The manufacture of nitric acid is very often combined with that of suIphuric, especially in those factories where the former is employed in making the vitriol. Large quantities, however, are produced for general manufacturing purposes.
Practically all nitric acid is now made by treating sodium nitrate (p. 119), with sulphuric acid, in cast-iron retorts. The reactions are as follows:-
1) KaX03 + H2S04 = NaHS04 + I-IK03•
2) 2 NaNOa +H2S04 = Na2S04 +2 HR03•
In practice, the quantities of material used do not correspond with either of these equations, but the charge is so regulated that a mixture of acid and neutral sulphates of sodium, which remains liquid at the temperature employed, is left in the retort. If reaction (1) were followed, too much sulphuric acid would be used for proHtable working, except in soda works, where the resulting acid sulphate might be used in the salt-cake furnace. If reaction (2) is carried out, the temperature must be very high, the neutral sulphate solidifies in the retort and is difficult to remove, and the resulting nitric acid may be partly decomposed by the heat before it can escape from the retort, thus causing a diminished yield and a product discolored by the oxides of nitrogen produced.
The sulphuric acid employed is usually that from the lead pan evaporation (sp. gr. 1.70), but for nitric acid above 1.38 sp. gr., oil of vitriol of 66° Be. is used, although this decomposes part of the nitric acid formed. The sodium nitrate used is the purified CI~ili saltpetre, containing from 98 to 99 per cent of NaNOs, when dried. It should be free from sodium chloride to avoid contaminating the nitric acid with hydrochloric acid. The size of the charge depends on the capacity of the plant, but in some more modern factories it amounts to as much as 1200 pounds of nitrate, with somewhat more than an equal weight of sulphuric acid (sp. gr. 1.70).
'1'he-old style of plant consisted of a horizontal cast-iron cylinder, 5 or 6 feet long, set over a fireplace in such a way that the flames played over the sides and top, heating all parts to a high temperature. Another and better form of plant is shown in Fig. 49, in which the cast-iron retort (A) is entirely surrounded by the flames from the grate. Cast iron is but little attacked by concentrated nitric acid or its vapors, and it is important to keep the retort hot enough in all parts, to prevent condensation of the acid. A more modern retort is shown in Fig. 50. In the lower part of the retort is a pipe, by which the melte!l residue of "nitre cake" is run off, after the reaction is finished. :For condensing the acid vapors which escape from the retort, a series of glass or earthen ware 'Woulfe bottles (bombonnes) (B, B, lTig.49) are employed. The first two or three of these bottles are generally placed over the flue by which the fire gases pass to the chimney. Being thus warmed, there is less danger of breakage by the high heat of the vapors from the retort. At the end of the series is usually placed a coke towel', fed with water or concentrated sulphuric acid, to condense the fumes escaping fro111the bombonnes. Usually, no water is admitted to the bombonnes unless a dilute acid is required; but they are sometimes placed en cascade, to allow the condensed acid to flow through the series in a direction opposite to the movement of the acid vapors.
The most concentrated acid is condensed in the first two or three bomboHnes, but is contaminated with sulphuric acid and nitrogen oxides. The last of the series contains dilute acid, which is C011- taminated with chlorine. In the middle bottles is a pure acid of moderate strength. Owing to more or less reduction of the nitric acid in the retort, the condensed acid has a yellow or red color, due to the absorbed nitrous vapors. These are undesirable in a commercial acid, and must be removed by "bleaching "; the acid is heated to about DO° C., and warm ail' blown in, which cal'l'ics away the nitrogen oxides, and is then passed through the coke tower for their recovery. Guttmann's apparatus * is more modern. The large cast-iron retort (Fig. 51) is made in three pieces and is entirely surrounded by the flames from the grate. The gases from the retort pass into a system (Fig, 52) of vertical earthenware pipes (AA), having very thin walls and joined at the top by 1800 bends, while they open at the bottom into a nearly horizontal collecting pipe (B B), which is divided into chambers by diaphragms. These chambers are joined by U-tubes, passing under the diaphragms. The diaphragms force the acid vapors to pass up one pipe and down of the vertical pipes allow very efficient cooling by exposure to the air alone, or they may be placed in a tank of cold water, as represented in the figure. By this very rapid cooling, the acid vapors are condensed quickly. Hot air at 80° C. is injected from (F) into the outlet: pipe (D), where it converts some of the nitrous vapors to nitric acid, increasing the yield materially. (The uncondensed nitrous vapors pass into the Lunge-Rohrmann plate tower (E) (p. 58), where the nitrogen oxides are absorbed in sulphuric acid or water.) If the vapors remain in the retort too long, part of the acid is decomposed, and nitrogen peroxide is formed, and absorbed by the condensed acid, to which it imparts a red color. But since there is a good draught through the apparatus, the vapors are drawn out of the retort very soon after they are evolved, and are at once condensed. Thus very little peroxide is formed, and a light colored, concentrated acid is obtained directly. It is claimed that acid of 40° Be. (1.38 sp. gr.), requiring no "bleaching," may be thus made, and that, with water-cooled pipes, 98 per cent of the theoretical yield is obtained as concentrated acid, while 2 per cent condenses in the Lunge-Rohrmann tower.
Hart's tube condenser (Fig. 53), for nitric acid, is a new form of apparatus. It is made of glass and earthenware tubes, and is placed above the brick arch covering the retort, thus occupying but little floor space. The vapor from the retort I III A (A) passes into the pot (8), and thence through the vertical earthenware tube (C). From (C) to (D) extend a number of glass tubes, which FIG. r;.~. are slightly inclined towards (C), and which are cooled by jets of water from the perforated pipe (EE). From (D), the uncondensed vapors pass to a Lunge tower or a coke tower. The acid condensed in the glass tubes flows back into (C), and then into (8), thus coming into contact with the hot vapors from the retort. This heats the acid so hot, that all the the U-tube (F) into bottles or carboys. In this apparatus, the acid is condensed very quickly and but little nitrogen peroxide is formed. If frothing occurs in the retort, the overflow is caught in the pot (B), and may be removed without difficulty. The flow of water from (E) can be so regulated that all or nearly all of it is evaporated on the s11rface of the glass tubes, thus securing the greatest cooling effect, with small consumption of water. Any condenser water not evaporated is caught in the trough (G). The chief repairs are of broken tubes, which are cheaply and easily replaced. The strength of the nitric acid produced in any apparatus depends upon the strength of the sulphuric acid, on the temperature of the retort, and on the purity of the sodium nitrate. "With sulphuric acid of 1.71 sp. gr., the nitric acid varies from 1.38 to 1.42 sp. gr: (400to 42° Be.). If the sodium nitrate contains chlorides, some of the nitric acid is decomposed by the hydrochloric acid produced; thus:-
HN03 + HCl = H,O +NO,+ Cl.
lt is claimed that with Guttmann's apparatus, an acid of 1.5 to 1.52 sp. gr. (50° Be.), containing about 95 per cen~ HX03, can be made.
For chemically pure acid, perfectly pure materials should be used, although formerly the common acid was purified by treating with silver and barium nitrates, and redistilling. But concentrated acid cannot be distilled without some decomposition, and the product must be "bleached" by heating and blowing in pure air. Fuming nitric acid is a solution of nitrogen peroxide in concentrated
nitric acid. lt is red in color and has a specific gravity of 1.55 to 1.62. To make this, perfectly dry sodium nitrate amI oil of vitriol (1.84 sp. gr.) are used. The reaction is carried so far that neutral sulphate of sodium is formed by the action of the acid sulphate on the nitrate: -
2:NaN03 +2 NaHS04 = 2:Na,S04 +2 :NO, + H,O +O.
The nitrogen peroxide formed dissolves in the nitric acid to form the fuming acid. A little powdered starch is sometimes added to assist in the reduction of "~henitric acid. An impure fuming acid is sometimes prepared by distilling a mixture of concentrated nitric and sulphuric acids.
Nitric acid is largely used in the manufacture of explosives; for parting gold and silver; in the manufacture of coal-tar dyes; as a various metallic nitrates. It is a colorless liquid, boiling at S(i° C., but with decomposition .. (See :above.) The pure acid also decomposes on exposure to strong light and becomes yellow (X02). "Then it acts on metals, the hydrogen liberated at once reduces some of the nitric :tcid itself, setting free various oxides of nitrogen, of which nitrie oxide is the most prominent. Ordin:try commercial nitric acid (1.42 sp. gr.) distills at 12:30 C., and contains :tbont GSto (iD per cent lINOs, and c01'l'esponds nearly to the formula 2 HXOs+3 H20. The very concentrated acid of 1.50 sp. gr. contains about 9.1 pel' cent HNOs' 'rhe acid sodium sulphate left in the retort after making nitric acid IS called" nitre cake," and is often used in the charge for making sulphate in the Leblanc process.
Various other processes for making nitric acid have been suggested, but owing to the low price of Chili saltpetre, none of them are now in use. An ingenious proposal to use the" still liquors" from the chlorine manufacture, instead of sulplnll'ic acid for decomposing sodium nitrate, is based on the following reactions:-
1) 10 )faNOs + 5l\InC12 = 2 :\[nO + 3 ::\ln02 +10 NaCl +10N02 + O2,
2) 10 N02 + O2 +4 H20 = SHN03 + 2 NO.
3) 2:N0 + 3 0 (ail') + H20 = 2 lINOs'
This permits the manganese to be recovered in a form suitatble for use in the chlorine stills again. The "still liquor" is evaporated to dryness, the pulverized residue mixed with dry sodium nitrate, and the mixture heated to about 230 C. in a retort. Reaction (1) takes place, and the gases, consisting of a mixture of nitrogen peroxide and oxygen, are led into a tower and condensed with water as per reaction (2). The nitric oxide produced is treated with air and steam and condensed according to reaction (3).