Polyaluminium chloride (PAC), also known as basic aluminum chloride, has the chemical formula ALn(OH)mCL3n-m. PAC is a multivalent electrolyte that significantly reduces the colloidal charge of clay-like impurities (multiple negatively charged) in water. Due to the large molecular mass and strong adsorption capacity, the formed flocs are larger, and the flocculation and sedimentation performance is superior to other flocculants.
The PAC has a high degree of polymerization, and rapid stirring after the addition can greatly shorten the formation time of the flocs. The PAC is less affected by the water temperature, and the effect is also good when the water temperature is low. It has a lower pH value for water and a wide pH range (can be used in the range of pH=5 to 9), so it is possible to dispense an alkaline agent. The dosage of PAC is small, the amount of mud produced is small, and it is convenient to use, manage and operate, and it is less corrosive to equipment and pipelines. Therefore, PAC has a tendency to gradually replace aluminum sulfate in the field of water treatment, and its disadvantage is that the price is higher.
In addition, from the point of view of solution chemistry, PAC is a kinetic intermediate of the hydrolysis-polymerization-precipitation reaction of aluminum salts, which is thermodynamically unstable. Generally, liquid PAC products should be used within half a year. The addition of certain inorganic salts (such as CaCl2, MnCl2, etc.) or polymers (such as polyvinyl alcohol, polyacrylamide, etc.) can improve the stability of the PAC and at the same time increase the cohesive ability. In terms of production process, one or several different anions (such as SO42-, PO43-, etc.) are introduced in the manufacturing process of PAC, and the structure and morphology distribution of the polymer can be changed to some extent by the polycondensation. The stability and efficacy of PAC; if other cationic components, such as Fe3+, are introduced during the manufacture of PAC, Al3+ and Fe3+ are interlaced and hydrolyzed to produce a composite flocculant polymerized aluminum-iron.
The content of aluminum oxide is a measure of the active ingredient of the polyaluminum chloride. In general, the higher the density of the flocculant product, the higher the content of aluminum oxide. In general, the higher the degree of alkalization, the better the adsorption capacity of the polyaluminum chloride for bridging, but the precipitation is likely to occur due to the proximity of [Al(OH)3]n, so the stability is also poor.