This must be constructed in such a manner that the bicarbonate,
which always contains some ammonium salts, is first freed
from these by moderate heating (of course taking care that the
ammonia is completely recovered), and later on, by raising the
temperature, it is decomposed into solid sodium carbonate and
gaseous carbon dioxide. The former needs only grinding to
constitute the final product, ammonia- soda ash; the latter is
again employed in the process of treating the ammoniacal salt
solution with carbon dioxide. Various forms of apparatus are
employed for this treatment of the crude bicarbonate--sometimes
semi-circular troughs with mechanical agitators on the
principle of the Thelen pan (see above)--all acting on the
principle that the escaping ammonia and carbon dioxide must
be fully utilized over again. The soda-ash obtained in the
end is of a high degree of purity, testing from 98 to 99%
Na2CO3, the remaining 1 or 2% consisting principally of NaCl.
A very important part of the process has still to be described,
viz. the recovery of the ammonia from the mother-liquor
coming from the vacuum filters and various washing liquors.
Unless this recovery is carried out in the most efficient
manner, the process cannot possibly pay; but so much progress
has been made in this direction that the loss of ammonia
is very slight indeed, merely a fraction per cent. The
ammonia is for the major part found in the mother-liquor as
ammonium chloride. A smaller but still considerable portion
exists here and in the washings in the shape of ammonium
carbonates. These compounds differ in their behaviour to
heat. The ammonium carbonates are driven out from their
solutions by mere prolonged boiling, being thereby decomposed
into ammonia, carbon dioxide and water, but the ammonium
chloride is not volatile under these conditions, and must
be decomposed by milk of lime: 2NH4Cl + Ca(OH)2 = 2NH3
+ CaCl2 + 2H2O. The solution of calcium chloride is run
to waste, the ammonia is re-introduced into the process.
Both these reactions are carried out in tall cylindrical columns
or ``stills,'' Consisting of a number of superposed cylinders,
having perforated horizontal partitions, and provided with
a steam-heating arrangement in the enlarged bottom portion.
The milk of lime is introduced at a certain distance from the
bottom. The steam causes the action of the lime on the
ammonium chloride to take place in this lower portion of the
still, from which the steam, mixed with all the liberated
ammonia, rises into the upper portion of the column where its
heat serves to drive out the volatile ammonium carbonate. Just
below the top there is a cooling arrangement, so that nearly
all the water is condensed and runs back into the column, while
the ammonia, with the carbon dioxide formerly combined with
part of it, passes on first through an outside cooler where
the remaining water is condensed, and afterwards into the
vessels, already described, where the ammonia is absorbed by
a solution of salt and thus again introduced into the process.
The reversible character of the principal reaction has
the consequence that a considerable portion of the sodium
chloride (up to 33%) is lost, being contained in the waste
calcium chloride solution which issues from the ammonia
stills. This is, however, not of much importance, as it
had been introduced in the shape of a brine where its value
is very slight (6d. per ton of NaCl). It is true that all
the chlorine combined with the sodium is lost partly as NaCl
and partly as CaCl2; none of the innumerable attempts at
recovering the chlorine from the waste liquor has been made to
pay, and success is less likely than ever since the perfection
of the electrolytic processes. (See CHLORINE.) For all
that, especially in consequence of the small amount of fuel
required, and the total absence of the necessity of employing
sulphur compounds as an intermediary, the ammonia-soda process
has supplanted the Leblanc process almost entirely on the
continent of Europe and to a great extent in Great Britain.
III. ELECTROLITIC ALKALI MANUFACTURE
In theory by far the simplest process for making alkalis together
with free chlorine is the electrolysis of sodium (or potassium)
chloride. When this takes place in an aqueous solution, the
alkaline metal at once reacts with the water, so that a solution
of an alkaline hydrate is formed while hydrogen escapes. The
reactions are therefore (we shall in this case speak only of the
sodium compounds): (1) NaCl = Na + Cl, (2) Na + H2O = NaOH + H.
The chlorine escapes at the anode, the hydrogen at the
cathode. If the chlorine and the sodiun hydrate can act upon
each other within the liquid, bleach-liquors are formed: 2NaOH
+ Cl2 = NaOCl + NaOH + H2O. The production of these for the
use of papermakers and bleachers of textile fabrics has become
an important industry, but does not enter into our province.
If, however, the action of the chlorine on the sodium hydrate is
prevented, which can be done in various ways, they can both be
collected in the isolated state and utilized as has been previously
described, viz. the chlorine can be used for the manufacture of
liquid chlorine, bleaching-powder or other bleaching compounds,
or chlorates, and the solution of sodium hydrate can be sold
as such, or converted into solid caustic soda. Precisely the
same can be done in the electrolysis of potassium chloride.
There is a third way of conducting the action, viz. so that
the chlorine can act upon the caustic soda or potash at a
higher concentration and temperature, in which case chlorates
are directly formed in the liquid: KCl + 8H2O = KClO3 +
8H2. This has indeed become the principal, because it is the
cheapest, process for the manufacture of potassium and sodium
chlorate. Perchlorates can also be made in this way.
In all these cases the chlorine, or the products made from
it, really play a greater part than the alkali. From 58.5
parts by weight of NaCl we obtain theoretically 23Na = 40NaOH =
53Na2CO3, together with 35.5 Cl, or 100 bleaching-powder.
As the weight of bleaching-powder consumed in the world is at
most one-fifth of that of alkali, calculated as Na2CO3, it
follows that only about one-tenth of all the alkali required
could be made by electrolysis, even supposing the Leblanc process
to be entirely abolished. The remaining nine-tenths of alkali
must be supplied from other sources, chiefly the ammonia-soda
process. As long as the operation of the Leblanc process is
continued, it will supply a certain share of both kinds of
products. Trustworthy statistics on this point cannot be
obtained, because most firms withhold any information
as to the extent of their production from the public.
The first patents for the electrolysis of alkaline chlorides
were taken out in 1851 and several others later on; but
commercial success was utterly impossible until the invention
of the dynamo machine allowed the production of the electric
current at a sufficiently cheap rate. The first application
of this machine for the present purpose seems to have been
made in 1875 and the number of patents soon rapidly increased;
but although a large amount of capital was invested and
many very ingenious inventions made their appearance, it
took nearly another twenty years before the manufacture of
alkali in this way was carried out in a continuous way on a
large scale and with profitable results. A little earlier
the manufacture of potassium chlorate (on the large scale
since 1890) had been brought to a definite success by H.
Gall and the Vicomte A. de Montlaur; a few years later the
processes worked out at the Griesheim alkali works (near
Frankfort) for the manufacture of caustic potash and chlorine
established definitely the success of electrolysis in the
field of potash, but even then none of the various processes
working with sodium chloride had emerged from the experimental
stage. Only more recently the manufacture of caustic soda
by electrolysis has also been established as a permanent and
paying industry, but as the greatest secrecy is maintained
in everything belonging to this domain, and as neither patent
specifications nor the sanguine assertions and anticipations
of interested persons throw much real light on the actual
facts of the case, nothing certain can be said either in
regard to the date at which the profitable manufacture
of caustic soda was first carried out by electrolysis, or
as to what extent this is the case at the present moment.
We shall here give merely an outline of those more important processes
which are known to be at present working profitably on a large scale.
(1) The Diaphragm process is probably the only one employed
at present for the decomposition of potassium chloride, and
it is also used for sodium chloride. A hot, concentrated
solution of the alkaline chloride is treated by the electric
current in large iron tanks which at the same time serve as
cathodes. The anodes are made of retort-carbon or other
chlorine-resisting material, and they are mounted in cells
which serve as diaphragms. The material of these cells
is usually cement, mixed with certain soluble salts which
impart sufficient porosity to the material. The electrolysis
is carried on until about a quarter of the chloride has
been transformed; it must be stopped at this stage lest
the formation of hypochlorite and chlorate should set
in. The alkaline liquid is now transferred to vacuum pans,
constructed in such a manner that the unchanged chloride,
which ``salts out'' during the concentration, can be removed
without disturbing the vacuum, and here at last a concentrated
pure solution of KOH or NaOH is obtained which is sold in
this state, or ``finished'' as solid caustic in the manner
described in the section treating of the Leblanc soda.
(2) The Castner-Kellner process employs no diaphragm, but a
mercurial cathode. The electrolysis takes place in the central
compartment of a tripartite trough which can be made to rock
slightly either to one side or the other. The bottom of the
trough is covered with mercury. The sodium as it is formed
at the cathode at once dissolves in the mercury which protects
it against the action of the water as long as the percentage
of sodium in the mercury does not exceed, say, 0.02%. When
this percentage has been reached, the cell is rocked to the
other side, so that the amalgam flows into one of the outer
compartments where the sodium is converted by water into sodium
hydrate. At the same time fresh mercury, from which the
sodium had been previously extracted, flows from the other
outside compartment into the central one. After a certain time
the whole is rocked towards the other side, and the process
is continued until the outer compartments contain a strong
solution of caustic soda, free from chloride and hypochlorite.
(3) Aussig process.--Here the anode is fixed in a bell,
mounted in a larger iron tank where the cathodes are
placed. The whole is filled with a solution of common
salt. As the electrolysis goes on, NaOH is formed at the
cathodes and remains at the bottom. The intermediate layer
of the salt solution, floating over the caustic soda solution,
plays the part of a diaphragm, by preventing the chlorine
evolved in the bell from acting on the sodium hydrate formed
outside, and this solution offers much less resistance to
the electric current than the ordinary diaphragms. This
process therefore consumes less power than most others.
(4) The Acker-Douglas process electrolyses sodium chloride
in the molten state, employing a cathode consisting of molten
