The Process of Making and Refining Steel from Raw Iron
Previously, we had posted an article here, stating the process of steel manufacturing from iron ore. In continuation, today we will touch upon the subject of refining steel as well.
The process of refining a metal consists of purifying an impure sample of metal, wherein the final material has similar chemical properties to the original material, but is purer in its composition. Pyro metallurgical and hydrometallurgical techniques are popular processes of refining.
Iron ore is utilized in the production of steel- the process of steel-making involves the removal of impurities from raw iron and the addition of alloying components to produce the desired quantity of steel.
There are two types of the modern steel-making process:
- Primary Steel-making- This process chiefly utilizes new iron samples as the feedstock from a blast furnace.
- Secondary Steel-making- The primary raw material in this procedure is scrap steel and is performed in an electric arc furnace.
The second step in producing steel is associated with the removal of impurities like sulfur, phosphorus and additional carbon from the iron used as raw material. Following this step, alloying elements like manganese, chromium, nickel and vanadium are added.
Dephosphorization
The process of removing phosphorous should ideally be performed after the removal of silicon.
This is a crucial step in the making of steel as phosphorous tends to form iron phosphide, which is brittle in nature.
As the stabilities of iron and phosphorous have similar temperature dependence (these two elements are close to each other in the Ellingham diagram), lime is added to the mixture to adjust the pH. Lime increases the basicity and effectively removes any phosphorous that might be present.
Removal of Sulfur
Reduction of silicon is lowered when the temperature in the blast furnace is lower. The slag thus formed as a result is more viscous in nature.
Desulfurizing reagents like calcium carbide and lime are used here to reduce the sulfur content to as low as 10ppm (0.001%).
Removal of Silicon
When the temperature of the blast furnace is high, the content of silicon increases in the grey pig iron- the reason for this is the increased reduction of silicon at high temperatures.
The process of de-siliconization is done using an oxygen lance in a transfer ladle.
Nowadays, silicon is removed through the process of ‘Torpedo’ ladle.
In this technique, hot metal is taken in a torpedo ladle and scrap is added to it. The chemical reaction following this step leads to the formation of slag. This slag is then skimmed off.
It needs to be noted that the process of refining steel differs from other processes like calcining and smelting from the aspect that the latter involve a chemical change in the raw material.
