| High-phosphorus oolitic hematite cannot be beneficiated by conventional mineral processing methods since its unique internal structure. Recent researches showed that Direct Reduction Iron (DRI) with low content of phosphorus can be obtained from high-phosphorus oolitic hematite ore by coal-based direct reduction-magnetic separation technology. However, most of these studies were conducted with the powder of the iron ore, coal, and additives while most of the roasting equipments for coal based direct reduction technologies charge pellet or lump ore. Therefore, it’s necessary to study the properties of the high-phosphorus oolitic hematite-coal composite briquette.In the present study, the High-phosphorus oolitic hematite ore obtained from Western Hubei Province was briquetted with coal and additives, and then the composite briquette was submitted to reduction roasting followed by magnetic separation. The Ca(OH)2and Na2CO3were used as additives in the reduction roasting. The effects of types and dosages of additives, coal dosages, roasting temperature and roasting time were investigated. It has been observed that DRI with93.28%Fe, and0.07%P can be obtained with a recovery of92.30%by addition of15%Ca(OH)2and3%Na2CO3.The mixtures of iron ore, coal, and additives were briquetted by a twin-roller briquette machine, and the strength of briquette and its performance at high temperature were studied. The experimental results showed that the drop strength and compress strength of the composite briquettes prepared without additional binder were of4.4and152.8N per briquette due to the carbonate bonding of additives. The strength of the composite briquettes can be increased by increasing the pressure of the briquette machine or decreasing the particle sizes of iron ore and coal. The reduction-magnetic separation results showed that the iron recovery and iron grade of DRI increased with decreasing of particle size of iron ore, however the phosphorus content of DRI increased sharply when the particle size of iron ore is too small (-0.1mm). On this basis, the addition of Na-CMC, molasses, sodium silicate, bentonite, and starch can improve the strength of the composite briquette. However, the compressive strengths of all of those composite briquettes were decreased sharply during the initial reduction process due to the decomposition of the binder, the gasification of coal, the transportation of by-product gases, the volume expansion with phase transition from Fe2O3to Fe3O4and the thermal stresses. Additionally, when anthracite was used as reductant, the composite briquettes swelled catastrophically when reduced at1200℃. This phenomenon was caused by the formation of large amounts of low-melting materials at the initial stage of reduction which hindered the transportation of by-product gases. This catastrophic swelling can be eliminated by decreasing the particle size of coal.Thermodynamic Software FactSage, X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were employed to investigate the mechanism of direct reduction-magnetic of composite briquette. It showed that the addition of15%Ca (OH)2and3%Na2CO3not only inhibit the reduction of fluorapatite but also promote the growth of metallic iron grains by forming a certain amount of liquid slag. The reduction of fluorapatite in the iron ore was activated when using-0.1mm iron ore, which resulted in the increase of P content of DRI. The reduction of iron oxides can be enhanced by using the coal with higher reactivity or decreasing the particle size of coal, as a result the iron recovery increased. On the other hand, the iron grade of DRI decreased since smaller amount of low-melting substances was formed which hindered the growth of metallic iron grains. The effects of type and particle size of coal on the P content of DRI were complex. |
PDF Full Text Request