Production of sodium trimetaphosphate and of polyphosphate products containing the same

摘要

PICT:1029261/C1/1 An inorganic polyphosphate product containing at least 25% by weight of sodium trimetaphosphate is produced by calcining at above 300 DEG C., preferably 350 DEG C., but below the melting point of said polyphosphate product, a molecularly hydrated alkali metal phosphate feed material, at least 90% by weight of said alkali metal being sodium, there being present during said calcining in contact with said feed material a completely molecularly dehydrated, water-soluble alkali metal polyphosphate, preferably in amount of at least 1% by weight of the feed material, which acts as an accelerator. The accelerator may be a crystalline water-soluble alkali metal polyphosphate having at least two phosphorus atoms (joined in a P-P linkage) in the molecule, and containing substantially no water of constitution, e.g. sodium tripolyphosphate, sodium tri- and tetrametaphosphate and potassium tripolyphosphate. Non-crystalline materials which can be converted into one of the above crystalline polyphosphates on heating below 450 DEG C. may also be used, e.g. sodium hexametaphosphate, Graham's salt and Kurrol's salt. The accelerator should be blended with the phosphatic feed material before the temperature of the resulting mixture is raised above 300 DEG C. At least one of the inorganic phosphate salts must contain water of constitution. The calcining may be carried out in the calcining apparatus shown in Fig. 1. Phosphate feed material enters calciner 13 via pipe 11 and passes to hot bed 17 of sodium trimetaphosphate through holes 19. Evaporation of water, followed by rotation of the calciner causes a solid mixture to pass from upper section 21 at 110-350 DEG C. to lower section 25 through dividing ring 23. Hot gases 28, which cause conversion in section 25, at a temperature of 350 DEG to 620 DEG C. flow from 27 countercurrently with respect to the flow of solids. The desired product 33 flows to 35, a portion of which is withdrawn through 37, the remainder being recycled to section 21 via pipes 39, 41 and 49. In Fig. 2 (not shown), the two sections of the calciner are maintained separately and simultaneously.