PARTICLE CLASSIFICATION

Abstract: A classifier for separating particulate matter into heavy and light fractions, which classifier includes an inlet for particulate matter entrained in a fluid, a classification chamber, an outlet for the removal of heavy fraction of particulate material, an exhaust for the removal of light fraction of particulate material, and an inlet for a further fluid wherein the inlet for the further fluid includes a wall having a plurality of small holes or mesh for directing further fluid to entrain light particles and transport the light particles to the exhaust for the removal of the light fraction.

Description: PARTICLE CLASSIFICATION The present invention relates to a method and apparatus for classifying, particulate matter into heavy and light fractions. There are many applications that require particulate matter to be classified into light and heavy fractions. In some applications the heavy fraction is the desired product and contamination of the heavy fraction with excess quantities of light particles may be undesirable. Such applications include the separation of seed from husk in the grain processing industry. Other industries require the light fraction, and it is desired that the light fraction does not contain excess quantities of heavy particles. For example, in the production of electricity from coal, the coal is first comminuted to provide a small particle size and thus increase the efficiency of combustion. The comminuted coal is then passed through a classifier and the light, efficiently combustible fraction is separated for combustion. The heavy fraction from the classifier is returned to the coal feedstock and subjected to a subsequent comminution. Returning excess fines with the oversize coal to the coal feedstock results in decreased efficiency. The increased volume of coal to be processed requires the comminutor to have an increased capacity. The comminutor also consumes more energy in the comminution process in order to pulverise the increased volume of coal to be processed. Inertial classifiers have been used to classify light and heavy fractions of particulate matter. Inertial separations operate by permitting the inertia of particles of the heavy fraction to direct those particles into a first collection stream. The particles of the light fraction have less inertia and are redirected by a lateral force into a second collection stream. Various designs and configurations of inertial classifiers have been used which employ a single classification force on the particulate matter. Cyclones are generally used to collect particulate matter from a fluid, whether the fluid is gas (standard cyclone) or a liquid (a hydrocyclone). Cyclones are used primarily as collecting devices. The collection efficiency of a cyclone will generally depend on a number of parameters such as the configuration of the inlet including the aspect ratio of the inlet and the angle of entry, relative dimensions of the cyclone cylinder, outlet cylinder and frustroconical cone. The method of discharge provides a fluid seal for the pressure drop across the cyclone. In continuous processes, it can take the form of a mechanical device such as a rotary feeder or by an aperture of fixed or continuously varying size. In batch processes, an air tight container can be used. We have now found a method and apparatus for separating, particulate matter into heavy and light fractions which provides a second classification whereby the amount of light fraction contained in the heavy fraction is reduced. Accordingly, in a first aspect the present invention provides a classifier for separating particulate matter into heavy and light fractions, which classifier includes an inlet for particulate matter entrained in a fluid, a classification chamber, an outlet for the removal of heavy fraction of particulate material, an exhaust for the removal of light fraction of particulate material, and an inlet for a further fluid wherein the inlet for the further fluid includes a wall having a plurality of small holes or mesh for directing further fluid to entrain light particles and transport the light particles to the exhaust for the removal of the light fraction. In a second aspect of the present invention there is provided a method for classifying particulate matter into heavy and light fractions including feeding particulate matter entrained in a fluid into a classification chamber and directing further fluid into an inlet for the further fluid through a wall having plurality of small holes or mesh whereby said further fluid entrains light particles and transports to the light particles to an exhaust for the removal of the light fraction. Particulate materials suitable for use in the present invention include a wide variety of materials. Typically, the particles entrained in the fluid are in the form of a suspension. Many types of suspensions (gaseous or liquid) which are capable of separation by a centrifugal separation may be classified by the method and apparatus of the present invention. Typically, gaseous suspensions may include particles from 5000 μm to particles in the low microns (<1 μm). Liquid suspensions may contain particles with a different proportion of size ranges. Centrifugal or inertial separation techniques may operate over a range of densities, preferably the method and apparatus of the present invention find application where there is significant difference in density between the fluid and the particle for separation. The classification of particles in accordance with the present invention is applicable to a variety of industries, including industries for producing power, cement, mineral processing, pozzolanic (fly ash recovery of unburnt carbon, upgrading fly ash), coal washing, pharmaceuticals, grains (separation of seed from husk), and petroleum refining. In order that the present invention, and its application, may be more readily understood, the classification of coal for use in the production of power will be specifically described. In the power generation industry, the top size of coal feed to the pulveriser (communitor) is generally less than 75mm although there are presently some trends towards increasing the top size. There are three types of pulverisers in the power industry: a vertical spindle where the crushing action is due to a thin layer of particles subjected to high pressure under the rolling action of a ball or a cylinder; a ball or tube mill where balls are contained in a rotating cylinder and the centrifugal action of the rotation of the cylinder lifts the balls and the free fall of the balls on the coal layer provides the crushing mechanism; and an attritor pulveriser where coal particles are imparted high kinetic energy which on impact against the hard surfaces provides the crushing mechanism. Typical ranges of the size distributions of the coal to the classifier and the product going to the burners are shown below. 10 20 45 75 150 250 500 1000 3000 5000 Size.um Size distributions: Feed to classifier after crushing Product after classification Generally, fluids for entraining the particulate material may be liquids or gases. Preferably, the fluid is selected to have a density significantly less than that of the particulate material for classification. For example, air is a convenient fluid for use in entraining particulate coal-based materials prior to combustion. In the power industry with the pulverisers of the type described above, air with some quantity of water from the drying of the coal is the normal medium. This fluid is normally in the temperature range of 50 to 110°C although the temperature at the inlet could be upto 350°C. Generally, the temperature and pressure of the fluid are not narrowly critical. The fluid for use in entraining the light particles is conveniently the same^ as the fluid used to entrain the particulate material fed into the classification chamber. In some circumstances it may be advantageous to use a different fluid for entraining the light particles. For example, where a fluid which is very efficient in classifying the light and heavy fractions is costly, a less costly fluid may be used for entrainment of the particulate material for feeding into the classification chamber. A lesser volume of the more efficient fluid may then be utilised for entraining the light particulate material. The classification chamber may be any convenient configuration, such as that used for centrifugal or inertial classification. In a preferred embodiment the classification chamber includes a cyclone. Accordingly, in a preferred embodiment of the present invention provides a cyclone classifier for separating particulate material into heavy and light fractions including a cyclone chamber including a cylindrical section of upper part and a reversed frustroconical section of lower part, the bottom of the reversed frustroconical cone being open and terminating in an outlet for the removal of heavy fraction, an aperture for the intake of particulate matter entrained in a fluid wherein said fluid directed substantially tangentially to the wall of the cyclone chamber whereby the heavy fraction is precipitated from the fluid, a light fraction exhaust for removal of the entrained light fraction wherein the cyclone classifier further includes a wall having a plurality of small holes or mesh for directing further fluid to entrain additional light particles and transport the additional light particles to the exhaust. The present invention also provides a method for classifying particulate matter into heavy and light fractions including feeding particulate matter entrained in a fluid into a classification cyclone chamber including a cylindrical section of upper part and a reversed frustroconical section of lower part, the bottom of the reversed frustroconical cone being open and terminating in an outlet for the removal of heavy fraction, directing said fluid through an intake aperture substantially tangentially disposed to the wall of the cyclone chamber whereby the heavy fraction is precipitated from the fluid, directing a further fluid through a plurality of small holes or mesh in a wall of the cyclone classifier to entrain additional light particles, removing the light fraction through a light fraction exhaust and removing the heavy fraction through outlet for the removal of the heavy fraction. In the preferred embodiment of the present invention, a conventional cyclone may be converted into a classifier by introducing a further fluid into selected part of the cyclone such as where a layer of the collected heavy particulate material exists. The further fluid may be introduced by making the outlet for the heavy fraction porous. Alternatively, the mesh or holes may be provided in the wall of the frustroconical section. Porosity can be achieved in a variety of ways including using sintered material or perforations, or with packing material. The selection will depend on the application and the desired size of the pores through which the further fluid enters the outlet. By controlling the velocity of the fluid through the layer, particles of different sizes are re-entrained into the outgoing fluid. Thus it is possible to perform an additional classification to separate the incoming stream of heavy particles in two streams, ie a stream consisting of light or lighter particles and a stream consisting of a coarse or heavier fraction. By control of the velocity of the further fluid it is possible to vary the collection efficiency of the cyclone over its entire range. The present invention is applicable to a nominally high efficiency cyclone, and is equally applicable to cyclonic classifiers to provide a second classification thereby improving the overall classifier efficiency. The inlet for particulate matter entrained in a fluid for a cyclonic classification chamber may be of any type, including single, substantially tangential entry, volute type entry, or multi-vane entry. The main advantage of these above geometries is that they provide a thin continuous layer of particulate solids generally heavy from which separation of additional fines (or light materials) can occur. The further fluid can be introduced using a number of methods. A continuous porous medium, such as sintered brass, stainless steel or plastic or a woven fabric may be used. Alternatively, rolled perforated sheets (Stainless steel, mild steel, plastic) or a combination of perforated and packing materials may be used. The selection will depend on the temperature, supply pressure and flow rate. The location of the plurality of small holes or mesh (such as a porous surface) will depend on the nature of the access to the surface, although the preferred location is on a discharge cylinder forming part of the outlet for the heavy fraction. In order that this invention may be more easily understood and put into practical effect, reference will now be made to the accompanying drawings that illustrate a preferred embodiment of the invention: Figure 1 shows a cross section of a nominally efficient cyclone modified to form a classification apparatus of the present invention. Figure 2 shows the results of Example 1. Figure 3 shows a cross-section of a cyclone classification apparatus of the preferred embodiment of the present invention. Figure 4 shows a cross-section of a cyclone classification apparatus of the preferred embodiment of the present invention. Figure 5 shows the results of Example 2. Figure 6 shows the effect of velocity on collection efficiency as a function of particle size in Example 3. Figure 7 shows the effect of velocity on collection in Example 4. Figure 8 shows the effect of velocity on collection efficiency as a function of particle size in Example 5. Figure 1 shows a standard experimental cyclone (1) having a 25 mm tangential inlet (2). Example 1 The tangential inlet (2) feeds into a cylindrical upper section (4) having a diameter of 140 mm. A frustroconical section (5) leads down to a porous stainless steel cylinder (6) which forms the outlet of the cyclone (1). A conical seal (7) provides the required pressure differential to obtain flow from the porous stainless steel cylinder (6) to the exhaust (3). The product is discharged in to a closed container. The further fluid, or control fluid, was supplied via a porous stainless steel cylinder at the outlet before the container. For test purposes the cyclone was operated under suction with the discharge gases passing through a vacuum cleaner. Compressed air was used as the control fluid. When there is no flow of the classifying fluid, the collection efficiency was close to 100%. As the classifying flow was increased, the fraction of the material collected by the cyclone decreased while the mass collected by the vacuum cleaner increased. The results demonstrate the ability of the method and apparatus of the present invention to classify particles even in a narrow range. Figure 3 shows a cyclone classifier (10) of the present invention. Raw coal is fed through a central feed shaft 11 onto a rotating table 12. Crushing balls 13 pulverise the raw coal and the pulverised raw coal is then forced by a flow of primary air 14 into the cyclone 15 through the classifier 16. A preliminary classification is provided where a heavier proportion of the crushed coal precipitates from flow of primary air 14 and falls onto the rotating table 12. The rotating table 12 crushes this heavier proportion and the crushed material is then reentrained into the primary airflow 12. Veins 17 provide the cyclonic airflow and primary classification. A proportion of the light fraction of crushed coal is entrained with the air exhausted through exhausts 18. The heavy fraction and un-entrained lighter fraction passes through the frustroconical section 19 and into the reject coal outlet 20. As the reject coal passes over the small holes or mesh 21 through which further fluid 22 is passed, additional light fraction coal is entrained in the further fluid and exhausted through the exhausts 18. The reject coal is returned to the rotating table 12 for further pulverisation by the crushing balls 13. Figure 3 shows a cyclone classifier similar to that shown in Figure 2, but with the further fluid 30 being provided through small holes or mesh 31 in the base of the coal feed shaft 32. Example 2 Figure 4 shows the results of tests conducted in an operating unit. The classifier was about 2 m in diameter and was fitted with a standard design of turning vanes to provide the primary classification. For test purposes a concentric cylinder type classification system was installed. Compressed air was supplied as the classifying fluid. Tests were conducted by placing the test pulveriser in a manual mode to ensure stable operation. Isokinetic samples of the pulverised fuel were taken from all the fuel pipes associated with the test pulveriser. The results shown in Figure 4 demonstrate that the size distribution at the outlet of the pulveriser can be controlled. Examples 3 and 4 Figures 5, 6 and 7 shows the results of tests in 0.8 m and 1.1 diameter cyclonic classifiers. The smaller classifier is a scaled version of the 2 m classifier used for Example 2. The second classifier is a prototype for an application. These classifiers were operated in a batch mode under suction. A known mass (up to 500 g) of coal particles in a narrow size range prepared by sieving were allowed to entrain and the collection efficiency was measured from the mass of particles collected in the container. Metered classifying air was supplied at the base via a fan. It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as is herein set forth.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A classifier for separating particulate matter into heavy and light fractions, which classifier includes an inlet for particulate matter entrained in a fluid, a classification chamber, an outlet for the removal of heavy fraction of particulate material, an exhaust for the removal of light fraction of particulate material, and an inlet for a further fluid wherein the inlet for the further fluid includes a wall having a plurality of small holes or mesh for directing further fluid to entrain light particles and transport the light particles to the exhaust for the removal of the light fraction. 2. A classifier according to claim 1 wherein said particulate matter is selected from the group consisting of cement, minerals, pozzolanic , coal, pharmaceuticals, and grains. 3. A classifier according to either claim 1 or claim 2 wherein the fluid and further fluid are gasses. 4. A classifier according to any one of claims 1 to 3 wherein the fluid and further fluid are gasses selected from the group consisting of air, carbon dioxide, and nitrogen. 5. A classifier according to any one of claims 1 to 4 wherein the fluid and further fluid are air. 6. A classifier according to claim 5 wherein the particulate matter is a coal-base material. 7. A cyclone classifier for separating particulate material into heavy and light fractions including a cyclone chamber including a cylindrical section of upper part and a reversed frustroconical section of lower part, the bottom of the reversed frustroconical cone being open and terminating in an outlet for the removal of heavy fraction, an aperture for the intake of particulate matter entrained in a fluid wherein said fluid directed substantially tangentially to the wall of the cyclone chamber whereby the heavy fraction is precipitated from the fluid, a light fraction exhaust for removal of the entrained light fraction wherein the cyclone classifier further includes a wall having a plurality of small holes or mesh for directing further fluid to entrain additional light particles and transport the additional light particles to the exhaust. 8. A classifier according to claim 7 wherein the fluid and further fluid are air. 9. A classifier according to claim 8 wherein the particulate matter is a coal-base material. 10. A classifier according to any one of claims 7 to 9 wherein the plurality of small holes are provided by continuous porous medium, rolled perforated sheets or a combination of perforated and packing materials. 11. A method for classifying particulate matter into heavy and light fractions including feeding particulate matter entrained in a fluid into a classification chamber and directing further fluid into an inlet for the further fluid through a wall having plurality of small holes or mesh whereby said further fluid entrains light particles and transports to the light particles to an exhaust for the removal of the light fraction. 12. A method for classifying particulate matter according to claim 11 wherein said particulate matter is selected from the group consisting of cement, minerals, pozzolanic , coal, pharmaceuticals, and grains. 13. A method for classifying particulate matter according to either claim 11 or claim 12 wherein the fluid and further fluid are gasses. 14. A method for classifying particulate matter according to any one of claims 11 to 13 wherein the fluid and further fluid are gasses selected from the group consisting of air, carbon dioxide, and nitrogen. 15. A method for classifying particulate matter according to any one of claims 11 to 14 wherein the fluid and further fluid are air. 16. A method for classifying particulate matter according to claim 15 wherein the particulate matter is a coal-base material. 17. A method for classifying particulate matter into heavy and light fractions including feeding particulate matter entrained in a fluid into a classification cyclone chamber including a cylindrical section of upper part and a reversed frustroconical section of lower part, the bottom of the reversed frustroconical cone being open and terminating in an outlet for the removal of heavy fraction, directing said fluid through an intake aperture substantially tangentially disposed to the wall of the cyclone chamber whereby the heavy fraction is precipitated from the fluid, directing a further fluid through a plurality of small holes or mesh in a wall of the cyclone classifier to entrain additional light particles, removing the light fraction through a light fraction exhaust and removing the heavy fraction through outlet for the removal of the heavy fraction. 18. A method for classifying particulate matter according to claim 17 wherein the fluid and further fluid are air. 19. A method for classifying particulate matter according to claim 18 wherein the particulate matter is a coal-base material. 20. A method for classifying particulate matter according to any one of claims 17 to 19 wherein the plurality of small holes are provided by continuous porous medium, rolled perforated sheets or a combination of perforated and packing materials. 21. A method for classifying particulate matter according to any one of claims 7 to 10 wherein the velocity of the further fluid is controlling whereby the size of the entrained particles is controlled. DATED THIS 15 th DAY OF JUNE 2001 SIGMA PROCESS SOLUTIONS PTY LTD BY PIZZEYS PATENT & TRADE MARK ATTORNEYS

Inventors: SINGH BALDEV, SINGH, BALDEV
Applicants: SIGMA PROCESS SOLUTIONS PTY LT, SINGH BALDEV, SIGMA PROCESS SOLUTIONS PTY LTD, SINGH, BALDEV
Application year: 2001
Main classification: B04C3
IPC codes: B02C15, B02C23, B04C5, B04C9