Plasma induction furnace is a combination of ordinary induction furnace and plasma arc heating device. It avoids the shortcomings of cold slag and unprotected atmosphere of ordinary induction furnace, thereby significantly improving the purification capacity of induction furnace. Plasma smelting can flexibly change the working gas, Therefore, during smelting, protective gas and reactive gas beams can be introduced to achieve the purpose of removing C and B elements. Europe uses plasma smelting to purify metallurgical grade silicon in the ARTIST project. This technology uses high-purity metallurgical silicon as raw material, and melts the silicon material under the joint heating of a plasma gun and a medium-frequency electromagnetic induction heating device. The plasma gun emits plasma under heating conditions and uses an inert gas as a carrier to introduce reactive gases such as H2 and O2 to react with non-metallic impurities such as B and C on the surface of the silicon melt to generate gases such as BH, BOH, BO, and CO. Evacuate the vacuum system. An intermediate frequency induction coil is arranged outside the crucible, and at the same time of induction heating, electromagnetic stirring is generated for the silicon melt, which increases the reaction rate and accelerates the discharge of the generated gas. For different impurity elements in polysilicon, corresponding reaction gas can be introduced to achieve the purpose of impurity removal, but the main elements in the gas are H and O.
This method was developed in the 1960s, and by the 1970s, the plasma induction furnace with a capacity of 2t was built and usually used a special plasma arc heating gun body as the heat source. In these gun bodies, the plasma arc is compressed by the hot wall, air or water flow, and magnetic field, so as to obtain a compressed arc with high temperature and high energy density. 3000K (the arc core temperature of the general free arc is usually 5000~6000K) or more.
The gun body uses tungsten or tungsten alloy as the electrode, and the gun body and nozzle are cooled by circulating water. The nozzle at the lower part of the gun body has a wall compression effect on the arc, and the gas flowing between the nozzle and the tungsten electrode has a thermal compression effect, so the formed plasma arc has a high temperature and energy density. Due to the high energy required for smelting, the arc current is usually relatively large, often reaching several thousand amperes.
The graphite electrode (or water-cooled copper electrode) required for conduction is installed at the lower part of the furnace body of the plasma arc furnace, and a transfer arc is formed between the furnace body and the plasma arc gun body during the smelting process. When the material to be smelted is a non-metallic material, even if the charge is not conductive, the non-transferred arc can be directly used for smelting. In the smelting process, it is usually necessary to pass hydrogen gas as a protective gas. For this reason, in order to ensure the stability of the atmosphere in the furnace, a sealed furnace is usually required. In addition, due to the uneven heat provided by the plasma arc, the phenomenon of uneven heating of the furnace is prone to occur, so the energized winding is installed at the bottom of the furnace, and the magnetic field generated by the winding interacts with the magnetic field caused by the current passing through the furnace, making the silicon liquid sports.