The chlor-alkali industry – industrial process

1. Introduction

• The chlor-alkali industry produces chlorine $(Cl₂),$ hydrogen $(H₂),$ and sodium hydroxide $(NaOH)$ through the electrolysis of brine ($NaCl$ solution).
• Three methods are used:
  • Membrane cell
  • Diaphragm cell
  • Mercury cell

2. Comparison of the Three Industrial Processes

Feature	Membrane Cell	Diaphragm Cell	Mercury Cell
Purity of $Cl₂$ produced	High	Medium ($NaOH$ contamination)	Low
Purity of $NaOH$ produced	High	Medium (contains $NaCl$)	Low
Energy Consumption	Lowest	Relatively low	Very high
Environmental Impact	Lowest (no harmful by-products)	Medium (waste brine)	High (toxic mercury pollution)
Health Risks	Lowest	Asbestos risk (if asbestos diaphragm used)	High (mercury poisoning risk)

3. The Membrane Cell Process

• Most commonly used method due to low environmental impact.

• Uses a cationic membrane that allows only positive ions $(Na⁺)$ to pass.

• Reaction at the electrodes:

  • Anode (oxidation): $2Cl^- \rightarrow Cl_2 + 2e^-$

  • Cathode (reduction): $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$

  • Overall reaction: $2NaCl + 2H_2O \rightarrow 2NaOH + H_2 + Cl_2$

• Advantages:

  • Produces pure NaOH.
  • No mercury or asbestos risks.
  • Lowest energy consumption.

4. Key Takeaways

• The membrane cell is the most environmentally friendly method.
• The mercury cell is highly toxic and rarely used today.
• Chlorine, hydrogen, and $NaOH$ are all industrially valuable products.
• Electrolysis of brine is the core process of the chlor-alkali industry.