Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SPECIFICATION
[Title of the invention]
Method of Pretreating Gold Ore
(0001)
[Technical field]
The present invention relates to a method of pretreating gold ore for
hydrometallurgically recovering gold from gold ore which contains pyrite.
(0002)
As a method for recovering gold from sulfide ore containing gold, a technique
relying
on the hydrometallurgical process is known. Traditionally, the leaching of
gold from
the sulfide ore into a solution has been conducted by using reagents such as
cyanide,
thiourea, thiosulfate, halogen gas or the like. Recently, a gold-leaching
solution
containing chloride ions, iron ions, copper ions and bromide ions is proposed
as a less
toxic leaching solution as described in
(Patent document 1) and Japanese Patent
Application Publication No. 2009-235525 (Patent document 2).
(0003)
Further, as a pretreatment for facilitating the leaching of gold from sulfide
ore, a
method of subjecting sulfide ore to oxidizing roasting is known and, recently,
a
pretreatment method comprising the oxidizing roasting process combined with
one or
other processes has been proposed. For example, Japanese Patent Application
Publication No. 2010-235999 (Patent Document 3) proposed a method of
subjecting
copper sulfide ore to leaching treatment at a temperature below the melting
point of
the sulfur, allowing the resulting fine sulfur particles and remaining non-
leached
sulfide particles to float up from the leached residue through utilization of
the
difference in their hydrophobicity from other iron oxide and gangue
components, and
separating iron oxide or gangue by precipitation or as ore tailings, whereby
the gold
contained in the residual solution is concentrated. Thereafter, the condensed
components containing gold is subjected to sulfur removal and then to the
oxidizing
roasting so as to transform the iron component into iron oxide (hematite)
which, in
turn, is dissolved in sulfuric acid, whereby the residue containing the
concentrated
gold is recovered.
(0004)
With respect to pyrite only, it has been known that pyrite decomposes into
pyrrhotite, which is readily-soluble to acid, and sulfur. Japanese Patent
Application
Publication No. 2005-042155 (Patent document 4) suggests utilizing the
reaction to
remove pyrite from the residue obtained after leaching copper sulfide ore
containing
pyrite and to enrich the noble metal.
(0005)
[Description of prior art]
Patent document 1: Japanese Patent Application Publication No. 2008-106347
Patent document 2:-Japanese Patent Application Publication No. 2009-235525
Patent document 3: Japanese Patent Application Publication No. 2010-235999
Patent document 4: Japanese Patent Application Publication No. 2005-042155
(0000
The method disclosed in Japanese Patent Application Publication No. 2009-
235525
(Patent document 2), which does not use highly toxic cyanide, thiourea,
thiosulfate,
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halogen gas, or the like and facilitates leaching of the gold contained in
copper sulfide
ore, is highly practical for leaching the gold included in the copper sulfide
ore.
However, when this method is applied to the pyrite ore, the gold-leaching
speed is not
sufficient.
(0007)
As such, a pretreatment which uses the oxidizing roasting by supplying oxygen
as
disclosed in Japanese Patent Application Publication No. 2010-235999 (Patent
document 3) is considered to remove sulfur in advance and to facilitate iron
leaching.
(0008)
However, if the method of the oxidizing roasting of sulfide ore, including the
method
disclosed in the Patent document 3, is adopted, the following chemical
reactions,
2CuS+302---2Cu0+2S02, 4CuFeS2+1302-4Cu0+8S02+2Fe203 and
4FeS2+1102¨,2Fe203+8S02
will occur predominantly, and accordingly the problem of generation of sulfur
dioxide
(S02), which is known as environmental contaminant, cannot be avoided. In
particular, as the content of pyrite in the gold ore is higher, the amount of
generation
of sulfur dioxide becomes larger. Accordingly there is a problem to be solved
in terms
of practical use.
(0009)
As for pretreatment for enhancing the gold-leaching speed, it is desirable,
from the
aspects of the safety and the protection of the environment, to decrease the
sulfur
dioxide which is generated during a treatment method of ores for gold-
leaching,
thereby enhancing the safety and reducing the influence on the environment. It
is
assumed that a pretreatment applicable to gold ore containing a lot of pyrite,
which
has been considered difficult to put to practical use, would greatly
contribute to the
progress of gold mining development.
(0010)
The Patent document 4 is related to a process predicated on recovering noble
metal
with pyrometallurgy in view of the problem residing in recovering noble metal
with
hydrometallurgy. As such, there is no supposition that noble metal should be
leached
with a hydrometallurgical process (see paragraphs 0007-0008, 0078 of the
patent
document 4). Further, it does not suggest the effect achieved by utilizing a
hydrometallurgical process at all.
[Summary of the invention]
(0011)
Therefore, the present invention has been invented under the above-mentioned
situations, and has an object of providing a method for pretreating gold ore
for
hydrometallurgically recovering gold from gold ore containing pyrite, and has
an
object of enhancing the gold-recovering speed while the generation of sulfur
dioxide is suppressed.
(0012)
The present invention, in one aspect, provides a method of pretreating gold
ore for
hydrometallurgically recovering gold from gold ore which contains pyrite
(FeS2), the
method comprising a step of converting pyrite contained in the gold ore to
iron
compound soluble to hydrochloric acid.
(0013)
In one embodiment of the method of pretreating gold ore according to the
present
invention, the method includes a step of converting the pyrite such that a
ratio of a
content of Fe soluble to hydrochloric acid (Fesoi) contained in the gold ore
after
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pretreatment with respect to a content of total Fe (Feaii) contained in the
gold ore
after pretreatment becomes 0.6 or more.
(0014)
In another embodiment of the method of pretreating gold ore according to the
present
invention, the iron compound soluble to hydrochloric acid contained in the
gold ore
after pretreatment is sulfide.
(0015)
In a further embodiment of the method of pretreating gold ore according to the
present invention, a content of the pyrite in the gold ore prior to
pretreatment is 5 to
80 mass%.
(0016)
In a further embodiment of the method of pretreating gold ore according to the
present invention, S (mass %)/Au (mass ppm) in the gold ore prior to
pretreatment is
1 to 20.
(0017)
In a further embodiment of the method of pretreating gold ore according to the
present invention, the pretreatment involves heat treatment.
(0018)
In a further embodiment of the method of pretreating gold ore according to the
present invention, the heat treatment comprises heating the gold ore to 450 C
or
more under a non-oxidative atmosphere.
(0019)
In a further embodiment of the method of pretreating gold ore according to the
present invention, the heat treatment is carried out under the conditions of
retaining
the gold ore at 600-750 C for 5-60 minutes.
(0020)
By conducting the pretreatment according to some embodiments of the present
invention
on the gold ore containing the pyrite ore and then conducting a
hydrometallurgical process,
improved gold-recovering speed may be attained, while the generation of
noxious sulfur
oxide may be suppressed. Especially, improvements in gold leaching speed may
be seen
when using a particular gold-leaching solution according to some embodiments
of the
present invention. In other words, some embodiments of the present invention
may provide
a practical gold-leaching method in the safety and preservation of the
environment.
(0021)
[Brief explanation of the drawings]
Fig. 1 is a graph showing the relation between the leaching time and the Au
grade in
the residues with respect to Example and Comparative Example.
Fig. 2 is a TG/DTA curve obtained during thermal analysis under a nitrogen
atmosphere for ground pyrite concentrate used in Example 1.
(0022)
[Mode of practicing the invention]
The present invention will be explained in details in the following.
(0023)
1. Pretreatment
One embodiment of pretreatment of gold ore for hydrometallurgically recovering
gold contained in the gold ore containing pyrite (FeS2) according to the
present
invention includes a step of converting pyrite contained in the gold ore to
compound
soluble to hydrochloric acid. In a preferred embodiment of pretreatment of
gold ore
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according to the present invention includes a step of converting the pyrite
such that a
ratio of a content of Fe soluble to hydrochloric acid (Fest>) contained in the
gold ore
after pretreatment with respect to a content of total Fe (Fean) contained in
the gold
ore after pretreatment becomes 0.6 or more.
(0024)
(1) Gold ore
The object of the present invention is gold ore containing pyrite. This is
because the
present invention aims at the enhancement of the leaching ratio of gold in the
pyrite,
which is difficult to dissolve and has a low gold-leaching ratio. However, the
other
conditions, such as the concentration of gold in the ore, for example, are not
questioned. The gold ore, which is the object of treatment, may be those
having been
subjected to conventional beneficiation such as floatation or gravity
separation. It is
also possible to grind the ore to smaller particle sizes so that the contact
of gold-
leaching solution with gold within the ore is facilitated. The gold
concentration of the
gold ore is typically in the order of 0.1-100 ppm by mass, and more typically
in the
order of 1-20 ppm by mass.
(0025)
In addition to pyrite, the gold ore may contain chalcopyrite, galena,
sphalerite,
arsenopyrite, antimonite, and pyrrhotite. In a typical example of the present
invention, gold ore containing at least 5 mass% of pyrite, more typically at
least 10
mass %, and yet more typically at least 30 mass% of pyrite, is used. In this
type of
gold ore, as the ratio of sulfur content to gold content (S/Au) in the ore
becomes
higher, it is generally difficult to efficiently recover gold. Therefore, by
using such
gold ore having a high concentration of pyrite, the effect of the pretreatment
of the
present invention is remarkably achieved. Specifically, S (mass%)/Au (mass
ppm) is 1
to 20, preferably 1.5 to 20, more preferably 1.5 to 10. There is no particular
upper
limit to the content of the pyrite in the gold ore and 100 mass% is allowable
but
typically the content is at most 80 mass%.
(0026)
(2) Conversion step
In the conventional technique, the ores were subjected to oxidizing roasting
under
the presence of the oxygen or air, and hence the sulfur contained in the
sulfide ores
was combined with the oxygen, resulting in generation of sulfur oxide. In the
present
invention, such oxidizing roasting is not carried out substantially. In the
present
invention, the pretreatment preferably converts pyrite to iron sulfide soluble
to
hydrochloric acid while still remaining iron sulfide in terms of suppression
of sulfur
oxide formation. If the iron in the ore exists as iron sulfide soluble to
hydrochloric
acid in a certain amount or more, a remarkably improved leaching speed of gold
can
be achieved in the subsequent leaching step.
(0027)
While pyrite (FeS2) is insoluble to hydrochloric acid, the iron sulfide
soluble to
hydrochloric acid is expected to improve a recovering speed of gold when
hydrometallurgically recovering gold. Especially, it was found that it showed
remarkable effect for a particular leaching solution. The particular leaching
solution
will be explained later in "2. Hydrometallurgical step".
(0028)
According to the research by the present inventors, supposed that a content of
total
Fe contained in the gold ore after pretreatment is represented by Feat' and a
content
of Fe soluble to hydrochloric acid is represented by Fes.' contained in the
gold ore
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after pretreatment, the "certain amount or more" should mean that Fesoi/Fean
is 0.6 or
more, preferably 0.8 or more, more preferably 0.9 or more. When pyrite is
completely
converted, Fesoi/Fean reaches 1.0, which is the upper limit.
(0029)
In the present invention, Fean is calculated according to the following
procedure. 0.2g
of gold ore after pretreatment, 4g of sodium peroxide, lg of sodium carbonate
are
charged in a crucible made of zirconium and heated with a gas burner for
alkali
fusion. After the crucible is cooled with water, 30mL of 35 mass% hydrochloric
acid
is charged for leaching of the melt. The post-leaching solution is subjected
to
determination by ICP-AES (in Example, Model:SPS4000 available from Hitachi
High-
Technologies Corporation (formerly SIT) was used.). Based on the determined Fe
concentration, the amount of liquid and the amount of ore, Fean is calculated.
Specifically, it is represented as follows: Feau = determined Fe concentration
(g/L) x
liquid amount (30mL) sample amount (0.2g).
(0030)
In the present invention, Fes.' is calculated according to the following
procedure. 50g
of the gold ore after pretreatment is subjected to leaching at 85 C for 180min
with
agitation in 1L of hydrochloric acid (1.0mo1/L) containing lmol/L of Fe3+,
which is
then filtered. The Fe concentration in the filtrate is determined by ICP-AES
(in
Example, Model:SPS4000 available from Hitachi High-Technologies Corporation
(formerly SID was used.) (Fe initially contained in the hydrochloric acid
should be
deducted.). Based on the determined Fe concentration, the amount of liquid and
the
amount of ore, Fesot is calculated. Specifically, it is represented as
follows: Fes.' =
(determined Fe concentration ¨ initial Fe concentration) (g/L) x liquid amount
(1L)
ore amount (50g).
(0031)
In the conversion step of pyrite, it is desirable that the pretreated pyrite
remain
sulfide. When pyrite is converted into oxide, the formation of sulfur oxide is
unavoidable, the amount of which is so large that simple means for its removal
such
as a shower tower is not sufficient and a device enabling sufficient removal
is
necessary. Accordingly, heat treatment by which pyrite still remains sulfide
after
pretreatment is desirable.
(0032)
The step of conversion can be carried out by heat treatment. From the
standpoint of
suppressing the generation of sulfur oxides, it is preferable to conduct the
conversion
step in a condition that oxygen feed is suppressed (in a non-oxidative
atmosphere).
The condition of such suppressed oxygen feed means in the present invention
that the
molar ratio of oxygen/pyrite ore = 1/ 2 or less. Also, the non-oxidative
atmosphere
means that the molar ratio of oxygen /pyrite = 1/5 or less, preferably 1/10 or
less.
(0033)
If the mixing of oxygen is suppressed, the amount of sulfur oxide generation
is low
and accordingly there is no necessity of installing a separate sulfuric acid
production
facility. A shower tower will be enough to remove it. If the non-oxidative
atmosphere
is used, even the shower tower may be dispensed with.
(0034)
The gold ore after the conversion step exhibits remarkably enhanced solubility
into
the gold leaching solution as will be explained hereafter and the leaching
speed of
gold may increase by approximately ten times more than the case without the
conversion step. It was quite amazing that such remarkable result has been
attained.
6
(0035)
As the non-oxidative atmosphere for conducting the conversion step, reductive
atmosphere such as ammonia, carbon monoxide and hydrogen sulfide, and inert
atmosphere such as rare gas (e.g. argon or helium), nitrogen and carbon
dioxide
may be cited. Among them, inert atmosphere is preferable in terms of
preventing
unexpected reaction to occur. Alternatively, the exhausted gas used in the
pyrolysis may be reused by recycling.
(0036)
During the conversion step, it is necessary to maintain the temperature of the
gold ore at least 450 C, preferably at least 550 C and more preferably at
least
650 C to enhance the pyrolysis of pyrite. Also, it is preferable to keep the
retention temperature for at least 5 minutes, preferably for at least 15
minutes.
This is to sufficiently progress the pyrolysis reaction. However, if the
temperature of the gold ore is excessively high, the energy for heating the
ore and
the processing time become too excessive, and accordingly the retention
temperature is preferably 800 C or less, and more preferably 750 C or less.
Similarly, the time for maintaining the retention temperature is preferably
120
minutes or less, more preferably 60 minutes or less.
(0037)
Although there is no particular restriction to the type of the heating furnace
for
the conversion step, a tubular furnace or a rotary kiln, for example, may be
used.
(0038)
The elemental sulfur generated by pyrolysis of pyrite has been gasified in the
high temperature furnace and accordingly the elemental sulfur can be subjected
to solid/gas separation and then can be delivered together with the
atmospheric
gas to a venting system. However, if the elemental sulfur is sent to the
venting
system, the sulfur will deposit with the decreasing temperature and may cause
trouble such as clogging of the gas flue. Therefore, it is desired to recover
the
sulfur with a wet scrubber. Alternatively, the gaseous elemental sulfur may be
cooled together with the pyrrhotite generated in the conversion step. In this
case,
they are recovered as solids, which in turn are sent together to the gold-
leaching
step. The elemental sulfur is separated in the leaching step as leaching
residue
without interfering with the leaching of gold. In this case, this method is
economical because the wet scrubber becomes unnecessary.
(0039)
Depending on the operational limitation, there may be a case where pyrolyzed
gold ore and unpyrolyzed gold ore are comingled and subjected to the iron-
leaching step and subsequent steps. However, even in such case, the gold ore
which has been subjected to the pyrolysis step is contained and accordingly
such
embodiment, too, belongs to the technical scope of the present invention.
(0040)
2. Hydrometallurgical process
The effect of the present invention can be exerted by recovering gold from the
pretreated gold ore through hydrometallurgical process. Hydrometallurgical
process includes, but not limited to, gold leaching in a cyanide bath combined
with autoclave treatment and gold leaching in an acidic bath.
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6a
(0041)
Gold leaching using a cyanide bath generally includes reacting gold ore
containing
pyrite with water and oxygen in a pressure-resistant container at a high
temperature
under a high pressure (e.g. 200 C, 30atm) to convert iron sulfide into iron
oxide, then
,
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leaching gold. The process is called "autoclave treatment" as an autoclave is
used as
the pressure-resistant container.
In case where the pretreatment is not conducted, the oxidation reaction of
iron
sulfide is represented by the following formula.
4FeS2 +1502 +8H2 0 2Fe2 03 +8E12 SO4 ¨ (1)
On the contrary, in case where the pretreatment is conducted, oxidation of
sulfide
generates sulfuric acid, which can leach iron compound soluble to acid,
enabling
reduction of reaction time.
(0042)
Also, in the gold leaching using an acidic bath, it is generally important to
bring
gold locked in the iron sulfide ore into contact with a leaching solution. The
pretreatment according to the present invention can bring gold in the iron
sulfide ore
into contact with the leaching solution in a shorter period since it can
convert pyrite
in the gold ore to iron sulfide soluble to acid.
(0043)
Though the time required for the hydrometallurgical process following the
pretreatment can be reduced in either hydrometallurgical process, the gold
leaching
using an acidic leaching solution is advantageous since it can be conducted
under a
mild operational condition (under atmospheric pressure, less than 100 C) and
without
toxic cyanide. The gold leaching using an acidic bath will be hereafter
explained in
detail.
(0044)
Types and steps for performing gold leaching using an acidic bath for the
pretreated
gold ore are not restrictive but the following gold leaching step, which
includes
contacting with a gold-leaching solution containing halide ions, copper ions
and iron
ions while supplying an oxidant, thereby to leach gold component in the gold
ore, can
be mentioned as a gold leaching step exhibiting a great effect.
(0045)
The leaching of gold proceeds as follows. The dissolved gold reacts with
halide ions,
particularly chloride ions or bromide ions, to form a gold halide complex,
particularly
chloride complex or bromide complex of gold. Though chloride ions may be
singly used
as the halide ions in the gold-leaching solution, the combined use of chloride
and
bromide ions allows formation of a complex at a lower oxidation-reduction
potential,
thereby enhancing the leaching efficiency of gold. Further, iron ions in the
form of
ferric ions formed under supply of oxidant, or ferric ions from the beginning,
function
to oxidize the gold. The gold-leaching solution preferably contains copper
ions.
Although the copper ions do not directly participate in the reaction, the
oxidation of
the iron ions is accelerated in the presence of the copper ions.
(0046)
As the source of chloride ions, though there is no particular restriction,
hydrogen
chloride, hydrochloric acid, metal chloride and chorine gas, etc. may be cited
for
instance. From the aspects of economy and safety, it is preferable to feed the
ions as
metal chloride salt. Cited as metal chloride salts are copper chloride
(cuprous
chloride, cupric chloride), iron chloride (ferrous chloride, ferric chloride),
chloride of
alkaline metal (lithium, sodium, potassium, rubidium, cesium, francium),
alkaline
earth metal (beryllium, magnesium, calcium, strontium, barium, radium) can be
cited. Sodium chloride is preferred from the standpoints of cost and easy
availability.
It is also preferable to use copper chloride and iron chloride because they
are utilized
also as sources of copper ions and iron ions.
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(0047)
As the source of the bromide ions, although there is no particular
restriction,
hydrogen bromide, hydrobromic acid, metal bromide and bromine gas can be
cited. As
metal bromide, copper bromide (cuprous bromide and cupric bromide), iron
bromide
(ferrous bromide, ferric bromide), bromide of alkaline metal (lithium, sodium.
potassium, rubidium, cesium and francium), bromide of alkaline earth metal
(beryllium, magnesium, calcium, strontium, barium, radium), and from the
economical standpoint and easy availability, sodium bromide is preferred.
Also,
copper bromide and iron bromide are preferred because they can be also used as
sources of copper ions and iron ions.
(0048)
Copper ions and iron ions are usually supplied in the form of their salts, for
example, halide salts. The copper ions are preferably supplied in the form of
copper
chloride and/or copper bromide, and the iron ions are preferably supplied in
the form
of iron chloride and/or iron bromide, from the standpoint that they can be
also used
as sources of chloride ions and/or bromide ions. As the copper chloride and
iron
bromide, it is preferable to use cupric chloride (CuC12) and ferric chloride
(FeCl3),
respectively, but cuprous chloride (CuC1) and ferrous chloride (FeC12) may
also be
used because they are respectively oxidized into cupric chloride (CuC12 ) and
ferric
chloride (FeCl3) by supplying oxidant to the leaching solution.
(0049)
The concentration of the chloride ions in the gold-leaching solution used in
the gold
leaching step is preferably 30g/L-180g/L. The concentration of the bromide
ions in the
gold- leaching solution is preferably 1g/L-100g/L from the standpoints of the
reaction
rate and the solubility, and more preferably 10g/L-40g/L from the economical
standpoint. And, the total concentration of the chloride ions and bromide ions
is
preferably 120g/L-200g/L. Also, the weight ratio of bromide ions to chloride
ions in
the gold- leaching solution is preferably at least 1.
(0050)
The oxidation-reduction potential (reference electrode is Ag/AgC1 electrode)
of the
leaching solution at the beginning of the gold leaching step (right before
contacting of
the ores with leaching-solution) is preferably at least 550mV, more preferably
at least
600mV, from the standpoint of acceleration of the gold-leaching. Also, during
gold-
leaching process, it is preferred to maintain the potential at 550mV or more
and more
preferably at least 600 mV. Also, to promote the gold-leaching, the pH of the
leaching
solution is preferably maintained at 2.0 or less and preferably 1.8 or less.
The
temperature of the gold-leaching solution is preferably at least 45 C, and
more
preferably at least 60 C from the standpoint of acceleration of the gold-
leaching.
However, excessively high temperature will cause evaporation of the leaching
solution or increase the costs for heating, and accordingly 95 C or less is
preferable
and 85 C or less is more preferable.
(0051)
Accordingly, in a preferred embodiment of the present invention, a mixed
solution
containing at least one of hydrochloric acid and hydrobromic acid, at least
one of the
cupric chloride and cupric bromide, and at least one of ferric chloride and
ferric
bromide may be used as the gold-leaching solution in the gold leaching step on
the
condition that both of chloride ions and bromide ions are contained in the
leaching
solution.
(0052)
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The oxidation-reduction potential is controlled by supplying the oxidant while
conducting the gold-leaching step. If the oxidant is not supplied, the
oxidation-
reduction potential will be decreased and thus the leaching reaction will not
proceed.
Though there is no particular restriction to the oxidant, oxygen, air,
chlorine,
bromine and hydrogen peroxide or the like may be cited. An oxidant having
excessively high oxidation-reduction potential is not necessary and the air is
sufficient. The air is preferred from the standpoint of the cost and safety.
(0053)
After pretreatment but before the gold-leaching step, various treatments for
removing impurities in the gold ore may be performed. For example, elemental
sulfur
can be removed by heating the pretreated gold ore to a temperature at which
the
elemental sulfur is molten and then separating the elemental sulfur and gold
by
filtration.
(0054)
After the leaching of gold and the subsequent solid/liquid separation, gold
can be
recovered from the resulting gold solution. Although there is no particular
restriction
to the method for recovering the gold, adsorption on activated carbon,
electrowinning,
solvent extraction, reduction, cementation and ion exchange or the like may be
utilized. Sulfur component may remain as sulfate, sulfide and elemental sulfur
in the
post gold-leaching solution but the gold leached in the solution can be
separated from
them by solvent-extraction.
(0055)
Further, it is also effective to recover gold during the leaching reaction,
whereby the
concentration of gold in the leaching solution is lowered, and as a result the
leaching
ratio of gold is increased. This can be performed, for example, by introducing
activated carbon with or without lead nitrate into the gold-leaching solution
during
the leaching reaction.
(0056)
[Examples]
In the following, the present invention will further be specifically explained
by way
of working examples. It should be noted that the present invention is not
restricted to
the examples. The analysis of the metals used in the working examples was
performed according to ICP-AES. However, the analysis of the gold used in the
examples was conducted according to ICP-AES for quantitative analysis after
causing
deposition of gold in the specimens by cupellation process (JIS M8111).
(0057)
[Comparative Example 1]
Pyrite ore concentrate (produced in Papua New Guinea) was prepared as gold
ore.
The content of pyrite in this pyrite ore concentrate was determined by XRD and
chemical analysis, and 17 mass% of pyrite was confirmed. In addition, the
ratio of S
(mass%)/Au(mass ppm) in the concentrate was 1.4. Fesoi/Feati was determined to
be 0
by the method explained earlier.
(0058)
The pyrite ore concentrate was milled and ground in a ball mill to adjust the
particle
size to 50p.m at the particle size d80, namely, the particle size at which the
cumulative weight becomes 80% in the distribution curve of cumulative weight
particle sizes. The d80 was the average of three measurements which were
conducted
using the laser diffraction particle size distribution analyzer (Shimadzu
Corporation
Model No. SALD2100). Subsequently, leaching operation was conducted on the
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ground pyrite ore concentrate (200g), using a hydrochloric acidic gold
leaching
solution having the composition as listed in Table 1, with pulp concentration
of 100
g/L at a temperature of 85 C for 90 hrs. Air was blown in (0.1 L/min per 1L of
the
concentrate) during the leaching operation with continuous agitation and the
oxidation-reduction potential (ORP: vs. Ag/AgC1) was maintained at 530mV or
higher.
Also, during the leaching, the pH of the gold leaching solution was maintained
at 1.0-
1.1 by appropriately adding hydrochloric acid.
(0059)
TABLE 1
Gold leaching solution
FeCl3-6H20(g/L) 10
CuC12-2H20(g/L) 48
NaCl(g/L) 25
NaBr(g/L) 103
All chloride ions(g/L) 40
All bromide ions(g/D 80
ORP(mV) 717
(vs.Ag/AgC1)
pH 1.52
(0060)
During the leaching test, samples of the leaching residue were periodically
taken
and the Au grade was determined. Fig. 1 shows the relation between the
leaching
time versus Au grade in the residue obtained from the test. Refer to the
plotting of
FeS2 (refer to the plot of "without FeS2 pyrolysis" in Fig. 1). From this
result, it is
ascertained that it took 90 hours for the Au grade in the residue, which was
approximately 6g/t at the start, to decrease to 0.9g/t.
(0061)
<Example 1>
The ground pyrite ore concentrate (1.5kg) which was identical with that of
Comparative Example 1 was charged in a tubular furnace and one hour was spent
to
raise the temperature to 700 C (the rate of temperature increase=10 C /min)
under
the nitrogen atmosphere. It was thereafter heated for one hour. After allowing
it to
cool to a room temperature, it was confirmed that the peak of FeS2 contained
in the
original ore had disappeared and a peak of FeS had been found by the XRD
analysis
before and after the heat treatment, the elemental sulfur resulting from the
heat
treatment was naturally removed from pyrite ore by solid-gas separation.
(0062)
For the pyrite concentrate after the heat treatment, Fesot/Feall was
determined to be
0.98 by the method explained earlier.
(0063)
Subsequently, using a hydrochloric acidic solution of the gold-leaching
solution
having the same composition as in Comparative Example 1, leaching was
conducted
on the heat-treated pyrite ore concentrate with pulp concentration of 100g/L
at the
solution temperature of 85 C for 18 hours. Air was blown during the leaching
(at the
rate of 0.1L/min per 1L of the concentrate) while agitation was kept and the
oxidation-reduction potential (ORP:vs Ag/AgCD was maintained at least 400mV.
During the leaching, hydrochloric acid was appropriately added to keep the pH
of the
CA 02898986 2015-07-22
11
gold-leaching solution at 1.0-1.1.
(0064)
During leaching test, samples of the leaching residue were periodically taken
and
the Au grade was determined. Fig. 1 shows the relation between the leaching
time
versus Au grade in the residue obtained from the test (refer to the plot of
"with FeS2
pyrolysis" in Fig. 1). From this result, it is ascertained that it took only
12 hours for
the Au grade in the residue, which was approximately 6g/t at the start, to
decrease to
0.6g/t. Incidentally, when using a gold leaching solution with no bromide
ions,
roughly similar results were obtained though the Au leaching speed was lower
than
the case with bromide ions.
(0065)
<The change in Fesoi/Feall caused by pyrolysis condition>
Using 1.5 kg of the ground pyrite ore concentrate used in Example 1, the
change of
Fesot/Fean was investigated when the retention temperature and the retention
time
was changed as shown in Table 2. The value of Fesoi/Feau was determined in the
same
procedure as in Example 1. The test was conducted using a tubular furnace
under the
nitrogen atmosphere. The elemental sulfur generated by pyrolysis was
evaporated
and purged by a nitrogen stream. The temperature was increased at a rate of
C/min for all tests. Cooling was conducted by allowing it cool to a room
temperature. The results are shown in Table2.
(0066)
Table 2
Heating Condition Feso/Fean
Retention Retention
temp.( C) time(min)
Before heat treatment 0
550 60 0.48
550 120 0.54
600 5 0.49
600 30 0.61
600 60 0.76
650 60 0.95
700 60 0.98
(0067)
From the result shown in Table 2, it is understood that the retention
temperature of
650 C or more and the retention time of 60 minutes or more is most preferable
as
Fesol/Fean reaches close to 1.
(0068)
<Example 2: The temperature at which pyrolysis occurs.>
On the ground pyrite ore concentrate used in Example 1, the weight change and
the
endothermic/exothermic heat at respective temperatures were monitored, using
the
thermal analysis device (Model TG/DTA6300 manufactured by Seiko). The results
are
shown in Fig. 2. From the fact that the mass decrease begins at 450 C and
simultaneously the change of calorific value is observed. It is confirmed that
the
pyrolysis of the pyrite begins. Under the nitrogen atmosphere, pyrolysis does
not
occur until the temperature reaches 450 C. It should be noted that, from the
results
CA 02898986 2015-07-22
12
of the XRD analysis, a long period of time is necessary for pyrolisys and
accordingly
heat treatment at 600 C or higher is desirable.
