Phase diagram for hydrothermal alkali activation of kaolin and quartz: Optimal digestion for the synthesis of zeolites

• Published in: Materials chemistry and physics 2022-10, Vol.290, p.126570, Article 126570
• Main Authors: Tang, Liang-Jie, Xie, Xiu-Zhen, Huang, Ya-Xi, Pan, Yuanming, Mi, Jin-Xiao
• Format: Article
• Language: English
• Subjects: Alumina and silica; Analcime; Hydrothermal alkali activation; Kaolin and quartz; Phase diagram
• ISSN: 0254-0584; 1879-3312
• DOI: 10.1016/j.matchemphys.2022.126570

Low-temperature activation of quartz and kaolin, two of the most abundant and affordable raw materials as the sources of alumina and silica for the production of zeolites, is highly desirable but is hampered by several challenges such as incomplete activation and the formation of acid-resistant aluminosilicates. In this paper, the hydrothermal alkali activation processes of kaolin and quartz with different Si/Al ratios have been studied in detail, aiming to establish a phase diagram for the optimal digestion of silica and alumina. The effects of the Si/Al ratio, reaction time, reaction temperature, alkali concentration, and doses of hydrothermal alkali activation have been thoroughly investigated and optimized. Sodalite, hydroxycancrinite and analcime are found to be the dominant aluminosilicates as activation products. This is the first demonstration that the molar ratio of NaOH/(Si + Al) is the key factor in controlling the formation of aluminosilicates. The establishment of this phase diagram allows us to avoid the formation of troublesome analcime, and it can be used for effectively converting most natural aluminosilicate materials into sol-gel as platform chemicals for the synthesis of zeolites and other aluminosilicate derivatives. New activation phase diagram of silica and alumina guide for avoiding the formation of analcime and more effective activation of natural aluminosilicate materials. [Display omitted] •The hydrothermal alkali activation of kaolin and quartz has been investigated.•Various activation conditions (temperature, time, NaOH) have been optimized.•The NaOH/(Si + Al) molar ratio has been shown as a key factor for the first time.•A new activation phase diagram of aluminosilicates has been established.•The phase diagram is useful for avoiding the formation of troublesome analcime.