Ferro-alloy furnaces are electrothermal in nature where heat is produced by supplying electric current through the carbonaceous electrodes to the working space of the furnace. The properties necessary for the satisfactory performance of an, electrothermal electrode at elevated tempe-ratures are
(1) high electrical conductivity,
(2) slow rate of oxidation (high apparent density),
(3) high: mechanical strength (particles well bonded together i.e. suitable aggregate having thoroughly mixed)
(4) low heat conductivity.
Two types of electrodes are generally in use i.e. carbon electrodes and graphite. Carbon elec-trode again are of two kinds i.e.
(I) soderberg electrodes of continuous and self baking type
(II) pre-baked carbon electrodes.
Continuous self-baking electrodes are widely used at present. Carbon electrodes are very rarely used in the ferro-alloy industry while graphitised electrodes are used in the production of decarbur-ised ferro-chrome, metallic manganese and crystal silicon in submerged arc furnaces (SAF). Graphite electrode find place in submerged arc furnaces' of such capacities where the electrode diameter is limited to 28". The electrode size for a submerged arc furnace is selected on the curr-ent density for the operation. Economics has dictated the highest possible current density to keep capital cost to a minimum. Selecting electrodes by current density may be satisfactory, since it tends to provide good electrode performance. Ferro-alloy furnaces which are to a large extent continuously operating units need high current (upto 50000 amperes in threephase).For the supply of high current, electrodes of a large' diameter, (20" to 55") are required. But the difficulties of producing quality electrodes grow' with their diameter. In the ferro-alloy 'industry, therefore continuous self leaking electrodes are used exclusively; in the heated up condition they differ very little in their physical properties from the usual carbon electrodes.
The self-baking electrodes are cheaper than the usual pre-baked carbon electrodes. They can be made on the spot and the expense in setting up a section to produce the electrode mixture is small compared with that of a carbon electrode factory. Moreover, no fuel is needed to heat the electrodes and there is no need of giant cranes to transport and screw the electrodes. The manufacturing process of both the type of electrodes is similar which comprises of drying, crushing and grind- ing of the base raw materials and then mixing with a suitable binder. The paste thus formed, in the case of prebaked electrodes is extruded in the cylindrical form and baked by calcining at about 1200 o -1300℃ in reduced atmosphere. During the process a dense, fully shrunk product is obtained to give prebaked carbon electrodes. In case of continuous or self baking electrodes, the green paste is either directly placed into the furnace' or moulded and kept in storage for use when needed. For grap-hitised electrodes, the prebaked carbon electrodes after tar impregnation and rebaking, graphitised by heating in the temperature range 2400- 2600℃ n reducing atmosphere.
