Obtaining NPK fertilizers using decomposition of phosphate rock by sulphuric acid

In this study a method for obtaining granular NPK fertilizers of various grades has been established. Kola apatite or Baltic phosphorites were dissolved in 57% H₂SO₄; the resulting slurries were then ammoniated, evaporated and mixed with urea or ammonium nitrate and potassium salts. In some experiments micronutrients (B, Cu, Mo, Co or Mn) were added. The mixtures were dried and granulated. When using urea, granulation was carried out by heating the mixture until partial melting occurred, but the mixtures with ammonium nitrate had to be granulated with the addition of water. The products contained 29–38% fertilizer nutrients (N soluble in water, P₂O₅ soluble in citric acid and K₂O soluble in water) and had a ratio of over 90% of P₂O₅ soluble in citric acid to the P₂O₅ total. The granules were hygroscopic, but little caking occurred. The urea-containing materials produced better granules. Adding micronutrients usually resulted in an improvement in the product quality (an increase in the granule hardness and phosphorus solubility).