I. When the unit operates continuously under negative pressure, the colloid remains stably suspended over the long term.

Silicate colloids are nanoscale particles with a homogeneous negative charge on their surfaces; electrostatic repulsion between the particles prevents them from clumping together under normal conditions.

Under conditions of sustained boiling at low temperature and negative pressure, the circulation pump creates stable liquid-phase turbulence, continuously generating water vapor bubbles within the chamber and keeping the entire fluid in constant motion. The continuous shear force of the water flow prevents colloidal particles from coming into close contact with one another or depositing and adhering to the heat exchange plates.

At the same time, during the continuous concentration process, residual complexing agents and trace organic compounds in the feed solution adsorb onto the surface of the silica gel particles, forming a protective layer that further enhances colloidal stability and inhibits particle adhesion.

Even as the solution’s salinity continues to rise, as long as a flowing, boiling environment is maintained, the colloids remain largely dispersed and suspended, preventing the rapid formation of scale deposits on the plates. This is evident in the slow rate of scaling during operation.

II. After Shutdown and Stagnation, Multiple Conditions Change, Leading to the Rapid Collapse of the Colloidal Stability System

Flow disturbance completely disappears, and gravitational settling takes effect

After shutdown, the circulation pump stops operating, the liquid phase comes to a complete standstill, and there is no longer any water flow to continuously disperse the colloidal particles. Nano-silica particles slowly settle downward, gradually aggregating at the bottom of the chamber and in the lower half of the heat exchange plates, where they accumulate in large quantities to form flocculent silica sludge.

The temperature gradually decreases, disrupting the colloidal charge stability system.

After the shutdown and heat retention period ends, the feed solution cools down overall. The hydration film on the surface of the colloidal particles thins, weakening electrostatic repulsive forces; effective collisions between particles become more likely, and condensation reactions occur between silanol groups. Small particles interlock to form larger flocs, which eventually precipitate out of the water.

High salt ion concentrations continuously compress the double layer, leading to flocculation during static holding

Following continuous concentration, the mother liquor contains very high salt levels, and a large number of metal ions continuously compress the outer charge barrier of the colloidal particles. During operation, fluid flow delays the flocculation process; once fluid agitation ceases, the silica gel particles with compressed charges rapidly lose stability, resulting in large-scale flocculation and precipitation.

Local concentration gradients exacerbate colloidal aggregation

Stagnant liquids naturally undergo concentration stratification, with higher salt and colloid concentrations at the bottom, making gelation more likely to occur. The precipitated silica flocs are sticky and slippery with strong adhesion; after restarting the system, simple circulation and flushing alone are often insufficient to completely disperse them. With prolonged and repeated periods of stagnation, the soft silica sludge gradually dehydrates and hardens, transforming into difficult-to-remove silica scale.

III. Distinguishing Easily Confusable On-Site Phenomena

Many operators mistakenly believe that the evaporation process does not produce silica impurities and that silica scale forms only during shutdowns. In reality, silicate colloids are present in the solution throughout the entire process; continuous flow merely delays aggregation and deposition, rather than eliminating the colloids. During shutdowns, the absence of flow disturbance combined with temperature changes disrupts the metastable equilibrium, causing aggregation and precipitation to manifest concentratedly.

IV. Hazards Arising from Derived Operating Conditions

Silica sludge accumulated during repeated shutdowns and standstill periods remains partially suspended in the liquid phase upon restart, clogging spray lines; another portion continues to adhere to heat exchange plates, forming an insulating scale layer that reduces heat transfer efficiency. Furthermore, silica scale readily forms composite scale with salt crystals and organic impurities, significantly increasing the difficulty of removal via acid washing. At the same time, fine silica fibers rise with the airflow, continuously exacerbating clogging of the demister screens and issues with mist carrying solids.

V. Directions for Operational Optimization

Avoid prolonged periods of static idling with the vacuum broken whenever possible; during nighttime shifts, prioritize low-speed circulation mode to maintain temperature and negative pressure, thereby sustaining basic fluid agitation; do not increase the concentration ratio indefinitely to prevent a continuous rise in ionic strength; and enhance flocculation and filtration in the front-end pretreatment stage to reduce the introduction of ultrafine silicate colloids into the evaporator at the source.