Hydrophobicity Of Coal Surface Research Articles

Wettability alteration and dynamic wetting behavior of coal during geologic CO2 sequestration using LF-NMR technology

As CO2 has a strong affinity to coal surface, coal seams are considered as an attractive geological formation for CO2 sequestration. Naturally occurring coal seams are often saturated with free water in the cleats as well as moisture. Hence, the wetting behavior of coal by water is an important aspect in the study of coal properties. However, carbon dioxide, which replaces methane on the surface of coal, interacts with water and coal for a long time. Geochemical reactions of CO2-H2O-coal system can change the surface properties and pore structure of coal. Studies on wettability of coal altered by CO2-H2O-coal interaction are rare. This paper explored the wettability alteration and wetting dynamic behavior of two bituminous coals in Huainan and Huaibei coalfield using low-field nuclear magnetic resonance (LF-NMR) combined with CO2-H2O-coal interaction experiments. The results show that dynamic wetting of water in the coal mainly depends on the pores structure. The geochemical reaction changed the pore structure of coal reservoir, further promoted the hydrophobicity of coal surface, and thereby affected the dynamic wetting process of water. Moreover, the wetting of large pores in coal is not significantly affected by the weakening of hydrophilicity, whereas wetting in small pores is obviously affected by the enhanced hydrophobicity, which may be beneficial to the adsorption and storage of CO2 in the coal.

Revisiting the enhanced selectivity in low-rank coal flotation by composite collector comprising kerosene and sorbitan mono oleate

In this study, the impact of KS collectors (blends of kerosene and span80) on flotation selectivity and the underlying mechanism were investigated to comprehensively understand the non-ionic composite collectors in low-rank coal flotation. Results demonstrate that Gaudin's selectivity indices are in the range 3.19–3.61 for kerosene at dosages in the range 2–10 kg/t, but 6.58–10.81 for KS collectors at dosages in the range 2–6 kg/t. This improved flotation selectivity was because the increase of gangue recovery was overshadowed by that of combustible recovery with the KS collectors. Results also revealed that, owing to span80, coal surface hydrophobicity was enhanced, thereby improving the true flotation recovery. Meanwhile, as a result of strengthened froth stability by span80, both water recovery and the degree of entrainment increased, resulting in the increased gangue recovery. In addition, proper usage of span80 for low-rank coal flotation was also highlighted in this study.

Changes in Surface Hydrophobicity of Coal Particles and the Formation of Coarse Particle–Bubble Clusters in the Process of High-Intensity Conditioning

The mechanism of high-intensity conditioning (HIC) has not been thoroughly revealed, and therefore this work investigates the effect of HIC on the surface hydrophobicity of coal with different particle sizes and the possible formation of particle–bubble clusters. The results show that different HIC conditions are required for coarse and fine particles. Coarse particles (+75 μm) require a higher turbulence intensity to increase collector dispersion, thereby increasing the adsorption of the collector. Fine particles (−75 μm) require a lower turbulence intensity to reduce the desorption of the collector. In this study, the optimum HIC conditions for coarse and fine particles are “2200 rpm + 1 min” and “1300 rpm + 1 min”, respectively. Interestingly, it seems that the adsorption capacity between fine particles and the collector is weaker than that for coarse particles. A non-enclosed HIC system produces up to 1.78 × 104/g bubbles in coarse particle–bubble clusters, and the mean bubble diameter is approximately 87 μm. The cluster achieves pre-mineralization and increases the apparent particle size, which is expected to improve flotation.

Qualitative surface characterization of Indian Permian coal using XPS and FTIR

ABSTRACT This paper reports on the surface properties associated with high-ash Indian Permian coal, which primarily depend on its geochemical compositions. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) techniques have been used to assess surface properties. The results confirm the presence of different elements, mineral phases, and organic functional groups in coal. The XPS survey confirms the presence of different elements on the surface of the coal. Some of them are C 1s (286.0 eV), O 1s (534.5 eV), N 1s (399.9 eV), S 2p (158.2 eV), such as SiO2 and Al2O3, which correspond to Si 2p (102.7 eV) and Al 2p (75.6 eV) with other clay minerals. XPS spectra of C 1s confirm the presence of aliphatic and aromatic carbon, which are hydrophobic compared to hydrophilic groups such as alcoholic, carboxylic, and phenolic. FTIR also validates the existence of hydrophobic functional groups as being higher than hydrophilic groups. Proximate and ultimate analysis results also confirm the presence of less moisture and a higher carbon percentage in the coal samples. Hence, the inherent hydrophobicity of the coal surface is excellent, and the surface hydrophobicity of coal can also be characterized using XPS and FTIR techniques. This information about the coal properties will be very beneficial for coal beneficiation and utilization.

Insight into the low-rank coal flotation using amino acid surfactant as a promoter

Low-rank coal flotation faces challenges due to its poor surface hydrophobicity. Massive oily collector, e.g. kerosene and diesel, are needed to improve its flotation performance, causing huge cost input. This study experimentally demonstrated that Sodium Cocoyl Glycinate (SCG), an amino acid surfactant, could help with obtaining higher efficiency when adopted in low-rank coal flotation. Results showed that compared with the condition using kerosene (5000 g/t) alone, a tiny amount of SCG (50 g/t) increased the concentrate yield by about 12%, while its ash content roughly remained the same. Effects of SCG on low-rank coal flotation was established from coal surface hydrophobicity, solution surface tension and the forth layer. The amphipathic groups of Sodium Cocoyl Glycinate would bond both with the hydrophilic and hydrophobic sites on the coal surface through hydrogen bonding, leaving the hydrophobic end exposed, which shorten the bubble-particle attachment time to about 100 ms. The decrease of solution’s surface tension was conducive to the emulsification of kerosene, making droplets diameter distribution into a smaller range of 0.26 to 10 μm. Meanwhile, lower surface tension offset the negative impact of oily collector, providing a more stable froth layer. Considering its hypotoxicity, biodegradability, environmental friendliness, Sodium Cocoyl Glycinate could be a green flotation promoter assisting with the conventional oily collectors.

Surface physico-chemistry governing microbial cell attachment and biofilm formation on coal

An essential but missing component in our understanding of microbial interactions leading to coal bed methane (CBM) formation lies in the relationship between coal and coal-adhering microorganisms. Here, we explored the influence of surface physico-chemistry on the microbial cell attachment on coal, utilising a known coal-oxidising bacterium P. fluorescens Pf-5 as a model. Based on the interfacial free energy of adhesion calculation and direct observation of cell adhesion on coal, natural lignite and subbituminous coals offered the most conducive surface for cell attachment driven by favourable hydrophobic and acid-base interactions. Coal pretreatments dramatically decreased coal surface hydrophobicity and increased its electron-donating potential (γ−), resulting in unfavourable interaction with cells that were hydrophilic and possessed high γ−. An exception to this was peroxide treatment, which maintained a low γ− of the coal surface and allowed favourable coal-cell interaction to occur. Cell colonisation on lignite appeared to be the strongest amongst all coal types, likely due to the relatively high van der Waals surface energy component that influence the adhesion strength in favourable cell-coal interactions. This would partly explain the higher methanogenic potential found in lignite compared to subbituminous coal in previous studies, as more adsorbed cells potentially lead to greater liberation of small organic compounds. Although our cell enumeration results generally correlated with the thermodynamic data, some exceptions were present that signify the biological role in cell-to-coal attachment and biofilm formation. Overall, these findings provide insights into the fundamental parameters that are involved in cell-coal adhesion dynamics at the molecular level.

Chemical and physical modification of low rank coal floatability by a compound collector

Effective collectors are essential for improving coal floatability. In this investigation, contact angle, X-ray photoelectron spectroscopy (XPS), optical micrograph, field emission scanning electron microscope (FE-SEM), and flotation tests were used to characterize chemical and physical modification of low rank coal floatability by a compound collector consisting of coal tar and diesel. XPS showed that the C-C/C-H content on coal surface after compound collector adsorption was higher than that on coal surface after diesel adsorption. Optical microscope image results showed that compound collector produced more flocs than diesel, which increased flotation efficiency. Contact angle results showed compound collector enhanced coal surface hydrophobicity more significantly than diesel. FE-SEM images illustrated that coal surface after compound collector adsorption was rougher than that after diesel adsorption, which accelerated the rupture of liquid film between coal particle and bubble. Therefore, coal floatability after compound collector adsorption was much better than that after diesel adsorption.

Role of dodecane on coal particle-bubble interaction in aqueous phase

Even if coal particles are hydrophobic, interaction between these particles and air bubbles can be enhanced by nonpolar oil in froth flotation. The role of a nonpolar liquid dodecane on coal particle-bubble interaction in aqueous phase was investigated and compared to that of a solid analogue, paraffin wax via a combined application of water contact angle and particle-bubble stability measurements, and molecular dynamics simulations. It was found both dodecane and paraffin wax can increase hydrophobicity of coal surface and attachment stability of coal particle-bubble aggregates when they covered the coal particles. Despite similar chemical structure and hydrophobicity, dodecane was more effective than paraffin wax. Molecular simulations confirmed that, comparing to the unmodified coal particle, both dodecane and paraffin wax models could increase the attachment strength of coal particle. However, different from immobile solid paraffin wax, we demonstrated that the liquid like properties of the dodecane layers present on the coal particle surfaces play an essential role in the particle-bubble interaction. When the coal particle covered by dodecane arrived at gas−water interface, the dodecane molecules will migrate and rearrange to form a new film at this interface. In addition to hydrophobic attraction of the gas bubble, the dodecane film will provide additional adhesion towards the coal particle due to oleophilicity of the coal particle, enhancing the stability of bubble-particle aggregate.

Comparison of pyrolysis and oxidation actions on chemical and physical property of anthracite coal surface

Hydrophobicity of coal surface is determined by its physical property and chemical composition. Once coal was oxidized or spontaneously ignited under natural conditions, the surface properties would change greatly, which inevitably affect the hydrophobicity of coal surface. To reveal the changing regulations of coal surface’s hydrophobicity after oxidation and pyrolysis treatment, in this paper, two conditions (pyrolysis and oxidation) of heating treatment were used to produce a series of coal surfaces, and then characterizing their physical and chemical properties through SEM and XPS. Meanwhile, induction time and flotation tests were performed to measure the hydrophobicity of coal. Results showed that the pores and cracks were well developed on the coal surface after both oxidation and pyrolysis processes. But they contributed to opposite aspects later in the flotation test. Additionally, the content of oxygen-containing functional groups on the surface decreased after pyrolysis, whereas having a sharp increase after the oxidation process, which predicted that the hydrophobicity of coal surface under two heating conditions would be completely different. Coal after pyrolysis owned a very short induction time, and its flotation recovery was high. For oxidation process, the coal surface became more hydrophilic and their induction time could reach at about 4000 ms, very hard to float.

Flotation of low-rank coal using sodium oleate and sodium hexametaphosphate

A mixture of low-ash coal and gangue particles was used to investigate the effects of sodium oleate (NaOL), kerosene, and sodium hexametaphosphate (SHMP) on low-rank coal flotation. X-ray photoelectron spectroscopy etching revealed that NaOL was adsorbed on the coal surface effectively. Furthermore, zeta potential results confirmed the adsorption of NaOL and SHMP on the coal surface and gangue surface. The yield of clean coal increased sharply with increasing NaOL dosage, while the clean coal ash content initially decreased with increasing NaOL dosage and then increased. Compared to the use of kerosene alone, the addition of 3000 g/t sodium oleate can increase the yield of clean coal by about 50%. Compared to treatment without SHMP, the ash content of clean coal can be reduced by about 8%, the volatile matter and fixed carbon content clean coal can be increased by about 3% and 5% than using 500 g/t SHMP. The fixed carbon content of clean coal decreased slightly with the SHMP dosage above 500 g/t. The contact angle results showed that NaOL enhanced the hydrophobicity of coal surface, while SHMP did not significantly affect coal surface hydrophobicity. SHMP increased the electronegativities of both coal and gangue, and enhanced the electrostatic repulsion between the coal and gangue particles. Scanning electron microscopy/energy-dispersive X-ray spectroscopy showed that the SHMP reduced the amount of gangue particles coated on the coal surface, which reducing the ash content of clean coal. The attachment time results showed that the coal particles adhered to the bubbles and agglomerated easily, which greatly increasing the carrying capacity of single bubble to the froth layer and enhancing low-rank coal flotation in the NaOL system. This study can recover combustibles from coal slime waste, which can not only reduce the environmental pollution of slime, but also increase economic benefits.

Investigation of nonylphenol ethoxylate on the surface characteristics of low rank coal

The surface characteristics of sub-bituminous coal have been determined in the presence of nonylphenol ethoxylate with 9 ethylene oxide groups (NP-9). The surface characterization techniques of zeta potential measurement, contact angle measurement, and surfactant adsorption have been used. Addition of NP-9 causes the zeta potential profiles to become less negative resulting from the surfactant molecules shielding the negative charge of the sub-bituminous coal. Changes in the sub-bituminous coal surface hydrophobicity are assessed using contact angle measurements. As the concentration of NP-9 gradually increases, the contact angle first increases and then decreases with the maximum contact angle at about critical micelle concentration (CMC) of NP-9. FTIR and XPS results show that oxygen functional groups on the low-rank coal surface were significantly covered by the NP-9, which indicates more hydrophobicity for the low-rank coal after adsorbing surfactant. The surfactant adsorption has been investigated as a function of concentration and temperature. The adsorption isotherms are well fitted by the Langmuir equation. The adsorption data are utilized to calculate thermodynamic parameters. The thermodynamic functions indicate that the whole adsorption process is exothermic, being driven by enthalpy-entropy synergically.

Density Function Study of the Interaction of a Surface Modifier with the Oxidized Coal Surface Model.

A density function approach has been used to screen an appropriate surface modifier for oxidized coal to enhance its hydrophobicity in a flotation process. Two oxidized coal surface models, coal–COOH and coal–COONa, based on the substitution of 10-fused benzene rings with COOH and COONa functional groups, have been constructed to mimic the surface hydrophilic sites at acidic and alkaline pHs, respectively. A nonpolar molecule and five polar candidate molecules with different functional groups have been examined on each oxidized coal model surface. Our present study indicates that octane is ineffective toward increasing the surface hydrophobicity for both coal–COOH and coal–COONa models due to its preferential adsorption on hydrophobic aromatic sheet, although it can spontaneously bind to the coal model surfaces at 298 K. Unlike octane, 4-pentylpyridine will present the preferred hydrophobic conformation on both models. However, its adsorption process is favorable energetically only on the coal–COOH model. The optimized geometries of all four oxygen-containing molecules (1-methoxyheptane, 1-octanol, octanal, and octanoic acid) show that directional hydrogen bonds will be formed between their oxygenated groups and the COOH group of coal–COOH model. This results in the protrusion of the hydrocarbon chain toward the water phase, which is beneficial for increasing coal surface hydrophobicity. However, the calculated Gibbs free energies suggest that octanoic acid is the best candidate. The adsorption of all four oxygen-containing molecules on the coal–COONa model is a spontaneous process. However, only sodium octanoate can be regarded as the effective surface modifier according to its optimized adsorption conformation at alkaline pH.

Surface physicochemical properties of semi-anthracitic coal from Painan-Sumatra during air oxidation

Painan coals of West Sumatra were selected as semi-anthracitic coal sample for studying the physicochemical properties such as measurement, evaluation and description of the changes of surface characteristic of coal sample and their oxidation in the atmospheric air at a temperature ranging from 105 to 400 °C for 30 min. Several methods are adopted to analyze and discuss several phenomena of the oxidized Painan coal surface during oxidation process for the change in the physicochemical properties as determined by Atomic Force Microscope (AFM), contact angle, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Thermal Analysis (TG–DTA) analyses as well as other supporting analytical equipment. AFM analyses revealed some changes in adhesion force and surface morphology with more adhesion force available between 0.6 and 8.6 nN on polished coal surfaces due to the increased oxidation temperature. The study revealed that the extent of hydrophobicity of coal surface decreased with the increased of oxidation temperature expressed as contact angles at about 80° and 20°. Another phenomenon occurred during the experiment was hydrophilicity index of coal surface increase at approximately 1.3 and 2.9. Oxidation of coal that occurred with increased temperature also indicated an increase in oxygen content from 3.8% to 22.9 wt%. Increased oxygen functional group also noted that oxidation of coal took place during the treatment. We also found that oxidation treatment also affected the combustion properties of coal: decreasing ignition temperature between 452.9 and 317.6, lowering the reactivity of coal at maximum combustion rate temperature, and reflecting their char characteristics as burnt out, ranging from 652.3 to 648.5 °C.

Analysis of coal wettability by inverse gas chromatography and its guidance for coal flotation

In coal flotation, the hydrophobicity of fine coal is a crucial factor that affects coal flotation recovery and performance. How to accurately characterize the surface hydrophobicity of coal becomes a research hot spot in coal science. This paper used a novel characterization technology, namely inverse gas chromatography (IGC) to characterize the wettability and hydrophobicity of fine coal particles by analyzing their surface energy. Five coals with different coal ranks were used as coal samples in this investigation, and the flotation results of five coal samples were obtained under the same flotation conditions. Throughout this paper, a good correlation between flotation recovery and surface energy was found and the hydrophilicity index of coal was calculated using the ratio of the specific component of surface energy to the total surface energy. A higher hydrophilicity index of coal leads to a lower flotation recovery. The flotation recovery is not governed by the coal rank but is determined by the surface energy of coal particles, i.e. the hydrophilicity index and the work of adhesion. The IGC is demonstrated to be a potential method for predicting the flotation recovery of fine coal in the coal preparation industry.

Effect of middle-temperature pyrolysis on the surface hydrophobicity of sub-bituminous coal

ABSTRACTMiddle-temperature pyrolysis is widely used to convert sub-bituminous coal into gas/liquid products and the coal char, which benefits the utilization of low rank coal resources. However, the coal char usually contains high-ash content because the volatile components in coal release from coal particle forming gas/liquid products while most of high-ash mineral components remain in the coal char. Therefore, the upgrading of the coal char is usually required to meet the requirement of calorific value for burning. It is necessary to find out the effect of middle-temperature pyrolysis on the surface hydrophobicity of coal. In this study, the effects of pyrolysis temperature (700, 800, and 900°C) and pyrolysis time (30 and 90 min) on the surface hydrophobicity of sub-bituminous coal were comprehensively investigated. X-ray photoelectron spectroscopy (XPS), attachment time, and flotation tests were used to reveal the changes of surface hydrophobicity and floatability of sub-bituminous coal before and after middle-temperature pyrolysis. The XPS results indicated the content of hydrophilic oxygen-containing functional groups was reduced while the content of hydrophobic functional groups on coal surface was increased after the pyrolysis. The attachment time of coal particle-bubble was reduced while the flotation recovery of coal was increased after the pyrolysis. The surface hydrophobicity and floatability of sub-bituminous coal were enhanced by middle-temperature pyrolysis, which makes the upgrading of the coal char feasible.

The role of surface cleaning in high intensity conditioning

Slime coating, defined as a layer of fine or ultrafine hydrophilic particles coated on the larger value mineral surface, has been recognized to have an important effect on froth flotation. These slimes on the mineral surface prevent value mineral particles from contacting with collectors and/or air bubbles, consequently resulting in a lower flotation recovery. It is demonstrated that the high intensity conditioning is an effective way to improve flotation performance. However, the relative significance of its mechanisms on high intensity conditioning still needs to be explored, especially the surface cleaning. In this work, the role of surface cleaning in high intensity conditioning was investigated by carrying out a series of tests including conditioning-flotation, microscope visualization, laser particle size analysis and Focused Beam Reflectance Measurement. The combustible matter recovery is employed to evaluate the flotation performance. The results show that the high intensity conditioning greatly improves the hydrophobicity of coal surface by producing a complete clean surface and consequently enhances the adhesion between coal and collector, which results in a significant increase of the combustible matter recovery no matter with or without the collector. It is also found that surface cleaning is an important factor in the reduction of the reagent consumption. In addition, it is noted that surface cleaning is the internal driving force for the significantly increased recovery of the coarse coal particles. Based on the findings in this work, a prospective high intensity conditioning-flotation process is presented.

Enhancing flotation cleaning of intruded coal dry-ground with heavy oil

Intruded coal is one type of unconventional coal resources in many coal fields of China. The research into the flotation of intruded coal has not received enough attentions. Plenty of intruded coal fines are produced during coal mining, screening and gravity separation. However, the intruded coal fines are difficult to float using common oily collectors because the intruded coal surface is porous and naturally hydrophilic. In this investigation, heavy oil was used as a collector associated with dry-grinding process to enhance the flotation recovery of intruded coal. The flotation recovery of intruded coal dry-ground with heavy oil was 82.40% which was higher than that (71.28%) of coal dry-ground with kerosene. The mechanism of heavy oil enhancing the flotation recovery of intruded coal was fully discussed by a combination of Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), attachment time measurements. FTIR results showed the heavy oil contained more unsaturated hydrocarbon and oxygen containing groups than the kerosene. The coal-bubble attachment was greatly improved because the attachment time was reduced after the dry-grinding with collectors. The intruded coal surface hydrophobicity was effectively improved by heavy oil through the adsorption of heavy oil drops, which made bubble catch intruded coal particles readily. In addition, the pores of intruded coal surface were filled by plenty of crushed coal fines during the dry-grinding process, which enlarged the contacting area for coal-bubble attachment.

Applications of micro-FTIR technique in studying hydrophobicity of coal

This paper investigates the application of micro-FTIR technique in predicting hydrophobicity of coal inferred from contact angle measurements. Two ranks of coal were selected: Pennsylvanian high-volatile bituminous coal from the Illinois Basin, USA (Ro 0.59–0.63%) and Cretaceous medium-volatile bituminous coal from British Columbia, Canada (Ro 1.19–1.50%). Sessile drop and captive bubble techniques were used to measure contact angle on coal surfaces, and those surfaces were then analyzed with micro-FTIR to correlate contact angle with chemistry. In addition, an image analysis system was used to add the maceral composition effect on chemistry and hydrophobicity. The results show that low- and high-rank coal samples have distinct and opposite trends between functional groups semi-quantitative ratios and contact angle, reflecting rank-dependent effects of chemistry on coal surface hydrophobicity. This work demonstrates that micro-FTIR can be a valuable tool to investigate the hydrophobicity of coal and shows that tracking correlations between chemical groups on the surface of individual macerals, and relating them to hydrophobicity is an effective way to build understanding of the effects of coal chemistry on flotation.

Optimization of grinding pretreatment for effective flotation of oxidized coal

ABSTRACTOxidized coal is difficult to float with common oily collectors because oxidation processes occurring on the coal surface reduces the surface hydrophobicity of coal. Grinding pretreatment has been widely applied in the improvement in the floatability of oxidized coal. However, there is no sufficient investigation into the optimization of grinding conditions for the effective flotation of oxidized coal. This paper is to reveal the effects of grinding conditions, such as grinding time, pulp density, and feed amount on the flotation behavior of oxidized coal. The optimum grinding conditions are as follows: the feed amount is 70 g, the pulp density is 40%, and the grinding time is 15 min. Under the optimum grinding conditions, 62.53% combustible matter recovery with 12.99% clean coal ash content is obtained.

Aggregation Mechanism of Particles: Effect of Ca2+ and Polyacrylamide on Coagulation and Flocculation of Coal Slime Water Containing Illite

Illite is one of the main components in coal slime water and it sometimes makes the water extremely difficult to clarify. In this study, the aggregation mechanism of coal and illite particles was investigated using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and settling experiments of slime water containing coal and illite. The results show that electrostatic energy plays a dominant role and manifests repulsive force in the long-range (>4 nm). However, the leading role becomes a hydrophobic force determined by the polar surface interaction energy in the short-range (<4 nm). A coagulant (Ca2+) can lower the surface electric potential to make all particles easier to coagulate, while the surface hydrophobicity of coal and illite determines whether the particles aggregate. Cationic polyacrylamide (CPAM) can promote the sedimentation of particles flocs effectively and makes the supernatant clearer due to attractive electrostatic forces while anionic polyacrylamide (APAM) flocculates particles through the bridge mechanism. Although the hydration shell on the hydrophilic surface of illite appears to be harmful for either coagulation or flocculation, illite can accelerate sedimentation when it is attached to the coal due to its higher density.