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.