Combined |Mixed Blowing Practices

● The processes have been developed to obtain the combined advantages of both LD and OBM to the extend possible. Therefore the parameters on which this can be done are :

1. Iron content of the slang as a function of carbon content of bath.
2. Oxidation levels in slag and metal.
3. Manganese content of the bath at the turndown.
4. Desulphurisotion efficiency in terms of partition coefficient.
5. Dephosphorisatio efficiency in terms of partition coefficient.
6. Hydrogen and nitrogen contents of the bath at turndown.
7. Yield of liquid steel.

● It should be borne in mind that the performance of each process at a particular shop which is different from similar one at the other. But the general performance date can be chosen for the sake of comparison and evaluation. Broadly three hybrid processes have been selected as typical examples for comparison with the two extreme ones. Such as LD and OBM.

1. Classical LD operation as (LD)
2. Top oxygen blown plus bottom inert gas stirred (LBE)
3. Top and bottom oxygen but no lime powder. (LD-OB)
4. Top and bottom oxygen with bottom lime powder. (K-BOP)
5. Classical oxygen bottom blown (OBM)
Relation between carbon and manganese at blow end
Relation between C and Mn at the blow end

● It is clear that for the same carbon content of the bath both the iron content of the slag and the oxygen content of the metal, decrease as the stirring increases from LBE to LD-OB to K-BOP.  The hybrid processes perform in between the  LD on one side and IBM on the other. Similarly the manganese content of the bath at the end of the blow increases from LBE to K-BOP. The generally less over oxidised slag and metal tend to help desulphurisation as bath agitation increases if lime is blown from bottom alongwith. The LBE type processes do not indicate any appreciable desulphurisation because mere stirring dose not help it much. If sulphur is to be removed then it is better to adopt K-BOP type process.

Dephosphorisation :

Dephosphorisation is a more complex reaction and is affected by many factors which vary in these processes, likes top Lance height, purging gas flow rate, lime injection from bottom and so on. The inert gas purging rate is also varied during the blow with respect to the lance height to achieve desired dephosphorisation. Vigorous stirring in the beginning is followed by low stirring when at the end when temperature favours phosphorus removal.

Bottom Stirring :         
Sulphur partition in basicity of slag - combined blowing steel making
Sulphur partition in function of basicity
  Argon stirring dose help in reducing the hydrogen contents of the bath because of the purging effect wherein the dissolved hydrogen diffuses in the rising argon bubbles.

        Nitrogen content of steel bath in almost all hybrid processes is practically the same except when nitrogen is used as a purging media and due argon rinse is not given in the end.
● The top blowing to any hybrid blowing there is a net decrease in slopping and oxidation potential of the slag and the bath and, hence the yield increases by 0.3 to 1.0% depending upon the operational details.
● It is obvious that the performance of LBE type processed is more closer to LD and that of K-BOP type processes is more closer to OBM and the LD-OB lying in between.

Future of Hybrid Processes :

● One or more hybrid process will only be the choice for a new Greenfield installations. The choice may depend upon the cost and the products to be made. Facility of more argon blowing from bottom may be found as cheapest.

● The existing LD shops can be readily converted into LBE type process using argon purging as an additional facility at relatively less cost and easy-to-operate modification. This has found a very large applicability all over the world in the last the years or so, also because of widely adopted classical LD plants all aver the world in the decades of 60's, 70's and 80's.
● The decarburization techniques using oxygen injection is quite common and as stainless steel production is rapidly going up, this may also be made more readily using a hybrid process in near future.

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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