Study On New Technology Of Microwave Heating Based On Dielectric Properties Of Metallurgical Materials

As an emerging green metallurgical method, microwave metallurgy has developed into a compelling interdisciplinary; microwave heating is different from conventional heating, it penetrates into the object, where it is transformed into heat by dielectric loss, thus microwave heating has the advantages of selective heating, fast temperature rising, high heating efficiency, reduction of the reaction temperature, easy to automatic control. Microwave heating was widely used in plenty of metallurgical process due to the specific advantages, such as:microwave assisted griding, microwave drying, microwave carbon thermal reduction, microwave roasting, microwave assisted leaching. The research of the interactions between microwave and metallurgical materials microwave metallurgy has an important guiding role for the development of new technology and new equipment in the field of microwave metallurgy. The complex permittivity, dielectric constant and loss factor are the basic parameters to describe the interactions between microwave and material. The complex permittivities of metallurgical materials vary nonlinely with temperature and composition. The viations of complex permittivity with temperature affect the distributions of temperature field and electric field inside the sample during microwave heating. Study of the variations of dielectric constant and dielectric loss factor with temperature, moisture content, density, the change of the chemical composition for choosing appropriate microwave heating process and design of the corresponding microwave heating system has an important guiding significance.In this thesis, the author has developed a dielectric parameters measurement system, aim to measure metallurgical powder materials. The dielectric properties of typical metallurgy materials under different density, temperature, moisture content and chemical composition were measured by this system. In the low temperature microwave heating filed, the dielectric properties of the titanium metallurgy materials were studied, a low temperature microwave drying method based on low microwave power was designed for the high titanium slag and petroleum oil. According to the poor microwave absorbing abilities, a high temperature microwave drying method based on high microwave power was choosen for the molecular sieve. In the high temperature microwave heating, according to the medium microwave absorbing abilities, a high temperature microwave roasting method based on high microwave power was designed for the zinc oxide dust. The three microwave heating processes parameters were optimized by the RSM method. The specific research results are as follows:(1) Based on the powder metallurgy materials and principle of resonant cavity perturbation method test, a set of high temperature dielectric parameters measurement system was chosen, the temperature measuring range is room temperature to1000, the testing range: permittivity εr’:2to100, loss tangent tan8:5x10-3-2. The test error: room temperature:|△ε’/ε’|≤<2.0%,|△tanδ|≤10%tan δ+3×10-3; high temperature:|△ε’/ε’|≤4.5%,|△tanδ|≤15%tanδ+5×10-3(2) The effect of moisture on the dielectric properties of titanium metallurgy materials was studied. Sodium chloride powder is the low microwave absorbing material, the dielectric constant, the dielectric loss and loss tangent have the highly linear relationship with the moisture content. The best microwave drying thickness is3cm according to the penetration depth. Both the high titanium slag and petroleum coke are high microwave absorbing materials. The dielectric constant of petroleum coke decreases linearly with moisture content between5%and10%, loss factor increases linearly with moisture content between3%and10%, dielectric constant and dielectric loss factor of petroleum coke increase linearly with the temperature. The best thickness for microwave drying of petroleum coke is less than3cm. (3) The effect of chemical compositions on the dielectric properties was studied. The dielectric properties of rutile and titanium concentrate from room temperature to1000℃.The dielectric constant of rutile has a positive linear relationship with temperature range in400-1000℃, loss factor has a quadratic polynomial function with temperature in400-1000℃. Penetration depth of rutile rise is in37-330cm. The dielectric constant of titanium concentrate increases with the temperature, and falls in400℃. Rutile and titanium concentrates both are high absorbing mineral.(4) Dielectric properties of molecular sieve change linearly with moisture content. The best material thickness for microwave drying is less than2.5cm. Based on molecular sieve dielectric characteristic parameters with variations of moisture contents, high temperature microwave drying based on the high power can achieve rapid drying rate and relative dehydration rate. The microwave drying process parametes were optimized by response surface method and the parameters are as follows:drying temperature93℃, the drying time20min, material thickness18mm. Under the conditions, the relative dehydration rate was99.37%.(5) Based on the dielectric properties of high titanium slag and petroleum coke change with moisture content, to improve the efficiency of microwave drying, a low microwave power density and temperature control drying process was designed. Two mathematical models were established and analysed using RSM to describe the relationship between the microwave drying conditions and the responses, moisture content and dehydration rate. Based on the RSM analysis, the optimum process conditions of microwave drying of high titanium slag are:microwave drying temperature of76℃, drying duration of10sec, and sample mass of45g, with the resultant moisture content being0.28at a dehydration rate of1.44g/min. Based on the RSM analysis, the optimum values of the process of microwave drying petroleum coke variables were obtained as: microwave drying temperature75℃, drying time10S, and sample mass60g, at these conditions, the moisture content was0.34, dehydration rate was2.94. The of low power density and temperature control microwave drying was process used in the industrial production, compared with the electric vibration fluidized bed drying, the tons of energy consumption cost is only1/3of the electric vibration fluidized bed drying, saving more than75%energy; Compared with the steam vibrating fluidized bed drying, the energy cost of microwave drying is also1/3of steam vibrating fluidized bed drying; Compared with the infrared drying, microwave drying energy costs by20%, significantly reduces the cost of energy.(6) The dielectric constant and loss factor is increase linearly with the bulk density of zinc oxide dust. According to the dielectric property of zinc oxide dust with temperature and density, a high microwave power cavity and microwave roasting processing was designed for the removal of fluorine chlorine from zinc oxide dust, and the processing parameters were optimized by the RSM method. Under the condition of air flow rate300L/h, the optimum technique parameters were roasting temperature of650℃, holding time of30min, stirring speed of60r/min, and the predicted value of the dechlorination rate from zinc oxide dust is97.13%, which is quite close to the actual experimental value (95.66%). The results show that the zinc oxide dusts treated by microwave roasting could definitely satisfy the demands of chlorine content in the electrodepositional process of zinc, and the chlorine concentration is12.39mg/L in the leaching liquid.(7) Based on the measurements of metallurgical material dielectric parameters, a dielectric parameter database system of metallurgy materials was established. The dielectric results were retrieved according to the names of different mineral grade levels, temperatures, moisture contents, and an orderly arrangement method was approved.