even be sent out in tank wagons, similar to those which are
employed for carrying sulphuric acid, holding 10 tons each.
Sometimes the chlorine is employed directly for bleaching
purposes, especially for some kinds of paper. A number of
organic chlorinated products are also produced on a large
scale. But most of the chlorine is utilized for the production
of bleaching- powder, of bleach-liquor, and of chlorate of potash.
Bleaching-powder is a compound obtained by the action
of free chlorine on hydrated lime, containing a slight
excess of water at ordinary temperatures or slightly above
these. Its composition approaches the formula CaOCl2,
and it is regarded as a double salt of calcium chloride and
hypochlorite, which by the action of water splits up into
a mixture of these salts. It always contains a certain
quantity of chemically combined water and also an excess of
lime. Usually this lime is regarded only as mechanically
mixed with the bleaching-compound, CaOCl2, but some chemists
adopt formulae in which this lime is equally represented.
For the manufacture of bleaching-powder, limestone of high
degree of purity (especially free from magnesia and iron) is
carefully burned so as to drive out nearly all the carbon dioxide
without overheating the lime. The quick-lime is then slaked
with the requisite quantity of water; the product is passed
through a fine-meshed wire sieve and is spread in layers of 2
or 3 in. at the bottom of large boxes, the ``bleaching-powder
chambers,'' made of lead, or sometimes of cast-iron protected by
paint, of slate or even of tarred wood. Chlorine, generated
in an ordinary or a Weldon still, is passed in and is rapidly
absorbed. When the absorption becomes slow, the gas is cut
off and the chamber is left to itself for twelve hours or more,
when it will be found that all the chlorine has been taken
up. Now the door of the chamber is opened, the powder lying
at the bottom is turned over and the treatment with gas is
repeated. Sometimes a third treatment is necessary in order
to get the product up to the strength required in commerce,
viz. 35% of ``available'' chlorine. The finished product is
packed into wooden casks lined with brown paper. The work of
packing is a most disagreeable and unhealthy operation which
is best relieved by erecting the chambers at a higher level
and placing the casks underneath, communication being made by
means of traps in the chamber-bottom. so that the packers can
do their work outside the chambers. The bleaching-powder casks
must be kept in a dry place, as cool as possible, and never
exposed to the direct rays of the sun, in order to prevent a
decomposition which now and then has even led to explosions.
The weak chlorine from the Deacon process cannot be treated in
this manner, as chambers of impossibly large dimensions would be
required. Originally the absorption of the Deacon chlorine took
place in a set of chambers, constructed of large slabs of stone,
containing a great many horizontal shelves superposed over one
another. About sixteen such chambers were combined in such
manner that the fresh gas passed into that chamber which had
been the longest time at work and in which the bleaching-powder
was nearly finished, and so forth until the gas, now all but
entirely exhausted, reached the last-filled chamber in which
it met with fresh lime and there gave up the last of the
chlorine. These ``Deacon chambers'' occupied a large space,
besides being expensive to build and difficult to keep in repair.
They are now mostly replaced by an apparatus, the invention of R.
Hasenclever, consisting of four horizontal cast-iron cylinders
with internal stirring-gear. The fresh lime is continually
charged into the top cylinder, is gradually moved towards the
other end, falls down into the next lower cylinder and thus
gradually makes its way to the lowest cylinder. The weak chlorine
gas from the Deacon apparatus travels precisely the opposite
way, from the bottom upwards, the result being that finished
bleaching- powder is continually discharged at the bottom
and air free from chlorine leaves the apparatus at the top.
Bleaching-powder is manufactured to the extent of several
hundred thousands of tons annually, almost entirely for the
use of papermakers and cotton bleachers. Smaller quantities
are used for disinfection and other purposes. It is usually
sold in ``tierces,'' that is, casks containing about 10 cwt.
Bleach-liquors.--If the chlorine is made to act on cream of
lime, care being taken that the temperature does not rise
above 35 deg. and that the chlorine is not in excess, a solution
is obtained containing a mixture of calcium chloride and
hypochlorite which is a very convenient agent for bleachers,
but which does not bear the expense of carriage over long
distances. Similar liquids are obtained with a basis of sodium
(``eau de Javel''), by passing chlorine into solutions of sodium
carbonate. The former kind of bleach-liquor is mostly used
in the industry of cotton, the latter in that of linen.
Chlorate of Potash.--Formerly all chlorate of potash, as
some is still, was obtained by passing chlorine into milk
of lime, allowing the temperature to rise almost to the
boiling-point, and continuing until the bleaching-solution,
originally formed, is converted into a mixture of calcium
chlorate and chloride, the final reaction being 6Ca(OH)2
+ 6Cl2 = 5Cacl2 + Ca(ClO3)2 + 6H2O. On adding to this
solution, after settling out the mud, a quantity of potassium
chloride equivalent to the calcium chlorate, the reaction
Ca(ClO3)2 + 2KCl = CaCl2 + 2KClO3 is produced, the ultimate
proportions thus being theoretically 2KClO3 to 6CaCl2, though
in reality there is rather more calcium chloride present.
When this solution is concentrated by evaporation and cooled
down, about five-sixths of the chlorate of potash crystallizes
out. It is purified by redissolving and crystallization, and
is sold either in the state of crystals or finely ground.
During these operations care must be taken lest a spark should
produce the inflammation of the chlorate on contact with any
organic substance. Large quantities of potassium chlorate
exposed to strong heat in contact with the wood of casks
or the timber of a roof have produced violent explosions.
Most of the chlorate of potash is now prepared by electrolysis
of potassium chloride (see below). It is employed for
fire-works, for some descriptions of explosives, for safety
matches and as an oxidizer in some operations, especially
in dyeing and tissue printing. For the last-named purpose
it is sometimes replaced by sodium chlorate. The chlorates
are usually sold in wooden kegs containing 1 cwt. each.
5. The Manufacture of Soda-ash from Salt-cake by the Leblanc
process.--This process consists in heating a mixture of commercial
FIG. 6.--Black-ash Furnace and Boiling-down Pan. Scale
[14051]
sulphate of soda (salt-cake) with about the same weight of
crushed limestone and half its weight of coal, until the
materials are fluxed and a reaction has taken place, the
principal phase of which is expressed by the equation Na2SO4
+ CaC03 + 2C = 2CO2 + Na2CO3 + CaS. A number of secondary
reactions, however, occur, owing partly to the excess of
calcium carbonate and coal and partly to the impurities
present, so that the solid product of the process, which is
called ``black-ash,'' has a somewhat complicated composition.
Its principal constituents are always sodium carbonate
and calcium sulphide, which are separated by the action of
water, the former being soluble and the latter insoluble.
The furnace in which the reaction takes place is shown in
fig. 6 in a sectional plan. It is called a ``black-ash''
furnace, and belongs to the class of reverberatory furnaces.
A large fire-grate (ab), having a cave (c) to facilitate
stoking and stepped back at (d), is bounded on one side by a
fire-bridge (e); on the other side of this, separated by an
air-channel (g), there is first the proper fluxing bed (h),
and behind this the ``back-bed'' (i) for pre-heating the
charge. The flame issuing from the furnace by (o) is always
further utilized for boiling down the liquors obtained in
a later stage, either in a pan (p) fired from the top and
supported on pillars (qq) as shown in the drawing, or in
pans heated from below. The charge of salt-cake (generally
3 cwt.), limestone and coal is roughly mixed and put upon
the back-bed; when the front- bed has become empty it is
drawn forward and exposed to the full heat of the fire,
with frequent stirring. After about three- quarters of an
hour the substances are so far fluxed or softened that the
reaction now sets in fully, as shown by the copious escape of
gas. This is at first colourless carbon dioxide, but later
on inflammable gases come out of the mass, which at this stage
has turned into a thicker, pasty condition, showing that the
end of the reaction is near. The inflammable gas is carbon
monoxide, which, however, does not burn with its proper purple
flame, but with a flame tinged bright yellow by the sodium
present. This carbon monoxide is formed by the action of
coal on the lime formed at this stage from the original
limestone. When the ``candles'' of carbon monoxide appear,
the pasty mass is quickly drawn out of the furnace into iron
``bogies,'' where it solidifies into a grey, porous mass, the
``black-ash.'' Care must be taken to heat it no longer than
necessary, as it otherwise turns red and yields bad soda.
The hand-wrought black-ash furnace has been mostly superseded
in the large factories by the revolving black-ash furnace,
shown in fig. 7. These furnaces possess a large cylindrical
shell (e), lined with fire-bricks, and made to revolve
round its horizontal axis by means of a toothed wheel fixed
on its exterior; (ff) are tire-seats holding tires (gg),
which work in friction rollers (h). The flame of a fixed
fireplace (a) enters through an ``eye'' (b) in the centre
of the front end of the cylinder and issues in the centre of
FIG. 7.--Revolving Black-ash Furnace. (Elevation.) Scale
the back end, first into a large dust-chamber (m.) and
then over or under boiling-down pans (p.) These mechanical
furnaces do the work of from four to ten ordinary furnaces
according to their size. with comparatively very little
expense for labour, but they must be very carefully managed
and the black-ash from them is more difficult to lixiviate
than that from hand-wrought furnaces, because it is less
porous. The lixiviation of the black- ash requires great
care, as the calcium sulphide is liable to be changed into
soluble calcium compounds, which immediately react with sodium
carbonate and destroy a corresponding quantity of the latter,
rendering the soda weaker and impure. This change of the
calcium sulphide may be brought about either by the oxidizing
action of the air or by ``hydrolysis,'' produced by prolonged
contact with hot water, the use of which, on the other hand,
cannot be avoided in order to extract the sodium carbonate
itself. The apparatus which has been found most suitable
for the purpose was devised by Professor H. Buff of Giessen,
and first practically carried out by Charles Dunlop at St
Rollox. It consists of a number of tanks or ``vats,'' placed
at the same level and connected by pipes which reach nearly to
the bottom of one tank and open out at the top into the next
tank. The vats are also provided with false bottoms, outlet
cocks, steam pipes and so forth. Tepid water is run in at one
end of the series, where nearly exhausted black-ash is present;
