Coal Recovery Research Articles

Comparative Study of Separation Performance of a Spiral and Dense-medium Cyclone on Cleaning Coal

ABSTRACT Spiral separators and dense-medium cyclones (DMCs) are widely used for separation of −1 + 0.125 mm fine coal. In this study, testing was carried out using −1 + 0.125 mm fine coal with low near density material to produce a concentrate, using both a spiral and DMC. Clean coal with 8.31% ash content was obtained with the spiral at 37.39% yield, 47.76% combustible recovery and 0.150 Ep value. For the DMC, the combustible recovery of clean coal at the ash of 8.31% ash was improved to 80.84% at 62.98% yield, and 0.118 Ep value. The advantage of the DMC is further proved by analysis using Fuerstenau upgrading curves.

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%.

A Comprehensive Overview of CO2 Flow Behaviour in Deep Coal Seams

Although enhanced coal bed methane recovery (ECBM) and CO2 sequestration are effective approaches for achieving lower and safer CO2 levels in the atmosphere, the effectiveness of CO2 storage is greatly influenced by the flow ability of the injected CO2 through the coal seam. A precious understanding of CO2 flow behaviour is necessary due to various complexities generated in coal seams upon CO2 injection. This paper aims to provide a comprehensive overview on the CO2 flow behaviour in deep coal seams, specifically addressing the permeability alterations associated with different in situ conditions. The low permeability nature of natural coal seams has a significant impact on the CO2 sequestration process. One of the major causative factors for this low permeability nature is the high effective stresses applying on them, which reduces the pore space available for fluid movement with giving negative impact on the flow capability. Further, deep coal seams are often water saturated where, the moisture behave as barriers for fluid movement and thus reduce the seam permeability. Although the high temperatures existing at deep seams cause thermal expansion in the coal matrix, reducing their permeability, extremely high temperatures may create thermal cracks, resulting permeability enhancements. Deep coal seams preferable for CO2 sequestration generally are high-rank coal, as they have been subjected to greater pressure and temperature variations over a long period of time, which confirm the low permeability nature of such seams. The resulting extremely low CO2 permeability nature creates serious issues in large-scale CO2 sequestration/ECBM projects, as critically high injection pressures are required to achieve sufficient CO2 injection into the coal seam. The situation becomes worse when CO2 is injected into such coal seams, because CO2 movement in the coal seam creates a significant influence on the natural permeability of the seams through CO2 adsorption-induced swelling and hydrocarbon mobilisation. With regard to the temperature, the combined effects of the generation of thermal cracks, thermal expansion, adsorption behaviour alterations and the associated phase transition must be considered before coming to a final conclusion. A reduction in coal’s CO2 permeability with increasing CO2 pressure may occur due to swelling and slip-flow effects, both of which are influenced by the phase transition in CO2 from sub- to super-critical in deep seams. To date, many models have been proposed to simulate CO2 movement in coal considering various factors, including porosity, effective stress, and swelling/shrinkage. These models have been extremely useful to predict CO2 injectability into coal seams prior to field projects and have therefore assisted in implementing number of successful CO2 sequestration/ECBM projects.

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.

Quantifying the benefits of coal mine methane recovery and use projects: Case study on the application of in-mine horizontal pre-drainage boreholes at gassy coal mines in India and the optimization of drainage system design using reservoir simulation

Abstract This article aims to illustrate, through the presentation of case studies, the application of coal mine methane (CMM) drainage technology at two gassy coal mines in India, and to quantify the associated economic and environmental benefits. This work, which is based on two studies developed by the U.S. Environmental Protection Agency (USEPA) through the Global Methane Initiative (GMI), demonstrates that the use of in-mine horizontal boreholes for pre-drainage, capture, and utilization of CMM is both technically and economically feasible. Furthermore, the results show reservoir simulation is a useful tool for optimizing in-mine horizontal pre-drainage borehole placement. The future development and implementation of CMM drainage at each mine has the potential to lower or offset electricity costs, reduce greenhouse gas emissions, unlock coal reserves, and improve mine safety and energy security.

Mine Size Effects on Coal Pillar Stress and Their Application for Partial Extraction

Coal is a nonrenewable resource. Hence, it is important to improve the coal recovery ratio and ensure the stability of coal mines for sustainable development of mining cities. Partial extraction techniques, such as strip pillar mining or room-and-pillar mining, are efficient methods to extract coal. Pillar stress is a critical property for pillar design and for the assessment of mine stability after partial extraction. Current pillar stress calculation methods can sometimes overestimate the pillar stress and unnecessarily large coal pillars may be left underground, which leads to a waste of coal resources. In this paper, the size effects of mining activity on the maximum vertical pillar stress were investigated using numerical simulations. Both strip pillar mining and room-and-pillar mining were considered as possible mining scenarios at different mining depths. The results show that the maximum pillar stress of a mine is primarily controlled by four factors: the mine size to mining depth ratio, the mining width to pillar width ratio, the overburden elastic modulus, and the mining depth. The maximum pillar stress of a mine gradually increases to an ultimate value as the mine size increases. Simplified formulas and methodology have been derived for stress calculations under consideration of mine size effects and, therefore, can reduce the waste of coal resources from the overestimation of pillar stress.

The influence of different diffusion pattern to the sub- and super-critical fluid flow in brown coal

Sub- and super-critical CO2 flowing in nanoscale pores are recently becoming of great interest due to that it is closely related to many engineering applications, such as geological burial and sequestration of carbon dioxide, Enhanced Coal Bed Methane recovery ( ECBM), super-critical CO2 fracturing and so on. Gas flow in nanopores cannot be described simply by the Darcy equation. Different diffusion pattern such as Fick diffusion, Knudsen diffusion, transitional diffusion and slip flow at the solid matrix separate the seepage behaviour from Darcy-type flow. According to the principle of different diffusion pattern, the flow of sub- and super-critical CO2 in brown coal was simulated by numerical method, and the results were compared with the experimental results to explore the contribution of different diffusion pattern and swelling effect in sub- and super-critical CO2 flow in nanoscale pores.

Experimental and numerical investigation on coal drawing from thick steep seam with longwall top coal caving mining

Steep coal seam mining activities will frequently occur during the next few decades in China. In this study, both experimental and numerical methods are employed to investigate the coal drawing from thick steep seam with longwall top coal caving mining. A series of analyses is performed to investigate the features of the drawing body, the distribution of top coal recovery ratio and the shape of the rock flow under steep conditions. The results indicate that the drawing body of top coal develops prior to upper side of the panel face obviously, and the top coal in the central part of the panel has a higher recovery ratio than that in the lower and upper parts in steep coal seam with caving mining method. The flow paths of the fragmented top coal are nearly straight lines moving towards the drawing window, and the fastest path maintains a constant angle with the plumb line. The spatial shape of the rock flow indicates “bidirectional asymmetry,” which results from the presence of the shield beam and dip angle of the coal seam; thus, this is the root cause of the appearance of the drawing body’s prior development towards the upper side of the panel. The field observation data indicates the same distribution of top coal recovery as that in the physical experiment and numerical simulation. Furthermore, suggested measurements are proposed to improve top coal recovery in steep seam mining based on the engineering practice of Dayuan coal mine.

Mathematical model simulating the ignition of a droplet of coal water slurry containing petrochemicals

Global problems of effective coal and oil processing waste recovery can be solved by making use of these wastes as the main fuel components for coal water slurries containing petrochemicals (CWSP). Until now, no predictive models have been developed that would simulate the sustainable ignition of CWSPs based on components with highly different properties, such as ash, moisture, and volatile content, heat of combustion, etc. This is exactly the type of model we are presenting in this paper. In order to gain a greater insight in the process under study, the experimental research has been conducted. We have created an experimental database with the main characteristics of CWSP ignition, namely the duration of stages, gas-phase and heterogeneous ignition delay times, maximum combustion temperatures, and minimum sufficient oxidizer temperatures. A mathematical model has been developed predicting the conditions and characteristics of CWSP droplet ignition. The signature feature of the model is that it accounts for all the main heat and mass transfer processes and chemical reactions in the solid fuel – liquid fuel – water system under study. This mathematical model can serve as the basis for estimating and comparing the ignition characteristics of different CWSPs.

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.

Flotation Performance of Low-rank Coal in the Presence of Cetyltrimethyl Ammonium Bromide

ABSTRACT The flotation enhancement mechanism and flotation kinetics of low-rank coal in the presence of cetyltrimethyl ammonium bromide (CTAB) were investigated. The contact angle of the coal sample increased by 46.07% and the attachment time between coal particles and a single air bubble decreased by 85.79% with increasing the concentrations of CTAB from 0 to 1 × 10–4 M. Low concentrations of CTAB enhanced the flotation performance of low-rank coal, and the concentrate yield significantly increased by up to 8.59 percentage points with increasing the concentrations of CTAB from 0 to 1 × 10–5 M at the cost of a lower increase in the ash content. However, the selectivity was reduced at high CTAB concentrations (5 × 10–5 M and 10–4 M). Six kinetic flotation models were employed to fit the flotation test data using the 1stOpt analysis software; the flotation rate constant (k), the maximum combustible recovery (ε ∞), the correlation coefficient (R 2) and the modified flotation rate constant (k m) were calculated. Most models showed that the modified rate constant and the maximum combustible recovery of low-rank coal increased with the increasing concentrations of CTAB.

Highwall mining of thick, steeply dipping coal–a case study in geotechnical design and recovery optimization

Highwall mining of thick (up to 30.48 m) steeply dipping (20° or more) coal seams provides many challenges, both geotechnically and operationally, as seam dips near or in excess of highwall mining machine capabilities are encountered. Maximizing coal recovery while maintaining highwall stability requires innovative techniques with regard to web and barrier pillar layout, depth of penetration, and choice of mining horizon within the seam. Stability of highwall mining slopes, openings, and pillarsare typically analyzed using the ARMPS-HWM program, as well as LAMODEL, UDEC and Slope-W modeling. Highwall stability can be maintained, and highwall mining production optimized by applying design criteria in creative ways, including alternating miner penetration depths and initiating mining of thick seams toward the bottom of the seam. Highwall mining of thick, steeply dipping coal requires careful planning and execution, including close cooperation between geotechnical design engineers, the mining company, and the highwall mining contractor. This paper describes the application of creative design techniques to a specific pit arrangement at the Westmoreland Kemmerer Mine, Kemmerer, Wyoming. Highwall mining was accomplished by UGM ADDCAR Systems, LLC on a contract basis.

Recovery of clean coal from tailings by flotation with aid of ultrasonic

ABSTRACTIn this paper, clean coal was recovered from tailings by flotation with the aid of ultrasonic irradiation. Flotation test, coal surface scanning and collector adsorption measurement were carried out. The results show that ultrasonic alleviated slime coating and decreased collector adsorption on coal surfaces. Meanwhile, the combustible recovery increased while the recovery of ash-formed minerals decreased. Particles with different size and density fractions have different flotation behaviors, which were also analyzed by measuring size fraction distribution and density fraction distribution of clean coal. This practice saved coal resources and benefitted environment.

Stress distribution characteristics in the vicinity of coal seam floor

Although longwall top-coal caving (LTCC) has been a popular, more productive and cost-effective method in recent years, roadway floor heave and rock bursts frequently appear when exploiting such coal seams with large dip angle. This paper proposes addressing this problem by adopting three-dimensional roadway layout of stagger arrangement (3-D RLSA). In this study, the first step was to analyse the stress distribution characteristics in the vicinity of coal seam floor based on the stress slip line field theory. In the second step, numerical calculation using FLAC3D was conducted. Finally, an evaluation of the 3-D RLSA for solving this particular issue was given. Results indicate that for this particular mine the proposed 3-D RLSA results in 24% increase in the coal recovery ratio and a modest reduction in excavation and maintenance costs compared to the conventional LTCC method.

Effect of middle-temperature pyrolysis on the surface hydrophobicity of sub-bituminous coal

ABSTRACTMiddle-temperature pyrolysis is widely used to convert sub-bituminous coal into gas/liquid products and the coal char, which benefits the utilization of low rank coal resources. However, the coal char usually contains high-ash content because the volatile components in coal release from coal particle forming gas/liquid products while most of high-ash mineral components remain in the coal char. Therefore, the upgrading of the coal char is usually required to meet the requirement of calorific value for burning. It is necessary to find out the effect of middle-temperature pyrolysis on the surface hydrophobicity of coal. In this study, the effects of pyrolysis temperature (700, 800, and 900°C) and pyrolysis time (30 and 90 min) on the surface hydrophobicity of sub-bituminous coal were comprehensively investigated. X-ray photoelectron spectroscopy (XPS), attachment time, and flotation tests were used to reveal the changes of surface hydrophobicity and floatability of sub-bituminous coal before and after middle-temperature pyrolysis. The XPS results indicated the content of hydrophilic oxygen-containing functional groups was reduced while the content of hydrophobic functional groups on coal surface was increased after the pyrolysis. The attachment time of coal particle-bubble was reduced while the flotation recovery of coal was increased after the pyrolysis. The surface hydrophobicity and floatability of sub-bituminous coal were enhanced by middle-temperature pyrolysis, which makes the upgrading of the coal char feasible.

Exploration on the mechanism of oily-bubble flotation of long-flame coal

The interaction between long-flame coal particles and oily bubbles controls the crucial processes of attachment and mineralization in oily-bubble flotation thus dominates the recovery of coal. In this study, the interaction energy between long-flame coal particles and diesel oily bubbles was estimated by both DLVO and EDLVO theories, while the attachment of coal particles to oily bubbles was evaluated by induction time measurements and oily-bubble flotation tests. The results indicated that the isoelectric points of oily bubbles and coal particles were observed at about pH 3.5 and 2.3, respectively. Over the entire range of pH 2–11, the attachment between coal particles and oily bubbles was confirmed by the results of the induction time and oily-bubbles flotation tests. Additionally, it was found that the flotation recovery of coals was inversely related to the induction time. The attachment could be accurately predicted by the EDLVO theory over the tested pH range. On the contrary, it showed a great deviation for the DLVO theory to predict the attachment over the tested pH range except at pH 3. The reason for the deviation in prediction may be that the hydrophobic force on the interface of coal particles and oily bubbles is too great to be ignored.

Fine coal recovery from washery tailings in Turkey by oil agglomeration

In Turkey, coal washeries discharge huge amount of fine coal containing tailings. Loss of economically valuable energy source and negative environmental effects of accumulated tailings necessitate the recovery of fine coals from these tailings. In the present study, the tailings sample from Tunçbilek Coal Washery of Turkish Coal Enterprise was subjected to oil agglomeration process in order to investigate the possibility of fine coal recovery. Free available waste sunflower oil was extensively used as bridging liquid. Effects of various parameters including solid ratio, oil dosage, agitation rate and time, slurry pH, amount of washing water, particle size, and oil type on deashing and desulphurization were investigated. Optimally, 84.1% of ash and 57.6% of the sulphur was removed from the tailings by combustible recovery of 46.8%. A clean coal with 29.9% ash and 1.3% sulphur was produced from the tailings containing 54.6% ash and 3.0% sulphur. In addition to ash and sulphur analyses, SEM-EDS analysis and polished section examination were also undertaken to reveal the performance of the process.

Washability Study on El-Maghara Coal Mine Reject

Egypt is not a coal-rich country; however, El-Maghara coal seams in Sinai (52 million tons with about 27 million tons as minable reserves) is the only coal reserve. Due to the importance of coal as a source of energy, this work deals with an attempt to retrieve escaped coal (gangue) as much as possible using Sink-and-float technique. A bulk sample was collected from the waste dumps of El-Maghara coal mine. The results were characterized by measuring yield recovery and ash contents. Sample was prepared by successive stages of crushing through jaw and roll crushers. Crushed sample was introduced to the screening process using a set of screens (6, 4, 2, 1 and 0.5 mm). Organic liquids such as Bromoform (CHBr3), Carbontetrachloride (CCl4) and Gasoline with specific gravities of 2.6, 1.59 and 0.721 respectively were used. Several bathes of various specific gravities (1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 and 2.0 g/cc) were prepared from organic liquids. The obtained results indicated that, we can achieve about 18.94% clean coal yield recovery with ash content of 2.58%, washing degree of 18.10 and separation efficiency of 95.53% respectively at heavy medium of 1.3 specific gravity. Also, the obtained results at 1.4 specific gravity were 19.78 % clean coal yield recovery with ash content of 2.61%, washing degree of 18.88 and separation efficiency of 95.47%.

Preliminary Beneficiation and Washability Studies on Ghouzlou's Low-Ash Coal Sample

In the present research work, a low-ash coal, from Ghouzlou deposit in Iran, with an average ash content of 12% was subjected to some beneficiation experiments such as heavy media separation and flotation. Sieve analysis showed that 62.3% of the coal sample with the size of +2 mm had around 7.3% ash contents. Also, heavy media tests carried out on five size fractions revealed that by setting the separation density at 1.4 g/cm3 for the coarse fraction (+1 mm), a 5% ash product with more than 70% coal recovery was obtainable. Samples with lower ash content ( 5%) based on the Mayer curves to produce a 5% coal product. Moreover, flotation tests on -1 mm fraction could reduce the ash content from more 13.2% to 10.4%.

Effect of a Dispersant Agent in Fine Coal Recovery from Washery Tailings by Oil Agglomeration (Preliminary Study)

Among the fine coal cleaning methods, the oil agglomeration process has important advantages such as high process recovery, more clean product, simple dewatering stage. Several coal agglomeration studies have been undertaken recently and effects of different variables on the process performance have been investigated. However, unlike flotation studies, most of the previous agglomeration studies have not used dispersing agents to minimize slime coating effects of clays. In this study, agglomeration process was applied for recovery of fine coals from coal washery tailings containing remarkable amount of fine coal. Negative effect of fine clays during recovery was tried to be eliminated by using dispersing agent instead of de-sliming. Although ash reductions over 90 % were achieved, performance remained below expectations in terms of combustible matter recovery. However, this study is a preliminary one. It is considered that more satisfied results will be obtained in the next studies by changing the variables such as solid ratio, oil dosage, dispersant type and dosage.