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

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

Solvay Process

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) + CO₃^2-_(aq) → CaCO_3(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, CaCO_3(s), in the lime kiln:

   CaCO_3(s) → CO_2(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:

   CO_2(g) + H₂O_(l) HCO₃^-_(aq) + H⁺_(aq)

   
   The ammonia in the brine reacts with H⁺ to form ammonium ions:

   NH_3(aq) + H⁺_(aq) NH₄⁺_(aq)

   
   The HCO₃^- then reacts with the Na⁺ to form a suspension of sodium hydrogen carbonate:

   HCO₃^-_(aq) + Na⁺_(aq) NaHCO_3(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:

   NH_3(aq) + CO_2(g) + NaCl_(aq) + H₂O_(l) → NaHCO_3(s) + NH₄Cl_(aq)

   
   The net ionic equation for the formation of sodium hydrogen carbonate in the carbonating tower is:

   NH_3(aq) + CO_2(g) + Na⁺_(aq) + H₂O_(l) → NaHCO_3(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 300^oC to produce sodium carbonate:

   2NaHCO_3(s) → Na₂CO_3(s) + CO_2(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) → CaCl_2(aq) + 2H₂O_(l) + 2NH_3(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.