INVESTIGATION OF HYDROGEN REDUCTION FOR LOW-GRADE BANDED IRON ORE

Abstract

The present study investigates the utilization of low-grade iron ores for iron recovery by using hydrogen as reducing agent. With the growing demands of steel and with the depletion of high grade iron ore reserves, the need for processing of alternative iron resources is inevitable. The low grade iron ores are one of the most readily available alternate iron ore reserves. The beneficiation process of low grade iron ores is quite different from the high grade ores, mainly due the presence of complex mineralogical association between iron values and impurities. The current study investigates hydrogen reduction of low-grade iron ore (~37% Fe) containing dispersed bands of hematite and jasper quartzite phases. Detailed investigation about the effect of time, temperature, particle size and gas flow rate is carried to optimise the reduction process for recovery of iron values. Hydrogen was used as reductant mainly to make the process environmentally friendly by reducing GHG emissions produced during conventional carbothermal iron and steel making. Preliminary experiments were carried out on ore fines (-150µm) to understand the effect of time and temperature on process responses like: DOR, DOM, FeM % and saturation magnetization values of the magnetic concentrate. Complete reduction of hematite was achieved in 2 h at 600C however the separation of quartz from the reduced ferrite was inefficient. Increasing temperature above 600 °C had minimal effect on DOR and DOM, however the product purity improved significantly with increasing temperatures. The association of impurities with hematite does not effect the reduction rate however, the trapped quartz particles in the reduced phases deteriorates the magnetic separation efficiency. The reduction temperature (300-600 °C), time (30-90 min), particle size (0.5-3.3 mm) and hydrogen flow rate (0.5-1.5 LPM) were optimized by statistical design of experiments to maximize the saturation magnetization, degree of reduction and metallization. The impact sequence of the experimental parameters was found out to be: Temperature >> time > particle size > flow rate. Temperature and time synergizes the reduction rate; improving reduction and metallization degree meanwhile, flow rate and particle size had a minimal effect. The ferrite formation was observed at temperatures above 450 C and below this temperature the reduction was restricted to magnetite. Under the optimal conditions; 600 ºC, 60 min, 0.5 LPM H2 and 3.32 mm particle size, the ferrite concentrate possessing saturation magnetization of 134.6 emu/g (68% FeM) , 97.9% DOR, 88.8% DOM and 47.3% yield was obtained. At higher temperatures the reduction of hematite results in the formation of micro-cracks and porous ferrite morphology.