There are numerous methods for synthesizing polyaluminum chloride (PAC); based on the raw materials utilized, these methods can be broadly classified into the metallic aluminum method, active aluminum hydroxide method, aluminum oxide method, aluminum chloride method, alkali dissolution method, and others.
① Metallic Aluminum Method: The primary raw materials employed in the metallic aluminum method for PAC synthesis are by-products from aluminum processing, such as aluminum chips, aluminum ash, and aluminum slag. The process typically involves slowly adding hydrochloric acid to aluminum ash-at a specific ratio and under continuous agitation-to initiate a reaction. Following a period of maturation and polymerization, the mixture undergoes sedimentation to yield liquid PAC; subsequent steps involving dilution, filtration, concentration, and drying are then performed to obtain the final product. From a process engineering perspective, this method can be further categorized into three variations: the acid method, the alkali method, and the neutralization method. The acid method primarily utilizes HCl, though controlling product quality can be challenging. The alkali method presents greater technical complexity in production, requires substantial investment in equipment, consumes large quantities of alkali, and entails higher costs due to the material-intensive nature of pH control. The neutralization method is the most widely adopted approach; provided that the reactant ratios are precisely controlled, the resulting product generally meets national quality standards.
② Aluminum Hydroxide Method: Since aluminum hydroxide powder possesses a relatively high degree of purity, the PAC synthesized via this method exhibits low levels of toxic impurities-such as heavy metals. Production typically employs a process involving acid dissolution under conditions of elevated temperature and pressure. While this process is relatively straightforward-benefiting from high-purity raw materials and resulting in PAC that has a minimal impact on water quality-the resulting PAC typically exhibits a low basicity. Consequently, the process is often modified to incorporate a two-step sequence: acid dissolution of aluminum hydroxide under heat and pressure, followed by a neutralization step utilizing calcium aluminate mineral powder.
③ Aluminum Oxide Method: Raw materials containing aluminum oxide (Al₂O₃) include gibbsite, bauxite, kaolin, and coal gangue. This production process is divided into two distinct stages: the first stage involves obtaining crystalline aluminum chloride, while the second stage entails converting this intermediate into PAC through either a thermal decomposition method or a neutralization method.
④ Aluminum Chloride Method: This method utilizes aluminum chloride powder as the primary raw material for the production of PAC. It is currently the most widely practiced synthesis method. The process typically involves subjecting crystalline aluminum chloride to fluidized-bed thermal decomposition at a temperature of 170°C, followed by the addition of water to facilitate maturation and polymerization; the final product is then obtained through subsequent solidification and drying steps.
⑤ Alkali Dissolution Method: This method begins by reacting aluminum ash with sodium hydroxide to generate a sodium aluminate solution. Subsequently, hydrochloric acid is added to adjust the pH level, thereby yielding a PAC solution. Products obtained using this method exhibit superior color and appearance, as well as lower levels of insoluble matter; however, they are characterized by a high sodium chloride content, high raw material consumption, and a low alumina concentration in the solution, resulting in substantial costs for industrial-scale production.
