the weak liquor takes up more soda from the intermediate tanks
and at last gets up to full strength in the last tank, charged
with fresh black-ash and kept at a higher temperature, viz.
60 deg. C. When the first tank has been quite exhausted, the
water is turned on to the next, the first tank is emptied by
discharging the ``alkali- waste,'' and is filled with fresh
black-ash, whereupon it becomes the last of the series. In
spite of all precautions a certain quantity of impurities is
always formed, but this should be kept down as much as possible
by strictly watching the temperature in the vats and by taking
care that the black-ash in the wet state is never exposed to the
air. The unavoidable contamination with muddy particles
of vat-waste is removed by allowing the vat- liquor to rest
for some hours in a separate tank and settling out the mud.
The clear vat-liquor, if allowed to cool down to ordinary
temperature, would separate out part of the sodium carbonate
in the shape of decahydrated crystals. As these do not
come out sufficiently pure, they would not be marketable and
therefore they are not allowed to be formed, but the liquid,
while still hot, is either run into the boiling-down pans, or
submitted to one of the purifying operations to be described
below. If it is boiled down without further purification,
the resulting soda-ash is not of the first quality, but it is
sufficiently pure for many purposes. The boiling down is most
economically performed by means of large iron pans covered
with a brick arch and heated from the top by the waste flame
issuing from the black-ash furnaces (see figs. 6 and 7). It
is continued until the contents of the pan have been converted
into a thick paste of small crystals of monohydrated sodium
carbonate, permeated by a mother-liquor which is removed by
draining on perforated plates or by a centrifugal machine,
and is always returned to the pans. The drained crystals are
dried and heated to redness in a reverberatory furnace; when
``finished,'' the mass is of an impure white or light yellow
colour and is sold as ordinary ``soda-ash.'' It is not easy
to make it stronger than 92% of sodium carbonate, which is
technically expressed as ``52 degrees of available soda'' (see next
page). If purer and stronger soda-ash is wanted, the boiling
down must be carried out in pans fired from below, and the
crystals of monohydrated sodium carbonate ``fished'' out as
they are formed, but this is mostly done after submitting the
liquor to the purifying operations which we shall now describe.
The dried or ``finished'' soda-ash is ground to a pretty fine
powder and is packed into wooden casks or ``tierces,'' holding from
10 to about 20 cwt. each, according to the way of filling them.
The principal impurities of crude vat-liquor are sodium
hydrate and sulphide, the latter of which always leads to
the formation of soluble double sulphur salts of sodium and
iron. The other impurities are of minor importance.
The sulphides can be removed by ``oxidizing'' them into
thiosulphates by means of atmospheric air, with or without
the assistance of other agents, such as manganese peroxide;
or by ``carbonating'' them with lime-kiln or other gases
containing carbon dioxide; or by precipitating them with
lead or zinc oxide. The last mentioned is the best but
costliest method, and is employed only in the manufacture
of the highest strengths of caustic soda. The most usual
process, where soda-ash is to be made, is the ``carbonating.''
This is usually effected either by forcing lime-kiln gas
through the liquor, contained in a closed iron vessel, or by
passing the gases through an iron tower filled with coke or
other materials, suitable for subdividing the stream of the
gases and that of the vat-liquor which trickles down in the
tower. The same apparatus is used for ``oxidizing'' by means
of atmospheric air passed through by means of an injector;
sometimes both air and carbon dioxide are passed in at the same
time. The operation is finished when all the sodium sulphide
has been converted into normal sodium carbonate, partly also
into acid sodium carbonate (bicarbonate) NaHCO3; at the
same time a precipitate is formed, consisting of ferrous
sulphide, alumina and silica, which is removed by another
settling tank, and the clear liquor is now ready either
for boiling down in a ``fishing-pan'' for the manufacture
of white soda-ash, or for the process of causticizing.
Soda-ash (as well as caustic soda) is sold by degrees of
``available soda.'' This means that portion which neutralizes the
acid employed for testing, and the degrees mean the percentage
of Na2O thus found, whether it be present as Na2CO3, NaOH,
or sodium aluminate or silicate. The purest soda-ash, equal
to 100% Na2CO3, would be 58 1/2 degrees of available soda. The
ordinary commercial strength of Leblanc soda-ash is from 52 to
54 degrees (in former times much was sold in the state of 48%).
6. Manufacture of Caustic Soda.--Most of the Leblanc liquor
is nowadays converted into caustic soda, as white soda-ash
is more easily and cheaply made by the ammonia-soda process.
We shall therefore in this place describe the manufacture
of caustic soda. This is always made from the carbonate by
the action of slaked lime: Na2CO3 + Ca(OH)2 = CaCO3 +
2NaOH. The calcium carbonate, being insoluble, is easily
separated from the caustic liquor by filtration. But as
this reaction is reversible, we must observe the conditions
necessary for directing it in the right sense. These are:
diluting with water so as not to exceed 10% of sodium carbonate
to 90% of water; boiling this mixture; and keeping it well
agitated. At the best about 92% of the sodium carbonate
can be converted into caustic soda, 8% remaining unchanged.
The operation is performed in iron cylinders, provided
with an agitating arrangement. This may consist of a steam
injector by means of which air is made to bubble through the
liquid, which produces both the required agitation and the
heating, and at the same time oxidizes at least part of the
sulphides; but this method of agitation causes a great waste
of steam and at the same time a further dilution of the
liquor. Many, therefore, prefer mechanical stirring by means
of paddles, fixed either to a vertical or to a horizontal
shaft, and inject only sufficient steam to keep the mass
at the proper temperature. Some heat is also gained by
the slaking of the caustic lime within the liquor. After
from half an hour to a whole hour the conversion of sodium
carbonate into sodium hydrate is brought about as far as is
practicable. The whole mass is now run into the filters,
which are always constructed on the vacuum principle. They
are iron boxes, in which a bed is made of bricks, above
them gravel, and over this sand, covered on the top by iron
grids. The space below the sieve thus formed is connected
by means of an outlet tap with a closed tank, and this again
communicates with a vacuum pump. By this means the filtration
is quickened by the atmospheric pressure, and goes on very
rapidly, as also does the subsequent washing. The filtered
caustic liquor passes to the concentration plants; the
washings are employed for diluting fresh vat-liquor for the
next operation, or for dissolving solid soda-ash for the same
purpose. The washed-out calcium carbonate, which always contains
much calcium hydrate and 2 or 3% of soda in various forms,
usually goes back to the black-ash furnaces, but it cannot be
always used up in this way, and what remains is thrown upon a
heap outside the works. Attempts have been made to use it in
the manufacture of Portland cement, but without much success.
The clear caustic soda liquor must be concentrated in such
a way that the caustic soda cannot to any great extent be
reconverted into sodium carbonate, and that the ``salts''
which it contains, sodium carbonate, sulphate, chloride, &c.,
can be. separated during the process. Formerly the most usual
concentrating apparatus was the ``boat-pan'' (fig. 8). This is an
FIG. 8.--Caustic Soda Concentration Boat-pan. (Sectional Elevation.) Scale
oblong iron pan, the bottom of which slopes from both sides
to a narrow channel. The latter rests on a brick pillar;
the remaining part of the sloping bottom is heated, either
by the waste fire from a black-ash furnace or by a special
fireplace. This arrangement has the effect that the salts, as
they separate out, slide down the sloping part and arrive in
the central channel, which is not exposed to the fire-gases,
so that they quietly settle there, without caking to the pan,
until they are fished out by means of perforated ladles. These
boat-pans were for many years almost everywhere employed, and
did their work quite well, but rather expensively. At many works
they have been replaced by either Thelen pans or vacuum pans.
The ``Thelen pan'' (thus named from its inventor, a foreman
at the Rhenania works near Aachen) is a mechanically worked
fishing-pan, which requires considerably less labour and
coal than ordinary boat-pans. It is a long trough, of nearly
semicircular section, the whole bottom being exposed to the
fire- gases. A horizontal shaft runs length-ways through the
trough, and is provided with stirring blades, arranged in
such a manner that they constantly scrape the bottom, so that
the salts cannot burn fast upon it, and are at the same time
moved forward towards one of the ends of the trough where
they are automatically removed by means of a chain of buckets.
The most efficient evaporating apparatus, as far as economy
of fuel is concerned, is the vacuum-pan, of which from two to
five are combined to form a set, but it has the drawback that
the removal of the salts is much more difficult than with the
older pans, described above. In this apparatus only the first
of the pans is heated directly, usually by means of ordinary
boiler- steam circulating round a number of pipes, containing
the liquid to be concentrated. The steam rising from the latter
is passed into a similar pan, in which it circulates round
another set of pipes, but as it could not bring the liquid in
the latter to boil under ordinary conditions, the second pan
is connected with a vacuum-pump so that the boiling-point of
the liquid in this pan is lowered. This pan may be followed by
a third pan, in which a stronger vacuum is maintained, and so
forth. By this means the latent heat of the steam, issuing
from all pans but the last, is utilized for evaporating
purposes, and from half to three-fourths of the fuel is saved.
After being concentrated up to a certain point, and after
the separation of nearly all the salts, the caustic liquor
is transferred to cast-iron ``finishing-pots'' (fig. 9),
holding from ten to twenty tons. Here it is further boiled
down until the greater part or nearly all of the water
has been removed, and until the salts on cooling would
set to a solid mass. This requires ultimately a good red
heat. Before the mass has reached that point the sulphides
still present have been destroyed, either by the addition of
solid nitrate of soda or by blowing air through the red-hot
melt. Before finishing, the molten mass must be kept at a quiet
FIG. 9.--Caustic Soda ``Finishing-pot.'' (Sectional Elevation.) Scale
heat for some hours in order to settle out the ferric oxide which
it always contains, and which becomes insoluble (through the
destruction of the sodium ferrite) only at high temperatures.
When it has completely cleared, the liquid caustic is ladled
or pumped out into sheet-iron drums, holding about 6 cwt. each,
