For arc heating, the electrodes are made of

For arc heating, the electrodes are made of

Right Answer is:

Graphite

SOLUTION

Electrodes used in an arc furnace

The following three types of electrodes are used in arc furnaces:

(i) Carbon electrodes:

  • These electrodes are used with small furnaces for the manufacture of ferroalloys, aluminum, calcium carbide, phosphorus, etc.
  • These are made of anthracite coal and coke. 
  • These are very cheap and cost less than one-half as much for the same weight ah graphite electrodes.
  • The large area of carbon electrodes allows for more uniform heating.

(ii) Graphite electrode :

  • These electrodes are obtained by heating the carbon electrodes to a very high temperature.
  • Owing to the lower resistivity of graphite (one-fourth that of carbon), graphite is required half in size for the same current resulting in easy replacement.
  • Graphite begins to oxidize at about 600°C whereas carbon at about 400°C. Under average conditions, the consumption of graphite electrodes is about one-half that of the carbon electrode.

(iii) Self-baking electrodes:

  • These electrodes are employed in Ferro-alloys and electrochemical furnaces and the electrolytic production of aluminum. 
  • These electrodes are made of special paste, whose composition depends upon the types of process for which it is used, it is contained in the thin steel cylinder. The flow of current produces heat and the paste is baked and formed into an electrode.

Features of carbon and graphite electrodes are:

  • Resistivity: The graphite electrodes have low-specific resistance than the carbon electrodes, so the graphite required half in size for the same current resulting in easy replacement.
  • Oxidation: Graphite begins to oxides at 600°C whereas carbon at 400°C.
  • Electrode consumption: For steel-melting furnaces, the consumption of the carbon electrodes is about 4.5 kg of electrodes per tonne of steel and 2.3-to 6.8 kg of electrodes per tonne of steel for the graphite electrodes.
  • Cost: The graphite electrodes cost about twice as much per kg as the carbon electrodes. The choice of electrodes depends chiefly on the question of the total cost. In general, if the processes require large quantities of electrodes, carbon is used but for other processes, the choice depends on local conditions.
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