Hybrid Refining Process

In LD or OBM processes mild bottom stirring resulting in good mixing and increase mass transport in two phase stimulated systems. This finally led to the development, which known as bath agitation process or BPA in short. This is brought about by introducing a small amount of nitrogen or argon gas either through a porous plug situated at the bottom or by a sub lance introduced from top. Then oxygen blowing only from the top or bottom alone and thereby developing the hybrid processes of steelmaking .

In bath agitated processes bath stirring is caused by a neutral gas introduced below the metal bath either through porous refractory plugs or tuyeres fitted in the bottom refractories or through auxiliary lance or pipes introduced from top.

Combined Blowing Processes :

The processes wherein gas phase is introduced from both top and bottom of the vessel are known as hybrid process or combined blowing processes. However combined blowing name may be given when oxygen is blown both from top and bottom and bath agitated processes, if inert gas stirring is carried out from bottom only with top oxygen blowing.

In combined blowing processes oxygen is introduced from both top and bottom, bottom oxygen being protected by hydrocarbon shield for refining, causing the necessary stirring for refining. The proportion of oxygen blown from each end varies from process to process.

● Hybrid process in a term applied to any such mixed blowing process. Nitrogen or argon may be chosen as the inert stirring media separately or together judiciously. It is introduced from bottom through porous refractory plugs or tuyeres.

Hybrid Process (combined blowing) Characteristics :

1. A small amount of inert gas, about 3% of the volume of oxygen blown from top, introduced from bottom, agitates the bath so effectively that slopping is almost eliminated.

2. However for the near equilibrium state of the system inside the vessel a substantial amount of gas has to be introduced from the bottom.

3. If 20 to 30% of the total oxygen, if blown from bottom can caused adequate stirring for the system to achieve near equilibrium conditions. The increase beyond 30% therefore contributes negligible addition of benifits.
Flow rate of bottom in hybrid process
Flow rate of bottom

4. However at 30% oxygen blowing from bottom leads to formation of very dry slag and possibility of its ejection during refining unless it is accompanied by lime also.

5. The more the oxygen fraction blown from bottom the less is the post combustion of CO gas and consequently less is the scrap consumption in the charge under identical condition of processing.

6. Blowing of inert gas from bottom has chilling effect on the bath and hence should be minimum.

7. As compared to top blowing, the hybird blowing eliminates the temperature and concentration gradient and effects improved blowing control, less slopping and higher blowing rates. It also reduces over oxidation and improve the yield. It is better to helps in effective dephosphorisation and desulphurisation also ability to make very low carbon steels.

8. As compared to bottom blowing the bath agitation gives the advantages of bottom blowing in a top blown process, to the extend of gas used for purging.
Combined blowing - Bath agitation blowing
Combined and Bath agitation Process

● Out of all these processes it is most easier to convert classical LD in to bath agitated process typically represented by the LEB design. This is more popular because of the following :

1. Existing LD finds it necessary to improve upon its drawbacks without much alternative and capital expenditure.

2. Availability of porous plugs and ease with which it can be fitted in a LD vessel.

3. A general reluctant to adopt complicated bottom oxygen blowing with shrouded hydrocarbons. The amount of top blowing and their types are above.

Bottom Gas Purging Design :

● The easiest to fit and operated the bottom gas purging, A variety of devices have been developed to introduce inert gas from bottom in an classical LD vessel. These devices are simple and rugged tolast for almost the life of the wall lining of the the vessel during refining.
Bottom injection for hybrid ld process
Bottom injection for hybrid LD processes

● They allow to vary the stirring times and it extend by controlling the gas flow. In emergency if it fails to operate, the vessel can still operate as classical LD style to continue steel production. The supporting pressure required to purge gas through is 2 to 3 bar only. The mass flow permeable element is maintain at around 4m.m/min to inject 1.7 to 2.0m.m.m of inert gas per tonne of liquid steel.

● If the bottom flow rate requirement is high then it is better to adopt oxygen tuyeres rather than these permeable elements. Similarly if the lime powder is to be injected from bottom then also the permeable elements are of no use and oxygen tuyeres is the only alternative.

Hybird Blowing Design :

● In hybird processes the top and the bottom blowing have to be so adjusted as to obtained stirring without and adverse effect on the refining reaction and yield. The top and the bottom blowing are designed to obtain optimum condition of overall blowing. The factor are :

1. The inclination of the nozzle to the vertical axis in the multi hole Lance : no of nozzles as well
2. Usual bath diameter inside the vessel
3. Usual height of the Lance in blowing position.
4. Oxygen pressure used for blowing.
5. Position, size and number of purging gas inlets at the bottom
6. Pressure of bottom gas used for blowing.

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

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