Desulphurization mechanism

Desulphurization can be carried out by injecting lime based powder, for example  or  . The injection rate varies between 2 -4 kg/ton of melt. When slag forming materials are injected into melt, they melt and the molten slag particles begin to rise and accumulate at the top surface of the melt. The desulphurization reaction occurs in two ways:

• During contact between rising molten slag particles and the melt. In this mechanism of desulphurization it is important that the powder becomes molten on injection. Residence time of the rising particles in the melt is also important, which means that the gas powder injection velocity must be suitably selected. Powder melts and the rising gas imparts mixing in the melt. This mechanism is known as transitory contact.

• Contact between top slag and the melt. As the molten slag particles rise they accumulate at the top surface of the melt and after a while top slag also takes part in the desulphurization. In this mechanism slag/metal interface area is important. Gas injection rate may be suitably selected to produce and entrain slag droplets into the melt for the faster rates of reaction. Once all the powder is injected, reaction between top slag and sulphur of melt governs the final sulphur content of steel. This mechanism is known as permanent contact

It must be noted that methods for injection of powder must also be developed. The slag forming materials are lighter than steel and deep injection would be required for the efficiency of the reaction. Powder can be injected either through cored wire or pneumatic transport. Figure 23.1 shows arrangement of ladle desulphurization carried out either by injecting cored wire or by pneumatic injection through a top lance. In both argon is bubbled through a porous plug fitted at the bottom of the ladle. The desulphurization efficiency can be estimated by the mass balance.