Improvements in Amalgamating and Extracting Gold from Refractory and other Ores.
Year: 1890

page_text: Date of Application, 5th Mar., 1890
Complete Specification Left, 4th Dec., 1890-Accepted, 10th Jan., 1891
PROVISIONAL SPECIFICATION.
Improvements in Amalgamating and Extracting Gold from Refractory
and other Ores.
I, WILLIAM CROOKES, F.R.S.,. of 7. Kensington Park Gardens, in the County of
Middlesex, do hereby declare the nature of this invention to be as follows:-
The present method of extracting gold from refractory ores, (such as sulphides,
tellurides, arsenio-sulphides &c., of zinc, copper, iron, bismuth, antimony &c.,) by
direct amalgamation with mercury is very imperfect as the sulphides, arsenio-
sulphides and other minerals in the ore have a deleterious action on the mercury,
causing it to ". flour" and "sicken" so that its fluidity is destroyed and it
becomes either a tenacious mass or assumes a powdery character. In either
case its amalgamating action is almost entirely destroyed, with the result that
from 30 to 80 or more per cent. of gold escapes the action of the mercury, being
lost in the tailings, while large quantities of mercury are also carried off in the
washings. The, "flouring" of the mercury is due to a film or tarnish from some
of the sulphides, arsenio-sulphides, &c., coating the mercury, and preventing the
separate globules trom uniting when once divided by trituration or grinding. The
trituration in the machines therefore constantly subdivides the mercury more and
more until it becomes a black slimy mass which will not settle in water for a
long time and is carried off in the washings and lost; the separation of this
metallic slime from the heavier portions of the ore being almost impossible.
ور
The "sickening" of the mercury is not quite so serious as its "flouring."
Sick mercury has taken up some of the constituents of the minerals present, and
has lost its fluidity and will not flow with a bright surface, neither will it touch
gold except with great difficulty. "Sick" mercury can generally be restored
by distillation in which case not much is lost, the chief drawback being that
sick" mercury will not take up gold from the ore; but floured" mercury is
not only lost of itself but it carries away with it all the gold which it may already
have taken up. But supposing the mercury neither "flours nor sickens
there is another serious loss when working with ores containing sulphides;
tellurides; arsenio-sulphides &c. Even when the mercury retains its bright
metallic condition and is in its usual active state it will seldom take up more
than half or two thirds of the gold present in the ore owing to the coat of tarnish
which is almost always on the surface of the gold, a coating which can only
be removed by chemical or mechanical means, the long grinding required to
remove the coating mechanically then tending to "flour"
the mercury.
"
Another method of extracting gold from the above named ores is by roasting
till free or almost free from sulphur and then treating with a solvent for gold:
The precious metal is then precipitated from its solution by appropriated
means. Chlorine is usually employed for this purpose but it is expensive. In
some cases cyanide of potassium or a soluble cyanide is used instead of chlorine
to dissolve the gold. This reagent has however, a very feeble solvent action
on gold and I can imagine few cases in which a sufficient amount of gold would
be dissolved to repay for the expense of the solvent.
It has been proposed to remove the tarnish or film (supposed to be sulphide of
gold) by the action of weak nitric acid, nitrate of mercury being present in the
bath for the purpose of amalgamating with the particles of gold as they are
cleansed from the film or tarnish. The drawback to such a process is that even
though the nitric acid solution be a weak one it will tend to decompose the more
easily acted on metallic sulphides present such as lead iron zinc and the like
sulphides and thus liberate sulphuretted hydrogen which in turn will recoat the
particles of gold with a sulphide coating.

page_text: It has also been proposed to use solutions of cyanide of potassium, and other
soluble salts both, with and without a further treatment by the electric current
but so far as I am aware it has always been proposed to use such substances as
a solvent for the gold which is subsequently recovered from its solution by a
further process.
Various proposals have also been made with regard to the use of an electric
current both for the purpose of assisting the amalgamating and for preventing
the "flouring or "sickening" of the mercury.
"
The object of my invention is to obviate the disadvantages inherent in the
above process and to this end it consists in submitting the refractory or other
ore to the action of a solution preferably of cyanide of mercury either in the
commercially pure state or prepared at the time by double decomposition between
'a soluble salt of mercury such as the sulphate and a soluble cyanide or fero-
cyanide. The process is so conducted that the mercurial cyanide shall decom-
pose the film or tarnish on the gold the cleansed particles of which become amal-
gamated with the mercury as distinguished from becoming dissolved in the solu-
tion the cyanide of mercury not being a solvent for pure gold. In lieu of the
cyanide the sulphate of mercury can be employed.
In many cases I also propose to use in conjunction with the solution of any
mercurial salt the current from any alternating dynamo machine.
In carrying out my combined process I proceed as follows:
1 take the gold ore, tailings, sulphurets &c. reduced to powder in the usual
manner and mix with them a solution of sulphate, nitrate, chloride, cyanide or
any soluble salt of mercury, and then pass a rapidly alternating current of elec-
tricity through the mass either when in a state of rest or when agitated or stirred
in any convenient manner. The bulk of the mass is not a very good conductor
of electricity while the fine particles of gold sparsely disseminated through the
mass are (as is well known) excellent conductors. iron carbon, or other con-
venient conductors may be used as the poles. Each pole is alternately cathode
and anode. If water is decomposed by such a current the gases being able to
escape do so, and a mixture of hydrogen and oxygen come from each pole. If
however the results of the decomposition are fixed, such as acid and metal, the
decomposition by the current in one direction is reversed by the following cur-
rent in the opposite direction and practically little or no decomposition results.
Assume that sulphate of mercury is the mercurial salt chosen then when the
pole with which it is in contact becomes positive, sulphuric acid is there
liberated whilst mercury is liberated at the opposite pole, but in a fraction of a
second the poles are reversed, the opposite decomposition is effected and the
sulphuric acid just liberated finds itself in presence of nascent mercury with which
it immediately combines. At the other pole the mercury liberated by the first
current finds a molecule of sulphuric acid ready to reunite with it, and so at
each pole the mercurial salt is decomposed only to be immediately reunited.
Every particle of gold in the wet mass through which the alternating current
is passing is a much better conductor of electricity than the surrounding mass,
and the equi-potential lines of force will converge towards them with the result
that more of the electric current passes through them than through the rest of
the mass and the two sides of each particle of gold act alternately, as anode and
cathode. On one side (the anode for the time being) sulphuric acid is liberated,
and on the other side (the cathode for the time being) mercury is liberated but
the affinity of gold for mercury is so great that the two metals instantly unite and
the gold becomes amalgamated. At the next alternation the side of the gold
particle which had the sulphuric acid liberated on it now has the mercury pre-
cipitated on it.
The affinity of gold for mercury here again comes into play, and instead of
reuniting with the sulphuric acid the mercury prefers to attack the gold. Thus
each alternation precipitates mercury on alternate faces of the particles of gold
in the mass of ore without requiring the gold to be in metallic communication
with either pole of the dynamo, as the extra conductivity of the gold draws to it,

page_text: the lines of force. The sizes of the particles is not essential as the finest "flour "
and float" gold will be amalgamated and consequently weighted as easily as
the largest pieces. Neither does it matter to what degree of coarseness the ore
is crushed so long as the solution of the mercurial salt penetrates to one part of
the piece of gold locked up in the ore for the action will then take place, and
the metal will become amalgamated, and in the subsequent amalgamating opera-
tions dissolved in the mass of mercury.
A further advantage incidental to the use of an alternating current is that the
sudden and violent decompositions and recompositions alternating with great
rapidity cause the mass to become hot, thereby greatly facilitating the amal-
gamation.
Any ordinary alternating current from an alternating dynamo or other source
of electricity will produce the desired effect. But the most efficient action is
dependent on the right adjustment of several variable factors such as 1. Current
density. 2. Area of electrodes. 3. Rate of alternation per second. 4. Electric
conductivity of the wet mixture of crushed ore and liquid. The latter varying
with every different kind of ore, it is evident that the others will have to be
adjusted in each separate case in order to get the maximum effect, and that
therefore no general directions can be here given which will meet all cases.
When it is impracticable to use an electric current I propose to employ a
solution of cyanide of mercury or a sulphate of mercury either of which solutions
will remove the film or tarnish and superficially amalgamate the particles of
gold the process will not however be so rapid as it is when the alternating electric
current is used although it may be hastened somewhat by heating the mass.
When carrying out the process in this manner I propose to use a solution com-
posed of two or three pounds of cyanide of mercury or of sulphate of mercury
dissolved in 80 to 100 gallons of water for each ton of ore but special circum-
stances may render it advisable to vary these quantities.
When the action is complete the mercury solution may be drained or drawn
off or removed in any other convenient way and the ore treated in amalgamating
pans or machinery or in other appropriate manner.
In some cases I find it an advantage. to add to the mercury used for the subse-
quent amalgamation process a little amalgam of sodium as described in my
British Specification No. 391 of 1865 or some of the modified amalgams described
by me in my British Specification No. 2229 of 1865.
It will be evident that the mercurial salt either alone or in conjunction with
the amalgams just referred to and with or without the use of the alternating
electric current may be employed in the grinding and amalgamating apparatus and
in this case the cleansing of the gold from the tarnish or film and the collection
and amalgamating of the cleansed particles may go on simultaneously. When
this is desired fresh mercury should be added to the mass and thoroughly mixed
or incorporated therewith.
The solution of mercurial salt and an alternating current may also be advan-
tageously employed in concentrating and vanning machines such use causing the
gold particles to unite more readily with the mercury.
Dated this 5th day of March 1900
COMPLETE SPECIFICATION.
0 Improvements in Amalgamating and Extracting Gold from Refractory
and other Ores.
I, WILLIAM CROOKES, F.R.S. &c. of 7 Kensington Park Gardens in the County of
Middlesex, 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
5 the following statement:-

page_text: 66
flour
"and
66
"
The present method of extracting gold from refractory ores, (such as sulphides,
tellurides, arsenio-sulphides, &c. of zinc, copper, iron, bismuth, antimony, &c.) by
direct amalgamation with mercury is very imperfect as the sulphides, arsenio-
sulphides and other minerals in the ore have a deleterious action on the mercury,
causing it to
sicken so that its fluidity is destroyed and it
becomes either a tenacious mass or assumes a powdery character. In either
case its amalgamating action is almost entirely destroyed, with the result that.
from 30 to 80 or more per cent. of gold escapes the action of the mercury, being
lost in the tailings, while large quantities of mercury are also carried off in the.
washings. The flouring" of the mercury is due to a film or tarnish from some
of the sulphides, arsenio-sulphides, &c., coating the mercury, and preventing the,
separate globules from uniting when once divided by trituration or grinding. The
trituration in the machines therefore constantly subdivides the mercury more and.
more until it becomes a black slimy mass which will not settle in water for a,
long time and is carried off in the washings and lost, the separation of this
metallic slime from the heavier portions of the ore being almost impossible.
<<
66
29
"
66
The sickening" of the mercury is not quite so serious as its "flouring."
Sick mercury has taken up some of the constituents of the minerals present, and
has lost its fluidity and will not flow with a bright surface, neither will it touch,
gold except with great difficulty. "Sick" mercury can generally be restored
by distillation in which case not much is lost, the chief drawback being that
"sick" mercury will not take up gold from the ore; but floured mercury is
not only lost of itself but it carries away with it all the gold which it may already.
have taken up. But supposing the mercury neither flours nor sickens
there is another serious loss when working with ores containing sulphides,
tellurides, arsenio-sulphides &c. Even when the mercury retains its bright
metallic condition and is in its usual active state it will seldom take up more
than half or two thirds of the gold present in the ore owing to the coat of tarnish
which is almost always on the surface of the gold, a coating which can only
be removed by chemical or mechanical means, the long grinding required to
remove the coating mechanically. then tending to "flour
the mercury.
Another method of extracting gold from the above named ores is by roasting
till free or almost free from sulphur and then treating with a solvent for gold.
The precious metal is then precipitated from its solution by appropriate
means. Chlorine is usually employed for this purpose but it is expensive. In
some cases cyanide of potassium or a soluble cyanide is used instead of chlorine
to dissolve the gold. This reagent has, however, a very feeble solvent action
on gold and I can imagine few cases in which a sufficient amount of gold would
be dissolved to repay for the expense of the solvent.
"
It has been proposed to remove the tarnish or film (supposed to be sulphide of
gold) by the action of weak nitric acid, nitrate of mercury being present in the
bath for the purpose of amalgamating with the particles of gold as they are
cleansed from the film or tarnish. The drawback to such a process is that even
though the nitric acid solution be a weak one it will tend to decompose the more
easily acted on metallic sulphides present such as lead, iron, zinc and the like
sulphides and thus liberate sulphuretted hydrogen which in turn will recoat the
particles of gold with a sulphide coating.
It has also been proposed to use solutions of cyanide of potassium, and other
soluble salts both with and without a further treatment by the electric current
but so far as I am aware it has always been proposed to use such substances as
a solvent for the gold which is subsequently recovered from its solution by a
further process.
Various proposals have also been made with regard to the use of an electric
current both for the purpose of assisting the amalgamating and for preventing
the "flouring" or "sickening of the mercury.
The object of my invention is to obviate the disadvantages inherent in the
above processes and to this end it consists in submitting the refractory or other
ore to the action of a solution preferably of cyanide of mercury either in the

page_text: commerciany pure state or prepared at the time by double decomposition between
a soluble salt of mercury such as the sulphate and a solubie cyanide of ferro-
cyanide. The process is so conducted that the mercurial cyanide small decom-
pose une him or tarnish on the goid the cleanseu particies of which become amal-
gamated with the mercury as uisunguished from becoming dissolved in the solu-
on the cyanide of mercury not being a solvent for pure gold. In lieu of the
cyanide the sulphate of mercury can be employed.
In many cases I also propose to use in conjunction with the solution of any
mercurial salt the current from any alternating dynamo machine.
in carrying out my combined process 1 proceed as follows:-
I take the gold ore, tailings, sulphurets &c. reduced to powder in the usual
manner and mix with them a solution of sulphate, nitrate, chloride, cyanide or
any soluble salt of mercury, and then pass a rapidly alternating current of elec-
tricity through the mass either when in a state of rest or when agitated or stirred
in any convenient manner. The bulk of the mass is not a very good conductor
of electricity, while the fine particles of gold sparsely disseminated through the
mass are (as is well known), excellent conductors. iron, carbon, or other con-
venient conductors may be used as the poles. Each pole is alternately cathode
and anode. If water is decomposed by such a current the gases being able to
escape do so, and a mixture of hydrogen and oxygen come from each pole. If
however the results of the decomposition are fixed, such as acid and metal, the
decomposition by the current in one direction is reversed by the following cur-
rent in the opposite direction and practically little or no decomposition results.
Assume that sulphate of mercury is the mercurial salt chosen, then when the
pole with which it is in contact becomes positive, sulphuric acid is there
liberated whilst mercury is liberated at the opposite pole, but in a fraction of a
second the poles are reversed, the opposite decomposition is effected and the
sulphuric acid just liberated finds itself in presence of nascent mercury with which
it immediately combines. At the other pole the mercury liberated by the first
current finds a molecule of sulphuric acid ready to reunite with it, and so at
each pole, the mercurial salt is decomposed only to be immediately reunited.
Every particle of gold in the wet mass through which the alternating current
is passing is a much better conductor of electricity than the surrounding mass,
and the equi-potential lines of force will converge towards them with the result
that more of the electric current passes through them than through the rest of
the mass and the two sides of each particle of gold act alternately as anode and
cathode. On one side, (the cathode for the time being,) sulphuric acid is liberated,
and on the other side, (the cathode for the time being.) mercury is liberated; but
the affinity of gold for mercury is so great that the two metals instantly unite and
the gold becomes amalgamated. At the next alternation the side of the gold
particle which had the sulphuric acid liberated on it now has the mercury pre-
cipitated on it.
The affinity of gold for mercury here again comes into play, and instead of
reuniting with the sulphuric acid the mercury prefers to attack the gold. Thus
each alternation precipitates mercury on alternate faces of the particles of gold
in the mass of ore without requiring the gold to be in metallic communication
with either pole of the dynamo, as the extra conductivity of the gold draws to it
the lines of force. The sizes of the particles is not essential as the finest "flour"
and "float" gold will be amalgamated and consequently weighted as easily as
the largest pieces. Neither does it matter to what degree of coarseness the ore
is crushed so long as the solution of the mercurial salt penetrates to one part of
the piece of gold locked up in the ore for the action will then take place, and
the metal will become amalgamated, and in the subsequent amalgamating opera-
tion dissolved in the mass of mercury.
A further advantage incidental to the use of an alternating current is that the
sudden and violent decompositions and recompositions alternating with great
rapidity cause the mass to become hot, thereby greatly facilitating the amal-
gamation.

page_text: Any ordinary alternating current from an alternating dynamo or other source of
electricity will produce the desired effect. But the most efficient action is
dependent on the right adjustment of several variable factors such as 1. current
density, 2. area of electrodes, 3. rate of alternation per second, 4. electric con-
ductivity of the wet mixture of crushed ore and liquid. The latter varying with
every different kind of ore, it is evident that the others will have to be adjusted
in each separate case in order to get the maximum effect, and that therefore no
general directions can be here given which will meet all cases.
When it is impracticable to use an electric current propose to employ a
solution of cyanide of mercury or a sulphate of mercury either of which solutions
will remove the film or tarnish and superficially amalgamate the particles of
gold the process will not however be so rapid as it is when the alternating electric
current is used although it may be hastened somewhat by heating the mass.
When carrying out the process in this manner I propose to use a solution com-
posed of two or three pounds of cyanide of mercury or of sulphate of mercury
dissolved in 80 to 100 gallons of water for each ton of ore but special circum-
stances may render it advisable to vary these quantities.
When the action is complete the mercury solution may be drained or drawn
off or removed in any other convenient way and the ore treated in amalgamating
pans or machinery or in other appropriate manner.
In some cases I find it an advantage to add to the mercury used for the subse-
quent amalgamation process a little amalgam of sodium as described in my
British Specification No. 391 of 1865 or some of the modified amalgams described
by me in my British Specification No. 2229 of 1865.
It will be evident that the mercurial salt éither alone or in conjunction with
the amalgams just referred to and with or without the use of the alternating
electric current may be employed in the grinding and amalgamating apparatus and
in this case the cleansing of the gold from the tarnish or film and the collection
and amalgamating of the cleansed particles may go on simultaneously. When
this is desired fresh mercury should be added to the mass and thoroughly mixed
or incorporated therewith.
The solution of mercurial salt and an alternating current may also be advan-
tageously employed in concentrating and vanning machines such use causing the
gold particles to unite more readily with the mercury.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is-
-
1st. The improvements in amalgamating and extracting gold hereinbefore set
forth which consist in submitting the ore to the combined action of a solution
of a mercurial salt and an alternating electric current substantially as described.
2nd. The improvement in amalgamating and extracting gold hereinbefore set
forth which consists in submitting the ore to the action of a solution_of_cyanide
or sulphate of mercury substantially as and for the purposes described.
3rd. The improvements in amalgamating and extracting gold hereinbefore set
orth which consist in employing in the final amalgamating process a solution of
cyanide or sulphate of mercury or an alternating electric current in conjunction
with a solution of a mercurial salt substantially as described.
Dated this 4th day of December 1800.
PHILIP M. JUSTICE,
55 & 56, Chancery Lane, London, Applicant's Agent.