Manufacture of Nitric Acid from Sodium Nitrate

The decomposition of sodium nitrate (Chili saltpetre) still remains one of the most important processes for the manufacture of nitric acid. The complete reaction shown by the equation

2NaNO₃ + H₂SO₄ = Na₂SO₄ + 2HNO₃

is not carried out in practice for a number of reasons. The high temperature necessary for the reaction (900° C.) involves high fuel consumption, excessive wear and tear of plant, and decomposition of nitric acid, which causes the product to be dilute, and to contain a high percentage of nitrous acid, according to the following reactions: -

4HNO₃ = 2H₂O + 4NO₂ + O₂;
2HNO₃ = H₂O + NO₂ + NO + O₂;
H₂O + 2NO₂ = HNO₃ + HNO₂.

Further, it would not be possible to remove the residual sodium sulphate at all readily from the retort owing to its high melting-point (860° C.). In consequence, nearly double the quantity of sulphuric acid is used,

NaNO₃ + H₂SO₄ = NaHSO₄ + HNO₃,

as the reaction represented by the above equation proceeds at a relatively low temperature (200° C.), and the residual sodium bisulphate (nitre cake) can be run off from the retort.

The plant consists of a retort, condenser, receiver, and absorbing system for the oxides of nitrogen.

This is a large cast-iron cylindrical vessel. As a typical example the Waltham retort may be mentioned, which has a cubical capacity of about 350 cubic feet, is 8 feet 6 inches by 6 feet 9 inches, and takes a charge of two tons of sodium nitrate. The quantity and concentration of sulphuric acid depend upon the dryness of the nitrate, the proportions being approximately 85 of nitrate to 80 of acid. The retort is surrounded by brickwork 18 inches thick, and is fired by coal, coke, oil, or gas. Whatever fuel is used, the flues are so arranged that the hot gases circulate round the retort so as to produce even heating.

The vapours from the retort are conducted through a silicon iron or fused silica pipe to the condensers. The conduit pipe is made of as simple form as possible in order to provide an easy flow. Various forms of condensers are in use at the present time, and some characteristics of these will be given.

Guttmann Condenser

This condenser consists of perpendicular pipes cooled in water, and was in extensive use at one time. Owing to high working costs and general inflexibility of the plant, however, this system is not used much in modern practice.

Hart Condenser

The outstanding feature of this system is the employment of glass tubes fitted between standards of pottery or silicon iron. The tubes, 6 feet long and 1¼ inches (English) or 3 inches (American) diameter, are slightly inclined to the horizontal, and are cooled by water trickling over the outside. Excellent condensation is effected and the progress of distillation can be watched; further, broken tubes can be replaced without interrupting the operation. On the other hand, the system is of an intricate nature, and its maintenance is expensive owing to the large number of joints.

Valentiner Condenser

This system differs from all others inasmuch as it works under reduced pressure. The chief condensation takes place in coils totally immersed in water, but some condensation also occurs in air-cooled Woulff's bottles, which act as receivers. All joints have to be carefully watched owing to the reduced pressure of the distillation, consequently high labour costs are involved.

Hough Condenser

Condensation of the hot vapours is effected by passing through a series of double U-tubes contained in a rectangular box and cooled by water. Acid-resisting iron is used, and this condenser is both compact and efficient.

S-pipe Condenser

Modern practice favours a relatively simple type of air-cooled condenser in the form of a series of S-bends. Fused silica ware or silicon iron is used, and one advantage seems that such a type requires but little attention.

These vessels are made of various materials - depending on the strength of acid to be collected and other conditions - but earthenware seems to be the only really safe material. Fume-tight lids have to be fitted, and of course provision has to be made for conveying fumes into the absorption system.

An efficient absorbing system is imperatively required, as often as much as 10 per cent, of the original nitrogen content of the sodium nitrate passes through the condensers as nitrogen peroxide, NO₂. Nitric oxide, NO, nitrosyl chloride, NOCl, chlorine, nitric acid vapour and water-vapour are also present. Absorption of nitrogen peroxide in water proceeds according to the equation

3NO₂ + H₂O ⇔ 2HNO₃ + NO. (1)

This nitric oxide, together with that produced during the distillation, has to be oxidised to the peroxide again:

2NO + O₂ = 2NO₂; (2)

which means that an adequate air supply must be introduced into the absorbing systems. Two further points of great importance are the retarding of the absorption of nitrogen peroxide by the presence of nitric oxide (equation (1) is reversible), and also the fact that reaction (2) requires appreciable time for completion.

The fume main from the condensers and receivers enters at the bottom of one of a series of towers, and the nitrous gases are pumped to the top, meeting a stream of water. The tower is filled with quartz packing, so as to provide as large a surface as possible for absorption by the water. The dilute nitric acid thus produced is pumped to the top of the next tower, while the gases are led in at the bottom, and the absorption and oxidation are repeated. The strongest acid obtainable is 60 per cent., and when this concentration is reached, the acid is drawn off and the absorption cycle continued with fresh water.

The strength of the nitric acid collected in the receivers depends upon the dryness of the sodium nitrate and the concentration of the sulphuric acid. It is possible to obtain nitric acid of an average strength of 90 per cent, by using dry nitrate and sulphuric acid of 90 to 94 per cent, concentration. The total yield of nitric acid is smaller, however, as the nitre cake retains some of the nitrogen compounds, and the resulting acid has a high nitrous acid content. A greater amount of nitric acid of average strength of 80 per cent, is obtained by using wet nitrate (2 to 2.5 per cent, water content) and Glover sulphuric acid (78 per cent. H₂SO₄ content). Nitric acid of 99.6 per cent, concentration is obtained by distilling weaker acid with concentrated sulphuric acid. The retort, such as is used in the manufacture, can be used as a still, and condensing and absorbing systems similar to those described above are used in conjunction with the distillation, which is carried out at about 135° C. Acids ranging in concentrations from 100 per cent, downwards may be obtained by running off from the receivers at various stages.

A process of "bleaching" is generally employed to remove nitrous acid in the form of oxides of nitrogen from the commercial acid. Generally this is brought about by blowing a stream of hot air through the acid, although in some plants the bleaching process is incorporated with the condensing operation, so that the distillate encounters the hot retort gases on the counter-current principle, with the result that not more than 1 per cent, of nitrous acid remains.

Commercial nitric acid contains as impurities hydrochloric acid, sulphuric acid, iodine, iron salts, and nitrogen peroxide. The hydrochloric acid is derived from the chlorides present in the nitrate (0.5 per cent.), and a certain amount is oxidised to chlorine by the nitric acid, as well as to nitrosyl chloride, NOCl. Traces of sulphuric acid are carried over from the retort, and the iodate in the nitrate is responsible for the small amounts of iodine. The iron salts, of course, are derived from the apparatus.