Insight of iron ore-coal composite reduction in a pilot scale rotary kiln: A post-mortem study

For complex mineral compositions and ultra-fine grain embedded structure, beneficiation of iron from low-grade ore resources has been a realistic problem. In this work, an innovative technology named rapid but low-temperature direct reduction followed by magnetic separation was developed to process this kind of refractory iron ore. Ore-coal composite prepared by a typical low-grade iron ore (silica: 41.81%; iron: 31.24%) was reduced by lignite at 960 °C for 35 min, and crud iron powder assaying 83.10% of total iron can be obtained with recovery of 81.48% by subsequent magnetic separation based on laboratory tests. However, there is no precedent for an industrial reduction method to process either low-grade ore or ore-coal composites. Thus, a pilot scale rotary kiln (φ1.5 m × 15 m) reduction test was carried out for months to explore a possible approach for processing this type of iron ore. Reduced samples with a metallization ratio of 85.15% were obtained under stable operating conditions using this method. A post-mortem study was then carried out to better understand and optimize this new reduction process. The results demonstrated that the minerals evolution course during rotary kiln reduction can be summarized as follows: hematite was reduced to magnetite, wustite, and therefore partial quartz transformed to fayalite as materials were preheated within 15–11.5 m position → reduction occurred rapidly in 11.5–5.5 m zone, magnetite and wustite were reduced to iron, fayalite transformed to iron and amorphous silica → migration and growth of metallic iron particles in the length of 4.5–0 m. Rapid reduction at low temperature lessened the fayalite formation, creating favorable beneficiation conditions for iron element.