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Purification principle of coal washing plant wastewater using polyaluminum chloride PAC


Coal washing wastewater is the wastewater produced during the wet coal washing process in coal mines, coal washing wastewater contains a large amount of slime and sediment.
It contains a certain amount of hydrocarbons, which need to be purified before being discharged. However, it is difficult for the coal washing wastewater to settle under static conditions.
Suspensions have good stability and need to be added with coagulants and flocculants to aid coagulation and sedimentation. This method is currently the most common method for wastewater treatment applications in coal washing plants.

The slime water from the coal washing plant is directly discharged into the slime sedimentation tank for purification treatment, the clarified water can be recycled or discharged because the fine slime particles and clay in the coal washing wastewater are difficult to settle.

The slime particles are continuously refined during the circulation process, which increases the SS concentration of the circulating water and will destroy the coal preparation process.
Therefore, the method of using high-concentration coal-washing wastewater by diluting with clean water and circulating water is not feasible.

Purification and treatment of coal-washing wastewater is carried out through the concentrated sedimentation method. Processes such as sedimentation tanks and thickeners are mostly used. After the slime water enters the thickener through the fishing pit, its overflow solid content does not exceed 10g / L [5]. The overflow water is further processed by the sedimentation tank.
The sedimentation method is used to treat coal washing wastewater, which has a low surface load and a large floor area. The suspended solids concentration after wastewater treatment is still high, and it cannot meet the wastewater discharge standards.

The coagulation sedimentation method mainly uses chemical coagulants and flocculants to purify coal washing wastewater. At present, PAC polyaluminum chloride and PAM polyacrylamide are the most commonly used coagulants and flocculants in the purification of coal washing wastewater.
According to the different underground coal seams in different areas, the properties of the coal washing wastewater produced are also different, and tests need to be performed to determine the type of flocculant required.
PAC polyaluminum chloride coagulant for coal washing wastewater treatment, commonly used drum-type polyaluminum chloride content of 26/28/30% three options, the higher the alumina content is between 26-30%, the higher the content, the higher the price of PAC polyaluminum chloride.

The PAC polyaluminum chloride used in coal washing wastewater is mainly based on the principle of compressing the electric double layer, the suspended particles in the coal washing wastewater are destabilized to produce coalescence, forming small alum flowers. The density of the PAC polymerized aluminum chloride slime mass will decrease, the cohesive force between the slime particles will increase, and high-power particles will be generated. Such particles have a large porosity, which leads to a decrease in the effective density e of the slime granulation.

The turbidity of effluent SSt decreased significantly with the increase of PAC dosage, and the effluent changed from turbidity to clear; when PAC increased to 2.6 mg / L, the change in SSt was small and basically stable.
The increase of PAC will increase the destabilization of fine particles in coal washing wastewater, increase the cohesion, and decrease the SSt sharply.
When the amount of polyaluminum chloride reaches a certain value, most of the particles in the wastewater are almost completely destabilized and agglomerated to achieve the best effect, at this time, there is no need to increase the dosage of PAC polyaluminum chloride, and the amount of coagulant is reasonably controlled to save water treatment costs.

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