Phase transformation mechanism of boiler ash roasted with sodium salt for vanadium extraction

Boiler ash, produced by burning heavy fuel oil (HFO) in electrical power plants, is one of the most serious environmental problems facing oil-rich countries. However, boiler ash also contains some important industrial elements like V, Ni, Zn, etc., and has great potential for positive utilization. The present work aims to maximize the possible utilization of Egyptian boiler ash for vanadium extraction and to explain the mechanisms of phase transformation during the salt roasting process. The effect of roasting and leaching variables on vanadium extraction from boiler ash, such as roasting temperatures, NaCl dosages, roasting time, and solid:liquid ratios has been studied. The prepared Boiler ash-NaCl mixture has been roasted in the temperature range 700-1000 celcius, and then, the obtained roast has been leached using water leaching process. Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) have been used for describing the thermal changes resulted during the reactions between the raw ash samples with sodium chloride mixture. Additionally, microstructural and morphology changes of the salted roasting products and leaching residue have been characterized using X-ray diffractometry (XRD-XRF) and Scanning Electron Microscopy (SEM-EDX). The results indicated that the highest recovery of vanadium oxide of 95% has been achieved from the boiler ash roasted with the addition of 20 wt.% NaCl dosage at 850 degrees C for 2.5 h and leached at 1:10 solid:liquid ratio. Thermodynamic calculations show that the decomposition of VOSO4 is favored thermodynamically in a priority to the decomposition of NiSO4 in the boiler ash sample. Oxidation of V3+ to V4+ in vanadium spinel (Fe2VO4) is easier to occur than that of V4+ to V5+, and the formations of sodium vanadates (NaVO3) are more negative and stable than the formation of Na4V2O7 at temperature above 800 degrees C. During roasting process, the sodium chloride contained in the prepared roasting mixture reacts with the vanadium-bearing minerals and transfer them into water-soluble sodium vanadates. Contrariwise, the vanadium leaching efficiency has been decreased due to the formation of refractory compounds at high temperature (1000 degrees C) which prevent the formation of water-soluble compounds during the salt roasting process. [GRAPHICS]