What are the main characteristics of polymeric aluminum chloride as a coagulant
2019-08-07
Polymeric aluminum chloride, also known as alkali aluminum chloride, is also known as polymeric hydroxy aluminum chloride. It is an inorganic polymeric multivalent polymeric electrolyte flocculant, divided into two types of polymeric hydroxy aluminum chloride and polymeric hydroxy aluminum sulfate. And polymeric hydroxy aluminum chloride is a kind of hydrolysis polymerization product between aluminum trichloride and aluminum hydrogen chloride, commonly known as alkaline aluminum chloride, English name PAC.
The color of polymeric aluminum chloride varies with the size of the salt base. Salinity, also known as alkalinity, is equal to [OH]/3 [Al], which is the isoelectronic molar (equivalent) ratio of hydroxyl and aluminum in polymeric aluminum chloride. When the salinity is in the range of 40%-60%, the polymeric aluminum chloride is light yellow and transparent, and above 60%, it gradually becomes colorless and transparent, with sour taste and astringent. The shape of solid polymeric aluminum chloride also varies with the salinity. When the salt base is below 30%, it is crystal-like; 30%-60% is colloidal; above 60%, it gradually becomes resin-like; below 70%, it is easy to absorb moisture and liquefy; above 70%, it is not easy to deliquescence. When polymerized aluminum chloride reacts with acid, depolymerization reaction occurs; the degree of polymerization and salinity decreases, forming positive aluminum salt, and the flocculation effect decreases. When reacting with alkali, it can increase its polymerization degree and salinity, which will further generate aluminum hydroxide precipitation and aluminate. If mixed with aluminum sulfate or other multivalent salts, it is easy to produce co-sedimentation and reduce or completely lose flocculation performance. When the temperature is heated above 110℃, decomposition will occur, releasing hydrogen chloride gas one after another, and finally decomposing into alumina. And the content of alumina is the measure of the effective composition of polymeric aluminum chloride.
Generally speaking, the higher the relative density, the higher the alumina content. Salinity is another important quality index of polymeric aluminum chloride, which is the determining factor of the product's structural form, polymerization, flocculation ability, storage stability and pH value. Under the same alumina content, the viscosity of polymeric aluminum chloride is lower than that of aluminum sulfate, therefore, it is beneficial to transport and use. Freezing temperature: The precipitation temperature of polymeric aluminum chloride is lower than that of aluminum sulfate, which is favorable for use and storage in low temperature areas. pH: Compared with other flocculants, the pH value of polymeric aluminum chloride is the highest at the same concentration. Therefore, its corrosion is the least. Filtration performance: In water treatment, the precipitated water treated with polymeric aluminum chloride is less turbid than that treated with aluminum sulfate, and the flocs are larger, so they are generally retained in the surface layer of the filter layer when filtering, which can shorten the flushing time and easy to flush clean. Polymeric aluminum chloride not only has good flocculation performance, but also when the turbidity of raw water increases, the increase of its dosage is less than that of ferric chloride, and the higher the turbidity, the more significant the difference. If compared with aluminum sulfate, the dosage of polymeric aluminum chloride is smaller than that of aluminum sulfate at any turbidity. Polymeric aluminum chloride can be applied to the treatment of highly turbid water in the Yellow River. Within a certain range, the higher the turbidity of raw water, the greater the range of turbidity changes, and its superior performance is obvious.
Polymeric aluminum chloride is added to water and then hydrolysis and polymerization reaction occurs, generating a series of aluminum salt hydrolysis polymer, which is then removed by flocculation precipitation and filtration, and this effect is similar to aluminum sulfate and ferric chloride flocculants. According to the toxicity test, it is non-toxic, no obvious accumulation in animals, no teratogenicity and mutagenicity, no cancer risk, and the tap water purified by it meets the standard of drinking water. Polymeric aluminum chloride flocculant has the advantages of rapid formation of flocs, large floc particles, good mechanical strength and high settling speed. It can be used in high turbidity water, low temperature low turbidity water, colored water and polluted water to achieve good flocculation effect, and has strong adaptability to changes in pH, temperature, turbidity, alkalinity, organic matter, etc. of raw water. If used in combination with organic polymers to deal with high turbidity water, the effect is better.
Under the condition of achieving the same flocculation effect, according to the calculation of Al2O3, the dosage for low turbidity water is only 50%-75% of aluminum sulfate; for high turbidity water, it is about 30%-40% of aluminum sulfate. It has the advantages of low dosage and low cost of water purification, so that polymeric aluminum chloride plays an important role in the development of water treatment industry.
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