典型冶金物料的微波及组合干燥应用研究

【作者】 李雨；

【导师】 彭金辉；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2011， 博士

【摘要】 干燥工艺是冶金工业的重要工序,大多采用热风、燃气、蒸汽(过热蒸汽)、电加热等方式来进行加热,热源和物料之间的温度梯度是能量传递的原始推动力,这种由表及里的传热方式在脱除水分的同时需要对全部物料进行加热；而传质的推动力是物料内部与表面之间水分浓度的梯度,水分的蒸发从物料表面开始,内部的水分慢慢从内部扩散至表面,为加快干燥速率,则需要较高的外部温度来形成温度梯度。微波加热具有内部加热、选择性加热等优势。由于水分子的介电常数ε’高达60-78,远大于一般矿物及化合物,在微波干燥过程中,水分子总是优先吸收大量微波,在短时间内形成较大蒸汽压,迅速逸出物料表面,显著减少了干燥处理时间。这种干燥方式减少了对物料的整体加热,极大地降低了能耗；同时,由于内部加热的特点,在物料中形成由内而外的温度梯度,其方向与水分传输反向相同,极大的促进了水分子的传质过程,提高了干燥效率,因此,微波干燥工艺日益受到重视并得到快速发展。本论文以物料吸波性能的强弱,选取了钛精矿、无烟煤和高纯石英砂三种典型的冶金颗粒物料,开展了微波及微波-热泵组合干燥工艺研究,具体内容如下：(1)首先在20-100℃的干燥温度范围内,利用终端开路同轴反射法测定了三种典型颗粒物料无烟煤、钛精矿与高纯石英砂的反射系数的幅值和相位,并运用遗传算法和有限元分析计算得到三种物料的介电常数(ε)、介电损耗因子(ε)和损耗角正切(tanδ),研究结果表明：钛精矿的介电常数在6.23-13.24,介电损耗因子在0.79-6.56,属于强吸波性物料；无烟煤的介电常数在2.56-12.65之间变化,介电损耗因子在0.32-5.36,属于中强吸波性物料；高纯石英砂的介电常数在3.98-5.38之间变化,介电损耗因子在0.26-0.40,属于弱吸波性物质。并根据其吸波特性的不同,探索了其相适合的干燥设备和方法。(2)研究了微波干燥钛精矿的新工艺。在单因子实验的基础上,建立了可信的各因子与响应值的数学模型为：脱水率=100.11+26.40-1.56x2-11.66x3-1.75x,x2+14.00x1x3+1.OOx2x3-18.20x12-1.06x22-7.77x32并应用响应面法对微波干燥钛精矿的工艺条件进行了优化,分析结果表明：微波时间、物料量及料层厚度对无烟煤干燥影响的显著性顺序为微波时间>>物料量>料层厚度；微波时间,物料量,料层厚度对脱水率的影响不是简单的线性关系,一次项和二次项对脱水率影响显著,交互项影响较小。响应面法优化微波干燥钛精矿的最佳工艺条件为：微波时间135s,物料量164g,料层厚度20mm。此时脱水率的实际得率为99.99%。此时实验值与预测的偏差为0.01%。(3)研究了微波干燥无烟煤的新工艺。在单因子实验的基础上,建立了可信的各因子与响应值的数学模型为：脱水率=24.51+6.37x1+18.19x2-6.55x3+5.44x1x2+3.30x1x3-0.23x2x3-0.61x12+2.78x22+7.69x32并应用响应面法对微波干燥无烟煤的工艺条件进行了优化,由响应面分析表明：微波功率、微波时间及物料量对无烟煤干燥影响的显著性顺序为微波时间>>物料量>微波功率；结果表明微波时间,微波功率,物料量对脱水率的影响不是简单的线性关系,一次项和二次项对脱水率影响显著,交互项影响较小。微波干燥参数优化表明：在脱水率达到要求的条件下,在设计水平内预获得较高的干燥效率,其最佳工艺参数为微波功率682.07W、微波时间2.98min、物料量49.19g。此时的处理效率最高为1.452kg/kW·h。此时实验值与预测的偏差为+0.57%。(4)研究了微波-热泵联合干燥高纯石英砂的新工艺。开展了高纯石英砂的微波干燥条件试验,考察了物料量、初始含水率及微波功率对脱水率、湿基含水率和干燥效能的影响；开展了高纯石英砂的空气源热泵干燥条件试验,考察了物料量、风速、及初始含水率对脱水率、湿基含水率和干燥效能的影响；在上述微波、热泵单独干燥的基础上,建立了可信的各因子与响应值的数学模型为：湿基含水率=0.39-0.91x1-0.44x2+0.69x3+0.34x1x2-0.096x1x3-0.091x2x3+0.45x12+0.10x22+0.40x32能效比=0.65-0.21x1-0.021x2+0.24x3-0.002x1x2-0.065x1x3+0.003x2x3+0.059x12-0.00922-0.072x32并应用响应面法对微波-热泵联合干燥高纯石英砂的工艺条件进行了优化,结果表明各影响因子对湿基含水率与能效比的影响是复杂的非线性关系,并得出联合干燥的最佳工艺参数：物料量620g,微波处理时间为4.6minute,热风处理时间为9minute,此时实验值与预测的偏差为0.01%。(5)进行了无烟煤、钛精矿和高纯石英砂的微波干燥动力学研究。得到无烟煤的微波干燥速率常数与功料比的关系为：k=0.06102-4.09204 m/p+140.05833(m/p)2-1691.86413(m/p)3干燥活化能为31.30629W/g,指数前因子A0为0.04285；得到钛精矿的微波干燥速率常数与功料比的关系为：k=0.0652-1.35849m/p+11.64559(m/p)2-30.57185(m/p)3；干燥活化能为6.9600 W/g,指数前因子A0为0.0362。得到高纯石英砂的干燥活化能为6.9600W/g,指数前因子A0为0.0362。在功率密度大于1(1.000-4.167)的条件下活化能为0.279W/g,在功率密度小于1(0.429-1.000)的条件下活化能为3.493W/g。研究了微波干燥高纯石英砂的能效比,实际值和理论值都是随着含水率的降低而减少,所获得的能效比的实际值和理论值分别为3.92和7.14MJ/kg水。(6)结合微波、热泵干燥的特点,提出了高纯石英砂的微波-热泵组合干燥的技术构想,在热工计算的基础上设计与试制了高纯石英砂的微波-热泵组合干燥系统实验样机。实验研究结果表明,组合干燥的脱水能效可达0.7kg/kW·h以上,远高于单一热源干燥及常规干燥的能效。更多还原

【Abstract】 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.更多还原