LIQUEFIED GASES Liquid Oxygen Explosives consist of mixtures of liquid oxygen with combustible substances. The combustible substances (lamp-black, wood pulp, cork dust, hydrocarbons absorbed in inert materials, or even pulverized metals) are packed in absorb ent paper or cloth cartridges, and immediately before use the cartridges are soaked in liquid oxygen until saturated. The cart ridges are removed from the liquid and fired with a detonator in a manner similar to that employed with ordinary dynamites. In the process of explosion an extremely rapid reaction takes place between the liquid oxygen and the combustible material, with the production of a large quantity of gas at high temperature and pressure. The mechanism of the explosion is similar to that tak ing place in other explosive mixtures, except that the oxygen supply is obtained from liquid oxygen instead of from chemically combined oxygen. The advantages of these explosives are that neither the cartridge nor the liquid oxygen are explosive until brought together and, due to the low temperature ( —183° C) of liquid oxygen, the oxygen completely evaporates in a relatively short time after the cartridges have been removed from the liquid, and they are then once more non-explosive. This latter fact gives rise to the disadvantage, however, that the explosive strength di minishes as the oxygen evaporates, and if too long a time elapses before the cartridge is fired, insufficient oxygen is available for complete combustion to (carbon dioxide), and large quanti ties of toxic CO (carbon monoxide) may be formed. This consti tutes a hazard in the use of this explosive in underground mining. Under favourable conditions liquid oxygen explosives are con siderably cheaper than dynamite.
Practical Applications.—In practice, liquid oxygen is manu factured at the mine or quarry by liquefaction of air and subse quent separation of the nitrogen by fractional distillation. Liquid oxygen is a bluish liquid and boils at —183° C. It cannot be stored under pressure but is kept in double-walled vacuum-in sulated metal containers and allowed to evaporate slowly to corn pensate for the heat conduction from the surroundings. These containers have been built with capacities as high as i,000 gal., but the ordinary sizes hold 4 to 25 gallons. The cartridges are soaked in insulated dipping containers just before use, and are then loaded in the bore-holes and fired as soon as possible.
An investigation of liquid oxygen explosives by the U.S. Bureau of Mines has shown that the packing density, or weight of com bustible in unit volume of the cartridge, is one of the most im portant factors in determining its explosive characteristics. Lamp black or gas-black was shown to be the optimum material for most purposes. The available life of a cartridge, or time after removal from the liquid during which it could be exploded effec tively, was found to augment with increasing cartridge diameter; e.g., a cartridge 12 in. in diameter had a life of about 3o min., whereas one 6 in. in diameter had a life of several hours. The explosive strength of the cartridge was found to bear a direct relation to the amount of oxygen present per unit weight of com bustible, and greater explosive effect was obtained by increasing the packing density.
The explosive strength of the cartridge was found to be a vari able quantity which depended upon the composition of the cart ridge material, density of packing, size of cartridge and time of evaporation after cartridges are removed from the liquid and before firing. Cartridges begin to lose strength almost from the moment they are removed from the liquid. It has been shown by tests with the ballistic pendulum (see BALLISTICS) at the explo sives experiment station of the U.S. Bureau of Mines that II in. cartridges packed with gas-black, fired 5 min. after removal from the dipping container, may have a strength 1o% greater than 40% gelatin dynamite, volume for volume; but no cartridge material has been found which will develop a greater relative strength than this. If fired after io min. evaporation, the strength is about 95% that of dynamite. On the other hand, if longer life is desired, a cartridge packed to a lower density will retain enough oxygen for complete combustion for io min., but will have an explosive strength less than the more dense cartridge. After 5 min. evaporation the strength of such a cartridge will be about 99% that of 4o% gelatin dynamite, and after io min. evaporation about 8o% the strength of dynamite. The rate of detonation of liquid oxygen explosives ranges from 3,00o to 6,000 metres per second, depending on the composition of the com bustible material.
History.—The possibility of using liquid oxygen as one of the components of an explosive was perhaps first suggested by Spren gel, in 1873, who stated that the beau ideal of a detonating ex plosive was a mixture of eight parts of liquid oxygen and one part of liquid hydrogen. The possibilities were foreseen at this early date, but obviously they could not be given serious con sideration until liquid oxygen could be produced in commercial quantities. In 1895, Linde evolved a method of producing liquid air commercially, and in 1897 he introduced a liquid air explosive "oxyliquit." This was first tried out in 1895 in a cross-cut of the Penzberg coal mine in upper Bavaria; but the first large-scale blasting was not done until 1899, when liquid oxygen explosives were used in driving the 12 m. Simplon tunnel between Italy and Switzerland. Little further work was done until the beginning of the World War, when the need for conserving glycerine and ni trates forced Germany to look for substitutes for use in industrial explosives. Under this stimulus, the use of liquid oxygen explo sives grew until in 1917 there was a total of 16o coal, metal, salt and potash mines using liquid oxygen explosives where the plants had an hourly production of 4,86o litres. The use has dropped off to some extent since 1918, but in 1926 there was still more than 1,000,000 litres of liquid oxygen produced in Germany for blast ing purposes. The explosive was used with some success after the war in the iron mines of Lorraine, and is now used to a limited but increasing extent in open-cut blasting in the United States (mainly coal) and in Chile (copper ore).
(a C. T P1 Any substance that changes chemically and with great rapidity from its natural state to that of a large volume of gases, accom panied with heat, is classed as an explosive in America. The transportation of explosives in the United States is regulated by the Interstate Commerce Commission with the co-operation of the Bureau for the Safe Transportation of Explosives and the Institute of Makers of Explosives. In the United States explosives may be classified as follows : blasting powders, dynamites, other high explosives, initiating explosives (detonators), military ex plosives, shotgun, rifle and common powders.
Blasting Powders.—Black powder for blasting purposes con tains sodium nitrate instead of the more expensive potassium nitrate, together with charcoal and sulphur. This is known as "B" blasting powder. About 8o% of the blasting powder manufactured in the United States is used in the mining of coal. The granula tions of black blasting powder vary widely. Its strength is com paratively low and is the same for all granulations, but as the finer granulations burn faster than the coarser ones their effect is a trifle more shattering. Black powder containing potassium nitrate, instead of sodium nitrate, and known as "A" blasting powder, is used for black sporting powders, for the manufacture of safety fuse, for pyrotechnics and, to a very limited extent, in quarrying dimension stone. In recent years a mixture of various grains of black blasting powder has been used successfully, under certain conditions, for column-loading in deep-hole blasting in quarries, in conjunction with Cordeau-Bickford detonating fuse extending the entire length of the bore-hole. Granulated black powder is also compressed into cylindrical pellets having a central perfora tion, which are wrapped in cartridge paper and fired by a fuse.
Dynamites.—The majority of dynamites in the United States consist of an explosive compound, usually nitroglycerine, mixed with an absorbent composed of sodium nitrate and wood meal or other carbonaceous material. The proportions are so chosen that the mixture will have an excess of available oxygen on ex plosion and therefore will produce a minimum of carbon monoxide and other poisonous gases. The use of Kieselguhr as an absorbent was abandoned many years ago in the United States. A dynamite containing no nitroglycerine, but nitrostarch and a mixture of oxidizing and carbonaceous ingredients, finds considerable use in the United States.
It has the advantage of not freezing at any temperature and of not producing headaches, which are often caused by nitroglycerine dynamites. Dynamites are divided into the following types: straight nitroglycerine, ammonia, gelatine, ammonia gelatine, blasting gelatine and permissible dynamites. All dynamite, with the exception of the larger diameters, is packed in cartridge form by large, semi-automatic machines. Straight nitroglycerine dyna mites are made in strengths from i 5% to 6o%. Their percentage strength represents the equivalent of the same proportion of liquid nitroglycerine. The nitroglycerine is mixed with a combustible absorbent which contributes to the force of the explosion.
Straight nitroglycerine dynamites have a relatively high rate of detonation, and theref ore are used wherever a quick action or a shattering effect is required. They resist water fairly well. Prac tically all dynamites made in the United States, shown in the above classification, with the exception of blasting gelatine, contain low freezing ingredients which prevent freezing at prevailing winter temperatures. These low-freezing ingredients are either tetra nitrodiglycerine, nitro-compounds, or both.
Ammonia dynamites, the most widely used, contain in place of part of the nitroglycerine ammonium nitrate. This salt slows down the velocity of detonation somewhat, and makes the action less shattering. They are safer to handle than straight nitroglycerine dynamites, and their gases are less objectionable.
Gelatine dynamites contain, instead of nitroglycerine, a colloidal solution of nitrocotton in nitroglycerine, absorbed in a mixture of sodium nitrate and wood meal. Their plastic consistency allows them to be extruded through a nipple of the required diameter into paraffin-paper shells to form cartridges. Gelatine dynamite is water-proof, and is used for wet work and under-water blasting. The strength of these dynamites ranges from 20% to 90%. They produce the least amount of noxious gases of any type of explosive.
Ammonia gelatine dynamites resemble the above except that a portion of the nitrocotton-nitroglycerine colloid is replaced by ammonium nitrate. Their explosive characteristics resemble gela tine dynamite.
Blasting gelatine consists of nitroglycerine in which has been dissolved nitrocotton to form a jelly, together with a small propor tion of an antacid, such as chalk. It is a tough, rubbery material upon which water has no effect, and is only used where the greatest possible concentration of energy is required. Because nitrogly cerine has a better colloiding action on nitrocotton than the low freezing tetranitrodiglycerine, the latter is not used in blasting gelatine. Blasting gelatine is the strongest and one of the quickest explosives manufactured for commercial use.
Permissible Explosives.—This is one which has passed certain tests of the U.S. Bureau of Mines to determine its safety for use in coal mines, and is permissible when used in accordance with conditions prescribed by the bureau.
The principal characteristics of permissible dynamites are that the flame of explosion is relatively cool, small and of short dura tion. There are three types of permissible explosives, viz., nitroglycerine, ammonium nitrate and gelatine. Very little of the nitroglycerine type of permissible explosive is made. The principal type permissible contains about io% nitroglycerine and a large proportion of ammonium nitrate, together with a small amount of wood meal or other carbonaceous ingredient. The relatively cool flame from the explosion of ammonium nitrate does not ignite gas or dust in a coal mine when used under conditions prescribed by the Bureau of Mines. Gelatine permissible dynamites contain flame-quenching salts.
A recent advancement in the explosives industry is the produc tion of bulky dynamites. One hundred pounds of straight nitro glycerine dynamite contains approximately 200 cartridges of the i in. by 8 in. size. Between 6o% and 7o% of all dynamite used in the United States is of this standard size. Because of the desirability in many operations of spreading the explosive force along the bore-hole instead of concentrating it at the back, ammo nium nitrate dynamites have been increased in bulk until certain of them are now manufactured to contain Soo of the above size cartridges to the hundred pounds. The advantages attained are a less shattering action and a resulting economy. In recent years considerable improvement has been made in the reduction of the amount of poisonous gases evolved on explosion of dynamites. This has been accomplished mainly by improving the oxygen bal ance of the ingredients and reducing the weight of the paper wrapper.
Other High Explosives.—Practically all nitroglycerine made in the United States consists of a mixture of nitroglycerine and tetranitrodiglycerine, or a mixture of nitroglycerine and nitrated sugar. These are made by nitrating a mixture of glycerine and diglycerine, or a solution of sugar in glycerine. Liquid nitrogly cerine transportation over common carriers in the United States is forbidden. Other than its use in the manufacture of explosives, its only application is in oil-well shooting. Very small quantities of nitroglycerine are used for medicinal purposes in glonoin, a dilute solution of nitroglycerine in ethyl alcohol.
Cordeau-Bickford (detonating fuse) consists of a small-bore lead tube made in lengths of several hundred feet and containing compressed trinitrotoluene (T.N.T.). When a blasting cap is detonated at one end of Cordeau-Bickford, it initiates a detonation of the trinitrotoluene, which detonation travels along the ex plosive at a high uniform velocity. This detonating fuse finds a wide use in simultaneously firing a series of loaded bore-holes, for which purpose it is put into the bore-hole with the dynamite and extended to a main line above ground. When the main line of fuse is detonated, the explosive wave travelling along detonates each branch leading to the several bore-holes, and thus detonates all of the charges in rapid, almost simtiltaneous succession. (Liquid oxygen [L.O.X.] and liquid carbon dioxide as explosives are described above.) Initiating Explosives.—Initiating explosives comprise many organic compounds and inorganic salts, only a few of which have obtained commercial importance as constituents of detonator compositions. The most important and the most widely used at present is mercury fulminate (q.v.). This is very sensitive to heat, shock and friction and detonates with great violence. When used in detonators it is usually mixed in the proportion of 8o parts of fulminate to 20 parts of fine, dry potassium chlorate. The or dinary No. 6 detonator contains one gram of this mixture. Lead azide (lead trinitride), although not used in the United States, is another important initiating explosive.
It is used in blasting caps, but only as a priming or top charge over organic explosives which do not by themselves ignite by the flame from a fuse. It is frequently mixed with lead trinitroresor cinate to improve the ease of ignition by a fuse. Organic explosives not detonated directly by the spark from a fuse, such as tetryl (tetranitromethylaniline), T.N.T. (trinitrotoluene), picric acid (trinitrophenol) and nitromannite are frequently used in deto nators as base charges beneath an initiating charge of fulminate of mercury.
Picric acid, or trinitrophenol (q.v.), for use as a shell filler, has been largely displaced by T.N.T. and amatol, principally on account of inconvenience in the manufacture of picric acid and the fact that it is likely to form, with the metal of the shell, highly sensitive salts which might cause premature detonation. Like amatol, picric acid requires a booster charge for proper detona tion. Ammonium picrate was used during the World War as a shell filler for very large shell. Gun cotton (q.v.) was once used in torpedoes and submarine mines, but has been rendered obsolete, in the United States, by T.N.T. and amatol. Nitrostarch explo sives were used extensively during the World War for filling hand and rifle grenades.
Single base powders are made by colloiding nitrocotton of a suit able degree of nitration with a solvent, forming into grains of various sizes and shapes and dried. Double base powders are made by colloiding nitrocotton with nitroglycerine and a solvent and forming into grains. As a rule smokeless powders for large guns are made in the form of cylindrical pellets containing several longitudinal perforations. The object of these perforations is the attainment of lower breech pressure and higher average pressure during the travel of the projectile along the barrel. This is effected by a uniformly accelerated burning speed, due to the increasing area of the internal perforations as combustion proceeds.
(N. D. R.)