Obtaining spheroidized fused-periclase powder using a high-frequency induction plasmotron

Abstract

In recent years, fused periclase (MgO) and the products based on it have been finding ever increasing applications in high-temperature technology as efficient refractories and electrical insulating materials. However, the specifications concerning the composition, the properties, and the structure of periclase and the demand for high-quality peric!ase products are not completely met by modern production technology because of the differences in the nature of the raw materials used, the specific features of the melting practice, and the distribution of the products in the block [i].In particular, the specifications concerning periclase used in tubular electrical heaters (TEH) as a particulate electrical-insulating material are extremely stringent. The reliability and the stability of the insulation characteristics of TEH depend on the quality of the insulating particulate charge. It must have a high electrical resistivity (1.2"i0v-5.5"I0 ~ ~-cm), a high dielectric strength (1.1-1.3 kV/mm), and satisfactory flowability [2]. The electrophysical properties of periclase depend on the impurities and their distribution in its lattice, its electrical resistivity is decreased by impurities such as Fe=Os, MnO, and AI203 contained in the raw materials and introduced during the crushing operation on the production cycle. The electrophysical properties (electrical resistivity and stability of the electrical parameters) of periclase depend to a significant extent on its hydration resistance, which can be improved by creating hydrophobic particle surfaces in the powder [2].It is desirable that the periclase grains havenot only a specific size but also spherical shape in order to ensure high flowability of the powder and, consequently, a high packing density of TEH.We studied the production technology of spheroidized periclase in the discharge of a high-frequency induction (HFI) plasmotron (Fig. I).Grade-3 electrical-engineering periclase (according to COST 13236-83) was used as a raw material.A particulate charge having a size dispersion of 100-2500 ~m was fed from the bunkers of the feeders into the zone of plasma discharge through the water-cooled probes fitted in the transition assembly located below the discharge chamber. The probes used for feeding the charge were shifted so that the particles of the powder-gas mixture move towards the inflowing plasma-forming gas, lose their speed, and move along with the gas. Such a scheme of charging made it possible to increase the duration of residence of the particles of the experimental material in the high-temperature zone. Furthermore, it did not disturb the stability of the burning process of the plasma discharge. The following parameters were maintained: voltage across the anode U a = 34-36 kV; current I a = 5.3-5.5 A; consumption of the plasma-forming gas Gp~ = 10-12 m3/h; and consumption of the transporting gas Gtr = 0.4-0.5 m3/h.After the introduction of the electrical-engineering periclase into the high-temperature zone of the plasma discharge, heating up of the particles and...