The apparatus is shown in Fig. 2O, side elevation, and in Fig. 21 a plan of the nitrating vessel and its accessories is given. In Fig. 20 is shown in sectional elevation one of the trough devices for enabling liquids to be added to those in the nitrating vessel without substantial disturbance.
[Illustration: FIG. 20.—SECTIONAL ELEVATION OF THOMSON'S APPARATUS, a, Tank; b, False Bottom; c, Bottom; c', Ribs; d, Draining Outlet; e, Grid; f, Troughs, with Aprons g; h, Pipe, with Branches h', leading to Troughs, f; k', Outlet Pipe of the Sulphuric Acid Tank k; l, Water Supply Pipe; m, Pipe to supply of Nitrating Acids; o, Perforations of Trough f; p, Cock to remove Acid.]
In carrying out this invention a rectangular lead-lined or earthenware tank a is employed, having a false bottom b, supported by ribs c', over the real bottom c, which slopes down to a draining outlet pipe d, provided with a perforated grid or plate e, adapted to prevent choking of the outlet. Suitably supported near the top of the vessel a are provided two troughs, f having depending aprons g, a pipe h has two branches h', leading to the troughs, f. This pipe h is adapted to be connected by a rubber pipe either to the outlet pipe k' of the sulphuric acid tank k or the water supply pipe l. The nitrating acids are supplied through the pipe m. A charge of mixed nitrating acids is introduced into the vessel a say up to the level n, and the dry cellulose thrown into the acids in small quantities at a time, being pushed under the surface in the usual way.
[Illustration: FIG. 21.—PLAN OF THOMSON'S APPARATUS, a, Tank; b,
False Bottom; c', Ribs; e, Grid; f, Troughs; g, Aprons; h and
h', Pipes to Troughs f; k, Sulphuric Acid Tank; m, Pipe to
Nitrating Acids Tank; o, Perforations of Troughs; p, Cock to remove
Acid.]
A thin layer, say half an inch, of a suitable liquid, preferably sulphuric acid, of a gravity not exceeding that of the waste acid to be produced, is run carefully on the top of the acids by means of the troughs f, which are perforated as shown at o, so that the sulphuric acid runs down the aprons g, and floats on the nitrating acids. The whole is then allowed to stand till nitration has been completed. Water is then supplied to the troughs by way of the pipes l, h, and h', and is allowed to float very gently over the surface of the sulphuric acid, and when a sufficient layer has been formed, the cock p at the bottom of the apparatus is opened, and the acid slowly drawn off, water being supplied to maintain the level constant. It is found that the rate of displacement of the acids is a factor which exerts a considerable influence on the properties of the resulting nitro-cellulose, and affords a means of regulating the temperature of displacement. A rate of displacement which has been found suitable is about two inches in depth of the vessel per hour when treating highly nitrated celluloses, but this rate may, in some cases, be considerably increased. The flow of water at the top of the apparatus is regulated so that a constant level is maintained. By this means the water gradually and entirely displaces the acids from the interstices of the nitro-cellulose, the line of separation between the acids and the water being fairly sharply defined throughout. The flow of water is continued until that issuing at the bottom is found to be free from all trace of acid. The purification of the nitro-cellulose is then proceeded with as usual, either in the same vessel or another.
In the process above described, the object of the introduction of a small layer of sulphuric acid is mainly to prevent the fuming which would otherwise take place, and is not essential, as it is found it can be omitted without any deleterious effect. In order to use the mixed acids in the most economical manner, the waste acid from a previous operation may be used for a first nitration of the cellulose; being afterwards displaced with fresh acids which carry the nitration to the required degree before they are in turn displaced by water. The apparatus may be used merely for the removal of the acid, in which case the nitration is carried out in other vessels in the usual way, and the nitro-cellulose removed to the displacement apparatus where it is just covered with waste acid, and the displacement then proceeded with as above described. In some cases the process is carried out in an ordinary nitrating centrifugal, using the latter to effect preliminary drying after acid extraction. This gives a great advantage over the usual method of working ordinary centrifugal nitrating apparatus, because the acid being removed before the centrifugal is run, practically all danger of firing therein disappears, and a greater proportion of the waste acid is recovered.
In some cases the acids and water may be supplied by perforated pipes, lying along the edges of the nitrating vessel, and these edges may, if desired, be themselves made inclined, like the sides of the troughs f. In the case of effecting nitration in centrifugals as above, the displacing sulphuric acid and water may thus be supplied round the edges of the machines, or removal troughs such as f may be used. It will be obvious that any inert liquid of suitable specific gravity may be used instead of sulphuric acid, as a separation layer.
~Moulding.~—By means of the small measuring tank above referred to, the gun-cotton pulp is drawn off from the stuff chest, and run into moulds of the shapes and sizes required. Thence a large proportion of the water is drawn off by means of tubes connected with the vacuum engine, the moulds having bottoms of fine wire gauze, in order to prevent the pulp from passing through. Hydraulic pressure of about 34 lbs. on the square inch is then applied, which has the effect of compressing the pulp into a state in which it has sufficient consistency to enable it to be handled with care, and also expels a portion of the remaining water.
~Compressing.~—The moulded gun-cotton is now taken to the press house, which is situated at some distance from the rest of the factory. Here the moulds are subjected to powerful hydraulic pressure, from 5 to 6 tons per square inch, and is compressed to one-third of its previous bulk. The slabs or discs thus formed are kept under pressure for a short time, not exceeding a minute and a half, to give the requisite density. It should, when removed, be compact, and just sink in water, and should perceptibly yield to the pressure of the fingers. There are perforations in the press blocks, to allow of the escape of gases, if formed, by reason of sufficient heat being generated. The men working the press are placed under cover, behind strong rope mantlets having eye tubes which command a view of the press.
~Packing.~—The finished slabs and discs are dipped into a solution of soda and carbolic acid, and packed in special wood metal-lined cases. When it is to be sent abroad, the metal lining, which is made of tinned copper, is soldered down, but both the outer wooden and inner metal cases are fitted with air-tight screw-plugs, so that when necessary water can be added without unfastening the cases.