Note: Descriptions are shown in the official language in which they were submitted.
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TITI.E: TREATMENT OF ORES
INVENTORS: ARTHUR E. COBURN
ROGER DEAL
This invention relates to the field of the recovery
of metals from pyrite, and more particularly to the
efficient recovery of valuable metals from mine tailings
and their separation into marketable metals in a way
which is both economically and ecologically sound.
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I.arge quantities of solid waste material known as
tailings are generated in mining, concentrating and smelt-
ing ore. Many mines dispose of such tailings by impound-
ing these materials ln settling ponds or basins. Such
natural settling types of systems require large quantities
of water and often involve pumping both the tailings and
the water long distances. The creation of dumps means
problems of possible stream pollution and has negative
effects on farms, real-estate and land values.
It is well known that valuable metals can be
recovered from waste by leaching. Tailings which were
discarded in the past, in some cases may contain metal
values eccnomically recoverable under changed market
conditions. Such tailings may be reground, subjected to
a flotation process and then leached. However a potential
environmental hazard always exists due to the possibility
that the leach solutions in the remaining waste could
enter the ground water system from the storage location.
In addition, many jurisdictions now have laws governing
the composition of waste products which are to be stored
or disposed of.
It has now been found that pyrite may be almost
completely separated into its valuable components, using
a minimal amount of water and leaving a minimal amount of
waste to be disposed of. Furthermore, said waste is in
an ecologically acceptable form. So far as is known, no
prior procedures have been known or utilized in which pyrite
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has been almost compLetely separated into its useful
components economically and with minimal -~ater require~
ments and waste production~
This invention provides an improved closed loop
waste treatment process for the recovery of metals from
pyrite which minimizes the water required for the
recovery and eliminates ecological hazards resulting from
the storage of mine tailings. Because of its continuous
nature, this invention does not require the treatment of
water as the water can be reused due to the fact that
essentially all of the metals are removed.
The present invention provides a process for the
recovery of one or more of the metals gold, silver, copper,
zinc, vanadium, germanium and cobalt from pyrite concen-
trate, in which:
(a) the pyrite concentrate containing one or more
of the metals gold, silver, copper, zinc,
vanadium, germanium and cobalt is mixed with
cyanide solvent so that the metals dissolve
in the solvent;
~b) the solvent and pyrite mixture is subjected to
treatment with reagents in a suitable apparatus to
yield pyrite free of said metals and a cyanide
solvent containing said metals;
(c) the pyrite is separated from the cyanide solvent;
I (d) a first stream of the cyanide solvent is
recycled to step (a);
~ej a second stream of the cyanide solvent is
passed through activated charcoal so that the
gold and some of the silver are adsorbed on
the charcoal leaving the solvent barren in gold;
(f) at least one of the metals remaining in the
solvent is extracted by means of ion e~change
providing cleaned solvent and an eluate of
cyanide waste;
~g) the cyanide waste is neutralized or eliminated;
and
(h~ the solvent from step (f) is recycled to step
(a)-
This invention will be better understood from a
study of the following disclosure in which reference is
directed to the attached drawing.
Figure 1 is a generally diagrammatic flowsheet
illustrating the process of the invention, including
optional steps.
The first step consists in dissolving the metals
which arecontained by the pyrite. The pyrite concentrate
in semi-dry or wet form, is preferably introduced into
a flotation cell containing a cyanide solvent. Any
cyanide compound such as potassium cyanide which releases
cyanide ions when added to water may be used in a~ueous
solution as the solvent. The mixture is agitated and then
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the entire mixture is separated into solids and a liquid ln
order to separate the now barren pyrite from the cyanide
solvent which contains the dissolved metals. The separa-
tion or so-called dewatering may be accomplished by using
conventional devices and processes such as cyclones,
gravity settling and overflow, plate separators, tube
separators or filters. The barren pyrite may be stock-
piled to allow the trace cyanide residues to decompose and
may be later roasted to provide sulphur dioxide gas and
iron oxide ore.
It is to be noted that conventionally cyanidation
of ores is accomplished by pumping the ore and acqueous
reagents into tanks with impellers for prolonged contact.
Although such a method could be used in the present inven-
tion, it has been found that the utilization of a flota-
tion cell to effect contact of the pyrite concentrate
with the cyanide allows for more rapid removal of the
valuable metals with improved efficiency. It is emphasized
that such cells are not utilized in this process step for
conventional froth flotation~
The pregnant cyanide solvent is transferred to a primary
holding tank. From this holding tank a first stream of
the cyanide solvent is circulated back into the flotation
cell for contact with fresh metal-bearing pyrite concen-
trate. Thus, the quantity of metal in the cyanidesolvent is increased resulting in a more concentrated
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solution thus producing higher yield of metals per solvent
processed. Furthermore, the recirculation of the solvent
decreases the quantity of solvent required. It should be
noted, however, that loss of solvent may occur in the
removal of the now barren pyrite and accordingly the water
and chemicals may have to be adjusted in the primary holding
tank. The preferred cyanide solvent is an aqueous solution
of sodium cyanide, sodium carbonate and hydrogen peroxide
and the preferred composition to be maintained is 2~ sodium
cyanide, 2% sodium carbonate and 0.6% hydrogen peroxide,
by weight, in aqueous solution. Furthermore, the preferred
ratio cf pyrite to solvent in the slurry is 65:35. So far
as is known, the preferred composition of the solvent used
in the present invention is novel as is the use of hydro-
gen peroxide as an oxidant in cyanidation.
An optional concurrent step consists of processinga side stream of the cyanide solvent from the primary
holding tank by ion exchange to remove calcium and magnesium.
This step is performed in order to avoid interference
which may occur later in the process when there is a con-
siderable amount of calcium andjor magnesium ~uild-up in the
water used. The necessity of performing this step will
vary with the degree of hardness of the water involved
and usually this step will not be required in small batches.
Conventional equipment and method may be used for this
process step. For example, the equipment may consist of
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a hydrogen cation~exchan~er unit containing a cation-
exchange resin in bead form, a dilute acid tank and a
strong acid tank. Cations of calcium and magnesium are
removed by exchanging hydrogen cations for them. Regen-
eration is effected with a dilute acid such as sulfuric.
After being passed through the ion exchange equipment,
the cyanide solvent is returned to the primary holding tank.
An essential step, which is performed concuxrently
with the recirculation of the cyanide solvent from the
primary holding tank to the flotation cell, is the
transporting of the main stream of the cyanide solvent
for the purpose of removing the dissolved valuable metals
from the solvent. The major step in this process involves
passing the cyanide solvent through activated charcoal
according to conventional methods. Any commercial grade
of activated charcoal may be used. The charcoal adsorbs
the gold and silver cyanide compounds present in the
solvent. When the charcoal's adsorption capacity is
reached, the gold and silver are removed from the charcoal,
for example by elution with an ethanolic solution of sodium
hydroxide.
After being passed through the activated charcoal,
the cyanide solvent may be subjected to further optional
processes for the recovery of metals such as copper and
zinc and additional silver.
Alternatively, any silver still retained by the
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cyanide solvent may be removed by introducing the solvent
into a tank. The addition of a soluhle chloride salt
precipitates the silver ion as silver chloride The
cyanide solvent may then be passed on to the next step.
Metals including copper and ~inc may be removed from
the cyanide solvent by passing the cyanide solvent through
an electrolytic cell.
Following the above two optional steps or immediately
upon the exit of the cyanide solvent from the activated
charcoal, the solvent may be stored in a secondary holding
tank. Concurrent streams of the solvent, now barren of
gold, silver and most metals, lead out of this secondary
tank. The first stream returns the solvent to the primary
holding tank thereby closing the loop. The second stream
lS is passed through an ion exchange unit for removal of trace
quantities of any metals, such as vanadium, germanium and
cobalt, remaining in the solvent. The solvent which passes
through the ion exchange unit is returned to the primary
holding tank for reuse.
The recovery of the trace metals by ion exchange
results in the generation of a small quantity of cyanide
waste which is destroyed or neutralized by conventional
processes for environmentally safe discharge. For example,
the pH of the cyanide waste may be increased and hydrogen
peroxide added to produce carbon dioxide and nitrogen gas.
It is anticipated that this invention will be used
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primarily to recover metals remaining in tailings of
mineral dressing and similar processing of ores. Accord-
ing]y, the feed pyrite for the process is not likely to
consist of pyrite concentrate but rather of raw tailings
containing pyrite. Under such circumstances additional
steps, priorto the mixing of the pyrite with the cyanide
solvent, are necessary for efficient use of the process.
These steps are identified as Stage 0 in Figure 1. In
the first step of Stage 0, the tailings containing pyrite
are screened and/or ground, formed into a slurry in water
and introduced into a gravity separation device such as a
sluice box. The preferred yravity separation device is
a moving bed sluice device. The device separates the
heavy mineral pyrite fraction from the liqhter silica
and granite. The silica and granite are discarded while
the pyrite fraction, still in slurry form, is passed to a
concentrating or separation table. The concentrating
table effects separation of the pyrite from the heavier
mineral forms such as cassiterite and galena. The pyrite
is then dewatered using a conventional device such as a
cyclone with the water being cycled back to the gravity
separation device for reuse with new feed. The pyrite
which remains is relatively pure and is in moist form,
ready to proceed to Step 1. This feed material usually
has been ground to less than 60 mesh (Tyler standard scale~.
~ The invention will be better understood by reference
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to the follo~7ing example.
Example:
The feed material used was ten pounds of copper
tailings, that is the waste residue remaining after the
copper minerals have been removed from ore. The tailings
were less than 80 mesh (Tyler standard scale) in size.
These tailings were formed into a 50~ water slurry and
passed through a moving bed sluice device. One pound of
pyrite was collected and the remainder of the material
discarded.
The one pound of pyrite, still in an approximately
50% water slurry, was passed over a separation table to
remove any lead or tin bearing minerals present. No such
minerals were present in this run.
The pyrite was then dewatered by leaving the pyrite
sl~lrry in a beaker for a period of five minutes to allow
natural settling of the pyrite fraction and decantation of
the water fraction. The water decanted was reserved to
be used in forming a subsequent batch of tailings into a
water slurry.
One pound of wet pyrite remained in the beaker.
This pyrite was added to a flotation cell containing 500 ml
of an aqueous solution consisting of 2% sodium cyanide,
2% sodium carbonate and 0.6% hydrogen peroxide by weight.
The material in the flotation cell was agitated for five
~ minutes after which the entire mixture was transferred into
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a beaker and allowed to settle. After several minutes,
the solution containing the dissolved metals was decanted
off frorn the pyrite.
Thls solution was then pas~ed through a column of
16 mesh activated charcoal and the raffinate
retained for further extraction. The column was
subsequently eluted with an ethanolic solution of sodium
hydroxide. An assay of thQ eluate showed a recovery
of silver and gold corresponding to 1.02 oz./T and 0.04 oz./T
respectively, when extrapolated to ton quantities.
The raffinate from the previous step, was then
passed through an ion exchange column containing IRA-400
(Rohm & Haas) resin and subsequently returned to the
flotation cell. The column was eluted with 2N NaCN
solution. The eluate, when analysed, was found to
contain 87 ppm of copper as well as traces of zinc,
cobalt and nickel.
