U.S. patent application number 12/225172 was filed with the patent office on 2009-12-03 for processing of metal values from concentrates.
Invention is credited to Victor John Ketcham, Karel John Osten, Ian Christopher Ritchie.
Application Number | 20090293680 12/225172 |
Document ID | / |
Family ID | 38540713 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090293680 |
Kind Code |
A1 |
Ritchie; Ian Christopher ;
et al. |
December 3, 2009 |
Processing of Metal Values from Concentrates
Abstract
The present invention relates to an improved method for the
recovery of metal values, in particular copper and gold, from a
metal value-bearing material containing arsenic and/or antimony and
a source of sulphate ions, by means of a high temperature pressure
oxidation process followed by cyanidation of the resultant high
temperature pressure oxidation residue.
Inventors: |
Ritchie; Ian Christopher;
(Western Australia, AU) ; Ketcham; Victor John;
(Western Australia, AU) ; Osten; Karel John;
(Western Australia, AU) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
38540713 |
Appl. No.: |
12/225172 |
Filed: |
March 23, 2007 |
PCT Filed: |
March 23, 2007 |
PCT NO: |
PCT/AU2007/000370 |
371 Date: |
June 30, 2009 |
Current U.S.
Class: |
75/744 ;
75/743 |
Current CPC
Class: |
Y02P 10/236 20151101;
C22B 15/0071 20130101; C22B 15/0089 20130101; C22B 11/04 20130101;
C22B 1/00 20130101; C22B 11/08 20130101; Y02P 10/216 20151101; C22B
3/08 20130101; Y02P 10/234 20151101; Y02P 10/20 20151101; C22B
30/00 20130101 |
Class at
Publication: |
75/744 ;
75/743 |
International
Class: |
C22B 3/04 20060101
C22B003/04; C22B 15/00 20060101 C22B015/00; C22B 11/00 20060101
C22B011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2006 |
AU |
2006901575 |
Claims
1. A method for the recovery of metal values from a metal
value-bearing material containing arsenic and/or antimony and a
source of sulphate ions, comprising the steps of: (a) providing a
feed stream comprising a metal value-bearing material containing
arsenic and/or antimony and a source of sulphate ions; (b)
subjecting the feed stream to oxidative conditions under elevated
temperature and pressure conditions thereby forming a slurry
comprising a metal value-containing leach solution and a solid
residue; (c) separating the metal value-bearing leach solution from
the solid leach residue; (d) recovering the metal value(s) from the
metal value-bearing leach solution; and (e) recovering any precious
metal values in the solid leach residue by cyanide leaching.
2. The method according to claim 1, wherein the slurry from step
(b) is maintained at a temperature in the range of from about
70.degree. C. to about 100.degree. C. for a period in the range of
from about 15 minutes to about 4 hours prior to separating the
metal value-containing solution from the solid leach residue.
3. A method for the recovery of metal values from a metal
value-bearing material containing arsenic and/or antimony and a
source of sulphate ions, comprising the steps of: (a) providing a
feed stream comprising a metal value-bearing material containing
arsenic and/or antimony and a source of sulphate ions; (b)
subjecting the feed stream to oxidative conditions under elevated
temperature and pressure conditions in the presence of at least one
component selected to decrease the effective free acid
concentration during the pressure oxidation step and promote the
formation of pH-stable iron(IH) sulphate products, thereby forming
a slurry comprising: (i) a metal value-containing leach solution
and a solid residue containing pH-stable iron (Ill) sulphate
products; and (ii) environmentally stable iron-arsenic and
iron-antimony products, (c) separating the metal value-bearing
leach solution from the solid leach residue; (d) recovering the
metal value(s) from the metal value-containing leach solution; and
(e) recovering any precious metals in the solid leach residue by
cyanide leaching.
4. The method according to claim 3, wherein the oxidation
conditions in the vessel used in step (b) provide the slurry, in at
least a first part of the vessel, with an Oxygen Reduction
Potential (ORP) of below about 425 mV, when measured with a
standard platinum (Pt) electrode against a standard silver/silver
chloride (Ag/AgCl) electrode, and a soluble ferric to ferrous molar
ratio of below about 1:1, and wherein the oxidation conditions
provide the slurry, in at least a second part of the vessel, with
an OPR of above about 425 mV and the soluble ferric to ferrous
molar ratio of above about 1:1, to facilitate the precipitation of
the pH-stable iran(HI) products and oxidation of the sulphide
sulphur to sulphate.
5. The method according to claim 4, wherein the ORP in the reaction
slurry in said first part of the vessel is below about 400 mV.
6. The method according to claim 4, wherein said first part of the
vessel encompasses up to about 50% of the total volume of the
vessel used in step (b).
7. The method according to claim 4, wherein said second part of the
vessel encompasses up to about 50% of the total volume of the
vessel used in step (b).
8. The method according to claim 4, wherein the oxidation
conditions are controlled by limiting the rate of oxygen injection
into the first and/or second part of the vessel.
9. The method according to claim 4, wherein the vessel of step (b)
is a pressure vessel, preferably an autoclave, and more preferably,
a substantially continuously operated autoclave.
10. The method according to claim 3, wherein the slurry from step
(b) is maintained at a temperature in the range of from about
70.degree. C. to about 100.degree. C. for a period in the range of
from about 15 minutes to about 4 hours prior to separating the
metal value-containing solution from the solid leach residue.
11. A method for the recovery of metal values from a metal
value-containing feed material containing arsenic and/or antimony
and a source of sulphate ions, the method comprising the steps of:
(a) providing a feed stream comprising a metal value-bearing
material containing arsenic and/or antimony and a source of
sulphate ions; (b) subjecting the feed stream to oxidative
conditions under elevated temperature and pressure conditions in
the presence of certain iron-containing compounds and/or other
chemical agents selected to decrease the effective free acid
concentration during the pressure oxidation step and promote the
formation of pH-stable iron(III) sulphate products, thereby forming
a slurry comprising: (i) a metal value-containing leach solution
and a solid residue containing pH-stable iron (III) sulphate
products; and (ii) environmentally stable iron-arsenic and
iron-antimony products; (c) separating the metal value-bearing
leach solution from the solid leach residue; (d) recovering the
metal value (s) from the metal value-containing leach solution; and
(e) recovering any precious metal values in the solid leach residue
by cyanide leaching.
12. The method according to claim 11, wherein the oxidation
conditions in the vessel used in step (b) provide the slurry, in at
least a first part of the vessel, with an Oxygen Reduction
Potential (ORP) of below about 425 mV, when measured with a
standard platinum (Pt) electrode against a standard silver/silver
chloride (Ag/AgCl) electrode, and a soluble ferric to ferrous molar
ratio of below about 1:1, and wherein the oxidation conditions
provide the slurry, in at least a second part of the vessel, with
an OPR of above about 425 mV and the soluble ferric to ferrous
molar ratio of above about 1:1, to facilitate the precipitation of
the pH-stable iron(III) products and oxidation of the sulphide
sulphur to sulphate.
13. The method according to claim 12, wherein the ORP in the
reaction slurry in said first part of the vessel is below about 400
mV.
14. The method according to claim 12, wherein said first part of
the vessel encompasses up to about 50% of the total volume of the
vessel used in step (b).
15. The method according to claim 12, wherein said second part of
the vessel encompasses up to about 50% of the total volume of the
vessel used in step (b).
16. The method according to claim 12, wherein the oxidation
conditions are controlled by limiting the rate of oxygen injection
into the first and/or second part of the vessel.
17. The method according to claim 12, wherein the vessel of step
(b) is a pressure vessel, preferably an autoclave, and more
preferably, a substantially continuously operated autoclave.
18. The method according to claim 11, wherein the slurry from step
(b) is maintained at a temperature in the range of from about
70.degree. C. to about 100.degree. C. for a period in the range of
from about 15 minutes to about 4 hours prior to separating the
metal value-containing solution from the solid leach residue.
19. The method according to claim 11, wherein the chemical agents
added to the material in the feed stream include metal salts,
preferably soluble alkali metal ion salts, more preferably sodium,
potassium and ammonium salts.
20. The method according to claim 11, wherein the chemical agents
added to the material in the feed stream include a source of
soluble sulphate salts, preferably magnesium and/or zinc
sulphate.
21. The method according to claim 20, wherein the source of soluble
sulphate salts include carbonate and/or hydroxide salts of
magnesium and/or zinc formed in situ under the oxidative conditions
of step (b) or by the leaching of zinc sulphide minerals present in
the material in the feed stream.
22. The method according to claim 11, wherein the chemical agents
added to the material in the feed stream include a base and/or
carbonate, preferably limestone or lime.
23. The method according to claim 3, wherein the pH stable iron(IH)
sulphate products formed are composed of one or more jarosite-type
minerals including hydronium, sodium, potassium or ammonium
jarosite, preferably hydronium and/or sodium jarosite.
24. The method according to claim 1, wherein the metal
value-bearing material containing arsenic and/or antimony is a
copper-bearing material containing arsenic and/or antimony,
preferably a copper sulphide containing arsenic and/or antimony,
and more preferably a mixed copper-gold sulphide containing arsenic
and/or antimony.
25. The method according to claim 1, wherein the metal
value-containing material is an ore or ore concentrate that
contains arsenic and/or antimony, and includes one or more
recoverable metals selected from the group consisting of copper,
nickel, cobalt, zinc, palladium and platinum.
26. The method according to claim 1, wherein the metal
value-containing material is an ore or ore concentrate that
includes one or more recoverable precious metals, preferably gold
and silver.
27. The method according to claim 1, wherein die material in the
feed stream includes iron compounds, preferably iron (IU)
compounds.
28. The method according to claim 27, wherein the molar ratio of
Fe:(As+Sb) in the material in the feed stream in step (b) is
greater than about 1:1, and preferably greater than about 2:1.
29. The method according to claim 27, wherein the iron compounds
are derived from pyrite, preferably calcined pyrite produced under
conditions that favour the formation of more solubilzable forms of
iron compounds including FeS, FeO, Fe.sub.3O.sub.4 or
gamma-Fe.sub.2O.sub.j over the formation of
alpha-Fe.sub.2O.sub.3.
30. The method according to claim 1, wherein prior to the step of
recovering the metal value(s) from the metal value-containing leach
solution, the pH is reduced to a pH of less than about pH2.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an improved process for the
recovery of metal values, in particular copper and gold, from
metal-bearing concentrates.
[0002] The present invention also relates more particularly to an
improved process for the recovery of metal values, in particular
copper and gold, from metal-bearing concentrates by means of a high
temperature pressure oxidation process followed by cyanidation of
the resultant high temperature pressure oxidation residue.
[0003] The present invention also relates. more particularly but
not exclusively to a process of maximising copper and gold
extraction from metal-bearing concentrates that also contain
significant amounts of-arsenic and/or antimony, and that
substantially simultaneously results in the formation of
environmentally stable iron-arsenic and/or iron-antimony compounds
in the process residues that can be discharged to tailings dams or
the like such that strict environmental regulations are complied
with.
[0004] The present invention also relates more particularly but not
exclusively to a high temperature pressure oxidation process in
which there is controlled oxygen addition to the first compartment
of a pressure vessel such as a substantially continuously operated
autoclave, and also more particularly relates to controlled oxygen
addition to approximately the first 50% of the total volume of the
continuously operated autoclave.
[0005] The present invention also relates more particularly but not
exclusively to a high temperature pressure oxidation process in
which the Oxygen Reduction Potential (ORP) of the reaction slurry
in the first compartment, and typically approximately the first 50%
of die total volume, of a pressure vessel such as a substantially
continuously operated autoclave, is kept below about 425 mV, and
preferably below about 400 mV, when measured with a standard
platinum (Pt) electrode against a standard silver/silver chloride
(Ag/AgCl) electrode, and the soluble ferric to ferrous molar ratio
is below about 1:1. The ORP and ferric and ferrous iron assays
referred to above are those obtained by rapid cooling to room
temperature of a sample of slurry withdrawn from the autoclave
within one hour and then filtered for assay purposes.
BACKGROUND OF THE INVENTION
[0006] In this specification, where a document, act or item of
knowledge is referred to or discussed, this reference or discussion
is not an admission that the document, act or item of knowledge or
any combination thereof was at the priority date:
(i) part of common general knowledge; or (ii) known to be relevant
to an attempt to solve any problem with which this specification is
concerned.
[0007] Many base metals are. sourced from sulphide ores. For
example, copper sulphide minerals such as chalcopyrite
[CuFeS.sub.2] contribute to the majority of global copper
production. There are also many other deposits that contain copper
in the form of arsenic-bearing minerals, primarily enargite
[Cu.sub.3AsS.sub.4] and tennantite [Cu.sub.12As.sub.4S.sub.13],
and/or antimony-bearing minerals such as tetrahedrite
[Cu.sub.12Sb.sub.4S.sub.13]. Included in such deposits is the
enargite-containing copper-gold resource at Chelopech,
Bulgaria.
[0008] Processes that involve the recovery of metal values from
arsenic-containing sulphide minerals such as those indicated above
generally require consideration of the form or forms in which the
arsenic component reports, and the environmental impact upon the
disposal of such arsenic-containing residues. Relevant national and
international discharge regulations specify the maximum allowable
dissolution of arsenic from such arsenic-containing residues under
appropriate disposal regimes.
[0009] Pyrometallurgical treatment of arsenic-containing metal
sulphide minerals is generally regarded as technically and
economically undesirable, as most of the arsenic reports as a flue
dust and as a speiss phase. Safe disposal of these
arsenic-containing materials involves considerable cost and
technical disincentives.
[0010] By contrast, many hydrometallurgical processes for treating
copper sulphide minerals that also contain arsenic are directed
teds the generation of an acidic copper sulphate solution
containing soluble copper, which is typically recovered therefrom
by a combination of solvent extraction and electrowinning. The
arsenic component of the feed material is converted into an
insoluble arsenic-containing phase such as hydrated ferric arsenate
[FeAsO.sub.42H.sub.2O]. This particular phase also occurs in nature
as the mineral scorodite. The hydrated ferric sulphate produced by
the hydrometallurgical processes can be safely disposed of in a
conventional tailings impoundment. Most of the hydrometallurgical
processes for treating copper sulphide minerals generally fall
within the general designation of pressure oxidation processes.
[0011] The kinetics of the copper leaching stage of many such
pressure oxidation processes are frequently slow and there is
generally co-precipitation of an iron-capper-arsenate-sulphate
compound or compounds, leading to copper losses to the leach stage
solid residue and thus to the overall process. Various means have
been proposed to overcome the slow leach kinetics, including finer
grinding of the feed material, although these sometimes result in
substantially increased capital and operating costs.
[0012] Many copper sulphide materials that contain arsenic and/or
antimony often also contain metal values including precious metal
values such as those of gold and/or silver, and any process to
treat such materials must also employ economically viable treatment
stages to recover the geld and/or silver contents. In the pressure
oxidation process described above, the gold and/or silver generally
report to the solid residue generated by the leach process. The
gold and/or silver are usually recovered by repulping the residue
and cyanide leaching under the appropriate alkaline pH conditions.
Meta-stable iron compounds such as basic ferric sulphate
[Fe(OH)SO.sub.4] and any copper-containing precipitates such as an
iron-copper-arsenate-sulphate in the residue will decompose (break
down) under the alkaline pH conditions required for gold/silver
cyanidation and thus bring about an increase in the lime and
cyanide consumption, thereby decreasing the economic efficiencies
of the overall process. In other words, the abovementioned solid
components present in the leach residue break down during the
cyanidation step, generating excess acid and reactive sulphate
compounds that must be subsequently neutralised.
[0013] In summary, many of the hydrometallurgical processes
currently employed to treat arsenic- and/or antimony-containing
copper sulphide materials suffer from unacceptable copper losses to
the leach residue. Moreover, if the feed material also contains
precious metal values such as those of gold and/or silver, then
current processing conditions also lead to the generation of solid
residues that result in unacceptably high lime and cyanide
consumption.
[0014] The present invention seeks to overcome at least. some of
the aforementioned disadvantages.
SUMMARY OF THE INVENTION
[0015] In the following description of the invention, except where
the context requires otherwise due to express language or necessary
implication, the words "comprise" or variations such as "comprises"
or "comprising" are used in an exclusive sense, ie., to specify the
presence of stated features, but not to preclude the presence or
addition or further features in various embodiments of the
invention.
[0016] Before the invention and preferred embodiments thereof are
described, it is to be understood that this invention is not
limited to the particular materials described, as these may vary.
It is also to be understood that the terminology used herein is for
the purpose of describing particular embodiments only, and is not
intended to limit the scope of the present invention in any
way.
[0017] It must also be noted that as used herein, the singular
forms of "a", "an" and "the" include plural reference unless the
context clearly dictates otherwise. Unless defined otherwise, all
technical and scientific terms herein have the same meanings as
commonly used by one of ordinary skill in the art to which this
invention belongs.
[0018] According to one aspect of the present invention there is
provided a method for the recovery of metal values from a metal
value-bearing material containing arsenic and/or antimony and a
source of sulphate ions such as a sulphide ore or concentrate, the
process comprising the steps of: [0019] (a) providing a feed stream
comprising a metal value-bearing material containing arsenic and/or
antimony and a source of sulphate ions; [0020] (b) subjecting the
feed stream to oxidative conditions under elevated temperature and
pressure conditions thereby forming a slurry comprising a metal
value-containing leach solution and a solid residue; [0021] (c)
separating the metal value-bearing leach solution from the solid
leach residue; [0022] (d) recovering the metal value(s) from the
metal value-bearing leach solution; and [0023] (e) recovering
precious metal values such as gold and/or silver values, if
present, in the solid leach residue by cyanide leaching.
[0024] The slurry from step (b) may be maintained at a temperature
in the range of from about 70.degree. C. to about 100.degree. C.
for a period in the range of from about 15 minutes to about 4 hours
prior to separating the metal value-containing solution from the
solid leach residue.
[0025] According to another aspect of the present invention there
is provided a method for the recovery of metal values from a metal
value-bearing material containing arsenic and/or antimony and a
source of sulphate ions such as a sulphide ore or concentrate, the
process comprising the steps of: [0026] (a) providing a feed stream
comprising a metal value-bearing material containing arsenic and/or
antimony and a source of sulphate ions; [0027] (b) subjecting the
feed stream to oxidative conditions under elevated temperature and
pressure conditions in the presence of at least one component
selected to decrease the effective free acid concentration during
the pressure oxidation step and promote the formation of pH-stable
iron(III) sulphate conditions, thereby forming a slurry comprising:
[0028] (i) a metal value-containing leach solution and a solid
residue containing pH-stable iron(III) sulphate products; and
[0029] (ii) environmentally stable iron-arsenic and iron-antimony
products, such oxidative conditions comprising the provision that
the Oxygen Reduction Potential (ORP) of the reaction slurry in at
least part of the vessel used for step (b) is kept below about 425
mV, when measured with a standard platinum (Pt) electrode against a
standard silver/silver chloride (Ag/AgCl) electrode, and the
soluble ferric to ferrous molar ratio is below about 1:1, and
wherein in at least another part of the vessel the OPR is allowed
to increase above about 425 mV and typically substantially above
about 425 mV so that the soluble ferric to ferrous molar ratio is
above about 1:1 and typically substantially above about 1:1 to
facilitate the precipitation of the pH-stable iron(III) products
and ensure a substantial proportion, and preferably substantially
all, of the sulphide sulphur is oxidised to sulphate; [0030] (c)
separating the metal value-bearing leach solution from the solid
leach residue; [0031] (d) recovering the metal value(s) from the
metal value-containing leach solution; and [0032] (e) recovering
precious metals such as gold and/or silver values, if present, in
the solid leach residue by cyanide leaching.
[0033] The vessel of step (b) will typically be a pressure vessel
such as an autoclave, and more typically a substantially
continuously operated autoclave.
[0034] Up to approximately the first 50% of the total volume of the
vessel of step (b) may be kept below about 425 mV and typically
below about 400 mV. Up to approximately 50% of the remaining volume
of the vessel of step (b) may be allowed to increase above about
425 mV and typically substantially above about 425 mV.
[0035] The slurry from step (b) may be maintained at a temperature
in. the range of from about 70.degree. C. to about 100.degree. C.
for a period in the range of from about 15 minutes to about 4 hours
prior to separating the metal value-containing solution from the
solid leach residue.
[0036] According to another aspect of the present invention there
is provided a method for the recovery of metal values from a metal
value-containing feed material containing arsenic and/or antimony
and a source of sulphate ions such as a sulphide ore or
concentrate, the method comprising the steps of: [0037] (a)
providing a feed stream comprising a metal value-bearing material
containing arsenic. anchor antimony and a source of sulphate ions;
[0038] (b) subjecting the feed stream to oxidative conditions under
elevated temperature and pressure conditions in the presence of
certain iron-containing compounds and/or other chemical agents
selected to decrease the effective free acid concentration during
the pressure oxidation step and promote the formation of pH-stable
iron(III) sulphate conditions, thereby forming a slurry comprising:
[0039] (i) a metal value containing leach solution and a solid
residue containing pH-stable iron(III) sulphate products; and
[0040] (ii) environmentally stable iron-arsenic and iron-antimony
products, such oxidative conditions including the provision that
the Oxygen Reduction Potential (ORP) of the reaction slurry in at
least part of the vessel used for step (b) is kept below about 425
mV, when measured with a standard platinum (Pt) electrode against a
standard silver/silver chloride (Ag/AgCl) electrode, and the
soluble ferric to ferrous molar ratio is below about 1:1, and
wherein in at least another part of the vessel the OPR is allowed
to increase above about 425 mV, and typically substantially above
425 mV, so that the soluble ferric to ferrous molar ratio is above
about 1:1, and typically substantially above 1:1 to facilitate the
precipitation of the. pH-stable iron(III) products and ensure a
substantial proportion, and preferably substantially all of the
sulphide sulphur is oxidised to sulphate; [0041] (c) separating the
metal value-bearing leach solution from the solid leach residue;
[0042] (d) recovering the metal value(s) from the metal
value-containing leach solution; and [0043] (e) recovering precious
metal values such as gold ardor silver values, if present, in the
solid leach residue by cyanide leaching.
[0044] The vessel of step (b) will typically be a pressure vessel
such as an autoclave, and more typically a substantially
continuously operated autoclave.
[0045] Up to approximately the first 50% of the total volume of the
vessel of step(b) may be kept below about 425 mV and typically
below about 40 mV. Up to approximately 50% of the remaining total
volume of the vessel of step A) may be allowed to increase above
about 425 mV and typically substantially above about 425 mV.
[0046] The slurry from step (b) may be maintained at a temperature
in the range of from about 70.degree. C. to about 100.degree. C.
for a period in the range of from about 15 minutes to about 4 hours
prior to separating the metal value-containing solution from the
solid leach residue.
[0047] The terms "pressure oxidation" or "pressure oxidation step"
or "oxidative conditions under elevated temperature and pressure"
used herein refer to a high temperature/high pressure leach process
operated under acidic oxidising conditions.
[0048] One particular aspect of the present invention is based upon
the realisation that it is possible to adjust the processing
conditions such that they prevent the formation of insoluble
copper-containing precipitates during the high temperature pressure
leaching process to extract metal values such as copper from a
metal value-containing material such as a sulphide ore that also
contains arsenic and/or antimony.
[0049] Another particular aspect of the present invention is based
upon the realisation that it is possible to adjust the processing
conditions to promote the formation of solid iron(III) sulphate
containing-products in the residue derived from the pressure
leaching process that are stable under the alkaline pH conditions
at ambient temperature that are used to recover the gold and/or
silver values from the said residue. For convenience, this solid
iron(III) sulphate containing-product is referred to as a "pH
stable iron(III) sulphate". Included in the means of promoting the
formation of the pH stable iron(III) sulphate product are means of
controlling (decreasing) the free acid generated during the
pressure oxidation step by addition of certain additives and/or
control of the slurry ORP in typically about the first 50% of the
total volume of the vessel such as a continuous autoclave used for
the pressure oxidation step. This latter means is achieved by
limiting the rate of oxygen injection into about the first 50% of
the total volume of the continuous autoclave.
[0050] The result of the correct selection of the high
temperature/high pressure leaching conditions for treating metal
value-bearing materials containing arsenic and/or antimony is that
the majority of the arsenic and/or antimony reports to a solid
residue as an environmentally stable mixed iron-arsenic and/or
iron-antimony solid species mixed with pH stable iron(III) sulphate
products. In addition, copper losses to the residue are minimised
by prevention of precipitation of a copper-iron-sulphate-arsenate,
while cyanidation of the gold and/or silver content of the leach
residue is enhanced because of the promotion of precipitation of pH
stable iron(III) sulphate products such as jarosite-type minerals
rather than basic iron sulphate.
[0051] The present invention is accordingly concerned with the
development of economically viable conditions that can at least
partially achieve one or more of (a) minimizing copper losses to
the leach residue, (b) ensuring that the arsenic and/or antimony
components of the feed material report to the residue in an
environmentally stable form, and (c) preventing the formation of
solid residues that break down during the gold and/or silver
cyanidation step and a concomitant increase in lime and cyanide
consumption in the case where the initial feed material contains
recoverable gold and/or silver.
[0052] Preferably, the metal value-bearing material containing
arsenic and/or antimony is a copper-bearing material containing
arsenic and/or antimony, in particular a copper sulphide containing
arsenic and/or antimony, and even more particularly a mixed
copper-gold sulphide containing arsenic and/or antimony. Typically
the metal value-containing material is an ore or concentrate that
contains arsenic and/or antimony, and include but is not limited
to: [0053] (a) an ore or concentrate that contains recoverable base
and other metals including but not limited to copper, nickel,
cobalt, zinc, and the platinum group metals; [0054] (b) an ore or
concentrate that contains recoverable precious metals, especially
gold and silver, [0055] (c) an ore or concentrate that contains
recoverable base and other metals including but not limited to
copper, nickel, cobalt, zinc and the platinum group metals, as well
as precious metals, especially gold and silver.
[0056] Typically, the pH stable iron(III) sulphate product formed
in the abovementioned pressure leach step is composed of one or
more jarosite-type minerals, such as hydronium, sodium, potassium
or ammonium jarosite. In one preferred embodiment of the present
invention, the pH sable iron(III) sulphate product is hydronium
and/or sodium jarosite.
[0057] While it is common for a metal value-bearing material
containing arsenic and/or antimony such as a copper sulphide ore or
concentrate containing arsenic and/or antimony to also contain at
least trace amounts of iron compounds, the inventors have
advantageously found that the presence of additional iron compounds
in the feed material subjected to the pressure leaching process
also promotes the formation of copper-free secondary ferric
sulphate minerals that also contain arsenic and/or antimony.
Preferably, the molar ratio of Fe:(As+Sb) in the feed material to
step (b) of the preferred embodiments described above is greater
than about 1:1, and. more preferably greater than about 2:1. Thus
by ensuring that the Fe:(As+Sb) molar ratio in the feed material to
step (b) is greater than about 1:1 and preferably greater than
about 2:1, the bulk of the arsenic and/or antimony in the feed
material reports to the residue as an environmentally stable
iron-arsenate and/or iron-antimonate phase, rather than as a
copper-iron-sulphate-arsenate/antimonate.
[0058] The inventors have found that the iron compounds suitable
for the abovementioned modification to the Fe:(As+Sb) molar ratio
in the feed material are such compounds that are readily
solubilised under the acidic high temperature/high pressure leach
conditions of the invention. The particle size of the suitable iron
compounds will typically be such that the solubilisation kinetics
are compatible with the retention time if the high temperature/hi
pressure leach stage.
[0059] Provided that the requirement for rapid solubilisation under
the high temperature/high pressure leach conditions is met, the
chemical valency of the iron compounds added to the feed material
to adjust the Fe:(As+Sb) molar ratio to the required level is not
thought to be critical. This is because, under the operating
conditions of the high temperature/high pressure leach step (b),
substantially all ferrous [Fed(II)] will be rapidly oxidised to the
ferric [Fe(III)] state. In other words, the iron compounds may be
ferrous or ferric compounds, or mixed ferrous/ferric compounds.
However, it is preferred that the iron compounds are in the ferric
state since this reduces the Len consumption during the high
temperature/high pressure leach step.
[0060] In one preferred embodiment of the invention, the iron
compounds are derived from pyrite, in particular calcined pyrite
produced under conditions that favour the formation of FeS, FeO,
FeO.sub.4 or gamma-Fe.sub.2O.sub.3 over the formation of
alpha-Fe.sub.2O.sub.3, since the former iron compounds are more
readily solubilised compared with the latter iron compound.
[0061] During the high temperature/high pressure leach step there
are many competing chemical reactions relating to the formation and
precipitation of different iron-containing species, such as, for
example, basic ferric sulphates, hematite, and jarosite. Promotion
of precipitation of jarosite and/or hematite aver basic iron
sulphate is favoured by the presence of suitable reactions that
decrease the effective concentration of free acid generated during
the high temperature/high pressure leach step.
[0062] In one preferred embodiment of the invention, the chemical
agents added to the feed material being subjected to the high
temperature/high pressure leach step comprise metal salts which
directly participate in the formation of jarosite-type compounds,
in particular soluble alkali metal ion salts such as those of
sodium or potassium, and ammonium salts, all of which form stable
jarosite-type minerals of the general formula
MFe.sub.3(SO.sub.4).sub.2(OH).sub.6 where M=Na, K and NH.sub.4,
respectively. The formation of these jarosite-type minerals
deceases the effective concentration of free acid under the
prevailing high temperature/high pressure leach conditions. The
addition of such soluble alkali metal ion salts also increases the
temperature at which jarosite-type minerals tend to form in
preference to basic iron sulphate type minerals during the pressure
oxidation process at any given acid concentration. The ability to
operate at higher temperatures while promoting the formation of
pH-stable iron(III) sulphate products over basic iron sulphate type
minerals provides economic advantages in the form of enhanced
leaching reaction kinetics and shorter required residence
(retention) times.
[0063] In another preferred embodiment of the invention, the
chemical agents also comprise soluble sulphate salts whose cations
are merely spectator ions and as such do not participate in any
precipitation reactions. The preferred chemical reagents
particularly include magnesium and/or zinc. Addition of a suitable
soluble sulphate increases the concentration of the bisulphate ion
present in the high temperature/high pressure leach slurry and
decreases the effective concentration of free acid at temperature
from that which would otherwise be experienced at a given feed
solids composition and concentration (% solids). The soluble
sulphate salts may be added directly to the high temperature/high
pressure leach step or generated by reacting carbonate and/or
hydroxide salts of magnesium and/or zinc in the high
temperature/high pressure leach step. In another preferred
embodiment of the invention, the soluble zinc salt may be
introduced by the leaching of zinc sulphide minerals that may be
present in the feed material.
[0064] In a further preferred embodiment of the invention, the
chemical reagents may also comprise bases or carbonates, in
particular limestone or lime, which directly consume acid and
decrease the effective concentration of free acid in the high
temperature/high pressure leach step.
[0065] In the case of copper sulphides containing arsenic and/or
antimony, copper dissolution in the high temperature/high pressure
leach step is optimised by addition of iron compounds to the
reaction vessel, typically an autoclave, in sufficient quantities
to favour precipitation of environmentally stable secondary
iron-arsenate and/or iron-antimonate and/or iron-arsenic-sulphate
and/or iron-antimonate-sulphate phases within the autoclave rather
than the precipitation of copper-containing arsenate-antimonate
residues, thereby limiting the copper content of the leach residue
and maximising the soluble copper content of the resultant liquid
stream available for copper recovery by a combination of solvent
extraction and electrowinning or by means of another suitable
recovery method.
[0066] By means of limiting the copper content of the solid leach
residue and efficient separation of the soluble copper from the
solid leach residue, the economics of gold and/or silver recovery
from the leach residue by cyanidation is enhanced as the extent of
the reaction between copper and the cyanide leachant is
significantly reduced, thereby lowering the overall cyanide
consumption.
DETAILED DESCRIPTION OF THE INVENTION
[0067] In accordance with various aspects of the present invention,
a metal value-bearing material containing arsenic and/or antimony
that also constitutes a source of sulphate ions is provided for
processing. The metal value-bearing material may be an ore,
concentrate, or any other material from which metal values, in
particular copper and gold and/or silver values, may be recovered.
The invention is equally applicable to other metal value-bearing
materials containing arsenic and/or antimony such as ores and
concentrates containing other valuable metals such as nickel,
cobalt, zinc and the platinum-group metals.
[0068] For convenience, however, the description of the preferred
embodiments of the invention is restricted to copper-containing
materials that also contain arsenic and/or antimony. The
copper-containing material is preferably a copper sulphide ore or
concentrate that contains arsenic and/or antimony, and particularly
applies to ores and/or concentrates that contain tennantite
(Cu.sub.12As.sub.4S.sub.13), enargite (Cu.sub.3AsS.sub.4) and
tetrahedrite (Cu.sub.12Sb.sub.4S.sub.13), and to other ores or
concentrates containing copper sulphide minerals such as, for
example, chalcopyrite (CuFeS.sub.2), chalcocite (Cu.sub.2S),
bornite (Cu.sub.5FeS.sub.4) and covellite (CuS), when contaminated
with arsenic- and/or antimony-bearing material.
[0069] Geologically gold and/or silver are frequently associated
with metal sulphide ores such as, for example, pyrite,
chalcopyrite, galena, arsenopyrite and stibnite. Gold and/or silver
are also often present in sulphide concentrates produced from such
ores. Accordingly, a preferred embodiment of the present invention
is particularly advantageous in connection with the recovery of
copper and gold and/or silver from mixed gold/silver/copper ores or
concentrates containing arsenic and/or antimony. Thus, the metal
value-bearing material is preferably a mixed gold/silver/copper ore
or concentrate containing arsenic and/or antimony. Typically the
mixed gold/silver/copper ore is a
tennantite-enargite-calcopyrite-pyrite ore.
[0070] The metal value-bearing material typically undergoes
comminution, flotation, blending and/or slurry formation, as well
as chemical and/or physical conditioning to afford a feed stream
which, in turn, is subjected to a high temperature/high pressure
oxidative leach step and a series of downstream unit stages to
afford recovery of the contained metal values.
[0071] The specific conditions applicable to the comminution,
flotation and conditioning stages are determined by the chemical
and physical properties of the metal value-bearing ore material. As
a general rule, these specific conditions are designed to yield a
concentrate that optimises recovery versus grade. These specific
conditions do not have a direct bearing on the application of the
preferred embodiments of the present invention. As such, the
present invention is primarily concerned with the treatment of a
dewatered concentrate exiting the comminution, flotation and
conditioning circuits.
[0072] After the metal value-bearing concentrate stream has been
suitably prepared as a slurry, the slurry is fed to an agitated
pressure vessel, preferably an autoclave, and subjected to pressure
oxidation. Typically the high temperature/high pressure leaching
process is carried out at a temperature in the range of from about
180.degree. C. to about 250.degree. C., preferably from about
190.degree. C. to about 230.degree. C. The optimum temperature
depends on many factors including, but not limited to, the
mineralogical composition of the feed, the sulphide sulphur content
of the feed, the particle size distribution of the feed, and the
pulp density. As a general rule, the higher temperatures in the
above ranges provide for shorter retention times and/or a reduction
and/or elimination of the need for regrinding of the feed material
prior to the high temperature/high pressure leach step.
[0073] The high temperature/high pressure leaching process is
typically carried out at a total pressure sufficiently high to
provide an oxygen partial pressure inside the autoclave of between
about 100 kPa and about 1500 kPa, preferably in the range of from
about 400 kPa to about 1000 kPa, taking into account the partial
pressure of steam and other ton-condensable gases within the
autoclave such as nitrogen and carbon dioxide. Oxygen is typically
delivered to the autoclave by bottom entry spargers entering
beneath the autoclave agitators at a pressure above that inside the
autoclave. The autoclave agitators are designed to maximise oxygen
mass transfer from the gas phase to the feed slurry.
[0074] In one of the preferred embodiments of the present
invention, it has been found advantageous to control the Oxygen
Reduction Potential (ORP) of the slurry in the first compartment of
the autoclave and more preferably in approximately the first 50% of
the total autoclave volume, to a value below about 425 mV, and more
preferably below about 400 mV, when measured against a standard
platinum (Pt) electrode against a standard silver/silver chloride
(Ag/AgCl) reference electrode. In this instance, the ORP is
recorded within one hour using a filtered slurry sample withdrawn
from the autoclave that had been rapidly cooled to ambient
temperature. Control of the ORP is achieved by limiting the rate of
oxygen injection into the first compartment and more preferably
approximately the first 50% of the total autoclave. volume. In the
remaining autoclave compartments and/or approximately the second
50% of the total autoclave volume the ORP is allowed to increase
above about 500 mV by increasing the rate of oxygen injection into
the autoclave. The inventors have found that control of the ORP in
the above manner permits regulation of the oxidation of ferrous
iron to the ferric state as the slurry moves through the autoclave
and assists in the generation of solid pH stable iron(III) sulphate
products.
[0075] The high temperature/high pressure leach step is typically
conducted over a period of from about 20 minutes to about 4 hours,
and more preferably to about 2 hours, with higher operating
temperatures and a finer feed particle size facilitating shorter
reaction times.
[0076] Under the high temperature/high pressure leaching process
conditions, solid metal sulphide minerals within the feed material
are oxidised to the corresponding soluble metal sulphates. That is,
the metal values are released into solution. The actual
oxidation/dissolution reactions for each metal sulphide mineral are
a reflection of the chemical composition of that mineral as well as
the temperature and free acidity of the leach slurry, but the
overall reaction can be simplified as shown in reaction (1).
MS(solid)+2O.sub.2(gas).fwdarw.MSO.sub.4(solution) (1)
[0077] The arsenic and antimony components of the feed material are
oxidized to the arsenate (ASO.sub.4.sup.3) and antimonite
(SbO.sub.4.sup.3-) species, respectively.
[0078] Some of the solubilised metal values then re-precipitate
within the autoclave and report to the solid phase component of the
autoclave slurry as metal oxides and/or metal mixed
hydroxyl-sulphates and/or metal-sulphate-arsenate-antimonate
species.
[0079] Iron may report to the solid phase component of the
autoclave slurry as one or more different iron-containing compounds
during the high temperature/high pressure leach process, the
identity of such phases being determined by a specific set of
operating conditions. For example, the formation of basic iron
sulphate is favoured by high operating temperatures and high free
acid conditions. Under such conditions, the oxidation of pyrite
(FeS.sub.2), a significant component of many metal sulphide
concentrates, can be represented by reaction (2).
4FeS.sub.2+15O.sub.2+6H.sub.2O.fwdarw.4Fe(OH)SO.sub.4+4H.sub.2SO.sub.4
(2)
[0080] The reaction of pyrite to form hematite (alpha
Fe.sub.2O.sub.3) is favoured by high temperatures and low free
acidity concentrations according to reaction (3).
4FeS.sub.2+17O.sub.2+8H.sub.2O.fwdarw.2Fe.sub.2O.sub.3+8H.sub.2SO.sub.4
(3)
[0081] The formation of jarosite is favoured by low operating
temperatures and the presence of cations such as Na.sup.+, K.sup.+
or NH.sub.4.sup.+, according to reaction (4) where M=Na, K or
NH.sub.4.
12FeS.sub.2+45O.sub.2+30H.sub.2O+2M.sub.2SO.sub.4.fwdarw.4MFe.sub.3(SO.s-
ub.4).sub.2(OH).sub.6+18H.sub.2SO.sub.4 (4)
[0082] Hydronium jarosite, in which M=H.sub.3O.sup.+, the hydronium
ion of free acid, takes the place of Na, K or NH.sub.4 is also
favoured by low operating temperatures in the absence of such
cations.
[0083] Arsenate and antimonate species formed by the oxidation of
the arsenic and antimony components of the feed material may
precipitate as the respective iron(III) arsenate and iron(III)
antimonate phases, but may also substitute for sulphate in, for
example, the jarosite phase. The precipitation of arsenate as
hydrated iron(III) arsenate, FeAsO.sub.42H.sub.2O, also known as
scorodite, and the partial replacement of sulphate by arsenate in
various jarosite phases is well documented in the scientific
literature. Jarosite is sometimes referred as a scavenger for both
arsenate and antimonate. The formation of hydrated iron(III)
arsenate and/or arsenic-containing jarosite materials in the
present invention is of considerable environmental benefit since
these materials are known to be environmentally stable and can be
safely discharged into and stored in conventional residue storage
impoundments.
[0084] Under typical prior art operating conditions for the high
temperature/high pressure leaching of mixed copper/gold metal
sulphide concentrates containing arsenic and/or antimony, formation
of basic iron sulphate and hematite are favoured. The basic iron
sulphate and hematite report to the solid residue resulting from
the high temperature/high pressure leach process. when the solid
residue is washed, repulped and then subjected to cyanidation in
order to extract the gold and/or silver values therein, there is an
uneconomically high consumption of lime and cyanide. This is
because the lime reacts directly with the basic iron sulphate
during the adjustment of the pH to a value of 10 or higher that is
required for the gold and/silver cyanidation step.
[0085] In the present invention, additional iron compounds are
added to the feed material to the high temperature/high pressure
leach step in order to promote the formation of jarosite rather
than basic iron sulphate. Under these conditions the jarosite phase
acts as an efficient scavenger for any soluble arsenate and/or
antimonate formed during the pressure oxidation reactions.
Moreover, the jarosite phase does not itself react with lime when
the gold and/or silver are recovered from the leach residue by
cyanidation.
[0086] Preferably, the total iron content of the feed material to
the high temperature/high pressure leach process is such that the
molar ratio of Fe:(As+Sb) is greater than about 2:1 and more
preferably at least about 4:1. Apart from facilitating the
formation of arsenic- and antimony-containing jarosite phases which
do not react with lime during cyanidation, the high Fe:(As+Sb)
molar ratio reduces and/or prevents the formation and precipitation
of a mixed copper-iron-arsenate-antimonate-sulphate phase
[0087] The iron compounds added to the metal value-bearing feed
material in order to adjust the molar ratio of Fe:(As+Sb) to the
desired level are of a mineral/chemical composition and particle
size such that they are readily solubilised under the acidic high
temperature/high pressure leach conditions.
[0088] The valency of the iron in the iron compounds is not thought
to be critical because under the operating conditions of the high
temperature/high pressure leach process, substantially all iron(II)
will be oxidised to iron(III). In other words, the iron compounds
may be ferrous or ferric compounds or mired ferrous/ferric
compounds, provided that they are soluble under the high
temperature/high pressure leach conditions. However, it is
preferred that the iron compounds are pre-treated to maximise the
ferric content and minimise any sulphide content in order to lower
the overall oxen consumption required during the high
temperature/high pressure leach step.
[0089] In a preferred embodiment of the present invention the iron
compounds are derived from pyrite, in particular calcined pyrite
produced by oxidative conditions with the calciner operated in such
a fashion as to produce a calcined pyrite with a significant
portion of the iron present in a form readily capable of being
solubilised in the autoclave under the high temperature/high
pressure conditions, such as for example, FeS, FeO, Fe.sub.3O.sub.4
or gamma-Fe.sub.2O.sub.3, rather than alpha-Fe.sub.2O.sub.3
produced in a conventional pyrite roaster, or the higher sulphide
containing FeS.sub.2 or uncalcined pyrite.
[0090] In a further preferred embodiment of the present invention,
the iron compounds may be sourced from recycled process solutions
containing iron sulphate, preferably in the ferric form, although
the process solutions may also carry minor amounts of ferrous iron
as well. Alternatively, the iron compounds may be iron-containing
precipitates from various other parts of the overall process, such
as the iron-containing precipitate produced during minor impurity
removal ahead of or subsequent to metal value recovery steps such
as copper recovery by a combination of solvent extraction and
electrowinning.
[0091] The iron compounds may be mixed with the metal value-bearing
feed stream before it is transferred to the high temperature/high
pressure autoclave leach vessel, or the iron compounds may be
separately transferred to the autoclave before or after
introduction of the feed stream to the autoclave.
[0092] One of the preferred embodiments of the present invention
incorporates the addition of specific chemical agents which
decrease the effective. concentration of free acid generated during
the high temperature/high pressure leaching process thereby
affording the precipitation of pH stable iron(III) sulphate
compounds and avoiding the precipitation of a basic ferric
sulphate. One group of chemical agents includes metal salts that
directly participate in the formation of jarosite-type compounds,
in particular sodium, potassium and ammonium jarosites. Such metal
salts include soluble alkali metal (sodium and potassium) and
ammonium sulphate. Typically the molar ratio of the added metal
salt per mole of iron present in the feed should be at least 1:3
and preferably at least about 1:2, that is, an excess of metal salt
above the stoichiometric requirement.
[0093] Another group of chemical agents that have the ability to
decrease the effective concentration of free acid generated during
the high temperature/high pressure leaching process comprise
soluble sulphate salts whose cations are merely spectator ions and
which do not participate in any precipitation reactions. Addition
of soluble sulphate increases the concentration of the bisulphate
ion present at the operating high temperature and decreases the
effective concentration of free acid that would otherwise be
expected at the given temperature, feeds solids composition and
pulp density.
[0094] The soluble sulphate salts may be directly added to the high
temperature/high pressure leaching step or generated by reacting
carbonate or hydroxide salts of the appropriate metals. The
inventors have established that the appropriate metal sulphate
salts include those of magnesium and zinc. Typically, magnesium is
added as magnesium carbonate (magnesite), magnesium oxide,
dolomite, or mixtures thereof.
[0095] The soluble sulphate salts, once added to or generated by
the overall process, may be conveniently recycled in process water
used for feed preparation and/or autoclave quench water once the
copper or other dissolved metal values have been recovered from the
leach solution.
[0096] The chemical agents may also comprise carbonates and other
bases, in particular limestone and lime, which directly consume
acid and decrease the effective concentration of free acid during
the high temperature/high pressure leach process. Typically, bases
are added in an amount necessary to yield less than about 60 g/L
sulphuric acid in solution in the product from the high
temperature/high pressure leach step, as measured by titration of
slurry samples at ambient temperature.
[0097] The chemical agents may be mixed with the feed stream before
it is transferred to die autoclave for the high temperature/high
pressure leach step, or the chemical agents may be separately
transfer to the autoclave before or after introduction of the feed
stream to the autoclave.
[0098] During the high temperature/high pressure leach step metal
values, in particular copper, may be solubilised to form a metal
value-containing solution. It is envisaged that the metal values
will be recovered from the metal value-containing solution by well
understood methods and techniques. For example, where the metal
value is copper, copper is typically recovered from the
copper-containing solution by a combination of solvent extraction
and electrowinning. However, other metal recovery processes such as
cementation or precipitation of an intermediate product such as a
hydroxide or sulphide could be employed. In a preferred embodiment
of the present invention it has been found to be advantageous to
maintain the slurry discharged from the autoclave at a temperature
above about 70.degree. C., and preferably in the range of about
85-100.degree. C. for a period in the range of from about 15
minutes to about 4 hours in an agitated tank or series of tanks
before it is subsequently cooled to ambient temperature and
subjected to solid/liquid separation by counter current decantation
and thickening ahead of the metal recovery from solution and gold
and/or silver recovery from the solid residue. This compares with
prior art that incorporates rapid cooling of the autoclave
discharge slurry to ambient temperature by means of a series of
flash vessels and subsequent solid/liquid separation processed
generally conducted below about 70.degree. C. The advantage of this
slow cooling or digestion-conditioning step disclosed in the
present invention relates to the fact that any remaining basic
ferric sulphate and/or copper-iron-sulphate-arsenate-antimonate in
the leach slurry will be converted into a pH stable iron(E)
sulphate and/or redissolve, which in the case of
copper-iron-sulphate-arsenate-antimonate will release soluble
copper, respectively. By this means, the lime consumption required
and, in the case of copper-containing feed materials, the cyanide
consumption required for gold and/or silver cyanidation should be
reduced, while any copper losses to the solid leach residue should
also be reduced.
[0099] Precious metal values such as gold and/or silver values
contained in the feed material will report to the solid residue
formed during the high temperature/high pressure leach process. It
is envisaged that the gold and/or silver values will be recovered
from the solid residue by washing to remove entrained acid and
soluble metal values, repulping and treating the consequent slurry
by a combination of conventional cyanidation, activated carbon,
stripping, electrowinning and smelting techniques.
[0100] By application of the preferred embodiments of the present
invention, copper recoveries in excess of 95% and lime consumption
of less than 15 kg/t of solid residue can be expected form a wide
range of copper/gold sulphide ores and concentrates that also
contain appreciable arsenic and/or antimony contents.
[0101] In summary, the advantages of the present invention compared
with prior art include but are not limited to the following: [0102]
(a) enhanced recovery of metal values, typically copper and gold,
by preventing the co-precipitation of metal values in the solid
residue discharged from the high temperature/high pressure leach
step; [0103] (b) prevention of the formation of unstable basic iron
sulphate species in the solid residue discharged from the high
temperature/high pressure leach step that consume excessive lime
during the recovery of the gold and/or silver by cyanidation; and
[0104] (c) generation of arsenic- and/or antimony-containing
residues that can be stored in conventional residue storage
impoundments without causing unacceptable environmental
outcomes.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0105] Preferred embodiments of the present invention are now
described by reference to the following example. The process
conditions reflected therein are intended to exemplify various
aspects of the invention, and are not intended to limit the scope
of the claimed invention. Numerous variations and modifications
will suggest themselves to persons skilled in the relevant art, in
addition to those already described, without departing from the
basic inventive steps. All such variations and modifications are to
be considered within the scope of the present invention, the nature
of which is to be determined from the foregoing description.
EXAMPLE
[0106] This example outlines the general scope of the preferred
embodiments of the present invention as applied to the continuous
processing of a run-of-mine tennantite-enargite-chalcopyrite-pyrite
ore containing on average 1.5% Cu and 3.8 g/t gold derived in from
the Chelopech (Bulgaria) resource. A simplified flowsheet of one
preferred embodiment of the present invention is shown in FIG.
1.
[0107] After crushing and grinding, a copper concentrate typically
containing 15.5% Cu, 24.8% Fe, 38.1% S, 4.7% As and 30 g/t Au is
produced by rougher, scavenger and cleaner flotation banks using
the appropriate flotation reagent regime. The copper concentrate is
directed to a copper concentrate dewatering circuit where the free
moisture is reduced to about 10%.
[0108] The copper concentrate is repulped in neutral barren
solution (NBS) derived from the downstream copper recovery circuit
(solvent extraction and electrowinning) that typically contains
about 42 g/L MgSO.sub.4 and 15 g ZnSO.sub.4 at pH 8.5, prior to
regrinding to a P.sub.80 of 25 micron. The reground concentrate is
thickened to approximately 55% solids and transferred to the
agitated autoclave feed tank. To this tank are added controlled
amounts of underflows from the final impurity (IR) stages of the SX
raffinate and mine water treatment circuits, as well as a limestone
slurry sufficient to achieve the desired carbonate-sulphur ratio in
the feed. The relative amounts of limestone slurry and impurity
removal underflow added to the reground concentrate are controlled
to ensure that the free acidity and Fe.(As+Sb) molar ratio of the
feed slurry are sufficient to prevent the precipitation of unstable
basic ferric sulphate and copper-iron-sulphate-arsenate phases in
the autoclave discharge slurry. The solid component of the blended
reground concentrate, limestone. and impurity removal slurry
typically contains about 8.5% Cu, 13.1% Fe, 24.5% S, 2.7% As and
8.8 g/t Au, which is pumped into the high temperature/high pressure
leach autoclave as a 45% solids slurry.
[0109] The combined slurry is directed to the first component of a
multi-compartment high pressure autoclave fitted with a plurality
of agitators by means of a centrifugal pump feeding a positive
displacement, piston driven diaphragm pump at an operating pressure
of over the steam saturation pressure at the operating temperature,
which will generally be over 2000 kPa.
[0110] High pressure steam is supplied to the autoclave for initial
heat-up and on as-needed basis.
[0111] Each compartment of the autoclave is fitted with a quench
water system by which a controlled flow of quench water, typically
neutral barren solution (NBS), can be directly injected into each
compartment such that the desired operating temperature, typically
in the range of from about 190.degree. C. to about 230.degree. C.,
is continuously maintained. The use of NBS as quench water assists
with maintaining the overall process flowsheet water balance, and
since it also contains appreciable magnesium and zinc sulphate
contents, also assists with the control of the autoclave slurry
chemistry.
[0112] Oxygen at 94% or greater purity is delivered from a
cryogenic oxygen plant to the autoclave by bottom entry spargers
entering beneath each of the autoclave agitators at a pressure
greater than about 2000 kPa. The bottom impeller on the agitators
is of the Rushton turbine design in order to maximize oxygen mass
transfer to the feed slurry.
[0113] The rate of oxygen injection into the first 50% of the total
autoclave volume is controlled such that the Oxygen Reduction
Potential (ORP), as previously defined and measured, is maintained
at or below about 400 mV. The rate of oxygen injection into the
remaining 50% of the total autoclave volume. is increased so that
the ORP increases to above about 500 mV to enhance the oxidation of
ferrous iron to the ferric state.
[0114] Following the required retention time, typically about 60-80
minutes, the processed slurry is discharged from the autoclave via
a single stage flash vessel at approximately 100.degree. C. Flashed
slurry flows by gravity through two agitated discharge tanks
connected in series with a total retention time of about 2 hours,
where the temperature is maintained at 85-100.degree. C. From there
the conditioned autoclave slurry is subjected to solid/liquid
separation via a series of five conventional counter current
thickeners.
[0115] The thickened underflow is washed to remove entrained leach
solution, washed and the resultant cake forwarded to a conventional
gold cyanidation circuit.
[0116] The final thickener overflow contains the dissolved content
of the feed and is directed to a primary neutralisation (PN)
circuit as pregnant leach solution (PLS). As the PLS contains a
relatively high sulphuric acid concentration, typically 30-60 g/L,
excess acid is neutralised by addition of a limestone slurry to
achieve a final PLS free acidity of about 2-5 g/L (pH approximately
1.5). After solid/liquid separation to remove precipitated solids,
principally gypsum, the neutralised PLS is clarified before the
copper is recovered by conventional solvent extraction and
electrowinning techniques.
[0117] The raffinate from the solvent extraction circuit is then
subjected to an impurity removal (IR) step by addition of limestone
and lime slurries. After removal of the precipitated solids, which
are recycled to the autoclave feed slurry preparation circuit, the
clarified neutral barren solution (NBS) is used in a variety of
appropriate duties noted above, including repulping of the incoming
dewatered concentrate and as autoclave quench water.
[0118] Modifications and improvements to the invention will be
readily apparent to those skilled in the art. Such modifications
and improvements are intended to be within the scope of this
invention.
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