Metallurgy and Industrial Extraction of Chlorine (Cl)

Natural Occurrence & Major Sources

Chlorine is a highly reactive halogen and does not occur in its elemental form in nature. It is found exclusively as chloride ions (Cl⁻) in various ionic compounds.

1. Seawater: The most abundant source, containing approximately 2.7% (w/v) sodium chloride (NaCl) along with smaller amounts of other metal chlorides (e.g., MgCl₂, CaCl₂).
2. Brine Wells: Underground deposits of concentrated salt solutions.
3. Rock Salt (Halite): Solid deposits of sodium chloride (NaCl).
4. Carnallite: A double salt, KCl·MgCl₂·6H₂O.
5. Sylvite: Potassium chloride (KCl).

For industrial extraction of elemental chlorine, sodium chloride is the primary raw material due to its abundance and ease of electrolysis.

Concentration of the Source Material

Unlike metallic ores that require physical concentration methods, the “concentration” of chloride sources primarily involves obtaining a pure, concentrated brine solution suitable for electrolysis.

1. From Seawater/Brine Wells:

   • Solar Evaporation: Seawater or dilute brine is allowed to evaporate in large shallow ponds, leading to the crystallization of crude sodium chloride.
   • Brine Purification: The crude salt is then dissolved in water or direct brine from wells is purified. This step is crucial to remove impurities like Ca²⁺, Mg²⁺, and Fe³⁺ ions. These ions can interfere with the electrolytic process (e.g., by precipitating as hydroxides, fouling membranes, or forming undesirable byproducts).
   • Chemical Treatment: Impurities are typically removed by adding sodium carbonate (Na₂CO₃) to precipitate calcium carbonate (CaCO₃) and sodium hydroxide (NaOH) to precipitate magnesium hydroxide (Mg(OH)₂). Filtration follows to remove these precipitates.

2. From Rock Salt:

   • Rock salt deposits are mined and then dissolved in water to form a saturated brine solution, which is subsequently purified as described above.

Reduction to Elemental Chlorine (Industrial Extraction)

Elemental chlorine is industrially produced almost exclusively by the electrolysis of aqueous sodium chloride solution (brine) or molten sodium chloride. The process is commonly known as the Chlor-Alkali Process, as it co-produces chlorine and an alkali (sodium hydroxide).

There are three main types of electrolytic cells used:

1. Diaphragm Cell (e.g., Nelson Cell, Hooker Cell)

• Principle: Electrolysis of aqueous NaCl solution using a porous diaphragm (historically asbestos, now often polymer-based) to separate anode and cathode compartments, preventing mixing of products.
• Setup: Anode compartment contains graphite or dimensionally stable anodes (DSA, e.g., RuO₂/TiO₂ coated titanium) immersed in brine. The cathode is a perforated steel plate. The diaphragm separates the two compartments.
• Reactions:
  • At Anode (Oxidation): Chloride ions are oxidized. 2Cl⁻(aq) → Cl₂(g) + 2e⁻
  • At Cathode (Reduction): Water is reduced in preference to Na⁺ ions (due to high overpotential for Na deposition in aqueous solution). 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
  • Overall Reaction: 2NaCl(aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2NaOH(aq)
• Products: Chlorine gas (at anode), hydrogen gas (at cathode), and a dilute solution of sodium hydroxide (containing unreacted NaCl). The NaOH solution requires further concentration and purification to remove salt.
• Advantages: Less energy-intensive than mercury cells; avoids mercury pollution.
• Disadvantages: Produces less pure and dilute NaOH solution, requiring further processing.

2. Mercury Cell (Castner-Kellner Cell)

• Principle: Electrolysis of aqueous NaCl solution using a flowing mercury cathode, which forms an amalgam with sodium. This separates the production of NaOH from the main electrolytic cell, allowing for higher purity products.
• Setup: Anodes (graphite or DSA) are suspended in brine above a flowing layer of mercury acting as the cathode. The cell is tilted slightly.
• Reactions:
  • At Anode (Oxidation): 2Cl⁻(aq) → Cl₂(g) + 2e⁻
  • At Mercury Cathode (Reduction): Sodium ions are reduced and dissolve in mercury to form sodium amalgam. Na⁺(aq) + e⁻ + Hg(l) → Na-Hg(l) (sodium amalgam)
  • Decomposer (Secondary Cell): The sodium amalgam flows into a separate decomposer, where it reacts with water over a graphite catalyst to produce high-purity NaOH and H₂. Mercury is regenerated and recycled. 2Na-Hg(l) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + 2Hg(l)
• Products: High-purity chlorine gas, high-purity and concentrated sodium hydroxide solution, and hydrogen gas.
• Advantages: Produces very pure and concentrated NaOH.
• Disadvantages: High energy consumption; significant environmental concerns due to mercury leakage, leading to its phasing out in many regions.

3. Membrane Cell (Ion-Exchange Membrane Cell)

• Principle: Utilizes a cation-exchange membrane (e.g., Nafion) to selectively allow Na⁺ ions to pass from the anode compartment to the cathode compartment, while preventing the migration of Cl⁻ and OH⁻ ions. This ensures separate, high-purity products.
• Setup: The cell is divided into two compartments by a cation-exchange membrane. Brine is fed to the anode compartment (DSA anode). Pure water is fed to the cathode compartment (nickel cathode).
• Reactions:
  • At Anode (Oxidation): 2Cl⁻(aq) → Cl₂(g) + 2e⁻
  • At Cathode (Reduction): Water is reduced. 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
  • Membrane Function: Na⁺ ions migrate through the membrane from the anode compartment to the cathode compartment, where they combine with OH⁻ ions to form NaOH.
  • Overall Reaction: 2NaCl(aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2NaOH(aq)
• Products: High-purity chlorine gas, high-purity and concentrated sodium hydroxide solution, and hydrogen gas.
• Advantages: Most environmentally friendly and energy-efficient method. Produces very pure and concentrated NaOH. Widely adopted in modern industrial plants.

Deacon’s Process (Historical/Minor)

• This process involves the catalytic oxidation of hydrogen chloride (HCl) gas with air (oxygen) to produce chlorine.
• 4HCl(g) + O₂(g) ⇌ 2Cl₂(g) + 2H₂O(g)
• Catalyst: CuCl₂ at 400-450 °C.
• This is not a primary method for extracting chlorine from its natural sources but rather a way to recover chlorine from byproduct HCl.

Refining and Purification of Chlorine

Chlorine gas directly obtained from electrolytic cells often contains impurities, primarily moisture and sometimes hydrogen gas (especially from diaphragm cells). It needs to be purified and dried for industrial use and safe storage.

1. Cooling: The chlorine gas is first cooled to condense most of the water vapor present.
2. Drying: The cooled chlorine gas is then passed through towers packed with ceramic rings, where it flows counter-currently against concentrated sulfuric acid (H₂SO₄). Concentrated sulfuric acid is a strong dehydrating agent and effectively removes residual moisture from the chlorine gas.
3. Separation of Hydrogen (if present): After drying, the chlorine gas can be further purified. If hydrogen is present, it is usually separated through liquefaction.
4. Liquefaction: The dry chlorine gas is compressed and further cooled to about -40 °C (or lower, depending on pressure) to liquefy it. Hydrogen (and other non-condensable gases) remains gaseous and is separated.
5. Storage: Liquefied chlorine is stored in steel cylinders, containers, or railway tank cars under pressure for transport and industrial use.