Chromite and Ferrochrome Processing

Open pit mining operations

Most chromite extraction is through open pit mining operations. As with any open pit operation inflows of water have to be managed and controlled, and overburden, waste rock and the ore have to be removed. Then ore is then crushed to release the chromite. Usually a “jaw crusher” is employed. Initial processing of chromite sorts ore into “lumpy” ores and fines, and uses heavy media or gravity separation of finer ores to remove gangue or waste materials and to produce upgraded ores or concentrates. Magnetic separation and froth flotation techniques have also been applied in some cases. “Lumpy ore” requires little beneficiation and is effectively quarried. In order to be considered “lumpy ore”, the chromite must have a grain over 6mm, so that it can be fed directly to the ferroalloy smelter.

The Big Daddy Deposit in the Ring of Fire may be as much as 100% lumpy ore. Micon estimated that the Big Daddy Massive Chromite Domain contains an indicated resource of 16.3 million tonnes averaging 40.66% chromium oxide (Cr2O3), and an additional inferred resource averaging 39.09% Cr2O3. By comparison, Outkumpu’s Kemi Mine in Finland has ore reserves of 41.1 million tonnes averaging 24.5% Cr2O3. It should be noted that indicated and inferred resources have not been analyzed for economic viability.

Converting to Ferrochromium

Before the chromite can be used for steel making, it has to be converted to ferrochromium. The International Chromium Development Association provides a description of the process of producing ferrochromium alloys from chromite. (see a further description in Appendix A) Chrome ore in various sizes is typically charged into a submerged AC electric arc furnace and reductants (coke, coal and quartzite) are added. The smelting process is energy intensive, requiring up to 4,000 kWh per tonne of material. Slag is separated from the liquid ferrochrome and tapped into ladles for further processing. Liquid ferrochrome is then poured into moulds and after cooling crushed into sizes as required by the customers. Crushed ferrochrome is railed to final customers or harbours for shipment.