This report describes the proposed processing of Willa mineral through the purchased MAX milling complex, based on 2004 testwork, historical operating data, and equipment assessments.
Report context
The 2011 technical report for the WILLAMAX project documents the assessment of the MAX Mine and flotation milling complex, purchased by Discovery for processing Willa mineral. The MAX mill, constructed in 2007/08 and operated from 2008 to 2011, was originally designed to treat molybdenum ore and produce a molybdenite concentrate. The report presents process design criteria, equipment assessments, and cost projections developed from metallurgical testwork and historical MAX mill operating data.
Processing route
Proposed design basis
The process design criteria for a conventional copper mineral milling circuit were developed from testwork data. The Willa mineral responds well to typical grinding and flotation conditions. Testwork data are considered adequate for process design purposes, provided the data are interpreted conservatively and flexibility is incorporated into the flotation circuit to permit easy circuit changes. Gravity concentration is also required to recover the coarse gold content.
Pyrite flotation is included in the proposed design to produce a rougher pyrite concentrate, which would desulphurise the tailing product and offers potential for recovery of additional values. Bench-scale scavenger flotation testwork conducted in 1996 produced a pyrite concentrate assaying 95 to 98% pyrite, with insignificant sulphides remaining in the final tailings discharge. Confirmatory testwork will be conducted prior to the production phase.
Crushing and grinding
The MAX crusher building measures 15 m long, 12 m wide, and 15 m high, with concrete foundation, steel I-beam frame with wood and tin siding. The installed crushing equipment is considered ideally suited for the Willa mineral. When the MAX mine was in production, the crushing plant operated between four and five hours per day to produce the 500 tpd required for treatment.
The crushing facility includes a coarse ore bin, apron feeder, Traylor jaw crusher (36” x 42”, 91 cm x 107 cm) with coolant fan and tank heater, three conveyors, permanent magnet, screen deck (5’ x 10’, 1.5 m x 3 m), and a Symons 5.5 ft (1.67 m) short head cone crusher.
The main mill building measures 49 m long, 24.4 m wide, and 12 to 15 m high, built on three levels. Grinding equipment includes two 7’ x 7’ (2.13 m x 2.13 m) Marcy grate-discharge ball mills in series, each with discharge boxes, SRL pumps, and glandwater pumps with standby. Two 10” (25 cm) Krebs cyclones are installed. A 7’ x 10’ ball mill is installed but not in use.
Flotation and concentrating
The rougher flotation circuit includes a conditioner tank and six Gardner Denver #24 rougher flotation cells, with SRL pumps (one on standby). The cleaner flotation circuit has four Gardner Denver #18 cells, two Gardner Denver #15 cells, and two Sperry automatic plate-and-frame concentrate filters, each with 35 sq m capacity.
While the rougher flotation capacity is more than adequate, a small number of additional cleaner cells are required. The tonnage of copper concentrate compared to molybdenite concentrate is expected to be higher by a factor of 2.25. Sixteen Gallagher 4’ x 4’ x 4’ (1.2 m x 1.2 m x 1.2 m) flotation cells and twelve Wemco 5’ x 5’ x 4’ (1.5 m x 1.5 m x 1.2 m) flotation cells are installed but not in use.
Rougher concentrate regrinding is provided by a 4’ x 7’ (1.2 m x 2.1 m) regrind ball mill, with SRL pumps and one 4” (10 cm) Krebs cyclone.
Concentrate handling
Concentrate thickening uses one 18’ diameter x 8’ deep (5.5 m x 2.4 m) thickener, with a 30’ diameter x 10’ deep (9 m x 3 m) thickener installed but not in use. The 5.5 m diameter thickener has a surface area of over 23 sq m and has been used for thickening molybdenum concentrate. Test work on thickening or filtering of copper concentrate was not conducted. Design parameters were assessed by Winckers Associates based on operating data from comparable applications, assuming 0.5 sq m/t/day. The 5.5 m diameter thickener has a capacity equivalent to 46 t/d, providing a safety factor of over 150%.
Concentrate filtering is provided by two automatic plate-and-frame filter presses, each with 34 sq m surface area. Based on an estimated required filter unit area of 400 kg/sq m/day, the two filter presses have a total filter area of 70 sq m and are capable of handling 28 tpd of concentrate. Projected daily copper concentrate production is expected to be in the range of 17 tpd, providing a safety factor of 65%.
Additional concentrate handling equipment includes a concentrate stock tank, Wilden 3” (7.5 cm) air pump, concentrate screw conveyor (9” x 19’, 23 cm x 5.8 m), and concentrate hopper with scale.
Required additions
The main addition required is a gravity concentrator with high centrifugal speed (Falcon Concentrator) for maximizing recovery of coarse gold. A half-size Deister table is specified for producing an ultra-high grade coarse gold concentrate for on-site smelting. Table tails are estimated at 0.75 tpd and would be combined with the copper concentrate.
Other additions required include:
- A lime circuit for maintaining flotation pH in the range of 10.5 to 11.0
- A sulfuric acid dosing system to reduce pH to within allowable discharge limits
- Additional cleaner flotation cells for producing copper concentrate
- Six 6” (15 cm) Krebs cyclones (smaller cyclones for fine overflow product are preferred)
- Additional rougher flotation cells for producing a potential auriferous pyrite concentrate (presently on site)
- An on-site smelter for producing gold bullion
- A high-density polyethylene-lined short-term pyrite concentrate holding pond
Operating cost estimates
The projected milling cost is estimated at $40/t, based on MAX mine and mill operating data for the 2009-2010 production period, corrected for inflation by a factor of 1.12. On-site supervision and technical costs are estimated at $4.30/t, including additional personnel for metallurgical and assay lab. Hourly-paid labour is estimated at $7.45/t milled, based on a base rate of $45/hr to account for inflation.
Mill consumables are based on past production usage for crushing liners, grinding liners and balls. Reagents have been altered for the change-over from flotation of molybdenum ore to copper and for lowering final tailings pH. The total projected reagent unit cost is estimated at $7.77/t.
Other items, including mill maintenance supplies, surface services, front end loader expenses, and local expenses, total $5.31/t. The total MAX unit milling cost is estimated at $39.83, excluding concentrate shipment. Shipping to Vancouver adds $2.91/t for a total of $42.75/dmt.
Power is supplied by three 1,000 kVA diesel generators, with two in operation and one spare. When the MAX mine was in operation, average power consumption was in the range of 1,450 kVA. Monthly power consumption is projected at 700,000 kWh, with unit power costs estimated at $15.00/t based on a fuel price of $1.20/L.
Water requirements
The drainage from the MAX mine has supplied mill needs in the past and will continue to do so. The mine drainage far exceeds the MAX mill requirements. Environmental test work will be conducted during the pre-production phase to determine the effect of quick lime on the sulphate content of the mine discharge water, as this may precipitate out as gypsum and reduce dissolved sulphate content.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Projected mill throughput | 500 tpd | Willa mine capability |
| Projected gold recovery | 82.0% | Flotation plus Falcon gravity |
| Projected copper recovery | 90% | Reduced from PRA projection |
| Projected silver recovery | 82% | PRA results |
| Copper concentrate production | ~17 dmtpd | Based on 500 tpd mill feed |
| Copper concentrate grade (combined) | 24% dry | After blending table tails |
| Required flotation pH | 10.5 – 11.0 | Chalcopyrite/pyrite flotation |
| Thickener capacity | 46 t/d | 5.5 m diameter unit |
| Concentrate filter capacity | 28 tpd | Two plate-and-frame presses |
| Filter safety factor | 65% | vs. 17 tpd projected |
| Milling unit cost | $39.83/t | Excluding concentrate shipment |
| Shipping to Vancouver | $2.91/t | Per tonne processed |
| Total milling cost | $42.75/dmt | Including shipping |
| Power consumption | 700,000 kWh/month | Projected |
| Power cost | $15.00/t | Based on $1.20/L fuel |
| Reagent cost | $7.77/t | Based on CCC Chemicals quote |
| On-site supervision | $4.30/t | Updated from MAX data |
| Hourly labour | $7.45/t milled | Base rate $45/hr |
| Other operating costs | $5.31/t | Maintenance, services, local |
| Pyrite concentrate grade | 95 – 98% pyrite | 1996 testwork |
| Concentrate tonnage increase | Factor of 2.25 | Copper vs. molybdenum concentrate |
Project website: https://www.mining.com/discovery-ventures-to-acquire-max-mine-and-mill-complex-77450/
Technical qualifications
Metallurgical test samples taken on behalf of Northair (1985-1988) and Bethlehem (2004) were restricted to samples from the West Zone only, as this zone was being actively drilled or developed at the time. The report states there is no reason to suspect that recoveries will be different in the East Zone or above the 1100 L.
Process tests not covered in past studies include thickening, filtering, and environmental test work. Filtering estimates were based on operating data from comparable operations rather than direct testwork. Pyrite flotation testwork was based on pyrite content in the range of 10%, but the average may vary; confirmatory test work will be conducted prior to the production phase. Test work will be conducted immediately after commencement of production with the purpose of reducing the selenium content of the concentrate by addition of reagents specifically designed for that purpose.
The potential for increased gold recovery from the pyrite concentrate has not been conclusively proven and has not been taken into account in the cash flow analysis. Assumed gold recovery of 82.0% using the Falcon concentrator is an adjustment from the 81.4% projected by PRA, while copper recovery has been reduced from 93.2% to 90%.
Source: 2011 Technical Report, Discovery Ventures Inc., Sections 17.0 to 17.6.7.
