Process of Extracting Precious Metals from their Ores.
Year: 1894

page_text: Date of Application, 22nd Dec., 1894-Accepted, 6th July, 1895
COMPLETE SPECIFICATION.
Process of Extracting Precious Metals from their Ores.
I, MIDDLETON CRAWFORD, residing at Colorado Springs, in the State of
Colorado, United States of America, Mining Engineer, do hereby declare the
uatue of this invention and in what manner the same is to be perforined, to be
particularly described and ascertained in and by the following statement •—
My invention relates to an improved process for the extraction and recovery of
precious metals by means of the solvent action upon said metals of the salts of
cyanogen, commonly known as the cyanide process.
Generally speaking, the cyanide process consists, first, in lixiviating the
pulverized ores of precious metals with a solution of cyanide of potassium, or
other similar salt, the result being the formation of a soluble auiocyanide of
potassium, or other corresponding salt, which is taken up in the solution, and,
second, in separating the precious metals from this solution either by electrolysis or
by passing the solution over zinc plates or shavings, whereby the precious metals
are precipitated, or by other appropriate means.
The difficulties incunted in the cyanide process have resulted from the great
length of time required to fully dissolve the precious metal in the cyanide solution,
and also from the expense entailed by reason of the large consumption of the
cyanide required for the formation of fresh solutions, the old solutions, either by
eason of their instability or by reason of the nature of the process employed for
extracting the precious metals from the same, being incapable of continuous use.
In my improved process I overcome both these difficulties.
In descubing my process I will specifically describe its application to the
extraction of gold fiom its ores, it being understood that it is equally applicable to
silver and other precious metals.
It has been recognized hither to that the solution of gold in cyanide of potassium,
and the consequent formation of an amocyanide, requires the presence of oxygen,
the formula by which the reaction takes place being expressed as follows
4An+8KC5 + 20 = 4KAuCy, + 2K,O
And many attempts have been made to supply this extra molecule of oxygen, with,
however, but slight success, owing to the difficulty of getting the oxygen in a
sufficiently stable condition to be inade practically available.
I have discovered methods of supplying this necessary oxygen in a condition to
so readily perform its part in facilitating the solution of the gold as to grently
reduce the time required to effect such solution, and also in a sufficiently stable
form to enable me to use the same solution over and over again in a continuous
process A further difficulty which has been experienced in the attempt to
discover a substance, which when added to the cyanide solution will afford the
requisite excess of oxygen, has resulted from the fact that the substances so added
have been accompanied by greater or less quantities, either, on the one hand, of
acid, or, on the other hand, of alkali, both of which substances I have found are
highly detrimental to, if they do not substantially prevent the proper canying out
of the process, for the reason that either an acid or alkaline excess in the solution
tends to oxidize any base metals which may be present in the ore, so that the
uconido attacks them in nation to the cold and fou the fauchow wannan that anx

page_text: Hydrocanic sem. Dy my process i have succeeded in avoiding this
difficulty also.
I will now describe three different methods by which I am able to impart to the
cyanide solution the requisite excess of oxygen in such a way as to avoid the
difficulties which have been above mentioned.
The first of these methods consists in the direct addition to the solution of
cyanide of potassium of a proper proportion of cyanate of the same, or a similar,
base. In carrying out this process in the preferred manner J proceed as
follows.-
-
I take an ordinary cyanide solution-cyanide of potassium being usually the
most convenient salt for the purpose. The strength of this solution will vary in
accordance with the nature of the ore to be treated, but under ordinary circum-
stances about one per cent. solution is to be preferred. To this solution I add and
thoroughly combine therewith cyanate of potash or other base, preferably, of
course, the same base as that present in the cyanide. To obtain good results not
less than ten per cent. of cyanate should be used. It is important, of course, that
the cyanate which is employed for the process should be free from deleterious
substances, such as if accidentally present would be capable of producing the
injurious effects above noted as incidental to the presence of an alkaline or acid
excess
With a solution prepared as above the powdered one containing the gold is
lixiviated, and if necessary agitated. Although under ordinary circumstances, and
in accordance with methods hitherto commercially employed, the agitation of ore
with the cyanide solution requires to be callied on for many days before the
precious metal is completely dissolved, I und that by reason of the combination of
a cyanate with the cyanide solution I am able to so facilitate the lixiviating process
that it practically occupies only about an hour.
The second of these methods consists in obtaining the same result by converting
a proper proportion of the cyanide solution into cyanate. This may be done by
chemical processes, and so far as these processes do not incidentally develop
deleterious substances or other wise umpan the efficiency of the remaining cyanide,
the solution obtained by means of then use is efficient for the purposes of my
process; but the preferred method which is employed by me for converting a
portion of the cyanide into a cyanate is an electrolytic one, and depends upon the
conversion of a cyanide solution into a cyanate at the positive pole of the battery
when such a solution is treated electrolytically. By reason of this conversion at
the positive pole, I find that the mere passage of a cutient of electricity through a
cyanide solution will somewhat increase its gold-dissolving capacity, but as the
action of the negative pole to a certain extent reverses the action of the positive
pole upon such a solution, I find it preferable to carry out my invention by
separating the two poles by means of a porous diaphragm, dividing the solution
into two distinct parts. Under these cucumstances that portion of the solution
within which the positive pole is situated rapidly increases its gold-dissolving
capacity, and, as I have stated above, I believe this result to be due to the
formation from the cyanide of a certain proportion of the coresponding cyanate,
which thereby supplies the necessary oxygen.
In the practical carrying out of this process I employ an electrolytic tank
contaming two compartments, separated from each other by a porous diaphragm.
This may be accomplished either by the use of a porous partition, or, more
conveniently still, by the insertion to the tank of a porous cup.
Within the
larger of these compartments is situated a carbon anode, and within the smaller an
iron or copper cathode. The composition of the electrodes may, however, be
varied. Both compartments are then filled with a solution of cyanide of potassium
and the current passed continuously through them In a short time, varying with
the strength of the current used and other conditions, the cyanide solution
surrounding the anode has very largely increased its gold-dissolving capacity.
When the proper point has been reached, which may be ascertained by appropriate

page_text: tests, the solution is drawn off from this compartment and used for purposes of
lixiviation, as above described. In place of the cyanide solution in the cathode
compartment, other solutions, such as sulphate of soda, may, if desired, be
employed.
The third of these methods consists in the addition to the cyanide solution of
certain chemicals which contain an excess of oxygen, which are sufficiently stable
to answer the purposes of my process, and which are free from either excess of acid
or alkali, and thereby avoid the difficulties which, as above set forth, result from
tho presence of such excess. It is quite likely that these chemicals do, to a certai
extent, convert a portion of the cyanide solution into cyanate, in which case, to
this extent, they come within the limits of the serond method above described.
The fact, however, of this conversion, and the extent of it, is very difficult to
demonstrate, and I therefore simply state the chemical substances and the method
of their preparation, which, when added to the cyanide solution, greatly increase its
gold-dissolving capacity, without reference to the theoretical question whether, by
their addition to the cyanide, they do or do not effect the formation of a cyanate
therefrom.
Of the chemical compounds which possess the qualifications just described, one is
prepared as follows. I agitate ethei with binoxide of barium, and add thereto.
gradually, small quantities of very dilute hydrochloric acid, the precipitated
chloride of barium 18 removed and all traces of acid neutralized by adding silicate
of soda or other suitable salt.
Another chemical compound possessing the same qualifications is prepared by
mixing peroxide of sodiuin with an equal quantity by weight of sulphuric acid in
water, the acid being added first and the peroxide afterwards, very slowly. To
the clear solution thus formed, sufficient silicate of soda, or other corresponding
salt, is added to render the solution perfectly neutial.
Both of these compounds contain an excess of oxygen in a condition ready to be
made use of for the purpose of facilitating the formation of an aurocyanide, and
they are funtherinore perfectly neutral. In carrying out my process by the third
method, I therefore add to the cyanide solution a suitable quantity of one on the
other of these compounds, the quantity being regulated to a certain extent by the
nature of the ore which is to be treated, and with the solution thus formed lixiviate
the powdered ore, as before described.
After the oic has been lixiviated with the cyanide solution prepared by either of
the three methods above described, that is, either by the addition of a cyanate, or
by the subjection to the positive pole of an electric current, or by the addition of
the chemical compounds above described, the gangue and also the solution are
withdrawn from the lixiviating vessel.
The cyanide process has hitherto been usually applied to what are called tailings,
that is, to gangue which has already been treated by an amalgamating process, the
amalgamating process, as is well known, being very efficient for the removal of the
larger particles of gold, but failing more or less to remove the microscopic or float
gold, or gold contained in concentrates, which latter forms are particularly
amenable to the cyanide process. In accordance with my invention, however,
I prefer to reverse this order and to subject the pulverized one as it comes from
the mine, first to lixiviation with a cyanide solution, as above described, and
subsequently to an amalgamating process. I, therefore, allow the gangue and its
accompanying solution to pass from the lixiviating vessel to an amalgamating tank,
where the comparatively coarse particles of metal, which are incapable of solution
by a cyamide process under any circumstances, within any reasonable time, are
reclaimed. I find that the amalgamation of the coarser particles 18 much facilitated
by subjecting the ore to the action of the cyanide solution previously to amalgama-
tion, by reason of the fact that the solvent liquid readers the surface of these
particles bright and clean, in which condition they are more readily taken up by
tbe mercury.
After the gangue and the solution have passed over the amalgamating surface,

page_text: the tailings are separated mechanically from the solution and the latter run into
the tank, where the gold is removed from it, preferably by an electrolytic process.
After the gold has been thus removed the residuam of the cyamide solution is run
into a storage reservoir and used over again for purposes of lixiviation
In so far as my process is carried out by the addition to the cyanide solution of a
proportion of cyanate, or by the conversion of a portion of the cyanide into
cyanate, it is evident that the requisite excess of oxygen in the solution prior to the
lixiviating process, is present in the cyanate, but when the gold has been taken up
and a double cyanide of gold and potassium formed, it is present in the form of a
corresponding quantity of caustic potash or similar oxide of whatever base has been
used; and then again, after the electrical separation has proceeded, it recurs in the
cyanate, which alternation of form explains the continuity of my process and the
stability of the solution formed by it Upon this hypothesis the formula repre-
senting the recurrent change is as follows:-
t
Although the solution used by me and prepared as above described is much more
stable than any which have hitherto been used, ret it is unavoidable that a certain
amount of degeneration of the solvent liquid should occur in the course of its
progress though the lixiviating, amalgamating and separating process This,
however, is readily made up by the addition from time to time, to the storage
eservoir above described, of a small quantity of cyanide and by subjecting the
solution in said reservoir to the action of the positive pole of an electric cunicnt
whereby any loss of oxygen, is made up, or the loss of oxygen may, if preferred,
be made up by the addition of the oxygen-impaiting chemical substances which
I have described. It is, however, more convenient to have the storage reservoir
itself divided by a porous partition into an anode and cathode compartment with
permanent electrical connections. In the anode compartment the cyanide solution
is originally treated electrically prior to its employment as a lixiviating agent, and
to this compartment the solution continually retuius after a cycle of the process,
where any impaniment of its efficiency is made good.
The advantages of this process are obvious. The comparatively short time
requned for the lixiviating process enables the ore to be rapidly run through, so
that the quantity of ore which may be treated by a given amount of cyanide is
very largely increased. This rendeis it practicable to apply the cyanide process,
not simply to tailings, but to the ore as it comes from the mine, and by combining
the process with an amalgamating process in the order I have desenbed, the
additional advantage is gamed of facilitating the amalgamation by the cleansing of
the gold The stability of the solvent liquid enables me to use it over and over
again continuously, thus greatly reducing the cost of the preparation of the solution.
By the combination of these advantages 1 am able to extract a very much larger
proportion of precious metal from a given quantity of ore than has hitherto been
done by the use of two separate processes Furthermore, I find that with a
solution prepared as I have described, while the facility with which the soluble
salts of the precious metals are formed is greatly increased, there is no corresponding
increase, but rather a decrease, in the facility with which various cyanide salts of
the base metals are formed, and consequently there is less waste of the cyanide and
also a purer metal as the result of the final electrolytic separation than would be
the case under similar conditions were the cyanide solution not treated by my
process. This is due, in the case of the chemical compounds which I add to the
solution, to their absolute neutiality.
Having now particularly described and ascertained the nature of my said invon-
tion, and in what manner the same is to be performed, 'I déclare that what '1
claim is:
'1. 'The process'of-preparing'a cyanide solution for the purpose of lixiviating the

page_text: ores of precious metals, which consists in combining a cyauate with said cyanide
solution.
2. The process of extracting precious metals from their ores, which consists in
hiviating the ore with a solution containing a cyanide and a cyanate.
3. The process of extracting precious metals from then ores, which consists in
fist xiviating the ore with a solution containing a cyanide and a cyanato, and,
second, electrolytically separating the dissolved metal from the solution, whereby
the lixiviating liquid is restored to its initial state
4 The process of extracting precious metals from their ores, which consists, first,
in exposing a cyanide solution to the passage of an electric curient; secondly, in
lixiviating the ore with that portion of the solution which is in the region of the
positive pole; and, thirdly, in reclaiming the precious metal from this solution by
electrolysis or other process.
5. The process of preparing cyanide solution for the purpose of lixiviating the
ores of precious metals with a view to extracting the metal from the ore, which
consists in converting a portion of the cyanide into a cyanate by exposure to an
electric cuiient in the region of the positive pole
6. The process of preparing cyanide solution for the purpose of lixiviating the
ores of precious metals, with a view to extracting the metal from the one, which
consists in passing an electric current through the cyanide solution with a porous
diaphragm interposed between the poles thereof, and then employing for the
pun pose of lixiviation that portion of the solution only which is taken from the
compartment containing the positive pole.
7. The process of extracting precious metals from then oies, which consists in,
first, lixiviating the one with a cyamide solution which has been exposed to the
action of the positive pole of an electric current, secondly, in subjecting the
lixiviated ore and accompanying cyanide solution to an amalgamating process;
and, thudly, in withdrawing the solution from the tailings and precipitating the
precious metals therefiom by electrolysis or other means
8. The process of extracting precious metals from their ores, which consists in
first, lixiviating the one with a cyanide solution which has been exposed to the
action of the positive pole of an electric current; second, in separating the
precious metal from such solution electrolytically, and, third, in ie-subjecting
the cyanide solution, from which the precious metals have been thus separated, to
the positive pole of an electric current for the purpose of using it over again as
before
9. The process of restoring the efficiency of a cyanide solution in which
precious metal has been dissolved and subsequently separated therefrom, which
consists in subjecting such solution to the action of the positive pole of an electric
curient.
10. The process of preparing cyanide solution for the purpose of lixiviating the
oles of precious metals with a view to extracting the metal from the ore, which
consists in adding to such evanide solution a substantially neutral substance
containing a permanent excess of oxygen
11. The process of preparing cyanide solution for the purpose of lixiviating the
ores of precious metals with a view to extracting the metal from the ore, which
consists in adding to such cyanide solution a chemical substance prepared by
agitating ether with bioxide of barium, adding gradually thereto small quantities
of dilute acid and neutralizing by silicate of soda
12. The process of preparing a cyamide solution for the purpose of lavinting
the ores of precious metals with a view to extracting the metal from the ore,
which consists in adding to such cyanide solution a chemical substance prepared
by mixing peroxide of sodium with dilute acid and neutralizing by silicate of soda
Dated this Fourteenth day of December 1894.
MIDDLETON CRAWFORD.