Key factors causing abnormal solid-liquid separation, failure to separate, suspended flocs, and a sticky, turbid residue in the solid-liquid separation of heavy metal chelation wastewater treated by low-temperature evaporation

1. The chelating agents themselves have a strong stabilising effect, preventing the precipitation of heavy metal hydroxides

EDTA, DTCR, organophosphates and polymeric chelating agents in heavy metal chelation wastewater form stable, water-soluble chelate complexes with heavy metals.

Conventional alkali precipitation allows for separation, but low-temperature evaporation merely concentrates the water content without breaking the complexes;

the complexes remain in a dissolved or colloidal state and do not spontaneously aggregate and settle, naturally preventing normal solid-liquid separation.

2. Gentle low-temperature evaporation: no thermal complex breakdown, no conditions for flocculation and maturation

Conventional high temperatures can partially thermally decompose weak chelates, but **low temperatures of 30–55°C** are too mild:

Insufficient to break the chelate bonds

Lacks the high temperatures required to promote floc aggregation and dense maturation

It merely evaporates the water, making the colloids increasingly stable as they become more concentrated

The more concentrated it becomes, the more it behaves as a uniform colloidal suspension, never settling or separating.

3. A sharp increase in salt content after concentration leads to colloidal protection and steric hindrance

Following evaporation and concentration, the total dissolved salt content rises significantly. A high-salt environment will:

Provide a double-layer protection for colloidal particles

Increase repulsive forces between particles, preventing them from aggregating into large flocs

Keep fine particles suspended indefinitely, preventing sedimentation and compaction

This manifests as a thick, viscous paste with no supernatant and an indistinct solid-liquid interface.

4. Residual organic polymers form a viscous network structure that traps the solid phase

Residual flocculants, chelating polymers and surfactants in the wastewater form an organic gel network structure upon concentration,

physically trapping heavy metal particles and salt crystals, suspending them like jelly and completely preventing natural stratification and sedimentation.

5. Localised supersaturation during evaporation leads to the formation of ultrafine crystals that are difficult to settle

Low-temperature evaporation involves slow, gradual crystallisation, resulting in nano- and micron-scale ultrafine crystals.

These particles are extremely fine and settle at a very slow rate; coupled with the protective effect of chelated colloids, they remain suspended for extended periods without stratification, preventing the formation of a compact sludge layer.

6. Concentration of the pH buffering system disrupts the optimal pH range for sedimentation

The chelated wastewater possesses inherent pH buffering properties; upon concentration, the pH shifts and buffering capacity increases,

deviating from the optimal pH range for the precipitation of heavy metal hydroxides. This results in loose, lightweight and fluffy flocs with extremely poor sedimentation compactibility and abnormal stratification.