Electrochemistry in Industry: Refining, Plating, Aluminium, Chlorine and Hydrogen

Electrolysis built the modern world

Look around: the copper in your house wiring, the aluminium in a pop can or a bike frame, the chlorine that keeps your tap water safe, the shiny chrome on a tap. Each was made or finished by electrolysis. In this lesson you will apply the last two lessons, Electrolytic Cells and Electrolysis and Faraday's Law: The Stoichiometry of Electrolysis, to the big industrial processes, and weigh their benefits against their health and environmental costs.

Electroplating

To coat an object with a metal:

• Make the object the cathode (connected to the negative terminal). Metal ions are reduced onto it.
• Make a bar of the plating metal the anode. It dissolves, replacing the ions that are plated out, so the solution concentration stays nearly constant.
• Use a solution of the plating metal's ions.

Plating protects against corrosion (tin on steel food cans, zinc on steel parts), improves appearance (chrome, silver-plated cutlery), and improves electrical contact (gold-plated connectors in phones and computers). Faraday's law lets the plater control the thickness exactly: current × time sets the mass.

Refining copper

Copper from a smelter is about 99% pure: not good enough for electrical wire, where impurities raise resistance. Electrorefining fixes this:

• Anode: a thick slab of impure copper. Copper is oxidized and dissolves: Cu → Cu²⁺ + 2e⁻.
• Cathode: a thin sheet of pure copper (or stainless steel). Cu²⁺ ions are reduced onto it: Cu²⁺ + 2e⁻ → Cu.
• The voltage is kept low (a fraction of a volt), so only the easiest reactions happen.

What about the impurities? Metals that are stronger reducing agents than copper (iron, nickel, zinc) are oxidized at the anode too, but their ions are weaker oxidizing agents than Cu²⁺, so they stay in solution and are not plated. Metals that are weaker reducing agents than copper (silver, gold, platinum) are not oxidized at all. They fall to the bottom as anode mud, which is collected and refined, often paying for much of the process. Sudbury's nickel-copper ores, for example, also contain platinum-group metals.

A related process, electrowinning, recovers a metal directly from a solution of its ions (for example, zinc or copper dissolved from ore by acid) using an inert anode, so the metal ions are used up and oxygen forms at the anode.

Aluminium: the Hall–Héroult process

Aluminium is the most abundant metal in Earth's crust, but it is locked in very stable compounds. Al³⁺ (−1.66 V) cannot be reduced from water, and pure aluminium oxide melts at over 2000 °C. In 1886, Charles Martin Hall (in the United States) and Paul Héroult (in France) independently found the solution: dissolve aluminium oxide in molten cryolite, Na₃AlF₆, at about 950 °C, and electrolyze it.

• Cathode (carbon lining of the cell): Al³⁺ + 3e⁻ → Al(l). Molten aluminium collects at the bottom.
• Anode (carbon blocks): oxide ions are oxidized, and the oxygen reacts with the carbon, which is slowly used up: C(s) + 2O²⁻ → CO₂(g) + 4e⁻.
• Overall: 2Al₂O₃ + 3C → 4Al + 3CO₂.

The process needs huge currents (hundreds of thousands of amperes per cell) and lots of electricity, so smelters are built near cheap power. Most of Canada's aluminium is made in Québec using hydroelectricity, which gives it a much smaller carbon footprint than aluminium made with coal-fired electricity. The carbon anodes still release CO₂, and companies are developing inert anodes that would release oxygen instead. Recycling aluminium takes only a small fraction (roughly 5%) of the energy needed to make new metal, which is why returning cans pays off.

Chlorine, sodium hydroxide and hydrogen: the chlor-alkali process

Electrolysis of brine (concentrated NaCl solution) gives three products at once:

• Anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine forms rather than oxygen because of oxygen's overvoltage)
• Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
• Overall: 2NaCl(aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2NaOH(aq)

Modern plants use a membrane cell: a membrane lets Na⁺ ions through but keeps the Cl₂ and the OH⁻ apart, because they would react to make bleach. Chlorine disinfects drinking water and makes PVC; sodium hydroxide is used in pulp and paper, soap-making and aluminium-ore processing; hydrogen is a fuel and a raw material. Older plants used mercury cathodes, which released mercury into the environment; under the international Minamata Convention on Mercury, these are being phased out worldwide.

Green hydrogen

Hydrogen made by electrolysis of water (2H₂O → 2H₂ + O₂) using renewable or nuclear electricity is often called green hydrogen. Most hydrogen today is made from natural gas, which releases CO₂. Electrolytic hydrogen can replace it in making ammonia fertilizer, refining and steelmaking, and it can fuel fuel-cell vehicles (see the lesson Fuel Cells, Electric Vehicles and Electrochemical Technologies). Its main drawbacks are cost and the large amount of electricity needed (about 50 kW·h per kilogram of hydrogen in practice, as you can estimate with Faraday's law).

Health, safety and the environment (F1.2)

• Electroplating baths can contain cyanide (in some silver, gold and copper plating) and hexavalent chromium, Cr(VI), in hard-chrome plating. Cr(VI) is a known human carcinogen, and its compounds are strictly regulated in workplaces. Wastewater must be treated before release: cyanide is oxidized to cyanate, and Cr(VI) is reduced to the much less toxic Cr(III).
• Chlorine is a toxic gas, so chlor-alkali plants and water-treatment plants follow strict storage and transport rules.
• Aluminium smelting emits CO₂ and small amounts of very potent fluorinated greenhouse gases (perfluorocarbons) when a cell is upset, and fluoride emissions must be controlled.
• Electricity use is the biggest environmental factor in all of these processes: the same plant has a much smaller footprint when powered by hydro, nuclear, wind or solar electricity.

Careers (A2.1): electrochemical engineers, metallurgists, plating technicians, chemical-process operators, water-treatment operators and environmental compliance specialists all use this chemistry. Many of these jobs are in Ontario and Québec, in mining, refining, automotive and water treatment.