I. Residual Organic Polymer Decolorizing Agents (The Primary Cause of Wall Adhesion)

The chemical itself possesses a highly viscous, long-chain structure.

Industrial-grade decolorizing agents are essentially high-viscosity liquid cationic polymers with extremely long molecular chains rich in amino and quaternary ammonium cationic groups; the viscosity of the undiluted solution can reach tens of thousands of cps at room temperature. During normal coagulation, these agents adsorb dyes, causing them to aggregate and settle; However, if sedimentation in the settling tank is incomplete, trace amounts of the polymer dissolve into the effluent and enter the evaporator for continuous concentration. As water continuously evaporates, the polymer loses the restraint of the aqueous solvent, causing the molecular chains to intertwine and cross-link, directly forming a gel-like, viscous paste that adheres firmly to the walls of the heat exchanger cylinders and spray pipes—unlike inorganic salts, which form loose crystals.

Forming stable, viscous complexes with dyes and colloids

The cationic chains of the decolorizing agent form electrically neutral complexes with negatively charged reactive/acidic dyes, sizing agents, printing colloids, and oils in the wastewater; at low concentrations, these do not settle but remain uniformly dispersed in the water as nanocolloids. During evaporation and concentration, the concentration of these complexes continues to rise, gradually transforming from a sol to a gel state. They exhibit extremely strong adhesion; once adhered to the surface, they form a soft, sticky sludge layer that cannot be removed by circulation and flushing, accumulating to an ever-increasing thickness.

Low-temperature evaporation conditions accelerate the solidification of polymer gels

Low-temperature evaporation (50–85°C) does not reach the polymer decomposition temperature (above 280°C), so the chemical does not thermally decompose but continues to dehydrate and polycondense; the liquid film on the heat exchange surface is thin and locally supersaturated, causing the gel to precipitate preferentially on the heat transfer surface; the liquid level at the wet-dry interface repeatedly concentrates, leading to continuous gel accumulation and the formation of band-like slime deposits on the walls.

Even trace residues can trigger a chain reaction of increased adhesion

Excessive dosing of pre-treatment decolorizing agents, insufficient settling time, and floating sludge can result in effluent containing only tens of mg/L of residual polymer. When this enters the evaporation system and is concentrated dozens of times, the system viscosity rises sharply, the flowability of the circulating fluid deteriorates, the boundary layer on the tube walls thickens, and the rate of fouling accelerates exponentially.

II. Residual inorganic coagulant-decolorizing agents (PAC, polymeric ferric sulfate) synergistically intensify fouling

Hydrolysis forms a viscous floc base of hydroxides

Residual PAC and iron salts entering the evaporator undergo continuous hydrolysis under heat, generating amorphous colloids of Al(OH)₃ and Fe(OH)₃. These hydroxides are inherently paste-like, viscous precipitates that first form a thin, sticky base layer on metal surfaces.

Acting as an inorganic framework, they trap organic colloids

Aluminum and iron hydroxide flocs are porous and adsorb organic decolorization gels, dye residues, and printing pastes, forming organic-inorganic composite sludge that combines gel-like viscosity with inorganic hardness. This cannot be completely removed by simple water washing; microcrystals of inorganic salts are embedded within the gel, preventing the scale layer from drying, cracking, or peeling off, allowing it to continue growing and adhering to the walls.

Metal ions intensify interfacial adsorption

Aluminum and iron cations neutralize the weak negative charge on stainless steel surfaces, reducing the repulsive force between the gel and the metal and significantly increasing adhesion strength.

III. Secondary Adhesion Problems Caused by Decolorizing Agent Residues

Foam Entrainment Exacerbates Scale Spread

Residual polymers are strong foam stabilizers. During evaporation, they generate large amounts of fine foam. This foam carries colloidal matter and salt crystals to the vapor zone at the top of the drum. When the foam bursts, the colloidal matter adheres directly to the top drum wall and baffles, causing widespread fouling throughout the entire chamber.

Nozzle Blockage and Flow Imbalance Exacerbate Localized Wall Adhesion

Gel residue adheres to spray nozzles, causing flow deviation and localized dry walls; at these dry wall areas, the gel dehydrates and hardens under high temperatures, forming hard, adhesive scale that cannot be removed by subsequent rinsing.

Continuous Deterioration of Concentrated Mother Liquor Flowability

Polymer gels increase the overall viscosity of the mother liquor, reducing the intensity of circulation turbulence. Insufficient scouring force on the pipe walls prevents newly formed sludge from being carried away by the fluid, leading to continuous deposition and thickening.

IV. Comparison of Water Quality Differences Without Residual Decolorizing Agents

If the wastewater contains no residual decolorizing agents, evaporation will only precipitate loose inorganic salt crystals. These crystals are brittle and easily washed away by the water flow, rarely causing extensive wall adhesion;

However, if the effluent contains trace amounts of decolorizing polymers or flocculated particles, the inorganic salts become encapsulated by colloidal matter, forming plastic sludge. This manifests as a thick layer of viscous, dark brown residue on the tank walls, which adheres in a mud-like state during shutdown cleaning. This is a characteristic phenomenon directly caused by residual decolorizing agents.