Improvements in the Electrolysis of Solutions of Alkali Salts, especially Alkali Chlorides.
Year: 1898

page_text: This Print is in accordance with the Original Specification as it stands after
Amendment (under Section 11 of the Patents, &c. Act, 1883) as required by
the decision of the Comptroller General of the 9th day of February 1899.
Date of Application, 23rd July, 1898-Accepted, 15th Oct., 1898
COMPLETE SPECIFICATION (AMENDED).
Improvements in the Electrolysis of Solutions of Alkali Salts,
especially Alkali Chlorides.
I, OLIVER IMRAY, of Birkbeck Bank Chambers, Southampton Buildings, in
the County of London, Chartered Patent Agent, do hereby declare the nature
of this invention (as communicated to me from abroad by Oesterreichischer
Verein für Chemische und Metallurgische Production, of Aussig on the Elbe,
Bohemia) and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement:-
―
The application of electrolytic decomposition for the production of chemicals
on a manufacturing scale has been the subject of much study and of many in-
ventions especially has it been sought to manufacture caustic alkali and chlorine
by electrolysis of alkali chlorides in the simplest manner and with the least
possible expenditure of electrical energy.
For this purpose inventors have followed two directions the one has for its
object the separation of the alkali metal as quickly as possible from the circuit
and to allow the reaction of the metal with water to occur outside the decom-
posing cell so as to reduce to a minimum the formation of alkali hydroxide in
the electrolyte. In processes of this class the use of mercury as cathode is the
most general device. The other direction has for its object to keep within the
smallest possible limits the interaction of the cations with the anions and the
consequent formation of secondary products and loss of current the use of appro-
priate diaphragms or a separation of the solution surrounding the cathode from
that surrounding the anode based upon the different specific gravities of these
solutions has been generally applied with greater or less success. Processes of
both classes so far as they have been disclosed in the literature of the subject
suffer from more or less important defects which detract from their technical
value. In the relatively more common class in which a mercury cathode is used
the alleged satisfactory utilisation of the current and the great purity of the
alkali obtained are partly discounted by the complexity of the apparatus and the
use of the costly mercury. On the other band processes of the other class have
realised only partially the object sought namely a favourable utilisation of the
current and a uniform yield of products of good quality in an apparatus of simple
arrangement and requiring little attention.
My foreign correspondents have devised as a result of many experiments a
process which practically avoids the disadvantages and defects apparently inherent
to the processes at present employed on a large scale and utilises the known
phenomena arising from the migration of the ions and the difference of specific
gravity of the liquids surrounding the anode and cathode respectively developed
during the electrolysis; they are able to achieve in this way a favourable utilisa-

page_text: tion of the current in a continous process yielding products of good quality
with an apparatus of the simplest possible construction.
Of the class of processes depending upon the principle of the separation of the
products of electrolysis by the difference of specific gravity of the solutions con-
taining them the oldest is that described by Elisha Cutten in his Austrian Patent
dated 6th October 1892. The apparatus for this process ought to render possible
a continous yield and therefore the layers of the solutions of different specific
gravity should be preserved, however, this is not achieved because owing to the dis-
position of the anodes and the manner in which the electrolyte is fed in, the layer
formation is destroyed, moreover owing to the projection of the anodes beyond
the bell which covers them the room for the formation of the layers is most un-
favourably contracted.
The process described by Gibson Baily and Malcolm Gutterie described in the
Specification No. 15610 of 1893 seeks to effect the separation of the anode and
cathode products by interpesing between the anode and cathode a pair of very
porous or sieve formed screens enclosing a space into which the electrolyte is fed. It
is thus intended that the liquid surrounding the anode should flow in one direc-
tion and that surrounding the cathode in the opposite direction. It is obvious
that this is not practicable or is so only when the current of liquid is so rapid that
the separation of layers of different specific gravity which must inevitably begin
to occur cannot be completed so far as this rapidity of flow can be admitted with-
out sacrificing the alkali solution there must inevitably occur an intermixture of
the anode and cathode liquids with all its consequences; for the desired vertical
position of the layers cannot be maintained owing to the tendency for formation
of horizontal layers by the liquids of different specific gravity.
The process of l'erd. Hurter, H. Auer and E. K. Muspratt described in the
Specification No. 19791 of 1893 is intended to utilise the separation of the solu-
tion of the cathode product from that of the anode product by taking advantage
of the difference of specific gravity, in an apparatus, however, which as con-
structed must hinder the formation of layers inasmuch as owing to the position
of the anodes and the use of the whole of the cell wall as cathode the evolution
of gas from anode and cathode must inevitably generate a movement of the liquid
and destroy the layer separation. Moreover the process is primarily devised for
discontinuous working and since it is remarked in the specification that the use
of the apparatus is possible for continuous working it must be supposed that the
electrolyte is to be introduced into the open cathode compartment for in the
apparatus shown no arrangement is provided for charging liquid into the closed
anode compartment this alone must inevitably destroy the layer formation and
has the further objection that the anode liquid becomes progressively poorer in
electrolyte and finally is so dilute that a secondary decomposition of the water
sets in entailing useless consumption of current.
The process of J. C. Richardson (Amer. Patent No. 508241 of 7th November
1893) is for discontinuous working and consequently valueless in practice. This
process involves a mode of disposing the anode and cathode which is not favour-
able to the collection of the alkali solution formed around the cathode at the
lowest part of the cell owing to its comparatively high specific gravity, and it
follows that when the fresh electrolyte is run in the alkali solution which has
been already formed is only partially removed.
Dr. Willy Bein has described in his German Patent No. 84547 of 22nd October
1893 a discontinuous process in which he uses an apparatus for the purpose of
utilising the difference of specific gravity between the anode and cathode liquids-
in this apparatus the two electrodes are contained in special divisions of the de-
composing cell arranged side by side and communicating under the surface of
the liquid and there is a device by which it is possible when the electrolysis has
proceeded so far that the cathode liquid it is about to enter the anode compart-
iment the two layers of liquids can be separated and the operation ended.
Obviously such a process requires careful supervision which enhances the diffi-

page_text: culty of working moreover the apparatus possesses complications entailing
important technical difficulties.
In a later patent (Amer. Patent No. 591730 of 12th October 1897) Dr. W. Bein
has described an arrangement for a continuous process in which at the place
where the valve for separating the liquids is situated in the earlier apparatus are
substituted means for continuously supplying fresh electrolyte to the cell and in
this way a separating layer is maintained between the anode and the cathode
liquids, moreover the electrolyte thus introduced causes the anode and cathode
liquids to move towards their respective outlets at a rate proportional to its flow,
The advantage which it was hoped to attain by this apparatus namely a con-
tinuous working is not realised since the admission of fresh electrolyte into the
space between the anode and cathode is not favourable for the formation of layers
and it is technically impossible to so regulate the efflux from the anode and
cathode compartments that the quantity flowing away from each shall be in a
constant proportion so that the working is very irregular. If the influx into the
anode compartment be restricted the liquid therein soon reaches such a stage of
dilution in the course of the electrolysis that secondary decomposition of the
water occurs and there is useless consumption of current.
Carmichael in his Gerinan Patent No. 87676 of 25th April 1894 had already
adopted the idea underlying this last patent of Dr. W. Bein. He uses a
diaphragm of special construction which leaves a free communication between
the anode and cathode compartments. By supplying fresh electrolyte between
the anode and the diaphragm, the alkali solution which has passed through to
the upper part of the diaphragm is perpetually made to flow back through the
communication in the diaphragm into the cathode compartment and so to the
outlet. This process apart from the necessity for a diaphragm and communica-
tions which complicate the apparatus suffers from the defect that the separation
of the layers of different specific gravity is destroyed by the admission of the
electrolyte into the space between the electrodes and that the impoverishment
of the anode liquid in electrolyte and the consequent useless expenditure of
current are not avoided.
By critical examination of the foregoing process and careful consideration of
Hittorp observations on the migration of the ions my foreign correspondents
have devised a process which avoids the defects inherent in those above men-
tioned and by means of a simple apparatus requiring no special supervision
effects a continuous electrolysis of alkali salts yielding an alkali solution as con-
centrated as possible.
It may be observed that in consequence of the migration of the ions during the
electrolysis of an alkali salt, particularly of an alkali chloride a constant passage
of hydroxyl ions into the anode compartment occurs, while the product of the
reaction of the cation with the water collects as an alkali solution around the
cathode and gradually spreads to the anode compartment. When the alkali solu-
tion has a higher specific gravity than that of the electrolyte and than that of the
liquid in the anode compartment it is obvious that the anode should be arranged
above the cathode whereby full use is made of the difference of specific gravity
between the anode and cathode liquids for separating them and with them, of
course, the products of the electrolysis.
In such an arrangement if this destructive influence of the evolution of gas
is avoided by proper arrangement of the cathode with regard to the anode it soon
becomes obvious that the specifically heavier layer of alkali solution which
collects around the cathode is separated from the anode liquid by a layer of
gradually decreasing strength of alkali. By continuing the electrolysis this
nearly neutral zone as the alkali solution continues to increase extends more and
more towards the anode liquid and finally the alkali contained in it enters into
a secondary reaction with the anion forming chlorate which constitutes a new
electrolyte and gives rise to useless' consumption of current and to attack of
the carbon anode.

page_text: It is possible to counteract the passage of this neutral zone or alkali layer into
the anode compartment by the continuous admission of fresh electrolyte into the
cell from the anode and to so regulate this admission and the corresponding
efflux of cathode liquid that the separating neutral layer may be kept constantly
in a certain position; moreover by admitting fresh electrolyte above the anode
the impoverishment of the anode liquid in electrolyte may be prevented and thus
useless consumption of current be avoided.
It is essential to the maintenance of a constant position of the neutral layer
separating the anode liquid from the cathode liquid that the stream of electrolyte
should be admitted in a direction opposite to that of the migration of the hydroxyl
or chlorine ions and therefore the apparatus must be constructed to fulfil this
condition.
As a result of their researches my foreign correspondents find that the simplest
and most practical apparatus for their purpose is that shown in the accompanying
drawing:
The anode a is situated within a bell the lower edge of which is considerably
below the under surface of the anode, and on the outside of which the cathodes c
are disposed. In the space between the under surface of the anode and the
edge of the bell, which space is indispensable for the success of the process, there
is formed in the course of the electrolysis a layer of liquid in such a way that the
anode liquid saturated with the anion is superposed above a certain part of the
cathode liquid, which beginning with a minimum strength of alkali becomes
more concentrated and specifically heavier from this point downwards and at the
lower edge of the bell has its maximum strength of alkali which is identical
with that contained in the liquid throughout the cathode compartment outside
the bell. As the electrolysis proceeds the separating zone between the two liquids
is continually approaching the anode. To prevent this fresh electrolyte is admitted
through the pipe e into the anode compartment either continuously or at short
intervals of time but in a manner totally different from that adopted by the
authors cited above. Whereas previous inventors have attempted to prevent the
intermixture of the anode and cathode liquids and to keep the fresh electrolyte
as a neutral layer (that containing neither free chlorine nor caustic alkali) the
greatest stress is laid in this process on intimately mixing the fresh
electrolyte with the anode liquid (which is attained by admitting the
electrolyte above the anode and allowing the evolved gas
the mixing, and thus keeping it of constant concentration instead of letting it
become weaker during the electrolysis. If the admission of electrolyte is regu-
lated so that the speed with which the anode liquid passes downwards is nearly
equal to that with which the alkali tends to pass upwards which is ultimately
connected with the quantity of current used it is possible to maintain constant
the level of the zone where the anode and cathode liquids meet without in any
way destroying the line of demarcation, moreover the caustic alkali solution is
caused to leave the cathode compartment through the overflow pipe f at the same
rate as that at which the fresh electrolyte is admitted into the anode compartment.
The possible passage of the anode liquid saturated with chlorine from the anode
compartment to the cathode compartment is attended with comparatively little
disadvantage since free chlorine is very slightly soluble in a strong solution of
alkali chloride. The chlorine is conducted through the pipe g and the hydrogen
from the cathodes escapes into the air.
to
effect
It is obvious that the form of apparatus here described is only a variant of
other possible forms; the mouth of the bell may be closed by a diaphragm if
this is sufficiently porous not to obstruct appreciably the current and direction of
the liquid. Furthermore any form of this apparatus may easily be closed and
worked under diminished pressure if this be necessary for facilitating the separa-
tion of the gaseous anion.
The herein described apparatus although possessing a general similarity to
those referred to at the beginning of this specification differs essentially from all

page_text: known arrangements for the electrolysis of alkali salts in the following points
namely from Cuttens process (Austrian Patent 42/2704) in that the electrolyte
is admitted above the anode so that the separating layer is kept at a constant
level in spite of the opposing current-from Gibson Baily and Male. Gutterie's
process in that the anode is above the cathodes and that the porous, diaphragm
is not necessary; from Huster & Aners process in that the position of the anode
with respect to the lower edge of the bell ensures continuous working by main-
taining constant the separating layer and in that the concentration of the anode
liquid is kept constant by admitting the electrolyte above the anode; from the
process of Dr. Willy Bein (German Patent No. 84547) in that the process is
continuous owing to the constant separating layer and the constant concentration
of the anode liquid in electrolyte and from his later process (American Patent
No. 591730) in that there is a free efflux of liquid from the cathode compartment
and the liquid contains as much alkali as is possible; from J. C. Richardsons
process (Amer. Pat. No. 508241) in that the working is continuous the impoverish-
ment of the anode liquid is avoided and the cathode liquid is as alkaline as pos-
sible; finally from Carmichaels process in that a diaphragm is not necessary
and that the separation of the layers is maintained by a current moving in the
direction opposite to that of the migrating ions.
Having now particularly described and ascertained the nature of this invention
and in what manner the same is to be performed, I declare that I am aware of
the Specification of the Letters Patent Number 2297 A.D. 1890 granted to
James Charles Richardson and Trevenen James Holland, and of the Specification
of the Letters Patent Number 12149 A.D. 1896 granted to Trevenen James
Holland, and I wish it to be understood that I do not claim anything described
and claimed therein. What I claim is:-
In the electrolysis of solutions of alkali salts the arrangement wherein the
electrodes are placed side by side and if necessary at different levels the anode
is covered by a bell which extends below it the electrolyte is continuously fed
into the anode compartment above the anode the alkaline lye produced is con-
tinuously removed from the cathode compartment and the layer formation of
the anode and cathode liquids is maintained without the use of a diaphragm so
that the products formed at the electrodes are continually separated by the said
layer substantially as described.
Dated this 23rd day of July 1898.