Coal washing wastewater is the wastewater produced in the wet process of coal washing in coal mines, containing a large amount of slurry and silt.
It contains a certain amount of hydrocarbons and needs to be purified before it can be discharged. However, coal washing wastewater is difficult to precipitate under static conditions.
Suspensions have good stability, and coagulants and flocculants need to be added to help coagulation and precipitation. This method is currently the most commonly used in coal preparation plant wastewater treatment applications.
The fine slurry particles and clays in the coal washing wastewater do not settle easily, so the clarified water can be recycled or discharged, thus purifying the slurry water from the coal washing plant.
The slurry particles continue to refine as they are recycled, increasing the concentration of suspended solids in the recycled water and disrupting the coal preparation process.
Therefore, it is not feasible to dilute high coal-washing wastewater with fresh water and recycled water.
It is not feasible to dilute high concentration coal washing wastewater with fresh water and circulating water. Sedimentation ponds and thickeners are often used. After the slurry water enters the thickener through the pit, the slurry water's overflow solid content does not exceed 10g/L . A sedimentation pond further treats the overflow water.
The sedimentation method treats coal washing wastewater, which has a small surface load and a large footprint. The concentration of suspended solids in the treated wastewater is still very high and cannot reach the wastewater discharge standard.
The coagulation and sedimentation method mainly uses chemical coagulants and flocculants to purify coal washing wastewater. At present, PAC poly aluminium chloride and PAM polyacrylamide are the most commonly used coagulants and flocculants in coal washing wastewater purification.
The nature of coal washing wastewater varies according to the different underground coal seams in different areas, so it is necessary to conduct tests to determine the type of flocculant needed.
Polyaluminium Chloride coagulants are used in coal washing wastewater treatment. There are three options for the common barrel content of PCA. The higher the alumina content between 26-30%, the higher the content and the higher the PAC coagulant price.
The PAC PCA used in coal washing wastewater is mainly based on double electric layer compression. The suspended particles in the coal washing wastewater are unstable and agglomerated, forming a small alum flower. The PAC PCA sludge density will be reduced, and the cohesion between the sludge particles will be increased, resulting in high-power particles. The porosity of these particles is large, resulting in a decrease in the sludge particles' effective density.
As the amount of PAC increases, the turbidity of wastewater SST decreases significantly. The change in SST from turbid to clear is small and basically stable when the amount of PAC increases to 2.6 mg/L.
PAC's increase will increase the destabilization of fine particles in coal washing wastewater, increase cohesion, and the SST drops sharply.
When PAC's dosage reaches a certain value, most of the particles in the wastewater will almost completely destabilize and agglomerate, thus achieving the best results. There is no need to increase the amount of polymeric aluminum chloride, so the amount of coagulant can be reasonably controlled, thus saving the cost of water treatment.