Utilization of low grade chromite ore fines to recover chromium as charge-chrome

Abstract

Charge chrome is normally produced from lump chromite ores in the electric arc furnace. Majority of the lump ore is very lean (~30% Cr) in nature. Production of a better quality ferro-chrome requires using high-grade chromite ores; but these are fragile and tend to form fines during handling. These fines may flue off from electric arc furnace during charging; therefore agglomeration of these fines is needed before charging in electric arc furnace. Normal chromite agglomeration processes include briquetting, pelletizing or sintering. Presently many ferro-chrome plants make pellets utilizing the friable chromite ore fines by grinding it to suitable finer size (say 60% < 45 μm) and pelletize them. This is followed by the firing of the green pellets at 1250 - 1350 C in a rotary kiln and their smelting in a submerged arc furnace (SAF) to make the ferrochrome. However, pelletizing is only suitable for vary fine feed that requires energy intensive grinding, expensive binders, and energy-intensive induration of pellets using liquid or gaseous fuel. Overall, the pelletization process is very complex and cost-intensive. In some plants, briquetting is also considered as an alternative agglomeration technique with the advantages of being a cold process and environment friendly. However, briquettes have inferior high temperature performance to pellets and sinter and the process involves with costly binders. To alleviate the above draw backs of pelletisation and briquetting, present study aims at producing sinter directly from the available chromite ore fines(-10 mm) without any grinding in presence of coke breeze ( like iron ore sintering). Sintering was carried out in 10 kg scale in a pot sinter. Result indicated that a good amount of (18-20%) molten phase formed for bonding. Sintering time was 25-30 min and maximum flame temperature reached greater than 1600 C in pot sinter. It decreased the spinel phases of prefused chromite fines which will reduce faster in subsequent electric arc furnace smelting. The molten phases comprised of Fe2SiO4, Ca2Al2SiO7, MgCa2Si2O7 and the unmelted phases was (Mg, Fe)(Cr, Al)2O4 as evident from the XRD study. Chemical composition of produced chromite sinter was 36% Cr2O3, 20% FeO, 8% SiO2, 9% MgO, 11% Al2O3, 6% CaO & others. Sintered chromite mass was next smelted in graphite lined 50 KVA electric arc furnace at 1600 – 1800 oC with coke and fluxes. Charge-chrome of composition 50-54%Cr, 30-35% Fe, 7-8%C, 1.5% Si was produced with chromium recovery above 90%.