In all coal-based DR processes, sized iron ore (pellets can of course be used if desired) and a coarse fraction of noncoking coal are fed into the rotary kiln from the inlet end in the required proportions. The coal not only acts as a reducing agent, but it also supplies the heat required for maintaining the temperature profile of the charge within the kiln. A finer fraction of coal is also often introduced from the discharge end of the kiln to help complete the reduction process. The temperature of the charge bed inside the kiln is confined to a maximum of around 950 to 1050°C so that the entire reduction occurs in the solid state. Noncoking coal contains sulfur, which can be partially absorbed in sponge iron during the course of reduction — to control this, a flux such as limestone or dolomite must be added to the coal charged into the kiln. The product discharged from the kiln is indirectly cooled in a rotary cooler to room temperature, so that no water normally comes in direct contact with the reduced product, which, being a nonequilibrium material, is prone to reoxidation. Figure 3.1 shows a simplified flow sheet of direct reduction in rotary kilns. Basic principles of sponge iron making in rotary kilns. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315138220/6ba18297-7766-4c2a-9901-d50d96c5995a/content/fig3_1.tif"/>