The steel manufacturing process

Iron ore is converted to steel via two main steps. The first involves the production of molten iron and the second is that of actual steel manufacture. The details of these steps are outlined below.

Step 1 – The production of molten iron

The Primary Concentrate is mixed with limestone and coal and heated. The iron oxides are reduced in the solid state to metallic iron, which then melts, and the impurities are removed either as slag or gas.

The multi-hearth furnaces

There are four multi-hearth furnaces, each of which feeds a rotary kiln. The furnaces preheat the materials fed into the rotary kiln and reduce the amount of volatile matter present in the coal from about 44% to about 9%. This is important because the large volumes of gas produced during the emission of the volatile matter would otherwise interfere with the processes in the rotary kiln.

There are 12 hearths in each furnace and the feedstock passes down through these. In the first three hearths, hot gases from the lower stages preheat the material in the absence of air to about 450°C. Air is introduced in hearths 4 to 9 to allow combustion of the volatile material, so as to increase the temperature to about 650°C. The supply of air is adjusted to control the percentage of residual volatiles and coal char in the product. In the final hearths (10–12) the char and the primary concentrate equilibrate and the final temperature is adjusted to 620°C. The total residence time in the multi-hearth furnace is 30–40 minutes.