Improvements relating to the Extraction of Chromium by Electrolysis.
Year: 1898

page_text: Date of Application, 1st Sept., 1898
Complete Specification Left, 30th May, 1899-Accepted, 5th Aug., 1898
PROVISIONAL SPECIFICATION.
Improvements relating to the Extraction of Chromium by
Electrolysis.
We, JULIUS FRANZ LUDWIG MOELLER, of 14, Palmer Street, London, Director,
and ERNEST AUGUSTE GEORGE STREET, of 60, Boulevard Haussmann, Paris, in
the Republic of France, Engineer, do hereby declare the nature of this invention
to be as follows:-
In establishing a process for the extraction of chromium by electrolysis, it is
necessary to consider, from an industrial point of view and apart from the motive
force, two elements which might have considerable influence upon the cost price
of the metal, these two elements are: 1st The selection of the anodes employed,
and 2nd. The nature and the quantity of the bye products coming from the
electroly sei
As regards the anodes, it 18 well known, that carbon anodes cannot be employed
in practice as they are deteriorated rapidly by the chromic acid formed in the
baths, thereby necessitating continual renewals which are very costly, and which,
as they wear, foul the electrolyte owing to the foreign matter they contain in
greater or less quantities. Platinum anodes only are therefore employed.
As will be readily understood, the employment of platinum for a daily pro-
duction of any magnitude would require the locking-up of a very large amount
of capital.
On the other hand, as regards the bye-products issuing from the electrolyser,
as the proportion of metal extracted in comparison with the mass of the electrolyte
employed is relatively small, it is obvious that their nature and quantity, accord-
ing to whether they are marketable or the reverse, or to whether they are capable
of being readily regenerated in order to again enter the cycle of operations, 18
an equally important consideration with regard to the cost of extracting the
metal.
We have therefore endeavoured to discover an industrial process for the
extraction of chromium by electrolysis which, on the one hand would enable the
use of platinum anodes to be discontinued, and on the other hand would yield
the minimum of bye-products, but which latter should be of a marketable
character.
The process which we have discovered and which enables us to attain this two-
fold object is based essentially upon the employment, as chromium salts to
electrolyse, of salts having a sulphate base, the electrolyser being made either
with or without porous partitions.
The salts of sulphate of chromium are the only salts which permit of the
employment of lead anodes, on condition that liquids not too heavily charged
with sulphuric acid are used. It is preferable not to exceed the proportions
admissible for lead plate accumulators
Under these conditions the anodes
become peroxidised and have a very long life
In cases in which a porous
partition 18 employed, care should be taken not to allow the concentration
of HSO to increase beyond a certain value, which is readily obtained by pro-

page_text: viding a water supply which may be regulated once for all for the working
conditions, and an overflow to enable the anolyte to run off
If a porous partition is not employed, the degree of acidity should be main-
tained, for the purpose of obtaining a deposit of metal, at a relatively low
point; it is therefore unnecessary in this case to take special precautions for
the purpose of preserving lead anodes
In the first place, therefore, we succeed, by the employment of salts having
a base of sulphate of chromium. in obviating the necessity for using platinum
anodes which we replace by lead anodes in the conditions indicated.
We will now explain in what manner we succeed in obtaining the minimum
of bye products coming from the electrolyser.
In order that this explanation may be readily understood, we will take as an
example the manufacture of electrolytic chromium by means of alum of chromium
either of potash or of soda We, however, prefer this latter because it is more
soluble than the alum of potash, and will now take it by way of example but
without in anyway limiting ourselves thereto
We will assume that an electrolvser having porous partitions is being employed.
The liquid of the cathode compartment is composed for example, as follows:
Water
Alum of chromium and soda
Sulphate of soda
1 500 kilo.
1 500 kilo.
1.500 kilo
The alum of chromium and of soda being assumed to be neutral, that is to
say without free sulphuric acid, the water intended for dissolving the salts
should be acidulated with about 5% by weight of sulphuric acid at 66° Beaume
The liquid of the anode compartment 18 constituted by water acidulated to 5%
by weight at 66° Beaume.
The cathode is formed by a sheet of copper or of brass rolled into tubular
form, and the anodes are of lead
An electrolyser constructed under these conditions and raised to a temperature
of about 70° C will vield a production of chromium per ampere-hour of about
0gr 20 if the apparatus is traversed by a continuous electric current the density
of which with respect to the cathode is about 40A per square decimetre
If what takes place is noted it will be seen that at the same time that the
metal becomes deposited, the liquid of the anode compartment becomes enriched
in sulphuric acid In addition to this, the liquid in this compartment gradually
becomes coloured and takes the characteristic tint of chromic acid solutions.
This arises from the fact that the sulphate of chromium traverses the porous
partition and becomes divided in the anode compartment into sulphuric acid and
chromic acid under the influence of the oxidising agents which it encounters.
An analysis made of the anode liquid of a certain electrolyser has shown that at
the end of 24 hours, for a production of metallic chromium of 23 500 kilos
there was a production of 0562 kilos. of chromic acid Cro and exosmose of
0963 kilos of sulphate of soda (NaOSO³) These figures naturally vary with the
nature of the porous partition
As regards sulphuric acid, the consideration of the chemical formula of sulphate
of chromium Cr²0 380* + NAq leads to the conclusion that for 104 grammes
of chromium extracted, 294 grammes of sulphuric acid must be liberated Con-
sequently a manufactory where one ton of chromium per 24 hours was manu-
factured, would produce as a bye product about 3 tons of sulphuric acid H³SO+
(at 66° Beaume), with the additional disadvantage that this quantity of acid
instead of being at 66° is diluted with a considerable quantity of water and is
mixed, in addition, with the other products which have been mentioned
In order to obtain an idea of the mass of liquid in question and taking as
concentration of the liquid of the anode a mean value of 10% of sulphuric acid,
30 tons of liquid would have to be dealt with. It will be understood that the

page_text: evaporation of 27 tons of water and the separation, if it were possible, of the
bodies other than sulphuric acid, would give 11se to operations which if not
impossible, would be of very little commercial value.
It is our opinion, that such operations could not be carried out, and we there-
fore endeavoured to discover a process which would cause not only the sulphuric
acid to re-enter the cycle of electrolytic operations, but also the products with
which it is mixed owing to osmotic phenomena, and that exactly in the same
proportions as those in which the sulphuric acid and such bodies leave the
electrolyse; the supply of active salts of chromium being proportional to the
weight of the metal deposited.
We operate in the following manner:
It has been stated that the liquid of anode contains sulphuric acid, chromic
acid and sulphate of soda. This latter is no disadvantage because its presence
in the supply liquid of the cathode compartment will only compensate the losses
due to the osmotic phenomena
As regards the chiomic acid, we convert it into sulphate by washing it with
gaseous sulphuric acid This transformation having been affected we boil in
order to expel the excess of sulphurous acid and concentrate the liquid at a
suitable point say 30° Beaume.
To this liquid we add a quantity of bichromate of soda such that, independently
of the sulphate of soda formed, it contains an equivalent of chiomic acid Cr³0°,
that is to say 2 (C10³) for two equivalents of sulphuric acid H³SO'
This liquid is allowed to fall in more or less thm streams into a lead vessel
which invariably contains an excess of melted sulphur which is maintained
at 130° C The chromic acid oxidises the sulphur and sulphate of chromium
(C120 380³) 18 formed owing to the two equivalents of fiee sulphuric acid
previously mentioned This sulphate with the sulphate of soda previously
toimed at the expense of the bichiomate of soda gives us alum. A small quantity
of water 18 again added, filtration is effected in order to separate the excess of
sulphur which 10-enters the cycle, and there results a solution, of alum of
chromium with which the electrolyser is fed proportionately to the metal
deposited.
The previous action of the sulphurous acid may be avoided by titrating the
liquid with chromic acid, taking this quantity of chromic acid into account when
adding the bichromate
If we now consider what takes place in the cathode compartment, we shall find
that the liquid gradually becomes poorer in sulphate of chromium as the metal
1s deposited, whilst at the same time remaining practically constant as regards
the proportion of alkalme sulphate which it contains
It will readily be understood that if such a bath is supplied with alum of
chromium and of soda the proportion of sulphate of soda will be increased to
such a point as to render further operations impossible.
This will be so much the more the case that solutions having a base of alum
of chromium, in order to obtain a suitable yield, should be titrated with cale
as regards the free acid and should contain a suitable proportion of alum with
respect to what we will call the auxiliary salts which are generally speaking,
alkaline sulphates. The deformation of the electrolyte, or an electrolyte which
18 not exactly in the proper condition from the start or which no longer gives a
deposit of chromium, produces the following effects:
1. No deposit 18 produced In this case the bath 18 too acid The current
should be allowed to pass; at the end of some time the bath decreases in acidity
in the cathode compartment, attains the appropriate degree and the deposit of
metal commences. The acidity may also be reduced immediately by means of a
base or of an alkaline carbonate or by means of hydrated oxide of chromium.
2. Oxide of chromium 18 produced upon the cathode. This is the symptom
of a bath which is too neutral. In order to obtain the deposit it is only necessary
to again gradually add sulphuric acid.
J

page_text: By feeding the electrolyser with alum to bring it back to its titrage in chromium
salt, it is absolutely necessary to introduce into it continuously increasing
quantities of alkaline sulphates, thereby modifying the constitution of the
electrolyte and changing the working conditions
its
it is therefore necessary to remove the excess of sulphate of soda as
formation takes place We attain this result by causing the liquid of the cathode
compartment to circulate in a continuous manner. This liquid on leaving the
electrolyte, where it is at a temperature of about 70°, passes into vessels where
it 18 cooled to a certain temperature, by means for example, of lead coils unmeised
in this liquid and traversed themselves by a curient of cold water. The lowering
of the temperature causes the crystallisation of a certain quantity of sulphate
of soda and the liquid of the compartment of the cathode, deprived of a certain
amount of its sulphate of soda, passes into the electrolyser after having been
suitably enriched in alum and chromium.
It will readily be understood that for a definite rate of current to the electro-
lysels, a suitable supply of alum and a suitable cooling surface, there will be a
certain speed of circulation of the cathode Liquid to which will correspond a con-
stituent quantity, always the same, of sulphate of soda.
In short, it is apparent that, with our process, not only do we utilise the sub
products to the best advantage but we also obtain a continuous action of the
electiolysers.
If instead of supplying the electrolyser with alum of chromium, sulphate of
chromium is employed, an increase in the constituent quantity of sulphate of soda
18 avoided and uniform action 18 obtained in a more simple manner.
The sulphuric acid produced in the anode compartment corresponds to the
metal deposited, if we saturate this acid by hydrated oxide of chromium, we
prepare a feed liquid which is so much the more perfect that we are intio-
ducing again into the cathode compartment the sulphate of soda which the
liquid of the anode contains, the presence of which is due to the phenomenon
of osmose.
In this case we heat the liquid of the anode with sulphurous acid in ordei
to transform the chromic acid into sulphate of chromium we raise to the point
of ebullition so as to expel the excess of sulphurous acid and to concentrate,
and we saturate with hydrated oxide of chromium.
Among the processes for the manufacture of this oxide, we prefer the
following.
We treat a solution of bichiomate of soda with an alkaline or terreous-alkaline
sulphide. These sulphides we dissolve in water and the filtered solution 18
brought into contact with the dissolved bichromate. The reaction takes place at
ebullition and the whole of the chromium 18 precipitated in the oxide state if
the liquids are suitably concentrated and if care has been taken to dissolve in the
sulphide a certain quantity of sulphu This process gives the maximum yield
of oxide of the first operation. It then only remains to pass through the filter
press in order to separate the oxide of the solution which
contains an excess of sulphide, from the sulphate and the various sulphites.
Crystallisation 18 then allowed to take place and the crystals are treated with
carbon in suitable ovens for the purpose of again forming sulphide to serve for a
fresh operation. The oxide of chromium retained in the filter press is washed
until the water used for the purpose gives no trace of sulphate. It is then.
collected with a view to the manufacture of sulphate of chromium It has
already been stated that the anode compartment of the electiolyser gradually
becomes enriched in sulphuric acid and in chromic acid. The quantity of
sulphuric acid corresponds to the sulphate of chromium decomposed and if we
saturate it with hydrated oxide, we shall have reconstituted a quantity of sulphate
corresponding to that decomposed For this purpose the liquid of the anode
compartment 18 fist of all treated by a curient of sulphurous acid with the
object of transforming the chromic acid which it contains into sulphate of

page_text: chromium, it 18 then brought to a suitable degree of concentration and sub-
sequently saturated with hydrated oxide of chromium whilst in a heated state
A liquid suitable for supplying the cathode compartiment of the electrolyser lb
thus obtamed. When the oxide of chromium 18 obtained by the employment of
ten cous-alkaline sulphides, of sulphide of bannum for example, the sulphate of
buyta formed remains upon the hiter with the oxide of chromium. After wash-
ang with water, the whole is again treated with the sulphuric acid of the anode
as above mentioned, but a fresh filtration is effected in order to separate the
sulphate of cluomium from the sulphate of baita This latter mixed with
carbon 18 1e-tiansformed into sulphide of barium
These operations as a whole enable the sub-product resulting from the electio-
lysis to be reintroduced into the cycle with the minimum of operations. They
also present the advantage of a leady control over the working of the electrolyseis,
the supply taking place with a substance which is always identical with itselt
(neutral sulphate of chromium) that 18 to say without free acid, which may be
subsequently brought to the exact degree of acidity required for continuous
working giving a good yield.
We reserve the night in this process of manufacture to substitute for alkaline
or teueous-alkaline sulphides any other bodies which will enable us to obtam
hydrated oxide of chromium whilst at the same time giving as bye products
compounds resulting from the oxidation, of the body substituted for the sulphide,
by the chromic acid of the bichromate.
We have hither to considered only the case of electrolysets having poious
partitions. The process of obtaming electrolytic chromium in an electiolyser
without a porous partition presents certain dimculties The electrolyte becomes
more and more acid as the deposit takes place, 100 grammes of chromium
deposited libciating about 300 grammes of H³SOʻ. On the other hand, the
sulphate of chromium, as we have stated, becomes divided upon the anode into
sulphuric and chiomic acid
We combat the increase of acidity in H2SO4 by means of suitable additions of
hydrated oxide of chromium. As regards the distribution of the chiomic acid
the presence of which in the bath is prejudicial to the progress of the operation,
we effect this by means of appropiate introduction of alkaline or terreous-alkaline
sulphides.
In the case of alkaline sulphides, the progress of the operation is sufficient, but
gradually the bath becomes deformed by the increase in the constituent quantity
of alkaline sulphate. In the case of teireous-alkaline sulphides such as sulphide
of banum, the bath remains practically constant and sulphate of baryta 18 pre-
cipitated which it is necessary to remove hom time to time.
•
The method by means of which no foreign element need be introduced into
the bath, 18 to deoxidise the chromic acid by means of sulphurous acid o1 of
sulphide of chromium, which may be so much the better effected in that there
18 always present in the electrolyte a certain quantity of free sulphuric acid which
rests upon the insoluble sulphide of chromium in forming sulphate of chromium
and hydrosulphuric acid which reacts in its turn upon the chromic acid and
finally gives sulphate of chromium.
It will be seen that by means of this last process the electrolyte is not modified
as regards auxiliary salts and the continuous operation may be voiy readily and
certainly assured.
We have also remarked in the course of our experiments that, given a definite
bath of chromium and a constant intensity of curent, by causing the surface of
the cathode to vary, the nature of the deposit is caused to vary likewise.
For example, in the same bath and with the same current; For a 1od of 8 m/m
diameter, a deposit of oxide is obtained, for a rod of 15 m/m diameter a good
deposit is obtained, thickening with a suitable yield; for a 1od of 18 in/m
diameter a deposit of chiomium which does not thicken is obtained and finally
for a rod of 20 m/m. there is no deposit. It will of course be understood that

page_text: the above figuies are given only by way of example They vary infinitely
according to the composition of the bath and the amount of free acid which it
contains.
It will thus be understood that with a given bath receiving a current in
amperes determined upon for giving, for example, a good deposit upon a 1od
of 15 m/m diameter say, a diameter of 10d may be found (an indicating 10d)
which placed in the same bath, arranged in parallel with the first and receiving
definite current will only rise to a chomage, that is to say to a deposit which
will not thicken.
If the acidity of the bath increases the indicating rod will not give rise to any
deposit, the bath will be brought to a suitable degree of acidity by one of the
methods indicated above until a slight deposit again appears.
For
It will be understood that the diameter of the indicating iod may be determined
and also the current that it receives in such a manner as to cause the acidity to
тагу within sufficiently close limits for obtaining a good average yield The indi-
cating rod may be made to automatically furnish the necessary indication
this purpose it is only necessary to suspend it to one of the aims of a balance,
the other arm being provided with a contact closing the cucuit of an electric
bell when the indicating iod has been freed from the deposit with which it 18
In order to
covered, that is to say when the quantity of acid present increases
cause the bell to cease ringing, all that is required is to suitably neutralise or
feed the bath.
The same indicating-1od device may be employed for the purpose of preventing
a diminution of acidity
In this case the indicating 10d 18 regulated as regards dimensions and as regards
curent, in such a manner that for normal working there takes place a simple
chromage, that 18 to say a deposit which does not thicken
If the bath becomes more neutral, the deposit will increase, the 1od will
increase in weight and this variation in weight may be used to cause a signal to
It will only be necessary to suitably acidify the bath in order to cause
the bell to cease inging
act
The process thus enables the state of the electrolyte to be controlled in the
most precise manner, and thus ensures a continuous uniform working.
Dated this 1st day of September 1898.
HASELTINE, LAKE & Co,
45, Southampton Buildings, London, W.C.,
Agents for the Applicants.
COMPLETE SPECIFICATION
Improvements relating to the Extraction of Chromium by
Electrolysis.
We, JULIUS FRANZ LUDWIG MOELLER, of 14, Palmer Street, London, Dnector,
and ERNEST AUGUSTE GEORGE STREET, of 60, Boulevaid Haussmann, Pans, in
the Republic of Fiance, Engineer, do hereby declare the nature of this invention
and in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement.-
-
In establishing a process for the extraction of chromium by electrolysis, it is
necessary to consider, from an industrial point of view and apart from the motive
force, two elements which might have considerable influence upon the cost price

page_text: of the metal; these two elements are: 1st The selection of the anodes employed,
and 2nd. The nature and the quantity of the bye products coming from the
electrolyser.
As regards the anodes, it is well known that carbon anodes cannot be employed
In practice as they are rapidly deteriorated by the chromic acid formed in the
baths, thereby necessitating continual renewals which are very costly, and which,
as they wear, foul the electrolyte owing to the foreign matter they contain in
greater or less quantities. Platinum anodes only are therefore employed
As will be readily understood, the employment of platinum for a daily pro-
duction of any magnitude would require the locking-up of a very large amount
of capital.
On the other hand, as regards the bye-products issuing from the electrolyser,
As the proportion of metal extracted in comparison with the mass of the electrolyte
employed is relatively small, it is obvious that their nature and quantity, accord-
ing to whether they are marketable or the reverse, or to whether they are capable
of being readily regenerated in order to again enter the cycle of operations, is
an equally important consideration with regard to the cost of extracting the
metal.
We have therefore endeavoured to discover an industrial process for the
extraction of chromium by electrolysis which, on the one hand would enable the
use of platinum anodes to be discontinued, and on the other hand would yield
the minimum of bye-products, but which latter should be of a marketable
character.
The process which we have discovered and which enables us to attain this two-
fold object is based essentially upon the employment, as chromium salts to
electrolyse, of salts having a sulphate base, the electrolyser being made either
with or without porous partitions.
The salts of sulphate of chromium are the only salts which permit of the
employment of lead anodes, on condition that liquids not too heavily charged
with sulphuric acid are used. It is preferable not to exceed the proportions
admissible for lead plate accumulators. Under these conditions the anodes
become peroxidised and have a very long life In cases in which a porous
partition is emploved. care should be taken not to allow the concentration
of HSO to increase bevond a certain value, which is readily obtained by pro-
viding a water supply which may be regulated once for all for the working
conditions, and an overflow to enable the anolyte to run off
If a porous partition is not employed, the degree of acidity should be main-
tained, for the purpose of obtaining a deposit of metal, at a relatively low
point; it is therefore unnecessary in this case to take special precautions for
the purpose of preserving lead anodes.
In the first place, therefore, we succeed, by the employment of salta having
a base of sulphate of chromium, in obviating the necessity for using platinum
anodes which we replace by lead anodes in the conditions indicated.
We will now explain in what manner we succeed in obtaining the minimum
of bye products coming from the electrolyser
In order that this explanation may be readily understood, we will take as an
example the manufacture of electrolytic chromium by means of alum of chromium
either from potash or of soda We, however, prefer this latter because it is more
soluble than the alum of potash, and will now take it by way of example, but
without in any way limiting ourselves thereto
We will assume that an electrolyser having porous partitions is being employed.
The liquid of the cathode compartment is composed for example, as follows:
Water
Alum of chromium and soda
Sulphate of soda
1.500 kilo.
1 500 kilo
1.500 kilo.
The alum of chromium and of soda being assumed to be neutral, that is to

page_text: nay without free sulphuric acid, the water intended for dissolving the salts
should be acidulated with about 5% by weight of sulphuric acid at 660 Baume.
The liquid of the anode compartment is constituted by water acidulated to 5%
by weight at 66° Baume.
The cathode is formed by a sheet of copper or of brass rolled into tubular
form, and the anodes are of lead.
An electrolyser constructed under these conditions and raised to a temperature
of about 70° C. will yield a production of chromium per ampere-hour of about
Ogr. 20 if the apparatus is traversed by a continuous electric current the density
of which with respect to the cathode is about 40A. per square decimetre
If what takes place is noted it will be seen that at the same time that the
metal becomes deposited, the liquid of the anode compartment becomes enriched
in sulphuric acid In addition to this, the liquid in this compartment gradually
becomes coloured and takes the characteristic tint of chromic acid solutions.
This arises from the fact that the sulphate of chromium traverses the porous
partition and becomes divided in the anode compartment into sulphuric acid and
chromic acid under the influence of the oxidising agents which it encounters.
An analysis made of the anode liquid of a certain electrolyser has shown that at
the end of 24 hours, for a production of metallic chromium of 23.500 kilos
there was a production of 0 562 kilos. of chiomic acid Cros and exosmose of
0 963 kilos of sulphate of soda (NaOSO³). These figures naturally vary with the
nature of the porous partition.
As regards sulphuric acid, the consideration of the chemical formula of sulphate
of chromium CPO 380 + NAq leads to the conclusion that for 104 grammes
of chromium extracted, 294 grammes of sulphuric acid must be liberated Con-
sequently a mannfactory where one ton of chromium per 24 hours was manu-
factured, would produce as a bye product about 3 tons of sulphuric acid HPSO*
(at 66° Baume), with the additional disadvantage that this quantity of acid
instead of being at 660 is diluted with a considerable quantity of water and is
mixed, in addition, with the other products which have been mentioned
In order to obtain an idea of the mass of liquid in question, and taking as
concentration of the liquid of the anode a mean value of 10% of sulphuric acid,
30 tons of liquid would have to be dealt with. It will be understood that the
evaporation of 27 tons of water and the separation, if it were possible, of the
bodies other than sulphuric acid, would give rise to operations which, if not
impossible, would be of very little commercial value.
It is our opinion that such operations could not be carried out, and we there-
fore endeavoured to discover a process which would cause not only the sulphuric
acid to re-enter the cycle of electrolytic operations, out also the products with
which it is mixed owing to osmotic phenomena, and that exactly in the saine
proportions as those in which the sulphuric acid and such bodies leave the
electrolyser, the supply of active salts of chromium being proportional to the
weight of the metal deposited
We operate in the following manner.
It has been stated that the liquid of the anode contains sulphuric acid, chromio
acid and sulphate of soda This latter is no disadvantage because its presence
in the supply liquid of the cathode compartment will only compensate the losses
due to the osmotic phenomena.
As regards the chromic acid, we convert it into sulphate by washing it with
gaseous sulphurous acid This transformation having been effected we boil in
order to expel the excess of sulphurous acid and concentrate the liquid at a
suitable point, say 30° Baume.
To this liquid we add a quantity of bichiomate of soda such that, independently
of the sulphate of soda formed, it contains an equivalent of chromic acid Cr³0,
that is to say 2 (Cro³) for two equivalents of sulphuric acid H°SO*
This liquid 18 allowed to fall in more or less thin streams into a lead vessel
which invariably contains an excess of melted sulphur which is maintained

page_text: at 130° C. The chromic acid oxidises the sulphur and sulphate of chromium
(Or³ ³ 3S0) 18 formed owing to the two equivalents of free sulphuric acid
previously mentioned This sulphate with the sulphate of soda previously
formed at the expense of the bichromate of soda gives us alum. A small quantity
of water is again added, filtration 18 effected in order to separate the excess of
sulphur which re-enters the cycle, and there results a solution of alum of
chromium with which the electrolyse 18 fed proportionately to the metal
deposited.
The
previous action of the sulphurous acid may be avoided by titrating_the
liquid with chromic acid, taking this quantity of chromic acid into account when
adding the bichromate
If we now consider what takes place in the cathode compartment, we shall find
that the liquid gradually becomes poorer in sulphate of chromium as the metal
18 deposited, whilst at the same time remaining practically constant as regards
the proportion of alkaline sulphate which it contains
It will readily be understood that if such a bath is supplied with alum of
chromium and of soda the proportion of sulphate of soda will be increased to
such a point as to render further operations impossible
This will be so much the more the case that solutions having a base of alum
of chromium, in order to obtain a suitable yield, should be titrated with cate
as regards the free acid and should contain-a suitable proportion of alum with
respect to what we will call the auxiliary salts which are generally speaking,
alkaline sulphates. The deformation of the electrolyte, or an electrolyte which
18 not exactly in the proper condition from the start or which no longer gives a
deposit of chromium, produces the following effects
1. No deposit 18 produced In this case the bath is too acid The current
should be allowed to pass; at the end of some time the bath decreases in acidity
in the cathode compartment, attains the appropriate degree and the deposit of
metal commences. The acidity may also be reduced unmediately by means of a
base or of an alkaline carbonate or by means of hydrated oxide of chromium
2 Oxide of chromium is produced upon the cathode This 18 the symptom
of a bath which is too neutral. In order to obtain the deposit it is only necessary
to again add gradually sulphuric acid
By feeding the electrolyser with alum to bring it back to its titrage in chiomium
salt, it is absolutely necessary to introduce into it continuously increasing
quantities of alkaline sulphates, thereby modifying the constitution of the
electrolyte and changing the working conditions
It is therefore necessary to remove the excess of sulphate of soda as its
formation takes place. We attain this result by causing the hquid of the cathode
compartment to circulate in a continuous manner. This liquid on leaving the
electrolyser, where it is at a temperature of about 70°, passes into vessels where
it 18 cooled to a certain temperature, by means for example, of lead coils immersed
in this liquid and traversed themselves, by a current of cold water The lowering
of the temperature causes the crystallisation of a certain quantity of sulphate
of soda and the liquid of the compartment of the cathode, deprived of a certain
amount of its sulphate of soda, passes into the electiolyser after having been
suitably enriched in alum of chromium
It will readily be understood that for a definite ate of curient to the electio-
Ivsers, a suitable supply of alum and a suitable cooling surface, there will be a
certain speed of circulation of the cathode liquid to which will correspond a con-
stituent quantity, always the same, of sulphate of soda.
In short, it is apparent that, with our process, not only do we utilise the sub
products to the best advantage but we also obtain a continuous action of the
electrolysers
If instead of supplying the electrolyser with alum of chromium, sulphate of
chromium 16 employed, an increase in the constituent quantity of sulphate of soda
18 avoided and uniform action is obtained in a more simple manner

page_text: The sulphuric acid produced in the anode compartment corresponds to the
metal deposited; if we saturate this acid by hydrated oxide of chromium, we
prepare a feed liquid which is so much the more perfect that we are intro-
ducing again into the cathode compartment the sulphate of soda which the
liquid of the anode contains, the presence of which is due to the phenomenon
of osmose
In this case we treat the liquid of the anode with sulphurous acid in vider
to transform the chromic acid into sulphate of chromium, we raise to the point
of ebullition so as to expel the excess of sulphurous acid and to concentrate.
and we saturate with hydrated oxide of chromium.
Among the processes for the manufacture of this oxide, we prefer the
following:
con-
It has
We treat a solution of bichromate of soda with an alkaline or earthy-alkaline
sulphide These sulphides we dissolve previously in water and the filtered' solution 18
brought into contact with the dissolved bichiomate. The reaction takes place at
ebullition and the whole of the chuomium is precipitated in the oxide state if
the liquids are suitably concentrated and if care has been taken to dissolve in the
sulphide a certain quantity of sulphu This process gives the maximum yield
of oxide in the first operation. It then only remains to pass through the
filter press in order to separate the oxide of the solution which
tains an
excess of sulphide, from the sulphate and the various sulphates.
Crystallisation 18 then allowed to take place and the crystals are treated with
carbon in suitable ovens for the purpose of again forming sulphide to serve foi a
fresh operation The oxide of chromium retained in the filter piess is washed
until the water used for the purpose gives no trace of sulphate It is then
collected with a view to the manufacture of sulphate of chromium.
already been stated that the anode compartment of the electrolyser gradually
becomes enriched in sulphuric acid and in chiomic acid. The quantity of
sulphuric acid corresponds to the sulphate of chromium decomposed and if we
saturate it with hydrated oxide we shall have reconstituted a quantity of sulphate
corresponding to that decomposed For this purpose the liquid of the anode
compartment is first of all treated by a current of sulphurous acid with the
object of transforming the chromic acid which it contains into sulphate of
chromium, it is then brought to a suitable degree of concentration and sub-
sequently saturated with hydrated oxide of chromium whilst in a heated state.
A liquid suitable for supplying the cathode compartment of the electrolyser is
thus obtained When the oxide of chromium is obtained by the employment of
earthy-alkaline sulphides, of sulphide of barium for example, the sulphate of
After wash-
baryta formed remains upon the filter with the oxide of chromium
ing with water, the whole 18 again treated with the sulphuric acid of the anode
as above mentioned, but a fresh filtration 18 effected in order to separate the
sulphate of chromium from the sulphate of baryta This latte mixed with
carbon 18 re-transformed into sulphide of barium
These operations as a whole enable the sub-products resulting from the electro-
lysis to be reintroduced into the cycle with the minimum of operations They
also present the advantage of a ready control over the working of the electrolysers,
the supply taking place with a substance which is always identical with itself
(neutral sulphate of chromium) that is to say without fiee acid, which may be
subsequently brought to the exact degree of acidity required for continuous
working giving a good yield
We reserve the right in this process of manufacture to substitute for alkaline
01 earthy-alkaline sulphides any other body which will enable us to obtain
hydrated oxide of chromium whilst at the same time giving as bye products
compounds resulting from the oxidation, of the body substituted for the sulphide,
by the chromic acid of the bichromate
We have hitherto considered only the case of electrolysers having porous
partitions The process of obtaining electrolytic chromium in an electiolyser

page_text: without a porous partition presents certain difficulties. The electrolyte becomes
more and more acid as the deposit takes place, 100 grammes of chromium
deposited liberating about 300 grammes of H° SO On the other hand, the
sulphate of chromium, as we have stated becomes divided upon the anode into
sulphuric acid and chromic acid.
We combat the increase of acidity in H2 SO* by means of suitable additions of
hydrated oxide of chromium. As regards the destruction of the chromic acid
the presence of which in the bath is prejudicial to the progress of the operation,
we effect this by means of appropriate introductions of alkaline sulphides, or
earthy-alkaline sulphides.
In the case of alkaline sulphides, the progress of the operation is sufficient, but
gradually the bath becomes deformed by the increase in the constituent quantity
of alkaline sulphate In the case of earthy-alkaline sulphides such as sulphide
of barium, the bath remains practically constant and sulphate of baryta is pre-
cipitated which it is necessary to remove from time to time
The method by means of which no foreign element need be introduced into
the bath is to deoxidise the chromic acid by means of sulphurons acid or of
sulphide of chromium, which may be so much the better effected in that there
1s always present in the electrolyte a certain quantity of fiee sulphuric acid which
reacts upon the insoluble sulphide of chromium in forming sulphate of chromium
and hydrosulphuric acid which reacts in its turn upon the chromic acid and
finally gives sulphate of chromium
It will be seen that by means of this last process the electrolyte is not modified
as regards auxiliary salts and the continuous operation may be very readily and
certainly assured
We have also remarked in the course of our experiments that, given a definite
bath of chromium and a constant intensity of current, by causing the surface of
the cathode to vary, the nature of the deposit is caused to valy likewise.
For example, in the same bath and with the same current: For a rod of 8 m/m
diameter, a deposit of oxide is obtained, for a 1od of 15 m/m. diameter a good
deposit 18 obtained, thickening with a suitable yield, for a 1od of 18 m/m.
diameter a deposit of chromium which does not thicken is obtained and finally
for a rod of 20 m/m there is no deposit. It will of course be understood that
the above figures are given only by way of example They vary infinitely
according to the composition of the bath and the amount of hee acid which it
contains.
It will thus be understood that with a given bath receiving a current in
amperes determined upon for giving, for example, a good deposit upon a 10d
of 15 m/m. diameter say, a diameter of 1od may be found (an indicating rod)
which placed in the same bath, arranged in parallel with the first and receiving
definite current will only give rise to a chromage, that is to say to a deposit which
will not thicken.
If the acidity of the bath increases the indicating rod will not give rise to any
deposit, the bath will be bi ought to a suitable degree of acidity by one of the
methods indicated above until a slight deposit again appears
It will be understood that the diameter of the indicating rod may be determined
and also the current that it receives in such a manner as to cause the acidity to
vary within sufficiently close limits for obtaining a good yield. The indi-
cating rod may be made to automatically furnish the necessary indication. For
this pui pose it 18 only necessary to suspend it to one of the arms of a balance,
the other arm being provided with a contact closing the circuit of an electric
bell when the indicating rod has been freed from the deposit with which it 18
covered, that is to say when the quantity of acid present increases
In order to
cause the bell to cease ringing, all that is required is to suitably neutralise or
feed the bath.
The same indicating-rod device may be employed for the purpose of preventing
a diminution of acidity

page_text: In this case the indicating rod is regulated as regards dimensions and as regards
current, in such a manner that for normal working there takes place a simple
chromage, that is to say a deposit which does not thicken.
If the bath becomes more neutral, the deposit will increase; the iod will
increase in weight and this variation in weight may be used to cause a signal to
Act It will only be necessary to suitably acidify the bath in order to cause
the bell to cease ringing.
The process thus enables the state of the electrolyte to be controlled in the
most piecise manner, and thus ensures a continuous uniform working.
Having now particularly described and ascertained the nature of our said
invention and in what manner the same is to be performed, we declare that what
we claim 18.-
1. A process for the extraction of chromium by the electrolysis of salts having
a sulphate of chromium base, the object being to utilise all the sub-products
present in the anode compartment, that 19 to say; sulphuric acid, chromic acid
and sulphate of soda, by causing them to enter the cathode compartment integrally
that is to say in the form of alum of chromium and of soda and of sulphate of
soda, the said process consisting. (a) In cases in which the bath 18 fed with
alum of chromium, in causing to fall in more or less small streams into a vessel
containing melted sulphur, the liquid leaving the anode compartment of the
electrolyser which has been previously treated with a current of sulphurous acid
in order to transform the chiomic acid into sulphate, and mixed with biohromate
of soda in such quantity that it contains one equivalent of free chromic
acid, CO, that is to say 2 (Cr 0³) for two equivalents of fiee sulphuric
acid H SO, so as to form sulphate of chromium which, with the sulphate of
Boda previously formed at the expense of the bichiomate, constitutes the alum
intended to supply the electrolyser, this complex liquid being intended to feed
the cathode compartment which continuously becomes enriched with an excess of
sulphate of soda, is submitted at the same time, during working, to a circulation
and to a reduction of temperature which frees it by crystallisation from this
excess of sulphate of soda, and thus enables the composition of the liquid in
the cathode compartment to be kept constant (b) In cases in which the bath 18
supplied with sulphate of chromium, in treating the liquid leaving the anode
compartment of the electrolyser with a current of sulphurous acid, then in
saturating with hydrated oxide of chromium, this complex liquid being intended
for feeding the cathode compartment.
2. In combination with the process above described the manufacture of oxide
of chionum by the treatment of a solution of bichromate of soda by means of an
alkaline or earthy-alkaline sulphide, as above described, the regeneration of the
alkaline or carthy-alkaline sulphide being obtained by the action of carbon, in
suitable ovens, upon the alkaline or earthy-alkaline sulphate formed at the same
time as the oxide of chromium
3 In the process above described, the employment of an indicating rod mounted
in parallel upon the encuit traveising the 1od or 1ods upon which the metallic
deposit takes place and in the same bath as these latter, in conditions as regards
intensity of curent per unit of surface of the indicating rod such that it under-
goes merely a chiomage with the bath at its normal degree of acidity; the said
indicating 1od being capable of being checked by inspection on withdrawing it
from the bath, or being capable of automatically giving the desired indications
on being connected with one of the arms of a balance the other arm of which
is provided with a contact closing the circuit of a bell, either when the indicating
rod arm of the balance rises owing to the disappearance of the chiomage of the
indicating rod arising from the increase of the acidity of the hath or when the
indicating-1od aim falls owing to the mcrease of the deposit upon the indicating
rod arising from the diminution of the acidity of the bath, as above specified
4 In combination with the process described above, the employment of lead

page_text: anodes which, whether porous partitions are used or not, are preserved from the
attack of the sulphuric acid owing to the provision of a water supply and of an
overflow which are regulated once for all in such a manner as to prevent the con-
centration of the acid from exceeding the limits permitted for secondary batteries
having lead plates
20+1 J
of Mar 1800
HASELTINE, LAKE & Co,
45, Southampton Buildings, London, W.C,
Agents for the Applicants