【作者】 江津河；

【导师】 李建隆；

【作者基本信息】 青岛科技大学 ， 化学工程， 2008， 博士

【摘要】 粉体制造时,分级是决定最终粉体产品粒度组成、级别宽窄、粗细颗粒含量的关键,对窄级别或极窄级别超细粉体的制造,分级显得比粉碎更重要。为此,国内外开展了大量的理论和实验研究,以解决分级下限、分级效率和能耗三大问题。传统的干式分级由于受流场稳定性、物料分散等方面的制约,在解决窄粒度分布、切割清晰度和高回收率上存在困难。湿式分级虽然流场稳定性优于干式分级,物料分散的难度低,但存在后续干燥,甚至需要二次粉碎等弊端,故主要用于某些贵重粉体的分级。基于现有干式分级和湿式分级各自存在的缺陷,通过详细分析超细颗粒之间的作用力和颗粒流态化后的扬析、夹带及粗细颗粒之间存在的相平衡现象,借鉴上升流分级原理和液体精馏原理,作者在此提出了多级多尺度散式流态化精密分级的干式分级方法和概念。即分级塔内设置多层筛板,每两层筛板间空域即为一分级区,靠近上层筛板的下方为稀相区,以含细颗粒为主,靠近筛板的上方为浓相区,浓相区颗粒及颗粒聚团靠倾斜滑动进入塔板低位端设置的粗颗粒聚集槽内由轴流风机引入下层筛板上的空间实现再分散,稀相区颗粒随上升气流上升至上层筛板。粉料进入塔前经过高速气流的预分散,在塔内呈散式流态化状态,小颗粒聚团和被大颗粒携带的小颗粒在下落被循环的过程中经轴流风机多次打碎分散,从而产生稀浓相颗粒在流体中的浓度平衡,使细颗粒获得多次分离,这样通过多个流化层对细颗粒的多次分离就产生出多级流态化分级效果;通过计算颗粒的终端速度,调节塔内风速的大小,可以满足分离不同粒径颗粒的分级要求,得到不同尺度要求的高纯超细粉体。经过一系列的试验、改进和优化。证明,该实验设备具有分级精度高,粒级效率高,分级下限低,易于控制等优点。作者以超细颗粒之间的作用力理论为基础,建立了干燥条件下分级过程中超细颗粒凝并模型,并进行了定量计算,得到了本分级塔实验条件下的不同粒径石英砂颗粒凝并后的颗粒当量直径。计算结果表明,直径1μm石英砂颗粒凝并后的颗粒聚团的当量直径可达1480μm,而直径10μm石英砂颗粒凝并后的颗粒聚团的当量直径只有279μm,对直径1~10μm石英砂颗粒,扩散力最小,可被忽略,范德华力与静电吸引力占绝对统治地位,从理论上定量解释了超细颗粒干式分级实验结果中粒级效率所呈现的鱼钩效应;另外,以超细颗粒凝并后的当量直径和流态化条件下颗粒的扬析与夹带理论为基础,建立了超细颗粒(属C类颗粒)的粒级效率模型,计算结果与实验结果基本吻合。借助于先进的模拟软件FLUENT6.2,对分级塔进行了气流流场和颗粒轨迹的模拟计算,流场模拟计算显示:精密分级塔内静压分布基本呈阶梯状分布,沿Z轴方向,由下至上呈递减的趋势,经过每一块塔板时就产生一次显著压降,从减压分级原理上讲有利于颗粒进行分级操作;气速沿Z轴方向向上穿过整个塔体时有一个速度递减趋势,即流态化精密分级塔所采用的速度梯度原理与精馏中的温度梯度原理相对应;气流穿过塔板时速度增大,其湍动强度远远大于其它空域的湍动强度,并在匀风板上达到最大;塔板与塔板间空域处,流场规整无涡旋,速度分布均匀,与上升流分级机流场相似。颗粒轨迹模拟结果显示:在塔内风速为0.100m/s的条件下,大颗粒群均沉降到塔的底部,小颗粒群大多数从塔顶溢出,极少数被引出气流引入下层塔板重新分离,中等粒径的颗粒均被引出气流引入下层塔板;粒径大的颗粒在沉降过程中,其动量也普遍大,但一部分经过塔板的碰撞和引出气流的吸引,动量减少,分离出的细颗粒无论粒径大小,其动量基本相同,表明了颗粒越小,其对气流的跟随性越好,越容易被分离出;颗粒粒径越大,其湍动度也相对较大,并且经过塔板时的湍动度达到最大;塔板通过的颗粒越多,颗粒的湍动度越大。模拟计算结果验证了分级塔塔板设计、粗颗粒引出系统、分级区设计和在塔内产生速度梯度的必要性,为精密分级塔总体结构上的改进和操作上的优化提供了坚实的理论依据。在本实验装置上的分级实验结果表明:以滑石粉颗粒为原料进行分级,将塔内风速控制在0.122 m/s,粒径10μm颗粒的粒级效率达到了78.78%,d95达到12.84μm;以石英砂颗粒为原料进行分级,将塔内风速控制在0.139 m/s,粒径10μm颗粒的粒级效率达到了92.80%,d95达到12.58μm;在操作条件允许的条件下,加料量越少,分级效率越高;粗颗粒循环系统循环风速,在本实验中由上至下为:0.5m/s、1.0m/s、1.5m/s、2.0m/s和2.5m/s较为合适;本实验条件下,回流细粉量越大对获得更纯细颗粒越有利,但也使分级效率下降,对分级产生不利影响。分级实验中的最佳风速与计算得出的理想风速是有差别的,对于粒径10μm颗粒的计算分离风速,通过分级性能趋势图可看出,要小于实际风速,但随着风速的提高,溢流产品中粗颗粒的返混率会不断增大,严重影响分级精度,对于不同的物料,基于颗粒本身的性质(颗粒的形状,密度,各种结合力等)与颗粒群的性质,其理想风速与实际最佳风速亦有不同。更多还原

【Abstract】 In the process of powders preparation technique, Classification was the final program to determine particle average diameter, particle efficiency and particle size distribution. For narrow-level or extreme narrow-level distribution ultra fine powders manufacturing technology, classification technology was even more important than grind technology. Because of the importance of classification technology, there were a number of theoretical and experimental studies at home and abroad, also there were a large number of patents, technologies and equipments. The majority researches were concentrated on the dry type classification, in order to solve the lower limit of classification, particle efficiency and energy efficiency issues. Because traditional dry type classification was confined by the flow stability and material dispersion, there were many real difficulties in solving the particle size distribution and particle composition. Although the stabilization of wet type classification was more than the dry type classification, there were a follow-up dry and a second problem of comminution, so the economic augmentation was brought. Except to classify some valuable powders, the wet type classification was not been used in mass classification process.Based on the actuality of classification, relied on the foundation of the physical properties and the entrainment and elutriation theory of particles, used for reference of ascending flow classification and rectification principles, a new multilevel multi-scale particulate fluidization precision classification tower was designed. Multi-trays were equipped in the tower, which could be formed several fluidization layers. Underside of upper layer was particle dilute phase. Upside of lower layer was particle dense phase. So the multi-fluidization and classification effect was produced. The feed particles were dispersed by air flow before entered into the tower. Accumulation of little particles and schlepped little particles by large particles were shattered by axial-flow blowers in the drop process, so the particulate fluidization effects was produced. Different diameter particles could be obtained through adjusting the air rate in tower. After a series of tests and improving, testified in experiments that the classification equipment had high particle efficiency, narrow-level particle size distribution and lower limit of classification.Based on the forces among the ultra fine particles, an agglomeration mathematical model of ultra fine particles in dry condition was established, and the qualitative and quantitative calculation results were made out. Agglomerated equivalent diameters of different diameter SiO2 particles were calculated out in the first. The results indicated that the agglomerated equivalent diameter of 1μm particles could reached 1480μm, while the agglomerated equivalent diameter of 10μm particles was 279μm. For the particles of diameters from 1μm to 10μm, diffusion force was so small that could be neglected, electrostatic force and Van Der Waals force were the master forces. It could be explained the experimental result of particle efficiency’s“fish hook effect”through this theory. Based on the entrainment and elutriation theories, the separation mathematical model of C type particles was established. The results of calculation were in concordance with the experimental results.Numerical simulation of flow and particle tracks in classification tower were been made depending on the advanced simulating software FLUENT6.2. These simulation results of flow revealed that the distribution of static pressure was a ladder line. The magnitude of static pressure was presented to a decreasing trend from the bottom to the crest of classification tower, which was similar to the grads of temperature in distillation column. This phenomena was in concordance with the distillation principle which was adopted by us. The turbulent intensity in the board was stronger than any other position. The turbulent intensity of air flow reached to the max. when it was passed the distribution sheet. The simulating results of particle tracks showed that the big particles subsided on the bottom of the tower, while the small particles escaped from the outlet at the air rate of 0.100m/s. A little portion of small particles and the medium of particles were pulled to the next sheet by circulation air flow. The bigger particles in the processing of sedimentation had bigger momentum, but some taken place collision with the boards and allured by circulation air flow whose momentum were decreased. The fine particles hold the similar momentum, which indicated that the fine particles had better following behaviors for air flow. The Reynolds number became big when the diameter of particle became big, the maximum turbulent intensity taken place in the board, which indicated the force between the coarse particle and the air flow became bigger than the fine particle. That confirmed the separating effect of the board. The numerical simulation results also validated the design validity of the sheets, circulation system of bigger particle and the rate gradient in the tower.This paper presented experimental data classified for classification tower. Previous laboratory results with an experimental tower classifying talc and quartz sand powder showed its superiority of classification. The grade efficiency of the diameter of 10μm talc particle was achieved 78.78% at velocity of 0.122 m/s while d95 of fine talc particle reached 12.84μm. Also the grade efficiency of diameter of 10μm quartz sand particle was achieved 92.80% at velocity of 0.139 m/s while d95 of fine quartz sand particle was reached 12.84μm. Grade efficiency became large while the feed rate reduced. Circular air rate was suitably at 0.5m/s、1.0m/s、1.5m/s、2.0m/s和2.5m/s from upside to underside. Grade efficiency had a decrease trend while regurgitant rate increased. The calculation air rate of diameter 10μm particle was smaller than the actual air rate. These results illustrated the distinctness between the optimal experiment velocity and the calculation velocity. And also could be interpreted that different materials had different properties (density, shape of particle, several combined force et al.), so the errors were different. It was indicated clearly that the new equipment had a good future in classification application field.更多还原