Process for preparing a sodium tripolyphosphate hexahydrate powder

#A process is provided for preparing a sodium tripolyphosphate hexahydrate composition which can be ground such that less than 50% will be retained on a 100 mesh sieve.

#Sodium tripolyphosphate (STPP) is useful as a builder in synthetic detergent formulations particularly for use in heavy duty detergents. Anhydrous grade STPP is satisfactory when the detergent is to be prepared by spray drying but usually is not employed for detergents formulated by dry mixing. A more absorbent product is required. U.S. Pat. No. 3,469,938 teaches a process to produce an absorbent (light density) granular STPP as the hexahydrate by hydrating a granular STPP containing at least 10% fines (less than 100 U.S. Standard mesh). The patent teaches the function of the fines in the process is to produce a cement when combined with the water used to hydrate the particles and produce low bulk density "granules," irregularly shaped agglomerates having a large surface area.

#Recently, liquid detergents have become popular because of their convenience. STPP is insufficiently soluble in aqueous solution to provide sufficient builder activity if formulated into a true solution. However, a slurry of finely powdered STPP hexahydrate would clearly overcome this solubility problem and permit formulating a detergent slurry which could contain sufficient STPP to provide effective builder activity. In general, a composition is considered to be a powder if at least 50% of the particles by weight pass through a 100 mesh sieve.

#The best mode of practising the invention will be clear to one skilled in the art from the following nonlimiting examples.

#STPP hexahydrate was prepared by the process of U.S. Pat. No. 3,469,938. After the hydrating and drying steps the intermediate product was screened and the oversize portion ground in a conventional hammer mill and combined as "product" with the screened product from the dryer. The particle size distribution of the feed and product are presented as Table I as % by weight.

#Granular STPP hexahydrate produced by the process of U.S. Pat. No. 3,469,938, Example A, was fed to a Mikro-Pulverizer high speed hammer mill. This style mill is particularly useful for producing very fine powders. The mill became overloaded and shut down. Examination showed the STPP hexahydrate had caked on the internals.