Conclusion: This will significantly alter the foaming characteristics of the solution; long-term recirculation of acid pickling waste liquid will inevitably exacerbate foaming and the carryover of mist and foam.

I. Acid pickling waste liquid contains a large amount of surfactants, which increase foam stability.

Citric acid, oxalic acid, aminosulfonic acid, and various corrosion inhibitors and penetrants used in cleaning are all low-surface-tension additives:

When the waste liquid is recirculated into the stock tank, the surface tension of the system drops significantly, making it extremely easy for fine, stable, and persistent microfoam to form during boiling;

While foam from ordinary inorganic salts breaks easily, the foam generated after mixing with acid pickling agents is highly elastic, maintains its height for a long time, and easily overflows the liquid surface to surge toward the demister screen.

II. Colloidal scale particles dissolved and stripped off during acid pickling become the foam skeleton

Acid washing dissolves silicate gels, organic flocs, and fine sludge of metal hydroxides on plate surfaces and tank walls:

Once these micron-sized suspended particles enter the stock solution, they become embedded in the foam liquid film, acting as a stabilizing framework for the foam; the foam does not collapse on its own but continues to accumulate and thicken, significantly increasing the risk of overflow and entrainment.

III. Severe pH fluctuations induce colloidal destabilization and foaming

Electroplating and chemical process solutions have their own stable pH ranges, whereas acid washing waste liquid is strongly acidic:

The overall pH of the process solution drops abruptly, causing residual complexing agents and hydroxide colloids in the water to destabilize and precipitate, simultaneously generating a large number of microscopic foaming nuclei;

Toward the end of the staged concentration process, as salinity increases, the foaming effect caused by acid-base imbalance is further amplified, leading to massive carryover of material even with slight fluctuations in vacuum pressure.

IV. Increased Total Ion Concentration Further Strengthens the Foam Film

Metal ions such as calcium, iron, nickel, and silicon are stripped off during acid washing, compounded by the continuous accumulation of acid anions:

A high-ion-strength environment compresses the colloidal double layer, making the foam liquid film more robust, significantly slowing down defoaming, and causing the foam layer inside the chamber to remain persistently high.

V. Long-Term Accumulation Leads to Irreversible Worsening of Foaming Issues

A small amount of single-use recirculation has limited impact, but after repeated reuse of acid washing waste liquid: corrosion inhibitors, organic acids, and silica sludge continue to accumulate;

Even if waste liquid is no longer recirculated, residual chemicals in the original solution will maintain strong foaming properties for an extended period, causing the demister load to consistently exceed standards and leading to a continuous increase in salt and organic matter levels in the condensate.

VI. Derived Chain Reaction of Failures

Foam continuously entrains salt crystals and colloids, clogging the demister mesh; this increases gas-phase resistance and causes frequent vacuum fluctuations;

Large amounts of foaming media enter the vacuum piping and activated carbon adsorption tanks, accelerating carbon bed saturation and contaminating the condensate;

Foaming droplets are drawn into the vacuum pump, where organic acids corrode the pump body while simultaneously emulsifying and damaging the vacuum pump oil.