Surface Of Coal Particles Research Articles

Solvent recovery from solvent-fine coal slurries by filtration and steam stripping

Dewatering of fine coal is a significant industrial challenge with economic and environmental implications. Due to the lack of suitable dewatering technologies, fine coal particles are often discarded to waste impoundments, leading to substantial loss of valuable natural resources while creating environmental problems. The hydrophobic-hydrophilic separation (HHS) process is a unique solution to this problem. In this process, a recyclable solvent is used to simultaneously remove inorganic impurities (ash) and water from a run-of-mine fine coal slurry. A small amount of recyclable oil (or solvent) is added to a fine coal slurry so that the solvent can spontaneously displace the water from the surface of coal particles. The spent solvent is subsequently recovered and recycled in a closed loop. Here, we report the results obtained using two different solvents, i.e., pentane and hexane, to de-ash and dewater ultrafine coal and recover the spent solvent by filtration, followed by steam stripping. Most of the spent solvent can be recovered during the filtration step at 20 psig N₂ and at a 60 s filtration time. The residual solvent left in the cake was then recovered using steam under different conditions. The results showed that the residual solvent concentration could be reduced to <1,400 ppm after 10 s of steam stripping at 150°C and 15 psig.

Influence of ball materials on the surface activation behavior of coal ash particles during a mechanochemical process

In this study, the surfaces of coal ash particles were activated via a planetary ball mill-assisted mechanochemical process by utilizing balls that were made from different materials such as zirconia, alumina, and iron. The influence of ball materials on particle morphology, crystallinity, chemical structure, and elution of Si and Al ions in alkali solvents were systematically investigated for the first time, during when the impact energy and friction force induced on the coal ash surface was also clarified in details via the assistance of a self-constructed system. It was found that the mechanochemical process performed with zirconia and iron balls featured a preferred “grinding effect” with decreasing particle size and increasing specific surface area. This was due to the efficiently impact energy induced. In contrast, alumina balls featured an “activation effect” by inducing crystal distortion on the surface and thereby, promoting the ion elution within the alkaline solvents.

Research of the coal surface wettability by filtering the liquid through a porous layer

Many technological processes occurred upon the extraction and primary processing of coal (dust suppression, grouting, hydraulic fracturing, wet enrichment, etc.) depend on the wettability of the coal surface, determined by the physicochemical properties of the interacting media. The filtration properties of a fractured-porous coal massif significantly depend on the wettability of the surface. We present the results of studying the wettability of the coal surface with water and its filtration through a layer of coal powder. It is shown that the increased humidity of coal contributes to an increase in the wetting and filtration properties of the coal layer in relation to water due to the creation of a hydrate shell at the contact surface. It is revealed that the method of coal sample preparation significantly affects the functional composition and hydrophilicity of the outer surface of coal particles. Grinding coal in the presence of oxygen in air contributes to the formation of polar oxygen groups (hydroxyl, carboxyl) on the surface of coal particles, which leads to an increase in the hydrophilicity and filtration properties of coal. The results obtained can be used to predict the wettability of coals with process fluids, to improve technologies for mining, enrichment and processing of coals.

Flotation response of a bituminous coal sample in presence of a pyrolitic oil recycled from used car tires

ABSTRACT In this research work, the role of a pyrolitic oil (PO) produced by pyrolysis of used car tires in coal flotation was evaluated using an extended full factorial design. Effects of operating parameters including pulp solid content and the concentration of pine oil, diesel oil, and pyrolitic oil on ash content and yield of final concentrate as process responses were assessed using experimental and statistical analyses. Fourier Transform Infrared Spectroscopy (FTIR) technique revealed that PO consists of hydrocarbons and their hydrophilic oxides. Analysis of variance (ANOVA) results indicated that the main effect of all parameters on flotation responses was significant. Experimental results showed that pyrolitic oil increased both the ash content and total yield through a nonlinear trend in ranges of 5% and 40%, respectively. Nonselective interaction between pyrolitic oil and pine oil molecules, and competitive adsorption of PO and diesel oil on the surface of coal particles were found to be the chief reasons behind the observed curved behavior of flotation measures. The negative effect of PO on ash content of final concentrate was also ascribed to the interaction between hydrophilic groups in PO structure and the oxide nature of noncombustible materials in coal particles.

The effect of inorganic salt modification of sludge on the performance of sludge-coal water slurry

Coal water slurry (CWS) is a coal-based liquid fuel with the characteristics of high efficiency and low pollution. Municipal sludge, as a substantial solid waste, is of increasing attention due to the severe environmental pollution and difficult disposal. In this study, sludge is modified by inorganic salt solution and then used to prepare salt-modified sludge coal water slurry (SSCWS). The results showed that the sludge modified by KCl and AlCl3 can reduce the viscosity of CWS. While low concentration of CaCl2 tends to slightly increase the viscosity of CWS and higher concentration of CaCl2 can reduce the viscosity. It was found that among the three salts used in the experiment, the higher the cation valence state is, the easier and more the dispersant molecules are adsorbed on the surface of coal or sludge particles to improve the dispersion of coal and sludge particles, which was the main reason for improving the slurryability of SSCWS.

Influence of waste brake oil on the rheological properties of coal-sludge water slurry.

In this work, the effect of waste brake oil (WBO) on the dispersion property of coal-sludge oil slurry (CSOS) was investigated and comprehensively analyzed. Apparent viscosity of CSOS decreases from 997 to 276mPa.s with the increasing of SS content from 0 to 12% under the calorific value (CV) of 17.70MJ/kg and oil/sludge ratio (O/S) of 2.0. The combination of sewage sludge (SS) and WBO could decrease the apparent viscosity of CSOS, and SS content and O/S are two important parameters to affect the apparent viscosity. WBO mainly contains the hydrocarbon compound and a lot of oxygenated functional groups according to its GC/MS results. WBO could cover on the surface of SS and coal particle, and reduce their hydrophobic character and the adsorbance amount of dispersant based on the FTIR, XPS and adsorption performance results. In addition, the mechanism of WBO on slurry characteristics of CSOS was investigated. With the low content of WBO, the liquid bridges from WBO force the solid particles to adhere together and form a stable network structure, thus reducing the flow ability of the suspension. With the increasing of WBO, the network structures could be broken into cells, resulting in a dramatic decrease in viscosity. This research provides a potential way to dispose the SS and waste industry oil.

New insight into the role of the emulsified diesel droplet size in low rank coal flotation

Low-rank coal (LRC) is difficult to float using oily collectors due to poor floatability resulted from its abundant oxygen-containing groups. In recent years, numbers of emulsified collectors have been used to enhance LRC flotation. In this study, diesel-Span80 mixed collectors were added to high shear disperser producing emulsified diesel (ED) with a droplet size ranging from 12.51 to 4.58 µm to discuss the effects of droplets size on LRC flotation. The adsorption and spreading of ED on coal particle surfaces was evaluated by total organic carbon analyzer (TOC) and fourier-transform infrared spectroscopy (FTIR). The interaction between the oil droplets and coal particles was conducted using adhesion force measurement system. The flotation results indicated clean product obtained by using ED as the collector had the higher combustible recovery and lower ash content than conventional diesel (CD). It was interesting to find an optimal droplet size (7.68 μm) at which the ED flotation performance was maximized and beyond which the flotation performance deteriorated. The FTIR, adsorption amount and adhesion force results were all well consistent with the flotation results. The effect of ED droplet size on LRC flotation is controlled by ED dispersibility and concentration of adsorbed surfactant molecules on each ED droplet surface, playing a critical role in determining the optimal droplet size which delivers the largest hydrophobicity, maximum adsorption amount, highest maximum adhesion force and hence the best flotation performance.

Study on Dispersion and Mixing Mechanism of Coal Slime Particles in Jet Mixing Flow Field

The jet flow field is characterized by the dispersion and mixing of multiphase flow, which is widely used in the field of coal slime flotation. In this paper, the behavior of coal slime surface modification, the behavior of material suspension and the effect of different jet fluids on coal slurry blending were studied. Based on the action mechanism of the jet flow field, a jet device suitable for coal slime graded mixing was proposed, and the mixing effect of the jet device was tested. The results show that the jet flow field has strong effects on material dissociation and dispersion, and the mixing effect of a single jet is equivalent to that of the laser particle size analyzer’s own agitation device after stirring for 2.5 min at 500 r/min speed. The SEM test of material surface morphology and the changes of Al and Si elements measured by EDS show that the jet flow field can effectively remove the fine mud wrapped on the surface of coal particles. The precondition of material suspension is to have the just-suspended capacity. The critical jet velocity of coal slime suspension is in the range of 6 m/s~9 m/s. The mixing ability of the jet stream has certain limitations. The increase or decrease of the jet height will cause the decrease of the suspension percentage of sampling points in the tank. The gas jet mode can promote the reagent acting on the surface of the bubble liquid film to form oil bubbles, which is more suitable for hydrophobic mineral flotation. The optimal speed of the gas jet is 0.86 m/s, and the shortest cycle period is 1.0 T (T is one material cycle period). The flotation perfection index of 0.5–0.25 mm and less than 0.075 mm coal slime increased by 2.67% and 26.78%, respectively, indicating that the overall idea of the jet mixing device proposed based on the experimental conclusion is feasible.

Bonding mechanism of binder and low-rank coal during carbonization

Low-rank coal is rarely used in the industry of carbonized briquette due to its poor cohesiveness. In order to replace lump coal and utilize low-rank pulverized coal as much as possible in a carbonized briquette process, washing oil residue (WOR) was used as an enhanced binder to enhance the bonding strength of resulted carbonized briquette. The effects of blending ratio and carbonization temperatures on binding strength were investigated, and moreover, a reasonable bonding mechanism was deduced. The results showed that the best crushing strength was obtained when the weight ratio of WOR and low-rank coal is 3:7 at 800 °C, and its crushing strength of M25 (M25) can reach to 97%, while the thermoplastic properties of WOR is thought to be responsible for the obtained good crushing strength, where WOR can be softened and coated on the surface of coal particles during carbonization, and then a coal-binder interface can be formed, resulting in the loose inert coal particles can be combined and the strength of coke is improved significantly.

Insight into the internal relevance of three-phase products in supercritical water gasification of coal

• Internal relevance of three-phase products of coal gasification was investigated. • Insight into the formation mechanism of products in the micro dimension. • Solid-liquid reaction model in coal particle surface was established. • Control steps for decreasing the irreversibility of reaction process was summarized. Supercritical water gasification (SCWG) is a promising technology for coal utilization and has attracted more attentions. This paper is aimed to obtain coal gasification behavior and the internal relevance of products from the perspective of micro level. Evolution characteristics of three-phase products were employed in high heating rate batch reactor. As a result, removal of functional groups in solid particles led to the decrease of the gasification reactivity of char (R coal > R 20s > R 60s > R 100s > R 1200s ). The hydrogen bonds formed by water forced the coal structure decomposition, in which the CO and CO 2 gas were generated from the deetherization and decarboxylation reaction. H 2 and CH 4 primarily came from the collision of free radicals and polycondensation reaction. Liquid intermediate components were detected by high-performance liquid chromatography quantitatively. The content of phenol and naphthalene was up to 280.8 ppm and 3647.4 ppm, and the mutual conversion mechanism of organic matters and polycyclic aromatic hydrocarbons (PAHs) was represented. Effectively controlling the ring growth of PAHs and the polycondensation of solid fragments is the primary goal to inhibit char formation, which is of great significance to decrease the irreversibility of chemical reaction process and realize the large-scale industrial production.

Research on Mechanisms of Improving Flotation Selectivity of Coal Slime by Adding Sodium Polyphosphate

A high percentage of high-ash fine slime materials can deteriorate flotation selectivity through surface covering. This covering of the surface is one of the issues that need to be addressed for efficient flotation processing of difficult-to-separate and high-ash coals. In this study, we investigated the depression effect of SPP on high-ash fine mud by flotation kinetic tests. We also revealed the mechanism of SPP depression of fine slime flotation and enhanced flotation selectivity of difficult-to-separate and high-ash coals by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Atomic Force Microscopy (AFM) analyses. The results showed that under the best condition of SPP dosage, clean coal with 9.75% ash content and 76.76% yield was obtained. Compared to the blank group, the ash content of the clean coal decreased by 2.39%, while the yield was only reduced by 2.18% in the presence of SPP. The reason for this was that the addition of SPP enhanced the stripping and dispersion of the gangue from the coal particle surface. The result was a reduction in the cover of the coal grain surface and an increase in the hydrophobic sites on the coal surface, thereby depressing the non-selective flotation of the gangue and enhancing the adsorption of the collector on the coal surface. The ash content of the flotation concentrate decreased, but the yield remained almost unchanged, which was the main reason for the better performance of SPP as a depressant compared to conventional depressants.

Biomass-coal reburning: Competitive mechanism of gas-solid product activation coal char

Reburning technology combining coal and biomass can effectively reduce NOx emissions and promote carbon neutrality in coal-fired systems. This work is based on the flat-flame burner test bench that decoupled the process of biomass-coal reburning effect on coal in the initial stage. The changes in surface functional groups and carbon skeleton structure of coal char were analyzed using FTIR, XPS, and Raman detection methods. The char's combustion reactivity and NO reduction ability were analyzed based on the micro-reactive fluidized bed test bench. The results showed that biochar and syngas would react with oxygen in the transport medium, and the resulting reactive species would activate the char. The activated effect of syngas is significantly better than that of biochar. The syngas reacts with oxygen while protecting the unnecessary consumption of the coal particle surface, and the generated active substances are enriched on the surface of the coal char to enhance its reactivity. The disordered structure of char was enhanced, and the number of small aromatic ring clusters increased by about 28%. The total alkane group content on the char surface increased by nearly 140%, and the total oxygen-containing group content decreased by nearly 17.8%. The combustion reactivity of char is increased by nearly 44.6%, and the reduced ability of NO is increased by nearly 23.4%. Theoretically, the reburned pulverized coal is reduced by 23.4%.

Effects of nonionic collectors with oxygen-containing functional groups on flotation performance of low-rank coal

This study aims to investigate the influence of nonionic collectors (diesel, methyl linoleate, oleic acid, and castor oil acid) on the flotation response of low-rank coal particles using X-ray photoelectron spectroscopy analysis, particle-bubble adhesion tests, and froth floatation measurements, in addition to density functional theory, molecular dynamics, and grand canonical Monte Carlo simulations. The results of the XPS analysis and the particle-bubble adhesion index showed that methyl linoleate, oleic acid, and castor oil acid could effectively adsorb onto the surfaces of low-rank coal particles and upgrade their surface hydrophobicity. The greatest content of hydrophobic functional group CC/CH and the flotation yield obtained by adding methyl linoleate during the flotation process were 78.88% and 86.95%, respectively. The electron-absorption induction effect of oxygen-containing groups and large π bonds of methyl linoleate considerably improved the wetting process on the surface of low-rank coal particles compared with that of dodecane, which contains a saturated bonding structure. The highest average adsorption capacity was obtained when methyl linoleate was applied to the surface of low-rank coal particles. The results suggest that ester surfactants such as methyl linoleate can substantially enhance the flotation response of low-rank coal.

Study on micro-energy consumption model of ultrafine grinding coal particles

Based on the particle size distribution and surface physicochemical characteristics of coal particles, a micro-energy consumption model of coal particles in the ultrafine grinding process was proposed. This model explained the microscopic mechanical behavior of the ultrafine grinding process by defining factors such as agglomeration between particles and the change in particle surface properties as the microscopic constraint energy of particles. Under the multi-domain fractal law of particle size distribution, the truncated particle size was introduced to eliminate the mutual interference between size grades, and the expression of the micro-energy consumption law in the ultrafine grinding process of coal particles was derived by combining with the surface area theory of macroscopic crushing. Through scanning electron microscopy and infrared spectroscopy analysis, it was confirmed that the evolutionary process of coal particle surface morphology and the fracture law of chemical bonds were in line with the viewpoint of the micro-energy consumption model.

Thermogravimetric analysis of the combustion of Tuva coals before and after their carbonization

The process of combusting Tuva coals before and after their carbonization was studied using the methods of thermogravimetric analysis and electron microscopy. Coal samples were subjected to thermomechanical and elemental analysis, which revealed a higher content of volatile substances in Kaa-Khem coal (47.5 %) compared to Chadan coal (10 %). Following carbonization, a decrease in volatile substances to 11.5 % and 9.3 %, respectively, was observed. The conducted thermogravimetric analysis showed the ignition temperature of the coke residue of the Kaa-Khem and Chadan coal samples to increase by 76 °C and 90 °C, respectively, after carbonization. The burnup temperature of the coke residue after carbonizing (723 °C) Kaa-Khem coal samples remained effectively the same, while the Chadan coal showed an increase from 704 °C to 727 °C. The carbonization of coals was established to decrease the maximum reaction rate from 19 % per min to 10% per min for Kaa-Khem coal and from 26 % per min to 11 % per min for Chadan coal. The process of combusting the coke residue after coal carbonization was found to shift into the region of higher temperatures: from 448–723°C to 524–724°C for Kaa-Khem coal and from 436–704 °C to 526–727 °C for Chadan coal. A morphological analysis of the surface of coal particles after carbonization showed the appearance of larger-size pores and cracks on the surface of carbonates compared to coal before carbonization. The conclusion is made that the content of volatile substances, rather than the developed pore structure, comprises the main factor in improving the combustion characteristics of Tuva coals under the conditions of non-isothermal heating before and after their carbonization.

Research of reagent interaction on induction time during bubble–particle interaction

ABSTRACT This paper focuses on the influence of reagent interaction on induction time in the process of bubble–particle interaction. Induction time instrument was used to investigate the influence of single reagent and reagent interaction on induction time. Meanwhile, the influence mechanism of reagent interaction on coal particle surface wettability, surface tension, and surface functional groups was studied combined with contact angle and FTIR. The results showed that when the collector interacted with the frother, the addition of the frother reduced the induction time and increased the hydrophobicity of the coal particle surface. Compared with raw coal, the surface contact angle of collector-frother-reacted coal particles increased, and its change law was inconsistent with that of collector-reacted coal particles. The larger the contact angle, the lower the surface tension, the higher the interfacial surface tension between coal particles and water, the lower the wettability and the more hydrophobic the particle surface. FTIR spectra indicated that the influence of collector–frother interaction on the surface functional groups of coal particles was mainly reflected in the absorption peaks at 3436 cm−1 and 1400 ~ 1000 cm−1. The effect of interaction varied with the structure of the reagent. 2-Octanol can produce multi-point adsorption on the surface of coal particles and promote the hydrophobic modification of coal particle surface. When ether alcohols interacted with arene, reverse adsorption occurred. Throughout this study, the mechanism affecting the adhesion characteristics of coal particles and bubbles caused by reagent interaction was clarified.

Kinetics and oxidation pathways of Fe3+-catalyzed carbon-assisted water electrolysis for hydrogen production

Carbon-assisted water electrolysis can significantly reduce the electricity consumption of electrolytic hydrogen production, but its slow kinetics limit its further development. In-depth investigation of the oxidation pathways and control steps of coal particles during electrolysis can help to improve the reaction rate in a more targeted manner. In this study, the electrochemical activities of several different ranks of coal, activated carbon, and graphite are compared, and the electrooxidation process is disentangled to investigate the kinetic characteristics of the oxidation of solution catalysts, the reaction of coal with solution catalysts and the electrooxidation of coal. Structural characterization is used to investigate the key factors affecting the electrochemical activity. The results show that coal particles mainly undergo indirect oxidation during electrolysis, and direct oxidation only accounts for a small fraction of oxidation; the oxidation of the solution catalyst on the electrode surface is reversible, and the controlling step in the whole process is the redox reaction between the solution catalyst and coal particles. A variety of oxygen-containing functional groups are generated on the surface of coal particles after electrolysis, among which the C-O group plays a more critical role in the oxidation activity.

A study on the ignition of different metamorphic grade coal particles by microsecond laser pulses in the free-running mode

ABSTRACT The ignition of different metamorphic grade coal particles by microsecond laser pulses (laser wavelength 1064 nm, pulse duration 120 μs, maximum energy per pulse 1.3 J) was studied. Three stages with characteristic ignition thresholds and durations of combustion were revealed. The fraction of the laser pulse energy absorbed by flame and thermal decomposition products of coal particles was determined; reflection and absorption coefficients of coal particles were measured at each ignition stage. Ignition thresholds of coal particles were established taking into account the fraction of laser pulse energy absorbed by flame and thermal decomposition products, and the reflection coefficient. At the first stage, reactive microprotrusions ignite on the surface of coal particles. A model was proposed to describe the processes occurring at the second and third stages of ignition.

Comparative study on distribution characteristics of anionic dispersants in coal water slurry

The anionic dispersants, i.e., sodium lignosulfonate (SLF), naphthalene sulfonate formaldehyde (NSF) and sodium polystyrene sulfonate (PSS), were commonly used to prepare a high concentrated coal water slurry (CWS). In order to reveal the distribution characteristics of the dispersants in CWS system and their adsorption behaviors on coal particle surface, the adsorption experiments, surface tension measurements, contact angle measurements and XPS analysis were conducted in this study. The saturation adsorption amount of dispersants on the coal surface was SLF > NSF > PSS. The fitting results showed that the three anionic dispersants were all monolayer adsorption with a chemical reaction or electron exchange. Surface tension measurements indicated that most SLF molecules were still dissolved in the liquid phase because of the lowest adsorption quantity and packing densities at the liquid-air interface. Contact angle measurements suggested that the hydrophilic groups of the dispersants were stretched to the solution. The adsorbed layer thickness of coal with different dispersants was uniform with the adsorption amount on the coal-liquid interface, which was opposed to the order of the adsorption amount on the liquid-air interface. It was concluded that there were competitive adsorptions between the liquid-air interface and the coal-liquid interface.

Study of the effect of sample preparation on the chemical composition and hydrophobicity of the coal surface

Sample preparation for determination of the chemical and technological properties of coals is carried out using standard methods when coal samples come into contact with air. However, the outer surface of the resulting small particles is inevitably oxidized during grinding by the molecular oxygen present in air. Therefore, the physicochemical properties of the laboratory sample do not match the properties of the original native coal or coal of larger fractions. We present the results of studying the laboratory samples of low- metamorphized coal. Preparation of the native coal sample was carried out in a pure inert atmosphere (nitrogen) of a glove box. Methods of IR- and EPR- spectroscopy, chemical functional analysis of oxygen groups, determination of specific surface area, as well as methods for determining the wettability (determination of the wetting edge angle by a drop of liquid and the rate of absorption of a drop into a coal briquette) were used to characterize changes in the surface of coal particles. It is shown that the method of coal sample preparation significantly affects the functional composition and hydrophilicity of the outer surface of coal particles. The results obtained can be used in determination of the properties of native stratal coals, improvement of the technology of coal sample preparation, as well as in enrichment and processing of coals.