Part 4 (1/2)
A mixture of ground c.o.ke and lime is introduced into the furnace through which an electric arc has been drawn. The materials unite and form an ingot of very pure carbide surrounded by a crust of less purity. The poorer crust is rejected in breaking up the ma.s.s into lumps which are graded according to their size. The largest size is 2 by 3-1/2 inches and is called ”lump,”
a medium size is 1/2 by 2 inches and is called ”egg,” an intermediate size for certain types of generators is 3/8 by 1-1/4 inches and called ”nut,”
and the finely crushed pieces for use in still other types of generators are 1/12 by 1/4 inch in size and are called ”quarter.” Instructions as to the size best suited to different generators are furnished by the makers of those instruments.
These sizes are packed in air-tight sheet steel drums containing 100 pounds each. The Union Carbide Company of Chicago and New York, operating under patents, manufactures and distributes the supply of calcium carbide for the entire United States. Plants for this manufacture are established at Niagara Falls, New York, and Sault Ste. Marie, Michigan. This company maintains a system of warehouses in more than one hundred and ten cities, where large stocks of all sizes are carried.
The National Board of Fire Underwriters gives the following rules for the storage of carbide:
Calcium carbide in quant.i.ties not to exceed six hundred pounds may be stored, when contained in approved metal packages not to exceed one hundred pounds each, inside insured property, provided that the place of storage be dry, waterproof and well ventilated and also provided that all but one of the packages in any one building shall be sealed and that seals shall not be broken so long as there is carbide in excess of one pound in any other unsealed package in the building.
Calcium carbide in quant.i.ties in excess of six hundred pounds must be stored above ground in detached buildings, used exclusively for the storage of calcium carbide, in approved metal packages, and such buildings shall be constructed to be dry, waterproof and well ventilated.
_Properties of Acetylene._--This gas is composed of twenty-four parts of carbon and two parts of hydrogen by weight and is cla.s.sed with natural gas, petroleum, etc., as one of the hydrocarbons. This gas contains the highest percentage of carbon known to exist in any combination of this form and it may therefore be considered as gaseous carbon. Carbon is the fuel that is used in all forms of combustion and is present in all fuels from whatever source or in whatever form. Acetylene is therefore the most powerful of all fuel gases and is able to give to the torch flame in welding the highest temperature of any flame.
Acetylene is a colorless and tasteless gas, possessed of a peculiar and penetrating odor. The least trace in the air of a room is easily noticed, and if this odor is detected about an apparatus in operation, it is certain to indicate a leakage of gas through faulty piping, open valves, broken hose or otherwise. This leakage must be prevented before proceeding with the work to be done.
All gases which burn in air will, when mixed with air previous to ignition, produce more or less violent explosions, if fired. To this rule acetylene is no exception. One measure of acetylene and twelve and one-half of air are required for complete combustion; this is therefore the proportion for the most perfect explosion. This is not the only possible mixture that will explode, for all proportions from three to thirty per cent of acetylene in air will explode with more or less force if ignited.
The igniting point of acetylene is lower than that of coal gas, being about 900 degrees Fahrenheit as against eleven hundred degrees for coal gas. The gas issuing from a torch will ignite if allowed to play on the tip of a lighted cigar.
It is still further true that acetylene, at some pressures, greater than normal, has under most favorable conditions for the effect, been found to explode; yet it may be stated with perfect confidence that under no circ.u.mstances has anyone ever secured an explosion in it when subjected to pressures not exceeding fifteen pounds to the square inch.
Although not exploded by the application of high heat, acetylene is injured by such treatment. It is partly converted, by high heat, into other compounds, thus lessening the actual quant.i.ty of the gas, wasting it and polluting the rest by the introduction of substances which do not belong there. These compounds remain in part with the gas, causing it to burn with a persistent smoky flame and with the deposit of objectionable tarry substances. Where the gas is generated without undue rise of temperature these difficulties are avoided.
_Purification of Acetylene._--Impurities in this gas are caused by impurities in the calcium carbide from which it is made or by improper methods and lack of care in generation. Impurities from the material will be considered first.
Impurities in the carbide may be further divided into two cla.s.ses: those which exert no action on water and those which act with the water to throw off other gaseous products which remain in the acetylene. Those impurities which exert no action on the water consist of c.o.ke that has not been changed in the furnace and sand and some other substances which are harmless except that they increase the ash left after the acetylene has been generated.
An a.n.a.lysis of the gas coming from a typical generator is as follows:
Per cent Acetylene ................................ 99.36 Oxygen ................................... .08 Nitrogen ................................. .11 Hydrogen ................................. .06 Sulphuretted Hydrogen .................... .17 Phosph.o.r.etted Hydrogen ................... .04 Ammonia .................................. .10 Silicon Hydride .......................... .03 Carbon Monoxide .......................... .01 Methane .................................. .04
The oxygen, nitrogen, hydrogen, methane and carbon monoxide are either harmless or are present in such small quant.i.ties as to be neglected. The phosph.o.r.etted hydrogen and silicon hydride are self-inflammable gases when exposed to the air, but their quant.i.ty is so very small that this possibility may be dismissed. The ammonia and sulphuretted hydrogen are almost entirely dissolved by the water used in the gas generator. The surest way to avoid impure gas is to use high-grade calcium carbide in the generator and the carbide of American manufacture is now so pure that it never causes trouble.
The first and most important purification to which the gas is subjected is its pa.s.sage through the body of water in the generator as it bubbles to the top. It is then filtered through felt to remove the solid particles of lime dust and other impurities which float in the gas.
Further purification to remove the remaining ammonia, sulphuretted hydrogen and phosphorus containing compounds is accomplished by chemical means. If this is considered necessary it can be easily accomplished by readily available purifying apparatus which can be attached to any generator or inserted between the generator and torch outlets. The following mixtures have been used.
”_Heratol,_” a solution of chromic acid or sulphuric acid absorbed in porous earth.
”_Acagine,_” a mixture of bleaching powder with fifteen per cent of lead chromate.
”_Puratylene,_” a mixture of bleaching powder and hydroxide of lime, made very porous, and containing from eighteen to twenty per cent of active chlorine.
”_Frankoline,_” a mixture of cuprous and ferric chlorides dissolved in strong hydrochloric acid absorbed in infusorial earth.
A test for impure acetylene gas is made by placing a drop of ten per cent solution of silver nitrate on a white blotter and holding the paper in a stream of gas coming from the torch tip. Blackening of the paper in a short length of time indicates impurities.
_Acetylene in Tanks._--Acetylene is soluble in water to a very limited extent, too limited to be of practical use. There is only one liquid that possesses sufficient power of containing acetylene in solution to be of commercial value, this being the liquid acetone. Acetone is produced in various ways, oftentimes from the distillation of wood. It is a transparent, colorless liquid that flows with ease. It boils at 133 Fahrenheit, is inflammable and burns with a luminous flame. It has a peculiar but rather agreeable odor.
Acetone dissolves twenty-four times its own bulk of acetylene at ordinary atmospheric pressure. If this pressure is increased to two atmospheres, 14.7 pounds above ordinary pressure, it will dissolve just twice as much of the gas and for each atmosphere that the pressure is increased it will dissolve as much more.