The trace amounts of silicate colloids in the feed solution do not immediately form hard scale during continuous low-temperature evaporation operation; however, once the unit is shut down and left to stand, they precipitate and form scale in large quantities. The key factor is that the stability conditions of the colloids are completely altered between the operational and stationary states—the colloids are not actually removed through evaporation, but are merely suppressed and kept in suspension.

During the continuous negative-pressure evaporation phase of the unit, the feed solution is constantly circulated and sprayed. The continuous fluid flow creates shear agitation, keeping the silicate colloidal particles uniformly dispersed throughout the feed solution system, making it difficult for them to collide, agglomerate, and settle. Even as the solution is continuously concentrated, the colloidal particles remain in constant motion, making it difficult for them to adhere and solidify on the heat exchange plates or chamber walls. At the same time, the system contains some metal ions and a small amount of complexing agents, which adsorb onto the surfaces of the colloidal particles, forming a hydrated protective film. This film prevents the colloids from sticking together, allowing them to remain suspended in the concentrated solution for an extended period.

Under low-temperature, vacuum evaporation conditions, the system temperature is lower than that of high-temperature evaporation. The polymerization reaction rate of silicon is inherently slow, and the dynamic circulation conditions further delay the process of siloxane bond polymerization leading to the formation of silica scale. Although the concentration continues to rise and the colloidal particles accumulate in increasing numbers, remaining in the liquid phase as a suspended slurry, no obvious precipitates are sometimes visible to the naked eye, and no significant scaling is observed on the heat exchange surfaces. This can lead to the mistaken belief that silicon has not precipitated. In reality, however, the silica colloid in the liquid phase is already in a supersaturated state; it simply has not completed precipitation and solidification due to insufficient kinetic conditions.

When the equipment is shut down and left to stand, with the circulation pump stopped and flow shear completely eliminated, the previously suspended colloidal particles lose fluid agitation and can freely collide and come into contact with one another. The temperature of the feed solution gradually drops from the evaporation operating temperature to room temperature, causing a shift in the system’s chemical equilibrium and a decrease in the stability of the hydrating protective film on the colloidal surfaces. The supersaturated silicate colloids begin to polymerize; tiny particles agglomerate and grow larger, transitioning from a suspended state to precipitation, where they settle directly and adhere to the heat exchange channels, the bottom of the chamber, and the inner walls of the spray pipes.

After shutdown, the pH of the feed solution also undergoes a slight change. When the vacuum is released and the solution comes into contact with air, it absorbs carbon dioxide from the atmosphere, causing the pH to drop slightly. This further accelerates the destabilization and polymerization of the silicate colloids, leading to the formation of large amounts of siliceous scale. This scale is hard in texture and has strong adhesion; once formed, it is difficult to flush back into the liquid phase solely through circulation after restarting the equipment, and it adheres firmly to the inner walls of the equipment.

A very common phenomenon observed on-site is that during continuous batch processing, the equipment’s pressure differential and heat transfer conditions remain relatively normal. However, whenever the system is shut down and left idle overnight, a layer of grayish-white silicate scale is found on the chamber and plate surfaces upon restarting the next day. Many operators mistakenly believe this scaling occurs during operation, when in fact it forms exclusively during the shutdown phase.

To address these operating conditions, minimize prolonged periods of static holding with material inside the equipment. Do not leave concentrated solution in the equipment overnight after a batch is complete; discharge the material as soon as concentration is finished, and perform immediate flushing if necessary. If a brief shutdown is unavoidable, maintain low-speed circulation of the material to prevent it from becoming completely stationary, thereby inhibiting the aggregation and precipitation of silicate colloids.