Study On The Mechanism Of Shaft Furnace Direct Reduction Of Oxidized Pellet By Microwave Heating

Microwave heating relies on the material's dielectric loss in the microwave field to generate thermal effects, with the advantages of selective heating, uniform heating, fast and clean. Given to the problems of long reaction time and uneven heat mass transfer in traditional direct reduction, the coal-based shaft furnace direct reduction of oxide pellets by microwave heating was developed. But the mechanism of reduction by microwave heating is not definitely, in this paper the reduction results and microstructure evolution behavior of the oxide pellet by microwave heating and conventional heating were compared, and the mechanism of microwave on direct reduction was discussed.Temperature rising characteristic of raw materials with small industrial microwave oven have been studied, researches show that, temperature rising rate of raw materials increases with the rise of microwave power, when the microwave power is 1300w, the average temperature rising rate of oxide pellet is 0.32℃/s, that of coke is 0.38℃/s. Heating rate of reduction materials (oxide pellets and coke mixed by C/Fe is 0.354) in the microwave shaft furnace is rapid, when microwave power is 10.5kw, the average temperature rising rate is 0.21℃/s.Effects of direct reduction by microwave heating and conventional heating are compared, the results show that:the reduction time can be shortened by microwave heating, when reduction temperature is 1050℃, it needs 120min for the metallization of oxide pellets reaches approximate 80% by conventional heating, while to get the same level it only needs 74min by microwave heating. Micro-hardness of metallic iron particles in the above two processes are compared with Micro-Vickers hardness tester, micro-hardness of metallic iron particles by microwave heating is 169.73kg/mm2, is much larger than that of conventional heating (126.64kg/mm2). Microstructure of pellets for the above two processes of direct reduction are also compared with scanning electron microscope, the characters in microwave heating process is:as the reduction rate is very fast, in the initial stage, the iron oxides turn from the original plate into "dendritic structure". Due to the coke in the furnace become thinner, the pressure in the furnace increases, the carbon gasification rate slows down, wustite appears "turbid bald structure", fayalite appears liquid phase, and warp parts of the wustite, inhibiting the reduction of wustite, reducing the rate of the reaction, internal microstructure of pellets appear obvious stratification. When metallization is more than 60%, the internal microstructure of pellets from outside to inside is divided into "metallic iron layer", "hoop slag layer " and "wustite layer".In order to enhance direct reduction of oxide pellets in microwave shaft furnace, inserting a diameter of a 20mm, length of 3m stainless steel tube into slow cooling section from the roof of the furnace, so that the excess gas in the furnace released in time, the pressure in the furnace decreased, which accelerating the reaction rate of the carbon gasification, thereby speeding up the rate of the entire reduction reaction. After the enhancement of direct reduction in microwave shaft furnace, the pellets will not occur "round spherical" wustite, the content of fayalite is significantly reduced, reduction efforts are significantly improved. Microwave irradiation can promote the migration, aggregation, growth of the metallic iron and wustite, and speed up the reduction of wustite and gasification of carbon. Results of microwave enhanced pellets direct reduction are compared with that of conventional heating, researches show that:at the temperature of 1050℃, the metallization of oxide pellets reaches 92.67%in 65min by microwave heating, while the time needed by conventional heating is about 150min; compressive strength of pellets can reach 1445N/a pellet, which is nearly 2 times as that of conventional heating (742N/a pellet); micro-hardness of metallic iron particles is 197.39kg/mm2, which is nearly 1.5 times as that of the conventional heating(136.84kg/mm2).