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Description Coal is one of the worlds major sources of energy. Coal is used to produce nearly one-third of all the electrical energy that is generated and used in the United States. Coal is a very complex and diverse energy resource that can vary greatly, even within the same deposit. In general, there are four basic varieties of coal, which are the result of geologic forces having altered plant material in different ways.
Lignite: Increased pressures and heat from overlying strata causes buried peat to dry and harden into lignite. Lignite is a brownish-black coal with generally high moisture and ash content and lower heating value. However, it is an important form of energy for generating electricity. Significant lignite mining operations are located in Texas, North Dakota, Louisiana and Montana.
Subbituminous: Under still more pressure, some lignite is changed into the next rank of coal, subbituminous. This is a dull black coal with a higher heating value than lignite that is used primarily for generating electricity and for space heating. Most subbituminous reserves are located in Montana, Wyoming, Colorado, New Mexico, Washington and Alaska.
Bituminous: Even greater pressure results in the creation of bituminous, or soft coal. This is the type most commonly used for electric power generation in the U.S. It has a higher heating value than either lignite or subbituminous, but less than that of anthracite. Bituminous coal is mined chiefly in Appalachia and the Midwest. It is also used to make coke for steel production.
Anthracite: Sometimes called hard coal, anthracite forms from bituminous coal when great pressures develop in folded rock strata during the creation of mountain ranges. Anthracite occurs only in limited geographic areas in the U.S.primarily the Appalachian region of Pennsylvania. Anthracite has the highest energy content of all coals and is used for space heating and generating electricity.
300TPH Coal Mining Plant in Ethiopia Project: coal mining plant Material: coal Capacity: 100TPH Country: Ethiopia Raw mineral description: contain limestone, shale. shown in figure 2 Customers requirements: increase the coal burning heat. ---Read more---
After removing the coal from the ground, the miners may send it to a preparation plant near the mining site. The plant cleans and processes coal to remove rocks, dirt, ash, sulfur, and other unwanted materials. This process increases the heating value of the coal.
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Economic and operating conditions make it important to provide a simple, low cost, efficient method for recovering fine coal from washery waste. Not only is the water pollution problem a serious one, but refuse storage and disposal in many areas is becoming limited and more difficult. Many breakers and washeries efficiently handle the coarser sizes, but waste the coal fines. This problem is assuming major importance due to the increase in the amount of coal fines being produced by the mechanization of coal mining.
Flotation offers a very satisfactory low-cost method for recovering a fine, low ash, clean coal product at a profit. Often this fine coal, when combined with the cleaned, coarser fractions, results in an over-all superior product, low in ash and sulphur, giving maximum profit returns per unit mined.
Generally a very simple flotation flowsheet, as illustrated above, will be suitable for recovering the lowash coal present in waste from coarse recovery washeries.Assuming the fines are approximately all minus 20 mesh and in a water slurry of about 20% to 25% solids, the first step is to condition with a reagent which will promote flotation of the fine coal particles. Kerosene, fuel oil, coal tars and similar hydrocarbons will accomplish this effectively when added to thecoal slurry in a (Patented) Super Agitator and Conditioner. A frothing agent such as pine oil, alcohol frother, or cresylic acid added to the slurry as it discharges from the conditioner is also used. The separation between low ash coal and high ash refuse is efficiently accomplished in a Sub-A Flotation Machine. As the amount of clean coal floated represents a high percentage of the initial feed, provision is made to remove the cleaned coal from both sides of the cell. Fine coal is dewatered with a Disc Filter, as the Flotation Machine can usually be regulated to produce a product low in ash and with proper density for direct filtration.
It is highly desirable to extend the range of coal flotation to include the coarser Sizes. Not only will this simplify general washery practice but will result in a superior product having desirable marketing characteristics for metallurgical and steam power plant uses. It is now possible to efficiently recover coal by flotation through the entire size range beginning at about 4 mesh down to fines, minus 200 mesh.
With the flowsheet as outlined for coarse coal recovery, the feed is first deslimed for removal of high ash slimes and excess water. The hydroclassifier underflow is conditioned at 40% to 45% solids with kerosene or fuel oil and diluted with water to 20%-25% solids prior to flotation. If pyrite and coarse high ash material are present, it is often helpful to pass the conditioned pulp over a Mineral Jig for removal of a portion of these impurities. Hindered settling in the jig against a rising pulsating water column classifies out the high gravity impurities and eliminates them from the flotation circuit. Water requirements are low and feed density to flotation can easily be maintained at the proper level.
The Sub-A (Lasseter Type) Flotation Machine has proved successful for treating coarse coal with the flowsheet as indicated. A frother of the alcohol type is generally added to the flotation feed after conditioning with kerosene. Floated coal will collect in a heavy dense matte at the cell surface and as raked off, will contain up to 60% solids. Mechanical dewatering is usually not necessary. Natural drainage, dewatering on porous bottom screw conveyors, and vibrating screen dewatering are all being used successfully in coarse coal recovery circuits.
Flotation, with the Sub-A gravity flow principle, provides the ideal way to treat coal fines even as coarse as 3/16 top size. According to reports from plants operating for the production of metallurgical coke, each percent ash in the coal carries a penalty of 2$ per ton of coal. Thus there is a considerable margin for operating costs in a fine flotation cleaning method that will materially lower the ash of the cleaned coal. Further convincing evidence that ash removal from coal is of major importance is found in the weekly magazine of metal working, Steel, January 29, 1951, reporting on a modern coal preparation plant. The report states that a 1% reduction in ash content of coal means a reduction of 30 cents in cost of pig iron. One large plant reduces the ash from 7% to 3.5% by cleaning, thus cutting the cost of producing pig iron a dollar or more per ton.
A coal flotation machine must not only be able to handle a coarse as well as a fine feed, but it must also be simple to operate. Gravity circulation permits the treatment of difficult unclassified feeds.
High cost of mining makes it very important from a profit standpoint to recover all of the low ash coal, both coarse and fines. With the present trend toward mechanization, more fines are produced in mining. In many operations it is no longer economical to discard these fines to waste even though ash contiminants render the fines unmarketable without additional cleaning.
Water conservation, stream pollution and refuse storage are also factors which must be taken into consideration along with marketing requirements for the clean coal product. Flotation offers an efficient and low-cost method for recovering coal fines at a profit. In many cases floated coal fines can be blended with the coarser fractions without affecting ash, moisture or size limitations. This is being done successfully in coking coal operations. Fine coal is also being used extensively in steam plants for electric power generation.
The above flowsheets are based on existing small coal flotation plants. They illustrate clearly the simplicity and feasibility of adding Sub-A Coal Flotation as an additional process to small washing plants.
Because of its limited output, treatment must be very simple and operating costs kept to a minimum. At the washery, illustrated by flowsheet A, the entire mine output is sold for coking coal. Mining the relatively narrow seam produces a product with 15 to 20% ash, although the coal when cleaned will carry only 3 to 3% ash. This low ash coal brings a premiumprice, so it is an economic necessity to remove the impurities.
The mine run coal is crushed to a size for coking coal requirements. The entire production is treated over a coal jig which removes as waste primarily the coarse refuse. The coarse clean coal passes over the jig along with the fines and is elevated to a wedge bar stationary screen with 1 millimeter openings for dewatering. The coarse clean coal passing over the screen discharges by gravity into a storage bin. The fine coal, along with clay and its high ash fractions and water averaging 15 to 18% solids, discharges by gravity into a (Patented) Super Agitator and Conditioner. Kerosene and pine oil are added and the conditioned slurry or pulp then is introduced into the Sub-A Coal Flotation Machine.
The low ash coal product removed from the Sub-A Coal Flotation Cells contains 35 to 40% solids and is transferred to the coarse coal storage bin through a Vertical Concentrate pump. The flotation coal mixes with the coarse product which allows for adequate drainage and minimum loss of fines.
In the operation as illustrated by flowsheet B, approximately 15 tons per hour of coal flotation concentrate are produced. This installation requires more control to meet specifications and consequently a more elaborate system is necessary.
Screen undersize and water containing fines from the gravity separator are thickened in a centrifugal or cyclone separator to give the proper water-to-solids ratio for subsequent treatment. The effluent from the cyclone contains collodial slimes and high ash fines in addition to the bulk of the water from screening and gravity systems. Thickened coal fines from the cyclone pass over a Mineral Jig which removes a high ash refuse and free pyrite down as fine as 150 to 200 mesh.
The coal fines passing over the Jig are conditioned with reagents in the (Patented) Super Agitator and Conditioner and subjected to flotation treatment in a 6-cell Sub-A Coal Flotation Machine at approximately 20-25% solids. Double overflow of froth is used due to the low ratio of concentration and the high weight percentage of floatable coal recovered by flotation.
The coal flotation product at 35% solids is dewatered by a Disc Filter. Coarse coal from the gravity section and fine coal from the flotation section are blended and transferred by rail to the coke plant.
In some cases the coarse and fine coal are dewatered by Dillon Vibrating Screens. The coarser fractions of coal are first added to the screen to form a bed and flotation fines are added on top of this bed for dewatering. Where operating conditions are favorable, this system is preferred to other means of dewatering as it assures a well blended product low in moisture and uniform in ash content.
Effluent from the cyclone, high ash jig refuse and flotation tailing refuse are thickened in a Thickener to conserve and re-use water. Thickener refuse is disposed of without contaminating local streams.
Sub-A Coal Flotation with its gravity flow principle and selective action makes it possible to recover low ash coal from 1/8 down to minus 200 mesh. If an appreciable amount of recoverable coal is plus 20 mesh in size, the Sub-A Lasseter Type Coal Flotation Machine should be used. It is no longer necessary to use a complex system for fine coal recovery. Flotation will effectively handle the entire fine size range at low cost and produce a low ash marketable product.
In the washing of coal the problem exists in having to clean the fines in an economical and efficient manner without an excessively complex flowsheet. Mechanized mining creates fines not considered as problems in older methods of selective mining and underground loading. In many cases the minus 1/8 inch fines require cleaning to lower the ash content and frequently it is also necessary to reclaim all of the water for re-use in the washing system. Most plants use a closed water system to conserve water and comply with anti-stream pollution regulations.
Flotation offers a means for handling the entire size range minus 1/8 inch x 0. Efficient recovery of the fines at a low ashcontent is accomplished in a relatively simple flowsheet. Thesubstantial amount of coarser sizes in the concentrates aids in subsequent dewatering either by vacuum filters or dewatering screens.
In the flowsheet shown mine run coal after proper size reduction treatment is passed over heavy duty screening equipment to removethe minus 1/8 inch fines. Wet screening down to 10 or 12 mesh offers no particular problem. Water sprays are generally employed to thoroughly wash the fines from the coarse coal and prepare it for treatment. A surge tank or a thickener ahead of the conditioning and flotation section may be necessary to provide a uniform feed rate both as to solids content and density.
The coarse coal is washed and up graded in a conventional manner through heavy media or coal jigs to produce a clean coal and a coarse refuse. Any fines due to degradation through the coarse cleaning system is collected, partially dewatered and combined with the fines from the screening section.
Minus 1/8 inch x 0 coal fines are conditioned with the required amount of fuel oil or kerosene (approximately 1 to 3 lbs/ton) to thoroughly activate the low ash coal particles and render them floatable. Density in the conditioner should be as high as possible; however, for the open circuit system as shown it very likely will be maintained between 20 to 25% solids. A Super Agitator and Conditioner is preferred for this service since any froth accumulation on the surface is drawn down the standpipe and thoroughly dispersed throughout the pulp. This also aids in the most effective use of reagents.
The discharge from the conditioner at 20% solids is floated in a Sub-A Flotation Machine of the free flow type for handling coarse solids. Some dilution water may be necessary to maintain the feed density at 20% solids. A frother such as pine oil, cresylic acid, or one of the higher alcohols is added to the head of the flotation circuit at the rate of about 0.5-1.5 lbs/ton.
In the primary flotation section a high recovery of the coal fines minus 28 mesh is secured. In addition some of the more readily floatable coarse coal, low in ash, is also recovered. However, ability of the machine to handle all 1/8 inch feed permits recovery of coal over wide range of mesh sizes, thus improving filtering and handling characteristics. This coal, if not clean enough, is refloated in cleaner cells and middlings are recycled back to the feed. Clean coal will contain about 35% solids which is ideal for vacuum filtration. A Agitator Type Disc Filter is used as solids are effectively kept in suspension giving uniform distribution of cake for greater dewatering.
Generally the refuse from the primary flotation cells will contain a very high ash content in the -28 or-35 mesh size fraction. By screening the refuse the excess water and undersize high ash fines are eliminated while screen oversize is re-treated by flotation. This screening need not be highly efficient since only a partial sizing is satisfactory. Handling the coal in this manner reduces size degradation to a minimum.
The coarse coal from the foregoing dewatering and screening step is repulped to about 40% solids and conditioned with reagents. The conditioned pulp after dilution to 25 to 28% solids is floated in a second bank of flotation cells. The coarse coal in the absence of fines will form a dense, heavy matte at the surface of the cells. For this type of flotation, slow moving rakes are provided to remove the coal as final concentrate. This clean coal will generally contain over 50% solids, thus making it ideal for dewatering over vibrating screens or on a horizontal or top feed vacuum filter. In some plants where moisture is not too critical a screw conveyor with wedge bar bottom sections is used for the dewatering step.
The refuse from the coarse coal flotation cells may still contain some coal notresponsive to flotation recovery but low enough in ash to be saved. In such cases the refuse can be screened and the oversize fraction jigged or tabled. The tonnage at this point is usually only a very small percentage of the initial fines so the equipment requirements for this gravity section are moderate.
All refuse in the 1/8 inch x 0 coal recovery section is collected in a thickener for water reclamation. The thickened refuse or sludge underflow may be pumped to waste ponds, or if water is in short supply, filtration of this refuse may be necessary.
Coal flotation concentrates produced in this primary section are filtered direct and the filtrate is re-cycled back to the flotation cells for re-use. This filtrate is high in reagent content and is particularly useful as dilution water. Generally the density of the coal from the primary cells will contain about 35% solids and thus does not require thickening ahead of filtration.
Coal from the coarse flotation and the gravity section, if employed, can be readily dewatered over screens or horizontal or top feed filters. In some cases it may be possible to divert part or all of this coal to the filter handling the fines provided it is equipped with proper agitation equipment and a high displacement vacuum system. Some of the new synthetic filter bag fabrics such as Saran and nylon materially aid in securing high filter rates and low final moisture content.
Sub-A Coal Flotation Systems have been successful for recovery of both coarse and fine coal. It is important, however, to employ a two-stage circuit for maximum efficiency in saving the plus 28 mesh fraction which is normally the most difficult to float. The development of the free flow and Type M flotation cells offers a means for efficiently handling coarse coal in a size range heretofore reserved for other more complex systems.
Ash and sulphur content is desired to be as low as, or lower than, for regular lump coal. Generally, for anthracite, not over 13 per cent ash is desired. Bituminous coal operations usually limit ash to not more than 8 per cent in the fines.
Flotation or gravity concentration are generally applied only to washery fines that otherwise would not be saleable and which generally have to be impounded to prevent stream pollution. Because of the low price secured, the expense of treatment must be held to a minimum. Pyrite and coarser ash-forming content may need to be removed by gravity treatment.
Kerosene or fuel oil with pine oil, or alcohol, frother are the more common reagents used. Cresylic acid frother may sometimes be advantageous. Fine pyrite, if free, may be rejected with the high-ash refuse by addition of lime to the flotation feed.
Under proper conditions, coal as coarse as 10 mesh maybe effectively floated with kerosene and pine oil. For this coarse flotation it is generally necessary to classify out the high-ash 200 mesh slimes ahead of flotation.
The life cycle assessment of the cobalt extraction route is carried out.Blasting and electricity consumption in cobalt mining is damaging to the environment.Eutrophication and global warming are the most affected impact categories.Carbon dioxide and nitrogen dioxide emission are highest from cobalt mining.Alternative energy sources for electricity generation would enhance sustainability.
This paper presents the results of an investigation carried out on the impacts of cobalt extraction process using a life cycle assessment by considering a cradle-to-gate system. Life cycle inventory data was collected from the EcoInvent and Australian Life cycle assessment database (AusLCI) and analysis were performed using SimaPro software employing the International Reference Life Cycle Data System (ILCD) method, and Cumulative Energy Demand method (CED) for per kg of cobalt production. Several impact categories are considered in the analysis i.e. global warming, ozone depletion, eutrophication, land use, water use, fossil fuels, minerals, human toxicity, ecotoxicity, and cumulative energy demand. The analysis results indicate that among the impact categories, eutrophication and global warming impacts are noteworthy. Medium voltage electricity used in cobalt production and the blasting operation appears to be causing most of the impact and emission into the environment. The sensitivity analysis was carried out using three different case scenarios by altering the electricity generation sources of UCTE (Synchronous Grid of Continental Europe) to investigate the proportional variation of impact analysis results. Furthermore, the impacts caused by cobalt production are compared with nickel and copper production processes to reveal their relative impacts on the environment and ecosystems.
Coal and gas outburst is the extreme instability caused by stress, gas, and coal. In this review article, dominant factors and inducing factors of outburst were summarized; geologic features of typical outburst cases and the effects of tectonic movement on outbursts were analyzed; the outburst stages with considerations to geologic factors were divided. It was found that inducing factors, including buried depth, tectonic movement, gas composition, coal seam conditions, overlying/underlying rock conditions, and mining mode, control the outburst by influencing the dominant factors (stress, gas, and coal). Among them, tectonic movement is the key of outburst. Influenced by tectonic movement, the primary structure of coals is damaged/pulverized due to the tectonic stress and unique tectonic mode, resulting in the formation of tectonic coals. When external dynamic factors are changed, tectonic coals are crucial to outburst control for its evolution of porous structure as well as the unique mechanical behaviors and gas flowing responses. Besides, the preparation stage of outburst includes the tectonic process and mining process. The former one refers to the restructuring process of the original coal-bearing strata by tectonic movement, while the mining process is the prerequisite of outburst and it refers to the disturbance of human mining activities to the initial coal seams. It is suggested that more work is required on geological factors of outburst, and a few research areas are proposed for future research.
Coal and gas outbursts (hereinafter referred to as outbursts) are a kind of sudden dynamic disaster in underground coal mines which have complicated processes [1, 2]. Since the catastrophe process is accompanied by considerable failure effect, outbursts are a great threat to safety exploitation of coal resources, thus becoming a wide concern around the world . In particular, outbursts during coal mining in China are more serious, which is determined by the extremely complicated occurrence environment of coal resources in China . Meanwhile, high-intensity exploitation (>30 billion tons) over years leads to gradual exhaustion of shallow coal resources, thus increasing the coal mining depth year by year . Increasing mining depth implies the continuous worsening of coal mining environment. Coal mining is going to face with a series of challenges, such as more complicated tectonic movements, high gas, high ground stress, low strength, and low-permeability coal mass. Therefore, studying the basic theory and control technology of coal and gas outbursts is still one of the important links in safety production of coal mines [6, 7].
Due to the complicated mechanism of outbursts, mechanisms of outbursts under different geological conditions are difficult to be explained by the uniform theory. Many studies have summarized the mechanism of outbursts as the collaborative consequence of stress, gas, and properties of coals [8, 9]. Moreover, the process of outbursts is divided into the outburst preparation stage, sudden instability stage of coal mass, continuous instability development stage toward deep coal mass, coal transportation stage in the mining space, and outburst termination stage [10, 11]. In addition, many studies focus on the role of stress and gas in coal instability, continuous instability development, and coal transportation  but pay few attentions to the effect of properties of coals on the outburst process, especially on outburst preparation, continuous instability, and coal transportation. Nevertheless, many cases of outbursts have demonstrated that most outburst accidents occur in tectonic zones (e.g., fault, fold, and overthrusting tectonics), which is attributed to the following reasons. These tectonic zones change the stress and gas environment in coal seams [2, 1416]. More importantly, tectonic zones alter microstructures (e.g., matrix, pore, and fracture) of coal, thus changing properties of coals. Influenced by tectonic movements, soft and highly crushed tectonic coals are observed in many fields of outburst accidents .
Under one-stage or multistage tectonic movements, the primary structure of coals develops different degrees of embrittlement, crushing or ductile deformation, or overlying failure, thus leading to the sharp reduction of matrix size of coals and even causing changes in internal chemical composition and structure . As a result, tectonic coals are formed. Moreover, a small-sized matrix of tectonic coals undergoes the secondary shaping under the effect of ground stress, forming polymeric tectonic coals . Tectonic coals are compacted under the in situ stress state, manifested by high closure of the fracture-pore system and extremely low permeability of coals . According to field statistical data, permeability of tectonic coals in typical tectonic coal mines of China is lower than 0.1mD [14, 1921]. Such low-permeability coal mass restricts gas flow in coal seams, thus resulting in uneven distribution of gases and local gas enrichment in coal seams . Nevertheless, tectonic coals show extremely weak antidisturbance, and their mechanical behaviors and gas flow become unique upon changes of external factors. Under this circumstance, tectonic coals are extremely easy to be crushed into abundant scattered small particles, in which gas is released at extremely high speeds .
Existing studies on geologic causes of outbursts mainly focus on statistical analysis of cases. However, it lacks a systematic study on the changes of the ground stress field and gas field in the original coal-bearing stratum caused by tectonic movements and the physical and chemical changes that coals have undergone under the effect of tectonic movement to form tectonic coals.
In this study, relevant factors were summarized and analyzed systematically. The specific topics reviewed in this study include the following: (1)Dominant factors and inducing factors of coal and gas outbursts as well as relations of these causes(2)Geologic features of typical outburst cases and the effects of tectonic movement on outbursts(3)Reconstruction of the stress field, gas field, and coal structure of the original coal seams caused by tectonic movement(4)The outburst process and stage division with considerations to geologic factors
Due to complexity and variability of outburst, there are many influencing factors of outbursts. Moreover, influencing factors vary for different outburst cases. However, studies over years demonstrate that there are three dominant factors of outburst, namely, stress, coal seam gas, and properties of coals.
Most outbursts occur at ends of roadways or working faces, where there are mining disturbances and prevalent stress concentration . Stress is the power and energy source of coal/rock failure in the outburst process. Stress participates in preparation, triggering, and development stages of outbursts [9, 11]. In the preparation stage of outburst, stress (tectonic stress) is the dominant factor that determines the occurrence state of coal seams, and it is also an important influencing factor of gas occurrence in coal seams [24, 25]. In the triggering stage of outbursts, it is generally necessary to damage rock pillar/coal pillar in front of outburst coals, which is caused by stresses . In addition, damage of outburst coals before separation in the development stage is also mainly attributed to the dominant role of stress [11, 27]. Therefore, the risk of outbursts is increased significantly in some regions with abnormal stresses or in high-stress environment in the deep mining stage.
Stress which dominates the outburst includes not only in situ stress of the coal seam but also disturbed stress. Specifically, in situ stress is the collaborative consequence of tectonic stress, gravity or thermal effect, and residual stress of tectonic effect . These components cannot be distinguished independently during practical measurement of in situ stress. Disturbed stress involves additional stress caused by mining activities (e.g., vibration and blasting). More importantly, the stress balance of primary rocks is broken by mining activities, thus resulting in stress transfer and forming stress concentration .
According to the literature review, the stress state at any point in underground coals and rocks can be determined by three principal stresses, and there are two principal stresses on or close to the horizontal plane in most regions . Besides, the maximum horizontal principal stress in shallow strata is generally higher than the vertical stress, which is attributed to the control of tectonic stress. Liu et al.  carried out a statistical analysis on in situ stress in coal seams in 74 formations in the Huanan Coal Mine in the buried depth of 350-1100m. The maximum principal stresses of 59 formations were on the horizontal plane, but maximum principal stresses of only 15 formations were on the vertical plane, and the measuring points with the maximum horizontal stress higher than the vertical stress accounted for 79.7%. Obviously, in situ stress of coal seams in the Huainan Coal Mine is controlled by tectonic stress significantly.
Li et al.  measured in situ stress (buried depth: 230-424m) of coal seams surrounding the Mafangquan fault in the Jiulishan Coal Mine, Jiaozuo Mine. They found that the maximum and minimum principal stresses at 5 measuring points were 12.4-18.3MPa and 4.9-9.2MPa, respectively. The maximum and minimum principal stresses at all measuring points were nearly horizontal, while the middle principal stress was nearly vertical. Due to the existence of the Mafangquan fault, the direction of in situ stress was changed. Specifically, the maximum principal stress far away from the fault zone was NNW-strike, while the maximum principal stress near the fault turned to NS-strike. Besides, Han et al.  measured in situ stress of coal seams in several typical outburst mine areas in China through the stress relief method and found that the maximum and minimum principal stresses were on the horizontal plane, but middle stress was on the vertical plane. The maximum and minimum principal stresses in these mining areas were significantly higher than those in other mining areas, and the tectonic stress in these mining areas was remarkable. Hence, the control effect of tectonic stress on outbursts cannot be ignored under current coal mining depth.
Moreover, the initial balance state of in situ stress is broken due to the sudden unloading on one direction and disturbed stress on other directions caused by mining activities . However, these stresses do not disappear but transfer to coals/rocks in front of the exposed surface, thus causing redistribution of the stress state on coals/rocks . Generally, redistribution of stress shows two different laws along the mining direction (radial) and perpendicular to mining direction (tangential) . Firstly, stress along the horizontal mining direction (radial) increases from 0 to the original state gradually. Secondly, stress perpendicular to the mining direction (tangential) transfers inward from the exposed surface, and a stress concentration occurs at a position in front of the exposed surface. Such stress distribution mode, composed of radial stress unloading and tangential stress concentration in coals/rocks in front of the exposed surface, favors the coal/rock damage, which is also vital to the occurrence of outbursts .
As another power and energy source of outbursts, coal seam gas (hereinafter referred to as gas) is also crucial to the occurrence of outbursts. Effects of gas on outbursts are manifested by two aspects. Firstly, gas can change mechanical properties of coals and participates in mechanical failure of coal masses [37, 38]. Secondly, gas provides power to the transportation of outburst coals/rocks, which is one of the requirements for continuous development of outbursts . Generally, gas which participates in outbursts is divided into free gas and adsorbed gas. The former one exists in coal fractures and large pore spaces as free phases, while the latter one is adsorbed onto the pore space surface of coals [42, 43]. Due to different occurrence states of free gas and adsorbed gas in coals, the complexity of their participation mode and processes in outbursts varies significantly, thus making it greatly difficult to determine their proportions in outbursts [44, 45].
Free gas participates in outbursts through expansion energy and decreasing effective stress of coal mass . Free gas changes mechanical properties of coals since it decreases effective stress of coal mass, which can be explained by Terzaghi effective stress laws . With the increase in free gas pressure, effective stress of coal mass decreases gradually, thus decreasing confined pressure of coal mass and thereby weakening mechanical strength of the coal .
According to literatures [49, 50], free gas in fractures and large pore spaces makes expansion energy immediately once pressure of external gas declines. However, there is still dispute over whether the expansion process of the free gas is an adiabatic process or an isothermal process . Scholars who agree that the expansion process is an adiabatic process pointed out that outbursts occur in a very short period, during which heat exchange between coal-gas and external world is very rare. Hence, the outburst process can be viewed as an adiabatic process . Scholars who advocate the isothermal process deemed that heat exchange in the outburst process cannot be ignored since the heat exchange area between coal-gas and external world is large, but temperature changes of coals and gas in this process can be ignored . At present, most scholars accept that the expansion energy is an adiabatic process and the theoretical formula of swelling energy of gas has been deduced.
The participation mode and process of adsorbed gas in outbursts are more complicated than those of free gas . Some stated that adsorbed gas decreases the surface energy of coal mass, thus changing mechanical properties of coals . The gas adsorption on coals is essentially attributed to surface energy of coals. The gas adsorption is a process that coals change from an unstable state to a stable state, during which surface energy of coals is decreasing gradually [54, 55]. According to the crack growth theory of Griffith, the critical stress for crack expansion declines with the reduction of surface energy, thus decreasing mechanical strength of coals [56, 57].
Moreover, the prerequisite for adsorbed gas to participate in outburst is that adsorbed gas is transformed to free gas quickly during the occurrence of outburst. The participation process of adsorbed gas in outburst involves three stages: desorption of adsorbed gas from the pore surface, diffusion of gas in pores, and gas seepage in the fracture system . These three stages are controlled by different influencing factors, and correlations among these factors are very complicated [58, 59]. Hence, it is very difficult to determine practical amount of adsorbed gas participating in outbursts. Nevertheless, some scholars have concluded that adsorbed gas makes remarkable contributions to outbursts, and it is an important energy source of outbursts [53, 60]. Sobczyk  carried out a simulation test of outbursts based on N2 and CO2 which have different adsorbability. He found that CO2 with the stronger adsorbability could induce outbursts under a relatively low gas pressure and the outburst duration in the CO2 test was longer than that in the N2 test under the same gas pressure. Test results reflected the effects of adsorbed gas on critical condition and strength of outbursts intuitively. Gale  believed that adsorbed gas in coals contained a lot of outburst potentials, but the amount of adsorbed gas which could be released to participate in outbursts was determined by a series of factors. From the perspective of energy transformation of outbursts, Zhao et al.  found that limited free gas was inadequate to meet energy demands for outbursts and rapid desorption of adsorbed gas in a short period was an essential condition for continuous development of outbursts. They also found that in the Zhongliangshan outburst, the adsorbed gas had to supply about 5.61108J expansion gases for transportation of outburst coals/rocks, which was nearly 6.3 times of energies that free gas could provide.
With respect to gas participation in damage of outburst coals, some study has pointed that gas dominates tensile fracture of coal masses in the outburst process [62, 63]. Based on an outburst simulation test and theoretical calculation, Shi Ping et al. believed that sudden reduction of gas pressure on exposed surface may cause gas seepage in the side coals, forming a gas pressure gradient and generating a tensile stress to destroy coal mass . Ding et al.  found that initial failure and continuous failure of coals were mainly caused by seepage of free gas. Hu  pointed out that there is a great gas pressure gradient near the exposed surface after sudden exposure of coals, which was related to the gas flow in coal pores and fractures. Such gas pressure gradient would generate a drag force to coals, so that coal powders which were not bonded with the coal matrix tightly would be brought out of coals along the gas flowing path. Additionally, some studies mentioned that crushed coals would be further smashed by the expansion energy of desorbed gas during transportation, which, however, lacks more detailed studies .
As the main carrier of coal and gas outbursts, coal is the acting object of all outburst energies . Therefore, the importance of properties of coals in outbursts is beyond doubt. Properties of coals are also one of dominant factors that control outbursts. Nevertheless, there are a lot of properties of coals. According to literatures [2, 65], mechanical properties, pore structural features, and gas occurrence/flow characteristics of coals are major control factors of outbursts.
Mechanical properties of coal are important features that determine the risk of coal outbursts. Mechanical strength and failure mode of coals are major parameters that influence outbursts . However, physical properties (e.g., elasticity, plasticity, brittleness, and creep properties) vary significantly in different coals due to the structural differences of coals. Moreover, mechanical strength also is significantly different among different coals . According to literature records, an in situ strength test and laboratory strength test are the main testing method of mechanical strength of coals at present [67, 68]. According to test results, uniaxial mechanical strength of tectonic coals with reconstruction structure is generally lower than 3MPa , but the uniaxial compressive strength of intact coals with relatively complete structure is higher than 10MPa [68, 73, 74].  pointed out that the width of the stress releasing zone in front of the exposed surface increases with the decrease in coal strength when advancing into the roadway. On the one hand, widening the stress releasing zone could increase the resistance of outbursts. On the other hand, it could lead to rapid releasing of adsorbed gas. However, rapid releasing of adsorbed gas often occupied the dominant role in the occurrence of outbursts, thus resulting in extremely high risk of outbursts in the soft coal seams.
Moreover, macroscopic morphological differences after the failure also influenced outburst risks of coals. The mechanical strength of intact coals is generally high, accompanied by strong brittleness [75, 76]. Hence, intact coals mainly develop shear failure along to a certain angle of shear fracture along the native weak surface. The lumpiness of broken blocks is relatively large [75, 76]. Nevertheless, tectonic coals show strong plasticity under high confined pressure and damage expansion after failure, and tectonic coals are extremely easy to be broken into abundant pieces with small size [69, 77]. Therefore, tectonic coals are extremely easy to be broken during the occurrence of outbursts, and broken tectonic coals are easier to release adsorbed gas quickly [69, 77]. The existence of tectonic coals can decrease conditions for the occurrence and development of outbursts.
As a complicated porous medium, coal not only contains a great deal of micropores to provide a large specific surface area for gas adsorption but also has big pores and microcracks as channels for flowing of gases in coal seams . The formation process, occurrence environment, and experienced tectonic movements all vary among different coals, thus resulting in different pore sizes and morphologies [17, 7981]. It has been proven by literatures that pore structure of coals has different variation laws due to influences by metamorphism and tectonic effects . According to pore classification standards proposed by IUPAC, volume and specific surface areas of big pores and mesoporous in coals are generally high at ends and low in middle with changes of vitrinite reflectance . Besides, volume of larger pores in coals increases significantly under strong tectonic effect. However, smaller pores in primary pores are damaged by tectonic effect, thus decreasing pore volume accordingly for smaller pores. This explains to some extent that tectonic effect promotes fracture of macromolecular chains or the aromatic layer greatly, which is conducive to disordering development of molecular structures in coals, finally increasing pore volume [18, 23, 85, 90, 91].
Pore structural features of coals are decisive factors of adsorption/desorption, diffusion, seepage, and other laws. According to literatures, the Langmuir volume of coals to methane adsorption presents a trend of high at ends and low in middle with the increase in the metamorphic grade of coals [88, 9295]. In other words, the Langmuir volume decreases from low-rank coals to middle-rank coals, and it reaches the minimum in the low-rank coals. Subsequently, it increases gradually from the middle-rank coals to the high-rank coals.
Pore structures and crushing degree of coals are developed and increased as a result to the tectonic effect, which brings a larger adsorption space to the coals and makes coals reach the adsorption balance state more quickly [93, 96]. In addition, since diffusion mainly occurs in pores of the matrix, different pore structures of coals cause different diffusion properties of gas in the matrix. According to existing research conclusions, no quantitative relations have been established yet between the pore structure of coals and gas diffusion characteristics [97, 98]. According to testing results of different scholars, the discreteness of the methane diffusion coefficient of coals is very high, crossing several orders of magnitudes . Influenced by tectonic effect, the methane diffusion coefficient of coals generally increases . On the one hand, this determines that coals which undergo tectonic effect often have high initial ability to quickly desorb gas [107, 108].
Moreover, gas occurrence capacity in coals determines the outburst gas potentials of coals . Coals shall be able to release gas very quickly in order to transform outburst gas potentials into effective outburst energy during the occurrence of outbursts [49, 109]. Hence, initial gas desorption capacity of coals is an important guarantee to gas supply in the outburst process.
Based on the above literature review, three dominant factors of outbursts, namely, stress, gas, and properties of coals, are also influenced by some factors. These factors might influence a dominant cause independently or together. These influencing factors generally are inducing factors of outbursts. Combined with the literature review, this section summarizes important inducing factors and their influences on outbursts.
According to literature records [110, 111], mining activities of human nowadays still concentrate in strata where the buried depth is lower than 800m. However, there are some mines with mining depth higher than 1000m. For example, the mining depth of the Suncun Coal Mine of the Shandong XinWen Mining Group reaches 1350m. Variation of buried depth will surely trigger changes of occurrence environment for coal seams. In particular, it can influence stress environment of coal seams [30, 112]. Influenced by overlying rock thickness, vertical stress of coal seams increases significantly with the increase in buried depth [30, 113].
Variations of occurrence environment/stress environment in coal seams have been proven an important influencing factor of outburst risks in coal seams. However, it has been reported by literatures [1, 110, 114117] that the lowest buried depth of outbursts is 80m (outbursts in the Cezar Zofia Mine in Poland, 1894) and the highest buried depth is higher than 1100m (outbursts in the Yubari Mine in Japan, 1981). So far, most outbursts occur in the buried depth of 300~700m. It seems that there are no general laws of the relationship between outbursts and buried depth [16, 116, 118]. Some scholars believe that outburst frequency increases with the increase in buried depth, while some scholars get the opposite conclusions. This might be because that occurrence environment/stress environment of coal seams is influenced by factors except buried depth more significantly .
The mechanisms of gas outbursts, under different geology backgrounds and mining conditions, are still unresolved, but it is generally recognized that stress, gas, and properties of coal play an important role in outburst . Meanwhile, these three factors are controlled by other factors, such as tectonic movement, mining method, and coal formation process [110, 119].
Shepherd et al.  integrated the novel aspects of stresses, gas, and geological structure in a worldwide review of the problem of outbursts and considered that tectonic movement is the main factor of outburst. He also pointed out that thrust, fault, and fold are especially outburst prone, while these structures cause tectonic (sheared) coal to be present due to differential compaction. Taylor  observed that north England coalfield outbursts occurred in the immediate vicinity of tectonic disturbances and that the outburst coal comes from the soft coal seam. Yan et al.  discussed the relationship between gas outburst in E9-10 coal seams and tectonic movement in the Pingdingshan 10th Coal Mine and found that there were thick tectonic coals as well as uneven gas distribution due to the control effect of NW-NWW-strike folded tectonic belts.
Therefore, geologic factors play an important role in the formation process of outbursts. However, unfortunately few geological details were given for many outbursts, even though these were premier information for the study of outburst that occurred. Table 1 summarizes the limited record for outbursts and their geological information from different countries in literatures. These literatures [14, 16, 110, 114, 117, 119, 122126] show that geological factors (such as fault, fold, thrust, roll, coal seam thickness changes, and dip) are a common feature associated with outburst, and some of them point that outbursts occur in the vicinity of tectonic (soft) coal pinched by geological factors.
Effects of these geologic factors on outbursts are manifested in two aspects . Firstly, geologic factors influence the occurrence and geometric shape of coal seams, including variation of coal seam thickness and inclination [1, 110]. This is mainly manifested by changes of stress environment, increasing gravity effect, gas, and energy gathering. Secondly, geologic factors can cause sudden disturbances to coal seams [1, 110]. This is mainly manifested by changes in geometric forms of coal seams and physical-chemical properties of coals, which directly favor the occurrence of outbursts and undergo sudden changes. Specifically, the existence of tectonic coals (soft coal) in many geological tectonic zones deserves extensive attentions.
Tectonic coals are the product of geologic factors (especially tectonic movement). Under extremely high stress conditions, tectonic coals are the product of continuous pulverization of primary coals and changes of physical-chemical properties under high stress conditions (Figure 1) . The distribution of coal seams depends on complicated geological genesis. Bed sliding is the main controlling factor for the regional distribution and stratified distribution of tectonic coal, which are mostly caused by folds (Figure 1(b)) and bedding faults (Figure 1(c)). The local distribution is mostly caused by faults cut by vertical coalbeds, as shown in Figure 1(d). Tectonic coals show significantly different macroscopic mechanical properties, microscopic pores, and fracture structures with intact coals . The formed tectonic coals have extremely low fracture resistance, which is similar to reconstructed coals . Compression of fractures leads to low permeability of coals, which creates conditions for gas enrichment . The low permeability of tectonic coal seams has been proven in many engineering cases of difficult gas extraction in tectonic coal seams [128, 129]. However, tectonic coals can be broken into abundant pieces with small size after failure, during which gas is released very quickly [105, 130]. Therefore, the tectonic movement zone is extremely more susceptible to outbursts than normal zones due to the unique properties of coals, high stress, and gas concentration.
Under general conditions, coal seam gas is the generic term of toxic and harmful gases, mainly CH4. These CH4-centered coal seam gases are mainly formed by decomposition of cellulose of ancient plants and organic matters by anaerobion in early piling and formulation of coals, or CH4 is generated continuously from physical and chemical effects of coals in high-temperature and high-pressure environments during the formation of coals . CH4-centered coal seam gases have very extensive distribution in coal seams. Most coal seams are mainly occupied by CH4. As a result, most coal and gas outbursts which have been reported are mainly caused by CH4-centered gases in coal seams.
However, high-concentration CO2 is the major gas component in some unique coal seams. The no. 2 coal seam of the Yaojie Coal Mine in Gansu Province, Northwest China, is a typical CO2-occupied coal seam. The CO2 concentration in coal seam gases in the eastern region reaches as high as 18.79-96.6% . The CO2 concentration in the no. 2 coal seam of the Haishiwan Coal Mine in deep regions of the Yaojie coalfield reaches 34.1-98.64%, and coal seam gas pressure is 1.0-7.5MPa . According to literatures , high-concentration CO2 in coal seams has several potential sources, including magmation, mantle degassing, and spontaneous combustion of coal seams. Among them, volcanic activities and magmation are important sources of high-concentration CO2. A lot of CO2 are generated due to thermometamorphism of carbonates during volcanic activities and magmation [134, 135]. Meanwhile, fault serves as pipelines, barriers, or mixed effect against transportation and sealing up of CO2 in coal seams [136, 137]. In particular, fault plays an important role in the formation of high-concentration CO2 in coal seams.
High-concentration CO2 can induce outbursts as well. The Yaojie coalfield has experienced several CO2 outbursts since 1977 , and similar CO2 outbursts have been reported in the Cevennes Basin (France) and Sydney and Bowen Basin in Australia [1, 117].
However, CH4 outburst and CO2 outburst cannot be compared directly due to complexity and nonrepeatability of outbursts. According to the laboratory test, the CH4 adsorption capacity of coals is lower than the CO2 adsorption capacity . Due to different adsorption capacities, the swelling effect of coals after CO2 adsorption saturation is significantly stronger than that after CH4 adsorption saturation , which can influence permeability and mechanical properties of coals [58, 140, 141]. Besides, gas potentials of coals after CO2 adsorption are higher than those after CH4 adsorption under the same gas pressure . These potentials will contribute to the occurrence of outbursts once there are conditions for releasing. However, the relationship between the main components of coal seam gas and outbursts still has to be discussed deeply by using similar simulation test means.
The influence of the occurrence environment of the coal seam on outburst is complex, which is necessary to comprehensively consider the coal seam dip, thickness, and overlying/underlying rock conditions.
Due to different sedimentary environments and different geological movements in the late period, occurrences and thickness vary significantly in different coal seams. According to the coal seam dip, existing coal seams can be divided into near-horizontal coal seams (<8), gently inclined coal seams (8-25), inclined coal seams (25-45), and sharply inclined coal seams (>45) . Nevertheless, the coal seam dip often changes suddenly and even there is inversion of coal seams during mining practices. According to literatures , influences of gravity stress of coal seams on outbursts are intensified with the increase in the coal seam dip, thus increasing the risk of outbursts in coal seams. Sudden changes of the coal seam dip or inversion of coal seams is generally related to tectonic activities, and these regions are often high-risk regions of outbursts . Moreover, thickness of the coal seam also can influence outburst risks. Some outbursts may occur in positions with sudden thickening of coal seams or in the soft layer of coal seams .
Mudstone or various sandstones are common overlying/underlying rock properties of coal seams. Since these differences of gas permeability and thickness of rock strata can influence the occurrence of coal seam gases, some unique overlying/underlying rock strata can influence outburst risks in coal seams significantly [145, 146].  studied influences of overlying red bed in the Xutong and Zhaoji Coal Mine of the Huaibei Coalfield on the occurrence of coal seam gases. They found that red bed was a kind of porous layer with high permeability and high diffusion and it was inferior to other rock strata in terms of sealing capacity of coal seam gases. As a result, coal seam gases can escape through the overlying red bed. The overlying red bed can be viewed as a symbol of low coal seam gas.
Additionally, a magmatic stratum is viewed as the main controlling factor of several outbursts in the Haizi Coal Mine and Wolonghu Coal Mine in the Huaibei Coalfield, China. Specifically, there were 11 outbursts below the 120m thick magmatic rocks in the Haizi Coal Mine and 2 outbursts in the circular closed zone of magmatic rocks in the Wolonghu Coal Mine . The control effect of the magmatic stratum over outburst also exists in the Tiefa coalfield in Northeast China . Such control effect is mainly manifested in influences on coal seam gases. The magmatic stratum has thermal evolution and sealing effects . Firstly, the thermal evolution of the magnetic stratum promotes secondary hydrocarbon generation of coal seams and changes the metamorphism degree of coal seams and pore/fracture structure of coals, thus increasing content and pressure of coal seam gases . Secondly, the significantly thick and dense magmatic rocks can seal up coal seam gases and prevent escaping of coal seam gases effectively .
Mining activity influences outburst as a man-made additional factor. Mining activities not only provide additional energy to outbursts but also break the original balance of coal seams, thus inducing the occurrence of outbursts . According to statistics on frequency and strength of outbursts, rock crosscut coal uncovering is the most favorable condition for the occurrence and development of outbursts compared to coal seam roadway advancing, raise advancing, downhill advancing, and coal face, thus increasing numbers of typical high-intensity outbursts . Mining techniques (e.g., blasting and fully mechanized coal mining) and mining speed are often viewed as influencing factors of outbursts. Outbursts have been reported for several times after blasting of the working face . Excessive mining speed is very easy to break the dynamic balance of coal/rock, stress, and gas, thus making it easy to induce outbursts.
Besides, the advancing direction of the working face and time-space relationship of the primary rock stress field in coal seams are very important to outbursts.  carried out a numerical simulation of outburst risks on the working face of roadways under different primary rock stresses by using FLAC3D. When the maximum principal stress is perpendicular to the axis of the roadway, the roadway is the easiest to suffer outburst. Moreover, the outburst risk of the roadway reaches the peak when the recovery cycle is in the position of stress concentration peak. Advancing safety in the roadway is improved when the maximum principal stress is parallel to the roadway .
Outburst is the extreme instability under the collaborative effect of stress, gas, and coals. Stress, gas, and properties of coals are all vital to outbursts. Stress and gas are the power and energy source of outbursts. The former one dominates coal-rock damage, while the latter one provides supply to transportation of coals and rocks and intensifies coal-rock damage. Coal is the acting object of power and energy of outbursts, and it determines the power and energy requirements of outbursts. All five inducing factors influence outbursts by changing stress and gas of coal seams or properties of coals. For example, vertical stress of coal seams increases with the increase in buried depth. Tectonic stress formed by tectonic movement is an important factor of outbursts, while increasing the coal seam dip can intensify influences of gravity stress of coal seams on outbursts. With respect to the occurrence of coal seam gas, gas is difficult to escape due to the thickening overlying strata caused by the increase in buried depth. Gas sealing condition was created by tectonic movement and the existence of the overlying thick and dense magmatic stratum. Heat effect of the overlying/underlying magmatic stratum is going to increase gas generation. Gas occurrence capacity of coals is increased by changing structures of coals. Moreover, tectonic movement also can trigger restructuring of coal seams, thus changing the basic properties of coals.
According to comparison of five inducing factors, there are no general laws in outburst and buried depth. The increasing vertical stress of coal seams and overlying strata thickening caused by the increase in buried depth can seal up gases, which influences the outburst risks of coal seams to some extent. Field conditions of CO2 outburst and CH4 outburst are not comparable. At present, it can only conclude according to experiments that CO2 outburst might be more serious. Hence, the key inducing factors of outburst mainly include tectonic movement, occurrence condition of coal seams, and mining mode. Among them, mining mode is an artificial factor caused by mining activities of human. Rock crosscut coal uncovering, blasting, and high mining speed often cause insufficient unloading of stress and insufficient release of gas, thus inducing outbursts. Unreasonable design of the mining direction may intensify stress concentration. However, tectonic movement is often viewed as closely related to outbursts, and it is the key factors that influence the occurrence of outbursts and together variations of stress, gas, and properties of coals. Moreover, it has been proven that the thick and compact magmatic stratum in overlying/underlying rock strata in coal seams can control some outbursts.
A serious coal and gas outburst occurred in the Xiangshui Coal Mine, Guizhou Province, on November 24th, 2012. This outburst occurred on the working face of the 1135 transport roadway. The elevation and buried depth of the outburst point were +1225m and about 203m, respectively. The outburst occurred on the no. 3 coal seam with an average thickness of 2.67m and a coal seam dip of 10~30. The measured gas and gas pressure at the no. 3 coal seam were 13.9m3/t and 1.65MPa, respectively (elevation of the measuring point was +1237.5m).
According to field investigation, the outburst hole was at about downward inclined 45 in front of the 1135 transport roadway advancing face, which had a small mouth and large cavity. The mouth width of the outburst hole was 3.58m, and the depth was about 12~15m.
There were bedding faults at two sides of coal seams in the 1135 transport roadway, and there was obvious rock bed dislocation at the left and right sides of the outburst hole. In addition, the coal seam near the outburst point was thickened, accompanied by evident changes in properties and structure of coals. Coals, which are 163m away from the outburst point, are bright and hard and presented characteristics of intact coals. However, coals became dark and softened near the outburst point and presented typical characteristics of tectonic coals, as shown in Figure 3.
In a word, stress conditions of the outburst point, gas conditions, and properties of coals determined the occurrence of this outburst together. However, it is difficult to have stress conditions in the outburst region for high smashing or pulverization of intact coals during the outburst process according to the buried depth . Moreover, coal restructuring is the most prominent changes of coal seams near the outburst point, and there are abundant tectonic coals near the outburst point. Such tectonic coals were crushed/pulverized continuously in the tectonic movement process (mainly bedding fault), thus requiring no abundant outburst energies in the outburst process. Once outburst conditions were met, tectonic coals were outburst by the collaborative effect of gas and stress. Hence, tectonic movement played the key role in this outburst.
A serious coal and gas outburst occurred in the Bailongshan Coal Mine of Yunnan Diandong Energy Company on September 1st, 2013. This outburst occurred on the gas drainage roadway advancing face in the 17+805 working face floor. The outburst coal seam was no. C7+8 coal seam (thickness of coal seam: 2.76m), which was at 6.9~7.0m on the roof of the gas drainage roadway. The buried depth and vertical ground stress at the outburst point were about 500m and 12.5MPa, respectively. Besides, gas pressure and gas in the no. C7+8 coal seam were 1.57MPa and 16.42m3/t, respectively. Protodyakonovs coefficient (f) of coals was 0.3.
According to field investigation (Figure 4), the gas drainage roadway in the 17+805 working face floor had complicated geological conditions, and more than 10 minor faults were discovered during advancing in the roadway. Besides, a reverse fault with a drop height of 6m was found at the corresponding outburst position of the gas drainage roadway in the 17+803 working face floor. Moreover, a fault with a drop height of 20m was found between the 23307 geological prospecting hole and 4233 geological prospecting hole, which was adjacent on the gas drainage roadway advancing face in the 17+805 working face floor.
According to comprehensive deduction, there is a reverse fault with a drop height of 4-6m in front of the gas drainage roadway advancing face. Strike, tendency, and dip of this fault were 110, 20, and 30, respectively (Figure 4). Influenced by faults, the positional relation between the advancing face and no. C7+8 coal seam changed. The no. C7+8 coal seam of the hanging wall of fault lifted up by extrusion, while the no. C7+8 coal seam of the footwall of fault moved downward and approached to the advancing face suddenly. Besides, the tectonic fracture zone was observed near the advancing face, and rock strata on the roof of the roadway 10m away from the working face were smashed. The outburst coals at the external side of the mouth were crushed and even pulverized.
To sum up, the coal seams near the outburst point in this case had high gas and high ground stress, which were important factors to the occurrence of outburst. Nevertheless, tectonic movement was crucial to this outburst. The reverse fault changed the positional relation between the advancing face and no. C7+8 coal seam, thus making the co. C7+8 coal seam move downward and approach to the advancing face suddenly. This created conditions for disclosing the outburst coal seams. Besides, the tectonic fracture zone was developed near the outburst point due to tectonic effect, thus resulting in breakages of rock strata on the roadway roof. As a result, coals in the outburst coal seams were crushed and even pulverized, showing low strength. Therefore, these changed the initiation conditions of outburst.
An outburst occurred after blasting in the belt roadway of the 11090 working face in the Xinyi Coal Mine on May 22nd, 2018. In this outburst, 1917t of highly pulverized coals was outburst. According to field investigation, the buried depth of the 11090 working face was 361.9~400.6m. In this region, there are simple tectonic movement and development of small folds, generally showing monoclinic morphology. The outburst hole was on the left side of the belt roadway, and it was blocked by pulverized coals. The coal seam roof in the outburst hole was flat, and no roof failure was observed (Figure 5). According to these phenomena, there are no sufficient overlying stress and tectonic stress near the outburst point. In addition, the existing gas data showed that the measured maximum gas pressure of the outburst coal seam (no. 21 coal seam) was 1.40MPa (buried depth of measuring point: 701.71m) and the maximum gas was 12.84m3/t (buried depth of measuring point: 700m). Combined with relevant research conclusions, coal seam gas near the outburst point cannot meet energy conditions for high pulverization of intact coals .
Therefore, properties of coals play the key role in this outburst. This outburst is a typical outburst case in the tectonic coals. The no. 21 coal seam in the 11090 working face belongs to III-V coals and tectonic coals. Besides, the outburst point was in the coal seam thickening zone, and there is a synclinal axis on the left floor of the belt roadway along the advancing direction, thus making tectonic coals highly crushed/pulverized under the original state.
Combined with key factors of coal and gas as well as geological features of the typical outburst case, tectonic movement can control outburst, as shown in Figure 6. This is mainly realized by changing three dominant factors of outburst. (1)Tectonic movement will change ground stresses in the tectonic zone. Therefore, ground stresses in the tectonic zone are sensitive to tectonic stress in addition to overlying stress. In particular, tectonic stress influences ground stresses significantly at present mining depth for most coal resources. Ground stresses are changed with tectonic movement, thus resulting in abnormal ground stresses in the region and forming high ground stresses(2)Some unique tectonic movements formed by tectonic effects can control the occurrence and migration of coal seam gases. For example, a closed fault can seal up coal seam gases, and a compact magmatic stratum can block the migration of coal seam gas. In addition, high ground stresses in the tectonic zone induce low permeability of coal seams, restricting the migration of gas along the coal seam which can lead to the abnormal occurrence of coal seam gas in the zone, thus forming gas enrichment(3)Nevertheless, attentions shall be paid to influences of microstructure changes of coals caused by tectonic movement on outburst. In the long-time tectonic movement process, the primary structure of coals is damaged/pulverized due to the high tectonic stress and unique tectonic mode, which decreases the matrix size of coals sharply. In addition, pulverized coals are compacted by tectonic stress to form tectonic coals. Physical structures of tectonic coals are significantly different from those of intact coals. As a result, mechanical behavioral responses of tectonic coals to changes of external stress are altered. Tectonic coals are generally damaged and expanded under low stress conditions, which can influence the matrix-fracture structure of tectonic coals significantly. During the formation of tectonic coals, internal micropores of coals vary as a response to decreasing matrix size of tectonic coal particles, so that pore structure in coals is further developed, thus influencing the occurrence and diffusion space of gas in the pore structure. Therefore, tectonic coal particles have high initial gas desorption capacity
Tectonic movement will change ground stresses in the tectonic zone. Therefore, ground stresses in the tectonic zone are sensitive to tectonic stress in addition to overlying stress. In particular, tectonic stress influences ground stresses significantly at present mining depth for most coal resources. Ground stresses are changed with tectonic movement, thus resulting in abnormal ground stresses in the region and forming high ground stresses
Some unique tectonic movements formed by tectonic effects can control the occurrence and migration of coal seam gases. For example, a closed fault can seal up coal seam gases, and a compact magmatic stratum can block the migration of coal seam gas. In addition, high ground stresses in the tectonic zone induce low permeability of coal seams, restricting the migration of gas along the coal seam which can lead to the abnormal occurrence of coal seam gas in the zone, thus forming gas enrichment
Nevertheless, attentions shall be paid to influences of microstructure changes of coals caused by tectonic movement on outburst. In the long-time tectonic movement process, the primary structure of coals is damaged/pulverized due to the high tectonic stress and unique tectonic mode, which decreases the matrix size of coals sharply. In addition, pulverized coals are compacted by tectonic stress to form tectonic coals. Physical structures of tectonic coals are significantly different from those of intact coals. As a result, mechanical behavioral responses of tectonic coals to changes of external stress are altered. Tectonic coals are generally damaged and expanded under low stress conditions, which can influence the matrix-fracture structure of tectonic coals significantly. During the formation of tectonic coals, internal micropores of coals vary as a response to decreasing matrix size of tectonic coal particles, so that pore structure in coals is further developed, thus influencing the occurrence and diffusion space of gas in the pore structure. Therefore, tectonic coal particles have high initial gas desorption capacity
Besides, physical structure of tectonic coals determines their low permeability under high ground stress , which restricts gas flow in tectonic coals and results in abundant gas residues in tectonic coals. However, the matrix-fracture structure of tectonic coals is changed after stress disturbance, and gas flowing in fractures is accelerated. Therefore, rapid gas releasing becomes one of the outburst energy sources.
The outburst process is generally divided into the preparation stage, triggering stage, development stage, and termination stage. Based on previous studies, the relationship between tectonic factors and outburst was further reviewed, and the outburst process was further described (Figure 7).
The preparation stage includes the tectonic process and mining process. The former one refers to the restructuring process of the original coal-bearing strata by tectonic movement. In the long-time geological process, coal-bearing strata might experience one-phase or multiphase tectonic movements, which change ground stresses and gas conditions in original coal-bearing strata, resulting in local high ground stresses and gas enrichment. Tectonic coals are the accompanying products of tectonic movement. Physical structures not only determine the properties of tectonic coals but also cause unique mechanical behaviors of tectonic coals and gas flow when external factors change. These are crucial to the preparation of outburst. Nevertheless, the mining process is the prerequisite of outburst, and it refers to the disturbance of human mining activities to the initial coal seams. The mining process breaks the structural balance of initial ground stresses of the coal seam, gas, and coals, causes migration of stress, gas flow, and structural evolution of coals, triggers quasistatic damage of coals, and reaches the critical conditions of outburst gradually. In other words, the preparation stage includes that gas pressure reaches the critical threshold for the occurrence of outburst. (2)Triggering stage
This stage is a process of instantaneous instability when a critical condition is met under the collaborative effect of stress, gas, and properties of coals. As the triggering point of outburst, the triggering stage often can cause rapid failure and throwing of outburst obstacles (e.g., rock pillar and coal pillar) and form the initial outburst hole and initial exposed surface. These provide a large space conditions for the subsequent development of outburst. Besides, more coals with high stress and gas pressure will be exposed after the formation of initial pores, thus providing favorable conditions for continuous failures. (3)Development stage
The development stage starts after the triggering stage, and it contains the whole process from acceleration to stability and finally to attenuation. Spatially, it can be divided into the inside and outside of the hole. Mechanical failure of coals in the outburst hole involves the following processes: (1) concentrated stress dominates fracture development of coals and the expanded plastic failure process. (2) A high gas pressure gradient is formed near the exposed surface, which causes tensile fracture of coals and makes fractures expand through the spalling parallel to the exposed surface. (3) Given the continuous influences of high-pressure gas, spalling breaks and then is separated and outburst. (4) The exposed surface migrates to the deeper positions, and it enters into the next failure cycle.
Outburst coal migration by gas flow occurs outside of the outburst hole, during which outburst coal experiences acceleration, deceleration, and final static under the power and frictional resistance provided by gas. In this process, outburst coal is further crushed and even pulverized due to friction, impact, and swelling effect of desorption gas. (4)Termination stage
With the development of outburst, gas energy and stress energy of outburst are consumed continuously. Meanwhile, a lot of outburst substances block the outburst hole and mining space, thus increasing the outburst resistance. Finally, the outburst energy and resistance are inadequate to maintain the outburst, and the outburst terminates. When the outburst energy gathers again and it is enough to overcome the outburst resistance, twice outburst might occur, or the outburst terminates directly. Coal-rocks return to the static balance.
Based on the previous analysis of effects of tectonic factors on outburst, there might be potential theoretical and experimental research directions with respect to geologic aspects and process of outburst (Figure 8).
In the geological aspects of outburst, the effects of tectonic factors on the ground stress field and gas field of the original coal-bearing strata have to be further studied. Attentions and further studies are needed on physical and chemical changes that coals have undergone during tectonic movement to form tectonic coals as well as the relations among tectonic movements, tectonic strength, and types of tectonic coals.
For properties of tectonic coals, a deep study on macro/microphysical structures of tectonic coals is the premise to master the basic characteristics of tectonic coals. The unique mechanical behaviors and gas flow of tectonic coals when external factors change as well as instability mode of tectonic coals under the coupling effect of stress and gas also shall be further explored.
In addition, the internal relations between tectonic coals and preparation and development of coal and gas outburst shall be studied deeply. The occurrence mechanism of coal and gas outburst in tectonic coals is built, and predictive indexes of outburst in tectonic coals are established to evaluate outburst risks. Specific multigrade and multilevel measures shall be adopted to eliminate outburst risk of tectonic coals, which also needs to be further studied.
Finally, it is expected to further disclose the formation mechanism and process of outburst under tectonic movement. It has important scientific significance to study the outburst mechanism and control.
(1)Outburst is the extreme instability caused by stress, gas, and coal. Inducing factors, including buried depth, tectonic movement, gas composition, coal seam conditions, overlying/underlying rock conditions, and mining mode, control the outburst by influencing the dominant factors. Tectonic movement will change the stress field, gas field, and structural characteristics of coals in the tectonic zone, and it is the key of outburst. Most outburst cases have proven the close relationship between outburst and tectonic movement(2)Influenced by tectonic movement, tectonic stress can change ground stresses and gases in the tectonic zone, thus resulting in abnormal ground stress and gas enrichment in the region. The original structure of coals is damaged/pulverized due to the tectonic stress and unique tectonic mode, resulting in a sharp reduction of matrix size. Crushed/pulverized coals are compacted and form tectonic coals again. The microstructure of tectonic coals determines their differences from intact coals in terms of porous structure, mechanical properties, and gas occurrence/flowing features. When external dynamic factors are changed, tectonic coals are crucial to outburst control for its evolution of porous structure as well as the unique mechanical behaviors and gas flowing responses(3)The outburst process is generally divided into the preparation stage, triggering stage, development stage, and termination stage. The preparation stage of outburst includes the tectonic process and mining process. The former one refers to the restructuring process of the original coal-bearing strata by tectonic movement, while the mining process is the prerequisite of outburst and it refers to the disturbance of human mining activities to the initial coal seams(4)Ground stresses and gases of the original coal-bearing strata change with tectonic movement. However, it still lacks a systematic study on physical and chemical changes that coals have undergone during tectonic movement to form tectonic coals and relations among tectonic structural form, tectonic strength, and types of tectonic coals. The unique mechanical behavior and gas flowing of tectonic coals when external factors change are the key to study the occurrence mechanism and process of coal and gas outburst
Outburst is the extreme instability caused by stress, gas, and coal. Inducing factors, including buried depth, tectonic movement, gas composition, coal seam conditions, overlying/underlying rock conditions, and mining mode, control the outburst by influencing the dominant factors. Tectonic movement will change the stress field, gas field, and structural characteristics of coals in the tectonic zone, and it is the key of outburst. Most outburst cases have proven the close relationship between outburst and tectonic movement
Influenced by tectonic movement, tectonic stress can change ground stresses and gases in the tectonic zone, thus resulting in abnormal ground stress and gas enrichment in the region. The original structure of coals is damaged/pulverized due to the tectonic stress and unique tectonic mode, resulting in a sharp reduction of matrix size. Crushed/pulverized coals are compacted and form tectonic coals again. The microstructure of tectonic coals determines their differences from intact coals in terms of porous structure, mechanical properties, and gas occurrence/flowing features. When external dynamic factors are changed, tectonic coals are crucial to outburst control for its evolution of porous structure as well as the unique mechanical behaviors and gas flowing responses
The outburst process is generally divided into the preparation stage, triggering stage, development stage, and termination stage. The preparation stage of outburst includes the tectonic process and mining process. The former one refers to the restructuring process of the original coal-bearing strata by tectonic movement, while the mining process is the prerequisite of outburst and it refers to the disturbance of human mining activities to the initial coal seams
Ground stresses and gases of the original coal-bearing strata change with tectonic movement. However, it still lacks a systematic study on physical and chemical changes that coals have undergone during tectonic movement to form tectonic coals and relations among tectonic structural form, tectonic strength, and types of tectonic coals. The unique mechanical behavior and gas flowing of tectonic coals when external factors change are the key to study the occurrence mechanism and process of coal and gas outburst
The authors are grateful for the support from the National Natural Science Foundation of China (No. 52004008), the Natural Science Foundation of Anhui Province (Nos. 2008085QE260 and 2008085QE222), and the Independent Research Fund of the State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines (Anhui University of Science and Technology) (No. SKLMRDPC19ZZ07).
Multotec specialises in coal beneficiation equipment with over 45 years of global experience and leading technology. Our team of metallurgists and engineers will partner with you to customise your equipment, helping your process run more efficiently while lowering your overall cost per ton. All Multotec equipment used in this process is shown in green on this flowsheet and is for illustrative purposes only; click on the red pins to find out more about each product, download PDForcontact usfor more information.
Multotec specialises in coal beneficiation equipment with over 45 years of global experience and leading technology. Our team of metallurgists and engineers will partner with you to customise your equipment, helping your process run more efficiently while lowering your overall cost per ton.
All Multotec equipment used in this process is shown in green on this flowsheet and is for illustrative purposes only; click on the red pins to find out more about each product, download PDForcontact usfor more information.
Firstly, large coal-bearing stones are crushed by coal crushers to a size of 20 to 25 mm , and then they are fed to coal bunkers. Through the vibrating screen, those crushed coal stones are continuously enter the coal mill. Coal gets ground by grinding machines in the coal plant. Coal particles get re-circulated around four times before achieving required fineness.
Classifier allows finer particles to escape mill outlet while coarse particles are returned to mill for further grinding. Primary air is supplied by fan through air pre heaters to coal mill or in some boilers, fan pressurize hot air received from air pre heaters and then supply to mills.
Hot primary air drives moisture from coal and acts as carrier to transport pulverized coal to the boiler through pipes. However, the above can be considered as the brief overview of coal mining process flowchart.Get in Touch with Mechanic