Abstract:

Roasting process with sulfuric acid plays an important role in extracting Al₂O₃ from coal fly ash. The results indicate that Al conversion rate increases with appropriately increased acid concentration, the mass ratio of acid to ash and the roasted temperature, while the effect of roasted time is relatively small. The Al conversion rate can reach 92%-93% at recommended processing parameters: sulfuric acid concentration 80%, acid-ash mass ratio 1.5:1, roasted temperature 270℃ and roasted time 60 min. An analysis for TG/DSC and XRD data demonstrates that the roasting process can be divided to 3 steps: the first step is Al(HSO₄)₃ formation and water evaporation; the second the formation of Al(HSO₄)₃ and Al₂(SO₄)₃·H₂O, and the third Al₂(SO₄)₃ formation from decompositions of Al₂(SO₄)₃·H₂O and Al(HSO₄)₃. The values of apparent activation energy for the three steps are 52.61, 74.11, and 96.08 kJ·mol^-1 respectively, an activation energy average of values obtained by Kissinger and Ozawa methods. Using the kinetics parameters estimated, such as activation energy, frequency factor, reaction order, three kinetic equations which correspond to three thermo-chemical reaction steps in the roasting process are established. These fundamental data and theoretical guideline provided in this paper could be used for the process design and optimization of sulfuric acid roasting process to extract alumina from coal fly ash.

Key words: sulfating roasting, coal fly ash, alumina, reaction kinetics, chemical process

摘要：

粉煤灰浓硫酸焙烧法提取Al₂O₃工艺中焙烧过程十分重要,研究发现适当提升硫酸浓度、酸灰质量比和温度可增大Al转化率,而延长焙烧时间影响并不显著;在硫酸浓度80%、酸灰质量比1.5:1、温度270℃和时间60 min条件下,Al转化率可达92 %～95%。TG/DSC与XRD分析表明,焙烧过程反应主要分为3个阶段:80～206℃时形成中间产物Al(HSO₄)₃及H₂O气化;206～241℃时生成Al(HSO₄)₃及Al₂(SO₄)₃·H₂O;241～304℃时由Al₂(SO₄)₃·H₂O和Al(HSO₄)₃二者分解转化成Al₂(SO₄)₃。由Kissinger微分法与Ozawa积分法分别计算出3个阶段表观活化能后取二者平均值,分别为52.61、74.11、96.08 kJ·mol^-1,并获得频率因子、反应级数和动力学方程,所获结果可为粉煤灰硫酸焙烧法提取Al₂O₃焙烧工艺设计和过程优化提供理论参考和基础数据。

关键词: 硫酸化焙烧, 粉煤灰, 氧化铝, 反应动力学, 化学过程