Ostwald process

The Ostwald process denotes the industrial manufacture of nitric acid through catalytic oxidation of ammonia over a platinum-rhodium gauze, followed by absorption of nitrogen oxides in water. Developed by Wilhelm Ostwald at the turn of the twentieth century, this route accounts for virtually all nitric acid production worldwide. Ammonia reacts with atmospheric oxygen at elevated temperature to yield nitrogen monoxide (NO), which is subsequently oxidised to nitrogen dioxide (NO2) and absorbed to form HNO3. Higher-strength acids require extractive distillation using sulphuric acid or magnesium nitrate.

Operating principles and stages

The oxidation reaction takes place over catalytic gauzes made of platinum-rhodium alloy. Air, previously filtered and compressed, is blended with gaseous ammonia in carefully controlled proportions to prevent flaming. The mixture passes through several layers of gauzes arranged in series within a converter. The nitrogen monoxide formed then undergoes secondary oxidation to nitrogen dioxide, before being absorbed in water or dilute acid columns.

Process condensates and dilute nitric acid from the catalytic oxidation, oxidation and post-oxidation stages are recycled within the process. However, condensate formation exceeds the requirements for concentrated acid production; a portion must therefore be valorised in other product lines or treated as effluent^[1]. For higher-strength acids, indirect processes based on extractive distillation employ sulphuric acid or magnesium nitrate as dehydrating agents. With sulphate, dilute nitric acid is preheated then distilled in the presence of H2SO4; with magnesium nitrate, a Mg(NO3)2 solution extracts water from the nitric acid. The dehydrating agents are regenerated under vacuum, whilst process condensates generated by this concentration step require appropriate wastewater treatment^[1].

Environmental aspects and emission abatement

The Ostwald process ranks among the leading industrial sources of nitrous oxide (N2O) emissions, a greenhouse gas of particular concern for global warming. Secondary N2O abatement technologies have undergone industrial development documented in emission reduction projects^[2]. Selective catalytic reduction (SCR) has also been implemented to curtail nitrogen oxides (NOx) emissions from nitric acid plant tail gases^[2]. European regulation strongly encourages adoption of these abatement techniques.

Applications and value chain

Nitric acid from the Ostwald process chiefly supplies two major sectors: nitrogen fertilisers and industrial explosives. In fertilisation, it serves as feedstock for producing ammonium nitrate, nitrogen solutions and NPK fertilisers via the nitrophosphate route. Nitric acid also enters into the manufacture of organic nitrated compounds and various speciality chemicals. Neutralisation of nitric acid with ammonia in a tubular reactor can directly yield ammonium nitrate solution (ANS) at 97% without further concentration, when 63% acid is preheated to 60 °C^[3]. For solid products obtained by prilling or granulation, the solution is typically concentrated by evaporation to a moisture content below 1%^[3].

Integration in the fertiliser industry

Nitric acid units are most commonly integrated on ammonia and fertiliser production sites, enabling energy optimisation and reduced logistics costs. Steam produced during nitrogen oxide condensation can be recovered and reused in downstream processes, notably for evaporating nitrate solutions^[3]. Merchant nitric acid, non-integrated, represents a marginal share of international trade compared with derivative products such as ammonium nitrate. Transport constraints arising from acid corrosiveness limit long-haul movements.