It has been proposed to free nitric acid from the oxides of nitrogen by blowing compressed air through it, and thus driving the gases in solution out. The acid was contained in a closed lead tank, from which the escaping fumes were conducted into the chimney shaft, and on the bottom of which was a lead pipe, bent in the form of a circle, and pierced with holes, through which the compressed air was made to pa.s.s; but the process was not found to be of a very satisfactory nature, and it is certainly better not to allow the formation of these compounds in the manufacture of the acid in the first instance. Another plan, however, is to heat the acid gently, and thus drive out the nitrous gases. Both processes involve loss of nitric acid.

Having obtained nitric and sulphuric acids as pure as possible, the next operation is to mix them. This is best done by weighing the carboys in which the acids are generally stored before the acids are drawn off into them from the condensers, and keeping their weights constantly attached to them by means of a label. It is then a simple matter to weigh off as many carboys of acid as may be required for any number of mixings, and subtract the weights of the carboys. The two acids should, after being weighed, be poured into a tank and mixed, and subsequently allowed to flow into an acid egg or montjus, to be afterwards forced up to the nitrating house in the danger area. The montjus or acid egg is a strong cast-iron tank, of either an egg shape, or a cylinder with a round end. If of the former shape, it would lie on its side, and upon the surface of the ground, and would have a manhole at one end, upon which a lid would be strongly bolted down; but if of the latter shape, the lid, of course, is upon the top, and the montjus itself is let into the ground. In either case, the principle is the same. One pipe, made of stout lead, goes to the bottom, and another just inside to convey the compressed air, the acids flowing away as the pressure is put on, just as blowing down one tube of an ordinary wash- bottle forces the water up the other tube to the jet. The pressure necessarily will, of course, vary immensely, and will depend upon the height to which the acid has to be raised and the distance to be traversed.

The mixed acids having been forced up to the danger area, and to a level higher than the position of the nitrating house, should, before being used, be allowed to cool, and leaden tanks of sufficient capacity to hold at least enough acid for four or five nitrations should be placed in a wooden house upon a level at least 6 or 7 feet above the nitrating house.

In this house also should be a smaller lead tank, holding, when filled to a certain mark, just enough of the mixed acids for one nitration. The object of this tank is, that as soon as the man in charge knows that the last nitration is finished, he refills this smaller tank (which contains just enough of the mixed acids), and allows its contents to flow down into the nitrating house and into the nitrator, ready for the next nitration.

The nitration is usually conducted in a vessel constructed of lead, some 4 feet wide at the bottom, and rather less at the top, and about 4 feet or so high. The size, of course, depends upon the volume of the charge it is intended to nitrate at one operation, but it is always better that the tank should be only two-thirds full. A good charge is 16 cwt. of the mixed acids, in the proportion of three to five; that is, 6 cwt. of nitric acid, and 10 cwt. of sulphuric acid, and 247 lbs. of glycerine.

Upon reference to the equation showing the formation of nitro-glycerine, it will be seen that for every 1 lb. of glycerine 2.47 lbs. of nitro- glycerine should be furnished,[A] but in practice the yield is only a little over 2 lbs., the loss being accounted for by the unavoidable formation of some of the lower nitrate of glycerine (the mono-nitrate), which afterward dissolves in the washing waters. The lead tank (Fig. 5) is generally cased in woodwork, with a platform in front for the man in charge of the nitrating to stand upon, and whence to work the various taps. The top of the tank is closed in with a dome of lead, in which is a small gla.s.s window, through which the progress of the nitrating operation can be watched. From the top of this dome is a tube of lead which is carried up through the roof of the building. It serves as a chimney to carry off the acid fumes which are given off during the nitration. The interior of this tank contains at least three concentric spirals of at least 1-inch lead pipe, through which water can be made to flow during the _whole_ operation of nitrating. Another lead pipe is carried through the dome of the tank, as far as the bottom, where it is bent round in the form of a circle. Through this pipe, which is pierced with small holes, about 1 inch apart, compressed air is forced at a pressure of about 60 lbs. in order to keep the liquids in a state of constant agitation during the whole period of nitration. There must also be a rather wide pipe, of say 2 inches internal diameter, carried through the dome of the tank, which will serve to carry the mixed acid to be used in the operation into the tank.

There is still another pipe to go through the dome, viz., one to carry the glycerine into the tank. This need not be a large bore pipe, as the glycerine is generally added to the mixed acids in a thin stream (an injector is often used).

[Footnote A: Thus if 92 lbs. glycerine give 227 lbs. nitro-glycerine, (277 x 1)/92 = 2.47 lbs.]

[Ill.u.s.tration: FIG. 5.--TOP OF NITRATOR. _A_, Fume Pipe; _B_, Water Pipes for Cooling; _C_, Acid Mixture Pipe; _E_, Compressed Air; _G_, Glycerine Pipe and Funnel; _T_, Thermometer; _W_, Window.]

Before the apparatus is ready for use, it requires to have two thermometers fixed, one long one to reach to the bottom of the tank, and one short one just long enough to dip under the surface of the acids. When the tank contains its charge, the former gives the temperature of the bottom, and the latter of the top of the mixture. The glycerine should be contained in a small cistern, fixed in some convenient spot upon the wall of the nitrating house, and should have a pipe let in flush with the bottom, and going through the dome of the nitrating apparatus. It must of course be provided with a tap or stop-c.o.c.k, which should be placed just above the point where the pipe goes through the lead dome.

Some method of measuring the quant.i.ty of glycerine used must be adopted. A gauge-tube graduated in inches is a very good plan, but it is essential that the graduations should be clearly visible to the operator upon the platform in front of the apparatus. A large tap made of earthenware (and covered with lead) is fixed in the side of the nitrating tank just above the bottom, to run off the charge after nitration. This should be so arranged that the charge may be at option run down the conduit to the next house or discharged into a drowning tank, which may sometimes be necessary in cases of decomposition. The drowning tank is generally some 3 or 4 yards long and several feet deep, lined with cement, and placed close outside the building.

The apparatus having received a charge of mixed acids, the water is started running through the pipes coiled inside the tank, and a slight pressure of compressed air is turned on,[A] to mix the acids up well before starting. The nitration should not be commenced until the two thermometers register a temperature of 18 C. The glycerine tap is then partially opened, and the glycerine slowly admitted, and the compressed air turned on full, until the contents of the apparatus are in a state of very brisk agitation. A pressure of about 40 lbs. is about the minimum (if 247 lbs. of glycerine and 16 cwt. of acids are in the tank). If the glycerine tube is fitted with an injector, it may be turned on almost at once. The nitration will take about thirty minutes to complete, but the compressed air and water should be kept on for an additional ten minutes after this, to give time for all the glycerine to nitrate. The temperature should be kept as low as possible (not above 18 C.).

[Footnote A: At the Halton Factory, Germany, cylinders of compressed carbon dioxide are connected with the air pipes so that in the event of a failure of the air supply the stirring can be continued with this gas if necessary.]

The chief points to attend to during the progress of the nitration are--

1. The temperature registered by the two thermometers.

2. The colour of the nitrous fumes given off (as seen through the little window in the dome of the apparatus).

3. The pressure of the compressed air as seen from a gauge fixed upon the air pipe just before it enters the apparatus.

4. The gauge showing the quant.i.ty of glycerine used. The temperature, as shown by either of the two thermometers, should not be at any time higher than 25 C.

If it rises much above this point, the glycerine should be at once shut off, and the pressure of air increased for some few minutes until the temperature falls, and no more red fumes are given off.

The nitration being finished, the large earthenware tap at the bottom of the tank is opened, and the charge allowed to flow away down the conduit to the next building, i.e., to the separator.

The nitrating house is best built of wood, and should have a close-boarded floor, which should be kept scrupulously clean, and free from grit and sand. A wooden pail and a sponge should be kept in the house in order that the workman may at once clean up any mess that may be made, and a small broom should be handy, in order that any sand, &c., may be at once removed. It is a good plan for the nitrator to keep a book in which he records the time of starting each nitration, the temperature at starting and at the finish, the time occupied, and the date and number of the charge, as this enables the foreman of the danger area at any time to see how many charges have been nitrated, and gives him other useful information conducive to safe working. Edward Liebert has devised an improvement in the treatment of nitro-glycerine. He adds ammonium sulphate or ammonium nitrate to the mixed acids during the operation of nitrating, which he claims destroys the nitrous acid formed according to the equation--

(NH_{4})_{2}SO_{4} + 2HNO_{3} = H_{2}SO_{4} + 2N_{2} + 4H_{2}O.

I am not aware that this modification of the process of nitration is in use at the present time.

The newly made charge of nitro-glycerine, upon leaving the nitrating house, flows away down the conduit, either made of rubber pipes, or better still, of woodwork, lined with lead and covered with lids made of wood (in short lengths), in order that by lifting them at any point the condition of the conduit can be examined, as this is of the greatest importance, and the conduit requires to be frequently washed out and the sulphate of lead removed. This sulphate always contains nitro-glycerine, and should therefore be burnt in some spot far removed from any danger building or magazine, as it frequently explodes with considerable violence.

[Ill.u.s.tration: FIG. 6.--SMALL NITRATOR. _N_, Tap for Discharging; _P_, Water Pipes; _T_, Thermometer; _W_, Windows; _P'_, Glycerine Pipe.]

In works where the manufacture of nitro-glycerine is of secondary importance, and some explosive containing only perhaps 10 per cent. of nitroglycerine is manufactured, and where 50 or 100 lbs. of glycerine are nitrated at one time, a very much smaller nitrating apparatus than the one that has been already described will be probably all that is required. In this case the form of apparatus shown in Fig. 6 will be found very satisfactory. It should be made of stout lead (all lead used for tanks, &c., must be "chemical lead"), and may be made to hold 50 or 100 lbs. as found most convenient. This nitrator can very well be placed in the same house as the separator; in fact, where such a small quant.i.ty of nitro- glycerine is required, the whole series of operations, nitrating, separation, and washing, &c., may very well be performed in the same building. It will of course be necessary to place the nitrator on a higher level than the separator, but this can easily be done by having platforms of different heights, the nitration being performed upon the highest. The construction of this nitrator is essentially the same as in the larger one, the shape only being somewhat different. Two water coils will probably be enough, and one thermometer. It will not be necessary to cover this form in with woodwork.

~The Nathan Nitrator.~[A]--This nitrator is the patent of Lt. Col. F.L.

Nathan and Messrs J.M. Thomson and W. Rintoul of Waltham Abbey, and will probably before long entirely supersede all the other forms of nitrator on account of its efficiency and economy of working. With this nitrator it is possible to obtain from 2.21 to 2.22 parts of nitro-glycerine from every 1 part of glycerine. The apparatus is so arranged that the nitration of the glycerine, the separation of nitro-glycerine produced, as well as the operation of "after-separation," are carried out in one vessel. The usual nitrating vessel is provided with an acid inlet pipe at the bottom, and a gla.s.s separation cylinder with a lateral exit or overflow pipe at the top.

This cylinder is covered by a gla.s.s hood or bell jar during nitration to direct the escaping air and fumes into a fume pipe where the flow of the latter may be a.s.sisted by an air injector. The lateral pipe in the separation cylinder is in connection with a funnel leading to the prewash tank. The drawing (Fig. 7) shows a vertical section of the apparatus; _a_ is the nitrating vessel of usual construction, having at the bottom an acid inlet pipe with three branches, one leading to the de-nitrating plant, _c_ leading to the drowning tank, and _d_, which extends upwards and has two branches, _e_ leading to the nitrating acids tank, and _f_ to the waste acid tank. On the sloped bottom of the nitrating vessel _a_ lies a coil _g_ of perforated pipe for blowing air, and there are in the vessel several coils _h_, three shown in the drawing, for circulation of cooling water. At the top of the vessel there is a gla.s.s cylinder _i_, having a lateral outlet _j_ directed into the funnel mouth of a pipe _k_ leading to the prewash tank. Over the cylinder _i_ is a gla.s.s globe _l_, into which opens a pipe _m_ for leading off fumes which may be promoted by a compressed air jet from a pipe _r_ operating as an injector. Into an opening of the gla.s.s dome _l_ is inserted a vessel _n_, which is connected by a flexible pipe _p_ to the glycerine tank, and from the bottom of _n_, which is perforated and covered with a disc perforated with holes registering with those through the bottom, this disc being connected by a stem with a k.n.o.b _q_ by which it can be turned so as to throttle or cut off pa.s.sage of glycerine through the bottom. _s_ is a thermometer for indicating the temperature of the contents of the vessel.

[Footnote A: Eng. Pat. 15,983, August 1901.]

[Ill.u.s.tration: FIG. 7.--NATHAN'S NITRATOR FOR NITRO-GLYCERINE. (_a_) Nitrating Vessel; (_b_) to Separating Vessel; (_c_) to Drowning Tank; (_e_) Nitrating Acids enter (_f_) to the Waste Acids; (_g_) Coils for Compressed Air; (_h_) Pipes for Cooling Water; (_i_) Gla.s.s Cylinder; (_j_) Outlet to _k_; (_k_) leading to Prewash Tank; (_l_) Gla.s.s Dome; (_m_) Pipe to lead off for Escape of Fumes; (_n_) Vessel; (_p_) Pipe conveying Glycerine; (_q_) k.n.o.b to turn off Glycerine; (_r_) Compressed Air Jet; (_s_) Thermometer.]

In operating with this apparatus the nitrating acid is introduced into the nitrating vessel by opening the c.o.c.k of the pipe _e_. The glycerine is then run in by introducing _n_ and opening the valve at its bottom, the contents of the vessel being agitated by air blown through the perforations of the pipe _g_. When the glycerine is all nitrated and the temperature has slightly fallen, the circulation of the water through the coils _h_ and the air-stirring are stopped, and the glycerine supply vessel _n_ is removed. The nitro-glycerine as it separates from the acids is raised by introducing by the pipe _f_ waste acid from a previous charge, this displacing the nitro-glycerine upwards and causing it to flow by the outlet, _j_ and pipe _k_ to the prewash tank. When nearly all the nitro-glycerine has been separated in this manner the acids in the apparatus may be run off by the pipe _b_ to an after separating vessel for further settling, thus leaving the apparatus free for another nitration, or the nitrating vessel itself may be used as an after separating bottle displacing the nitro-glycerine with waste acid as it rises to the top, or skimming off in the usual manner. When the separation of the nitro- glycerine is complete the waste acid is run off and denitrated as usual, a portion of it being reserved for the displacement of the nitro-glycerine in a subsequent operation.

In a further patent (Eng. Pat. 3,020, 1903) the authors propose with the object of preventing the formation and separation of nitro-glycerine in the waste acids, after the nitro-glycerine initially formed in the nitrating vessel has been separated and removed, to add a small quant.i.ty of water to the waste acids; this is carried out as follows. A relatively small quant.i.ty of water is added, and this prevents all further separation of nitro-glycerine, and at the same time the strength of the waste acids is so slightly reduced that their separation and re-concentration are not affected. "After-separation" is thus done away with, and the nitro- glycerine plant simplified and its output increased. After nitration separation is commenced at a temperature such that when all the displacing acid has been added, and the separation of the nitro-glycerine is complete, the temperature of the contents of the nitrating vessel shall not be lower than 15 C. A sufficient quant.i.ty of the displacing acid is then run off through the waste-acid c.o.c.k to allow of the remaining acids being air-stirred without splashing over the top. A small quant.i.ty of water, from 2 to 3 per cent. according to strength of acid; if waste consists of sulphuric acid (monohydrate), 62 per cent.; nitric acid (anhydrous), 33 per cent. and water 5 per cent.; temperature 15 C., then 2 per cent. of water is added; if waste acids contain less than 4 per cent. of water of temperature lower than 15 C., from 3 to 5 per cent. of water may have to be added. The water is added slowly through the separator cylinder, and the contents of the nitrator air-stirred, but not cooled, the temperature being allowed to rise slowly and regularly as the water is added--usually about 3 C. for each per cent. of water added.

When air-agitation has been stopped, the acids are kept at rest for a short time, in order to allow of any small quant.i.ty of initially formed nitro-glycerine adhering to the coils and sides of the vessel rising to the top. When this has been separated by displacement, the acids are ready for denitration, or can be safely stored without further precaution.

~Separation.~--The nitro-glycerine, together with the mixed acids, flows from the nitrating house to the separating house, which must be on a lower level than the former. The separating house contains a large lead-lined tank, closed in at the top with a wooden lid, into which a lead pipe of large bore is fixed, and which is carried up through the roof of the building, and acts as a chimney to carry off any fumes. A little gla.s.s window should be fixed in this pipe in order that the colour of the escaping fumes may be seen. The conduit conveying the nitro-glycerine enters the building close under the roof, and discharges its contents into the tank through the pipe G (Fig. 8). The tank is only about two-thirds filled by the charge. There is in the side of the tank a small window of thick plate gla.s.s, which enables the workman to see the level of the charge, and also to observe the progress of the separation, which will take from thirty minutes to one hour.

The tank should be in connection with a drowning tank, as the charge sometimes gets very dangerous in this building. It must also be connected by a conduit with the filter house, and also to the secondary separator by another conduit. The tank should also be fitted with a compressed air pipe, bent in the form of a loop. It should lie upon the bottom of the vat. The object of this is to mix up the charge in case it should get too hot through decomposition. A thermometer should of course be fixed in the lid of the tank, and its bulb should reach down to the middle of the nitro-glycerine (which rests upon the surface of the mixed acids, the specific gravity of the nitro-glycerine being 1.6, and that of the waste acids 1.7; the composition of the acids is now 11 per cent. HNO_{3}, 67 per cent. H_{2}SO_{4}, and 22 per cent. water), and the temperature carefully watched.

[Ill.u.s.tration: FIG. 8.--SEPARATOR. _A_, Compressed Air Pipes; _G_, Nitro- glycerine enters from Nitrator; _N_, Nitro-glycerine to _P_; _L_, Lantern Window; _W_, Window in Side; _S_, Waste Acids to Secondary Separator; _T_, Tap to remove last traces of Nitro-glycerine; _P_, Lead Washing Tank; _A_, Compressed Air; _W_, Water Pipe; _N_, Nitro-glycerine from Separator.]

If nothing unusual occurs, and it has not been necessary to bring the compressed air into use, and so disturb the process of separation, the waste acids may be run away from beneath the nitro-glycerine, and allowed to flow away to the secondary separator, where any further quant.i.ty of nitro-glycerine that they contain separates out after resting for some days. The nitro-glycerine itself is run into a smaller tank in the same house, where it is washed three or four times with its own bulk of water, containing about 3 lbs. of carbonate of soda to neutralise the remaining acid. This smaller tank should contain a lead pipe, pierced and coiled upon the bottom, through which compressed air may be pa.s.sed, in order to stir up the charge with the water and soda. After this preliminary washing, the nitro-glycerine is drawn off into indiarubber buckets, and poured down the conduit to the filter house. The wash waters may be sent down a conduit to another building, in order to allow the small quant.i.ty of nitro-glycerine that has been retained in the water as minute globules to settle, if thought worth the trouble of saving. This, of course, will depend upon the usual out-turn of nitro-glycerine in a day, and the general scale of operations.

[Ill.u.s.tration: FIG. 9.--FILTERING AND WASHING PLANT. _W_, Lead Washing Tank; _WP_, Water Pipe; _L_, Lid; _S_, Nitro-glycerine from Separator; _A, B, C_, Filtering Tanks; _B2_, Indiarubber Bucket.]

~Filtering and Washing.~--The filter house (Fig. 9), which must of course be again on a somewhat lower level than the separating house, must be a considerably larger building than either the nitrating or separating houses, as it is always necessary to be washing some five or six charges at the same time. Upon the arrival of the nitro-glycerine at this house, it first flows into a lead-lined wooden tank (W), containing a compressed air pipe, just like the one in the small tank in the separating house.

This tank is half filled with water, and the compressed air is turned on from half to a quarter of an hour after the introduction of the charge.

The water is then drawn off, and fresh water added. Four or five washings are generally necessary. The nitro-glycerine is then run into the next tank (A), the top of which is on a level with the bottom of the first one.

Across the top of this tank is stretched a frame of flannel, through which the nitroglycerine has to filter. This removes any solid matters, such as dirt or sc.u.m. Upon leaving this tank, it pa.s.ses through a similar flannel frame across another tank (B), and is finally drawn off by a tap in the bottom of the tank into rubber buckets. The taps in these tanks are best made of vulcanite.

At this stage, a sample should be taken to the laboratory and tested. If the sample will not pa.s.s the tests, which is often the case, the charge must be rewashed for one hour, or some other time, according to the judgment of the chemist in charge. In the case of an obstinate charge, it is of much more avail to wash a large number of times with small quant.i.ties of water, and for a short time, than to use a lot of water and wash for half an hour. Plenty of compressed air should be used, as the compound nitric ethers which are formed are thus got rid of. As five or six charges are often in this house at one time, it is necessary to have as many tanks arranged in tiers, otherwise one or two refractory charges would stop the nitrating house and the rest of the nitro-glycerine plant.

The chief causes of the washed material not pa.s.sing the heat test are, either that the acids were not clean, or they contained objectionable impurities, or more frequently, the quality of the glycerine used. The glycerine used for making nitro-glycerine should conform to the following tests, some of which, however, are of greater importance than others. The glycerine should--

1. Have minimum specific gravity at 15 C. of 1.261.

2. Should nitrify well.

3. Separation should be sharp within half an hour, without the separation of flocculent matter, nor should any white flocculent matter (due to fatty acids) be formed when the nitrated glycerine is thrown into water and neutralised with carbonate of soda.

4. Should be free from lime and chlorine, and contain only traces of a.r.s.enic, sulphuric acid, &c.

5. Should not leave more than 0.25 per cent. of inorganic and organic residue together when evaporated in a platinum dish without ebullition (about 160 C.) or partial decomposition.

6. Silver test fair.

7. The glycerine, when diluted one-half, should give no deposit or separation of fatty acids when nitric peroxide gas is pa.s.sed through it.

(Nos. 1, 2, 3, and 5 are the most essential.)

The white flocculent matter sometimes formed is a very great nuisance, and any sample of glycerol which gives such a precipitate when tried in the laboratory should at once be rejected, as it will give no end of trouble in the separating house, and also in the filter house, and it will be very difficult indeed to make the nitro-glycerine pa.s.s the heat test. The out- turn of nitro-glycerine also will be very low. The trouble will show itself chiefly in the separating operation. Very often 2 or 3 inches will rise to the surface or hang about in the nitro-glycerine, and at the point of contact between it and the mixed acids, and will afterwards be very difficult to get rid of by filtration. The material appears to be partly an emulsion of the glycerine, and partly due to fatty acids, and as there appears to be no really satisfactory method of preventing its formation, or of getting rid of it, the better plan is not to use any glycerine for nitrating that has been found by experiment upon the laboratory scale to give this objectionable matter. One of the most useful methods of testing the glycerine, other than nitrating, is to dilute the sample one-half with water, and then to pa.s.s a current of nitric peroxide gas through it, when a flocculent precipitate of eladic acid (less soluble in glycerine than the original oleic acid) will be formed. Nitrogen peroxide, N_{2}O_{4}, is best obtained by heating dry lead nitrate (see Allen, "Commercial Organic a.n.a.lysis," vol. ii., 301).

When a sample of nitro-glycerine is brought to the laboratory from the filter house, it should first be examined to see that it is not acid.[A] A weak solution of Congo red or methyl orange may be used. If it appears to be decidedly alkaline, it should be poured into a separating funnel, and shaken with a little distilled water. This should be repeated, and the washings (about 400 c.c.) run into a beaker, a drop of Congo red or methyl orange added, and a drop or so of N/2 hydrochloric acid added, when it should give, with two or three drops at most, a blue colour with the Congo red, or pink with the methyl orange, &c. The object of this test is to show that the nitro-glycerine is free from any excess of soda, i.e., that the soda has been properly washed out, otherwise the heat test will show the sample to be better than it is. The heat test must also be applied.

[Footnote A: A. Leroux, _Bul. Soc. Chim. de Bel._, xix., August 1905, contends that experience does not warrant the a.s.sumption that free acid is a source of danger in nitro-glycerine or nitro-cellulose; free alkali, he states, promotes their decomposition.]