In several former notes and articles in these pages, we have spoken of
the severe crisis through which the old established, or "Leblanc,"
process has now for some years been passing. It is, in fact, pushed
well nigh out of the running by the newer process, known as the
"ammonia-soda" process, and would have had to give up the battle
before now were it not for the fact that one of its by-products,
bleaching powder, cannot, so far, be produced at all by the
ammonia-soda works. The bleaching powder trade has thus remained in
the hands of the workers of the Leblanc process, and its sale has
enabled them to cover much of the loss which they are suffering on the
manufacture of soda ash and caustic soda.
In brief outline, the old Leblanc process consists in the following
operations: Salt is decomposed and boiled down with sulphuric acid.
Sulphate of sodium is formed, and a large amount of hydrochloric acid
is given off. This is condensed, and is utilized in the manufacture of
the bleaching powder mentioned above. The sulphate of sodium, known as
"salt cake," is mixed with certain proportions of small coal and
limestone, and subjected to a further treatment in a furnace, by which
a set of reactions take place, causing the conversion of the sulphate
of sodium of the "salt cake" into carbonate of sodium, a quantity of
sulphide of calcium being produced at the same time. The mass
resulting from this process is known as "black ash." It is extracted
with water, which dissolves out the carbonate of sodium, which is sold
as such or worked into "caustic" soda, as may be required. The
insoluble residue is the "alkali waste," which forms the vast piles,
so hideous to look at and so dreadful to smell, which surround our
large alkali works.
The sulphuric acid required for the conversion of the salt into "salt
cake" is made by the alkali manufacturer himself, this manufacture
necessitating a large plant of "lead chambers" and accessories, and
keeping up an immense trade in pyrites from Spain and Portugal. The
development of the alkali trade in this country has been something
colossal, and the interests involved in it and connected with it are
so great that anything affecting it may safely be said to be of truly
national importance, quite apart from what technical interest it may
possess.
The "ammonia-soda" process, which has played such havoc with the old
style of manufacture, proceeds on totally different lines. Briefly
stated, it depends on the fact that if a solution of salt in water is
mixed with bicarbonate of ammonium, under proper conditions, a
reaction takes place by which the salt, or chloride of sodium, is
converted at once into bicarbonate of sodium, the bicarbonate of
ammonium being at the same time converted into chloride of ammonium.
The bicarbonate of sodium settles out at once as insoluble crystals,
easily removed, marketable at once as such, or easily converted into
simple carbonate of sodium, and further into caustic soda, as in the
ordinary "old" process. The residual chloride of ammonium is
decomposed by distillation with lime, giving ammonia for reconversion
into bicarbonate of ammonium, and chloride of calcium, which is a
waste product.
The maker of "ammonia" soda works direct on the brine, as pumped from
the salt fields. His plant is simpler and less costly, and he arrives
at his first marketable product much more rapidly and with very much
lower working costs than the maker of Leblanc soda, in spite of all
the great mechanical improvements which have of late years been
introduced into the old process, and which have cheapened its work.
The original patents on the use of ammonium bicarbonate have, we
understand, long since expired. But the working details of the process
and much of the most successful apparatus have undergone great
development and improvement during late years, all the important
points being covered by patents still in force, and mainly, if not
wholly, in the hands of the one large firm which is now carrying on
the manufacture in this country, and is controlling the market.
The one weak spot of the ammonia-soda process, as we mentioned before,
is its inability to supply hydrochloric acid or chlorine, and so allow
of making bleaching powder. Time after time it has been announced
positively that the problem was solved, that the ammonia-soda makers
had devised a method of producing hydrochloric acid or chlorine, or
both, without the use of sulphuric acid. But the announcements have so
far proved baseless, and at present the Leblanc makers are getting
incredulous, and do not much excite themselves over new statements of
the kind, though they know that if once their rivals had this weapon
in their hands the battle would be over and the Leblanc process doomed
to rapid extinction.
Such is at present the state of the struggle in this great industry,
and the above outline sketch of the two processes is designed to give
some idea of the conditions to such of our readers as may not have any
special knowledge of these manufactures.
At the present moment great interest is being taken in a new process,
about to be put to work on a large scale, which is designed to take up
the cudgels against the ammonia process and enable the Leblanc makers
to continue the fight on something more like equal terms.
We allude to the process proposed and patented by Messrs. Parnell &
Simpson, and about to be worked by the "Lancashire Alkali and Sulphur
Company," at Widnes. We recently had the opportunity of inspecting
fully the plant erected, and of having the method of procedure
explained to us. We look upon the new process as such a spirited
attempt to turn the tide of a long and losing battle, and as so very
interesting on its own merits, that an account of it in these pages
will be thoroughly in place.
The main idea of the process is to combine the "Leblanc" and the
"ammonia-soda" manufacture. But in place of using caustic lime to
decompose the ammonium chloride and get back the ammonia, the "alkali
waste" spoken of above is employed, it being found that not only is
the ammonia driven off, but that also the sulphur in the "waste" is
obtained in a form allowing of its easy utilization, it and the
ammonia combining to form ammonium sulphide, which passes over in
gaseous form from the decomposing apparatus. This ammonium sulphide
is, as we shall see, quite as available for the working of the
ammonia-soda manufacture as pure and simple ammonia, and all the
sulphur can be obtained from it.
In outline the process is as follows: We will suppose that a quantity
of bicarbonate of sodium has been just precipitated from a brine
solution, and we have the residual ammonium chloride to deal with.
This is decomposed by "alkali waste," giving a final liquor of calcium
chloride, which is run to waste, and a quantity of ammonium sulphide
gas. This latter is led at once into a solution of salt in water, till
saturation takes place. Into this liquor of brine and ammonium
sulphide _pure_ carbonic acid gas is now passed. The ammonium sulphide
is decomposed, pure sulphureted hydrogen gas is given off, which is
conducted to a gas holder and stored, while ammonium bicarbonate is
formed in the liquor, which brings about the conversion of the salt
into bicarbonate of sodium, ready for removal and preparation for the
market.
It will be observed that we printed the word _pure_ in italics in
speaking of the carbonic acid used. This is one of the great points in
the process, as in order that the sulphureted hydrogen gas obtained
shall be concentrated and pure, only pure carbonic acid can be used in
liberating it. The apparatus employed in its preparation is perhaps
the most ingenious part of the works, and well worthy of attention by
others besides alkali makers. The method is based on the fact that if
dilute impure carbonic acid is passed into a solution of carbonate of
sodium, the carbonic acid is absorbed, bicarbonate of sodium being
formed, and the diluting gases passing away.
The bicarbonate of sodium on heating gives up the extra carbonic acid,
which can be collected and stored pure, while the liquor passes back
to simple carbonate of sodium, to be used over again as an absorbent.
This is not at all new in theory, of course, nor is this the first
proposal to use it commercially; but it is claimed that this is the
first successful working of it on a large scale.
The gases from a large limekiln supply the dilute carbonic acid gas,
which contains 25 per cent. to 30 per cent. of pure gas, the principal
diluting gas being, of course, nitrogen. This kiln gas is drawn from
the kiln by a blowing engine, and is first cooled in two large
receivers. It is then forced into the solution of sodium carbonate in
the absorption tower, 65 ft. high by 6 ft. diameter, filled with the
liquor. The tower has many diaphragms and perforated "mushrooms," to
cause a proper dispersion of the gases as they ascend through the
liquor. The strength of liquor found best adapted for the work is
equal to a density of about 30° Twaddell. After saturation the mud of
bicarbonate of sodium is drawn off and passed into the "decomposer," a
tower 35 ft. high by 6 ft. 6 in. in diameter, with perforated shelves,
into which steam is blown from below, the liquor passing downward. The
bicarbonate is decomposed, pure carbonic acid being given off. This is
passed through a scrubber and into a gas holder ready for use. The
liquor, which has now returned to the state of simple carbonate of
sodium, only requires cooling to be ready to absorb a fresh lot of
carbonic acid gas. The cooling is effected in a tower packed loosely
with bricks, the hot liquor trickling down against a powerful current
of air blown in from below. Liquor has been cooled in this way, in
once passing through the tower, from 220° Fahr. to 58° Fahr., but of
course the exact cooling obtained depends more or less on the
temperature of the atmosphere.
The next stage of the process, if we follow on after the preparation
of the pure carbonic acid, is the employment of the gas for the
decomposition of the ammonium sulphide absorbed in a brine liquor as
above explained. The brine and ammonium sulphide are contained in what
is known as a "Solvay tower," provided with proper means for
dispersion and absorption of the carbonic acid gas. The precipitated
bicarbonate of sodium is removed and washed, and prepared for the
market in whatever form is required, the sulphureted hydrogen gas
being led to a holder and stored, as before stated.
The decomposition of the ammonium chloride by means of "alkali waste"
is carried out in a specially designed still. This is a tower 45 ft.
high by 8 ft. diameter, divided by horizontal plates into compartments
of about 3 ft. 8 in. in height. These compartments communicate with
one another by means of pockets, or recesses, in the shell of the
tower. A vertical shaft, with arms, revolves in the tower. The "waste"
is fed in at the top by means of hopper and screw feed. The liquor is
heated by steam blown in to over 212° Fahr. The ammonium sulphide is
led direct into an absorbing vessel full of brine.
It now only remains to see how it is proposed to deal with the
sulphureted hydrogen gas which represents the sulphur recovered from
the waste. It can be burnt direct to sulphurous acid and utilized for
the production of vitriol perfectly pure and free from arsenic,
commanding a special price. But Messrs. Parnell & Simpson state that
by a method of restricted combustion they are able to obtain nearly
all the sulphur as such, and put it on the market on equal terms with
the best Sicilian sulphur. We did not gather that this has yet been
done on the working scale, however.
It will be seen that it is proposed that a Leblanc alkali maker shall
continue to produce a portion of his make by the old process, but
shall erect plant to enable him to make another portion by the Parnell
& Simpson method, using his Leblanc "waste" in place of the caustic
lime now employed by the ammonia soda people. He is thus to have the
benefit of the cheaper process for, say, half his make, while he
further cheapens the ammonia method by saving the cost of lime and by
recovering the sulphur otherwise lost in his waste.
The saving in lime is stated to be one ton for each ton of sodium
carbonate produced, or in cash value about 10s. per ton at Widnes,
while the sulphur saved is estimated to be 6 cwt. per ton of sodium
carbonate. We reproduce these figures with all reserve, not being
ourselves sufficiently specialists to judge of them. But we were
assured that they represent the minimum expected, and reasons were
given to us to show that they would probably be exceeded.
Another gain for the Leblanc maker would be that he will escape the
cost of removal and disposal of a portion of his refuse or waste.
The plant now erected was calculated for a yield of one hundred tons
carbonate of sodium and about thirty-five tons of sulphur per week,
but it now appears likely that this will be exceeded; while the
carbonic acid plant was supposed to be equal to a yield of 6 tons of
pure gas per day, and is now found capable of doing twice as much.
A few weeks will now bring this new combination process into the
active and crucial test of the markets. Chemists and chemical
engineers have all along taken a keen interest in the ingenious ideas
of Parnell & Simpson. Commercial men are no less interested in the
financial result of the experiment about to be tried at the expense of
a few gentlemen of Liverpool and district. So far as we can learn,
opinions are to some extent divided, though many good judges are very
hopefully inclined. For our own part, speaking with diffidence, as
being a little off our regular track of work, we will only say that we
were favorably impressed with what we saw and heard; and we certainly
wish the venture that full success which its cleverness and its pluck,
as well as its great importance at this crisis, deserve for
it.--_Engineering_.
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