Direct Reduction Units
These units constitute one of the intermediate parts of the process of steelmaking, and thus they are very important. In these units, reduction gas is used to reduce iron ore pellets into sponge iron. Then, the resulting sponge iron is sent to the electric arc furnaces in the steelmaking section to charge these furnaces South Kaveh Steel Company has two direct reduction units based on the on the MIDREX process, with a total annual capacity of 1.8 million tons. The operations of these units started in 2011 and 2012. In the MIDREX process, the iron ore pellets produced in the pellet-making units will be sieved and coated by lime, and then they are sent using a conveyor belt to the feeding storage unit on top of the furnace. Then, they are distributed into the direct reduction furnace in a smooth manner. The reduction furnace is a cylindrical furnace with two sections, i.e., the top and bottom sections.
The top section of the furnace is the main reduction area, where the pellets are reduced using a reduction gas stream consisting of hydrogen and carbon monoxide. The reduction gas produced in the reformer will be injected into the top section of the furnace with a temperature ranging from 780 to 870 degrees Celsius depending on the production rate and the type of the pellet. While moving upward, the gas is contacted to the iron oxide pellets moving in the opposite direction of the gas, i.e., from the top section of the furnace downward. Due to this contact, the oxygen content of the iron ore pellet is absorbed, making the pellets porous. The final product of this process is called sponge iron. This product, which has a very high temperature, is cooled down at the bottom of the furnace using natural gas until it reaches the ambient temperature. Ultimately, it exits the furnace, and it is sent to the steelmaking plant for consumption.
The second part of each MIDREX unit consists of a reactor for producing the reduction gas, a.k.a. a reformer. In this reactor, a mixture of natural gas and the gas returned from the furnace is converted into reduction gas in the range of 1070 to 1100 degrees Celsius in the presence of a catalyzing agent. In order to supply the heat required for breaking the gas, there are torches installed in the bottom section of the reformer. These torches burn a mixture of the gas returned from the furnace and natural gas. The gases obtained from the combustion in the reformer are collected by a number of ducts, and after exiting the reformer with a temperature ranging from 1070 to 1100 degrees Celsius, the gases are directed toward the heat recovery or recuperator.
The recuperator is in fact the third part of the MIDREX process. In the recuperator, there are grids of steel pipes, called tube bundles, which act as a heat exchanger. They use the heat from the gases created by the combustion to preheat the consumed air in the reformer torches, natural gas, and the gas returned from the furnace. The gases obtained from the combustion exit this section after heat exchange in the recuperator, and they are discharged into the air through the main stack.