What is the difference between poly aluminum chloride and aluminum chloride?

Polyaluminum chloride (PAC) and aluminum chloride (AlCl₃) are both aluminum-based coagulants used in water treatment, but they have distinct differences in their chemical composition, properties, and applications. Understanding these differences is crucial for selecting the appropriate coagulant for specific water treatment needs.

Chemical Composition and Structure

Polyaluminum Chloride (PAC): PAC is a group of salts that contain multiple aluminum ions connected by hydroxide groups. It has a general formula of [Aln(OH)mCl3n-m]x, indicating that the aluminum ions are polymerized with varying degrees of polymerization. The structure of PAC can vary widely, which affects its coagulation efficiency and the specific water treatment applications it is best suited for.

Aluminum Chloride (AlCl₃): Aluminum chloride is a simple salt formed by the reaction of aluminum and chlorine. It is typically seen in its anhydrous form as a white or pale yellow powder, but it can also form a hexahydrate (AlCl₃·6H₂O). Unlike PAC, aluminum chloride is not polymerized and consists of single aluminum ions.

Coagulation Mechanism

PAC: Due to its polymeric structure, PAC can form larger and more stable flocs in the water treatment process. This results in more efficient removal of contaminants, turbidity, and organic matter. PAC is also more effective over a broader range of pH levels compared to aluminum chloride.

Applications

PAC: Due to its versatility and efficiency, PAC is widely used in drinking water treatment, wastewater treatment and industrial water treatment. Its ability to operate efficiently over a wide pH range makes it suitable for a variety of applications, including the treatment of low temperature, low turbidity and even high color water.

Aluminum Chloride: Aluminum chloride is primarily used in applications where lower pH levels are present, such as in certain industrial wastewater treatment processes. It may also be used in the manufacture of other chemicals and as a catalyst in organic synthesis.

Environmental Impact and Safety 

PAC: PAC generally has a lower impact on water pH and reduces the need for pH adjustment chemicals. It also tends to produce less sludge compared to aluminum chloride, which can be beneficial from an environmental and disposal standpoint.

Aluminum Chloride: The use of aluminum chloride can lead to a more significant drop in the pH of treated water, requiring the addition of alkaline chemicals to adjust the pH. The handling and storage of aluminum chloride, especially in its anhydrous form, need to be managed carefully due to its corrosive nature.

Conclusion

Choosing between PAC and aluminum chloride depends on specific water treatment requirements, including the nature of the water to be treated, desired outcomes, and environmental considerations. PAC's polymeric structure and flexibility make it a more versatile and widely applicable coagulant for various water treatment scenarios, while aluminum chloride may be preferred in specific situations where its particular characteristics align with the treatment goals.