PROCESS FOR PRODUCING POLYPHOSPHATES OF ALKALI METALS

摘要

1414369 Alkali metal polyphosphates RHONE-PROGIL 30 July 1973 [31 July 1972 14 June 1973] 36145/73 Heading C1A A process for preparing alkali metal polyphosphates having a predetermined crystalline form comprises, in a first stage, joining a mixture of alkali metal orthophosphates into spherical granules in the presence of water, the granules obtained having a molar ratio of alkali metal oxide/P 2 O 5 of from 1 to 2, drying the spherical granules at a temperature less than 120‹ C. and, in a second stage, heating the granules to form polyphosphates. It is preferred to prepare the granules from a mixture of mono and disodium orthophosphates, having a molar ratio of Na 2 O/P 2 O 5 close to 5/3, using a 50% by wt. aqueous solution of these salts to give granules of an average grain size of 0À1-3À15 mm. which are dried at 100‹ C. in, for example, a fluidized bed. The second stage of the process in which polyphosphates are formed is carried out either by (a) heating the dried granules in a fluidized bed or (b) introducing the dried granules into a fluidized bed by means of a pneumatic conveyer at a high temperature, in the presence in both cases of water vapour. In the case of processes (a) the average residence time of the granules in the bed is 5-30 mins. At a temperature of 300-400‹ C. in the bed granules of phase II tripolyphosphates are attained, whilst at temperatures of at least 480‹ C. phase I tripolyphosphates are obtained. The use of intermediate temperatures allows the formation of both phases in a predetermined manner. In the case of process (b) the dried granules are introduced into a pneumatic conveyer at a high temperature, e.g. 550‹ C. to give a residence time of a few seconds in the conveyer and 1-10 mins. in the following fluidized bed. The ranges of temperature in which phase I and phase II are formed are less high than in process (a). The water vapour content of the bed gases is preferably 14-100% by weight.