Pilot plant scale evaluation of beneficiation potential of Donkin-Morien coal, N.S.

As part of a program for evaluation of the beneficiation potential of eastern Canadian coals, Donkin-Morien coal was selected for pilot plant scale evaluation. A bulk sample (74t) was made available for testing at the Coal Research Laboratory facilities at Devon, Alberta. The objective of the study was to determine the limitation and potential of reducing the sulphur content in relation to mass and thermal recoveries. Two flowsheet options were selected to achieve three objectives a) Maximum thermal recovery at limited sulphur removal h) lower sulphur in clean coal to 2% or less, and c) maximum sulphur removal at acceptable mass recovery. Three clean coal samples (approximately 5t each) with different ash and sulphur levels based on the above objectives were made available for testing at the Canadian Combustion Research Laboratory in Ottawa. Donkin-Morien coal is relatively low in ash (8.24%) but high in sulphur (3.40%) of which 55.3% (of the total sulphur) is pyritic. Moderate ash and limited sulphur liberation were observed when the raw coal was reduced in top size from 50 to 10 mm by crushing. The coal is relatively hard; however, it is expected to generate 19.5% by mass of minus 0.6 mm due to degradation during mining, handling and processing. Washability data indicated that the coal is amenable to significant sulphur removal (50-65%) by density separation at low cutpoints. Laboratory characterization also indicated that the coal is easy to float with conventional reagents resulting in more than 45% sulphur removals in the clean coal. Pilot plant runs produced clean coal products with sulphur removals of 33.9 to 65.1% at thermal recoveries of 95.6 and 64% respectively. The performance of the heavy medium cyclone proved its capability to achieve very low cutpoints (1.28-1.33 dp) with efficient separation. However the Ratac jig and Water-Only Tricone cyclone performance were below what might be expected partly due to the low ash content of the feed. The pilot plant flotation performed similar to the bench scale tests i.e. showed good results. The water recovery circuit produced clarified water for recirculation at minimum addition of flocculants with no change in the water pH due to sulphur leaching. Results indicate that Donkin-Morien Coal is amenable to limited sulphur liberation by crushing. However, this advantage may be offset by less efficient separation of finer particle sizes. As a result, it is recommended that the raw coal not be crushed finer than 50 mm top size. The heavy medium cyclone system is the most suitable means to remove 50 to 65% of the sulphur and give a clean coarse coal with 1.3-1.9% ash and 1.6-1.9% sulphur. An option that should he considered is a three-product separation to maximize both sulphur removal and thermal recovery. This option would include a two-stage system for coarse cleaning to produce a premium product (1.6-1.9% sulphur) and a secondary product relatively high in ash and sulphur. The fines would be treated by a combination of a water-only system (cyclones or spirals) and froth flotation.