Based upon diamond drilling of 100 holes the reserve is estimated at 2,300,000 tonnes containing 8% graphite. The ore is found in two zones. Both ore bodies open at depth and outcrop at the surface. They are separated by about 2000 ft. and occur in clearly defined zones on site as bands as wide as 70 feet but averaging over 20 feet dipping approximately 70°.
Pilot Scale Test Set-Up
The flowsheet tested incorporated the following unit processes:
– Autogenous Milling
– Flotation
– A Tabling Circuit
– A Secondary Regrind and Flotation Circuit
Autogenous Milling. The primary autogenous milling circuit incorporated a 1.58m dia x 0.58m long Nordberg Autogenous Mill. The net power drawn was determined from a compound input-output efficiency curve. To produce minus 0.42 mm (35 mesh) material from minus 200 mm (8″) feed it consumed 7.3 kilowatt per metric tonne in a closed circuit operation. This is equivalent to Bond Work Index of 11.2.
The Flotation Circuit. The primary flotation circuit consisted of a:
- 400 L capacity unit cell
- 0.76 diameter Sweco screen to classify the unit cell flotation concentrate. The oversize was fed to the gravity concentration (tabling) circuit while the undersize was fed to a regrind flotation circuit.
Secondary Regrind and Flotation Circuit. The secondary circuit incorporated:
A 1.8 m x 0.9m diameter rubber lined Denver ball mill. It was used to regrind the undersize (0.4 mm) material produced by screening the unit cell flotation concentrate. The mill was charged with 450 kg of minus 3.8 cm steel balls which is equivalent to 25% of a full charge. The mill was operated in open circuit. Cleaner tails recycled from the flotation circuit were included with the new feed. Since the cleaner tailings were not thickened the mill was operated at a low pulp density (20% solids).
Pilot Plant Test Results
Assay results showed:
– 10.3% graphite carbon and
– 3.7% carbonate
Primary Flotation. The minus 10 mesh (-1.68 mm) autogenous mill product was submitted to flotation. The +1.68 mm fraction was recirculated in the autogenous mill. The unit cell flotation tailings were screened on a 35 mesh (0.42 mm) DSM screen. The minus 35 mesh material was discharged as tailings while the plus 35 mesh material was recirculated to the autogenous mill.
Secondary Regrinding and Flotation. The formal regrind and flotation circuit tests to upgrade test products were conducted as follows:
The total 0.42 mm undersize fraction of the unit cell concentrate was fed as wet filter cake to the regrind mill operated in closed circuit with a rougher and 2 cleaner flotation stages. The cleaner 2 concentrate was screened on a 100 micron rating (150 mesh) DSM Rapifine screen to collect the oversize as a final product while the screen undersize was recirculated together with Cleaner 1 flotation tailings back to the regrind mill.
Full Scale Mill Design
As the heart of the mill design would be the autogenous mill, it was contemplated that some used equipment would be procured. A 17 ft. diameter x 5 ft. long 600 hp unit was selected. In part, the overall mill feed rate of 500 tpd was influenced by the most common mill size available on the market commensurate with product market requirements.
The Primary mill is fed by apron feeder and belt conveyor. The mill is in closed circuit with two 5 ft x 10 ft 10 mesh vibrating screens. There are four DSM Screens to classify the unit cell tails at 14 mesh and 35 mesh. Middlings are also recirculated from the concentrating tables to the primary mill. Mill discharge is handled by a pair of 8″ x 5″ rubber-lined slurry pumps.
Plant Layout
Special attention is paid to equipment access and plant clean up. Also extra electrical protection is included to account for the conductive nature of graphite.
Orrwell Energy Corporation intended to begin production in 1984. Uncertainty of the economy has made financing of the mine and plant difficult, however, in spite of a favourable feasibility study; it is foreseen that with the proposed modernization of the U.S. industry this project may be considerably accelerated in the meantime.
