Kok ve linyit kömürü tozu içeren manyetit peletlerinin redüksiyon davranışı

Abstract

Use of prereduced pellet in the production of steel is increasing rapidly. As blast furnace feed, it offers much scope for improving both productivity and economy of coke consumption. By using the prereduced pellet, an increase in production of at least 25-30% can be obtained in the existing blast furnace. In electric arc furnaces, prereduced pellets have proved an adequate substitute for steel scrap. The term Composite pellet describes a pellet consisting of a mixture of fines iron oxide and carbonaceous material ( coal, coke, char). The increased demand of raw material for the iron and steel industry has resulted in mechanization of mining operations, leading to increased production of fines. These fines cannot be charged directly into blast furnace. The pellet should have sufficient strength to withstand high temperature and stresses during reduction in a furnace. For great utilization of low grade ores, benefication after crushing and grinding has become an essential step. Such operation yield concentrate in a finer form. The fines produced during sizing of coke are also not utilized in the blast furnace. Composite pellets have some advantages given below; Use of noncoking coal as reductant, Coking operations are costly and environmentally problematic. In the process of composite pellets at which noncoking coal can be used these deleterious effects can be decreased. Utilization of fines without high-temperature burden preparation, such as sintering, heat hardening of pellets, and cokemaking. Composite prereduced pellets are produced by single stage firing, where sintering of fine particles as well as prereduction takes place. The production cost of composite pellets is lower due to the single stage firing. Whereas pellets which has been used in industry firing and reduction are carried out in two stage. Coking coal reserve in all over the world has been gradually decreased. Unfortunately our country has not rich coking coal mines. On the other hand, significant amount of lignite reserves are present. By developing the composite pellet process the use of coke fines and lignite have been possible. In this study, the reduction of magnetite pellets containing solid carbon was investigated. Fine coke powder (-200 mesh ) and lignite ( -200 mesh) were used as the solid reductants and also magnetite as iron ore, Magnetite - coal ( coke powder or lignite) mixed pellets were reduced isothermal condition 900, 1000 and 1100°C under constant flow rate of nitrogen gas. The effects of Magnetite/Carbon ratio, and heating time on the extent of the reduction of pellets have been studied. The other series of experiments was also carried out at 900, 1000 and 1100'C with coke and lignite in order to examine the effects of reductant type and reduction time on degree of reduction. 75% of the magnetite ore was under 325 mesh. In order to obtain the desired Ccn^/FeaCU ratio, magnetite ore was mixed thoroughly with coal fines of - 200 mesh in a ceramic container. Then 1% bentonite and 10% distilled water were added to the mixture and mixed thoroughly. vi The chemical composition of the magnetite ore is given on table below; Rııhatances Total Fe 69.50 Si02 0.90 S 0.49 P 0.008 Na20 0.054 K20 0.091 MgO 0. 76 CaO 0.06 Zn 0.008 A1203 0.64 The following table shows the chemical composition of the coke and the lignite used in eperiments.-. Coke Lignite (%) 41.00 10.25 47.75 1.00 By using these mixtures, pellets having a diameter of 1.1 cm and a height of 1.1 cm were obtained. Reduction experiments were carried out in a tube furnace under isothermal conditions at three different temperatures ranging from 900 to 1100 °C for 5, 10. 20, 40 and 60 minutes. The reduction degrees of the pellets are calculated according to formula below; Degree of reduction = weight of oxygen removed from iron ore / weight of removable oxygen VI i Weight of oxygen removed = total weight loss - weight loss due to VM of coal -weight loss due to combustion gasification of fixed carbon Weight loss due to gasif ication= total weight of fixed of carbon carbon in pellet- weight of carbon left in pellet after reduction The results obtained from the experiments are given below; - There is an increase of the rate of reduction of iron oxide with increasing temperature, owing to the higher rate of reaction between iron oxide and carbon or carbonmonoxide produced during reduction. As the temperature increases for a given time and a C c«i9t>/Fe30*» ratio, a remarkable increase was observed at the degree of reduction. - For each C< c±x-> /Fe3CU ratio, the graphics plotted ^Reduction versus time showed that the curves remains constant at a specific time(40 min.). It can be thought that at the initial stages of the reaction, there is a high reaction rate caused from the pyrolysis of the remaining volatile matter in the coal. As the reduction time increases the reduction rate almost remains constant, it has been thought that the decreased diffusion rate of CO and C02 in to and out of the pellets caused from the forming of the dense ( having less porosity) layer of metallic iron as a result of the reduction. - The effect of C<ırı»e> /Fe3CU ratio on the reduction degree was also studied by using three different temperatures in the range of 900-1100 °C for three different ratios ranging from 0.2 to 0.5. With increasing C/Fe304 ratio, an increament in reduction degree was observed. However, further increase in C/Fe3CU ratio had no significant effect on the reduction degree. This is because the forming of the dense layer of metallic iron prevents the gas diffusion. Vlll For example, when the Ctn^o/FesCU ratio was increased from 0.3 to 0.4 at 1000'C for 20 min., the reduction degree of %20 was obtained. Under the conditions, on the other hand, increasing the Ccrijo/FeaCU from 0.4 to 0.5 caused in a reduction degree of 6%. - The effects of reductant type on reduction degree were also investigated by using lignite and coke powder as reductant at different temperatures for time varying from 0 to 60 min. It can be seen from the experiments that the highest degree of reduction was obtained using lignite and lower ones by using coke. The maximum degree of reduction observed is 91% at 1100 "C for 60 min. and the ratio of C(n»o/Fe304=0.5. By using coke under the same conditions, the degree of reduction obtained is %86. When we use lignite as the reducing agent, pellet disintegration was observed. The pellets containing coke as the reducing agent did not show any sign of disintegration after reduction.