Nitric acid is currently almost exclusively produced by the catalytic oxidation of ammonia using the Ostwald process (1902). The reaction of sodium nitrate (Chile niter, the only nitrate occurring naturally in large quantities) with sulfuric acid, operated at the turn of the century, has not been economic since the emergence of the Haber-Bosch ammonia synthesis process shortly before World WarⅠ. The previously developed direct synthesis process from nitro-gen(Ⅱ) oxide produced from nitrogen and oxygen at temperatures >2000 °C (arc process, thermal NO-synthesis) is no longer operated industrially.

The process for manufacture from ammonia consists of three exothermic reaction steps as shown schematically in Fig. 1.4-5:

The overall reaction corresponds to:



The three reaction steps are based on very different physical chemical relationships which appreciably influence the operation of the process. The third step is normally followed by tail gas purification, to prevent the emission of nitrous gases.

Catalytic Combustion cfilmmonia to (Ⅱ)oxide:

The oxidation of ammonia (combustion) with (excess) atmospheric oxygen to nitrogen(Ⅱ) oxide (NO) is carried out in the presence of a catalyst at 820 to 950 °C either at atmospheric pressure or at pressures up to 12 bar:

This reaction is one of the most efficient catalytic processes in industrial chemistry, having an extremely short reaction time (10^-11s) and a high selectivity. The oxidation of ammonia benefits slightly from pressure reduction, since less nitrogen and dinitrogen(Ⅰ) oxide (N₂O) is then produced in side reactions:

The adverse influence of pressure, necessary in the case of reduced apparatus size (to reduce investment costs), upon yield, can to some extent be compensated by increasing the combustion temperature, but with increased catalyst losses. The yield is generally 94 to 98% (e.g. 97 to 98% at 1 bar, 95 to 96% at 5 bar, 94% at 8 to 10 bar). The combustion mixture contains up to 13% by volume of ammonia, being below the lower explosion limit for ammonia-air mixtures (15.5% by volume at 1 bar). At higher operating pressures the concentration of ammonia in the combustion mixture is lower still (below 11%), since the lower explosion limit decreases with increasing operating pressure.

The ammonia oxidation catalyst is usually a platinum alloy gauze containing 5 to 10% rhodium, or additionally with 5% palladium, with a diameter of up to 4 m (with 1024 meshes/cm2 and a wire thickness of 0.06 to 0.076 mm, the latter for higher pressures). The higher the pressures and flow rates the larger the number of gauzes incorporated into the reactor (up to 50 one above another). The ammonia combustion plants operate with bright red glowing platinum gauzes, which leads to precious metal loss e.g. as a result of evaporation of platinum dioxide, which is formed as an intermediate, or by mechanical abrasion (ca. 0.05 to 0.35 g per t of 100% HNO₃). The higher the operating pressures and temperatures in the reactor, the higher these losses. Up to 80% of this precious metal can be recovered by adsorption on marble chips or on palladium-gold gauzes.

Oxidation of Nitrogen(Ⅱ) Oxide to Nitrogen(Ⅳ) Oxide and Dinitrogen(Ⅳ) Oxide:

The hot nitrogen(Ⅱ) oxide-containing gas from the combustion step (e.g. with ca. 10 to 12% NO) is cooled, the heat content being utilized for steam production or waste gas-heating. It is then reacted with additional atmospheric oxygen (secondary air) to nitrogen(Ⅳ) oxide (NO₂):

This reaction is favored by low temperatures, the temperature coefficient of the rate constant being negative, and still more strongly by increased pressure due to the volume reduction during the reaction. Dimerization to dinitrogen(Ⅳ) oxide is also promoted by low temperatures and high pressures.

The nitrogen(Ⅱ) oxide oxidation takes place partly in the waste heat boiler, due to reaction with the excess oxygen present in the combustion gases from the ammonia oxidation, and partly (after addition of secondary air) in the lowest stage of the absorption columns (mostly operated at high pressures) or in an oxidation tower before the absorption column. The higher the pressure in the combustion step the greater the amount of nitrogen(Ⅳ) oxide formed during the cooling of the combustion gases. This reacts with the reaction water forming nitric acid, the HNO₃ concentration in these so-called acid condensates being 2 to 50%.

Conversion of Nitrogen(Ⅳ) Oxide into Nitric Acid

The gas mixture obtained by oxidation of nitrogen(Ⅱ) oxide, containing nitrogen(Ⅳ) oxide and dinitrogen(Ⅳ) oxide (so-called nitrous gases), is reacted in the third reaction step with water as follows:



to nitric acid, nitrogen(Ⅱ) oxide and nitrous acid. The nitrous acid is further oxidized to nitric acid by the (atmospheric) oxygen present, either in the liquid or vapor phase.

The absorption of the nitrous gases in the process water is favored by low temperatures, high pressures and longer contact times. The quantity of process water, of which the acid condensate is a part, is dependent upon the required nitric acid concentration. Higher pressures permit the production of higher nitric acid concentrations (up to 70% HNO₃), since under pressure almost complete absorption of nitrous gases can be attained in a small quantity of process water with low emission of residual gas. Only 45 to 50% nitric acid can be produced at atmospheric pressure.