Phosphate rock is solubilised in nitric acid. The solution contains Ca(NO3)2 and H3PO4. Ca(NO3)2 is converted into KNO3 by reacting the solution with a potassium-loaded ion exchanger. The ion exchanger is regenerated with a KCI-solution giving a CaCl2-effluent. The product solution containing KNO3 and H3PO4 is used as an intermediate in the manufacture of chloride-free NPK fertilisers.
This procedure makes it possible to produce Cl-free NPK using KCI as a raw material, and at the same time it removes calcium from phosphate rock, i.e. it produces phosphoric acid.
If only about half of the Ca(NO3)2 is convened into KNO3, the residual Ca can be precipitated from the solution together with the major part of the P2O5 in the form of CaHPO4 of feed grade quality, and the mother liquor can be used for making Cl-free NPK.
This means that P2O5 from the phosphoric acid produced in the ion exchange process combines with P2O5 from phosphate rock to make a product, which commands an even higher price per unit of P2O5 than does the phosphoric acid.
K C Knudsen, MSc (ChemEng), Superphos a/s, Denmark.
12 pages, 2 figures.