~The Waste Acids.~—The waste acids from the separating house, from which the nitro-glycerine has been as completely separated as possible, are run down the conduit to the secondary separator, in order to recover the last traces of nitro-glycerine that they contain. The composition of the waste acids is generally somewhat as follows:—Specific gravity, 1.7075 at 15° C.; sulphuric acid, 67.2 per cent.; nitric acid, 11.05 per cent.; and water, 21.7 per cent., with perhaps as much as 2 per cent. of nitric oxide, and of course varying quantities of nitro-glycerine, which must be separated, as it is impossible to run this liquid away (unless it can be run into the sea) or to recover the acids by distillation as long as it contains this substance. The mixture, therefore, is generally run into large circular lead-lined tanks, covered in, and very much like the nitrating apparatus in construction, that is, they contain worms coiled round inside, to allow of water being run through to keep the mixture cool, and a compressed air pipe, in order to agitate the mixture if necessary. The top also should contain a window, in order to allow of the interior being seen, and should have a leaden chimney to carry off the fumes which may arise from decomposition. It is also useful to have a glass tube of 3 or 4 inches in diameter substituted for about a foot of the lead chimney, in order that the man on duty can at any time see the colour of the fumes arising from the liquid. There should also be two thermometers, one long one reaching to the bottom of the tank, and one to just a few inches below the surface of the liquid.
The nitro-glycerine, of course, collects upon the surface, and can be drawn off by a tap placed at a convenient height for the purpose. The cover of the tank is generally conical, and is joined to a glass cylinder, which is cemented to the top of this lead cover, and also to the lead chimney. In this glass cylinder is a hole into which fits a ground glass stopper, through which the nitro-glycerine can be drawn off. There will probably never be more than an inch of nitro-glycerine at the most, and seldom that. It should be taken to the filter house and treated along with another charge. The acids themselves may either be run to waste, or better treated by some denitration plant. This house probably requires more attention than any other in the danger area, on account of the danger of the decomposition of the small quantities of nitro-glycerine, which, as it is mixed with such a large quantity of acids and water, is very apt to become hot, and decomposition, which sets up in spots where a little globule of nitro-glycerine is floating, surrounded by acids that gradually get hot, gives off nitrous fumes, and perhaps explodes, and thus causes the sudden explosion of the whole. The only way to prevent this is for the workman in charge to look at the thermometers frequently, and at the colour of the escaping fumes, and if he should notice a rise of temperature or any appearance of red fumes, to turn on the water and air, and stir up the mixture, when probably the temperature will suddenly fall, and the fumes cease to come off.
The cause of explosions in this building is either the non-attention of the workmen in charge, or the bursting of one of the water pipes, by which means, of course, the water, finding its way into the acids, causes a sudden rise of temperature. If the latter of these two causes should occur, the water should at once be shut off and the air turned on full, but if it is seen that an explosion is likely to occur, the tank should at once be emptied by allowing its contents to run away into a drowning tank placed close outside the house, which should be about 4 feet deep, and some 16 feet long by 6 feet wide; in fact, large enough to hold a considerable quantity of water. But this last course should only be resorted to as a last extremity, as it is extremely troublesome to recover the small quantity of nitro-glycerine from the bottom of this tank, which is generally a bricked and cemented excavation some few yards from the house.
It has been proposed to treat these waste acids, containing nitro- glycerine, in Mr M. Prentice's nitric acid retort. In this case they would be run into the retort, together with nitrate of soda, in a fine stream, and the small quantity of nitro-glycerine, coming into contact with the hot mixture already in the retort, would probably be at once decomposed. This process, although not yet tried, promises to be a success. Several processes have been used for the denitration of these acids.
~Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and
Gun-Cotton.~—The composition of these acids is as follows:—
Nitro-glycerine and Gun-cotton
Waste Acid.
Sulphuric acid 70 per cent. 78 per cent.
Nitric acid 10 " 12 "
Water 20 " 10 "
The waste acid from the manufacture of gun-cotton is generally used direct for the manufacture of nitric acid, as it contains a fairly large amount of sulphuric acid, and the small amount of nitro-cellulose which it also generally contains decomposes gradually and without explosion in the retort. Nitric acid may be first distilled off, the resulting sulphuric acid being then added to the equivalent amount of nitrate of soda. Nitric acid is then distilled over and condensed in the usual way. Very often, however, the waste acid is added direct to the charge of nitrate without previously eliminating the nitric acid. The treatment of the waste acid from the manufacture of nitro-glycerine is somewhat different. The small amount of nitro-glycerine in this acid must always be eliminated. This is effected either by allowing the waste acid to stand for at least twenty- four hours in a big vessel with a conical top, where all the nitro- glycerine which will have separated to the surface is removed by skimming; or, better still, the "watering down process" of Col. Nathan may be employed. In Nathan's nitrator every existing trace of nitro-glycerine is separated from the acids in a few hours after the nitration, and any further formation of nitro-glycerine is prevented by adding about 2 per cent. of water to the waste acids, which are kept agitated during the addition. The waste acid, now free from nitro-glycerine, but which may still contain organic matter, is denitrated by bringing it into contact with a jet of steam. The waste acid is passed in a small stream down through a tower of acid-resisting stoneware (volvic stone), which is closely packed with earthenware, and at the bottom of which is the steam jet. Decomposition proceeds as the acid meets the steam, nitric and nitrous acids are disengaged and are passed out at the top of the tower through a pipe to a series of condensers and towers, where the nitric acid is collected. The nitrous acid may be converted into nitric acid by introducing a hot compressed air jet into the gases before they pass into the condensers. Weak sulphuric acid of sp. gr. 1.6 collects in a saucer in which the tower stands, and is then passed through a cooling worm. The weak sulphuric acid, now entirely free from nitric and nitrous acids, may be concentrated to sp. gr. 1.842 and 96 per cent. H_{2}SO_{4} by any of the well-known processes, e.g., Kessler, Webb, Benker, Delplace, &c., and it may be used again in the manufacture of nitro-glycerine or gun-cotton.
Two points in the manufacture of nitro-glycerine are of the greatest importance, viz., the purity of the glycerine used, and the strength and purity of the acids used in the nitration. With regard to the first of these, great care should be taken, and a complete analysis and thorough examination, including a preliminary experimental nitration, should always be instituted. As regards the second, the sulphuric acid should not only be strong (96 per cent.), but as free from impurities as possible. With the nitric acid, which is generally made at the explosive works where it is used, care must be taken that it is as strong as possible (97 per cent. and upwards). This can easily be obtained if the plant designed by Mr Oscar Guttmann[A] is used. Having worked Mr Guttmann's plant for some time, I can testify as to its value and efficiency.
[Footnote A: "The Manufacture of Nitric Acid," Jour. Soc. Chem. Ind.,
March 1893.]