Lignite Surface Research Articles

Facilitated bioreduction of nitrobenzene by lignite acting as low-cost and efficient electron shuttle

The searching for efficient and economical redox mediators to promote the treatment of wastewater containing recalcitrant organic compounds is greatly needed. In this study, the redox mediator activities of four different lignite samples to facilitate the bioreduction of nitrobenzene by Shewanella oneidensis MR-1 were tested for the first time. The initial nitrobenzene reduction rate was increased by 40.4%–90.3% in the presence of 50 mg/L of different lignite samples. Lignite collected from Xinjiang (XJL) having more oxygenated groups performed better in enhancing nitrobenzene bioreduction. The stimulating effects increased with the increase of lignite dosage (0–200 mg/L) and the decrease of lignite particle size (150–0.1 μm). However, the pristine XJL samples with assorted sizes of particles exhibited better stimulating effects than size-fractionated ones, implying that different-sized XJL particles might have synergetic effects on the bioreduction process. When humic acid or iron was removed from XJL, its promoting effects were decreased, demonstrating the crucial roles of both components in lignite-enhanced nitrobenzene bioreduction. Nitric acid treatment could form more oxygenated moieties on lignite surface, which played vital roles in promoting nitrobenzene bioreduction. The initial nitrobenzene bioreduction rate in the presence of HNO3-treated XJL was 80.8% higher than that obtained with pristine XJL. This study proposed an effective and readily available redox mediator that could be applied to promote the bioreduction of recalcitrant electrophilic pollutants.

Dielectric properties and microwave upgrading characteristics of Zhaotong lignite

ABSTRACT The dielectric properties of lignite were studied by using the resonant cavity perturbation method. The effect of temperature, particle size, moisture content, and microwave frequency were investigated. Results showed that the variation of dielectric parameters could be divided into four stages: Stage I (25–200°C), Stage II (200–550°C), Stage III (550–800°C), and Stage IV (800–1000°C). The dielectric properties increased with particle size due to its high void ratio. The higher moisture content leads to the higher dielectric properties. The frequency of 915 MHz was more appropriate to dispose the tonnage material in high power industrial than that of 2450 MHz. Additionally, the micro-structure of lignite was analyzed by the scanning electron microscope (SEM) and the fourier transform infrared spectroscopy (FT-IR). The characterization analyses demonstrated that a large quantity of pores and cracks were generated in lignite surface because of the microwave pumping effect. The polar substances in sample selectively absorbed and transferred microwave into heat and then evaporated water to steam, causing an instantaneous increase in internal pressure. There was a uni-directional mass and heat transfer process from the interior to the surface under the steam driving force, which caused a great number of pores and cracks emergence to substantially facilitate the removal of moisture content and accelerate heat dissipation. And the apparent aromaticity of lignite dried by microwave could be increased and the capability of potential re-absorption could be decreased. Meanwhile, the mechanisms of microwave heating and generation of cracks and pores were analyzed in detail. These results could provide a theoretical basis for microwave processing of lignite.

Investigation of Occupational Accidents in Western Lignite Corporation by Using the Efficiency Assessment Approach

The mining sector, which has an important place in terms of raw material and energy supply, is referred as a sector with one of the highest fatal accidents in comparison with other sectors. In this century, where occupational health and safety awareness has increased, it is important to review the past accident statistics and to determine the current deficiencies by using the information in the inventory. In this study, the Malmquist total factor productivity index (MPI) has been utilized to investigate the changes in the work accidents occurring in the Western Lignite Corporation (Turkey), where underground and surface production methods are applied, and to determine the measures to be taken. According to the results obtained, both the decrease in production amount and the increase in the number of accidents caused inefficiencies during the period in concern due to the expected 10-year economic life of the mechanized system operating in the underground production panel that came to an end in 2007 and the flooding took place in early 2017. Compared with the underground operations, it has been determined that production can be carried out at the appropriate scale despite the heavy working conditions encountered due to the changing climatic conditions and the progress of production in the surface operations.

Molecular dynamics simulation of a lignite structure simplified model absorbing water

ABSTRACTTo study the formation mechanism of hydration films, a lignite molecular model based on FTIR analysis was established. Both the positive and negative extreme values (±184.76 kJ/mol) of the electrostatic potential appear around oxygen-containing functional groups. Water molecules are mainly adsorbed near oxygen-containing functional groups. This paper studies the absorption of lignite for water molecules using MD simulation. With the addition of water molecules, the number of polar groups increases, and the electrostatic potential level of the system increases, causing an increase in the number of active sites. When the amount of water is inadequate for covering the surface of lignite, water molecules adsorb at active sites and places with lower z values. As the number of water molecules increase, the average length of hydrogen bonds between water molecules and that between lignite and water are close to the set values (1.97 and 1.8 Å, respectively). The short-range order structure of the interface has little effect on the average bond length of water molecules, but it reduces the average bond angle by 1°~2°. After a hydration film forms, the increase to the thickness of the hydration film is primarily attributed to hydrogen-bond interaction between water molecules.

Effects of pre-wetting time on surface topography and floatability of lignite particles

The laser particle size analyzer, NMR, SEM/EDS, induction time tests and surface roughness analysis were employed in the mechanism analysis of this investigation. The floatability of lignite was decreased with the pre-wetting increasing from 0 – 5min because the pores and fractures of lignite surface can be filled with water. However, the floatability of lignite was increased with the pre-wetting from 5 – 480min. NMR and SEM indicate that a long pre-wetting process (>5min) polished and rubbed the surface of lignite reducing the amount of macropores and fractures as well as the porosity. The rough lignite surface was also polished to be smooth. As the pre-wetting time increases, the topography of lignite surface, such as porosity, macropores, fractures and roughness decreased, which results in the decrease of water filling negative effects, and hence the floatability of lignite gradually increased with the pre-wetting time over 5min.

Car-Parrinello molecular dynamics study on the interaction between lignite and water molecules

Water-desorption from lignite surface is an important issue in energy usage, however, corresponding researches are inadequate. Herein, in order to comprehensive understand the process of water-desorption from lignite surface, the interaction between water molecules and carboxyl group of lignite were investigated systematically from 298.15 K to 440 K, by using Car–Parrinello molecular dynamics (CPMD) method. The result indicated that in the key carboxyl group, the hydrogen site (H51) showed stronger adsorption capacity for water molecules than the oxygen site (O23) of the carboxyl group. Based on the statistical result, the water molecules adsorbed at H51 and O23 were considered as removed after 360 K. Vibration analysis was performed to acquire stretching frequencies of carboxyl group before and after water adsorption. The NBO analysis was used to study the reorientation of water molecules in a 0.14 ps trajectory when they replace with each other. The NBO orbitals obtained show that when water molecule is replacing, the two lone pair orbitals of O31 and O32 interact with the σ antibonding orbital of O24H51 bond at the same time. It enhances the formation of new H-bond and facilitates the fracture of old H-bond. This work can not only reveal the mechanism of water desorption from lignite, but also lay valuable foundations for efficient utilization of lignite resource.

Water Clusters in Lignite and Desorption Energy Calculation by Density Functional Theory.

The interaction of water and hydrophilic sites with hydroxyl, carboxyl, and multiple oxygen-containing functional groups (OFGs) in lignite molecules was studied by density functional theory. The adsorption of water molecules on the lignite surface initially resulted in the formation of hydrogen bond-driven stable rings by three to four water molecules, followed by the formation of three-dimensional water clusters like a ″patchwork″. Aqueous layer thickness obtained from the water cluster size was 0.4–0.6 nm, which was consistent with the experimental data. Thus, pore-filling water beyond this range was less affected by the OFGs on the surface. Calculation of the adsorption energy predicts that the water clusters were primarily formed in the hydrophilic sites with three OFGs (site 1, including a carbonyl group, an alcoholic hydroxyl group and an etheroxy group in tetrahydropyran), then in COOH, and in O–H. For isolated hydroxyl groups, the interaction between the hydroxyl group and water molecules was weaker than that between the water molecules. When the water cluster was located at the hydrophilic sites with two or more OFGs, the adsorption energy of lignite–water interaction was higher than that of water–water interaction. Investigating the thermodynamics of the adsorption process at a molecular scale will help in understanding both drying and resorption process of dried lignite during industrial production.

Effects of ultrasonic pretreatment on particle size and surface topography of lignite and its relationship to flotation response

ABSTRACT The aim of this investigation was to study the effects of different ultrasonic powers and treatment times on particle size and surface topography of low ash fine lignite (<0.125 mm) combined with flotation, wet-screening, surface roughness, scanning electron microscopy (SEM), and induction time tests. Flotation results show the concentrate yield gradually decreased with the increase of ultrasonic power and treatment time except 18 W. The wet-screening results indicate that increasing ultrasonic power and treatment time had more significant crushing effect on lignite particles and produced more fine particles (<0.045 mm), which was harmful for the improvement flotation performance of lignite. SEM results show that crack was gradually expanded into a gap as well as some new cracks and cavities on the surface of lignite are produced with the increase of ultrasonic power. The generation of these gaps, new cracks, and cavities increases the roughness of lignite surface. The higher the ultrasonic power is, the rougher the lignite surface becomes, which promotes its surface to entrap more water during the flotation process resulting in a formation of thick hydration shell. The induction time increased as ultrasonic power and treatment time increased except 18 W. Hence, there will be a negative impact on lignite flotation if the lignite particles are treated by higher ultrasonic power for longer. The findings of this paper may provide some guidance to understand the effects of ultrasound on surface topography of lignite and makebetter use of ultrasound to assist coal flotation.

Impact of ultrasonic pretreatment on the flotation of lignite particles of different size fractions

ABSTRACT The purpose of this study was to investigate the impacts of ultrasonic pretreatment on flotation of lignite particles of different size fractions. Wet-screening, laser particle size analyzer, energy dispersive spectrometer (EDS), mercury porosimeter, attachment time and flotation tests were employed to reveal the changes of particle size, surface properties and floatability of different size fraction lignite before and after ultrasonic pretreatment. Flotation results show the combustible matter recovery of coarse particles (0.25–0.125 and 0.125–0.074 mm size fractions) after ultrasonic pretreatment was significantly improved whereas that of fine particles (0.074–0.045 and −0.045 mm size fractions) was decreased. Wet-screening and laser particle size analyzer results reveal that more breakage occurred in 0.125–0.074 mm size fraction using ultrasound than in other size fractions. In addition, coarse particles were crushed to fine particles using ultrasound benefitting improvement of the flotation performance. EDS and mercury porosimeter results indicate ultrasonic pretreatment removed some high ash minerals embedded in lignite surface through surface cleaning and cracking to reduce some pore/cracks of all size fractions except −0.045 mm, which made an increase in the hydrophobicity and floatability of lignite. Therefore, ultrasonic pretreatment was an efficient method to enhance flotation performance of 0.25–0.074 mm size fractions lignite.

Effect of Nitric Acid Modification on Characteristics and Adsorption Properties of Lignite

The objective of this research was to explore the changes of the pore structure and surface properties of nitric-modified lignite and base the adsorption performance on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, several lignite samples were treated with different concentrations of nitric acid in order to get different pore structure and surface chemistry adsorbent levels. A common heavy metal ion contaminant in water, Pb2+, served as an adsorbate probe to demonstrate the change of modified lignite adsorption properties. The pore structure and surface properties of lignite samples before and after modification were characterized by static nitrogen adsorption, X-ray diffraction, Scanning electron microscope, Fourier transform infrared spectroscopy, zeta potential, and X-ray photoelectron spectroscopy. The experimental results showed that nitric acid modification can increase the ability of lignite to adsorb Pb2+. The adsorption amount of Pb2+ increased from 14.45 mg·g−1 to 30.68 mg·g−1. Nitric acid reacted with inorganic mineral impurities such as iron dolomite in lignite and organic components in coal, which caused an increase in pore size and a decrease in specific surface areas. A hydrophilic adsorbent surface more effectively removed Pb2+ from aqueous solution. Nitric acid treatment increased the content of polar oxygen-containing functional groups such as hydroxyl, carbonyl, and carboxyl groups on the surface of lignite. Treatment introduced nitro groups, which enhanced the negative electrical properties, the polarity of the lignite surface, and its metal ion adsorption performance, a result that can be explained by enhanced water adsorption on hydrophilic surfaces.

Drying of lignite during beneficiation in the air dense medium fluidized bed under mild conditions

High water content and ash content are major challenges in lignite utilization. The beneficiation processes, between the mining and upgrading, can achieve deashing and dewatering. An air dense medium fluidized bed integrated drying and beneficiation of lignite was designed in the present research. The drying characteristics of lignite in designed fluidized bed under mild conditions were investigated. Results show that for refractory lignite with diameters from 13 mm to 25 mm, the surface water removal efficiency reaches up to 91% and the adhesion efficiency of dense medium decreased to 0.16% at 85 °C for 2 min with original surface water content of 1.2% and air velocity of 3.5 cm/s. The probable error of separation Ep is 0.048 g/cm3 which is much lower than that of common air dense medium fluidized bed. Based on momentum transfer, heat transfer and mass transfer between moist lignite, adhered dense medium, fluidized dense medium and hot air in the fluidized bed, the drying process in lignite beneficiation was detailed explained. During drying in beneficiation, the dense medium adhered on lignite surface play a significant role since they helped moisture transfer from lignite surface to the hot air, realizing the removal of surface water.

Decrease in hydrophilicity and inhibition moisture re-adsorption of lignite using binary surfactant mixtures with different hydrophilic head-groups

Effect of binary surfactant mixtures on hydrophobic modification and moisture re-adsorption inhibition of lignite are investigated, along with that of dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfonate (SDS) and dodecyl nonaethoxyl ether (C12(EO)9). Decrease in hydrophilicity of lignite is measured by determination of wetting heat and moisture re-adsorption rate of lignite before and after adsorbing surfactant. The differences in hydrophilicity change of lignite by using single and binary surfactant mixtures are discussed to disclose the synergistic or antagonism effect on hydrophobic modification of lignite. The experimental results show that, with adsorption of both single and binary mixtures of surfactants, lignite surface become hydrophobic. Compared to single component, DTAB-C12(EO)9 and DTAB-SDS display a higher efficiency on the hydrophilicity reduction and moisture re-absorption inhibition of lignite, while C12(EO)9-SDS does not. The calculated results indicate that there exists intermolecular synergism in the mixtures of DTAB-C12(EO)9 and DTAB-SDS, with forming synergetic aggregates “co-surfactant” DTAB-C12(EO)9 and DTAB-SDS, and C12(EO)9-SDS acts no synergism. These experimental results correspond to the calculation results of the properties of surfactant solution. Therefore, the solution properties of binary surfactant mixtures may provide a theoretical guiding in binary surfactant selection for reducing the hydrophilicity of lignite.

Adsorption of ammonia nitrogen on lignite and its influence on coal water slurry preparation

Using lignite as an adsorbent and subsequently as a material for coal water slurry (CWS) preparation represents an alternative method for coal chemical wastewater treatment. However, the components in these wastewaters could affect the CWS performance, especially ammonia nitrogen (NH4+-N). The influencing mechanism of NH4+-N was studied through adsorption method. Associated with adsorption reaction model, Fourier transform infrared spectrum and zeta potential detection, it was found that the adsorption reaction of NH4+-N, driven by ion-exchange interactions with H+ of –OH and –COOH on the lignite surface, displayed a typical monolayer mode and was much faster than that of NSF which displayed a double-layer mode and was adsorbed preferentially at hydrophobic sites. In binary adsorption, the adsorption amount of NSF was increased remarkably by adding NH4+-N. This was attributed to the modified lignite surface and hydrophilic group by NH4+-N that decreased the surface electronegativity as well as the electrostatic repulsion among –SO3− in NSF molecules. The enhanced adsorption of NSF resulted in capturing more water molecules in hydration film surrounding the particles in CWS. The hydration film was then thickened and stabilized, thus leading to a decrease in free water for CWS flowing. As a result, the apparent viscosity was increased, and CWS tended to be dilatant. In addition, the thickened and stabilized hydration film also increased the yield stress of CWS prepared with NH4+-N, resulting in improved stability.

Influence of surfactant for improving dewatering of brown coal: A comparative experimental and MD simulation study

Abstract Experimental method and molecular dynamics (MD) simulations were used to investigate the filtration dewatering process of brown coal in the presence of the surfactant TRITON X-100 (TX-100). A significant reduction in filter cake moisture content was observed upon addition of TX-100 in the dewatering experiments. It was observed that a narrow TX-100 concentration range of below the critical micelle concentration (CMC) was best for dewatering. The enhancement in filtration dewatering characteristics could be attributed to the wetting characteristic changes of the brown coal surface caused by surfactant adsorption, as indicated by measurement of contact angles. Monolayer adsorption of TX-100 at the lignite surface was investigated by MD simulations. The TX-100 molecules repelled water molecules near the coal surface and adsorbed at the water-lignite interface with the ethoxylate closer to the coal surface than the octylphenol. The mobility of water molecules was enhanced in presence of TX-100 according to the results of mean square displacement (MSD) and diffusion coefficient. It was inferred that adsorption of TX-100 resulted in a more hydrophobic brown coal surface, in agreement with the dewatering and contact angle experiments.

A series of nonionic surfactants for lignite flotation and adsorption performance

ABSTRACT Nonionic surfactants have been applied to improve the lignite floatability in the flotation process. In this paper, Inner Mongolia lignite was pretreated by three type surfactants, including sorbitan monolaurate, sorbitan monostearate, and sorbitan monooleate, respectively. It was investigated that the effect of the length and saturability of hydrocarbon chain on the collectivity and selectivity of surfactant. The flotation results indicated that the optimum efficiencies were found at the dosages of about 35 kg/t for the three surfactants. However, the recovery of combustible material of lignite pretreated by sorbitan monolaurate is comparatively low, which may relate to the weak collectivity of this shorter hydrocarbon chain surfactant. On the other hand, XPS results showed that the C1s can be fitted to C-C, C-H, C-O, and COO- with the contents of 10.99%, 9.19% and 2.11% on the raw lignite surface, respectively. After the pretreatment, the contents of hydrophobic C-C, C-H were increased to 13.53%, 16.07% and 17.18% for the three surfactants. Obviously, the hydrophobic functional groups were much higher after pretreated by sorbitan monooleate, which was corresponded with the better flotation results. Therefore, the longer and unsaturated hydrocarbon chain in sorbitan monooleate molecule is beneficial for the collectivity and selectivity for lignite.

Effects of hydrophilic groups of nonionic surfactants on the wettability of lignite surface: Molecular dynamics simulation and experimental study

Abundant oxygen-containing functional groups make lignite high moisture content, which reduces the utilization efficiency of lignite. The decrease in hydrophilicity of lignite surface can be achieved by treating with surfactant. In the present work, two kinds of nonionic surfactants with different hydrophilic groups, n-dodecyl β-D-maltoside (C12G2) and dodecyl hepta glycol (C12E7), were selected to modify the wettability of lignite surface by molecular dynamics simulation. Because of the drastic differences in compositions and structure of their headgroups, different behaviors were observed. The adsorption results of simulation indicate that polyhydroxy surfactant, C12G2, adsorbs strongly on lignite surface as a comparison to the poly ether surfactant, C12E7. However, the extent of hydrophobicity of modified lignite surface by these surfactants is inconsistent with their adsorption capabilities. Compared to the raw lignite, the hydrophilicity of lignite significantly decreases by adsorption of C12E7, while the C12G2 makes the lignite even more hydrophilicity. The strong polar oxygen-containing functional groups of lignite surface are covered by ethers in C12E7 with weaker polarity, which weakens the interaction between water and lignite. The hydrophilicity of lignite adsorbed C12G2 is strengthened due to the increase in surface polarity by the introduction of polar hydroxyl groups. The results of simulations are in accord with the available experimental data.

Interfacial water at the low-rank coal surface: an experiment and simulation study

Interfacial water at the low-rank coal (LRC) surface plays an important role in many technological processes, including flotation, slime water settling and dewatering. In this work, molecular dynamics simulations were performed for a series of model systems consisting of a model surface of Wender lignite and 100–600 adsorbed water molecules. A number of equilibrium properties, including density profiles, radial distribution functions (RDFs) and binding energies, are reported to explain the characteristics of interfacial water. The RDF results demonstrate that the order degree of the water molecules decreased with increasing distance from the LRC surface, although the structure of interfacial water at the LRC surface is substantially similar to that in bulk water. In addition, the results also present that the four groups affecting interfacial water at the LRC surface follow the order of carboxyl > hydroxyl ≫ ether > carbon chain. The density profiles of water molecules are insufficient to give a description of interfacial water in detail, but it can be speculated from the density profiles and RDFs that water molecule adsorption at the LRC surface is preferentially fixed around the polar functional groups when there are fewer water molecules. With increasing number of water molecules, water gathers at the hollows. Finally, the water molecules cover the whole surface of the LRC. The calculated binding energies increase monotonically with increasing number of water molecules, while the unit binding energies show the opposite trend. Furthermore, a common non-ionic surfactant, dodecyl 7-ethoxyl ether (C12EO7), was selected to modify the LRC surface. The hydrogen bonding, density profile and binding energy are reported to provide a microscopic understanding of the behaviour of interfacial water before and after surfactant adsorption. The results indicate that surfactant adsorption reduces the difference between interfacial water and bulk water. The simulation results are in accordance with the available experimental data.

Structure and dynamics of water adsorbed on the lignite surface: Molecular dynamics simulation

Structure and dynamics of water adsorbed on the lignite surface: Molecular dynamics simulation

Further exploring on aqueous chemistry of micron-sized lignite particles in lignite–water slurry: Effects of humics adsorption

The aqueous chemistry of micron-sized lignite particles has essential influences on the dispersion stability of water-lignite slurry. In this work, the adsorption behaviors of dissolved humics on lignite surface in lignite–water slurry were studied through batch adsorption experiments. The influences of adsorbed humics on the surface properties, particle size distribution and dispersion stability of lignite particles in water-lignite slurry were examined by advanced analysis techniques, EDLVO theory and settlement tests. The adsorption results indicate that the adsorption capacity of humics on lignite increases from 4.11 mg·g−1 to 8.25 mg·g−1 with the humics concentration increasing from 100 mg·L−1 to 200 mg·L−1. The adsorption process can be well described by the pseudo-second order kinetic model. The adsorbed humics significantly improves the hydrophilicity and electronegativity of lignite surface and reduces the apparent particle size of lignite. The EDLVO calculation and settlement tests reveal that the total interaction energy of the humics-adsorbed lignite particles at 18 nm of separation distance is enhanced from 0.85 × 10−17 J to 1.45 × 10−17 J with the initial humics concentration increasing from 100 mg·L−1 to 200 mg·L−1. The dispersion stability of micron-sized lignite particles is improved with humics adsorption on lignite surface.

Experimental study on co-pyrolysis and gasification behaviors of petroleum residue with lignite

Co-processing of petroleum residue (deoiled asphalt, DOA) with lignite was investigated in this paper. The co-pyrolysis of three lignite with DOA was studied by a fixed bed reactor and the gasification reactivity of the co-pyrolysis chars was evaluated by a thermogravimetric analyzer. The results showed that no synergetic effect was observed on the char yield during co-pyrolysis of the lignite and DOA, while the DOA melted and stuck to the lignite surface and caused the agglomeration of the co-pyrolysis char. The XRD analysis reflected that the graphitization degree of the co-pyrolysis char’s became higher than that of pure lignite, but the mineral form did not changed. The gasification reactivity of the DOA in the co-pyrolysis char increased greatly with the addition of the lignite. Among the three lignites, Wenshan lignite which is rich in Ca and Fe and poor in Si and Al showed the best catalytic effect on DOA gasification. The co-gasification of lignite char with DOA char showed that only Wenshan char had an obvious catalytic effect on DOA gasification though the catalytic effect is lower than that in the co-pyrolysis char. For the co-processing of DOA and lignite, it’s better to mix the raw materials together before they are pyrolyzed, since the co-pyrolysis char shows higher gasification reactivity than that of the pure char blends.