GB742623A - Production of pyrophosphates and polyphosphates - Google Patents

Abstract

<PICT:0742623/III/1> Finely divided calcined orthophosphate is transported by hot gas through a long stationary pipe, the gas temperature, the length of the pipe and the flow velocity being such that conversion into pyro- or poly-phosphate takes place, preferably completely. The production of tetrasodium pyrophosphate from disodium orthophosphate, sodium tripolyphosphate or a mixture of sodium tripolyphosphate and tetrasodium pyrophosphate from sodium orthophosphate, acid calcium pyrophosphate from monocalcium orthophosphate, neutral calcium pyrophosphate from dicalcium orthophosphate, and tetrapotassium pyrophosphate from dipotassium orthophosphate is mentioned. Heated air, nitrogen or other inert gases, or combustion gases may be used and gas entering into chemical reaction with the starting or intermediate products may be used with the hot gas, examples being hydrochloric acid gas (to give with disodium orthophosphate, acid pyrophosphate), and oxidizing gases e.g. nitrogen oxides to destroy impurities in the starting product. If desired a part of the waste gas or a part of the finished product may be recycled. Hot waste gases may be used to heat the reaction apparatus or, e.g. as the hot gas in a spray tower for producing the calcined orthophosphate. To avoid sintering, a pipe is used of which the first part is wider than the rest, whereby sintering and caking take place before the particles touch the wall of the pipe. In the Example, disodium orthophosphate the majority of which has a particle size of less than 150 microns, is fed through worm 1 (see Figure), into a pipe 14.5 m. long and having a diameter of 100 m., and heated by means of illuminating gas burnt in burners B1, B2 and B3 whereby temperatures ranging between 250 DEG C. at t2 and 195 DEG C. at t3 and 370 DEG C. at t9 are obtained in the pipe. The flow velocity is about 5 m/sec. The resultant tetrasodium pyrophosphate is removed by cyclone separator 2, the gases are metered through gas flow indicator 3 and pass through a throttle valve 4 before being extracted by fan 5. If desired the diameter of the pipe could vary along its length so as to maintain the gas velocity constant.