Keywords: prevention and control of environmental impact of acid mine water in coal mining activities
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Coal is the main energy source in China, accounting for more than 76% of China's disposable energy, so it is necessary to carry out a large number of coal mining. In the process of coal mining, the coal seam environment is destroyed, and its original reducing environment has become an oxidizing environment. Coal generally contains about 0.3% ~ 5% sulfur, mainly in the form of pyrite, accounting for about 2/3 of the sulfur content in coal.
After coal mining, it is in an oxidizing environment. After the flowing iron ore comes into contact with mine water and air, through a series of oxidation and hydrolysis reactions, sulfuric acid and iron hydroxide are generated, which makes the water acidic, that is, acidic mine water is produced. Mine water with PH below 6 is called acidic mine water. Acidic mine water is widely used in some coal mines in China, especially in southern coal mines. The pH value of mine water in southern China is generally 2.5 ~ 5.8, sometimes reaching 2.0. The low pH value is closely related to the high sulfur content in coal. The formation of acidic mine water has caused serious pollution to groundwater, at the same time, it will corrode underground equipment such as pipes, pumps, rails and concrete shaft walls, and also seriously pollute surface water and soil, making fish and shrimp in river water extinct, soil hardening, crops withering and affecting human health.
1, the harm of acidic mine water
When the pH value of mine water is lower than 6, it is acidic and corrosive to metal equipment. When the pH value is lower than 4, it is highly corrosive and will cause serious harm to the safe production and ecological environment in the mining area. Specifically, there are the following aspects:
1 & gt； Corrosion of underground rail, wire rope and other coal mine transportation equipment. For example, rails and wire ropes are affected by pH value.
2> When exploring and discharging old empty water with low pH value, iron-controlled water pipes and gates corrode quickly under the scouring of water flow, which makes the water discharge out of control and brings disaster;
The content of SO _ 42-in 3> acid mine water is very high, and it interacts with some components in cement to generate hydrated sulfate crystals. These salts expand in volume when they are formed. According to the measurement, when SO _ 42- generates CaSO4.2H2O, its volume doubles. When MgSO4.7H2O was formed, the volume increased by 430%. The increase of volume makes the concrete structure loose, the strength decreases and it is destroyed.
4> Acid mine water is also an environmental pollution source. Acid mine water is discharged into rivers, and when the pH value is less than 4, fish will die. Acidic mine water is discharged into the soil, which destroys the aggregate structure of the soil, hardens the soil, yellows crops and reduces production, affecting the relationship between workers and peasants; Acidic mine water is not suitable for human consumption. Long-term contact will make people's hands and feet break, their eyes itch, and enter the human body through the food chain, affecting human health.
2. The causes of acid mine water
Most coal measures strata were formed in reducing environment, and coal seams containing pyrite (FeS2) were formed in strong reducing environment. Coal generally contains about 0.3% ~ 5% sulfur, mainly in the form of pyrite, accounting for about 2/3 of the sulfur content in coal. After coal mining, it is in an oxidizing environment. After the flowing iron ore comes into contact with mine water and air, sulfuric acid and iron hydroxide are generated through a series of oxidation and hydrolysis reactions, which makes the water acidic, that is, acidic mine water is generated. The main reasons for the formation of acidic mine water are the following main chemical reactions:
1 & gt； Pyrite is oxidized to produce free sulfuric acid and ferrous sulfate;
2fe S2+7o 2·2h2o 2·h2so 4·2fe so 4
2 > ferrous sulfate is converted into ferric sulfate under the action of free oxygen;
Ferrous sulfate +2H2SO4+O22Fe2(SO4)3+2H2O
3> In mine water, the oxidation of ferrous sulfate sometimes does not necessarily require sulfuric acid:
12fe S2+3o 2·6h2o 4·Fe 2(SO4)3+4Fe(OH)3
Iron sulfate in 4> mine water can further dissolve various sulfide minerals;
Fe2(SO4)3+MS+H2O+3/2O2M SO4·2feso 4+h2so 4
5> ferric sulfate is hydrolyzed in weakly acidic water to produce free sulfuric acid;
Fe2(SO4)3·6h2o 2·Fe(OH)3+3h2so 4
6> When the content of hydrogen sulfide in deep mine is high, free sulfuric acid can also be produced in mine water rich in ferrous sulfate under reducing conditions:
2 ferrous sulfate +5 ferrous sulfate +3S+ ferrous sulfate +4 H2O
The properties of acid mine water are not only related to the sulfur content in coal, but also related to the geological conditions and mining methods such as mine water inflow, closed state, air circulation, coal seam dip angle, mining depth and area, water flow path and so on. When the mine water inflow is stable, the acidity of water is stable; Poor sealing and good air circulation will make the water more acidic and contain more Fe3 ions. On the contrary, the acidity is weak and there are more Fe2 ions. The deeper the mining, the higher the sulfur content of coal; The larger the mining area, the longer the water flow path, the more fully the oxidation and hydrolysis reactions are carried out, and the stronger the acidity of water is, the weaker the acidity is.
3. Prevention and control of acid mine water
3. 1 prevention and control of acid mine water
According to the conditions and causes of acid mine water formation, its harm can be prevented or reduced from three aspects: reducing the source, reducing the quantity and reducing the time.
1 & gt； Source reduction: picking and utilizing acid-making minerals to turn harm into benefit. The main acid-producing minerals in coal deposits are pyrite nodules mixed in coal seams and sulfur content of coal itself. The low recovery rate of coal, the loss of residual coal pillar or floating coal, pyrite nodules abandoned in underground goaf and soaked in water for a long time are important sources of acidic water. Reducing the loss of floating coal in working face and actively picking and utilizing pyrite nodules can reduce the substances that produce acidic water. Intercept surface water and reduce infiltration. For example, backfill gangue, control the roof, and prevent surface water from soaking into the old goaf along the collapse cracks. In underground, especially in old wells or abandoned closed alleys, appropriate bacteriostatic agents are applied to mine water to inhibit or kill the activity of microorganisms or reduce the number of microorganisms in mine water. By reducing the effective action of microorganisms on sulfide, the purpose of controlling the production of acidic mine water is achieved.
2> Reduce water discharge: Set up a special drainage system to centrally discharge acidic water and store it on the surface for evaporation and concentration before treatment to avoid pollution.
3> Reducing the discharge time of acid water: Reducing the retention time of mine water underground can reduce the oxidation of sulfide in coal by microorganisms to a certain extent, thus helping to reduce the formation of acid mine water. For shallow coal seams with high pyrite content, high sulfur content and good surface water leakage conditions, or old pits with strong acid water, the advantages and disadvantages should be weighed and overall arrangements should be made in the development and layout. It is not suitable for mining or exploration in the early stage of mine, so it should be left to the later stage of mine water treatment to avoid long-term discharge of acidic water.
3.2 Treatment of Acid Mine Water
Under certain geological conditions, sulfuric acid in acidic water can react with calcareous rocks or other alkaline minerals to reduce acidity. The amount of caustic soda used as neutralizer is small, and the sludge is small, but the total hardness of water is often high. Although the acidity of water has decreased, the hardness has increased and the cost is high, so it is basically not used now. At present, the treatment methods include lime milk neutralizer, limestone neutralizer, limestone-lime method, microorganism method and wetland treatment method. Lime milk neutralizer treatment method is suitable for treating mine water with strong acidity and small water inflow; Limestone-lime method is suitable for all kinds of acidic mine water, especially when there are more Fe2 ions in acidic mine water, and the amount of lime can be reduced. The basic principle of microbial method is to remove iron by oxidation with iron oxide bacteria. This kind of bacteria can absorb iron from aquatic environment, and then precipitate iron in the form of iron hydroxide in their mucus secretion, which can transform low iron in acidic water into high iron precipitation, and then neutralize free sulfuric acid with limestone, which can reduce investment and precipitation. Wetland method, also known as shallow swamp method, has the advantages of low cost, easy operation and high efficiency, and the specific methods are not detailed here.
conclusion
Most coal measures strata are formed in reducing environment, and coal seams are in oxidizing environment after mining. After the flowing iron ore comes into contact with mine water and air, after a series of reactions such as oxidation and hydrolysis, the water becomes acidic and forms acidic mine water. It will have a certain environmental impact and damage to the environment and facilities such as groundwater, and will also have a certain impact on human health. By analyzing the causes of the formation of acidic mine water and taking certain preventive measures, the pollution of acidic mine water to groundwater, the harm to other environments and facilities and the impact on human health can be reduced.
References:
1 edited by Wang, Hydrogeological Basis, Geological Publishing House, Beijing.
2. Yuan Qishun, Abstract of Research Papers on Environment and Groundwater Hydraulics, Journal of Yangtze Research Institute,1994,3.
3 Lin Nianfeng, Li Changhui, Tian Chunsheng, et al., Environmental hydrogeology, Beijing, Geological Publishing House,1990,21.
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