Aluminum is very reactive metals, to prevent the surface oxidation or affected by the surrounding environment caused by the defects, before using the aluminum surface treatment. By processing can form a protective film in the surface of aluminum, the protective film is beautiful, corrosion resistance, improve the mechanical strength and prolong the service life of the role in [1]. Need to consume large amounts of water on the surface of the aluminum processing, per ton aluminum surface treatment usually consumes water 50 ~ 80 t. Aluminum surface treatment of waste water is mainly for acid and alkali waste water, which contains a lot of Al3 +, SO42 - and a small amount of Ni2 +, Sn2 +, Cr3, +, F -, etc., for the waste water usually adopts the method of acid and alkali neutralization process. The general acidic wastewater, alkaline neutralization, in the process of neutralization Al3 + and Ni2 +, Sn2 +, Cr3, + form hydroxide in the process of adsorption and precipitation has been removed; And SO42 - and F - anion in the process of coagulation with cations such as Ca2 +, Fe2 + generated precipitation, partial removal. Aluminum surface treatment of the main water consumption link for degreasing, alkali corrosion, and after washing, such as oxidation process link, the link to the water quality requirement of the water is not too high, in addition to skim, neutralization, oxidation after washing with water pH greater than 2, alkali corrosion after washing with water pH should be less than 12, turbidity is lower than 50 ntu, Al3 +, SO42 - less than 500, 2000 mg/L respectively can meet the requirements (2). Thus, using the combination of coagulation and microfiltration membrane separation process in order to achieve the purpose of recycling aluminum surface treatment of waste water.

This study first by coagulation precipitation test to determine the optimum coagulant of combination, coagulant aid and dosing quantity, and then examines the microfiltration membrane operation combination as well as the influence factors on the properties of membrane filtration process of overall effect, and the technical and economic analysis. The combined technique can achieve the goal of save water, reduce pollution, with social, environmental, and economic multiple benefits.

1, experimental materials and methods

1.1 test with water

Testing equipment used by directly on an aluminum factory, foshan city, guangdong province aluminium profile surface treatment line comprehensive wastewater adjusting beside the pool, the factory in addition to the waste water containing chromium, nickel and other pollutants like separate into regulating pool after processing, the rest of the waste water directly into adjusting pool), used in waste water directly from the comprehensive waste water regulating pool, the water quality: Al3 + 500 ~ 600 mg/L, SO42-2000 ~ 3000 mg/L, turbidity, 300 ~ 500 ntu, pH3 ~ 5.

1.2 test apparatus

Test apparatus is shown in figure 1.

Aluminum surface treatment of wastewater treatment and reuse technology

Chinalco's net

Concrete pool, the effective volume of inclined tube sedimentation tank and filter tank respectively, 30, 50, 80 l, microfiltration membrane by Japan's mitsubishi company production of polyethylene hollow fiber membrane, membrane aperture 0.1 mu m, membrane wire diameter 0.27 mm, outside diameter is 0.42 mm, membrane area is 2.0 m2. Membrane module directly placed in the filter tank, setting of its lower part of the trachea, air scour the role of membrane module. Membrane module outlet pipe at both ends of electromagnetic valve to control system of time controller water with aeration.

1.3 test method

1.3.1 coagulant screening test

In order to select suitable flocculant and coagulant aid, in the exploratory stage by sequencing batch type test for a variety of commonly used coagulant coagulation effect were compared. According to the characteristics of the aluminum surface treatment of waste water and wastewater treatment solutions need, choose turbidity experiments, and the removal rate of Al3 +, SO42 - flocculation sedimentation performance as a measure of body, to find the optimum coagulant and dosing quantity.

Take some 1000 ml beaker, the add in 500 ml of comprehensive adjustment pool waste water (water temperature and room temperature are at 28 ℃ or so), join the coagulant dose, adding NaOH powder to pH 8.0, and then respectively to join in the beaker of metering coagulant aid PAM; In six league mixer on the first 1 min to 360 r/min mixing, and stir to 150 r/min 2 min. Let stand for 30 min after that test supernatant fluid flocculation precipitation effect.

1.3.2 coagulation + microfiltration membrane separation combination experiment

Flocculant and coagulant aid solution respectively through the metering valve to enter into the pump suction piping, and raw water through the impeller after rapid mixing into concrete pool; After slow mixing into inclined tube sedimentation tank; Spate separation after the supernatant fluid into the filter tank, the suction pump suction under the action of the membrane filter to get out of the water. Inlet valve and outlet valve under the control of the time controller work alternately. According to the content of suspended solids in water coagulation system, every 6 ~ 8 cycles to eliminate heavy mud at the bottom of the filter 1 times.

2, the results and discussion

2.1 the coagulation precipitation test

This study chose FeCl2, FeCl3, Fe2 (SO4) 3, AlCl3, Ca (OH) 25 kinds of coagulants, in order to increase the degree of dense flocs, PAM as coagulant aid. Coagulant dosing mass concentration respectively 100, 150, 200, 150 mg/L; Coagulant aid additive mass concentration respectively 2, 4, 6, 8 mg/L. Through contrast experiments on various kinds of coagulants/the best dosing quantity and coagulant aid flocculation effect, the results are shown in table 1.

Chinalco's net

Can be seen from table 1, the Ca (OH) 2 / PAM coagulation effect is better than other kinds of coagulants. When the Ca (OH) 2 additive mass concentration of 200 mg/L, PAM dosing mass concentration of 4 mg/L, after coagulation precipitation over 50 ntu, the effluent turbidity water Al3 +, SO42 - less than 18, 1500 mg/L respectively, show that the single coagulation precipitation aluminum surface treatment integrated wastewater treatment, reluctant to skim, alkali corrosion, neutralization, such as oxidation process to the water quality requirement of the water, but the large amount of coagulant dosing, and the effluent is stable enough.

The coagulation sedimentation process by adding the full amount of coagulant and coagulant aid in order to make the water suspension colloid particles sedimentation performance good to get rid of the floc. Combination with coagulation + microfiltration membrane separation process, the microfiltration membrane separation technology can achieve 0.1 mu m order of magnitude of solid-liquid separation, dosing of coagulant and coagulant aid only make colloidal particles by compressed double electric layer stability can, without form depending on the gravity sedimentation particle size, it can not only reduce the dosage of the flocculant and coagulant aid, and a single coagulation precipitation increases the range of contaminants can be removed. Test in Ca (OH) 2 additive quantity (200 mg/L) unchanged, but the coagulant aid PAM dosing only half dose (2 mg/L) under the condition of combination of coagulation and microfiltration membrane separation process of aluminum processing waste water treatment effect.

2.2 cycles of microfiltration membrane aeration effect on the performance of membrane filtration

This study USES the average flux as the membrane filtration performance evaluation index. In order to reduce the degree of membrane fouling, maintain the stability of the membrane filtration performance test cycle the aeration operation mode used to remove the mud cake deposition on the membrane surface layer, the aeration pressure is 0.15 MPa, the aeration time every time for 3 min. Membrane filtration under the conditions of different cycle change results show that the performance of the aeration membrane cycle can be greatly to remove mud cake layer deposited on the membrane surface, reduce the membrane fouling, recovery of membrane flux, effectively maintain the stability of the membrane filtration performance. Using filtering 30 min, the aeration 3 min run, cycle water rate can be stable in about 40 l/h. (m2).

Coagulation + 2.3 the performance of the combination microfiltration membrane separation process

With Ca (OH) 2 and PAM as flocculant and coagulant aid, additive concentration, 200, 2 mg/L, respectively at 30 min filtering, 3 min aeration way of operation, in the average flux control in 40 L/(m2 · h) under the condition of stable operation for 2 weeks. Combination of coagulation and microfiltration membrane separation process of aluminum surface treatment integrated wastewater treatment effect as shown in figure 2 and figure 3.

Chinalco's net

Chinalco's net

The figure 2 shows that after microfiltration membrane filtration, the final effluent turbidity for around 10 ntu, fully meet the degreasing, alkali corrosion, neutralization, oxidation process after the water washing links such as water requirement for turbidity.

The figure 3 shows that although the system fill SO42 - volatile emissions (mainly composed of irregular parts degreasing bath or out of the light), but in the end the water stability of SO42 - about 1000 mg/L, the generation of calcium sulfate precipitation and aluminum hydroxide precipitation of SO42 - adsorption is likely to be the main way for the system to remove sulfuric acid root; Combination after coagulation + microfiltration membrane separation process after processing, the system of the water Al3 + stable at about 8 mg/L. After the combination process, the system fully satisfy the degreasing, alkali corrosion, neutralization, water oxidation process after the water washing links such as water to the requirement of Al3 + and SO42 -.

3, technical and economic analysis

Compared with the conventional process, coagulation and microfiltration membrane separation combined process aluminum surface treatment wastewater treatment has the stable discharging water, cover an area of an area small, infrastructure investment province, a high degree of automation, convenient operation and management (3), and effluent can reach the purpose of recycling.

At 1000 t, treatment and recycle waste water as an example, the combination of coagulation and microfiltration membrane separation process for technical and economic analysis: infrastructure investment is 150000 yuan, equipment cost price 150 yuan/m2 (membrane) is 186000 yuan, depreciation cost (in 3 a film life, the other is 20 a) is 0.192 yuan/m3, 0.4 yuan/m3 (electricity energy consumption by 0.8 yuan/(kW · h) meter), agent fee is 0.133 yuan/m3, artificial cost is 0.15 yuan/m3, the system total cost is 0.875 yuan/m3. Compared with the current water cost 2.5 yuan/m3 (where the water is 1.3 yuan/m3, waste water disposal 1.3 yuan/m3, the pollutant discharge fee RMB 0.8 / m3), economic benefit is obvious.

4, the conclusion

Combination with coagulation + microfiltration membrane separation process aluminum surface treatment integrated wastewater treatment and reuse of process conditions: with Ca (OH) 2 as coagulant and PAM as coagulant aid, Ca (OH) 2 and PAM dosing mass concentration, respectively, 200, 2 mg/L, microfiltration membrane separation system for 30 min filtration, 3 min run against aeration. 2 weeks on this condition, the system stable running, while water quality change is bigger, the water turbidity steady at around 10 ntu, Al3 + and SO42 - stability in 8 mg/L and 1000 mg/L, effluent water quality fully meet the degreasing, alkali corrosion, neutralization process after washing, oxidation etc.) of water requirement. Compared with the conventional treatment process, combination of coagulation and microfiltration membrane separation technology has obvious advantages in technology, feasible in economy.