The stones mixed in the coal seam contain a small amount of combustibles and are not easy to burn.It is commonly known as "gangue".Crushed stone extracted from underground or mixed into ore during mining.The thin rock stratum in the middle of the coal seam is called "rock intercalation".The gangue heap on the ground of the mine is called "gangue hill".Some gangue can be used as raw materials for building materials.[1]
Chinese name
waste
Foreign name
rock refuse
Application
Production of agricultural fertilizer, concrete, etc
Hazards
Land occupation, spontaneous combustion, rain, mud, etc
type of shipping
Automobiles, trains, trams and aerial ropeways
solid waste
Rock roadway, hand dressing gangue, tailing of coal preparation plant, etc
Coal gangue is a kind of solid waste discharged in the process of coal mining and washing.China's coal gangue has accumulated more than 5 billion tons, and is still increasing at a rate of 300 million to 350 million tons per year. It is estimated that by 2020, the annual discharge of coal gangue in China will increase to 729 million tons.
The massive stockpiling of coal gangue not only wastes land resources, but also causes spontaneous combustion, rain, mud and other situations, which seriously harms the environment.However, from the perspective of resource attribute, coal gangue is a valuable secondary resource. Its resource utilization is a necessary measure to prevent environmental disasters, and is also one of the important work to achieve ecological civilization and circular economy development in China's coal industry.[2]
The solid waste of the gangue hill includes stripped gangue, gangue in coal lane, gangue in rock lane, gangue from hand dressing, tailings from coal preparation plant, etc.Spontaneous combustion, explosive blowout, landslide and leaching water of gangue hill all pollute the environment of the mining area.
Transportation and stacking of gangue
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China's coal mines use vehicles, trains, tramcars and aerial ropeways to transport gangue.Gangues are mostly piled up in wasteland in plain areas, or used to fill coal mine subsidence areas for reclamation, or piled up into conical or ridge shaped gangue hills.In mountainous areas, the waste rock will roll down freely along the hillside and be discharged, filling the gully and forming a flat roof waste rock hill.Due to the different waste discharging machinery and stacking process, the shape of the waste dump is different, and the grain size structure of the waste dump is also different.The gangue dump with different particle size structure has different aerodynamic properties.The cone shaped hills with spontaneous combustion tendency have the highest spontaneous combustion rate, followed by the ridge shaped and flat roof gangue hills.[3]
Analysis of environmental hazards of coal gangue
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Occupy a large amount of cultivated land
The coal gangue is mostly hoisted by winch and dumped by dumper, naturally piled in piles and stacked in the open air, which occupies a lot of land.According to incomplete statistics, China's coal gangue dump covers an area of nearly 15000 hmtwoMoreover, with the annual increase of coal gangue emissions, the phenomenon of farmland encroachment will further deteriorate, which will further exacerbate the shortage of land resources in China.In addition, due to spontaneous combustion and other phenomena, the coal gangue hills have low vegetation coverage and large areas of black ground exposure, which seriously affect the geological and geomorphological landscape.[2]
air pollution
In many areas of China, the coal gangue group is composed of sandstoneSandy conglomerateTherefore, when coal gangue is piled up in the open air, it is easy to be weathered into fine particles and brought to the atmosphere. The combustible hydrocarbons in these fine particles are oxidized, decomposed, dehydrogenated, polycondensation and a series of complex reactions under high temperature to form granular suspensions such as carbon black, fly ash and haze.[2]
At the same time, the carbon content of coal gangue is 5%~45%, and a large amount of SO will be produced when the coal gangue spontaneously combuststwo、CO、COtwo、NOxAnd other toxic and harmful gases and smoke and dust, seriously polluting the environment of the mining area.
Liu Ruhai and others analyzed the relationship between the carbon content, sulfur content, and the proportion of volatile matter in the coal gangue and whether the coal gangue can spontaneously ignite, taking the coal gangue in northeast China as the research object, and reported the accidents of pollution and casualties caused by coal gangue spontaneous combustion.[2]
heavy metal pollution
Among the pollutants contained in coal gangue, heavy metal elements have become a research hotspot at home and abroad because of their non degradability, long retention period in the environment, and strong "three send" characteristics.
Many scholars worry that the heavy metals in the coal gangue will pollute the surrounding soil, ground and groundwater when the location of the coal gangue dump is not good and the anti-seepage measures are not in place.Fanfani et al. studied the heavy metal pollution of a mine tailings dump in the west of Montevecchi, Italy, to the soil water environment from the perspective of leaching element solubility.Panov et al. investigated the environmental chemistry of heavy metals in the Bass coalfield in Roston, Russia, and found that the contents of Hg, As, Pb, Zn and Cd in many soil samples in this area exceeded the standard.Chinese scholars have measured the heavy metal elements such as Hg, Cr, Pb, Cu, As, Zn, etc., which are harmful to the environment, and conducted a lot of research on the pollution of heavy metal elements in the coal gangue dump and its surrounding soil.[2]
According to the secondary standard of paddy field (pH=5.5~6.5) for agricultural land in the Environmental Quality Standard for Soil (GB1561-2008), the heavy metals in the soil around the coal gangue dump are evaluated. Except Cr and Zn, most of the heavy metals in the soil around several typical coal gangue dump are higher than the standard requirements,It indicates that the coal gangue dump has certain heavy metal pollution to the surrounding environment, which may be due to the enrichment and migration of heavy metals in the mining area soil, so it is urgent and necessary to control it.[2]
Geological hazard
Due to historical reasons, China's coal gangue hills are basically not designed, and the stacking of coal gangue is extremely irregular, loose in structure and poor in stability.Under the condition of disordered excavation and natural rain wash, landslide, collapseDebris flowSuch geological disasters pose a serious threat to the lives and property safety of local people.[2]
Application of coal gangue in agriculture
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land reclamation
Because coal gangue hills occupy a large amount of land, it is an important task to use coal gangue to fill and reclaim land, and to reclaim and afforest coal gangue hills.Pei Zongyang systematically studied the background theory of ecological restoration of coal gangue hills and the site conditions of coal gangue hills, and pointed out that the ecological restoration of coal gangue hills should focus on the construction of vegetation restoration system, site condition system, succession law of vegetation restoration of coal gangue hills and the construction of evaluation system of ecological governance.The coal gangue reclamation technology can consume a large amount of coal gangue, reduce the environmental pollution caused by the open-air stacking of coal gangue, and improve the ecological environment of the mining area.[2]
Production of agricultural fertilizer and soil improvement
The utilization of coal gangue in agriculture is mainly reflected in the production of agricultural fertilizer and soil improvement.Coal gangue, especiallyHigh sulfur coalIn gangueIron sulfideIt is oxidized at high temperature to generate sulfur dioxide, and then synthesized with ammonia to generateammonium sulphate。As early as 1992, Liu Daman reported this application of coal gangue, and found that the ammonium sulfate produced by the test is a good fertilizer.
Generally speaking, the newly produced coal gangue is slightly alkaline, while the pH value of the aged coal gangue is generally low. Gao Guoxiong and others have studied the improvement effect of coal gangue sand barrier on sandy soil, and found that coal gangue can increase fine matter, increase soil moisture content, improve soil structure, and increase the content of organic matter, total N, available P, and available K in the soil,It can be used as an amendment of sandy soil, but when coal gangue is used in agricultural production, special attention should be paid to the problem of pollution diffusion.[2]
Application of coal gangue in road engineering
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Coal gangue contains a certain amount of alumina andSilicon oxideComposition.In the utilization of coal gangue, the mass ratio of aluminum to silicon (w (AltwoOthree）/w（SiOtwo））Coal gangue greater than 0.5 is used as raw material for manufacturing wall bricks, advanced ceramics, calcined kaolin and molecular sieve;Cement clinker is produced from coal gangue (mainly sandstone coal gangue) with aluminum silicon mass ratio less than 0.5 and appropriate aluminum iron correction raw materials;The content of aluminum and silicon (w (AltwoOthree+SiOtwo））High coal gangue can be used to produce cement after proper activation treatment.[2]
Coal gangue has a certain calorific value, which can save some fuel when used to produce cement and clinker.However, the quality of coal gangue fluctuates greatly, which is easy to affect the quality of cement clinker. Moreover, the content of hard sandstone in coal gangue is high, and the grinding energy consumption is large when it is used in cement production, so the technology promotion of coal gangue used in cement production is relatively slow.[2]
However, the use of coal gangue in road construction has a long history abroad.According to relevant literature, France and the United States use burned coal gangue slag as road filling materials, Russia's Baston Coal Mine uses processed spontaneous combustion coal gangue under asphalt concrete pavement as subbase material, and Britain mixes coal gangue and bauxite in a ratio of 4:1 to make a simple anti-skid pavement.
In 1998, Beijing Fuzhou Expressway paved a 10km section with coal mine stones. In 2003, there was also a 10.2km section of Pingdingshan Linfen Expressway filled with coal gangue. These roads are still in good condition.The surface of coal gangue particles is rough and angular. After rolling, they are closely embedded with each other, producing a certain amount of friction force. They bond with cement and have better road performance.The physical and chemical properties of coal gangue particles, such as composition, expansibility, compactness and water stability, are excellent, indicating that coal gangue can be used in road engineering.[2]
Gangue concrete
Wang Qing et al. evaluated the chloride ion permeability of coal gangue concrete by orthogonal test and electric flux method.findWater glueThe ratio of coal gangue concrete has a more significant impact on the chloride ion penetration resistance, the silica fume content has a greater impact, and the coal gangue content and water reducer content have a smaller impact on the chloride ion penetration resistance of concrete.Prepare the coal gangue concrete combination with the best resistance to chloride ion penetration. The water binder ratio is 0.30, the content of coal gangue is 40%, the content of water reducer is 0.75%, and the content of silica fume is 7%. At this time, the 6h electrical flux of coal gangue concrete is 943.25C, which has a strong resistance to chloride ion penetration.
Zhang Yuqing prepared permeable ecological concrete with different amount of coal gangue aggregates.With the increase of coal gangue replacement, the 28d compressive strength and flexural strength of permeable ecological concrete gradually decrease, and there is an approximate linear relationship between them.When the replacement amount is not more than 40%, the permeable ecological concrete of coal gangue aggregate can meet the application requirements of pavement.[2]
Chuedan et al. made coal gangue cement based foam concrete with Shaanxi Tongchuan coal gangue as the main raw material and cement as the binder.The results show that the addition of PVA can significantly improve the pore structure of foam concrete, and its distribution is more uniform. The dry density and water absorption are reduced. When the PVA content is 2%, the 28d compressive strength and flexural strength are 6.72 and 2.04 MPa, respectively, and the thermal conductivity is relatively low.[2]
Wang Qing and others studied the influence of coal gangue replacing crushed stone on the frost resistance of concrete, and improved the frost resistance of concrete by using mineral admixtures and admixtures.The results show that the frost resistance decreases gradually with the increase of the replacement amount of coal gangue, and the suitable content is less than 60%;The addition of fly ash, superplasticizer and air entraining agent can improve the pore structure of concrete, slow down the deterioration rate of pores with the increase of freezing and thawing times, and significantly improve the frost resistance of concrete.The optimum content of fly ash is 10% of cementitious material;The composite use of water reducing agent and air entraining agent has better effect. When the content is 0.40% and 0.02% respectively, the frost resistance of concrete is the strongest.
The research of coal gangue asphalt concrete has begun to rise in China.Niu Cunliang and others paved the first warm mixed WAC-25C coal gangue asphalt concrete pavement bottom layer in Danxia Road, Beijing. The mix ratio of this project is: limestone (10~30mm): limestone (10~20mm): coal gangue (5~10mm): machine-made sand: mineral powder=26 ∶ 22 ∶ 21 ∶ 29 ∶ 2, and the optimal oil stone ratio is 4%. The test technical indicators are excellent, including rutting test (60 ℃)The dynamic stability is 1968 times/mm, the residual Marshall stability is 93.2%, and the freeze-thaw splitting residual strength ratio is 81.7%.In this project, a total of 31898.71m coal gangue asphalt concrete with a thickness of 7cm is pavedtwo417m coal ganguethree。The test and practice show that warm mixed coal gangue asphalt concrete is a low-carbon and environment-friendly pavement bottom surface material.[2]
Coal gangue asphalt concrete can be prepared from coal gangue and successfully used in road construction.[2]
Gangue lightweight aggregate concrete
In the 1980s, China began to develop coal gangue ceramsite lightweight aggregate concrete, but at present, coal gangue lightweight aggregate concrete is still in the industrial test stage. Most enterprises use the rotary kiln method to produce expansive coal gangue ceramsite, and concrete cracking is still common in actual concrete projects.
Therefore, it is generally forced to take technical measures to increase the amount of cementitious materials, add high-efficiency shrinkage reducing agents and active mineral admixtures to prevent cracking, but it also causes obvious shrinkage, cracking and durability decline of concrete, which seriously restricts its application in structural engineering.[2]
Therefore, improving the volume stability of coal gangue lightweight aggregate concrete and reducing shrinkage cracking are the key to determine the application of coal gangue lightweight aggregate concrete.Qiu Jingping carried out the research on the influence of shrinkage reducing agent and ceramsite pre wetting on the volume stability of coal gangue ceramsite lightweight aggregate concrete, and found that increasing the amount of shrinkage reducing agent and improving the degree of pre wetting of coal gangue ceramsite would cause the shrinkage rate of coal gangue ceramsite lightweight aggregate concrete specimens to decline significantly and the volume stability to improve significantly.The combined application of the two is more conducive to reducing the shrinkage rate of the specimen and improving the stability of the specimen.[2]
Application of coal gangue in aluminum extraction industry
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Coal gangue contains about 15%~35% AltwoOthreeIf this part of aluminum can be utilized, it will produce huge economic benefits.The research on the production of aluminum containing products from coal gangue has always been a hot spot in the chemical utilization of coal gangue.[2]
Relationship between aluminum content in coal gangue and leaching rate
Comparing the extraction rate of aluminum in fly ash after calcination of different types of coal gangue, it can be seen that the aluminum content extracted from different types of coal gangue samples is quite different.calcinationAfter activation, aluminum is extracted by acid leaching. The aluminum leaching rate will not increase or decrease with the increase of aluminum content in coal gangue, but the leaching rate will always remain in the range of 60%~70% when the aluminum in coal gangue is calcined and activated under the same test conditions.This shows that the leaching rate of aluminum in coal gangue has nothing to do with the source of coal gangue, which also provides a theoretical basis for extracting aluminum from low aluminum coal gangue.[2]
Extraction of aluminum hydroxide from coal gangue
The main mineral phase in coal gangue is kaolinite (AltwoOthree·2SiOtwo·2HtwoO) And quartz (SiOtwo）Kaolinite has a single-layer network structure, and the single-layer network is connected by hydrogen bond, so the activity of alumina is poor. It is very difficult to extract alumina from kaolinite.To extract from coal ganguealuminaThe coal gangue needs to be mineral modified to make the alumina in kaolinite become active alumina.
Ren Xuejiao uses coal gangue as raw material to extract aluminum through mechanical activation, thermal activation and acid leaching, and the acid leaching solution uses Fe3+、A13+Hydrolyze the difference of pH value to separate aluminum and iron, and prepare aluminum hydroxide. The optimal process conditions were determined as follows: particle size 0.178mm, calcination temperature 750 ℃, calcination time 120min, leaching temperature 95 ℃, leaching time 4h, liquid-solid mass ratio 3, and sulfuric acid mass fraction 40%.Al in coal gangue under this conditiontwoOthreeThe leaching rate reached 81.8%.Geng Xuewen et al. extracted aluminum hydroxide from high alumina coal gangue by alkali lime sintering method. The quality ratio of aluminum to silicon in the test raw materials was increased from 0.86 to 2.21 after alkaline pretreatment.When the sintering temperature is 1200 ℃, the amount ratio of Na2O/Al2O3 is 1.0, and the amount ratio of CaO/SiO2 is 2.1, the sintering quality of clinker is the best, and the leaching rate of alumina reaches 94%.The dissolution solution obtained under the optimal conditions takes gibbsite as crystal seed, decomposes for 5h at 30 ℃ after adding methanol, and the decomposition rate of alumina in sodium aluminate solution reaches 88.9%;scanning electron microscopeThe results show that the generated aluminum oxide hydrate grows in sheet shape.[2]
Lu Shuzhen prepared high-purity ultrafine aluminum hydroxide powder from coal gangue by carbonization method, and ingeniously used CtwoThe volume expansion caused by the transformation of S crystal phase results in the pulverization (self pulverization) of dicalcium silicate together with other minerals, which eliminates the traditional grinding process. This can not only save electric energy, but also the powder formed by chemical pulverization is finer than that produced by mechanical pulverization, which is conducive to the extraction of aluminum components in the form of NaAlO2, and the preparation of ultra-fine aluminum hydroxide by carbonization with efficient dispersant.In the development process of extracting aluminum component from coal gangue self pulverizing material in the form of NaAlO2 with 8% Na2CO3 solution and preparing ultrafine aluminum hydroxide powder with high-efficiency dispersant carbonization method, the best conditions for preparing ultrafine aluminum hydroxide powder with high-efficiency dispersant carbonization method were found, which opened up a new way for high value utilization of coal gangue.[2]
Preparation of flocculant polyaluminum from coal gangue
Ironflocculant It has the characteristics of low production cost, non-toxic, high efficiency, fast settling speed, etc. The product has great market potential and can be widely used for wastewater treatment in petrochemical, printing and dyeing, paper making, metallurgy and other industries.The preparation of polymeric aluminum ferric sulfate from coal gangue is mainly based on the reaction of Al2O3 and Fe2O3 in coal gangue with sulfuric acid to produce Altwo（SOfour）3 and Fetwo（SO4）threeAnd then polymerize to produce polyaluminum iron.After adding polymerization agent, [Altwo（SO4）threeAnd [Fetwo（SOfour）threeStep by step polycondensation to polymer [Altwo（OH）n（SOfour）3－m/2]nAnd [Fetwo（OH）n（SOfour）3－n/2]mIt is the main component of water purification agent.[2]
Kong Deshun et al. prepared a highly efficient inorganic macromolecule flocculant poly aluminum ferric sulfate (PAFS) from the acid leaching solution of high iron gangue.The optimized process conditions for preparing polymeric aluminum ferric sulfate are as follows: the total concentration of iron ion and aluminum ion is 0.5mol/L, the mass ratio of iron ion to aluminum ion is 0.25, the pH value of the system is 0.8, the polymerization time is 8h at 80 ℃, and the curing time is 24h at room temperature.The polymerized aluminum ferric sulfate prepared under the optimized conditions was characterized by X-ray diffraction (XRD) and infrared spectrum (IR). It was shown that the product was polymerized by iron and aluminum partially.
Luo Daocheng uses coal gangue as raw material to prepare high efficiency coagulant polysilicate aluminumcoagulantThe treatment effect of industrial wastewater was compared with that of polymeric ferric sulfate. The results showed that the removal rate of COD and chroma in the effluent was increased by about 20% and 25%, respectively, and the removal rate of SS was increased by about 10%. The wastewater treatment effect was good, and the coagulant had good industrial application prospects.[2]
Great progress has been made in the research of coal gangue resource utilization, but its comprehensive utilization rate is still not high, and its utilization potential is huge. The later application needs to pay attention to the two-way development and utilization of coal gangue products with large consumption and high added value on the basis of being harmless to the environment, and finally realize the "zero discharge" of coal gangue in China's mining areas, and build China's green and ecological civilized mines.
The specific work in the later stage is as follows.1) Further carry out continuous research on environmental hazards of coal gangue, establish a real-time monitoring system for soil, water and air pollution caused by coal gangue, and explore technical approaches for safety control.
2) The coal gangue has good road performance. In the later research, it is necessary to increase the research on the processing technology of rocky coal gangue, and increase the research on the application of coal gangue in asphalt concrete, in order to industrialize and promote the application of coal gangue in road use as soon as possible.
3) Coal gangue is rich in SiO2, A12O3, Fe2O3, CaO and carbon, which provides a basis for its application in chemical industry.
However, the structure of natural coal gangue is dense, so it must be modified first to improve the activity of coal gangue.In the later stage, further research on the modification of coal gangue, especially chemical modification, is needed.[2]
Application of gangue in coal mining
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Direct filling mining method
The filling coal mining method of disposing coal gangue in the goaf of the working face can not only reduce the discharge of coal mine solid waste, but also reduce the mining subsidence disaster and improve the recovery rate of mine resources. It is one of the key technical ways to achieve green mining in coal mines. At present, a technical framework for direct filling coal mining with gangue has been formed, including the comprehensive mining gangue filling technologyCommon mining gangue filling technology and roadway excavation filling technology.[4]
Fully mechanized coal mining with filling
Longwall fully mechanized mining is a mining method widely used in medium thick and thick coal seam mining with high degree of mechanization in China at present. Fully mechanized mining and filling mining is developed based on longwall fully mechanized mining.At present, this mining technology has been successfully applied in Xingtai, Xinwen and other mining areas in China.[4]
Common mining (or blasting) and filling mining method
Conventional mining (or blasting mining) is still a common coal mining process in China at present. Research and development of high-efficiency gangue filling technology equipment and process matching with longwall conventional mining is crucial to the overall promotion of gangue filling mining technology.At present, this mining technology has been successfully applied in Xinwen and other mining areas in China.[4]
expectation
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Great progress has been made in the research of coal gangue resource utilization, but its comprehensive utilization rate is still not high, and its utilization potential is huge. The later application needs to pay attention to the two-way development and utilization of coal gangue products with large consumption and high added value on the basis of being harmless to the environment, and finally realize the "zero discharge" of coal gangue in China's mining areas, and build China's green and ecological civilized mines.
1) Further carry out continuous research on environmental hazards of coal gangue, establish a real-time monitoring system for soil, water and air pollution caused by coal gangue, and explore technical approaches for safety control.
2) The coal gangue has good road performance. In the later research, it is necessary to increase the research on the processing technology of rocky coal gangue, and increase the research on the application of coal gangue in asphalt concrete, in order to industrialize and promote the application of coal gangue in road use as soon as possible.
3) Coal gangue is rich in SiOtwo、A1twoOthree、FetwoOthreeCaO and carbon, which provides a basis for its application in chemical industry.
However, the structure of natural coal gangue is dense, so it must be modified first to improve the activity of coal gangue.In the later stage, further research on the modification of coal gangue, especially chemical modification, is needed.[2]
