Kinetics and phase transformation of low-calcium coal fly ash (CFA) under hydrofluoric acid leaching

Abstract

ABSTRACT The content and depolymerization degree of the glass phase in coal fly ash (CFA) is positively correlated with the activity of CFA, which is one of the key factors in the comprehensive utilization of this ash. At present, the most common and accurate method for quantitative analysis of fly ash is selective dissolution. However, more experimental data are needed to specify the effect of HF concentration on reaction kinetics, the dissolution rate of amorphous phases, and the formation of new amorphous and crystalline phases. In this study, the effects of reaction temperature, reaction time, and hydrofluoric acid concentration on the leaching rate of CFA were investigated with a single-factor condition experiment and orthogonal experiment. Phase composition and transformation of CFA under different conditions were observed with XRD and SEM. The results show that with increasing reaction time (5–120 min), temperature (20–80°C), and HF concentration (5% to 30%), the leaching rate of CFA also increased from 29.4% to 90.2%. The phase transformation of CFA in acid solution is as follows: in 5% hydrofluoric acid concentration solution, the original glass phase disappears totally and a new glass phase is formed; in 15% hydrofluoric acid concentration solution, quartz disappears and a new phase named ralstonite [Na0.5Mg0.5Al1.5F4(OH)2 · (H2O)] is formed; and when the hydrofluoric acid concentration reaches 30%, mullite disappears and a new phase named potassium aluminum fluoride (KAlF4) is formed. Also, when the temperature rises to 80°C, the f-CaO in CFA participates in the reaction to form creedite (Ca3Al2SO4F7.5(OH)2.5 · 2H2O) and fluorite (CaF2). Reaction kinetics demonstrate that the leaching reaction to the glass body of CFA is a diffusion-controlled process, and the apparent activation energy of the Si and Al leaching reaction in glass phase is 11.62 kJ/mol and 26.039 kJ/mol, respectively.