Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
[Title of the invention]
Pretreated Gold Ore
(0001)
[Technical field]
The present invention relates to a pretreated gold ore suitable for
hydrometallurgically recovering gold from gold ore which contains pyrite. The
present
invention also relates to a method of pretreating the gold ore.
(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 Japanese Patent Application
Publication No.
2008-106347 (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)
[Prior art literatures]
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
(0006)
[Summary of the invention]
[Problem to be solved by the invention]
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The method disclosed in Japanese Patent Application Publication No. 2009-
235525
(Patent document 2), which does not use highly toxic cyanide, thiourea,
thiosulfate,
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+1302-44Cu0+8S02+2Fe203 and
4FeS2+1102-42Fe203+8S02
will occur predominantly, and accordingly the problem of generation of sulfur
dioxide
(SO2), 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
hydro metallurgical process at all.
(0011)
Therefore, the present invention has been invented under the above-mentioned
situations, and has an object of providing a pretreated gold ore suitable for
hydrometallurgically recovering gold from gold ore containing pyrite. The
present
invention also has an object of providing a pretreatment method effective for
obtaining such gold ore that can enhance the gold-recovering speed while the
generation of sulfur dioxide is suppressed.
(0012)
[Means for solving the problem]
The present invention, in one aspect, provides a pretreated gold ore for
hydrometallurgically recovering gold from ggld ore which contains pyrite
(FeS2),
wherein it has an accumulative pore volume for pores having a diameter of 3 to
5 pm
that is twice or more times larger than prior to pretreatment.
(0013)
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In one embodiment of the pretreated gold ore according to the present
invention,
wherein 60% or more of pyrite contained in the gold ore prior to pretreatment
has
been converted to pyrrhotite represented by a following formula: Fei-.S (where
x=0-
0.2).
(0014)
In another embodiment of the pretreated gold ore according to the present
invention,
a content of the pyrite in the gold ore prior to pretreatment is 5 to 80
mass%.
(0015)
In a further embodiment of the pretreated gold ore according to the present
invention, S (mass %)/Au (mass ppm) in the gold ore prior to pretreatment is 1
to 20.
(0016)
In a further embodiment of the pretreated gold ore according to the present
invention, the pretreatment involves non-oxidative roasting.
(0017)
In a further embodiment of the pretreated gold ore according to the present
invention, the non-oxidative roasting is performed at a temperature of 550 C
or more
for 5 to 120 minutes.
(0018)
In a further embodiment of the pretreated gold ore according to the present
invention, the pretreated gold ore is to be subjected to gold leaching in a
subsequent
step.
(0019)
The present invention, in another 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 gold ore such that 60% or more of
pyrite
contained in the gold ore prior to pretreatment is converted to pyrrhotite
represented
by a following formula: Fei-.S (where x=0-0.2) and an accumulative pore volume
for
pores having a diameter of 3 to 5 um becomes twice or more times larger than
prior to
pretreatment.
(0020)
[Effect of the invention]
By conducting a hydrometallurgical process for the pretreated gold ore
according to
= an embodiment of the present invention, improved gold-recovering speed
may be
attained, while the generation of noxious sulfur oxide may be suppressed.
Especially,
the improved gold leaching speed is remarkable when using a particular gold-
leaching solution according to some embodiments of the invention. In other
words,
some embodiments of the present invention provide a highly practical gold-
leaching
method which excels 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.
Fig.3 is an XRD chart prior to heat treatment with respect to the pyrite ore
concentrate used in Example 1.
Fig.4 is an XRD chart after heat treatment with respect to the pyrite ore
concentrate
used in Example 1.
Fig.5 is a SEM image of iron sulfide (pyrite) in the gold ore prior to
pretreatment.
Fig.6 is a SEM image of iron sulfide in the gold ore after pretreatment
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(0022)
[Mode of practicing the invention]
The present invention will be explained in details in the following.
(0023)
(1) Gold ore prior to pretreatment
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.
(0024)
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. The
majority of
iron sulfide contained in the gold ore to be treated by the present invention
is pyrite.
For instance, among iron sulfides contained in the gold ore, normally 80 mass
% or
more is pyrite, typically 90 mass % or more is pyrite, more typically 99 mass
% or
more is pyrite. 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%.
(0025)
(2) Gold ore after pretreatment
2-1) Iron sulfide having pores
In the normal state of gold ore (concentrate) without any pretreatment, no
pores can
be seen in particles of pyrite contained in the ore as shown in the SEM image
of Fig.
5. The present inventors paid attention to the state and presumed that
leaching of
ore may be performed with an improved leaching speed if an iron-containing
particle
after pretreatment have pores. After conducting research assiduously, the
present
inventors have been able to obtain the particles of iron compound having pores
as
shown in Fig. 6.
The present inventors have analyzed pore volume distribution of the gold ore
before
and after the pretreatment with a mercury intrusion method and have found that
there occurs a characteristic change in pores having a diameter of 3 to 5 pm,
for
which an accumulative pore volume will remarkably increase by conducting the
pretreatment.
(0026)
Determination of pore volume distribution by the mercury intrusion method,
which
is carried out with respect to the entire ore, gives only the total value
including not
only for changed pyrite but also for other gangues in the ore. However,
according to
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the research by the present inventors, it has been found that the change in
the
accumulative pore volume within the mentioned diameter range is remarkable for
pyrite. Further it has been found that when the twice or more times of the
volume
change is observed before and after the pretreatment, desirable change of
pyrite has
sufficiently occurred irrespective of ore type. Further, it is believed that
increase of
pore volume will give an advantage that gold leaching solution can easily
percolate to
the interior of the gold ore.
(0027)
The accumulative pore volume for pores having a diameter of 3 to 5 pm is
preferably
2.5 or more times, more preferably 3 or more times larger than prior to
pretreatment.
However, the ratio is affected by the content of pyrite contained in the gold
ore prior
to pretreatment and it is about 20 times even in case where the content of
pyrite is
close to 100 mass%. For the gold ore with less pyrite content, the ratio
becomes not so
high. Accordingly, it is typically 15 or less times, more typically 10 or less
times, yet
more typically 5 or less times.
(0028)
2-2) Composition of iron sulfide
The pretreated gold ore according to the present invention is desirably the
one in
which 60% or more of pyrite (FeS2) contained in the gold ore prior to
pretreatment is
converted to pyrrhotite represented by the following formula: Fei-.S (where
x=0-0.2).
This reaction is typically expressed by the formula FeS2¨>FeS+S.
Pyrrhotite is a kind of iron sulfides having Fe:S=0.8 to 1:1 in the
stoichiometric
ratio. In the present invention, the conversion of Fe in the pyrite to
pyrrhotite is
judged by XRD analysis. Namely, when a peak derived from Fei-xS is recognized
through XRD analysis, it is judged that the pretreated ore includes pyrrhotite
and
that Fe in the pyrite is converted to pyrrhotite. The degree of conversion can
be
assessed to some extent by whether the peak derived from FeS2 exists certainly
or at
the minimum. The condition for XRD analysis is set at starting (20) angle of
30,
finishing (20) angle of 90 , sampling width of 0.02 , scanning speed of 4
/min,
divergence slit of 1 , scattering slit of 10, receiving slit of 0.3mm,
divergence
longitudinal limitation slit of lOmm, voltage of 40kV, and electric current of
20mA. In
Examples, Rint Ultima2200 available from Rigaku Corporation (formerly Rigaku
Denki) was used. The XRD result for the ore prior to conversion is compared
with
that after conversion. If the intensified peak corresponds to that for
pyrrhotite, it is
considered that pyrrhotite is observed.
(0029)
The conversion rate is acceptable if it is 60% or more, preferably 80% or
more, yet
preferably 90% or more, and yet further preferably 95% or more. In the present
invention, the conversion rate is calculated according to the following
formula.
Conversion rate = Fe amount derived from pyrrhotite in the gold ore after
pretreatment/Fe amount derived from pyrite in the gold ore prior to
pretreatment.
The Fe amount derived from pyrrhotite in the gold ore after pretreatment is
calculated according to the following procedure. 50g of the pretreated gold
ore after
pretreatment is subjected to leaching at 85 C for 180min with agitation in 1L
of
hydrochloric acid (1.0mol/L) containing lmol/L of Fe3+, which is then
filtered. The Fe
concentration in the filtrate is determined by ICP-AES (in Example,
Mode1:SPS4000
available from Hitachi High-Technologies Corporation (formerly SIT) was used.)
(Fe
initially contained in the hydrochloric acid should be deducted.). The Fe
concentration after deduction is assumed as all derived from pyrrhotite. Since
pyrite
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is insoluble to hydrochloric acid, such assumption is permitted. Based on the
determined Fe concentration, the amount of liquid and the amount of ore, Fe
amount
is calculated. Specifically, it is represented as follows: Fe amount derived
from
pyrrhotite = (determined Fe concentration ¨ initial Fe concentration) (g/L) x
liquid
amount (1L) ore amount (50g)
The Fe amount derived from pyrite in the gold ore prior to pretreatment is
calculated according to the following procedure. 0.2g of gold ore prior to
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, M0de1:SPS4000 available from Hitachi High-Technologies Corporation
(formerly SII) was used.). Based on the determined Fe concentration, the
amount of
liquid and the amount of sample, Fe amount is calculated. Specifically, it is
represented as follows: Fe amount derived from pyrite = determined Fe
concentration
(g/L) x liquid amount (30mL) sample amount (0.2g).
(0030)
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.
(0031)
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.
(0032)
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. In the pyrolysis according to the present invention, since
most pyrite
(FeS2) is converted to pyrrhotite (iron (II) sulfate) and not converted to
hematite
(Fe203), it was anticipated that the ratio of gold-leaching would not be
sufficient.
Therefore, it was quite amazing that such remarkable result has been attained.
(0033)
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.
(0034)
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 pyrotite.. 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
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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.
(0035)
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.
(0036)
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.
(0037)
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.
(0038)
(3) Hydrometallurgical process
The effect of the present invention can be exerted by recovering gold from the
pretreated gold ore according to the present invention 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.
(0039)
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
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 +81{2 SO4 - (1)
On the contrary, in case where the pretreatment is conducted, the oxidation
reaction
of iron sulfide is represented by the following formula.
4FeS+902+4H2 0 ¨> 2Fe2 03 +4H2 SO4 ¨ (2)
Though not intending to limit the invention by any theory, if the reaction
occurs in a
container, the reaction (2) is considered to more easily proceed than the
reaction (1)
since the reaction (2) produces less product than the reaction (1) for one
equivalent of
iron sulfide.
(0040)
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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.
(0041)
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.
(0042)
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.
(0043)
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. More specifically, after
the
pretreatment of the present invention, sufficiently high gold leaching speed
can be
obtained only with chloride ions. The gold leaching speed is comparative to or
more
than the case where the pretreatment is not conducted and chloride ions and
bromide
ions are both used. When the pretreatment according to the present invention
is
conducted and chloride ions and bromide ions are both used, the gold leaching
speed
is dramatically improved. 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.
(0044)
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.
(0045)
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
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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.
(0046)
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
(FeC13),
respectively, but cuprous chloride (CuCO 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.
(0047)
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.
(0048)
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.
(0049)
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.
(0050)
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.
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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.
(0051)
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.
(0052)
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.
(0053)
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.
(0054)
[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).
(0055)
[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 and the proportion of
pyrite in the
iron sulfide of the concentrate was determined by XRD and chemical analysis,
and 17
mass% and 95 mass % or more was confirmed, respectively. In addition, the
ratio of S
(mass%)/Au(mass ppm) in the concentrate was 1.4. The pyrite ore concentrate
was
milled and ground in a ball mill to adjust the particle size to 5011m 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 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
CA 02899053 2015-07-22
11
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.
(0056)
TABLE 1
Gold leaching solution
FeC13-6H20(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/L) 80
ORP(mV) 717
(vs.Ag/AgC1)
pH 1.52
(0057)
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.
(0058)
<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.
(0059)
For the pyrite concentrates prior to and after the heat treatment, particles
containing iron sulfide were searched and it was confirmed that pores have
generated
in them through the observation with SEM. The SEM images are shown in Fig. 5
(prior to pretreatment) and Fig. 6 (after pretreatment). In addition, pore
volume
distribution was obtained by the mercury intrusion method and the change of
accumulative pore volume for pores having a diameter of 3 to 5 pm before and
after
the heat treatment was investigated. It was 0.02cc/g prior to pretreatment and
increased to 0.04cc/g after pretreatment. The determination of pore volume
distribution was carried out under the following conditions.
Measurement device: Pore Master 60-GT (available from Quantachrome)
Sampling amount: 0.5 to 1.0g
Sample cell: small cell (10cpx3Omm)
Measurement range: high pressure range
Measurement range: pore diameter of 0.0036pm to 10pm
Purity of mercury: Special grade (99.9999 mass%)
CA 02899053 2015-07-22
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Mercury contact angle: 140deg
Mercury Surface tension: 480dyn/cm
(0060)
For the pyrite concentrates prior to and after the heat treatment, the
presence of
Fei-,S was confirmed by the XRD analysis according to the condition explained
earlier. For the pyrite concentrate prior to the heat treatment (see Fig. 3),
there was
no peak for Fei-.S but there was a peak for FeS2. In contrast, for the pyrite
concentrate after the heat treatment (see Fig. 4), the peak for Fei,S was
confirmed
but the peak for FeS2 was not confirmed. Accordingly, it was determined that
the
pyrite was converted to pyrrhotite. The conversion rate was calculated as 98%
or
more according to the method explained earlier. In addition, the proportion of
pyrite
among the iron sulfides contained in the concentrate after pretreatment was
2mass%
or less.
(0061)
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/AgC1) was maintained at least 400mV.
During the leaching, hydrochloric acid was appropriately added to keep the pH
of the
gold-leaching solution at 1.0-1.1.
(0062)
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.
(0063)
<Investigation of influence on pore volume caused by heating condition>
A sample of pyrite ore concentrate with a different origin from Example 1 was
prepared. A pyrite reagent sample with little impurity was also prepared for
reference purpose. For the pyrite ore concentrate sample with a different
origin from
Example 1, the proportion of pyrite among the iron sulfides contained in the
sample
was 95mass% or more, the content of pyrite in the sample was 64 mass%, and
S(mass%)/Au(mass ppm) was 3.2. For the pyrite reagent sample, the proportion
of
pyrite among the iron sulfides contained in the sample was 100mass%, the
content of
pyrite in the sample was 95 mass% or more, and Au was not contained. After
grinding, each sample (1.5kg) was charged in a tubular furnace and heat-
treated
under the conditions shown in Table 2. For these samples after the heat
treatment,
pore volume distribution was obtained by the mercury intrusion method and the
change of accumulative pore volume for pores having a diameter of 3 to 5 pm
before
and after the heat treatment was investigated. The result is shown in Table 2.
(0064)
CA 02899053 2015-07-22
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TABLE 2
Sample type Heating Conversion rate Accumulative pore vol
condition (%) ume for 3 to 511m pore
diameter (cc/g)
Same as Example 1 Before heating 0.019
Same as Example 1 Nitrogen atmosphere 98% or more 0.041
700 Cx30min
Different origin from Before heating 0.021
Example 1
Different origin from Nitrogen atmosphere 98% or more 0.064
Example 1 700 Cx30min
Pyrite reagent Before heating 0.003
Pyrite reagent Nitrogen atmosphere 98% or more 0.055
700 Cx30min
(0065)
<The change in Fel-S and conversion rate caused by pyrolysis condition>
Using 1.5 kg of the ground pyrite ore concentrate used in Example 1, the
presence of
Fei-.S and conversion rate were investigated when the retention temperature
and the
retention time was changed as shown in Table 3. The presence of Fel-xS and the
conversion rate 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 10 C/min for all tests. Cooling was
conducted by allowing it cool to a room temperature. The results are shown in
Table3.
(0066)
TABLE 3
Heating Condition Presence of Conversion
Retention Retention Fei-xS rate
temp.( C) time(min) %)
Before heat treatment 0
550 60 X 48
550 120 A 54
600 5 A 49
600 30 A 61
600 60 o 76
650 60 o 95
700 60 o 98
(0067)
The standards for "Presence of Fei-xS" are as follows.
o: Fei-xS peak is confirmed but FeS2 peak is not confirmed or is at minimum.
A: Fei-xS peak and FeS2 peak are both confirmed.
X: Fei-,S peak is not confirmed.
(0068)
From the result shown in Table 3, the presence of Fel-xS, which was confirmed
CA 02899053 2015-07-22
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when heated to 550 C or more, shows pyrolysis of crystalline pyrite. 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 the conversion rate exceeds 80%.
(0069)
< 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
of the XRD analysis, a long period of time is necessary for pyrolisys and
accordingly
heat treatment at 600 C or higher is desirable.
