Improvements, in Extracting Metals.
Year: 1894

page_text: Date of Application, 23rd July, 1894:
Complete Specification Left, 22nd Apr., 1895-Accepted, 27th July, 1895.
Improvements, in Extracting Metals.
I, JOHN JAMES HOOD, of 1 Fenchure Avenue in the City of London, Chemist,
do hereby déclare the nature of this invention to be as follows:-
It is a well known chemical fact that some of the heavy metals will displace, or,
precipitate others from their compounds or salts, the former taking the place of the
latter combining with the.acid radicle to form.equivalent compounds.. As instances
of this kind of double decomposition which are largely employed in metallurgical
operations may be cited the extraction of metallic copper from the aqueous
solutions of its salts by metallic iron as may be represented by the chemical
equation CuSO4 + Fe =FeSO + Cu, Another analogous reaction is largely
employed in the refining of silver wherein metallic copper is caused to act upon a
solution of sulphate of silver, forming metallic silver and sulphate of copper, thus:
Ag₂SO + Cu CuSO +2 Ag, or again the well known reaction whereby.
metallic gold is precipitated from its solutions.by metallic zinc.
In these several reactions it, is seen that, the object of the metallurgist is to
recover, or precipitate, in the metallic form a more valuable metal from its dilutę
solutions at the expense of a less valuable one. But it is evident that in certain
cases the operations might be looked upon as reversed. If metallic copper should
at any time become more valuable than silver the former might readily be extracted
from its ores when existing in the. metallic state by first washing the ore with a
solution of nitrate or sulphate of 'silver, this metal being precipitated as such
throughout the. mass of ore whilst the metallic copper would go into solution, as
sulphate or, nitrate from which metallic copper could easily be precipitated by iron,
Or if it were necessary to extract finely divided, metallic iron from an ore the
metallurgist would simply have to wash, the material with a solution, of a copper
salt which would give him a solution of sulphate of iron at the expense of an
equivalent of copper deposited.
The improvements, described herein relate to such metallurgical processes, and
have for their object not primarily the precipitation. of a more valuable metal as
such from its solution at the-expense of a less valuable one; but the solution or
dissolving of a more valuable one by the precipitation, of a less valuable metal-
and such improvements have regard more particularly to the precious metals gold
and silver which as is well known occur frequently in their ores in the metallic state.
There is an old; statement and one that has been much controverted that metallic
gold per se is soluble in cyanide of potassium as well as in ferrocyanide, of potassium.
Finely divided gold precipitated from its solutions. is to̟, a very minute extent
dissolved by a solution, of cyanide of potash but the same gold-melted, into a globule
and placed in cyanide solution is not dissolved. But as metals when precipitated
from their solutions are very apt to carry down small portions of basic salts or even
of their oxides,and as such. bodies in the case of gold are soluble in cyanide and-are
decomposed at a bright red heat these two facts may account for the, anomaly.
That metallic gold per se is not soluble in cyanide is easily proved by adding one
part of gold as chloride to a large excess of cyanide, and placing a piece of zinc in
solution when the gold is immediately' deposited as a, bright yellow,coating upon
the zinc and will so remain although the quantity of cyanide is many times greater
than is required to dissolve. the gold. The zine at the same time showing no
appreciable loss of weight by solution in the craṇide.

page_text: Based upon the old statements regarding the solubility of gold in cyanide of
potash a large industry has been developed for the extraction of gold from its ores
by washing the ores with potassium cyanide solution and precipitating the gold
from its solution by metallic zinc. But gold is known to occur not only in the
metallic state but also as telluride, selenide, and sulphide, and as most compounds
of gold are soluble in potassium cyanide particularly the sulphide and as a solution
of sulphide of gold in a cyanide is precipitated in the metallic state by zinc it is
probable that the success of the process is largely due to the ores containing
sulphide of gold.
Many ores do undoubtedly contain gold and silver in the metallic state and it is
evident that if a salt of a cheap metal were known which by double decomposition
would dissolve the precious metal whilst the cheaper-metal mas precipitated it
would be a simple matter to extract finely divided gold from its ores by washing
the ore with such a salt solution.
The subject of this Patent is to describe several such solutions of metallic salts
which by double decomposition will dissolve gold whilst the baser metal is precipi-
tated, how the solutions are prepared and worked and the method of recovering the
gold from its solutions.
If metallic gold or natural auriferous ores be treated with certain solutions of the
salts of mercury or lead and some other base metals the gold is dissolved with
great ease the base metal being precipitated. The metal 1 prefer to employ is
mercury but lead also acts well. This preference is because the base metals like
lead tend to form a hard coating upon the particles of gold and so renders the
further solution of the precious metal slow whereas in the case of mercury as it is
precipitated on the particles of gold it remains fluid and hardly if at all retards the
solvent action of the solution employed."
To prepare my active solution I take sulphate, chloride or other compound of
mercury or the oxide of mercury either in solution or finely ground and dissolve
the compound in a solution of soda a potash cyanide or mixtures of these, a
considerable amount of alkali either as caustic or carbonate is also added, by
preference caustic soda, any insoluble matters are settled out and the clear solution
evaporated to dryness for export. The quantities of the ingredients will vary
according to the nature of the auriferous ore that is to be treated. One mixture
that answers well consists of 2 parts cyanide of soda or potash 1 part mercuric
chloride or its equivalent of sulphate whilst the caustic soda varies from to 2 parts.
If a small quantity of this solution be placed upon gold, mercury is quickly
deposited and gold goes into solution. Estimating the amount of gold dissolved for
a given amount of mercuric chloride the reaction appears to be in its first stage
represented thus: 2 Au +3 HgCl, = 3 Hg + Au,Cl, or roughly two parts of
mercuric chloride employed on a quantity of auriferous quartz with an alkaline
cyanide solution, will dissolve one part of gold assuming that there is more than
this one part of gold in the quantity of ore employed.
If pure gold be treated with the above solution and kept warm a secondary
reaction takes place resulting in the amount of gold dissolved being greater than
the weight of mercuric chloride employed. This may be explained by the solvent
action of the auric salt acting on the metallic gold to form an aurous salt in the
presence of the alkaline cyanide, just as a ferric salt will dissolve metallic iron to
form a ferrous salt.
To prepare the other base metals for dissolved gold, for instance lead, I take one
of its compounds, as ordinary litharge boil it with caustic alkali to dissolve as much
as possible and mix this alkaline lead solution with the cyanide and employ the
mixed solution as in the case of mercury. Such a solution poured upon metallic
gold soon gives a black deposit of metallic lead and gold in solution.
These gold solvents may be converted into solid masses and packed for export or
they may be prepared as required at the mines from the constituents.
I am aware that the employment of caustic soda has been patented in the
ordinary cyanide processes, as worked at present, in order to avoid the loss of

page_text: cyanide due to the acidity of many ores, but in: the process herein described the
alkalinity that is claimed for the solutions is for.quite another purpose and in fact
is necessary. For if the above described solution is slightly acid it scarcely acts
upon the gold and if such alkaline solution after it has dissolved gold be rendered
acid as it may be in passing over ores which are of a bighly acid character nearly
all the gold already dissolved is precipitated as a complex yellow compound so that
it would he lost.
Instead of preparing the mercury compound in the manner above indicated
cyanide of mercury may be prepared by reacting upon a soluble cyanide with a
compound of mercury and crystallizing the mercury cyanide employing this material
alone in conjunction with caustic alkali or what yields better results with a
proportion of alkali cyanide.
To employ any of the above described solvent solutions the ore containing
metallic gold is ground in the usual fashion and placed inside tanks of wood, or if
of metal the inside and all the metal connections are coated with paint or varnish
to prevent the mercury salt acting upon them, and the mass of ore is washed to free
it from acid bodies which would precipitate the mercury, or the washing may be
done simultaneously with the grinding. The alkaline solution is poured over the
ore and allowed to run through, it is again poured over the ore and allowed to rest
for several hours and it is essential that. the solution should still be alkaline as it
runs from the ore.
The ore is next lixiviated and the solution and washings are treated in either of
the following methods.
The auriferous solution is acidified with sulphuric or other acid and run over iron
filings or finely divided iron which precipitates the gold; or instead of finely
divided metallic iron I employ the highly active galvanic couple devised by the
chemists Gladstone and Tribe known as the copper-zinc couple prepared according
to the methods described by them either as a wet or dry couple.
The latter I prefer as it is more suitable for export and is made by agitating and
gently heating a mixture of finely divided copper with zinc. This highly reducing
agent plays a double office when brought into contact with the auriferous solution
as it not only reduces the gold to the metallic state but also any excess of mercury
that may be in the solution and is recovered.
Instead of exporting the material above described manufactured as above it may
be made directly at the mines by dissolving a mercury or lead compound in alkaline
cyanide just as required employing about 2 ozs. or more of the mercury salt for
each ounce of gold the cyanide of potash or soda being about twice or more that
required to keep the mercury in solution and the alkali being sufficient to maintain
the solution and washings alkaline during all the time of contact with the ore.
Dated this 23rd day of July 1894.
Improvements in Extracting Metals.
I, JOHN JAMES HOOD, of 1 Fenchurch Avenue, in the City of London, Chemist,
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 improvements described herein relate to metallurgical processes for the
extraction of valuable metals from their ores and have for their object not primarily
the precipitation of a more valuable metal from its solution at the expense of a less
valuable metal, but the solution or dissolving of a more valuable metal by the
precipitation of a less valuable one. Such improvements have regard more

page_text: sparticularly to the precious metals gold and silver which as is well known'occur
frequently in their ores in a metallic state.
Iemploy for the extraction of sucha metals assolution of a salt or compound of a
baser metal whichton being brought into the presence of the precious metal (whether
duannetallo state or othwise) causes it pass into solution by reason of sits
displacing the baser metal instheesolution. The extraction of the precious metals
is effected by washing the ground ores with such a solution.
Taking gold as an example I will now describe several such solutions of metallic
salts as will dissolve gold while the baser metal is precipitated and will set forth
how the solutions are prepared and worked and the method of recovering the gold
from the solutions.
If metallic gold or natural auriferous ores be treated with certain solutions of
salts of mercury or lead or other base metals (the gold is dissolved with great lease
the base metal-being precipitated.
The metal I prefer to employ is inercury but lead 'does well, and several other
metals will do though less conveniently. This preference is because the base metals
like-lead tend to form a hard covering upon the particles of gold and so to render
the further solution of the precious metal slow-whereas in the case of mercury,as
-it is precipitated on the particles of gold it remains-fluid and hardly if at all retards
the solvent-action of the solution employed.
To prepare such an active solution in one way I take the cyanide-sulphate
chloride or oxide-of mercury or other compound of mercury soluble in an alkaline
cyanide and dissolve it in a solution of a suitable alkaline cyanide-preferably soda
or potash cyanide or mixtures of these. Alkali either as caustic or carbonate
(by preference caustic soda) it will be well in most cases in practice to add. If the
-material is being prepared for export-anylinsoluble matters are allowedu tossettle
out and the clear solution is evaporatedsto dryness. This can readily belonerfor
the presence of the metallic salt prevents loss of cyanide during theusevaporation.
A mixture that answers wall consists of two parts by weight of cyanide of potassium
(or its equivalent of cyanide of sodium) ione part. of mercuric chloride. or its
equivalent of sulphate or other mercury salt and from one half to two parts of
-caustic soda.
If a small quantity of this solution be placed upon gold, mercury is quickly
deposited and gold, goes into solution.
Estimating the amount of gold-dissolved fur-aigiven amount of mercuric chloride
the reaction appears to be in its first stage represented-by 21Au+ 3Hg01,3Hg
+AuCl or roughly-two-parts- by weight of mercuric chloride employed on a
quantity of auriferous quartz with an alkaline-cyanide solution will dissolve cone
part of gold.
If pure gold be treated with the above solution and-kept warm a secondary
reaction, probably takes place resulting in the amount of gold dissolved being
greater as compared with the mercuric chloride than the above proportion. This
may possibly be explained by the solvent action of the nuric salt acting on the
metallic gold to form an aurous salt in the presence of alkaline cyanide, just as a
ferric salt will dissolve metallic iron to form a férrbustsalt.
To prepare another base metal for dissolving gold, for instance lead, I take one
of its compounds suclifastofdinttryolitharge,boil it with caustic alkali to dissolve as
much as possible and mix this alkaline lead solution with the alkaline cyanide and
employ the mixed solution as in the case of mercury.
Such a solution poured upon metallic gold soon gives a black deposit of metallic
leadvand gold goes into solution.
These gold solvents may be converted into solid masses and packed for export, or
they may be prepared as required at the mines from the constituents.
Although I have mentioned certain salts of baser-metals in the preparation of my
solution I do not restrict myselfotonthese salts. mentionsthem because theycare
merely convenient and cheap-salts-toises in the preparation of my solution which
when prepared consists essentially of the cyanide of an alkali with some salt ofca

page_text: baser metal in solution; and in all these cases the precious metal when being
dissolved is acted upon by a solution which contains a baser metal or soine salt of
baser metal in solution in the presence of an alkaline cyanide, and all the solutious
act upon the precious metals in the same way, that is to say, by the precious metal
displacing the baser metal in that solution.
Instead of preparing this metallic compound in the manner above indicated io
which caustic alkali is employed I may omit the caustic at this stage and proceed
by reacting upon a soluble cyanide with a compound of mercury soluble therein and
if desired crystallizing it. The material thus obtained is employed in conjunction.
with caustic alkali, or what yields better results with a solution of alkaline-
cyanide.
:
If the process be performed at the mines without the assistance of the compound
previously prepared, the solutions may be formed in the vat itself and in the
presence of the ore. Thus, if to an ore lying in a solution of alkaline cyanide a
mercury or lead salt soluble in such solution be added the gold will pass into
solution displacing the baser metal; or the material may be made directly at the
mines by dissolving a mercury or lead compound in alkaline cyanide just as
required employing about two ounces of the mercury salt for each ounce of gold;
or more must be added if the ore contaius other bodies which throw the mercury
out of solution.
To employ any of the above described solvent solutions the ore is ground in the.
usual fashion and placed inside tanks of wood or if the tanks be of metal the inside
and all the metallic connections are coated with paint or varnish or otherwise.
protected so as to prevent the mercury salt acting upon them and the mass of ore
is washed to free it from acid bodies which would precipitate the 'mercury or the
washing may be done simultaneously with the grinding. The alkaline solvent
solution is poured over the ore and allowed to run through. It is again poured
over the ore and allowed to rest for several hours and it is essential that the solution
should still be alkaline as it runs from the ore.
The ore is next washed and the solutions and washings are treated in either of the
following methods :-
The auriferous solution is then treated by any well known method such as
precipitation with zinc to recover the gold and mercury. I prefer however to
employ the highly active galvanic couple devised by the chemists Gladstone and
Tribe, known as the copper zinc couple, which may be prepared according to the
methods described by them or otherwise either as a wet or dry couple. The latter
I prefer as it is more suitable for export and is made by agitating and gently heating
a mixture of finely divided copper with zinc.
This highly reducing agent plays a double office when brought into contact with
the auriferous solution as it not only reduces the gold to the metallic state but also
any excess of mercury that may be in the solution. The zinc copper couple acts
well even in an acid solution.
In the practical working of the process two rules are to be observed: Firstly,
the solution is to be alkaline until the gold has displaced the baser metal and the
solution has been run off the ore; and secondly the alkalinity during the precipita-
tion by the zinc copper couple must not be so great as to prevent the precipitations
taking place freely.
The alkalinity may be regulated in the ordinary way by the addition of a
sufficient quantity of an alkali such as caustic soda or by partially neutralizing by
acid.
The above process is applicable to ores where the gold is in a metallic state as
well as to those in which it is in chemical composition with other substances.
Here I would remark that by the terms "base metal" and "baser metal " I mean
to denote mercury, lead and such other metals as are displaced by metallic gold
from their solutions in alkaline cyanide.
If and so far as the gold is in the form of a sulphide in the ore the use of the
metallic salt is not required as the sulphide of gold will pass into solution in an

page_text: alkaline cyanide.without any other metallic salt, being, present. I do not therefore
claim the above method as applied to the purpose of the extraction of gold when in
the form of a sulphide of gold which may be extracted or got rid of in any way
desired but only sofan as the gold is in the shape of metallic gold or compound
other than the sulphide; but subject to, the above limitation. I would have it
understood that what I claim fs-
1. In the extraction of precious metals from their ores the employment of a
substance formed by the action of a solution of an alkaline-cyanide upon a salt or
compound of 'a baser metal soluble therein so as to produce a-displacement of the
båser metal by the precious-metal which thereby passes into solution.
2. In the extraction of precious metals from their ores the employment of a.
salt or compound of a baser metal in a solution of an-alkaline cyanide in which it is
soluble so as to produce a displacement of the baser metal by the precious metal
which thereby passes into solution:
3 In the extraction of precious metals from their ores dissolving the precious
metals by means of a solution containing both a cyanide of potassium or sodium
and a salt or compound of a baser : metal in solution therein so as to produce a
displacement of the baser metal by the precious metal which thereby passes into
solution:
4. The preparation and use in the extraction of precious metals from their ores
of solvents substantially as described:
5. In the extraction of precious metals from their ores, the use of a zinc copper
couple for the precipitation of 'precious metals from solution:
Dated this 20th, day of April 1895.
JOHN JAMES HOOD.