Introduction to wastewater treatment in aluminum alloy surface treatment

     The clean production plan for the surface treatment of aluminum alloys has been briefly introduced, but regardless of the specific approach taken, wastewater management remains a critical issue. While detailed monographs on wastewater treatment are available, this section will focus on key aspects and provide an explanation.

    The wastewater generated during the surface treatment of aluminum alloys can generally be categorized into several types: acid-base wastewater, chromium-containing wastewater, nickel-containing wastewater, dyeing wastewater, and fluorine-containing wastewater. Each type requires different treatment methods to ensure environmental compliance and resource efficiency.

    Dyeing wastewater comes from the coloring process and can be treated using ozone or other oxidizing agents to remove color before being sent to the general acid-base wastewater pool for neutralization and precipitation. This helps reduce the overall contaminant load in the system.

    Fluorine-containing wastewater typically originates from acid etching processes. It is first filtered to remove suspended solids, then treated with calcium compounds such as calcium fluoride or calcium hydroxide to precipitate the fluorine. The resulting calcium fluoride is of high purity and can be safely discharged into the acid-base wastewater pool for further treatment.

    Nickel-containing wastewater comes from electroless nickel plating, electroplating, and sealing processes. This type of wastewater can be treated through ion exchange systems for recovery, followed by neutralization and precipitation in the acid-base wastewater pool. If complexing agents are involved, additional treatment may be required, which should be addressed according to specific guidelines.

    Chromium-containing wastewater, especially from hexavalent chromium, is highly toxic and requires specialized treatment. Due to its complexity, a separate discussion will follow later to cover the most effective and safe disposal methods.

    General acid-base wastewater includes all other types not covered above. If no nickel plating is involved, wastewater from the sealing process can often be combined with the main acid-base pool. However, it’s recommended to recover nickel through ion exchange before discharge. This ensures better resource utilization and reduces environmental impact.

    For the treatment of metal ions in acid-base wastewater, the choice of precipitant plays a key role. Sodium hydroxide is commonly used, leading to good metal ion precipitation. However, due to high sulfate and phosphate concentrations in the wastewater, significant dilution may be needed, increasing the risk of eutrophication. Alternatively, calcium oxide can be used to precipitate both metals and phosphates/sulfates, reducing the need for dilution and enabling reuse in pre-treatment processes. However, this method produces more sludge, requiring careful cost-benefit analysis.

    In some cases, secondary precipitation is applied. This involves first treating the wastewater with sodium hydroxide to remove metals, then using calcium-based reagents to target phosphates and sulfates. Although this increases construction costs, it allows for better classification of sludge, facilitating recycling and waste management.

    To achieve more precise separation of sludge, it's advisable to treat each type of wastewater separately based on the metal content. This enhances resource recovery and supports sustainable practices. While initial investment and management costs increase, if the sludge can be reused, it can offset processing expenses and contribute to a near-zero emission system.

    A comprehensive approach to wastewater treatment in aluminum alloy surface treatment is illustrated in Figure 7-23.