Design of a Novel Fertilizer Made of Steelmaking Slag Using the Glassy Phase of the CaO-SiO2-FeO System. Part I: Optimization on the Slag Chemistry and Microstructure

摘要

Fertilizers made of steelmaking slag are important inexpensive materials for recovering degraded paddy fields. The results of our previous works showed that essential Ca and Si elements were effectively supplied by the dicalcium silicate phase (C2S) while the glassy phase of the CaO-SiO2-FeO system served as an Fe source. In this study, we have developed a slag containing both the C2S and glassy phases that are capable of simultaneously supplying Ca, Si, and Fe elements to paddy fields. To determine the optimum conditions for supplying Fe from the glassy phase, the molten slag of the CaO-SiO2-FeO system was quenched from 1573 K, and the upper limits of its FeO content and basicity for glass formation were obtained. After that, slags consisting of the glassy phases with various compositions and the C2S crystal phase were synthesized and subjected to a leaching test. By analyzing the dissolution behaviors of different phases, a kinetic model was established to estimate the dissolved amount of each element and determine the optimum slag composition, which was obtained at a quenching temperature of 1573 K, basicity of 1.6, and FeO concentration of approximately 25 mass. The dissolved amounts of Fe and Si produced by this slag exceeded those of commercial fertilizers prepared from steelmaking slag by factors of 6 and 2-3, respectively, while their respective amounts of supplied Ca were close to each other.