Effect of sintering temperature of potassium feldspar-limestone/dolomite on composition and microstructure of silicate fertilizers

Abstract

Acid soils call for a fertilizer providing base cations (Ca, Mg, K, et al.) and releasing nutrients with a gradual rate. The dissolution rate from soil minerals such as potassium feldspar (KAlSi3O8) is believed to be too slow to provide agronomic benefits. Using limestone (CaCO3) and/or dolomite (CaMg(CO3)2) as the additives, crystalline structure breakdown of potassium feldspar is achieved by high-temperature treatment and a K-containing multi-nutrient silicate fertilizer is thus produced. Effect of sintering temperature on the available nutrient contents, mineral phases, and microstructure was investigated. The evolution of K-containing minerals and their relative dissolution rate were discussed. The dissolution behaviors of other calcium/magnesium silicates were also analyzed. After sintered at 1220°C, the plant-available Si, Ca, K, and Mg contents in the fertilizer were 15.1%, 21.6%, 5.4%, and 2.4%, with the proportion of 45% potassium feldspar, 33% limestone, and 22% dolomite. Kalsilite (KAlSiO4) was proved to be the most effective one among potassium feldspar (KAlSi3O8), leucite (KAlSi2O6), and kalsilite (KAlSiO4), which was totally soluble after 30 minutes extraction in 0.5 mol/L HCl acid. Pseudwollastonite (Ca3Si3O9) was totally soluble while gehlenite (Ca2Al2SiO7) and akermanite (Ca2MgSi2O7) could only be dissolved after extraction of several times. Chemical reaction processes between KAlSi3O8 and CaO/MgO were discussed, and dicalcium silicate (Ca2SiO4, C2S) should be avoided in the reaction system.