Tracked changes of dolomite into Ca-Mg-Al layered double hydroxide

• Published in: Applied clay science 2018-06, Vol.159, p.25-36
• Main Authors: Mao, Ning, Zhou, Chun Hui, Keeling, John, Fiore, Saverio, Zhang, Hao, Chen, Liang, Jin, Gui Chen, Zhu, Ting Ting, Tong, Dong Shen, Yu, Wei Hua
• Format: Article
• Language: English
• Subjects: Ca-Mg-Al layered double hydroxide; Dolomite; Hydrolysis; Hydrothermal treatment; Mechanism
• ISSN: 0169-1317; 1872-9053
• DOI: 10.1016/j.clay.2017.06.011

Dolomite is a widespread carbonate mineral that has been investigated extensively over the past two centuries. Despite the high level of investigation, aspects of the environmental conditions of crystallization, diagenesis and dissolution remain elusive and the transformation of dolomite into other value-added products is still challenging. This work shows a novel, cleaner method to dissolve dolomite by taking advantage of the acidity from the hydrolysis of Lewis acid AlCl3 and as such the mineral can be efficiently converted into Ca-Mg-Al layered double hydroxide (LDH). The dolomite and the resulting Ca-Mg-Al LDH samples were characterized by powder X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), thermogravimetric analysis (TG), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composition of the products was measured by energy dispersive spectrometer-mapping (EDS-mapping), X-ray fluorescence (XRF) and atomic absorption spectroscopy (AAS). Particle sizes of the products were measured by a dynamic light scattering method. It is documented, for the first time, that dissolution of dolomite in AlCl3 aqueous solution is facilitated by hydrothermal treatment. Well-crystallized Ca-Mg-Al LDH crystals are achieved under (Ca+Mg): Al molar ratio=1:1, pH=10.5 via a simple co-precipitation method. The maximum utilization efficiencies of Ca2+, Mg2+ from dolomite to LDH are 36.3% and 95.5%, respectively. The possible mechanisms of LDH formation were: 1) the dolomite dissolved in the strong acid solution due to the hydrolysis of AlCl3 and thus provided Ca2+ and Mg2+; 2) with addition of NaOH, Al3+ cations precipitated to form Al(OH)3 (at 8.3≤pH≤9.3) and then part of the Al(OH)3 reacted with Ca2+ and Mg2+ to form the Ca-Mg-Al LDH; 3) at pH=10.5, the Al(OH)3 was transformed as [AlO(OH)] and meanwhile Ca2+, Mg2+ and Al3+ took part in the formation of Ca-Mg-Al LDH; 4) when pH=10.6, [AlO(OH)] reacted with NaOH to dissolve into [Al(OH)4]− and the [Al(OH)4]− finally co-precipitated with Ca2+ and Mg2+ to form the Ca-Mg-Al LDH during heating for drying. [Display omitted] •Synthesis of Ca-Mg-Al LDH using dolomite as Ca, Mg source and AlCl3.•Dissolution of dolomite in AlCl3 solution is facilitated by hydrothermal treatment.•The utilization efficiency of Ca2+ and Mg2+ from dolomite reach 36.2% and 95.3%, respectively.•LDH formation involve multiple Al3+ species and their interaction with Ca2+ and Mg2+.