There are many methods for synthesizing polyaluminum chloride (PAC). Based on the different raw materials, they can be divided into the metallic aluminum method, the activated aluminum hydroxide method, the aluminum oxide method, the aluminum chloride method, and the alkali solution method.
① Metallic Aluminum Method: The raw materials for synthesizing PAC using the metallic aluminum method are mainly aluminum processing by-products, such as aluminum shavings, aluminum ash, and aluminum slag. Hydrochloric acid is slowly added to aluminum ash in a certain proportion under stirring to react, followed by aging polymerization and sedimentation to obtain liquid PAC. This is then diluted, filtered, concentrated, and dried. The process can be divided into three types: acid method, alkali method, and neutralization method. The acid method mainly uses HCl, and the product quality is difficult to control; the alkali method has a higher production difficulty, requires larger equipment investment and a large amount of alkali, and pH control is expensive due to raw material consumption, resulting in higher costs; the neutralization method is the most commonly used, and as long as the proportions are well controlled, it can generally meet national standards.
② Aluminum Hydroxide Method: Aluminum hydroxide powder has relatively high purity, and the synthesized PAC has low content of heavy metals and other toxic substances. It generally uses a heated and pressurized acid dissolution production process. This process is relatively simple, uses high-purity raw materials, and produces polyaluminum chloride (PAC) with minimal impact on water quality. However, the basicity of the produced PAC is low, so it generally involves two steps: acid dissolution under heat and pressure with aluminum hydroxide, followed by neutralization with calcium aluminate powder.
③ Alumina Method. Raw materials containing aluminum oxide mainly include gibbsite, bauxite, kaolin, and coal gangue. This production process can be divided into two steps: the first step is to obtain crystalline aluminum chloride, and the second step is to obtain PAC through pyrolysis or neutralization.
④ Aluminum Chloride Method. This method uses aluminum chloride powder as raw material to process PAC. This method is the most widely used. Crystalline aluminum chloride can be boiled and pyrolyzed at 170℃, then polymerized by adding water, followed by solidification and drying.
⑤ Alkali Dissolution Method. First, aluminum ash is reacted with sodium hydroxide to obtain a sodium aluminate solution, then the pH is adjusted with hydrochloric acid to obtain a PAC solution. The product produced by this method has a better color and appearance and less insoluble matter, but it has a high sodium chloride content, high raw material consumption, low alumina content in the solution, and high industrial production costs.
