*Article* **The Influence of Hydrothermal Temperature on Alumina Hydrate and Ammonioalunite Synthesis by Reaction Crystallization**

**Junkai Wang, Laishi Li \*, Yusheng Wu \* and Yuzheng Wang**

School of Material Science and Engineering, Shenyang University of Technology, Shenyang 110870, China **\*** Correspondence: lilaishi@sut.edu.cn (L.L.); wuyus@sut.edu.cn (Y.W.)

**Abstract:** With the rapid development of the alumina industry and the shortage of bauxite, highalumina coal fly ash (HACFA) has attracted more and more attention as a potential alternative alumina resource. In order to extract alumina from HACFA with newly developed technology, the investigation of the crucial step, the reaction between NH4Al(SO4)2·12H2O and NH3·H2O, is necessary and valuable. Thermodynamic analyses have shown that four kinds of alumina hydrate (boehmite, diaspore, gibbsite, and bayerite) might be formed at 120–200 ◦C, and ammonioalunite might be formed at temperatures over 180 ◦C. A hydrothermal reaction crystallization method was applied to this reaction. The experimental results showed that boehmite (AlOOH) could be formed at 150 ◦C and 200 ◦C after 12 h and NH4Al3(SO4)2(OH)6, an unstable intermediate, is formed during the initial stage and transformed into boehmite, eventually. The higher temperature (200 ◦C) was more energetically favorable for the formation of NH4Al3(SO4)2(OH)6, and the crystallinity of the products was better. More importantly, the sheet-like structure of boehmite (AlOOH) could be formed at 150 ◦C after 24 h of reaction time. The SEM results proved that the sheet-like structures evolutionary process of boehmite.

**Keywords:** alumina hydrate; boehmite; ammonioalunite; evolutionary process; reaction crystallization
