SEPARATION OF PARTICLES OF DIFFERING DENSITIES

Abstract: Particulate material to be fractioned is fed from a tube (16) on to a revolving porous plate (12). Air is passed upwards through the plate to fluidise the particles which are entrained in the rotating body of air. Particles of different densities are centrifugally separated to different radial distances from the axis of rotation. Suction pipes (21 to 28) remove the different fractions. The plate (12) is conical so that the particles are separated by the combination of centrifugal and gravitational forces. A flat porous plate can be used and the different fractions collected at different circumferential positions about the periphery of the plate.

Description: SEPARATION OF PARTICLES OF DIFFERING DENSITIES This invention relates to a method of and apparatus for the separation of particles of differing densities. It has application, for example, in the separation of components of pulverised fuel ash (PFA . Some of the components of PFA, if separated, have a substantially enhanced commercial value compared with raw PFA. One particular fraction which is of commercial interest comprises the particles known as cenospheres. These particles are of low density and may be used for example for making concrete having a positive buoyancy or as protective coatings or shields, particularly against thermal radiation or for refractory materials or may be used as a filler in protective paints. Hitherto it has been the practice to separate these low density particles hvdraulicallv. Such a wet process is expensive in energy, particularly if the material has to be dried. It is one of the objects of the present invention to provide an improved process for separation, from PFA, of a fraction containing a higher proportion of cenospheres. However, the invention is also applicable to other materials where it is desired to separate particles of differing density; one example is the separation of magnetite from PFA. Magnetite particles are of high density and may be used, for example, in the dense media separation of coal or for the making of fefro-cement, used as a nuclear radiation shield. Heretofore magnetite has been separated from PFA by magnetic means. According to one aspect of the present invention a method of separating particulate material into fractions of differing particle density comprising the steps of feeding the material on to a surface of a porous element which is rotated about a substantially vertical axis whilst a fluid is passed upwardly through the porous element to give the particles mobility to move radially and collecting particles at two or more positions at different radial distances from the axis of rotation or at two or more different circumferential positions. The fluid is preferably gaseous, conveniently air. The element may be a flat plate which is substantially horizontal and in this case the less dense particles would move further outwardly radially compared with more dense particles. The plate may be a flat disc in this case or it may be an annular element. The device may be operated, in this case, as a batch separating device, a batch of the oarticulate material, e.g. PFA particles, being fed on to the porous element and fractionated according to the density of individual particles before being removed. In this case, the feed may reside on the plate for a plurality of revolutions of the plate. Continuous operation is often convenient, the particulate material being fed continuously on to the element at one radial distance from the axis and collected at two or more other locations differently displaced circumferentiallv from the feed point. In this case the collection locations may be at the same radial distance from the axis. Then the particles can remain .in the device for a time which may be up to the duration of one revolution. Removal may conveniently be effected by suction from the surface of the element and it is readily possible to have a plurality of suction devices at differing radial distances from the axis of rotation. Very conveniently however the porous element is of conical or bowl shaped form. It may be arranged, in this case, so that the particles move either inwardly or outwardly according to density under the action of both centrifugal and gravitational forces. Such an annular element may be of conical form or may have a bowl shaped surface. The pores in the porous element preferably are smaller than the smallest particles of the input feed to be ractionated. The gaseous fluid is conveniently fed to the underside of the porous element through a hollow shaft used for rotating the element. Conveniently the porous element forms part of a drum or like structure into which the gaseous fluid is fed. The rate of flow of gaseous fluid passing" through the porous element is preferably sufficient to fluidise the feed material at least over part of the surface of the element. The invention also includes within its scope apparatus for separating particulate material into fractions of differing particle density comprising a porous element with means for rotating said element about a substantially vertical axis, said element having an upper surface with means for feeding the particulate material on to said upper surface, means for forcing a fluid upwardly through the porous element and means at at least two differing radial distances from the axis of rotation for removing particulate material from said surface. The upper surface of said element may be substantially horizontal but preferably is of conical or bowl shaped form. The element may be an annular element. Vacuum means may be provided for removing particles from said surface. In a further aspect, the invention provides apparatus for separating particulate material into fractions of differing particle density comprising a porous element with means for rotating said element about a substantially vertical axis, said element having an upper surface with means for feeding the particulate material on to said upper surface, means for forcing a fluid upwardly through the porous element and means at at least two different circumferential positions for collecting particulate matter from the surface. Preferably, said means for feeding is arranged to feed particulate material on to said surface at a predetermined location which is fixed relative to and radially displaced from the axis of rotation. Then, the porous element may be circular and said means for collecting may be located outside the outer circumference of the circular element to collect material spiralling off the rotating element. In the following description of examples of the invention, reference will be made to the accompanying drawings in which: - Figure 1 is a sectional elevation of one embodiment of apparatus for fractional separation according to particle density of PFA; Figure 2 is a sectional elevation through another embodiment of apparatus for fractional separation of PFA; Figures 3, 4, 5, 6 and 7 are diagrams illustrating further forms of apparatus for fractional separation of PFA; "" Figure 8 is a graphical diagram showing results obtained using the method of the present invention to separate PFA into eight fractions; Figure 9 is a plan view of a different embodiment of the invention; and Figure 10 is a sectional elevation of the embodiment of Figure 9. Referring to Figure 1 there is shown diagrammatically a drum 10 with a vertical axis and mounted on a bearing 11 for rotation about that axis. Driving means (not shown " ) are provided for rotation of the drum. The upper part of the drum is closed partly by an annular porous element 12 of conical form with the central region having a cover plate 13. The porous element, in this particular embodiment, is a sintered acrylic material having a hard surface and a typical pore size of 20 micron. Air is fed from a pump or other air supply source 14 througn a hollow shaft 15 supporting the drum and enters the closed region within the drum to pass through the porous element 12. Pulverised fuel ash is fed, as shown at 16, on to this element 12 and is fluidised by the air stream passing through the element which air stream makes the particles on the upper surface of the element 12 mobile. They are thus free to move radially across the surface of this element under the action of centrifugal forces and gravity. The degree of mobility given to each particle is dependent on the density of that particle and the particles with lower density have greater mobility than those with a higher density. If it were required only to separate the material into two fractions, the apparatus could be used with a continuous flow of feed stock and continuous removal of separated fractions, the lower density fractions passing over the outer rim of the- element 12. The rotational speed and the rate of gas flow may be adjusted to give the required separation conditions for any particular material. In the particular apparatus shown in Figure 1, separation into eight fractions is achieved by using vacuum extraction pipes 21 to 28 positioned at differing radial distances from the axis of rotation, these pipes being provided with suction means for removing material from the surface of the element 12. It will be appreciated that, although in Figure 1 the eight pipes 21 to 28 are shown in a single radial plane, they may conveniently be disposed in differing radial positions with respect to the axis of the drum. In this particular arrangement, it is possible to operate by batch processing, a batch of material being fed on to the element 12 and, after separation into fractions, the different grades of material are removed through the various suction pipes 21 to 28. The use of such an annular vessel however favours a continuous flow process. Figure 2 illustrates diagrammatically a modification of the arrangement of Figure 1 in which the separation surface is a horizontal surface of a flat porous plate 30. In this arrangement the separation forces are centrifugal without any gravitational component. The material is fed in at the centre as shown at 31 and separate fractions may be removed at differing radial positions as shown for example at 32, 33 and 34. As shown in Figure 3, using a horizontal plate 35, it may be convenient to have this plate of annular form since the centrifugal forces are small towards the centre. In Figure 3 the material is fed in near the inner periphery as shown at 36 and may be removed at a plurality of positions at differing radii fram -the "axis, as shown at 37, 38, 39. Figure 4 illustrates a porous element 40 of dish shaped form with a feed 41 at' the centre of the dish and extraction means 42, 43 and 44 at differing radial distances outwardly from the centre. Figure 5 illustrates a modification of the arrangement of Figure 4 in which the porous element 50 is dish shaped but of annular form. In this embodiment there is shown a feed 51 and extraction means 52, 53. Figure 6 shows a conical porous element 60 with feed means 61 and extraction means 62, 63 and 64. Figure 7 illustrates a modification of the arrangement of Figure 6 with an annular porous element 70 with feed means 71 and extraction means 72, 73 and 74. Figure 8 is a graphical diagram showing the bulk density of collected fractions at the eight different positions between the outer periphery and the centre of the porous element in an arrangement in which the feed was to the inner edge of an annular porous element such as is illustrated in Figure 7. In the above described embodiments, the separate fractions of the particulate material are removed from the rotating porous element at different radial distances from the axis of rotation. If the porous element is flat, as for plates 20 or 35, the particulate material fed on to the plate becomes entrained in the revolving mass of fluidising air and tends to spiral outwards due to centrifugal action. If the material is not removed from the porous plate, it will spiral to the outer circumference of the plate and then "spill" off the edge. Referring to Figures 9 and 10, an embodiment is illustrated which avoids the need for the radially distributed suction tubes and instead collects the * material fractions at different positions around the periphery of the revolving plate. In these Figures, the particulate material to be fractionated is fed to the surface of the revolving porous plate 80 by a pipe 81 at a location which is fixed relative to and radially spaced from the axis of rotation. Surrounding the periphery of the circular plate 80, there are a number of collection boxes (three in the drawings, 82, 83 and 84), located to collect particulate material spilling off the edge 85 of the plate. Each collection box is arranged to collect material spilling off the edge of the plate over a selected arc above the periphery of the plate at a fixed circum erential distance from the feed pipe 81. In operation, the particles of greatest density in the material fed on to the plate 80 tend to spiral outwards quickest, e.g. as indicated by spiral 86, and so are collected by box 82 which is first about the periphery of the plate 80 from the feed pipe 81. Less dense particles are swept around the disc in the fluidising air for longer and may follow spiral 87 into the second box 83, whereas the least dense particles may follow spiral 88 into the third box 84. It will "be appreciated that more than three boxes may be provided if a greater number of fractions is required. Further, the speed of rotation must be such that all particles have spiralled off the plate within about one revolution.

Claims: CLA IMS 1. A method of separating particulate material into fractions of differing particle density comprising the steps of feeding the material on to a surface of a porous element which is rotated about a substantially vertical axis whilst a fluid is passed upwardly through the porous element to give the particles mobility to move radially and collecting particles at two or more positions at different radial distances from the axis of rotation or at two or more different circumferential positions. 2. A method as claimed in claim 1 wherein the fluid is gaseous. 3. A method as claimed in claim 2 wherein the gaseous fluid is air. 4, A method as claimed in either of the preceding claims wherein said element is a flat plate which is substantially horizontal. 5. A method as claimed in claim 4 wherein the element is a flat disc. 6. A method as claimed in claim 4 wherein the element is an annular element. 7. A method as claimed in any of claims 1 to 3 wherein the porous element is of conical or bowl shaped form so that the radial movement of the particles occurs due to both centrifugal forces and gravity. 8. A method as claimed in any of the preceding claims utilising batch separation, a batch of the particulate material being charged on to the porous element and fractionated according to their density before being removed. 9. A method as claimed in any of claims 1 to 7 with continuous operation, particulate material being fed continuously on to the element at one radial distance from the axis and collected at two or more other locations differently displaced circum erentially from the feed point. 10. A method as claimed in any of claims 1 to 7 with continuous operation, particulate material being fed continuously on to the element at one radial distance from the axis and collected at two or more other locations at differing radial distances and differently displaced circum erentially from the feed point. 11. A method as claimed in claim 10 wherein removal is effected by suction from the surface of the element. 12. A method as claimed in any of the preceding claims wherein the pores in the porous element are smaller than the smallest particles of the material to be fractionated. 13. A method as claimed in any of the preceding claims wherein the fluid is fed through a hollow shaft used for rotating the element. 14. A method as claimed in any of the preceding claims wherein the porous element forms part of a drum or like structure into which the fluid is f ed 15. A method as claimed in any of the preceding claims wherein the rate of flow of fluid passing through the porous element is sufficient to fluidise the material at least over part of the surface of the element. 16. A method of separating particulate material into fractions of differing particle density substantially as hereinbefore described with reference to the accompanying drawings. 17. Fractionated particulate material made by the method of any preceding claim. 18. Apparatus for separating particulate material into fractions of differing particle density comprising a porous element with means for rotating said element about a substantially vertical axis, said element having an upper surface with means for feeding the particulate material on to said upper surface, means for forcing a fluid upwardly through the porous element and means at at least two differing radial distances from the axis of rotation for removing particulate material from said surface. 19. Apparatus as claimed in claim 18 wherein vacuum means are provided for removing particles from said surface. 20. Apparatus for separating particulate material into fractions of d-iffering particle density comprising a porous element with means for rotating said element about a substantially vertical axis, said element having an upper surface with means for feeding the particulate material on to said upper surface, means for forcing a fluid upwardly through the porous element and means at at least two different circumferential positions for collecting particulate matter from the surface. 21. Apparatus as claimed in claim 20 wherein said means for feeding is arranged to feed particulate material on to said surface at a predetermined location which is fixed relative to and radially displaced from the axis of rotation. 22. Apparatus as claimed in claim 21 wherein the porous element is circular and said means for collecting are located outside the outer circumference of the circular element to collect material spiralling off the rotating element. 23. Apparatus as claimed in any of claims 18 to 22 wherein the upper surface of said element may be substantially horizontal. 24. Apparatus as claimed in any of claims 18 to 24 wherein the element is of conical or bowl shaped form. 25. Apparatus as claimed in anv of claims 18 to 24 wherein the element is an annular element. 26. Apparatus for separating particulate material into fractions of differing particle density substantially as hereinbefore described with reference to Figure 1 or Figure 2 or Figure 3 or Figure 4 or Figure 5 or Figure 6 or Figure 7 of the accompanying drawings.

Inventors: SMITH IAN E, STAMP RICHARD J, SMITH, IAN, E., STAMP, RICHARD, J.
Applicants: CENTRAL ELECTR GENERAT BOARD, CENTRAL ELECTRICITY GENERATING BOARD
Application year: 1986
Main classification: B03B4
IPC codes: B03B4, B07B13