Note: Descriptions are shown in the official language in which they were submitted.
I
-- 2
FIELD OF THE INVENTION
This invention relates to the treatment of metallurgical
residues and the recovery of valuable components therefrom in
a manner which is both faster and ecologically safer than
conventional processes. More particularly, this invention
describes a close-loop hydrometallurgical process for the recovery
of valuable components such as gold, silver and selenium by
cyanidation from residues such as anode mud, anode slime or
tailings.
BACKGROUND OF THE INVENTION
.
During the refining of metals, such as copper and other
non-ferrous metals, the electrolytic process produces a residue
known as anode mud or anode slime. The mud (slime) typically
contains valuable elements such as gold, silver and selenium.
In addition, there are commonly practiced hydrometallurgical
extracting processes where the leaching of base metals leads to
precious metal bearing residues which can be further treated for
gold and silver recovery. These metallurgical residues are
in some instances further treatable with acid leaching in the
presence of very strong oxidizing agents to recover the valuable
metals. This however, inevitably leads -to dissolution of nearly
all the metals and hence to expensive separation processes.
A further complication is that a separate process is required
-to recover the selenium contained in the residue. Tailings
from mineral processes could also bear precious metals which are
economically recoverable.
Lo
DISCUSSION OF PRIOR AT
A well known process for the recovery ox gold and silver prom
residues is a cyanidation process which is usually carried out
in mildly alkaline solutions to avoid the escape of toxic
hydrogen cyanide. Such a process is taught in U.S. 702,305
wherein -the cyanidation of precious metals is conducted in the
presence of potassium carbonate but without the addition of any
oxidant. In this process no consideration is given to the
toxic nature of the effluent. Another process taught by U.S.
718,633 describes the cyanide leaching of gold and silver in the
presence of hydrate of calcium and carbon dioxide; and with
compressed air to oxidize the metals to lead to soluble cyanide
complex formation. U.S. 1,198,011 teaches eyanidation conducted
in a special vessel where compressed air is introduced for
oxidation, and above atmospheric pressure is maintained in the
vessel. U.S. 4,~01,46~ teaches another cyanidation process
for the precious metals contained in sulphidic and oxidic ores
and residues in the presence of lime or sodium hydroxide and
carried out in special apparatus wherein the slurry is subjected
to compressed air, in a vessel which is equipped with a nozzle.
The vessel taught has a recycling unit preceding the liquid
solid separation of the process to prolong the action of
compressed air. There is no attempt in this process to reuse
the cyanide after the complexes precious metals have been removed,
nor to avoid contamination of the environment by the toxic reagents
in the effluent.
~3~7
SUMMARY OF INVENrrION
An improved method has now been found for extracting gold,
silver together with selenium, from metallurgical residues such
as anode mud or slime, precious metal bearing leach residues and
tailings, by solubilizing in cyanide the valuable metals contained
therein in an alkaline solution stabilized with alkali carbonate,
and with controlled oxidation with hydrogen peroxide, and
wherein the cyanidation is accomplished in an agitated vessel.
The pregnant solution resulting from the cyanidation is when
separated from the insoluble solids, and the gold, silver and
selenium are recovered while the cyanide reagent is regenerated.
The cyanide solution is subsequently cleaned by known methods,
brought up to the desired cyanide and carbonate concentration and
returned to leach fresh residue.
By another aspect of the invention -the rate of completing
by cyanide is enhanced thereby diminishing the attendant oxidation
of the lea chant and allowing repeated use of the cyanidation
reagent by recycling.
Another aspect of the invention is to separate the precious
metal components from base metals contained in metallurgical
residues and to return the hose metals for further processing.
Yet another aspect of this invention is to produce high purity
selenium for use in the electronics industry.
This invention will be better understood from the following
disclosure and the attached drawings.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a diagrammatic flow sheet describing the steps
of the close loop cyanidation process of the invention.
Figure 2 shows a minor variation of the same process.
I
-- 5
RETAILED DESCRIPTION OF PREFERRED EMBODIMENT
.
Cyanidation or the extraction of gold, silver and other
metals has been a long practiced process within the metallurgical
industry. The advantages of preferentially solubilizing gold and
silver and thereby separating the valuable metals from the base
metals are, however, considerably diminished by the toxic effects
of the reagents and the costs of avoiding damage to the
environment.
The chemical reactions in cyanidation of precious metals
are summarized as follows:
2 A + 4NaCN + 1/2 2 + H20 - Nixon + Noah (1)
2 A + 4NaCN + 1/2 2 Jo H20~ 2NaAu(CN)2 + Noah (2)
Base metals which accompany the gold and silver, also react
with cyanide; these are however, precipitated by the excess
of this reagent.
Selenium will also be solubilized by cyanide according to
the reaction described in various -textbooks:
So -I Nan ~NaSeCN I
This is only a broad indication of the reaction and the exact
mechanism is not yet known.
Under oxidizing conditions the aqueous cyanide will become
Senate, described by the reaction;
Nan 1/2 2 ~NaCN0 (4)
Senate is less harmful and toxic but is not capable of solubilizing
precious metals.
The conventional cyanidation processes are carried out without
added oxidants and with little agitation, in mildly alkali
solutions to reduce toxicity, and for prolonged reaction periods,
such as 20 - 30 hours. In addition, after the dissolved silver
I
6 --
and gold are recovered from the pregnant solution, the reagents
in the solution are too dilute to be reused and their elimination
from the effluent adds to the cost of the cyanidation process. A
process which would syllables only gold and silver and selenium,
without oxidizing the reagents, hence allowing the latter to be
reused without discharging in the effluent, and which could be
carried out at a rate which is economical, would enable the
metallurgical industry to utilize the advantages of cyanidation
without its inherent disadvantages.
According to the present invention, by controlling the conditions
of cyanidation, a fast method of extracting gold, silver and
selenium has been found which also allows the recycling and
reuse of the reagent, thus avoiding harmful effluents. Furthermore,
all the cyanide that is separated as precipitate, is either
destroyed or regenerated for further use.
The process described below can be employed to extract gold
silver, selenium from precious metal bearing metallurgical residues;
such as anode mud (anode slime) which collects as very fine
deposit during the electrolytic refining of copper, nickel and
cobalt, or similar metals. Residues bearing gold, silver and
selenium which result from leaching operations, can also be
treated by this method. Gold, silver and any selenium present
in various flotation tailings will also respond to the present
treatment. -it should be clear to those familiar with the art,
that the specific reagent ratios and the amounts of oxidant required
in the process described hereinbelow will be dictated by the
nature and origin of the residue, the amount of base metals
accompanying the gold, silver and selenium, and other economic
and marketing considerations.
.
_ 7 _ ~3~77
The process is diagrammatically represented in Figure 1, and
the steps comprising it will be described in detail in the
following sections.
A) The residue is reacted with alkali cyanide and alkali
carbonate, in an agitated container and with oxidation, with
an oxidant solution added in the requisite amounts.
The stoichiometric requirements of alkali cyanide for the
leaching of silver, gold and selenium contained depend on the
nature of the residue, as discussed above, and is also
dependent upon the feed rate and on the pulp density. At
least 20~ above stoichiometric requirements is maintained to
ensure constant excess and allow for feed metal concentration
variations. The amount of alkali carbonate required is also
dictated by the nature of the ore; in general it should be
sufficient to maintain the pi of the slurry in excess of 10.
The preferred alkalinity is around a pi 11, and generally a
10% carbonate solution strength has been found satisfactory in
the process.
The oxidant is added in the form of aqueous solutions,
such as 35% hydrogen peroxide. The rate of addition of the
oxidant relative to the concentration in the leach liquor is
very important, and is critical to the successful extraction,
and utilizing of the cyanide. Hydrogen peroxide is destroyed
Go consumed by cyanide. Therefore -the addition must be
on a minimal yet continuous basis to ensure that the bulk of the
atomic oxygen enters directly into the desired cyanidation
reaction. The solution concentration utilized in the present
process is generally in the range of .9 to 1.2% for corresponding
pulp densities of 25 - 50%, It should be understood however,
that this again depends on the nature of the residue to be
extracted.
~3S5~7
The residue,slurried in the solution bearing the above
reagents and the oxidant is reacted in an agitated reactor.
Flotation cells, such as a Denver 18 or a Denver used without
any froth removal, were found to be most useful for the leaching
but any other apparatus operating on the same principle can be
used. Under such conditions gold, silver and selenium are
solubilized to a very high degree, within 10 - 20 minutes.
Market considerations will dictate the limit of precious metal
extraction for a given residue, but to achieve the maximum
economic extraction with the least oxidation of the reagent, a
second extraction period with fresh reagent in a second cycle,
may be beneficial. It was found that leaching gold, silver and
selenium in two 10-minute reaction periods with agitation and
fresh reagents, will produce close to 99% extraction of the
available value metals; this was found to preferential to one
20-minute leaching period. The exact condition for achieving
optimum results have to be decided for each metallurgical residue
to be handled.
The leach steps can be carried out continuously or in batch
manner. In the continuous mode the residue, and lea chant are
fed into the reactor at a fixed rate. The oxidant is added to
the system at the point of contact between lea chant and feed.
This reagent is wholly consumed and hence it is continuously
added at a fixed rate during the leach step.
B) The leaching step is followed by a solid-liquid separation
step. The slurry of the pregnant solution and the non-leachable
solids from the reactor, is passed through a filtration unit.
The solid products usually consist of the primary base metal
constituents present in the feed and include residuals of gold,
silver, selenium and the platinum group metals; if the residue
treated originated as anode mud. Silicates, sulfides, barren
rock constituents, and similar compounds which are not leachable
by cyanide will also be retained in the solids. These solids
are washed and the wash water treated separately to extract
valuable metals contained, and to regenerate cyanide. The
separated solids are usually treated by conventional pyrometallurgical
processes for base metal recovery. If the residue treated
originates as tailings, the washed solids can be discarded.
C) The pregnant solution bearing the valuable components
is then passed to the recovery step In the embodiment shown
in Figure 1, the recovery from clarified pregnant solution is
carried out in one or more electrolytic cells ion series The
gold, silver and selenium are reduced to the zero oxidation state
at the cathode, to be subsequently recovered either singularly
or as a conglomerate product for further refining and processing.
In the electrolytic reduction, the cyanide ions forming the
metallic complexes are released at the anode, thus are regenerated
for reuse in subsequent leaching. The regenerated cyanide bearing
spent electrolyte is removed from the cell, usually by
means of an overflow.
In another embodiment of this process shown in Figure 2, the
recovery of the valuable metals and the regeneration of the
cyanide reagent are conducted in an ion-exchange recovery unit.
D) With the bulk of the gold, silver and selenium values
removed, the barren solvent passes through a polishing ion
exchange system where trace metal residues, solvent degradation
products and soluble heavy metal complexes are removed. The ion
exchange system that can be usefully employed in this step may
I be a combination of fixed-bed and moving bed arrangement, as
- 1 o - I
required. The trace metal residuals could be combined with a
metallic product from the electrolytic step or treated separately,
as dictated by the composition and volumes. The degradation waste
products are neutralized with regards to toxicity and then
discarded. These are usually small volume waste residues.
A commercially available strong-base ion exchange resin may
work satisfactorily in this step.
In cases of relatively low residual and heavy metal
concentrations in the regenerated cyanide solutions, only a bleed
is taken through the polishing ion exchange system. Such
modification is shown in Figure 2.
The wash water of the non-leachable solids from the filtration
step, may also be treated in this polishing inactions unit.
E) The totally barren and now cleaned solution from the
polishing ion-exchange unit of the previous step, is made up to
original strength by the addition of the appropriate chemicals,
for completing the loop of the cyanidation cycle described.
Chemical replenishment or addition is conducted in a mixing tank
equipped with the appropriate analytical sensing devices. This
tank will also serve as a buffer container for fluctuations or
flow rate changes in the entire cycle. The rejuvenated solution
is then passed to step A, to leach fresh residue.
EXAMPLE 1
A commercially available sample of anode mud containing
250 oz./T gold and 11,750 oz./T silver, along with copper,
selenium, lead, silicon together with the usual impurities, was
slurries in a solution containing about 6.7 White sodium cyanide
and 3.4 White sodium carbonate.
I
-- 11 --
The anode mud slurry was mixed in a ratio of 1 ton of mud to
200 gallons of solution. The pit of the slurry was ho A solution
containing 35% hydrogen peroxide was added to this slurry in an
amount sufficient to maintain an average of 0.12% hydrogen peroxide
concentration, based on the total volume of the solution. The
leaching was carried out in a Denver 18 flotation cell without
froth removal, and continuously agitated during the 10 minutes
residence time. The slurry was then removed, filtered and the
solids, consisting mainly of base metal oxides and precipitated
cyanides, were washed and treated for base metal recovery in a
conventional pyrometallurgical manner. Filtration was carried
out with ease, without slime formation, followed by washing of
the solids.
The pregnant solution from the solid-liquid separation was
passed through a conventional electrolytic cell wherein the gold,
silver and selenium were deposited at the cathode and the cyanide
was regenerated on a large surface anode. The flow-rate, as
those familiar with the art will know, was adjusted to the
geometry of the cell, the total anode surface, and the cyanide
concentration The deposited metal and selenium were collected
for purification in a conventional manner.
The regenerated cyanide solution was passed through an ion
exchange system where the soluble copper and iron cyanide complexes
were removed together with the residual gold, and silver cyanides
which had not been precipitated by the preceding electrolysis.
The metal complexes in the so-called polishing step were collected
at intervals and subsequently treated for separate metal and
cyanide recovery.
Cyanide complexes contained in the wash water from the solid
filtration step were also treated separately in the polishing
ion-exchange columns.
The cleaned solution was then passed to a vessel where its
concentration with respect to sodium cyanide and sodium carbonate
was adjusted to the level in the cyanide leach liquor and then
passed to the Denver cell to leach fresh anode mud.
The recovery of precious metals and selenium in this simple
circuit was 70% for silver, 38% for gold and in the order of
40~ for selenium.
EXAMPLE 2
.
The solids from the liquid solid separation of Example 1
following the leaching of the anode mud, were passed into another
Denver 18 flotation cell without froth collection, for a
second leach with the rejuvenated solution of Example 1. 35~
hydrogen peroxide was again added to give a 0.1% Loveland the
solid content of the slurry was adjusted to have similar
ratio to that of Example 1. This slurry was leached with
agitation and in a residence time of 10 minutes.
The filtration steps and the subsequent process steps were
-the same as Example 1. In the present example, only a bleed
of approximately 1/3 of the total volume of the regenerated cyanide
solution was passing through the polishing ion-exchange resin.
The regenerated cyanide solutions were mixed together and made
up to contain the required cyanide and carbonate levels for
subsequent use in leaching fresh mud.
I
- 13 --
The electrolytic products of both Examples 1 and 2
were combined and analyzed, giving a total recovery of gold and
silver in excess of 99% based on the initial gold and silver
contents of the anode mud The total selenium recovery was
around 50%. The gold, silver and selenium were further separated
and purified in the conventional manner.
Our cyanidation process is described above in such terms
as to enable those skilled in the art to which it pertains to
understand and practice it.
