Research status and prospects on overall mineral use of mixed-dimensional attapulgite clay

In recent years, the high-efficiency and high-value development and use of mixed-dimensional clay minerals with large natural reserves have attracted great attention with the over-exploitation and rapid consumption of high-quality clay mineral resources. The reserves of mixed-dimensional attapulgite clay ore in China exceed 1 billion tons. Their main mineral component is attapulgite, but they also contain illite, chlorite, kaolinite, illite-smectite mixed-layer minerals, and other minerals. At the same time, the phenomenon of isomorphic substitution is common, and part of the octahedral sheet may be replaced by Al³⁺, Fe³⁺, and other metal ions; therefore, the mixed-dimensional attapulgite clays are mostly brick-red, gray, or khaki. The relative complexity of associated minerals and darker color restrict the large-scale development of the industry. Scientific and technological innovation to realize the “best use” of mixed-dimensional clay mineral resources to develop high-value-added products will be the most important development trend in the future. However, basic theoretical research on the high-value application of mixed-dimensional clay minerals is lacking, so new ideas, approaches, and methods are urgently needed to solve the difficulties and sore points of industrial development. On the basis of a deep understanding of the microstructure of clay minerals, progress has been made in using the total mineral resources of mixed-dimensional attapulgite clay, which has preliminarily formed a comprehensive utilization system of “simultaneous transformation of associated minerals—acid leaching for whitening—functional material construction—recovery of acid leaching metal ions—clay mineral biochar—create red pigments based on the red appearance—quartz sand utilization”. Specifically, using mixed-dimensional attapulgite clay to construct various porous silicate materials by structural reorganization; developing a series of methods, including wet, semi-dry, and microwave techniques, to achieve efficient whitening-variegated mixed-dimensional attapulgite clays without damaging the aspect ratio of attapulgite nanorods and the properties of other associated minerals; using the whitened mixed-dimensional attapulgite clays containing one-dimensional fibrous attapulgite and two-dimensional layered clay minerals to construct nanocomposites, such as films and plastics, showing the advantages of mixed-dimensional attapulgite clays in improving mechanical properties and realizing the high value of associated minerals; recovering iron, aluminum, magnesium, and other metal ions from the wastewater of acid leaching attapulgite clay to synthesize layered double hydroxide materials to adsorb dyes, antibiotics, and other organic matter, then producing mineral biochar materials by anaerobic calcination for heavy metal-contaminated soil remediation; using the iron-rich properties of mixed-dimensional attapulgite clays to construct nanohybrid iron-red mineral pigments, which significantly improve the weather resistance of iron-red pigments; and employing quartz sand obtained by dry separation from clay minerals to construct cobalt-blue hybrid pigments via mechanochemical method. The long-term practice has shown that whitening is the premise for the high-value use of mixed-dimensional attapulgite clays, and overall mineral use is the key to industrial development. Therefore, an effective way to simultaneously transform associated clay minerals into attapulgite or other single minerals should be further explored. The structural regulation and transformation mechanism of mixed-dimensional attapulgite clays should be clarified to open a new approach for the functional application of mixed-dimensional attapulgite clays. In addition, on the basis of the acid leaching for whitening, the differences and complementarities of one-dimensional rod crystals and two-dimensional layered structures in constructing functional composite materials must be systematically studied, and the foundation for mixed-dimensional nanorod crystal/layered structures in functional materials must be laid, finally providing a feasible and reliable approach for the comprehensive use of mixed-dimensional attapulgite clay resources.