The extraction and utilization of coal inevitably lead to the generation of significant solid waste, including coal gangue. This byproduct is rich in kaolin mineral resources, and their effective utilization presents a critical strategy for the management of such a bulk solid waste, offering substantial environmental and economic benefits. The processing methodologies are often hindered by the inherent mineralogical properties of the materials. Kaolinite minerals are central to the chemical reactivity in calcined coal kaolin clay, the content and characteristics of kaolinite can vary significantly in different coal kaolin sources due to the diversity in coal-forming geological conditions. It is thus necessary to clarify the mineralogical properties of kaolinite from various sources for the innovative application of coal gangue. In this paper, a high-iron coal-series kaolin clay was analyzed. In addition, the impact of mineralogical properties on the thermal activation and preparation of calcined kaolin clay was also investigated.

A high-iron coal-series kaolin clay was obtained as a representative sample from Sichuan, China. The chemical composition, phase composition, microstructure, and thermal behavior of the high-iron coal-series kaolinproperties were determined by X-ray fluorescence spectrometry (XRF), X-ray powder crystal diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetry-differential scanning calorimetry (TG-DSC).

The results reveal that the chemical composition of Yibin coal-series kaolin clay is predominantly composed of SiO₂, Al₂O₃ and Fe₂O₃, with small contents of CaO, MgO, SO₃ and TiO₂, accounting for a total oxide mass of 71.84%. This coal-series kaolin clay could be categorized as high-iron coal-series kaolin clay because the content of Fe₂O₃ is 8.54%. The coal-series kaolin clay calciated exhibits a loss mass of 28.14%, which is attributed to the volatile matter and fixed carbon (15% in the mass). The principal minerals of high-iron coal-series kaolin clay along with their respective mass fractions are low-crystallinity kaolinite (56%), quartz (12%), and iron-bearing minerals such as goethite (Gth, 3%), hematite (Hem, 3%), siderite (Sd, 3%), and pyrite (Py, 2%). Furthermore, a small amounts of calcite (Cal, 3%) and anatase (Ant, 1%) also appear. The micro-morphology of the kaolinite is thin flakes or tightly stacked layers of minerals with a diameter of less than 5 μm, having a distinct laminar dissolution, and edges encrusted with small particles of calcite, quartz, anatase and other minerals.

In the calcination process, kaolinite initiates the loss of hydroxyl groups at 413 ℃, and the crystalline layers progressively become disordered at 550 ℃. It completely transforms into amorphous metakaolin at 830 ℃, accompanied by a total weight loss of 26.24%. Hematite is ultimately formed through the oxidation of pyrite and siderite, releasing volatile components such as SO₂ and CO₂. Goethite also converts to hematite, leading to an increased hematite content in the calcined product, which remains more stable at <900 ℃. In the thermal activation experiment, the kaolinite in coal gangue kaolin clay undergoes a maximal transformation into amorphous metakaolin, entailing the removal of a substantial number of hydroxyl groups and the disruption of the alumina octahedral sheet structure. This results in a shift of aluminum coordination to five-fold (Al^V), creating amorphous Al₂O₃, SiO₂, and numerous broken bonds, significantly enhancing chemical reactivity. At 750 ℃, the dissolution rate of Al₂O₃ peaks is 68.47% by mass. These results imply that the structural evolution of kaolinite at elevated temperatures is closely related to the activation process of kaolinite in Yibin coal gangue kaolin clay.

The results showed that heat treatment could transform Yibin coal gangue kaolin clay into calcined kaolin clay as a resource. This synthetic pozzolanic material exhibited favorable hydration properties, rendering it suitable for the fabrication of high-performance building materials, such as thermally insulated walls, thereby contributing to the conservation of resources and energy. This material could also have a potential for the partial substitution of cement in cementitious systems like concrete and geopolymers. This study could provide a mineralogical foundation and offer a developmental guidance for the comprehensive utilization of high-iron coal-series kaolin clay.