Zero-emission recycling technology for GER anodization production lines

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The last edit to this post was made by beyond81566 on 2018-7-12 at 19:26. The application of NF+RO technology in the treatment and reuse of aluminum anodization wastewater involves the use of a water treatment process based on nanofiltration and reverse osmosis membrane technologies to treat the aluminum anodization wastewater from an electronics factory in Suzhou. This study examines the effectiveness of this process in removing CODcr, hardness, and high conductivity from such wastewater, as well as the operational performance of the system. After 10 days of operation, the water quality of the water produced by the system was good, and the operation was stable. The average removal rates of the nanofiltration membrane for CODcr, hardness, and conductivity are 85%, 100%, and 90%, respectively, while the RO system achieves a removal rate of 94% for conductivity. Moreover, the concentrated water from the system can meet the filtration standards. Aluminum alloy anodized coatings are widely used for corrosion protection and surface decoration. In industrial and consumer aluminum structures and products, about 65% require surface treatment. Aluminum components and parts used in the construction industry, as well as in the aviation and aerospace industries and the automotive manufacturing industry, are almost all subjected to anodizing or coating treatments. Currently, aluminum alloy anodization generally employs the sulfuric acid anodization process. However, regardless of the process used, large amounts of wastewater and waste liquid are generated, causing environmental pollution to varying degrees. Therefore, the treatment of these wastewater and waste liquids is an essential part of the aluminum alloy anodization production process. The common practice in the industry today is to discharge the bath solution from each processing step when it does not meet the production requirements, collect it in a discharge tank, and then add an appropriate amount of acid or base for neutralization in order to adjust the pH to within the allowable discharge standards. During the neutralization process, a large amount of sediment is generated; due to the difficulties involved in dealing with it, most manufacturers discharge it without treating it, resulting in severe environmental pollution. In recent years, as environmental protection measures have been strengthened, the discharge of wastewater from various industrial sectors has been restricted; as a result, advanced treatment of wastewater and its maximum possible recycling have become priorities. Membrane separation technology is energy-efficient, requires low investment, occupies little space, is easy to operate, and has high treatment efficiency. It can also achieve deep purification of wastewater, and when appropriately combined, it can be applied to the treatment and reuse of various types of wastewater. It is currently being increasingly adopted in the field of industrial wastewater treatment and reuse. Taking into account the characteristics of the water quality and the requirements for reuse, a combined process centered on nanofiltration (NF) + reverse osmosis (RO) membrane technology was selected to treat the aluminum anodization wastewater from an electronics factory in Suzhou. As a pretreatment for reverse osmosis feedwater, nanofiltration can optimize the requirements for reverse osmosis feedwater, reduce the operating pressure of reverse osmosis, decrease the frequency of backwashing of sand filters, and increase the system’s recovery rate. The treated pure water is returned to various processes, enabling water recycling and reducing water consumption. This not only holds economic significance but also responds to the calls for energy conservation and emission reduction during the 11th Five-Year Plan period; it thus has great strategic importance for the development of China’s aluminum industry. 1 Process flow and treated water quality 1.1 Treated water quality The water quality characteristics of the comprehensive wastewater treated in this project are shown in Table 1. As shown in Table 1, the challenge of this project lies in the very high conductivity (salt content) of the wastewater. Table 1 Water quality parameters of the combined wastewater Sequence Number Parameter Value 1 Inlet water flow rate 50 m3/h 2 pH value 6–8 3 CODcr ≤120 mg/l 4 Total hardness 400–900 mg/l 5 Conductivity 2500–16000 μs/cm 1.2 Considering factors such as the overall water quality and the requirements for the effluent, a combined process based on nanofiltration and reverse osmosis membrane technologies was selected to treat the wastewater from aluminum anodization processes. This process consists of four stages: pretreatment, nanofiltration, reverse osmosis, and ion exchange resins. The specific process flow is shown in Figure 1. 1.2.1 Pretreatment section The pretreatment system consists of a raw water tank, a lift pump, a quartz sand filter, an activated carbon filter, and a safety filter. The wastewater flows from the reservoir, through a lift pump, into the quartz sand filter, where most of the solid suspended particles are removed ； It then passes through an activated carbon filter, which can absorb organic substances, oils, and residual chlorine from the wastewater, while also removing odors and color from it ； Security filters prevent particles in the pipeline from entering the membrane system, thereby avoiding damage to the membrane modules. All pre-treatment processes are designed to maximize the prevention and delay of pollutant deposition on the membrane surface, to prevent clogging caused by colloids and solid suspended particles, as well as damage to the membrane caused by organic substances, microorganisms, oxidizing agents, etc.; they also serve to slow down the hydrolysis process of the membrane, thereby ensuring that the membrane system operates in optimal condition. file:///C:/Users/ADMINI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image002.gif Figure 1: Comprehensive wastewater treatment process flow. 1.1.2 After pre-treatment, the wastewater from the NF system is pumped into the nanofiltration unit using a high-pressure pump. The system uses 65 polyamide anti-contamination membrane elements produced by Hydranautics of the United States, with a treatment capacity of 48 m3/h per unit (for produced water), and a desalination rate of over 90%. Through the treatment by this system, 75% of the water in the wastewater is separated, while the vast majority of the dissolved components with a relative molecular weight between 200 and 1000 are retained in the concentrate by the membrane with a high retention rate, resulting in a concentration factor of 4. The performance parameters of the NF membrane modules are shown in Table 2. Table 2 Performance parameters of NF membrane modules Material Membrane shape Inlet pH Desalination efficiency per module Operating pressure Polyamide composite membrane Spiral-wound type 3.0～10.0 75～97% 0.5～0.8 MPa 1.1.3 The permeate from the RO system and NF system enters the RO unit via a high-pressure pump. The system uses 50 polyamide high-desalination membrane elements from Hydranautics of the United States, with a treatment capacity of 36 m3/h per unit (for produced water), and a desalination rate of over 98%. After being processed by this system, it was concentrated another 4 times. The performance parameters of the RO membrane module are shown in Table 3. Table 3 Performance indicators of RO membrane modules
Material, Desalination rate (%), Operating pressure (MPa), Water production rate (m3/h), Membrane area (m2): Polyamide, High-desalination membrane – 99.7%, <4.14 MPa, 41.6 m3/h, 37.2 m2. 1.1.4 The ion exchange system: A mixed-bed is short for an ion exchange column; it is typically placed after the reverse osmosis unit to further desalt water, thereby producing high-purity water. In this system, the water treated by reverse osmosis is pumped into a mixed-bed reactor for further desalination. Regeneration is carried out by using reverse osmosis produced water to clean the resin for about 30 minutes. The ultimately produced high-purity water enters the water production tank and is reused in the water-consuming sections, thus forming a sustainable closed-loop water system for clean production. 2 Results and Discussion 2.1 Removal of CODcr by the system When the inlet CODcr concentration was 60–120 mg/L, the variation of CODcr concentration over time is shown in Figure 2. file:///C:/Users/ADMINI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image004.gif Figure 2: Curve showing the variation of CODcr concentration over time. As can be seen from Figure 2, even when the CODcr level in the influent water fluctuates within a certain range, the CODcr level in the effluent water remains relatively stable, staying below 20 mg/l. The average removal rate of CODcr is around 85%, meaning that the quality of the effluent water meets the standards for reuse. Nanofiltration membranes (NF) exhibit a very significant effect in removing CODcr. The removal of organic substances by NF is the result of both pore size screening and the Donnan effect, depending on the structural characteristics of the organic substances (such as molecular weight and polarity) as well as the interaction between the organic substances and the membrane. The pore size of the NF surface layer is in the nanometer range, allowing it to retain small organic molecules with a molecular weight of several hundred. During the operation of this project, the water fed into the nanofiltration equipment has been treated through quartz sand filters and activated carbon filters, resulting in almost zero SS levels; this facilitates further treatment by the nanofiltration membranes. 2.2 The system’s removal of hardness: When the total hardness of the nanofiltration feed water is in the range of 400–900 mg/l, the variation of total hardness over time is shown in Figure 3. As can be seen from Figure 3, the nanofiltration membrane (NF) exhibits a very significant effect in removing hardness; the hardness of the effluent is 0, with a removal rate of 100%, and the quality of the effluent meets all the requirements for reuse. The salt rejection performance of nanofiltration membranes (NF) is primarily attributed to the electrostatic interactions between ions and the membrane. The charge strength of salt ions varies, which in turn affects their rejection rate. In systems containing ions of different valencies, the penetration of multivalent ions is hindered due to the influence of ion radius and electrostatic repulsion; as a result, NF has a higher capacity to reject divalent and higher-valency ions compared to monovalent ions. The retention rates of anions by nanofiltration membranes increase in the following order: NO3-, Cl-, OH-, SO42-, CO32-- ； The retention rate for cations increases in the following order: H+, Na+, K+, Mg2+, Ca2+, Cu2+. Therefore, in this project, the NF membrane exhibits a high retention capacity for divalent ions such as Ca2+ and Mg2+ that cause hardness. file:///C:/Users/ADMINI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image006.gif Figure 3: Curve showing the variation of total hardness over time. 2.3 Removal of conductivity by the system: The inlet conductivity of the NF system varied within the range of 2500–16000 us/cm, and the change in conductivity over time is shown in Figure 4. In this project, the water produced by the NF system flows directly into the RO system. When the conductivity of the water from the NF system is in the range of 200–600 us/cm, the conductivity of the RO system changes over time as shown in Figure 5. file:///C:/Users/ADMINI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image008.gif Figure 4: Curve showing the change in conductivity of the nanofiltration system over time. file:///C:/Users/ADMINI~1/AppData/Local/Temp/msohtmlclip1/01/clip_image010.gif Figure 5: Curve showing the change in conductivity of the reverse osmosis system over time. As can be seen from Figure 4, the conductivity of the water output by the NF system remains relatively stable at around 500 us/cm, with a removal efficiency of approximately 90%. However, due to the high conductivity of the feed water, the conductivity of the nanofiltration output water remains high, and the quality of this output water does not meet the standards for reuse. Therefore, the produced water still requires further desalination using an RO system in order to reduce its conductivity. As can be seen from Figure 5, after reverse osmosis treatment, the conductivity of the effluent **decreases**, remaining at around 20 us/cm; the removal rate of conductivity is approximately 94%. The water produced undergoes further deep desalination through an ion exchange mixed-bed process, resulting in effluent that fully meets the requirements for reuse. 2.4 Analysis of the equipment’s operational performance: The entire system began operating on June 30, 2010, and after 10 days of continuous operation, it was found that its performance met the design requirements to a large extent. ⑴The system operates stably and has strong shock resistance; when the conductivity of the feed water is 15,900 us/cm, the conductivity of the water produced by the NF system is 600 us/cm, while that of the RO system is 26 us/cm. ⑵The operating pressure differences of the NF and RO systems remain relatively constant throughout the process; the operating pressure difference for the NF system is approximately 0.2 MPa, while that for the RO system is around 0.1 MPa. ⑶The fluxes of the NF and RO systems have been corrected to those at 25°C, while the fluxes of the membrane systems are represented by the daily average flux. The flux of NF and RO membrane systems gradually decreases as operating time increases. Once it drops to a certain level, a stable phase is reached; this is because the formation of deposits on the membrane surface is a dynamic process, and when this process reaches equilibrium, the water permeation resistance of the membrane stabilizes, and thus the membrane flux also stabilizes. The pollution status of NF and RO membranes as well as their cleaning cycles will be further studied in the future. ⑷There are still some issues with the equipment during operation. At present, the main problem is that the security filter elements in the NF system need to be replaced frequently, on average every 48 hours. Analysis suggests that it is due to the plant adding large amounts of chemical agents such as Ca(OH)2, PAM, and PAC during the original wastewater treatment process, which caused blockages in the filter elements. 2.5 The economic analysis results during the initial stable operation phase of the economic analysis system are shown in Table 4. Table 4 Analysis of equipment operation costs Item Sulfuric acid (L/t) Redox potential dosage (mL/t) Scale-inhibiting dosage (mL/t) Power consumption (KWh) Dosage: 0.2, 3, 5, 24 As can be seen from Table 4, the system exhibits a certain degree of economic viability during its initial stable operation: (1) Concentrated sulfuric acid is added to the raw water tank to adjust the pH of the discharged water; at the beginning of operation, the dosage of sulfuric acid is maintained at 0.2 L/t. ⑵To prevent the NF membrane from oxidizing, it is required that the redox potential (ORP) of the water entering the nanofiltration equipment not exceed 200 mV, with a dosage maintained at 3 mL/t. ⑶Maintaining a scale inhibition dose of 5 mL/t can prevent scaling of nanofiltration and reverse osmosis membranes during equipment operation. ⑷The power consumption of the equipment during operation is 24 KWh. 3 Conclusions and Recommendations This project employs a combined process based on NF and RO membrane technologies to treat aluminum anodization wastewater. Through the analysis of data from the initial operation period, the following conclusions were drawn: (1) The average removal rates of NF membranes for CODcr, hardness, and conductivity are 85%, 100%, and 90%, respectively, while the average removal rate of RO membranes for conductivity is 94%. Ultimately, the CODcr, hardness, and conductivity of the produced water remained relatively stable at 20 mg/L, 0, and 20 μS/cm, respectively, meeting the requirements for reuse. ⑵During operation, the flux of the NF and RO membranes generally decreases as operating time increases, but for a certain period, the membrane flux remains relatively stable. ⑶The addition of chemical agents such as Ca(OH)2, PAM, and PAC can easily shorten the lifespan of the security filter elements, thereby affecting the performance of the entire system. It is recommended that the chemicals added to the membrane system be as efficient, minimal in quantity, and environmentally friendly as possible, so that each stage of the system can handle its corresponding load, perform its proper function, and enhance the efficiency of the system.