Oxyacid of nitrogen is available in the form of nitric acid, Its consumption have increased significantly in manufacturing of some organic and inorganic chemicals like fertilizers, explosives, adipic acid, nitrobenzene, sodium nitrate, potassium nitrate and calcium nitrate.
Three methods have gain fame in nitric acid production in large scale:
- Chile saltpetre or Nitrate process
- Birkland and Eyde process or Arc process
- Oxidation of ammonia or Ostwald’s process
From the above all process we can go through Chile Saltpetre or Nitrate process in detail. This is old process used back in 1920’s which may now been replaced with new technologies but even though the chemical process and technology involved in it will help to understand the nitric acid creation. Chile saltpetre is material which contains sodium nitrate NaNO3 with percentage around 35-60%, and remaining percentage compounds with KNO3 and NaCl. This raw Chile saltpetre is concentrated by crystallization in pre-treatment of ore to attained 95% NaNO3 and remaining KNO3 as feed raw material.
Sulphuric acid with 93% is mixed with the refined Chile saltpetre material as per the ratio required as per stoichiometry and send into a retort which is made with cast iron and the mixture is heated to 200oC with help of furnace flue gases and coal fire. Thus at this temperature the following reaction is carried forward for nitric acid production as vapours.
NaNO3 + H2SO4 → NaHSO4 + HNO3
All hot vapours of nitric acid are sent to cool down in water circulated cooled silica pipes. Condensed HNO3 is collected in receiver which have material resistant to nitric acid. Uncondensed gas which escapes from the collector is scrubbed with cooled water in packed bed tower to collect nitric acid in dilute format. Liquid sodium bisulphate is collected from the bottom outlet of the retort.
Based on the concentration of nitric acid, grades are provided for easy reference which are given below:
|95%||Technical, C.P or USP|
|80%HNO3 and15%H2SO4||Nitrating or Mixed acid|