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Recovering zinc from galvanising waste water

Selective precipitation by pH, complex ions, amphoteric hydroxides, electrolytic recovery

Waste water from a galvanising plant contains 0.0100 mol L

^{-1}

\ce{Zn^2+}

and 0.0100 mol L

^{-1}

\ce{Mg^2+}

. The company wants to remove the zinc for recovery while leaving the magnesium in solution.

K_s

\ce{Zn(OH)2}

3.0\times10^{-17}

;

\ce{Mg(OH)2}

5.60\times10^{-12}

. p

K_a

(

\ce{NH4+}

) = 9.24.

E°

(V):

\ce{Zn^2+/Zn}

−0.76;

\ce{H2O/H2}

−0.83;

\ce{O2/H2O}

+1.23.

Facts you may need

Ks and Q
pH ↔ [OH⁻]
complex ions
electrolysis E° arguments

(a)

Calculate the pH at which each hydroxide begins to precipitate.

(b)

The pH is adjusted to 8.50. Show that

\ce{Mg(OH)2}

does not precipitate, and calculate the concentration and percentage of

\ce{Zn^2+}

remaining in solution.

(c)

Evaluate a technician's proposal to hold the pH at 8.50 with an

\ce{NH3}

\ce{NH4Cl}

buffer: calculate the

\ce{NH3}

\ce{NH4+}

ratio required, then judge whether the method would achieve the separation.

(d)

Describe what is observed as NaOH(aq) is added drop by drop until it is in large excess, writing equations. Explain why controlling the pH carefully matters even without ammonia.

(e)

The

\ce{Zn(OH)2}

is redissolved in acid and zinc metal is recovered by electrolysis with inert electrodes. Discuss, using

E°

values, which species is reduced at the cathode, write the electrode equations, and calculate the minimum voltage required.