Electric furnaces for making pig iron are meeting with some success in centers where electricity is cheapest and where iron ore also is available; this is possible in localities in the Scandinavian countries and in California.
Fig. 29 is a diagram of an electric furnace making pig iron; it is the combination Arc-and-resistance type, continuously operated. The construction of this furnace Section at AB SECTION AT 'CD' differs somewhat from that of others which are in use but the principles and chemistry involved are the same in all and will be obvious after having studied the iron blast furnace. A furnace like that of Fig. 29 is heated by the current through the electrodes as indicated in the cut. The coke fed in with the ore effects the main part of the reduction; gases formed by the combustion may be circulated up through the column of ore to give as much pre-heating and reduction as possible.
Electric furnaces for making steel excel in that no current of gas has to pass through or over the metal, and in the high temperature which can be obtained readily. Only where electricity is wonderfully cheap, do these furnaces prove economical in the reducing of iron from its ores, in the melting of cold metal, or in the comparatively low temperature heating and slagging period which is necessary for the removal of phosphorus from the metal.
Fig. 30 is a section through the Stassano pure-arc type of furnace. The electrodes are entirely above the charge which is heated by radiation. Mechanical arrangement provides for turning the furnace about to give motion to the bath. Quite a number of these furnaces are in use.
Fig. 30. Section of Pure-Arc Type of Electric Steel Furnace Courtesy of the "Engineering and Mining Journal".
The furnace most used resembles the type of Fig. 31, which is of Heroult make. It is obviously a tilting furnace. No electrodes are shown in place in the picture but are to be inserted in the two large holders suspended above the furnace. Such furnaces require many thousands of amperes, with the voltage usually less than 100; the electricity is for heating only, the heat is partly from the arc between the electrode and the slag and partly because of the resistance through the bath. The current comes in at one electrode and goes out at the other.
The electric furnace has a real field of usefulness in the final removal of sulphur from a partly purified bath, because we can get a very liquid slag and a very basic slag by using sufficiently high temperature, such as cannot be obtained in any other type of furnace. Electric furnaces, of course, may be lined with either an acid or a basic refractory, depending on whether we wish to make an acid or a basic slag. As intimated by the possibility of sulphur removal just mentioned, most furnaces now are lined with basic material.
The most common type of electric furnace is the combination arc-resistance type in which the heat is partly developed by the arc between electrode and slag and partly by the resistance which the slag and metal offer to the passage of the current. Extremely high temperatures thus are attainable, and thinly fluid slags often containing much calcium carbide are obtained, which would be absolutely infusible in any other type of furnace. Such a high-lime slag is very efficacious in fluxing off even small amounts of sulphur from the metal. Otherwise, the chemistry of the electric furnace is no different from that of other types of furnaces. Carbon is easily introduced by adding varying amounts of pig iron, or is burned out by adding iron ore.
The tonnage of electric steel is increasing year by year, and furnaces holding 20 tons in one charge are now operating. Of the 215 electric steel furnaces in the world (1915) 75 are of the type of Fig. 31. All the furnaces now in the world have a combined yearly capacity of over a million tons of steel.
Fig. 31. Heroult Type of Electric Steel Furnace Courtesy of the "Iron Age".