The decomposition of a solution of common salt, and its conversion
into chlorine gas and caustic soda solution by means of an electric
current, has long been a study with electro-chemists. Experimentally
it has often been effected, but so far as we are aware, the success of
this method of production has never until now been demonstrated on a
sound commercial basis. The solution of this important industrial
problem is due to Mr. James Greenwood, who has been engaged in the
development of electro-chemical processes for many years. The outcome
of this is that Mr. Greenwood has now perfected an electrolytic
process for the direct production of caustic soda and chlorine, as
well as other chemical products, the operation of which we recently
inspected at Phoenix Wharf, Battersea, London. One of the special
features in connection with Mr. Greenwood's new departure is the novel
and ingenious method by which the electrolyzed products are separated,
and their recombination rendered impossible. This object is attained
by the use of a specially constructed diaphragm which is composed of a
series of V-shaped glass troughs, fitted in a frame within each other
with a small space between them, which is lightly packed with asbestos
fiber. Another important feature of the apparatus is a compound anode
which consists of carbon plates, with a metal core to increase the
conductivity. The anode is treated in a special manner so as to render
it non-porous and impervious to attack by the nascent chlorine evolved
on its surface. No anode appears ever to have been invented that is at
all suitable for working on a large scale, and the successful
introduction of this compound anode, therefore, constitutes a marked
advance in the apparatus used in electrolytic methods of production.
The apparatus by which the new process is being successfully
demonstrated on a working scale has been put up by the Caustic Soda
and Chlorine Syndicate, London, and has been in operation for several
months past. The installation consists of five large electrolytic
vessels, each of which is fitted up with five anodes and six cathodes
arranged alternately. The anodes and cathodes are separated by the
special diaphragms, and each vessel is thus divided into ten anode or
chlorine sections and ten cathode or caustic soda sections. The anodes
and cathodes in each vessel are connected up in parallel similar to an
ordinary storage battery, but the five electrolytic vessels are
connected up in series. The current is produced by an Elwell-Parker
dynamo, and the electromotive force required to overcome the
resistance of each vessel is about 4.4 volts, with a current density
of 10 amperes per square foot of electrode surface. The anode
sections, numbering fifty altogether, are connected by means of tubes,
the inlet being at the bottom and the outlet at the top of each
section. The whole of the cathode sections are connected in the same
manner. In commencing operations, the electrolytic vessels are charged
with a solution of common salt, through which a current of electricity
is then passed, thus decomposing or splitting up the salt into its
elements, chlorine and sodium. In the separation of the sodium,
however, a secondary action takes place, which converts it into
caustic soda. An automatic circulation of the solutions is maintained
by placing the charging tanks at a slight elevation, and the vessels
themselves on platforms arranged in steps. The solutions are pumped
back from the lowest vessel to their respective charging tanks, the
salt solution to be further decomposed and the caustic soda solution
to be further concentrated. The chlorine gas evolved in the fifty
anode sections is conveyed by means of main and branch tubes into
several absorbers, in which milk of lime, kept in a state of
agitation, takes up the chlorine, thus making it into bleaching or
chlorate liquor as may be required. If the chlorine is required to be
made into bleaching powder, then it is conveyed into leaden chambers
and treated with lime in the usual manner. The caustic soda formed in
the fifty cathode sections is more or less concentrated according to
the particular purpose for which it may be required. If, however, the
caustic soda is required in solid form, and practically free from
salt, then the caustic alkaline liquor is transferred from the
electrolytic vessels to evaporating pans, where it is concentrated to
the required strength by evaporation and at the same time the salt
remaining in the solution is eliminated by precipitation.
Such is the method of manufacturing caustic soda and chlorine by this
process, which will doubtless have a most important bearing upon many
trades and manufactures, more particularly upon the paper, soap, and
bleaching industries. But the invention does not stop where we have
left it, for it is stated that the process can be applied to the
production of sodium amalgam and chlorine for extracting gold and
other metals from their ores. It can also be utilized in the
production of caustic and chlorate of potash and other chemicals,
which can be manufactured in a state of the greatest purity. A very
important consideration is that of cost, for upon this depends
commercial success. It is therefore satisfactory to learn that the
cost of production has been determined by the most careful electrical
and analytical tests, which demonstrate an economy of over 50 per
cent. as compared with present methods. Highly favorable reports on
the process have been made by Dr. G. Gore, F.R.S., the eminent
authority on electro-chemical processes, by Mr. W.H. Preece, F.R.S.,
and by Messrs. Cross & Bevan, consulting chemists. Dr. Gore states
that the chemical and electrical principles upon which this process is
based are thoroughly sound, and that the process is of a
scientifically practical character. Should, however, the economy of
production even fall somewhat below the anticipations of those who
have examined into the process very carefully, it can hardly fail to
prove as successful commercially as it has scientifically.
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