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To study the variation laws of the mechanical properties of coal with different oxidation degrees, coal samples were subjected to constant temperature oxidation at 80°C, 120°C, 160°C and 200°C by using a coal oxidation spontaneous combustion test system. Through a uniaxial compression experiment, the variation laws of mechanical parameters, failure characteristics and energy dissipation law of coals with different oxidation degrees were analyzed and obtained. The study shows that the stress – strain curves of coal with different oxidation degrees vary in different stages. The peak stress, elastic modulus, secant modulus and Poisson's ratio appear to decrease to different degrees with the deepening of oxidation, and the peak strain increases. The cracks on the surface of the coal samples increase and become more with the deepening of oxidation, the exothermic characteristics of the coal samples decrease, and the failure characteristics of the coal samples change from single shear-type failure characteristic to conjugate shear-type characteristic and then to shatter-type failure characteristic. With an increasing degree of oxidation, the total energy required for coal destruction gradually decreases, the conversion rate of total energy into elastic energy decreases, the dissipation energy gradually increases, and the release rate of elastic energy and the increase rate of dissipation energy gradually slow down after the peak strain. [ABSTRACT FROM AUTHOR]

The conversion of coal to a hydrophilic state presents a significant challenge for process engineers, especially when treating oxidized coals in flotation-based beneficiation plants. Flotation of oxidized coal often requires a substantial consumption of oily collectors, resulting in unacceptably low yields. To address this, several pre-processing methods have emerged for coal deoxidation, including mild grinding, surface activation, and ultrasonic or microwave treatment. However, their effective application depends on a thorough understanding of the degree of oxidation in the coals. Therefore, it is crucial to gain insights into the nature of oxidized coal before initiating the deoxidation process. This present study provides various characterization techniques such as XRD, SEM-EDS, FTIR Spectroscopy, Raman Spectroscopy, TGA, and Zeta Potential Analysis employed on oxidized low-grade coals. The study revealed that the proximate analysis of low-grade coals KC, DC, RC, and PC contains varying ash percentages of 32.65%, 44.47%, 25.44%, and 37.53%, respectively, with SiO2 and Al2O3 comprising over 80% of the total ash. XRD characterizations identified quartz and kaolinite as major mineral phases in all samples, while SEM-EDS confirmed rough coal surfaces with peeling, off-white aggregations, and surface pores. HGI value ranging from 80 to 91 was attributed to partial decomposition and surface cracking due to oxidation. FTIR spectra revealed pronounced oxidation behavior, with strong −OH peaks in the 3600–3700 cm−1 range. Zeta potential analysis affirmed the oxidized nature of the coals with a pH below 3.1 and an extremely negative zeta potential beyond 3.1. The findings mentioned above showcase the distinct characteristics of oxidized low-grade coals, which serve as crucial indicators for exploring coal beneficiation, especially through the flotation process. [ABSTRACT FROM AUTHOR]

To explore the surface properties of oxidized coal and reduce its risks of spontaneous combustion, the pore structure development of oxidized coal and its effect on oxygen adsorption capacity were studied by using liquid nitrogen adsorption and oxygen adsorption analysis techniques. The results show that the pore structure development of oxidized coal increases the pore volume and specific surface area. Micropore surface area is the main component of the specific surface area of coal. The mesopores of oxidized coal break into micropores and small pores. Among them, micropores are the most developed to generate more active adsorption sites, and the order of micropore growth is Coal 1 lignite>Coal 3 fat coal>Coal 2 gas coal. A deeper oxidation degree of coal corresponds to a greater number of active adsorption sites generated on the oxidized coal surface, which promotes the adsorption of oxygen on the surface. The order of oxygen adsorption capacity growth is Coal 1 lignite>Coal 3 fat coal>Coal 2 gas coal. The specific surface area is positively correlated with the maximum oxygen adsorption capacity. The order of sensitivity of the maximum oxygen adsorption capacity to the growth of specific surface area is Coal 1 lignite>Coal 2 gas coal>Coal 3 fat coal. The pore structure development could promote the generation of active adsorption sites and enhance the oxygen adsorption capacity of the oxidized coal surface. [ABSTRACT FROM AUTHOR]

With the deterioration of the high-grade coal seams and the inadequate adsorptions noticed with the oily collectors, along with the cost escalations in petroleum-based products, there is a potential for alternative collectors. For this cause, bio-based green flotation reagents and their conjugations are gaining more concern on account of their reduced cost and sustainability. In this paper performance efficiency of oily collectors, Vegetable oil (Crude Palm Oil), and their combinations are compared for high-ash oxidized Indian coal considering their fatty acid adequacies and availability in the local markets. The adsorption mechanisms of both single and mixed collectors on coal surfaces were conducted with the aid of Fourier Transform Infrared Spectroscopy, Gas Chromatography-Mass Spectrometry, X-ray photoelectron spectroscopy, and contact angle measurements. Different surface characterization results divulged that the arrangement of more oxygenated functional groups in the POC and KP could ameliorate flotation efficiency through dipole attraction or hydrogen bonding, in addition to the Van der Waals forces. A maximum yield of 72.84% could be obtained with KP collectors with reduction ash of up to 20.44% (from feed ash of 35.63%). The present biodegradable collector and the mixtures lay an essential pathway for the effective separation of oxidized medium volatile coking coals. [ABSTRACT FROM AUTHOR]

Numerous oxygen elements are present on the oxidized coal surfaces, resulting in the difficulties for flotation. In this study, the enhancement of oxidized coal flotation using the impact flow conditioning pulp was investigated, and the impact flow velocity was optimized. The enhancement mechanism of the impact flow conditioning pulp was investigated via a series of experiments. The results showed that the use of impact flow conditioning pulp could improve the cumulative combustible recovery and flotation kinetics of oxidized coal, and these increased with the increasing impact flow velocity. In the experimental range, an optimum flotation result was obtained at an impact flow velocity of 5 m/s. The SEM analysis showed that the impact flow conditioning pulp could remove the aggregations on the oxidized coal surface, exposing a fresh surface. The XPS analysis confirmed that the impact flow conditioning pulp could increase the carbon contents and decrease the oxygen contents on the oxidized coal surface. Further, the impact flow conditioning pulp could significantly enhance the hydrophobicity and floatability of oxidized coal. [ABSTRACT FROM AUTHOR]

The effect of pH on zeta potential, combustible matter recovery, and Fourier transform infrared (FTIR) spectrum of oxidized coal was studied, and also the effects of Na+, Mg2+, and Fe3+ on oxidized coal flotation at different pH were investigated. The optimal floatability of oxidized coal was explored through the conditions of pH and inorganic salt ion. Both combustible matter recovery and zeta potential decreased as pH increased. The maximum combustible matter recovery and approximate isoelectric points (IEP) were obtained at pH 3. The adsorption peaks of oxygen-containing functional groups and hydrophobic groups in FTIR spectrums were, respectively, lowered and enhanced while pH varied from 7 to 3. Oxidized coal flotation was improved by Na+, Mg2+, and Fe3+ in acidic condition but depressed in alkaline condition. Compared with the combustible matter recovery in the absence of inorganic salt ion, Na+ presented the maximum increase rate of combustible matter recovery at pH 6 but also the minimum at pH 12. The combustible matter recovery for Na+ gradually increased while that for Mg2+ and Fe3+ generally decreased with the increase in ion concentration. The optimal floatability was gained at pH 3 and in the presence of Na+ with a concentration of 1600 g/t. [ABSTRACT FROM AUTHOR]

High-rank coals are generally more floatable than low rank or oxidized coals due to their higher hydrophobicity. Surface oxidation of coals due to the exposure to atmospheric oxygen renders the surface some hydrophilic character. As a result, the low rank or oxidized coals do not effectively attach to air bubbles and result in lower flotation yields. Nonpolar collectors like diesel and other commercial collectors when used in a flotation process do not effectively attach to the hydrophilic sites of the oxidized coal surface leading to lower yields. Researchers have generally tried pre-treatment with reagents to improve the floatability of these types of coals. In the present work, we have synthesized two collectors, tetrahydrofurfuryl oleate based (C1) and tetrahydrofurfuryl ether based (C2), which contain both hydrophilic and hydrophobic components in some rationale so that they can attach to hydrophobic as well as hydrophilic sites of the oxidized coal. The attachment of synthesized collectors to the oxidized coal can provide better particle-bubble attachment which was evident from the improvement in contact angle from 73.4° to a maximum of 88.54°. Flotation results showed that the yield increased from 8% in absence of any collectors to 40% in presence of collector 1. The flotation performance was further enhanced by the synthesis of a novel frother and the synergistic effect of developed collectors and frothers produced higher yield (53%) and lower ash (10%) clean coal fines compared to some of the well-known collectors like diesel, polyethylene glycol and frothers like Methyl Isobutyl Carbinol (MIBC) used in the study. [ABSTRACT FROM AUTHOR]

Nowadays, the flotation of oxidized coal attracts more and more attention in China. In this study, a conventional flotation column with a novel of forth feed was used to beneficiate the oxidized coal. Exploratory experiments showed that the froth feed flotation can improve the overall combustible recovery by 22.25% with only 1.19% increase in ash content compared to conventional pulp feed flotation. Box-Behnken design (BBD), a response surface method, was used to design the flotation tests and model the effect of variables on responses. Four operating parameters, including frother concentration, superficial gas velocity, feed height and wash water rate as well as two response factors, namely ash content and combustible recovery were investigated. Meanwhile, their interactions were also described with three-dimensional response surface. Results showed that the models obtained for both ash content and combustible recovery were statistically significant. Additionally, the effect of wash water rate on ash content was found to be more significant than those of other parameters, while the combustible recovery depended more on the frother concentration. At last, the predicted optimal results were obtained within the experimental ranges, which well responded to the verification experiments. [ABSTRACT FROM AUTHOR]

In this study, coal tar, an industrial by-product of the carbonization/gasification of coal, was adopted as an economical flotation collector for recovering clean oxidized coal. The characteristics of high-temperature coal tar (HTCT), its performance as a flotation collector, and the interaction mechanism between HTCT and oxidized coal were analyzed by gas chromatography/mass spectrometry, flotation experiments, X-ray photoelectron spectroscopy, and dynamic contact angle measurements. The results revealed that HTCT has a strong selective collection ability because it contains a large number of aliphatic and aromatic compounds with oxygen-containing functional groups. At HTCT dosage of 3000 g/t, 79.23% of combustible matter with an ash content of 9.99% was obtained. The major mechanism responsible for the enhanced oxidized coal separation is that, with the adsorption of HTCT onto oxidized coal, the C–O hydrophilic functional groups were covered and an increased amount of the C–C/C–H hydrophobic functional groups of the oxidized coal surface was exposed, resulting in improved surface hydrophobicity and floatability of the oxidized coal. This study provides valuable guidelines for the re-utilization of coal tar and the flotation upgradation of oxidized coal. [ABSTRACT FROM AUTHOR]

An oxidized coal surface is more negatively charged and hydrophilic than that of fresh coal because of the introduction of polar oxygen functional groups. During flotation, the decreased attractive hydrophobic force between bubbles and coal particles and the increased repulsive electrostatic force result in a low recovery for oxidized coal. In the present study, preconditioning with positive-charged microbubbles (PCMs), generated using a microbubbles generator and a cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB), was proposed to enhance oxidized coal flotation. Conventional column flotation tests were also carried out for comparison. The results show that many oxygen functional groups (-OH and C-O) existed on the oxidized coal surface as indicated by X-ray photoelectron spectroscopy (XPS). A flotation recovery of 84.33% with an ash recovery of 82.97% was obtained when PCM was used, much higher than that of conventional flotation (a recovery of 17.82% with an ash recovery of 9.64%). PCM preferably adsorbed at the negative-charged hydrophilic sites on oxidized coal in the preconditioning stage. An additional attractive electrostatic double-layer force between oxidized coal and negative flotation bubbles was introduced, enhancing PCM preconditioning. However, the worst flotation selectivity was observed using PCM compared with that of conventional flotation based on Fuerstenau upgrading curves analysis. [ABSTRACT FROM AUTHOR]

Oxidized coal is difficult-to-float using conventional diesel as the collector due to the abundant oxygen-containing groups on coal surface. In this study, a short-chain cationic amine, n-octylamine, was added into diesel in 1:4 ratio for enhancing oxidized coal flotation. X-ray photoelectron spectroscopy (XPS) was used to identify the interaction mechanism between diesel-n-octylamine mixture (DO) and oxidized coal. The results showed that the flotation yield using 500 g/t DO mixture was much higher than that of using 2000 g/t diesel under a similar ash content of clean coal. The addition of n-octylamine was proven to be an effective way to lower the oily collector consumption in oxidized coal flotation. XPS showed that electrostatic bonding between n-octylamine and negative-charged hydrophilic sites on oxidized coal surface was responsible for the enhancement of flotation recovery using DO as the collector. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Oxidized 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. [ABSTRACT FROM PUBLISHER]

Colloid mill attrition can remove the oxidized layer from oxidized coal surface. The effects of colloid mill gap space and milled pulp density were investigated according to the flotation response. It was found that both particle size reduction and flotation recovery enhancement were higher at higher pulp density and smaller gap space. When colloid mill worked at higher pulp density and smaller gap space, oxidized layers of coal particles could be removed more thoroughly. In addition, the greater reduction in particle size at higher pulp density and smaller gap space should be conductive to the flotation of oxidized coal because large particle size of oxidized coal is considered not suitable for its flotation. Because colloid mill was successfully applied in improving oxidized coal flotation, the results of this paper will promote the industrial application of colloid mill into effective flotation of oxidized coal in coal preparation plants. [ABSTRACT FROM AUTHOR]

In this investigation, the flotation behavior of oxidized coal affected by the conditioning time separated into two periods: prewetting time and collector conditioning time, were discussed. The prewetting time is 0 min, 1 min, 2 min, 3 min, 4 min, and 5 min, respectively. The collector conditioning time is 1 min, 2 min, 3 min, 4 min, 5 min, and 6 min, respectively. Fourier-transform infrared was used to analyze the chemical property of oxidized coal and fresh coal. The results show that the oxidized coal surface is more hydrophilic than the fresh coal. The results of flotation tests show that combustible matter recovery will be higher while the prewetting time is shorter. The mechanism of effects of conditioning time on the flotation of oxidized coal was discussed. [ABSTRACT FROM PUBLISHER]

In this investigation, a comparison was done between the use of dodecane and oxidized diesel oil as collectors to float oxidized coal. FTIR was used to determine the chemical properties of oxidized diesel oil and oxidized coal, which showed that oxidized coal has less hydrophobic functional groups while oxidized diesel oil has high hydrophobic groups that will enhance oxidized coal flotation. Oxidized coal was dry ground for different times, namely, 0 min, 40 min, and 80 min and the flotation results showed that, compared to the dodecane as the collector, the oxidized diesel provides much higher (40% to 70%) combustible recovery. Grinding of coal and the use of oxidized diesel oil improved the flotation of oxidized coal due to the hydrogen bonding with the polar part of oxidized coal surface and oxidized diesel oil. [ABSTRACT FROM AUTHOR]

Oxidized coal is difficult to float using either kerosene or diesel. In this investigation, we compared the flotation results obtained using biodiesel with those using diesel as a collector. It was found that the biodiesel could improve flotation of oxidized coal. FTIR study results show that the biodiesel contains much more abundant unsaturated fatty acids than diesel and the oxidized coal has much less hydrophobic functional groups than fresh coal. [ABSTRACT FROM PUBLISHER]

Two preconditioning methods, namely, ultrasonication and high-intensity conditioning in an agitated vessel, were compared as a means for improving the flotation of oxidized coal. The use of preconditioning by ultrasound or high-intensity conditioning significantly improved the flotation of the oxidized coal in batch-type laboratory tests, particularly with respect to the flotation rate and the recovery of combustible matter. In addition, the rate and recovery of the smaller size fractions were enhanced considerably. The beneficial effects of preconditioning on the batch flotation of oxidized coal were clearly reflected in smaller bubble size distributions, higher froth loadings, and greater stability of the flotation froths. [ABSTRACT FROM AUTHOR]