A process for the production of granular perborates

摘要

Granular perborates are formed by partially melting a perborate containing water of crystalization by heating under pressure and thereafter pressure moulding the softened mass into structures which possess small size in at least one dimension, and finally hardening by cooling the structures so obtained. The softening is suitably done in a jacketed extrusion press and the temperatures employed are below 100 DEG C., and preferably below 75 DEG C. The bulk density of the product may be from 500 to 800 grams per litre. As shown in the drawings, Fig. 2, an extrusion press is provided with an endless screw 12 the pitch of which is preferably smaller in the direction of movement and the outlet from the press is closed by a matrix 14, through the openings of which the perborate is extruded. The extrusion press is provided with jackets 9, 10, and 11 through which a heating fluid may be passed and in addition the matrix 14 may be provided with an electric heater. The shafts 13 carrying the screw 12 may be hollow and also provided with heating or cooling means. The endless screw 12 may, if desired, be broken up into blades separately attached to the hollow shaft and the housing may carry kneading teeth. The apertures in the matrix are preferably such that no perborate particle is removed further than 1.5 mm. from the wall of the extruded granule. Alternatively a granulating drum as shown in Fig. 5 may be employed, where the matrix 14 is replaced by a perforated drum 19 forming an extension of the cylinder of the extrusion press. Strippers 20 are attached to and rotate with the shaft 13 to brush off the surface of the granulating drum and thus cut the emerging perborate ropes. The perborate granules are cooled by means of a cold gas such as air at a temperature below 50 DEG C. as soon as possible after extrusion. If desired the perborate ropes emerging for example from the matrix 14 may be passed on to cooled conveyer belts, the speed of which may be greater or smaller than that of the extruded ropes thereby either folding the perborate rope into coils or stretching it to break it. According to the preferred embodiment the cooling of the formed perborate granules is effected by means of a fluidized bed. As shown in Fig. 7, the granulating drum 33 of an extrusion press is provided with a hood 34 and the extruded granules pass down a tube 35 into a fluidized bed resting on a perforated support 28. Here the particles are fluidized by a stream of cold air, the cooling action of which is assisted by a cooling jacket 29. The perborate granules are removed from the fluidized bed through a tube 36 and passed to storage at 39. Alternatively a multi-bed system may be employed as shown in Fig. 8, where beds 48, 49 and 50 are placed above one another and are provided with down-comers 46 and 47. The cooled granules may be crushed if necessary and their bulk density may be further reduced by removing water of crystallization at a temperature below the softening point, or by treatment with organic solvents which are miscible with water or are able to absorb water such as alcohols, ketones or ethers. According to a typical example, sodium perborate tetrahydrate was passed through an extrusion press the jacket of which was heated to about 55 DEG C. and the matrix was heated to the same temperature. The perborate ropes emerging were fed to the apparatus of Fig. 7 after a preliminary crushing and were fluidized with air at room temperature. The cooling jacket 29 was full of water at 12 DEG C. The perborate granules removed from the apparatus had a bulk density of 711 grams per litre.