AU6356399A - Method and apparatus for coating granules - Google Patents

Description

WO 00/24504 PCT/GB99/03524 -1 Method and Apparatus for Coating Granules This invention relates to a method and apparatus for coating granules, and in particular for coating granules in the manufacture of waste water treatment media. Waste water can be treated by gasification, for example by the aeration or oxygenation of sewage or other waste water containing organic matter degradable by the action of oxygen thereon. A wide range of treatment methods and apparatus have been used or proposed. Oxygen does not dissolve easily or quickly in water and it is therefore in principle desirable to utilise fine bubble aerators wherein the bubbles are less than 2 mm and desirably less than 1 mm in diameter. Smaller bubbles have a larger specific surface area for oxygen transfer to liquid, and also move more slowly through the liquid to give a longer time for the oxygen to transfer before the bubble reaches the liquid surface. It has been proposed, for example in our International Patent Publication No. WO 96/25367 that a treatment plant be provided comprising a treatment vessel containing a layer of loose particulate material and a number of gasifiers so that the loose particulate material acts as a fluidizable bed driven into circulation by the gas bubbles from the gasifiers. The particulate material provides a habitat for microorganisms effective in waste water treatment and additionally oxygen bubbles contacting the surface of the particles tend to stick to the surface thereof temporarily, increasing the length of time taken by the oxygen bubbles to reach the surface of the water and so increasing the amount of oxygen dissolved into the waste water. In WO 96/253 67 it is taught that a particularly desirable particulate material for use in waste water treatment is formed by granules of plastics material each having a WO 00/24504 PCT/GB99/03524 -2 plurality of grains of a substantially inert material coated thereon, the granules having a predetermined particle size range and the grains having a predetermined particle size range and being disposed at a predetermined packing density range on the granules and the granules having concavities formed on their surfaces between the grains to increase the surface area of the particles. In WO 96/25367 it is taught that the particles can be formed by contacting a mixture of grains of the inert material and a soluble substance at an elevated temperature with the granules of plastics material to coat with granules with the mixture and subsequent dissolving the soluble substance grains from the coating to provide granules coated with the grains of substantially inert material and having concavities between the grains of inert material and the use of sand as the inert material and salt as the soluble material is suggested. In British Patent Publication No. GB 2 302 293 a method of producing particulate material of this type is described in which the sand and the salt are mixed in a tumbling mixer and heated to a temperature in the range 215 to 225'C and then tipped into a separate tumbling mixer containing granules of polyethylene at ambient temperature. The softening temperature of polyethylene is in the range of about 180 to 200*C so as the polyethylene granules are tumbled and the heated grain mixture is poured as a stream into the tumbling polyethylene granules the heated grains are contacted with the polyethylene granules so that the granules become coated with a layer of sand/salt mixture on and partially embedded in their outer surfaces.

WO 00/24504 PCT/GB99/03524 -3 Tumbling of the coated granules then continues until the combined mixtures cool to a temperature which the granules are no longer significantly soft. In practice it has been found that there are a number of problems with this method of producing coated granules. Firstly, this method of production is a batch process requiring, for each production cycle, the heating and subsequent cooling of a large mass of material in addition to the material forming the coated granules. As a result, production using this method is relatively slow and difficult to speed up by expanding the size of the plant used because the task of uniformly heating and mixing and pouring the large volumes of material becomes increasingly inconvenient. Further, this method requires a relatively large amount of manual control and input to operate and because it is a batch process it is difficult and expensive to reduce this manual input by automating the process. Finally, because this method is a batch process any error in setting the parameters of the process or carrying them out will generally result in wastage of the entire produced batch of particles. This is because the large number of particles required in any commercial waste water processing system is such that it is not practical to individually check the particles. As a result, if an unacceptable percentage of the particles are not properly formed the entire batch must be rejected. As a result of these problems the price of the particulate material produced by this method is relatively high. In fact, the price of the particulate material produced by this method is so high as to limit the commercial exploitation of the waste water treatment method.

WO 00/24504 PCT/GB99/03524 -4 The present invention is intended to overcome the disadvantages of the prior art, at least in part. In a first aspect, this invention provides a method of manufacturing loose particulate material in the form of granules of plastics material carrying grains of another material which is stable to a temperature higher than the softening temperature of the plastics material, said method comprising tumbling and heating a mixture of grains and plastics materials granules to a temperature not less than the softening temperature of the plastics materials so that the plastics material granules become coated with the grains and then tumbling and cooling the coated granules. In a second aspect, this invention provides apparatus for-manufacturing loose particulate material in the form of plastics material carrying grains of another material which is stable to a temperature higher than the softening temperature of the plastics material, said apparatus comprising means for tumbling and heating a mixture of grains and granules to a temperature not less than the softening temperature of the granules so that the granules become coated with the grains and means for tumbling and cooling the coated granules. The inventive method and apparatus allows the coating of granules by a continuous process. The process is relatively quick and can be easily and simply scaled up to produce any desired volume of material. The process requires little manual control and can be easily automated, reducing production costs. Since production is a continuous process, if the produced coated granules are not of acceptable quality the operating parameters can be quickly altered to restore acceptable quality with little wastage.

WO 00/24504 PCT/GB99/03524 -5 A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic figures, in which: Figure 1 shows a partially cut away view of a first embodiment of apparatus according to the invention; Figures 2a to 2c show an end view of a tube of the apparatus of figure 1 at different angular positions; Figure 3 shows a partially cut away view of a second embodiment of apparatus according to the invention. Referring to the figures an apparatus and method for producing particles according to the invention will be described. In a first embodiment of the invention the apparatus comprises a cylindrical hollow tube 1 mounted for rotation about its axis through two, spaced apart, bearings 2. Preferably the tube should have a length to diameter ratio of about 5 to 1, but this is not essential. The bearings 2 are supported by a supporting structure (not shown) so that the axis of the tube 1 is inclined to a horizontal at a shallow angle. A pulley 2 is mounted on the outside of the tube 1 and the tube 1 is rotated by a motor (not shown) through a belt 4 passing around the pulley 2. It will be understood that other driving methods could be used. A band heater 5 surrounds a central section of the tube 1. A longitudinal fin or rib 6 projects radially inwardly from the inner surface of the tube 1 and extends along its entire length. It is preferred that the rib 6 project inwards by a distance in the range 1/3 to 1/10 of the diameter of the tube 1 and best results have been obtained with a rib 6 projecting by 1/5 of the diameter. However, this is not essential.

WO 00/24504 PCT/GB99/03524 -6 A first sand/salt mixture feed hopper 7 and a second plastics granules feed hopper 8 are arranged to feed sand/salt mixture and plastics granules respectively onto a chute 9 and into the raised end of the tube 1. The first and second hoppers 7 and 8 have respective metering mechanisms 7a and 8a driven by respective motors 7b and 8b to control the supply of the sand/salt mixture and plastics granules respectively. At the lower end of the tube 1 a sieve 10 is located. The apertures in the sieve 10 are sized so that the sand/salt mixture falls through the sieve 10 while the coated particles do not. The sieve 10 is inclined at an angle so that the particles 10 can move across the sieve 10 into a particle output hopper 11. The sand/salt mixture passing through the sieve 10 falls into a mixture collection hopper 12. At the base of the mixture collection hopper 12 a worm pump 12a is located and driven by a motor 12b to drive the sand/salt mixture through a return pipe 13 and return it to the top of the first hopper 7. In operation the first hopper 7 is charged with a mixture of sand and salt grains. As will be explained in more detail below the ratio of sand to salt and the sizes of the sand and salt grains can be varied as desired to control the characteristics of the finished particles. A mixture of equal amounts by volume of ordinary table salt (sodium chloride) and washed sharp sand is one example of a suitable mixture. The second hopper 8 is charged with polyethylene granules having a diameter in the size range 4 to 6mm. Such granules can be readily produced. The band heater 5 is then activated to begin heating the tube 1. When the tube 1 has reached its operating temperature, the tube 1 is rotated, and the motor 7b is activated to begin delivering the sand/salt mixture through the metering unit 7a from the WO 00/24504 PCT/GB99/03524 -7 first hopper 7 through the chute 9 into the upper end of the tube 1. After a short interval the motor 8b is also activated to begin delivering plastics granules through the metering unit 8a from the second hopper 8 through the chute 9 into the upper end of the tube 1. This ensures that the polyethylene granules cannot directly contact the tube walls without being in intimate contact with the sand/salt mixture. The band heater 5 heats the tube walls in the central region of the tube 1 directly exposed to the band heater 5 to a temperature above the softening point of the polyethylene granules, i.e. higher than about 200'C and preferably in the range 215 to 225 C. This heat is conducted along the walls of the tube 1. Further, because of the inclination of the tube 1 a convection current of air flows from the lower end of the tube 1. The apparatus is arranged so that the maximum temperature of the upper end of the tube 1 where the sand/salt mixture and plastics granules enter is below the softening point of the polyethylene granules, for example around 80*C. Conductive heating along the walls of tube 1 together with the flow of convection air ensures that the walls of the tube 1 are heated along their entire length and the temperature of the sand and salt grains and plastics granules begins to rise as soon as they enter the tube 1. In practice, there may also be some slight heating by the convection air flow while the grains and granules are in the chute 9 but this source of heating is generally too slight to be of any practical significance. The mixture of grains and granules passes along the bottom of the tube 1 sliding due to gravity along the bottom of the rotating tube 1. Only a relatively small angle of inclination is necessary to cause this movement. Because of the rotation of the tube 1 the mixture is tumbled and the angle of inclination of the tube can be WO 00/24504 PCT/GB99/03524 -8 considerably smaller than the angle at which the grains and granules would form a stable slope on a stationary surface and still cause them to slide along the tube 1. As the tube rotates the mixture of grains and granules is periodically lifted, poured and mixed by the action of the rib 6. As the mixture of grains and granules moves downwards along the tube it picks up heat from the tube 1 and the convection air flow and steadily increases in temperature until the mixture reaches the region within the band heater 5 where their temperature is raised sufficiently for the outer surfaces of the granules to soften and partially melt so that the grains embed themselves in the surfaces of the granules. As the grains and granules are mixed in this region the granules become completely coated with an adhered coating of the grains. The mixture of grains and granules then passes out of this maximum heat zone within the band heater 5 and then cools as it passes on down the tube towards the lower end of the tube. In this region the convection air flow is of course cooling the granules and grains. The length of the lower part of the tube 1 is arranged so that the granules are essentially solid by the time they reach the bottom end of the tube 1 where the mixture of grains and granules is decanted from the tube 1 onto the sieve 10. The sieve 10 is inclined so that the mixture of grains and granules tends to slide across it away from the end of the tube 1 and has apertures sized so that the grains pass straight through the sieve 10 into the recycling hopper 12 while the granules move across the surface of the sieve 10 and are deposited into the output hopper 11.

- WO 00/24504 PCT/GB99/03524 -9 Optionally, the sieve 10 may be rocked or vibrated to encourage the grains to pass through the sieve 10 and the granules to roll across the sieve 10. The grains in the recycling hopper 12 are driven through pipe 13 by the worm pump 12a driven by motor 12b and back into the top of the first hopper 7 for re-use. The presence of the rib 6 or some alternative mixing structure on the internal surface of the tube 1 is important. Because of the difference in density and size between the grains and the granules, if the mixture of grains and granules were allowed to pass down an inclined rotating tube 1 having no integral mixing structures the grains and granules would be tumbled but would tend to separate out. This separation would allow the granules to come into prolonged direct contact with the walls of the tube 1 and with one another resulting in the granules melting excessively in the region of the tube within the band heater 5 so that the granules would stick to the wall of the tube 1 or stick together to form large masses of plastics material. However the action of the rib 6 is to prevent such separation by stirring and re-mixing the mixture of grains and granules at each rotation. Views along the tube 1 are shown in figures 2a to 2c at different points in the rotation of the tube 1. In these figures the tube 1 is rotating in a anticlockwise direction as shown by the arrow. Initially, as shown in figure 2a substantially all of the mixture of grains and granules is captured and lifted by the rib 6. As a tube 1 continues to rotate the angle of the rub 6 changes so that the mixture of grains and granules can no longer be securely held between the rib 6 and the WO 00/24504 PCT/GB99/03524 -10 wall of the tube 1 and the mixture of grains and granules begins to slide off the rib 6 into the bottom of the tube 1 as shown in figure 2b. Eventually, as the tube 1 continues to rotate, as shown in figure 2c the angle of inclination of the rib 6 becomes so great that essentially all of the mixture of grains and granules slips off the rib 6 and into the bottom of the tube 1. This stirring and mixing action prevents the grains and granules separating out in the tube 1. An alternative apparatus providing a second embodiment of the invention is shown in Figure 3. This comprises a cylindrical hollow tube 1 mounted on bearings 2 at a shallow angle to the horizontal and driven by a belt and pulley arrangement similarly to the first embodiment. The tube 1 is heated by three separate band heaters 15a, 15b and 15c spaced along the tube 1. Use of three separate band heaters 15a to 15c in place of the single heater 5 of the first embodiment allows the temperature gradients within the tube 1 to be more accurately controlled by varying the temperatures of the band heaters 15a to 15c. This also allows the proportion of the tube 1 which is heated above the softening point of the granules and the temperature gradients within the tube 1 to be varied in order to allow different sizes of granules and different material flow rates through the tube 1 to be correctly processed. The tube 1 has an internal rib 6 similarly to the first embodiment but the rib 6 stops short of the upper end of the tube 1 so that the granules and grains entering the tube 1 pass along tube 1 for a short distance before being stirred and mixed by the rib 6. The provision of this short ribless entry section of the tube 1 helps to ensure a steady WO 00/24504 PCT/GB99/03524 -11 flow of the grains and granules into the tube 1 and allows the chute 9 to project into the upper end of the tube 1 in close proximity to the side wall so that the mixture of grains and granules can be fed into the tube 1 with minimal spillage. However, the provision of such a ribless section is not essential. In practice, it has been found that with a tube diameter of 381 mm (15in) and a rib projection height of 76 mm (3in) a ribless section of the tube 1 around 50-76 mm (2-3in) in length is useful. However, it is believed that the length of the ribless section could be varied quite widely provided that the temperature of the tube 1 did not reach the softening point of the polyethylene granules within the ribless section. At the downstream end of the tube 1 the sieve 10 of the first embodiment is replaced by a cylindrical sieving section 19. The cylindrical sieving section 19 is co axial with the tube 1 and is attached to the downstream end of the tube 1 to form a continuous cylindrical structure. The cylindrical sieving section 19 is a cylindrical tube perforated by a large number of apertures 19a sized to allow the grains to pass freely therethrough but to be too large to allow the granules or coated particles to pass through or into them. As explained above the cylindrical sieving section 19 is co-axial with the tube 1 so that it is at the same angle to the horizontal as the tube 1. Accordingly, as the sieve section 19 rotates the mixture of grains and granules slides along the lower surface of the sieving section 19. As this movement continues the mixture of grains continuously falls through the apertures 19a in the sieve section 19 and into the hopper 12. The granules slide across the lower surface of the rotating sieve section 19 until they drop out of the bottom end and into the output hopper 11.

WO 00/24504 PCT/GB99/03524 -12 In the described embodiment the sieve section 19 is a separate element to the tube 1 and is secured to it, for example by welding. It would be possible to form a tube 1 having an integral sieve section 19 by forming apertures in a section of the downstream end of the tube 1 from which the mixing rib 6 was omitted but this would generally be more complex to manufacture. Baffle 16 is provided at the bottom ends of the sieve section 19, leaving sufficient clearance for the granules to pass out of the sieve section 19. The baffle 16 reduces the convection airflow through the tube 1. It has been found that the heat loss from the apparatus due to the convection air flow through the tube 1 if no baffle 16 is provided can be so high as to significantly effect the heating requirements of the apparatus and this problem tends to become greater as the diameter of the tube 1 gets larger. Additional baffles could be placed at the upper end of the tube 1 and sieve section 19 if necessary. The mixture of unused grains passing through the sieve 10 is recycled through a tube 13 into the first hopper 7 for re-use similarly to the first embodiment. In the second embodiment the granules passing through the sieve 10 are carried by the walls of a hopper 12 into the pipe 13 where they are blown along the pipe 13 by an air flow generated by a fan 17. A filter 18 is provided at the top of the first hopper 7 to allow the air to escape. The filter 18 is sized to block the grains carried by the air flow along pipe 13 so that the grains are deposited in the first hopper 7 for recycling. It will be appreciated that the different technical features of the first and second embodiments could be exchanged and used in different combinations as desired.

WO 00/24504 PCT/GB99/03524 -13 In both embodiments the use of ring heaters to heat the tube 1 is referred to. Although the use of ring heaters is convenient to ensure even heating of the tube 1 the heating could instead be provided by an array of separate heaters spaced around the circumference of the tube instead of a continuous ring heater. In principle, provided that the speed of rotation of the tube 1 is sufficiently high the use of single or multiple element band heaters is not essential since a single heater could be used instead. However, it is preferred to use single or multiple element band heaters. It is most important that once the tube 1 has been brought to its operating temperature by the band heater or heaters the granules are not allowed to be fed into the tube 1 without the grains. If this occurs the granules will stick to the walls of the tube 1 and to one another in the region of the tube with the band heater or heaters generating a large block of molten or semi molten plastics material. Accordingly, it is prudent to ensure that the grains begin flowing into the tube before the granules are supplied as set out above, although strictly it is only essential that the granules not be supplied before the grains. An alternative arrangement would be to feed the recycled mixture of sand and salt grains along the tube 13 into the chute 9 and thus essentially directly back into the upper end of the tube 1. Under some circumstances heating and cooling of salt or a sand/salt mixture can result in the salt or the mixture solidifying when cool, depending upon the humidity, the temperature change and, in the case of a mixture, the sand to salt ratio, among other things. In order to avoid this problem it is necessary for the operating parameters of the apparatus shown in figure 1 to be carefully selected to avoid combinations of operating WO 00/24504 PCT/GB99/03524 -14 parameters which result in the salt or salt/sand mixture solidifying within the apparatus and specifically within the first hopper 7. Alternatively, a suitable anti-coagulant can be added to the salt to prevent solidification or the salt or mixture not recycled, but this last option is undesirable on cost grounds. If the return pipe 13 is fed directly back into the upper end of the tube 1 or chute 9 it can be arranged for the temperature of the grains to be maintained at a elevated temperature throughout the cycle so that possible solidification of the grains is a potential problem only when the apparatus is shut down at the end of a production operation. If necessary, the danger of solidification can be overcome at this point by simply clearing all of the mixture from the apparatus at the end of the production operation as well as by the methods discussed above. This arrangement also provides the advantage of reducing the total energy requirement of the system by placing the grains back in the tube 1 before they have fully cooled, thus reducing the heating energy requirement. This disadvantage of this arrangement is that the tube 1 must be heated and the grains passed through it probably for at least one full cycle of grains around the apparatus and possibly for several before the temperature of the grains at each point in the system stabilises so that the granules can be added and reliable coating started, so the time required to start up the apparatus will be increased. In such a system a basic load of sand and salt grains will be circulated continuously through the apparatus and additional grains need only be added at a rate sufficient to replace those actually taken out of the apparatus attached to the coated particles. Further, since there is no requirement to recycle the sand/salt grains mixture WO 00/24504 PCT/GB99/03524 -15 through the first grain hopper 7 separate hoppers for sand and salt could be used, although it will normally be necessary to include a mixed grain hopper to act as a header tank to allow for variations in the return rate of the grains along the tube 13. It will be appreciated that the straight radial rib described above could be replaced by a rib inclined to the radial direction or by a plurality of circumferentially spaced apart ribs or by one or more helical ribs. Similarly, there is no requirement for a single continuous rib. A plurality of axially and/or circumferentially spaced apart or staggered ribs could be used. Finally, studs, spikes or other projections to be used in place of ribs. In practice, although a single straight rib is the simplest configuration to manufacture it may be advantageous to employ a plurality of circumferentially spaced ribs or a helical rib in order to make the torque required to rotate the tube 1 constant. Where a single rib only is provided the torque required to rotate the tube 1 will vary in dependence upon the angular position of the rib 1. Although only a single mixed grains hopper 7 is shown, additional separate hoppers for sand and salt grains respectively would normally be provided feeding into the mixed grains hopper 7. In theory it would be possible to place the heater or heaters at the upper end of the tube 1 and simply drop the mixture of grains and granules directly into the hottest region of the tube 1 within the band heater 5. It has been found that the use of gradual heating in a region of the tube 1 leading towards the maximum temperature region within the band heater 5 reduces the power requirement for heating due to the connection transfer of heat from the cooling mixture towards the lower end of the tube WO 00/24504 PCT/GB99/03524 -16 to the mixture being heated in the upper end of the tube and also results in more consistent coated particle production. It is believed that the more consistent particle production is due to the gradual heating of the grains and granules in tumbling intimate contact so that immediately the surface of a granule begins to soften grains begin to embed whereas if the grain and granule mixture is dropped directly in the hottest region the much quicker heating can result in some granules or parts of granules being heated while in momentary contact with other granules or with the tube wall so that they stick to the tube wall or to one another. In addition to tumbling and mixing the grains and granules and assisting transport of the grains and granules along the tube 1 rotation of the tube also tends to spread the grains and granules across a larger portion of the curved bottom of the tube 1 than would otherwise be the case, so ensuring good thermal contact between the grain and granule mixture and the tube 1, allowing the heating and cooling of the grain and granule mixture to progress quickly and evenly. In an alternative embodiment, where the mixture of sand and salt granules is supplied from the first hopper 7 to the upper end of the tube 1 and recycled back into the hopper 7 rather than being recycled directly to the inlet of the tube 1 it may be convenient to preheat the salt and sand mixture in the hopper to accelerate the heating process within the tube 1. Similarly, the granules within the second hopper can also be preheated. Such preheating should only be to a temperature well below the melting point of the granules. For example, half the proposed peak temperature.

WO 00/24504 PCT/GB99/03524 -17 The precise temperature required in the tube within the heated region would depend upon the material of the granules. The precise dimensions of the apparatus and rotation rates will depend upon the materials and ratios used and production rates required and can easily be arrived at empirically by the person skilled in the art on a case by case basis. In use the coated granules are washed with water to dissolve the salt leaving the plastics granule with embedded sand grains and recesses where the embedded salt grains have been dissolved out. Advantageously this washing process can be carried out immediately the particles have been produced so that the residual warmth of the particles assists in dissolving the salt. If desired the resulting brine can be recycled to extract the salt for re-use. The material and size of the granules can be varied as necessary. It has generally been found that a granule size range in the range 3 to 10 mm, preferably 4 to 8 mm and most preferably about 4 to 6 mm is suitable. The plastics material used can be for example polyethylene or rubber and other plastics materials would also be suitable. One particular advantage of using rubber is that rubber chips produced by recycling tyres can be used. Such recycled rubber chips are readily and cheaply available because of the large number of tyres recycled and the relative lack of demand for the recycled rubber. Similar polyethylene chips are also available, for example as recycled chips from reject plastics mouldings, but these are generally more expensive than rubber chips because they are in greater demand. The sand and salt grains can be of any convenient type and it is generally preferred for the grains to have a small grain size for example for 0.1 to 3.5 mm and WO 00/24504 PCT/GB99/03524 -18 preferably by 0.1 to 2.5 mm. Ordinary table salt having a grain size of about 0.15 mm or water softening salt having a grain size of about 3.3 mm can both be conveniently used. Another advantage of the method and apparatus according to the invention is that the tumbling and mixing of the granules within the tube 1 in their softened state tends to cause the granules to become more nearly spherical if their initial shape is irregular. The mixture of grains of sand and salt can be in any ratio from all sand through to all salt. The ratio of the number of sand grains on the surface of each granule to the number of salt grains on the surface of each granule is the same as the ratio of sand to salt in the grain mixture and as a result once the particles have been washed the ratio of sand grains to recesses in the granules and thus, if the grain sizes are equal, the packing ratio of the sand grains is the same as the sand to salt ratio in the grain mixture. In generally the density of the plastics material is less the density of the sand with the density of the plastics material being less than that of water and the density of the sand being greater than that of water. As a result the density of the washed particles can be selected over a wide range by varying the sand to salt grain ratio. The surfaces of the grains embedded in the granules and.also the surfaces of the recesses formed by dissolving the salt grains both provide additional surface area on the granule for bacterial growth. Further, where the sand and salt grains are the same size the average surface area added to the granule by a recess is the same as that added by an embedded sand grain. As a result, the use of sand and salt grains of equal size is advantageous because this allows the density of the produced particles to be varied by WO 00/24504 PCT/GB99/03524 -19 changing the sand to salt ratio in the mixture of grains without altering the average surface area of the particles. This greatly simplifies the task of producing particles having a specified average density or density range. It will be realised because the sand and salt grains are coated onto the surface of the plastics granules, the amount of sand and salt in the finished particles is much smaller than the amount of plastics granules. However, in order to ensure reliable consistent particle production it is necessary to have a much greater quantity of sand and salt grains in the mixture in the tube than would be required to coat the granules in the mixture. Generally, it is best to have several times the coating volume of sand and salt grains in the mixture relative to the volume of plastics granules and advantageously is approximately one to four parts sand and salt grains to one part plastics granules by volume. It has been found that the size and shape of the produced particles is consistently derived from the original shape of the plastics granules. The shape of the produced particles are generally derived from the shape of the plastics granules rather than being identical to the granules because there is usually some change in shape due to the softening and re-hardening of the plastics material unless the plastics granules are spherical. For example, if the plastics granules are long cylinders the coated particles produced tend to be shorter and fatter cylinders. However, the shape of the finished product is consistent provided that the shape of the plastics particles used is consistent. Accordingly, where the granules are to have a controlled density, for example to allow use as described in WO 96/25367, it is necessary to use plastics granules having a consistent shape and size because otherwise the ratio of plastics material to WO 00/24504 PCT/GB99/03524 -20 inert grains in the granules produced, and thus the density, cannot be kept constant. However, if particles having a varying density are acceptable, for example for use in a fluidised bed, the use of irregular chips as the plastics material would be acceptable. The temperature and plasticity of the produced particles leaving the tube 1 can be varied as required. Normally, it is preferred to ensure that the temperature of produced particles is low enough by the time they are collected in the output hopper 11 that the particles will not deform under their own weight as they are stacked in the hopper 11 or subsequently transported and handled. However, it may be preferred to have the produced particles leave the tube 1 at a high enough temperature that they are still soft so they deform into lenticular shapes under their own weight when piled in the output hopper 11 in order to further increase the surface area to volume ratio of the produced particles. Alternatively, post processing means such as press or roller arrangements could be provided downstream of the tube 1 to press the produced particles into a desired shape before they are piled in the hopper 11. Such post processing is cheapest and simplest to carry out when the particles are already soft following heating in the tube 1. The post processing can be used to obtain shapes for the produced particles which are otherwise impractical because of the changes in particle shape when heated within the tube 1. It would be possible to employ a forced air flow or forced heated air flow along the tube in place of the natural convection heating air flow or to have essentially no or a minimal air flow by having a continuous chute 9 sealed to the upper end of the tube 1. Further, heaters could be placed inside tube 1 if desired.

WO 00/24504 PCT/GB99/03524 -21 In all of the above embodiments the unused grains of the sand/salt mixture are recycled. It would of course be possible not to do this and to simply employ fresh sand and salt grains at all times but this would significantly increase costs, particularly in view of the requirement to have an excess of sand and salt grains.in the tube 1. In the above description the use of sand grains and salt grains has been discussed with the salt grains being dissolved out of the completed particles using water. It would of course be possible to use alternative non-soluble grains in place of the sand, alternative soluble grains in place of salt and/or an alternative solvent in place of water provided that the basic requirements that the soluble and non-soluble grains were both thermally stable at the operating temperature of the apparatus at which they became embedded in the granules and that the soluble grains dissolved in the solvent while the unsoluble grains and the granules did not. Finally, to allow use in waste water treatment the material of the granules and the solid grains will of course have to be unaffected by immersion in water and be essentially biologically inactive and unaffected by the pollutants expected to be found in the specific waste water cleaning application. In the described embodiment the tube 1 is rotated continuously in one direction. It would of course be possible to rotate the tube 1 sequentially in opposite directions or to rotate the tube 1 with a rocking motion through partial rotations in alternate directions without actually completing a full rotation. In the above description the activation and operation of various motors has been described. These can be controlled manually or by an automatic control system based upon appropriate sensor information.

WO 00/24504 PCT/GB99/03524 -22 The above description is intended to be exemplary only and the skilled person will understand that various substitutions and modifications can be made within the scope of the invention as defined by the accompanying claims.

Claims (17)

1. A method of manufacturing loose particulate material in the form of granules of plastics material carrying grains of another material which is stable to a temperature higher than the softening temperature of the plastics material, said method comprising tumbling and heating a mixture of grains and plastics material granules to a temperature not less than the softening temperature of the plastics material so that the plastics material granules become coated with the grains and then tumbling and cooling the coated granules.

2. A method according to claim 1 in which the tumbling and heating and tumbling and cooling of the mixture takes place within a heated rotating hollow tube.

3. A method according to claim 2 in which the tumbling and heating and tumbling and cooling of the mixture takes place within a heated rotating hollow tube having at least one internal projection.

4. A method according to claim 3 in which the internal projection is an internal rib.

5. A method according to any preceding claim in which the tumbling and cooling is continued until the coated granules have cooled down to a temperature at which the WO 00/24504 PCT/GB99/03524 -24 granules are no longer significantly soft, whereby they can then reside in quantity without significant mutual deformation under their own weight.

6. A method according to any preceding claim in which the mixture contains an excess of grains over the amount required to coat the granules.

7. A method according to claim 6 in which the mixture contains at least as much grains as granules by volume.

8. A method according to any preceding claim in which the granules are of rubber.

9. A method according to any preceding claim in which the granules are of polyethylene.

10. A method according to any preceding claim in which the grains are inert material grains.

11. A method according to any preceding claim in which the grains consist of a mixture of grains of inert material and grains of a soluble substance.

12. A method according to claim 11 in which the grains consist of a mixture of grains of sand and grains of sodium chloride. WO 00/24504 PCT/GB99/03524 -25

13. A method according to any preceding claim in which the grains not coated onto the granules are separated from the mixture and re-used.

14. Apparatus for manufacturing loose particulate material in the form of plastics material carrying grains of another material which is stable to a temperature higher than the softening temperature of the plastics material, said apparatus comprising means for tumbling and heating a mixture of grains and granules to a temperature not less than the softening temperature of the granules so that the granules become coated with the grains and means for tumbling and cooling the coated granules.

15. Apparatus according to claim 14 in which the means for tumbling and heating and the means for tumbling and cooling is a rotating heated tube.

16. Apparatus according to claim 15 in which the tube has an internal projection.

17. Apparatus according to claim 16 in which the internal projection is an internal rib.