ACID
AND
ALKA: ' I MANUFACTURE 43 reactions induced in this way, practically the whole of the again for absorbing nitrous vapours or any other purpose S02 is ultimately converted into sulphuric acid, and at desired. Since that time, in every properly appointed the same time the nitrogen oxides are always recovered sulphuric acid manufactory, the following cycle of operawith comparatively very slight losses and made to serve tions is canied out. Jo begin with, in the burners pyrites over again. (or, as the case may be, brimstone or blende) is made to The reactions taking place in the vitriol chambers are yield hot burner-gas, containing about 7 per cent, (in the very complicated, and have been explained in many differ- case of brimstone 10 or 11 per cent.) of S09. This, after ent ways. The view now accepted by most chemists is having been deprived of most of the flue-dust, is passed that developed by Lunge, according to which there are through the “ Glover tower,” i.e., an upright cylindrical or two principal reactions succeeding each other, it may be square tower, consisting of a leaden shell lined with heatin quite contiguous places, but under different conditions. and acid-proof stone or brick, and loosely filled or “ packed ” Where the nitrous fumes prevail and there is less water with the same material, over which a mixture of acid from present, sulphur dioxide combines with nitrous acid and the Gay-Lussac tower and from the chambers trickles oxygen to form nitroso-sulphuric acid, a crystalline sub- down in such proportions that it arrives at the bottom as stance of the formula S02(0H)(0N0). The reaction is denitrated acid of from 78 to 80 per cent. The gases therefore:—S02 + 0 + N02H = S05NH. The solid sub- then pass on to the “chambers,” structures of lead, stance is, however, only exceptionally met with, as it at generally about 20 feet wide, 18 feet high, and 100 or once dissolves in the mist of sulphuric acid floating in 150 feet long. Several such chambers are usually comthe chamber, and forms “nitrous vitriol.” Wherever this bined to a “set,” with a cubic capacity of as much as nitrous vitriol comes into contact with liquid water {not 150,000 cubic feet or even more. Here the gases meet steam), which is also present in the chamber in the shape with more nitrous vapours, and with steam, or with water, of mist, and practically as dilute sulphuric acid, it is converted into a fine dust or spray. Here the reactions decomposed into sulphuric and nitrous acid, thus: — sketched above take place, so that “ chamber-acid ” as S02(0H)(0N0) + H20 = H2S04 + N02H. The re-formed already described is formed, while a mixture of gases nitrous acid, although not stable, any~ more than is its escapes containing all the atmospheric nitrogen, some anhydride, N203, is nevertheless the “oxygen carrier” in oxygen in excess, about 0‘5 per cent, of the total SO„, and question, as the products of its spontaneous decomposition, some oxides of nitrogen. This gas is now passed through when meeting with other compounds, always react like nitrous the Gay-Lussac tower, which somewhat resembles the acid itself and thus may transfer an indefinite quantity of Glover tower, but is usually filled with coke, over which oxygen to the corresponding quantities of S02 and H20, sulphuric acid of about 80 per cent., H2S04, trickles with the corresponding formation of H2S04. This theory down in sufficient quantity to retain the nitrous vapours. at once explains, among other things, why the acid formed Ultimately the waste gas is drawn off by a chimney, in the vitriol chambers always contains an excess of water or sometimes by mechanical means. (the second of the above-quoted reactions requiring the Of course a great many special improvements have been mass action ” of this excess), and why the external cool- made in the plant and the working of chamber systems; ing produced by the contact of the chamber sides with the of these we mention only some of the most important air is of great importance {liquid water in the shape of By judiciously watching all stages of the process, by a mist of dilute sulphuric acid being necessary for the obseiving the draught, the strength of the acid produced, process). the temperature, and especially by frequent analyses of The commercial production of sulphuric acid impera- the gases, the yield of acid has been brought up to 98 per tively requires that the nitrogen oxides (which originally cent, of the theoretical maximum, with a loss of nitre were always introduced in the shape of nitric acid) should sometimes as low as two parts to 100 of sulphur burned. be available as long as possible, before being lost mechanic- The supply of the nitric acid required to make up this ally or by reduction to the inactive forms of nitrous oxide loss is obtained in England by “potting,” that is, by or elementary nitrogen. The first step towards securing decomposing solid nitrate of soda by sulphuric acid in a this requirement was taken as early as 1827 by Gay- flue between the pyrites burners and the chambers. On Lussac, who discovered that the nitrous fumes, otherwise the Continent makers generally prefer to employ liquid carried away from the lead chambers by the waste atmo- nitric acid, which is run through the Glover tower tospheric nitrogen and oxygen, could be retained by bringing gether with the nitrous vitriol. Although this method the gases into contact with moderately strong sulphuric appears more troublesome, it allows the amount of nitre acid, the result being the formation of nitroso-sulphuric to be more easily and more accurately regulated. The size acid : 2H2S04 + N203 = 2S02(0H)(0N0) + H20, and the of the Glover towers, and more especially that of the Gaylatter remaining dissolved in sulphuric acid as “ nitrous Lussac towers, has been progressively increased, and vitriol.” But this important invention was of little use thereby the cube of the lead chambers themselves has until John Glover, about 1866, found that the nitrous been diminished to a much greater extent. By improved vitriol could be most easily reintroduced into the process “ packing,” the towers have been rendered more durable, by subjecting it to the action of burner-gas, before this and in the case of the Gay-Lussac tower, the loss of nitre enters into the lead chambers, preferably after diluting it has been diminished by avoiding the use of a coke packwith chamber acid, that is, acid of from 65 to 70 per cent., ing, which acts upon that substance as a reducing agent. H2S04, as formed in the lead chambers. The reaction is Many attempts have been made to reduce the chamber then : 2S02(0H)(0N0) + S02 + 2H20 = 3H2S04 + 2NO ; space by apparatus intended to bring about a better that is to say, all the “ nitre ” is returned to the chambers mixture of the gases, and to facilitate the interaction of in the shape of NO; the sulphuric acid employed in the the misty particles of nitrous vitriol and dilute acid floatGay - Lussac process is not merely recovered, but an ing in the chamber with each other, and with the chamber additional quantity is formed from fresh S09 ; as the heat atmosphere. The earliest really successful, and still the of the burner-gases also comes into play, much water is most generally applied apparatus of this kind, is the evaporated which supplies part of the steam required for Lunge-Rohrmann “plate columns” or “reaction towers” the working of the chambers; and the acid issues from placed between the chambers, but though this and similar the apparatus in a “ denitrated ” and sufficiently concen- apparatus has proved to be very useful in the later stages trated state (78 to 80 per cent. H2S04) to be used over of the process, it has not been found practicable to do
