A slow-release K+ fertilizer from residues of the Brazilian oil-shale industry: synthesis of kalsilite-type structures

The preparation of a potential slow-release potassium fertilizer (SRKF) fro m discharge from the Brazilian oil-shale industrialization process (PETROSI X) is described in this study. Thermogravimetric analysis (TG), differentia l scanning calorimetry (DSC), and electron paramagnetic resonance (EPR) spe ctroscopy data were used to indicate the best temperature for the synthesis of the fertilizer (900 degreesC). X-ray powder diffraction and Fourier-tra nsformed infrared spectroscopy (FTIR) data are compatible with a structure belonging to the metastable silicate kalsilite group, alpha -K2MgSi3O8. The solubility of the product, expressed as a percentage of K2O, was 30.3% (in HCL 0.5 mol/l), 23.2% (in citric acid 0.1 mol/l), and 6.9% (in H2O). These results are similar or better than those reported for other SRKFs prepared by alternative methods. The use of oil-shale fines and lime shale as start ing materials for the synthesis of soil fertilizers may help to avoid waste disposal problems and lower production costs of the Brazilian oil-shale in dustry.