Note: Descriptions are shown in the official language in which they were submitted.
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TREATMENT OF RECLAIM WATER FOR USE IN METALS RECOVERY
FIELD OF THE INVENTION
[0001] This invention generally relates to gold recovery
and the treatment and use of reclaim water in gold-bearing
recovery circuits having carbon-in-leach or carbon-in-pulp
operations.
BACKGROUND OF THE INVENTION
[0002] Gold is frequently recovered from gold-bearing
materials by leaching the gold into a leachate solution, for
example, by contacting the gold-bearing material with a
lixiviant. One method conventionally employed for recovering
gold from gold-bearing materials is leaching with cyanide and
adsorption of gold-cyanide onto activated carbon.
[0003] In modern cyanidation circuits, the dissolved
gold is typically adsorbed onto particles of activated carbon,
either during a cyanide leach itself by carbon-in-leach (CIL)
or following the leach by carbon-in-pulp (CIP). An alternate
method of recovering gold from cyanide leach solutions is
through zinc cementation and variations of the Merrill-Crowe
process.
[0004] While cyanide leaching has been proven over
decades as effective and environmentally safe when practiced
under accepted handling and destruction procedures, millions
of dollars of gold is still lost to "preg robbing." In
carbonaceous ores, preg robbing occurs as active carbon
indigenous to the ore has the ability to rob gold from the
cyanide bearing leach solution, reducing recovery.
[0005] Pressure oxidation, as described by Thomas et al.
(U.S. Pat. Nos. 5,074,477 and 5,785,736) and incorporated in
their entirety herein by reference, can partially deactivate
the indigenous carbon. Much of the carbonaceous ore is
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unaffected, however, allowing it to adsorb gold from cyanide
solutions. Also, CIL has been successful for mildly preg
robbing ores, as the activated carbon that is added to the
slurry possesses adsorption kinetic characteristics superior
to those of the indigenous carbon, allowing the gold to load
onto the added carbon as soon as it is leached and before it
can load onto the carbon in the ore. Other methods of
reducing the preg robbing effect include destroying the preg
robbing carbon with a roaster or with chlorine, using an
alternative lixiviant such as thiosulfate, using ion exchange
resins to load the gold, and oxidizing extremely fine ore
under extreme oxidizing conditions in a low chloride
environment.
[0006] To initially form the gold-bearing slurry, the
gold-bearing material is crushed and wet milled prior to a
liquid/solid separation stage using liquid recovered from the
system. This liquid typically comprises so-called "make-up"
or "recovery" water and is also utilized, e.g., in post-
autoclaving cooling and neutralization. The liquid, which is
held in a tailings pond until such use, further comprises a
quantity of cyanide (CN-) and thiocyanate (SCN-) ions. As used
in the art and throughout this application, the liquid going
into, held in, and coming from the tailings pond is typically
called simply "water" even though it is actually water with a
quantity of CN-, SCN-, and other compounds.
[0007] When CN- or SCN- is present in the gold-bearing
slurry, it can form soluble Au complexes, which are then
unintentionally loaded onto indigenous during the pre-
cyanidation treatment. Accordingly, there are gold losses due
to the presence of CN- or SCN- in the gold-recovery operation
prior to the gold-recovery stage itself.
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SUMMARY OF THE INVENTION
[0008] Among the several objects of the present
invention, therefore, is the provision of a process for
recovering gold from comminuted ores, concentrates, or other
feed materials from which gold has been leached wherein the
concentration of CN- and SCN- in the water taken from the
tailings pond is reduced prior to introduction into the gold-
recovery process, thereby allowing a greater quantity of gold
to remain in the solution and, ultimately, to be recovered.
[0009] Briefly, therefore, the invention is directed to
a process for recovering gold from carbonaceous ore in a
cyanide-based gold recovery operation, the process comprising
directing tailings from the cyanide-based gold-recovery
operation to a reservoir, the tailings comprising CN- and SCN-;
holding the tailings in the reservoir; removing an initial
water effluent comprising an initial total concentration of CN-
and SCN- from the reservoir; treating the initial water
effluent to oxidize CN- and SCN- to yield a treated water
effluent having a treated total concentration of CN- and SCN-
which is at least 30a below said initial total concentration
of CN- and SCN- in the initial water effluent; and introducing
said treated water effluent into a gold-recovery operation to
recover gold from carbonaceous ore with preg robbing
tendencies.
[0010] In another aspect the invention is directed to a
process for treating water for use in a gold-recovery system,
wherein the water has an initial total concentration of CN- and
SCN-, the process comprising adding H2SO5 to the water to yield
treated water having a total treated concentration of CN- and
SCN- which is at least 30o less than said initial total
concentration of CN- and SCN-, wherein the initial total
concentration of CN- and SCN- is above about 20 ppm.
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[0011] These and other objects, features, and advantages
of the invention will become apparent from the following
detailed description.
BRIEF DESCRIPTION OF THE DRAWING
[0012] Fig. 1 is a flow diagram illustrating the overall
process for one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] In accordance with this invention, water
introduced to a cyanide leaching gold recovery process is
treated so as to decrease the concentration of CN- and SCN-,
thereby diminishing the preg robbing phenomena and improving
the gold-recovery yield from carbonaceous ore. One preferred
gold recovery process to which the invention is applicable
are shown in Figure 1.
[0014] According to such a process, ore is crushed
during a grinding step performed by a milling process, such as
one or more semi-autogenous (SAG) mill/ball mill circuits or
other grinding circuits, after which the ground ore slurry is
screened for trash or tramp material. Lime is optionally
added during the grinding step to adjust the working pH of the
slurry as desired, commonly, for example, between about 9 and
about 9.5. The ground ore slurry is thickened, if necessary,
by removal of excess water in a solid-liquid separation
operation. The thickened slurry may comprise any suitable or
desired portion of solids, with between about 50% and about
70% or between about 55% and about 60% as common slurry
compositions. The excess water is stored and reused in the
grinding step.
[0015] The slurry is directed to acidulation tanks where
a sufficient amount of sulfuric acid is added to dissolve
carbonate materials. This process yields liberated C02, which
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is subsequently volatilized as much as possible. Thereafter,
the slurry is pumped to the acid pressure oxidation vessels to
break down the sulfides and liberate the encapsulated gold.
There, oxygen and steam are introduced to the pressure vessel
to attain the necessary temperature and desired degree of
sulfide oxidation. Energy from the exothermic pressure
oxidation is recovered by heat exchange between the oxidized
slurry and acidulated feed.
[00161 After it is partially cooled, the oxidized slurry
is further cooled and passed directly to a neutralization
operation. Here, a base, such as milk of lime, is added to
adjust the pH to allow for subsequent cyanide leaching.
Typically, the pH value is adjusted to between about 10 and
about 10.5. Gold is recovered from the neutralized, oxidized
slurry by any known means such as, for example, CIL
cyanidation in a continuous countercurrent system. The barren
slurry, known as tailings, is directed to and held in a
reservoir (tailings pond).
[00171 The tailings typically comprise, e.g., recovery
water, residual lixiviant, neutralizing agent, and comminuted
ore. When cyanide is used as the lixiviant, untreated
tailings comprises residual CN- and SCN- at a concentration
between about 15 ppm and about 40 ppm. According to common
practice in the art, the tailings is treated to reduce the CN-
concentration. This treatment typically occurs either before
or shortly after entry into the tailings pond. Typically,
oxidizers such as hydrogen peroxide or Carols acid
(peroxymonosulfuric acid) can be used to reduce the
concentration of free CN-, weakly acid dissociable (WAD)
cyanides, and highly complexed cyanides. While these forms of
cyanide are reduced, they are not completely eliminated in the
traditional practice because any amount remaining in the
tailings will reduce the amount of CN- that must be added after
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water from the tailings pond is reintroduced into the gold-
recovery system.
[0018] In accord with this invention, after a period of
time during which the solids settle to the bottom of the
tailings pond, a water effluent is removed from the tailings
pond and is reintroduced into the gold-recovery process.
Typically, this water comprises some residual lixiviant and
neutralizing agent. in a typical gold-recovery system wherein
cyanide is used as the lixiviant, this water comprises, for
example, CN-, usually as a WAD cyanide, in a concentration
between about 5 ppm and about 20 ppm and SCN- in a
concentration of between about 10 ppm and about 30 ppm. The
total concentration of CN-and SCN- is more than about 20 ppm,
typically more than about 25 ppm, and often more than 30 ppm
or 40 ppm.
[0019] In accord with this invention, the water effluent
is treated with Caro's acid (peroxymonosulfuric acid, H2SO5) to.
reduce the concentration of both CN- and SCN- by at least 30%.
In certain embodiments it is reduced by more than 40 or 500,
such as by more than 70%. Generally, Caro's acid is added to
the water in an amount sufficient to reduce the total
concentration of CN- and SCN-. For example, Caro's acid is
added to the water in an amount sufficient to reduce the total
concentration of CN- and SCN- to below about 20 ppm, such as to
below about 10 ppm. In one preferred embodiment, the total
concentration of CN-and SCN- is below about 5 ppm. In the
most preferred embodiment, there is no detectable CN- or SCN-
present in the water reintroduced into the gold-recovery
system.
[0020] In accordance with this invention, the amount of
Caro's acid necessary to obtain the desired total
concentration of CN- and SCN- can be determined by calculating
the stoichiometric requirement of the amount of Caro's acid
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necessary to react with the concentration of CN- and SCN- in
the untreated tailings. In general, the amount of Caro's acid
added to the untreated tailings is greater than about 100% of
this stoichiometric requirement. For example, the amount of
Caro's acid added is typically greater than about 1500, 175%,
or 200% of the stoichiometric requirement. In one preferred
embodiment, the amount of Caro's acid added to the recovery
water is greater than about 125% of the stoichiometric
requirement.
[0021] In accordance with this invention, the Caro's
acid may be added to the water at any point after it leaves
the tailings pond and before introduction into the gold-
recovery process. In one preferred embodiment, the Caro's
acid is added to the water after reentry into one of the gold-
recovery system components, e.g., a holding tank, directly
before the water is introduced to a particular process step.
Regardless of when the Caro's acid is added to the tailings,
it may be added continuously, at regular intervals, or when
necessary as determined by monitoring the CN- and SCN-
concentration. Furthermore, the treated water may be
introduced at any process step utilizing water, e.g., to
grinding, lime slakers, exhaust scrubbers, and pump gland
seals.
[0022] Upon introduction to the water, Caro's acid
reacts very quickly, first with CN- and then with SCN-, to
reduce their concentration. The Caro's acid reacts with the
CN- according to the following reaction:
H2SO5 + CN- - - - - - - - > H2SO4 + OCN-
With thiocyanate, the Caro's acid oxidizes sulfur and
liberates free cyanide according to the following reaction:
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3 HZSO5 + SCN-' + H20 ------- > OCN- + 4 H2SO4
Then, the Caro's acid oxidizes the liberated free cyanide to
obtain the following overall reaction:
4 H2SO5 + SCN-- + H20 ------- > OCN- + 5 H2SO4
Unlike many other commonly used acids, Caro's acid is strong
enough to oxidize the SCN- after an amount has reacted with the
CN-.
[0023] Without being bound to a particular theory, the
present invention improves the final yield of a gold-recovery
process by limiting the preg robbing effect of CN- and SCN-
exposed to the ore prior to the autoclave process. By
comparison, conventional gold-recovery processes comprise
treatment of the tailings to control only the concentration of
the CN-. In such processes, control of or even complete
elimination of CN- has not proved successful in reducing the
preg robbing phenomena. In accord with the present invention,
successfully improving the yield of the gold recovery process
depends on reducing the recovery water's SCN- concentration in
addition to reducing the CN- concentration. Under acidic
conditions, SCN- can leach gold and contribute to the preg
robbing effect during the gold-recovery process. Furthermore,
when SCN- oxidizes under conditions such as those surrounding
the autoclave, it can form free CN-, which will leach gold and
cause preg robbing. Furthermore, in trying to limit the SCN"
concentration, oxidation is difficult, such that most
oxidizers are not powerful enough to reduce the SCN-
concentration. But by introducing Caro's acid, a strong
oxidizer, just before reintroduction of the recovery water
into the gold-recovery system, the SCN- concentration is
efficiently controlled. Also, merely modifying the known
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practice by adding Caro's acid to the tailings before entry
into the tailings pond to reduce the CN- and SCN- concentration
is nearly impossible and, therefore, commercially impractical.
This is because (1) the tailings comprises several other
constituents that would react with the Caro's acid, such as
the comminuted ore solids, reducing the Caro's acid
effectiveness in oxidizing the SCN-, and (2) SCN- is difficult
to oxidize, such that an excessive amount of Caro's acid would
be required in view of the large volume of the pond.
[0024] In general, Caro's acid is added to the tailings
in an amount sufficient to sufficiently oxidize CN- and SCN-,
to decrease preg robbing tendencies, and thereby improve the
overall recovery of gold from the gold-recovery process by at
least about 1% as compared to a gold-recovery process using
tailings water treated only prior to or in the tailings pond
for environmental reasons (conventional process). For
example, the overall recovery of gold from the gold-recovery
process is improved by at least about 2% as compared to a
conventional process. In one preferred embodiment, the
overall recovery of gold from the gold-recovery process is
improved by at least about 3% as compared to a conventional
process. With ores that have greater preg robbing tendencies,
the improvement is typically greater.
[0025] In addition to the Caro's acid, a base may
optionally be added to the water from the tailings pond to
adjust the pH of the water. One base commonly used is milk of
lime. The base is added in an amount sufficient to bring the
pH of the water between about 8 and about 11 prior to
reintroduction into the gold-recovery process. For example,
the pH of the water is between about 9 and about 10, such as
about 9.5. The precise amount of base necessary will depend
on the strength thereof.
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[0026] In accord with a further optional step of the
invention, the treated recovery water may be clarified or
strained to remove any solids therefrom before reintroduction.
These solids are removed to prevent problems such as plugging
or scaling of processing equipment at the processing steps
where the water is reintroduced. The clarification or
straining can be accomplished by any means known in the art,
such as, for example, a clarifier, filter, screen, or cyclone.
EXAMPLES
[0027] Further illustration of the invention is provided
by the following examples:.
EXAMPLE 1:
[0028] An experiment was conducted using an
semi-continuous autoclave (SCAC) oxidation procedure, rather
than a batch test.
[0029] SCAC tests were performed in a semi-continuous
2-liter agitated pressure vessel'at about 225 C under about
0.69 MPa oxygen overpressure with a pulp density of about 400
solids. Approximately every 6 minutes, a quarter of the
slurry volume was withdrawn from the vessel and replaced with
the same volume of fresh, untreated slurry. To ensure that
the process had stabilized, the first eight slurry
withdrawals, equivalent to two autoclave volumes, were
discarded. Thereafter, each withdrawal from the autoclave was
collected and composited for the'bottle roll test. The
nominal retention time for the SCAC test is about 24 minutes.
The average residence time is about 23.2 minutes after three
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volume turnovers and about 23.8 minutes after four volume
turnovers.
EXAMPLE 2:
[0030] Ore samples that had been oxidized using the SCAC
procedure described in Example 1 were bottle-roll leached with
cyanide for about 16 hours in the presence of approximately 12
g of virgin activated carbon, with a pulp density of about 400
solids. The slurry was then subjected to the gold reclaim
process in three settings: using untreated recovery water,
using recovery water treated with Caro's acid, and using DI
water.
[0031] For the treated recovery water trial, Caro's acid
and milk of lime were added simultaneously to the reclaim
solution, such that the pH was maintained at about 9.5 to 10.
Caro's acid was added in an amount equivalent to about 12501 of
the stoichiometric requirement. After approximately 20
minutes of agitation, the Caro's acid was able to reduce the
concentrations of SCN- and WAD cyanide to 0.13 and 0.8 ppm,
respectively.
[0032] The data below shows the results of gold recovery
processes for three different carbonaceous ore samples. The
process using DI water is the control for these trials because
of the absence of both CN- and SCN-. In each trial, the
treated recovery water yielded better gold recovery than the
untreated recovery water, with the improvement ranging between
about 1.1o to about 2.9%.
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Caro's acid Head Tails Au Rec
% Stoich q/t Au cr/t Au %
0 5.97 1.06 82.2
125 5.97 0.994 83.3
DI 5.97 1.06 82.2
0 4.70 0.789 83.2
125 4.70 0.651 86.1
DI 4.70 0.651 86.1
0 6.51 1.27 80.5
125 6.51 1.13 82.6
DI 6.51 1.10 83.2
[0033] When introducing elements of the present
invention or the preferred embodiment(s) thereof, the articles
"a," "an," "the," and "said" are intended to mean that there
are one or more of the elements. The terms "comprising,"
"including," and "having" are intended to be inclusive and
mean that there may be additional elements other than the
listed elements.
[0034] The foregoing relates to a limited number of
embodiments that have been provided for illustration purposes
only. it is intended that the scope of invention is defined
by the appended claims and there are modifications of the
above embcdiments that do not depart from the scope of the
invention.
