Effect of ultrasound on the true flotation of lignite and its entrainment behavior

In this study, the ultrasound was fixed in the pulp zone of flotation cell and its effect on the true flotation of lignite was analyzed. Flotation results indicated that the simultaneous ultrasound treatment increased the concentrate yield and decreased the concentrate ash content. Screening analysis of flotation products revealed that the ultrasound could crush coarse coal to fine coal and scanning electron microscopy (SEM) tests indicated that the ultrasound could reduce the coverage of high-ash coal fines on the coarse particle surface. Thus, the flotation recovery of coarse lignite particle was increased. In addition, the true flotation and entrainment of ?0.074 mm fine particles were studied by the sink-float test and the method of Trahar. It was found the ultrasound significantly enhanced the true flotation of fine particles and improved the overall water recovery in lignite flotation.     

Improving the flotation performance of coking coal using the reverse-direct flotation process

In order to improve the flotation performance of the coking coal particles, the flotation tests of the coking coal particles were conducted in the direct flotation, reserve flotation, and reverse-direct flotation processes. It was found that the concentrate ash content of coking coal particles was higher than 20%, which cannot be effectively reduced using the direct and reserve flotation processes. However, the flotation concentrate with the ash content of 12.53% can be obtained from the reverse-direct flotation process. In the reverse-direct flotation process, the surface hydrophobicity was reduced with the dextrin and 1-dodecylamine (DDA) addition at the reverse flotation process stage. For the addition of diesel collectors at the direct flotation process stage, the surface hydrophobicity of the coking coal samples was improved.     

A new process based on a combination of gravity and flotation for the recovery of clean coal from flotation tailings

Beneficiation of clean coal from flotation tailings with a high intergrown ash content is gaining popularity in China. It not only improves the utilization rate of coal resources but also reduces environmental pollution. In this paper, a novel gravity–flotation process is proposed to concentrate low-ash fine coal and includes the disposal of tailings via gravity, grinding liberation, and froth flotation. The experimental results show that the clean coal had an ash content of 12.33% and the yield from the flotation tailings was 32.24% with an ash content of 46.39%, which has economic value in the current market.     

Flotation kinetics of the removal of unburned carbon from coal fly ash

Unburned carbon (UC) in coal fly ash indicates the waste of energy source and is an obstacle to the utilization of coal fly ash. The purpose of this study was to investigate the flotation kinetics of the removal of UC from coal fly ash. The collector and frother dosage are key factors for coal fly ash flotation. Six flotation kinetic models were applied to fit the flotation kinetic tests data. The fitted results showed that the classical first-order model had better consistency with the experimental data. The contact angle of flotation concentrates decreased as the flotation time was extended which revealed the decrease of flotation rate.     

Contribution of particle shape and surface roughness on the flotation behavior of low-ash coking coal

In this study, the influence of particle shape and surface roughness on the flotation behavior of +0.25–0.5 mm low-ash coking coal particles was investigated. The low-ash coking coal particles with different particle shape and surface roughness obtained through grinding or crushing were measured, calculated, and analyzed using an optical microscope associated with Image J software. The flotation kinetics tests were conducted in a 0.75 L XFD flotation cell with the presence of frother and in the absence of collector in order to investigate the natural floatability of the low-ash coking coal particles. The flotation kinetics constant of low-ash coking coal particles was calculated through the first-order rate equation. The experimental results illustrated that the flotation kinetics constant increased with increasing the aspect ratio and roughness, while the particle owning high roundness and circularity value led to smaller flotation kinetics constant. Finally, the quantitative contributions of particle shape and roughness of low-ash coking coal particles on flotation performance were established.     

Recovering clean low-rank coal using shale oil as a flotation collector

Shale oil (SO) was adopted as flotation collector to recover clean low-rank coal. Gas chromatography-mass spectrometry, flotation, X-ray photoelectron spectroscopy and contact angle measurements were performed to identify the chemical constituents and flotation performance of SO, and the surface properties of low-rank coal. The results indicated that the long-chain hydrocarbon and high viscosity of SO may lead to the higher combustible matter recovery and lower ash content of clean coal. The adsorption of SO significantly enhanced the carbon-bearing content and decreased the C?O content, which was favorable to enhance the hydrophobicity, and thus improve the flotability of low-rank coal.     

Investigation of the energy-size reduction and mineral liberation of raw coal in the Hardgrove mill

Raw coal was ground in a Hardgrove mill, and mineral liberation from coal was investigated by float-sink test and electron probe microanalyzer (EPMA). Based on EPMA, associated conditions of mineral were determined. Analyses of energy-size reduction show that energy efficiency decreases with time. For +0.074 mm progenies, yields of particles in ?1.5 g.cm^?3 decrease with the increase of size and time, and products in +1.8 g.cm^?3 show the opposite trend. EPMA of 0.25–0.125 mm, 1.5–1.8 g.cm^?3 products ground for 10 min indicates that only part of minerals are liberated. For ?0.074 mm products, the yield of particles in ?1.5 g.cm^?3 decreases by 23.36% in last several minutes, but ash content of +1.8 g.cm^?3 also decreases by 13.18%. Since breakage of raw coal in last 9 min does not reduce particle size obviously, and liberation degree cannot be improved dramatically, closed grinding of 10 min in Hardgrove mill is proper to balance the size reduction and mineral liberation.     

Segregation of coal particles in air classifier: Effect of particle size and density

To investigate the effect of size and density on the segregation of particles in the static classifier of vertical spindle mill, classification tests of coal were conducted in a lab-scale classifier, and its performance was evaluated by classification efficiency. Results show that yield and fineness of pulverized fuel? (PF), as well as the classification efficiency of the whole size, would be improved by increasing the air volume. While the comprehensive classification efficiency reaches around 40% and keeps stable. Meanwhile, the separation phenomenon in the classification process was revealed by comparing the ash content between the feed and PF. Results show that ash content of each size fraction decreases after classification and presents growing trend with the decrease of particle size in PF. Separation effect is especially obvious for coarse particles, and its intensity increases as air volume reduces. Furthermore, such a phenomenon is also verified by industrial data, which proves again that minerals with high density would accumulate in the classifier reject. Deviations in bypass fraction, actual cut size, and comprehensive efficiency between regarding coal as homogeneous or heterogeneous were calculated. Thus, to be more accurate, density should be considered in the investigation of the coal particle behavior within classifier.     

Effects of calcium chloride and sodium chloride on the flotation removal of unburned carbon from coal fly ash

The effects of calcium chloride and sodium chloride on the coal fly ash flotation were investigated by studying the surface properties of coal fly ash. X-ray photoelectron spectroscopy and scanning electron microscopy showed the presence of many lime particles in the coal fly ash, which, together with Ca(OH)₂(s), could be adsorbed on the rough surface of unburned carbon particles. The flotation results indicated that calcium chloride reduced the performance of unburned carbon removal from the coal fly ash, whereas sodium chloride increased froth stability thereby improving the flotation performance.     

A new process for the production of medium quality fuels from coal washing plant coarse tailings

Turkey’s proven bituminous coal reserve is very low, and it is about 1.3 billion tons. The annual bituminous coal production is 2 million tons, and the annual import amount is 30 million tons. Turkey is a foreign-dependent country in its bituminous coal requirement. In this respect, the highest recycling of coarse and fine plant tailings is important in respect of the efficient use of limited natural resources. In this study, a novel process was developed for medium quality fuel production from coarse plant tailings. By the developed process, from coarse plant tailings with an ash content of 78.21%, medium quality fuel was produced which has ash content between 29.20 and 44.38% and which has economic value in the current market. The upper calorific values of these fuels change between 5620 and 4350 kcal/kg. The developed process basically includes the stages of micronized grinding and then froth flotation applied to the obtained powder tailing material.     

Effects of oxy-fuel condition on morphology and mineral composition of ash deposit during combustion of Zhundong high-alkali coal

Zhundong coalfield is a super-large coal reserve, with high-alkali feature exacerbating ash deposition. Oxy-fuel combustion technology could propel the clean utilization of Zhundong high-alkali coal. While the ash deposition behavior of high-alkali coal under oxy-fuel condition has yet to be sufficiently investigated. The present study compared the differences of ash deposits between oxy-fuel and air combustion, and also examined the effects of oxygen content on ash deposition mechanism, employing a drop-tube furnace equipped with a specially designed sampling probe and some analysis methods, such as X—ray diffraction equipment, simultaneous thermal analyzer, etc. Experimental results indicated that ash deposition was weaker, with fewer contents of sodium chloride, calcium sulphate and less agglomeration ash in oxy-fuel atmosphere compared to the air case with same oxygen content. The content of the ash particle distributed in the range of 0–40 μm was up to 60% under oxy-fuel condition. The first weight loss of ash deposits, around 850 °C, was put down to the decomposition of carbonate and the second one, about 1150 °C, was ascribed to the decomposition of the sulphate minerals in the thermal process. Ash deposition worsened with more large particles (>120 μm), as the oxygen content rose. Sodium chloride content reached 9.7% with 50% oxygen content. The present study not only focuses on the morphology and chemical components, but also probes into the thermal volatility of ash deposits, which benefits the further understanding of the ash deposition mechanism and utilization of Zhundong high-alkali coal during oxy-fuel combustion.     

Study on Al<Subscript>2</Subscript>O<Subscript>3</Subscript> extraction from activated coal gangue under different calcination atmospheres

Coal gangue was calcinated under air, nitrogen, carbon dioxide, air–hydrogen, and hydrogen atmospheres. The effects of different calcination temperatures and atmospheres on the mineral composition of activated coal gangue were investigated by X-ray diffraction. Moreover, the acid leaching kinetics of aluminum oxide from coal gangue was investigated with sulfuric acid. It showed that the air atmosphere promoted kaolinite decomposition during coal gangue calcination. The hydrogen atmosphere promoted the activation and decomposition of kaolinite at reaction temperatures exceeding 650°C. The carbon dioxide atmosphere eliminated the influence of residual carbon on coal gangue. When the ratio of acid/coal gangue was 1.5 and reaction temperature was 650°C, the sulfuric acid leaching rate under air, air-hydrogen, carbon dioxide, hydrogen and nitrogen atmospheres were 93.66%, 90.90%, 84.06%, 81.91% and 77.54% respectively. The acid leaching reaction process conformed to unreacted shrinking core model of particle unchanged, and was controlled by the interfacial chemical reaction. The reaction kinetic equation for the leaching process was 1-(1-x)1/3=kt with an apparent activation energy of 48.97 kJ/mol.     

Biodiesel as a renewable collector for coal flotation in the future

Biodiesel is a renewable and clean energy which is being developed day by day. However, oil and coal reserves are decreasing due to an increase in energy demand for the development of social economy. Additionally, fine coal is usually beneficiated by flotation using diesel or kerosene as collectors, which may be a waste of energy. Therefore, using biodiesel as a collector for coal flotation has its potential value. In this investigation, biodiesel was used as a collector for coal flotation. Three bituminous coal samples with different coal ranks were selected, that is, gas coal, coking coal, and lean coal. FTIR results showed that biodiesel contained much more abundant unsaturated fatty acids than diesel. Flotation results showed that the flotation performances of three coal samples using biodiesel as collector were similar to that using diesel as collector. Even better, the concentrate yields of three coal samples using biodiesel as collector were a little higher than that using diesel as collector, while their concentrate ash contents were similar. Throughout this paper, it was proved that biodiesel could have a potential application in coal flotation.     

Effect of temperature on Lu’an bituminous char structure evolution in pyrolysis and combustion

In the process of pyrolysis and combustion of coal particles, coal structure evolution will be affected by the ash behavior, which will further affect the char reactivity, especially in the ash melting temperature zone. Lu’an bituminous char and ash samples were prepared at the N₂ and air atmospheres respectively across ash melting temperature. A scanning electron microscope (SEM) was used to observe the morphology of char and ash. The specific surface area (SSA) analyzer and thermogravimetric analyzer were respectively adopted to obtain the pore structure characteristics of the coal chars and combustion parameters. Besides, an X-ray diffractometer (XRD) was applied to investigate the graphitization degree of coal chars prepared at different pyrolysis temperatures. The SEM results indicated that the number density and physical dimension of ash spheres exuded from the char particles both gradually increased with the increasing temperature, thus the coalescence of ash spheres could be observed obviously above 1100°C. Some flocculent materials appeared on the surface of the char particles at 1300°C, and it could be speculated that β-Si₃N₄ was generated in the pyrolysis process under N₂. The SSA of the chars decreased with the increasing pyrolysis temperature. Inside the char particles, the micropore area and its proportion in the SSA also declined as the pyrolysis temperature increased. Furthermore, the constantly increasing pyrolysis temperature also caused the reactivity of char decrease, which is consistent with the results obtained by XRD. The higher combustion temperature resulted in the lower porosity and more fragments of the ash.     

Experimental studies of ash film fractions based on measurement of cenospheres geometry in pulverized coal combustion

In pulverized coal particle combustion, part of the ash forms the ash film and exerts an inhibitory influence on combustion by impeding the diffusion of oxygen to the encapsulated char core, while part of the ash diffuses toward the char core. Despite the considerable ash effects on combustion, the fraction of ash film still remains unclear. However, the research of the properties of cenospheres can be an appropriate choice for the fraction determination, being aware that the formation of cenospheres is based on the model of coal particles with the visco-plastic ash film and a solid core. The fraction of ash film X is the ratio of the measuring mass of ash film and the total ash in coal particle. In this paper, the Huangling bituminous coal with different sizes was burnt in a drop-tube furnace at 1273, 1473, and 1673 K with air as oxidizer. A scanning electron microscope (SEM) and cross-section analysis have been used to study the geometry of the collected cenospheres and the effects of combustion parameters on the fraction of ash film. The results show that the ash film fraction increases with increasing temperature and carbon conversion ratio but decreases with larger sizes of coal particles. The high fraction of ash film provides a reasonable explanation for the extinction event at the late burnout stage. The varied values of ash film fractions under different conditions during the dynamic combustion process are necessary for further development of kinetic models.     

准东煤掺烧煤矸石安全性与经济性分析

高岭土与低钠煤可以有效抑制燃用准东煤时炉内的沾污结渣问题,但由于其价格升高,为降低电厂燃煤成本,因此提出将煤灰成分相似的煤矸石作为准东煤防结渣添加剂.分析煤矸石作为添加剂抑制炉内沾污结渣的可行性,根据入炉煤灰中折算Na2O含量低于3.0％,确定掺烧比例,并在某150 MW机组上进行实炉掺烧试验.通过掺烧试验分析可见,煤...     

An improved sessile drop method for assessing the wettability of heterogeneous coal surface

To assess the wettability of heterogeneous coal surface, a new modified sessile drop technique named volume–length method is proposed to calculate the contact angle according to the spreading ability of droplet on the coal surface. The tested contact angles were compared with the predicted contact angles using Cassie–Baxter model. Low ash coal and kaoline powders are mixed with different mass ratios and mixing degrees. The mixtures were pressed to plates for sessile drop contact angle tests. The surface chemical component and nonuniform degree determine the droplet spreading on the coal surface, thereby affecting the contact angle.     

Characterizing surface properties of oxidized coal using FTIR and contact angle measurements

This paper compares FTIR (Fourier Transform Infrared Spectroscopy) and contact angle measurements to characterize surface properties of the oxidized coals with different ash contents. FTIR analysis was employed to investigate functional chemical groups of the oxidized coals. Contact angle measurements were utilized to characterize the hydrophilicity of the oxidized coal. The results showed that contact angle measurements were the same regarding of the type of the oxidized coal. However, FTIR results show that the coal samples with lower ash content had more hydrophobic groups and less hydrophilic groups than the coal samples with higher ash content. This paper demonstrated that FTIR technique is more sensitive than contact angle measurements in studying surface properties of oxidized coal samples.     

粗煤泥回收工艺及其设备应用

随着国内外采煤机械化程度的提高,洗煤厂入选原煤中的末煤含量也随着增高,若是粗煤泥不进行再次分选回收,则会造成精煤损失,如果直接掺入精煤,就会导致总精煤灰分升高,使重选和浮选为其＂背灰＂,从而导致总精煤产率降低;而且再次分选的方法也是提高精煤产率和经济效益的关键。     

Experimental study on ash deposition of Zhundong coal in oxy-fuel combustion

To facilitate the large-scale utilization of high-alkali and -alkaline earth metals (AAEMs) coals in power generation, the ash deposition behaviors of a typical Zhundong coal in oxy-fuel combustion were experimentally investigated using a drop tube furnace. A wall-temperature-controlled ash deposition probe by which the bulk gas temperature could be measured simultaneously was designed and employed in the experiments. The deposition tendencies, ash morphologies, chemical compositions of deposited ash particles were studied respectively under various oxygen concentrations, bulk gas temperatures, probe surface temperatures and probe exposure times. The experimental results revealed that the oxygen concentration had a significant influence on the deposition behavior during oxy-fuel combustion of high-alkali coal. Compared with air case, more fine ash particles were generated during the combustion of Zhundong coal in 21% O₂/79% CO₂ atmosphere but the deposition tendency was weaker. However, a higher oxygen concentration could aggravate the tendency of ash deposition. The high contents of iron (Fe), calcium (Ca), sulfur (S), and sodium (Na) in Zhundong coal could result in the generations of low-melting point compounds. Calcium in flue gas existed as CaO and was captured prior to SO₃ by the probe surface during the ash deposition process. At the initial 30 min of the ash deposition process, the dark spherical fine ash particles rich in Fe, Na, oxygen (O), and S were largely produced, while in the range of 60–90 min the light spherical fine ash particles with high contents of Ca, barium (Ba), O, and S were generated on the other hand. The deposition mechanisms at different stages were different and the melted CaO (BaO)/CaSO₄ (BaSO₄) would give rise to a fast growth rate of ash deposit.