where it solidifies and forms the caustic soda known to commerce.
The best caustic soda tests from 75 to 76 degrees of
``available soda''; this is only a few per cent removed
from the composition of pure NaOH, which would be = 77.5
degrees Na2O. Most of the caustic soda is sold at a
strength of 70 degrees, sometimes as low as 60 degrees.
Caustic soda is used in very large quantities in the
manufacture of soap, paper, textile fabrics, alizarin
and other colouring matters, and for many other purposes.
7. Soda-Crystals.--Another product made in alkali works is
soda-crystals. Their formula in Na2CO3, 10H2O, corresponding
to 37% of dry sodium carbonate. They are made by dissolving
ordinary soda-ash in hot water, adding a small quantity of
chloride of lime for the destruction of colouring matter and the
oxidation of any ferrous salts present, carefully settling the
solution, without allowing its temperature to fall below the
point of maximum solubility (34 deg. C.), and running the clarified
liquid into cast-iron crystallizers or ``cones,'' where, on
cooling down, most of the sodium carbonate is separated in
large crystals of the decahydrated form. This process lasts
about a week in winter, and up to a fortnight in summer. In
France the crystallization of soda is performed not in large
tanks but in sheet-iron dishes holding only about 1/4 cwt.,
and requires only from 27 to 48 hours in the cool season; it
is not carried on at all in warmer climates during the summer
months. The mother-liquor, drained from the soda-crystals, on
boiling down to dryness yields a very white, but low-strength
soda-ash, as the soluble impurities of the original soda-ash
are nearly all collected here; it is called ``mother-alkali.''
Although the soda-crystals contain the alkali combined
with such a large quantity of water, they are made in large
quantities, because their form, together with their complete
freedom from caustic soda, makes them very suitable for domestic
purposes. Hence they are best known as ``washing-soda.''
Sometimes they are made, not from soda-ash, but from Leblanc
soda-liquor before ``finishing'' the ash, or from the crude
bicarbonate of the ammonia-soda process by prolonged boiling,
until nearly half of the carbonic acid has been expelled.
Formerly bicarbonate of soda was made from Leblanc soda-
crystals by the action of carbonic acid, but this article
is now almost exclusively made in the ammonia-soda process.
8. The Recovery of Sulphur from Alkali-waste.--For many
years all the sulphur used in the Leblanc process in the
shape of sodium sulphate, and originally imported into the
manufacture in the shape of brimstone or pyrites, was wasted
in the crude calcium sulphide remaining from the lixiviation of
black-ash. This ``alkali-waste,'' also called tank-waste or
vat- waste, was thrown into heaps where the calcium sulphide
was gradually acted upon by the moisture and the oxygen of the
air. The sulphur was by these converted partly into gaseous
sulphuretted hydrogen, partly into soluble polysulphides,
thiosulphates and other soluble compounds, and in all shapes
caused a nuisance which became more and more intolerable as
the number and size of alkali works increased. Both the air
and the water in their neighbourhood were contaminated thereby.
Both this nuisance and the loss of the sulphur (whose cost
sometimes amounted to more than half of the total cost of the
soda-ash) led to many attempts at extracting the sulphur from the
alkali-waste. This was first done with a certain amount of
success by the processes of M. Schaffner (1861) and L. Mond
(1862), but as these required the use of hydrochloric acid,
and as they only recovered about half of the sulphur, they
were superseded by another--a process which had been originally
proposed by W. Gossage in 1837, but has been made practicable
only by the inventions of C. F. Claus, in 1883, and from 1887
onward by the technical skill of Messrs Chance Brothers, of
Oldbury. The Claus-Chance process, as it is called, comprises
the following operations. The wet alkali-waste as it comes
from the lixiviating vats, is transferred into upright
iron cylinders in which it is systematically treated with
lime-kiln gases until the whole of the calcium sulphide has
been converted into calcium carbonate, the carbon dioxide of
the lime-kiln gases being entirely exhausted. The sulphur
issues as sulphuretted hydrogen, mixed with the nitrogen of the
air. It is mixed with fresh air containing sufficient oxygen
for the combustion of the hydrogen, and the mixture is passed
through red-hot iron oxide (burnt pyrites) which by its
catalytic action causes the reaction H2S + O = H2O + S to take
place. By cooling the vapours the sulphur is condensed in a
very pure form, and about 85% of the whole of it is recovered,
the remaining 15% escaping in the shape of sulphur dioxide
(SO2) and H2S. Unfortunately it has been hitherto found
impossible to deal with these gases in any profitable way.
It should be noted that this ``recovered sulphur,'' which is equal
in purity to the ``refined brimstone'' of commerce, has a far
higher value than the sulphur contained in the originally employed
pyrites, so that the recovery is a paying process, in spite of
the somewhat considerable cost of the plant and of the working
operations. It has been introduced at most large Leblanc alkali
works, and has, so to say, given them a new lease of life.
II. THE AMMONIA-SODA PROCESS
In spite of the great improvements effected during recent
times the Leblanc process cannot economically compete with
the ammonia-soda process, principally for two reasons. The
sodium in the latter costs next to nothing, being obtained
from natural or artificial brine in which the sodium chloride
possesses an extremely slight value. The fuel required is less
than half the amount used in the Leblanc process. Moreover,
the ammonia process has been gradually elaborated into a very
complicated but perfectly regularly working scheme, in which
the cost of labour and the loss of ammonia are reduced to a
minimum. The only way in which the Leblanc process could still
hold its own was by being turned in the direction of making
caustic soda, to which it lends itself more easily than the
ammonia-soda process; but the latter has invaded even this
field. One advantage, however, still remained to the Leblanc
process. All endeavours to obtain either hydrochloric acid
or free chlorine in the ammonia- soda process have proved
commercial failures, all the chlorine of the sodium chloride
being ultimately lost in the shape of worthless calcium
chloride. The Leblanc process thus remained the sole
purveyor of chlorine in its active forms, and in this way
the fact is accounted for that, at least in Great Britain,
the Leblanc process still furnishes nearly half of all the
alkali made, though in other countries its proportional
share is very much less. The profit made upon the chlorine
produced has to make up for the loss on the alkali.
The ammonia-soda process was first patented in 1838 by H. G.
Dyar and J. Hemming, who carried it out on an experimental
scale in Whitechapel. Many attempts were soon after made
in the same direction, both in England and on the continent
of Europe, the most remarkable of which was the ingenious
combination of apparatus devised by J. J. T. Schloesing and E.
Rolland. But a really economical solution of the problem
was first definitely found in 1872 by Ernest Solvay,
as the result of investigations begun about ten years
previously. The greater portion of all the soda-ash of
commerce is now made by Solvay's apparatus, which alone we
shall describe in this place, although it should be borne
in mind that the principles laid down by Dyar and Hemming
have been and are still successfully carried out in a number
of factories by an entirely different kind of apparatus.
The leading reaction of this process is the mutual decomposition
of ammonium bicarbonate and sodium chloride: NaCl + NH4HCO3
= NaHCO3 + NH4Cl. It begins, however, not with ready-made
ammonium bicarbonate, but with the substances from which
it is formed--ammonia, water and carbon dioxide--which are
made to act on sodium chloride. In practice the process
is carried out as follows. A nearly saturated solution of
sodium chloride is obtained by purifying natural or artificial
brine, i.e. an impure solution of common salt, especially
removing the alkaline earths and so forth by addition of
sodium or ammonium carbonate and settling out the precipitate
formed. This solution is saturated with ammonia, produced
in the recovery plant (see below), in vessels provided with
mechanical agitators and strongly cooled by coils of pipes through
which cold water is made to flow. These vessels, as well as
all others which are used in the process, are not open to the
air, but communicate with it through washers in which fresh
salt solution is employed for retaining any escaping vapours of
ammonia. The ammoniacal salt solution is now saturated with
carbon dioxide. This is employed in the shape of lime-kiln
gases, obtained in a comparatively pure and strong form (up
to 33% CO2), in very large kilns, charged with limestone and
coke. The kilns are closed at the top, and the gases are drawn
out by powerful air-pumps, washers being interposed between the
kilns and the pumps for the purpose of purifying and cooling the
gas. The heat evolved by the compression in the air-pumps
(which rises to four atmospheres or upwards) is again removed
by cooling, and the gas is now passed upwards in the ``Solvay
tower'' (fig. 10). This is a tall iron erection, built up from
superposed cylinders, which are separated from one another by
perforated horizontal diaphragms, constructed in such a way
that the gases are over and over again subdivided into many
smaller streams and are thus thoroughly brought into contact
with the ammoniacal salt solution with which the tower is about
two-thirds filled. There the reaction mentioned above takes
place, and owing to the concentration of the liquid the sodium
bicarbonate formed is to a great extent precipitated in the
shape of small crystals, forming with the mother-liquor a thin
magma. This takes place with considerable evolution of heat
which is removed by internal and external cooling with water.
The temperature must not be allowed to rise beyond a certain
point, for the reaction NaCl + NH4HCO3 = NaHCO3 + NH4Cl is
reversible, and at a temperature of about 60 deg. or 70 deg. C. it
is in fact practically going the wrong way, viz. from right to
left. On the other hand the cooling must not be carried too
far, for in this case the crystals of sodium bicarbonate
become so fine that the muddy mass is very difficult to
filter. The best temperature seems to be about 30 deg. C.
Either at certain intervals, or continuously, a portion of
the contents of the tower is withdrawn and fresh ammoniacal
salt solution is introduced higher up. The muddy liquid
running out is passed on to the vacuum filters (Z, fig.
10). Here a separation takes place between the crystals
of sodium bicarbonate and the mother-liquor. The former
are washed with water until the chlorides are nearly
removed, and are then carried into the drying apparatus.
From Thorpe's Dictionary of Applied Chemistry,
by permission of Longmans, Green & Co.
FIG. 10.--Ammonia - soda Carbonating Towers and Filters.
(Sectional Elevation.) Scale 1/100. AA, Tower; B, ammoniacal brine
main; E, gas-inlet; Z, vacuum filter; V, pipe to air-pump.
