I. Dilute Solution Stage: Chelating Agents Inhibit Crystallization and Reduce Short-Term Hard Scale

When the original solution is at a low concentration, trace amounts of chelating agents such as EDTA, tartaric acid, and citric acid preferentially form stable, water-soluble chelates with metal ions such as nickel, copper, calcium, and magnesium. This significantly reduces the concentration of free scale-forming ions, lowers the solution’s supersaturation, and makes it difficult for salt crystals to nucleate and grow rapidly.

At the same time, chelating agents adsorb onto the surfaces of nascent microcrystals, causing lattice distortion; as a result, the crystals cannot grow in an orderly manner and are less likely to aggregate and deposit. Furthermore, electrostatic repulsion keeps the fine crystal particles suspended in the liquid phase, making it difficult for them to adhere to the heat exchange plates. This significantly delays the formation of hard salt scale in the plate-and-frame channels and slightly extends the cleaning cycle.

II. Mid-to-Late Concentration Stage: Once salt saturation is reached, the complexation equilibrium is disrupted, rapidly accelerating wall scaling.

As water continues to evaporate, the salinity of the mother liquor rises steadily, and the ionic strength of the system increases sharply. High concentrations of chloride ions and sodium salts compress the colloidal double layer, disrupting the stable structure of the complexes. A large number of trapped metal ions are released, instantly reaching severe supersaturation, leading to an explosive formation of crystal nuclei within a short period.

This forms sticky, composite soft scale with extremely strong adhesion.

The desorbed salt crystals become entangled with organic molecules from the complexing agents and silicate colloids, forming a slippery, viscous composite organic-salt scale. Unlike simple inorganic salt crystals, which are easily washed away by circulating water flow, this scale adheres firmly to dead corners of corrugated heat exchangers and the wet-dry interface at the liquid surface, accumulating to ever-greater thickness.

Scale layers continue to thicken at the wet-dry interface

In the area where the liquid level fluctuates, the scale layers intermittently dehydrate and solidify, gradually transforming the soft sludge into dense, composite hard scale that clogs the narrow flow channels of the plates. The rate of pressure drop increase is significantly higher than that of the parent solution without chelating agents.

Scale formation worsens significantly during shutdown and standstill

During shutdown, the absence of water flow and shear forces eliminates the dispersing and suspending effects, causing complex flocs to rapidly settle and accumulate at the bottom of the chamber and on the lower half of the heat exchange plates. These deposits are difficult to completely flush out upon restart, leading to a continuous increase in scale thickness due to long-term accumulation.

III. Associated Secondary Issues: Increased Foaming Indirectly Exacerbates Material Loss and Secondary Scaling

Trace amounts of chelating agents are organic surfactants that significantly increase foam stability. The fine, dense foam generated by vacuum boiling is difficult to break up and easily overflows the demister screen, resulting in salt-laden mist entrainment.

This salt-containing organic mist is carried into the condensation system. When the condensate is reused and returned to the feed tank, the chelating agent continues to circulate and accumulate within the system. In this vicious cycle, wall scaling and foaming problems will continue to worsen over time. At the same time, organic tail gas enters the vacuum pump, accelerating the emulsification and degradation of the pump oil. Fluctuations in vacuum further intensify liquid surface turbulence, once again increasing the likelihood of crystal adhesion.