Clean Coal Ash Research Articles

Separation of super clean coal with two-stage cyclones and related characteristics

ABSTRACT To recover super clean coal from raw coal slime of 28.53% ash, in this study, single factor experiment and response surface optimization experiment was used to design and optimize the design parameters of the two-stage cyclones for the separation and recovery of super clean coal. A quadratic mathematical model between the ash of super clean coal and the design parameters of the cyclones was established. Considering the minimum ash content as the optimization index, the following optimal experimental conditions were obtained: minimum ash content could reach 2.55%, which represented 5.35% recovery of the whole sample. The migration law of particles of different sizes, densities in classification and separation process, and the principle of preparation of super clean coal were explored under optimal conditions. Through the separation effect of the second-stage, overflow with particle size ≥ 0.125 mm could effectively recover super clean coal. However, the separation density was increased, and the separation effect was weakened when the particle was small. Therefore, to effectively recover super clean coal, sieving and desliming are necessary.

Selective screening of coal to improve the washability characteristics at different levels of size reduction

ABSTRACT In coal beneficiation, ash content is the main focus area as it plays a vital role in controlling the yield and quality of the washed clean coal. Indian sub-bituminous and bituminous coals are of low-grade with intrinsically distributed ash-bearing minerals, which makes it difficult to segregate the low ash coal. Different stages of crushing and grinding techniques have been used to produce the desired feed size of coal washeries, i.e., −13 mm. During blasting and crushing, cracks propagate through the weaker interfaces within the coal particles. Petrographic analysis suggests the formation of good amount of naturally liberated maceral-rich particles of −13 mm size at different stages. These particles generated at mine site and primary crusher are subjected to further crushing during subsequent stages of secondary and tertiary crushing, resulting in more fine generation and higher energy consumption. Hence, an attempt has been made to check the washability characteristics of coals at different stages of crushing and effect of prescreening of coarse (−13 + 0.5 mm) and fine (−0.5 mm) fraction on washability characteristics of coal and overall clean coal yield. It was observed that −13 mm particles generated at early stage of crushing show good washability characteristics compared to the crushed lump coal (+13 mm) for the separation of 17.8–18% ash clean coal; and hence, prescreening of −13 mm prior to tertiary crushing improves the process efficiency.

Correction of froth gray value in the prediction of clean coal ash content in coal flotation

ABSTRACT The correlation between froth properties and the quality of the froth product is the key to predict flotation performance and improve the automatic control of flotation. It is known that froth gray value increases with the clean coal ash content and is often used in the prediction of coal flotation performance. But the accuracy of ash content prediction is usually unsatisfactory because the froth gray value can be easily interfered by the circumstance. In this study, the average gray value of the black experimental table was selected as the reference block to correct the froth gray value. The inaccurate froth gray value was corrected by the initial gray value and the gray value fluctuation of the reference block. The R2 of the linear fitting between froth gray value and clean coal ash content increased from 0.64 to 0.93 after the correction. This work showed that the correction method can significantly improve the accuracy of flotation performance prediction by gray value.

Sustainability of coal mines: Separation of clean coal from the fine-coal rejects by ultra-fine grinding and density-gradient-centrifugation

Due to the availability of high ash feedstocks and drift origin, Indian coal washeries face tremendous pressure in maintaining the clean coal yield and handling huge quantities of rejects. Since flotation rejects (tailings) discharged in the form of fine-sized (−500 μ) coal-water-slurry, handling and disposal is an environmental concern and it impacts mine sustainability. As conventional methods for extracting clean coal from tailings reported poor yields, a two-stage process i.e. wet-grinding followed by density-gradient-centrifugal separation was explored and optimized for recovering >80% of the carbon values. Clean coal of 19.2%–21.4% ash with >40% yield was obtained by density-gradient-centrifugation of different ultrafine size (−75 μ) coal-water-slurries at 50% pulp density. However, further reduction in clean coal ash by maintaining the yield was limited due to the hindrance in the movement of the high-ash coal particle caused by particle-particle interactions in a thick slurry. Hence, the pulp density of coal-water-slurry was optimized with the addition of extra water, which resulted in 15.1%–17.3% ash clean coal at an average yield of 54–58% from coal tailings having 34.9%–39.3% ash content. Optimization and kinetics of grinding and density-gradient-centrifugation were studied to check the effect of pulp density and other operating conditions. Ash reduction was mainly due to the removal of silica-based materials, where alumina to silica ratio was increased from 0.394 to 0.475. The petrographic analysis showed that the product coal was rich in macerals with ~60% of vitrinites and ~ 30% of inertinites.

On-line prediction of clean coal ash content based on image analysis

Image analysis was adopted to estimate the ash content of clean coal, which contains approximately 3% ash. Seven features were identified based on the gray histogram of coal pictures. The Pearson correlation coefficients showed that most of the features had a good linear or quadratic relationship with the ash content. The polynomial regression (PR) method, polynomial regression after feature selection (PRFS), and the support vector machine optimized by particle swarm optimization (PSO-SVM) were employed for modeling. The PRFS model was selected owing to its best performance on the test data set after 10-fold cross-validation. The model was presented, and an average relative error of 4.16% was obtained for the training data set. Finally, the model was validated by the pilot run as the relative errors ranged from 0.61% to 5.57%, and the predicted ash content values matched the quick ash measurements.

Effective Utilization of Coal Processing Waste: Separation of Low Ash Clean Coal from Washery Rejects by Hydrothermal Treatment

ABSTRACT Handling and disposal of high volume of fine coal tailings is a biggest challenge for coal washeries. Separation of clean coal by density separation and flotation shows limited yields of 23–28% and 13–14% respectively. Chemical leaching process has a challenge with the formation of insoluble salts. The present investigation aimed to separate clean coal by a combination of chemical-physical separation technique, i.e. alkaline hydrothermal process followed by density separation. Hydrothermal extraction of humic substance causes dissociation of coal aggregated structure (Mineral-Maceral-Pore structure), thus improves the separation. Experiments performed as per design of experiments and reduced design of experiments, where 100 µ particle size, 50% (w/w) pulp density, 4.5% KOH (w/w), and 140°C reaction temperature found to be optimum, at which 16.2% ash clean coal at 40.7% yield and humic acid by-product at 18.3% yield were obtained. Use of swelling agent such as tetrahydrofuran shown negative impact on clean coal recovery. Material balance and cost data were presented to show the techno-commercial viability of this technology.

Effects of the operating parameters on the separation results of a three-stage cone water-only cyclone

ABSTRACT In this study, a three-level Box-Behnken design of experiments coupled with response surface methodology was applied to investigate the effects of the operating parameters (solid content of the feed, feed pressure and insertion depth of the vortex finder) on the separation results (ash content and yield of the overflow) of a three-stage cone water-only cyclone. The results showed that the solid content of the feed and insertion depth of the vortex finder impacted the ash content and yield of the overflow positively, while the feed pressure behaved negative affection. The reasons behind the operating parameters as they affected the separation results were explained via simulations. Then, the effects of the interactions of the feed pressure and solid content of the feed on the separation results were evaluated. The affecting degrees of the feed pressure were positively influenced by the solid content of the feed, while such degrees of the solid content of the feed were negatively impacted by the feed pressure. Finally, the optimal operating parameters combination (clean coal ash content was required as 10%) was determined as 10.85% solid content of the feed, 0.056 MPa feed pressure and 49.50 mm insertion depth of the vortex finder, respectively.

Taguchi based Grey relational analysis for optimization of design parameters of a 100 mm water-only cyclone

ABSTRACT Performance of the 100 mm water-only cyclone and its design parameters in cleaning minus 3.0 mm Indian coal was successfully evaluated via Taguchi based Grey-Relational-Analysis (GRA) using L16 (Single-level) orthogonal array experimental layout. The Taguchi method integrated with GRA was used as a performance index in analyzing multiple responses i.e., clean coal yield (%yield), ash content (%ash), and recovery of non-ash (%RNA). The parameters chosen were vortex finder diameter (VFD) and spigot diameter (SPD). The study revealed the subsequent optimum parameter levels: VFD of 50 mm (level 4) and SPD of 22 mm (level 1). The goodness of fit confirmed that using the GRG regression model shows a decent level of the prediction within the range of variables selected (correlation coefficient of 0.84). Experimental results obtained through this approach also showed that the parameter vortex finder diameter has the foremost vital impact on the multiple performance characteristics. Analytical results from two confirmation experiments conducted at the optimal parameter combinations further confirmed that the above-mentioned performance characteristics can be effectively improved through this approach.

Characterization and numerical simulation on preparation of super-low ash clean coal by two-stage cyclones

ABSTRACT This study used two-stage cyclones combined with a full cylinder and a short cone cyclone to recover super-low ash clean coal from the coal slime to solve problems of coal slime such as large production, difficult recovery, and low value-added used. After two-stage cyclones separation and screening, super-low ash clean coal with an ash content of 2.74% was obtained, and the mass yield was 10.56% via a simple and easy method that realized high value-added utilization of coal slime. The separation test indicated that the grading effect caused the first-stage cylinder to remove a large amount of high-ash fine mud in the coal slime and water, decreased the complexity of the particle size, and provided favorable conditions for the density separation of the second-stage cyclone. Through density separation effect in the second-stage cyclone, the recovery of −1.3 g/cm3 density grade in Cyc.O/F is 84.15%, which led to effective coal recovery. Additionally, its cutting density exceeds 2 g/cm3. Computational fluid dynamic simulations indicate that the high turbulence zone corresponds to the main gradation and separation action area and the high turbulence due to the existence of circulating flow, due to its circulating flow, particle separation and grading is obtained.

The use of release analysis to show the selectivity of reagents towards differently sized feeds of the same coal

ABSTRACT Release analysis is used to characterize the cleaning ability of coal fines. It defines the limit of separation at any target level of ash. Since flotation depends upon many factors like feed size and its composition, surface properties, and type of reagents, it is important to characterize the optimum boundary of separation. Release analysis following the BS7503 procedure was used to characterize a − 0.5 mm composite feed and three sized feeds, i.e., −0.5 + 0.25, −0.25 + 0.1, and −0.1 mm using different reagent combinations. Coal response reflected by release curves and corresponding batch flotation results for various reagent combinations was found to be different for all three feeds. No single combination of collector and frother could deliver the maximum separation over the entire clean coal ash range for all three feeds. Significant yields ranging from 35% to 86% could be obtained at the 13% to 17% ash level for the medium volatile metallurgical coal investigated. Release curves and subsequent batch flotation results were found to be sensitive to feed size, feed size distribution, and reagent combination types with their corresponding dosage indicating that the clean coal ash requirement would dictate the application of reagents. The investigation results further show that release analysis results can be reproduced by batch flotation.

Separation of the coal-quartz mixture using tribo-electrostatic separator: Effect of surface pretreatment

The present study illustrates the effect of physical (thermal) and chemical surface treatment on the separation of the coal-quartz synthetic mixture in a tribo-electrostatic separator. The work also reports the effect of applied voltage and splitter position on the tribo-electrostatic separation of surface treated and untreated samples. Different reagents used in chemical conditioning of the synthetic mixture are kerosene, acetic acid, acetone, ethanol, phenol, toluene, and aniline. The study establishes that both chemical and heat treatment improves the tribo-electrostatic separation efficiency of coal and quartz particles. The applied voltage and splitter position affect the product grade and yield of treated and untreated samples. The ash removal and combustible separation efficiency of the synthetic mixture improved after chemical conditioning of the synthetic mixture with ethanol, acetone, acetic acid, and aniline. However, aniline treatment has produced the best separation result. It is possible to achieve 9 and 14% ash clean coal with 40% yield each from 53% ash synthetic mixture at 15 kV applied voltage after aniline and heat treatment, respectively. The ash removal and combustible separation efficiency of the tribo-electrostatic separator for the aniline and heat-treated synthetic mixture are 96, 60% and 90, 70%; respectively.

Improving the yield of ultra clean coal by adding acetic acid and propionic acid in selective agglomeration

ABSTRACTIn this study, ash content lower than 1%, the ultraclean coal, using anthracite coal, is prepared by selective agglomeration flotation. ultraclean coal with lower ash is needed as electrode material and solid fuel, aim of obtaining lower ash content of ultraclean coal, dispersant is usually used to reduce ash content, but the yield will decrease. Therefore, clean coal yield can be further increased by regulator. For this purpose, the effects of stirring speed; dispersant (sodium hexametaphosphate) dosage, kerosene dosage, organic regulator (butyric acid; propionic acid; acetic acid; fumaric acid; maleic acid; ethylenediaminetetraacetic acid) on yield and ash content of clean coal were investigated in the experiments. The analyses of zeta potential, contact angle, x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and Fourier transform infrared spectroscopy (FTIR) were used to illustrate the performance of the process. The results showed that the sodium hexametaphosphate and stirring speed had a significant effect on the reduction of clean coal ash content. Furthermore, adding acetic acid and propionic acid can improve the clean coal yield without increasing the clean coal ash content. Acetic acid was used as regulator, the yield of clean coal increased by 19.42%. At the same time, acetic acid and propionic acid are proposed as a method to produce microbubbles. This important discovery provides an optional method to obtain more target products for fine mineral flotation.

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.

A simple and efficient process for small-scale cleaning of poorly liberated, difficult-to-wash low-rank coal

ABSTRACT Low rank coals are often characterized by high amounts of ash forming and sulfur-bearing minerals, poor liberation characteristics, and difficult washability properties. Low-rank coal of Punjab (Pakistan) inherits all these attributes. In this study, the run-of-mine coal sample was crushed down to 9 mm and sieved into six fractions. Coarser fractions were characterized by float-sink test and ultrafine fraction (−0.25 mm) by release analysis. Laboratory-scale shaking table tests were performed on the coarser fractions to evaluate the effects of table tilt and water flowrate on clean coal yield, ash, and sulfur. Likewise, batch flotation tests were conducted on −0.25 mm size fraction to assess the effects of collector dosage, impeller speed, and frother dosage on the same product-quality parameters. Based on the results of these tests and coal-production rate, a low-capacity processing flowsheet was proposed which can generate concentrate and tailing products suitable for cement and brick industry respectively, thus eliminating the requirements of waste disposal. Economic calculations indicate that the mining industry can earn a profit of approximately 60.9 million rupees by processing coal based on the suggested flowsheet.

Interaction characteristics between diesel and coal with different hydrophilicity: Kinetic and force effects

Collector adhesion and spreading on the surface of coal plays a critical role in increasing the floatability and flotation efficiency of coal. In this work, the interaction characteristics, including kinetic and force effects, between the collector and coal surface with different hydrophilicity was comprehensively investigated using collector adsorption tests, induction time measurements, the atomic force microscope (AFM) colloidal probe technique, and molecular dynamics simulation (MDS). The results showed that the process of diesel adsorption on the coal surface conformed to the pseudo first-order kinetic model. As coal hydrophilicity increased, the maximum capacity exhibited a non-linear exponential decrease while the rate exhibited a linear decrease, in accordance with the induction time of coal particle-oil bubble. Force effects results showed that the adhesion force and acting distance between model diesel and coal decreased as coal hydrophilicity increased. MDS results further showed that hydrocarbon collectors were adsorbed on the hydrophobic sites of coal, but not on the hydrophilic sites. Therefore, the number of hydrophobic sites for non-polar diesel adsorption and the adhesion force between diesel and coal particles decrease as hydrophilicity increases, which in turn causes a reduction in the collector adsorption rate and maximum adsorption capacity. Furthermore, flotation tests were conducted to validate the kinetic and force effects and to indicate that both the combustible matter recovery and clean coal ash content of coal samples significantly decrease from 98.49% to 58.44% and from 3.02% to 2.65%, respectively, with increasing coal hydrophilicity.

Separation performance of mechanical flotation cell and cyclonic microbubble flotation column: in terms of the beneficiation of high-ash coal fines

ABSTRACT Separation performance of mechanical flotation cell and cyclonic microbubble flotation column (FCMC) was compared in terms of the beneficiation of high-ash coal fines that has the characteristics of the content of <0.045 mm accounting for more than 50% of the flotation feed and the ash content of <0.045 mm being more than 50%. The optimal experimental results of the mechanical flotation were determined as the flotation perfection index η of 47.37%, the clean coal yield of 39.19% and the clean coal ash content of 10.78% at the feed pulp concentration of 40 g/L through twice explorative flotation experiments and the orthogonal experiment. It was found that the ash content of the clean coal had a strong dependence on the feed concentration. In contrast, the yield of the concentrate of the column flotation was increased to 43.26% with the lower ash content of 9.47% and the higher η of 54.51% even at higher feed concentration of 60 g/L. The remarkable separation efficient of the FCMC flotation column for the difficult-to-float coal slime that has the characteristics of high content of fine particles with high ash was attributed to the presence of the centrifugal force field, more microbubbles generated by hydrodynamic cavitation, the form of countercurrent mineralization, and thicker froth layer.

Preparation and characterization of silicon nanomaterials with different morphologies

Coal combustion produces a large number of pollutants, leading to serious air pollution. The development of efficient ash removal technology from coal is imminent. Coal is a mixture of carbonaceous organic matter and inorganic mineral ash. Usually, the clean coal and inorganic mineral ash are separated by flotation according to the Hydrophobicity of carbonaceous organic matter and the hydrophilicity of inorganic mineral ash. Hydrophilic silica nanorods we prepared have high surface energy and chemical activity, which is easy to adsorb inorganic mineral ash, and then be floatated by flocculation. In this paper, a series of silicon nanomaterials with different morphologies were prepared by microemulsion method. On the basis of hydrophobic difference between coal and mineral ash, The inorganic ash minerals are adsorbed on the nanomaterials, and then separated by conventional flotation. At the same time, we used XRD and SEM to characterize the silicon nanomaterials.

Beneficiation of coal fines using the Reflux™ Classifier

Reflux™ Classifier (RC), one of the advance gravity separator in coal and mineral processing industries, is a combination of liquid fluidized bed, autogenous dense medium and lamella settler. In the previously published literature, the feasibility of the RC for beneficiating low ash Australian coals was analyzed. However, the sensitivity analysis of RC for various operating conditions was not critically assessed for the high ash coking coal fines (0.25–2 mm). Moreover, the clean coal ash and yield relationship is also not established. The parametric evaluation of the effects of important parameters such as feed flow rate, bed setpoint and fluidization water velocity of RC on the concentrate ash, size and yield is investigated in the current work. The separation performance of RC is described through the partition curves, in terms of separation density (ρ50) and Ecart probable error (Ep). The variation in separation density is also quantified using the slip velocity model. In the laboratory experimentation, the clean coal having ash in the range of 7.9 to 20% was produced from a feed containing around 28% ash content on dry basis. The yield obtained varied from 31 to 64%. It was also observed that the separation density (ρ50) varied from 1.34 to 1.7 and the Ecart probable error (Ep) varied from 0.07 to 0.1, depending on the operating conditions.

Generation of low ash fine clean coal powder by autogenous grinding process powder technology

Crushing and grinding process in mineral processing industry is high energy consuming but it is essential for generating required sized ore with improved liberation, which enhances the process efficiency of physical cleaning of ore. Due to the presence of various minerals in lump of ore, breakage mechanism plays a major role in liberation of different materials, which is solely depends on the type of force applied for crushing and grinding. In conventional crushing and grinding equipment, high impact and attrition forces are applied on the ore lump, which breaks the grain mixture homogeneously, but mineral cleaning enhances when there is a breakage through grain boundaries and not through the grains.Coal is a carbonaceous ore contains maceral and minerals with different properties. Due to the presence of high mineral content, the mineral beneficiation process is adopted where the liberation of inorganic minerals plays key role. As coal is a soft material, use of impact mills generates coal with homogeneous distribution of minerals. In this paper, Autogenous grinding method explored for coal to study the effect on liberation of coal and it was observed that, autogenous grinding helps in improving the liberation of minerals. Experiments also revealed that, the autogenous grinding produces clean coal fines with low ash percentage(16.5% ash clean coal from 24.4% ash feed coals) directly without washing. It was also observed from petrographic study that liberation of coal improved in comparison with conventional grinding process. This is due to the breakage of the coal particles through the coal and mineral boundary.

Integrated estimation model of clean coal ash content for froth flotation based on model updating and multiple LS-SVMs

Integrated estimation model of clean coal ash content for froth flotation based on model updating and multiple LS-SVMs