Nitric acid (HNO3) is a colorless, highly corrosive, and toxic strong mineral acid that is normally colorless but tends to acquire a yellow cast upon long storage due to accumulation of oxides of nitrogen. Ordinary nitric acid has a typical concentration of 68%. Solutions containing over 86% nitric acid are referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming act is characterized as white fuming nitric acid (WFNA) or red fuming nitric acid (RFNA), with concentrations above 95%.
Nitric acid is produced by 2 methods. The first method utilizes oxidation, condensation, and absorption to produce a weak nitric acid with concentrations ranging from 30-70 % nitric acid. The second method combines dehydrating, bleaching, condensing, and absorption to produce a high-strength nitric acid from a weak nitric acid. High-strength nitric acid generally contains more than 90% nitric acid.
1a) Weak Nitric Acid Production
1ai) Primary oxidation (formation of nitric acid)
Oxidation of ammonia is carried out in a catalyst chamber in which one part of ammonia and eight parts of oxygen by volume are introduced. The chamber temperature is approximately 600oC, and it contains a platinum gauze which serves as a catalyst. In primary oxidization, 95% of ammonia is converted into nitric oxide (NO).
4NH3 + 5O2 ↔ 4NO + 6H2O
1aii) Secondary oxidation (formation of nitrogen dioxide, NO2)
Nitric oxide gas obtained by the oxidation of ammonia is passed through a heat exchanger where the temperature of nitric oxide is reduced to 150oC. It is then transferred to another oxidizing tower where it is oxidized to NO2 at about 50oC
2NO + O2 ↔2 NO2
1aiii) Absorption of NO2 (formation of HNO3)
Nitrogen dioxide from the secondary oxidation chamber is introduced into a special absorption tower. NO2 gas passed through the tower and water is showered over it. Nitric acid is obtained via absorption.
3NO2 + H2O ® 2HNO3 + NO
Nitric acid (HNO3) is recycled in the absorption tower so that more NO2 gets absorbed. After recycling, HNO3 becomes about 68% concentrated.
1b) High-Strength Nitric Acid Production
A high-strength nitric acid (98-99% concentration) can be obtained by concentrating the weak nitric acid (30-70% concentration) using extractive distillation. The nitric acid concentration process involves feeding strong sulfuric acid (typically 60% concentrated) and nitric acid (55-65%) to the top of a packed dehydrating column under atmospheric pressure. The acid mixture flows downward, countercurrent to ascending HNO3 vapors. H2SO4 acts as a dehydrating agent and absorbs water from HNO3 and concentrated nitric acid leaves the top of the column as 99% vapor concentrated HNO3.
Nitric acid is mainly used in the manufacturing of fertilizers such as calcium nitrate and ammonium nitrate.
Used in organic oxidation to manufacture terephthalic acid and other organic compounds.
Nitric acid can be used to manufacture explosives such as trinitrotoluene (TNT), nitroglycerin, nitrocellulose.
Its aqueous blend is used for cleaning food and dairy equipment where it remove precipitated calcium and magnesium compounds.
It is used in for steel and brass pickling.
It is used to oxidize KA-oil in the manufacture of nylon.