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Solvay Chemistry

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Soda Ash Production
Solvay process
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Solvay Chemistry
Salt + limestone → Na2CO3

The Solvay process makes sodium carbonate from salt and limestone by passing carbon dioxide through ammoniated brine, so that sodium bicarbonate precipitates, filtering it out and heating it to about 200 °C to give soda ash. Overall it is 2NaCl + CaCO3 → Na2CO3 + CaCl2, but the real reactions run through ammonia, which is recovered with lime and recycled. The carbon dioxide comes from the lime kiln, and its balance closes: the kiln gives one mole of CO2 and the heating step another, together the two moles the precipitation needs.

Part of Industrial Chemistry

The sequence of reactions in ammonia-soda manufacture and the material balances that make it a closed loop.

Why The Direct Reaction Does Not Work

Salt and limestone do not react to soda ash directly. The carrier’s route is indirect, using ammonia as a carrier:

  1. Dissolve salt in water to make brine.
  2. Bubble ammonia through the brine to make ammoniated brine.
  3. Bubble carbon dioxide through it. Sodium bicarbonate precipitates because it is less soluble than the other products.
  4. Filter the bicarbonate and heat it to drive off CO2 and water, leaving sodium carbonate.
  5. Recycle the CO2 to step 3.
  6. Treat the remaining liquor with lime to recover the ammonia, and recycle that to step 2.

Written as equations (the equations are general chemistry, consistent with the carrier’s description):

  • NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl
  • 2NaHCO3 → Na2CO3 + CO2 + H2O (heated, the carrier says to about 200 °C)
  • CaCO3 → CaO + CO2 (the kiln, above 900 °C per the carrier; see Limestone Burning)
  • CaO + H2O → Ca(OH)2 (slaking; Slaking)
  • 2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O (ammonia recovery)

Add the five and cancel the intermediates and you obtain 2NaCl + CaCO3 → Na2CO3 + CaCl2.

What You Need

  • Saturated brine, about 26% salt by weight (the carrier’s figure for brine in its electrolysis section).
  • Ammonia and a way to recover it (Simplified Ammonia).
  • A lime kiln for CO2 and quicklime (Lime Kiln).
  • Cooling for the carbonation step: the carrier says to keep it below 15 °C if possible, since bicarbonate precipitates better when cooler.
  • Filters, a bicarbonate kiln, and the equipment in Equipment Needed.

Method: The Loop In Order

Step 1: Make brine and saturate it with ammonia. Keep it cool; the ammonia dissolving gives heat.

Step 2: Carbonate with CO2 from the kiln in a cooled tower.

Step 3: Filter the bicarbonate and send the filtrate on.

Step 4: Heat the bicarbonate to soda ash and return the CO2.

Step 5: Treat the filtrate with slaked lime, boil off the ammonia and return it.

Step 6: Discharge the calcium chloride waste as set out in Waste Treatment.

Worked Example: The CO2 Balance

Per mole of soda ash made:

  1. Two moles of sodium bicarbonate must precipitate (one per sodium in Na2CO3), so the carbonation needs 2 mol CO2.
  2. Heating the 2 mol of bicarbonate gives 1 mol of Na2CO3 and releases 1 mol CO2, which is recycled. Still needed: 2 − 1 = 1 mol.
  3. The kiln burns 1 mol of CaCO3 to give 1 mol CO2 and 1 mol CaO. That is the missing 1 mol.
  4. The CaO slakes to 1 mol of Ca(OH)2, which recovers the ammonia: 2NH4Cl needs 1 Ca(OH)2, and 2 NH4Cl is just what 2 NaHCO3 made.

So the kiln gives both the missing CO2 and the lime for ammonia recovery, as the carrier says, and the loop balances. In mass terms: 2 mol NaCl = 117 g and 1 mol CaCO3 = 100 g go in; 106 g of Na2CO3 and 111 g of CaCl2 come out.

Common Mistakes

  • Letting the carbonation tower warm up. Less bicarbonate precipitates.
  • Losing ammonia in the filtrate rather than recovering it.
  • Under-heating the bicarbonate and shipping a damp mixture.
  • Skipping the CO2 count and starving the tower.

Safety

Ammonia, carbon dioxide and a very hot kiln share one site

Ammonia vapour is toxic and corrosive; CO2 pools in pits and displaces air; the lime kiln runs above 900 °C and slaked lime is caustic. Keep the ammonia circuit tight, ventilate towers and pits, and never enter a tank or pit without a rope and a second person. See Toxic Gases.

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