Tharisa's processing stages comprise:
Producing chrome concentrate
The run-of-mine (ROM) is crushed and then milled to -0.8mm. It is then separated in gravity spirals into metallurgical grade chrome concentrate and the tailings of this spiral plant become the feedstock for the PGM flotation plant.
Milling/pelletising/sintering
The chrome concentrate is initially milled into a fine powder. This fine powder is then put through a pelletising machine, producing pellets with a particle size of 15mm. The concentrate pellets are then sintered. Sintering improves furnace operation, increases chrome recoveries from the furnace and reduces smelting power consumption.
Proportioning
The sintered chrome concentrate is then stored in proportioning bins alongside the reductants (coke, coal and anthracite) and the fluxes (quartz and limestone) required for smelting. From these proportioning bins, materials will be withdrawn in the correct ratio, or recipe, on to a conveyor system that feeds the furnace building.
Furnace
The mixture is then passed through to the electric arc furnaces. The furnace process melts and reduces the oxides of chrome and iron, producing the ferrochrome alloy. The ferrochrome alloy and slag separate in the furnace and the molten ferrochrome and slag are then tapped from the furnace. Each furnace will be tapped at least nine times a day for both ferrochrome alloy and slag at a rate of around 50 tonnes per tap.
Liquid transfer/crushing
It is envisaged that up to 80% of the molten ferrochrome could be tapped into ladles, ready for liquid transfer to the Baosteel Descheng Stainless Steel Plant while the balance would be cooled into ferrochrome ingots. The ingots will be processed at the crushing plant, after which the ferrochrome will be ready for dispatch to customers of the Group.
Processing flow chart
