高铁型煤系高岭土矿物学属性及其煅烧相变与活化效能

Mineralogical Properties of Iron-Rich Coal-Measuries Kaolin Clay and Its Phrase Transform and Reactivity Activation in Calcination

煤矸石含有丰富的高岭土矿物资源,对其进行资源化利用是处置这类大宗固体废弃物的重要途径,但其加工方式常受到矿物学属性的制约。以产自四川宜宾的高铁型煤系高岭土为例,采用X射线荧光光谱法、X射线粉晶衍射、扫描电子显微镜和热重-差示扫描量热法等分析测试手段研究了该煤系高岭土的化学成分、物相组成、微观形貌和热效应特征,探讨了矿物属性对热活化制备偏高岭土的影响。结果表明,宜宾煤系高岭土的主要组成矿物及其质量分数为低结晶度的高岭石(56%)、固定碳(15%)、石英(12%)和含铁矿物(11%)等;在煅烧过程中,高岭石从413℃开始脱除羟基,晶层在550℃逐渐变得无序,受其他组分影响,830℃时完全转变为非晶态的偏高岭石,质量损失合计26.24%;同时,黄铁矿和菱铁矿发生氧化,煅烧产物中出现含量较高的赤铁矿。此煤系高岭土经煅烧后活性显著增强,Al2O3溶出率在750℃时达到最高(68.47%,质量分数)。本工作可为高铁型煤系高岭土资源的综合利用提供矿物学依据和开发指导。

Introduction 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.Methods 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 determinedby X-ray fluorescence spectrometry (XRF), X-ray powder crystal diffraction (XRD), scanning electron microscopy (SEM), andthermogravimetry-differential scanning calorimetry (TG-DSC).Results and discussion The results reveal that the chemical composition of Yibin coal-series kaolin clay is predominantlycomposed of SiO_(2), Al2O3 and Fe_(2)O_(3), with small contents of CaO, MgO, SO3 and TiO_(2), 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 Fe_(2)O_(3) is 8.54%. Thecoal-series kaolin clay calciated exhibits a loss mass of 28.14%, which is attributed to the volatile matter and fixed carbon (15% in themass). The principal minerals of high-iron coal-series kaolin clay along with their respective mass fractions are low-crystallinitykaolinite (56%), quartz (12%), and iron-bearing minerals such as goethite (Gth, 3%), hematite (Hem, 3%), siderite (Sd, 3%), andpyrite (Py, 2%). Furthermore, a small amounts of calcite (Cal, 3%) and anatase (Ant, 1%) also appear. The micro-morphology of thekaolinite 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 progressivelybecome disordered at 550 ℃. It completely transforms into amorphous metakaolin at 830 ℃, accompanied by a total weight loss of26.24%. Hematite is ultimately formed through the oxidation of pyrite and siderite, releasing volatile components such as SO2 andCO2. Goethite also converts to hematite, leading to an increased hematite content in the calcined product, which remains more stableat <900 ℃. In the thermal activation experiment, the kaolinite in coal gangue kaolin clay undergoes a maximal transformation intoamorphous metakaolin, entailing the removal of a substantial number of hydroxyl groups and the disruption of the alumina octahedralsheet structure. This results in a shift of aluminum coordination to five-fold (AlV), creating amorphous Al2O3, SiO_(2), and numerousbroken bonds, significantly enhancing chemical reactivity. At 750 ℃, the dissolution rate of Al2O3 peaks is 68.47% by mass. Theseresults imply that the structural evolution of kaolinite at elevated temperatures is closely related to the activation process of kaolinitein Yibin coal gangue kaolin clay.Conclusions The results showed that heat treatment could transform Yibin coal gangue kaolin clay into calcined kaolin clay as aresource. This synthetic pozzolanic material exhibited favorable hydration properties, rendering it suitable for the fabrication ofhigh-performance building materials, such as thermally insulated walls, thereby contributing to the conservation of resources andenergy. This material could also have a potential for the partial substitution of cement in cementitious systems like concrete andgeopolymers. This study could provide a mineralogical foundation and offer a developmental guidance for the comprehensiveutilization of high-iron coal-series kaolin clay.