Dosing device for pulverulent material
This invention relates to a device for dosing a pulverulent material into an open vessel. Continuous dosing of pulverulent materials to a reaction liquid or to other solid substances has long been known in chemical engineering. The apparatus used for this purpose are normally dosing screws or conveyor belt scales. There are numerous materials which when in the pulverulent state tend to lose their free-flowing quality and cake together when subjected to pressure and friction. Such a substance then sticks to the metal or plastic surfaces of the dosing device and dosing becomes very inaccurate. In many cases, pulverulent materials have the added disadvantage of being hygroscopic. These materials absorb water vapor or solvent vapor from the reaction solution during rest periods so that they become less pourable. Dosing devices involving shaker conveyor troughs or conveyor belt scales cannot be used for such materials because the absorption of moisture indirectly affects the dosing process. 
     It is therefore an object of the present invention to provide a continuously operable dosing device which will operate without troubles and with great accuracy over long periods and give consistently reproducible results even when used for the difficult pulverulent material, mentioned above. The dosing device is designed to dose the pulverulent material into an open vessel underneath it. 
     According to the invention there is provided a device for dosing a pulverulent material into an open vessel, comprising a funnel-shaped storage container for pulverulent material, the container having an outlet aperture at its lower end, a plate located beneath the said aperture and a distance therefrom to that in use a standing column of material is formed between the plate and the aperture, a dosing rod which is displaceable or rotatable in a horizontal plane which passes through the column of material, and means for periodically passing the rod through the column of material thereby to convey at each such passage a quantity of material into the open vessel which is located beneath the dosing device. 
     The open vessel is advantageously designed as a continuous flow reaction vessel so that the pulverulent material can be continuously reacted with a solution in the vessel underneath it. 
     To prevent small residues of substance being left on the dosing rod after its passage through the column of material, a stripper is advantageously arranged above the plane of the dosing rod to carry the residues into the open vessel underneath after each passage of the rod. 
     The dosing device according to the invention doses with a high degree of accuracy even those pulverulent materials which tend to cake and stick to the device. Moreover, the dosing device is simple in construction and inexpensive to manufacture. One major advantage of the dosing device is that it does not subject the material to high frictional forces or pressure. The substance is therefore reliably prevented from forming lumps. The dosing device according to the invention is also particularly suitable for dosing hygroscopic materials. In contrast to shaker conveyor troughs and conveyor belt scales known in the art, the rate at which it doses is not affected by water absorption by the material. In the case of shaker conveyor troughs, the rate of dosing is to a large extent determined by the flow properties of the substance, which in turn often depends on the amount of water absorbed by it. In the case of conveyor belt scales the quantity of water absorbed directly affects the dosing rate. 
     The dosing rate can easily be adapted to any given requirements within wide limits by adjusting the frequency of operation of the dosing rod. In addition, the dosing range can be varied by adjusting the height of the column of material above the plate and the diameter of the dosing rod. 
     One particular application of the device is the dosing of pulverulent sodium dithionite for the continuous desilvering of spent bleach-fixing baths in the photographic industry. 
    
    
     The invention will now be explained in more detail with reference to an example shown in the drawing which is a perspective view of the dosing device according to the invention. 
    
    
     The pulverulent material, for example sodium dithionite, stored in a funnel-shaped container 1 trickles from the container on to a flat plate 4 under the funnel outlet 3, where it collects to form a standing column 5 of material. The surface of the flat plate 4 is generally larger than the cross-section of the outlet aperture 3, that is to say the plate 4 extends outside the limits of the aperture 3 in the horizontal direction. The column 5 is therefore formed by part of the pulverulent material 2 collecting on the plate 4 before it can trickle over the edge of the plate. 
     A dosing rod 6 which is rotatable in the horizontal plane is arranged at the same level as the column of material 5. The rod 6 is driven by the motor 7. The diameter of the rod 6 is smaller than or at the most equal to the height of the column 5 or the flat plate 4. With each rotation of the rod, it passes once through the column of substance and thereby conveys an approximately constant quantity of substance into the reaction vessel 8 underneath it. The rate of dosing is proportional to the speed of rotation of the dosing rod 6. 
     To ensure quantitative transfer of the material on the dosing rod 6 into the reaction vessel 8, a stripper 9, for example a rubber strip, is arranged slightly above the dosing rod. Viewed in the direction of rotation of the rod, it is fixed behind the container 1. The pulverulent material 2 normally drops unguided into the reaction vessel 8 below it. After each dosing operation, more material slips down from the container 1 to refill the space between the plate 4 and outlet aperture 3. 
     If the dosing device is used for a continuous process, the reaction vessel 8 must be a continuous flow vessel. 
     EXAMPLE OF DIMENSIONS 
     
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Distance between plate 4 and outlet aperture 3                            
                            : 4 mm                                        
Diameter of outlet aperture 2                                             
                            :20 mm                                        
Diameter of dosing rod      : 2 mm                                        
Diameter of plate 4         :40 mm                                        
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     Under these conditions, for example, the quantity of sodium dithionate dosed into the reaction vessel 8 is 1.6 g with each passage of the rod. 
     The continuous rotation of the dosing rod 6 could be replaced by a reciprocating movement. 
     The motor 7 would in that case be a reversing motor, for example of the kind used for windscreen wipers. The dosing device could also be designed with a dosing rod 6 executing a translational reciprocating movement through the column 5.