carbonate can be extracted by exhausting it with water.
Leblanc himself for a time carried out his process on a
manufacturing scale, but he was ruined in the political troubles
of the time and died by his own hand in 1806. His invention
was, however, at once utilized by others in France; and in
Great Britain, after a few previous attempts on a small scale,
it was definitely introduced by James Muspratt (q.v.) in
1823. From that time onward the Leblanc process spread more and
more, and for a considerable period nearly all the alkali of
commerce was made by it. The rise of the ammonia-soda process
(since 1870) gradually told upon the Leblanc process, which
in consequence has been greatly restricted in Great Britain
and Germany, and has become practically extinct in all other
countries, except as far as its first part, the manufacture
of sodium sulphate and hydrochloric acid, is concerned.
The production of alkali in Great Britain, soon after the
introduction of the Leblanc process, became the most extensive
in the world, and outstripped that of all other countries put
together. With the rise of the ammonia-soda process, for
which the economic conditions are nearly as favourable in other
countries, the predominance of Great Britain in that domain
has become less, but even now that country produces more alkali
than any other single country. Most of the British alkali
works are situated in South Lancashire and the adjoining part of
Cheshire, near the mouth of the Tyne and in the West of Scotland.
Various industries are carried on in Leblanc alkali works, as follows:--
1. Manufacture of sodium sulphate.
2. Manufacture of hydrochloric acid.
3. Preparation of chlorine.
4. Employment of chlorine for the manufacture
of bleaching- powder and of chlorates.
5. Manufacture of ordinary alkali from sulphate of soda.
6. Manufacture of caustic soda.
7. Manufacture of soda crystals.
8. Recovery of sulphur from alkali waste.
1. Manufacture of Sodium Sulphate.--This is commercially
known as salt-cake, and is made by decomposing common
salt with sulphuric acid of about 80%, the reaction being
2NaCl + H2SO4 = Na2SO4 + 2HCl. This reaction proceeds
in two stages. At first principally acid sodium sulphate,
NaHSO4, is formed together with some normal sulphate;
later, when the temperature has risen, the NaHSO4 acts
with more NaCl so that nearly all of it is converted into
Na2SO4. The gaseous hydrochloric acid evolved during
all this time must be absorbed in water, unless it is
directly converted into chlorine (see below, 2 and 3).
The process is carried out either in hand-wrought furnaces, or
mechanical furnaces, both called ``decomposing'' or ``salt-cake
furnaces.'' In the former case, the first reaction is produced
in cast- iron pans or ``pots,'' very heavy castings of circular
section, fired from below, either directly or by the waste
heat from the muffle- furnace. The reaction is completed
in a ``roasting- furnace.'' The latter was formerly often
constructed as a revereratory funace, which is easy to build
and to work, but the hydrochloric acid given off here, being
mixed with the products of the combustion of fuel, cannot
be condensed to strong acid and is partly, if not entirely,
wasted. It is, therefore, decidedly preferable to employ
``muffle-furnaces'' in which the heating is performed from
without, the fire-gases passing first over the arch and
then under the bottom of the muffle. This requires more
time and fuel than the work in ``open'' furnaces, but in
the muffles the gaseous hydrochloric acid is separated
from the fire-gases, just like that evolved in the pot, and
can therefore be condensed into strong hydrochloric acid,
like the pot-acid. This roaster-acid is, however, of less
value than the pot-acid, as it contains more impurities.
It is not easy to keep the muffles permanently tight, and as soon
as any leakages occur, either hydrochloric acid must escape into
the fire-flue, or some fire-gases must enter into the muffle.
The former is decidedly more objectionable than the latter, as
it means that uncondensed hydrochloric acid is sent into the
air. This drawback has been overcome by the construction of
``plus-pressure'' furnaces (figs. 1 and 2), where the fire-grate
is placed 11 ft. below the top of the muffle. In consequence
the fire-gases, when arriving there by the chimney shaft
(a), have already a good upward draught, and when circulatung
round the muffle are at a lower pressure than the gases within
the muffle, so that in case of any cracks being formed, no
hydrochloric acid escapes into the fire-flues, but vice versa.
Since the work with ordinary hand-wrought salt-cake furnaces
is disagreeable and costly, many attempts have been made
to construct mechanical salt-cake furnaces. Of these J.
Mactear's furnaces (fig. 3) have met with the greatest
success. They consist of a horizontal pan, 17 ft. wide,
which is made up of a central pan (e), and a series
of concentric compartments (C1), (C2), (C3), and
which is supported on a frame (d d), revolving round a
perpendicular axis on the wheels (n n). It is with an
arch and heated on the top from one side (l), either by an
ordinary coal-grate or by a gas-producer. A set of stirring
blades carried in the frame (b b), and driven by gearing,
FIGS. 1. and 2.--Salt-cake Furnace. (Sectional Elevation and Plan.) Scale
Figs. 1-9 from Lunge's Handbuch der Soda-Industrie,
by permission of Friedr. Vieweg u. Sohn.
passes through a gap in the arch in such a manner that the gases
cannot escape outwards. The salt is conveyed to the furnace
by a chain of buckets running on the pulley (g), and passing
into the hopper (h), and through the pipe (i) is mixed
with the proper amount of acid supplied by the pipe ( f.)
The mixture is fed in continuously to the central pan (e.)
whence it overflows into the compartments (c1), (c2), (c3)
successively until it reaches the circumference, where it is
discharged continously by o and p into the collecting-box
(q), being now converted into salt-cake. This furnace acts
very well, and has been widely introduced both in Great Britain
and in other countries, but it has one great drawback, apart
from its high cost, viz. that all the hydrochloric acid gas
gets mixed with fire-gases, and consequently is condensed
in a weaker and less pure form than from ordinary pots and
muffles. This has led some factories which had introduced
such furnaces to revert to hand-wrought muffle-furnaces.
Much was expected at one time from the.``direct salt-cake
process'' of Hargreaves and Robinson, in which common salt
is subjected in a series of large cast-iron cylinders to
the action of pyrites-burner gases and steam at a low red
heat. The reaction going on here is: 2NaCl + SO2 + O
+ H2O = Na2SO4 + 2HCl. This means that the previous
manufacture of sulphuric acid in the vitriol-chambers is
done away with, but this apparently great simplification is
balanced by the great cost of the Hargreaves plant, and by
the fact that the whole of the hydrochloric acid is mixed
with nine or ten times its volume of inert gases. Owing to
this, it is practically impossible to condense the gaseous
hydrochloric acid into the commercial acid, although this
acid may be obtained sufficiently strong to be worked up in
the Weldon chlorine process (see below, 3). Therefore the
Hargreaves process has been introduced only in a few places.
Although the consumption of salt-cake for the manufacture of
alkali is now much less than formerly, since the Leblanc alkali
process has been greatly restricted, yet it is largely made and
will continue to be made for the use of glassmakers, who use it
for the ordinary description of glass in the place of soda-ash.
Nor must it be overlooked that salt-cake must be made as long
FIG. 3.--Mechanical Salt-cake Furnace. (Sectional Elevation.) Scale
as there is a sale for hydrochloric acid, or a
consumption of the latter for the manufacture of chlorine.
2. Manufacture of Hydrochloric Acid (commercially also
known as ``muriatic acid''). This unavoidable gaseous
bye-product of the manufacture of salt-cake was, during
the first part of the 19th century, simply sent into the
air. When its deleterious effects upon vegetation, building
materials, &c., became better known, and when at the same
time an outlet had been found for moderate quantities of
hydrochloric acid, most factories made more or less successful
attempts to ``condense'' the gas by absorption in water. But
this was hardly anywhere done to the fullest possible extent,
and in those districts where a number of alkali works were
located at no great distance from one another, their aggregate
escapes of hydrochloric and other acids created an intolerable
nuisance. This was most notably the case in South Lancashire,
and it led to the passing of Lord Derby's ``Alkali Act,'' in
1863, supplemented by further legislation in 1874, 1881 and
later. There is hardly another example in the annals of
legislative efforts equal to this, in respect of the real
benefit conferred by it both on the general public and
on the manufacturers themselves. This is principally the
consequence of the exemplary way in which the duties of
inspector under these acts were carried out by Dr R. Angus
Smith (1817-1884) and his successors, who directed their
efforts not merely to their primary duty of preventing
nuisance, but quite as much to showing manufacturers how to
make the most of the acid formerly wasted in one shape or
another. Not merely Great Britain but all mankind has been
immensely benefited by the labours of the British alkali
inspectors, which were, of course, supplemented by the work of
technical men in all the countries concerned. The scientific
and technical principles of the condensation of hydrochloric
acid are now thoroughly well understood, and it is possible
to recover nearly the whole of it in the state of strong
commercial acid, containing from 32 to 36% of pure hydrochloric
acid, although probably the majority of the manufacturers are
still content to obtain part of the acid in a weaker state,
merely to satisfy the requirements of the law prescribing the
prevention of nuisance. The principles of the condensation,
that is of converting the gaseous hydrochloric acid given
off during the decomposition of common salt into a strong
solution of this gas in water, can be summarized in a few
words. The hydrochloric acid gas, which is always diluted
with air, sometimes to a very great extent, must be brought
into the most intimate contact possible with water, which
greedily absorbs it, forming ordinary hydrochloric acid,
and this process must be carried so far that scarcely any
hydrochloric acid remains in the escaping gases. The maximum
escape allowed by thc Alkali Acts, viz. 5 % of the total
hydrochloric acid, is far above that which is now practically
attained. For a proper utilization of the condensed acid it
is nearly always imperative that it should be as strong as
possible, and this forms a second important consideration
in the construction of the condensing apparatus. Since the
solubility of hydrochloric acid in water decreases with the
increase of the temperature, it is necessary to keep the
latter down--a task which is rendered somewhat difficult
both by the original heat retained by the gases on their
escape from the decomposing apparatus, and by the heat given
off through the reaction of hydrochloric acid upon water.
Very different methods have been employed to effect all the
above purposes. In Great Britain Gay-Lussac's coke-towers,
