Activity Of Coal Gangue Research Articles

Synergistic performance of microwave-activated coal gangue with limestone in low-carbon cement

This study aims to apply microwave-activated coal gangue (MCG) to develop limestone calcined clay cement (LC3). The impact of microwave activation schemes (microwave temperature and holding time) on the pozzolanic activity of coal gangue were explored. After that, LC3 cements were prepared by replacing cement clinker with limestone powder (LS) and MCG at different ratios. The uniaxial compressive strength, phase composition, microstructure, and pore structure of the MCG-based LC3 cements were analyzed. The results indicate that the maximum volcanic ash activity index is achieved when the material is microwave-activated at 800 °C with a holding time of 20 min. The MCG based LC3 cements achieve the highest strength at an MCG content of 20 % with an MCG:LS ratio of 2. Then, the response surface methodology was employed to predict the optimal mixture proportions for achieving the best strength of the material. The life cycle analysis shows that the carbon emission of the MCG-based LC3 material is 30 % lower than that of Ordinary Portland Cement. This study develops a new high-performance material with low carbon emission for construction industry.

Analysis of calcination activation modified coal gangue and its acid activation mechanism

Acid activation and utilization of coal gangue is of great significance for effective treatment of bulk solid waste in the environment. However, most gangue is quite inert and requires calcination to induce acid activation. In this paper, coal gangue was calcined, and the optimal calcination temperature for activation of the coal gangue acid was explored. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry/thermal differential analysis (TG/DTG) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the crystal structures of the gangue samples formed at different calcination temperatures, which showed that increases in the amorphous content and structural degradation of the coal gangue samples occurred during calcination. The solubility of silicon and aluminum in acid solution was demonstrated with inductively coupled plasma optical emission spectroscopy (ICP‒OES). XRD and TG/DTG were used to characterize the acid activation products formed at different calcination temperatures. The results show that with the increase of calcination temperature, the compressive strength first increases and then decreases, and reached the maximum value at 650 °C; the compressive strengths at 7 d, 28 d and 90 d were 1.32 MPa, 3.22 MPa and 5.14 MPa. At 650 °C, the kaolinite in the gangue was converted into metakaolin, the aluminum-oxygen octahedra had the hydroxyl groups removed, the silicon-oxygen tetrahedra were depolymerized, the absorption bands for Si–O, Si–O–Al and Al–O bonds were broadened, the Al2p and Si2p binding energies were reduced, the splitting were increased, and more Al dissolved in the acid solution, so it has the highest acid reactivity.

Activation of Low-Quality Coal Gangue Using Suspension Calcination for the Preparation of High-Performance Low-Carbon Cementitious Materials: A Pilot Study

Although the calcination-based activation of coal gangue is important for its valorization in the form of cementitious materials, the related works mainly focus on high-quality coal gangue, neglecting its low-quality counterpart. To bridge this gap, we herein conducted the pilot-scale suspension calcination of low-quality coal gangue; explored the effects of calcination temperature, particle size, and O2 content on the phase composition of the calcined product, kaolinite decomposition, decarbonization, and silica/alumina dissolution; and evaluated calcination-product-based cementitious materials. Under optimal conditions (temperature = 875–900 °C; particle size = 39.71–46.84 μm; and O2 content = 12–14%), the carbon content of the calcined product equaled 1.24–1.87 wt%, and the dissolution rates of activated alumina and silica were 77.6–79.5% and 49.4–51.1%, respectively. The 28 d compressive strength (50.8–55.7 MPa) and true activity index (98.8–108.4%) of the cementitious material prepared at a calcination product dosage of 30–38 wt% met the standard of 42.5 grade cement. This study demonstrated the suitability of suspension calcination for the preparation of high-performance low-carbon cementitious materials from low-quality coal gangue, thus providing a basis for further industrialization and technological development.

The influence of NaOH concentration on the strength and microstructure of ceramic binders prepared from coal gangue through geopolymerization

The efficient utilization of coal gangue plays an indispensable role in reducing environmental pressure, improving resource utilization efficiency and promoting green development. As a kind of solid waste rich in silicon and aluminium, coal gangue can be used to prepare ceramic materials. Therefore, a new ceramic preparation process was proposed in this study to prepare ceramic binders for grinding wheels by geopolymerization. The SiO2-B2O3-Al2O3-RO-R2O ceramic binder, a green and low-cost material, was successfully prepared using this method. The effect of the concentration of NaOH on the mechanical properties and microstructure of the ceramic binder was also studied. The strength and microstructure of different ceramic binders were characterized by XRD, TG-DTG, SEM-EDS and FT-IR. The results show that the bending strength of the geopolymer ceramic binder increases proportionately with the increase of NaOH concentration. When the concentration of sodium hydroxide is 15 M, the bending strength reaches 19 MPa. N-A-S-H gel and zeolite formed in the geopolymerization reaction and the pores formed in the sintering process have a significant effect on the bending strength of the ceramic binder. This technology can significantly stimulate the chemical activity of coal gangue and provide innovative ideas for efficient utilization of coal gangue.

Kill two birds with one stone: Contribution of steel slag on enhancing the performance of coal gangue-based cementitious materials and simultaneous sulfur fixation

The thermal activation of coal gangue (CG) is essential for its application in cement system; however, controlling the sulfur emission and improving the hydration activity to CG during activation are considerably challenging. Steel slag (SS), a potential additive, can simultaneously enhance cementitious material strength while attaining sulfur fixation. This research tries to introduce SS to solve CG dual puzzle and reached “kill two birds with one stone”. The results indicate that the calcination behavior of CG can be divided into three stages, with most calcination occurring in the second stage and calcination inhibited during the main weight loss stage. CG primarily releases sulfur in the form of SO2 and H2S during the combustion. Sample G2S1(CG:SS = 2:1) showed the highest sulfur fixation efficiency, with 67.87% for total sulfur gas, 94.73% for H2S and 66.91% for SO2. SS reduced the required average activation energy by nearly half, indicating that SS has a significant impact on lowering the energy barrier and enhancing the overall activity. Together with sulfur fixation, G2S1 calcined at 600 °C considerably improved the performance of cementitious materials, with the compressive strength reaching 82.67 MPa. The mechanism of achieving the "kill two birds with one stone" objective can be attributed to the introduction of SS, which provides an abundant supply of calcium. On the one hand, it facilitates the sulfur fixation process by forming CaSO4. On the other hand, it contributes to the availability of CaO in the cementitious material, accelerating the formation of C–S–H gel.

Study on the Reactivity Activation of Coal Gangue for Efficient Utilization.

In this study, the research aim is to enhance the activity index of activated coal gangue and study its activation mechanism. The activation process of coal gangue was optimized through orthogonal tests, and the Back-Propagation (BP) neural network model was improved using a genetic algorithm. With the effects of grinding duration, calcination temperature, and calcination duration, the morphological changes and phase transformation processes of coal gangue were studied at the micro and meso levels to clarify the activation mechanism. The results indicated that the effect of calcination temperature on the strength activity index of coal gangue was most significant, followed by grinding duration and calcination duration. The potential activity of coal gangue can be effectively stimulated through mechanical and thermal activation, and the content of potential active minerals in coal gangue powders was also increased. The activation process of coal gangue for the optimal scheme was obtained as grinding at 76 min first and thermal treatment at 54 min at 749 °C. As the thermal activation under 950 °C, some unstable external hydroxyls, and internal hydroxyls in kaolinite from coal gangue were removed, the AlⅥ-O octahedron was destroyed, and kaolinite was transformed into spatially disordered metakaolinite with very high activity.

Thermal behavior of Al(NO3)3·9H2O and its application in preparing Al2O3 and regenerating HNO3

The new process ‘coal gangue/fly ash – HNO3 pressure leaching (NAPL) –low-temperature pyrolysis’ was proposed by our team, which aimed to achieve the transformation of coal-based solid wastes from harmless to Al resource. For cost control, the regeneration of HNO3 at a low cost was crucial for the process. Following NAPL of coal gangue and fly ash, massive amounts of Al(NO3)3·9 H2O were collected, with a decomposition temperature as low as 523 K. The energy consumption for low-temperature pyrolysis could be supplied by the combustion of residual carbon (6.96 wt%) during the thermal activity of coal gangue, allowing HNO3 to be regenerated and recycled without extra heat supply. However, little research has been done on the pyrolysis behavior and kinetics of Al(NO3)3·9 H2O, which was examined in this study. The pyrolysis characteristics of Al(NO3)3·9 H2O were assessed by TG-DSC analysis. Further experiments and XRD/FTIR/MS analysis revealed that phase evolution of Al(NO3)3·9 H2O followed the sequence of Al(NO3)3·9 H2O→ Al(NO3)3·nH2O→ Al(NO3)3·3Al(OH)3·5/2 H2O→ AlOOH→ Al2O3. Subsequently, the thermodynamic (∆G, ∆H, ∆S) and kinetic (Ea, A) parameters were calculated based on the thermogravimetric data by the iso-conversional methods. And the mechanism models and functions were identified via the Malek method. Finally, the economic evaluation of pyrolysis indicated that 5.04 × 10–3 Nm3 of natural gas was consumed during pyrolysis of one mole Al(NO3)3·9 H2O, equivalent to 44.33 $ per ton of Al2O3. As a basic research of Al(NO3)3·9 H2O pyrolysis, this paper enhances related theories of the NAPL process.

Adsorption performance of calcined coal gangue for ethyl mercaptan

ABSTRACT The modified coal gangue with different structures were used as the adsorbents which were prepared via the calcination process with the different temperature, time and atmosphere. The calcined coal gangues were characterized by XRD, FT-IR, TGA, SEM-Mapping, EDS, nitrogen adsorption-desorption isotherms and XPS. The adsorption performance of calcined coal gangue was evaluated via the adsorption of ethyl mercaptan in methane gas. The results show that the internal structures of coal gangue are affected by the calcination temperature and calcination time. The main components in the coal gangue are changed from kaolin to amorphous metakaolin, and the adsorption activity of coal gangue is greatly improved. The carbon content, especially the content of functional groups C-O and C=O in the coal gangue is changed in the difference of calcination atmosphere. When the calcination temperature, time and air content are set at 600°C, 3 h and 0.1 respectively, the breakthrough adsorption capacity of calcined coal gangue for ethyl mercaptan is 39.32 mg·g−1. The coal gangue modified by a simple calcination method has strong adsorption desulfurization performance and this is a typical example of treating waste by waste.

Experimental Research on Improving Activity of Calcinated Coal Gangue via Increasing Calcium Content.

In this investigation, non-spontaneous combustion coal gangue was activated by two methods: (1) low-temperature calcination and (2) calcium addition. Differences in the activity of the activated coal gangue were studied at various calcination temperatures and amounts of calcium addition. Meanwhile, the cementation activity of the activated coal gangue was evaluated according to the activity effect analysis. Furthermore, the influences of the activated coal gangue on the cementation activity of cement were investigated. The results indicated that the activities of the activated coal gangue increased at a temperature between 500 °C and 700 °C. The calcium addition method can also increase the activity of coal gangue, with the effect being better when the gangue is mixed with slag. The addition of calcium and the calcination of coal gangue can promote the production of active minerals such as metakaolin, which is the main reason for the increased cementation activity.

Thermal Activation of Coal Gangue with Low Al/Si Ratio as Supplementary Cementitious Materials

To effectively utilize coal gangue (CG) with low Al/Si ratio, the thermal activation method was used. The activated CG, as supplementary cementitious materials (SCMs), was added into ordinary Portland cement (OPC) to study its physical properties. The XRD results show that CG undergoes a phase transition from kaolinite to metakaolinite during activation. The NMR tests reveal that the low polymerization state Q3 is continuously broadened, and the Al coordination gradually changes from Al VI to Al V and Al IV. The CG particles are scale-like and glassy with a loose structure. By mixing the activated CG (under 800 °C) with cement (mass ratio = 3:7), the water demand of normal consistency increases by 7.2% and the initial and final setting times extend by 67 min and 81 min, respectively. The rough surface and loose structure of activated CG are the main factors contributing to the higher water demand of normal consistency. The micro-aggregate effect of the activated CG reduces the contact rate between the cement particles and water, and the interparticles, thus slowing down the process of hydration reaction, and leading to longer setting times.

Activation Mechanism of Coal Gangue and Its Impact on the Properties of Geopolymers: A Review.

Coal gangue is one of the industrial solid wastes that may harm the human body through the ecosystem for a long time. Using coal gangue in geopolymer preparation can effectively reduce cement output and meet the sustainability requirements. In this paper, the physical and chemical characteristics, including the heavy metal content, of coal gangue from different producing areas are described. Then, the mechanism of physical activation (mechanical and thermal activation), chemical activation, and compound activation of coal gangue are illustrated. The machinability, as well as the mechanical, microscopic, and toxicity consolidation properties of geopolymers prepared from coal gangue, are summarized and analyzed. The results indicate that the coal gangue geopolymers can have higher mobility and mechanical strength than cement-based composites by adjusting high calcium element material, alkali activator content, Na2SiO3 modulus, and curing condition. After physical activation, coal gangue is used in geopolymer preparation with a chemical activator (alkali excitation agent), which effectively forms a three-dimensional silicon aluminate polymer network. The pore structure is dense, the physical fixation and chemical bonding are strengthened, and the solidification and adsorption of heavy metal ions are improved. Further, it can also be applied to solidifying radioactive waste, which is following the future development direction.

Thermal behavior and chemical reactivity of coal gangue during pyrolysis and combustion

The efficient extraction of Al from coal gangue plays a significant role in achieving the high-value utilization of coal gangue and alleviating the shortage of bauxite resource. Coal gangue is usually subjected to thermal activation to stimulate its chemical reactivity before extracting Al through acid method. The thermal-activation atmosphere is a key ingredient influencing the activation effect and is thus worthy of in-depth study. Herein, a series of comparative studies were performed to gain insights into the thermal behavior and chemical reactivity of coal gangue in nitrogen atmosphere (pyrolysis) and air atmosphere (combustion). First, the pyrolysis and combustion characteristics of coal gangue were determined by thermogravimetric analysis at various heating rates of 5, 10, 15, 20, and 25 °C/min. Second, the kinetics of the two thermal activation processes was analyzed using the iso-conversional method and model-fitting method. Results showed that the pyrolysis and combustion reactions of coal gangue conformed to the nucleation-growth model, and the mechanism functions were determined as A1/2 and A2/3, respectively. We found that the intense combustion of combustible substances did not change the reaction mechanism, but reduced the apparent activation energy required for thermal activation from 177.47–185.17 kJ/mol to 140.24–144.39 kJ/mol. Phase-evolution analysis also revealed that combustion lowered the temperature required for inert kaolinite activation from 550–650 °C to 450–550 °C, indicating that combustion effectively promoted the activation of coal gangue. Finally, HNO3 was used to establish the correlation between the thermal activation and chemical reactivity of coal gangue in view of its significant advantage of easy regeneration. Leaching results further demonstrated that combustion presented obvious benefits for the efficient extraction of Al and the effective inhibition of impurity Fe.

Thermal activation and structural transformation mechanism of kaolinitic coal gangue from Jungar coalfield, Inner Mongolia, China

The benefits of industrial solid waste as a supplementary cementitious material in the manufacture of eco-efficient cement are evident in the context of sustainable and green development. Coal gangue from the Jungar coalfield, Inner Mongolia, China, was characterized and activated thermally at 500 °C–1000 °C. This study investigated the effects of thermal activation conditions on the phase and structural changes of the minerals in coal gangue. The pozzolanic activity of the thermally activated coal gangue was evaluated using both direct and indirect methods. The experimental results show that the coal gangue had a silicoaluminous nature mainly comprising of kaolinite. The pozzolanic activity of coal gangue depends on the phase and structural transformation of kaolinite during calcination. Highest pozzolanic activity is reached when calcined at 800 °C for 2 h, with lime consumption and strength activity index up to 1039 mg g−1 and 1.1, respectively. The high pozzolanic activity of metakaolinite was attributed mainly to the formation of a five-coordinated Al sheet and the less polymerized Si sheet after dehydroxylation under calcination.

Aluminum extraction from activated coal gangue with carbide slag

A new coal gangue(CG) activation additive was studied in this paper, carbide slag(CS) was used as an auxiliary agent on the extraction of Al2O3 from coal gangue activated by Na2CO3 through acid leaching. The dissolution rate of Al2O3 was determined by Inductively Coupled Plasma-Atomic Emission Spectrometer(ICP-AES) and calculated using industrial standard SN/T 1599–2005. The results showed that compared with the direct activation of coal gangue with Na2CO3, the addition of carbide slag could reduce the dosage of Na2CO3 by 60% and the acid leaching time by 1/3. The alumina dissolution rate reaching 94.58% when calcined at 800 °C for 120 min, m (CG:Na2CO3:CS) = 1:0.4:1.2, the liquid-solid ratio was 15 mL·g−1, and the acid leaching time was 60 min. Simultaneous thermal analyzer (TG-DSC), fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope(SEM) analysis showed that calcination of coal gangue with Na2CO3 produces sodium aluminosilicate, sodium silicate and nepheline, making aluminum in coal gangue easier to be leached by acid. The carbide slag was involved in the activation process of coal gangue by Na2CO3 at 740–847 °C, which destroyed the Si-O structure and Al-OH structure in coal gangue, generated calcium silicate with more developed pore structure consumes part of SiO2, and reduced the consumption of Na2CO3 during the calcination process by reaction with SiO2, and shortened the reaction time between minerals and hydrochloric acid.

Performance and Hydration Mechanism of Modified Tabia with Composite-Activated Coal Gangue

The feasibility of modified tabia (MT) with composite-activated coal gangue (CACG) as the subgrade material of low-grade highways was experimentally investigated. A composite activation method was employed to improve the pozzolanic activity of coal gangue. The effect of CACG content on the mechanical properties of MT was investigated through a series of experiments. It was found that the pozzolanic reactivity of coal gangue was remarkably enhanced by the composite activation method. Compared with traditional tabia (TT), the unconfined compressive strength, splitting strength, and flexural tensile strength of the MT with 50% of CACG content increased by 5.03 times, 9.71 times, and 1.50 times, respectively. The impermeability of specimens with CACG significantly improved. Furthermore, the mass loss rate of MT was less than 2.83%, while it reached up to 34.20% in TT after being conditioned to 40 freeze–thaw cycles. Finally, the microstructure change and hydration mechanism of MT are discussed and revealed.

Experimental Study on the Strength of Coal Gangue Aggregate Concrete with Basalt Fiber

In order to study the effect of basalt fiber content, basalt fiber length and coal gangue activation on compressive strength and splitting tensile strength of concrete, 24 groups of orthogonal experiments were designed. The results show that the influence of various factors on the compressive strength and splitting tensile strength of concrete is coal gangue activation > basalt fiber content > basalt fiber length. The activation of coal gangue is conducive to the increase of concrete strength; the influence of fiber content on strength increases first and then decreases; the increase of fiber length is beneficial to compressive strength and unfavorable to splitting tensile strength; the best ratio of engineering practice is fiber content 0.10 %, fiber length 18 mm, activated coal gangue.

Mechanism on Activation of Coal Gangue Admixture

Coal gangue, an industrial waste, is rich in silicon and aluminum phase and may be used as a mineral admixture in concrete after moderately stimulating activity, allowing for efficient solid waste utilization. This study used a mortar strength and activity evaluation method to investigate single or compound activation methods to find the optimum activation method of coal gangue. FLIR, XRD, and SEM were used to investigate the activation mechanism of different modes, providing a theoretical foundation for the study of coal gangue as a concrete admixture. Results showed that mechanical ball milling, microwave, and chemical activator could activate coal gangue, and the composite activation effect was the best. The fineness of the coal gangue powder was more than 300 mesh, according to the optimal compounding method. Accordingly, the particle surface was smooth, the internal defects were reduced, and the microwave irradiation temperature was 700°C–800°C, causing the coal gangue particles to form a bonding surface and gradually agglutinate and densify. Meanwhile, the layered structure of kaolin minerals was destroyed, and a significant amount of glassy active SiO2 and Al2O3 was produced, enhancing the gel ability and activity of coal gangue. Finally, 8% Ca(OH)2 was added in the production of mortar specimens, which increased the alkalinity of the slurry, stimulated the rapid cracking and secondary hydration of the coal gangue, and enhanced the strength of mortar. At this time, the activity rate of coal gangue powder reached the highest, which was 90.5%.

Experimental Study on the Characteristics of Activated Coal Gangue and Coal Gangue-Based Geopolymer

Coal gangue-based geopolymer (CGGP) is one of the hot spots existing in the recycling of coal gangue resources due to its good comprehensive mechanical properties. However, the coal gangue structure is stable and reactivity is poor, so the coal gangue needs to be activated before utilization. In this paper, the microstructure changes of activated coal gangue by different mechanical and thermal activation methods, as well as the mechanical properties and microstructure changes of the CGGP specimens were studied by experimental investigation. The results indicated that mechanical activation and thermal activation were two effective methods to change the reactivity of coal gangue, which consisted of destroying the stable kaolinite structure and improving the activity of coal gangue. Conversely, part of the amorphous structure in coal gangue was destroyed when the activation temperature reached 900 °C, which was not conducive to the further enhancement of coal gangue activity. For the CGGP prepared by thermally activated coal gangue and modified sodium silicate alkali solution, the uniaxial compressive strength of the CGGP specimens decreased with thermal activation temperatures of the raw coal gangue materials at 700 °C, 800 °C, and 900 °C. The main reason for this was the lower amount of the active metakaolin structure in coal gangue at 900 °C, which was not conducive to the geopolymerization process.

Unprecedented catalytic activity of coal gangue toward environmental remediation: Key role of hydroxyl groups

Heavy metal ions, especially the Cr(VI) ions, cause serious environmental problems, which endows attractive efforts on the Cr(VI) removal with high efficiency. Basically, the Cr(VI) removal in the presence of organic acids under light irradiation presents a relatively slow process. In this regard, this work found that natural coal gangue (CG) without any chemical modification could extraordinarily enhanced the removal rate of Cr(VI) in the presence of tartaric acid (TA) under visible light irradiation. The Cr(VI) removal efficiency could be modulated by calcination treatment. Mechanism investigation uncovered that the abundant hydroxyl groups on natural CG surfaces contributed mostly to the excellent adsorption performance toward O2 and TA and thus facilitated the generation of O2−, which played the dominant role in the photocatalytic process. In addition, Fe species in natural CG also acted as active sites and partially participated the Cr(VI) reduction process. This work would offer persuasive evidence of significant roles for natural solid wastes in catalytic process.

Microwave activation of coal gangue for Al compound

The coal gangue was activated by microwave. From the results of XRD and 27AlMAS NMR, we can speculate the reactions occurred and new species were generated during the microwave activation process. Effects on power grades of microwave oven, kinds of additives, time of microwave activation, temperatures of water bath and ratio of coal gangue to additives were investigated. Much higher extraction percent of Al (>50) has been obtained with KOH as additive, ratio of coal gangue to additive is 1:2, power of microwave oven is 595W and time of microwave activation is 20 min.