Introduction

In steelmaking the impurities in hot metal like carbon, silicon, manganese, phosphorus and sulphur are removed through oxidation and slag formation so as to produce steel of desired chemistry and cleanliness. For this purpose oxygen is supplied and slag of desired chemistry is formed. When oxygen is supplied, oxidation of all impurities of hot metal including iron begins simultaneously.

To understand the conditions favourable for the removal of an impurity, we will first consider oxidation of an individual impurity. We will be using principles of thermodynamics to obtain the optimum conditions for the removal of an impurity. Note the following

• Carbon can oxidize to CO and CO₂ but at high temperature carbon oxidation to CO is highly probable. We will consider oxidation of C to CO.
  
  
• In expressing activity of solutes in molten steel, Henry’s law is used by using 1 weight % standard state. Raoult’s law is used to express activity of solutes in slag.
  
  
• Since impurities are dissolved in molten metal, reactions between impurity and oxygen occur with dissolved oxygen.
  
  
• Square brackets  [ ]  in a reaction denote impurity in metal, round brackets  ()  in slag and curly {}  in gas.