| Drying treatment using hot air, combustion gas, steam (superheated steam) or electric heating as heat source, is a very important process of metallurgy industry. The driving force for energy transfer is the temperature gradient between heat source and materials. Due to the conventional heating taking from the outside into the inside, it needs to heat up the whole material to remove water from the inside of material during drying process. The driving for mass transfer is the moisture concentration gradient between the inside and outside of material. The moisture is transferred from the inside to the surface of material, and then evaporated into atmosphere. For conventional drying process, it needs high external temperature to form a temperature gradient to speed up the drying process. Microwave irradiation has advantages for the internal and selective heating. The water specific dielectric constantεof 60-78 is much higher than common minerals or compounds, so the moisture always quickly absorb electromagnetic wave and is rapidly transferred to vapors. Hence, it can significantly reduce the drying time. The selective heating reduces the thermal loss of material integral heating and can significant reduce the energy consumption. Apart from this, an inside to outside temperature gradient is formed due internal heating characteristic of the microwave drying process. The temperature gradient has a same direction as that of moisture diffusion, which can promote the mass transfer and improve the drying efficiency. Therefore the microwave drying technology has been received much attention and obtained rapid development.In this thesis, three typical particle materials, i.e ilmenite concentrate, anthracite and silica sand assorted according to different microwave absorbing characteristic, were selected as experimental raw material. Microwave drying and microwave-heat pump combined drying were conducted, and the experimental details are as follow:(1) Firstly, Terminal Open Coaxial Reflection Method (TOCRM) was employed to measure the amplitude and phase of reflection coefficient of three particle materials, i.e. ilemenite concentrate, anthracite and silica sand, and then calculate the Specific Inductive Capacity (SIC)ε, Dielectric Dissipation Factor (DDF)εand Loss Tangent (LT) tanδof them through Genetic Algorithm (GA) and Finite Element Analysis (FEA). The experimental temperature range is from 20 to 100℃. The results show that ilmenite concentrate is a strong absorbing material, and theεandεare 6.23～13.24 and 0.79～6.56, respectively; the anthracite is a medium absorbing material, and theεandεare 2.56～12.65 and 0.32～5.36, respectively; the silica sand is a weakly absorbing material, and theεandεare 3.98～5.38 and 0.26～0.40, respectively. Different drying equipments and methods were explored according to different microwave absorbing characteristic. (2) Secondly, a new process of microwave drying of ilmenite concentrate was studied. In the basis of single factor experiments, a mathematical model was developed according to factors and response, as: Dehydratnratio=10011+2640x1-1.56x2-11.66x3-1.75x2x2+14.00x1x3+1.00x2x3-18/.20x12-1.06x22-7.77x23 The microwave drying process conditions were optimized by using Response Surface Method (RSM). The analysis results show that the significance of three variables are in reverse order:drying time>>sample mass>material thickness. The relation between microwave irradiation time, material thickness and sample mass with dehydration ratio is not a simple linear relation:the first and second order terms have significant effect on dehydration rate, and the interaction terms have negligible effect. The optimized conditions were in the following:microwave irradiation time of 135s, sample mass of 164g, and material thickness of 20mm. The maximal dehydration ratio was 99.99% under the optimized conditions, which has only a 0.01% deviation between experimental and predicted value.(3) Thirdly, a new process of microwave drying of anthracite was studied. In the basis of single factor experiments, a mathematical model was developed according to factors and response, as: Dehydratioratio=24.51+6.37x1+18.19x2-6.55x3+5.44x1x2+3.30x1x3-0.23x2x3-0.61x21+2.78x22+7.69x23 The microwave drying process conditions were optimized by using Response Surface Method (RSM), the analysis results show that The significance of three variables are in reverse order:drying time>>sample mass>microwave power. The relation between microwave irradiation time, material thickness and sample mass with dehydration ratio is not a simple linear relation; the first and second order terms have significant effect on dehydration rate, and the interaction terms have negligible effect. According to the required dehydration, a high drying efficiency should be achieved. The optimized conditions were in the following:power level of 682.07 W; drying time of 2.98 min; and sample mass of 49.19 g. The maximal effectiveness ratio is 1.452 kg/kWh under the optimized condition. The verification experiment indicated that the experimental results were in good agreement with the predicted values, which has only a+0.57% deviation.(4) Fourthly, a new process of microwave-heat pump combined drying of silica sand was studied. Condition experiments of microwave and heat pump drying of silica sand were conducted. The effect of sample mass, initial moisture content, microwave power level, hot air velocity on dehydration ratio, wet basis moisture content and drying efficiency were studied respectively. In the basis of single drying experiment by microwave irradiation or heat pump hot air, mathematical models were developed according to factors and response, as: moistureotent=0.39-0.91x1-0.44x2+0.69x3+0.34x1x2-0.069x1x2-0.091x2x3+0.45x21+0.10x22+0.40x23 and effective(?)srati=0.65-0.2x1-0.011x2+0.24x3-0.002x1x2-0.065x1x3+0.003x2x3+0.059x21-0.009x22-0.072x23 The combined drying process conditions were optimized by using Response Surface Method (RSM). The analysis results show that the relations between factors and responses are complex nonlinear relations. The optimized conditions were in the following:sample mass of 620g, microwave irradiation time of 4.6 min, heat pump drying time of 9 min. The obtained wet basis moisture content and effectiveness ratio are 0.7% and 0.85kg/kw.h, respectively, which has only a 0.01% deviation between experimental and predicted value.(5) Fifthly, microwave drying characteristic and kinetics of three typical powder materials were studied. The anthracite microwave relationship between drying rate constant and the Power ratio was:κ=0.06102-4.09204m/p+140.05833 (m/p)2-1691.86413 (m/p)3, drying activation energy is 31.30629W/g, pre-exponential factor A0is 0.04285; obtained ilmenite microwave relationship between drying rate constant and the Power ratio was: k=0.0652-1.35849m/p+11.64559(m/p)2-30.57185(m/p)3,drying activation energy is 6.9600W/g, pre-exponential factor A0is 0.0362; obtained high purity silica sand activation energy is 6.9600W/g, and pre-exponential factor A0 is 0.0362. If the power density is greater than 1 (1.000-4.167, the activation energy is 0.279W/g; if the power density is less than 1 (0.429-1.000), the activation energy is 3.493W/g. Due to microwave drying energy efficiency of high purity silica sand, the actual value and the theoretical values are lowered as the moisture content is reduced, and the EER obtained by the actual value and the theoretical values were 3.92 and 7.14MJ/kg, respectively.(6) To fully take advantage of microwave and heat pump, a novel combination drying system with microwave and air-source heat pump is also designed and built. A series of drying experiments on high purity silica sand were performed. The drying experiments of damp arenaceous quartz were also carried out. Experimental results indicate that the drying energy efficiency of the microwave and air-source heat pump is as high as 0.7 kilogram water per kW-h.
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