engineering asPects – Furnaces LOW COST FERROALLOY EXTRACTION IN DC-ARC FURNACE AT MIDDLEBURG FERROCHROME

Middleburg Ferro-Chrome (MFC), a company in Samancor Chrome, commissioned a new Ferro-Chrome smelter in March 2009. This is a 60 MW DC-arc furnace with a DC power supply for the smelting of Transvaal chromite ore fines. With the benefit of 12 years experience with this technology while operating a 44 MW DC-arc furnace, MFC mitigated all known challenges and considered up scaling risk in the new plant, which had a rapid ramp up to full power in Q2 of 2009. Good operating practices, as made possible by robust equipment selection, including a thyristor rectifier set of proven technology, and operation with optimal slag chemistry, leads to a safe and effective plant operation. With the optimized furnace design and a correctly sized power conditioning system, using only standard (static) harmonic filters and a correctly sized smoothing reactor, the arc furnace is not a ‘dirty load ’ for the utility, staying well within IEC limits for power quality. A patented Arc Compensation system is installed and allows for full arc position control (to compensate for magnetic field induced arc deflection) so that a symmetrical crucible is maintained, significantly benefitting refractory life. The principle benefits gained from the MFC process is the fact that ore fines are processed, without metallurgical coke, and higher chrome recoveries are achieved compared to other smelting technologies when using a DC open arc for smelting.