adapted by W. Gossage to the condensation of hydrochloric
acid, are still nearly everywhere in use, frequently combined
with a number of stone tanks through which the gas from
the furnaces travels before entering the towers, meeting
on its way the acid condensed in the tower. This process
is excellent for effecting a complete condensation of the
hydrochloric acid as prescribed by the Alkali Acts, and for
recovering the bulk of the acid in a tolerably strong state,
but less so for recovering nearly the whole of it in the
most concentrated state, although even this is occasionally
attained. On the continent of Europe, where the last-named
requirement has been for a long time more urgent than in Great
Britain, another system has been generally preferred, namely,
passing the gas through a long series of stoneware receivers,
and ultimately through a small tower packed with stoneware or
coke, making the acid flow in the opposite direction to the
gas. Great success has also been obtained by ``plate-towers''
made of stoneware, which allow both the coke-towers and
most of the stoneware receivers to be dispensed with.
3. Preparation of Chlorine.--In this place we speak only
of the preparation of chlorine from hydrochloric acid by
chemical processes; the electrolytic processes will be treated
hereafter. It is clear that free chlorine must be prepared
from hydrochloric acid by oxidizing the hydrogen. This can
be done most easily by ``active'' oxygen, such as is present
in the peroxides, in chromic or permanganic acid. Practically
the only agent employed in this way, and that already by C. W.
Scheele, the discoverer of chlorine, in 1774, is the peroxide
of manganese (manganese dioxide), found in considerable
quantities in nature as ``manganese ore'' (the purest of which
is called pyrolusite), and also artificially regenerated from
the waste liquors of a former operation. Even now, where
chlorine is required for immediate use in some other chemical
operations on a comparatively small scale, it is obtained by
the action of hydrochloric acid on native manganese dioxide,
according to the equation: Mno2 + 4HCl = MnCl2 + Cl2 +
2H2O. This action must be promoted by heating the mixture,
but even then nothing like all of the hydrochloric acid
employed is made to act as above, because the attack on the
manganese ore requires a certain minimum concentration of the
acid. Formerly, instead of free hydrochloric acid a mixture
of common salt and sulphuric acid was sometimes employed, but
this is never done on a manufacturing scale now. Owing to
the impossibility of employing any metal in contact with the
acid, the ``chlorine stills,'' where the above reaction is
carried out, must be made of acid-proof stones or ``chemical''
stoneware. This process is very costly, as much of the
acid and all of the manganese is wasted. Moreover it is
of a most disagreeable kind, as the waste ``still-liquor,''
containing very much free hydrochloric acid and even some free
chlorine, forms a most deleterious impurity when finding its
way into drains or water- courses, apart from the intolerable
nuisance caused by the escapes of chlorine from the stills
and otherwise, which cannot be at all times avoided.
Many endeavours were made to avoid the loss of the
manganese in this operation, but with only partial or no
success. The difficulty was only overcome by the Weldon
process, being the inventions of Walter Weldon from 1866
onwards, and his process up to this day furnishes the greater
proportion of chlorine manufactured in the world. It begins
with ``still-liquor,'' obtained in the old way from native
manganese ore and hydrochloric acid. This liquor is first
treated with carbonate of lime (ground chalk or limestone)
in a ``neutralizing-well,'' made of acid-proof material and
provided with wooden stirring-gear. Here the free hydrochloric
acid is converted into calcium chloride, and at the same time
any ferric chloride present is converted into insoluble ferric
hydroxide: 2FeCl3 + 3CaCO3 + 3 H2O = 2Fe(OH)3 + 3CaCl2 +
3CO2. The sulphuric acid present is mostly precipitated as
calcium sulphate. The mud thus formed is settled out, and
the clear liquor, which is now quite neutral and contains
both manganese and calcium chlorides, is mixed with cream of
lime and treated by a strong current of air, produced by a
blowing-engine. This is done in a tall iron cylinder, say
9 ft. wide and 30 ft. high, called the ``oxidizer.'' The
air-pipe goes right to the bottom of the cylinder and there
branches out into perforated side-pipes, so that the mass
is thoroughly stirred up all the time. The first action of
the lime is to convert the manganese chloride into manganous
hydrate (Mn(OH)2) and calcium chloride; then more lime
is added which greatly promotes and hastens the oxidizing
process. The object of the latter is to convert the
manganous hydroxide by the atmospheric oxygen into manganese
dioxide, but this would take place much too slowly if there
was not an excess of lime present ready to combine with the
manganese dioxide to form a calcium manganite. Only so much
lime is used that an acid manganite is formed corresponding
to one molecule of calcium oxide to two of manganous
oxide. This additional lime, which is called the ``basis,''
certainly takes up hydrochloric acid in the next stage of
the process, but that causes no more waste of acid than
the incomplete action on native manganese ore, mentioned
before. The product obtained, called ``Weldon mud,'' is of
such fine texture that it acts immediately with hydrochloric
acid when mixed with it in the ``Weldon stills'' (fig.
4), and that this acid can be almost entirely neutralized
thereby. The new still liquor formed in this manner is treated
as above, so that the manganese does its work over and over
again. There is only a slight mechanical loss, which
is reduced in the best managed works to about 2 parts of
manganese dioxide to 100 of bleaching- powder. There are
also other advantages of this process which explain its wide
extension, in spite of the fact that only from 30 to 35
parts of the hydrochloric acid employed is converted into
chlorine, the remainder ultimately leaving the factory in the
shape of a harmless but useless solution of calcium chloride.
Weldon's later attempts at superseding his classical process
by other inventions which utilize a larger proportion
of the chlorine, introduced as hydrochloric acid, have
not been successful in the long run, although some of
them were aided by the great technical skill of A. R.
Pechiney. But the Deacon process, the invention of Henry
Deacon (who was greatly aided by his chemist Dr Ferdinand
Hurter), carried out since 1868, has attained to better,
although nothing like complete, success in that direction.
The Deacon process, like the Weldon process, effects its object
by the oxidizing action of atmospheric air, but in a very
different manner. Weldon retained the principle of the Scheele
FIG. 4.--Weldon Chlorine Still. (Sectional Elevation.) Scale
C, Stone steam column resting in stone socket K.
process by employing the active oxygen of manganese dioxide
to convert hydrochloric acid into free chlorine, and he
employed the atmospheric oxygen only indirectly, for the
recovery of manganese dioxide from the manganese chloride
formed. But Deacon worked on the direct reaction: 2HCl +
O = H2O + Cl2. This reaction in ordinary circumstances
is so slow as to be practically useless. If, however, a
``contact-substance'' is employed and that at the proper
temperature, the process goes on at an immensely quickened
rate and can even be carried out as a continuous operation.
The only substance which possesses sufficiently strong
catalytic properties for the reaction is cupric chloride.
If pieces of porous clay are soaked in a solution of this
salt and dried and kept at a temperature of 450 deg. C. (in
practice it is necessary to go to a rather higher temperature),
it is possible continuously to convert a united stream of
hydrochloric acid and atmospheric air, passed through the
contact- substance in a ``decomposer'' (fig. 5), to a larger
extent into chlorine and water, of course mixed with the excess
of oxygen and all the nitrogen of the air. On a small scale
it is possible to push the decomposition as far as 90% of the
hydrochloric acid, but on the large scale only at most 60% is
reached. The mixture of hydrochloric acid and air is taken
directly from the ``decomposing-pan'' of an ordinary salt-cake
furnace, is first cooled down in pipes sufficiently to
condense most of the moisture present (together with about
8% of the hydrochloric acid), and then passed through a
cast-iron superheater and from this into the ``decomposer.''
The gaseous mixture, issuing from the latter, is washed with
water in the usual condensing apparatus, to remove the 40 or
50 parts of hydrochloric acid left unchanged, and can then be
immediately employed for the manufacture of chlorate of potash.
Where (as is the more usual case) the chlorine has to serve for
the manufacture of bleaching-powder, it must first be deprived
of the great amount of moisture which it contains, by means of
FIG. 5.--Deacon ``Decomposer.'' (Sectional Elevation.)
Scale 1/40. a,a, Upright cast-iron cylinders; b,b,
brick jacket; c,c, flues; d,e, iron plates arranged
like venetian blinds, between which the contact-substance
is contained; f, charging hole; g, discharging hole;
h, entrance pipe for gas; i, exit pipe for gas.
coke-towers fed with moderately strong sulphuric acid. As
the gas issuing from these contains only about 5 volumes % of
hydrochloric acid, it cannot be made to act upon lime in the
ordinary bleaching-powder chambers, but specially constructed
chambers must be provided (see fig. 4). The movement of
the gases through all this complicated set of apparatus is
produced by a Root's blower placed at the end of it all.
The Deacon process makes cheaper chlorine than the Weldon
process, but the plant is complicated and costly and the
working requires a great deal of attention. In skilled
hands it has been proved to yield excellent results.
The hydrochloric acid from the calcining-furnaces or
``roasters'' cannot be employed immediately for the Deacon
process, as the sulphuric acid always contained in the roaster
gases soon ``poisons'' the contact-substance and renders it
inoperative. This acid must, therefore, be condensed in the
ordinary way into liquid hydrochloric acid and formerly could
be worked up only by the Weldon process. R. Hasenclever
has overcome this drawback by running this impure acid into
moderately strong sulphuric acid (140 deg. Twaddell), blowing in
air at the same time. This produces a mixed current of pure
hydrochloric acid gas and air, which is carried into a Deacon
decomposer where it acts in the usual manner. The sulphuric
acid, of which 6 or 7 parts are used to one of impure liquid
hydrochloric acid, is always reserved for use in the same
process, by driving off the excess of water in a lead pan,
fired from the top, so that the principal expense of the
process is that of the fuel required for the last operation.
4. Applications of Chlorine.--Some of the chlorine manufactured
(practically only such as is obtained by the electrolysis
of chlorides) is condensed by cold and pressure into liquid
chlorine. If this is anhydrous, as it must be in any case for
this purpose, it does not act upon the metal of the compressors,
nor upon the iron bottles in which it is sent out. It may
