Heliyon (Jun 2024)
Spectroscopic characterizations of silicate fertilizers prepared by chemical deashing of coals
Abstract
Quality silicate fertilizers should be in great demand, and yet the production has been limited due to strict regulations on heavy metals, despite many raw materials and activation methods being used. In the chemical deashing of coals for the production of ultraclean coals, the silica gels of high purity were precipitated with little heavy metals from the acid deashing solutions, which could be used to produce quality silicate fertilizers by pulping with CaO or MgO under mild conditions. By varying the Ca/Si molar ratios, silicate fertilizers with different chemical compositions were prepared, and the active silica contents were measured and validated by ICP and colorimetric methods. For the curve of the active silica contents versus the Ca/Si molar ratios, four regions could be clearly marked with unique patterns, and quality silicate fertilizers occurred with the Ca/Si molar ratios from ∼1.10 to ∼3.50. The pH values of the silicate fertilizers could also be divided into the same four regions with respect to the Ca/Si molar ratios, and the highest active silica content occurred at the pH value of ∼11.30 with the Ca/Si molar ratio of ∼1.50. With the XRD investigations of the silicate fertilizers selected from the four regions, the water-insoluble 1.5CaO·SiO2•xH2O was identified as the contributor of active silica in the silicate fertilizers. By replacing full or part of CaO with MgO in the preparation of silicate fertilizers, the silica gels were found to preferably react with CaO, and the active silica contents grew with the increase of CaO. By referring to the model silicate fertilizers prepared in this work by varying the (Ca + Mg)/Si molar ratios, 1.5CaO·SiO2•xH2O was also identified as the dominant in one commercial slag silicate fertilizer. Silicate fertilizers by silica gels can be helpful for secondary pollution elimination and cost reduction of coal deashing.
