Formation of anode spots on rotating graphite crucible in the plasma arc furnace was investigated. The size and temperatures of the spots were determined by means of photographs and optical pyrometer. Composition of the plasma in anode spot region has been calculated. Cylindrical crucible of 100 mm in diameter with an axial hole of 40 mm in diameter and 28 mm in depth was rotated with the speed of ~100 rpm. Cylindrical cathode of 50 mm in diameter had an axial channel of 24 mm in diameter. Argon was fed through the channel as plasma forming gas. Crucible and cathode were located in the steel water cooled chamber with a volume of 150 liters. The arc was several centimeters long, arc current was 560-630 A. It was shown experimentally that 23-25 kW argon plasma arc can forms contracted or diffuse anode spot. The current density of contracted spot is 1800 A/cm2, while that of diffuse spot is ~250 A/cm2. Contracted spot formation occurs on a cold crucible and accompanied by intensive evaporation of graphite in a spot region. The carbon vapor decreased ionization potential of plasma near the anode and promoted contraction. A uniform heating of the hole in crucible up to 2700 K is needed for transformation of the contracted spot into diffuse one. Such heating was attained by rotation of crucible. Thermal isolation of crucible also promotes diffuse spot formation. Poisson’s equation was solved, and the voltage falls in space charge zones were calculated for contracted and diffuse spots as 10.7 and 1.5 V respectively. It was assumed that the lesser voltage fall promotes the diffuse spot formation.
Comparison of Copper and Graphite Crucibles for Si Extraction from TiO2 - SiO2 System at Plasma-Arc Heating
