Improved Process for Preparing Insoluble or Hardly Soluble Salts from Metallic Anodes by Electrolysis of Aqueous Solutions con- taining Two Salts.
Year: 1897

page_text: Date of Application, 17th Nov., 1897-Accepted, 5th Mar., 1898
Improved Process for Preparing Insoluble or Hardly Soluble Salts
from Metallic Anodes by Electrolysis of Aqueous Solutions con-
taining Two Salts.
We, Dr. OTTO CARL STRECKER, No. 26/28, Christoph Strasse, and Dr. HANS
HERMANN STRECKER, No. 9, Limburger Strasse, both of Cologne, in the German
Empire, Chemists, 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:-
The present invention relates to a process for the preparation of insoluble or
slightly soluble salts from metallic anodes by means of the electrolysis of aqueous
solutions containing two salts.
Our invention, in contradistinction to those hitherto known is devised to regulate
the relations of the salts to one another in such a manner that the consumption of
energy shall be as small as possible.
For this purpose, the metals which form the anodes are dissolved by electrolysis
with the aid of a first salt and the dissolved metals precipitated in the same vessel
permanently and immediately as insoluble or slightly soluble salts in fine division.
The latter operation is performed by means of a second salt whose component
forming part of the insoluble salt is continuously fresh supplied to the electrolyte.
The salts are so mixed that the electrolysis requires only a relatively slight.
consumption of energy.
For the carrying out of our new processes electrolytes are made use of in which
two salts are always dissolved in such proportions the one to the other, that the
amount of that salt through whose anion the metal of the anode is brought into
solution, should as accurately as possible correspond to the maximum of the con-
ductivity of this solution, whilst the amount of the second salt whose anion causes
the precipitation of the dissolved metal, is made use of in the minimum of its
electrochemical equivalent in relation to the current strength. Thus the conduc-
tivity of the electrolytes is considerably increased and thereby energy economised.
The quantity to be used of that salt whose anion is to effect the solution of the
metallic anode must consequently be chosen according to the maximum of its
conductivity, whilst the quantity of the other salt to be employed whose anion
effects the precipitation of the dissolved metal as an insoluble salt is obtained by
calculation according to the electrochemical equivalents.
Thus for instance, suppose the conductivity of chlorate of sodium be established
at 7 to 100 then one can calculate for instance in order to produce white lead the
quantity of carbonate of soda according to the following method;
In the quantity of liquid which exists between the electrodes, there must so many
precipitation anions be present in order to precipitate the metal converted into the
ion condition at any given moment. The quantity of the precipitating salt must
consequently be so calculated that one may consider the same as dissolved in the
liquid between the electrodes and utilise it so that it suffices in order to precipitate.

page_text: the metal dissolved in a minute. Then the electrolyte possesses a maximum con-
ductivity and the anodes remain bright.
Assuming that the bath contains 60 litres of electrolyte of seven parts by weight
of chlorate of sodium to 100 parts by weight of water 10 anodes and 11 cathodes
each with a surface of 2 × 20 x 30 cm. at a distance of 1.5 cm. from one another.
Then the quantity of liquid between each pair of electrodes will be 20 × 1.5 × 30
equals 900 ccm. Consequently for 10 pairs of electrodes 20 × 900 equals 18,000
ccm. These 18 litres must consequently contain that quantity of precipitating salt.
which will precipitate the metal passing into solution per minute. The anodes
have a surface of 120 square cm., which results from the proportions already given;
when calculated for the above conditions and applied to the production of white
lead there will then be rendered soluble with a current of 0.5 ampere per square cm.
of anode surface, consequently 60 amperes 3.852 grammes of lead per minute.
The anode has 2 cm. x 3 cm. =6 square cm. On both sides 6 square cm., are
equal altogether to 12 square cm. Consequently 10 anodes have 10 times 12 square
cm. equals 120 square cm. and require 120 × 0.5 ampere equals 60 amperes.
3.852 grammes of lead require for conversion into basic carbonate of the formula
2 (Pb CO3) Pb (OH) 2.
1315 grammes of carbonate of soda and 0'496 grammes of sodic hydrate or if
calculated entirely as carbonate of soda equal to 1972 grammes of carbonate of
soda. Suppose these 1'972 grammes of carbonate of soda be dissolved in the above-
mentioned 18 litres of liquid, then a solution of carbonate of soda will be obtained
of G011 %. The total solution of the bath should consequently be equivalent to
a content of 0011 per cent of carbonate of soda. The salt to which the duty of
the dissolving of the metal comes (the dissolving salt) must therefore be a salt
whose anion easily dissolves the metal of the anode whereas the second salt (the
precipitating salt) which serves for the precipitation of the metal must always
contain an anion which precipitates the respective metal as an insoluble salt. For
the purpose of solution alkaline chlorates are specially suited, for the purpose of
precipitation preferably salts of carbonic acid, chromic acid, phosphoric acid and
other acids with the alkalies according to the precipitate required. The reactions
take place on these principles with all salts which are not more or less decomposed
by electrolysis; excepting however for the preparation of special insoluble salts,
the salts of other bases and acids than those indicated offer no particular advantages.
The acids withdrawn from the electrolytes during the process must be continuously
freshly supplied, because the composition and density of the electrolytes must be
kept constant in order. not to alter the conductivity, and in order to maintain the
electrochemical equivalent proportion of the two salts, otherwise there is a loss
of energy.
The salts are precipitated in this process pure and finely divided without
adhering to the anode and shall be used as colours and the like according to their
respective properties. For instance: preparation of white lead. A vessel con-
tains 60 litres of 7 per. cent. solution of chlorate of sodium and 0011 per cent. of
carbonate of sodium. 10 anodes and 11 cathodes having a surface of about 20-30 cm.
are suspended in the bath at a distance of 15 cm. from one another. The addition
of the precipitating salts is calculated here as indicated above. During the
electrolysis carbonic acid is carefully blown into the fluid from fine tubular points
of insulating material, whereby on the one hand the circulation of the fluid and on
the other hand the renewal of the precipitating salt is continuously effected. There
is then with a current strength of 05 ampere per square cm. anode, a tension of
about 125 to 13 volts at a temperature of from 15 to 17 degrees.
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 is: -
Process for the preparation of salts, which are insoluble or hardly soluble in.
water or salt solutions, by means of electrolysis from metallic anodes, characterised

page_text: by the mixing of the solutions of two salts used as the electrolyte in such a manner,
that the electrolyte contains the one salt (the dissolving salt) in the proportion of
the greatest conductivity of the said salt and the other (the precipitating salt) in
the proportion of the electrochemical equivalent of the current density employed.
Dated this 17th day of November 1897.
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C., Patent Agents for the Applicant.