Manufacture of dicalcium phosphate, dihydrate

Dicalcium phosphate dihydrate, CaHPO4 2H2O, is produced from phosphoric acid and/or a phosphoric acid salt, a calcium compound and, if desired, a basic compound, by a process comprising introducing the calcium compound and the phosphoric acid and/or phosphoric acid salt, in the presence of water, into a reactor, and rapidly and intimately mixing them therein at temperatures below 70 DEG C., adjusting the reaction mixture to a pH value of 2.0 to 6.0 and maintaining the pH value during the greater part of the reaction, by regulating the introduction of the feed compounds so that the pH value is maintained, wherein to produce CaHPO4.2H2O formed of relatively small grains within a given range of grain, the reaction is carried out at a relatively high pH within the range specified, and for larger grains it is carried out at a lower pH value within that range, and wherein after the introduction of all of the phosphoric acid and/or all the phosphoric acid salt, the balance of the other starting materials is added to the reaction mixture while increasing the pH value of the said mixture, which is then treated for the separation of CaHPO4.2H2O from the accompanying liquid. For instance, grain sizes within the range 5 to 40m are obtained by conducting the reaction at a pH of about 3 and grain sizes smaller than 15m are obtained when the pH is 5. Specified feed compounds are thermal phosphoric acid, potassium diphosphate, CaCO3, Ca(OH)2, CaCl2 and Ca(NO3)2. The optional basic compound is e.g. an alkali metal or ammonium hydroxide, carbonate or orthophosphate. Stabilizers such as tetrasodium pyrophosphate, magnesium pyrophosphate, sodium tripolyphosphate, disodium metasilicate and disodium metaborate may be incorporated in the product by introduction at any stage of the process before final separation of the product. Examples are given of the production of CaHPO4.2H2O by discontinuous and continuous processes employing as the feed compounds (i) CaCl2, H3PO4 and NaOH, (ii) CaCO3 or Ca(OH)2, and H3PO4, (iii) CaCO3, CaCl2, H3PO4 and NaOH, (iv) CaCl2, H3PO4, monosodium tripolyphosphate and NaOH or (v) Ca(NO3)2, H3PO4 and aqueous ammonia.