Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE OF THE INVENTION
An improved closed loop method for gold and silver extraction by halogens
FIELD OF THE INVENTION
[0001] The present invention relates to method for gold and
silver extraction by
halogens. More specifically, the present invention is concerned with an
improved closed loop method
for gold and silver extraction by halogens.
BACKGROUND OF THE INVENTION
[0002] The extraction of gold and silver from polymetallic ores
using halogens in brine
(NaCl/NaBr in water) has been reported (U.S. Patent 7,537,741).
[0003] A closed loop method for extracting precious metals from
an ore, using
halogens has also been developed, where the halogens (chlorine and bromine)
are used as free
halogens at the time of extracting the precious metals and are recycled by
electrolysis of the barren
solution in the form of hypohalides, i.e. sodium hypochlorites Na0C1 and
sodium hypobromites Na0Br
(US Patent application 2013/0074655). The free halogens are liberated from the
hypohalide state by
operating the leaching under acidic conditions. The following equation
summarizes the main steps of
this closed loop method, which is illustrated in Figure 1.
[0004] Liberation of free halogens from hypohalides
[0005] 2 OCI-/2 OBr + 2 H+ .. C12/Br2 + H20
[0006] Gold extraction with halogens mixture
[0007] 3 Br2 + Ore(Au) ¨> 2 AuBr3 + Ore (depleted)
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[0008] 3 AuBr3 + 3 Cl2 AuCI3 + 3 Br2
[0009] Collection of gold
[0010] 2 AuCI3 + reducer 2 Au + 6 C1
[00111 Regeneration of hypohalides from barren solution
[0012] 6 Cl- + electrooxidation 6 OCI-
[0013] 6 Br- + electrooxidation 6 OBr
[0014] In these various reactions, the anions, either halides or
hypohalides, are carried
by sodium as cations, i.e. NaCI, NaBr, Na0Br or Na0C1.
[0015] The implementation of this method involves the recycling of
halogens,
particularly the more expensive bromine. The liberation of the free halogens
under acidic conditions
generates an atmosphere rich in chlorine and bromine. The operation being
conducted at atmospheric
pressure, the excess halogens must be scrubbed, typically with sodium
hydroxide NaOH, in order to
recover these halogens for recycling. Vapors that accompany the filtration of
the pregnant solution
may also contain a small but significant amount of halogens that must be
recovered for economic and
environmental reasons. This is the first instance where addition of a chemical
such as sodium
hydroxide NaOH is required in the method.
[0016] A second instance of chemical addition in the method occurs at
the time of
collection of the gold dissolved in the pregnant solution, when a reducer is
used to bring down the
trivalent gold to an elemental state. This operation is achieved by reducing
the Oxidation-Reduction
Potential (ORP) from values in the range of 1000 mV to less than 400 mV vs a
Ag/AgCI reference
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electrode. The reducer, either sodium sulfite Na2S03 or sulfur dioxide gas
S02, involves sodium
addition directly from the Na2S03 or from the neutralization of the excess and
oxidized SO2 (to H2SO4)
with sodium hydroxide NaOH.
[0017] A third instance is after the gold collection, when the barren
brine which has
collected impurities, i.e. mainly iron, from the ore, must be purified before
recycling by electrolysis.
This purification is done by pH adjustment with sodium hydroxide in order to
precipitate iron and other
traces of base metals followed by a treatment with sodium carbonate Na2003 to
bring down the
calcium content to a level acceptable at the time of electrolysis.
[0018] Therefore at three steps in the above method, namely, at the
time of scrubbing
the off gases from the chlorination reactor, when precipitating the gold from
the pregnant solution and
at the time of the purification of the barren solution, the use of sodium
hydroxide leads to an
unavoidable accumulation of sodium. This sodium exists as sodium sulfate,
since the acid used in the
reactor is sulfuric acid H2S0 and the reducer for gold precipitation is sulfur
dioxide SO2 (see Figure 1).
[0019] Removing sodium sulfate by crystallization is a known process
but it is relatively
expensive in capital and operation costs. Moreover, such a step eliminates the
closed loop character
of the method. Therefore, it was found highly desirable to improve upon these
operational conditions
that led to sodium build-up.
[0020] There is still a need in the art for a method and system for
gold and silver
extraction using halogens.
SUMMARY OF THE INVENTION
[0021] More specifically, in accordance with the present invention,
there is provided a
method for extracting precious metals from a polymetallic ore, comprising a)
generating hypochlorites
from a salt brine; b) chlorination of the ore using hypochlorites under acidic
conditions; c) filtering to
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collect a pregnant solution and treating the pregnant solution for collection
of precious metals; d)
filtering to separate the precious metals and a barren brine; e) purifying the
barren brine; and f)
recycling halogens from the purified brine in the form of hypohalites formed
by electrolysis of the
purified brine, and comprising hypochlorite and hypobromite; step b) further
comprising scrubbing
excess halogens using calcium hydroxide; treating the pregnant solution in
step c) comprises reducing
an oxydo reduction potential of the pregnant solution and using calcium
hydroxide for neutralization;
and step e) comprising increasing the pH of the barren brine by addition of
calcium hydroxide.
[0022] Other objects, advantages and features of the present invention
will become
more apparent upon reading of the following non-restrictive description of
specific embodiments
thereof, given by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the appended drawings:
[0024] Figure 1 is a diagrammatic view of a method for gold extraction
using halogens
as known in the art; and
[0025] Figure 2 is a diagrammatic view of a method for gold/silver
extraction using
halogens according to an embodiment of an aspect of the present invention.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0026] In a method for gold and silver extraction using halogens, it
has been
discovered that sodium build-up could be avoided by using a base such as
calcium hydroxide Ca(OH)2
rather than sodium hydroxide NaOH i) at the time of scrubbing the off gases
from the chlorination
reactor, ii) when precipitating the gold/silver from the pregnant solution and
iii) at the time of the
purification of the barren solution, thereby providing a cost effective closed
loop method.
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[0027] In a sulfuric acid condition, the resulting neutralization
leads to calcium sulfate
(gypsum, CaSO4.2H20), which has a very low solubility in water and can be
filtered off easily.
[0028] The use of calcium hydroxide called for the preparation of a
slurry of this base
which has a tendency to settle, and since the resulting product, gypsum, is
not soluble, there was
always coating formation of this gypsum around non reacted calcium hydroxides
and other particulates
in the reaction mixture. This situation called for the use of excess amounts
of calcium hydroxide, since
a stoichiometric approach would lead to a base deficiency because of this
encapsulation of calcium
hydroxides or lime by gypsum. Also, encapsulation could prevent the reaction
of gold-bearing
particulates with chlorinating species.
[0029] Another matter of concern with the use of calcium hydroxide as
a base was the
possibility of co-precipitation of some of the bromide ion in the brine with
the gypsum. Such a
phenomenon, even at a very low level, would be highly detrimental to the
economics of the method
because of the relatively high cost of bromine or bromides.
[0030] Therefore, the use of calcium hydroxide, although desirable to
prevent ionic
build-up in the brine, had the potential drawback of preventing the efficient
gold / silver extraction and
the recycling of the bromides.
[0031] The three instances of use of a base in the method for gold
extraction using
halogens was thus examined, and conditions of its optimal use to avoid these
potential drawbacks
were determined.
[0032] First, at the time of scrubbing the halogens from the
chlorinating reactor, a
calcium hydroxide Ca(OH)2 slurry with Ca(OH)2 content in a range between 2 and
10% in weight,
preferably 5%, was used to recover the halogens values from the chlorination
reactor.
[0033] Such a recovery can be described by the following equations:
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[0034] 2 Br2 + 2 Ca(OH)2 Ca(OBr)2 + CaBr2 + 2 H20
[0035] 2 Cl2 + 2 Ca(OH)2 Ca(0C1)2 + CaCl2+ 2 H20
[0036] During this recovery of the halogens values from the
chlorination reactor, half of
the calcium involved thus exists as calcium hypohalides Ca(OBr)2, Ca(0C1)2.
These hypohalides can
be recycled to the chlorination reactor in order to recuperate the halogens
values. Surprisingly, it was
noted that in spite of the acid conditions in the chlorination reactor, the
hypohalides of calcium turned
out to be very efficient for the dissolution of gold. Therefore, half of the
scrubbed halogens values were
turned into useful hypohalides without need of electrolysis. There is thus
provided a first halogens
recycling step.
[0037] The anticipated potential problems of encapsulation were not
encountered,
most likely because of the grinding effect of the slurried and well-stirred
ore on the gypsum, which
appeared in the reactor, maintained acidic with sulfuric acid, at the time of
addition of the recycled
hypohalides from the scrubbing step described hereinabove. The barren ore,
along with the gypsum,
could be filtered off to obtain the pregnant solution with full gold and
halogen values and, interestingly,
without added sodium content.
[0038] Many ores contain carbonates and the acidic conditions in the
reactor liberate
carbon dioxide that is evacuated along with the halogens vapors. When scrubbed
with lime or calcium
hydroxide Ca(OH)2, these vapors yield water-soluble calcium halides or
hypohalides that can be
recycled to the reactor while calcium carbonate CaCO3, which is insoluble, can
be filtered off and
removed from the system. Thus, the use of calcium hydroxide as a base proves
to be much more
attractive than sodium hydroxide, which gives soluble carbonate and does not
allow separation of
carbonate from halogens salts.
[0039] Second, at the step of precipitation of gold, in order to
recover gold from the
pregnant solution, the ORP of this pregnant solution, which is in the range
between about 0.7 and
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about 1.2 V vs a Ag/AgCI reference electrode, must be reduced to values lower
than about 400 my in
order to bring the trivalent gold to elemental state. There is a variety of
reducers that can achieve this
ORP reduction.
[0040] Sulfur dioxide SO2, which is typically available on site
from the oxidation of the
starting ore, is the preferred reagent. This ORP reduction is accompanied by
an acidification of the
solution by the formation of sulfuric acid from the oxidation of the reducer
S02. The pH of the solution
has to be adjusted by neutralization. Calcium hydroxide can be used in molar
equivalent to excess
SO2, but precipitation of gypsum must not deprive the brine of its halides
components, i.e. NaCI and
NaBr, by coprecipitation. Appropriate rinsing, i. e. with at least a double
displacement (volume of water
twice the volume of the solid to be rinsed), prevents this undesirable effect.
Oxidized and excess sulfur
dioxide SO2 are removed as sulfate S042- and sulfite of calcium CaS03
respectively, these salts of low
solubility leaving a small background of calcium in the brine, i.e. down to
the solubility of CaSO4.
[0041] Thirdly, at the stage of purification of the barren
solution, the barren solution,
i.e. free of gold/silver, has to be purified before regeneration of
hypohalides by electrolysis. The main
impurity is iron collected at the stage of gold extraction. Also, the small
amount of dissolved gypsum,
typically around 600 ppm of Ca, must be reduced by a factor of ten in order to
preserve the
properties of the electrodes in the electrolytic cell.
[0042] The removal of the iron can be done by pH increase of the
brine to a range
between about 8 and about 9, by addition of calcium hydroxide. The other minor
contaminants,
essentially base metals, arsenic, antimony and bismuth, are also precipitated
by this treatment.
[0043] The reduction of the level of alkaline earth elements
(Ca, Mg) can be achieved
by carbonatation of the brine, these carbonates being in equilibrium with less
than 50 ppm of
Ca/Mg.
[0044] These interventions with Ca(OH)2 on the brine loop thus
prevent halogen
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losses, while maintaining the desired capabilities for gold/silver extraction.
[0045] With the use of calcium hydroxide rather than sodium
hydroxide as a
neutralizing agent, it has been possible to prevent the build-up of
undesirable ion in the brine, to
maintain the Cl/Br ratio in the brine, to retain the halogens from
precipitated phases and to maintain
the calcium level at values compatible with the electrochemical recycling of
the brine.
[0046] The following examples illustrate in a non
limitative manner the main features of
this invention.
[0047] Scrubbing chlorine, bromine and carbon dioxide with
calcium hydroxide.
[0048] A 200 ml solution of sodium hypochlorite Na0C1 (2 %)
and sodium hypobromite
Na0Br in a one-liter three necks flask was acidified to reach a pH comprised
in a range between 0.5
and 3, with a preferred value of 2, by slow addition of sulfuric acid H2SO4 40
% over a period of one
hour. During this period, a stream of nitrogen (100 ml/min) was circulated
through the flask and
directed towards a vertical column, 10 cm in diameter and two meters in length
through which a
sprayed slurry of calcium hydroxide at 10 % solid was circulated (2
liter/min), Gas analysis at the outlet
indicated a complete adsorption of chlorine, bromine and carbon dioxide while
in the slurry the
halogen existed as hypohalides and halides of calcium.
[0049] Gold extraction with hypohalides of calcium.
[0050] In the course of two simultaneous and otherwise
identical experiments, a gold
ore (11.25 g/t Au) was treated with i) sodium halide/hypohalide mixture and
ii) with calcium
halide/hypohalide respectively, using 8% brine with a NaCI to NaBr ratio in a
range between about 7/1
and about 7/3, 20% ore slurry at 30-40 C for four hours, i. e, using an amount
of active halogens in a
range comprised between about 0.5 and about 2 percent of the ore, the oxido-
reduction potential in
the reactor being in a range comprised between about 0.7 and about 1.2 V vs a
Ag/AgCI reference
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electrode. It was found that the gold recovery was identical in both
experiments, i.e. at the level of
about 95 %, this showing that the substitution of sodium by calcium in the
halide/hypohalide reagent
does not affect the gold extraction,
[0051] Purification of the barren brine prior to the
electrolysis.
[0052] A barren solution from gold recovery by chlorination
was showing the following
composition: CaSO4: 0.99 g/I; CuSO4: 1.637 g/I; Fe2(SO4)3: 15,98 g/I; H2SO4:
3.67 g/I; MgSO4: 0.10 g/I;
MnSO4: 0.018 g/I; NaBr: 0.97 g/I; NaCI: 44.8 g/I; Na2S03: 4.15 g/I; Na2SO4:
31.22 g/I; PbSO4: 0.094 g/I.
One liter of this barren solution was treated with an excess of calcium
hydroxide to pH 8.2. The
precipitated solid was filtered and the filtrate, after rinsings (two
displacements), contained the initial
amounts of NaCI and NaBr. Carbonatation of this purified brine brought down
the ratio Ca¨/Mg¨
below 50 ppm,
[0053] Thus, as illustrated in Figure 2, there is provided a
method for gold and silver
extraction, comprising generating hypochlorites from a salt brine (step 110);
chlorination of the ore
using hypochlorites under acidic conditions (step 120); filtering to collect a
pregnant solution (step
130); treating the pregnant solution for gold collection (step 140) and
filtering (step 142) to recover the
Au/Ag on the one hand and the barren brine on the other hand; purification of
the gold/silver depleted
brine, i. e. the barren solution (step 150), before circulating the purified
barren solution though an a
membrane less electrolytic cell for regenerating the halogens (160), yielding
a regenerated brine with
hypohalides. Calcium hydroxide is used instead of sodium hydroxide as a base
at three steps of the
method, as described hereinbelow.
[0054] In step 120, halogens from the chlorinating reactor are
scrubbed using a
calcium hydroxide Ca(OH)2 slurry with Ca(OH)2 content in a range between 2 and
10% in weight,
preferably 5%, is used to recover the halogens values from the chlorination
reactor (step 122). As
described hereinabove, the barren ore, along with gypsum, is filtered off
(step 126) to yield a solution
with gold/silver and calcium hypohalides Ca(OBr)2, Ca(0C1)2, which is
recirculated to the chlorination
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reactor (step 124), and calcium carbonate CaCO3, which is removed from the
system.
[0055] At the step of precipitation of gold, in order to
recover gold from the pregnant
solution, the ORP of the pregnant solution, which is in the range between
about 0.7 and about 1.2 V vs
a Ag/AgCI reference electrode, must be reduced to values lower than about 400
my in order to bring
the trivalent gold to elemental state, There is a variety of reducers that can
achieve this ORP
reduction. In step 140, sulfur dioxide SO2, which is typically available on
site from the oxidation of the
starting ore, is used as a reducing reagent. This ORP reduction is accompanied
by an acidification of
the solution by the formation of sulfuric acid from the oxidation of the
reducer S02. The pH of the
solution has to be adjusted by neutralization. Calcium hydroxide is used in
molar equivalent to excess
SO2, but precipitation of gypsum must not deprive the brine of its halides
components, i.e. NaCl and
NaBr, by coprecipitation. Appropriate rinsing, i. e. with at least a double
displacement (volume of water
twice the volume of the solid to be rinsed), prevents this undesirable effect.
Oxidized and excess sulfur
dioxide SO2 are removed as sulfate S042- and sulfite of calcium CaS03
respectively, these salts of low
solubility leaving a small background of calcium in the brine, i.e. down to
the solubility of CaSO4.
[0056] Step 150 comprises increasing the pH of the brine with
calcium hydroxide to a
value in a range between about 8 and about 9 which precipitates the iron, and
other minor
contaminants, essentially base metals, arsenic, antimony and bismuth, and
carbonatation of the brine,
which reduces the level of alkaline earth elements (Ca, Mg).
[0057] There is thus provided, in a closed loop gold
extraction method using sodium
chloride/sodium bromide brine as carrier for hypohalides oxidizers, an
improvement which consists of
substituting sodium hydroxide by calcium hydroxide for scrubbing and
neutralization purposes, thus
preventing sodium build-up in the loop while preserving the halogens content
of the brine.
[0058] In the present method, calcium hydroxide is
substituted to sodium hydroxide for
the scrubbing of the halogens vapors and carbon dioxide emissions, thus
allowing the recycling of
halogen values to the loop while discarding the carbon dioxide as calcium
carbonate.
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[0059] Calcium hydroxide is substituted to sodium hydroxide in
order to collect the
extracted gold, said calcium hydroxide precipitating the reducing agent as
insoluble calcium
sulfite/calcium sulfate after reduction of gold chloride to elemental gold.
[0060] Calcium hydroxide is substituted to sodium hydroxide in
order to remove
impurities from the barren brine by pH adjustment, said precipitated
impurities, mainly iron with traces
of other base metals, being filtered off and residual alkaline earth ions
reduced to a level below 50
ppm by carbonatation.
[0061] Calcium hydroxide is used as a slurry in water
containing from 1 to 20 A by
weight of calcium hydroxide with a preferred value at 5 %.
[0062j As people in the art will appreciate, there is thus
provided a method for gold and
silver extraction by halogens, chlorine and bromine, where the reagents are
recycled and reactivated
in a closed loop. In the present method, the steps where external adjunctions
of chemical reagents are
required to retain halogens in the loop, to collect the precious metals from
the pregnant solution and to
purify the barren solution prior to regeneration by electrolysis are achieved
using calcium hydroxide as
a base, without creating imbalance in the ionic composition of the brine in
the loop.
[0063] In a closed loop, the present gold/silver extraction
method comprises using
sodium chloride/sodium bromide brine as carrier for hypohalides oxidizers, and
calcium hydroxide for
scrubbing and neutralization purposes, thus preventing a sodium build-up in
the loop while preserving
the halogens content of the brine.
[0064] Calcium hydroxide is used for the scrubbing of the
halogens vapors and carbon
dioxide emissions, thus allowing the recycling of halogen values to the loop
while discarding the
carbon dioxide as calcium carbonate.
[0065] Calcium hydroxide is used in order to collect the
extracted gold, the calcium
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hydroxide precipitating the reducing agent as insoluble calcium
sulfite/calcium sulfate after reduction of
gold chloride to elemental gold.
[0066] Calcium hydroxide is used in order to remove impurities from the
barren brine
by pH adjustment, the precipitated impurities, mainly iron with traces of
other base metals, being
filtered off and residual alkaline earth ions reduced to a level below 50 ppm
by carbonatation.
[0067] Calcium hydroxide is used as a slurry in water containing from 1
to 20 % by
weight of calcium hydroxide with a preferred value at 5 %.
[0068] The scope of the claims should not be limited by the preferred
embodiments set
forth in the examples, but should be given the broadest interpretation
consistent with the description
as a whole.
