But this dephosphorization by oxidation can be carried out only in the case slag is basic. If it is acid, i.e.
The quantity of phosphorus in the pig iron is usually known accurately, and the dephosphorization takes place so regularly that the quantity of air which it needs can be foretold closely.
Such a slag not only corrodes the furnace lining, but also impedes dephosphorization, because it is irretentive of phosphorus.
At the end of the melting down, dephosphorization is then impeded by the temporary acidity of the slag, as just explained.
When the Heroult furnace is used for completing the purification of molten steel begun in the Bessemer or open-hearth process, and this is its most appropriate use, the process carried out in it may be divided into two stages, first dephosphorization, and second deoxidation and desulphurization.