SULPHUR TRIOXIDE.
Sulphur trioxide does not exist by itself in nature. Moreover it is but
little known even as an artificial product. It is not an article of
ordinary sale, though it is occasionally made by the chemist. Yet it
is a constituent part of one of the most important compounds known to
modern industry. That compound is sulphuric acid.
Sulphur trioxide is a white solid, but it cannot easily be kept so.
This is because it has very strong affinity for moisture. It fact it
readily absorbs that water-vapor which is distributed through the
atmosphere, even in dry weather and when the ordinary observer would
suppose that the air contained no moisture at all. When it absorbs
moisture it chemically combines with it, forming sulphuric acid.
The chemical change is represented by the following equation:
=SO₃= + =H₂O= = =H₂SO₄=
One molecule of One molecule of One molecule of
Sulphur trioxide, Water, Sulphuric acid,
80 18 98
parts by weight. parts by weight. parts by weight.
\____________________________________/ \_______________/
| |
98 98
On account of this reaction, sulphur trioxide is often spoken of as
sulphuric anhydride, the term anhydride being intended to suggest that
the substance so named is derived from an acid by the removal of water
from the latter. Thus sulphuric acid _minus_ water produces sulphuric
anhydride. And this harmonizes with what has before been declared,
namely, that sulphuric anhydride—or sulphur trioxide—_plus_ water
produces sulphuric acid.
Sulphuric Acid.
This substance is known to commerce chiefly under the name of oil
of vitriol. It is an oily liquid nearly twice as heavy as water. It
has very powerful chemical action upon most substances with which
it comes in contact. Moreover, its market price is very low, that
is, between one and two cents a pound at wholesale. To these two
facts last mentioned—that is, the marked chemical power and the low
price is referable the enormous demand for the substance. To be
sure, increase of demand and fall in price have a reciprocal action;
for even a slight cheapening of a substance widens considerably the
range of its possible uses and increases the amount consumed. Again,
increase of demand and consumption, lead manufacturers to increase
their production, a circumstance which is generally followed by lower
price. The manufacture of sulphuric acid exemplifies these well-known
principles of political economy. The manufacture of this substance has
risen within the last hundred years from almost nothing to a present
annual production of about nine hundred thousand tons in Great Britain
alone. The price meanwhile has fallen to about one-thirtieth of what it
was in the middle of the last century. At the present time the price
of oil of vitriol seems to be steadily decreasing, while the amount
produced is steadily increasing in England, France, Germany and the
United States—indeed in all countries pervaded by active industrial
enterprises. It will be generally admitted, as M. Dumas has said, that
the amount of sulphuric acid consumed affords a very precise measure
of the advancement in industrial arts of a given country or of a
historical epoch.
[Illustration: FIG. 48.—Section of chambers for manufacture of
sulphuric acid.]
Uses of Oil of Vitriol.
It would be difficult to enumerate the many industries that demand the
use of sulphuric acid. It must likewise be admitted that there are
but few manufacturing operations which do not directly or indirectly
involve its employment. The industries that stand in the front rank
as direct consumers of this acid are those that involve the following
processes, namely: the bleaching of cotton goods; the removal of
scale from iron in its various forms, such as castings, wire, etc.;
the changing of corn starch into the variety of sugar commonly called
glucose; the refining of bullion of gold and silver; the refining
of petroleum oil; last, but not least, the manufacture of chemical
fertilizers for agricultural use. Less directly, but still in enormous
quantities, it is used in the manufacture of soda-ash, and bleaching
powder already referred to as having reached an incredible consumption;
in the manufacture of alum; in the manufacture of both of the great
acids of commerce, chlorohydric acid and nitric acid, which must be
said to come next to sulphuric acid in usefulness; and finally, in
almost all the distinctly chemical industries.
Manufacture of Sulphuric Acid.
Notwithstanding the extremely low price of oil of vitriol and the
immense quantity of it manufactured, its production implies a series of
processes far more complicated than those involved in the preparation
of any other well-known acid. Moreover, although the various intricate
details of its preparation are matters of thorough _experimental_
knowledge to the producer, there are several steps which are not yet
clearly comprehended even by the most eminent chemists of the age.
The process of manufacture, as at present conducted, is properly
described as a continuous one. By this it is meant that the raw
materials are steadily introduced at one end of the apparatus used, and
the finished product is steadily drawn out at the other, the process
meanwhile going on without interruption, night and day, for years. In
order to a better comprehension of the process it is here described in
four stages.
In the first stage, sulphur is burned in a current of air. The material
employed is either partly refined Sicily sulphur, or what is largely
used at the present day, some mineral compound of sulphur, like the
iron and copper pyrites. In either case, sulphur dioxide (SO₂) is
formed. This is the well-known choking gas given out by a burning
sulphur match. As produced on a large scale the gas passes into a
series of enormous leaden chambers. These are, in fact, rectangular
rooms, often as large as one hundred and fifty feet long, twenty feet
wide and fifteen feet high. Generally at least three chambers are in
a series, connected by leaden pipes. Sulphur dioxide gas flows in a
steady stream into the series of chambers and toward the high chimney
of the works, whose draft produces the advance of gases through the
whole apparatus.
[Illustration: FIG. 46.—Section of building fitted for manufacture of
Sulphuric acid; _f_, furnace where sulphur is burned and oxides of
nitrogen are liberated; _k_, boiler from which steam is supplied to the
leaden chamber, _A_.]
The second stage is the most complicated one. It is the oxidizing of
the sulphur dioxide (SO₂) into sulphur trioxide (SO₃). This is indeed
accomplished by means of the oxygen of the air. But this oxygen is not
capable of _directly_ changing SO₂ into SO₃. Certain gaseous oxides of
nitrogen are forced into the chamber at this stage; and these have the
remarkable power on the one hand of taking oxygen to themselves from
the air, and on the other of imparting this oxygen to the compound SO₂
in such a way as to change it into the compound SO₃. Of course the air
is impoverished by the operation, a fact which necessitates a fresh
supply of it through the entire series of chambers.
The third stage is one whose principle has already been explained.
At various parts of the chamber, jets of steam are blown in. While
these aid mechanically in the progress of the gases through the entire
series, their main purpose is to furnish water which shall combine
with sulphuric anhydride to produce sulphuric acid.
Although this chemical change, represented by the following equation,
has been given before, it may not be improper to repeat it here:
=SO₃= + =H₂O= = =H₂SO₄=
One molecule of One molecule of One molecule of
Sulphur trioxide, Water, Sulphuric acid.
80 18 98
parts by weight. parts by weight. parts by weight.
\____________________________________/ \_______________/
| |
98 98
The effect of the steam is to give rise to a steady rain of oil of
vitriol in the chambers. Of course this liquid collects at the bottom.
Thence it is drawn off, for treatment in a fourth stage. It is plain
that up to this point the series of chemical reactions takes place in
what we may characterize as a vast but irregular tube, open at both
ends. This tube is enlarged here and there into great pockets which
constitute the chambers. It is bent into a form appropriate to the
conditions of the business. It is entered here and there by pipes for
introducing the agents whose proper interaction gives rise to the
product sought. It is also tapped for the purpose of drawing off the
acid generated. This open tube has its final exit into the atmosphere
through the tall chimney with which it is connected. It has its first
connection with the atmosphere at the open throat, which swallows at
once the vast volumes of sulphurous gas from the sulphur burned, and at
the same time levies upon the air to contribute its oxygen to produce
the substance which is the final purpose of the whole industry.
The fourth stage is the only one that may be properly said to be
disconnected from the others. The continuous process already described
cannot properly be made to produce acid of the strength demanded by
commerce. In the fourth stage then, the acid from the chambers is
boiled with a view of expelling some of the water in it, and thus of
producing a more concentrated product. This evaporation is itself
no inconsiderable portion of the business. It is conducted first in
shallow tanks of lead, and finally in costly stills of platinum. When
at length the acid in the platinum stills has attained the proper
degree of concentration, it is drawn out by means of a siphon tube,
and through a cooling tank of cold water, into the glass flasks called
carboys, in which it makes its appearance in commerce.
[Illustration: FIG. 47.—Section of apparatus used for concentrating
sulphuric acid. _A_, _A_, leaden pans in which the first evaporation is
conducted; _B_, platinum retort in which the concentrating is finished.]
Of course the account thus given is but a general sketch of this
great industry. Associated with the apparatus and the processes here
briefly described there are employed in actual working a multitude of
other devices and operations. Indeed it might be anticipated that the
successful conduct of a business of such magnitude and complexity would
draw upon the inventive resources of some of the best minds that have
been brought to bear upon chemical industries.
READING REFERENCES.
Sulphur Industry in Sicily.
=Barbaglia=, A.—Chem. News. xxxiv, 245; xxxv, 3, 28.
=Vincent=, C.—Am. Chem. Journal. vi, 63.
Sulphur, Extraction of.
=Sestini=, F.—Jour. of Chem. Soc. of London. xxviii, 335.
Sulphuric Acid.
=Affleck=, J.—Chem. News. xxxvii, 167, 192, 207.
=Hasenclever=, R.—Chem. News. xxxv, 48, 67, 88, 118, 183,
189, 214, 227.