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Solvay Process for the Production of Sodium Carbonate

Key Concepts

  • Sodium carbonate, Na2CO3, has a number of uses but its most common use is in the production of glass.

  • Since the 1860's, sodium carbonate has been produced using the Solvay Process.

  • The Solvay Process is a continuous process using limestone (CaCO3) to produce carbon dioxide (CO2) which reacts with ammonia (NH3) dissolved in brine (concentrated NaCl(aq)) to produce sodium carbonate.

  • The steps in the Solvay Process are:

  1. Brine Purification

  2. Sodium Hydrogen Carbonate Formation

  3. Sodium Carbonate Formation

  4. Ammonia Recovery

Properties and Uses of Sodium Carbonate

Sodium carbonate, Na2CO3, dissolves in water to form an alkaline solution.

Used as a base, sodium carbonate is cheaper and safer than sodium hydroxide.

Uses of Sodium Carbonate
Use Process Notes
Glass Making A mixture of Na2CO3, CaCO3 and SiO2 (silicon dioxide sand) is used for window or bottle glass.

Water Softening Agent CO32- from dissolved Na2CO3 can precipitate Mg2+ and Ca2+ ions from hard water as the insoluble carbonates, preventing them from forming a precipitate with soap resulting in scum.
For this reason, sodium carbonate is also known as washing soda.

Paper Making Na2CO3 is used to produce the NaHSO3 necessary for the sulfite method of separating lignin from cellulose.

Baking Soda Production Baking soda (or sodium hydrogen carbonate or sodium bicarbonate), NaHCO3, is used in food preparation and in fire extinguishers.

Sodium Hydroxide Production for Soaps and Detergents Na2CO3 is reacted with a Ca(OH)2, slaked lime, suspension.

Wool Processing Na2CO3 removes grease from wool and neutralises acidic solutions.

Power Generation Na2CO3 is used to remove SO2(g) from flue gases in power stations.

Solvay Process

The Solvay Process for the production of sodium carbonate is summarised in the flowchart below:

brine
NaCl(aq)
-----> ammoniated brine <----- ammonia
NH3
      |
|
    /\
|
 
limestone
CaCO3
    |
|
|
|
NaCl
H2O
NH3
  |
|
|
|
NH3
|
|
\/
    |
|
|
\/
    |
|
|
|
 
lime kiln CO2
----->
carbonating tower   |
|
|
 
 
H2O
|
CaO
    |
\/
    |
|
 
|
\/
|
\/
    filter
|
NH4Cl
--------->
ammonia recovery
lime slaker Ca(OH)2
------------|--------------->
  |
|
\/
  |
\/
  product
NaHCO3
  by-product
CaCl2
  |
300oC
|
\/
   
  product
Na2CO3
   

  1. Brine Purification


    Brine is concentrated by evaporation to atleast 30%
    Impurities such as calcium, magnesium and iron are removed by precipitation, eg,
    Ca2+(aq) + CO32-(aq) → CaCO3(s)
    Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)
    Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

    Brine solution is then filtered and passed through an ammonia tower to dissolve ammonia.
    This process is exothermic, releases energy, so the ammonia tower is cooled.

  2. Sodium Hydrogen Carbonate Formation


    Carbon dioixide is produced by the thermal decomposition of limestone, CaCO3(s), in the lime kiln:
    CaCO3(s) → CO2(g) + CaO(s)

    Carbon dioxide is bubbled through the ammoniated brine solution in the carbonating tower.
    The carbon dioxide dissolves to form a weak acid:
    CO2(g) + H2O(l) HCO3-(aq) + H+(aq)

    The ammonia in the brine reacts with H+ to form ammonium ions:
    NH3(aq) + H+(aq) NH4+(aq)

    The HCO3- then reacts with the Na+ to form a suspension of sodium hydrogen carbonate:
    HCO3-(aq) + Na+(aq) NaHCO3(s)

    NaHCO3 precipitates because of the large excess of Na+ present in the brine which forces the equilibrium position to shift to the right by Le Chatelier's Principle (NaHCO3 is quite soluble in water).
    The overall molecular equation for the formation of sodium hydrogen carbonate in the carbonating tower is:
    NH3(aq) + CO2(g) + NaCl(aq) + H2O(l) → NaHCO3(s) + NH4Cl(aq)

    The net ionic equation for the formation of sodium hydrogen carbonate in the carbonating tower is:
    NH3(aq) + CO2(g) + Na+(aq) + H2O(l) → NaHCO3(s) + NH4+(aq)
    where Cl- is a spectator ion

  3. Sodium Carbonate Formation


    Suspended sodium hydrogen carbonate is removed from the carbonating tower and heated at 300oC to produce sodium carbonate:
    2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g)

    The carbon dioxide produced is recycled back into the carbonating tower.

  4. Ammonia Recovery


    CaO is formed as a by-product of the thermal decomposition of limestone in the lime kiln.
    This CaO enters a lime slaker to react with water to form calcium hydroxide:
    CaO(s) + H2O(l) → Ca(OH)2(aq)

    The calcium hydroxide produced here is reacted with the ammonium chloride separated out of the carbonating tower by filtration:
    Ca(OH)2(aq) + 2NH4Cl(aq) → CaCl2(aq) + 2H2O(l) + 2NH3(g)

    The ammonia is recycled back into the process to form ammoniated brine.
    Calcium chloride is formed as a by-product of the Solvay Process.

Environmental Issues

  1. Solid Wastes

    Calcium chloride, CaCl2, is a by-product of the Solvay Process.
    There are a limited number of uses for CaCl2:
        - drying agent in industry
        - de-icing roads
        - an additive in soil treatment
        - an additive in concrete
    The rest must be disposed of either by pumping out to sea, or by evaporating to dryness and disposing of the solid.
    CaCl2 can not be pumped into rivers or lakes because it will raise the concentration of chloride ion to unacceptable levels.

    Other solid wastes include unburnt calcium carbonate, sand and clay from the kiln. It is possible that these could be used to make bricks, landfill or road base.

  2. Air Pollution

    Some ammonia is lost to the atmosphere during the Solvay Process. Ammonia is a toxic atmospheric pollutant.
    Ammonia losses are minimised to reduce plant operation costs.

  3. Thermal Pollution

    Some of the processes involved in the Solvay Process are exothermic, they release heat.
    Near the ocean, water used during the cooling processes can be released into the sea without causing disruption to aquatic organisms.
    Inland plants need to either release heated water slowly into rivers or lakes or cool the water first before releasing in order to prevent disruption to aquatic organisms.


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