Improving the performance of the HYL-III unit at OAO Lebedinskii GOK

Abstract

The cost of reduced pellets is associated with the quality and productivity of the shaft furnaces, which, in turn, depend on the properties of the initial roasted pellets—their structure and the type of binder. Methods for enhancing the quality of the roasted pellets used in shaft reduction furnaces were discussed in [1‐5]. The most interesting are associated with increase in the softening temperature of the binder, the hot strength, and the reducing properties [6‐8]. This is also associated with increase in temperature in the reduction zone, which provides the basis for the introduction of a technology such as oxygen injection in the shaft furnaces. The HYL-III unit at OAO Lebedinskii GOK has some scope for increase in the reducing-gas temperature, but its utilization is not always possible. Thus, increasing the temperature in the reducing zone above 855 ° C usually disrupts batch descent and discharge of the reduced pellets, on account of pellet deformation. The prevention of such disruptions, which are undesirable and in some cases dangerous, is mainly associated with the properties of the oxidized pellets: first, their degree of oxidation (their FeO content), which is determined by the heat treatment of the batch layer on the conveyer-type roasting machine and significantly influences the reduction process; and, second, the structure of the pellets and their binder, which must be difficult to melt and to reduce. The degree of pellet oxidization determines the minimum deformation of the reduced pellets under load, while the structure of the pellet binder determines how great a role it will play in low-temperature softening of the iron oxides. The effectiveness of this approach was confirmed in the startup of the HYL-III unit in 2001, using pellets with added bauxite. However, with the existing composition of the pelletization batch, whose use has allowed the unit to exceed its design parameters, it is impossible to increase the temperature in the reducing zone above 845‐847 ° C, on account of the high plasticity of the reduced pellets. Therefore, increasing the productivity of the HYL-III unit by further temperature rise in the reducing zone calls for two approaches: modification of the pelletization-batch composition, which permits the formation of stronger and more refractory binder; and optimization of the heat-treatment conditions, so as to form such binder and increase pellet oxidation. In the present work, we investigate these approaches. The role of the rock composition in forming the metallurgical properties of the pellets was outlined in detail in [1, 6‐8]. It is found that an effective means of optimizing the structure and metallurgical properties of the pellets for blast-furnace smelting and reduction is to modify the batch with additives that increase the melting point of the binder and facilitate the formation of more open porosity. The existing binder, corresponding to the Fe 2 O 3 ‐Al 2 O 3 ‐CaO‐SiO 2 system, is created by introducing bauxite as an additive in palletizing the concentrate [6‐8]. Using bauxite increases not only the pellet strength on reduction but also the oxidation rate on roasting and the rate of reduction. To increase the strength of the pellets on reduction, their structure and the binder properties are further modified by means of magnesium-bearing additives, to form the CaO‐MgO‐SiO 2 and MgO‐Al 2 O 3 ‐SiO 2 systems. In laboratory research, the batch for pellet production is modified not with chalk but with Mg-bearing serpentinite‐magnesite, of the following chemical composition: 3.76% Fe tot ; 0.99% FeO; 4.28% Fe 2 O 3 , 27.5% SiO 2 , 0.42% Al 2 O 3 , 5.69% CaO, 32.38% MgO, 0.012% TiO 2 , 0.172% MnO, 0.008% S, 3.21% C, 0.018% P 2 O 5 , 0.004% K 2 O, 0.016% Na 2 O, 22.28% calcination losses. In the laboratory apparatus, to obtain the masstransfer characteristics, characteristics such as the rate constant of pellet reduction at 900 ° C and the strength of pellets partially (up to ~30%) reduced at this temperature are determined. The influence of the batch composition on the shrinkage, sintering properties, and degree