OBM Process | Working Principle :

● The OBM vessel is essentially a Bessemer like converter fitted with a special bottom. The tuyeres are inserted from the bottom and the oxygen would be surrounded by a protective hydrocarbon gas like propane. On entry of propane cracks down in an endothermic reaction and takes up some of the heat generated by the entry of oxygen. The relatively feed rates of these two fluids are adjusted to obtain optimum temperatures at the tuyere tip and there by ensure it's reasonable life as well as speed of refining. The deposition of carbon cracking also helps to protect the bottom from heat generated due to the refining reactions at the tips of tuyeres.
Oxygen bottom blown converter
Oxygen bottom Blown converter

● The other product of cracking is hydrogen. The amount of hydrogen thus dissolved increases particularly towards the end of the blow when flow rate of hydrocarbon is increased relative to that of oxygen. A nitrogen rinse is given at the end of refining to desorb dissolved hydrogen. But these steels end up with high hydrogen contents as compared with Thomas steels. Nearly 50% of the hydrogen burns with dissolved oxygen and there by generated heat in this process.

● In order to promote turbulence in the bath and good slag metal contact, the tuyeres are arranged only on half the converter bottom. By this arrangement, it is ensured that the direction of metal circulation is upwards in tuyere half of the vessel, and downwards in the other half. This arrangement is also helpful in minimising the damage to tuyeres while charging scrap, since it can now be charged on that part where there are no tuyeres. Which increases the vessel capacity for the same inner volume as compared to Thomas vessel, since hot metal can be filled up to the almost half bottom area in an inclined position. The existing Thomas converters when converted to OBM, it increased their capacity up to 40%.

● The tuyeres arrangements is such coupled with the provision of hydrocarbon and oxygen for usual blowing, it can be used for preheating the scrap. The tuyeres are generally made of stainless steel pipes emvembed in magnesite bottom. This is essentially for consistent operation of the converter during its entire campaign. During blowing the pressure of the protective gas is about 20 to 50% below that of the oxygen.

OBM Operation :

● The operation of the vessel is more likely the Thomas process which has been for Thomas grade iron 1.8%P.  After charging scrap and hot metal, blowing is started. Lime is added immediately afterwards from an overhead chute. The blow is divided into three distinct parts. The first lasts for 16 to 17mins and at the end of which a high phosphoric slag, 18 to 22% P2O5 and 12 to 15% FeO is raked off.

● The carbon and phosphorus are analysed by taking a sample. The analysis is generally 0.3% C and 0.08% P at this stage. Fresh lime is added and blowing continued for about a minute when the carbon comes down to 0.1% and phosphorus to 0.025%. The final blowing is of nitrogen without protective gas and thereby the bath averages finally 0.0004% hydrogen and 0.005% nitrogen. The process is capable of producing all variety of plain carbon steel with equal ease, using all variety of hot metals.

Bottom Blowing Process :

● The refining reaction are extremely fast in OBM process. The stirring caused by the blowing action results in achieving near equilibrium condition with respect to the gas-metal and metal-slag interfaces. The system inside the vessel operates at near equilibrium. On the contrary in top blowing oxygen is supplied to the metal not directly as in OBM, but thought the a slag layer thereby over oxidising the slag beyond the equilibrium state.
OBM impurities elimination
Sequence of elimination of impurities in OBM

● The use of supersonic oxygen jet does not result in temperature and concentration gradients within the bath causing periodic slopping and ejections. Depending upon the top lance position some oxygen is always available for CO combustion above the bath and which contributes some heat to the process which in turn allows 3 to 6% additional scrap in the charge. Due to 98% oxygen being reacted with metal in OBM, hence that much scrap rate is lower in the OBM.

● The iron losses in the top blowing are nearly 5% more than OBM. This also leads to situation where in higher carbon levels can be obtained by 'catch carbon techniques' easily in LD than OBM, at low P contents. The stirring intensity, which is estimated to be nearly ten times more in OBM than in LD gives better partition of phosphorus and sulphur, higher manganese and lower oxygen at turndown resulting in better ferroalloys recovery.

● Because of hydrocarbon, the OBM steels have higher hydrogen content and even if agron rinse is adopted it is still on the higher side. The OBM is ideally suited for even low carbon steels at blowing rate of oxygen nearly 4 to 4.5 Nm3/min and scrap rate 4% lower then in LD. The scrap rate besides other factors primarily depending on silicon contents in iron. With rise of silicon from 0.5 to 1.8 the scrap may go up correspondingly from around 20 to 35%. It is easier to feed lime with oxygen in top blowing then in OBM. The thermal balance at the tuyere tip in OBM is very critical and has to be carefully adjusted, by the flows of oxygen, hydrocarbon and lime.

Mechanism :

OBM tuyere protection mechanism
OBM tuyere protection mechanism

As the hydrocarbon comes in contact with the molten metal, it gives rise to cracked mushroom growth and then perphas accumulation of carbon at the tuyere tip. This is perhaps followed by in tense combustion of this carbon with oxygen. The hydrogen from cracking may dissolvein the metal and which later combined with oxygen giving heat. The refractory performance of the main lining of OBM vessel should be comparable with that obtainable from conventional vessel. The unique bottom designe in OBM is the heart of process and can also become a limiting factor in vessel operation unless the life on the whole matches with that of other lining.

Advantage :

1. Vessel volume is less than the conventional LD converter as no foam is formed during blowing.
2. Blowing is guide than the top blown LD process. Hence better stratification of slag and metal.
3. Refining process is enhanced as hydrocarbon during blowing.
4. Cost of installation is less.

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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