BENEFICIATION STUDIES OF BANDED IRON ORES

Abstract

The present work investigates the utilization/beneficiation of low-grade iron ores to meet the steel vision 2030 of 300MT steel production. Due to limited rate of crude steel production and depletion of high-grade iron ores, processing of alternative iron resources such as low-grade iron ores is inevitable. The low-grade iron ores do not respond to conventional beneficiation techniques because of their complex chemical association. In this study detailed study of low-grade iron ores i.e., banded iron ores were carried out such as mineral identification, characterization studies, magnetic property studies, and wet chemical analysis to develop suitable beneficiation process. The processing of banded iron ores include first part deals with comminution studies (breakage rate characteristics through ball mill /Compression mode breakage) and second part deals with up-gradation (iron grade, yield, and iron recovery) of banded iron ores. In comminution studies, BHJ ore was found harder than BHQ and BMQ. Thermal treatment of banded iron ores causes massive random micro-cracks which enhances breakage kinetics. Compression testing of banded iron ores yielded finer size reduction than ball mill due to induced microcracks. But the introduction of compression comminution could not lead to any improvement in the concentrate through the downstream magnetic separation process. The up-gradation of banded iron ores included physical separation, microwave exposure, and carbothermal reduction (conventional/microwave). It was found that BHJ/BMQ can be upgraded and the concentrate can be used as feed for pellet making for blast furnace through microwave processing. The microwave exposed BMQ, produced a concentrate under optimal condition with 60.1% Fe grade, 99% Fe recovery at 71% yield in a single step. Whereas microwave exposed BHJ under optimal conditions, yielded an iron- concentrate with 61% Fe, 85% Fe recovery at 50% yield and BHQ yielded concentrate with FeG of 56.30%, FeR of 50.68% and a yield of ~27%. Effective conversion of the hematite phase to magnetite phase was observed in conventional carbothermal reduction process which helped ineffective separation of iron values from gangue. Carbothermal reduction was evaluated using Box Behnken statistical design and was found effective for iron enrichment. Under optimal condition, carbothermal reduction of BHQ produced a concentrate of FeG of 57.6%, FeR of 68% and a yield of 35.7%. Where as in BHJ concentrate enriched to FeG of 61.2%, FeR of 82% and a yield of 52% and coarser particle size produced the fused fraction consisting of ̴85%Fe with retained austenite and martensitic iron phases. In microwave carbothermal reduction the formation of spheroidal ferrite ball was observed within 10 min with the addition of a minimal amount of charcoal dosage at a microwave power of 900 W. Microwave carbothermal reduction of BHJ under optimal condition yielded a magnetic vi concentrate having ~61.6% Fe, ~73.4% recovery at a yield of 44% and can be used for blast furnace feedstock. Similarly, other experimental condition yielded a magnetic concentrate with ~49.1% Fe, ~89.3% recovery at a yield of 59.7% with significant amount of ferrite balls. The microwave carbothermal reduction of BHQ yielded FeG of 57.6%, FeR of 47% and a yield of 24% at optimum condition and other optimal condition yielded a less grade with a significant amount of ferrite balls. It was found that a small fraction of microwave irradiated ore-charcoal mixture was rapidly melted to produce pure ferrite balls with Fe ~93 %. The iron grade deteriorated at prolonged exposure due to the formation of fayalite phase. The optical micrograph of the ferrite ball reveals the presence of retained austenite and martensitic iron phase with a saturation magnetization of ~200 emu/g.