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Chloralkali Process

1890

The chloralkali process is an industrial method for the electrolysis of sodium chloride (NaCl) solution, known as brine. It is the primary source for producing chlorine (Cl₂), sodium hydroxide (NaOH), and hydrogen (H₂), which are essential commodity chemicals. Modern methods use a membrane cell to separate the anode and cathode products, ensuring high purity and efficiency.

The chloralkali process is one of the largest-scale applications of electrolysis, fundamental to the modern chemical industry. The overall reaction is [latex]2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2[/latex]. The process takes place in an electrolytic cell with a brine solution as the electrolyte. At the anode, chloride ions are oxidized to form chlorine gas: [latex]2Cl^- \rightarrow Cl_2 + 2e^-[/latex]. At the cathode, water is reduced to form hydrogen gas and hydroxide ions: [latex]2H_2O + 2e^- \rightarrow H_2 + 2OH^-[/latex].

A critical aspect of the process is keeping the products, chlorine and sodium hydroxide, separate, as they would otherwise react to form sodium hypochlorite and chlorate. Historically, this was achieved using mercury cells or diaphragm cells, but these have been largely phased out due to environmental concerns (mercury pollution) and lower efficiency. The modern standard is the membrane cell. This cell uses a cation-exchange membrane (typically made of a perfluorinated polymer like Nafion) that separates the anode and cathode compartments. The membrane is permeable to positive ions like Na⁺ but impermeable to negative ions like Cl⁻ and OH⁻. Sodium ions migrate across the membrane from the anode compartment to the cathode compartment, where they combine with the hydroxide ions produced at the cathode to form high-purity sodium hydroxide.

This technology significantly improved the energy efficiency and product purity of the chloralkali process while eliminating the environmental hazards associated with mercury. The co-production of three major chemicals makes the economics of the process complex, as the market demand for chlorine and caustic soda (NaOH) must be balanced.

UNESCO Nomenclature: 3305
– Chemical engineering

Type

Industrial Process

Disruption

Revolutionary

Utilisation

Widespread Use

Precursors

  • humphry davy’s initial electrolysis of brine
  • invention of the dynamo for large-scale electricity generation
  • development of early diaphragm and mercury cell technologies
  • faraday’s laws of electrolysis

Applications

  • production of pvc plastic (from chlorine)
  • water purification and disinfectants (chlorine and sodium hypochlorite)
  • pulp and paper manufacturing (bleaching)
  • production of soaps, detergents, and textiles (sodium hydroxide)
  • chemical synthesis for a vast range of products

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Related to: chloralkali process, electrolysis, chlorine, sodium hydroxide, caustic soda, brine, membrane cell, industrial chemistry

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Historical Context

(if date is unknown or not relevant, e.g. "fluid mechanics", a rounded estimation of its notable emergence is provided)

Related Invention, Innovation & Technical Principles

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