China Aluminum Industry Network: China currently has over 600 aluminum profile manufacturers, producing more than 2 million tons of aluminum profiles annually. However, this large-scale production also leads to significant environmental challenges, particularly in terms of wastewater discharge and pollution. The surface treatment process of aluminum profiles involves a substantial amount of water, resulting in high volumes of wastewater that often contain harmful substances. If not properly treated, this wastewater can severely impact the environment. Additionally, increasing environmental taxes on pollutants are adding to the financial burden of companies. Therefore, it is crucial for enterprises to consider both their social responsibility and long-term benefits by implementing effective wastewater treatment systems. Based on years of experience in process research and management, the author presents a practical and feasible wastewater treatment and reuse plan that can be referenced by industry peers.
1. Sources and Types of Wastewater from Aluminum Surface Treatment The wastewater generated during the surface treatment of aluminum profiles includes pre-treatment oil removal, post-acid washing, alkaline washing after alkali etching, acid washing after acid etching, and rinsing after oxidation. It also contains colored acid solutions containing Ni²âº, Sn²âº, acidic water, electrophoretic paint ion exchange waste, waste acid, waste alkali, and a small amount of electrophoretic paint wastewater. The mixed wastewater is typically acidic and contains cations such as Al³âº, Ni²âº, Sn²âº, Snâ´âº, Naâº, Cr³âº, and anions like SO₄²â», Fâ», NO₃â», Clâ», AlOâ‚‚â», Acâ», surfactants, acrylic acid, resins, and other organic compounds. The waste liquid may include sulfuric acid from degreasing, oxidation, and neutralization processes, as well as waste solutions from coloring, electrophoretic painting, and sealing stages containing Ni²âº, Fâ», and other contaminants.
2. Methods to Reduce Wastewater and Waste Liquid Discharge To minimize wastewater and waste liquid discharge, several measures can be implemented. These include optimizing water usage time, controlling the angle of material loading to reduce carry-over of solution, and using a three-stage counter-current rinsing system. For instance, the rinse water from etching and degreasing can be reused for alkali etching, while the rinse water from oxidation can be used for acid etching and degreasing. In addition, recovery devices should be integrated into the production line, such as acid and alkali recovery systems, anodizing recovery, RO (reverse osmosis) units for coloring and electrophoretic painting. These technologies significantly reduce wastewater volume and production costs while promoting sustainable practices.
3. Principles and Methods for Treating Aluminum Surface Treatment Wastewater Wastewater from aluminum surface treatment is primarily acidic, containing small amounts of harmful ions like Ni²âº, Sn²âº, and Fâ». The primary treatment method is acid-base neutralization, where alkali is added to raise the pH to around 7–8.5. During this process, cations such as Al³âº, Ni²âº, Sn²âº, and Cr³⺠form hydroxide precipitates. The neutralized wastewater is then transferred to a flocculation tank, where a polyamide flocculant is added to promote particle aggregation. After flocculation, the wastewater flows into a sedimentation tank, where methods such as inclined plate precipitation, air flotation, or centrifugal separation are used. Centrifugal precipitation is considered more stable. The clarified supernatant is then either discharged or sent for further treatment. For reuse, the water can be filtered through an RO unit to remove excess ions and organic matter. If the pH is too low, it can be adjusted using ion exchange equipment to meet reuse standards. The sludge produced during sedimentation is periodically removed and processed via a plate press or belt filter before being sent to an industrial waste treatment facility.
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