GB704768A - Improvements in or relating to a process for treating powdered water-soluble alkali metal or ammonium phosphate glass apparatus therefor, and the phosphate glass compositions so produced - Google Patents

Abstract

<PICT:0704768/III/1> Alkali-metal or ammonium phosphate glass having a molar ratio of alkali-metal or ammonium oxide to phosphorous pentoxide between 0.9 : 1 and 1.7 : 1, preferably 1.5 : 1 is rendered non-tacky and more easily soluble in water by dispersing the powdered phosphate glass in a gaseous humidifying zone such as superheated steam to render the surfaces of the individual particles tacky and then passing the tacky particles while still in gaseous dispersion into a contiguous gaseous drying zone, such as a current of heated air, having a temperature of from 130 DEG F. to 500 DEG F., to render the surface of the individual particles non-tacky and covered with a coating of an acidic reversion product and collecting the product of the process as a loose friable finely divided mass. A finely divided water-soluble ingredient compatible with the phosphate glass may be dispersed in the humidifying zone in admixture with or at least substantially simultaneously with the phosphate glass particles which, when rendered tacky, are coated by the ingredient and retain the coatings when rendered non-tacky. The ingredient may comprise a powdered water-soluble alkali metal compound or mixture of compounds such as alkali metal carbonate or bicarbonate or sulphite which releases a gas on reaction in acqueous solution with the acidic reversion product. The ingredient may also comprise a detergent material such as soaps, a wetting agent such as alkyl sulphonates, alkyl-aryl sulphonates and the polyalkyloxyalkyl phenols, an anti-foaming agent such as trioctyl phosphate, zinc stearate and mineral oils or a pH adjusting acid or alkaline agent. The glasses may contain the usual impurities such as water of constitution, boric oxide, silica, calcium oxide and magnesium oxide. As shown, the coating materials are fed from storage bins 10, 12 and proportions 11, 13 respectively to the outlet 14 of a phosphate glass storage bin 15 by a screw conveyer 16. The outlet 14 empties into screw conveyer 17 of pulverizer 18 in which the ingredients from bins 10, 12 and 15 are mixed and ground together to pass through a fine screen on to chain conveyer 19 which discharges on to a vibrating screen 20, the fine material passing through on to chain elevator 22 for conveyance to hopper 23 and the oversize particles being returned to the outlet 14 by screw conveyer 21. The hopper is vibrated to feed a uniform layer of the powdered material to a vibrating feeding pan 25 from the edge 39 of which it is fed in a film to drop across a jet of steam issuing from nozzle 26, e.g. at from 60ft. to 250ft. per second, by which it is carried to the centre of the chamber 27 and into a stream of upwardly moving warm air at from 135 DEG F. to 500 DEG F. from inlet 28. The descending powder is directed by the hopper bottom 29 and baffle 30 into shoot 31 leading to storage bin 32. The temperature of the steam is controlled by a super heater 26a located in the steam line just prior to the steam jet and a cyclone separator 26e removes excess moisture from the steam prior to entering the superheater. The powder entering the steam is cool relative to the steam, the degree of superheat being controlled between 0 and 250 DEG F. superheat to control the bulk density of the resulting product, e.g. of 35 to 60lb./cu. ft. The product is cooled before reaching the storage bin 32 by drawing a stream of cooling air through openings 35 in the sides of shoot 31 the air being removed through outlet pipe 36 branching into outlet pipe 33 for the steam and drying current of air for delivery to the cyclone separator 34.