carbide is heated to complete liquefaction and tapped at short
intervals. There is no unreduced material, and the process
is considerably simplified, while less expensive plant is
required. The run carbide, however, is never so rich as
the ingot carbide, since an excess of lime is nearly always
used in the mixture to act as a flux, and this remaining
in the carbide lowers its gas-yielding power. Many
attempts have been made to produce the substance without
electricity, but have met with no commercial success.
Properties of calcium carbide.
Calcium carbide, as formed in the electric furnace, is a
beautiful crystalline semi-metallic solid, having a density
of 2.22, and showing a fracture which is often shot with
iridescent colours. It can be kept unaltered in dry air,
but the smallest trace of moisture in the atmosphere leads
to the evolution of minute quantities of acetylene and gives
it a distinctive odour. It is infusible at temperatures up
to 2000 deg. C., but can he fused in the electric arc. When
heated to a temperature of 245 deg. C. in a stream of chlorine
gas it becomes incandescent, forming calcium chloride and
liberating carbon, and it can also be made to burn in oxygen
at a dull red heat, leaving behind a residue of calcium
carbonate. Under the same conditions it becomes incandescent
in the vapour of sulphur, yielding calcium sulphide and
carbon disulphide; the vapour of phosphorus will also unite
with it at a red heat. Acted upon by water it is at once
decomposed, yielding acetylene and calcium hydrate. Pure
crystalline calcium carbide yields 5.8 cubic feet of acetylene
per pound at ordinary temperatures, but the carbide as sold
commercially, being a mixture of the pure crystalline material
with the crust which in the electric furnace surrounds the
ingot, yields at the best 5 cubic feet of gas per pound under
proper conditions of generation. The volume of gas obtained;
however, depends very largely upon the form of apparatus
used, and while some will give the full volume, other
apparatus will only yield, with the same carbide, 3 3/4 feet.
Impurities.
The purity of the carbide entirely depends on the purity
of the material used in its manufacture, and before this
fact had been fully grasped by manufacturers, and only the
purest material obtainable employed, it contained notable
quantities of compounds which during its decomposition by
water yielded a somewhat high portion of impurities in the
acetylene generated from it. Although at the present time
a marvellous improvement has taken place all round in the
quality of the carbide produced, the acetylene nearly always
contains minute traces of hydrogen, ammonia, sulphuretted
hydrogen, phosphuretted hydrogen, silicon hydride, nitrogen and
oxygen, and sometimes minute traces of carbon monoxide and
dioxide. The formation of hydrogen is caused by small
traces of metallic calcium occasionally found free in the
carbide, and cases have been known where this was present in
such quantities that the evolved gas contained nearly 20% of
hydrogen. This takes place when in the manufacture of the
carbide the material is kept too long in contact with the
arc, since this overheating causes the dissociation of
some of the calcium carbide and the solution of metallic
calcium in the remainder. The presence of free hydrogen is
nearly always accompanied by silicon hydride formed by the
combination of the nascent hydrogen with the silicon in the
carbide. The ammonia found in the acetylene is probably partly
due to the presence of magnesium nitride in the carbide.
On decomposition by water, ammonia is produced by the action
of steam or of nascent hydrogen on the nitride, the quantity
formed depending very largely upon the temperature at which
the carbide is decomposed. The formation of nitrides and
cyanamides by actions of this kind and their easy conversion
into ammonia is a useful method for fixing the nitrogen
of the atmosphere and rendering it available for manurial
purposes. Sulphuretted hydrogen, which is invariably present
in commercial acetylene, is formed by the decomposition of
aluminium sulphide. A. Mourlot has shown that aluminium
sulphide, zinc sulphide and cadmium sulphide are the only
sulphur compounds which can resist the heat of the electric
furnace without decomposition or volatilization, and of
these aluminium sulphide is the only one which is decomposed
by water with the evolution of sulphuretted hydrogen.
In the early samples of carbide this compound used to be
present in considerable quantity, but now rarely more than
1/10 % is to be found. Phosphuretted hydrogen, one of the
most important impurities, which has been blamed for the
haze formed by the combustion of acetylene under certain
conditions, is produced by the action of water upon traces
of calcium phosphide found in carbide. Although at first it
was no uncommon thing to find 1/2% of phosphuretted hydrogen
present in the acetylene, this has now been so reduced
by the use of pure materials that the quantity is rarely
above 0.15%, and it is often not one-fifth of that amount.
Generation of acetylene from carbide.
In the generation of acetylene from calcium carbide and water,
all that has to be done is to bring these two compounds into
contact, when they mutually react upon each other with the
formation of lime and acetylene, while, if there be sufficient
water present, the lime combines with it to form calcium hydrate.
Calcium carbide. Water. Acetylene. Lime.
CaC2 + H2O = C2H2 + CaO
Lime. Water. Calcium hydrate.
CaO + H2O = Ca(HO)2
The decomposition of the carbide by water may be brought
about either by bringing the water slowly into contact with
an excess of carbide, or by dropping the carbide into an
excess of water, and these two main operations again may
be varied by innumerable ingenious devices by which the
rapidity of the contact may be modified or even eventually
stopped. The result is that although the forms of apparatus
utilized for this purpose are all based on the one fundamental
principle of bringing about the contact of the carbide with
the water which is to enter into double decomposition with
it, they have been multiplied in number to a very large
extent by the methods employed in order to ensure control in
working, and to get away from the dangers and inconveniences
which are inseparable from a too rapid generation.
Generators.
In attempting to classify acetylene generators some authorities
have divided them into as many as six different classes,
but this is hardly necessary, as they may be divided into
two main classes---first, those in which water is brought in
contact with the carbide, the carbide being in excess during
the first portion of the operation; and, second, those in
which the carbide is thrown into water, the amount of water
present being always in excess. The first class may again
be subdivided into generators in which the water rises in
contact with the carbide, in which it drips upon the carbide,
and in which a vessel full of carbide is lowered into water
and again with-drawn as generation becomes excessive. Some
of these generators are constructed to make the gas only
as fast as it is consumed at the burner, with the object
of saving the expense and room which would be involved by a
storage-holder. Generators with devices for regulating and
stopping at will the action going on are generally termed
``automatic.'' Another set merely aims at developing the gas
from the carbide and putting it into a storageholder with as
little loss as possible, and these are termed ``non-automatic.''
The points to be attained in a good generator are:--
1. Low temperature of generation.
2. Complete decomposition of the carbide.
3. Maximum evolution of the gas.
4. Low pressure in every part of the apparatus.
5. Ease in charging and removal of residues.
6. Removal of all air from the apparatus before generation of the gas.
When carbide is acted upon by water considerable heat is
evolved; indeed, the action develops about one-twentieth of
the heat evolved by the combustion of carbon. As, however,
the temperature developed is a function of the time needed
to complete the action, the degree of heat attained varies
with every form of generator, and while the water in one form
may never reach the boiling-point, the carbide in another
may become red-hot and give a temperature of over 800 deg. C.
Heating in a generator is not only a source of danger, but
also lessens the yield of gas and deteriorates its quality.
The best forms of generator are either those in which water
rises slowly in contact with the carbide, or the second main
division in which the carbide falls into excess of water.
Purification
It is clear that acetylene, if it is to be used on a large
scale as a domestic illuminant, must undergo such processes
of purification as will render it harmless and innocuous
to health and property, and the sooner it is recognized as
absolutely essential to purify acetylene before consuming it
the sooner will the gas acquire the popularity it deserves.
The only one of the impurities which offers any difficulty
in removal is the phosphuretted hydrogen. There are three
substances which can be relied on more or less to remove
this compound, and the gas to be purified may be passed
either through acid copper salts, through bleaching powder
or through chromic acid. In experiments with those various
bodies it is found that they are all of them effective in
also ridding the acetylene of the ammonia and sulphuretted
hydrogen, provided only that the surface area presented to
the gas is sufficiently large. The method of washing the gas
with acid solutions of copper has been patented by A. Frank
of Charlottenburg, who finds that a concentrated solution
of cuprous chloride in an acid, the liquid being made into
a paste with kieselguhr, is the most effective. Where the
production of acetylene is going on on a small scale this
method of purification is undoubtedly the most convenient
one, as the acid present absorbs the ammonia, and the copper
salt converts the phosphuretted and sulphuretted hydrogen
into phosphates and sulphides. The vessel, however, which
contains this mixture has to be of earthenware, porcelain
or enamelled iron on account of the free acid present; the
gas must be washed after purification to remove traces of
hydrochloric acid, and care must be taken to prevent the
complete neutralization of the acid by the ammonia present
in the gas. The second process is one patented by Fritz
Ullmann of Geneva, who utilizes chromic acid to oxidize
the phosphuretted and sulphuretted hydrogen and absorb the
ammonia, and this method of purification has proved the most
successful in practice, the chromic acid being absorbed by
kieselguhr and the material sold under the name of ``Heratol.''
The third process owes its inception to G. Lunge, who
recommends the use of bleaching powder. Dr P. Wolff has found
that when this is used on the large scale there is a risk
of the ammonia present in the acetylene forming traces of
chloride of nitrogen in the purifying-boxes, and as this is
a compound which detonates with considerable local force,
it occasionally gives rise to explosions in the purifying
apparatus. If, however, the gas be first passed through
a scrubber so as to wash out the ammonia this danger is
avoided. Dr Wolff employs purifiers in which the gas is
washed with water containing calcium chloride, and then passed
