Polyaluminium chloride, PAC for short, is an excellent water purifying agent. In recent years, aluminum sulfate and other traditional coagulant products have been gradually replaced in urban water supply and drainage purification in China due to low price, stable quality, outstanding effect, simple use and convenient transportation.
Preparation method of polyaluminium chloride
There are many synthetic methods of polyaluminium chloride. According to different raw materials, it can be divided into metal aluminum method, active aluminum hydroxide method, aluminum trioxide method, aluminum chloride method, alkali solution method, etc.
Metal aluminum method
Polyaluminium chloride synthesized by this method mainly uses leftover materials processed by aluminum, such as aluminum chips, aluminum ash and aluminum slag. Liquid polyaluminium chloride is prepared by slowly adding hydrochloric acid into aluminum ash according to a certain proportion under stirring for reaction, aging polymerization and sedimentation, and then diluted, filtered, concentrated and dried. The process can be divided into acid method, alkali method and neutralization method. The acid method mainly uses HCl, and the product quality is not easy to control; The alkali production process is difficult, the equipment investment is large, the amount of alkali is large, the pH control cost is high, and the raw material cost is high; The neutralization method is used most. As long as the ratio is well controlled, it can generally meet the national standard.
Aluminum hydroxide process
The purity of aluminum hydroxide powder is relatively high, and the content of toxic substances such as heavy metals in the synthesized polyaluminium chloride is low. Generally, the production process of heating and pressure acid dissolution is adopted. This process is relatively simple, but the alkalinity of polyaluminium chloride produced is low. Therefore, aluminum hydroxide heating and pressure acid dissolution and calcium aluminate ore powder neutralization are generally used.
Tri alumina process
Raw materials containing aluminum oxide mainly include gibbsite, bauxite, kaolin, coal gangue, etc. The production process can be divided into two steps: the first step is to obtain crystalline aluminum chloride, and the second step is to obtain polyaluminium chloride by pyrolysis or neutralization;
Aluminium chloride process
Polyaluminium chloride is processed with aluminum chloride powder as raw material. This method is most widely used. It can be prepared by boiling pyrolysis of crystalline aluminum chloride at 170 ℃, aging polymerization with water, curing and drying.
Alkali solution method
Firstly, aluminum ash is reacted with sodium hydroxide to obtain sodium aluminate solution, and then hydrochloric acid is used to adjust the pH value to prepare polyaluminium chloride solution. The product prepared by this method has good color and appearance, less water-insoluble matter, but high sodium chloride content, high raw material consumption, low alumina content in solution and large industrial production cost.