Improvements in and relating to the Extraction of Silver from its Ores.
Year: 1895

page_text: Date of Application, 25th June, 1895-Accepted, 27th July, 1895
COMPLETE SPECIFICATION.
Improvements in and relating to the Extraction of Silver from
its Ores.
I, JOHN JEREMIAH CROOKE, of No. 40 West 46th Street, New York City,
in the County and State of New York, United States of America, Chemist and
Metallurgist, do hereby declare the nature of this invention and in what manner
the same is to be performed, to be particularly described and ascertained in and
by the following statement :-
The present invention relates to the treatment of argentiferous ores and mattes
for the extraction of silver and any small percentage of gold contained therein, and
it may be regarded as an improvement on the well known Augustin process, which is
based on the conversion of the sulphides of silver to chlorides, by roasting the ores
with an alkaline chloride, chloride of sodium being generally used for this purpose.
The ores in the Augustin process, after roasting sweet, are leached with a hot
saturated solution of chloride of sodium and water, which dissolves the chlorides of
silver, and the silver is then recovered from the filtered solution by exposure to
metallic copper.
Although this process has been used for the extraction of silver from mattes and
certain varieties of ores, its application as a commercial method has been for serious
reasons very limited, some of the reasons being as follows:
The prolonged roasting necessary to convert all the contained silver to chloride
causes a loss of silver amounting to from ten to twelve per cent. Silver-bearing
ores are also generally associated with base metallic elements, especially lead and
iron, which are converted to a considerable extent, by the prolonged and intense
heat of the roasting operation, to chlorides which load and foul the solution, and
render it ineffective to dissolve the chloride of silver. These base metallic
compounds are also continually passing from a soluble to an insoluble condition and
are deposited upon the copper with the silver and gold, thus seriously interfering
with the recovery of the precious metals. These defects, especially the loss of
silver in the roast, have prevented the economic use of this process, particularly for
the treatment of the various diverse argentiferous ores which largely constitute the
sources of silver in this country.
It is one of the objects of the present invention to remedy these defects in the
above described process and adapt it to the treatment of all argentiferous ores and
mattes, as well as all argentiferous by-products, even when such ores and products
are largely associated with lead and other base metals; and to this end, I have
especially directed my attention to the improvement of the composition and
I am
management of the leaching solution, and have perfected the same so that I
able to materially diminish the heat and duration of the roasting step of the
process and thereby prevent the great loss of silver and gold and easily reduce
these metals to a soluble condition and recover thein from the solution, without
appreciable loss, by the action of metallic copper and iron, or other suitable
agents.
To accomplish this object I prefer to prepare the leaching solution as follows:
Chloride of sodium is dissolved in water to saturation; to every one thousand
gallons of this solution I add thirty pounds of a solution of nitrate of copper,
preferably of a specific gravity indicating 100 degrees by the ordinary salinometer,

page_text: or about 25 degrees by Baume's hydrometer, the brine solution and the nitrate of
copper solution being both preferably brought to same density.
I now add to every three thousand gallons of this solution twenty pounds of
sulphuric acid. This amount of acid will not precipitate the lead as sulphate in
the tailings, but will prevent the iron from becoming basic. The presence of the
nitrate of copper and sulphuric acid in their re-actions on the chloride of sodium
in the solution will furnish the necessary chlorine to keep the chloride of lead
soluble in the menstruum as plumbic chloride. This salt will pass the filter, and
will not be detained by metallic copper, if used, nor contaminate the silver and
gold. The lead can be removed from the filtrate by subsequent treatment more
fully hereinafter described.
The presence of nitrate of copper in conjunction with the chloride of sodium and
sulphuric acid in the solution is of the utmost importance, especially as affecting
the degree and manner of roasting the ores. The liberation in the solution of one
of the most powerful oxidizing agents known to science, the nitric peroxide or
peroxide of nitrogen, permits a superficial chlorination of the silver in the roasting
furnace. The heat need never be brought to redness. It should never even
approach the volatilizing point of chloride of silver, and the ores can be passed
through the furnace with the utmost rapidity. The nitric peroxide in the solution
will remedy all defects of furnace chlorination, and bring the silver and gold into
the condition of soluble chlorides without appreciable loss.
It is also the object of my invention to provide improved means for recovering
the silver and gold from the solution by passing it directly from the leaching tank
filter, through a series of filters or perforated holders carrying metallic copper
(preferably cement copper), which filters are arranged one above the other, so that
the filtrate from each upper filter will be discharged upon the next lower one of
the series, the filters being also removably and interchangeably arranged upon
their supporting frame or rack for the purposes hereinafter set forth. When the
solution carries gold the copper cement in the filter is supplemented with a few
strips of soft sheet iron. The iron will, when in contact with the solution, give off
hydrogen which, combining with the chlorine of the gold to form hydrochloric
acid, will allow the gold to precipitate with the silver in a metallic condition.
In the accompanying drawing I have shown a series of filters arranged in proper
relation to a leaching tank and a precipitating tank, the parts being shown in
vertical cross-section, in which
A represents a leaching tank in shape of a truncated cone, preferably made
steam and gas tight, and provided with a manhole a, and a pipe h for the admission
of steam, of a capacity of about 15 tons of roasted ores and an equal weight of
leaching solution, the tank being provided with a filter A¹ of any suitable con-
struction. B is a series of filters preferably four or more in number arranged
one above the other and in such relation to the leaching tank A that the solution
as it passes the filter A¹ of the leaching tank can be delivered directly upon the
filter of the series. The filters B may be of cheap construction, consisting of a
square or rectangular frame C, and stout canvas D well secured at its edges to
the frame, but sufficiently slack to permit it to bag down to form a receptacle for
the cement copper E. They are supported upon a rack F provided with brackets G
on which they can be readily slid to place them in working position or to remove
them for taking off the silver and gold or for cleaning and re-charging. About
forty pounds of cement copper and a small quantity of metallic iron should be
loading upon each filter, and the charge should be renewed as frequently as the
richness of the ores in silver and gold require. As a very large proportion of the
silver and gold in the solution will be deposited upon the upper filter of the series B,
this filter may be removed and the next lower filter, which is the next richest in
precious metals be substituted for it, all of the lower filters being moved up one
step, whereby the copper and iron are gradually replaced by the precious metals,
until the deposit of these metals upon the upper filter is sufficiently thick to be
readily removed as a coherent sheet of nearly pure metal for melting or casting

page_text: into bars. H is the exit of the leaching tank to which is attached a flexible hose
as a convenient means for closing and opening the exit. J is a receiving and
precipitating tank which is located directly beneath the series of filters B to
receive the solution as it passes the lower filter.
The preferred method of carrying out my process by the use of the means
described is as follows:
Raw ores, pulverized to from 40 to 50 mesh, are mixed with chloride of sodium,
the percentage depending upon the richness of the ores in silver, but from four to
eight per cent. of chloride of sodium to the weight of the ore is generally sufficiently
to convert the silver and the small quantity of gold in the ores to chlorides. This
mixture is calcined or roasted in any furnace adapted for the purpose, though I
prefer to use a furnace of the reverberatory type, inasmuch as it is more easily
controlled in the application of heat. As this process possesses the novelty of
requiring only a very low temperature for roasting or calcining the ores with
chloride of sodium, is absolutely necessary to have the heat under perfect control.
A forty foot furnace is capable of preparing twenty tons every twenty-four hours.
The roasted charge as it is drawn froin the furnace is preferably heaped upon a
cooling floor where it should be allowed to remain undisturbed until its temperature
has fallen to about 150 degrees Fahrenheit, and is then trammed to the leaching
tanks, where it is treated with an equal weight of a hot aqueous solution of chloride
of sodium, nitrate of copper and sulphuric acid, in the proportions heretofore
mentioned. About three thousand gallons of this solution is sufficient to treat
fifteen tons of the prepared and roasted ore. The solution, when laundered into
the lixiviation tank containing about fifteen tons of prepared ore, to cover the
charge and should be about twelve inches about its surface. I prefer that the
temperature of the solution should be about 170 degrees Fahrenheit, and that it
should stand on the charge about one hour. At the end of this time the hose
beneath the diaphragm of the tank is lowered, and the solution is discharged upon
the upper
filter of the series of filters D constructed as hereinbefore set forth,
containing cement copper and a few strips of iron (if the solution contains gold),
the filtrate passing through the entire series of filters from top to bottom with a
final delivery into the tank beneath. If the solution is properly prepared, all the
silver and gold will be deposited upon the cement copper and iron on the filters in
a nearly pure metallic condition, and the metal upon the upper filter will form a
sufficiently thick coherent coating to permit it to be readily rolled off from the
copper. As herein before described, the filters are made interchangeable in position
so that each may be enriched to a point where the coating of silver and gold can
be easily removed, and this also enables the enriched filters to be removed and
others substituted. The filtrate, after it has passed through the series of filters
and has been freed of its silver and gold, as above described, is delivered into the
receiving tank, which contains metallic iron, by which the copper (which has been
replaced by the silver and gold) is recovered, and can again be placed upon the
filters for re-use.
The iron employed for the precipitation of the copper, as also that derived from
the ores, gradually loads the solution with ferrous salts, but if the solution be kept
in an acid condition, it can be re-used an indefinite number of times without
materially affecting its efficiency to dissolve silver and gold chlorides.
Whenever the solution becomes loaded with sulphate of soda, iron salts, or
chloride of lead, to a degree to materially affect its efficiency as indicated by a
gravity of from 120 to 140 degrees brine test, these salts, or any of them, may be
removed by any means adapted to accomplish this result.
It is apparent that the practical value of the above described process largely
resides in keeping the solution in a state of permanent power to fully dissolve and
retain all the silver and gold as chlorides until these metals are recovered in their
metallic state. The method of treating the solution as detailed above will
effectually maintain its power for an indefinite period of time, and by the
practice of the process in a large way, and as above described, I have succeeded in

page_text: extracting more than 99 per cent. of the assay value of silver and gold in sulphide
ores and furnace mattes.
I do not claim that this process is applicable to ores very rich in gold, but that
it is especially advantageous to ores rich in silver, which generally carry from
one-tenth to one-half ounce in gold, and when such ores are treated in charges of
from seven to fifteen tons, from 99 per cent. to clean of all the silver and gold can
be extracted.
I do not desire to limit my invention to the use of any special form or con-
struction of roasting furnace, or lixiviating tanks or filters, as all these can be
varied materially from those described and shown in the drawings, without
departing from the principles of my invention, as is evident to those skilled in the
art of working silver ores; nor do I wish to limit the invention to the use of the
proportions of the ingredients set forth herein, as I am aware that good commercial
results can be obtained when the preferred proportions are materially changed.
Neither do I wish to limit my invention to the use of metallic copper for recovering
the silver from the solution, or metallic iron for recovering the gold therefrom,
as both the silver and gold can be recovered by any other means adapted for
this purpose.
I consider that a novel and important feature of my invention consists in the use
of nitrate of copper in the lixiviating solution, which acts with the chloride of
sodium and sulphuric acid of the solution to convert any metallic silver or gold
which have passed the roasting furnace into soluble chlorides, and thus makes it
practicable to greatly diminish the degree of heat and the length of time heretofore
employed in roasting the ores and the attendant loss of the silver and gold, and at
the same time to fully convert these metals into soluble chlorides.
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:-
1. The herein described process of extracting silver from its ores or mattes,
which consists in roasting the ores or mattes with chloride of sodium, treating the
roasted mass with a hot aqueous solution containing chloride of sodium, nitrate of
copper and sulphuric acid, and recovering the silver from the solution.
2. The herein described process of extracting silver from its ores, which consists
in roasting the ores with chloride of sodium, leaching the roasted mass with a hot
aqueous solution containing chloride of sodium, nitrate of copper and sulphuric
acid, and exposing the solution to the action of metallic copper, whereby the silver
contained in the ores is converted into a chloride, is dissolved by the solution and is
deposited upon the copper.
3. The herein described process of extracting silver from ores containing lead,
which consists in roasting the ores with chloride of sodium, leaching the roasted
mass with a hot aqueous solution containing chloride of sodium, nitrate of copper
and sulphuric acid, the quantity of the latter not being sufficient to precipitate the
lead as sulphate, whereby the lead will be converted into chloride and pass the filter
in that condition, and exposing the solution to the action of metallic copper to
deposit the silver therefrom.
4. The herein described process of extracting silver from ores containing lead,
which consists in roasting the ores with chloride of sodium, leaching the roasted
mass with a hot aqueous solution containing chloride of sodium, nitrate of copper
and sulphuric acid, the quantity of the latter not being sufficient to precipitate the
lead as sulphate, whereby the lead will be converted into chloride and pass the filter
in that condition, and exposing the solution to the action of metallic copper to
deposit the silver therefrom, and afterwards precipitating the lead from the solution
to prepare it for re-use.
5. In an apparatus for extracting silver from its chloride solution, a series of
filters or holders carrying metallic copper removably and interchangeably arranged
one above the other, in connection with a leaching tank and a receiving tank,

page_text: whereby the solution will pass from the leaching tank directly through the series of
filters and be discharged into the receiving tank, and the silver will be deposited
upon the copper in the filters, and the lower filters of the series be easily removed
and advanced upward, as set forth.
Dated this 25th day of June 1895.
HASELTINE, LAKE & Co.,
45 Southampton Buildings, London, W.C., Agents for the Applicant.