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Chemical synthesisHigh temperatureIndustrial scaleCO₂-emittingNobel Prize

Ostwald process

Catalytic oxidation of ammonia (from Haber-Bosch) on a platinum-rhodium gauze to produce nitric acid. Coupled with Haber-Bosch, it's the backbone of the fertilizer and explosives industry.

Molecular synthesis through controlled chemical reactions

Key reaction

4 NH₃ + 5 O₂ → 4 NO + 6 H₂O ; 2 NO + O₂ → 2 NO₂ ; 3 NO₂ + H₂O → 2 HNO₃ + NO

Operating conditions

Temperature
850-950 (gauze) ; 30-50 (absorption)°C
Pressure
4-12bar
Catalyst
Pt-Rh (90/10 ou 95/5) gauze tissée
Phase
gas + liquid

How it works

Schema coming soon

How it works

The Ostwald process, patented in 1902 by Wilhelm Ostwald (Nobel 1909), oxidizes ammonia to nitric acid in three sequential steps, all run in a single integrated unit. Its power comes from the elegance of the catalyst: a simple woven 'gauze' of platinum-rhodium alloy, exposed to an NH₃ + air gas flow, selectively oxidizes NH₃ to NO without burning it to N₂. (1) Catalytic combustion: NH₃ and air are blended (10 vol % NH₃) and pass over the Pt-Rh gauze at 850-950 °C, under 4-12 bar: 4 NH₃ + 5 O₂ → 4 NO + 6 H₂O. Selectivity exceeds 95 % for the target NO against 5 % toward parasitic N₂ + N₂O. The gauze-gas contact lasts < 1 millisecond — beyond that, kinetics flip back toward N₂. (2) Oxidation and absorption: outgoing gases at 900 °C are cooled in a waste-heat boiler (producing HP steam), then enter a stainless steel absorption tower. On cooling, NO + ½ O₂ → NO₂ (slow at 30-50 °C), then 3 NO₂ + H₂O → 2 HNO₃ + NO. NO formed as a byproduct returns to the top of the tower to be re-oxidized. The tower is 30-50 m tall, 3-4 m in diameter, with 30-50 trays or structured packing, run under pressure (4-12 bar) to favor solubility. (3) Concentration: outgoing acid is diluted to ~60-65 wt %. For common uses (fertilizer), that's enough. For technical uses (explosives, organic chemistry), it must be concentrated to 98-99 % by extractive distillation with H₂SO₄ or Mg(NO₃)₂. The process is globally exothermic (ΔH = −906 kJ/mol for the full chain) and each tonne of acid produces ~6 GJ of recoverable steam. World production ~75 Mt/year (2022), ~80 % dedicated to fertilizers (ammonium nitrate, urea-nitrate). Parasitic N₂O (5-10 g per tonne acid), a greenhouse gas 265× more potent than CO₂, has become the main environmental issue — hence the systematic addition of N₂O abatement catalysts downstream.

Key components

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

  • Pt-Rh gauze

    Selective oxidation catalyst NH₃ → NO. The heart of the process.

    Fine metal weave (wire Ø ~75 µm) of Pt-Rh alloy (90/10 or 95/5 depending on operating pressure), stacked as 5-30 overlaid grids. Specific surface ~1 m²/g. Selectivity > 95 % for NO. Pt losses by evaporation: ~0.1-0.3 g Pt per tonne acid produced — captured downstream on Pd gauzes that recover ~80 % of volatile Pt.

    Pt-Rh 90/10 ou 95/5 · 5-30 grilles · pertes Pt 0,1-0,3 g/t · récupération Pd

    See also :nh3

  • NO waste-heat boiler

    Cools NO gases from 900 °C to 200 °C while producing HP steam.

    Tubular heat exchanger in austenitic stainless steel 304L or 316L, designed to resist moist NOₓ corrosion. Steam produced ~40 bar / 380 °C. Gradual cooling to avoid premature water condensation (which would dissolve NO into unstable HNO₃ and cause local corrosion).

    Inox 304L/316L · vapeur 40 bar / 380 °C · refroidissement contrôlé

  • NO₂ absorption tower

    Converts cooled NOₓ to HNO₃ by counter-current contact with water.

    Cylindrical tower 30-50 m tall, 3-4 m in diameter, stainless steel, pressurized at 4-12 bar. Valve trays or structured packing (Sulzer Mellapak™) to maximize NOₓ/H₂O transfer. Water in at the top, NOₓ gas at the bottom, concentrated acid out at the bottom (60-65 wt %). External coil cooling to stay < 50 °C (NO₂ solubility falls exponentially with T).

    30-50 m · 4-12 bar · acide sortant 60-65 % · cooling externe

    See also :hno3

  • N₂O abatement catalyst

    Decomposes parasitic N₂O (strong greenhouse gas) before atmospheric release.

    Secondary catalytic bed (Fe-zeolite or mixed Co-Ce-Al oxide) downstream of the gauze, at 800-900 °C. Decomposes 2 N₂O → 2 N₂ + O₂ at 90-99 % efficiency. Made mandatory in the EU and under Kyoto (each tonne N₂O avoided counts as 265 t CO₂-eq carbon credit). Example: Yara Porsgrunn eliminated > 30 kt N₂O/year as early as 2009.

    Fe-zéolite ou Co-Ce-Al · 800-900 °C · efficacité 90-99 %

Physical and chemical principles

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

  • Short-contact selectivity (millisecond chemistry)

    On the Pt-Rh gauze, two competing reactions consume NH₃: oxidation to NO (desired) and combustion to N₂ (parasitic, irreversible). At 900 °C, kinetics toward NO are faster during the first millisecond of contact, then NO starts reducing back to N₂. The thin gauze (low mass, low volume) ensures residence time < 1 ms — the reason for choosing a woven grid over a granular fixed bed.

    Applies to components :gauze-pt-rh

  • Gas/liquid oxidation cascade

    Three equilibria stack up in the absorption tower: NO + ½ O₂ ⇌ NO₂ (gas, slow), 2 NO₂ ⇌ N₂O₄ (gas, fast), 3 NO₂ + H₂O → 2 HNO₃ + NO (gas/liquid interface). Regenerated NO rises and re-loops. High pressure (4-12 bar) accelerates all three steps. The engineer's art lies in dimensioning heights and temperatures to minimize NOₓ losses.

    Applies to components :tour-absorption-no2

Compounds involved

Input

Output

World production

75 Mt/yr
2022

Main applications

  • Fertilizers (ammonium nitrate NH₄NO₃, urea-nitrate)80 %
  • Explosives (TNT, nitroglycerin, ANFO)8 %
  • Organic chemistry (aromatic nitration)6 %
  • Metallurgy (passivation, etching), miscellaneous6 %

Fugitive N₂O and the cost of Pt

Before systematic abatement (post-2005), an HNO₃ plant emitted 6-15 g of N₂O per tonne of acid — about 5 % of global anthropogenic N₂O emissions. With secondary abatement, it has fallen below 0.5 g/t — one of the fastest industrial climate wins of the 21st century. The Pt issue remains: ~150 t of Pt are tied up in gauzes worldwide, with ~30 t/year evaporative losses of which only 24 are recovered on Pd grids. The process remains structurally dependent on platinum prices.
  • Catalyseurs d'abattement N₂O secondaire (Fe-zéolite) — −95 % d'émissions
  • Grilles de récupération Pd en aval — +5-10 % de Pt récupéré
  • Optimisation pression / sélectivité (procédés mono- et duo-pression)
  • Couplage HNO₃ vert avec NH₃ vert (Haber-Bosch électrolytique)

Similar or competing processes

Related industrial processes — alternative chemistry, alternative technology.

  • haber-bosch

    Mandatory upstream source — Ostwald NH₃ comes 100 % from Haber-Bosch.

History and discovery

Discovery year1902
First industrial deployment1908
Wilhelm Ostwald· Allemagne
Nobel Prize: 1909
Sources
  • Ullmann's Encyclopedia of Industrial Chemistry — Nitric Acid
  • EFMA — Best Available Techniques for Nitric Acid Production
  • IFA Nitric Acid Statistics
  • Thiemann, M. — Nitric Acid, Nitrous Acid and Nitrogen Oxides (Wiley)
Processes