Search Results (53)

Coal mining and washing processes generate substantial amounts of coal gangue, posing significant environmental challenges. Coal gangue as a solid waste is rich in SiO₂ and Al₂O₃, with the SiO₂/Al₂O₃ molar ratio closely aligned with the ideal composition of 4A molecular sieves. In this study, through a synergistic pretreatment process involving low-temperature oxidation and hydrochloric acid leaching, the Fe₂O₃ content in coal gangue was reduced from 7.8 wt% to 1.1 wt%, markedly enhancing raw material purity. The alkali fusion–hydrothermal synthesis parameters were optimized via orthogonal experiments—calcination (750 °C, 2 h), aging (60 °C, 2 h), and crystallization (95 °C, 6 h) to maintain cubic symmetry, yielding highly crystalline 4A zeolite. Characterization via XRD, calcium ion adsorption capacity, SEM, and FTIR elucidated the regulatory mechanism of calcination on kaolinite phase transformation and the critical role of alkali fusion in activating silicon–aluminum component release. The as-synthesized zeolite exhibited a cubic morphology, high crystallinity, and sharp diffraction peaks consistent with the 4A zeolite phase. The pH of the zero point charge (pH_ZPC) of the 4A molecular sieve is 6.13. The 4A molecular sieve has symmetry-driven adsorption sites, and the adsorption of Cu²⁺ follows a monolayer adsorption mechanism (Langmuir model, R² = 0.997) with an average standard enthalpy change of 38.96 ± 4.47 kJ/mol and entropy change of 0.1277 ± 0.0148 kJ/mol, adhering to pseudo-second-order kinetics (R² = 0.999). The adsorption process can be divided into two stages. This study provides theoretical and technical insights into the high-value utilization of coal gangue. Full article

Amending saline–alkali soils to improve agricultural productivity is critical for addressing global food security challenges. Biochar is a promising soil amendment, and its modified composites offer significant potential for soil remediation. In this study, we developed a novel phosphoric acid–mineral-comodified biochar composite for saline–alkali soil improvement. SEM and XRD analyses indicate that chemical interactions between phosphoric acid, minerals, and biochar result in the formation of distinct mineral phases on the composite surface. Furthermore, FTIR analysis reveals that these interactions give rise to functional groups such as Si-O-Si, and thermogravimetric analysis demonstrates that the modified biochar composite exhibited enhanced stability. Compared with raw biochar, the modified biochar composites exhibited significant decreases in pH, EC, and base cation content (especially Na⁺), with maximum reductions of 7.26 pH units, 639.5 μS/cm, and 3.69 g/kg, respectively. In contrast, the contents of P, Si, and Ca increased significantly, with maximum increases of 140.04 g/kg, 90.32 g/kg, and 114.27 g/kg, respectively. In addition, the specific surface area and pore volume of the modified biochar composite increased by up to 5.2 and 15 times, respectively. Principal component analysis indicates that mineral type was the primary factor influencing the properties of the composites: hydroxyapatite enhanced porosity and phosphorus levels, whereas kaolinite and montmorillonite increased silicon content. Pot experiments show that the modified biochar composite increased alfalfa plant height by 17.36–20.27% and shoot biomass by 107.32–125.80% in saline–alkali soils. Overall, the newly developed phosphoric acid–mineral–biochar composites were evaluated to have high application potential for saline–alkali soil amendment. Full article

The Vali–Janlou kaolin deposit is located in the northern part of the Urmia–Dokhtar magmatic belt, central-northern Iran, and is hosted by middle Eocene rhyodacitic volcanic rocks. The Vali–Janlou kaolin deposit is one of the most important sources of raw material for ceramics industries in Iran. No trace element geochemical characterizations of this deposit have been conducted in detail before, and this is the main objective of the current research work. Kaolinite and quartz are the major mineral phases present in this deposit, accompanied by some minor phases like illite, rutile, pyrophyllite, dickite, alunite, diaspore, and chlorite. The calculation of mass balance changes revealed that the kaolinization of the rhyodacitic rocks was accompanied by the enrichment of Sr, Zr, Hf, Ta, Nb, U, Th, Y, La, and Pr, leaching–fixation of Sm, Nd, and HREEs, and depletion of Rb, Cs, Ba, Pb, V, Cr, Zn, Eu, and Ce. The behavior of trace elements during kaolinization was controlled by factors such as variation in the pH and temperature of the hydrothermal fluids, the residual concentration, and the presence of mineral phases resistant to alteration. The occurrence of negative Eu anomalies during kaolinization indicates plagioclase destruction by high-temperature hydrothermal solutions and also the liberation of Eu²⁺ during a decreasing intensity of hydrothermal alteration. The presence of diaspore, dickite, and pyrophyllite together with the differentiation of HREEs from one to another, the occurrence of robust negative Ce anomalies, the strong positive correlation between P₂O₅ and LOI, and geochemical parameters like Ce + La + Y, Nb + Cr, Rb + Sr, and Y/Ho are all indicative of the effective role of hypogene processes in the evolution of this deposit. Full article

Composite materials based on diatomite (DT) with the addition of biochar (BC), dolomite (DL), and bentonite (BN) were developed. The effect of chemical modification on the chemical structure of the resulting composites was investigated, and their influence on heavy metal immobilization and the ecotoxicity of post-flotation sediments was evaluated. It was demonstrated that the chemical modifications resulted in notable alterations to the chemical properties of the composites compared to pure DT and mixtures of DT with BC, DL, and BN. An increase in negative charge was observed in all variants. The addition of BC introduced valuable chemically and thermally resistant organic components into the composite. Among the chemical modifications, composites with the addition of perlite exhibited the lowest values of negative surface charge, which was attributed to the dissolution and transformation of silicon compounds and traces of kaolinite during their initial etching with sodium hydroxide. The materials exhibited varying efficiencies in metal immobilization, which is determined by both the type of DT additive and the type of chemical modification applied. The greatest efficacy in reducing the mobility of heavy metals was observed in the PFS with the addition of DT and BC without modification and with the addition of DT and BC after the modification of H₂SO₄ and H₂O₂: Cd 8% and 6%; Cr 71% and 69%; Cu 12% and 14%; Ni 10% and Zn 15%; and 4% and 5%. In addition, for Zn and Pb, good efficacy in reducing the content of mobile forms of these elements was observed for DT and DL without appropriate modification: 4% and 20%. The highest reduction in ecotoxicity was observed in the PFS with the addition of DT and BC, followed by BN and DL, which demonstrated comparable efficacy to materials with DT and BN. Full article

A technology for the hydrochemical processing of the kaolinite fraction of bauxite has been developed, and it involves preliminary chemical activation in a sodium bicarbonate solution and alkaline leaching in a recycled high-modulus solution with the addition of an active form of calcium oxide. Chemical activation allows for the transformation of the difficult-to-explore kaolinite phase to form easily soluble phases of dawsonite, sodium hydroaluminosilicate and bemite. An active, finely dispersed form of calcium oxide was obtained as a result of CaO quenching in Na₂SO₄ solution at elevated temperature and pressure. Using a central composite design (CCD) via response surface methodology (RSM), the technological leaching mode was achieved. The influence on the leaching process was studied by adjusting the CaO/SiO₂ ratio, temperature, alkaline solution concentration and duration. It was found that the determining factors are the concentration of the leaching solution and the temperature. At a stable CaO/SiO₂ ratio, a combination of these two factors determines the duration of the process to achieve the predicted degree of recovery. The results of experiments carried out using the developed model of the leaching process confirmed the validity of the calculated indicators, with an error of 2.01%. In an optimal technological mode at a Na₂O leaching solution concentration of 260 g/L, a temperature of 260 °C, a CaO/SiO₂ ratio of 1.5 and a leaching time of 5 h, the extraction of Al₂O₃ into the solution was 89.7%, which is close to the value of 87.9% predicted by the model. Full article

In this study, the structure and phase transition of kaolin lithium clay at different calcination temperatures were studied and discussed; subsequently, the effects of Li leaching with sulfuric acid under various factors were investigated in detail. The experimental results indicated that an optimal Li leaching rate of 81.1% could be achieved when kaolin lithium clay was calcined at 600 °C for 1 h, followed by leaching with 15.0% sulfuric acid at 80 °C for 2 h. The TG-DSC, XRD, and SEM analyses showed that the layered structure of the clay was not destroyed during the leaching and calcination processes. During the process of calcination, kaolinite was converted to metakaolinite via dehydroxylation. During the process of leaching, the Al on the surface of the metakaolinite was dissolved by sulfuric acid, resulting in the destruction of the Al-O structure; then, Li⁺ was exchanged for H⁺ to the surface of the mineral and entered the solution under the action of diffusion. The leaching kinetics showed that the leaching process was controlled by a diffusion model, and the activation energy (E_a) was 41.3 kJ/mol. The rapid extraction of Li from calcined kaolin lithium clay with sulfuric acid leaching offers a high-efficiency, low-energy-consumption strategy for the utilization of new lithium resources. Full article

In natural kaolinite lattices, ${\mathrm{A}\mathrm{l}}^{3+}$ can potentially be substituted by cations such as ${\mathrm{M}\mathrm{g}}^{2+}$, ${\mathrm{C}\mathrm{a}}^{2+}$, and ${\mathrm{F}\mathrm{e}}^{3+}$, thereby influencing its adsorption characteristics towards rare earth elements like ${\mathrm{S}\mathrm{c}}^{3+}$. Density functional theory (DFT) has emerged as a crucial tool in the study of adsorption phenomena, particularly for understanding the complex interactions of rare earth elements with clay minerals. This study employed DFT to investigate the impact of these three dopant elements on the adsorption of hydrated ${\mathrm{S}\mathrm{c}}^{3+}$ on the kaolinite (001) Al-OH surface. We discerned that the optimal adsorption configuration for hydrated ${\mathrm{S}\mathrm{c}}^{3+}$ is ${\mathrm{S}\mathrm{c}\left({\mathrm{H}}_2\mathrm{O}\right)}_8^{3+}$, with a preference for adsorption at the deprotonated O_u sites. Among the dopants, Mg doping exhibited superior stability with a binding energy of −4.311 eV and the most negative adsorption energy of −1104.16 kJ/mol. Both Mg and Ca doping enhanced the covalency of the Al-O bond, leading to a subtle shift in the overall density of states towards higher energies, thereby augmenting the reactivity of the O atoms. In contrast, Fe doping caused a pronounced shift in the density of states towards lower energies. Compared to the undoped kaolinite, Mg and Ca doping further diminished the adsorption energy of hydrated ${\mathrm{S}\mathrm{c}}^{3+}$ and increased its coordination number, while Fe doping elevated the adsorption energy. This study offers profound insights into understanding the role of dopant elements in the adsorption of hydrated ${\mathrm{S}\mathrm{c}}^{3+}$ on kaolinite. Full article

In this paper, a new preparation technology is developed to make high-alumina coal gangue (HACG) auxiliary cementitious admixture by calcining HACG–Ca(OH)₂ (CH) mixture. HACG powders mixed with 20 wt.% CH were calcined within a temperature range of 600–900 °C, and the thermal transformation and mineral phase formation were analyzed. The hydration reaction between activated HACG–CH mixture and cement was also investigated. The results showed that HACG experienced a conventional transformation from kaolinite to metakaolin at 600 °C and finally to mullite at 900 °C, whereas CH underwent an unexpected transformation process from CH to CaO, then to CaCO₃, and finally to CaO again. These substances’ states were associated with the dehydroxylation of CH, the chemical reaction between CaO and CO₂ generating from the combustion of carbon in HACG, and the decomposition of CaCO₃, respectively. It is the formation of a large amount of CaO above 800 °C that favors the formation of hydratable products containing Al₂O₃ in the calcining process and C-A-H gel in the hydration process. The mechanical properties of HACG–cement mortar specimens were measured, from which the optimal calcination temperature of 850 °C was determined. As compared with pure cement mortar specimens, the maximum 28-d flexural and compressive strengths of HACG–cement mortar specimens increased by 5.4% and 38.2%, respectively. Full article

This paper presents and investigates the properties of concrete in which a portion of the cement is substituted with non-sintered Hwangto (NSH), a readily available building material in Asia. Given the inactive nature of NSH, this study aimed to determine the optimal cement replacement ratio and quantitative strength of the material. The unit weight, compressive strength, ultrasonic pulse velocity (UPV), and stress–strain of the NSH concrete (NSHC) were evaluated. Additionally, we developed a predictive model for determining compressive strength based on the regression analysis of compressive strength and UPV. The water-to-binder ratio was set to 0.41, 0.33, and 0.28, and the NSH replacement rates in the cement were set to 0%, 15%, 30%, and 45% for evaluating various strength ranges. The mechanical property measurements indicated reductions of 5.35% in unit weight, 35.62% in compressive strength, and 6.34% in UPV as the NSH was replaced. Notably, the smallest deviation from plain concrete was observed at a replacement rate of 15%. The scanning electron microscopy analysis results showed that the plain concrete exhibited a crystalloid structure; however, as the NSH replacement rate increased, the amorphous structure and pores increased while unreacted NSH particles were also observed. The X-ray diffraction analysis results demonstrate that the peak intensities for kaolinite and mullite increased as the NSH replacement rate increased, while those of C–S–H gel and CaO showed low peak intensities. Furthermore, the regression analysis concluded that an exponential function was suitable. Consequently, a compressive strength prediction model was developed, and in the error test, the NSHC model demonstrated an average error of <10%, with fewer errors at the lower compressive strength boundaries. Full article

Coal gangue (CG) and coal gasification coarse slag (CGCS) possess both hazardous and resourceful attributes. The present study employed co-roasting followed by H₂SO₄ leaching to extract Al and Fe from CG and CGCS. The activation behavior and phase transformation mechanism during the co-roasting process were investigated through TG, XRD, FTIR, and XPS characterization analysis as well as Gibbs free energy calculation. The results demonstrate that the leaching rate of total iron (TFe) reached 79.93%, and Al³⁺ achieved 43.78% under the optimized experimental conditions (co-roasting process: CG/CGCS mass ratio of 8/2, 600 °C, 1 h; H₂SO₄ leaching process: 30 wt% H₂SO₄, 90 °C, 5 h, liquid to solid ratio of 5:1 mL/g). Co-roasting induced the conversion of inert kaolinite to active metakaolinite, subsequently leading to the formation of sillimanite (Al₂SiO₅) and hercynite (FeAl₂O₄). The iron phases underwent a selective transformation in the following sequence: hematite (Fe₂O₃) → magnetite (Fe₃O₄) → wustite (FeO) → ferrosilite (FeSiO₃), hercynite (FeAl₂O₄), and fayalite (Fe₂SiO₄). Furthermore, we found that acid solution and leached residue both have broad application prospects. This study highlights the significant potential of co-roasting CG and CGCS for high-value utilization. Full article

Calcium oxide plays an important role in alumina production by binding SiO₂ from aluminosilicate raw materials (bauxite, nepheline, kaolinite, etc.) in aluminum-free compounds. The efficiency of the hydrochemical technology depends on the activities of calcium oxide or its compounds introduced into the alkaline aluminosilicate slurry. In this paper, we considered the effects of different calcium compounds (calcium carbonate CaCO₃, gypsum CaSO₄·H₂O, calcium oxide CaO and calcium hydroxide Ca(OH)₂), introduced during the hydrothermal stripping of aluminosilicates with alkaline solutions, on the degree of aluminum oxide extraction, with the subsequent production of fillers for composites. Ca(OH)₂ was obtained by the CaO quenching method. Extraction of Al₂O₃ in an alkaline solution was only possible with Ca(OH)₂, and the degree of extraction depended on the conditions used for CaO quenching. The effects of temperature and of the duration of CaO quenching on particle size were investigated. In potassium solution, the best results for Al₂O₃ extraction were obtained using CaSO₄·H₂O gypsum. The obtained solutions were processed using the crystallization method. Full article

CAN-zeolite was synthesized with a high purity from natural kaolinite via alkali fusion by hydrothermal treatment at a pressure of 1 kbar H₂O. It was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy and nitrogen adsorption at 77 K. The results show that after AK hydrothermal treatment (under specific conditions), the SBET increases from 5.8 m²g⁻¹ to 30.07 m²g⁻¹ which is six times greater. The AK which was a non-porous or macroporous solid (the nitrogen adsorption/desorption of AK is of type II) became mesoporous (N₂ adsorption–desorption isotherms exhibit typical hysteresis of type IV) with a pore size of 5.9 Å. XRD of AK shows the presence of quartz (Q) as impurities, and illite and kaolinite as major fractions; after hydrothermal treatment, the XRD diffractogram shows only fine pics related to CAN-zeolite (with a good crystallinity), confirming the success of the synthesized process. These results suggest that the synthesized CAN-zeolite has the potential to be tested in the removal of heavy metals from waste water as part of a remediation process. Batch reactors were used to evaluate the adsorption isotherms and kinetic studies of heavy metals, cadmium, and lead, by natural kaolinite clay (AK) and synthesized cancrinite zeolite (CAN-zeolite). The results show that the adsorption kinetics of the bivalent heavy metals cadmium and lead are extremely fast with either AK or CAN-zeolite. Equilibrium was reached within 2 min. Adsorption isotherms show that the synthesized CAN-zeolite has a higher adsorption capacity; the retention capacity of lead and cadmium was three times greater than that presented by the natural clay mineral. According to the findings, CAN-zeolite has a higher affinity for PbII (192 mg/g) compared to CdII (68 mg/g). The negative reactive surface sites interacting with these cationic heavy metals resulted in a higher amount of heavy metals adsorption than the cation exchange capacity (CEC). The adsorption information was analyzed using the Langmuir and Freundlich equations. The Langmuir model provided a good fit to the equilibrium data, indicating a monolayer adsorption mechanism. Full article

During the last decade, due to an increase in anthropogenic activities, a higher environmental accumulation of heavy metals has been found, which has resulted in disturbed biogeochemical balance. Many kinds of remediation techniques have been practiced to mitigate heavy metal toxicity in the aqueous phase; however, adsorption is the most commonly accepted technique for efficient heavy metal removal. In this study, conocarpus waste was pretreated with 0%, 10%, and 20% kaolinite and pyrolyzed at 600 °C for 1 h to synthesize biochars (BC, BCK10, and BCK20, respectively), while hydrothermalized at 200 °C for 6 h to synthesize hydrochars (HC, HCK10, and HCK20, respectively). After characterization, synthesized materials were employed for the removal of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) from contaminated water. Experimental data was further subjected to isotherm and kinetic models to estimate the adsorption mechanism. Among all the tested adsorbents, kaolinite-synthesized materials revealed comparatively higher adsorption compared to pristine materials. It was found that pH 7 was optimum for the maximum removal of tested heavy metals. Adsorption of tested heavy metals was well explained by Langmuir and Freundlich isotherms, while pseudo-second order and Elovich kinetics models fitted well for adsorption kinetics. The maximum adsorption capacity, as predicted by the Langmuir isotherm, was the highest for BCK20 (63.19 mg g⁻¹ for Cd, 228.05 mg g⁻¹ for Cu, 248.33 mg g⁻¹ for Pb, and 45.79 mg g⁻¹ for Zn) compared to the other tested materials, and for HCK20 (31.93 mg g⁻¹ for Cd, 181.78 mg g⁻¹ for Cu, 231.85 mg g⁻¹ for Pb, and 45.72 mg g⁻¹), it was higher than pristine HC. Isotherm and kinetics modeling data indicated that multiple mechanisms were involved in Cd, Cu, Pb, and Zn removal, such as chemisorption and electrostatic interactions. The amount of oxygen-containing surface functional groups and SiO₂ particles could be responsible for the maximum adsorption of heavy metals by BCK20 and HCK20. Our findings suggest that biochar, hydrochar, and their kaolinite-modified composites possess the excellent potential to remove heavy metals from contaminated aqueous media, and could be further applied to treat wastewater to mitigate heavy metal toxicity for a sustainable environment. Full article

The air quality of the Himalayan region of India is deteriorating due to the increasing load of particulate matter that is emitted from various local and regional sources, as well as to the transit of dust-related pollutants from the Indo-Gangetic Plain (IGP) and surrounding areas. In this study, the mineralogical characteristics of coarse mode particulate matter (PM₁₀) was analyzed using the X-ray diffraction (XRD) technique from January to December 2019 over Nainital (29.39° N, 79.45° E; altitude: 1958 m above mean sea level), a central Himalayan region of India. XRD analysis of PM₁₀ samples showed the presence of clay minerals, crystalline silicate minerals, carbonate minerals, and asbestiform minerals. It was shown that quartz minerals with significant levels of crystallinity were present in all the samples. Other minerals that are contributing to the soil dust were also observed in the analysis (CaFe₂O₄, CaCO₃, CaMg(CO₃)₂, calcium ammonium silicate hydrate (C-A-S-H), gypsum, kaolinite, illite, augite, and montmorillonite). The minerals ammonium sulphate, hematite, and magnetite were also found in the samples and are suggested to be from biogenic and anthropogenic activities, including biomass burning, fuel combustion, vehicle exhaust, construction activities, etc. This study indicated that the majority of the minerals in PM₁₀ that were present in this Himalayan region are from soil/crustal dust. Full article

Fourier transform infrared spectroscopy (FTIR) was used to study the molecular structure of four medium- and low-temperature heat-treated medium-rank coals. The FTIR spectral parameters, which consist of CH₂/CH₃, aromaticity (f_a), aromatic carbon rate (f_C), aromatic hydrogen rate (f_H), oxygen-containing (C–O) rate (IR), organic matter maturity (M), and the degree of aromatic condensation (Dc), indicate different characteristics, including changes in the aromatic hydrocarbon structure, fatty hydrocarbon structure, hydroxyl structure, and oxygen-containing functional groups of medium-rank coal. The results show that with the increase in heat treatment temperature, the sulfur content in coal gradually decreases, but the C/H ratio gradually increases. Meanwhile, the content of kaolinite and pyrite in coal gradually decreases, whereas the content of dolomite and hematite gradually increases. With the increase in heat treatment temperature, the relative content of ether oxygen hydroxyl groups in the hydroxyl structure significantly decreases, but the relative content of self-associated hydroxyl groups increases. The relative content of alkyl ether (C–O) in oxygen-containing functional groups gradually increases, whereas the relative content of aromatic nucleus C=C vibration presents a trend of first increasing and then decreasing. In addition, –CH₂– is the majority in the structure of fatty hydrocarbons, and the absorption peak intensity of asymmetric –CH₃ stretching vibration increases with the increase in heat-treated temperature. The structure of aromatic hydrocarbons mainly consists of four substituted benzene rings (except for R-303.15 K), in which the relative content of the trisubstituted benzene ring decreases with the increase in heat treatment temperature. With the increase in the heat-treated temperature of medium-rank coal, Dc, f_H, f_C, and f_a show a trend of first increasing and then decreasing, M and IR reveal a trend of first decreasing and then increasing, and CH₂/CH₃ present a gradually decreasing trend. In conclusion, during the increase in the heat treatment temperature of medium-rank coal, the length of the fatty side chains in the fatty hydrocarbon structure becomes shorter, the number of branch chains continuously increases, and the maturity and condensation degree of organic matter first increases and then decreases. On this basis, further research on the effect of coal gasification suggests combining various technologies such as ¹³C NMR, XRD, and TG-MS to obtain semi-quantitative structural information of molecules in coal from different perspectives. Full article