New progress in the research of aluminum-iron composite coagulant

[China Aluminum Network] Aluminum and iron have many similar properties, such as the atomic covalent radius. The ionic radii are relatively similar. Fe3+ and Al3+ all have the same charge. They are easily hydrolyzed and their salts are covalent. Therefore, they can be cross-copolymerized to form multiple cores and longer. The more stable molecular chain, the inorganic polymer composite coagulant with better coagulation effect—aluminum iron polymer. This type of composite coagulant combines the characteristics of polyaluminium and polyaluminum, which can overcome the aluminum salt treatment of the pods to produce slowly and lightly. The disadvantage of slow sedimentation can overcome the disadvantages of unclear water and high chroma. Usually based on aluminum salt, supplemented by iron salt, the price is slightly higher than PAC. In recent years, the development and application of such blends have become a hot spot and a clear trend for development. Our country is more active in this field of Z, and more papers and published patents have been published. The main raw materials are aluminum salts, iron salts and silicates, and minerals containing elements such as aluminum and iron. Slag waste, so its raw material source is wide, and the production process is relatively simple, which is conducive to the development and utilization. There are many types of aluminum limited composite coagulants that have been developed in China. We have divided them into three categories: aluminum-iron composite coagulants containing only two cations, aluminum-iron composite coagulants containing various anions, and Other cationic aluminum-iron composite coagulants.
1. Aluminum-iron multi-column coagulant with only two cations. In addition to aluminum and iron, this type of composite coagulant does not contain other metal ions (even if it is rare), and the anion is mainly based on one kind. It can be said that its composition is relatively simple and its purity is high. There is no doubt that this has a higher demand for raw materials. Since there are few impurities, especially less harmful substances, they are suitable for the treatment of drinking water. Research on this kind of coagulant is also in-depth, and some of these products have entered the domestic water treatment market. This type of coagulant mainly has the following types:
1.1 Polyaluminum Ferric Chloride (PAFC)
Usually, PAC reacts with FeCl3 (or FeCl2), or AlCl3 or oligoaluminum chloride reacts with iron, and then hydroxylation polymerization is carried out to obtain polyaluminum chloroaluminum chloride. Or put the crushed bauxite into a reaction kettle and react with hydrochloric acid, then pour it into a stirred tank, add high aluminum ash and water, stir for 3-5h, and the precipitation is the product. PAFC products are light yellow, dark yellow flakes, granular or powdery solids that are easily soluble in water and deliquescent in air. The liquid product is light yellow transparent or suspension, relative density> 1.2, w(Al2O3)=6.0%-6.5%, w(Fe2O3)=4.0%-4.3%, salt base degree 30%-50%, pH value (1% aqueous solution) is 2-3. This product is used for drinking water. Industrial water purification and various sewage chemical treatment. For example, an electronic tube factory uses PAFC to treat wastewater with a turbidity of 319 NTU. The dosage is 0.4 or 0.6 mg/L. The treated water is colorless and transparent, and the heavy metals meet the standards. In a certain factory, PAC and PFS were used to treat electroplating wastewater. The steel could not reach the standard. After the adoption of PAFC, all indicators met the standard. The use of PAFC to treat wastewater from the production of detergents is much better than using PAC, PFS and PFSC. Therefore, PAFC is an alternative product of polyaluminium and polyaluminium [1-7].
1.2 Polyaluminum Ferric Sulfate (PAFS)
The bauxite, high aluminum ash, and sulfuric acid were used as raw materials to prepare PAFS in the same process as PAFC. In the reactor, ferrous sulfate, aluminum sulfate, water, sulfuric acid and aluminum nitrate can also be added in sequence, and air is introduced and stirred. Oxidation, hydrolysis, and polymerization are performed for about 0.5 h to obtain a red-brown transparent liquid with a basic degree of alkalinity. 19%, ρ(Fe3+) 161.4 g/L, ρ(Al3+) 7.0 g/L. PAFS was used for the treatment of the industrial waste water in the refinery industry. Flotation and addition of PAFS (8 or 10 mg/L) were used. The oil removal rate was 99% and the COD was reduced by 88.5%. PAFS is used for sludge dewatering. It has the advantages of small dosage, low moisture content of filter cake, good transparency of filtrate and high clarification. The effect is significantly better than PAC and PFS [7-10].
1.3 Polyaluminum Ferrosilicate (PSAF)
Usually the high modulus (M ≥ 3.0) water glass is diluted, dilute sulfuric acid is added dropwise under rapid stirring to a pH of about 5.7, allowed to polymerize after standing for a period of time, and then an aluminum sulfate and iron sulfate solution is added to age. After some time, it is PSAF. It is a transparent, uniform, reddish-brown solution with good stability and can be placed for 2-3 months. Using PSAF with n(Fe)/n(Al)=1:3, n(SiO2)/n(Fe+Al)=l:3, the LAS removal rate in synthetic detergent wastewater can be achieved at pH=10. More than 90%. For turbidity removal of Songhua River, the residual turbidity after treatment is small (1.6 degrees), which is in full compliance with the hygiene standards for drinking water in China. Other documents also show that PSAF has better coagulation performance, and its use amount is less, the suitable pH value range is wider, the formation of pods is rapid, and the flocs are thick and solid, which is a promising water purifying agent [11- 16].

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