Combustible Recovery Research Articles

Improvement of coal flotation by exposure of the froth to acoustic sound

Simultaneously improving coal flotation performance and the subsequent froth breakdown is challenging. The present work addresses this challenge by developing and evaluating a non-chemical approach based on the stabilization effect of acoustic sound on flotation froth. The effectiveness of this approach for improving coking coal flotation at different frother concentrations (10–25 ppm MIBC), collector dosages (0–400 g diesel/t) and aeration rates (1.2–2.2 cm/s) was demonstrated at laboratory scale and in batch mode, with a loudspeaker being placed in air above the flotation cell lip. It was found that at any operating condition tested, the use of sound could improve the flotation performance including the final combustible recovery, apparent flotation rate constant and separation efficiency rate. There was no statistically significant difference in product ash content between the flotation tests with and without applying sound, except at the highest MIBC concentration tested. The images of the product froth taken before and 5-min after being discharged from the flotation cell indicated that the froth stabilization effect induced by sound would occur only within the sound field, demonstrating the potential of the approach in mitigating the overfrothing issue without sacrificing the flotation performance. The possible mechanisms of the improvement of flotation performance with applying sound and its potential of improving coal flotation practice through reducing reagent and air consumption were discussed.

Improving Low Rank Coal Flotation Using a Mixture of Oleic Acid and Dodecane as Collector: A New Perspective on Synergetic Effect

The mixed collector can improve low rank coal flotation efficiency, but its synergistic mechanism needs to be further explored. In this paper, oleic acid-dodecane (OA-D), oleic acid (OA), and dodecane (D) were employed to treat the low rank coal for revealing new synergistic mechanism of the mixed collector. First the surface free energy of the coal, the surface free energy of coal-water and coal-water-coal were calculated. Then wetting heat measurement, X-ray Photoelectron Spectroscopy (XPS) and FTIR were used to analyze synergistic mechanism of the mixed collector in depth. The results showed that OA-D obtained a higher combustible recovery than using OA and D, respectively. The essence of synergetic mechanism of OA-D was that they formed a relatively ordered “supramolecular structure” on the low rank coal surface, especially there were hydrophobic and van der Waals forces between the oleic acid chain and the dodecane chain that can promote the formation of a continuous collector film.

Effect of reagents on flotation kinetics of differently sized feeds

ABSTRACT The particle size and reagents are the most important parameter in flotation performance. Flotation kinetics models are used to define the rate of flotation and ultimate recovery. In this paper, flotation kinetics studies on a composite feed of −0.5 mm and three split-sized feeds including −0.5+0.25, −0.25+0.1, and −0.1 mm were conducted by using four different reagent combinations. The experimental data in terms of both combustible recovery – time and ash recovery – time were fitted with six different kinetic models to evaluate the best-fitted kinetics model. The feeds of different sizes responded differently to flotation kinetics with different reagent combinations. The combustible particles for all the sized feeds followed only first-order kinetics, whereas ash-bearing particles followed the second-order kinetics. Although coarsest particles have the lowest kinetics reported in the literature, synthetic collector and synthetic frother exhibited the highest kinetics by coarsest feed and lowest by intermediate-sized feed. Other reagent combinations show the highest kinetics by intermediate-sized feed. The selectivity of a particular reagent combination was found to be different for different sized feeds.

The Effect of Hydrothermal Treatment on Structure and Flotation Characteristics of Lignite and a Mechanistic Analysis.

Lignite is difficult to obtain highly efficient indexes by conventional flotation due to its poor surface hydrophobicity. Although the modification of flotation reagents has made some progress in improving the flotation performance, they all remain at the stage in the laboratory. Here, we proposed to improve the flotation performance by hydrothermal treatment dewatering (HTD) for lignite. Combined with the 13C NMR and FT-IR analysis, the impact of the HTD process on lignite’s chemical structural evolution and flotation performance was investigated. The results showed that the HTD process is an effective means for dehydration and deoxygenation to increase lignite quality and the metamorphic degree of coal. The content of oxygen-containing functional groups generally decreases during the HTD process, especially carboxyl acid and ether groups. Therefore, surface properties and wettability of HTD coal samples were changed, and the contact angle gradually increases with the HTD temperature increased, which enhance the hydrophobicity and decrease hydrophilia of the lignite surface. The HTD process effectively improves the flotation performance of lignite during the conventional flotation operation condition, even if the coal pulp after HTD was directly used as a flotation feedstock. Specifically, the coal oil and capryl alcohol were used as collector and forming agents, respectively, and the pulp concentration was adjusted to 60 g/L. The yield of the cleaned coal increased from 12.14% of the raw coal to 55.58% of HTD310, and combustible matter recovery increased from 13.83% of the raw coal to 65.17% of HTD310 by raw coal basis.

Effect of ultrasonic standing waves on flotation bubbles

Ultrasonic flotation was an effective method to float fine coal. In this study, the effects of the standing waves with different frequencies on ultrasonic flotation were investigated. The dynamic processes of bubble and coal-bubble were revealed by a high-speed camera. The results showed that under the action of Bjerknes force, bubble aggregates were formed within 450 ms and coal bubble aggregates were formed within 20 ms. The bubble aggregates were statistically analyzed by image processing method. The number of aggregates and small bubbles in the ultrasonic field at 100 kHz was greater than those at 80 and 120 kHz. Besides, 100 kHz ultrasonic flotation achieved the highest yields of clean coal (35.89%) and combustible recovery (45.77%). The cavitation bubbles acted as either a “medium” or an “inclusion”, entrapping and entraining the coal particles in the flotation pulp. It promoted the aggregation of bubbles with coal particles, so the flotation efficiency was effectively improved in the presence of ultrasonic standing waves.

Influence of hydrodynamic cavitation pretreatment on coal flotation

The flotation of fine coal particles has been a long-standing challenge in coal processing industry. In this study, the influence of hydrodynamic cavitation pretreatment (HCP) on coal flotation was investigated. A special HCP experiment system was designed for the conditioning of pulp based on the cavitation phenomenon through a venturi tube. Flotation results indicate that, compared with the conventional flotation method, HCP increases combustible recovery of concentrate by about 5%, besides, 50% collector dosage or 25% frother dosage can be saved with HCP under the same combustible recovery of concentrate. Meanwhile, HCP enhances the flotation rate constant by 9.41%. The sieving analysis of flotation products reveals that HCP flotation has a better response for fine coal particles relative to that of coarse coal particles. The bubble observation tests confirm the existence of micro-nano bubbles generated by HCP and the selective generation of micro-nano bubbles on the surface of coal. In addition, HCP leads to an obvious improvement of dispersion of collector. The aggregation of fine coal particles caused by HCP is another important contribution for the coal flotation. [Received: May 18, 2019; Accepted: March 28, 2020]

Effect of slurry preconditioning on the occurrence of major minerals in the flotation products of coal

ABSTRACT Organic and inorganic matters are two main compositions of coal, and their occurrence forms in flotation products are closely associated with the ultimate flotation performance. Considering that pulp-mixing stage can strongly affect particle surface properties, this study tentatively investigated the effect of slurry preconditioning on the element distribution of major minerals in flotation products of coal using a jet-flow mixer under various energy inputs. Measurements were obtained using X-ray diffraction, X-ray fluorescence, laser particle size analyzer, X-ray photoelectron spectroscopy, and a scanning electron microscope. The flotation results indicated that the optimal performance was obtained at 98.57 W/kg with a maximum combustible matter recovery (93.51%) and yield (85.56%). Throughout the flotation time, three variational stages of particle diameters were identified from relatively medium size to finer size and then to coarser size, corresponding to a progressive increase in the ash content of concentrates. Furthermore, the combined analyses of mineral phase and morphological characteristics revealed that kaolinite exhibited a higher removal rate than quartz in concentrates, for an appropriate conditioning intensity could detach fine slimes coating on coal surface but weakly influence the interlocked coal-mineral bodies. On the whole, an optimized preconditioning process could intensify the selective mineral migration to some extent.

Assessing the Applicability of Gravity Separation for Recycling of Non‐Metal Fraction from Waste Printed Circuit Boards

AbstractPrinted circuit boards (PCB) are one of the most studied electronic waste streams due to the presence of high‐value metals. The rejects from waste PCB recycling, also known as a non‐metal fraction (NMF), are usually sent to landfill. This work explores the applicability of gravity separation, widely used in the mineral processing industry, to separate organic and inorganic components in the NMF. The float‐sink test is conducted to understand the gravity separation performance, and the results show that a 47% yield at 86% organic content with 70% combustible recovery can be obtained using dense media separation. The washability behavior of NMF is assessed using different washability indices standard in the coal industry; and the material is classified as difficult‐to‐clean. The float‐sink products show the concentration of organic stream in −1.8 g cm−3 range, fiberglass in +2.0–2.5 g cm−3 range, and metals in +2.5 g cm−3 range. The recovery of these components will facilitate the concept of the circular economy and promote sustainability.

Effect of mixed cationic/anionic surfactants on the low-rank coal wettability by an experimental and molecular dynamics simulation

Mixed surfactants consisting of the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and the anionic surfactant sodium dodecyl sulfate (SDS) demonstrated high floatability of low-rank coal, and the optimal molar ratio of CTAB and SDS was 1:9. The surface and thermodynamic parameters (CMC, γCMC, Γmax, Amin, ΔGm0) obtained from surface measurements indicated higher surface activity and denser molecular arrangement in the mixed CTAB/SDS surfactants than in single CTAB or SDS. The contact angle of the coal sample increased from 38.9° to 73.7°, and the wetting heat of the coal became less negative, which showed a better positive effect on enhancing the contact angle and restricting the hydrophilicity of the coal surface. Molecular dynamics (MD) simulations were carried out to investigate the influence on the wettability of low-rank coal. The results indicated a significant increase in hydrophobicity of the low-rank coal. Furthermore, the combustible recovery was enhanced significantly by up to 47.67%, and the concentrated ash value was increased by up to 3.63%.

Optimizing oxidized coal flotation using impact flow conditioning pulp

ABSTRACT 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.

The application of combined lignite cleaning processes, bacterial leaching and flotation, for reducing higher ash and sulfur contents

ABSTRACT In this study, biological and physicochemical processes were combined to decrease the ash and sulfur contents of finely ground coal. Firstly, biodesulphurization experiments of lignites with high ash and sulfur contents were carried out with air-supply fiberglass bioreactor using Acidithiobacillus ferrooxidans. At the end of the 12-hour bacterial desulfurization experiments; ash content, sulfur content, calorific value, sulfur emission value, recovery of calorific value, and combustible recovery values were 33.88%, 3.97%, 3982 kcal/kg, 0.24% 91.22%, and 89.74%, respectively. Afterward, in the second stage experiments, flotation method was used to beneficiation Can/Canakkale lignites by physicochemical methods. In flotation experiments, the effects of collector (Fuel oil; 0–900 g/t) and frother (MIBC; 0–300 g/t) dosages on flotation were investigated. Optimum collector and frother dosages were determined as 750 g/t and 250 g/t in flotation experiments, respectively. In flotation experiments under optimum conditions ash content, sulfur content, calorific value, sulfur emission value, recovery of calorific value and combustible recovery values were 16.38%, 0.75%, 5166 kcal/kg, 0.03% 98.60%, and 96.11%, respectively. As a result of all experiments, ash and sulfur removal efficiencies were carried out 65.12% and 86.93% in Can/Canakkale lignites, respectively.

Influence of impeller speed and cell volume on coal flotation performance in a self-aerating flotation machine

The effect of impeller speed and cell volume on batch coal flotation performance in a self-aerating flotation machine is studied in the present investigation. Factorial designed experiments are carried out to establish the relationships between the manipulated variables (impeller speed, and cell volume, and their interactions) with the separation indicators (yield, ash, combustible recovery and entrained water recovery). Correlation coefficient values are used to select the best fit model, and an analysis of variance to identify the statistically significant components of the model. The results indicated that both the impeller speed and cell volume significantly influence the performance of coal flotation. Further, the influence of these variables and their significance is discussed by flotation rate constant.

Dry beneficiation of coal on KAT air table

ABSTRACT Dry gravity-based separation processes such as pneumatic jigs and tables generally use density stratification of feed material with light and dense particles to achieve separation. Therefore, it is necessary to enhance the stratification of the particles in the separation bed. The main purpose of this work was to evaluate the use of an intrinsic autogenous separation medium of feed coal for the dry separation process by increasing the separation bed height in a pneumatic table separator. Tests were conducted and the results showed that increased bed height and an autogenous separation bed resulted in a significant benefit for the separation efficiency. Bituminous coal samples with high-density impurities containing 48.9% feed ash were reduced to 7.7% ash content with a combustible recovery of 89.7%. In the case of higher density anthracite coal, the experimental separator was able to reduce feed ash 50–60% to 26–28% in the product.

Effect of frother addition mode on coal flotation in downflow flotation column

Using a downflow flotation column, we comparatively investigated the effects of two frother addition modes on clean coal production and bubble characteristics. These modes are traditional frother mode, which involves the direct addition of liquid Sec-Octyl alcohol (2-octanol) to slurry, and a novel addition mode, which injects 2-octanol vapor into the bubbles. Compared with the traditional mode, the vapor addition mode produced a higher concentrate yield, lower ash content, and higher combustible matter recovery. This result suggests that the vapor addition mode facilitates the larger product yield, producing a cleaner coal product. The consumption of 2-octanol under the liquid addition mode was reduced by 53.57% compared with that under the vapor mode to achieve 83% recovery of combustible matter. In the first 0.5 min and last 1.0 min of the 3 min flotation time, the vapor addition mode showed a slower flotation rate, whereas in the middle 1.5 min, the rate was faster. In addition, the vapor addition mode led to a smaller bubble size, critical coalescence concentration (CCC), and 23.21% reduction in 2-octanol consumption to achieve a constant bubble size. The higher capture probability of hydrophobic coal particles presented by the vapor addition mode is considered the fundamental cause for the enhanced clean coal yield and combustible matter recovery. With the increase in frother dosage, the particle–bubble collision probability was identified as the main factor determining the increase in the ash content of the concentrate at a concentration below the CCC. At a concentration above the CCC, the viscosity of bubble surfaces became the main factor.

Effect of pH on surface hydration of coal particles and its attachment with oily bubbles in flotation

ABSTRACT The pH of flotation slurry directly affects the surface potentials of mineral particles and bubbles, and therefore the hydration of coal surface and the mineralization process of flotation. In this paper, the effect of pH on the surface potential, hydration degree, and floatability of lignite particles was studied, wherein Einstein’s dispersion viscosity theory was adopted to calculate the hydration degree, and DLVO theory was used to estimate the interaction between coal particles and oily bubbles. The effect of pH on the lignite floatability was evaluated in terms of induction time and oily-bubble flotation results. The results showed that with increasing pH, the zeta potentials of coal particles and oily bubbles decrease, causing a 37.89% increase in the coal surface hydration from pH 3 to pH 11. As a result, the energy barrier for attachment between the coal particles and oily bubbles reaches the maximum value of 2.669 × 10−16 J at pH 11, resulting in low attachment efficiency. Correspondingly, the combustible matter recovery of oily-bubble flotation is significantly decreased from 57.76% to 28.00%.

Synergistic Adsorption Mechanism of Anionic and Cationic Surfactant Mixtures on Low-Rank Coal Flotation.

Mixed surfactants have a prominent synergistic effect and show advantages in many aspects. In this work, the effects of a mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) on the flotation of low-rank coal were studied from the wetting rate, contact angle, surface tension, and zeta potential. Furthermore, the adsorption configuration of the mixed surfactant on the surface of oxygen-containing graphite was simulated at the molecular level by molecular dynamics simulation. The experimental results show that the combustible matter recovery of low-rank coal flotation is improved using the mixed surfactant, and the contact angle test and wetting rate test confirmed the synergistic effect of the mixed surfactant. In the mixed surfactant system, the addition of SDS with an opposite charge to DTAB can reduce the mutual repulsion between DTAB molecules and enhance the degree of DTAB alignment in solution, which was analyzed by surface tension and zeta potential tests. Meanwhile, the simulation results reveal the adsorption behavior of anionic and cationic surfactants on the surface of oxygen-containing graphite from the molecular level and also verify the experimental results. This investigation provides a good understanding of the interaction mechanism of mixed surfactants in low-rank coal flotation.

Effect of Nanobubbles on the Flotation Performance of Oxidized Coal.

In this study, the effects of air bubbles and nanobubbles on flotation performance and kinetics of oxidized coal were investigated. The surface properties of the coal sample before and after oxidation were characterized by a scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The nanobubbles on highly oriented pyrolytic graphite (HOPG) were observed by an atomic force microscope (AFM). The interaction between coal and conventional bubbles in the absence and presence of nanobubbles was explained by induction time. Flotation results showed that oxidized coal flotation in the presence of nanobubbles resulted in 10% higher combustible matter recovery than conventional air bubble flotation. Moreover, it was found that the flotation of oxidized coal in the absence and presence of nanobubbles can be best described using the first-order model with the rectangular model. AFM images analysis showed that a large number of nanobubbles were produced and attached to the oxidized coal surface. The induction times of the oxidized coal in the absence and presence of nanobubbles were 1000 and 39 ms, respectively, indicating that the existence of nanobubbles effectively promotes the interaction between oxidized coal and macroair bubbles. In addition, the agglomeration between oxidized coal particles also occurred spontaneously in the presence of nanobubbles, which was helpful in improving the combustible matter recovery and flotation rate of oxidized coal.

A novel coal tar-based collector for effective flotation cleaning of low rank coal

Clean utilization of low-rank coal is an important problem in coal production and sustainable development. Flotation of low-rank coal is a major challenge for clean utilization of coal resources and environmental protection in the world. Common oily collectors were not effective for low-rank coal flotation, this paper proposed a novel collector named CTB collector, which is a mixture of diesel and extracted light components from coal tar using diesel as extractant. The Gas chromatography-mass spectrometry and Fourier transform infrared spectroscopy analyses revealed that the CTB collector contained more aromatic compounds and oxygen-bearing functional groups than the diesel. The non-polar sites on coal surface can bind with the aromatic compounds through Van der Waals force and π-π stacking interaction. The polar sites can interact with the oxygen-bearing functional groups through hydrogen bond. The adsorption of macromolecular aromatic compounds on non-polar sites can also cover the polar sites adjacently. The hydrophobicity and floatability of low-rank coal were improved through above adsorption. The flotation results showed that the combustible matter recovery of CTB collector was 21%∼34% higher than that of diesel and the CTB collector can save 1500 g/t dosage than diesel while the flotation performance was still better. The coal flotation efficiency index of the CTB collector was 130–240 higher than that of the diesel. The particle carried index was designed to evaluate the carrying ability of a single bubble to coal particles. The flocs were significantly formed during flotation using the CTB collector. These flocs enhanced the recovery of low-rank coal. The particle carried index of the CTB collector-treated coal particles and bubble was higher than that of diesel-treated coal particles and bubble. The induction time results showed that the CTB collector accelerated the adhesion between coal particles and bubble while slowing down the adhesion between gangue particles and bubble. This study can not only provide new strategies for the design of coal flotation reagents and the evaluation of the adhesion efficiency between particles and bubbles, but also be beneficial to the clean utilization of coal energy and environmental protection.

Improvement on combustible matter recovery in coal slime flotation with the addition of sodium silicate

The influence of an inorganic salt dispersant, sodium silicate (SS), on the removal of minerals in coal slime flotation was investigated. XRD, contact angle measurements, zeta potential measurements and EDLVO calculations were employed to reveal the interactions between SS and coal slime. The flotation experiments indicated that the combustible matter recovery of clean coal could be increased by 10.9 % with the addition of 1200 g/t SS. The reasons of SS on facilitating the separation of the combustible components from coal slime were governed by two factors, decreasing the hydrophobicity of minerals and enhancing particle dispersion, both of which are beneficial to the coal flotation. Furthermore, it was found that the effect of SS on decreasing the hydrophobicity of minerals was mainly attributed to the surface modification on kaolinite. It was concluded that SS could significantly improve the flotation recovery of coal slime especially under the condition of a high content of kaolinite in coal slime.

Gravity-desliming using cascading REFLUX™ Classifiers at full-scale

ABSTRACT A cascading sequence of two full-scale REFLUX™ Classifiers was tested using 1.0 mm top size coal. The gravity separation in the first stage gave a clean coal product down to about 0.03 mm. The second stage then deslimed this product with minimal coal loss. At volumetric feed rates up to roughly 100 m3/h (~ 50 t/h solids), 30–50 wt.% ash feeds were reliably cleaned to give product ash values less than 10 wt.% at overall combustible recoveries of 60–80 %. At higher feed rates up to 240 m3/h (~ 100 t/h solids), product ash values were in the range 8–13 wt.% at 43–67% combustible recovery. One run processed 150 m3/h of a very problematic high-slimes feed with 35 wt.% -0.038 mm slimes and 52 wt.% head ash, giving a 14 wt.% ash product at 56% combustible recovery. The deslime unit size partition Ep values increased exponentially with overflow rate, but remained less than 0.05 mm for overflow rates up to 100 m3/h. The cut point size increased with the deslime unit overflow flux. When the effects of cut point density and hindered settling were accounted for, there was good agreement with previous laboratory and pilot-scale test work.