Ultrafine Coal Research Articles

Ultrafine coal dewatering: Relationship between hydrophilic lipophilic balance (HLB) of surfactants and coal rank

Dewatering process is one of the most costly steps in mineral processing and coal washing plants. This fact becomes an important industrial challenge in lignite cleaning. The presence of micropores and relatively low hydrophobic nature of lignitic coal lead to trapping of large quantities of water in the coal matrix especially in fine sizes. Since lignites are used as a common fuel in power stations, they need to be dried to a certain moisture level before feeding to the combustion chamber. Dump feeding in power plants cause inevitable loss of efficiency and may be deleterious for combustion sections. It is thus generally wise to dewater coal as much as possible by physical means before it is dried. However, further dewatering cost in some cases may be prohibitive particularly for fine and hydrophilic particles.In this study, various type and amount of surfactants have been tested to increase the efficiency of coal dewatering. As the hydrophobic characteristic of coals varies and increases with increasing the coal rank, dewatering of bituminous and anthracite type coals can be accomplished using low HLB surfactants (Oily), however, these surfactants are not efficient in the case of low rank coals like lignites and brown coals. A special set-up has been designed to study the effect of high HLB surfactants on dewatering of ultrafine lignite particles. It is shown that, while surfactants with lower HLB number and oily characteristics are the most convenient dewatering reagents for slightly hydrophobic surfaces such as hard coals, surfactants with higher HLB values especially those with HLB of around 10 are the most efficient reagents for dewatering of ultrafine lignite and lower rank coals. The governing mechanisms are discussed in the light of hydrophilicity index and modification of physicochemical conditions.

Surface and bulk characterization of an ultrafine South African coal fly ash with reference to polymer applications

South African coal-fired power stations produce about 25 million tons of fly ash per annum, of which only approximately 5% is currently reused. A growing concern about pollution and increasing landfill costs stimulates research into new ways to utilize coal fly ash for economically beneficial applications. Fly ash particles may be used as inorganic filler in polymers, an application which generally requires the modification of their surface properties. In order to design experiments that will result in controlled changes in surface chemistry and morphology, a detailed knowledge of the bulk chemical and mineralogical compositions of untreated fly ash particles, as well as their morphology and surface properties, is needed. In this paper, a combination of complementary bulk and surface techniques was explored to assess the physicochemical properties of a classified, ultrafine coal fly ash sample, and the findings were discussed in the context of polymer application as fillers. The sample was categorized as a Class F fly ash (XRF). Sixty-two percent of the sample was an amorphous glass phase, with mullite and quartz being the main identified crystalline phases (XRD, FTIR). Quantitative carbon and sulfur analysis reported a total bulk carbon and sulfur content of 0.37% and 0.16% respectively. The spatial distribution of the phases was determined by 2D mapping of Raman spectra, while TGA showed a very low weight loss for temperatures ranging between 25 and 1000°C. Individual fly ash particles were characterized by a monomodal size distribution (PSD) of spherical particles with smooth surfaces (SEM, TEM, AFM), and a mean particle size of 4.6μm (PSD). The BET active surface area of this sample was 1.52m2/g and the chemical composition of the fly ash surface (AES, XPS) was significantly different from the bulk composition and varied considerably between spheres. Many properties of the sample (e.g. spherical morphology, small particle size, thermal stability) appeared to be suitable for its applicability as filler in polymers, although the wide variation in surface composition between individual particles may challenge the development of a suitable surface modification technique. The observation that the bulk and surface compositions of the particles were so intrinsically different, strongly suggested that surface characterization is important when considering compatibility between matrices when applying fly ash as filler in polymers.

Study of the Microstructure of Industrial Residues and its Application in High Performance Concrete

Along with the rapid development of China and its eye-catching economic take-off, industrial production has brought about increasingly enormous negative impact. Industrial residues (coal ash, iron-ore slag, phosphorous slag and ganister sand) have accumulated in large quantities, thus severely contaminating the environment and greatly restricting the sustainable economic development. This paper conducts research on microstructure of the industrial wastes that are massively applied in domestic architectural material industry, such as coal ash, (S95S105) slag powder, S75 phosphorous slag, micro-bead (ultra-fine coal ash) and the micro ganister sand so as to analyze its size distribution and particle appearance and introduce its functions and effect in concrete and other cement products.

Mesoporous Material from Fly Ash as a Catalyst for Stereoselective Reduction of Substituted Cyclohexanones

Mesoporous material synthesized using ultrafine coal fly ash as a source for silicon and aluminium with cetyl trimethyl ammonium bromide as the structure directing agent has been found to be a good support for enhancing the stereoselectivity of reduction of some substituted cyclohexanones. Exclusive formation of trans-4-t-butyl cyclohexanol and 4-methyl cyclohexanol is evidenced by the appearance of single peaks in 1HNMR spectra (δ = 3.5), which are due to axial C–H protons. In other cases, enhancement of stereoselectivity has been observed. Apparently, Lewis/Bronsted acid sites present in the mesoporous material activate such preference for the formation of isomers with an equatorially disposed OH group. A probable transition state has been suggested.

Mitigating acid rock drainage risks while recovering low-sulfur coal from ultrafine colliery wastes using froth flotation

Ultrafine coal wastes contain sulfide minerals, particularly pyrite, which oxidize and give rise to acid rock drainage (ARD) resulting in extensive and prolonged contamination of local ground and surface waters. Currently, mining operations emphasise an end-of-pipe approach to ARD management using costly chemical or biological treatment techniques, which do not address the long term problem of achieving sustainable closure solutions within the resource lifetime. Eliminating ARD potential before waste disposal would have a major beneficial impact on water quality and facilitate long term closure solutions. Recovering a saleable coal product would be an added economic incentive.This paper presents the results of an investigation to establish the feasibility of a two-stage flotation process to produce: (i) a low-volume sulfide-rich concentrate, (ii) a high-volume benign (low sulfur) tailings, and (iii) a coal concentrate. Laboratory-scale batch flotation experiments were carried out using an oily collector to float coal from samples of coal ultrafines and a xanthate collector to recover acid-generating sulfides. The results of acid generating potential tests (both static and biokinetic) indicate that a low-sulfur tailings with low ARD potential may be produced, together with a coal ultrafine concentrate stream with a low ash content.

Mineral microspherules in Chinese coal and their geological and environmental significance

Abstract Studies of finer-grained minerals in coal have great significance for both geology and the environment. Eleven coal samples of different rank were collected from different regions in China, and mineral microspherules were identified in these samples by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). The microspherules usually occur as regular spheres with smooth surfaces, and have an average size of less than 5 μm in diameter. Most microspherules are colloidal in origin, and include phases such as Fe-sulfides, glauconite, Fe-hydr/oxides, calcium oxalate and allophane. Allophane is a product of colloidal Al2O3 and SiO2, which, as a precursor of kaolinite, exists ubiquitously in the coals, as does kaolinite. Some microspherules, such as garnet and sphene, are mainly derived from detrital sources, and a few microspherules probably result from volcanogenic ash and/or from cosmic dust. Ultrafine Na-bearing minerals were also found in the coals. Such micro, sub-micro, and ultrafine minerals in coal may play an important role in the formation of ultrafine coal fly ash particles.

Experimental Research on Ultrafine Coal Powder for Methane Adsorption

A new adsorption material technology and its adsorption mechanism were studied in this paper. Anthracite and lignite were taken as raw materials in those experiments. Firstly, the coal was made into different sizes by ball mill then cooked in tube furnace so as to obtain the ultrafine coal material for methane adsorption. Secondly, the ultrafine coal powder which fully adsorbed methane was gone through the gas chromatography analysis to choose the best samples. Finally, the characterization of adsorption and adsorption mechanism were researched by the micro-morphology analysis, thermal analysis, X-ray diffraction analysis, BET surface area-aperture-volume analysis and infrared spectral analysis. The results of experiments showed that the ultrafine lignite powder which mainly consists of microporous, is more suitable for methane adsorption. Under the same treatment condition, the adsorption capacity of lignite series is better than that of anthracite series. After milling for 120 minutes and coking in medium temperature, the lignite could be used as the best material of ultrafine coal powder for methane adsorption.

Ultra-fine coal flocculation using dual-polymer systems of ultrasonically conditioned and unmodified flocculant

Despite the progress in chemical engineering, efficient dewatering of ultra-fine coal (<150 μm) from slurries is still one of the biggest challenges in coal-mines. This process consists of two consecutive steps: suspension flocculation and sludge filtration. Unfortunately a sort of paradox appears in this situation. On the one hand large flocs are desired in order to fast and efficiently separate solid particles from the suspension, on the other hand large and porous flocs have worse dewaterability characteristics than small and tight ones. In this paper solution to this problem is proposed. In order to create large and tight aggregates a dual-polymer system was used using unmodified and sonicated flocculant. The flocculation process was investigated on a special apparatus for on-line particle size distribution (PSD) measurements consisting of the laser particle sizer, baffled mixing tank with turbine mixer and peristaltic pump. Moreover, pressure filtration tests were made. Presented results proved that ultrasonically conditioned polymers could be an attractive alternative for standard dual-polymer systems. The application of polymer blends resulted in accelerated aggregation kinetics, whereas sequential addition allowed the system to obtain slightly larger aggregates. In high overdose region a synergistic effect was found – the size of aggregates was larger than in case of unmodified polymer optimal dosage both for sequential addition and polymer blend cases. Finally, enhanced dewaterability characteristics of received sludge were obtained.

The Benefication of Ultrafine Coal by Using the LM, Modified LM, and Conventional Flotation Cell

This article describes modification of the LM cell developed by Cheng Xinghua and using it to clean ultrafine coal. The major modifications involve replacing the column in a standard LM cell with a hydrocyclone and giving air into the system through a porous material. Thus, centrifugal forces are also incorporated into the system and a higher coal recovery is obtained. In addition, such important design and operating parameters as the apex diameter, gas flow rate, wash water rate, reagent dosage, and solid ratio were optimized, which affect the performance of the modified LM cell. Next, flotation experiments were carried out using the standard LM cell and conventional mechanical cell and the results were compared with those obtained from the flotation with the modified LM cell. It was established that the modified LM cell is an effective one for cleaning ultrafine coal and has a considerably high capacity. In fact, following the flotation of merely 60 sec, coals with an ash content of 46% yielded clean coals having a 10–15% ash content with 70–75% combustible recovery. The conventional flotation cell, on the other hand, produced a poor selectivity and lower coal yield. The separation efficiency of the modified LM cell was in the 55–60% range, whereas that of the conventional cell varied between 50–55%.

Nanobubble generation and its applications in froth flotation (part IV): mechanical cells and specially designed column flotation of coal

Coal is the world's most abundant fossil fuel. Coal froth flotation is a widely used cleaning process to separate coal from mineral impurities. Flotation of coarse coal particles, ultrafine coal particles and oxidized coal particles is well known to be difficult and complex. In this paper, the nanobubbles' effects on the flotation of the varying particle size, particle density and floatability coal samples were evaluated using a bank of pilot scale flotation cells, a laboratory scale and a pilot scale specially designed flotation column. The parameters evaluated during this study include the flow rate ratio between the nanobubble generator and the conventional size bubble generator, the superficial air velocity, collector dosage, frother concentration, flotation feed rate, feed solids concentration, feed particle size, and the superficial wash water flow rate, etc. The results show that the use of nanobubbles in a bank of mechanical cells flotation and column flotation increased the flotation recovery by 8%∼27% at a given product grade. Nanobubbles increased the flotation rate constants of 600∼355, 355∼180, 180∼75, and 75∼0 microns size coal particles by 98.0%, 98.4%, 50.0% and 41.6%, respectively. The separation selectivity index was increased by up to 34%, depending on the flotation feed characteristics and the flotation conditions.

An enhanced-gravity method to recover ultra-fine coal from tailings: Falcon concentrator

The Falcon concentrator is an enhanced-gravity separator used for the concentration of fine and ultra-fine minerals. This study was conducted to evaluate the effects of different process variables on the performance of the Falcon SB-40 concentrator for beneficiation of tailings to recover ultra-fine coal. Various operating and design conditions such as bowl speed (G force), water pressure, pulp solid ratio and pulp feed rate were investigated. A hydrocyclone was used for pre-enrichment with the Falcon concentrator. Operation parameters of the hydrocyclone, namely feed solids, inlet pressure, vortex finder and apex diameters were investigated. In order to produce fine coal concentrates, regression equations were derived by applying the least squares method using Minitab 15 software. Response functions were produced for the ash content and the recovery of the clean coal concentrates for the performance of the hydrocyclone and Falcon concentrator under different operating conditions. Predicted values were found with the experimental values giving R 2 values of between 0.73 and 0.58 for ash content and between 0.65 and 0.39 for recovery of the clean coal. It was shown that under optimized conditions the Falcon concentrator can produce a clean coal with an ash value of 36% from a feed coal of about 66% ash.

Removal of Cr (VI) from aqueous solution using ultrafine coal fly ash

Coal fly ashes WSRA and BQRA were ball milled for 5 h to produce their ultrafine coal fly ashes WSUA and BQUA, respectively. Batch kinetic, isotherm and pH effect on adsorption were studied to evaluate removal of Cr (VI) from aqueous solutions by ultrafine coal fly ashes comparing with raw coal fly ashes. The kinetics of adsorption indicates the process to be intraparticle diffusion controlled and follows the Lagergren first-order kinetics for all coal fly ashes. The first-order rate constants (k1) of Cr (VI) adsorption onto WSRA, WSUA, BQRA and BQUA are 1.981, 1.497, 2.119 and 1.500 (×10−2) min−1, respectively. The adsorption capacities of WSUA and BQUA are much better than those of WSRA and BQRA. Equilibrium adsorption data of all coal fly ashes well satisfy the Langmuir isotherm. The adsorbed amounts of Cr (VI) onto WSUA and BQUA decrease from pH 2 to pH 6 and then increase up to pH 12.

Considerations on the Kinetics of Froth Flotation of Ultrafine Coal Contained in Tailings

This article presents a kinetic evaluation of froth flotation of ultrafine coal contained in the tailings from a Colombian coal preparation plant. The plant utilizes a dense-medium cyclones and spirals circuit. The tailings contained material that was 63% finer than 14 µm. Flotation tests were performed with and without coal “promoters” (diesel oil or kerosene) to evaluate the kinetics of flotation of coal. It was found that flotation rates were higher when no promoter was added. Different kinetic models were evaluated for the flotation of the coal from the tailings, and it was found that the best fitted model was the classical first-order model.

Adsorption Behavior of Methyl Orange onto Modified Ultrafine Coal Powder

The adsorption of methyl orange onto ultrafine coal powder (UCP) and modified ultrafine coal powder (MUCP) from aqueous solutions were studied, in which the influence of contact time, dosage, temperature, pH, and methyl orange concentration in the solution were investigated. The adsorption kinetics of methyl orange by UCP and MUCP can be described by the Lagergren first-order and pseudo second-order kinetic models, respectively. The adsorption isotherms of methyl orange onto MUCP at 303, 313 and 323 K follow the Freundlich and Langmuir isotherm equation. Values of Δ G 0 for methyl orange adsorption onto MUCP are −22.55, −23.10 and −23.79 kJ·mol −1 at 303, 313, and 323 K, respectively. The values of Δ H 0 and Δ S 0 are −3.74 kJ·mol −1 and 61.99 J·mol −1, respectively. The adsorption process is spontaneous and exothermic.

An Evaluation of a Two-Stage Spiral Processing Ultrafine Bituminous Coal

Testing was conducted to evaluate the performance of a multistage Multotec SX7 spiral concentrator treating ultrafine bituminous coal. This spiral mimics a two-stage separation in that the refuse is removed after four turns, and the clean coal and middlings are repulped (without water addition) and then separated in the final three turns. Feed samples were collected from the spiral circuit of a coal cleaning plant located in southwestern Pennsylvania. The samples consisted of undeslimed cyclone feed (nominal − 0.15 mm) and deslimed spiral feed (nominal 0.15 × 0.053 mm). Testing was carried out to investigate the effects of slurry flow rate and solids concentration on spiral performance. Detailed size and ash analyses were performed on the spiral feed and product samples. For selected tests, float-sink and sulfur analyses were performed. In nearly all cases, ash reduction occurred down to approximately 0.025 mm, with some sulfur reduction occurring even in the − 0.025 mm interval. The separation of the + 0.025 mm material was not significantly affected by the presence of the − 0.025 mm material when treating the undeslimed feed. The − 0.025 mm material split in approximately the same ratio as the slurry, and the majority of the water traveled to the clean coal stream. This split ultimately increased the overall clean coal ash value. A statistical analysis determined that both flow rate and solids concentration affected the clean coal ash value and yield, though the flow rate had a greater effect on the separation.

Segregation of Ultrafine Particles in a Centrifugal Fluidized Bed Separator

The segregation of ultrafine particles in a centrifugal fluidized bed and the effect of a ragging ring and pulsed fluidized water on the segregation efficiency have been investigated. A mixture of ultrafine clean coal and silica sand used as feed materials and a pilot-scale continuous Knelson centrifugal fluidized bed separator were used to evaluate the effects of four main variables, i.e. centrifugal force, fluidized water flow rate, and discharge valve opening time and closing time, on the overflow product grade and recovery. A new design of fluidization distributor, ‘the ragging ring’, was used to generate homogeneous fluidized beds. An orthogonal statistical test programme was conducted on the new distributor and it was found that the ragging ring could not significantly improve the overall performance of the unit. The ragging ring could not increase the superficial water velocity. This eventually resulted in similar performance as the original superficial velocity ring achieved. Although different size glass beads were used, there were not significant differences in the separation performance because of their similar porosities. The optimum parameters were identified by using different ragging materials, pulsed water flow rates and pulsation frequencies. It was noted that the ragging ring with 5-mm glass beads generated the best results with a pulsed water flow rate of 18 l/min and 1-Hz pulsation. This was probably due to achieving the appropriate porosity as well as the packing of the bed inside the ring.

Optimization of multigravity separation for recovery of ultrafine coal

Enhanced gravity separation appears to be one of the most promising methods for processing all types of <200-µm coal feeds and tailings. Systems based on this technology are reviewed, and the results of investigations based on C900 Multi-Gravity Separator tests conducted to assess the treatment of <200-µm, extremely difficult-to-wash coal settling pond tailings are presented. Multi-Gravity Separator processing tests of ultra-fine tailings with a high clay particle size content (22.6% <10 µm and 60.1% <40 µm) and a 69.56% ash content show that this material can be effectively treated after desliming (<10 µm) of clay-size particles to produce a coal with a 20.6% ash at a separator combustible material recovery of 69.3%. Desliming is shown to be critical in reducing coal ash content and combustible material recovery. Similar improvements are anticipated in the recovery and separation processes for other enhanced gravity separation systems from the desliming of feeds. Although not a conventional form of treating Multi-Gravity Separator data, the existence of well-defined polynomial relationships based on the product of drum shake amplitude and shake frequency relative to product coal recovery, product ash content and yields at varying wash water flows is demonstrated. Low-amplitude and intermediate- to high-frequency bed agitation are shown to produce optimum ash reduction results.

Developing the enrichment of ultrafine coal

Developing the enrichment of ultrafine coal

Screen Bowl Centrifuge Dewatering Process: A Parameteric Study

Screen bowl centrifugation process is widely used for fine coal dewatering due to its relatively low cost, high capacity of providing low‐moisture content product as well as relative ease of operation and maintenance. However, screen bowl centrifuge tends to lose a significant amount of ultrafine clean coal to the main effluent and screen‐drain streams. This study aims at obtaining a better understanding of the parameter main effects and interaction effects with an ultimate goal of improving the dewatering performance of a screen bowl centrifuge. A three‐level factorial experimental design has been utilized to conduct a test program using a continuously operated screen bowl centrifuge having a bowl diameter of 0.5 meter. The results of this study indicate that moisture content of the clean coal product is affected by both feed solid content and the pool depth maintained in the centrifuge. On the other hand, clean coal recovery to the product launder was found to be a function of feed solid content and volumetric feed flow rate but independent of the pool depth. The interaction effect of feed flow rate and feed solid content also appeared to have a significant effect on the clean coal recovery.

Ultrafine ash aerosols from coal combustion: Characterization and health effects

Ultrafine coal fly-ash particles, defined here as those with diameters less than 0.5 μm, typically comprise less than 1% of the total fly-ash mass. These particles are formed primarily through ash vaporization, nucleation, and coagulation/condensation mechanisms, which lead to compositions notably different compared to other fine or coarse particle fractions formed by fragmentation. Whereas previous studies have focused on health effects of particulate matter with aerodynamic diameters less than 2.5 μm (PM 2.5) (including both vaporization and fragmentation modes), this paper reports results of interdisciplinary research focused on both characterization and health effects of primary ultrafine coal ash aerosols alone. Ultrafine, fine, and coarse ash particles were segregated and collected from a coal burned in a 20 kW laboratory combustor and two additional coals burned in an externally heated drop tube furnace. Extracted samples from both combustors were characterized by transmission electron microscopy (TEM), wavelength dispersive X-ray fluorescence (WD-XRF) spectroscopy, Mossbauer spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Pulmonary inflammation was characterized by albumin concentrations in mouse lung lavage fluid after instillation of collected particles in saline solutions and a single direct inhalation exposure. Results indicate that coal ultrafine ash sometimes, but not always, contains significant amounts of carbon, probably soot originating from coal tar volatiles, depending on coal type and combustion device. Surprisingly, XAFS results revealed the presence of chromium and thiophenic sulfur in the ultrafine ash particles. Although the single direct inhalation study failed to reveal significant health effects, the instillation results suggested potential lung injury, the severity of which could be correlated with the carbon (soot) content of the ultrafines. Further, this increased toxicity is consistent with theories in which the presence of carbon mediates transition metal (i.e., Fe) complexes, as revealed in this work by TEM and XAFS spectroscopy, promoting reactive oxygen species, oxidation–reduction cycling, and oxidative stress.