Study On Some Problems Of Heating Efficiency Of Microwave Metallurgical Furnace

Microwave metallurgy oven is one of key equipment for microwave metallurgy. However, for a long time, the heating efficiency of the microwave metallurgical furnace has played a plagued role in both academic and industrial field, which as well affects the application and promotion of this technology. Many researchers have improved it in the aspect of adjusting the metallurgical process parameters. Studies show that the heating efficiency of the microwave metallurgical furnace is closely related to absorbing properties of materials, wave-transparent properties of the furnace refractory lining and the incidence angle and polarization of electromagnetic waves, which affect the microwave heating efficiency in complex ways. Therefore, to improve the efficiency of microwave metallurgical furnace, the coupling of furnace lining, materials and electromagnetic and so on must be considered at the same time.The microwave energy conversion efficiency is the direct factor that influenced microwave metallurgy efficiency while factors that influenced microwave energy conversion efficiency mainly includes three aspects:First, the microwave absorbing properties of materials; the second is the polarization and incident angle of the electromagnetic wave; the last but not least is the microwave-transmission properties of lining and carrier. To improve the heating efficiency of the microwave metallurgical furnace, for essential factors that affect the microwave heating efficiency, This paper studies the influence of physical parameters, geometry and other factors on microwave absorbing properties of typical metallurgical materials and transparent wave properties of refractories used in metallurgy, and the influence of the incident frequency, polarization and incident angle of electromagnetic wave on the microwave absorbing properties of the typical metallurgical materials. Specific contents are as follows:(1) In order to study the distribution of thermal coupling with electromagnetic fields of the microwave metallurgical furnace inserted with linings and materials, based on multi-physics field coupling theory, the distribution of electromagnetic fields and temperature fields of microwave resonant cavity inserted with linings and materials has been calculated. And the dynamic changes of heat hots and the electromagnetic field have also been analyzed. The results show that:the electromagnetic parameters of linings and materials can seriously affect the distribution of both the electromagnetic field and the temperature field; Early period in the microwave heating, microwave energy mostly dissipated in the material, but lining absorb less microwave energy; at high temperature period, most of the microwave energy dissipated in the surface layer of the material, heating unevenness will become more apparent. Microwave penetration depth decreases with increasing temperature, microwave penetration depth of less than 10 cm after 8000 s. in the normal condition, the distribution of the electromagnetic field and the temperature field are the same, which high electromagnetic fields density is corresponding to High temperature, but with the temperature gets higher, the electromagnetic mode hopping phenomenon occurs in microwave heating, the temperature field changes follow the electromagnetic field, but is lagging behind.(2) For the issue that microwave heating efficiency heavily depends on the dielectric properties of materials, taking three vertical reflection materials as objects(sodium chloride, silica sand, petroleum coke), the influence of the water content, temperature, and materials thickness on absorbing properties have been studied based on the analysis of vertical reflection loss. The results showed that:Best heating process parameters of chloride material layer is temperature 60℃, material thickness 0.04m, this conclusion is consistent with the experimental results in the literature, reflection losses of materials with different moisture contents or temperature related to material thickness; reflection loss curve appears several absorption peaks with the increasing thickness, and absorption peaks shift as the change of the temperature and moisture content; high microwave absorption can only be achieved in the layer with a slight absorption peak shift (less than one-eighth wavelength) occurs, and the thickness ranges that obtained the maximum absorbing properties have been given.(3) In order to investigate the effects of the electromagnetic wave incident angle and polarization on the microwave heating efficiency, microwave oblique incidence reflection losses of sodium chloride, silica sand and petroleum coke have been calculated based on the analysis of oblique incidence reflection loss, and the influence of incident angle and polarization on the microwave absorption have also been analyzed. The results show that:the incident angle and polarization of the electromagnetic wave can seriously affect the microwave absorption of the materials; the maximum efficiency of microwave absorption can be achieved by changing the angle of incidence and polarization throughout the microwave heating process. Under the horizontally polarized electromagnetic wave (TE), the temperature is 20℃, the thickness of 0.143m, silica sand could achieve maximum efficiency when the incident angle of 8°; There is no rule that the effects of angle of incidence and polarization on absorbing properties of materials to follow, reflection losses need to be calculated and analyzed according to specific electromagnetic properties of materials. Finally, the incident angle ranges that could obtain the maximum absorbing properties have been given.In order to improve the transmission efficiency of microwave energy in refractories, power transmission coefficients of several microwave metallurgical refractories (mullite ceramics, alumina ceramics, silica ceramics, etc.) have been calculated. The influences of the microwave frequency, temperature and thickness on wave-transparent properties have been studied based on the analysis of power transmission coefficients. The results show that the transmission efficiencies of silicon dioxide are more than 70% within [0,0.1m] thickness range and the range of 20℃-1000℃; There are several transmission peaks in the PTC patterns. The transmission peak amplitude depends sensitively on the thickness of the refractory and the peak shifts towards a smaller thickness as the temperature of the refractory increases. Finally, selection principles and thickness ranges that could obtain the maximum wave-transparent properties have been given.