LD PROCESS BLOWING CHARACTERISTICS

LD Process Blowing 


LD process is unique in many ways which characteristics are :

1. As the oxygen jet travels through the converter atmosphere it carries some of ambient medium along with it. This entrainment is much less in the supersonic core but is substantial in the sonic portion of the jet. The composition of the jet where it strikes the bath, depends on the blowing rate, height of the Lance etc.

2. As the jet strikes the bath is results it stirring of the bath. It circulates radially outwards on the surface and upwards on the vertical axis. The stirring of bath coupled with exothermic refining reactions result in the dissolution of scrap, in the early part of the blow.
LD vessel oxygen Lance jet pressure
Supersonic jet on bath circulation

● The rate of dissolution of scrap depends primarily on its size, blowing conditions, vessel design, temperature and composition of the hot metal etc. Due to using of supersonic jet the kinetic energy of the jet is not enough to mixing of the slag and metal inside the vessel. By which the metal bath suffers from significant gradients in the temperature and composition, resulting for periodic ejections and slopping and thus loss of yield.

Refining Characteristics :

3. The rate of dissolution of lime in slag can be increased by increasing FeO content of the slag, retaining a part of the previous slag, adding a little of dolomite in the beginning and increasing the reactivity of lime. This leads the desulphurisation of the bath during refining.

4. The sequence of elimination of impurities during a LD blow was depends on analysising the bath samples. It revealed that the removal of silicon is quite fast and is more or less in about six minutes after the commencement of the blow. Manganese comes down concurrently with silicon up to a certain point and it remains is the rest of the blow except, that some reversion may take place towards the end of the blow. Carbon and the phosphorus oxidise together starts the process of oxidation after a few minutes of blowing till the ends.

5. The process control such a sequence of elimination of impurities from the bath is of limited significance to arrive at the desired end conditions when the blow is stopped. This led to the study of the dynamic nature of the process.

6. In the early days of LD the high refining rates of LD process were quite attributed to the formation of hot spot at the impact area of the jet. It has now been conclusively proved that the process is much more dynamic than earlier.

7. The high velocity jet tears off small droplets of metal from the bath and throw them up in the vessel atmosphere. The presence of thin slag-gas matrix inside the vessel leads to the formation metal-slag-gas emulsion. The formation of metal slag gas emulsion increases the interfacial areas of metal-slag and gas-slag systems tremendously. The metal droplets in the emulsion may have sizes around 14 to 100 mesh that is equivalent area of 10 to 20 cm.cm/g.
Ld blowing - eliminations of impurities
Sequence of elimination of impurities in a LD blowing

8. There are two distinct zones of refining in a LD vessel. The reactions in the emulsion and in the bulk phase. The contribution of bulk refining at which bulk slag-metal interface, is dominant in the beginning since emulsion is yet to from properly. The substantial decarburisation of droplets can occur because of its free exposure to an oxidising gas, particularly in the beginning. As the emulsion builds up the emulsion refining attains a dominant role. The bulk phase refining dominates again towards the end when the emulsion collapses.

9. The early exothermic oxidation of silicon and manganese, particularly in the emulsion phase, raises, the temperature of the slag and helps dissolution of lime. As the volume fluidity of the slag increases the volume of metal droplets in the emulsion increases and the decarburization of the droplets, because of the generation of CO gas which tends to stabilise the emulsion from within. It  is a case of mutual compound acceleration.

Decarburization in Blowing :

10. The residence time of the droplets in the emulsion is estimated to be nearly 2 to 3min half way through the blow. It is slightly more in the beginning due to the high viscosity of the early slag and is slightly less towards the end of refining since the slag is thin. The particle is repeated as long as emulsion is stable. In the early part of the blow, silicon and manganese are oxidised in some droplets where as in some others decarburization may have commenced.

● The decarburization of droplets is delayed due to lack of supersaturation of dissolved oxygen necessary for the reaction. The large silicon and manganese are almost completely oxidized in the 4 to 6 min of initial blowing. This is period is marked by :

* Firstly- Slag is formed due to the exothermic oxidation reaction and fluxing action of the oxide product in particular silica.

* Secondly- The amount of metal droplets increase in the slag phase and attain a peak value.

* Thirdly- The decarburization of droplets increase in increasing propertion as is evident from the build up of the decarburization rate curve.

11. Towards the end of refining the carbon concentration of the droplets in the emulsion approaches a value such that the rate of decarburization is no more controlled by the rate of supply of oxygen and instead, it is controlled by the carbon content which is low. The decarburization rate curve beings to fall and enough CO is formed. The drop in the decarburization rate towrads the end is quite rapid since now it is a case of mutual compound declaration. This tail of the decarburization rate curve is quite reproducible and formed the basic for dynamic control of the process in the production of soft steel.
LD blowing - decarburisation rate as per time
Decarburization rate as a function of time of blowing

Dephosphorisation in Blowing :

12. Condition for dephosphorisation is the slag should be basic, thin, oxidising and the temperature should be low. The slag is formed in ld process only after the initial 4 to 6 min of blowing. The rate of dephosphorisation picks up concurrently with the rate of decarburization. For efficient decarburization as well as dephosphorisation the slag should form as early as possible in the process. If a performed slag is present as in a double slag and the second slag is retained in the vessel in part or full, the decarburization rate curved rises more steeply in the beginning. In a LD practice adjusted to obtain early slag conductive to effective P removal.

13. Dephosphorisation is very rapid in the emulsion because of the increased interfacial area and efficient mass transport. Phosphorous should be fully eliminated before the emulsion collapses. If this is not achieved the heat will have to be kept waiting for dephosphorisation to take places. In bulk phase, it is extremely slow as compared to that in the emulsion. In general dephosphorisation should be over by the time carbon is down to 0.7 to 1.0%.

14. At the steady state the relative rates of dephosphorisation and decarburization are controlled to achieve the desired objective. This can be simply done in a given practice by adjusting the Lance height or adjusting the flow rate of oxygen. Raising the height of the Lance or decreasing the oxygen pressure decreases the gas-metal reaction in the emulsion that is decarburization, and vice versa. The phosphorisation reaction is thus relatively increased by the above change and vise versa.

Towards the end when temperatures is high the phosphorus reversion doesn't exist but it can be prevented by maintaining a high basicity of the slag.

15. The blowing strategy is decided in advance to obtained effective dephosphorisation well ahead of decarburization. Some opeartors prefer to alter the height of the Lance while maintaining the oxygen flow rate constant. Others keep the Lance height constant during the entire blow but vary the oxygen pressure to obtain the desired condition of blowing.

16. The decarburization path in practice have been observed to be different even with similar blowing practices. The physical distribution or scrap inside the vessel, in effect to cause baffles and delay the ignition may be responsible for such observed variations.

17. The Blowing condition have a major effect on manganese distribution. The initial rate of oxidation of manganese is around 0.25% per min but as the FeO content of the slag builds up towards the end it is reverted at the rate of around 0.03% per min.



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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