Mineral Transformation Law And Mechanism In Coal-based Direct Reduction Process Of Low-grade Limonite

Limonite ore is one of the important sources of iron ore,but the utilization efficiency of limonite ore is low at present.The iron grade of Tiekeng iron ore in Xinyu is 38.25%,and the iron ore is mainly limonite with complex impenetrable relationship and easy to slime in grinding,which belongs to low-grade refractory iron ore.At present,the whole magnetic process is adopted to recover limonite in Tiekeng iron ore,the grade of iron concentrate is only 50.45%,the recovery rate is only about 50%,thus the resource waste is serious.In order to utilize the ore efficiently,coal-based direct reduction-magnetic separation technology was employed to treat the ore to prepare direct reduced iron.The influence of roasting factors such as the type and amount of reducing agent,roasting temperature,roasting time,and additive types on the index of direct reduced iron(DRI)was studied.The results show that appropriately increasing the amount of reducing agent,increasing the roasting temperature and prolonging the roasting time can all improve the index of DRI in the absence of additives.The best roasting conditions are bituminous dosage of 20%,roasting temperature of 1200 ?,and roasting time of 30 min.As a result,the best index of DRI with iron grade of 85.77%and recovery of 85.87%was obtained,which is unsatisfactory.Adding proper amount of calcium carbonate,sodium carbonate and fluorite can improve the index of DRI,and fluorite had the best performance.When 2%fluorite was added,a DRI with iron grade of 93.46%and iron recovery of 92.88%was obtained,which lays the foundation for the efficient use of the ore.The quantitative conversion law of minerals during the coal-based direct reduction process of limonite was studied by MLA combined with the calculation by the thermodynamic software of FactSage.The results show that in the absence of additives,part of FeO reacts with quartz to form fayalite,and the amount of liquid in the roasting system is low which makes it difficult for the growth of iron particles.This is the main reason for the poor index of DRI.The addition of calcium carbonate,sodium carbonate and fluorite have great influence on the distribution of Fe in iron phase and the size of the iron particles in the calcined products.When the dosage of calcium carbonate was 10%,the distribution of iron in iron phase reached 92.41%,and the D50 of the iron particle was 106 ?m.When 1%sodium carbonate was added,the proportion of iron in iron phase increases to 92%,and the D50 value of iron particles increases to 68 ?m.When 2%fluorite was added,the proportion of iron in iron phase increases to 93.02%,and the D50 value of the iron particles is 106 ?m.The results of the growth kinetics of iron particles show that the order of growth rate constant(K0)of iron with different additives is fluorite>calcium carbonate>sodium carbonate>no additives.The K0 value of adding fluorite is always the maximum value,which means that the growth rate of iron particles is the fastest by adding fluorite,so fluorite has the best performance on promoting the index of DRI.