Formation of Plasma Arc Anode Spot on the Rotating Graphite Crucible

The formation of anode spots on a rotating graphite crucible in the plasma arc furnace is studied. The size and temperatures of the spots are determined by means of photographs and an optical pyrometer. The composition of the plasma in the anode spot region is calculated. The cylindrical crucible 100 mm in diameter with an axial hole 40 mm in diameter and 28 mm in depth is rotated at the speed of ~100 rpm. A cylindrical cathode 50 mm in diameter has an axial channel 24 mm in diameter. Argon is fed through the channel as a plasma-forming gas. The crucible and cathode are located in a steel water-cooled chamber with a volume of 150 L. The arc is several centimeters long and the arc current is 560–630 A. It is shown experimentally that an argon plasma arc at a power of 23–25 kW can form a contracted or diffuse anode spot. The current density of the contracted spot is 1800 A/cm 2 , while that of the diffuse spot is ~250 A/cm 2 . A contracted spot is formed on the cold crucible and it is accompanied by intense evaporation of graphite in the region of the spot. The carbon vapor reduces the plasma ionization potential near the anode and promotes contraction. Uniform heating of the crucible of up to 2700 K is needed for transformation of the contracted spot into a diffuse one. Such heating is attained by rotation of the crucible. Thermal isolation of the crucible also promotes the formation of a diffuse spot. The Poisson equation is solved, and the near-anode potentials in the space charge layer are calculated for contracted and diffuse spots as 10.7 and 1.5 V, respectively. It is assumed that the decrease in the near-anode potential in the diffuse regime promotes its formation.