Refining:

In the melting operation Refining is the elimination of unwanted impurities from the metal bath. The general kinds of impurities that might be present are

• dissolved gases
• dissolved non-gaseous elements, and
• suspended oxides and inclusions.

In non-ferrous melting no effort is made in the foundry to eliminate dissolved impurities. Dissolved gases are eliminated by one of the degassing method described above. A refining operation to eliminate entrapped oxides and inclusions is frequently essential in non- ferrous metals-especially in magnesium and aluminium alloys. These alloys easily make solid oxides known as dross which tend to remain suspended in the melt. Adequate time might be allowed for the particles to float to the surface in the furnace and ladle or they might be fluxed from the metal through stirring a reactive salt flux into the bath.

In the case of steel two general kinds of melting practices - acid practice and fundamental practice - are utilized. Acid melting involves the utilization of acid refractories & acid slags in contact along the metal. Such refractories and slags are high in SiO₂ content and low in MgO and CaO. Fundamental melting involves refractories and slags of relatively high MgO and CaO content. In acid melting, suspended non-metallics and dissolved gases are eliminated, and carbon composition is adjusted. However Phosphorus and sulphur contents are not influenced and as these are deleterious  and impurities to the mechanical and casting properties of steel, acid melting needs an initial charge low in these elements. Basic melting procedure the metallurgical advantages of acid melting, along the addition that phosphorus and sulphur might be decreased to any desired level by appropriately controlling the various melting variables involved, chiefly the concentration of lime and iron oxide in the slag. Therefore basic melting may employ a lower and cheaper grade of scrap like charge material.