Synthetic Slag

Introduction :

This practice is employed to obtain clean steels and to desulphurised molten steel. Synthetic slag practice is adopted to meet as

1. To cover molten steel for heat losses.
2. To avoid reoxidation of steel from atmospheric oxygen.
3. To remove inclusion from molten steel
4. Used for desired basicity and sulphide capacity deoxidized slag can be better for desulphurized.
5. Synthetic slag practice is attractive due to low capital cost on equipment.

Desulphurization of Steel :

Synthetic slag practice can desulphurized steel up to 50% to 60% of original sulphur in steel. The properties are

1. Slag should have high sulphide capacity.
2. Basic slag is required.
3. Slag should be liquid for faster reaction rates.
4. Slag should not cause excessive refractory wear.
5. Steel should be deoxidized and slag carry over should be minimise.

Design of Synthetic Slag :

The slag contain CaO, CaF2, Al2O3 and with small amount of SiO2. The principle component of synthetic slag is lime. Calcium fluoride increase the sulphide capacity of slag and help fluidizing the slag. Aluminum is present to deoxidized the molten steel since transfer of sulphur from molten steel to slag is followed by transfer of oxygen from slag to steel. Therefore deoxidation of steel is must for efficient desulphurization. In which slag contains 45 to 55% CaO, 10 to 20% CaF2, 5 to 16% Al, and 0 to 5% SiO2. This is the prefused slag in solid state.

Problem due to Synthetic Slag Practice

1. Desulphurization may varies from one heat to other if slag carry over from BOF/EAF is not controlled. Oxygen content of steel should be same for consistent.

2. CaO is the main component. It is hydroscopic and leads to hydrogen pick up.
3. Argon bubbling is done to stirring the bath. Temperature drop could be the order of 10℃ to 25℃ for 150 to 300ton heat. The temperature drop is resulting from radiation heat loss from surface and heat transfer due to argon bubbling.

4. The slag attacks the ladle refractory. Excessive amount of CaF2 result in refractory wear. Higher tap temperature increase refractory wear.

: Injection of Solid powder :

Injection technique have the advantage of dispersing the reactions in the steel bath and at the same time provide a large reaction surface area. The type of powder used is governed by the purpose of injections.

Dephosphorization ------- CaO + CaF2 + Fe2O3 (mill scale soda)

Desulphurization ---------- CaO + Al,  CaO + CaF2 + Al, CaC2, Mg + ( MgO, Al2O3, Chloride slag ) , CaC2 + CaCO3, CaO.

Alloying ------------------------ FeSi, CaCN2, NiO, MoO2, FeB, FeTi etc.

Deoxidation and inclusion shape control ---------- Al, Ca, Si, CaSi, Mn, AlCaSi and Ba.

Desulphrization Mechanism :

● Desulphurization can be carried out by injection lime base powder like 90% CaO + 10% CaF2 or 70% CaO + 20% Al2O3 + 10% CaF2. The injection rate varies between 2 to 4kg / ton of metal, when slag forming material are injected in to melt, by which they melt and the molten slag particle begin to rise and accumulate at the top surface. So the reaction that occurrs :
Injection of powder or wire in steel ladle
Ladle desulphurization

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

2. Contact between top slag and the melt. As the molten slag particles ruse 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 maybe suitably selected to produce and entrain slag droplets in to 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.

● The slag forming materials are lighter than steel and deep injection would be required for efficient of the reaction. Powder can be injected either through the cored wire or pneumatics transfer. Both amount of slag partition coefficient are important for desulphurization.

References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty

4. Sr. Professors from IIT's & BPUT. (INDIA)

Author :
Subir Kumar Sahu

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