MILD ALKALI
Caustic and Mild. There are two classes of alkalis distinguished by the
terms caustic and mild. If a piece of all-wool material is boiled with a
solution of caustic soda or potash, it dissolves completely, giving a
yellow solution. Mild alkali will not dissolve flannel, though it may
have some slight chemical action causing shrinkage. Partly for this
reason, and partly because commercial washing soda often contains a
little caustic soda, woollen garments must not be boiled or even washed
in hot soda water.
The disintegrating action of the caustic alkalis is also illustrated by
the use of caustic soda in the preparation of wood pulp for paper
making. Tree trunks are first torn up and shredded by machinery; but
notwithstanding the power of modern machinery, the fibre is not nearly
fine enough for the purpose until it has been “beaten” with a solution
of caustic soda, whereby the pulp is brought to a smooth and uniform
consistency like that of thin cream.
Mild Soda and Potash. Until the middle of the eighteenth century, it was
thought that the soluble matter extracted from the ashes of all plants
was the same. In 1752 it was shown that the substance obtained in this
way from plants which grew in or near the sea differed from that from
land vegetation by producing a golden yellow colour when introduced into
the non-luminous flame of a spirit lamp, while that from land plants
gave to the flame a pale lilac tinge. The former substance is now known
as mild soda, and the latter as mild potash.
At this point it is well to make it clear to the reader that there are
two bodies commonly called soda, and two called potash. One of each pair
is caustic and one mild.
By a simple chemical test it is easy to distinguish a mild from a
caustic alkali. When a little dilute acid is added to the former, there
is a vigorous effervescence caused by the escape of carbon dioxide, but
no gas is given off when a caustic alkali is treated in the same way.
The liberation of carbon dioxide on the addition of acids shows that the
mild alkalis are carbonates.
Washing Soda is so well known, that very little description of its
external characteristics is necessary. It is a crystalline substance,
easily soluble in water. The crystals, when freshly prepared, are
semi-transparent; but after exposure to air for some time, they are
found to lose their transparency and to become coated with an opaque
white solid which crumbles easily. This change in appearance is
accompanied by a loss in weight.
Crystals of soda melt very easily on the application of heat and, on
continued heating, the liquid seems to boil. When this operation is
carried out in a vessel attached to a condenser, the vapour that is
given off from the melted soda condenses to a clear colourless liquid
which, on examination, proves to be water. When no more water collects
in the receiver, the vessel contains a dry, white solid, which by any
chemical test that may be applied is shown to be the same as washing
soda, but it contains no water of crystallization and has a different
crystalline form. This substance is anhydrous sodium carbonate, or soda
ash as it is called in commerce. When soda ash is mixed with water, it
combines with about twice its own weight of that liquid, forming soda
crystals again.
Washing soda, then, contains nearly two-thirds of its weight of water.
Some of this water is given off spontaneously when the soda is exposed
to air; the water may even be said to evaporate. This accounts for the
loss of weight observed and also for the formation of the white layer of
partially dehydrated soda over the surface of the crystal. The property
of losing water in this way is common to most crystals containing a high
percentage of water of crystallization. The phenomenon is known as
“efflorescence.” It may here be observed that crystals of washing soda
which have become coated over in this way contain relatively more soda
than those which are transparent.
Natural Soda. In Egypt, Thibet, and Utah, there are tracts of country
where the soil is so impregnated with soda that the land is desert. The
separation of the soda from the earth is a simple operation, for it is
only necessary to agitate the soil with water and, after the insoluble
matter has settled down, to evaporate the clear solution until the soda
crystallizes out.
In addition to alkali deserts, there are also alkali lakes. Those in
Egypt are small, nevertheless, about 30,000 tons of soda per annum are
exported from Alexandria. Owens Lake in California is said to contain
sufficient soda to supply the needs of North America; while in the East
African Protectorate, beneath the shallow waters of Lake Magadi
(discovered in 1910), there is a deposit of soda estimated at
200,000,000 tons.
The Leblanc Process. At the present time, the greater part of the
world’s supply of soda is made from common salt by two processes. The
older of these, which is known as the Leblanc process, was introduced in
France towards the end of the eighteenth century. In those days soda was
very dear, for the main supply came from the ashes of seaweeds;
wherefore the French Academy of Sciences, in 1775, offered a prize for
the most suitable method of converting salt into soda on a manufacturing
scale. The prize was won by Nicholas Leblanc, who in 1791 started the
first soda factory near Paris. These were the days of the French
Revolution; the “Comité de Sûreté Général” abolished monopolies and
ordered citizen Leblanc to publish the details of his process.
[Illustration: Fig. 12. SALT CAKE FURNACE]
The first alkali works were established in Great Britain in 1814. The
total amount of soda now made in this country every year is about
1,000,000 tons, of which nearly one-half is still made by the Leblanc
process.
Salt Cake. The first stage of the Leblanc process consists in mixing a
charge of salt weighing some hundredweights with the requisite amount of
“chamber” sulphuric acid. The operation is carried out in a circular
cast-iron pan (D, Fig. 12) about 9 ft. in diameter and 2 ft. deep. The
pan is covered over with a dome of brickwork, leaving a central flue (E)
for the escape of hydrochloric acid gas which is produced. At first, the
reaction takes place without the application of heat, but towards the
end the mass is heated for about one hour. The contents of the pan are
then raked out on to the hearth of a reverberatory furnace (_a_, _b_)
and more strongly heated. More hydrochloric acid gas is given off, and
the reaction is completed. The solid product which remains is impure
Glauber’s salt (sodium sulphate), and is known in the trade as “salt
cake.”
Black Ash. In the second stage of the Leblanc process, salt cake is
converted into black ash. The salt cake is crushed and mixed with an
equal weight of powdered limestone or chalk and half its weight of coal
dust. This mixture is introduced into a reverberatory furnace (Fig. 13)
by the hopper K, and heated to about 1000° C. by flames and hot gases
from a fire at _a_. During this operation, the mass is kept well mixed,
and after some time it is transferred to _h_ where the temperature is
higher. The mixture then becomes semi-fluid and carbon monoxide gas is
given off.
[Illustration: Fig. 13. BLACK ASH FURNACE]
The formation of carbon monoxide within the semi-solid mass renders it
porous. This is an advantage, because it greatly facilitates the
subsequent operation of dissolving out the soluble sodium carbonate. The
appearance of the flames of carbon monoxide at the surface of the black
ash indicates the end of the process. The product is then worked up into
balls and removed from the furnace.
The chemical changes which take place in making black ash are probably
as follows: Carbon (coal dust) removes oxygen from sodium sulphate,
which is thus changed to sodium sulphide. This substance then reacts
with the limestone (calcium carbonate), forming sodium carbonate (soda)
and calcium sulphide.
Extraction of Soda. It now only remains to dissolve out the soda from
the insoluble impurities with which it is mixed in the black ash. It is
evident that the smaller the amount of water used for this purpose the
better, because the water has subsequently to be got rid of by
evaporation. The process of extraction is, therefore, carried out
systematically. The black ash is treated with water in a series of tanks
which are fitted with perforated false bottoms. The soda solution, which
is heavier than water, tends to sink to the bottom and, after passing
through the perforations, is carried away by a pipe to the second tank,
and so on throughout the series. The fresh water is brought first into
contact with the black ash from which nearly all the soda has been
extracted.
The method of finishing off the black ash liquor differs according to
the final product which the manufacturer desires to obtain, for the
liquor contains caustic soda as well as mild soda. For the present, we
will suppose that the end product is to be washing soda. In this case,
carbon dioxide is passed into the liquor to convert what caustic soda
there is into mild soda.
The clarified soda liquor is then evaporated until crystals of soda
separate out. The first part of this process is carried out in large
shallow pans (P. Fig. 13), using the waste heat of the black ash
furnace, and finally in vats containing steam-heated coils. As the
crystals separate out, they are removed, drained, and dried.
Alkali Waste. Black ash contains less than half its weight of soda, so
that for every ton of soda produced there is from a ton and a half to
two tons of an insoluble residue which collects in the lixiviating and
settling tanks. This residue is known as alkali waste.
Alkali waste is of no particular value. It is not even suitable as a
dressing for the land, and since it is not soluble in water there is no
convenient means of disposing of it. Consequently, it is just
accumulated at the works and, as the heap grows at an alarming rate, it
cumbers much valuable ground. Moreover, it contains sulphides from
which, under the influence of air and moisture, sulphuretted hydrogen is
liberated. Alkali waste, therefore, has a very unpleasant odour.
The whole of the sulphur which was contained in the sulphuric acid used
in the first stage of the process remains in the alkali waste, mainly as
calcium sulphide. A plant for the recovery of this sulphur is
established in some of the larger works. The alkali waste is mixed with
water to the consistency of a thin cream, in tall, vertical cylinders.
Carbon dioxide under pressure is forced into the mixture, and this
converts the calcium sulphide into calcium carbonate and sets free
hydrogen sulphide, which, when burnt with a limited supply of air,
yields sulphur.
By this process, the most unpleasant feature of alkali waste, namely,
the smell, is removed. The calcium carbonate which remains is of very
little value. Some of it is used in making up fresh charges for the
black ash process and some for preparing Portland cement, for which
finely-ground calcium carbonate is required; the remainder is thrown on
a heap.
Bicarbonate of Soda. Bicarbonate of soda can be easily distinguished
from washing soda. It is a fine, white powder similar in appearance to
the efflorescence on soda crystals. It does not contain any water of
crystallization.
When bicarbonate of soda is heated, it does not melt, and, as far as its
external appearance is concerned, it does not seem to undergo any
change. If, however, suitable arrangements are made, water and carbon
dioxide gas can be collected, and the sodium bicarbonate will be found
to have lost 36·9 per cent. of its weight. The substance which remains
is identical with that obtained by heating soda crystals, that is,
anhydrous sodium carbonate. Sodium bicarbonate is, therefore, a compound
of sodium carbonate and carbonic acid.
The most familiar use of this compound is indicated by its common names
“baking-soda” and “bread-soda.” It is mixed with dough or other similar
material in order to keep this from settling down to a hard solid mass
in baking. The way in which bicarbonate of soda prevents this will be
readily understood when it is remembered that an ounce of this substance
liberates more than 2,300 cu. in. of carbon dioxide when it is heated.
When the bicarbonate of soda is well mixed with the ingredients of the
cake or loaf and disseminated throughout the mass, each particle will
furnish (let us say) its bubble of gas. Since these cannot escape, a
honey-combed structure is produced.
[Illustration: Fig. 14. THE SOLVAY PROCESS]
Baking powder is a mixture of bicarbonate of soda and ground rice; the
latter substance is merely a solid diluent.
The Solvay Process. Soda ash is one of the principal forms of mild
alkali used in commerce. Large quantities of this substance are made by
heating bicarbonate of soda. We shall now consider another alkali
process in which this substance is the primary product.
For the greater part of the first century of its existence, the Leblanc
soda process had no rival, although another method, known as the
ammonia-soda process, was patented as early as 1838. In this case,
however, as in many others, expectations based on the experiments
carried out in the laboratory were not realized when the method came to
be tried under manufacturing conditions. It was not until 1872 that
Ernest Solvay, a Belgian chemist, had so far solved the difficulties,
that a new start could be made. In that year, about 3,000 tons of soda
were produced by the ammonia-soda or Solvay process, as it has now come
to be known. Since then, however, the quantity produced annually has
been steadily increasing, until at the present time it amounts to more
than half of the world’s supply.
The Solvay process is very simple in theory. Purified brine is saturated
first with ammonia gas and then with carbon dioxide. Water, ammonia, and
carbon dioxide combine, forming ammonium bicarbonate, which reacts with
salt (sodium chloride), producing sodium bicarbonate and ammonium
chloride.
The principal reaction is carried out in a tower (Fig. 14 (1), _a_, _a_)
from 50 to 65 ft. in height and about 6 ft. in diameter. At intervals of
about 3½ ft. throughout its length, the tower is divided into sections
by pairs of transverse discs, one flat with a large central hole, and
one hemispherical and perforated with small holes (Fig. 14 (2)). The
discs are kept in position by a guide rod G. Fig. 14 (3) shows a better
arrangement of the guide rods. In modern works, the space between the
discs is kept cool by pipes conveying running water. The ammoniated
brine is led into the tower near its middle point. The carbon dioxide is
forced in at E in the lowest segment, and as it passes up the tower it
is broken up into small bubbles by the sieve plates. Sodium bicarbonate
separates out as a fine powder, which makes its way to the bottom of the
tower suspended in the liquid.
The perforated plates are necessary for the proper distribution of
carbon dioxide through the brine. They are, however, a source of
trouble, because the holes quickly become blocked up with sodium
bicarbonate, and every ten days or so it is necessary to empty the tower
and clean it out with steam or boiling water.
Recovery of Ammonia. The production of 1 ton of soda ash by the Solvay
process involves the use of a quantity of ammonia which costs about
eight times as much as the price realized by selling the soda. It is
evident that the success of the process as a commercial venture depends
largely on the completeness with which the ammonia can be recovered.
During the process, ammonia is converted into ammonium chloride, which
remains dissolved in the residual liquor. From this ammonia gas is set
free by adding quicklime and by blowing steam through the mixture. It is
now claimed that 99 per cent. of the ammonia used in one operation is
recovered.
Soda Ash. The bicarbonate of soda produced by the Solvay process is
moderately pure. For all ordinary purposes, it is only necessary to wash
it with cold water to remove unchanged salt, and after drying, it is
ready to be placed on the market if it is to be sold as bicarbonate. The
greater part of the Solvay product, however, is converted into soda ash
by the application of heat. If soda crystals are required, the soda ash
is dissolved in water and crystallized.
In many ways, the Solvay process compares very favourably with the older
method. It is an advantage to start with brine, for that is the form in
which salt is very often raised from the mines. The end product is
relatively pure; moreover, it is quite free from caustic soda, which for
some purposes for which soda ash is used is a great recommendation.
There is no unpleasant smelling alkali waste. On the other hand, the
efficiency of the Solvay process is not high, for only about one-third
of the salt used is converted into soda. This would make the process
impossible from the commercial point of view were it not for the
cheapness of salt.
The Leblanc process, too, has its advantages. In the next chapter we
shall see that it is adaptable for the production of caustic as well as
mild alkali. The chlorine which is recovered in the Leblanc process is a
very valuable by-product. In the Solvay process, chlorine is lost, for
hitherto no practicable method has been found for its recovery from
calcium chloride.
The position with regard to the future supply of alkali is very
interesting. The competition between the Leblanc and the Solvay
processes for supremacy in the market is very keen. At the same time,
both processes are in some degree of danger of being supplanted by the
newer electrical methods, which will be mentioned in the last chapter.
The following table shows very clearly the rapid progress made by the
Solvay process in ten years. The quantities are given in _tonnes_ (1
tonne = 0·9842 ton).
1884. 1894.
_Leblanc _Solvay _Leblanc _Solvay
soda._ soda._ soda._ soda._
Great Britain 380,000 52,000 340,000 181,000
Germany 56,500 44,000 40,000 210,000
France 70,000 57,000 20,000 150,000
United States — 1,100 20,000 80,000
Austria-Hungary 39,000 1,000 20,000 75,000
Russia — — 10,000 50,000
Belgium — 8,000 6,000 30,000
545,500 163,100 456,000 776,000
Mild Potash. Potassium carbonate (mild potash) was formerly obtained
from wood ashes. The clear aqueous extract was evaporated to dryness in
iron pots, and the substance was on this account called _potashes_;
later, potash. A whiter product was obtained by calcining the residue,
and this was distinguished as _pearl-ash_. Chemically pure potassium
carbonate was formerly obtained by igniting cream of tartar (potassium
hydrogen tartrate) with an equal weight of nitre. It is for this reason
that potassium carbonate is sometimes called “salt of tartar.”
About the middle of last century, natural deposits of potassium chloride
were discovered in Germany. The beds of rock salt near Stassfurt are
covered over with a layer of other salts, and for many years these were
removed and cast aside as “waste salts” (_abraumsalze_). When at a later
date they were examined more carefully, they were found to contain
valuable potassium compounds, notably the chloride. After that
discovery, mild potash was made by the Leblanc process., and Germany
controlled the world’s markets for all potassium compounds.
At the outbreak of war, the German supplies of potassium compounds
ceased as far as the allied nations were concerned, and an older method
of making potassium chloride from _orthoclase_ or potash-felspar was
revived. This involves the heating of the powdered mineral to a high
temperature after mixing it with calcium chloride, lime, and a little
fluorspar. The potassium chloride is then extracted from the fused mass
with water. This method has been worked with great success in America,
and it is claimed that potassium chloride can be made in that country at
a cost which is lower than that formerly paid for the imported article.
Mild potash and soda are so very similar in chemical properties that in
most cases it is immaterial which compound is used. In all cases in
which there is this choice, soda is employed, both because it is cheaper
and because it is more economical, for 106 parts of soda ash are
equivalent to 138 parts of potash. There are, however, some occasions
when soda cannot be substituted, notably for the manufacture of hard
glass and soft soap, and for the preparation of caustic potash,
potassium dichromate, and other potassium salts.
Potassium Bicarbonate. This resembles the corresponding sodium salt in
nearly every respect. It is, however, much more readily soluble in
water, so much so, that it is not possible to obtain this substance by
the Solvay method. It is made from potassium carbonate by saturating a
strong aqueous solution of that substance with carbon dioxide.