Coal Flotation Research Articles

A comparison of ignition characteristics of slurry fuels prepared using coal processing waste and finely divided coal

The scientific novelty of the research is that for the first time differences in the conditions and characteristics of the ignition and burning of droplets of slurries prepared on the basis of coals and waste from their enrichment have been established. The practical significance of the research results is that they illustrate the prospects of utilization of the numerous coal enrichment wastes by combustion in the composition of aqueous slurries with the generation of a rather large amount of energy and a relatively small negative environmental impact. The most significant characteristics were compared: the limiting (minimum) temperature; the ignition delay times; the maximum combustion temperature; the concentration of the main gas anthropogenic emissions. It has been found that fuel mixtures prepared from wet waste of coal flotation are characterized by higher inertia and ignition temperatures compared to slurries with high-quality coal dust. However, the established differences considering the availability and low cost of filter cakes illustrate the prospects of waste derived fuel combustion. The combustion heat of the investigated slurries based on coal and filter cake with addition of petroleum products differs by no more than 5–30%. The average difference between the duration of ignition for fuel droplets based on dust and filter cake of coking and low-caking coals is about 20%. At that the addition of waste turbine oil (10% wt.) into the filter cake reduces the duration of ignition by 12–25% and the ignition temperature – by 10–15 °C without a significant increase in anthropogenic gas emissions. The difference between the minimum ignition temperatures of coal and waste coal based slurries was from 10 °C to 80 °C. On environmental and economic indicators, coal waste is more attractive than coal.

Improving coal flotation using the mixture of candle soot and hydrocarbon oil as a novel flotation collector

Abstract An effective flotation collector determines the flotation performance and efficiency of fine coal particles. Generally, hydrocarbon oils, such as diesel and kerosene are widely applied in the flotation circuit in coal preparation plants. However, hydrocarbon oils are not effective enough for difficult-to-float coals, such as low rank coal and difficult to float coking coal. In this study, the mixture of candle soot and hydrocarbon oil (dodecane) was used as a novel flotation collector in the flotation of coking coal and low rank coal. X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used to indicate the surface properties of candle soot. Scanning electron microscopy with energy dispersive spectrometer (SEM/EDS) and attachment time measurer were used to show the adsorption of candle soot on coal particle surface. XPS and contact angle results showed that the candle soot was very hydrophobic. SEM/EDS results showed the candle soot consisted of plenty of carbon nanoparticles which could attach and adsorb on the coal particle surface and hence shortens the attachment time of coal particle and bubble. The flotation recovery of both coking and low rank coals using the mixture of candle soot and dodecane as a collector were higher than that using solo dodecane as collector. The adsorption of candle soot and other ultra-fine particles on coal particle surface enhanced the surface hydrophobicity and floatability of coking and low rank coal particles.

Effect of Oleic Acid on Froth Properties and Reverse Flotation Performance of Thermal Coal

Reverse flotation was proposed as a potential method for the upgradation of low rank thermal coal. Higher combustible matter recovery in the float could be obtained using reverse flotation method compared to conventional flotation method. In order to reduce the entrainment of fine particles, oleic acid (OA) was used as froth regulator in coal reverse flotation process. Experimental data showed that the total volume of froth product in the absence of OA was almost 3 times more than that in the presence of 0.45 kg/Mg OA dosage. Reverse flotation results showed that the ash content of froth product increased with the increase of OA dosage which indicated that the entrained coal particles could be decreased by the addition of froth regulator OA.

The enhanced flotation of coal by nanosilica particles

ABSTRACT It is well known that the stability of hydration film on the particle surface affected the particle-bubble attachment in a flotation process. In this paper, the effect of attached nanosilica particles on the stability of liquid film around the coal particles was investigated by measurements of viscosity of the coal pulp and the bubble attachment time for coal particles. The interaction energy between a nanosilica particle and a coal particle was calculated using the EDLVO theory. Results showed that the nanosilica-coal attachment was driven by hydration attraction. And the stability of liquid film was weakened with the attachment of nanosilica particles, which is probably due to the change of surface roughness. In this case, the bubble-particle attachment time was reduced and further improved the flotation recovery of coal.

Analysis of coal wettability by inverse gas chromatography and its guidance for coal flotation

In coal flotation, the hydrophobicity of fine coal is a crucial factor that affects coal flotation recovery and performance. How to accurately characterize the surface hydrophobicity of coal becomes a research hot spot in coal science. This paper used a novel characterization technology, namely inverse gas chromatography (IGC) to characterize the wettability and hydrophobicity of fine coal particles by analyzing their surface energy. Five coals with different coal ranks were used as coal samples in this investigation, and the flotation results of five coal samples were obtained under the same flotation conditions. Throughout this paper, a good correlation between flotation recovery and surface energy was found and the hydrophilicity index of coal was calculated using the ratio of the specific component of surface energy to the total surface energy. A higher hydrophilicity index of coal leads to a lower flotation recovery. The flotation recovery is not governed by the coal rank but is determined by the surface energy of coal particles, i.e. the hydrophilicity index and the work of adhesion. The IGC is demonstrated to be a potential method for predicting the flotation recovery of fine coal in the coal preparation industry.

Oily bubble flotation technology combining modeling and optimization of parameters for enhancement of flotation of low-flame coal

There are abundant low-medium rank coal resources in China that provide an important guarantee for China's economic and social development as well as security of energy supply. However, it is difficult to achieve the economic recovery of low-medium rank coals by conventional flotation owing to its high natural surface hydrophilicity. In this study, the characteristics of low-medium rank coal samples were comprehensively investigated based on their size/density distributions, XPS, BET, and SEM analyses. An improved oily bubble flotation combining modeling and optimization of technological parameters was employed to enhance the flotation of low-medium rank coal samples. The results indicated that the studied samples were rich in oxygenated functional groups, pores, and cracks that existed on the surfaces of the coal samples, thereby making it difficult for them to float. As a result, the collector dosage was maintained at a high level ranging from 10 to 50 kg/t in the conventional flotation of the coal sample. However, the oily bubble flotation technology showed much better flotation efficiency in the case of low-medium rank coal, achieving a higher combustible matter recovery at a much smaller collector dosage. Additionally, various parameters, such as the collector dosage, impeller speed, flotation pulp concentration, and air flow rate, had a profound effect on the flotation performance in the oily bubble flotation of the coal samples. An optimization of the technological parameters was conducted regarding the flotation of oily bubbles of low-medium rank coals, and a quadratic function model for the combustible matter recovery was established. Finally, an optimization scheme was established attaining a combustible matter recovery of 86.32% for oily bubble flotation of coal samples, where the frother dosage, collector dosage, impeller speed, pulp concentration, and air flow rate were 0.4 kg/t, 2.32 kg/t, 2198.53 rev/min, 52.47 g/L, and 0.23 m3/h, respectively. As a consequence of the optimization scheme, the collector dosage was decreased by 95.36%.

Exploration on the mechanism of enhancing low-rank coal flotation with cationic surfactant in the presence of oily collector

It is difficult to yield a high combustible matter recovery in the flotation of low rank coal with common oily collectors. This study focuses on the enhancement in flotation performance of low rank coal by combining diesel oil with didodecyldimethylammonium bromide (DDAB), as well as its corresponding intensifying mechanism. The adsorption of DDAB and/or diesel oil on the pure coal and quartz surfaces was investigated based on the zeta potential and FTIR analyses, while the floatability of the coal and quartz samples before and after pretreatment was evaluated by the attachment time and flotation results. The results indicated that DDAB had great effects on the zeta potentials of coal and quartz, and the isoelectric points of these two samples were observed at the DDAB concentrations of about 0.007 and 0.006 g/L, respectively. Additionally, according to the FTIR analyses, the physical and chemical adsorptions were proposed to interpret the adsorption of DDAB, and diesel oil and mixed diesel oil/DDAB on the coal surface, respectively. Meanwhile, the diesel oil’s adsorption of on the coal surface was enhanced by the conditioning process with the mixed DDAB/diesel oil. In contrast, diesel oil adsorption on the quartz was very little in quantity. As a result, a great difference in surface hydrophobicity between the coal and the quartz was attained, and an excellent flotation performance was obtained by using the mixed DDAB/diesel oil. However, a too low surface tension of flotation pulp caused by the excessive DDAB seems to be unfavorable to the flotation of coal samples. In this case, water with less surface tension was readier to spread on the coal surface than diesel oil, thereby making the coal hydrophilic and meanwhile hindering the diesel oil’s spreading on its surface.

Enhancing low-rank coal flotation using a mixture of dodecane and n-valeraldehyde as a collector

Enhancing low-rank coal flotation using a mixture of dodecane and n-valeraldehyde as a collector

Molecular dynamics simulations of nonylphenol ethoxylate on the Hatcher model of subbituminous coal surface

Nonylphenol ethoxylate can be used to improve the performance of low-rank fine coal flotation. Herein, we utilized experimental and computational methods to investigate the adsorption of nonylphenol ethoxylate with 10 ethylene oxide units (NPEO-10) onto the surface of subbituminous coal, revealing that the adsorption could be described by a Langmuir-type isotherm, in agreement with the negative interaction energy calculated by molecular dynamics simulation. X-ray photoelectron spectroscopy analysis showed that the coverage of oxygen-containing surface groups by NPEO-10 improved the hydrophobicity of subbituminous coal. Moreover, the performed simulations indicated that in the presence of NPEO-10, water molecules were more strongly repelled by the coal surface due to its increased hydrophobicity, which was supported by the results of contact angle measurements at low surfactant concentration. Analysis of surfactant molecule aggregates by head and tail group density profiling revealed that these aggregates were attached to the coal surface via ethoxylate groups. The calculated dynamic properties of NPEO-10 showed that its diffusion was most pronounced in the direction perpendicular to the coal surface due to the hydrophobic interactions of alkyl chains.

Effects of Fine Coal Particles on the Recovery of Coarse Coal in Collectorless Coal Flotation

ABSTRACT The effects of fine coal (<74 μm) of four density fractions, namely <1.4, 1.4–1.6, 1.6–1.8, and >1.8 g/cm3, on the recovery of coarse coal (−500 + 250 μm) in collectorless flotation were studied in this article. It was found that the fine coal of all density fractions increased the coarse coal recovery except for the fine coal of <1.4 g/cm3 density fraction with the low mixing ratio in the feed. By analyzing the coarse coal yield, water recovery, concentrate weight composition, froth gray value, and froth height over weir, two mechanisms of increasing the coarse coal recovery by fine coal particles were, respectively, put forward for hydrophobic and hydrophilic particles. The hydrophobic particles mainly affected the dynamic froth stability by hydrophobicity and the overflowing froth volume by the amount of floatable particles, while the hydrophilic particles affected water drainage by the entrained particles in the froth network.

Performance Evaluation of Batch Coal Flotation from the Petrologic Point of View

ABSTRACT In this paper, an attempt was made to study the flotability of washery grade-IV (ash content between 28% and 35%) type of coking coal in terms of maceral components and to examine how each maceral group floats in the course of a time-wise batch flotation test. The objective of this paper was to evaluate the flotation characteristics of high-ash coking coal fines (of size <500 µm) in terms of the maceral components using different levels of collector (diesel oil) and frother (methyl isobutyl carbinol). The flotation test results were evaluated from the petrologic analysis, and the petrographic test results show that at equal level of collector and frother doses (0.5 kg/ton), 86% of the vitrinite maceral group floated into the concentrate in 120 s, and 77% of the mineral matter and 90% of pyrite were observed in the last two concentrates and flotation tailings. Proportionate distribution of the maceral components in flotation products was confirmed through the petrographic analysis. The different flotability of coal petrographic constituents was further investigated through surface characterization techniques such as contact angle, zeta potential, and Fourier transform-infrared spectroscopy.

The effect of froth on the dewatering of coals – An oscillatory rheology study

Dewatering of fine coal flotation products is discouraged by the presence of stable coal froth, which is becoming a common problem in coal preparation plants, however, the underpinning mechanism is still unclear. In this study, the dewatering behaviours of fine coal flotation products in both thickening and filtration were investigated and compared with those of coal slurry under the same condition. The mechanism that how coal froth influenced dewatering was investigated by analysing the froth properties using oscillatory rheology in conjunction with the characterization of particle arrangements on the air-water interface using Scanning Electron Microscopy (SEM). It was found that, in the presence of air, particle interactions in coal froth were stronger than those in coal slurry, which was caused by the capillary attractive force between particles on the air-water interface. In addition, particles in coal froth were arranged regularly, while particles in coal slurry were distributed randomly. Strong particle interactions and well organised structures in coal froth made the capillary radii smaller, and as a result, a large amount of froth stabilized on top of water during settling and the final moisture of filter cake was high in filtration.

Effects of Diesel and 2-Octanol on Water-carrying Properties and Ultrafine Coal Flotation

ABSTRACT The combustible recovery and water-carrying properties were characterized for fine coal flotation with focus on the relationship between water-carrying volume and fraction recovery of various size fractions. Diesel and 2-Octanol were used as the collector and frother, and the collector addition of 80 μL to 240 μL and the frother addition of 8 μL to 40 μL were studied using uniform experimental design method. The combustible recovery rate and water-carrying rate were found to be mainly determined by the frother but by the collector within the collector addition of 240 μL to 400 μL and the frother addition of 24 μL to 40 μL. The fraction recoveries of ultrafine particles less than 45 μm were found to linearly increase with the water-carrying volume but decrease with the particle size. The particle of smaller size was found to be more easily entrained by the water carried to the froth products than that of larger size.

Geochemistry of Toxic Elements and Their Removal via the Preparation of High-Uranium Coal in Southwestern China

High-uranium (U) coal is the dominant form of coal in Southwestern China. However, directly utilizing this resource can also harm the environment because this element is radioactive; it is, therefore, necessary to clean this kind of coal before burning. This research studied the geochemistry of toxic elements and their partitioning during the preparation of high-U coal in China. The results show that high-U coals are mainly distributed in Southwestern China and are characterized by a high organic sulfur (S) content and vanadium (V)-chromium (Cr)-molybdenum (Mo)-U element assemblage. These elements are well-correlated with one another, but are all negatively related to ash yield, indicating that all four are syngenetic in origin and associated with organic materials. A mineralogical analysis shows that U in Ganhe and Rongyang coal occurs within fine-grained anatase, clay minerals, guadarramite, and pyrite, while V occurs in clay minerals, pyrite, and dolomite, and Cr occurs in dolomite. Other elements, such as fluorine (F), lead (Pb), selenium (Se), and mercury (Hg), mainly occur in pyrite. By applying a gravity separation method to separate minerals from coal, the content of the enrichment element assemblage of V-Cr-Mo-U in Rongyang coal is still shown to be higher than, or close to, that of the original feed because this element assemblage is derived from hydrothermal fluids during syngenetic or early diagenetic phases, but other elements (beryllium [Be], F, manganese [Mn], zinc [Zn], Pb, arsenic [As], Se, Hg) can be efficiently removed. Once cleaned, the coal obtained by gravity separation was subject to a flotation test to separate minerals; these results indicate that while a portion of V and Cr can be removed, Mo and U remain difficult to extract. It is evident that the two most commonly utilized industrialized coal preparation methods, gravity separation and flotation, cannot effectively remove U from coal where this element occurs in large proportions. Finally, dilute hydrochloric acid (HCl) leaching experiments show that the majority of Mo and a portion of V, Cr, and U are adsorbed in clay minerals and organic matter and, therefore, exist in an adsorbed state. In this state, these elements can be removed using the acid method. Thus, as U cannot be fully removed from coal, the use of high-U coals is not recommended.

Optimization of RBFneural network used in state recognition of coal flotation

Optimization of RBFneural network used in state recognition of coal flotation

Contact angle and induction time of air bubble on flat coal surface of different roughness

The wettability of coal is an important parameter for representing the surface hydrophobicity and floatability of coal particle, and hence determines coal flotation behavior. The wettability of coal is primarily governed by the surface functional groups. However, the role of surface topography in the wettability of coal has not attracted enough attentions. This paper was to investigate the effect of roughness on the wettability of coal by measuring the contact angle between air bubble and flat coal surface. Besides, a new method to measure the contact angle is introduced. The shape of the bubble is fitted by both the circle and the ellipse. The lump coal particles were used as coal sample and polished by sand papers of different meshes to gain the specific flat coal surfaces of different roughness. The attachment time measurements and Wenzel theory were used to further explain why the surface roughness has significant effects on the contact angle of air bubble on coal surface. Throughout this paper, it was found that the wettability of coal was decreased with the increase of surface roughness. The water may be entrapped in the pores/cracks of rough coal surface which prevents the attachment between coal surface and bubble as well as the spreading of three-phrase contact line. Therefore, both the attachment time and contact angle were decreased with the increase of roughness.

Determining the Interactive Effects in CO2 Capture Integrated with Coal Flotation by High-Order Multifield Synergy Model

CO capture is very essential to mitigate the greenhouse gas control. However, the CO desorption is quite costly during CO capture. To reduce the cost, the CO desorption integrated with coal flotation was developed as a retrofitted process, which can simultaneously achieve the CO desorption and coal flotation. To determine the key interactive effects between coal, amine, and CO in the process, a high-order multifield synergy model was developed accordingly. According to the high-order multifield synergy model, the effects of coal particle diameter, coal weight fraction, CO loading, and amine on the CO desorption and coal flotation were analyzed in detail. It was found that there is almost twice the CO amount at coal weight fraction of 0.06 than that at a coal weight fraction of 0.02. The coal recovery rate at 0.35 mol/mol is twice of that at 0.15 mol/mol. The adsorption energy, adsorption layer thickness, and zeta potential determine the interactive effects between coal, amine, and CO. The coal, amine, and CO system increases the coal recovery rate by 25% and reduces the CO desorption energy by 20% with desorption temperature below 373 K. The high-order multifield synergy model shows the advantages of higher prediction accuracy and is easier to converge than the conventional multifield synergy model.

The effect of saline water on the critical degree of coal surface oxidation for coal flotation

In this work, the effects of water with different salinities on the flotation of oxidized coals were studied and the underlying mechanisms were revealed. Flotation tests were conducted using coal samples with different degrees of surface oxidation and in water of different salinities. A critical degree of coal surface oxidation, above which the coal particles cannot be recovered by true flotation, was observed in flotation with all types of water. It is interesting to find that the critical degree of surface oxidation increased with water salinity, suggesting that coal flotation in saline water had a better tolerance for surface oxidation. The effects of water salinity on the flotation of oxidized coals were studied by investigating the change of coal surface properties and froth stability. On one hand, the surface hydrophobicity of oxidized coals decreased in saline water, which was unfavorable for coal flotation. On the other hand, the compression of the electric double layer in saline water could increase the flotation efficiency by reducing the electrostatic repulsion between particle and bubble. In addition, smaller bubbles and a higher stability froth were generated in saline water, which could also increase the flotation efficiency.

Effect of Dodecane and Oleic Acid on the Attachment between Oxidized Coal and Bubbles

The objective of this study is to explain the different flotation responses observed in oxidized coal flotation when using a nonpolar flotation collector, dodecane, versus a polar flotation collector, oleic acid. Particularly, the effect of each flotation collector on bubble–coal particle attachment was investigated. Colloidal probe atomic force microscopy (AFM) was used to directly measure the force between a model coal surface and a model bubble in the presence of either dodecane or oleic acid. Pyrolytic graphite (PG) treated with oxygen plasma and a polymethylmethacrylate (PMMA) particle were selected to represent the model oxidized coal surface and model bubble. High speed visualization for bubble–oxidized coal attachment was used to monitor the attachment behavior between bubble and oxidized coal in presence of dodecane and oleic acid, respectively. It was found that the force between the oxidized PG and the PMMA particle in Milli-Q water was monotonically repulsive, illustrating that oxidized coal particles attach onto bubble surface with difficulty. The flotation recovery using a traditional hydrocarbon oil, dodecane, was always lower than when oleic acid was used at a low dosage (300–1100 g/t). The force measurements showed that an attractive hydrophobic force was introduced when a 0.01 mM oleic acid solution was used, while the force was still repulsive in presence of a 0.01 mM dodecane solution. The minimum contact time for successful attachment between oxidized coal surface and bubbles in the presence of 0.01 mM oleic acid is much shorter than that in 0.01 mM dodecane. However, a high flotation recovery of 85.81% was obtained when dodecane concentration was further increased to 1700 g/t. The significant jump into contact effect observed in the AFM force curves and the short induction time in the presence of 1 mM dodecane solution was responsible for this high flotation recovery.

Improving the quality of coal is one of the factors in intensification of combustion processes in coal-based energy

For flotation enrichment of coals, spent engine oils modified with ozone were used. Preliminary ozonation contributes to the increase in the share of low-boiling components in the compositions of spent oils, also it reduces the viscosity of reagents and improves their flotation properties when coals are enriched. By flotation of coal, using ozonized spent oils, the yield of concentrate significantly increases, ash content is reduced, mineralization of waste flotation is increased, and as a result, the selectivity of the process is increased as well.