GB723245A - Improvements in the manufacture of sodium tripolyphosphates - Google Patents

Abstract

<PICT:0723245/III/1> <PICT:0723245/III/2> <PICT:0723245/III/3> Polymeric sodium tripolyphosphate is obtained by neutralizing phosphoric acid with soda ash to form monosodium phosphate, filtering to remove impurities, dividing the filtrate into two parts substantially in the ratio of 2:1, neutralizing the larger part with either soda ash or caustic soda to form disodium phosphate, mixing the disodium phosphate with the remaining monosodium phosphate solution, spray drying the mixture, heating the product, coating and if desired, grinding. In the preferred embodiment illustrated by the flow sheet (Fig. 1), phosphoric acid containing about 30 per cent P2O5 by weight is continuously treated with a slight excess of soda ash in vessel 2 heated e.g. by steam coil 3 so as to have a final temperature of about 85 DEG C. and a pH of 4.3-4.5. Carbon dioxide is removed by pump 8. The sodium monophosphate solution is filtered, the residue is mixed with water in vessel 13 and disposed of, and the filtrate divided into two parts in the ratio 1:2 by a mechanical device such as a V-notch weir or a suitably arranged pump. The larger portion is neutralized with soda ash (or if a purer product is required, with caustic soda) in the vessel 20. Both neutralization stages are preferably controlled by electrometric pH determination. The portions are mixed in vessel 22 and pumped to a spray dryer 26 where the mixture meets gases at about 450 DEG C. from the kiln 32. Entrained particles are collected in cyclone 29 and bag filter 30 and join the main product in line 41, which leads into kiln 32 where the particles are maintained at a temperature of about 350-450 DEG C. for a time which may be as little as 15 mins. Finally the product passes to a rotary water cooler 38 and band conveyer 39, or is alternatively cooled and conveyed pneumatically in one operation. The preferred construction of kiln 32 is shown in Fig. 2 in which gas-firing is employed, the hot gases being at 800-850 DEG C. initially. The spray-dried material is passed via hopper 50 into the first of a series of covered troughs 34 in which it is propelled from one end to the other at the desired rate by means of adjustably inclined blades 37 (see also Fig. 4) mounted on rods 36. The last blade 37a is not inclined and pushes the powder into the adjacent troughs before discharge. The water driven off is vented into the hot gases, particles entrained therein being recovered during the spray drying.