Solvay Process

Why the Solvay process matters

• Soda ash is the largest single sodium chemical. ~75 Mt/yr globally; ~57 Mt of that comes from Solvay-style synthetic plants and ~18 Mt from natural trona mining (mostly in Wyoming, USA). Demand grows at 2-3% per year, tracking flat-glass and detergent markets.
• Glass making consumes ~50% of soda ash. Soda-lime-silica glass is ~74% SiO2, ~13% Na2O, ~10% CaO. The Na2O comes from soda ash, which lowers the melting point of silica from 1700 °C to ~1500 °C — a 200 °C reduction worth tens of GJ per tonne of glass.
• Detergents and chemicals. ~15% of output goes to powdered detergents (sodium-carbonate-based water softeners and pH boosters); ~15% to chemicals manufacture (sodium silicate, sodium dichromate, sodium phosphates, sodium percarbonate); ~10% to pulp and paper, water treatment, and minor flue-gas desulfurization.
• The ammonia loop is a textbook example of catalyst-style recycling. Stoichiometrically the process consumes NaCl and CaCO3 only; ammonia carries cations through the system but is recovered downstream and returned. Recovery efficiency ~99% means make-up NH3 is only ~3 kg per tonne of Na2CO3 — a critical cost lever.
• Replaced the polluting LeBlanc process. By 1900 LeBlanc plants — which emitted HCl gas and produced fetid CaS waste — were extinct outside France. Britain's 1863 Alkali Act was passed to control LeBlanc HCl emissions; Solvay needed no such regulation.
• Energy intensity ~10-13 GJ/t. About half goes to lime-kiln calcination of CaCO3 (CaCO3 -> CaO + CO2 needs ~178 kJ/mol = 1.78 GJ/t CaCO3), the other half to ammonia distillation, brine purification, and bicarbonate calcination. Modern plants integrate the kiln-CO2 directly with the carbonation tower for closure.
• Stabilizes coastal salt and limestone economies. Solvay plants are sited near salt-bearing aquifers (e.g. Solvay Green River, Wyoming; Tata Mithapur, India), salt domes (Bayer Wolfen, Germany), or coastal solar evaporation works (Magadi, Kenya — actually a natural-trona plant). Limestone quarries within 50-100 km supply the CaCO3.

Common misconceptions

• Sodium carbonate is the same as sodium bicarbonate. No. The Solvay process makes both, but its purpose is sodium carbonate (Na2CO3, soda ash), produced by calcining the intermediate sodium bicarbonate (NaHCO3, baking soda) at 200 °C. A small fraction of bicarbonate is sold directly as a baking and pharmaceutical product.
• Ammonia is consumed by the process. No — only ~3 kg/t Na2CO3 is consumed. Ammonia recycles via the lime-kiln loop. The economics depend on this; if NH3 were stoichiometrically consumed, soda ash would cost roughly 5x more per tonne and be unable to compete with natural trona.
• The lime kiln is just a heat source. The lime kiln is a chemical step — it provides both the CO2 for the carbonation tower (CaCO3 -> CaO + CO2) and the CaO for ammonia recovery (CaO + H2O -> Ca(OH)2; Ca(OH)2 + 2 NH4Cl -> 2 NH3 + CaCl2 + 2 H2O). Lose the kiln and you lose the loop.
• The process is essentially CO2-neutral. No. Net CO2 emissions are ~0.2-0.4 t CO2 per tonne Na2CO3 from kiln fuel and from the stoichiometric CO2 evolved during NaHCO3 calcination that escapes the loop. Modern plants are working on CO2 capture (the Solvay Lostock plant in the UK trialed CCS in the 2010s).
• Calcium chloride byproduct is a waste. Some, not all. Roughly half is sold — primarily as winter road de-icer (CaCl2 freezing-point depression beats NaCl below -10 °C), oilfield brine, and dust suppressant. The unsold remainder is impounded; Solvay plants typically maintain large CaCl2 settling lagoons.
• Hot temperature drives the precipitation. No — the carbonation tower is run cold (30-40 °C) to maximize NaHCO3 precipitation; sodium bicarbonate solubility is ~70 g/L at 0 °C versus ~160 g/L at 60 °C. Cooling the tower with refrigerated water is a standard energy line.

Process: the four interlocking towers

A Solvay plant is built around four columns and a kiln. (1) The brine purification step takes saturated NaCl brine (~310 g/L), removes Ca²⁺ and Mg²⁺ by adding Na2CO3 and NaOH (the calcium and magnesium would otherwise precipitate as carbonates inside the carbonation tower and foul it). (2) The ammonia absorber saturates the purified brine with gaseous NH3 from the recovery distiller, producing 'ammoniated brine' at ~85 g/L NH3 and ~260 g/L NaCl. (3) The carbonation tower (Solvay tower) is the heart of the plant: ammoniated brine descends counter-currently against rising CO2 (~40% from lime kiln, ~60% from bicarbonate calciner). NaCl + NH3 + CO2 + H2O → NaHCO3 (s) + NH4Cl. The slurry is cooled progressively from 60 °C at the top to 30 °C at the bottom to drive NaHCO3 to precipitate. (4) The slurry is filtered on rotary vacuum filters; the bicarbonate filter cake is washed and sent to the calciner. (5) The mother liquor (NH4Cl + residual NaCl) flows to the distiller, where steam and milk-of-lime liberate NH3 for return to the absorber. Calcium chloride exits as a brine bleed.

The lime kiln runs in parallel. It calcines limestone CaCO3 (~1.1 t per tonne Na2CO3) at ~1000 °C using coke or gas, producing CaO and CO2. The CO2 stream (~30-40% pure, the rest combustion gases) is washed and compressed; quicklime is slaked with water to milk-of-lime for the distiller. The bicarbonate calciner — a heated rotary or fluid-bed unit at ~200 °C — turns the wet cake into anhydrous Na2CO3 ('light ash'); the released CO2 (~50% pure) returns to the carbonation tower. Light ash typically has a bulk density of ~0.5 g/cm³; for glass-grade soda ash it is hydrated and re-calcined to give 'dense ash' at ~1.0 g/cm³, which flows better through glass-batching equipment.

Mass and heat balances: per tonne Na2CO3, you consume ~1.5 t NaCl, ~1.1 t CaCO3, and ~0.05 t make-up NH3 (the loss the 99% recycle does not catch); you produce ~1.0 t CaCl2 and ~10-13 GJ of energy, mostly as kiln fuel and distiller steam. Annual single-train capacity in modern plants ranges 300 kt-1.2 Mt; Tata Mithapur (India), Solvay Devnya (Bulgaria), and Ciech Inowrocław (Poland) operate in this band. Wyoming trona-based natural soda ash undercuts synthetic Solvay on energy by ~50% (no kiln) and dominates the U.S. market — but trona deposits exist nowhere else at the scale Wyoming offers.

Solvay vs LeBlanc vs Hou processes

Process	Era	Net inputs	Net outputs	Byproduct	Status
LeBlanc	1791-~1920	NaCl, H2SO4, coal, CaCO3	Na2CO3	HCl gas, CaS slag	Extinct — replaced by Solvay
Solvay (ammonia-soda)	1861-present	NaCl, CaCO3 (NH3 recycled)	Na2CO3	CaCl2	Dominant synthetic route, ~57 Mt/yr
Hou (dual alkali)	1933-present	NaCl, NH3, CO2	Na2CO3 + NH4Cl fertilizer	None significant	Major in China, ~10 Mt/yr
Trona mining (natural)	1938-present	Trona ore (Na3HCO3·CO3·2H2O)	Na2CO3	Insolubles overburden	~18 Mt/yr; dominant in U.S.
Lake Magadi solar	1911-present	Trona-rich brine + solar	Na2CO3	Insolubles	~0.5 Mt/yr (Kenya)
Caustic-CO2 (electrolytic)	1900s niche	NaOH (chlor-alkali), CO2	Na2CO3	None	Tiny niche; high power cost
Modified Solvay (CO2 capture)	2010s pilot	NaCl, CaCO3 + flue-gas CO2	Na2CO3 + sequestered CO2	CaCl2	Pilot scale; emerging climate option

Applications and case studies

• Glass making (~50% of demand). Soda-lime-silica float glass — by far the dominant flat-glass type — is batched as ~74% silica sand, 13% soda ash, 10% limestone, plus minor cullet, dolomite, salt cake. Pilkington and Saint-Gobain float lines each consume 100-300 kt/yr of soda ash. Without it, silica's 1700 °C melting point would make flat glass economically unviable.
• Tata Chemicals Mithapur, India. India's largest synthetic soda-ash producer at ~1.0 Mt/yr capacity, supplied by inland salt pans on Gujarat's Gulf of Kutch coast and limestone quarried within ~50 km. The plant exports ~30% to Southeast Asia.
• Solvay Lostock, UK. Operated 1874-2024, this Cheshire plant tapped underground brine wells via solution mining. At its 2010s peak it produced ~600 kt/yr soda ash. Closure in 2024 reflected European energy costs and competition from Chinese Hou-process imports.
• Lake Magadi, Kenya. Tata Chemicals Magadi mines naturally occurring trona-rich brine from the Rift Valley soda lake and solar-evaporates / re-crystallizes it; ~0.5 Mt/yr. The Magadi process is technically not Solvay (no ammonia loop) but competes head-to-head in African and Middle Eastern markets.
• Detergent powders. Sodium carbonate is the alkalinity builder in heavy-duty laundry powders at 5-25 wt%; it raises wash-pH to ~10, saponifies fatty soils, and complexes Ca²⁺/Mg²⁺ via sodium-tripolyphosphate or zeolite co-builders. Henkel, Procter & Gamble, and Unilever are top buyers in their own right.