Aluminum and iron have many similar properties, such as the covalent radii of atoms. The ionic radii are relatively close, Al3+ and Fe3+ have the same charge, they are easy to be hydrolyzed, and their salts are covalent. As a result, they can be formed by cross copolymerization to form a multi core and longer. More stable molecular chain, get better coagulation effect of inorganic polymer composite coagulant - polymeric aluminum iron. This kind of composite coagulant with poly aluminum and poly iron, which can overcome the aluminum salt treatment of alum, alum generated slow light. The settlement is slow, but also can overcome the iron water is not clear, the shortcomings of high chroma. Usually, the aluminum salt iron supplement, its price is slightly higher than that of PAC a little bit. In recent years, the development and application of this kind of mixing; coagulant has become a hot spot and the development of the obvious trend. Our country in this field is more active in the field of Z, published papers and published more patents. The main raw material is aluminum silicate and aluminum salt, ferric salt and iron and other mineral elements. Slag waste, so the raw material source is wide, the production process is relatively simple, is conducive to the development and utilization of. Our country has developed aluminum limited mixed more types of agent, we divided them into three categories: containing only two cationic aluminum iron composite coagulant, containing a variety of anionic aluminum iron composite coagulant, and contains other cationic aluminum iron composite coagulant.
1 aluminum and iron compound coagulant with two kinds of cations
In this kind of compound coagulant, except for aluminum and iron, there is no other metal ions (even if there is, but also very few). It can be said that the composition is relatively simple, high purity. There is no doubt that there is a high demand for raw materials. They are suitable for the treatment of drinking water, because of less impurities, especially less harmful substances. Research on this kind of coagulant is also more in-depth, some products have entered the domestic water treatment market. This kind of coagulant mainly has the following several:
 

1.1 polymeric aluminum ferric chloride (PAFC)
In general, PAC and FeCl3 (or FeCl2) reaction, or AlCl3 or low poly aluminum chloride and iron reaction, and then the hydroxyl polymerization, can be prepared by the polymerization of ferric chloride. Or will crush the bauxite into the reaction kettle with hydrochloric acid reaction, and then pour it into the mixing tank, and add high aluminum ash and water, stirring 3-5h, precipitation is the product. PAFC product is light yellow, dark yellow flake, granular or powdered solid, soluble in water, easy deliquescence in the air. Liquid product is a light yellow transparent or suspension, relative density >1.2, w (Al2O3) =6.0%- 6.5%, w (Fe2O3) =4.0%-4.3% and basicity is 30% - 50%, pH (1% aqueous solution) for 2-3. The product is used for drinking water. Industrial water purification and chemical treatment of all kinds of sewage. Such as a tube plant with PAFC treatment of turbidity of 319NTU wastewater, dosage of 0.4 or 0.6mg/L, after the treatment of water colorless transparent, heavy metal standards. The initial use of PAC and PFS treatment of electroplating wastewater, steel is always not up to standards, the use of PAFC after the indicators are standard. PAFC treatment with the production of detergent wastewater, compared with PAC, PFS and PFSC the effect is much better. Therefore, PAFC is an alternative to poly aluminum and iron [1-7].
 

1.2 polymeric aluminum ferric sulfate (PAFS)
With bauxite, high aluminum ash and sulfuric acid as raw materials, PAFS was prepared by the same process as PAFC. In the reactor, followed by adding ferrous sulfate, aluminum sulfate, water, sulfuric acid and aluminum nitrate, pass into the air and stirring, oxidation, hydrolysis and polymerization reaction about 0.5h, red brown and transparent liquid, basicity of up to 19%, P (Fe3 +) 161.4g/L, P (Al3 +) 7.0g / L. PAFS for refinery wastewater treatment, and the dosage of PAFS by flotation (8 or 10mg/L), the degreasing rate reaches 99%, COD decreased by 88.5%. PAFS is used for sludge dewatering, which has the best dosage, low filter cake moisture content, good light transmittance and high clarity. The results were significantly better than PAC and PFS[7-10].

1.3 polymeric aluminum ferric silicate (PSAF)
Usually high modulus (M is larger than or equal to 3.0) glass of water dilution, in rapid mixing dropping dilute sulfuric acid, to about pH 5.7, placed after a period of time in the polymerization, then add the aluminum sulfate and ferric sulfate solution, after a period of time of aging the PSAF. It is transparent, uniform, reddish brown solution, good stability, can be placed 2-3 months. N (Fe) /n () =1:3 (Al) n () (SiO2) /n (Fe+Al) =l:3 PSAF, under the conditions of pH=10, the synthetic detergent wastewater LAS removal rate reached more than 90%. For the Songhua River water turbidity treatment, after treatment of the residual turbidity is small (1.6 degrees), in full compliance with the Chinese sanitary standard for drinking water. Other documents that PSAF has better coagulation performance, less the amount, the suitable pH value range is wide, formation of alum rapidly, and flocs large and strong, is a promising water purification agent [11-16].