Production of Chloride-Free NPK Fertiliser and Feedgrade Dicalcium Phosphate

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.