Hydrophobicity Of Coal Research Articles

Optimization of grinding pretreatment for effective flotation of oxidized coal

ABSTRACTOxidized coal is difficult to float with common oily collectors because oxidation processes occurring on the coal surface reduces the surface hydrophobicity of coal. Grinding pretreatment has been widely applied in the improvement in the floatability of oxidized coal. However, there is no sufficient investigation into the optimization of grinding conditions for the effective flotation of oxidized coal. This paper is to reveal the effects of grinding conditions, such as grinding time, pulp density, and feed amount on the flotation behavior of oxidized coal. The optimum grinding conditions are as follows: the feed amount is 70 g, the pulp density is 40%, and the grinding time is 15 min. Under the optimum grinding conditions, 62.53% combustible matter recovery with 12.99% clean coal ash content is obtained.

Aggregation Mechanism of Particles: Effect of Ca2+ and Polyacrylamide on Coagulation and Flocculation of Coal Slime Water Containing Illite

Illite is one of the main components in coal slime water and it sometimes makes the water extremely difficult to clarify. In this study, the aggregation mechanism of coal and illite particles was investigated using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and settling experiments of slime water containing coal and illite. The results show that electrostatic energy plays a dominant role and manifests repulsive force in the long-range (>4 nm). However, the leading role becomes a hydrophobic force determined by the polar surface interaction energy in the short-range (<4 nm). A coagulant (Ca2+) can lower the surface electric potential to make all particles easier to coagulate, while the surface hydrophobicity of coal and illite determines whether the particles aggregate. Cationic polyacrylamide (CPAM) can promote the sedimentation of particles flocs effectively and makes the supernatant clearer due to attractive electrostatic forces while anionic polyacrylamide (APAM) flocculates particles through the bridge mechanism. Although the hydration shell on the hydrophilic surface of illite appears to be harmful for either coagulation or flocculation, illite can accelerate sedimentation when it is attached to the coal due to its higher density.

Effect of compound collector and blending frother on froth stability and flotation performance of oxidized coal

The beneficiation of oxidized coal is difficult to achieve by conventional flotation with oily collectors. Oxidized coal samples were prepared by the peroxide solution oxidation and a new type of oxygenated polar compound collector (CC) and a powerful blending frother (BF) were used to intensify oxidized coal flotation in the present study. The effects of the CC and BF on the froth stability and flotation performance (i.e., recovery and selectivity) were investigated. The results show that only 44.51% recovery with 9.71s dynamic froth stability index (DFS) was obtained for oxidized coal flotation by using conventional diesel and octanol as the collector and frother, respectively. Lots of oxygen functional groups (−OH and C-O) were introduced after peroxide oxidation indicated by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis, leading to poor hydrophobicity and low flotation recovery. An optimal separation result with 76.24% oxidized coal recovery and 19.43s DFS was obtained using the CC combined with octanol, which was attributed to the hydrogen bonding between the hydrophilic sites on coal surface and polar oxygen functional groups in CC. An identical DFS (19.43s) was obtained using conventional diesel combined with the BF, while with only 21.38% recovery. Diesel did not sufficiently render the coal hydrophobicity and as a result only a small amount of coal particles was collected by attaching to bubble surface in pulp phase. The Fuerstenau upgrading curves show that both the maximum recovery and the best flotation selectivity could be obtained using the CC combined with octanol.

FLOTATION OF LONG FLAME COAL PRETREATED BY POLYOXYETHYLENE SORBITAN MONOSTEARATE

In this investigation, polyoxyethylene sorbitan monostearate (Tween 60) was used to improve flotation of fine long flame coal. The flotation recovery of long flame coal could be increased when long flame coal was either pretreated or conditioned with Tween 60 in a flotation cell for a period before the addition of collector. Fourier Transform Infrared (FTIR) technique was used to indicate surface properties of long flame coal. The results of FTIR show that there are many oxygen functional groups on the sur- face. Contact angle measurements were used to indicate changes in hydrophobicity of coal surface before and after Tween 60 and/or diesel pretreatments. The results of contact angle measurements show that hydrophobicity of coal can be increased by Tween 60. Tween 60 can also enhance adsorption of diesel on the coal surface, and hence floatability of long flame coal can be further improved. Tween 60 primarily enhances the flotation recovery of low density coal fractions (<1.5 and 1.5-1.8 kg/dm 3 ). However, the increase in flotatation recovery is less significant with an excessive addition of Tween 60.

Adsorption of collectors on model surface of Wiser bituminous coal: A molecular dynamics simulation study

Coal flotation makes use of the hydrophobicity of coal surface. To enhance the hydrophobicity of coal particles, oily collectors are usually added. Although collectors have been widely applied in coal processing, the microscopic collection mechanism remains a formidable task due to the complexity of coal surface structure. In this work, different combined systems formed by collectors, water and a model surface of Wiser bituminous coal have been studied using molecular dynamics calculations. We modeled the interaction of each collector oil droplet with the coal surface in aqueous phase. In addition, water/collector model systems were also considered in order to reproduce the interaction of the different collectors with water molecules in the collector adsorption process. The results indicate that all three collectors, i.e. dodecane, nonylbenzene and nonylphenol, form spherical insoluble oil droplet in aqueous phase. After the addition of collector droplet over coal surface, all these collectors could thermodynamically repel the interfacial water molecules and spread over the coal surface, thereby reducing roughness of coal surface. The adsorption of nonylphenol on coal surface via both π–π stacking and hydrogen bonds is most favorable, followed by that of nonylbenzene and dodecane. The presence of collector on the coal surface could improve the hydrophobicity. The number of hydrogen bond between water and modified coal surface decreases and the mobility of water molecules increases compared to the original coal surface. However, the extent of hydrophobicity of modified coal surface by these collectors is inconsistent with their adsorption capabilities. Nonylphenol is the worst collector to improve the surface hydrophobicity of coal as part of nonylphenol molecules orient away from coal surface to form hydrogen bonds via their hydroxyl groups with water molecules. Dodecane should be the most effective collector as the hydration energy of the modified surface and the number of formed hydrogen bond between this surface and water molecules are the lowest, whereas the mobility of water molecules on this modified surface is the highest.

Comparison of surface properties of anthracite coals before and after high temperature heating process

Surface property plays a very important role on coal hydrophobicity which determines flotation behavior of fine coals. However, coal spontaneous combustion makes coal suffer both oxidation and high temperature heating processes. Coals from the outside of coal piles primarily suffer oxidation process (or combustion process) while coals from the inside of coal piles primarily suffer high temperature heating process. It is necessary to investigate the changes in surface properties of coal before and after high temperature heating process. In this investigation, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), surface area (BET) and contact angle measurements were used to indicate the changes in surface properties of anthracite coal before and after high temperature heating process. Throughout this study, surface properties of coal can be changed significantly after high temperature heating process. Meanwhile, coal hydrophobicity can be also reduced after high temperature heating process.

Changes in surface properties of anthracite coal before and after inside/outside weathering processes

XPS, SEM and contact angle measurements were used to indicate the changes in surface properties of anthracite coal before and after inside/outside weathering processes. Weathering processes of anthracite coal were conducted on the roof and in the room respectively. XPS results showed that the content of CC and CH groups on the surface of anthracite coal was decreased after weathering processes while the content of CO, CO and OCO groups was increased. SEM results showed that the surface roughness of anthracite coal was increased after the weathering processes. Contact angle measurements showed that the contact angle of weathered coal was lower than that of fresh coal. Throughout this paper, it was found that inside weathering processes were less intense than outside weathering processes. The hydrophobicity of anthracite coal was less changed by inside weathering processes than outside weathering processes. Hydrophobic coal surface should be more useful for the flotation processes of fine coals than hydrophilic coal surface. It was recommended that storing coal inside, such as in the room or in the raw coal barns could avoid the heavy weathering processes.

Changes in the hydrophobicity of anthracite coals before and after high temperature heating process

Coal spontaneous combustion can make coal suffer a high temperature heating process. This heating process can change physical and chemical properties of coal, but the changes in the hydrophobicity of coals before and after high temperature heating process have not been investigated adequately since the natural hydrophobicity of coal usually determines the flotation behavior of fine coals. Therefore, this paper is to investigate the changes in the hydrophobicity of coals before and after the high temperature heating process. X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used to indicate these changes. The heating temperature was 600°С. The heating times were 0 (original coal), 10, 20, 60, and 120min, respectively. An ultra-low-ash anthracite coal with ash content of 1.55% was used to conduct the high temperature heating process in the laboratory. Throughout this study, the hydrophobicity of anthracite coal can be reduced quickly after high temperature heating process since the content of hydrophilic functional groups on anthracite coal surface is increased while the content of hydrophobic functional groups is reduced by the high temperature heating process.

Adsorption of Paenibacillus polymyxa and its impact on coal cleaning

The adsorption of micro-organisms and bacteria on minerals surfaces depends mainly on the type of the bacteria used as well as the nature of the studied mineral surface. Such adsorption could change the surface properties of the mineral surface and leads to control its surface for increasing its separation selectivity from associated impurities or enhance/retard the adhesion with other substances. Therefore, in the current study, adsorption of Paenibacillus polymyxa (P. polymyxa) on coal was studied. Several methods were used such as: zeta potential, adsorption isotherms, adsorption kinetics and Fourier Transform InfraRed (FTIR). The main goal is to determine the difference in surface behaviour of coal particles before and after the treatment with P. polymyxa bacteria. The results showed that electrostatic interactions are insignificant in P. polymyxa adsorption on coal particles. The results suggest that the adsorption of bacteria on the coal particles is mainly physical and it depends on electrostatic forces, hydrogen bonding as well as the hydrophobic forces between the bacteria wall and the organic matter in the coal and coal hydrophobicity.

A short review of improvement in flotation of low rank/oxidized coals by pretreatments

Low rank/oxidized coals are difficult to float with oily collectors. The presence of polar hydroxyl, carbonyl, phenol groups and some peroxide type oxygenated moieties on the surface of low rank/oxidized coals is the main factor which decreases the coal hydrophobicity. Pretreatments may be used to enhance the flotation of low rank/oxidized coals due to improved hydrophobicity. The primary methods of pretreatments are grinding, premixing/preconditioning, ultrasound, thermal, microwave and direct contact mixing of the reagents with dry coal before wetting. Among these pretreatment methods, grinding, premixing/preconditioning may be considered to remove the oxide layer on the oxidized coal surface, which is also reflected by ultrasound treatment. Thermal and microwave treatments are attributed to the removal of pore water, hydration water and some hydroxy functional groups in low rank/oxidized coals. Direct contact procedure makes the oxidized coal avoid the prevention for the collectors absorbing on the coal surface. The prevention usually comes from hydration shell on wet coal surface while the coal is wetted by water. This review is to highlight the effect of different pretreatment methods on the flotation of low rank/oxidized coals.

Investigation of Bituminous Coal Hydrophobicity and its Influence on Flotation

The hydrophobicity of bituminous coal surface has a significant effect on the separation of coal and mineral matters by flotation process. In this work, water contact angles on bituminous coals displaying different brightness in terms of specular reflectance were measured to characterize their surface hydrophobicity. It is found that water contact angle on coal increases with specular reflectance for both bright and dull coals, and the bright coals have higher contact angle and specular reflectance than the dull coals. The oxidation of coal surface will render it less hydrophobic, but has little effect on the specular reflectance. Results from petrographic analysis show remarkable differences in volume percentage of maceral groups for bright and dull coals. Bright coals contain more vitrinite, less inertinite and mineral mater, than the dull coals. X-ray photoelectron spectroscopy (XPS) analysis indicates that the dull surface has more hydrophilic, oxygen-bearing functional groups. These results imply that vitrinite is more hydrophobic than inertinite, and should float better than the latter. The combustible recoveries of bench scale flotation tests increase with the corresponding contact angles and their relationship can be accurately described by a sigmoidal equation. The maximum combustible recovery for vitrinite is 96.76% under the current operation conditions. The results also indicates that in order to reach the targeted combustible recovery in flotation, the contact angle of coal particles needs to reach certain value.

Surface chemistry aspects of coal flotation in bore water

Several theories have been proposed to explain enhancement of coal flotation in salt solutions. In this paper, surface chemistry aspects of coal flotation in bore (hypersaline) water were examined using bubble-particle attachment time experiments, zeta potential measurements, cyclic measurements of contact angle and Atomic Force Microscopy (AFM). The attachment time experiments showed that the bubble-particle attachment in deionized water was instantaneous and independent of the particle size. The attachment in bore water required longer time, which increased with increasing particle size. The cyclic measurements of contact angle on a flat coal surface showed that the coal hydrophobicity as measured by the advancing (maximum) and receding (minimum) contact angle did not change in the presence of salt ions. The zeta potential measurements show that both the coal particles and air bubbles were negatively charged in bore water. The AFM studies showed that bore water reduced repulsive surface forces between the coal particles and air bubbles but had little effect on the force of adhesion. The overall results suggest that enhancement of coal flotation in hypersaline water is not entirely attributed to the surface chemistry aspects as previously proposed.

Study of the effect of ultrasonic treatment on the surface composition and the flotation performance of high-sulfur coal

This paper highlighted the use of X-ray diffraction, scanning electron microscopy and X-ray fluorescence spectroscopy to investigate the changes on the surface composition of high-sulfur coal and pyrite before and after ultrasonic conditioning. The results showed that ultrasonic conditioning resulted in a decrease in the contents of iron and sulfur in coal, an increase in the content of element carbon, and an increase in the purity of the coal. Conversely, ultrasonic conditioning led to an increase in the content of iron and sulfur in pyrite, a decrease in the impure content of calcium, and a relative increase in the purity of the pyrite after ultrasonic conditioning. This study verified that on the one hand, ultrasonic conditioning can promote the pyrite separation from the high-sulfur coal, with the separated pyrite taking the form of FeS; on the other hand, it can produce a cleaning effect on the surface of coal and pyrite with the consequent increase both in hydrophobicity of coal and hydrophilicity of pyrite. The paper introduced ultrasonic pre-treatment of the slurry and stepped froth removal tests of high-sulfur coal and the study on the yield, ash and sulfur content of clean coal in different phases. The results gave further evidence of the increases both in the rate and the selectivity of flotation. This study shows that ultrasonic conditioning can enhance the performance of de-sulphurization of high-sulfur coal flotation.

Study of Enhanced Fine Coal De-sulphurization and De-ashing by Ultrasonic Flotation

Abstract The feasibility of using ultrasound to enhance the performance of de-sulphurization and de-ashing during slime flotation was investigated. The Setaram C80 calorimeter, the contact angle gauge DCAT21 and an electrophoresis apparatus were used to study the surface nature of coal, pyrite and refuse before and after ultrasonic conditioning. The yield, ash and sulfur contents of equally sized coal slimes were also measured before and after ultrasonic conditioning. The results show that ultrasonic conditioning can drive the separation of pyrite and refuse from coal. After ultrasonic conditioning the hydrophobicity of coal and hydrophilicity of pyrite and refuse increase. The perfect index of flotation, the perfect index of de-sulphurization and the percentage of de-sulphurization increase by 22.51%, 25.36% and 2.49%, respectively. This study shows that ultrasonic conditioning can enhance the performance of de-sulphurization and de-ashing of coal flotation methods.

Coal dust dispersal around a marine coal terminal (1977–1999), British Columbia: The fate of coal dust in the marine environment

Abstract A 1999 assessment of sediments, adjacent to the Roberts Bank coal terminal in Delta, British Columbia, Canada, shows that the concentration of coal particles (reported as non-hydrolysable solids or NHS) has increased substantially since a prior study in 1977. NHS concentrations have doubled from a mean concentration of 1.80% in 1977 to 3.60% in 1999. Overall the dispersal distance of coal has not increased over the 22-year period but rather the abundance of coal in the surface sediment within the dispersal area has increased. Since 1977 the main deposition of coal has occurred in the vicinity of the coal-loading terminals, where concentrations of 10.5% and 11.9% NHS (non-hydrolysable solids = coal) occur. The settling properties of fresh and oxidized coal particles ( 2.36 mm) were examined in order to better understand the dispersal of coal in marine waters. No change in settling velocity of coal particles occurred with increasing oxidation. However, the proportion of buoyant coal particles decreases with oxidation in all size fractions, reflecting the decrease of coal hydrophobicity with oxidation. The distribution of coal around the terminals agrees with measured particle settling velocity and current velocity, with coal concentration decreasing rapidly away from the terminal. Coarser sediment fractions contain the highest coal (NHS) concentrations and carbon / nitrogen ration when compared to finer fractions. Coal particles with > 2.36 mm diameter (settling velocities ≤ 10.54 cm/s) settle out close to the terminal (depending on currents), whilst small ( Coal distribution would likely affect those benthic flora and fauna, most susceptible to coal dust coverage and possible anoxic conditions that might arise during coal oxidation within very close proximity (0–100 m) to the coal-loading terminal.

Coal–oil assisted flotation for the gold recovery

Using coal–oil agglomeration method for free or native gold recovery has been a research subject for many researchers over the years. In this study, a new approach “coal–oil assisted gold flotation” was used to recover gold particles. The coal–oil–gold agglomeration process considers the preferential wetting of coal and gold particles. The method takes advantage of the greater hydrophobicity and oleophilicity of coal and gold compared to that the most gangue materials. Unlike the previous studies about coal–oil–gold agglomeration, this method uses a very small amount of coal and agglomerating agents. Some experiments were conducted on synthetic gold ore samples to reveal the reaction of the coal–oil assisted gold flotation process against the size and the number of gold particles in the feed. It was observed that there is no significant difference in process gold recoveries for feeds assaying different Au. Although there was a slight decrease for coarse gold particles, the process seems to be effective for the recovery of gold grains as coarse as 300 μm. The decrease in the finest size (<53 μm) is considered to be the decrease in the collision efficiency between the agglomerates and the finest gold particles. The effect of changing coal quantity for constant ore and oil amounts was also investigated. The experiments showed that the process gives very similar results for both artificial and natural ore samples; the best results have been obtained by using 30/1 coal–oil ratio.

Determination of Free and Bound Water in Fine Coal Filter Cake

The bound water in fine coal filter cake represents the water that cannot be removed using mechanical dewatering techniques. The amount of bound water in a fine coal filter cake, in other words, represents the moisture content achievable by an ideal dewatering technique. In this study the dilatometer technique was used to measure the bound and free water in a fine coal filter cake. Results showed that addition of cationic (0.25 kg/t) and non-ionic (1 kg/t) surfactants reduced the bound water content in the filter cake from 13.73% to 3.72% and 8.28%, respectively. Decrease in bound water content was attributed to the adsorption of surfactants on the coal surface, which increased the coal hydrophobicity. Addition of 10 g/t of cationic and anionic flocculants also reduced the bound water content from 13.73% to 4.58% and 6.63%, respectively, which was attributed to the replacement of surface water molecules with the polymer. Based on the free and bound water data, ideal dewatering curves were developed for both surfactants and flocculants.

Research on Mechanism of Enhanced Desulfurizing Flotation of High Sulfur Coal with Electrolytic Reduction

Pretreatment of high sulfur coal by electrolytic reduction reduces the number of oxygen-functional groups on the coal surface and improves coal hydrophobicity, and reduces the initial oxidation products on the pyrite surface (e.g: polysulfide, free sulfur, etc.) increasing pyrite hydrophilicity. This improves the selectivity of the subsequent flotation process. The results indicate that the electrolytic reduction can significantly decrease the content of pyrite sulphur in the floatation concentrate. Using this method, 89% of pyrite in the Beisu coal could be rejected while only 54.5% of pyrite sulfur was rejected without the treatment.

Coal Reverse Flotation. Part I. Adsorption of Dodecyltrimethyl Ammonium Bromide and Humic Acids onto Coal and Silica

Adsorption of dodecyltrimethyl ammonium bromide (DTAB) and humic acids (HA) on bituminous, oxidized bituminous, and subbituminous coals, as well as on silica, was studied through direct adsorption, electrokinetic, and contact angle measurements. It was concluded that the adsorption of DTAB and HA on a hydrophobic coal surface takes place through hydrophobic interactions between the hydrocarbon chains of these compounds and the hydrophobic coal surface. The adsorbed DTAB molecules cause a gradual decrease of coal hydrophobicity due to their tail-to-surface orientation. The adsorption density of DTAB on a hydrophilic/oxidized coal is much higher than on a bituminous coal. The results indicate that the adsorption mechanism involves some strong interactions between the cationic head group of the surfactant and the negatively charged oxygen groups on the coal surface. Despite the apparent head-to-surface orientation, the adsorbed DTAB molecules do not render the surfaces hydrophobic; this probably results from the fairly chaotic orientation of DTAB molecules on the surface of oxidized coal. The well-ordered conformation of the DTAB ions adsorbed onto the negatively charged silica surface, combined with significant adsorption of this surfactant at the solution/air interface makes the silica surface very hydrophobic.

A new flotation scheme for a difficult-to-float coal using pitch additive in dry grinding

Unlike bituminous coals, lower rank coals are inherently hydrophilic and thus difficult to float with typical oily collectors in moderate additions. The poor floatability of Soma coal was enhanced through a new processing scheme involving dry grinding of coal in the presence of bituminous coal pitch followed by flotation with usual collectors. The floatability of the coal was further improved by heating the coal in the temperature range of 300–600°C. Addition of pitch in dry grinding is found to achieve remarkable increase in recoveries from 22.1 to 89.9 wt% and reduce ash levels from 11.8 to 10.1 wt%. The mechanism of this selective improvement in the hydrophobicity of coal is discussed on the basis of the experimental data.