Fundamental And Technical Research Of Iron Ore Pellets Coal-Based Direct Reduction By Microwave Heating

The traditional iron and steel production has the disadvantages of long process, great investment, high energy consumption and serious pollution issues. Further more, coking coal is increasingly exhausted. Therefore, the development of a new process, which makes the iron making process replacement of coke by coal, energy saving, environment protecting, high-efficiency and compact, has important strategic meanings. In this research, iron ore pellets and anthracite are taken as the objects to investigate the fundamental and technical research of pellets for coal-based direct reduction by microwave heating. The main innovations and conclusions obtained are as follows:The electromagnetic properties of iron ore pellets and coal, as well as the temperature rising characteristics of microwave heating were studied. The descending order of temperature rising rate of briquettes is Fe3O4>FeO≈Fe2O3≈oxide pellets>Fe. Phase and microstructure of iron ore pellets are changed by microwave heating, activity of iron oxide increases and reduction reaction is promoted. In microwave field, due to the strong absorption ability, anthracite has the dual role of both reducing agent and heating element in microwave field and gasification rate of fixed carbon is increased.The microstructure and physical phase of iron ore pellets during microwave heating coal-based direct reduction process were researched detailedly by using advanced testing methods, such as optical microscope, SEM, and XRD. It is found that fragmentations appear under the action of microwave and the reduction reaction of iron oxide is promoted. Inner stress is caused due to change of crystal lattice. And the volume of pellet expands. The expansion volume peaks when the reduction degree and temperature is43.06%and850℃and the lowest value of pellet compressive strength is320N/P. Metallic iron increases when the pellets are further reduced; followed with crystal grains migrate, gather and grow. The compressive strength of metallized pellets with a reduction degree of97%is1035N/P after50min of heating at 1050℃。The restrictive step of coal-based direct reduction of iron ore pellets under the action of microwave is revealed. Microwave heating coal-based direct reduction of oxide pellets is limited by internal diffusion of CO. When the reducing atmosphere is insufficient, FeO with an irregularly perfectly round is formed and grows since iron ore grain is not reduced rapidly after cracking. Lots of FeSiO3forms with the existence of SiO2, and reduction reaction is resisted.A new iron making process using pre-reduced iron ore pellets by microwave heating has been investigated. The pre-reduced pellet has loose structure, developed pore, small volumetric expansion and high compressive strength. The reduction of center of pre-reduced pellets with lower reduction degree (metallization rate<40%) is limited by the formation of metallic iron shell. The reduction of pre-reduced pellets with higher reduction degree (metallization rate>40%) is promoted because of the higher CO/CO2diffusion rate. The reduction degree and metallization rate are94.29%and88.23%when118g pre-reduced pellets with a reduction degree of66.90%are reduced at1050℃and the power of microwave is1.3kW and ratio of carbon/pellet is0.22.The study of microwave heating coal-based direct reduction of iron ore pellets was put forward using a φ120mm pilot-sized continuous microwave shaft furnace. The results indicate that the metallization rate and productivity of oxidized pellets is95.25%and4.08kg/h, respectively after65min of reduction at1050℃; the metallization rate and productivity of pre-reducted pellets with a metallization rate of42.85%is97.68%and6.33kg/h, respectively with41min of reduction at1050℃. The reduction time is shortened by36.92%and the productivity increased by55.15%.