Manufacturing of Neutral Sodium−Potassium Pyro- and Tripolyphosphates as Complexing Agents of Nutrient Microelements

Abstract

A low-temperature condensation process of a mixture of KH2PO4 and NH4H2PO4as a product of the conversion of potassium chloride with ammonium dihydrogen orthophosphatewas investigated. At the same time, NH4+ and H+ ions have been replaced with sodium ion (as Na2CO3) to obtain neutral sodium and potassium pyro- or tripolyphosphates. The influences of the process temperature (350−500 °C) and the type of initial mixture on the phase composition of the obtained polyphosphates, on their water solubilities and on the quantitative constitution of the P2O5 forms (orthophosphates, pyrophosphates, tripolyphosphates, and higher condensed phosphates) in the product, were determined. It was proved that under the assumed conditions of the polycondensation reaction of the orthophosphates mixture, soluble condensed phosphates were obtained as products, mainly consisting of pyro- and tripoly- or tripolyphosphates, and the created crystalline phases were the suitable solid solutions of potassium−sodium pyro- and tripolyphosphates: (K,Na)4P2O7, (K,Na)5P3O10. The approximate chemical compositions these solid solutions using X-ray diffraction analysis were determinated. Keywords: Hydrophosphates; condensation; pyrophosphates; tripolyphosphates