Explain the principle and procedure involved in the manufacture of washing soda by ammonia soda process. Sketch a well labelled diagram for it. How would you convert washing soda into i) soda ash…

Principle and Procedure of the Ammonia-Soda Process (Solvay Process)

Principle

The ammonia-soda process (also called the Solvay process) is used to manufacture washing soda (sodium carbonate decahydrate, Na₂CO₃·10H₂O) from brine (NaCl solution) and limestone (CaCO₃). The key principle relies on:

1. Solubility differences: Sodium bicarbonate (NaHCO₃) is less soluble in cold water than sodium chloride (NaCl), allowing its precipitation.
2. Recycling of ammonia (NH₃): Ammonia is reused in the process, making it economically viable.
3. Carbon dioxide (CO₂) supply: Obtained from thermal decomposition of limestone (CaCO₃).

Procedure

The process involves the following main steps:

1. Preparation of Brine Solution

   • A saturated solution of sodium chloride (NaCl) is prepared by dissolving rock salt in water.
   • The brine is filtered to remove impurities.

2. Ammoniation of Brine

   • Ammonia gas (NH₃) is passed into the brine solution to form ammonium chloride (NH₄Cl) and sodium hydroxide (NaOH) in solution: $$\mathrm{NH}{}_3+\mathrm{H}{}_2\mathrm{O}+\mathrm{NaCl}\to \mathrm{NH}{}_4\mathrm{Cl}+\mathrm{NaOH}$$
   • This step increases the concentration of OH⁻ ions, which react with CO₂ to form sodium bicarbonate (NaHCO₃).

3. Carbonation

   • Carbon dioxide (CO₂) is bubbled through the ammoniated brine. The CO₂ reacts with NH₄OH (formed from NH₃ + H₂O) to produce ammonium bicarbonate (NH₄HCO₃), which then reacts with NaCl to form sodium bicarbonate (NaHCO₃): $$\mathrm{NH}{}_3+\mathrm{H}{}_2\mathrm{O}+\mathrm{CO}{}_2\to \mathrm{NH}{}_4\mathrm{HCO}{}_3$$ $$\mathrm{NH}{}_4\mathrm{HCO}{}_3+\mathrm{NaCl}\to \mathrm{NaHCO}{}_3(\mathrm{precipitate})+\mathrm{NH}{}_4\mathrm{Cl}$$
   • Sodium bicarbonate (NaHCO₃) precipitates out due to its low solubility in cold water and is filtered.

4. Calcination of Sodium Bicarbonate

   • The filtered NaHCO₃ is heated strongly to decompose it into sodium carbonate (Na₂CO₃, soda ash) and carbon dioxide (CO₂): $$2\mathrm{NaHCO}{}_3\to \mathrm{Na}{}_2\mathrm{CO}{}_3+\mathrm{CO}{}_2+\mathrm{H}{}_2\mathrm{O}$$
   • The CO₂ produced is recycled back into the carbonation step.

5. Recycling of Ammonia

   • The NH₄Cl solution left after filtration is treated with lime (Ca(OH)₂), which is obtained from the decomposition of limestone (CaCO₃): $$\mathrm{CaCO}{}_3\stackrel{\mathrm{heat}}{\to }\mathrm{CaO}+\mathrm{CO}{}_2$$ $$\mathrm{CaO}+\mathrm{H}{}_2\mathrm{O}\to \mathrm{Ca}(\mathrm{OH}){}_2$$
   • Ca(OH)₂ reacts with NH₄Cl to regenerate NH₃: $$2\mathrm{NH}{}_4\mathrm{Cl}+\mathrm{Ca}(\mathrm{OH}){}_2\to 2\mathrm{NH}{}_3+\mathrm{CaCl}{}_2+2\mathrm{H}{}_2\mathrm{O}$$
   • The NH₃ is recycled back into the ammoniation step.

6. Crystallization of Washing Soda

   • The Na₂CO₃ obtained from calcination is dissolved in water and crystallized to form washing soda (Na₂CO₃·10H₂O).

Flow Diagram of the Ammonia-Soda Process

flowchart TD
    A["Brine (NaCl)"] --> B["Ammoniation (NH₃ added)"]
    B --> C["Carbonation (CO₂ added)"]
    C --> D["NaHCO₃ precipitation"]
    D --> E["Filtration"]
    E --> F["Calcination (heat) → Na₂CO₃"]
    F --> G["Crystallization → Washing Soda (Na₂CO₃·10H₂O)"]
    D --> H["NH₄Cl solution"]
    H --> I["Treatment with Ca(OH)₂"]
    I --> J["NH₃ regeneration"]
    J --> B
    E --> K["Limestone (CaCO₃) → CaO → Ca(OH)₂"]
    K --> I

Conversion of Washing Soda into Other Compounds

i) Conversion of Washing Soda into Soda Ash (Na₂CO₃)

• Washing soda (Na₂CO₃·10H₂O) is heated strongly to remove water of crystallization, yielding anhydrous sodium carbonate (soda ash, Na₂CO₃): $$\mathrm{Na}{}_2\mathrm{CO}{}_3\cdot 10\mathrm{H}{}_2\mathrm{O}\stackrel{\mathrm{heat}}{\to }\mathrm{Na}{}_2\mathrm{CO}{}_3+10\mathrm{H}{}_2\mathrm{O}$$
• This is a dehydration process where the hydrated form loses water molecules.

ii) Conversion of Washing Soda into Sodium Silicate (Na₂SiO₃)

• Washing soda reacts with silica (SiO₂) (sand) at high temperatures (~1400°C) to form sodium silicate (Na₂SiO₃): $$\mathrm{Na}{}_2\mathrm{CO}{}_3+\mathrm{SiO}{}_2\stackrel{\mathrm{heat}}{\to }\mathrm{Na}{}_2\mathrm{SiO}{}_3+\mathrm{CO}{}_2$$
• This reaction is used in the manufacture of glass and silicates.

Important Uses of Washing Soda (Na₂CO₃·10H₂O)

1. Detergent Industry: Used in the manufacture of soaps and detergents due to its ability to soften water by precipitating calcium and magnesium ions.
2. Glass Manufacturing: Acts as a flux in glass production, lowering the melting point of silica.
3. Paper Industry: Used in the paper-making process to adjust pH and remove impurities.
4. Textile Industry: Helps in boiling off sizes (removing starch and other impurities from fabrics).
5. Water Softening: Removes temporary hardness by precipitating calcium and magnesium carbonates.
6. Food Industry: Used as a food additive (E500) in baking powder and as a raising agent.
7. Chemical Manufacturing: Used in the production of borax, caustic soda (NaOH), and other sodium compounds.
8. Laboratory Reagent: Used in titrations and qualitative analysis due to its basic nature.