Research progress on hazards and treatment technology of mine wastewater

1. Sources and hazards of mine wastewater

1.1 Source of mine wastewater

Mine wastewater is a general term for the wastewater discharged from mining sites, concentrators, tailings DAMS, slag disposal sites and living areas within the scope of mines. According to the different pH value, the mine wastewater can be divided into acidic wastewater and alkaline wastewater. Acid wastewater is the waste water formed by oxidation, weathering and decomposition in the production process of ore mining, transportation, mineral processing, waste rock discharge and tailings storage. Alkaline wastewater is the wastewater produced by adding alkaline pH regulator to the pulp system in order to obtain the best separation effect when the ore is flotation.

1.2 Hazards of mine wastewater

The harmful substances in mine wastewater are mainly heavy metal ions and various organic and inorganic flotation agents used in ore flotation, in addition to sulfate, chloride or hydroxide of sodium, magnesium and calcium, as well as insoluble dispersed impurities. The harm caused by mine wastewater mainly has the following aspects

(1) corrosion of pipes, pumps, rail and other equipment and facilities, while directly threatening the safety and stability of pollution containment and pollution storage facilities (such as sewage DAMS, etc.).

(2) Mine wastewater containing heavy metal ions is discharged into farmland, which has a toxic effect on most plants, causing most plants to wither and die, and seriously affecting the yield and quality of crops. A small number of plants absorb heavy metals and endanger human health through the food chain.

(3) Mine wastewater is directly discharged into rivers, lakes or into the ground, resulting in deterioration of water quality and posing a great threat to fish, algae and humans.

2. Research progress of mine wastewater treatment technology

In view of the characteristics of large discharge, wide pollution range and complex composition of mine wastewater, acid-base neutralization method, coagulation sedimentation method, chemical oxidation method, constructed wetland method, biological method and other treatment technologies have been gradually developed.

2.1 Acid-base neutralization method

Acid-base neutralization method is to add a neutralizer to the wastewater, use the acid-base neutralization reaction to adjust the pH value of the wastewater, and remove the harmful substances in the wastewater. Adding alkaline neutralizer to acidic mine wastewater can, on the one hand, make the pH value of the wastewater close to neutral, and on the other hand, make the heavy metal ions in the wastewater chemically react with the neutralizer to form hydroxide or carbonate precipitation with low solubility and separate from the wastewater. The commonly used neutralizing agents of acidic wastewater are lime, slaking lime, limestone, etc. Fly ash, coal gangue, alkaline waste liquid or waste residue can also be used to neutralize acidic wastewater. Lime is the most commonly used neutralizer for treating acid mine wastewater because of its low price and low treatment cost.

Wang Hongzhong et al. studied the neutralization method to treat the acidic mine wastewater discharged into Xiaofu River. In the test, calcium carbonate was first used to neutralize the pH value of the wastewater to about 4.5, and then calcium hydroxide was used to continue to increase the pH value. The final effluent pH value reached 7.5, and the content of sulfate and total iron was trace, which met the discharge requirements. Li Xuejin et al. studied a two-stage neutralization method for the treatment of acidic wastewater from an iron ore tailings pond. Firstly, mineral or waste residue was used as a neutralizer, the pH value of the wastewater was adjusted to about 4, and then milk of lime was used for neutralization. The test results show that when the lime dosage is 0.5 ~ 0.8kg/m3 and the flocculant dosage is 2×10-6, the effluent pH value reaches 6 ~ 8 and the water quality meets the national discharge standard. The two-stage neutralization method saves lime consumption and treatment cost compared with the one-stage neutralization method. Feng D et al. used the alkalinity and adsorbability of iron and steel slag to neutralize acid mine wastewater and remove copper, lead and other heavy metal ions in the wastewater, and achieved success in the treatment of mineral processing wastewater of a gold mine in South Africa.

Acid-base neutralization is the most commonly used traditional method to treat acid mine wastewater. It has the advantages of simple process, convenient operation and low operating cost, but it also has some disadvantages such as serious scaling, large amount of sediment and easy to cause secondary pollution. In the treatment of acid mine wastewater, in order to improve the treatment effect, the neutralization method is often combined with oxidation method and other methods.

2.2 Coagulation sedimentation method

Coagulation sedimentation method is to destroy the stability of colloids by adding coagulant to wastewater, so that fine suspended particles and colloidal particles aggregate into larger particles and settle, so as to achieve the purpose of wastewater purification. Commonly used coagulants are alum, aluminum chloride, polyaluminum chloride (PAC), polyferric sulfate (PFs), polyferric chloride (PFC), polyacrylamide (PAM) and so on.