Chemistry Labs
Undergraduate · 25 min

The Solvay process: soda ash from brine

Follow the five stages of the ammonia-soda cycle: brine purification, ammoniation, carbonation, filtration of NaHCO₃ and calcination to Na₂CO₃ — with ammonia recycled.

Goal

Trace the mass flow of NaCl, NH₃ and CO₂ through the plant, explain why NaHCO₃ precipitates selectively, and identify the role of the ammonia recycle loop.

Apparatus and reagents

A model flow diagram rather than wet glassware: concentrated brine, ammonia and carbon dioxide (from limestone calcination), carbonation columns, filters and a calcining kiln.

Procedure

  1. Read the flow left to right: purified brine, ammoniation, carbonation, filtration and calcination. Each column is a stage, and the moving particles suggest material flowing through.
  2. Vary the flow-rate slider: faster throughput means less contact time per stage — in the real plant each column is sized so the reaction stays nearly complete.
  3. Track the chemistry: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl, then 2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O on heating.
  4. Identify the recycle: ammonia released from NH₄Cl (using lime) returns to the ammoniation column, and part of the calcination CO₂ feeds back to carbonation.

What to observe

  • Only stage 4 produces a solid: NaHCO₃ is the one low-solubility species in the ammoniacal brine, which is why it can be filtered off.
  • The last stage glows hot: calcination releases the CO₂ that carbonation captured, closing part of the gas loop.

Explanation

Ammonia raises the alkalinity of the brine so CO₂ is absorbed as bicarbonate; NaHCO₃ is the least soluble salt present, so it crystallises and is filtered. Calcination (about 200 °C) converts it to Na₂CO₃. The net input is salt and limestone — ammonia acts as a recyclable carrier — leaving CaCl₂ as the main by-product.

History of the experiment

Ernest Solvay patented his ammonia-soda process in 1861 and started production in 1863, replacing the costly, polluting Leblanc process. The trick — saturating brine with ammonia before carbonation so that NaHCO₃ precipitates while NH₄Cl stays in solution — made cheap soda possible for glass and soap industries, and the process still supplies most of the world’s synthetic Na₂CO₃.

Chemists behind it

Fritz HaberCarl Bosch

Related topics

Production of H₂SO₄, NH₃ (Haber–Bosch), HNO₃, NaOHReaction engineering, heat and mass transfer

Virtual experiment: a simplified model to build intuition. It does not replace real lab work or safety training; never repeat chemistry at home without supervision.