrate is then further treated in the refinery. The supply of water to the grinding circuit will come from the reservoir with barren solution or mill solution.
Thickener, leach and Merrill-Crowe circuits
The 35 ft diameter thickener overflow is fed into a buffer tank before the Merrill-Crowe precipitation circuit. The thickener underflow is fed into the first leaching stage. The thickened pulp remains about 15 h in the first leaching stage before being filtered. There are two drum filters of 11'6" diameter per 16 ft long. During filtration, the cake is washed with sterile solution. After filtration, the cake is re-pulped to 50% solid and more with barren solution. This slurry is pumped into the second leaching stage. The pulp stays about 15 h in retention in the second leaching stage before being filtered using two drum filters identical to those of the first stage. The cake is washed with barren solution.
The clarifier is fed from the buffer tank, which purpose is to filter the solution of all particles. The clarifier consists of large panels covered with a filter cloth on which is added a layer of diatomaceous earth which allows a total filtration of the solution. The clarified solution is pumped through a deaeration tower, which removes the air dissolved in the solution, to precipitate the gold. To enable a better precipitation of the gold, a solution of zinc powder and a little of lead nitrate is added in the outlet pipe of the deaeration tower.
Gold recovery and refinery
The gold precipitate passes through a filter press where it remains trapped while the sterile solution will be stored in the reservoir of sterile solution. When the filter press is full, it is air dried and emptied to collect the precipitate which will be put in a drying oven and further melted at the refinery to obtain a gold doré. The drying oven and furnace are also used to treat the shaking table concentrate. The doré ingots are stored in a safety vault.
Tailings and cyanide degradation
After filtration, the 35% solid pulp cake is returned with water to the polishing pond before being pumped on to the tailings pump box. The cyanide remaining in the slurry will be naturally degraded in the tailing pond.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Average production rate | Approximately 575 tpd | Design |
| Jaw crusher feed size | About 20" | Design |
| Jaw crusher feed rate | 175–200 m³/h | Design |
| Fine ore silo capacity | About 1,000 t each | Existing infrastructure |
| Conveyor feed rate to grinding | 32 t/h | Design |
| Rod mill dimensions | 8 ft × 12 ft | Existing infrastructure |
| Ball mill dimensions | 8 ft × 12 ft | Existing infrastructure |
| Thickener diameter | 35 ft | Existing infrastructure |
| Leaching retention time per stage | About 15 h | Design |
| Drum filter dimensions | 11'6" diameter × 16 ft long | Existing infrastructure |
| Cake solids after filtration | 35% | Design |
| Gravity circuit design capacity | 750 tpd | Design |
| Mine site power demand | 1,925 kW | Design estimate |
| Access road new construction | 8 km forestry road, 25 m bridge/culvert | Design |
| Access road upgrade | 10 km ice road segment | Design |
| Power line | Private 25 kV three-phase overhead | Design |
Project website: https://www.canadianminingjournal.com/news/gold-monarques-drills-46-6-gt-croinor-property/
Project website: https://www.cecorp.ca/en/critical-elements-corporation-blue-note-mining-inc-extend-completion-croinor-acquisition-2/
Technical qualifications
The recovery methods section describes the process equipment at the Beacon Mill taking into account metallurgical test work results thus far and the findings of preliminary economic trade-off studies. The report presents the modifications that are planned and discusses the gold recovery that could be obtained in this processing facility. The issuer has a certificate of authorization for mine operation at the Croinor Property, issued in September 2010, and a certificate of authorization for mill operation at the Beacon property, issued in February 2017. An authorization for mine dewatering and other accessory permits will be obtained prior to project start-up. Various permits and authorizations will also be obtained for the new transportation infrastructure. An updated closure plan for the Croinor Gold site was approved in January 2015; a further updated version must be tabled by 2019.
Source: NI 43-101 Updated Prefeasibility Study for the Croinor Gold Property, March 22, 2018, Sections: Recovery Methods, Project Infrastructures, Environment and Permitting, and Section 17 Recovery Methods.
