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As a type of coagulant, what are the main characteristics of polyaluminum chloride?

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Company News

2019-08-07

Polyaluminum chloride, also known as basic aluminum chloride or polyhydroxy aluminum chloride, is an inorganic polymeric polyelectrolyte flocculant. It is divided into two main categories: polyhydroxy aluminum chloride and polyhydroxy aluminum sulfate. Polyhydroxy aluminum chloride is a hydrolytic polymerization product intermediate between aluminum trichloride and aluminum hydrochloride; it is commonly referred to as basic aluminum chloride and is known in English as PAC.

The color of polyaluminum chloride varies with its basicity. Basicity, also known as alkalinity, is defined as the ratio of [OH] to 3[Al], representing the molar (equivalent) ratio of hydroxyl groups to aluminum in polyaluminum chloride. When the basicity ranges from 40% to 60%, polyaluminum chloride appears pale yellow and transparent; above 60%, it gradually becomes colorless and transparent, with a sour and astringent taste. The physical form of solid polyaluminum chloride also varies with its basicity: below 30%, it exists as crystalline particles; between 30% and 60%, it takes on a colloidal form; and above 60%, it progressively transforms into a resin-like structure. Below 70%, it readily absorbs moisture and may liquefy; above 70%, it resists deliquescence. Polyaluminum chloride undergoes depolymerization upon reaction with acids, reducing both its degree of polymerization and basicity, thereby forming simple aluminum salts and diminishing its flocculation efficiency. In contrast, when reacted with bases, its degree of polymerization and basicity increase, leading to the further formation of aluminum hydroxide precipitates and aluminates. If mixed with aluminum sulfate or other polyvalent acid salts, it tends to co-precipitate, significantly reducing or even completely eliminating its flocculation performance. When heated to temperatures above 110°C, polyaluminum chloride begins to decompose, sequentially releasing hydrogen chloride gas and ultimately breaking down into alumina. The alumina content serves as an important indicator of the effective composition of polyaluminum chloride.

Generally speaking, the greater the relative density, the higher the aluminum oxide content. Basicity is another important quality indicator of polyaluminum chloride; it determines the product’s structural morphology, degree of polymerization, flocculation capacity, storage stability, and pH value. At the same time aluminum oxide content, polyaluminum chloride has a lower viscosity than aluminum sulfate, which facilitates its transportation and use. Freezing point: The precipitation temperature of polyaluminum chloride is lower than that of aluminum sulfate, making it more suitable for use and storage in low-temperature regions. pH value: Compared with other coagulants, polyaluminum chloride has the highest pH value at the same concentration. Consequently, it exhibits the lowest corrosivity. Filtration performance: In water treatment, the effluent turbidity resulting from polyaluminum chloride treatment is lower than that from aluminum sulfate treatment, and the flocs formed are larger. As a result, during filtration, these flocs tend to remain on the surface of the filter media, shortening the backwashing time and making them easier to remove. Polyaluminum chloride not only demonstrates excellent flocculation performance but also shows a smaller increase in dosage requirement compared to ferric chloride when the raw water turbidity rises—this difference becomes even more pronounced as turbidity levels increase. Compared with aluminum sulfate, polyaluminum chloride always requires a lower dosage than aluminum sulfate regardless of the initial turbidity level. Polyaluminum chloride can be effectively applied to the treatment of highly turbid waters from the Yellow River. Within certain limits, the higher the raw water turbidity and the wider the range of turbidity variation, the more evident its superior performance becomes.

After being added to water, polyaluminum chloride immediately undergoes hydrolysis and polymerization reactions, forming a series of aluminum salt hydrolysis polymers. These polymers are then removed through flocculation, sedimentation, and filtration—similar to the mechanism employed by aluminum sulfate and ferric chloride flocculants. Toxicity tests have shown that polyaluminum chloride is non-toxic, does not exhibit significant accumulation in animal bodies, and lacks teratogenic or mutagenic effects. It also poses no carcinogenic risk. Tap water purified using polyaluminum chloride meets the standards for potable drinking water. Polyaluminum chloride flocculants offer several advantages, including rapid floc formation, large floc particle size, good mechanical strength, and high settling velocity. They deliver excellent flocculation performance when used with highly turbid water, low-temperature and low-turbidity water, colored water, and contaminated water. Moreover, these flocculants demonstrate strong adaptability to variations in the raw water’s pH, temperature, turbidity, alkalinity, and organic content. When used in combination with organic polymeric flocculants to treat highly turbid water, the treatment effect is even more pronounced.

Due to its high content of the active ingredient (Al2O3), low dosage requirements, and ease of transportation, storage, and use, under conditions achieving the same flocculation effect, when calculated based on Al2O3 content, the dosage required for treating low-turbidity water is only 50% to 75% of that needed for aluminum sulfate; for highly turbid water, the dosage is approximately 30% to 40% of that for aluminum sulfate. With its advantages of low dosage and reduced water treatment costs, polyaluminum chloride has thus assumed an important position in the development of water treatment technology.

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