Question 6 of 6: Electrochemical vocabulary and zinc electrowinning energy consumption

Problem 6 — Electrochemical vocabulary and zinc electrowinning energy consumption (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Definitions and industrial examples.

Electrorefining. Electrolytic purification of an already-metallic, impure anode: the impure metal dissolves anodically and re-deposits as pure metal at the cathode, leaving insoluble impurities as slimes. Example: copper electrorefining, impure blister/fire-refined Cu anodes → 99.99% LME-grade cathode copper.

Electrowinning. Electrolytic deposition of metal from a purified leach solution onto an inert or starter-sheet cathode, with an inert anode evolving a gas (usually O₂). Example: zinc electrowinning from purified ZnSO₄ leach liquor (Problem 6b), or copper electrowinning from dilute acid-leach/SX raffinate.

Electroplating. Depositing a thin, adherent, usually decorative or protective metal coating onto a (often dissimilar-metal) substrate cathode from a plating bath, typically at low current density for a smooth, uniform deposit. Example: chromium or nickel plating of automotive trim onto a steel substrate.

Overpotential. The extra potential, beyond the thermodynamic (Nernst) equilibrium value, that must be applied at an electrode to drive a given current — the sum of activation, concentration and (where relevant) resistance/IR-drop contributions. Example: the O₂ evolution overpotential at a Pb-alloy anode in zinc electrowinning is the single largest component of the cell voltage in Problem 6b, well above the thermodynamic $E_{cell}$.

Slimes (anode slimes/mud). The insoluble residue (noble metals, selenides/tellurides, etc.) that collects beneath the anode in electrorefining, since these impurities are not oxidized/dissolved at the operating anode potential. Example: copper-refinery anode slimes are the primary industrial source of by-product silver, gold and selenium.

Current efficiency. The fraction of total charge passed that actually deposits the desired product, versus charge lost to side reactions (chiefly H₂ evolution at the cathode in aqueous electrowinning). Example: the 95% current efficiency given for Problem 6b's zinc cell means 5% of the current is wasted evolving hydrogen rather than depositing Zn.

Current density. Current per unit electrode (usually cathode) area, $j=I/A$, the key design/operating variable controlling deposit quality, cell voltage and throughput. Example: zinc electrowinning cathodes typically run 400-600 A/m²; too high a current density degrades deposit morphology and current efficiency.

(b) Given.

Find. Specific energy consumption, in kWh per kg of Zn deposited.

Approach. Faraday's law gives the theoretical charge per mole of Zn as $nF$; a current efficiency below 100% means MORE charge than $nF$ must actually be passed to deposit one mole, so divide by $\eta$. Multiply by the cell voltage for energy, scale to a kilogram, and convert joules to kWh.

1. Actual charge per mole of Zn. $$Q_{\text{mol}}=\frac{nF}{\eta}=\frac{2\times96{,}485}{0.95}=203{,}126\ \text{C/mol}$$
2. Energy per mole, then per kg. $$E_{\text{mol}}=V\cdot Q_{\text{mol}}=3\times203{,}126=\boxed{609{,}378.9\ \text{J/mol}}$$ $$E_{\text{kg}}=E_{\text{mol}}\times\frac{1000\ \text{g/kg}}{M}=609{,}378.9\times\frac{1000}{65.4}=\boxed{9{,}317{,}721\ \text{J/kg}}$$
3. Convert to kWh/kg. $$E_{\text{specific}}=\frac{9{,}317{,}721}{3.6\times10^{6}\ \text{J/kWh}}=\boxed{2.59\ \text{kWh/kg Zn}}$$ This sits at the lower end of, but consistent with, typical reported industrial zinc electrowinning specific energy consumption (≈3.0-3.5 kWh/kg), which additionally accounts for busbar/rectifier losses and a somewhat higher practical cell voltage not included in this simplified calculation.