Influence of coal mining process on groundwater and its prevention

Coal is China's major energy, accounting for more than 76% of primary energy in the country, must be a lot of coal. During the coal mining process, the environment in which the coal seam is located is destroyed, and the original reducing environment is changed into an oxidizing environment. Coal generally contain from about 0.3% to 5% of sulfur, mainly in the form of pyrite, about 2/3 of the sulfur content of coal. After the coal seam is mined, it is in an oxidizing environment. After the iron ore mine is in contact with the mine water and air, it undergoes a series of oxidation, hydrolysis and other reactions to form sulfuric acid and ferric hydroxide, so that the water is acidic, that is, the acid mine water is produced. Mine water with a pH below 6 is called acid mine water. Acid mine water in some coal mines in China especially makes the southern coal mines more extensive. The pH value of mine water in China's southern coal mines is generally between 2.5 and 5.8, sometimes reaching 2.0. The reason for the low pH is closely related to the high sulfur content in coal. The formation of acid mine water causes serious pollution to groundwater, and also corrodes downhole equipment such as pipelines, pumps, rails and concrete well walls, and also seriously pollutes surface water and soil, so that fish and shrimp in river water are eliminated, soil compaction, and crops wither , affecting human health.

1 Hazard of acid mine water

Mine water pH of less than 6 i.e. acidic, corrosive to certain metals apparatus; i.e., a pH less than 4 with a strong caustic, cause serious adverse effects mining environment and safety in production. Specifically, there are the following aspects:

1> Corrosion underground mine rails, wire ropes and other coal transportation equipment. For example, steel rails and steel wire ropes are eroded by acid mine water with pH value <4, and their strength will be greatly reduced from ten days to several tens of days, which may cause transportation safety accidents;

2> Exploring the old empty water with low pH value, the iron water control pipeline and the gate are corroded quickly under the water flow, causing the water release to lose control and bring disaster;

3> The content of SO42- in acid mine water is very high, and it interacts with certain components in cement to form aqueous sulfate crystal. These salts expand in volume upon formation. It was determined that when SO42-generated CaSO4•2H2O, the volume doubled; when MgSO4•7H2O was formed, the volume increased by 430%; the volume increase caused the structure of the concrete structure to be loose and the strength was reduced and destroyed.

4> Acid mine water is also a source of environmental pollution. Acid mine water is discharged into the river. When the pH is less than 4, the fish will die. The acid mine water is discharged into the soil, destroying the agglomerate structure of the soil, causing soil compaction, crop yellowing, yield reduction, affecting the relationship between workers and farmers; acid mine water human Unable to drink, long-term contact, can make people's hands and feet rupture, itchy eyes, enter the human body through the food chain, affecting human health.

2 Reasons for the formation of acid mine water

Most coal-bearing strata are formed in a reducing environment, and coal seams containing pyrite (FeS2) are formed in a strong reducing environment. Coal generally contains about 0.3% to 5% sulfur, mainly in the form of pyrite, accounting for about 2/3 of the sulfur content of coal. After the coal seam is mined, it is in an oxidizing environment. After the iron ore mine is in contact with the mine water and air, it undergoes a series of oxidation, hydrolysis and other reactions to form sulfuric acid and ferric hydroxide, so that the water is acidic, that is, the acid mine water is produced. The main reason for the formation of acid mine water is the main chemical reactions:

1> Pyrite oxidation to free sulfuric acid and ferrous sulfate:

2FeS2+7O2+2H2O→2H2SO4+2FeSO4

2> Ferrous sulfate is converted to iron sulfate by the action of free oxygen:

4FeSO4+2H2SO4+O2→2Fe2(SO4)3+2H2O

3> In mine water, the oxidation of ferrous sulfate sometimes does not necessarily require sulfuric acid:

12FeS2+3O2+6H2O→4Fe2(SO4)3+4Fe(OH)3

4> Iron sulfate in mine water, which has the function of further dissolving various sulfide minerals:

Fe2(SO4)3+MS+H2O+3/2 O2→M SO4+2FeSO4+H2SO4

5> Ferric sulfate is hydrolyzed in weakly acidic water to produce free sulfuric acid:

Fe2(SO4)3+6H2O→2 Fe(OH)3+3H2SO4

6> In the deep hydrogen sulfide content of the mine, under the reducing conditions, the ferrous sulfate-rich mine water can also produce free sulfuric acid:

2FeSO4+5H2S→2 FeS2+3S+H2SO4+4 H2O

In addition to the sulfur content in coal, the nature of acid mine water is also related to geological conditions and mining methods such as mine water inflow, closed state, air circulation, coal seam dip, mining depth and area, and water flow path. If the mine water inflow is stable, the acidity of the water is stable; if the airtightness is poor and the air circulation is good, the acidity of the water is strong, and the Fe3+ ion content is high; on the contrary, the acidity is weak, and the Fe2+ ions are more; the deeper the mining, the coal contains The higher the amount of sulfur; the larger the mining area, the longer the flow path of water, the more fully the reaction of oxidation, hydrolysis, etc., the stronger the acidity of water, and vice versa.

3 Prevention and treatment of acid mine water

3•1 Prevention of acid mine water

According to the conditions and reasons for the formation of acid mine water, it can be prevented or mitigated from three aspects: source reduction, reduction, and time reduction.

1> Reduction source: Select and use acid-producing minerals to make harm to profit. The main acid-producing minerals in coal deposits are the sulphur content of pyrite nodules and coal itself in the coal seam. The coal mining rate is low, and the residual coal pillar or floating coal is lost. The pyrite nodules are discarded in the underground goaf and are soaked by the accumulated water for a long time, which is an important source of acidic water. Reduce the loss of working face floating coal, actively select and use pyrite tuberculosis, can reduce the production of acidic water. Intercept the surface water to reduce the amount of infiltration. For example, backfilling the meteorites and controlling the roof plate prevents the surface water from immersing into the old empty space along the collapsed fissures. In the underground, especially in old wells or abandoned closed shafts, apply appropriate amount of bacteriostatic agent to the mine water to inhibit or kill the activity of microorganisms or reduce the amount of microorganisms in the mine water. By reducing the effective action of microorganisms on sulfides, the purpose of controlling the formation of acid mine water is achieved.

2> Reduce the displacement: set up a special drainage system, concentrate the acidic water, and store it on the surface to evaporate, concentrate, and then treat it to avoid pollution.

3> Reduce the time of discharging acidic water: reduce the residence time of mine water in the well, which can reduce the oxidation of sulfides in coal to some extent, which helps to reduce the formation of acid mine water. For shallow coal seams with high pyrite content, high sulfur content, good surface water leakage conditions, or old hydrothermal water areas where strong acid water has been formed, we must weigh the pros and cons in the development layout, and make overall arrangements in the early stage of the mine. It will not be mined or explored, and will be treated at the end of the mine water 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 neutralize with calcareous rocks or other basic minerals to reduce acidity. The use of caustic soda as a neutralizing agent is less, and the sludge is produced less, but the total hardness of the water is often high. Although the acidity of the water is lowered, the hardness is increased, and the cost is high, and it is basically not used. At present, treatment methods include lime milk as a neutralizing agent, limestone as a neutralizing agent, and limestone-lime method, microbial method, and wetland treatment. The lime milk neutralizer treatment method is suitable for treating mine water with strong acidity and small water inflow; the limestone-lime method is suitable for various acid mine waters, especially when there are many Fe2+ ions in acid mine water, and Reduce the amount of lime; the basic principle of microbial method is to use iron oxide bacteria to oxidize and remove iron. This bacteria can take iron from the aquatic environment, and then ferric precipitates in their mucus secretions in the form of ferric hydroxide. The conversion of low iron into high iron precipitates, and then the neutralization of free sulfuric acid with limestone reduces investment and reduces sediment. The wetland method is also called the shallow swamp method. This method has the advantages of low cost, easy operation and high efficiency. The specific method is not detailed here.

in conclusion

Most coal-bearing strata form and reduce the environment. After the coal seam is mined, it is in an oxidizing environment. After the iron ore mine is in contact with the mine water and air, it undergoes a series of oxidation, hydrolysis and other reactions to make the water acidic and form acidic mine water. It will cause certain environmental impacts and damage to groundwater and other environments and facilities, and will have certain impact on human health. By analyzing the formation of acid mine water and taking certain prevention and treatment measures, it can reduce the damage caused by acid mine water to groundwater, other environment and facilities, and the impact on human health.

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