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This article was downloaded by Jilin University On 16 July 2011 At 04 11 Publisher Taylor 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 2 5 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 188D TAO ET AL Downloaded by Jilin University at 04 11 16 July 2011 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 25mm In other words this particular sample had a d50of smaller than 25mm 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 Table 1 Particle size by size weight and ash distribution Particle size mm IncrementalCum RetainedCum Passing Wt Ash Wt Ash Wt Ash 1501 836 531 836 53100 008 03 150 7410 215 5712 045 7198 178 06 74 633 196 4415 245 8787 968 35 63 446 637 3221 876 3184 768 42 44 3719 377 2841 246 7678 138 51 37 258 358 2949 597 0258 768 92 2550 419 03100 008 0350 419 03 Total1008 03 A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR RTS 189 Downloaded by Jilin University at 04 11 16 July 2011 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 100 F prior to the separation test 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 two high 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 to particle 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 Table 2 Density fractionation analysis of 325 mesh fine coal sample Incremental washability analysisCumulative Float Clean coal Particle density fraction Wt Ash Wt Ash Product Ash Feed Ash Combustible Rec 1 2 1 325 816 7025 816 7071 8126 56 1 3 1 443 886 9669 696 8673 5771 59 1 4 1 623 4510 6593 147 8283 7894 69 1 6 1 84 6218 0397 768 3088 9698 87 1 8 2 652 2454 29100 009 33100 00100 00 Total100 009 33 190D TAO ET AL Downloaded by Jilin University at 04 11 16 July 2011 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 motion trajectories 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 y the 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 1 Illustration of the rotary triboelectrostatic separator A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR RTS 191 Downloaded by Jilin University at 04 11 16 July 2011 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 particle traveling 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 x x of the moving particle is d2 x x dt2 E E q m 1 where m is the mass of particle x x the horizontal displacement vector t the time E E 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 d2 x x dt2 6p g m r d x x dt E E q m 2 Figure 2 Illustration of the separator chamber 192D TAO ET AL Downloaded by Jilin University at 04 11 16 July 2011 The solution to Equation 2 gives the speed of the particle as a function of time d x x dt E Eq 6pgr 1 e t m 6pgr 3 When t m 6pgr or t 1 the horizontal terminal velocity of particle is d x x dt horizontal E Eq 6pgr 4 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 d2y dt2 6pr g m dy dt g 5 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 y t g V0B eBt Bgt g V0B B2 6 where B 6prg m The particle trajectories can be obtained from Equations 4 and 6 under given conditions Figure 3 shows typical trajec tories for negatively charged 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 a A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR RTS 193 Downloaded by Jilin University at 04 11 16 July 2011 higher electric field intensity and greater particle charge density How ever the electric field intensity is limited by air ionization and is normally set at 300 000 500 000V m A potentially huge improvement in separ 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 5000rpm 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 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 Figure 3 Trajectories of particles of different sizes in an electric field 194D TAO ET AL Downloaded by Jilin University at 04 11 16 July 2011 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 3000rpm injection air velocity 2 5m s co flow air velocity 3 1m s feed rate 800g h separation voltage 22 5kV charger voltage 2 5kV temperature 24 C 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 400g h to 800g h the separation performance was essentially constant As the feed rate increased further to 1500 and 2000g 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 2000g h to 3600g h which suggests the maximum feed rate should be at about 2000g 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 2000g h A cleaner Figure 4 Effect of feed rate on triboelectrostatic separation curves left and separation efficiency right A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR RTS 195 Downloaded by Jilin University at 04 11 16 July 2011 product can be produced at the expense of combustible recovery For example a clean coal of about 4 5 ash which represents 30 ash reduction was achieved at 800g 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 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 5000rpm 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 4000rpm and a slightly lower efficiency was observed at 5000rpm It is clear from the data shown in Figure 5 that better separation was accomplished at a higher rotation speed of 4000 or 5000rpm 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 on triboelectrostatic separation curves left and separation efficiency right 196D TAO ET AL Downloaded by Jilin University at 04 11 16 July 2011 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 5kV to 5kV It is quite clear from both figures that the separation performance was significantly increased as the charging voltage varied from 5kV to 5kV It is inter esting to point out that compared to 0Vor no charging voltage that is the case with the conventional triboelectrostatic separator the separation at 5kV was substantially more efficient For example a product ash reduction of 30 could hardly be obtained at 0V charging voltage How ever it can be easily obtained at a 5kV charging potential with a com bustible recovery of more than 55 Comparing the separation curves at 0V and 5kV indicates that up to a 50 increase in combustible recov ery was achieved if the charging voltage was changed from 0V to 5kV 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 A NOVEL ROTARY TRIBOELECTROSTATIC SEPARATOR RTS 197 Downloaded by Jilin University at 04 11 16 July 2011 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 5 3 7m s injection velocity A velocity lower than 2 5m s or higher than 3 7m s resulted in poorer separation due to lower impact velocity and shorter residence time respectively Effect of Co Flow Rate The 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 5m s to 2 5m s the separation curve moved considerably toward the upper left corner indicating a better sep aration As the co flow rate further increased to 3 1m s and 3 7m 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 7m 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 3m s which is essentially the same as the optimum injection flow rate shown in Figure