Golder's 2012 mine feasibility study update for Fortune Minerals' Arctos anthracite project describes washing about 125 million tonnes of run-of-mine coal to a 10% ash product, with a projected average clean coal yield of 55.4%.
Arctos is an anthracite project reached by road and by rail along the Dease Lake extension grade, with clean coal planned to move 1,390 km by rail to the Ridley Coal Terminal at Prince Rupert for shipping to international markets. Golder Associates prepared the report, titled the Technical Report on the 2012 Update of the Arctos Anthracite Project Mine Feasibility Study and dated November 28, 2012, for Fortune Minerals Limited. The proposed production plan mines about 125 million run-of-mine (ROM) tonnes of the total reserve at an average ratio of 11.3 bank cubic metres of mine rock per tonne of clean coal. ROM coal goes to a coal preparation plant where it is crushed and processed to reduce the ash content to 10% on an air-dried basis (adb). The report calls the processing method standard and typical for the coal industry.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| ROM coal mined | About 125 | Mt | Total reserve plan |
| Clean coal produced | About 69.2 | Mt | 25 mine years |
| Plant capacity | 3.0 | Mtpa clean coal | Coal preparation plant |
| Average clean coal yield | 55.4 | % | Washability based |
| Product ash | 10 | % adb | Design |
| Product total moisture | 8 | % | Clean coal |
| Coarse coal circuit | 50 mm x 0 | Not applicable | Heavy media bath |
| Ultra-fine circuit | Minus 150 | µm | Froth flotation |
| Mine rock ratio | 11.3 | bcm per tonne clean coal | Average |
| Rail haul to port | 1,390 | km | To Ridley Coal Terminal |
| Unit train loadout | 12,500 | t in under 6 h | Rail loadout |
Overview
The preparation plant uses a gravity-based separation process that removes rock and non-carbonaceous material from ROM coal. It applies standard coal washing technology to make a clean metallurgical coal product with an average of 10% ash on an air-dried basis and 8% total moisture. The plant has a heavy media bath for coarse coal, froth flotation for ultra-fine material and a middlings regrind circuit. Coarse and fine rejects are taken to a designated rejects storage pile where they will be capped. The flowsheet is shown in the report's Item 13 as a simplified solids flowsheet.
Key Process Stages
Coal preparation
ROM coal is crushed at the preparation plant and then washed. The heavy media bath treats the coarse coal, defined in Item 17 as 50 mm by zero. The ultra-fine fraction, minus 150 µm, goes to froth flotation, and a middlings regrind circuit reprocesses middlings. The coarse circuit includes a middlings recovery step for material plus 6 mm, which is crushed and reprocessed in the appropriate fine coal circuit.
Predicting yield
The earlier owner, Gulf, ran more than 700 washability analyses on drill core from all seams and did extensive pilot processing and sampling of I Seam coal from the trial cargo pit, plus some H Seam coal from adits. The 2012 study was built on a single 10% ash (adb) product. Yield was estimated with a spreadsheet-based coal process simulation that adds out-of-seam dilution, applies plant efficiencies from partition curves for each separation process, and models three size fractions. For the fine fraction, froth flotation yield was calculated by ash balance, assuming a 10% product ash and a tailings ash of 52.73%. The raw coal size distribution used was 45.2% in 50 mm by 6 mm, 42.0% in 6 mm by 0.5 mm, 7.8% in 0.5 mm by 0.15 mm and 5.0% below 0.15 mm.
Washability data selection
Washability data from the 1985 to 1988 drilling programs were used, while 1982 to 1984 data were excluded because the top size of those tests was 10 mm, which could overstate product yield. Drill cores were accepted only if core or seam recovery exceeded 70%, undiluted specific gravity was below 2.0 and mineable thickness exceeded 0.6 m.
Projected product quality
The report projects an average ash of 10.0% and residual moisture of 0.9% for the clean coal. The annual schedule table shows yield by mine year, from 49.3% to 66.0%, with the thinner seams losing more at the roof and floor. Over the plan, 125.1 Mt of ROM coal yields 69.2 Mt of clean coal, with average ROM ash of 32.8%, sulphur of 0.57%, volatile matter of 7.4% and fixed carbon of 56.1%.
Additional Interesting Data and Summary
Recommended future work includes collecting representative bulk samples of the major seams in proportions consistent with the mining area, to confirm the flowsheet parameters and the benefit of the coarse middlings circuit, and to determine the top size of plant feed and the top and bottom size of the heavy media cyclone circuit. The report also calls for more flotation and dewatering test work to optimise the minus 0.5 mm clean coal product for recovery and moisture.
The mine infrastructure includes a work camp complex, coal processing plant, train loadout, administration and maintenance facilities. The processing plant capacity is 3.0 Mtpa of clean coal, and the loadout can load a 12,500 tonne unit train in under six hours.
The projected clean coal specification gives a maximum ash of 18.7% and a minimum of 7.4%, around the 10.0% mean, with a maximum residual moisture of 2.0%. Coal recovery varies by seam and is generally thickness based, because thinner seams lose more coal at the roof and floor during mining.
Key Processes
- Crushing of run-of-mine coal at the preparation plant
- Heavy media bath washing of coarse coal
- Froth flotation of ultra-fine coal
- Middlings crushing and regrinding
- Rejects storage with capping
- Rail loadout for unit trains
Source: Technical Report on the 2012 Update of the Arctos Anthracite Project Mine Feasibility Study, November 28, 2012.
