Process Control

● The process is always depends upon the composition and amount of charge material (input variables) and, operating variable like Lance heigh 'flow rate' pressure of oxygen,etc. Every care is taken to stick to the input variable, the parameters do change and the process needs to be controlled by charging the operating variables suitably. The term process control means the ability of adjusting the operating variables in order to obtain the desired turn-fown conditions, which is desireb composition and temperature on the bath, despite the variables in the quality and quantity of the charge materials.

● LD process is too fast to analyses of slag and metal during refining, adjust the slag composition. LD process control has much relied on the experience gained from blows of nominally similar charge compositions to produce steels of identical specifications. This procedure often results in off-temperature heats which lower the productivity, since corrective measures such as re-blowing to increase the temperature or adding coolants to decrease the temperature have to be adopted.

LD process control
The LD Process Control

● A positive and predictive control, which is flexible and versatile enough for various raw materials, operational characteristics and desired end product is prime requisite. Initially static control,based on thermal and material balancing of the charge,prior to the blowing, was developed to attain desired turn-down conditions. This is a predictive type of model and using mainly off-line computer decides the optimum weights of the materials to be charged and the amount of oxygen to be blown to obtain desired end end product. It may include, the prediction of a re- blow or cooling additions, required at the end.

● The operation is, increasingly being controlled automatically to improve upon the production rate by reducing the percentage off-grade heats. Several mathematical models have been developed to simulate the process to work out computer control program.

● If the Lance height is less than the correct value ; more metal is thrown in the emulsion and thereby, the true height tends to attain the correct value ; the converse us also true. The same will be observed if the pressure of oxygen varies at any fixed position of the Lance. This is, possible only within certain narrow limits. This in- built and self-catering balance allows variations around the standard state without affecting refining adversely and thereby make the process control of otherwise a dynamic process a relatively less difficult job.

Output :

● The blowing times in LD vessel are kept nearly the same irrespective of the sizes of the vessels. The tap to tap time, therefore increase with vessel size due to the increased time of charging, tapping. One of the fastest vessel of 150t capacity has a tap to tap time of just 30mins in which lancing time is 16mins.
LD process output rate - LD process
LD Process Output rates

● The lancing time can still be decreased but then there is not enough time for slag formation and adequate dephosphorisation and desulphurisation, which may not take place before carbon is brought down to the required level.

● For expansion of an existing shop prefer to add a third vessel in existing shop so that two work at any time out of three. This can be increase the production rate by 80%. Production rate has been increased by 60% by working both the vessels in a two vessel shop on week days and carrying repairs of one over the week ends.

Slag :

● In general the basic it is on the high side in most LD plants. Excess lime prolongs lining life and ensures complete dephosphorisation and maximum desulphurisation without the denger of their reversion. The slag emulsion all around the hot spot protect the lining from radiation. It improves thermal efficiency of the process.

● A typical composition of slag from a heat of dead soft steel at the time of tapping is :

CaO : 47 to 52%
MgO : 2 to 5%
P2O5 : 1 to 3%
SiO2 : 10 to 16%
MnO : 3 to 5%
Fe : 15 to 22%

● The iron ore content of the slag is lower in high carbon steel. If the phosphorus in the charge is more then intermediate slagging off is carried out. The first slag is very oxidised with low lime content, and the second slag is much more basic and oxidizing as compared to the single slag practice.

Yield :

The metallic yield in LD steel making is 90%, and ingots or slab yield of 87 to 95%. More of phosphorus and silicon contents in the hot metal generate large slag volume and yield decrease due to increased loss of iron in the slag.

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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