Studies on Potassium Magnesium Phosphate Fertilizers bu Using Sea Water Magnesia

The present authors had reported the result of the study on the magnesium hudrogen phosphate fertilizer (MgHPO₄·3H₂O) in this Journal, Vol.12, p.276 (1958), and then the further studies on the potassium magnesium phosphate fertilizer (KMgPO₄·6H₂O) and ammonium magnesium phosphate fertilizer (NH₄MgPO₄·6H₂O)(now in the contribution to the Journal of the Society of Chemical Industry, Japan).<BR>In the present paper, on continuing the above described studies, the synthesis of potassium magnesium phosphate fertilizer is carried out completely by the quite different method. Especially it is very interesting if the potassium dihydrogen phosphate fertilizer (KH₂PO₄·xH₂O) will be easily obtained from the double decomposition reaction between potassium chloride and phosphoric acid solution with the byproduct hydrochloric acid and potassium dihydrogen phosphate fertilizer as shown in the following formula: KCl+H₃PO₄=HCl+KH₂PO₄<BR>In the authors' previous report (loc. cit.), potassium magnesium phosphate fertilizer (KMgPO₄·6H₂O) was synthesized by the reaction between potassium hydroxide (KOH) solution and magnesium hydrogen phosphate (MgHPO₄) in excess phosphoric acid solution, but in the present study, this potassium magnesium phosphate was obtained by the reaction between sea water magnesium hydroxide (Mg(OH)₂) and dilute solution (1, 5, 10 and 20%) of potassium dihydrogen phosphate (KH₂PO₄) obtained above at several temperatures (15, 40, 60 and 80°C). This reaction can be writeen by the following either (1) or (2) formula:<BR>1: Mg(OH)₂+KH₂PO₄→KMgPO₄·6H₂O<BR>2: 3Mg(OH)₂+3KH₂PO₄→K₂Mg₃H (PO₄) ₃·9H₂O<BR>The prepared samples of potassium phosphate fertilizers were analysed and the main compositions of MgO, K₂O and P₂O₅ and the ratio of K₂O/P₂O₅ are shown in the following table and compared with those values of two compounds (1) KMgPO₄·6H₂O and (2) K₂Mg₃H (PO₄) ₃·9H₂O. From these results, it is clear that the obtained 7 samples (a) to (g) are the mixtures of (1) KMgPO₄·6H₂O and (2) K₂Mg₃H (PO₄) ₃·9H₂O. These potassium magnesium phosphate fertilizer samples were further studied on their solubilities of main fertilizer components MgO, K₂O and P₂O₅ in citric acid solution and the hygroscopic property. The results of X-ray analysis were also the clear dissolution, that these prepared samples are to be the mixtures of the two potassium magnesium phosphate compounds (1) KMgPO₄·6H₂O and (2) K₂Mg₃H (PO₄) ₃·9H₂O.