IN SITU DRYING AND PREHEATING INCLUDING GASEOUS PRE-REDUCTION OF CHROMITE ORE INSIDE CHARGING CHUTE OF SUBMERGED ARC FURNACE IN FERRO-CHROME PRODUCTION PROCESS

摘要

Gaseous pre-reduction of preheated chromite ore inside charging chutes of submerged Arc Furnace (SAF) followed by low temperature dissociation of carbon monoxide (co) gas catalysed by the freshly reduced iron leading to carburisation of the later and solid state reduction of chromite takes place in the production of Ferro chrome. The above objects are met by drawing the off-gas from a closed type SAF through its charging chutes by means of a high suction hot gas fan so that the outgoing high temperature gas, which is rich in carbon monoxide content; ( 80% CO.), comes in intimate contact with the incoming preheated charge material at low temperatures (300-500 deg.C). The iron oxide part of the chromite ore is pre-reduced by the CO gas, yielding lower oxides of iron as well as metallic iron. Further, the metallic iron acts as catalyst for the low temperature dissociation of CO gas, yielding finely divided carbon and carbon dioxide gas. The intimately distributed carbon combines with the fresh metallic iron to form iron carbide, which subsequently acts as an efficient reductant for the chromite ores at high temperatures.For the drying and preheating of the charge materials, the gas drawn above is combusted in a combustion chamber by preheated air supplied by an air blower in stoichiometric ratio. The gas drawn from the moving bed is initially cleaned in dry condition by means of an electrostatic precipitator. An online heat-exchanger is provided to reduce the gas temperature in order to increase efficiency of the hot gas fan and reduce its capital cost. The hot flue gases emanating from the combustion chamber are split into two streams. One part is used to dry the incoming feed materials in counter-current direction of the flows to maximize the heat and mass transfers. The other part is used to preheat the descending charge materials inside the charging tube in parallel flow mode to minimize the carbon gasification, which is quite common in case of a counter flow heat exchanger deployed currently for the purpose.