Industrial Wastewater Metal Precipitation Reactor

Influent Metal

Reactor Controls

Lime dose → reactor pH 9.0

Influent concentration 40 mg/L

Mixer speed (flocculation) 45 rpm

Effluent Quality

COMPLIANT

Residual dissolved metal

0.00 mg/L

Discharge limit

1.00 mg/L

Removal efficiency

0.0%

Sludge produced

0.0 kg/d

How it works

Dissolved heavy-metal ions in industrial effluent form insoluble hydroxide precipitates once enough hydroxide ion is present. Adding lime (Ca(OH)₂) or caustic raises the pH, which raises [OH⁻], which drives the equilibrium toward the solid phase — the working principle behind almost every industrial metal-finishing and mine-drainage treatment plant.

M(OH)₂ (s) ⇌ M²⁺ + 2 OH⁻      Ksp = [M²⁺][OH⁻]²
[OH⁻] = 10^(pH − 14)
[M²⁺]_residual = min( C₀ , Ksp / [OH⁻]² )
Removal efficiency = 1 − [M²⁺]_residual / C₀

Each metal has its own hydroxide Ksp, so the pH needed to reach a given discharge limit differs by ion:

Cu²⁺ — Ksp ≈ 2.2×10⁻²₀, precipitates readily above pH ≈ 6.5–7.
Zn²⁺ — Ksp ≈ 3.0×10⁻¹⁵, needs roughly pH 8–9, and can re-dissolve (amphoteric) above pH ≈ 10.5.
Ni²⁺ — Ksp ≈ 5.5×10⁻¹⁶, the hardest of the four to fully precipitate, typically needs pH > 9.5.
Cr³⁺ — trivalent, Ksp ≈ 6.3×10⁻³³ for Cr(OH)₃, precipitates efficiently near pH 8–9.

In the tank, dissolved ions (bright spheres) swirl with the mixer; once pH drives the local solubility below their concentration, a fraction of the swept particles nucleate into grey floc and settle under gravity into the sludge blanket at the bottom, exactly as a full-scale clarifier removes flocculated solids from the clear effluent above. The mixer speed controls how quickly ions are swept into contact with existing floc (flocculation kinetics), not the final residual concentration, which is set purely by pH and Ksp.

Dissolved metal ion Precipitated floc / sludge

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