An Improved Process for Obtaining Chemically Pure Gold from Fine Gold and Alloys Rich in Gold.
Year: 1896

page_text: Date of Application, 13th Apr., 1896
Complete Specification Left, 8th Jan., 1897-Accepted, 27th Feb., 1897
An Improved Process for Obtaining Chemically Pure Gold from
Fine Gold and Alloys Rich in Gold.
A communication by Dr. EMIL WOHLWILL, of 31 Elbstrasse, Hamburg, in the
Empire of Germany, Chemist.
I, WILLIAM PHILLIPS THOMPSON, F.C.S., M.I.M.E., of The Agency for
Foreign Patent Solicitors, 6, Lord Street, Liverpool, and 6 Bank Street,
Manchester, both in the County of Lancaster, 118 New Street, Birmingham
in the County of Warwick, and 322 High Holborn in the County of Middlesex,
Civil Engineer, do hereby declare the nature of this invention to be as follows:-
This invention has for its object a process for obtaining in the form of chemically
pure gold, the total gold contained in fine gold produced by affinage, or by other
methods, said fine gold still containing metallic admixtures of various kinds and
quantities, and also for similarly obtaining the total gold contained in auriferous
alloys. Whilst the above is effected in the chemical method by the impure metal
or alloy being first dissolved in a disintegrated condition in aqua regia and the gold
precipitated by means of precipitating substances from the dilute solution which is
cleared by settling-on the other hand in the electrolytic process, similarly to the
action in the electrolytic refining of copper and silver, the dissolving of the raw
material and the precipitation of the pure gold are effected in a single operation.
To this end it is necessary, on the decomposition by the electric current of the
solution of gold or electrolyte, that gold or mixtures of the same be introduced into
the solution to the same extent as pure gold is separated out at the cathode, under
the influence of the ions separated out at the anode, and accordingly with a
continual separation of pure gold, the amount of gold contained in the solution
remains approximately the same.
No chlorine is therefore given off in a gaseous condition during the electrolytic
action, if, (as is preferable) a solution of chloride of gold be employed as the
electrolyte, because gold or its admixtures corresponding to the quantities of
chlorine released or given off, would remain undissolved. When, however,
entirely pure gold as well as more or less impure gold is employed as anode in a
chloride of gold solution, there is found an essential variation from the behaviour
of copper and silver under similar processes for the electrolytic purification of these
metals. Whilst in this case for the continuous transfer of the anode constituents
in the solution, it is immaterial whether the solution be neutral, weak or strongly
acid, concentrated or moderately diluted, and also whether it be cold or warm-an
electro-chemical dissolution of gold as chloride does not take place at ordinary
temperatures, nor at higher temperatures in neutral or slightly acidulated solutions
of gold chloride, because the chlorine evolved at the anode under the above
mentioned circumstances escapes without acting on the metal of the anode.
Further researches have shown that this evolution of free chlorine may be entirely
suppressed even in solutions comparatively poor in gold; that is to say, that a
continuous dissolution of the anode-gold may be effected, if, whilst simultaneously
employing a high temperature, there be added to the electrolyte a considerable
excess of free hydrochloric acid proportioned according to the current density.
The purification of gold by the electrolytic process, employing as the electrolyte

page_text: a not solution of chloride of gold treated with an excess of hydrochloric acid as
the electrolyte, is therefore the main characteristic of the present invention.
To particularise: The temperature of the solution may preferably be maintained
at 60-70° C.; the gold contained may be in the proportion of 25-30 gr. of gold
per litre, the addition of free hydrochloric acid for one litre of the solution may
amount according to the current density employed to from 20 to 50 cubic
centimetres of fuming acid of 1.19 specific gravity. As in the course of the
operation, owing to evaporation and other consumption, the amount of hydrochloric
acid contained may vary in many ways, it may be noted, apart from the testing
from time to time of the acid contents, that when a suitable temperature is
maintained, a deficiency of free hydrochloric acid becomes evident by the
development of gaseous chlorine, and on noticing the smell of chlorine, and until
the same disappears, hydrochloric acid must be added to the solution after it has
first been established without doubt that no considerable diminution of temperature
has taken place.
If in order to explain these ascertained facts, it be assumed that the secondary
process by which the evolution of free chlorine at the anode, and correspondingly
the constant transfer of the constituents of the anode into the solution, consists in
the formation of a compound AaCl, +nHCl, it follows therefrom that the liquid
in close proximity to the anode must contain so much more free hydrochloric
anhydride, the greater the current density and correspondingly the greater the
quantity of chlorine to be retained at the surface of the anode, that is to say,
the more the anode diminishes in the course of the operation with a constant
current or as with a constant surface the strength of current is increased.
If the temperature be the correct one, and the amount of hydrochloric acid
contained be sufficient, the bulk of the impurities in the gold and among these the
otal amount of platinum and palladium contained, pass into the solution under the
action of the electric current, with the gold, the main part of the iridium and other
platinum metals remaining undissolved in a metallic condition, and the silver
contained being separated out with these in a solid condition in the form of chloride
of silver. The same applies to a portion of the lead, as lead chloride, when the
solution is entirely or approximately saturated with the same, and to bismuth as
oxychloride when the amount of hydrochloric acid contained in the solution is not
sufficient. With the insoluble mud separated out in this manner at the anode and
easily falling therefrom in small quantities apart from very small fractions of the
unchanged anode, there is regularly mixed an excess quantity (about 10% of the
weight of the anode) of extremely finely subdivided gold. The production or
formation of this gold deposit must be ascribed to a portion of the anode gold being
first converted, not into chloride, but into protochloride (AuC1) which immediately
after its formation however is again converted into chloride of gold and finely
subdivided gold is deposited.
Whilst in this manner the constituents of the anode are transferred, partly to the
solution, partly to the mud accumulating at the bottom, exclusively pure gold is
deposited at the cathode, even with an increase in the impurities in the solution,
and with a current density up to above 500 ampères per square metre. The
deposited gold is thus always replaced in the solution to a large extent by gold
dissolved at the anode, and partly by equivalent quantities of other metals, and
equally the separation of gold at the cathode corresponds to the conversion of
silver lead and bismuth in the form of solid chlorides; there takes place therefore
under the action of the current a constant impoverishment of the solution in gold,
which must be replaced by the addition of chloride of gold. The increase in the
amount of foreign constituents contained in the solution which simultaneously takes
place is, if care be taken to provide for the presence of the necessary amount of
gold, without influence on the purity of the gold even in the case of a considerable
excess of such foreign constituents; the presence of the foreign constituents in the
solution is in fact of assistance in the formation of the gold deposit, in that the
latter from a pure solution, particularly in the case of high current density is very
.

page_text: voluminous, whilst on the other band, without anecting its purity, it is deposited
the more compactly as the solution is the more charged with impurities.
From the above there results, as an essential advantage of the proposed process,
the possibility of a continuous working with a proportionately extremely moderate
consumption of acid. In the purification of gold by the chemical process for each
fresh quantity of the material to be separated which is to be dissolved, there must
always be again employed a corresponding quantity of hydrochloric acid and nitric
acid, and in a similar way, corresponding to the quantity dissolved, fresh depositing
substances must be added, whereas in this new process the small quantity of
hydrochloric acid employed for forming the electrolyte is always again available as
a solvent in consequence of the deposition of pure gold at the cathode, and
therefore precisely this small quantity of hydrochloric acid may be employed in a
large series of operations, both for dissolving the gold to be separated out, as well
as for depositing the pure gold contained therein.
With the advantage of a continuous working in the same solution, there is
combined as a further advantage the possibility of accumulating from large
quantities of the anode material the comparatively small amount of platinum
contained in the same, in such a way that (after separating out the gold) the
platinum may be directly recovered in the form of ammonio-chloride of platinum.
If for instance a gold be employed as anode material containing an average of
Toon of platinum, on the daily treatment of 20 kilos. of such material about
2 kilos. of platinum would be transferred to the gold solution at the end of
100 days.
As however according to observations made, an enrichment of the platinum
contents of the solutions up to double their gold contents is without influence on
the purity of the gold deposited, the re-employment of the same solution may be
continued without objection until it appears advantageous to remove the amount of
platinum contained for the purpose of further treatment thereof.
The amount of palladium in the solutions cannot be allowed to accumulate to the
same extent as the amount of platinum without influence on the purity of the gold;
whilst quantities of less than 5 gr. per litre do not appreciably affect the con-
stitution of the deposit, beyond that an excess of even a minimum of impurity
makes itself apparent by traces of palladium; as however the quantities of
palladium existing in gold are only usually a fraction of the amount of platinum,
the renewal of the solution may as a rule take place according to the enrichment
of the latter as herein before mentioned.
As regards the carrying out of the work the following may be stated in order to
complete what has been herein before stated;
In view of the high value both of the anode material, the product of the
electrolysis, and the solution, a treatment in the shortest time and in the smallest
space is required. Accordingly, the current density must be kept as high as
possible, and the thickness of the anode as small as possible. If anodes be
employed of 4 millimetres in thickness, the said anodes will, with an initial current
density of 400 ampères per square metre (of the anode) within 24 hours be so far
eaten away that the remainder (about 1 of the original weight of the anode)
must be recast for further treatment. The approximately equal amount of gold,
which is deposited at the bottom in the anode mud, is either removed from the
vessels after each separate operation, or after longer periods, in order according to
its constitution, to be again prepared either directly, or after preliminary
purification, for further treatment in the form of anodes.
Thin rolled fine gold obtained by electrolysis is employed as support for the
cathodes. The cathodes may be made considerably narrower than the anodes, if
about equal length, as, by the action of the current, an increase of size in all
directions very rapidly takes place, and with a suitably large initial current
density the deposit is precipitated as an adherent, (that is in a mass).
The last mentioned circumstance. makes it possible even with a high current
density to approach the electrodes to within about 3 centimetres of one another

page_text: without short circuiting, and without fear of the separated gold being rendered
impure. In the gold solutions at first employed, which are not as yet at all
impure, or only slightly contain foreign constituents the distances apart of the
electrodes must be made somewhat greater in view of the voluminous nature of the
deposit, or the cathodes must he more frequently changed.
The arrangement of the electrodes in separate baths, as well as the grouping of
several baths together will be readily understood by practical persons from what
has been herein before stated, without any further explanation.
For the vessels or cells, earthenware, porcelain, or other materials found to be
capable of resisting hot diluted hydrochloric acid must be employed.
The use of hot baths renders it necessary to provide the same with any ordinary
devices for automatically maintaining the level of the liquid. The vessels
employed for thus automatically refilling, are filled with the water used for
washing the finished deposit, the residue, and the mud, and, if there be a deficiency
of such washing water, with an addition of distilled water. In this way the
diluted parts of the gold solution are again always supplied to the baths from
which they were removed, and concentrated without any other apparatus by the
evaporation of the electrolytic solution.
The process herein before described may as is already evident be employed for
obtaining pure gold from auriferous alloys of all kinds. It is only necessary for
this purpose to bear in mind that on the chlorination of the non-gold constituents
of the anode, an equivalent quantity of gold is separated from the solution at the
cathode, which must be replaced by the addition of gold chloride. In the case of
such material it will therefore be necessary to complete to a certain extent the
dissolution of the gold by the electrolytic process by a further treatment with the
chemical process, and in so much greater proportion as the quantity of the foreign
constituents is greater and their electro-chemical equivalent is lower in comparison
to that of gold.
If the material which is to be separated contain a proportionately large quantity
of silver, the surface of the anode is covered with a layer of chloride of silver
which no longer drops off, but is adherent and at the same time non-conducting.
The diminution of the effective metal surface thereby produced causes the
separation of gaseous chlorine, and consequently the anode is attacked only to an
insufficient extent and the gold contents of the solution are precipitated. In such
alloys the interruption of the dissolving process may be entirely prevented by
removing periodically by mechanical means from the surface of the anode plates
the interfering layer and this more frequently the greater the amount of silver
contained and the greater the current density.
The disturbances by the accumulation of bismuth oxychloride, if such takes
place, may also be removed in a similar way.
If the anode alloy contains in addition to silver, or other metals, lead in any
considerable quantity, if a solution mixed only with hydrochloric acid is employed,
such solution soon becomes charged with chloride of lead. From this. period both
electrodes as well as the walls of the vessel and the surface of the liquid become
coated with crystallized chloride of lead, and even if it is possible to obtain the
gold, (recovered in the first operation) free from lead by repeated boiling with
distilled water, and to remove from solution when cooled a large part of the
chloride of lead by scooping out, the utilization for further treatment of a solution
thus charged with lead is very disadvantageous. It is possible to carry on a
continuous uninterrupted operation when treating alloys containing lead by an
addition of sulphuric acid to the solution. A quantity of concentrated sulphuric
acid is employed about equivalent to the free hydrochloric acid present. The lead
then forms on the anode almost insoluble layers of lead-sulphate, which either
gradually drop off, or where the alloys are very rich in lead must be mechanically
removed like the chloride of silver. It is of course evident that the amount of
sulphuric acid removed from the solution in the form of lead sulphate must be
replaced from time to time.

page_text: As the presence of sulphuric acid does not otherwise affect the operation, it is
preferable to add, to the gold chloride solution from the first, a mixture of
hydrochloric acid and sulphuric acid, in any case where gold containing lead is
employed as anode material, even though it only occasionally occurs.
Dated this 11th day of April 1896
An Improved Process Ior
Dotaining
Fine Gold and Alloys Rich in Gold.
A communication by Dr. EMIL WOHLWILL, of 31, Elbstrasse, Hamburg, in the
Empire of Germany, Chemist.
I, WILLIAM PHILLIPS THOMPSON F.C.S., M.I.M.E. of The Agency for
Foreign Patent Solicitors, 6, Lord Street, Liverpool and 6, Bank Street,
Manchester both in the County of Lancaster, 118, New Street, Birmingham in
the County of Warwick and 322, High Holborn in the County of Middlesex,
Civil 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 :-
This invention has for its object a process for obtaining in the form of chemically
pure gold, the total gold contained in fine gold produced by affinage or by other
methods, said fine gold still containing metallic admixtures of various kinds and
quantities, and also for similarly obtaining the total gold contained in auriferous
alloys. Whilst the above is effected in the chemical method by the impure metal
or alloy being first dissolved in a disintegrated condition in aqua regia and the gold
precipitated by means of precipitating substances from the dilute solution which is
cleared by settling on the other hand in the electrolytic process, similarly to the
action in the electrolytic refining of copper and silver, the dissolving of the raw
material and the precipitation of the pure gold are effected in a single operation.
To this end it is necessary, on the decomposition by the electric current of the
solution of gold that, under the influence of the ions separated out at the anode,
gold or mixtures of the same be introduced into the solution to the same extent as
pure gold is separated out at the cathode, and accordingly with a continual
precipitation of pure gold, the amount of gold contained in the solution remains
approximately the same. No chlorine is therefore allowed to be given off in a
gaseous condition during the clectrolytic action, if (for instance) a solution of
chloride of gold be employed as the electrolyte, because gold or its admixtures
corresponding to the quantities of chlorine released or given off, would remain
undissolved.
If the electrolytic process is to be adapted (as is intended by the present
invention) to industrially replace the chemical process, the following further
condition of its performance must be provided for, viz. :-the working up of a
given material must not be allowed to require more time than the processes
hitherto employed. It is therefore necessary to employ an electric current of a
density as high as possible for the electrolytical working up of the gold, because
it is only by means of such currents that it is possible to effect a sufficiently rapid
recuperation of the gold. The electrolyte must therefore have such quality or
property that, even in the decomposition by means of currents of the necessary
bigh density, a separation of gaseous chlorine does not take place at the anode and

page_text: that under the same circumstances the gold is precipitated in a perfectly pure
state and of a good coherency.
By the latter requirement particularly, the employment of a solution of cyanide
of gold is quite excluded, because in employing such solution nearly all metals
contained in the impure gold to be treated are not only dissolved together with
the gold at the anode but are also precipitated together with the same at the
cathode and in such case pure gold cannot be obtained.
If it be tried to dissolve electrolytically anodes of pure or impure gold in a
solution of gold-chloride by means of electrolytic currents of high density, the
behaviour of the gold in such process is essentially different from the behaviour of
copper and silver in the analogous methods for electrolytically purifying these
metals. Whilst as regards copper and silver it is quite unessential for continuously
transferring the constituents of the anode into the solution, whether the solution is
neutral, weakly or strongly acid, concentrated or moderately diluted, cold or warm,
these differences have a high influence upon the electro-chemical dissolving of
gold, especially in using electric currents of the said high density.. If a neutral
solution of pure gold-chloride be electrolytically decomposed between gold-
electrodes, with electric currents even of very small density (for instance of a
density less than 10 ampère per square metre) the chlorine separated at the anode
will escape without having acted upon the metal; such behaviour is not altered
by increasing the temperature. The same phenomenon is to be seen at somewhat
higher densities of the current (these densities being still much less than 100 ampère
per square metre) if as electrolyte a weakly acid solution be employed such as for
instance is obtained by evaporating a solution of gold in aqua regia. Therefore
solutions of the one or of the other kind cannot be used for the purpose of
electrolytically purifying gold. The further researches and experiments of the
inventor have shown, that the troublesome separation of gaseous chlorine can be
diminished and even fully avoided by adding a sufficiently excessive quantity of
hydrochloric acid. By these researches and experiments of the inventor it has
been shown on the one hand that at a definite temperature a maximum of the
current density corresponds to a definite amount of acid, at which maximum the
whole quantity of the separated chlorine is bound by the metal of the anode, and
on the other hand that this maximum of the current density and of the quantity of
chlorine to be bound increases if the amount of hydrochloric acid contained in the
Bolution be increased. A further important increase of this maximum at a
given degree of acidity can be obtained by increasing the temperature.
therefore at a definite temperature and at a definite degree of acidity the
density of the current be increased over the said maximum, thereby producing
a separation of gaseous chlorine, this separation can be fully avoided within
certain limits by increasing the amount of acid as well as by increasing
the temperature. By his experiments the inventor has also found that in this
manner by the combined action of excessive acid and of high temperature, the
continuous dissolving of the anode-gold can be performed even at current densities
of 1000 to 2000 ampères per square metre and in relatively weak solutions of
chloride of gold. The inventor has further found that by the employment of such
gold chloride solution as electrolyte also at the highest current-densities necessary
for industrial purposes, the gold is deposited on the cathode, in the form of a
coherent metal, which is free from any accessory or secondary constituents of the
solution, for instance also free from platinum and silver. Therefore as the main
feature of the present invention a process for the electrolytical purification of gold
is to be considered, which is performed with a high density of electric current with
an electrolyte consisting of a hot gold chloride solution mixed with a strong excess
of free hydrochloric acid (chloride of hydrogen) which excess is to be increased as
the density of the electric current to be used is increased.
If
To particularise: The temperature of the solution may preferably be maintained
at 60-70° C.; the gold contained may be in the proportion of 25-30 gr. of gold
per litre; the addition of free hydrochloric acid for one litre of the solution

page_text: may amount, according to the current density employed, to from 20 to 60 cubic
centimetres of fuming acid of 1-19 specific gravity. As in the course of the
operation owing to evaporation and other consumption, the amount of hydrochloric
acid contained may vary in many ways, it may be noted, apart from the testing from
time to time of the acid contents, that when a suitable temperature is maintained,
a deficiency of free hydrochloric acid becomes evident by the development of
gaseous. chlorine, and on noticing the smell of chlorine, and until the same
disappears, hydrochloric acid must be added to the solution after it has first been
established that no considerable diminution of temperature has taken place.
If in order to explain these ascertained facts, it be assumed that the secondary
process (by which the evolution of free chlorine at the anode is suppressed and the
constant transfer of the constituents of the anode into the solution correspondingly
effected) consists in the formation of a compound AuCl + HCl, it follows
therefrom that the liquid in close proximity to the anode must contain so much
more free hydrochloric acid, the greater the current density, and correspondingly
the greater the quantity of chlorine to be retained at the surface of the anode, that is
to say, the more the anode diminishes in the course of the operation with a constant
current or, as the strength of current is increased with a constant surface.
If the temperature be the correct one, and the amount of hydrochloric acid
contained be sufficient, the bulk of the impurities in the gold and among these the
total amount of platinum and palladium contained, pass into the solution under
the action of the electric current, with the gold, the main part of the iridium aud
other platinum metals remaining undissolved in a metallic condition, and the silver
contained being separated out with these in a solid condition in the form of chloride
of silver. The same applies to a portion of the lead, as lead chloride, when the
solution is entirely or approximately saturated with the same, and to bismuth as
oxychloride when the amount of hydrochloric acid contained in the solution is not
sufficient. With the insoluble mud separated out in this manner at the anode and
easily falling therefrom in small quantities hesides very small fractions of the
unchanged anode, there is regularly mixed an excess quantity (about 10% of
the weight of the anode) of extremely finely subdivided gold. The production or
formation of this gold deposit must be ascribed to a portion of the anode gold
being first converted, not into chloride, but into protochloride (AuCl) which
immediately after its formation however is again converted into chloride of gold
and finely subdivided gold.
Whilst in this manner the constituents of the anode are transferred, partly to
the solution, partly to the mud accumulating at the bottom, exclusively pure gold
is deposited at the cathode, even with an increase in the impurities in the solution,
and with a current density up to above 1000 ampères per square metre. The
deposited gold is thus always replaced in the solution to a large extent by gold
dissolved at the anode, and partly by equivalent quantities of other metals, and
equally the separation of gold at the cathode corresponds to the conversion of
silver lead and bismuth in the form of solid chlorides; there takes place therefore
under the action of the current a constant impoverishment of the solution in gold,
which must be replaced by the addition of chloride of gold. The increase in the
amount of foreign constituents contained in the solution which simultaneously
takes place is, if care be taken to provide for the presence of the necessary amount
of gold, without influence on the purity of the gold even in the case of a con-
siderable excess of such foreign constituents, the presence of the foreign constituents
in the solution is in fact of assistance in the formation of the gold deposit, in that
the latter from a pure solution, particularly in the case of high current density, is
very voluminous, whilst on the other hand, without affecting its purity, it is deposited
the more compactly as the solution is the more charged with impurities.
From the above there results, as an essential advantage of the proposed process,
the possibility of a continuous working with a proportionately extremely moderate
consumption of acid. In the purification of gold by the chemical process for each
fresh.quantity of the material to be separated which is to be dissolved, there must

page_text: always be again employed a corresponding quantity of hydrochloric acid and nitric
acid, and in a similar way, corresponding to the quantity dissolved, fresh depositing
substances must be added, whereas in this new process the small quantity of
hydrochloric acid employed for forming the electrolyte is always again available
as a solvent in consequence of the deposition of pure gold at the cathode, and
therefore precisely this small quantity of hydrochloric acid may be employed in a
large series of operations, both for dissolving the gold to be separated out, as well
as for depositing the pure gold contained therein.
With the advantage of a continuous working in the same solution, there is
combined as a further advantage the possibility of accumulating from large
quantities of the anode material the comparatively small amount of platinum
contained in the same, in such a way that (after separating out the gold) the
platinum may be directly recovered from the solution in the form of ammonio-
chloride of platinum. If for instance a gold be employed as anode material,
containing an average of robo of platinum, on the daily treatment of 20 kilos. of
such material about 2 kilos. of platinum would be transferred to the gold solution
at the end of 100 days.
As however according to observations made, an enrichment of the platinum
contents of the solutions up to double their gold contents is without influence on
the purity of the gold deposited, the re-employment of the same solution may be
continued without objection until it appears advantageous to remove the amount of
platinum contained for the purpose of further treatment thereof.
The amount of palladium in the solutions cannot be allowed to accumulate to
the same extent as the amount of platinum without influence on the purity of the
gold; whilst quantities of less than 5 gr. per litre do not appreciably affect the
constitution of the deposit, a greater quantity of palladium in the solution makes
itself apparent by influencing though in a very little degree, the purity of the
gold as however the quantities of palladium existing in gold are only usually a
fraction of the amount of platinum, the renewal of the solution may as a rule take
place according to the enrichment of the latter as hereinbefore mentioned.
As regards the carrying out of the work, the following may be stated in order
to complete what has been hereinbefore stated:
In view of the high value both of the anode material, the product of the
electrolysis, and the solution, a treatment in the shortest time and in the smallest
space is required. Accordingly, the current density (as has already been said)
must be kept as high as possible, and the thickness of the anode as small as
possible. If anodes be employed of 4 millimetres in thickness, the said anodes
will, with an initial current density of 400 ampères per square metre (of the
anode) within 24 hours be so far eaten away that the remainder (about of the
original weight of the anode) must be recast for further treatment. The approxi-
mately equal amount of gold, which is deposited at the bottom in the anode mud,
is either removed from the vessels after each separate operation, or after longer
periods, in order according to its constitution, to be again prepared either directly,
or after preliminary purification, for further treatment in the form of anodes.
Thin rolled fine gold obtained by electrolysis is employed as support for the
cathodes. The cathodes may be made considerably less broad than the anodes,
their length being about the same, as, by the action of the current, an increase of
size in all directions very rapidly takes place, and even with a suitably large initial
current density the deposit is precipitated as an adherent (that is in a mass).
The last mentioned circumstance makes it possible even with a high current
density to approach the electrodes to within about 3 centimetres of one another
without fear of short circuiting or of the separated gold being rendered impure.
In the gold solutions at first employed, which are not as yet at all impure, or only
slightly contain foreign constituents the distances apart of the electrodes must be
made somewhat greater in view of the voluminous nature of the deposit, or the
cathodes must be more frequently changed.
The arrangement of the electrodes in separate baths, as well as the grouping of

page_text: several baths together will be readily understood by practical persons from what
has been hereinbefore stated, without any further explanation.
For the vesselɛ or cells, earthenware, porcelain or other materials found to be
capable of resisting hot diluted hydrochloric acid must be employed.
The use of hot baths renders it necessary to provide the same with any ordinary
devices for automatically maintaining the level of the liquid. The vessels employed
for thus automatically refilling, are filled with the water used for washing the
finished deposit, the residue, and the mud, and, if there be a deficiency of such
washing water, with an addition of distilled water. In this way the diluted parts
of the gold solution are again always supplied to the baths from which they were
removed, and concentrated without any other apparatus by the evaporation of the
electrolytic solution.
The process herein before described may, as is already evident, be employed for
obtaining pure gold from auriferous alloys of all kinds. It is only necessary for
this purpose to bear in mind that on the chlorination of the non-gold constituents
of the anode, an equivalent quantity of gold is separated from the solution at the
cathode, which must be replaced by the addition of gold chloride. In the case of
such material it will therefore be necessary to aid to a certain extent the dissolution
of the gold by the electrolytic process by the treatment of another portion by the
chemical process, and in so much greater proportion as the quantity of the foreign
constituents is greater and their electro-chemical equivalent is lower in comparison
to that of gold.
If the material which is to be separated contain a proportionately large quantity
of silver, the surface of the anode will be covered with a layer of chloride of silver
which no longer drops off, but is adherent and at the same time non-conducting.
The diminution of the effective metal surface thereby produced causes the
separation of gaseous chlorine, and consequently the anode is attacked only to an
insufficient extent and the gold contents of the solution are precipitated. In such
alloys the interruption of the dissolving process may be entirely prevented by
removing periodically by mechanical means from the surface of the anode plates
the interfering layer and this more frequently, the greater the amount of silver
contained, and the greater the current density.
The disturbances by the accumulation of bismuth oxychloride, if such takes
place, may also be removed in a similar way.
If the anode alloy contains in addition to silver, or other metals, lead in any
considerable quantity, if a solution mixed only with hydrochloric acid is employed,
such solution soon becomes charged with chloride of lead. From this period both
electrodes as well as the walls of the vessel and the surface of the liquid become
coated with crystallized chloride of lead, and even if it is possible to obtain the
gold, (recovered in the first operation) free from lead by repeated boiling with
distilled water, and to remove from solution when cooled a large part of the
chloride of lead by scooping out, the utilization for further treatment of a solution
thus charged with lead is very disadvantageous. It is possible to carry on a
continuous uninterrupted operation when treating alloys containing lead by an
addition of sulphuric acid to the solution. A quantity of concentrated sulphuric
acid is employed about equivalent to the free hydrochloric acid present. The lead
then forms on the anode almost insoluble layers of lead sulphate, which either
gradually drop off, or where the alloys are very rich in lead must be mechanically
removed like the chloride of silver. It is of course evident that the amount of
sulphuric acid removed from the solution in the form of lead sulphate must be
replaced from time to time.
As the presence of sulphuric acid does not otherwise affect the operation, it is
preferable to add, to the gold chloride solution from the first, a mixture of hydro-
chloric acid and sulphuric acid, in any case where gold containing lead is
employed as anode material, even though it only occasionally occurs.
Finally it may be stated that in a similar manner as by the addition of hydro-
chloric acid (i.e. chloride of hydrogen) to the hot solution of gold chloride, the
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page_text: suppression of the evolution of gaseous chlorine at the anode and the consequent
continuous dissolving of the anode material may be effected by an equivalent
addition of sodium-chloride, or of other chlorides forming double salts with gold
chloride. The amount of such chlorides necessary for the purpose mentioned
increases (similarly to the hydrochloric acid) with the increasing current density.
In all cases therefore in which according to the above description the employment
of an addition of hydrochloric acid takes place for the preparation of the
electrolyte, this hydrochloric acid may be partially or wholly replaced by the
equivalent quantity of sodium chloride or of another chloride forming double salts
with gold chloride. If gold containing platinum is to be worked up there cannot
be employed such chlorides which (like chloride of potassium or of ammonium)
with platinum form hardly soluble double salts.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, as communicated to me by
my foreign correspondent, I declare that what I claim is :-
1. A process for producing perfectly pure gold from fine gold obtained by
refining or in other manner, consisting in the treatment by electrolysis with
currents of a high density at the anode as well as at the cathode and with an
electrolyte consisting of a hot solution of gold chloride containing an excess of
hydrochloric acid, substantially as hereinbefore described.
2. A process for producing perfectly pure gold from auriferous alloys, consisting
in the treatment by electrolysis with currents of high density at the anode as well
as at the cathode and with an electrolyte consisting of a hot solution of gold
chloride containing an excess of hydrochloric acid, the gold contents of the
solution being constantly replenished by gold chloride obtained by chemical means
substantially as described.
3. A process for producing perfectly pure gold from argentiferous gold,
consisting in the treatment by electrolysis with currents of high density at
the anode as well as at the cathode and with an electrolyte consisting of a hot
solution of gold chloride containing an excess of hydrochloric acid, the layer of
chloride of silver covering the anode being periodically removed by mechanical
meaus, substantially as described.
4. Á process for producing perfectly pure gold from plumbiferous gold,
consisting in the treatment by electrolysis with currents of high density at the
anode as well as at the cathode and with an electrolyte consisting of a hot solution
of gold chloride containing an excess of hydrochloric acid, and in adding to the
electrolyte a given quantity of sulphuric acid, and removing periodically by
mechanical means the lead sulphate accumulating on the anode substantially as
described.
5. In the herein described process for obtaining perfectly pure gold from fine
gold and alloys rich in gold, the addition to the electrolyte of sodium chloride or
of other chlorides combining with gold chloride so as to form double chlorides,
instead of the whole or a part of the hydrochloric acid substantially as herein before
set forth.
WM. P. THOMPSON & Co.,
Of 6 Lord Street, Liverpool, Agents for the Applicant.