This Preliminary Economic Assessment examines modifying the existing Bachelor mill from 750 tpd to 1200 tpd to process mineralized material from the Barry deposit.
Report context
The report was prepared in accordance with Canadian National Instrument 43-101, following discussions between Metanor and GoldMinds and a March 4th proposal for a Preliminary Economic Assessment (PEA) for mining and milling approximately 1200 to 1300 tonnes per day of mineralized material from the Barry deposit. The processing section was written by Gilbert Rousseau Eng. and describes the present Bachelor mill operation along with proposed changes to process the Barry material.
Processing route
Present milling operation
Metanor currently mines and mills approximately 750 tpd of Bachelor mine ore. The mill circuit comprises a 24” x 36” KueKen jaw crusher followed by a 4¼’ Simons standard cone crusher and then a 5½’ Simons short head cone crusher. Material crushed and screened to 3/8” is stored in a 1200 tonne fine ore bin.
From the fine ore bin, material is ground in a conventional grinding circuit comprising a primary 10’ x 14’, 400 HP rod mill followed by a 10½’ x 9’ x 10’, 525 HP tricone ball mill in parallel with two secondary ball mills: a 6’ x 10’, 150 HP mill and a 5’ x 10’, 100 HP mill. Feed F80 is 9,525 microns and final grind P80 is 75 μm. Classification is by a set of four 15-inch cyclones.
Cyclone overflow reports to a wood chips screen prior to gravity flow into a 40-foot single tray thickener. Thickener underflow is pumped to a bank of four 22’ x 22’ leaching tanks followed by four 22’ x 22’ CIP reservoirs, providing total leaching time of approximately 50 hours. Tanks are interconnected with launders allowing slurry to flow by gravity. Each tank has impeller mechanical agitators. Oxygen for leaching is provided by air sparging through the bottom of the agitator shaft. Adsorption tanks each have a woven wire inter-tank screen to retain carbon, with bypass facilities for maintenance.
Sodium cyanide solution is metered into the leach feed distribution box. Compressed air is distributed and sparged down agitator shafts to maintain a high dissolved oxygen profile. Fresh and regenerated carbon are returned to the circuit at CIP tank 4 and advance counter-current to slurry flow by pumping from tank 4 to tank 3 to tank 2, and so on. Loaded carbon is recovered from CIP tank No 1 using an air lift to a loaded carbon recovery screen. Leached tails pass through a safety screen before being pumped to the tailings pond.
The carbon elution and refining circuit comprises a loaded carbon tank, acid wash tank, carbon strip vessel, bank of electrowinning cells, and two Wabi type gold bullion furnaces. Stripped carbon is reactivated in a horizontal kiln, quenched, and classified. Classifier oversize is reused while very fine carbon particles are filtered and kept aside for shipment to a copper-gold smelter.
Tailings from the safety screen and other waste streams are combined in a tailings collection pump box and pumped to the tailings pond to settle. Supernatant water flows by gravity to a polishing pond, and clear water is pumped back to the process water reservoir.
Proposed changes for Barry mineralized material
The flowsheet will remain essentially the same with the following modifications. A new 90 tonne hopper and a 12” x 12” grizzly will be installed above the outside feeder to accept incoming material from the Barry deposit. Material will be brought to the ore hopper with the existing front end loader, and large chunks not passing the grizzly will be broken with a stationary hydraulic rock breaker.
The two small 150 HP and 100 HP ball mills will be removed and replaced with a single 500-550 HP ball mill to work in parallel with the existing tricone 525 HP mill.
Because of coarse gold particles within the Barry mineralized material, two new 40” Knelson concentrators (one in operation, one spare) and a shaking table will be added to the circuit at the grinding stage, prior to cyanide leaching.
Both 3 x 6 – 75 HP tricone mill discharge pumps will be replaced with two 6 x 6 x 15” SRL pumps with 100 HP motors.
To obtain approximately 40+ hours retention time in the leaching and CIP circuit, three new 30’ x 30’ tanks will be added.
The fine ore bin will be isolated from the main mill building to prevent heating of frozen material during winter and avoid condensation, ensuring free rock flow. The mill compressed air system is considered sufficient to aerate the new leaching tanks.
Anticipated gold recovery
Based on previous metallurgical testing, it is anticipated that 24% of the gold will be recovered at the gravity circuit. Out of the remaining 76%, another 71% will be recovered from leaching of the gravity tailings, for a total gold recovery of 95%.
Mill operation costs (OPEX)
Mill operating costs are for processing approximately 420,000 tonnes per year of Barry mineralized material. Cost estimation limits start at ROM storage and end at the polishing pond. Milling costs are based on salaries, consumption of reagents and other consumables, supplies, and power. Manpower costs include 35% fringe benefits and contingency allowances. Mill operation costs are considered to have an accuracy range of +/-35%.
Total mill operation cost is CAD $19.90 per tonne milled, comprising consumables and spare parts at $9.00, electric power at $3.18, salaries at $5.91, and miscellaneous at $1.81.
The mill workforce will remain essentially the same as August 2016 except for the addition of four loader operators and yard men, and two Knelson concentrator and table operators. Salaries include vacations and are based on 2,080 working hours per year. Total annual salaries including 35% fringe benefits and 10% overtime are CAD $2,483,271.65.
Consumables include electric power ($3.18/t), grinding media for rod mill ($1.00/t), grinding media for ball mills ($1.22/t), lime ($0.42/t), cyanide ($3.74/t), spare parts ($1.50/t), and other items at approximately 10% of the above ($1.11/t), for a total of $12.17 per tonne.
Capital expenditure (CAPEX)
Capital cost to increase Bachelor mill capacity from 800 tpd to 1200 tpd is estimated at CAD $1.1 million and is considered at a conceptual level with +/-30% accuracy. Major costs include: rock breaker ($110,000), shop-fabricated grizzly ($25,000), 90-tonne hopper and ramp ($50,000), two Knelson concentrators ($400,000), shaking table ($40,000), two tricone mill discharge pumps ($40,000), three leaching tanks ($300,000), isolation of fine ore bin ($20,000), and other costs at 10% ($100,000).
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Current mill throughput | 750 tpd | Historical operating data |
| Proposed mill throughput | 1200 tpd | Proposed design |
| Annual throughput (proposed) | 420,000 tonnes | Proposed design |
| Feed F80 | 9,525 microns | Historical operating data |
| Final grind P80 | 75 μm | Historical operating data |
| Crushed product size | 3/8” | Historical operating data |
| Total leaching time (current) | ~50 hours | Historical operating data |
| Target leaching time (proposed) | ~40+ hours | Proposed design |
| Gold recovery – gravity | 24% | Testwork (Chapter 13) |
| Gold recovery – leaching of gravity tails | 71% | Testwork (Chapter 13) |
| Total gold recovery | 95% | Testwork (Chapter 13) |
| Mill OPEX | $19.90 CAD/t | Estimate, +/-35% accuracy |
| Mill CAPEX | $1,085,000 CAD | Conceptual estimate, +/-30% accuracy |
| Primary grinding | 10’ x 14’, 400 HP rod mill | Historical operating data |
| Secondary grinding (current) | 10½’ x 9’ x 10’, 525 HP tricone ball mill; 6’ x 10’, 150 HP ball mill; 5’ x 10’, 100 HP ball mill | Historical operating data |
| Secondary grinding (proposed) | Existing 525 HP tricone ball mill in parallel with new 500-550 HP ball mill | Proposed design |
| Cyclone classification | 4 x 15-inch cyclones | Historical operating data |
| Leach/CIP tanks (current) | 4 leach + 4 CIP, 22’ x 22’ each | Historical operating data |
| Additional leach tanks (proposed) | 3 x 30’ x 30’ | Proposed design |
| Gravity concentration (proposed) | Two 40” Knelson concentrators + shaking table | Proposed design |
| Thickener | 40-foot single tray | Historical operating data |
| Fine ore bin capacity | 1,200 tonnes | Historical operating data |
Technical qualifications
Mill operating costs are considered to have an accuracy range of +/-35%. Capital cost estimates are at a conceptual level with +/-30% accuracy. The report notes that the idea of removing the two small ball mills and adding a single 500-550 HP ball mill, along with adding gravity concentration ahead of cyanidation, was at the suggestion of Goldminds. Anticipated gold recovery is based on previous metallurgical testing referenced in Chapter 13 of the report. The mill workforce projections are based on the August 2016 staffing levels with additions for Barry material processing.
Source: Technical Report – Preliminary Economic Assessment, Barry Gold Project, Quebec, Canada, Sections 17.1 through 17.7, GoldMinds Geoservices Inc.
