I. During continuous evaporation operations, three conditions prevent the formation of silica scale:

Continuous high-velocity shear maintains a stable colloidal dispersion.

The circulation pump continuously flushes the heat exchange plates and chambers at high speed; the fluid shear force breaks the hydrogen bonds between the hydroxyl-terminated silica colloids, preventing the colloidal particles from adsorbing onto each other and agglomerating. Micron-sized silica flocs remain suspended in the liquid phase and cannot adhere to the walls to coalesce into scale.

Negative Pressure and Low Temperature + Abundant Water Molecules Ensure a Stable Silica Hydration Structure

Evaporation occurs at low temperatures of only 40–60°C, significantly lower than those in high-temperature evaporation under atmospheric pressure; the system is filled with saturated water vapor, and the outer layer of silica colloids is enveloped by a complete hydration film. The shared negative charge creates electrostatic repulsion, preventing destabilization and flocculation.

At the same time, continuous steam generation maintains a uniform liquid-phase environment, eliminating localized areas of water loss and concentration.

Trace amounts of chelating agents provide a dispersing and protective effect

Residual chelating agents such as EDTA and citric acid in electroplating rinse water adsorb onto the surface of silica colloids, further preventing particle collisions, delaying the polymerization of siloxane bonds, and inhibiting the formation of silica gel.

II. After shutdown and standing, all stabilizing conditions disappear, leading to the concentrated and massive precipitation of silica scale

1. Water flow ceases, shear force completely disappears, and colloidal particles collide freely to polymerize

When the circulation pump stops, there is no flushing; silica colloidal particles continue to collide via Brownian motion. Their surface hydration films fuse together, and dehydration condensation occurs between Si-OH groups, rapidly forming three-dimensional, network-like silica gel flocs.

2. After breaking the vacuum, the liquid surface at atmospheric pressure continues to lose water, leading to localized supersaturation.

If the vacuum is broken after shutdown, the liquid surface comes into contact with air, causing the surface water to slowly evaporate. At the liquid surface, tank walls, and heat exchange plate interfaces, the silicon concentration instantly exceeds the threshold, leading to the preferential precipitation of a slippery silicon sludge;

even if negative pressure is maintained for thermal insulation, the absence of circulation will result in natural sedimentation, with colloidal particles slowly depositing at the bottom and on the plates.

3. Temperature Gradient-Induced Stratification, Exacerbating Silica Gel Instability

After shutdown, without forced circulation, a significant temperature difference develops between the upper and lower parts of the tank. In the lower-temperature regions, the solubility of silica decreases, making it easier for silica gel to precipitate from the liquid phase and adhere to the corrugated dead corners of the heat exchange plates, which are at lower temperatures.

4. Complexing Agent Degradation, Loss of Dispersion Protection

During the standstill period, small amounts of organic chelating agents slowly oxidize and decompose. The protective layer originally enveloping the silica particles disappears, causing the colloids to lose their electrostatic repulsion and rapidly flocculate into clumps.

5. High salt ions compress the double layer, accelerating silica gel precipitation

The concentrated mother liquor has high salinity. A large number of metal cations compress the negative double layer on the surface of the silica colloids, significantly reducing the repulsive forces between particles and making them highly prone to agglomeration, forming inorganic composite scale.

III. Distinguishing by Visible Phenomena

Continuous operation: The heat exchange plates are clean, the pressure drop rises slowly, and only a small amount of fine suspended flocs is present;

After standing idle overnight or for half a day: The chamber and plate heat exchange surfaces are covered with a layer of milky-white, slippery, soft silica gel scale that can only be removed by acid washing; upon restarting, flow imbalance and foaming are highly likely to occur.