Clean Coal Yield Research Articles

Interactions between Mg2+-doped kaolinite and coal: Insights from DFT calculation and flotation

In order to explore the micro-influence mechanism of clay surface/interface property differences on coal slime flotation based on impurity defects, the influence mechanism of Mg2+ doping on the microscopic interactions between particles of kaolinite and coal is preliminarily discussed by using DFT (density functional theory) and flotation. DFT results showed that different coal structural units adsorption on Mg(II)_K (Mg2+-doped kaolinite) surface mainly through electrostatic attraction, and the coal structural units can be stably adsorbed on Mg(II)_K surface in the case of competitive adsorption with H2O. When coal structural units and H2O competed for adsorption on Mg(II)_K surface, H2O promoted the adsorption of coal structural units on Mg(II)_K surface. Flotation results showed that with the increase of kaolinite mass percentage in mixed minerals, the clean coal yield significantly decreased, and the clean coal ash firstly decreased and then increased. The influence mechanism of Mg2+ doping on the interaction between kaolinite and coal was that Mg2+ doping significantly enhanced the interaction between Mg(II)_K surface and coal structural unit. The results revealed the complex interaction between fine particles in coal slime, paving the way for its efficient selective separation.

Effect and mechanism of microemulsion on low-rank coal flotation by mixing AEO-3 and diesel oil

ABSTRACT Low-rank bituminous coal has a low degree of metamorphism, a high oxygen content, and poor hydrophobicity. When using a conventional collector, it renders a low yield of clean coal, making it difficult to improve the quality via flotation. In this work, a type of diesel microemulsion (C1) was prepared using primary alcohol ethoxylate (AEO-3), which was adopted to conduct flotation. The results demonstrated that compared with a conventional diesel collector, the yield of clean coal was increased by 44.37%; the content of tailing ash was increased by 37.60%; the combustible recovery rate was improved by 55.65%, respectively; and the flotation kinetic constant was increased by 0.505. The addition of AEO-3 could significantly reduce the surface tension and generate smaller emulsion droplets, which could also adhere to coal particles, easily enhancing their hydrophobicity. An X-ray photoelectron spectroscopy analysis and its contact angle verify a significant enhancement in the hydrophobicity of the coal surface. More importantly, a molecular dynamic simulation revealed that adding AEO-3 could augment the electrostatic interaction between the collector (diesel) molecules and coal particles, as well as reduce the mean square displacement, i.e., the adsorption strength between the collector molecules and coal particles. On the other hand, the microemulsion could improve the hydrophobicity and floatability of coal particles and enhance the wrap angle of particle-bubble. And accelerating the generation of stable mineral flocculation, which was the main reason for the faster flotation process.

Intensification of coal slime separation in fluidized bed flotation column: change of turbulence

ABSTRACT Flotation cell and flotation column are widely used as the separation equipment for coal slime. In order to obtain better flotation results for different particle sizes, different turbulence intensity of flow field need to be created, but the turbulence intensity is not easy to adjust in conventional flotation equipment. In this paper, a new fluidized bed flotation column was designed to control turbulence intensity by the liquid velocity and static bed height. The effects of liquid velocities (0.198 m/s,0.226 m/s,0.254 m/s) and static bed heights (0.317 m,0.343 m,0.380 m) on coal slime flotation with different particle sizes were studied, which were evaluated by the combustible recovery, ash content, and the yield of clean coal. The research results showed that the flotation performance of coarse particles is better with low turbulence caused by low liquid velocity and great static bed height than that in high turbulence. Under the condition of high liquid velocity and low bed height, a high turbulence intensity environment is created, and the fine particle separation efficiency is better than that in low turbulence.

Investigation on mechanism of the oleic acid/methyl oleate/diesel ternary compound collector in low-rank coal flotation

Polar and compound collectors are current research hotspots in the field of low-rank coal (LRC) flotation. However, most studies only focus on the collecting ability of collectors while ignoring its selectivity. This study presents the design for a novel high-efficiency ternary compound collector (oleic acid (OA), methyl oleate (MO) and diesel) with great selectivity, strong collecting and spreading ability for Sujiagou LRC. In comparison to diesel, the clean coal yield is increased by 41.40%, and its ash content is reduced by 0.91% at optimal dosage of 2000 g/t. And we propose some new insights on the mechanism of ternary compound collector through density functional theory (DFT) calculations, focused beam reflectance measurement (FBRM), and molecular dynamics (MD) simulations. OA has the strongest collecting ability with the shortest adsorption distance and smallest frontier orbital gap, which ensures the flotation efficiency and collecting ability of ternary compound collector. The great selectivity of ternary compound collector owing to these two reasons: MO adsorbs more strongly on the LRC surface through the flat-lying adsorption at the junction of hydrophilic and hydrophobic parts; the “wasteful adsorption” on the gangue surface is prevented as the nonpolar parts of ternary compound collector attract polar collectors. Finally, a firm oil film with strong spreading ability is formed on the LRC surface, which can achieve a spread across the hydrophilic area even without the adsorption of polar collectors. The synergistic adsorption not only improves the collecting ability of the ternary compound collector but also ensures its selectivity.

Study on electrochemical improvement of flotation reagents for carbon recovery from coal gangue

ABSTRACT A novel collector to improve the collecting capacity of the coal gangue and to reduce the consumption of the medicament was proposed in this paper, which the potassium permanganate is used as a catalyst to carry out electrochemical catalytic oxidation modification on the kerosene. The effects of electrolysis time, electrolytic current density, catalyst dosage, electrolyte concentration, different catalysts and electrodes on the oxidation of the kerosene were investigated, and the compositions and properties of before and after kerosene oxidation employing FTIR and GC-MS. Results showed that the oxidation effect is the best which the acid value of the product is 7.32mgKOH/g when the electrolysis time is 40 minutes, the amount of catalyst is 500 mg/L, and the electrolytic current density is 2000 A/m2. The flotation results showed that the amount of collector could be saved by 200 g/t dosage and the frother can be saved by 80 g/t dosage by using oxidized kerosene as a collector when combustible matter recovery was 20−25% higher than that of common kerosene and clean coal yield was 8−10% higher than that of common kerosene. The FTIR and GC-MS analysis of the product revealed that some carbonyl compounds and unsaturated hydrocarbons are formed in the product after electrochemical treatment of kerosene, which enhances the collection performance of kerosene. At the same time, alcohol, ether, and ester-creating in the product make the product has been specific foaming performance. This work provides a new reagent for coal gangue carbon recovery.

Enhancement of Flotation Performance of Oxidized Coal by the Mixture of Laurylamine Dipropylene Diamine and Kerosene

Conventional hydrocarbon oil cannot adhere effectively to oxidized coal, resulting in a low yield of clean coal. In this study, a high-speed homogenizer was used to emulsify LDD (laurylamine dipropylene diamine) and kerosene, which enhanced the flotation efficiency of oxidized coal. The flotation results showed an increase from 4.12% (only kerosene) to 23.33% (emulsified oil). An increase in contact angle indicated that the mixture reagent can increase the hydrophobicity of coal particles, which is attributed to the adsorption of LDD onto the coal particle surface and the decrease of the oil droplet A lower surface tension of LDD allows it to produce a stable layer of froth than the layer generated by kerosene alone.

Integrated process for coal chemical demineralization and spent caustic regeneration- A pilot scale study

Indian coal washing plants face tremendous pressure in maintaining clean coal yield and handling the rejects. Conventional coal washing plants generate clean coal with 15–18% ash content and 35–50% yield by several physical beneficiation techniques. Handling and disposal of the rest of 50% reject is an environmental concern and hence, alternate processes for utilization of high-ash washery rejects are of high importance. In this work, the chemical demineralization process is explored at a pilot scale to produce the low ash clean coal from different washery rejects. 44.6%–67.8% demineralization is achieved with the single-stage alkali-acid leaching process and 68.4%–78.3% by multi-stage leaching at optimum operating conditions. However, the economics of the process depends on the regeneration of the spent alkali solution. Therefore, a two-step methodology for spent-alkali regeneration is developed. In the first step, carbonation of spent-alkali with CO2 separates almost >95% of the silica and alumina impurities. In the second step, 89.2% caustic soda is recovered by causticization using quick lime.

Effect of 0–1 mm pulverized coal on desulfurization and upgrading of low grade lignite in dry cascade separation bed with compound force field

Coal is the important pillar of national economic development. China has a rich variety of coal resources, of which low-grade lignite resources have huge reserves. As the consumption of high-quality coal resources intensifies and its output is declining, strengthening the efficient use of low-grade lignite is an important measure to ensure energy supply and safety, and to stabilize industrial energy production. This paper mainly studies the influence of the characteristics of 0–1 mm coal powder in the dry cascade separation bed with compound force field on the spatial migration law of low-grade lignite and the uniformity and stability of the density of the autogenous medium bed. The regional spatial distribution of sulfur content of low-grade lignite under different autogenous medium content was systematically studied. The emphasis is on the influence of the particle size and density composition of 0–1 mm coal powder on the uniform stability of the autogenous medium bed density. At the same time, the spatial distribution and cascade separation of low grade lignite in the variable distance regional autogenous medium bed are analyzed. The test results show that when the content is 12.5–20%, the sulfur content of the upper and lower materials is relatively stable in the high and low density material enrichment areas, and the sulfur content of the middle material has a large fluctuation range. At this time, a autogenous medium fluidized bed with uniform and stable density can be formed; The 0.125–0.25 mm coal powder particles are uniformly distributed along the XOY section and XOZ section, which is the dominant particle size fraction for forming an autogenous fluidized bed; Coal particles with a density of 1.6–1.8 g/cm3 can form a fluidized bed with stable density, which is the dominant density fraction for forming an autogenous medium bed. During the dynamic sorting process, the materials in zone I present a differentiated distribution, realizing the separation of low-density materials. The spatial distribution of materials in zone II fluctuates greatly, realizing the separation of medium and high-density materials. The materials are distributed relatively evenly in zone III, which realizes the conveying and separation of high-density materials. The yield of clean coal is 65.12%, the ash content is 9.89%, the sulfur content is 0.99%, the gangue yield is 34.88%, the ash content is 44.98%, and the sulfur content is14.89%. The probable error of E is 0.07 g/cm3, and the high-precision desulfurization, ash reduction and quality improvement of low-grade lignite by the dry cascade separation bed is realized.

Extraction of clean coal from washery rejects and its effect on coking properties: an approach toward sustainable development

ABSTRACT Since coal washeries generate huge quantities of high-ash rejects as waste, effective utilization of coal rejects help in sustainable coal preparation. Out of different coal rejects, tailings from froth-flotation process can be a potential material for the extraction of clean coal due to its fine size, availability in the slurry form, presence of good carbon values, and better liberation properties. In the present study, three different methodologies i.e., chemical method, chemico-physical method, and physical method are explored to separate clean coal with >49.8% yield for their utilization in coke making process. The chemical method of demineralization by alkali-acid leaching shows a high clean coal yield of 65.4%; however, the alteration of vitrinite-group macerals makes it unsuitable for coke making process. Chemico-physical method of clean coal separation by hydrothermal extraction followed by density separation results in clean coal at 49.8% yield with an increase in vitrinite group macerals, which shows a positive impact on rheological and coking properties. The physical separation method by ultrafine grinding followed by density-gradient-centrifugation resulted in 58.2% of clean coal yield with coking properties similar to the clean coal obtained in conventional coal beneficiation plants. By adopting chemico-physical method and physical method of coal cleaning, the overall clean coal yield of the washery can be improved by >5.1%. Also, there is no deterioration in coke properties such as coke strength after reaction and coke reactivity index by using the prepared clean coal in coal blend.

Эффективность обогащения угля и функция желательности Харрингтона

The technological efficiency of the coal washing process is traditionally assessed by the correlation between the practically achieved results of this coal washing at a coal dressing plant and the theoretically possible ones defined by the laboratory testing to define the coal washability. However, the laboratory assessment of coal washability is a labor-intensive process and cannot be used for on-line control. The article considers the possibility of introducing a performance evaluation indicator using parameters traditionally defined in operational control schemes. This indicator is based on the well-known function of qualitative process evaluation based on several parameters, i.e. the Harrington's desirability function. The proposed efficiency indicator was calculated based on the qualitative and quantitative parameters of washing thirty different coals using the conventional processing scheme. Evaluation of the proposed efficiency indicator was made by comparing it with the traditionally used efficiency indicators. The evaluation results showed that the proposed indicator is more informative, because it also allows to take into account the compliance of the ash content during washing with the specified parameter, as well as to evaluate the efficiency of the washing process more objectively than the traditionally used clean coal yield indicator. Within a given ash content of the clean coal there exists a correlation with the performance indicators recommended for assessment of the washing efficiency. Given the specific features of the desirability function, the indicator makes it possible to qualitatively assess the coal washing process and determine the optimal value..

Experimental study of coal washability using float and sink test

Pakistani coal is characterized by large amount of mineral matters especially pyrite, which restricts its use in power generation and process industry. High ash contents in coal create problems in terms of slagging, fouling, clinker formation and environmental pollution. High ash content of the coal will affect the coal applications, in order to reduce ash the contents of coal, there is need to clean the coal. In this experimental study effect of dense medium, specific gravity, settling time and particle size on coal washability have been carried out. Experimental study showed that maximum clean coal float fraction was separated out at particle size of 60 mesh. The maximum yield of clean coal is 86.1 % obtained in float and sink test. Settling time also influenced directly on float fraction, float and sink fraction became almost constant at 16 hours. In present research lowest ash contents of clean coal is reduced from 36.38 % to 11.1% which is acceptable for further utilization of process industry.

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.

Motion characteristics and density separation of fine coal in an inflatable-inclined liquid–solid fluidized bed

To improve the adaptability of fluidized beds for fine coal separation, a new type of liquid–solid fluidized bed was constructed, i.e., the inflatable-inclined liquid–solid fluidized bed (IILSFB). A combination of simulation analysis and separation experiments was used to analyze the fluidization characteristics and separation performance of the IILSFB. The results showed that there was upflow and downflow in the fluidized bed. The upflow was mainly composed of water flow, followed by light and heavy particles; on the other hand, the downflow was caused by the backflow of heavy particles that settled at the inclined section. In addition, the light particles that settled at the inclined section could return to the rising water flow under the action of secondary airflow. As the water velocity, separation time, and secondary gas velocity increased, the comprehensive separation efficiency of fine coal in the fluidized bed improved, while the value decreased as the feed quantity increased. This also indicated the order of importance for these four factors, i.e., water velocity, separation time, feed quantity, and secondary gas velocity, on fluidisation. Furthermore, the comprehensive separation efficiency of 0.1–1 mm fine coal varied significantly with various factors, while that of ∼0.1 mm and 1–3 mm fine coal was always at a low value. In the latter case, the classification process of the size fraction was significantly better than the separation process in the fluidized bed. Under optimal working conditions, an IILSFB was used to separate the fine coal (0.1–1 mm). The yield of clean coal was 37.95% with an ash content of 12.11%, and the possible error was 0.085 g/cm3, indicating that the IILSFB had good separation performance for 0.1–1 mm fine coal.

Assessment of coal sortability and washability using dual energy X-ray transmission system

ABSTRACT Sorting coal using x-ray transmission sensors has been explored since 1972 by Jenkinson et al. Coal sortability is assessed using washability derived from dual-energy X-Ray transmission (DE-XRT) system. Coal sorting is considered to be dry separation of coal that can be carried out before coal is send to the processing plant. It can act as deshaling/destoning process or it can also be used to selectively mine shallow coal seams to avoid overburden contamination hence dilution. There are a few commercial concepts in the coal industry which utilizes the XRT sensors to sort the coarse particle coal. In order to define final product quality in terms of ash as well the efficiency of any separation based on density such as yield, washability is used to determine the separation process parameter such as density of separation, e.g. specific gravity. Washability characteristics have been traditionally assessed using the float-sink (washability) tests. These analyses determine how much of coal can be separated from associated rock (mineral matter) in liquids of different densities. However, the test work requires a wet processing where organic liquids, referred to as heavy liquids are used. The test work is very time consuming as it requires significant amount of time to complete full washability for a wide spectrum of coal particles. To avoid using toxic chemicals and save time, the DE-XRT can easily become an alternative method to assess the washability of coarse coal before the coal is processed by sorting to establish separation parameters and obtain information about the yield of clean coal of desired quality in terms of ash content. In this project in order to assess effectiveness of DE-XRT for sorting and at the same time to derive the washability data for the coarse coal, the test work has been designed to test various lithotypes identified from coarse coal sample. As a result, the test work included classification of coal lithotypes (larger pieces of coal of certain petrographic composition), density measurement using both DE-XRT and float-sink tests and ash analysis. The results are compared to float-sink tests for coarse coal fractions obtained by proxy washability analysis derived from pycnometry. Similar to float-sink tests, the DE-XRT can estimate a relative density by measuring the attenuation of x-rays from two different energy levels. The paper explains the basics of the DE-XRT analysis and its data processing methods to determine the relative densities and how to derive washability curves and density of separation for coal sorting. The basic principle of dual-material decomposition is adopted from Dual-Energy CT in the medical industry where the sum of each vector provides a proportion of the two material.

Industrial demonstration of a sensor for monitoring coal flotation

This paper presents a simple new sensor for real-time monitoring of coal flotation. The sensor was developed based on measurement of the drag exerted by the overflowing froth onto a cantilever beam. The effectiveness of this sensor for flotation monitoring was demonstrated by an on-site trial in a coking coal flotation plant in Queensland, Australia. The trial was carried out using one of the full-scale column flotation cells in the plant for five days, with the froth depth being fixed at 1.0 m but frother dosage and aeration rate being varied over a wide range. The output of the drag sensor had a strong linear correlation with the clean coal yield (mass recovery), with the coefficient of determination being 0.97.

Effect of mixing ratio on the adsorption behavior of low-rank coal surface using mixed collectors: Experimental and molecular dynamics simulation study

ABSTRACT The effect mechanism of mixing ratio on the adsorption behavior of low-rank coal surface using mixed collectors has not been systematically studied. In this study, dodecane (D) and n-valeric acid (N) were mixed into different collectors in different ratios, and their adsorption mechanism on the surface of LRC was studied by experiment and simulation calculation. The flotation results indicated that the sequence of the flotation clean coal yield was 8:2 > 6:4 > 5:5 > 7:3 > 9:1. The measurement results of the heat of adsorption, total organic carbon, and liquid phase contact angle all responded to the flotation results as a whole. The molecular dynamics simulation results revealed that when an appropriate ratio was used to form a mixed collector, the N molecules could make the D molecules form oil molecular layer thicken, thus increasing the hydrophobicity of the surface of the LRC molecules. The optimal mixing ratio could also create a large interaction energy between the mixed collector and LRC. Therefore, D and N need to be mixed at a proper ratio to maximize the LRC flotation efficiency.

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.

Utilization of High Ash, Low Volatile Coking Coals of Jharia Coalfield, India for Coke Making

India is having limited resources of coking coal, an essential input for production of iron and steel especially through blast furnace route. The coking coal sources are presently from the lower seams of Jharia coalfield, which are high in ash content and low in volatile matter and difficult to wash. This paper describes the washability characteristics of a typical coking coal from the Eastern and Western Jharia coalfields aiming at 18% ash level in the clean coal mainly to meet the specifications of coke making for blast furnace use. Conventional float-and-sink testing was used to determine the yield of clean coal for the coarser fraction, while the coal fines was subjected to flotation. The theoretical recovery of clean coal for the coal tested from Eastern and Western Jharia coalfields are 27% and 19.7% at the stipulated ash content. The coking propensities and the petrographic analysis of the clean coal strongly support its use for coke making either directly or as a blend with imported coal.

Application of screening using a flip-flow screen and shallow groove dense-medium separation in a steam coal preparation plant

ABSTRACT Low-rank steam coal is an important coal resource, but has high water and ash content, which needs to be separated to improve its quality and calorific value. In this paper, the process of screening using a flip-flow screen and density separator using a shallow groove dense-medium separator for steam coal was proposed and applied to the Lijiahao coal preparation plant. Kinematics of the screen box and the surface of the flip-flow screen were investigated using an acceleration and laser displacement measurement and analysis system. The mechanism of the flip-flow screen to overcome aperture blocking and to enhance the screening performance for moist and fine coal was discussed. The results from an industrial experiment when screening at 3 mm showed that the screening efficiency was 82.51%, while the total misplaced materials were 5.91%. The commercial separation of a wide size range of coal using a shallow groove dense-medium separator was analyzed. The results showed that the system exhibited satisfactory separation performance for 100–3 mm coal, whereas the corresponding Ep value was found to be 0.06 g/cm3. Compared with the 6 mm separation system, the yield of clean coal increased by 8.50%, which remarkably increased the economic benefits for the company.

Adsorption of sodium oleate on coal surface with different oxidation degrees and its effect on flotation

As traditional hydrocarbon oils cannot interact effectively with oxidized coal surface, this paper studied the effect of sodium oleate (NaOL) on the flotation of coal with different oxidation degrees. X-ray photoelectron spectroscopy, ultraviolet spectrophotometer, contact angle and zeta potential results confirmed the effective adsorption of NaOL on oxidized coal surface. For 8 h oxidized coal, its contact angle increased from 65.8° to 75.7° and zeta potential decreased from −80.8 mV to -82.1 mV after the adsorption of NaOL. The NaOL mainly reacted with the oxygen-containing functional groups on oxidized coal surface by hydrogen bonding, thus the surface hydrophobicity and floatability of oxidized coal were significantly improved. The flotation yield of clean coal using NaOL as collector was higher than that using kerosene. With the combined usage of sodium hexametaphosphate (SHMP) and NaOL in oxidized coal flotation, the ash content of clean coal was significantly reduced to an acceptable quality.