In 250 tons EAF furnace, free board of 500 mm is given by designers.
In my opinion, rather than free board, ladle should be free from mouth jam in order to avoid wastage of wire. However, free board should be maintained 400-500 mm for 150 ton metal. Feeding rate greater than 250 m/min is good for 150 ton ladle, best recovery can be obtained at 300-350 m/min. This can be calculated on the basis of ladle height.
Feeding rate calculation can be done in the following way. Speed should be such that maximum it should reach to the 80% or bekow the ladle height. It ahould not be too high so that it will hit the ladle bottom. Wire cobble should also be considered while making calculations. Simple force or energy balance with restriction on travelling of wire, speed can be calculated.
Recovery of Ca-Si wire depends on several factors like bath temperature as boiling point of Ca is quite low, wire speed, amount of Ca in cored wire gm/m, %S, %O (amount of reducible oxides in slag, oxygen ingress during argon puring through slag eye) , %Al in bath, amount of slag, plume eye area, argon purging profile after Ca-Si treatment etc.
Clean steel definition is mainly based on its inclusion rating. Low inclusion rated steel considered clean. Inclusion rating consider the following parameters like inclusion size, its morphology, no. of inclusions, inclusion density. Apart from this it also includes, low concentration of S, P, N, H etc.
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