Heroult Electric Steel Process: Metallurgical and Chemical Principles

Posted By Richard Jefferson on 11 November 2014

Posted in The Vintage Machinery Almanac

This article was originally published in Electrochemical and Metallurgical Industry Publication of July 1907. Information within this article is therefore correct as of 1907. The publication of this material aims to provide historical insight on the subject and its place in industry.

As explained before the whole process of making the steel consists of two stages: first, treatment in the open-hearth furnace; second, treatment in the electric furnace, and the success, and especially the economy, of the electric treatment depends to a certain extent on the right way of carrying out the previous open-hearth treatment. The operation in the open-hearth furnace is carried out preferably further than is usual in ordinary open-hearth practice.

Now, it is well known that phosphorus is always eliminated after carbon. Hence, if we use the open-hearth furnace for reduction of phosphorus as far down as just stated, we necessarily also eliminate the carbon and at the same time highly oxidize the metal. This would be very undesirable in ordinary practice, but the product thus obtained from the open hearth is exceedingly suitable for subsequent treatment in the electric furnace.

What is introduced into the electric furnace from the open hearth is a molten, highly oxidized metal, exceedingly low in phosphorus and carbon. What the electric furnace must accomplish is essentially the deoxidation of the metal, its recarburization and the elimination of the sulphur. All this is attained successfully in the Heroult process, which yields a product containing any desired percentage of carbon, with less than 0.01 sulphur and phosphorus, free from oxide and slag.

The removal of sulphur is possible on account of the possibility of selecting slags which would not be sufficiently fluid at lower temperatures but which can be used at the high temperature of the Heroult furnace. There is no danger that the charge might be spoiled by this high temperature, since the charge is continually in rapid circulation and all particles in the bath are brought into contact with the slag, the highest temperature being concentrated in the layer of slag.

With respect to deoxidation it should be emphasized that this is obtained in ordinary practice only with very great difficulty and expense. However, if iron oxides are dissolved in the steel they cause pipes and blow-holes. To prevent them ferro-manganese and ferro-silicon are ordinarily added, but while largely successful, they have the disadvantage that the resulting oxides of manganese and silicon are solid and remain in the steel in a finely divided state in form of an emulsion. Further, as long as the slag contains iron oxides in solution they will react with the molten bath, and thorough deoxidation of the bath is impossible. The neutral slag used in the Heroult furnace is perfectly free from iron, and this is a very important point.

Ferrous oxide and iron carbide can exist together, and for this reason complete deoxidation is not possible under ordinary conditions by means of adding carbon, but this is possible with the Heroult process, the addition of carbon to the slag results in the formation of calcium carbide, and therefore the deoxidation can be carried on much further than is possible. For this reason only the least traces of ferrous oxide need be rendered harmless by means of manganese. The metallic manganese reduced by the carbon present reduces the last traces of ferrous oxide in the bath. No reoxidation is possible, since the bath is protected by the slag from the atmosphere.

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