Lowering the cost of iron produced by direct reduction

Abstract

In order to reduce the cost of hot metal, the operation of a system consisting of a roasting machine and a blast furnace was investigated in [1]. Approximate estimates indicate the effectiveness of hot pellets ‐ in particular, partially reduced pellets—in blast-furnace batch. However, there are fundamental difficulties in the operation of this system: the need to develop a hightemperature charging unit; the reduced effectiveness of hot pellets with cold sinter and coke; the need to separate the bottom component from the partially reduced pellets; and the different annual operating time of the blast furnace (360 days) and the roasting machine (330 days). In the present work, we investigate means of eliminating these problems. The basic goals are as follows. First, we need a reducing unit that operates on uniform batch—perhaps a unit for direct iron production based on natural-gas reforming or on solid fuel. Second, in the production of partially reduced pellets, it is expedient to use a technology without isolation and charging of the bottom component. The benefit of these methods is that they reduce the demand for natural gas, whose cost is steadily increasing. Thus, our analysis is based on two concepts: the use of hot oxidized pellets (more precisely, pellets that are not cooled in the roasting unit); and the use of partially reduced pellets. We now consider several systems that combine the production of raw material in pellet form and its subsequent coke-free processing. 1. A natural-gas system that includes a grating‐ tubular-furnace combination (with no annular cooler) and a Midrex shaft furnace. (Oxidized pellets at 850 ° C are employed.) One problem here is the considerable temperature difference (up to 100 ° C) between the batch at the center and periphery of the reducing zone. This is addressed by introducing oxy-burners, which increase the furnace productivity by 5‐7%. The benefits of charging hot pellets in the shaft furnace are as follows: reduced capital expenditures, thanks to the lack of an annular cooler; reduced likelihood of sinter formation in the production of partially reduced pellets in a rotary furnace; and higher productivity of the Midrex unit on account of increase in the reduction zone and equalization of the corresponding temperature field. This system necessitates the application of a protective coating to the pellets before charging in the reducing furnace. One possibility is to apply a coating to the raw pellets prior to roasting, but a detailed design workup is required here. 2. Combined operation of a grating‐tubular-furnace unit and a Corex smelter. The batch for pellet production contains solid fuel—for example, coal. After treatment in a rotary furnace, the degree of reduction of the pellets is 30‐50%; they are charged at 800‐850 ° C in the preliminary-reduction reactor of the Corex unit (Fig. 1).