Ultrafine Coal Research Articles

Separation density prediction of geldart A– dense medium in gas-solid fluidized bed coal beneficiators

Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) process is a crucial dry coal beneficiation fluidization technology. The work employs the GFBCB process alongside a novel Geldart A– dense medium, consisting of Geldart A magnetite particles and Geldart C ultrafine coal, to separate small-size separated objects in the GFBCB. The effects of various operational conditions, including the volume fraction of ultrafine coal, the gas velocity, the separated objects size, and the separation time, were investigated on the GFBCB's separation performance. The results indicated that the probable error for 6∼3 mm separated objects could be controlled within 0.10 g/cm3. Compared to the traditional Geldart B/D dense medium, the Geldart A/A– dense medium exhibited better size-dependent separation performance with an overall probable error 0.04∼0.12 g/cm3. Moreover, it achieved a similar separation accuracy to the Geldart B/D dense medium fluidized bed with different external energy for the small-size object beneficiation. The work furthermore validated a separation density prediction model based on theoretical derivation, available for both Geldart B/D dense medium and Geldart A/A– dense medium at different operational conditions.

Mechanistic study of the effects of ultrafine grinding on the surface properties of anthracite coal particles and interface regulation on froth flotation under multifactorial influences

Anthracite, a valuable and scarce coal, typically requires systematic liberation for deep deashing, wherein the particle size reduces to the micron level upon ultrafine grinding. Changes in the physicochemical properties of anthracite particles induced by ultrafine grinding significantly impact subsequent flotation. Accordingly, in this study, a scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDS) system, an Opton IPCAS maceral analyzer, a high-sensitivity microelectromechanical balancer, and an X-ray photoelectron spectrometer (XPS) were employed to investigate the changes in the physicochemical properties of the anthracite particle surface during ultrafine grinding. The flotation efficiency of ultrafine anthracite coal is determined by various factors such as the extent of sample liberation, surface wettability, and the state of inorganic mineral coatings. During the initial phase of ultrafine grinding, the comminution efficiency of the sample is high, leading to a progressive increase in the hydrophobicity. However, suboptimal flotation performance results from inadequate liberation. When the sample is liberated to an ultrafine scale, a higher degree of liberation is achieved. The particle surfaces become increasingly covered with inorganic minerals, accompanied by intensified oxidation, leading to a gradually reduced particle hydrophobicity that impaired the flotation performance. This study advances our knowledge on the mechanism by which ultrafine grinding alters the physicochemical properties of anthracite coal particles and affects the flotation processes. The theoretical framework of this study provides crucial insights for interface regulation in the preparation of ultra-low ash anthracite coal and ultrafine coal flotation.

Levitation of air-borne strong-absorbing nanoparticle clusters dominated by photophorestic force and migration behavior under thermophorestic force

In order to explore the levitation and migration behavior of strongly absorbing nanoparticle clusters in air by using laser technique, in this study trapping and levitating nanoparticle clusters is proposed based on the counter-propagated bi-Bessel beams, and then the clusters are released to observe and analyze their migration behaviors. Two Bessel beams are generated by a conical lens and polarizing beam splitter, arranged horizontally in reverse to form a three-dimensional optical trap. The stiffness of the optical trap can be controlled by adjusting the power ratio of the two Bessel beams. The particles in the levitation chamber are fluidized through weak airflow, and then captured and levitated by a light trap. A high-speed camera is used to record the levitation and migration process of clusters. The particle motion parameters can be obtained through image analysis. The strong-absorbing ultrafine coal particle clusters are first selected to conduct the experiments on their levitation and release migration. Then, the photophorestic force, gravity, buoyancy, drag force, and thermophorestic force acting on the clusters are calculated and analyzed. The experimental and computational results indicate that the photophorestic force of air-borne strong-absorbing nanoparticle clusters generated by laser illumination dominates the levitation; nanoparticle clusters can be stably levitated in a three-dimensional potential well formed by counter-propagated bi-Bessel beams, achieving dynamic equilibrium with gravity, buoyancy, drag, etc. by adjusting the levitation position. The relative instability parameter of levitation is used to evaluate the stability of air-borne strong-absorbing nanoparticle clusters, and the minimum relative instability of ultrafine coal particle clusters reaches 0.075. By analyzing the images of nanoparticle cluster recorded by high-speed camera after being released, the migration motion parameters of the cluster can be obtained, therefore the thermophorestic force acting on the cluster is accurately measured. For the ultrafine coal particle clusters with equivalent particle sizes in a range of approximately 13–21 μm, the magnitudes of their thermophorestic forces are in a range of 10<sup>–11</sup>–10<sup>–10</sup> N. As the cluster size increases, the thermophorestic force increases linearly, which is consistent with the theoretical calculation trend. The use of laser to levitate and release particles provides a novel approach for measuring and analyzing thermophorestic force, and also presents a novel manipulation tool for controlling and transporting particles in a gaseous medium.

The Exploration of the Characteristics of Concrete Incorporating Ultrafine Coal Bottom Ash and Spent Garnet

The building sector is a significant contributor to global carbon emissions, primarily due to the production of cement, a crucial construction material. The need for sustainable building practices has spurred research into alternative materials to traditional cement and sand. Two such materials, ultrafine coal bottom ash (uCBA) and spent garnet (SG), have shown potential as substitutes for cement and fine aggregate, respectively. This research examines the effectiveness of uCBA as a partial replacement for cement and SG as a partial replacement for fine aggregate in concrete, at varying levels of 3%, 6%, and 9% for uCBA and 10%, 20%, and 30% for SG. Initially, the chemical composition analysis was conducted using an X-ray fluorescence spectrometer (XRF), and the particle morphology of uCBA, OPC, and SG was examined through field emission scanning electron microscopy (FE-SEM). Then, the study evaluates the performance of these materials based on slump value, compressive strength, and splitting tensile strength, with specimens undergoing 28, 56, and 90-day curing periods. Results indicate that integrating uCBA and SG in concrete at levels of 3% and 10%, respectively, enhances compressive and splitting tensile strength while decreasing environmental impact. Moreover, workability improves with higher substitution levels. Overall, this research indicates that uCBA and SG hold great potential as replacements for cement and fine aggregate in concrete, providing a means to mitigate the environmental impact of construction.

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.

The increase of the performance of ultrafine coal flotation by using emulsified kerosene and the prediction of the flotation parameters by random forest and genetic algorithm

The increase of the performance of ultrafine coal flotation by using emulsified kerosene and the prediction of the flotation parameters by random forest and genetic algorithm

Fluidization expansion of novel generation dense medium and flow regime transition in gas-solid separation fluidized bed

Gas-solid separation fluidized beds are important for coal cleaning through the removal of ash-forming impurities. Homogeneous fluidization is considered as an ideal separation method because it results in fewer pressure fluctuations and smaller bubbles. And Geldart C ultrafine powder could further intensify the fluidization stability of Geldart A particles. Thus, the present work provides a breakthrough in the density adjustment method in the gas-solid separation field, namely, combining Geldart A magnetite particles and Geldart C fine coal particles as a novel dense medium. The results showed that the addition of ultrafine coal effectively increased the overall expansion of the dense phase by the adhesion of the coal particles on the surfaces of the magnetite particles. To comprehensively understand the difference in the dense phase expansion ratio between the Geldart B/D and Geldart A particles, the flow regime was investigated to determine the transition point of homogeneous expansion using various dense media. The propagation velocities of the shock and continuity waves were analyzed using the theory of elastic systems. A quantitative criterion is proposed to identify the transition point. Based on the error analysis, the available data in the literature and the present work gave an overall in 5 × 10−5 error range compared to the prediction data. Overall, this research provides a comprehensive understanding of homogeneous fluidization characteristics using a novel dense medium and a reliable quantitative transition criterion of the flow regime for Geldart B/D and Geldart A particles in a gas-solid separation fluidized bed.

Separation of pyritic sulfur from ultrafine coal by pyrolysis & wet high magnetic separation

ABSTRACT Coal plays a major role in the economic development of many countries, especially in metallurgical industries and conventional power generation plants. The removal of sulfur from coal has recently become even more critically important. Existence of sulfur compounds in coal limits its industrial applications due to environmental as well as technical problems. Thus, high sulfur coal is usually upgraded by desulfurization through physical, chemical, and biotechnological processes. In this study, lowering of pyritic sulfur took place after thermal treatment at 500–700°C (pyrolysis) using a wet high-intensity magnetic separation process. The samples were subjected to size reduction to −25 µm and characterized with X-ray diffraction (XRD), proximate and ultimate analysis, and differential thermal analysis (DTA). Then, the experiments were performed based on central composite rotatable design (CCRD) and response surface methodology (RSM) after thermal treatment of coal containing 3.30% total sulfur. Statistical analysis was performed to examine the significance of the effect of different operating parameters. A maximum sulfur rejection of 90.10% with the lowest sulfur content of 0.59 was obtained at 8.38% solid concentration, 4.59 g/min feed rate, and 7810 gauss magnetic field intensity. The confirmation tests at the optimum conditions produced 0.54–0.56% total sulfur with 90.25–92.33% sulfur rejection.

Geldart A− particles: Hydrodynamics of Geldart A magnetite powder with modulation by ultrafine coal particles in Gas-solid Fluidized Bed Coal Beneficiator (GFBCB)

The Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) is an important technology used in the dry coal beneficiation process, which requires lower bed density and very uniform density distribution. Geldart A− particles, Geldart A magnetite particles “modulated” by a small fraction of Geldart C ultrafine coal particles, is proposed. Fundamental studies are carried out to provide a comprehensive understanding of its hydrodynamics, including the minimum fluidization velocity, bed expansion, and spatial bed density distribution. By adding the ultrafine coal particles, the minimum fluidization velocity (Umf) is lowered, with a modified Umf correlation proposed, the bed expansion drastically increased (by up to 30%), with more gas being retained in the bed for improved flowability, and the corresponding bed density decreased to 1600–2000 kg/m3, with more uniform distribution, all contributing to a more desirable separation environment for coal beneficiation.

Efficient Separation of Ultrafine Coal Assisted by Selective Adsorption of Polyvinylpyrrolidone

The efficient separation of ultrafine coal is a challenging process due to the ultrafine particle size and gangue entrainment. In this study, a polymer, polyvinylpyrrolidone (PVP), was introduced as a regulator for ultrafine coal beneficiation. The addition of PVP improved the combustible recovery of clean coal and decreased the ash content. This effect was also presented by the selectivity index. The regulation mechanism of PVP was investigated using diverse methods. The adsorption tests performed demonstrated the adsorption amount of PVP on coal, kaolinite, and quartz, which were related to the increase in the separation efficiency. A zeta potential analyzer was employed to elucidate the effect of PVP on the electrical properties of ultrafine particles. The results revealed that the electrokinetic potential of mineral was sensitive to the varying PVP concentration. The particle size distribution was observed to value the influence of PVP on the particle behavior, which was tested by a laser particle size analyzer. X-ray photoelectron spectroscopy was used to investigate the surface elemental compositions of coal, kaolinite, and quartz, which were regulated by the adsorption of PVP. This research is beneficial to understanding the role of PVP as regulators and provides a basis for the efficient separation of ultrafine coal.

Compressive and Flexural Properties of Ultra-Fine Coal Gangue-Based Geopolymer Gels and Microscopic Mechanism Analysis.

Geopolymer gel that possesses advantageous features of fast setting, high strength, and good durability is increasingly used in civil engineering, including rapid retrofit projects, roadway, and other construction projects. Furthermore, geopolymer gel is also a green and economical material as it derives from solid wastes. In this study, activators with different sodium silicate modulus and alkali content were used to activate ultrafine coal gangue and slag powder to prepare coal-gangue-based geopolymers with high strength. To study the influence of slag powder content, sodium silicate modulus, and alkali activator content on strength, a two-stage design was adopted. In the first stage, the orthogonal test with three factors and four levels (10–40% slag, 0.4–1.0 modulus, 16–22%) was used to obtain the influence of each factor on the strength and select the design range of the specimen mix ratio with higher strength. In the second stage, based on the orthogonal experiment, the scope was narrowed to continue to find the optimal excitation scheme and the relationship between the influencing factors and strength. Further, mineral compositional, microstructural, functional group and elemental analyses were performed using X-ray diffraction technique, IR infrared diffraction, electron microscope observation and energy spectrum analysis to elucidate the mechanisms of the strength development. The results show that the factors affecting the geopolymer’s strength were in the order of slag content > alkali content > modulus. The optimum dosage of alkali activator was 18–20%, and the sodium silicate modulus was 0.6–0.8, and the compressive and flexural strength could reach above 40 MPa and 5.9 MPa, respectively. The compressive strength and modulus were in a parabolic relationship. Three types of cementing gels (N-A-S-H, C-A-S-H, and C-N-A-S-H) that were characterized with dense structure and high strength were identified from coal gangue and slag powder after alkali excitation.

New Process for Recovery of Ultra Fine Coal by Pre-Treatment with Surfactant

New Process for Recovery of Ultra Fine Coal by Pre-Treatment with Surfactant

Effect of Diesel Oil and Mixture of Alcohol-Glycol Ether on Colombian Ultrafine Coal Cleaning Using a Test-Rig Closed-Loop Flotation Column

A test-rig closed-loop flotation column was used to observe the effect of diesel oil (collector) and Flomin F-425 (frother) on mass yield and ash content for two Colombian coals: Caypa (northern zone) and Guachinte (southwestern zone). The coal samples of less than 38 µm (-400 M) were processed in a collector concentration range of 0,32 to 1,60 kg/ton of coal, as well as a frother concentration range of 10 to 50 ppm. The response surface methodology was used for the experimental test runs. The results showed that the maximum mass yield obtained by Caypa coal was 98,39% at 1,28 kg of collector/ton of coal and 40 ppm of frother concentration, whereas Guachinte coal obtained a maximum mass yield of 94,71% at 0,96 kg of collector/ton of coal and 30 ppm of frother concentration. In general, for Caypa coal, the mass yield tends to increase (low ash removal) with the collector and frother concentration increase; while the mass yield tends to decrease (high ash removal) for Guachinte coal when the collector concentration increases (low ash removal) at high frother concentrations. It is worth highlighting that the ash content of 0,65% obtained for Caypa coal is the lowest value reported in the literature while employing a test-rig loop flotation column in a single stage, which is considered to be an ultra-clean coal obtained by a physical cleaning process.

Impact of pH on interaction between the polymeric flocculant and ultrafine coal with atomic force microscopy (AFM)

Solution pH plays a vital role in the interactions between polymer flocculants and solid colloid particles which determines the flocculation performance. The influence of solution pH on the flocculation of Magnafloc5250 on ultrafine coal suspension and the interactions between the polymers and solid surfaces were investigated by using atomic force microscopy (AFM), hydrodynamic radius measurements, particle floc size analyses, and flocculation dewatering tests. Our results revealed that the settling velocity and supernatant transmittance of ultrafine coal suspension without flocculant were low in alkaline conditions due to the strong repulsion, which agreed with the extended DLVO theory and direct force measurements by AFM. In the presence of Magnafloc5250, the highest transmittance was observed at pH 4 due to the compressed electric double layer among the coal particles, while the settling velocity reached a maximum value at pH 10, which was consistent with the largest average size of flocs (41.80 µm) and more extended conformation of flocculant. The adhesion force measured by AFM was divided into two parts: flocculation force and bare force. And the flocculation force reached a maximum value with the broadest range of action at pH 10, which supported the settling tests and floc size measurements well. The strengthened bridging interaction (e.g., via hydrogen bonding) with increasing solution pH is attributed to the increase of effective binding sites between more extended polymer chains and coal particles, thus contributing to an enhanced flocculation behavior.

Aggregates characterizations of the ultra-fine coal particles induced by nanobubbles

Particle aggregates induced by nanobubble (NB) bridging has been recognized as one of the main contributors to improve the flotation performance of minerals in NBs-associated flotation. Despite the numerous investigations in particle-NB interaction, scarce research on revealing the properties of aggregates induced by NBs has been conducted, severally limiting the further understanding of the NB-assisted mineral flotation. In this study, NBs were first produced based on the principle of hydrodynamic cavitation. The aggregates of ultra-fine coal particles with different hydrophobicity induced by these tiny bubbles was investigated, with the size distribution, structure, and strength features of the aggregates systematically explained accordingly. In this study, NBs remained stable in the aqueous solution for more than one hour, and the size of these bubbles expanded from 240 to 650 nm within this period. The aggregates of coal particles was significantly enhanced, especially for the more hydrophobic ones, which could be well reflected by the right-shifted particle-size distribution curve, the newly formed chain-structure aggregates, and larger apparent viscosity and shear yield stress of the coal pulp. Moreover, the flotation performance (both flotation recovery and flotation kinetics) of ultra-fine coal particles was remarkedly improved in the presence of NBs, confirming that NBs did benefit the ultra-fine coal aggregates.

Evaluation of polymer binders in briquetting of coal fines for combustion applications

Coal briquetting traditionally makes use of high-dosage, nonpolymeric binders (such as coal tar pitch or molasses) or polyvinyl alcohol (PVA) at low dosages for the binding of fine and ultrafine coal. Due to constraints associated with the manufacturing and cost of PVA-bound briquettes, a novel polyacrylic binder was developed that could possibly be added in low dosages at room temperature. A set of formulations of two polyacrylic components (components A and B) was developed, from which an optimal binder mixture was determined. This formulation comprises 40wt.% component A and 10wt.% component B with a 0.04wt.% cross-linking additive, and the balance water. The uniaxial briquette compressive strength was used to obtain the optimal binder mixture. The binder mixture was added to the coal powder in dosages from 0.1 to 1.2wt.%. Mechanical tests and thermogravimetric analysis in an oxidized environment at a maximum temperature of 930°C were conducted. Binder-bound briquettes were compared to binderless briquettes and PVA-bound briquettes. Briquettes made from the polyacrylic formulation had uniaxial compressive strengths double that of the binderless briquette yield strength, as well as the minimum recommended compressive strength of 0.375 MPa. The organic binders did not affect the combustion of the briquettes, since the results compared well with the binderless briquettes. The polyacrylic formulation is considered to be a possible substitute for PVA in terms of mechanical and thermal properties and raw material cost.

Discrimination of Six Flotation Kinetic Models Used in the Conventional Flotation and Carrier Flotation of -74 μm Coal Fines.

In this study, experimental results of conventional flotation and carrier flotation were characterized by six commonly used flotation kinetic models. Two statistical criteria (coefficient of determination, R2, and root mean square error, RMSE) were used for comparison of fitting performance of different models. All kinetic models tested gave good levels of goodness of fit, but the second-order model with rectangular distribution (model 6) provided the best fitting performance for the experimental data of conventional flotation and carrier flotation. On this basis, two parameters, that is, modified flotation rate constant (Km) and selectivity index (SI), were used to evaluate the difference in flotation separation selectivity between conventional flotation and carrier flotation. Comparisons of Km and SI values indicated that carrier flotation significantly improved the flotation rate constant of combustible materials and flotation separation selectivity of ultrafine coal (−74 μm). In addition, measurements of average bubble size and water recovery indicated that both the coalescence of bubbles and the drainage of liquid in the froth were promoted when coarse coal particles (contact angle >90°) were employed as the carrier to assist the flotation recovery of ultrafine particles, which in turn favored the inhibition effect of the entrainment of gangue materials in carrier flotation compared to conventional flotation.

Reclamation of ultra-fine coal with scenedesmus microalgae and comprehensive combustion property of the Coalgae® composite

Combustion of South African discard ultra-fine coal (i.e. coal dust), charcoal, microalgae biomass, and composites of the three under air were studied. The study involves to find out the effect of Scenedesmus microalgae biomass on the comprehensive combustion characteristics of the ultra-fines. Coal dust is considered as waste material, but it could be modified and combusted for energy. The composites were designed with Design Expert, and unlike blending with the dry microalgae biomass, fresh slurry was blended with the ultra-fine coal and charcoal. Non-isothermal combustion was carried out at heating rate of 15 C/min from 40 – 900 ºC and at flow rate of 20 ml/min, O2/CO2 air. Combustion properties of composites were deduced from TG-DTGA and analysed using multiple regression. On combustion, the interaction of coal-charcoal-microalgae was antagonistic (b = - 1069.49), while coal-microalgae (b = 39.17), and coal-charcoal (b = 80.37), was synergistic (p = 0.0061). The coal-microalgae (Coalgae®) indicated first order reaction mechanism unlike, coal, and the charcoal. Comprehensive combustion characteristics index of Coalgae®, (S-value = 4.52E8) was superior relative to ultra-fine (S-value = 3.16E8), which indicated high quality fuel. This approach to combusting ultra-fine coal with microalgae biomass is partly renewable, and it would advance the production of heat and electricity.
 Key words: coal-dust, combustion, s-value, Coalgae®, renewable.

A novel flotation technique combining carrier flotation and cavitation bubbles to enhance separation efficiency of ultra-fine particles.

In this paper, a novel flotation technique that combines nano-scale bubbles generated by hydrodynamic cavitation (HC) and carrier flotation is proposed to promote the flotation efficiency of a high-ash (43%) ultra-fine coal sample (<45µm). We investigated the mechanism by which cavitation bubbles enhance the separation efficiency of carrier flotation using focused beam reflectance measurements, polarizing microscopy, and extended Derjaguin-Landau-Verwey-Overbeek theory. The carrier particles (polystyrene (PS)) and fine coal were pre-treated in a venturi tube and then floated in a laboratory mechanical flotation cell. The flotation results indicate that the presence of cavitation bubbles significantly improved the carrier flotation performance of high-ash ultra-fine coal. This improvement was attributed to the presence of highly hydrophobic PS, which creates additional gas nuclei in the flotation system. The nano-bubbles, which were produced by the venturi tube and adhered to the fine coal particle surfaces, were conducive to the agglomeration of fine coal particles into large aggregates. Moreover, the nano-bubbles functioned as "bridges" of interaction between the carrier particles and large aggregates of fine coal particles. This paper mainly focused on the effect of carrier (PS) and HC on high-ash fine coal. The influence of different HC intensities on carrier (PS) flotation was discussed. Two models for the interactions between the coal particles, nano-bubbles, and PS during cavitation were proposed and were proved using the E-DLVO theory.

Effect of aggloflotation of coal slimes by use Flomin C9606 as a collector

In many coal preparation concentrators, coal slimes form the major challenge due to difficulties in its processing and upgrading. Several techniques are currently being devised in coal industries to aid in beneficiation of the coals in their finer sizes. Ultra-fine coal particles are exclusively processed by oil agglomeration, selective flocculation or hydrophobic flocculation flotation techniques. In this research, the focus is on hydrophobic flocculation flotation to clean ultrafine coal particles. Collector dosage, agitation stirring speed and grinding fineness batch tests were conducted, followed by flocculation and froth flotation. The results showed that optimum collector dosage (Flomin C9606) was 10 kgs/ton, the impeller speed 1,500 rpm and 30 minutes achieved the best grinding fineness. Two stage flow sheet produced froth recovery of 86.28% and ash content was 6.39%. It was therefore concluded that Flomin C9606, as a collector, was capable of obtaining clean coal product from these results.