To share this content with an AI assistant
Chemical synthesisIndustrial scaleCO₂-emitting

Solvay process

Production of sodium carbonate (Na₂CO₃, 'Solvay soda') from brine (NaCl) and limestone (CaCO₃), with ammonia as a recycled intermediate. Has dominated soda ash production since 1865.

Molecular synthesis through controlled chemical reactions

Key reaction

2 NaCl + CaCO₃ → Na₂CO₃ + CaCl₂ (NH₃ recyclé en boucle)

Operating conditions

Temperature
30-60 (carbonatation) ; 200 (calcination NaHCO₃) ; 1000 (four à chaux)°C
Pressure
1-3bar
Catalyst
NH₃ (porteur recyclé à >99 %)
Phase
liquid + solid + gas

How it works

Schema coming soon

How it works

Sodium carbonate ('soda ash') is one of the most-consumed mineral chemicals in the world — glassmaking, detergents, paper, base chemistry. The Solvay process, patented by Ernest Solvay in 1861 and industrialized in 1865, makes it from two cheap and abundant raw materials: brine (saturated NaCl) and limestone (CaCO₃). The genius of the process is using ammonia (NH₃) as a catalytic carrier, recycled at over 99 %. The sequence has five main steps. (1) Limestone is calcined in lime kilns to release CO₂ and yield quicklime: CaCO₃ → CaO + CO₂. (2) Pretreated brine is saturated with ammonia in an absorption tower (NH₃ + H₂O → NH₄OH). (3) This ammoniated brine is then carbonated by CO₂ from the lime kiln, in the 'Solvay tower': NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl. The bicarbonate, poorly soluble in the presence of NH₄Cl, precipitates and is filtered. (4) The bicarbonate is calcined at 200 °C: 2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O — the released CO₂ is recycled to the Solvay tower. (5) Finally, the quicklime from step 1 is slaked into milk of lime Ca(OH)₂ and used to regenerate ammonia from NH₄Cl: 2 NH₄Cl + Ca(OH)₂ → 2 NH₃↑ + CaCl₂ + 2 H₂O. The ammonia returns to step 2, closing the loop. Net balance: 2 NaCl + CaCO₃ → Na₂CO₃ + CaCl₂. CaCl₂ is the only major byproduct — underused (road de-icing, desiccant) and often dumped at sea or stockpiled. World production ~62 Mt/year (2022), dominated by China (~50 %), then USA (natural Wyoming trona, which doesn't use Solvay), then Europe and India. China and India rely heavily on Solvay; the USA exploits natural trona ore, cheaper but geographically limited.

Key components

The role of each main part, and the elements / compounds it involves.

  • Lime kiln

    Calcines limestone to provide the CO₂ and quicklime needed for the rest of the process.

    Vertical shaft kiln (~25 m tall) or rotary kiln, fed with limestone and fuel (coke, natural gas) at 900-1100 °C. Produces two streams: hot CO₂ (~40 %) sent to the Solvay tower, and solid CaO sent to the ammonia regenerator. This is the largest direct CO₂ emitter in the process.

    900-1100 °C · CO₂ ~40 % · CaO chaud

    See also :caco3caoco2

  • Ammonia absorption tower

    Saturates the brine (NaCl) with ammonia before carbonation.

    Tray or packed tower where brine flows down counter-current to gaseous NH₃ from the regenerator. Outgoing ammoniated brine contains ~80 g/L NH₃ and is cooled to 10-15 °C before entering the Solvay tower (NaHCO₃ solubility drops in the cold, aiding precipitation).

    Contre-courant · ~80 g/L NH₃ · refroidissement à 10-15 °C

    See also :nh3nacl

  • Solvay tower (carbonation)

    Key reactor of the process — precipitates NaHCO₃ by injecting CO₂ into the ammoniated brine.

    Cylindrical tower 20-25 m tall, externally cooled by coils. Ammoniated brine enters at the top, CO₂ at the bottom. As they meet, NaHCO₃ progressively precipitates and the slurry is extracted at the bottom. Typical Na conversion: ~75 %. Yield is limited by residual NaHCO₃ solubility and NH₃ losses — precipitation kinetics governs productivity.

    20-25 m · contre-courant · conversion Na ~75 % · cooling externe

    See also :nahco3nh3co2

  • Bicarbonate calciner

    Decomposes filtered NaHCO₃ to pure Na₂CO₃, releasing CO₂ and H₂O that are reused.

    Horizontal rotary kiln at 200 °C or 'steam dryer' (indirect steam heating). Collected CO₂ is compressed and recycled to the Solvay tower — the second internal CO₂ source. The Na₂CO₃ obtained is 'light soda' (density 0.5) and can be densified into 'dense soda' by hydration/calcination.

    200 °C · four rotatif ou steam dryer · CO₂ recyclé

    See also :nahco3na2co3

  • Ammonia regenerator (distillation column)

    Recovers NH₃ from residual NH₄Cl via lime treatment, closing the catalytic loop.

    Distillation column where the residual liquor (NH₄Cl + NH₄HCO₃) meets hot milk of lime Ca(OH)₂ (~100 °C). Volatile NH₃ is steam-stripped, recompressed and returned to the absorption tower. The liquid residue CaCl₂ + H₂O is discarded. Typical NH₃ losses: ~3 kg/t Na₂CO₃ — economically decisive.

    Colonne de strippage à la vapeur · pertes NH₃ ~3 kg/t Na₂CO₃

    See also :nh3ca-oh-2cacl2

Physical and chemical principles

The fundamental laws that make this process possible — and the constraints they impose.

  • Recycled stoichiometric carrier

    Ammonia is used in stoichiometric (not strictly catalytic) amounts but fully regenerated at the end of the cycle. This is what sets Solvay apart from the Leblanc process it replaced: Leblanc consumed sulfuric acid and generated HCl + Na₂S waste, Solvay only consumes NaCl + CaCO₃ and produces only CaCl₂ as waste.

    Applies to components :regenerateur-nh3

  • Selective precipitation

    At 10-15 °C in the presence of NH₄Cl, NaHCO₃ becomes the least-soluble salt in the system (common-ion effect on HCO₃⁻ and lower solubility with T). This solubility gradient allows isolating the bicarbonate without solvent evaporation — a major energy saving for the process.

    Applies to components :tour-solvay

Compounds involved

Input

Intermediate

Output

By-product

Catalyst

World production

62 Mt/yr
2022

Main applications

  • Glassmaking (flat glass, container glass)50 %
  • Detergents and washing powders13 %
  • Metallurgy and mineral chemistry12 %
  • Pulp and paper5 %
  • Water treatment, food-grade, miscellaneous20 %

CaCl₂ waste and CO₂ footprint

The Solvay process emits ~0.5-0.8 t CO₂/t Na₂CO₃ (limestone calcination + energy), compared to ~0.3 t CO₂/t for natural Wyoming soda. More importantly, it produces ~1 t of CaCl₂ per tonne of Na₂CO₃ — i.e. ~50 Mt/year worldwide, of which only a fraction is valorized (de-icing, desiccant, concrete additive). Coastal plants often dump it at sea (local salinity impact). Improvement levers include energy integration of lime kilns, CaCl₂ valorization, and partial substitution by natural soda or glass recycling.
  • Récupération de chaleur des fours à chaux (préchauffage saumure)
  • Valorisation CaCl₂ en saumure de forage pétrolier
  • Substitution partielle par carbonate naturel (Wyoming, Turquie)
  • Intégration avec capture CO₂ industrielle (CO₂ devient input plutôt qu'émission)

Similar or competing processes

Related industrial processes — alternative chemistry, alternative technology.

  • haber-bosch

    Source of make-up ammonia — Solvay losses (~3 kg NH₃/t) are topped up with Haber-Bosch NH₃.

History and discovery

Discovery year1861
First industrial deployment1865
Ernest Solvay · Alfred Solvay· Belgique
Sources
  • Ullmann's Encyclopedia of Industrial Chemistry — Sodium Carbonate
  • USGS Mineral Commodity Summaries — Soda Ash
  • Kostick, D. — Soda Ash Production and Uses
  • Solvay SA — Histoire industrielle 1865-2025
Processes