Coal-series Kaolin Research Articles

Efficient extraction of rare earth elements from coal-series kaolin by combining alkali fusion and sulfamic acid leaching

As a solid waste produced by coal mining, the comprehensive utilization of coal-series kaolin is the focus of attention. In this paper, the extraction of rare earth elements (REEs) from coal-series kaolin was carried out by alkali fusion and sulfamic acid leaching. The occurrence modes of REEs in coal-series kaolin was analyzed by means of inductively coupled plasma-mass spectrometry analyse and sequential chemical extraction. The influence of alkali fusion conditions on the leaching of REEs was investigated, and the activation mechanism of alkali fusion was revealed by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and field emission scanning electron microscope (FESEM) analysis. In addition, the effect of key operating parameters on the leaching efficiency of REEs from coal-series kaolin was further investigated. The results show that REEs mainly existed in silicate & aluminosilicate state of coal-series kaolin. The mineral structure was destroyed by alkali fusion, and the leaching efficiency of REEs was increased by 59.08 %. At the sulfamic acid concentration of 0.5 mol/L, H2O2 concentration of 0.2 mol/L, solid to liquid ratio of 1/50, the leaching efficiencies of total rare earth elements, light rare earth elements and heavy rare earth elements were 84.91 %, 88.43 % and 71.03 %, respectively.

A feasible strategy for triboelectric separation of coal-series kaolin using a cascade electrostatic field

The coal-series kaolin in China possesses abundant resources, and its efficient utilization can be promoted by separation. In this paper, a cascade electrostatic field was designed for the triboelectric separation of coal-series kaolin. The motion behavior of charged particles with different size fraction in cascade electrostatic field was investigated by high-speed dynamic camera system, and the distribution characteristic of charge-to-mass ratio of products was analyzed by the coulometer. The influence of key operating parameters (e.g., feed speed, rotation speed and plate voltage) on product distribution were studied by electric separation test. The results indicated that the charge-to-mass ratio of the product after separation has the characteristics of cascade distribution. In addition, compared with 0.05–0.25 mm and 0.25–0.125 mm, the dispersion performance of 0.125–0.074 mm coal-series kaolin in the cascade electrostatic field was poor. For 0.5–0.25 mm, 0.125–0.25 mm and 0.074–0.125 mm coal-series kaolin, the decarburization efficiency after separation in cascade electric field was 47.09 %, 53.48 % and 44.63 %, respectively. Our study indicates that the setting of cascade electrostatic field achieved efficient purification and enrichment of coal-series kaolin.

Study of the Preparation and Performance of TiO2-Based Magnetic Regenerative Adsorbent.

Against the backdrop of "carbon neutrality", the green treatment of dye wastewater is particularly important. Currently, the adsorption method shows strong application prospects. Therefore, selecting efficient and recyclable adsorbents is of significant importance. TiO2 is an excellent adsorbent, but its difficult recovery often leads to secondary pollution. γ-Fe2O3-modified coal-series kaolin exhibits both excellent adsorption properties and rapid separation through magnetic separation technology. By utilizing the synergistic effects of both, TiO2/-γFe2O3 coal-series kaolin, magnetic adsorbent regeneration materials were prepared using coprecipitation method and characterized. The influencing factors of this functional material on the adsorption of Congo red dye and its regeneration performance are discussed. The experimental results indicated that the specific surface area, pore volume and Ms value of this functional material are 127.5 m2/g, 0.38 cm3/g, and 13.4 emu/g, respectively. It exhibits excellent adsorption characteristics towards Congo red, with an adsorption rate reaching 96.8% within 10 min, conforming to the pseudo-second-order kinetic model, and demonstrating Langmuir IV-type monolayer adsorption. After the adsorption of Congo red, magnetic separation shows superior efficiency. Furthermore, treatment of the adsorbed composite with EDTA allows for recycling, with adsorption rates still above 91% after three cycles, indicating an excellent regeneration capability.

Enhanced mortar strength and resistance to seawater degradation in the presence of calcined coal-series kaolin

The hydration process of blended mortars and its resistance to seawater degradation in the presence of calcined coal-series kaolin (CCK) was investigated in this study. The TG tests were used to quantify the effect of calcination temperature on the chemical-bound water content of the blended pastes while the XRD tests were applied to analyze the main crystalline hydration phases of the blended pastes, which were further quantified using the Rietveld-XRD refinement. In addition, microcalorimetric tests were used to characterize the degree of hydration. The hydration process of the blended mortars was further investigated in detail via combining SEM and pozzolanic activity tests. Based on this, the resistance to seawater degradation of the blended mortars with added CCK were verified via the expansion and strength loss tests, and the mechanism was analyzed via the electric flux and the total Cl− content tests. The optimal percentage of CCK (12%) was determined via a linear weighted sum method. The CCK obtained at 800 °C had the highest pozzolanic activity, promoting hydration reactions to form new gels (C-S-H, C-A-H and C-A-S-H) at active nucleation sites in mortars. These further enhanced both the flexural and compressive strengths (the compressive strength reached ∼53.7 MPa) of the blended mortars. However, CCK reduced the generation of Mg(OH)2, AFt and Friedel’s salt, decreasing seawater degradation to the mortars. Therefore, this study provides a new strategy to prepare high-strength mortar with high resistance to seawater degradation.

Effective decarbonization of coal-series kaolin by triboelectric separation based on different motion behaviors of charged particles in electrostatic field

In this work, a rotary triboelectric separation system was set up to study the motion characteristics of charged particles (i.e., kaolinite and coal) using Comsol software simulation and high-speed camera tracking. The results showed that the negatively charged kaolinite particles moved toward the positive electrode, while the positively charged coal particles moved toward the negative electrode. Meanwhile, in horizontal displacement increased with the increase of the charge-to-mass ratio of kaolinite and coal, which favored the decarbonization of coal-series kaolin. Therefore, the effective decarbonization of series-coal kaolin with different particle sizes was achieved via rotary triboelectric separation, in which the decarbonization efficiencies for 0.25–0.50 mm, 0.125–0.25 mm, and 0.074–0.125 mm coal-series kaolin were 41.86%, 45.38%, and 42.66%, respectively. This indicates that the efficient purification and upgrading of coal-series kaolin can be realized via rotary triboelectric separation.

Effect of Si/Al ratio on in-situ synthesis of Al2O3–β-Sialon composite ceramics for solar thermal storage by aluminothermic and silicothermic nitridation

In-situ Al2O3–β-Sialon composite thermal storage ceramics were one-step synthesized by aluminothermic and silicothermic reduction nitridation method, using solid waste coal-series kaolin, Al powder and Si powder as raw materials and firing at 1600 °C in N2 atmosphere. The effects of Si/Al ratio and firing temperature on the phase composition, structure and properties of fired samples were researched by XRD, SEM, DSC-TG, etc. The results show that increasing Si/Al ratio and firing temperature can improve the properties by increasing the content of β-Sialon. However, with the increasing Si/Al ratio, the deviations between actual and designed contents of β-Sialon increase due to Si volatilization, and the properties cannot be further improved when Si content is larger than 16.3 wt%. The fired samples with Si metal and Al metal mass ratio = 64.9% obtained optimal properties: water absorption 8.4%, bulk density 2.58 g·cm−3, room temperature bending strength 82.7 MPa, high-temperature bending strength 81.1 MPa, thermal expansion coefficient (room temperature-1000 °C) 5.06 × 10−6·°C−1, thermal conductivity (at room temperature) 7.77 W·(m·K)−1, specific heat capacity (at room temperature) 0.7 J·(g·K)−1, which mean that it is a good candidate for the thermal storage material.

Sintering inhibition and oil-absorption properties of calcined coal-series kaolin

AbstractIn this study, aluminium fluoride trihydrate (AlF3⋅3H2O) was used to inhibit the sintering of calcined coal-series kaolin (CCSK). In addition, the oil absorption properties of CCSK were studied. The particle-size distribution, specific surface area and porosity of the samples were investigated as a function of calcination temperature and the addition of AlF3⋅3H2O. Moreover, the ability of AlF3⋅3H2O to improve the oil absorption of CCSK was explored. The morphology, structure and phase composition of the specimens were investigated using scanning electron microscopy and X-ray diffraction. The phase transition during heating of the samples was studied using thermogravimetric analysis. The preparation with 10 wt.% AlF3⋅3H2O had the optimal sintering inhibition effect on CCSK at 1000°C. The release of SiF4 gas during heating and the formation of mullite whiskers on the particle surface caused by AlF3⋅3H2O moderated the formation of molten-phase liquid bridges between particles and inhibited sintering of the raw material powder. Furthermore, the formation of submicron mullite whiskers on the surface of the CCSK particles enhanced the oil-absorption properties of the sample significantly.

Surface modification to enhance the decarbonization performance of coal-series kaolin by triboelectric separation

As a vital non-metallic mineral resource, cleaning and purification of coal-series kaolin is of importance for the following processing and utilization. This research focused on surface modification to enhance the decarbonization performance of coal-series kaolin by triboelectric separation. The effect of citric acid or kerosene on the surface modification and charging performance of kaolinite and coal was explored through Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and charge-to-mass ratio measurements. The results indicated that owing to the adsorption of citric acid or kerosene, the decreased O content and the increased C content of coal enhanced the positive charge on the coal surface, while the decreased Al and Si content of kaolinite improved the negative charge on the kaolinite surface, which enlarged the charge-to-mass ratio gap between coal and kaolinite. Meanwhile, citric acid improved the charging performance of kaolinite and coal more significantly than kerosene. Furthermore, the influence of plate voltage, feeding speed, and friction wheel rotating velocity on the decarbonization performance of coal-series kaolin after surface modification was studied by triboelectric separation. The separation results indicated that surface modification with citric acid or kerosene improved the triboelectric separation efficiency of coal-series kaolin, and citric acid could realize the more effective separation of coal-series kaolin than kerosene. For 0.125–0.25 mm, 0.074–0.125 mm, and −0.074 mm coal-series kaolin, with citric acid modification, the valuable component recovery was 81.91%, 82.21%, and 82.83%, respectively, and the decarbonization efficiency was 44.66% (increased by 6.56%), 47.19% (increased by 7.00%), and 14.88% (increased by 2.53%), respectively. Our study provides vital guidance for enrichment and purification of coal-series kaolin resource via triboelectric separation.

In-situ synthesis of nitride whiskers-bonded SiAlON–Al2O3 ceramics for solar thermal storage by aluminothermic nitridation of coal-series kaolin

Nitride whiskers-bonded SiAlON–Al2O3 ceramics used for solar thermal storage were in-situ synthesized by aluminothermic nitridation of coal-series kaolin, an abundant solid waste in China. Mechanisms on the aluminothermic nitridation and in-situ synthesis of Si3N4 and SiAlON whiskers were studied by TG-DSC, XRD, SEM, TEM, etc. The effect of sintering temperature on the phase evolution, microstructure and properties of the ceramics was investigated, and the thermophysical properties were compared to those of the available solar thermal storage materials. Results indicated that Si3N4 and SiAlON whiskers could be in-situ synthesized simultaneously at 1300 °C via Vapor–Solid mechanism. Increasing sintering temperature decreased the amount of Si3N4 whiskers, but increased that of SiAlON whiskers, which transformed to be columnar SiAlON grains at 1550–1600 °C. By increasing the sintering temperature, the enhancing densification and the formation of the columnar SiAlON grains were in favor of improving the strength of the ceramics, and the increasing SiAlON content decreased the thermal expansion coefficient of the ceramics to 5.73 × 10−6 °C−1. Compared with other thermal storage materials, the advantages of the SiAlON–Al2O3 ceramics are their relatively higher thermal conductivity (6.34 W/m/K, at room temperature), higher specific energy density (2941 kJ/m3/K, at 900 °C), and better high-temperature thermal stability, making them competent for high-temperature thermal storage uses.

Microstructure and performances of corundum−mullite composite ceramics for heat transmission pipelines: Effects of Ho2O3 additive content

Ho 2 O 3 was employed to improve the microstructural densification and performances of pressureless sintered corundum–mullite ceramic composites. This study investigated the influences of Ho 2 O 3 addition on the microstructure, physical properties and thermal shock resistances of the composites. The results indicated that sample AH5 (80 wt% Al 2 O 3 , 20 wt% coal series kaolin, and 5 wt% additional Ho 2 O 3 ), which was sintered at 1550 °C, showed the best comprehensive properties. In this Al 2 O 3 -rich and SiO 2 -poor system, a reaction between the Ho 2 O 3 and Al 2 O 3 –SiO 2 system produced an Ho 2 O 3 –Al 2 O 3 –SiO 2 liquid phase. This liquid phase increased the microstructural densification and resulted in a lower sintering temperature. The generation of mullite and holmium disilicate during thermal shocks improved the thermal shock resistance. The high bending strength and satisfactory thermal shock resistance of the as-prepared corundum–mullite ceramic composites showed their potential for use in heat transmission pipelines.

Pozzolanic activity of mechanochemically and thermally activated coal-series kaolin in cement-based materials

The shortage of good quality supplementary cementitious materials (SCMs) for producing cement concrete eagerly demands an alternative pozzolan material. Thanks to the rich silica-alumina contents and large reserves in China, coal-series kaolin (CK) (a by-product of coal mining) is promising to be used as an effective SCMs in cement-based materials. This study investigated the effectiveness of using thermally activated CK coupled with mechanical grinding in ordinary Portland cement paste and mortar. The mineralogy, physicochemical structures, thermal decomposition, pozzolanic reactivity of this novel treated CK were evaluated and compared with two water-washed high-purity kaolinitic clays. The results showed that the large change of Al coordination environment in the thermally activated CK at a relatively low temperature (650 °C) rendered a high pozzolanic reactivity. The high content of kaolinite and reduced particle size by mechanical treatment were beneficial to enhancing the reactivity of thermally activated CK. With the use of thermoactivated CK, the early hydration of cement paste was significantly accelerated. The high degree of dihydroxylation, large specific surface area and high pozzolanic reaction of thermoactivated CK contributed to refining the pore structure and improving the compressive strength of blended mortar. The results of this study provided an implication of using treated CK as an alternative SCMs.

Analysis of flow-field and particle-motion characteristics in the dry separation process for coal-series kaolin

ABSTRACT Feng Xuan (FX) dry separators can effectively separate coal-series kaolinite. Coal-series kaolinite from Inner Mongolia (China) was selected as the sample. Computational fluid dynamics (CFD) and the discrete element method (DEM) were used to study velocity field, pressure field, turbulent kinetic energy field, and particle-motion characteristics of the FX dry separator. The results showed that the air velocity and pressure of the four air chambers symmetrically distributed from the centre to the surrounding area, and their values gradually decreased. The heavy products were located at the lower part of the material layer and moved horizontally along the sieve plate, the light products were located on the upper part of the material layer and moved obliquely along the sieve plate, and the intermediate products aggregated along horizontal and inclined directions, and their compaction effect on heavy products was obvious at the end of the sieve plate. This study provided a new way to optimise the structural parameters of FX dry separators.

Microstructure and performances of Gd2O3-added corundum–mullite ceramic composites for concentrated solar power applications

Gd2O3 was added to increase the properties of corundum–mullite ceramic composites by enhancing the microstructural densification using coal-series kaolin and α-Al2O3. The effect of Gd2O3 addition on the microstructure and properties of ceramic composites was explored. The results showed that the formation of Gd2O3–Al2O3–SiO2 liquid phase could improve the bending strength, hardness, and acid resistance, since such liquid phase promoted microstructural densification via a reaction between Gd2O3 and Al2O3–SiO2 binary system in this Al2O3-rich and SiO2-poor system. The precipitation of mullite and gadolinium disilicate during thermal shocks resulted in enhanced bending strength. In particular, sample AG5 (80 wt% Al2O3, 20 wt% coal-series kaolin, and 5 wt% additional Gd2O3) sintered at 1550 °C was considered as candidate for heat transmission pipeline for the optimum comprehensive performance.

Preparation and thermal stability investigation of Al2O3–mullite–ZrO2–SiC composite ceramics for solar thermal transmission pipelines

To obtain composite ceramics with excellent thermal shock resistance and satisfactory high−temperature service performance for solar thermal transmission pipelines, SiC additive was incorporated into Al2O3−mullite−ZrO2 composite ceramics through a pressureless sintering process. The effect of the SiC additive on thermal shock resistance was studied. Also, the variations in the microstructure and physical properties during thermal cycles at 1300 °C were discussed. The results showed that both thermal shock resistance and thermal cycling performance could be improved by adding 20 wt% SiC. In particular, the sample with 50 wt% Al2O3, 35 wt% Coal Series Kaolin (CSK), 15 wt% partially yttria−stabilized zirconia (PSZ), and 20 wt% SiC additional (denoted as sample A2) exhibited the best overall performance after firing at 1600 °C. Furthermore, the bending strength of sample A2 increased to 124.58 MPa, with an increasing rate of 13.63% after 30 thermal shock cycles. The increase in thermal conductivity and the formation of mullite were the factors behind the enhancement of thermal shock resistance. During the thermal cycles, the oxidation of SiC particles was favorable as it increased the microstructure densification and also facilitated the generation of mullite, which endowed the composite ceramics with a self−reinforcing performance.

Leaching kinetics of iron from coal-series kaolin by oxalic acid solutions

Leaching kinetics of iron from coal-series kaolin by oxalic acid solutions

Preparation and properties of resin coated ceramic proppants with ultra light weight and high strength from coal-series kaolin

In this study, low-cost coal-series kaolin and ZnO powders were employed to prepare petroleum fracturing proppants with low density and high strength. Furthermore, the method of resin coating was used to optimize the performance of ceramic proppants. The bulk density, apparent density and breakage ratio of specimens were systematically investigated as the function of sintering temperature and the content of ZnO. Moreover, the reason that epoxy resin coating could increase the strength and reduce the density has been discussed by means of 3D modeling. The morphology structure and phase composition of specimens were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The phase transition of kaolin was studied using thermal techniques (TG/DSC). The experimental results revealed that the proppants sintered at 1350 °C with the addition of 2 wt% ZnO had the best performance. The apparent density and breakage ratio under 35 MPa was 2.662 g/cm3 and 2.44%. The addition of ZnO could promote the densification behavior of the sample. The coated proppants with 12 wt% epoxy resin displayed excellent performance with 2.270 g/cm3 of apparent density and only 1.16% of breakage ratio under 69 MPa, which had great advantages in the application of deep well oil and gas development.

Enhanced removal performance for Congo red by coal-series kaolin with acid treatment

ABSTRACT In this study, acid treatment coal-series kaolin (AK) materials (named 2AK, 3AK, 5AK, 10AK) were synthesized by a facile method. Batch adsorption experiments were carried out using Congo red (CR) as a model dye pollutant in various experimental conditions. The samples were studied by XRF, XRD, FTIR, SEM, Zeta potential and N2 adsorption techniques. The results indicate that the acid treatment materials exhibit increased specific surface area and pore structures, and showed obvious effects on the adsorption performance of kaolin. Owing to the structural edges of kaolin receiving protons from the hydrochloric acid aqueous solution, the prepared 5AK exhibited excellent adsorption performance for Congo red (CR) anionic dye. The adsorption process followed a pseudo-second-order rate model and the Langmuir isotherm was found a better fit with a maximum adsorption capacity of 237.53 mg/g, which is very close to the experimental data. Thermodynamic studies showed Congo red adsorption on samples was exothermic and spontaneous in nature. The acid treatment coal-series kaolin shows significant potential applications in the fields of adsorption, pH-responsive delivery and other environmental remediation.

Comparing pozzolanic activity from thermal-activated water-washed and coal-series kaolin in Portland cement mortar

In this paper, we investigated the pozzolanic activity of thermal-activated coal-series kaolin and two types of water-washed kaolin, which were calcined at temperatures ranging from 600 to 700 °C for 3 h. Chapelle test and strength activity index were used to evaluate the pozzolanic activity of thermal-treated kaolin. Results indicated that thermal-activated water-washed kaolin presented a higher pozzolanic activity when calcined at same temperature, which is due to its well-defined morphology, relatively high specific surface area, and high amounts of active aluminum. The incorporation of 10 wt% of calcined kaolin significantly increased the amount of hydrate water and promoted strength development at 7 and 28 days. Moreover, the crystallinity of kaolinite showed obvious impacts on the dehydroxylation temperature and aluminum structure. The optimum dehydroxylation temperature is mainly dependent on the crystallinity of the kaolinite; in addition, the aluminium structure is closely related to the kaolinite crystallinity. Well-crystallized kaolin showed higher pozzolanic activity than poorly crystallized kaolin at the same calcination condition.

Optimization for the COD reduction and thermodynamics research of coal-series kaolin

Coal-series kaolin is the product of seam deposition in coal-accumulating basin, and may always contain reducing substances. The reducing substances such as carbon matter and pyrite could cause relatively high chemical oxygen demand (COD) of the raw kaolin, resulting in low quality kaolin products. In this study, narrow range classification and calcination were investigated to evaluate decreasing COD on coal-series kaolin from the Hubei Province. The kaolin and roasting products were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and thermogravimetry–differential scanning calorimetry (TG-DSC) analyses. The results showed that the COD value in kaolin ore (19,252 µg/g) was largely concentrated in the size fraction of – 150 to + 60 μm. The optimum conditions for reducing COD were identified to be – 74 to + 60 μm at a roasting temperature of 450 °C, and the COD value decreased from 27517 to 585 µg g−1. Moreover, the COD value and Ea showed linear correlation (R2 = 0.932) among different particle sizes, and G(α) = $$ \left[ {1 - \left( {1 - \alpha } \right)^{{\frac{1}{2}}} } \right]^{{\frac{1}{2}}} $$ could be regarded as the most possible mechanism function for the thermal decomposition of coal-series kaolin.

Influence of Replacement Level of Coal-series Kaolin on Hydration of Ordinary Portland Cement by X-ray Diffraction/Rietveld Method

The influence of replacement level of calcined coal-series kaolin (CCK) on hydration of ordinary Portland cement (OPC) was studied by X-ray diffraction(XRD)/Rietveld method. X-ray diffraction/Rietveld method was used to quantify the crystalline phase composition of the hydrated samples. Additionally, the morphology of hydrated samples was observed by scanning electron microscopy (SEM). The results showed that, calcium hydroxide (CH), ettringite (AFt) and amorphous phase content in hydrated samples decreased as the replacement level of CCK increased, while AFm and stratlingite increased, which was caused by the combination of dilute, physical and pozzolanic effects. The hydration of anhydrous cement phases was accelerated by physical effect but hindered by the retardation effect of CCK. The role of each effects was discussed in detail to analyze the mechanism of OPC hydration with CCK addition. The SEM images showed that the shortening of AFt at 1 day and the denser texture at 28 days was observed with CCK addition, which was caused by the physical and pozzolanic effects, respectively.