**1. Introduction**

Alumina (Al2O3), one of the most important powder materials obtained from calcined alumina hydrates, has been applied in various fields such as in the development of heat-resistant, abrasion-resistant, and corrosion-resistant materials [1,2]. The microscopic morphology of alumina powder is the same as one of its precursors, alumina hydrate [3]. Therefore, it is also necessary to research the synthesis of alumina hydrate. As an important alumina hydrate, boehmite is not only a precursor of alumina, but it also has wide applications in the context of catalysis, surfactants, and adsorbents due to its high surface area mesoporous structure [4,5].

It is well known that alumina is mainly extracted from bauxite; however, the shortage of bauxite is a growing problem in China. In recent years, the extraction of alumina from high-alumina coal fly ash (HACFA), which is the solid waste contained in aluminum resources, has stimulated great research interest [6–8]. A new process for alumina extraction was developed using HACFA as follows: (a) aluminum ammonium sulfate solutions containing impurities are firstly acquired via leaching HACFA and by using ammonium hydrogen sulfate solutions; (b) relatively pure solid aluminum ammonium sulfate is obtained via impurity removal by cooling crystallization; and (c) the alumina hydrate is prepared via the reaction of solid aluminum ammonium sulfate with ammonia (NH3·H2O). For aluminum resource extraction by leaching HACFA and ammonium hydrogen sulfate solutions, the preparation of alumina hydrate by reactive crystallization is a crucial step for controlling the morphology of alumina [9,10].

**Citation:** Wang, J.; Li, L.; Wu, Y.; Wang, Y. The Influence of Hydrothermal Temperature on Alumina Hydrate and Ammonioalunite Synthesis by Reaction Crystallization. *Crystals* **2023**, *13*, 763. https://doi.org/ 10.3390/cryst13050763

Academic Editor: James L. Smialek

Received: 10 April 2023 Revised: 25 April 2023 Accepted: 29 April 2023 Published: 4 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Hydrothermal methods can generate high-temperature and high-pressure environments, which can help insoluble or poorly soluble powders to achieve dissolution and recrystallization. The phase transformation and achievement of various microscopic morphologies can also be obtained using hydrothermal methods. Peng et al. [11] discovered that the different phase structures of MnO2 could be obtained using hydrothermal methods. As it is an important chemical synthesis method, reaction crystallization has been used widely in industrial fields. Moreover, hydrothermal reaction crystallization is a method through which crystallization takes place in hydrothermal reactors, and it allows one to obtain supersaturation via a coordination reaction [12]. Huang et al. [13] found that hierarchically boehmite and urchin-like hollow alumina microspheres with interconnected needle-like building blocks can be synthesized using hydrothermal methods. Therefore, the application of hydrothermal reaction crystallization to solid NH4Al(SO4)2·12H2O and NH3·H2O as a new process for extracting alumina from HACFA could lead to new findings.

Regarding the crystalline structure of alumina hydrate, there are two major categories: Al(OH)3 (gibbsite and bayerite) and AlOOH (diaspore and boehmite) [14]. The preparation of alumina hydrate plays a decisive role in determining the quality and properties of Al2O3. The microscopic morphology of the alumina powder inherits the morphology of its precursor, aluminum hydroxide powder, due to the topological conversion between aluminum hydroxide and alumina. For the reaction crystallization between solid NH4Al(SO4)2·12H2O and NH3·H2O, Ji et al. [15] revealed that pseudoboehmite was obtained through the reaction crystallization of a solution of ammonium aluminum sulfate [NH4Al(SO4)2 (aq)] and ammonia water [NH3·H2O (aq)]. Compared with liquid–liquid reactions, solid–liquid reactions represent a more energy-efficient option. In the liquid–liquid reaction, the solid NH4Al(SO4)2·12H2O was first dissolved into a solution system and then reacted with ammonia to produce alumina hydrate.

In this paper, we focused on the reaction crystallization process between the solid NH4Al(SO4)2·12H2O and NH3·H2O. A thermodynamic analysis was used to assess the feasibility of the reaction product at 120–200 ◦C. The samples were prepared at 150 ◦C and 200 ◦C with different reaction times. The phases of the samples were prepared at 150 ◦C and 200 ◦C, and the processes that led to the transformation to alumina hydrate and ammonioalunite products were investigated. Additionally, the morphology of the samples obtained at different reaction times was studied, and the evolution process that led to the creation of sheet-like boehmite structures was also investigated.
