Activate nutrients access to plants as multiphase slow-release fertilizers by sintering reaction in K2O–MgO–Al2O3–SiO2 system using K-feldspar as a major source

Abstract

The sintering reactions in K2O–MgO–Al2O3–SiO2 system using K-feldspar, magnesite, and potassium carbonate as the starting materials were studied to prepare potassium magnesium silicates and potassium aluminosilicates as multiphase slow-release fertilizers (SRFs). Effects of sintering temperature (850–1250 °C), time (1–5 h), and addition of K2CO3 (100 %–130%) on the reaction extents as well as the types of final products were investigated. Results showed that the multi-phases of K1.14Mg0.57Si1.43O4 and KAlSiO4 were prepared as stable products under the concerned conditions of 1050 °C for 2h, and K2MgSiO4 was obtained instead of K1.14Mg0.57Si1.43O4 under the situation of adding excessive K2CO3. Sintering experiments of five raw mixtures were designed to discuss reaction mechanism by comparing the Al-phases in final products. The results indicated that Al2O3 component favored to persist as potassium aluminosilicates (KAlSiO4 or KAlSi2O6), and three multiphase compounds, KAlSiO4 + K1.14Mg0.57Si1.43O4, KAlSiO4 + K2MgSi3O8, and KAlSi2O6 + K2MgSi3O8 were successfully prepared at optimal conditions and subsequently characterized. Solubility tests of sintered samples in water, 20 g/L critic acid solution, and 0.5 mol/L hydrochloric acid solution showed that the three samples exhibited increasing solubility orderly with consistent component soluble sequence of MgO > K2O ≥ Al2O3 > SiO2 in different solutions. K1.14Mg0.57Si1.43O4 is found to perform the best soluble behavior, while KAlSi2O6 shows a less soluble behavior. Present work proposed considerable approaches to make the inert elements in minerals (K-feldspar and magnesite) and the active element (K) in potassium carbonate accessible for plants in a slow-release route.