Normally, non-metallic wastes, such as sands, concrete and asbestos are regarded as electrically non-conductive materials. However, when the temperatures are increased up to the melting point, their electrical conductivities can be greatly improved, flowing arc current. Accordingly, these nonmetallic wastes can be efficiently treated by heating them up to the electrically conducting temperatures by using a non-transferred type plasma torch, and then, melting them completely with arc currents in transferred mode of plasma torch. For this purpose, we propose a convertible plasma torch consisting of three cylindrical electrodes (rear electrode, front electrode and exit nozzle). Compared with conventional plasma torch with two cylindrical electrodes (rear electrode and front electrode), the proposed plasma torch can provide more stable plasma jet in high powered and non-transferred mode due to the presence of exit nozzle, resulting in rapid heating of the non-conductive materials.

Nowadays, transferred type arc plasma torches have been widely present in industrial applications, in particular, using melting pool of electrically conducting materials such as arc furnace, welding and volume reduction of radioactive wastes. In these applications, the melting pools are normally employed as an anode, thus, heat flux distributions on anode melting pool need to be characterized for optimum design of melting pool system. For this purpose, we revisited the one-dimensional model of the anode boundary layer of arcs and solved governing equations numerically by using Runge-Kutta method. In addition, the direct melting process of non-combustible wastes in the crucibles were discussed with the calculation results.