Method of and apparatus for drying particulate material
FIELD OF THE INVENTION 
     This invention relates to a method of and apparatus for drying particulate material. The invention has been developed particularly for the drying of cenospheres and is herein described in this context, but it is to be understood that the invention may be applied to drying of other materials such as fine sand. 
     BACKGROUND OF THE INVENTION 
     Cenospheres are hollow spherical-form particles which are contained in fly ash. They have a white to light grey colour, are composed predominantly of silica (59%), alumina (28%) and iron oxide (5%), contain mainly nitrogen within the sphere, have a size from 45 to 150 micrometers, a specific gravity of 0.55 and a dry density (less than 0.5% moisture) of 330 Kg/m 3 . The cenospheres have application as aggregate for light-weight concrete and are suitable for use in refractories, plastics materials, paints, sound insulation material, general filler applications and thermal insulating materials. 
     The cenospheres are separated from the other constituents of fly ash by water flotation processes but subsequent drying of agglomerated cenosphere material has been found to be extremely difficult, using conventional drying apparatus, due to the low density of the material. 
     SUMMARY OF FIRST ASPECT OF THE INVENTION 
     The present invention seeks to avoid this difficulty by providing a drying apparatus comprising a retainer for moist particulate material, a screen located within the retainer and arranged to support the material when loaded into the retainer, means for extracting moisture from material located adjacent the screen, and means for vibrating the retainer whereby the material when at least partially dried is induced to flow through the screen. The means for extracting moisture comprises at least one heater element which is disposed in or adjacent the screen and a vent which extends upwardly from a position adjacent the screen. 
     The screen has a mesh size which is sufficiently large to pass the material in its dried or partially dried condition but which is not so large as to pass the material in its moist (agglomerate) state. A 1200 micrometer mesh screen has been found suitable for use in the apparatus when employed for drying cenospheres. 
     PREFERRED FEATURES OF THE INVENTION 
     The heater element may comprise an electrical heating element or a heat exchange type of heating element that employs, for example, hot fluid (liquid or gas) as a heating medium. However, the term &#34;heater element&#34; is to be understood in the context of this specification as excluding a burner that produces heat by combustion in the region of the screen and which would thereby cause a gas draft which would tend to carry dried material up the vent. 
     When the heater element comprises an electrical heating element, it may be embodied in the screen or it may be mounted in heat exchanging relationship with the screen. 
     The vibrator is preferably arranged to be energized periodically, but it may be energized continuously. The frequency and amplitude of vibration, the period between successive energizations of the vibrator, the period of vibration, the mesh size of the screen and the moisture extraction rate are all, at least to a certain extent, dependent variables and the best combination of these factors must be determined empirically for different types of material to be dried. 
     SUMMARY OF SECOND ASPECT OF THE INVENTION 
     As well as providing an apparatus, the present invention may be defined as providing also a method of drying moist agglomerated particulate material, the method comprising loading the material into a retainer and supporting the material upon a screen within the retainer, extracting moisture from the material whilst supported on the screen and vibrating the retainer. Vibration of the retainer may be effected either continuously or at periodic intervals of time. The moisture extraction is effected by applying heat to the material from at least one heating element that is disposed in or adjacent the screen and by venting liberated vapour through a vent which extends upwardly from a position adjacent the screen. 
     The invention will be more fully understood from the following description of a preferred embodiment of an apparatus, which is suitable for use in the drying of cenospheres, as shown in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a sectional elevation view of the drying apparatus, and 
     FIG. 2 shows a plan view of a retainer portion of the apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown, the apparatus comprises an inverted square-pyramidal-form retainer 10 into which moist cenospheres are loaded following their separation, by a flotation process, from fly ash. The walls 11 of the retainer define an included angle θ of approximately 90°, and the walls join a lower cylindrical outlet 12. 
     Located within the retainer is a screen in the form of a stainless steel mesh 13 (mesh size 1200 micrometers) which is arranged to support the material loaded into the retainer. 
     Four radially projecting electric heaters 14 are mounted to and in direct contact with the underside of the screen 13. The heaters each comprise a steel sleeved silica element. 
     A vent 15 rises from the underside of the screen 13 and projects above the level of the retainer, for removing water vapour generated by heating of the material by the elements 14. 
     A vibrator 16 is mounted to the retainer and is electrically energized by way of a controller 17. The controller is set to provide actuation of the vibrator at hourly intervals and for a period of 45 seconds. 
     In a typical drying apparatus of the type above described, the screen 13 is approximately 1.2 meters square and is located 0.75 meters below the rim of the retainer 10. The vent 15 has an internal diameter of 10 centimeters and has a total length of 1.05 meters, so that it projects above the rim of the retainer by a distance of approximately 0.3 meters. Each of the heaters has a heating capacity of 1.0 kilowatt, making a total heating capacity of 4.0 kilowatts. 
     In operation of the apparatus, dried cenospheres pass through the screen 13 and fall into collecting bins 18, movement of the dried material being induced by vibration of the retainer. 
     Drying of the cenospheres is effected by applying heat from the elements 14 to the screen 13 and hence to the material that lies adjacent the screen. The applied heat causes vaporisation of moisture in the material and liberated vapour tends to follow the path of least resistance by first moving downwards through the screen and into the zone of the retainer below the screen. Thereafter, the vapour rises and passes from the retainer by way of the vent 15. 
     The length of the vent is kept relatively short for two reasons. Firstly, to prevent the occurrence of a significant chimney affect which would produce an upward gas flow velocity of a magnitude sufficient to cause cenosphere entrainment and extraction, and secondly to militate against condensation within the vent of vapour that passes upwardly from the retainer.