Precipitation as Separation
Selective precipitation is one of the oldest and most reliable techniques in hydrometallurgy. By raising the pH of a multi-metal solution with lime or caustic soda, metals can be knocked out of solution one by one, exploiting the vast differences in their hydroxide solubility products. Iron, with its extremely insoluble Fe(OH)₃, falls out first at pH 3–4. Copper follows at pH 5–6, then zinc and nickel at pH 7–9. This pH staircase is the basis for purifying leach solutions before electrowinning.
Solubility Product Chemistry
The solubility product Ksp defines the equilibrium between a solid precipitate and its dissolved ions. For a metal hydroxide M(OH)n, Ksp = [M^n+][OH⁻]^n. This means the residual metal concentration in solution depends on [OH⁻] raised to the power n — trivalent metals like Fe³⁺ are far more sensitive to pH changes than divalent metals like Cu²⁺. The logarithmic relationship between pH and solubility creates the characteristic S-shaped precipitation curves shown in this simulation.
Industrial pH Windows
In zinc plant practice, the pregnant leach solution first passes through an iron precipitation stage at pH 3.5 using limestone, producing a jarosite or goethite residue. The iron-free solution then undergoes cementation (zinc dust purification) to remove Cu, Co, and Cd. Separate circuits use careful pH staging: copper at pH 2–3 with H₂S gas, cobalt at pH 4–5 with α-nitroso-β-naphthol. Each precipitation window must be precisely maintained to maximize selectivity and minimize co-precipitation losses.
Beyond Hydroxide Precipitation
While hydroxide precipitation is the most common method, sulfide precipitation offers superior selectivity. Metal sulfide Ksp values span an even wider range than hydroxides, enabling separation of metals with similar hydroxide precipitation pH values. Copper sulfide (Ksp ~10⁻³⁶) precipitates at pH as low as 1, far below zinc sulfide (Ksp ~10⁻²⁵). This simulator focuses on hydroxide chemistry but the principles extend directly to sulfide, carbonate, and phosphate precipitation systems.