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Comprehensive solution for process water generated during the passivation of aluminum surfaces – Author: Enviow- Chemi GmbH, Germany. The process fluids produced during metal surface treatment pose high demands on equipment technology due to their special properties; high concentrations of acids and bases, resulting large fluctuations in pH value, as well as water temperatures ranging from 70 to 96°C, all require engineers to consider various factors comprehensively when designing solutions that are as simple as possible. The use of modern techniques ensures the advantages of cost-effectiveness and environmental sustainability in the production process. Belgian company COIL is a firm that passivates the surface of aluminum sheets through anodization, and it is a leader in its industry across Europe. Due to the growing demand, the company decided to expand its production capacity. The new branch was located in Bernburg, in the state of Saxony-Anhalt in Germany, which offers a favorable investment environment. The entire factory was designed by the Industrial Project Services (IPS) design institute. Among them, the acid recycling and process water technologies were designed and constructed by ENVIRO-CHEMIE. The appearance of the equipment has the following requirements for the installations made by ENVIRO-CHEMIE: * Maintain the water quality in the anodization tank in order to ensure the quality of the product ； * Minimize the use of chemicals in the process ； * Make the reuse of waste materials as possible as possible through appropriate logistics separation ； * To effectively address indirect emission standards, this goal is achieved through 4 steps: 1. Regenerate sulfuric acid in the production process using an ion exchanger ； 2. A device for regenerating caustic soda ； 3. Use a water softening device and a reverse osmosis system to produce process water ； 4. And use a device to treat production wastewater. The entire factory was built in just one year. After the factory building was completed, the treatment unit designed by ENVIRO-CHEMIE had only 3 months to be installed and commissioned. Process flow: Reuse of acids and bases. A crucial aspect of the anodizing production process is to maintain constant stability in the quality of the fluid in the anodizing tank. For the anodization process, in order to achieve the best oxidation results, the aluminum concentration in the tank must be maintained within a constant range. Without acid recovery, the aluminum concentration in the tank will increase over time, which will also lead to higher energy consumption. Therefore, the acidification tank needs to be replaced from time to time. To ensure the economical operation of the plant, it is necessary to maintain the aluminum content in the tank within an optimized range, which can only be achieved through acid recovery (APU). APU makes it possible to separate aluminum sulfate from sulfuric acid, with the acid being returned to the production process after being recovered and purified. The main recovery process operates automatically without the addition of chemical agents, consuming only a small amount of electrical energy; system regeneration and acid reuse require only moderate-quality deionized water. The main component of the APU is a resin ion exchange column; during the adsorption phase as water flows upward, acidic molecules are adsorbed onto the resin, while metal salts pass through the resin bed unimpeded. This water stream, which contains a small amount of acid but high levels of salt, will be fed into the wastewater treatment facility. The regeneration process is a process in which water flows downward. Moderately deionized water can dissolve the acids on the resin body, thereby producing a solution with a higher acid concentration and less salt. This stream of water will return to the anodization tank in the production process. Adsorption and regeneration occur alternately, but to maintain a constant total volume of liquid in the anodization tank, the same volume is used each time. By reusing regenerated sulfuric acid, it is possible to reduce the waste generated during the production process, as well as to decrease the amount of sulfuric acid that needs to be added. It reduces energy consumption while also saving raw materials. Another component of the recycling scheme is the regeneration of alkali in the production process. To passivate the surface of aluminum sheets, alkali etching is required after degreasing. In the absence of alkali regeneration, in order to keep the dissolved aluminum and alkali in the erosion tank as constant as possible, a portion of the solution in the tank must be drained regularly and replaced with fresh alkali. The regeneration of the base is achieved through crystallization. Crystallization allows the aluminum dissolved during the erosion process to be continuously separated, thereby making it possible to reuse the alkali. This system mainly consists of a container for the generation and separation of aluminum crystals, a filter for removing water from the crystals, as well as pumps and containers. Under normal pool conditions, dissolved aluminum will precipitate in large quantities as aluminum hydroxide, forming scale on the container walls and heaters. To prevent this from happening, the precipitation of aluminum hydroxide is monitored and controlled in a separate tank. There, the aluminum concentration is kept below a certain threshold, which on one hand prevents excessive scaling and on the other hand enables the desired crystal structure to be obtained. Aluminum crystals are separated and can be reused after collection. The regenerated alkali will return to the erosion tank. Alkali regeneration can also help conserve resources and save costs: * It reduces the cost of alkali ； * **Reduced the amount of silt ； * A marketable by-product was obtained. Production water and wastewater: Drinking water from the municipal supply network cannot be used directly in production; it must first have its hardness reduced and it must be desalted before use. A water softening capacity of 30 m3/h can be achieved using an ion exchange unit. The softening device operates on a switching basis: one ion exchanger is always in operation, while the other is being regenerated or ready for use. Using softened water for maintenance ensures the smooth operation of the device. A portion of the water flow from the demineralized water, approximately 13 m3/h, will be further treated using reverse osmosis, as demineralized water is also required in production. Wastewater with different loads is generated in the 10 steps of the passivation process. They first converge into 4 buffer pools for homogenization and cooling. The buffered wastewater from every 2 pools is fed into an intermittent reaction tank for physicochemical treatment. In this sedimentation tank, wastewater from aluminum surface treatment processes is precipitated and flocculated. The scheme is designed such that two different treatment processes are carried out in the intermittent tank. The first process involves precipitating the aluminates and sulfates in the wastewater by adding lime water. The sludge resulting from this process is primarily composed of gypsum, which can be reused as a material in the cement construction industry. In the second process, heavy metals such as nickel are mainly removed. The silt generated here contains heavy metals and must be treated specially. The economic operation of the process, along with the techniques for recycling and water management, ensures the efficient operation of anodization. It helps to reduce energy consumption, as well as acid usage and waste generation. After treatment through wastewater treatment systems, the wastewater can meet the standards for indirect discharge. “From today’s perspective, the unit meets all the expectations regarding its operational and processing capabilities,” said Mr. TASIAUX, the manager of COIL Ltd. in Pörnbach, several weeks after the unit’s trial operation. PROCESS 《Process Industry》