Returning acid washing waste liquid directly to the original solution tank does indeed alter the foaming characteristics of the feed solution, inducing or exacerbating carryover during evaporation. This is not caused by the waste liquid itself directly producing foam, but rather by substances introduced by the waste liquid that alter the colloidal stability of the original solution. Under concentration conditions, foam forms more easily and the strength of the foam film increases; when the demister’s load exceeds its design capacity, liquid droplets are entrained in the vapor phase.

The acid washing process dissolves scale deposits from heat exchange surfaces and chamber walls. The waste liquid contains large amounts of dissolved metal ions—including iron, aluminum, calcium, and magnesium ions—as well as residual acid washing chemicals, corrosion inhibitors, and fine inorganic scale debris that has been stripped away. Once these substances enter the feed tank, they alter the electrical charge state of the colloidal particles within the feed solution. Colloidal suspensions that were originally stable or about to settle following pretreatment flocculation and sedimentation encounter large amounts of metal cations. The charges on the colloidal particles are neutralized, preventing direct sedimentation and instead leading to the formation of a large number of stable, finely dispersed particles. These particles accumulate at the gas-liquid interface, acting as the skeletal framework of the foam and enhancing the toughness of the foam liquid film, making the foam less likely to collapse on its own. When introduced into the vacuum evaporation environment of the evaporator, boiling generates bubbles, resulting in a fine, dense, and persistent foam that is resistant to collapse.

In normal feedstock, bubbles form and quickly collapse, so a thick foam layer does not form on the liquid surface; however, when acid washing waste liquid is mixed in, the resulting foam is dense and viscous. Even if the vacuum level and evaporation temperature remain unchanged, the thickness of the foam layer increases significantly. As the foam comes into direct contact with the demister, a large amount of liquid phase is entrained by the gas flow—a phenomenon known as “evaporation with entrained liquid.”

Residual acid washing corrosion inhibitors in the waste liquid are also a major contributing factor. Many acid washing formulations inherently contain surfactant-based corrosion-inhibiting components; even a small amount entering the feed solution reduces the liquid’s surface tension, further promoting foam formation. Even if a water rinse is performed after acid washing, trace amounts of the chemical are carried into the system. Over time, through repeated recirculation and accumulation, the chemical concentration gradually increases, causing the foaming problem to become progressively more severe.

Additionally, there is the continuous accumulation of fine solid debris. Tiny particles of silicates and carbonates shed from the plate surfaces remain suspended in the feed solution, acting as solid-phase stabilizers that reinforce the foam film. Simply adding defoamers has limited effectiveness against this type of foam, as it is not generated by a single surfactant but rather by a foam system jointly stabilized by colloidal and solid particles. Defoamers can only temporarily suppress the foam; after a period of operation, the foam will reappear.

This issue exhibits a time lag; material leakage does not occur immediately after a single recirculation. When small amounts of waste liquid are recirculated, impurities continuously accumulate in the feed tank, and the problem only becomes apparent after multiple evaporation cycles. On-site, it is common to observe that everything runs normally at the start of operation following acid washing, but after half a day to one or two days, foam levels gradually rise and suspended solids in the condensate increase. Often, the root cause—the recirculation of acid washing waste liquid—is not immediately identified.

As a secondary effect, repeated pH fluctuations can also exacerbate the problem. Since acid washing waste liquid is acidic, mixing it with the original solution causes the pH in the tank to drop. Subsequent addition of alkali to adjust the pH results in the formation of new hydroxide flocs through acid-base neutralization, introducing a large number of fine suspended particles that further exacerbate the foam stabilization effect. The repeated cycle of acid washing, recirculation, and pH adjustment causes impurities to continuously accumulate in the original solution tank, making foam and carryover increasingly difficult to control.

Many facilities fall into the misconception that acid-washing waste liquid is merely acidic wastewater that can be simply evaporated in an evaporator to remove water, overlooking the resulting changes in colloidal and foaming properties. Once such problems have arisen, it is difficult to eliminate carryover by merely adjusting the vacuum or evaporation temperature; it is necessary to partially drain and flush the stock tank to reduce the accumulated metal ions and fine solid impurities inside the tank. At the process operation level, acid pickling waste liquid should be treated as a separate waste stream and must not be returned to the feed tank. After acid pickling is complete, the chamber should be thoroughly rinsed; it is also not recommended to return the rinse water to the feed system. This prevents the continuous accumulation of impurities from altering the foaming characteristics of the feed solution at the source.