Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Smelting Method
[0001] This invention relates to a smelting method for the recovery of gold
from source
materials which contain gold.
Background
[0002] The process of gold recovery frequently involves a leaching step and
adsorption of
gold and other precious metals onto an adsorbent such as carbon or a suitable
synthetic
resin. Improvements in the adsorption process such as the carbon in column
(CIC),
carbon in leach (CIL) and carbon in pulp (CIP) processes have led to efficient
gold
recovery which in some cases have even justified reprocessing of mine
tailings.
Precious metals are stripped from the adsorbent by elution using suitable
solubilising
liquors for precious metals to form a strip liquor containing the precious
metals stripped
from the absorbent.
[0003] Precious metals including gold and silver may be recovered from the
strip liquor in
an electrowinning process in which the precious metals are deposited from the
strip
liquor onto the cathode of an electrowinning cell.
[0004] The electrode-associated material includes materials such the direct
cathode
deposits and electrode-associated sludge which may collect on or below the
cathode of a
gold electrowinning cell. At least some of the cathode associated material
generally has
the morphology of a fine wire or fine wire coating. This material forms clumps
comprising
elongated high surface area components rather than solid independent
particles.
[0005] The word clump and variation such as clumps is used herein to refer to
a cluster
or lump particularly a bunch filaments of elongated material such as obtained
from
cathode deposits of gold rich material derived from wire-type cathodes. Where
used
herein the term particle and variations such as particles and particulate are
intended to
include material in clump form.
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[0006] The cathode in this process is usually a high surface area cathode, and
may
comprise steel wool. Both the material deposited on the cathode (often called
wire-gold
where the cathode is steel wool) and cathode slimes (deposits which collect
beneath and
in association with the cathode) are rich in precious metals, and the next
step in the
precious metal recovery process usually involves acid treatment to remove
steel wool,
followed by smelting and bullion formation.
[0007] When copper is refined by electrolysis the anodes are frequently cast
from
processed blister copper placed into an aqueous solution of 3-4% copper
sulfate and 10-
16% sulfuric acid. Cathodes are often thin rolled sheets of highly pure
copper. At the
anode, copper and less noble metals dissolve. More noble metals such as silver
and
gold as well as selenium and tellurium settle to the bottom of the cell as
anode mud,
which forms a saleable by-product. The anode mud therefore includes the anode
associated gold.
[0008] Gravity gold, that is gold refined by a gravitational process such as
tabling and
spiral classification, is another gold rich processing product.
[0009] A common method used to process gold-containing material, particularly
gold
concentrates (material at least partly refined to enrich precious metal
content), involves
smelting that material. Smelting involves mixing flux with source material,
placing the
mixture in a graphite crucible and heating to about 1250 C. Some base metal
contaminants are collected in the floating slag layer that forms over the
molten precious
metals. After cooling the slag can be physically separated, leaving a dore
metal bar that
can be sold as is or further processed to obtain more highly purified gold.
[0010] There is an ongoing need for processes which increase the levels of
gold
recovery.
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Summary
[0011] There is provided a method of recovering gold from a gold containing
source
material comprising:
forming a molten pool comprising at least one collector metal selected from
the
group consisting of copper, silver, gold and platinum group metals; and
adding at least part of the source material into the molten pool of collector
metal.
[0012] In one set of embodiments the method comprises:
forming a molten pool of fluxing agent;
forming a molten pool of collector metal selected from the group consisting of
copper, silver, gold and platinum group metals, beneath the molten pool of
fluxing agent;
and
adding the gold containing source material to the molten pool of fluxing
agent.
The source material added to the fluxing agent typically becomes incorporated
in the
molten pool of collector metal.
[0013] In one set of embodiments there is provided a method of recovering gold
from a
gold-containing source material comprising:
treating the source material to at least partly remove base metals,
particularly lead
or iron, from the gold-containing source material;
forming a molten pool comprising at least one metal selected from the group
consisting of gold and metal which form an alloy with gold; and
adding at least part of the treated source material into the pool of molten
metal.
[0014] In one set of embodiments the molten pool is formed from a particulate
mixture
comprising particulate source material and particulate material comprising at
least one
collector metal selected from the group consisting of copper, silver gold and
platinum
group metals. In this set of embodiments a blend of particles of source
material and
particles of collector metal may be formed and added to a crucible in which
the molten
pool is formed by heating to the melting temperature of the collector metal.
The mixture
of the particulate source material and particulate collector metal is, in one
set of
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embodiments, gradually added or added in a discrete sequence of two or more
charges
to a heated crucible such that a molten pool is formed during addition and
further
particulate mixture is added to and becomes part of the molten pool.
[0015] In a particularly preferred set of embodiments the source material
comprises
cathode associated material which is deposited on, or collects below, the
cathode of a
gold electrowinning cell.
[0016] In one set of embodiments the method for recovering gold comprises
forming a
molten pool comprising source material and collector metal wherein the
collector metal
has a concentration of at least 80% by weight (preferably at least 90% and
more
preferably at least 95% and still more preferably at least 99% by weight) of
at least one
of copper, silver, gold and platinum group metals.
[0017] In a further set of embodiments the molten pool is formed from a
portion of
particulate source material enclosed in a sheet of collector metal. The sheet
may, if
desired, be added to a preformed molten pool of collector metal.
[0018] In another set of embodiments the molten pool is formed from collector
metal and
the source material is added to the molten pool. The manner and rate of
addition of the
material is preferably optimised. In one embodiment the material is added to
the molten
pool in a sequence of discrete charges. If the charges are too large, the
temperature
after charging is unduly reduced. If , on the other hand, the charges are too
small,
charged material tends to float on the molten phase rather than becoming
immersed
therein.
[0019] In one set of embodiments the particulate collector metal comprises
copper and
the copper is substantially free of oxidation reaction products and in
particular oxidation
products formed when copper metal is in contact with air. This is facilitated
if the crucible
is surrounded by an oxygen depleted gas, or if the copper is rapidly submerged
in a
molten phase such as a molten fluxing agent.
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[0020] In one set of embodiments the mixture of particulate source material
and
particulate collector metal further comprises particles of fluxing agent. The
fluxing agent
may be any suitable agent known to the person skilled in the art. For example,
the
fluxing agent may comprise at least 30% such as at least 50%, at least 80% by
weight
borax (more preferably at least 90% borax). The role of the fluxing agent is
to provide a
separate phase to receive impurities present in the source material. The
fluxing agent
comprising impurities solidifies at low temperature to form a slag phase which
can be
separated (for example mechanically separated) from the precious metal
containing
phase.
[0021] In one set of embodiments the smelting is conducted in a crucible which
comprises less than 10% by weight (preferably less than 5%) carbon and less
than 10%
by weight (preferably less than 5%) of carbides. The crucible may be a ceramic
vessel
and is preferably formed of clay.
[0022] In one set of embodiments there is provided a method of recovering gold
from a
gold-containing source material comprising:
treating the gold-containing source material to at least partially remove base
metals;
forming a molten pool comprising a collector metal selected from the group
consisting of gold, silver, copper and platinum group metals; and
adding the treated source material into the pool of molten metal.
[0023] In one example of this set of embodiments the molten pool is formed
from a solid
particulate mixture comprising particles of treated source material and
particles of
collector metal. In this set of embodiments the pool metal preferably
comprises silver,
copper or mixtures thereof.
[0024] The manner and rate of addition of the particulate mixture of the
treated source
material and collector metal may be optimised according to the specific nature
of the
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materials used. For example, the mixture may be gradually added to a preformed
molten
pool of fluxing agent. In this case the particulate mixture preferably
contains little or no
fluxing agent such as no more than 10%, no more than 5%, no more than 2% or no
more
than 1 % fluxing agent.
[0025] The particulate mixture of the treated source material and collector
metal is
preferably added to a heated crucible such that a molten pool is formed during
addition
and further particulate mixture is added to and becomes part of the molten
pool.
[0026] In one set of embodiments the smelting method comprises adding the
treated
source material to a previously melted pool of collector metal. The pool may
for example
initially contain at least 50%, such as at least 70%, at least 80% or at least
90 by weight
of the molten pool of the collector metal.
[0027] Throughout the description and the claims of this specification the
word "comprise"
and variations of the word, such as "comprising" and "comprises" is not
intended to
exclude other additives, components, integers or steps.
Detailed Description
[0028] During the process of smelting gold the present inventor has found that
a
significant amount of gold, frequently of the order of from 1 to 3% or even
more, is lost to
slag. Even when the slag is ground and reintroduced into an earlier part of
the gold
recovery circuit this slag associated gold may be substantially unrecoverable.
The
inventor has also found that mixing of source material and fluxing agents may
facilitate
the loss of gold values to slag.
[0029] In general it has been found that recovery of gold is improved by
forming a molten
pool comprising at least one collector metal selected from the group
consisting of copper,
silver, gold and platinum group metals; and adding at least part of the source
material
into the molten pool of collector metal.
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[0030] The method comprises forming a molten pool comprising at least one
collector
metal selected from the group consisting of gold and metal which form an alloy
with gold.
The pool may for example initially contain at least 50%, such as at least 70%,
at least
80% or at least 90 by weight of the molten pool of the collector metal. In a
further set of
embodiments the molten pool is formed only from collector metal comprising at
least one
of copper, silver, gold and platinum group metals and the gold containing
source material
is added to the molten pool formed from the collector metal.
[0031] It is preferred that the pool metal is formed from at least one metal
selected from
the group consisting of copper, silver, gold and platinum group metals and
more
preferably silver or gold. It is preferred that the pool metal is free of
iron.
[0032] The gold-containing source material is preferably added to the pool of
molten
metal in a particulate form such as fine particles of size passing through a
100 micron
sieve, more preferably 80% by weight passing through a 100 micron sieve.
[0033] In one set of embodiments the source material is in divided form such
as
particulate from and comprises a particulate mixture comprising particulate
source
material and collector metal. The collector metal is preferably in the form of
fine particles
of size passing through a 100 micron sieve, more preferably 80% by weight
passing
through a 100 micron sieve.
[0034] The smelting may be conducted in the presence of a fluxing agent, such
as a
fluxing agent comprising borax, or alternatively the smelting may be carried
out with a
composition substantially free of fluxing agent (e.g. less than 5% by weight
fluxing agent
such as less than 2% by weight fluxing agent or less than 1 % by weight of
fluxing agent
based on the total weight of smelt composition. A fluxing agent, when used,
preferably
comprises at least 30% borax, more preferably the fluxing agent comprises at
least 50%
by weight borax, more preferably 80% by weight borax and still more preferably
at least
90% borax.
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[0035] When fluxing agent is used in the set of embodiments which comprises
smelting
a mixture comprising particulate source material and particulate collector
metal it may be
preferred to form a molten pool of borax and add a mixture of particulate
source material
and material comprising at least one metal selected from the group consisting
of copper,
silver gold and platinum group metals to the molten borax.
[0036] In yet another set of embodiments a pool of molten collector metal is
formed and
particulate source material is added, preferably gradually or in discrete
sequential
charges, to the molten pool of collector metal.
[0037] In another set of embodiments the molten pool is at least partially
formed from a
portion of particulate source material enclosed in a sheet comprising
collector metal. In
this embodiment the sheet initially forms a barrier between the crucible and
particulate
source material and further source material is added, preferably gradually, to
a molten
pool formed of the sheet and particulate source material. Alternatively the
sheet
enclosing source material is added to a preformed molten pool of collector
metal.
[0038] As described above it is generally preferred that addition of gold-
containing source
material to the molten pool takes place in discrete sequential additions. The
discrete
sequential addition may for example be provided by adding gold-containing
source
material to the molten pool through a conduit, such as a ceramic pipe, that
guides said
material into the bulk phase of the molten pool.
[0039] Preferably the molten pool of collector metal to which the source
material is added
extends to the side wall of the crucible used in smelting.
[0040] In instances where the particulate collector metal comprises copper it
is
particularly preferred that the copper is substantially free of oxidation
reaction products
and in particular oxidation products formed when copper metal is in contact
with air.
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[0041] A range of types of crucible are known for use in smelting gold-
containing source
materials and may be used in the method described above. In a preferred set of
embodiments smelting is conducted in a crucible which comprises less than 10%
by
weight (preferably less than 5%) carbon and less than 10% by weight
(preferably less
than 5%) of carbides. Crucibles formed of ceramics, particularly clay are
generally more
preferred.
[0042] Preferably the collector metal composition has a melting point in
excess of 900 C.
[0043] In embodiments in which a collector metal is used, particularly copper,
it is
generally preferred that the pool of molten metal cover the internal bottom
wall of the
crucible so that source material added to the crucible will contact the molten
metal rather
than the surface of the crucible. If the bottom of the crucible is concave or
curved,
sufficient metal collector is preferably used to ensure that the molten pool
extends to the
side wall of the crucible. A conduit, as described above, may also be used to
substantially avoid direct contact of the added gold-containing source
material with the
crucible used in smelting.
[0044] In one set of embodiments the smelting of the gold rich source material
in a
crucible further comprises:
Forming a molten pool of a fluxing agent such as borax;
forming a molten pool of collector metal, comprising at least one metal
selected
from the group consisting of gold, silver, copper and platinum group metals,
beneath the
molten pool of fluxing agent; and
adding the gold rich source material to the molten pool of fluxing agent
wherein
the source material becomes incorporated in the molten pool of collector
metal.
[0045] On completion of the addition of the collector it is preferred that the
molten pool of
fluxing agent cover the molten pool of collector metal and preferably the
distance
between the top of the molten pool of collector metal and the top of the
liquid fluxing
agent is optimised to provide as shallow a layer of fluxing agent as is
practicable to
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ensure the molten pool of collector metal remains substantially covered during
the
introduction of gold-containing source material into the molten pool.
Preferably, this
distance is at least 1 cm.
[0046] In order to optimize recovery in this set of embodiments it may be
preferred to use
coarsely divided collector metal for addition to the pool of fluxing agent
preferably of
particle size of at least 1 mm. Without being bound by theory we believe the
use of
coarser particles minimized oxidation of the collector particularly where the
collector is
copper.
[0047] The molten pool of collector metal may be formed by addition of
collector metal to
the molten pool of fluxing agent and providing a temperature not less than the
melting
point of the collector metal.
[0048] The collector metal may, for example, be present in an amount of up to
5000 parts
collector metal per 100 parts source metal such as 5 to 5000 parts per 100
parts source
material.
[0049] In one set of embodiments the smelting method comprises adding the
treated
source material to a previously melted pool comprising at least one metal
selected from
copper, silver, gold and platinum group metals.
[0050] The method of recovering gold may further comprise treating the gold-
containing
source material to at least partially remove base metal prior to forming the
molten pool
comprising at least one metal selected from the group consisting of gold and
metals
which form an alloy with gold (preferably at least one of copper, silver, gold
and platinum
group metals).
[0051] Base metals referred to herein may be in the form of metal compounds or
other
metal moieties such as complexes or mixed valence species or mixed oxide
species.
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[0052] The removing of base metals preferably comprises leaching the gold-
containing
source material with an aqueous leach liquor to remove significant amounts of
base
metals from the gold-containing source material.
[0053] The removed base metal preferably comprises removal of lead and/or
iron.
[0054] The removal of base metals is preferably carried out at temperatures
sufficiently
low to prevent the formation of a full or partial molten environment in the
concentrate
prior to aqueous leaching to remove base metal moieties.
[0055] In one set of embodiments the gold-containing source material comprises
less
than 3ppm of platinum group metals (preferably less than 1 ppm).
[0056] In one set of embodiments of the invention sufficient base metals are
removed so
that slag formation in fluxless smelting conditions is less than 1%
(preferably less than
0.1 %) by weight of the molten pool. Slag formation can be determined by
observing the
presence of a distinct phase other than metal. The slag will typically contain
compounds
formed between metals and non metals particularly metal oxides.
[0057] Examples of source materials that are rich in precious metals include,
but are not
limited to:
a. cathode-associated material formed during electrolysis of a strip liquor.
The
strip liquor may arise when gold is stripped from an activated carbon;
b. anode-associated material formed during the electrolytic refining of copper
or
other base metal from a base metal cast anode; and
c. gravity gold.
[0058] The process of gold recovery may involve a leaching step and adsorption
of gold
and other precious metals onto and adsorbent such as carbon or a suitable
synthetic
resin. Improvements in the adsorption process such as the carbon in column
(CIC),
carbon in leach (CIL) and carbon in pulp (CIP) processes have led to efficient
gold
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recovery which in some cases have even justified reprocessing of mine
tailings.
Precious metals are stripped from the adsorbent by elution using suitable
solubilising
liquor to form a strip liquor containing the precious metals stripped from the
absorbent.
[0059] Precious metals including gold and silver may be recovered from the
strip liquor in
an electrowinning process in which the precious metals are deposited from the
strip
liquor onto the cathode of an electrowinning cell.
[0060] The electrode-associated material includes materials such the direct
cathode
deposits and electrode-associated sludge which may collect on or below the
cathode of a
gold electrowinning cell. The electrode-associated material may also comprise
anode
mud from a copper electro-refining process.
[0061] The cathode in this process is usually a high surface area cathode, and
may
comprise steel wool. Both the material deposited on the cathode (often called
wire-gold
where the cathode is steel wool) and cathode slimes (deposits which collect
beneath and
in association with the cathode) are rich in precious metals, and the next
step in the
precious metal recovery process usually involves acid treatment to remove
steel wool,
followed by smelting and bullion formation.
[0062] When copper is refined by electrolysis the anodes are frequently cast
from
processed blister copper placed into an aqueous solution of 3-4% copper
sulfate and
10-16% sulfuric acid. Cathodes are often thin rolled sheets of highly pure
copper. At the
anode, copper and less noble metals dissolve. More noble metals such as silver
and
gold as well as selenium and tellurium settle to the bottom of the cell as
anode mud,
which forms a saleable by-product. The anode mud therefore includes the anode
associated gold.
[0063] A common method used to process gold-rich electrode associated material
involves smelting that material. Smelting involves placing the source material
in a
crucible, adding fluxing agents and heating to about 1250 C. Base metal
contaminants
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are collected in the floating slag layer that forms over the molten precious
metals. After
cooling the slag can be physically separated from the dore metal bar and
further
processing can take place to obtain more highly purified gold.
[0064] When aqueous leach liquor is used to remove base metals the aqueous
leach
liquor may comprise at least one of an aqueous alkaline, aqueous acid, aqueous
reducing liquor leaching and an aqueous chelating agent for lead moieties and
mixtures
of two or more thereof.
[0065] Methods which can be used to at least partially remove base metals
include acid
leaching. The acid leaching may be conducted using an acid such as
hydrochloric acid,
nitric acid, citric acid. The leach liquor may also comprise chelating agents
such as the
agents described in US 5916534. Removal of base metals can be enhanced by
agitation
and in particular ultrasonic agitation.
[0066] It is preferred to conduct the leaching at a temperature above ambient.
It may be
preferred in some embodiments of the invention to conduct the leaching at a
temperature
of at least 60 C.
[0067] Leach liquors for removing base metals may also be reducing liquors. In
this set
of embodiments the reducing liquor may be provided by a reducing agent, by
contact
with a reducing electrode or combination of two or more thereof. The reducing
agent is
preferably compatible with aqueous liquor and may be metal containing or non-
metal
containing. Examples of suitable metal containing reducing agents include
metal
containing moieties in a valence state lower then the maximum stable valence
state
achievable in an aqueous solution. The more preferred metals may be chosen
from the
group consisting of chromium (Cr II), tin (Sn II), copper (Cu I) and titanium
(Ti II, Ti III),
most preferably tin (Sn II). In a preferred embodiment, the aqueous reducing
liquor
comprises stannous ion, for example stannous chloride.
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[0068] Examples of suitable non-metal containing reducing agents include
sulfites and
oxalic acid, formic acid, hydrazine, sulfite and dithionite. There may be two
or more
contacts between a leach liquor and the source material. In this set of
embodiments it is
preferred that the reducing liquor in at least one contact with source
material is acidic,
preferably the pH is less than about 1.5, more preferably less than about 1Ø
Preferably
the acid is a non-oxidising acid. Preferably the acid is hydrochloric acid.
The reducing
agent may be a regenerable reducing agent, for example a reducing agent which
can be
regenerated from the oxidised form produced as a result of the process by
electrolytic
regeneration of the reducing agent.
[0069] The removal of base metals may use a lead complexing or solubilising
agent.
Examples of lead complexing or lead solubilising agents may be aqueous liquors
comprising one or more selected from the group consisting of hydrochloric
acid, nitric
acid, alkaline material such as sodium hydroxide or other hydroxide moities or
other
water-compatible alkalis, ,lead acetate, ammonium chloride, chlorides,
carboxylic acids
and their salts, chelating agents, fluoro silicate, phenol sulfonate, peroxy-
disulfate and
any other agent that enhances the solubility of lead oxide moieties in water.
When the
lead complexing or solubilising agent is selected from carboxylic acids and
their salts or
chlorides it is preferred that (a) the carboxylic acids are selected from the
group
consisting of citric acid, lactic acid, acetic acid, formic acid, iso-butyric
acid, acetyl
salicylic acid and their salts such as the alkali and alkaline earth metal
salts and (b) the
chlorides are selected from the group consisting of ammonium chloride, sodium
chloride,
potassium chloride, calcium chloride and strontium chloride.
[0070] Preferably the contact step between source material and reducing
aqueous liquor
leads to a bleaching of the source material. The bleaching may be measured
using
quantitative colorimetric methods, such as the LAB method. The reducing leach
may
produce at least partial removal of a base metal from the source material.
Without being
bound by theory it is believed likely that the use of a reducing leach may
facilitate the
dissolution of moieties comprising Iron (III), and that these moieties are
responsible or
partially responsible for immobilizing gold. Evidence for the dissolution of
moieties
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comprising iron III includes decoloration of material after leaching. Leaching
may be
carried out in liquors comprising 1 % HCI and one or more reducing agents such
as tin (II)
chloride, chromium (II) chloride and oxalic acid. Based on the observed degree
of
decoloration the effectiveness of reducing agents decreases according to the
ranking tin
(II) chloride, >_ chromium (II) chloride > oxalic acid.
[0071] In one embodiment of the invention the contact between the source
material and
aqueous liquor is carried out in conditions that encourage the dislodgment of
refractory
material from the surface of the solid. Such conditions may include ultrasonic
agitation
and stimulation by time variant electrical and/or magnetic field.
[0072] In one set of embodiments the leaching treatment of the gold-containing
source
material comprises 2 or more leaching steps to remove or facilitate removal of
base
metals. Preferably at least one leaching step is a reducing leach and the
other is a nitric
acid leach. Preferably the nitric acid leach is a lower pulp density leaching
step where
the leach pulp density is less than 5% pulp in leach liquor, more preferably
less than 3%,
even more preferably less than 2% or even less than 1 %.
[0073] In one set of embodiments base metals are at least partly removed by an
aqueous
leach liquor comprising one or more agents selected from the group consisting
of
hydrochloric acid, nitric acid, alkali, lead acetate, chelating agents,
carboxylic acids and
their salts, chlorates, perchlorates, chlorides, fluorosilicate, phenol
sulfonate, and
peroxydisulfate.
[0074] The aqueous leach liquor may, for example, comprise aqueous acid
(preferably
hydrochloric acid or nitric acid, more preferably 0.5 to 5M hydrochloric acid
or nitric acid
0.5M to 10M and more preferably 1 to 5M nitric acid).
[0075] In one set of embodiments the method further comprises subjecting
leaching in an
aqueous reducing liquor and leaching the solid residue from leaching in
aqueous
reducing liquor to at least one leaching step in an aqueous liquors comprising
agents
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selected from the group consisting of hydrochloric acid, nitric acid, alkali,
lead acetate,
chelating agents, carboxylic acids and their salts, chlorates, perchlorates,
chlorides,
fluorosilicate, phenol sulfonate, and peroxydisulfate.
[0076] The reducing liquor comprises, in one set of embodiments, at least one
base
metal chelating agent, preferably selected from the group consisting of beta-
diketones,
amino polycarboxylic acids, salts of amino polycarboxylic acids, carboxylic
acids, salts of
carboxylic acids, and polyphosphonates.
[0077] The gold-containing source material, in one set of embodiments, is
leached with
an aqueous leach liquor comprising a reducing and/or acid leach liquor
followed by
leaching with an alkaline liquor preferably of pH greater than 13, more
preferably of pH
greater than 14 and most preferably aqueous sodium hydroxide of concentration
at least
5% by weight.
[0078] The leaching of gold-containing source material with an aqueous leach
liquor may
further comprise subjecting the cathode associated gold concentrates to
ultrasonic
radiation at a frequency in the range 10 - 60 kHz, preferably 20 - 45 kHz.
[0079] Preferably sufficient base metals are removed so that slag formation in
fluxless
smelt conditions is less than 1 % (preferably less than 0.1 %) by weight of
the molten pool.
[0080] The invention will now be described with reference to the following
examples. It is
to be understood that the examples are provided by way of illustration of the
invention
and that they are in no way limiting to the scope of the invention.
EXAMPLES
Example 1
Gold Gravity Concentrate (GGC) - Source material
[0081] Gold loaded carbon from the gravity gold circuit was stripped in
caustic cyanide
and the strip liquor processed in an electrowinning cell. Cathode material and
cathode
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sludge from the cell was aggregated and soaked in 25% HCI for 2 hrs, to leach
out steel
wool from the sample. The residual material was rinsed and dried to provide
12.5 kg of
source material.
[0082] This source material was homogenised by crushing and chopping, and
multiple
sub-samples of approx 10g were riffle split. Apart from the 10g sub-samples
the
remainder of the material was smelted using a standard process, and the
commercially
recoverable gold was found to be 60.4% gold.
[0083] A 10.06g sub-sample (particle size sub 250 microns) was added to a
500m1
beaker. Liquor comprising 8g stannous chloride dihydrate (dissolved), 100ml
concentrated HCI and 100 ml water was added to the beaker, and the beaker was
placed
in a heated ultrasonic bath (Soniclean, maximum power = 250W) at 60 C for 8
hours.
Ultrasonic agitation (60% max setting) was applied according to the following
schedule:
minutes initial sonication, 80 minutes pause, 10 minutes sonication, 80
minutes pause
and so on to the end of the 8 hour period. No mechanical agitation was used.
[0084] After 8 hours, the contents of the beaker were filtered (Whatman 40
ashless filter
paper, equivalent in filtration speed to Whatman 2) and the residue on the
filter paper
washed with water. The residue was then washed from the paper into another 500
ml
beaker, and care was taken to use less than 100ml of water to achieve this
transfer. The
water level in the beaker was made up to 100ml, and 100mis of 8% aqueous
sodium
hydroxide liquor was added to provide 4% final caustic leach liquor for the
second leach.
The beaker was placed in a heated ultrasonic bath and treated according to the
above
protocol. After filtration and water washing, the residue was dried in an oven
at 80 C
overnight. The residue cake was readily disrupted to make a fine powder by
simple
mechanical stimulus with a spatula.
[0085] Fine silver granules (plus 99.9% silver) were purchased from PW Beck &
Co silver
merchants of Adelaide, Australia. The granules were approximately 2mm in
diameter.
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Sheet silver (fine silver grade) of diameter 0.3mm, with each sheet weighing
10g was
also purchased from PW Beck & Co.
[0086] 100 g of the fine silver granules were placed in a 250 ml fire assay
crucible made
from clay purchased from Furnace Industries, of Perth Australia. The loaded
crucible was
placed inside an electric furnace and brought to 1220 C. Molten silver derived
from the
granules formed a small pool on the bottom of the crucible.
[0087] Dried residue derived from the caustic leach step described above was
folded into
a 10g piece of sheet silver. The hot crucible containing the silver pool was
withdrawn
from the furnace, and the silver sheet envelope was dropped into the crucible
directly
onto the molten silver pool. The sheet silver melted quickly and the contents
of the silver
sheet envelope became immersed in the silver pool without making contact with
the
sides of the crucible. The crucible was immediately returned to the furnace,
brought back
to 1220 C and retained at that temperature for 15 minutes. The molten contents
of the
hot crucible were poured into a hemispherical button mould, and allowed to
cool. The
button was dislodged from the mould and quenched in water, then allowed to
dry. The
approximate dimensions of the hemispherical button were: diameter 4cm, max
height
3cm. The button was drilled out to obtain approx 6g of shavings and burrs,
which were
sent for bullion assay by Umpire Assay Laboratories, in Perth Australia.
[0088] The initial 10.06g sub-sample comprised gold at 60.4% (multiple bullion
assay
results on replicate samples). The gold recovered from the button described
above was
6.16g, and 0.14g gold (total) was assayed on the filter papers used in the
acid and
alkaline leaching steps prior to smelting. This corresponds to a total of 6.3g
gold
recovered from the initial sub-sample, compared to 10.06 x 0.604 = 6.076g gold
expected from the bullion assay on the initial sub-sample. The 0.368g gold
increment
represents the benefit obtained by using the method of the invention.
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Example 2
Gold Gravity Concentrate (GGC) - Source material (a)
[0089] Gold loaded carbon from the gravity gold circuit. was stripped in
caustic cyanide
and the strip liquor processed in an electrowinning cell. Cathode material and
cathode
sludge from the cell was aggregated and soaked in 25% HCI for 2 hrs, to leach
out steel
wool from the sample. The residual material was rinsed and dried to provide
12.5 kg of
source material.
This source material was homogenised by crushing and chopping, and multiple
10g sub-
samples were riffle split. Apart from the 10g sub-samples the remainder of the
material
was smelted using a standard process, and the commercially recoverable gold
was
found to be 77.06% gold.
Gold Carbon in Pulp (CIP) Concentrate - Source Material (b)
[0090] Gold loaded carbon from the C-I-P circuit was stripped in caustic
cyanide and the
strip liquor processed in an electrowinning cell. Cathode material and cathode
sludge
from the cell was aggregated and soaked in 25% HCI for 2 hrs, to leach out
steel wool
from the sample. The residual material was rinsed and dried to provide 12.5 kg
of source
material.
[0091] This source material was homogenised by crushing and chopping, and
multiple
10g sub-samples were riffle split. Apart from the 10g sub-samples the
remainder of the
material was smelted using a standard process, and the commercially
recoverable gold
was found to be 35.04% gold.
[0092] Source material (a) and (b) as described above was then subjected to a
leaching
process comprising a combination of a reducing leach step, an alkaline leach
step and/or
a nitric acid leach step, prior to silver pool smelting.
The procedures adopted for each leach and the silver pool smelt are as follows
and the
results obtained are set out in Table 1 and Table 2.
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Reducing Leach Step
[0093] Take 10 g sub-sample and add to reducing liquor. The leaching process
as
described in the following:
One of the10g sub-samples was added to a beaker with liquor comprising 200 ml
of 50%
HCI and 8g stannous chloride. The contents of the beaker were heated to 80 deg
C and
after 5 minutes the beaker was placed in a "Soniclean 160T" ultrasonic bath
(bath water
at 60 deg C, frequency 40kHz, maximum power 250W, power setting 60% of 250W =
150W). After 5 minutes of ultrasonic agitation the beaker was re-heated and
the cycle
repeated 2 times. The residue was obtained by filtration, rinsed in water and
dried.
[0094] Note: If the reducing leach is not the first leaching step, use leach
residue from
the previous leaching step. Note that the 10 g sub-samples were used as source
material
in the initial leaching step.
Alkaline leach step
[0095] Take 10 g sub-sample and add to alkaline liquor. The alkaline leach is
as
described in the following:
Residue from the reducing leach step (described above)was added to 200 ml of a
10%
sodium hydroxide liquor, and taken to 80 deg C for 5 minutes, followed by 3
cycles of
ultrasonic agitation as described above. The resultant residue was obtained by
filtration,
rinsed in water and dried.
[0096] Note: If the alkaline leach is not the first leaching step, use leach
residue from the
previous leaching step. Note that the 10 g sub-samples were used as source
material in
the initial leaching step.
Leaching in 50% Nitric Acid
[0097] Take 10 g sub-sample and add to 200 ml of 50% Nitric acid liquor.
Perform
ultrasonic agitation, filtering, rinsing and drying as described in the
preceding section
"reducing leach step".
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[0098] Note: If the acid leach is not the first leaching step, use leach
residue from the
previous leaching step. Note that the 10 g sub-samples were used as source
material in
the initial leaching step.
Silver Pool Smelting
[0099] Take 100 g of fine silver granules, add to a crucible and heat to 1220
C in an
electric furnace. Take sheet silver of diameter 0.3 mm (fine silver grade, 10
g per sheet)
and wrap the sheet around the finely divided material to be smelted, (this
material is the
residue remaining after previous leaching steps on 10 g of sub-sample) to form
a silver
envelope. Remove the hot crucible containing molten silver from the furnace
and drop
the silver envelope into it. Immediately return the crucible to the furnace
and reheat to
1220 C for 15 minutes. Pour the molten contents of the hot crucible into a
button mold
and allow to cool. Remove slag and drill out the button to obtain shavings and
burrs for
bullion assay.
[0100] Residual gold on filter papers and in leach solutions are added to aqua
regia and
gold found in this way is added to gold bullion numbers.
Table 1
Commercially Gold recovered in silver
Source recoverable pool smelt after leach
material gold Leach sequence sequence
b 35.04% Reducing leach then caustic 36.37%
leach
b 35.04% Nitric leach then reducing leach 37.74%
then caustic leach
b 35.04% Reducing leach then caustic 37.22%
leach then nitric leach
b 35.04% Reducing leach then caustic 37.87%
leach then 2 X nitric leach
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Table 2
Commercially Gold recovered in silver
Source recoverable pool smelt after leach
material gold Leach sequence sequence
a 77.06% 8 x nitric leach, then reducing 77.64%
leach, then 2 x caustic leach
Example 3
[0101] 500.97 g of wire gold (referred to as CIP-2) was received from a CIP
plant
processing facility. This sample was taken by representative sampling from a
wire gold
production run after hydrochloric acid treatment to remove the cathode wire
and the gold
grade of the sample (calculated by commercial smelting of the production
sample with
gold determination of the bullion bar by bullion assay from the Perth mint).
The gold
content was found to be 35.04% by weight.
Pre-smelt treatment
[0102] 10 g of CIP-2 (representative subsample obtained by riffle splitting)
was added to
200m1 of 50% by volume conc. nitric acid in water in a 600 ml beaker. The
beaker was
placed in a heated ultrasonic bath at 60 C ("Soniclean" brand, maximum power =
250W)
and agitated at maximum power for one hour. The liquor was filtered off and
the residue
washed with water.
[0103] The water washed residue was added to a liquor comprising 8 g stannous
chloride
dehydrate (dissolved) 100ml conc. hydrochloric acid and 100 ml water in a 600
ml
beaker. The beaker was placed in a heated ultrasonic bath at 60 C ("Soniclean"
brand,
maximum power = 250W) and agitated at maximum power for one hour. The liquor
was
filtered off and the residue washed with water.
[0104] The water washed residue from the previous step was added to 200m1 of
50% by
volume nitric acid in water in a 600 ml beaker. The beaker was placed in a
heated
ultrasonic bath at 60 C ("Soniclean" brand, maximum power = 250W) and agitated
at
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maximum power for one hour. The liquor was filtered off and the residue washed
with
water, and dried.
[0105] All filter papers and liquors produced in the above operations were
assayed for
gold by standard techniques.
The dried residue was then smelted as described below in Table 3 and the
results set
out in Table 4 were achieved.
Table 3 - Smelt treatments
Sample Sample Particulate mixture Crucible
Number
CIP2 SLCIP2M19 2 g treated residue plus 2 gms silver Clay
powder plus 20 g borax
SLCIP2M20 2 g treated residue plus 2 gms copper Clay
powder plus 20 g borax
[0106] Note: In the 3 above smelts, the particulate mixture was placed in a
250 ml fire
assay crucible purchased from Furnace Industries of Perth, Australia. The
loaded
crucible was placed inside an electric furnace and brought to 1220 C and
retained at that
temperature for 15 minutes. The molten contents of the hot crucible were
poured into a
preheated hemispherical button mold and allowed to cool. The button was
dislodged
from the mold and quenched in water then allowed to dry. Slag was separated
and the
button was sent for bullion assay to Umpire Assay Laboratories in Perth,
Australia.
Table 4 - Total Gold Recovery (from smelt and solutions/filter paper)
Gold Recovered (g) Sample %
Gold Upgrade
Sample Initial Head from
Number Weight Smelt Solutions/filter TOTAL Grade commercial
(g) paper (%) smelt
results
SLCIP2M19 10.04 3.248 0.442 3.69 36.75 4.88
SLCIP2M20 10.18 3.279 0.402 3.68 36.15 3.17
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Comparative Example 1, Examples 4 and Example 5
Source Material
[0107] For the purposes of comparison the same source material was used for
Comparative Example 1, Examples 4 and Example 5.
Cathode associated wire gold was taken from an electrowinning cell. The feed
liquor in
the electrowinning cell was derived from the following process sequence:
Gravity gold concentrate is treated by cyanide leaching and the leach liquor
is contacted
with activated carbon.
The loaded carbon is stripped with caustic cyanide to provide the
electrowinning feed
liquor.
[0108] The cathode associated wire gold was submerged in 30% hydrochloric acid
for 2
hours to dissolve the wire component. The residue was washed and dried and the
complete washed/dried residue yield was homogenized with a blender comprising
a
rapidly moving blade. The resulting mixture comprised wire gold derived
particulate
material of size less than 5 mm and substantially less than 1 mm in size. The
complete
blended residue weighed 16.8 Kg. This was used as the source material in
subsequent
smelting examples (Comparative Example 1 and Examples 4 and 5)
Comparative Example 1 - Standard Smelt Process
[0109] 10.8 kg of the source material referred to above was taken for standard
smelting
using cone and quartering methods. The smelting of the 10.8 kg of material
took place in
a graphite crucible in a gas fired kiln to provide an ingot containing 252.577
Troy ounces
of gold (Perth Mint bullion assay).
The standard commercial process involved the use of a mixed flux formulation
containing
the following in the parts by weight specified:
borax (2 parts), sodium carbonate (1 part), silica flour (1 part) and sodium
nitrate (0.25
part) as a fluxing agent.
10.8 kg of the fluxing formulation was stirred together with 10.8 kg of wire
gold material.
A graphite crucible was heated to approximately 1220 C in a gas fired kiln and
multiple
charges of the mixed powder as described above were added. The weight of each
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charge was approx. 2 kg. The molten material was poured into a mold, cooled
and the
slag layer removed. The ingot contained 252.577 Troy ounces of gold (Perth
Mint bullion
assay). This corresponds to 2.3387 Troy ounces of gold per 100 g of wire gold
source
material.
Example 4
[0110] 400 g of source material was divided into 4 charges of 100 g by cone
and quarter
method.
1 kg of borax was added to a clay crucible in a digital electric furnace and
the
temperature was brought to 1220 C. At this temperature the borax was a
homogeneous
and fluid liquid. 200 g of copper powder was added to the molten borax and the
crucible
contents were restored to 1220 C. At this temperature the copper formed a
molten pool
underneath the molten borax. The first charge of 100 g of source material was
added to
the crucible and the source material descended through the molten borax into
the molten
pool of copper. After 10 minutes the temperature was restored to 1220 C.
Thereafter a
second charge of 100 g source material was added to the crucible. After 10
minutes a
third charge of 100 g source material was added and the process repeated once
more so
that the last of the four charges was added and any associated reaction was
complete.
The molten material was poured into a mould and allowed to cool. The slag
component
was removed and the ingot was found to contain 9.769 Troy ounces of gold
(Perth Mint
bullion assay). This corresponded to 2.4422 Troy ounces of gold per 100 g of
wire gold
source material. Relative to the standard smelt process described above, this
corresponds to a gold increment of 4.43%.
Example 5
[0111] 400 g of source material was divided into 4 charges of 100 g.
1 kg of borax was added to a clay crucible in a digital electric furnace and
the
temperature was brought to 1220 C. At this temperature the borax was a
homogeneous
and fluid liquid. Two charges of 200 g each of copper powder were added with a
10
minute interval to the molten borax and the crucible contents were restored to
1220 C. At
this temperature the copper formed a molten pool underneath the molten borax.
The first
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charge of 100 g of source material was added to the crucible and the source
material
descended through the molten borax into the molten pool of copper. After 10
minutes
the temperature was restored to 1220 C. Thereafter a second charge of 100 g
source
material was added to the crucible. After 10 minutes a third charge of 100 g
source
material was added and the process repeated once more so that the last of the
four
charges was consumed and any associated reaction was complete. The molten
material
was poured into a mould and allowed to cool. The slag component was removed
and the
ingot was found to contain 9.788 Troy ounces of gold (Perth Mint bullion
assay). This
corresponded to 2.4470 Troy ounces of gold per 100 g of wire gold source
material.
Relative to the standard smelt process described above, this corresponds to a
gold
increment of 4.63%.
