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DRY CLEANING OF PULVERIZED COAL USING A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR (RTS)D. TAO 1,2 , A. SOBHY1 , Q. LI1, R. HONAKER 1 , AND Y. ZHAO21University of Kentucky, Lexington, KY, USA 2School of Chemical Engineering, China University of Mining Technology, Xuzhou, Jiangsu Province, China Coal cleaning is often conducted using wet physical separation processes such as heavy medium vessels or spirals at coal prep- aration plants to remove impurities such as ash, sulfur, and mercury. However, the resultant clean coal product still contains a significant amount of impurity due to the fact that impurities are not well liber- ated from coal particles ranging from several millimeters to inches in size at which wet cleaning processes take place. A cleaner coal product can be obtained if a dry process is avaialble to further clean pulverized and thus better liberated fine coal at the power plant prior to its combustion.In this study, a novel rotary triboelectrostatic separator (RTS) was investigated for its application to dry cleaning of fine coal sam- ples acquired from the power plants in the state of Illinois. The pro- prietary RTS is characterized by an innovative high-efficiency rotary charger, charger electrification, laminar air flow, etc. Compared to existing triboelectrostatic separators, the RTS offers significant advantages in particle charging efficiency, solids throughput, The authors would like to sincerely acknowledge the funding of the Department of Commerce and Economic Opportunity of the state of Illinois through the Office of Coal Development and the Illinois Clean Coal Institute under the project Number of 08-1= 4.1A-1, which made this work possible. Technical assistance and professional advice from the project manager Joseph Hirschi is deeply appreciated. separation efficiency, applicable particle size range, etc. Important process parameters such as charger rotation speed, injection and co-flow rate, and feed rate were investigated for their effects on separation performance. Keywords: Fine coal; Liberation; Particle charging; Rotary charger; Triboelectrostatic separation INTRODUCTION Coal is a major source of energy in the United States and more than 51% of the electricity used in the country is generated from coal. Numerous advanced coal-cleaning processes have been developed in recent years to reduce ash, sulfur, and mercury contents. However, most of the processes involve the use of water as a medium and thus the clean coal products must be dewatered before they can be transported and burned at power plants. The high cost associated with fine coal dewatering makes it difficult to deploy advanced coal-cleaning processes in commer- cial applications. Dry beneficiation technique is an alternate approach to solving this problem. Recently, the literature on dry beneficiation methods for coals with specific reference to high-ash Indian coals has been summarized by Dwari and Rao 1. Triboelectrostatic process is one of the key dry pro- cess techniques to separate the ash-forming inorganic minerals from coal. Electrostatic separator with tribo-charging technique has great potential for coal preparation in fine sizes. The triboelectrostatic system can be divided into two major zones: a tribocharging zone to differentially charge a mixture of particles and a separation zone to physically separate charged particles. The coal- and ash-forming minerals are charged in the tribocharger based on their rela- tive work functions. After triboelectrification, the particles entering into the electric field get attracted towards a positive or negative electrode plate according to their charge polarity and magnitude. Several studies have shown that clean coal generally charges positively and ash-forming minerals or high-ash coals charge negatively 25. The present study was conducted to investigate the novel rotary tri- boelectrostatic separator (RTS) for its application to dry cleaning of fine coal samples acquired from a power plant. The pulverized fine coal con- tains well liberated ash and pyrite minerals and is an ideal feed to the tri- boelectrostatic separator for further cleaning without the use of water or any chemical reagents. The coal particles are positively charged while ash particles are negatively charged as a result of differential charging in the charging chamber.EXPERIMENTAL Fine Coal Sample The pulverized coal sample used in this study was acquired from a power plant that uses a wet pulverizer to reduce coal size feed to the boiler. The proximate analysis showed the coal sample had 16.32% moisture, 8.0% ash, 45.50% volatile matter, and 30.73% fixed carbon. Table 1 shows the size-by-size weight and ash distribution data of the coal sample. Approxi- mately 50.41% of the sample had a particle size less than 25 mm. In other words, this particular sample had a d50 of smaller than 25 mm. The washability data shown in Table 2 for a 325 mesh particle size fraction was obtained using a lithium metatungstate (LMT) solution as heavy medium. The fact that there is a very small amount of 1.6 density fraction indicates this sample is very difficult for further cleaning by any gravity-based separation method. Flotation release analysis was also per- formed to produce the best possible flotation performance with this parti- cular coal sample and the results will be presented in the next section. Triboelectrostatic Separation Tests A 100-gram sample was fed to the rotary triboelectrostatic separator (RTS) in each experiment using a vibratory feeder. Three products were collected from the first stage. Each product from the first stage was further processed to generate nine final products. Process parameters investigated in this study included feed rate, charger rotation speed, charger material, charging voltage, injection flow rate, and co-flow rate. The feed was dried over night in an oven at a temperature slightly above 100F prior to the separation test.Table 1. Particle size-by-size weight and ash distributionTable 2. Density fractionation analysis of 325 mesh fine coal sample The rotary triboelectrostatic separation (RTS) system shown in Figure 1 includes a vibratory sample feeder, a rotary charger or charge exchanger, a separation chamber, an injection gas unit, and twohigh-voltage DC supplies. Samples are fed by the feeder into the rotary charger. A small amount of transport gas is injected with the particles. The gas-particle flow interacts with the rotary charger. Due toparticle-to-charger or particle-to-particle collisions, particles become charged negatively or positively, depending on their work functions. The charged particles then pass through the separation chamber and report to one of three cyclones attached to the system. More details about the apparatus can be found in a previous publication 5.There are two high-voltage sources in the rotary separator system: one is for the particle charging, which is attached to the charger, and the other is for the separation of the charged particles, which is attached to the separation chamber. The most distinct feature of the rotary separ- ator is that particle charge density and polarity can be controlled by changing the applied voltage. This is an innovative concept that allows separation of multiple components at different stages with different applied voltages, which is analogous to adding different reagents at different stages in the flotation process.Figure 1. Illustration of the rotary triboelectrostatic separator.Description of Fundamentals of Triboelectrostatic Separation Triboelectrostatic separation is a dry separation process based on the fact that when two particles are rubbed against each other or a third object (referred to as charger), the particle with higher work function becomes negatively charged and the other positively charged. The work function / is defined as the minimum energy that must be supplied to extract an electron from a solid. It is a measure of how tightly electrons are bound to a material. The charged particles are subsequently sepa- rated in an external electric field as a result of their different motiontrajectories. A separation zone shown in Figure 2 can be used to describe moving particle trajectories in an electric field where x represents the horizon- tal direction and ythe vertical direction. When a charged particle enters the electric field, its trajectory is governed by the momentum of the gas flow and gravity in addition to the electric force. It deflects to a positive or negative electrode, depending on its charge polarity.Figure 2. Illustration of the separator chamber.If the high voltage electrodes are mounted vertically as shown in Figure 2, the electrostatic force will accelerate the particles horizontally. The particle residence time, that is, the time for a particletraveling through the separation chamber, is controlled by the particle vertical motion. However, the horizontal particle motion is controlled by electric field deflection. The law governing the horizontal displacement, xx, of the moving particle is: where m is the mass of particle, xx the horizontal displacement vector, t the time, EE the electric field intensity, and q the charge of particle. The charge-to-mass ratio, q=m, is referred to as particle specific charge. It is a very important parameter for the motion of the particle in the electrostatic separation process. If the resistance of air with viscosity g is also considered, the hori- zontal motion of a moving spherical particle of radius r is given by:A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR (RTS) The solution to Equation 2 gives the speed of the particle as a function of time:When or the horizontal terminal velocity of particle is: Under these conditions, the horizontal terminal velocity is independent of the mass. However, since time t is in milliseconds with a practical sep- arator, the mass does play an important role in determining the horizon- tal motion of the particle and, therefore, the resultant trajectory that affects the separation performance. Particle motion in the vertical direction is influenced by gravitational force and gas drag force. The governing equation is: where g is the dynamic viscosity of gas and g is the gravitational acceler- ation. For the initial conditions of t = 0, y(0) =0, and dy(0)/dt =V0 , Equation 5 can be solved as where B =6r/m. The particle trajectories can be obtained from Equations 4 and 6 under given conditions. Figure 3 shows typical trajec- tories for negatively harged particles of different sizes. They deflect right to the positive electrode and can be readily separated from posi- tively charged metal particles that deflect left to the negative electrode.The particle trajectory is affected by particle charge, mass of par- ticle, radius of particle, and electric field intensity, as indicated by Equa- tions 4 and 6. A larger difference in trajectories of different particles enhances separation efficiency. This may be achieved by the use of ahigher electric field intensity and greater particle charge density. How- ever, the electric field intensity is limited by airionization and is normally set at 300,000500,000 V=m. A potentially huge improvement insepar- ation efficiency can be achieved by enhancing particle charge density, which can be achieved using the RTS that takes advantages of high rotation speed and controlled potential of the charger. However, higher charger rotation speed increases the wear rate of the roller and thus it was limited to 5000 rpm in the study. The maximum applied charger volt- age was imposed since too high a voltage (either negative or positive) will cause electric sparks as a result of ionization of air molecules.Figure 3. Trajectories of particles of different sizes in an electric field.RESULTS AND DISCUSSION Several series of triboelectrostatic separation tests were conducted to evaluate the dependence of process performance on operating para- meters. The separation performance data is presented in terms of com- bustible recovery versus normalized product ash that is defined as the percentage of product ash to feed ash. The normalized product ash is utilized in this study since the wet and sticky feed sample was very diffi- cult to homogenize completely and significant fluctuations in feed ash were observed in the tests. In addition, the separation efficiency (defined as the combustible recovery minus ash recovery) of the first stage of sep- aration is used to show the effect of individual process parameters. Unless otherwise specified, all separation tests were conducted using the copper charger under the following conditions: charger rotation speed: 3000 rpm; injection air velocity: 2.5 m/s; co-flow air velocity: 3.1 m/s; feed rate: 800 g/h; separation voltage: 22.5 kV; charger voltage: 2.5 kV; temperature: 24C. Effect of Feed Rate Figure 4 shows the changes in separation curves (left) and separation efficiency (right) with feed rate. As the feed rate doubled from 400 g/h to 800 g/h, the separation performance was essentially constant. As the feed rate increased further to 1500 and 2000 g/h, the separation curve shown in Figure 4 shifted away gradually from the upper left cor- ner and the value of separation efficiency decreased gradually. A more significant decrease in separation efficiency was observed when the feed rate increased from 2000 g/h to 3600 g/h, which suggests the maximum feed rate should be at about 2000 g/h. The lower separation efficiency at higher feed rate is mainly caused by fewer contacts with the charger sur- face and thus lower charge density on particle surface. Figure 4 (left) shows that a product 13% cleaner or of 5.5% ash can be obtained at 80% combustible recovery at a feed rate of up to 2000 g/h. A cleaner product can be produced at theexpense of combustible recovery. For example, a clean coal of about 4.5% ash, which represents 30% ash reduction, was achieved at 800 g=h feed rate with about 33% combustible recovery. It should be noted that during the tests the lower ash product moved toward the negative electrode and the higher ash product was deflected to the positive electrode, suggesting that carbon particles were positively charged and minerals were negatively charged.Figure 4. Effect of feed rate on triboelectrostatic separation curves (left) and separation efficiency(right). Effect of Charger Rotation Speed The tribocharging is largely attributed to the relative speed between par- ticles and the rotary charger, with higher speed resulting in greater sur- face charge density. The easiest way to control tribocharging is perhaps to adjust the rotary charger rotation speed. The experimental results on the effects of rotation speed on fine coal separation are shown in Figure 5. Figure 5 indicates that the optimum rotation speed was 5000 rpm at which an ash reduction of more than 30% can be achieved at a combustible recovery of about 40%. The separation efficiency for the first stage of separation suggests that the maximum separation efficiency was achieved at 4000 rpm and a slightly lower efficiency was observed at 5000 rpm. It is clear from the data shown in Figure 5 that better separation was accomplished at a higher rotation speed of 4000 or 5000 rpm, which is consistent with the established theory that higher surface charge results from an increase in the relative motion speed between the charger and particles 6. Figure 5. Effect of charger rotation speed ontriboelectrostatic separation curves (left) and separation efficiency (right).Effect of Applied Charger Voltage. One of the unique features of the rotary triboelectrostatic separator is the applied potential to the charger to enhance the particle-charging pro- cess. Figure 6 shows the separation curves and separation efficiency at different charging voltages ranging from 5 kV to 5 kV. It is quite clear from both figures that the separation performance was significantly increased as the charging voltage varied from 5 kV to 5 kV. It is inter- esting to point out that compared to 0 V or no charging voltage that is the case with the conventional triboelectrostatic separator, the separation at 5 kV was substantially more efficient. For example, a product ash reduction of 30% could hardly be obtained at 0 V charging voltage. How- ever, it can be easily obtained at a 5 kV charging potential with a com- bustible recovery of more than 55%. Comparing the separation curves at 0 V and 5 kV indicates that up to a 50% increase in combustible recov- ery was achieved if the charging voltage was changed from 0 V to 5 kV, which clearly illustrates the great importance of controlling the applied charger potential. Effect of Injection Flow Rate It is known that the injection flow rate or velocity affects the particle speed and the residency time in the charging and the separation cham- ber, and thus the particle charge density and separation efficiency. A higher injection flow rate results in a faster velocity at which particles Figure 6. Effect of applied charger voltage on triboelectrostatic separation curves (left) and separation efficiency (right).Figure 7. Effect of injection flow rate on triboelectrostatic separation curves (left) and separation efficiency (right).struck the charger but it causes a shorter charging time and separation time. Figure 7 shows the separation curves and separation efficiency at different injection flow velocity. The best separation performance was obtained at about 2.53.7 m/s injection velocity. A velocity lower than 2.5 m/s or higher than 3.7 m/s resulted in poorer separation due to lower impact velocity and shorter residence time, respectively.Effect of Co-Flow RateThe gas flow that enters the separation zone on both sides of the connec- tor between the charging and separation chamber is referred to as co-flow. It is used to comb the misplaced particle and to force them to deflect to the desired product stream. Figure 8 shows the separation curves and efficiency, respectively at different co-flow rates. As the co-flow rate increased from 1.5 m/s to 2.5 m/s, the separation curve move considerably toward the upper left corner, indicating a better sep- aration. As the co-flow rate further increased to 3.1 m/s and 3.7 m/s, the separation shifted moderately lower toward the right side, suggesting a lower combustible recovery at a given product ash. However, the differ- ences in combustible recovery at 2.5, 3.1, and 3.7 m=s were quite small, especially at higher product ash values. The separation efficiency curve for the first stage of separation indicates that the optimum co-flow rate was at approximately 3 m=s, which is essentially the same as the optimum injection flow rate shown in Figure 7. This result is consistent with the previous study with a fly ash sample that has demonstrated that the opti- mum performance was achieved when the injection flow and co-flow rates are nearly equal to each other 5. It is believed that when two gas velocities are identical, the turbulence in the separation chamber is minimal, which creates a favorable condition for efficient particle separation.Figure 8. Effect of co-flow rate on triboelectrostaticseparation curves (left) and separationefficiency (right).Effect of Charger MaterialTo investigate the effect of charger material on triboelectrostatic separ- ation performance, two chargers made of copper and stainless steel were tested for fine-coal cleaning. Copper is known to be a good material for the charger while stainless steel is a common wear-resistant material. Figure 9 shows the separation curves obtained with both copper and stainless steel chargers. Obviously, the copper charger produced better separation than the stainless steel charger. The improvement in the sep- aration efficiency observed with the copper charger is moderate with this particular coal sample but much more significant improvement with other coal samples, as shown in Figure 10. Apparently, the importance of the charger material for the separation efficiency depends on the coal sample to be processed. The relatively small difference in separation per- formance between two different chargers in Figure 9 is because this coal sample contains a small amount of high-ash particles and thus more difficult for further cleaning.Figure 9. Effect of charger material on triboelectrostatic separation curve (Combustible Recovery vs. Product Ash/Feed Ash). Comparison of RTS Performance with Other Separation Methods In order to evaluate the performance of the RTS technology, its separ- ation performance curve is compared with the washability and releaseFigure 10. Effect of charger material on triboelectrostatic separation curve with another fine coal sample (Combustible Recovery vs. Product Ash/Feed Ash). Figure 11. Comparison of RTS separation results with washability and release analysis data.analysis data, as shown in Figure 11. Obviously, the RTS technology produced a much better separation than flotation release analysis. In comparison with a washability analysis, the RTS process is more efficient than the float-sink test in producing lower ash product but slightly less efficient if less than 27% ash reduction is needed.CONCLUSIONSBased on the above test results, the following conclusions can be drawn: 1. For the pulverized fine coal sample, the coal particles are positively charged while ash particles are negatively charged. 2. Process variables including feed rate, injection flow rate, co-flow rate, and charger potential all affect the separation efficiency. The appro- priate charger potential is critical for the separation efficiency. For the tested coal sample, 5 kV produced much better separation than 0 V or positive voltages. Other coal samples may require considerably different potential. The injection flow rate and co-flow rate should be equal to result in the most favorable flow condition in the separation chamber.3. The optimum operating conditions for this specific coal sample were 2000 g/h feed rate, 5000 rpm charger rotation speed, 5 kV charger potential, and injection and co-flow air rates of approximately 2.53 m/s. 4. The copper rotary charger performed better than the stainless steel rotary charger in upgrading the fine coal particles. The degree of improvement by the copper charger depends on the coal sample. 5. The RTS technology produced a separation performance that is con- siderably better than the flotation release analysis and comparable to washability analysis. 6. Separation samples need to be analyzed for sulfur and mercury contents to determine the reduction efficiency for both harmful components. The results will be reported in a future publication.REFERENCES1. Dwari, R. K., and K. Hanumantha Rao. 2007. Dry beneficiation of coal A review. Mineral Processing and Extractive Metallurgy Review 28: 177234.2. Carta, M., C. Del Fa, R. Ciccu, L. Curreli, and M. Agus. 1976. Technical and economical problems connected with the dry cleaning of raw coal and inparticular with pyrite removal by means of electric separation. In Proceedings of 7th International Coal Preparation Congress, ed. A. Partridge, 135. Australia: Adept Printing.3. Lockhart, N. C. 1984. Review paper: Beneficiation of coal. Powder Technology 40: 1742.4. Alfano, G., P. Carbini, R. Ciccu, M. Ghiani, R. Peretti, and A. Zucca. 1988. Progress in triboelectric separation of minerals. In Proceedings of XVI International Mineral Processing Congress, ed. E. Forssberg, 833844. Amsterdam: Elsevier.5. Tao, D., M. Fan, and X. Jiang. 2008. An Innovative Rotary Triboelectrostatic Separator for Fly Ash Purification, Annual SME Meeting. Preprint 08-008, Salt Lake City, Utah, February 2528, 2008.6. Hower et al. 1997. 202 D. TAO ET AL. Downloaded by新型旋转摩擦电选机在粉煤干法分选中的的应用摘要：目前使用的选煤方法是利用物理的湿法分离过程，例如现在选煤厂使用跳汰分选和重介质旋流器分选技术来除去如灰 、硫、和汞等杂质。然而，在湿法分选过程中，分选出来的精煤产品还包含一定因在细颗粒中没有完全解离而存在一定数量的杂质。一个清洁的煤炭产品是在发电厂燃烧前对细粒煤充分解离并用干法分选得到清洁的粉煤。这项研究是在伊利诺斯州的一座发电厂，运用了一种新型的旋转式摩擦电选机（即RTS）来对细粒煤煤样进行干法分选。RTS专有的特点是有一个创新高效旋转带电室,带电室电气化,形成层流气流。与现有的摩擦静电分离器相比，RTS具有的优势是粒子带电效率高，处理量大的特点。根据对重要工艺参数如带电室旋转速度，入料量大小和速度，以及协流量的研究，从而来确定对分离效率的影响。关键词：粉煤 解离 离子导电率 带电量 摩擦分选引言在美国，煤是一种主要的能源，超过51%的电能来自燃煤电厂。因此近年来研究开发了许多先进的煤炭清洁技术以减少灰尘、硫和汞含量。然而，大多数的技术是以水作为介质对煤进行分选,从而使得清洁煤炭产品必须脱水后才能运送到发电厂燃烧。但是成本过高的细粒煤脱水技术却很难在商业中获得应用与推广。而干法分选技术能很好解决这个问题。 最近，Dwari 和Rao在文献上发表了印度高灰煤的特定分选方法。摩擦电选过程是将煤和由无机矿物的形成的灰分分离的干法工艺的关键技术之一。静电分离器的摩擦带电技术具有很大的潜力来分选粉煤。摩擦电选系统可以分为两个主要部分：一个摩擦带电区域使混合物颗粒因电异性不同而分别带电，从而以物理方式在分离区分离带电粒子。煤和形成灰分的矿物根据在摩擦带电区域中所做的功不同而分别带不同电性的电荷。摩擦带电后，进入电场的粒子开始根据它们的电荷极性和幅值不不同分别吸引向正或负电极板。一些研究已经表明，清洁煤一般表面带正电而形成灰分的矿物或高灰分煤表面带负电。 本研究是对新型旋转摩擦带电分选机（RTS）在从电厂收集的优质煤炭样本进行干法洗选方面的应用进行研究。将粉碎的细煤和解离的灰分和黄铁矿矿物，以一定适合的给料速度给入的摩擦电选分离机中的摩擦带电室里，通过摩擦而使煤粒子带正电荷，灰分颗粒带负电荷。从而进行分选。同时不使用水和任何化学试剂。实验细粒煤样本研究中使用的粉煤样品是发电厂在燃烧前利用湿式粉碎机破碎的粉煤。其分析表明，水分为6.32，灰分为8.0，挥发分为45.50，和30.73固定碳的煤样。表1 为煤样粒度和灰分之间的关系表。其中粒度小于25mm的样品含量为50.41。换句话说，这个特定样品的D50小于25mm。使用锂作为重介质的偏钨酸铵（LMT）溶液，得到为325网目的粒度级可选性数据示见表2。事实上，在密度为+1.6g/cm含量较少，表明该样本运用任何重力分选方法都是非常困难的。通过浮选分析，这个特殊的煤样最佳浮选性能，其结果将在下一节中介绍。表1 粒子大小尺寸重量与灰分分布 增 量 筛 上 物 筛 下 物粒级（um） 重量% 灰分% 重量% 灰分% 重量% 灰分%+150 1.83 6.53 1.83 6.53 100.00 8.03150-74 10.2 5.57 12.04 5.71 98.17 8.0674-63 3.19 6.44 15.24 5.87 87.96 8.3563-44 6.63 7.32 21.87 6.31 84.76 8.4244-37 19.37 7.28 41.24 6.76 78.13 8.5137-25 8.35 8.29 49.59 7.02 58.76 8.92-25 50.41 9.03 100.00 8.03 50.41 9.03总计 100 8.03 表2 +325目细煤样的密度分离分析增量可洗性 分析 精 煤 累 积密度 重量% 灰分% 重量% 灰分% 重量% 灰分1.21.3 25.81 6.70 25.81 6.70 71.81 26.561.31.4 43.88 6.96 69.69 6.86 73.57 71.591.41.6 23.45 10.65 93.14 7.82 83.78 94.691.61.8 4.62 18.03 97.76 8.30 88.96 98.871.82.65 2.24 54.29 100.00 9.33 100.00 100.00总计 100.00 9.33摩擦分离试验实验中，使用振动给料器将100克的样品给到旋转摩擦电选机（RTS），第一阶段分离出三种产品。对第一阶段的每个产品进一步处理，从而以生成9个最终产品。在这项研究中研究的工艺参数包括给料速率，电选机转速，摩擦带电器材料，加电电压，喷射流量和协流量。在实验进行前，将煤样放入100oF的烘箱中干燥。 旋转摩擦分离系统（RTS）如图1，其包括一个振动样品给料机，旋转式带电器或加电热交换器，分离室，喷射气体单元，和两个高压的直流电源。样品由给料机输送到旋转带电室。少量的气体也被注入。气体颗粒流与旋转带电室发生交互运动。由于颗粒与带电室或颗粒与颗粒之间碰撞，颗粒被带负电荷或正的，这取决于它们的自我特性。带电粒子带电后，通过三个气旋分离室和通道从而进入到系统。在以前的相关文章中可以找到有关设备的更多细节。旋转分离系统有两个高电压源：一个是给粒子带电，它是连接到带电室，另一个是用于分离带电粒子的，它位于分离室。旋转分离器的最显着特征是，粒子的电荷密度和极性，可以通过改变所施加的电压而改变。这是一种创新概念，它有多个分离组件，它允许在不同阶段的施加不同的电压，这是类似于在浮选过程中的不同阶段中添加不同的试剂。图1 图示为旋转摩擦电分离过程摩擦电选分选原理摩擦电选是一种干燥的分离过程。其过程为：当两个颗粒彼此摩擦或与带电室摩擦（以下简称为带电室），变成带负电荷的颗粒具有更高的功，其它的就带正电荷。功的最小值被定义是固体颗粒中可得到的一个电子。其带电粒子在外电场分离后，从而使他它们有不同的运动轨迹。在分离区，如图2中所示，颗粒在电场中的运动轨迹如图，其中“X”表示在水平方向，“Y”表示在垂直方向。当带电粒子进入电场，其轨迹是由静电力和气体阻力以及自身重力势能所决定。而偏转到一个正的或负的电极，这取决于它的电荷极性。图2 分离室图示如果高电压电极垂直安装，如在图2中所示，静电力将使粒子得到一个水平加速度。颗粒在分离室的时间是由颗粒在垂直方向运动所决定的。但是，粒子水平方向的运动可以通过电场偏转控制。如果x表示水平位移，则粒子的运动方程如下： 方程1 其中m是粒子的质量，x的水平位移矢量，t为时间，E是电场强度，q为粒子的电荷量。q/m为电荷与电子质量比，被称为作为粒子荷质比。这是一个非常重要的参数，在静电分离过程中的对粒子的运动中，这是一个非常重要的参数。如果电阻与空气的粘度是已知的，半径为r的球形颗粒在水平方向运动的运动由下式给出：方程2、对方程2，当颗粒的速度是时间的函数时可得出如下方程：方程3 当或者则粒子的水平的终端速度是：方程4 在这些条件下，水平的终端速度是独立的量。然而，当粒子在水平方向速度一定时，由于时间t是与一个实际参量（以毫秒为单位）同时质量也起到了重要的作用，因此，在此条件下得到的轨迹对分离结果有一定的影响。粒子在垂直方向上的的运动受重力和气体阻力的影响。方程为：方程5 其中，是气体的动态粘度，g是重力加速度。当满足t=0， y（0）=0，dy(0)/dt =V0 时，方程5可变为：方程6 在给定条件下，可以从方程（4）和6中可得到的颗粒轨迹。如图3所示为带不同电荷量的负电荷颗粒的典型轨迹。他们偏转到正电极，从而与向左边负电极偏移的带正电荷金属离子分开。 其中粒子电荷量，颗粒的质量，粒子半径，和电场强度对颗粒轨迹的影响，如方程4和6。对不同的粒子的运动轨迹有较大的区别，从而提高了分离效率。这可以通过用一个高电场强度和具有更大的粒子电荷密度来实现。但是，空气电离的电场强度是有限的，一般为300000-500000 V /M。如使用的RTS，利用控制高速旋转的带电室来加强粒子的电荷密度，分离效率是可以大幅度的提高的。然而，较高的带电室的旋转速度增加辊子的磨损率，因此，在研究中它被限制到5000 rpm。最大带电电压是由于太高电压（无论是正电或负电）因空气分子的电离会引起电火花，因此也有一定的范围。图3。在电场中的不同尺寸的颗粒的轨迹曲线 结果与分析 通过一系列的实验，可得出各操作参数与工艺性能的关系。由分离特性的数据可可知燃体回收率随产品灰分变化而变化，其定义为产品灰分与入料灰分百分数的比值。在本次试验中，产品灰分是指定不变化的，但是由于潮湿和粘度的影响，入料煤样很难均匀化，因此，在实验中使用的煤样灰分有显著的波动。此外，分离的第一阶段的分离效率（由可燃物回收量减去灰分回收量表示）为单个过程参数的影响效果。除非另有规定，所有的分离试验在下列条件下进行了使用铜带电室：带电室旋转速度：3000rpm,空气流速：2.5m/s 混合空气流速：3.1m/s给料速度：800g/h分离电压：22.5kv带电室电压：-2.5kv温度：24oC.给料速度的影响 图4所示的在分离曲线（左）和分离效率（右）与进料速率的变化。给料量从400g/h 到800g/h,其分离效果基本不变化。当给料量进一步上升至1500g/h到2000g/h时，如图4中所示的分离曲线移离逐渐从左上角逐渐降低和分离效率逐渐变小。当给料量由2000g/h变到3600g/h时，分离效率明显减小，由此表明，最大给料量为2000g/h。较高的给料量而低的分离效率是由于给料量变大后，颗粒表面与带电室接触几率变小，从而降低粒子表面上的电荷密度。图4（左）表明在进料速率高达至2000g/h时，在得到80的可燃物回收率的前提下，产品产率为13的或者说得到灰分为5.5产品，可以在高的可燃物回收率下，可以得到一个好的清洁产品。图4擦电选分离曲线（左）和分离的给料速率对效率的影响效率（右）。如，在给料量为800g/h和33%的回收率时，可以得到灰分为4.5%的产品，其灰分减少为30%。应当指出，在测试过程中的低灰分的产品向负电极移动，和较高的灰分的产品向正电极移动，表明碳粒子带正电，带负电荷为矿物。带电室旋转速度的影响粒子表面电荷密度很大程度上归因于颗粒之间的相对摩擦和颗粒与旋转带电室的摩擦，因此，带电室具有更高的速度，从而导致颗粒具有更大的表面电荷密度。通过调整旋转带电室的旋转速度来控
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