Abstract:
An apparatus and a process for drying a suspension of a finely divided particulate solid material to produce the solid material in a substantially dry and unagglomerated form. The apparatus consists of a grinding chamber containing a particulate grinding medium such as silica having a diameter of 1-5 mm, a conduit for introducing the suspension into the grinding chamber and an impeller which rotates in the grinding chamber. The grinding chamber has a perforated base through which a heated gas is introduced to provide an upward flow of gas through the grinding chamber and through the grinding medium. As the particulate material is dried, fine particles of the dried material are carried upwards in the upflow of gas through the grinding chamber. A filter such as one or more cyclones and/or a bag filter assembly is located downstream of the grinding chamber in order to separate the particles of dried material from the gas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/435,587 filed May 5, 1995 and now abandoned, which application claims the foreign priority of U.K. application serial no. 94303280.5 filed May 6, 1994. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to a process and apparatus for drying suspensions of particulate solid materials. 
     BACKGROUND OF THE INVENTION 
     Many different types of apparatus are available for thermal drying of wet particulate materials, but most suffer from the disadvantage that the particles tend to agglomerate during the drying process and the particle size distribution of the dry product tends to be significantly coarser than that of the wet feed material. The problem is particularly evident when the particulate material is finely divided, for example when it has a particle size distribution such that more than about 60% by weight consists of particles having an equivalent spherical diameter smaller than 2 μm, and when the particulate material is highly concentrated in the aqueous medium, for example when the solids content of the suspension is in excess of about 50% by weight. For this reason, many finely divided particulate materials are made available commercially in the form of concentrated aqueous suspensions, and it is not commercially attractive to produce them in the form of a dried powder. 
     For many end uses, and especially when the particulate material is to be dispersed in a non-aqueous or non-polar medium, such as, for example, when it is to be used as a filler or extender in an organic polymer composition such as a rubber, plastics or non-aqueous paint composition, a finely divided particulate material in dry powder form would be commercially and technologically advantageous. 
     Transporting a particulate material in aqueous suspension form has the obvious disadvantage that a significant quantity of water is associated with the particulate material, which adds to the cost of conveying the material. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided an apparatus for drying a suspension of a finely divided particulate solid material in an aqueous or non-aqueous medium to thereby produce a substantially dry product, comprising: 
     (a) a grinding chamber containing a particulate grinding medium, said grinding chamber having an inner wall and a perforated base through which a heated gas is introduced to provide an upflow of gas through the grinding chamber and through the grinding medium, said perforated base having a central imperforate area which causes the gas to pass preferentially through a region near the walls of the grinding chamber; 
     (b) means for introducing the suspension into the grinding chamber; and 
     (c) an impeller capable of rotation in the grinding chamber, said impeller being provided with impeller bars. 
     The gas to be introduced through the perforated base of the grinding chamber is advantageously compressed by means of a fan and may be heated indirectly by passage through one side of a heat exchanger, or directly by contact with electric heating elements, or by passage through a combustion chamber in which a gaseous, liquid or solid fuel is burned. 
     The perforated base of the grinding chamber preferably has a central imperforate area which causes the gas to pass preferentially through the region near the walls of the grinding chamber. A top plate having a central opening is preferably positioned in the grinding chamber at a height above the perforated base which is not greater than one half of the diameter of the grinding chamber. The top plate is provided to prevent a bed of particulate solid material in the grinding chamber from extending upwards to an undesirable degree near the walls of the grinding chamber. 
     Classifying means for separating the substantially dry product into fractions of different particle sizes is preferably provided downstream of the grinding chamber. The classifying means may be separate from the grinding chamber and connected thereto by suitable conduits or may be mounted above the grinding chamber and may form an integral part therewith. The classifying means may conveniently be of the type in which a cylindrical or frusto-conical cage, comprising peripheral, longitudinally extending blades or vanes, is rotated about its longitudinal axis in a current of a suspension of particles in a gas. Relatively fine particles in the suspension pass between the blades or vanes of the rotating cage, while relatively coarse particles are deflected by the blades or vanes in the radially outward direction of the cage. 
     Advantageously, means are provided to inject additional gas into the mixture of gas and particles between the grinding chamber and the classifying means or into the classifying means itself. This arrangement makes it possible to adjust the flow rate of gas through the grinding chamber independently of the flow rate of gas through the classifying means, with the result that conditions in the grinding chamber can be adjusted to give an optimum product which will enable the classifying means to operate with the greatest precision and efficiency. Advantageously, a suction fan is provided downstream of the classifying means to provide negative pressure which will draw gas and suspended particles at the desired rate through the classifying means. 
     Filter means are also preferably provided downstream of the grinding chamber, (and of the classifying means, if used) to separate solid particles from the suspending gas. These means may be, for example, one or more cyclones and/or a bag filter assembly. 
     According to a second aspect of the invention, a process is provided for drying a suspension of a finely divided particulate solid material in an aqueous or non-aqueous medium to thereby produce a substantially dry product, comprising the steps of: 
     (a) introducing the suspension into a grinding chamber containing a bed of particulate grinding medium; 
     (b) rotating an impeller provided with impeller bars present in the grinding chamber so as to agitate the grinding medium; and 
     (c) introducing a heated gas through a perforate base of the grinding chamber such that it passes through the bed of the grinding medium, said substantially dry product being entrained by the gas and conveyed out of the grinding chamber. 
     Preferably, in practicing the process of the invention, the impeller will rotate in the grinding chamber at a peripheral speed in the range of about 5 to 20 m.s −1 , especially 8 to 11 m.s. −1 . Advantageously, the impeller will have a diameter d such that a gap in the range of about 0.01 d to 0.05 d is present between the ends of the impeller bars and the inner wall of the grinding chamber. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The grinding medium preferably comprises particles of diameter within the range from 0.5 to 12.5 mm. More preferably, the grinding medium comprises particles of diameter within the range from 1.0 to 5.0 mm. The grinding medium may comprise balls, beads or pellets of any hard mineral, ceramic or metallic material. Alternatively, particles of natural sand of appropriate size have been found to be very effective. 
     The process of the invention is particularly advantageous when the particulate solid material has a particle size distribution such that at least 60% by weight consists of particles having an equivalent spherical diameter smaller than 2 μm. The suspension of the solid material is preferably a concentrated aqueous suspension and the process is particularly advantageous when the suspension has a solids content in excess of about 50% by weight, based on the weight of the suspension. 
     The suspension may be fluid and of relatively low viscosity, in which case a dispersing agent would generally be used. The dispersing agent may be, for example, a water soluble condensed phosphate, a water soluble salt of a polysilicic acid or a polyelectrolyte, for example a water soluble salt of a poly(acrylic acid) or of a poly(methacrylic acid) having a number average molecular weight not greater than 20,000. The amount of the dispersing agent used would generally be in the range of from 0.1 to 2.0% by weight, based on the weight of the dry particulate solid material. The suspension may be introduced at a temperature within the range from about 4° C. to about 100° C. However, it is advantageous to pre-heat the suspension to a temperature within the range from 25° C. to 100° C. Alternatively, the suspension may be in the form of a cake such as is formed in high pressure filtration of a relatively dilute suspension of the particulate solid material, provided that the cake is non-sticky. If the suspension is of the fluid type, it may be introduced into the grinding chamber through a conduit provided with a suitable inlet nozzle. If the suspension is in the form of a cake, it may be introduced through suitable ducting, the grinding chamber end of which may be closed by a rotating valve to prevent gas from passing from the grinding chamber into the ducting. 
     The gas containing entrained particles leaving the grinding chamber is preferably passed through dry classifying means to remove any particles having diameters greater than the desired maximum particle size. Generally it is required that the substantially dry product is free of any particles having an equivalent spherical diameter greater than 10 μm, and the dry product is preferably free of any particles having an equivalent spherical diameter greater than 5 μm. 
     The gas passing through the perforated base of the grinding chamber preferably has a temperature such that the temperature of the gas and suspended particles leaving the grinding chamber is at least 70° C. It has also been found that a classifying means will not provide efficient separation if the gas which carries the particles to be separated is bone dry. Generally, for most efficient separation, it is required that the gas entering the classifying means has a relative humidity of at least 15%, but the relative humidity of the gas must not be so high as to cause condensation in later stages of the process. Generally the relative humidity of the gas should not be greater than about 55%. The particulate material is preferably dried to a water content of not more than 1% by weight, and preferably to a water content of not more than 0.2% by weight. 
     The fine particulate material which is dried in the grinding chamber may, at the same time be coated with a surface treatment agent which is conveniently introduced into the grinding chamber in particulate solid form through a duct which is provided at its grinding chamber end with a rotary valve. The surface treatment agent may be, for example, a fatty acid having from 12 to 24 carbon atoms in its hydrocarbon chain. Alternatively a liquid surface treatment agent may be used, such as a substituted silane substituted with at least one group having hydrophobic properties and at least one group which is compatible with the surface of the particulate solid material to be treated. Most suitably the substituted silane is of the type which is represented by the general formula:                           
     where R 1  is an aminoalkyl or mercaptoalkyl group, R 2  is a hydroxy, hydroxyalkyl or alkoxy group, and each of R 3  and R 4 , which may be the same or different, is a hydrogen atom or a hydroxy, alkyl, hydroxyalkyl or alkoxy group. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The accompanying drawing is a diagrammatic representation of the apparatus of the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWING 
     A grinding chamber  1  is defined by a casing  2 , a base plate  3 , which has a peripheral annular perforated area  4  and a central imperforate area  5  and a top plate  6 , the height of which above base plate  3  is not greater than one half of the diameter of grinding chamber  1 , and which is provided with a central aperture  7 . An impeller consisting of four cylindrical impeller bars  8  secured to a vertical shaft  9  rotates in a horizontal plane a small vertical distance above base plate  3 . Shaft  9  has a diameter d such that a gap in the range of about 0.01 d to 0.05 d is present between the ends of impeller bars  8  and the inner wall of grinding chamber  1 . 
     The impeller is driven by an electric motor  10  through a gear box  11  advantageously at a peripheral speed of about 5 to 20 m.s −1 , preferably 8 to 11 m.s −1 . The grinding chamber is charged with a grinding medium  12  which conveniently consists of grains of silica sand having diameters in the range of about 0.5 to 12.5 mm, preferably from 1 mm to 5 mm. 
     Heated air under pressure is introduced into a plenum chamber  13  immediately below base plate  3  by means of a fan  14 . Compressed air is blown into plenum chamber  13  through a heat exchanger  15 , to which hot gas is admitted through an inlet  16 , and from which it is exhausted through an outlet  17 . The volume of heated air entering plenum chamber  13  may be adjusted by admitting additional air through a vent  18 , the volume of air entering in this way being controlled by means of a valve  19 . 
     Central aperture  7  of top plate  6  of grinding chamber  1  is covered with a frusto-conical hood  20 . An aqueous suspension of a particulate material to be dried is introduced into the grinding chamber through a conduit  21  which passes through the side of this hood. An inlet duct  22  for charging solid materials into the grinding chamber passes through the top of the hood and is provided with a rotary valve arrangement  23  which seals the interior of grinding chamber  1  from the exterior. Solid surface treatment agents may be introduced into the grinding chamber through duct  22 . Alternatively, if the suspension of particulate material to be dried is in the form of a substantially non-sticky cake, this may be introduced through duct  22 . 
     Air containing entrained substantially dry fine particles leaves grinding chamber  1  through a duct  24  which leads to an air classifier  25 . Air classifier  25  comprises a rotating cage  26  made up of peripheral bars or vanes. Of the particles entrained in the air, those of sufficient fineness pass between the bars of rotating cage  26 , and are discharged from the classifier through a duct  27 , while any undesirably coarse particles present in the air stream are deflected by the bars of rotating cage  26  and are thrown to the outer wall of classifier  25  to be discharged through a chute  28  to be discarded or recycled to grinding chamber  1 . Generally not more than about 5% by weight of the particles entering classifier  1  are discharged as the coarse fraction. 
     The stream of air containing entrained fine particles passes through duct  27  to a cyclone  30  which separates the fine particles from the air, the fine particles being discharged as product through a rotary valve arrangement  31  at the base of cyclone  30 . Air which is almost completely freed from entrained fine particles passes through a duct  32  to a bag filter assembly  33  where the remaining finely divided material is separated from the air. Pulses of high pressure air are supplied through a conduit  34  to a plurality of inlets  35  communicating with the interior of filter stockings (not shown) in the bag filter to blow accumulated solid material off the outer surface of the filter stockings. The solid material falls to the base of bag filter assembly  33  whence it is discharged through a rotary valve  36 . Substantially clean air leaves through an outlet  37 . 
     The particle size distribution of the fine particles leaving air classifier  25  may be controlled by adjusting the speed of rotation of cage  26  and the volumetric flow rate of air and entrained particles through classifier  25 . For this latter purpose, additional air may be drawn into the stream entering classifier  25  through an inlet  38 , the flow of air being controlled by means of a valve  39 . Alternatively, the additional air may be drawn through an inlet  29  provided in the casing of air classifier  25 . The intake of air at one of these points is assisted by providing a reduced pressure in the bag filter assembly by means of a fan  40  which is connected to outlet  37  of bag filter assembly  33 . 
     The invention will now be described with reference to the following example. 
     EXAMPLE 1 
     A suspension containing 78% by weight of a natural marble which had been comminuted to give a product having a particle size distribution such that 90% by weight consisted of particles having an equivalent spherical diameter smaller than 2 μm was introduced through the inlet conduit  21  of a drying apparatus of the type illustrated in the Figure. The suspension also contained 0.7% by weight, based on the weight of dry marble, of a sodium polyacrylate dispersing agent. 
     The final product was found to contain 0.1% by weight of water. This product was tested for percentage reflectance to light of wavelength 457 nm and 570 nm, respectively, according to the procedure laid down in International Standard No. I.S.O. 2470. The product was also tested for the percentages by weight of the particles which had equivalent spherical diameters larger than 53 μm, larger than 10 μm, smaller than 2 μm and smaller than 1 μm, respectively. The size of the largest particles in the product after dispersion to break down any agglomerates present was also determined by means of a Hegman gauge. As a comparison, a sample of the feed material was also subjected to the same tests. 
     The results obtained are set forth in Table 1 below: 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 % reflectance to 
                 % by weight 
                 Hegman gauge 
               
             
          
           
               
                   
                 light of wavelength 
                 larger 
                 larger 
                 smaller 
                 smaller 
                 reading 
               
             
          
           
               
                 Product Source 
                 457 nm 
                 570 nm 
                 than 53 μm 
                 than 10 μm 
                 than 2 μm 
                 than 1 μm 
                 (μm) 
               
               
                   
               
               
                 Feed Suspension 
                 94.0 
                 95.0 
                 0.013 
                 0.3 
                 87 
                 61 
                 — 
               
               
                 Dry Product 
                 94.1 
                 95.0 
                 0.013 
                 0.1 
                 87 
                 60 
                 5