U.S. patent application number 15/760998 was filed with the patent office on 2018-09-20 for hydrometallurgical method for silver recovery.
The applicant listed for this patent is Eldorado Gold Corporation. Invention is credited to Roman BEREZOWSKY, Jinxing JI, Paul SKAYMAN.
Application Number | 20180265947 15/760998 |
Document ID | / |
Family ID | 60160577 |
Filed Date | 2018-09-20 |
United States Patent
Application |
20180265947 |
Kind Code |
A1 |
BEREZOWSKY; Roman ; et
al. |
September 20, 2018 |
HYDROMETALLURGICAL METHOD FOR SILVER RECOVERY
Abstract
A process for recovering silver from silver-bearing gold
concentrate or other silver-bearing material which may comprise
adding oxygen, water, and/or acid to an acidulated concentrate
slurry of an input silver bearing material and reacting them
together in an autoclave at an elevated pressure and temperature in
a pressure oxidation step; processing the oxidized concentrate
slurry in a post pressure oxidation conditioning step; applying a
first solid/liquid separation and wash step and a filter and wash
step to form a first washed slurry/solid and first acid-containing
solutions; reacting the first washed slurry/solid with sulfur
dioxide in a reductive leach step; applying a second solid/liquid
separation and wash step to form a second washed slurry/solid and
second acid-containing solutions; and applying an optional surface
cleaning step, to produce a free-milling silver-bearing material,
which is amenable to conventional cyanidation to recover the silver
therefrom.
Inventors: |
BEREZOWSKY; Roman; (St.
Albert, CA) ; SKAYMAN; Paul; (Vancouver, CA) ;
JI; Jinxing; (Burnaby, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Eldorado Gold Corporation |
Vancouver |
|
CA |
|
|
Family ID: |
60160577 |
Appl. No.: |
15/760998 |
Filed: |
April 28, 2016 |
PCT Filed: |
April 28, 2016 |
PCT NO: |
PCT/CA2016/000129 |
371 Date: |
March 16, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22B 3/22 20130101; Y02P
10/234 20151101; C01G 5/006 20130101; Y02P 10/20 20151101; C22B
11/04 20130101; C22B 3/44 20130101; C22B 3/08 20130101 |
International
Class: |
C22B 3/00 20060101
C22B003/00; C22B 3/22 20060101 C22B003/22; C22B 3/08 20060101
C22B003/08 |
Claims
1. A process for recovering silver from a silver-bearing material,
comprising: subjecting an aqueous slurry of the silver-bearing
material to a pressure oxidation step, wherein the aqueous slurry
of the silver-bearing material is reacted with oxygen and an acid
in an autoclave at an oxidizing pressure and at an oxidizing
temperature, to form an oxidized concentrate slurry; subjecting the
oxidized concentrate slurry to a post pressure oxidation
conditioning step, wherein the oxidized concentrate slurry is
discharged from the autoclave and maintained for several hours at a
temperature in a range of from about 50.degree. C. to about
100.degree. C., to form a conditioned slurry; subjecting the
conditioned slurry to a first solid/liquid separation and wash
step, wherein the first solid/liquid separation and wash step
comprises at least one conventional technique for facilitating the
separation of a slurry into solids and a solution, and washing the
resultant solids, to form a first washed slurry/solid and at least
one first acid-containing solution; subjecting the first washed
slurry/solid to a reductive leach step, wherein the first washed
slurry/solid is reacted with sulfur dioxide to form a leached
concentrate slurry; subjecting the leached concentrate slurry to a
second solid/liquid separation and wash step, wherein the second
solid/liquid separation and wash step is one of a number of
techniques for facilitating the separation of a slurry into solids
and a solution, and washing the resultant solids, to form a second
washed slurry/solid and at least one second acid-containing
solution; wherein the second washed slurry/solid is a free-milling
silver-bearing material that is amenable to leach treatment to
extract the silver.
2. The process of claim 1, wherein prior to the pressure oxidation
step, the silver-bearing material is subjected to an acidulation
step, wherein an acid is added to the silver-bearing material, to
form an acidulated concentrate slurry;
3. The process of claim 2, wherein in the acidulation step, the
acid is concentrated sulfuric acid or an aqueous solution of
sulfuric acid.
4. The process of claim 1, wherein in the pressure oxidation step,
the aqueous slurry of the silver-bearing material is reacted with
the oxygen and acid for from about 30 minutes to about 120 minutes
in the autoclave, and wherein the oxidizing pressure is in a range
of from about 200 psig to about 600 psig total pressure, the
partial pressure of oxygen is in the range of from about 15 psi to
about 250 psi, and the oxidizing temperature is in the range of
from about 190.degree. C. to about 240.degree. C.
5. The process of claim 4, wherein in the pressure oxidation step,
the aqueous slurry of the silver-bearing material is reacted with
the oxygen and acid for from about 60 minutes to about 90 minutes
in the autoclave, and wherein the oxidizing pressure is in a range
of from about 430 psig to about 530 psig total pressure, the
partial pressure of oxygen is in the range of from about 25psi to
about 100 psi, and the oxidizing temperature is in the range of
from about 220.degree. C. to about 230.degree. C.
6. The process of claim 1, wherein the first solid/liquid
separation and wash step, comprises one or more of: applying a
thickening step, to form an underflow stream of thickened oxidized
concentrate slurry and an overflow stream of a first
acid-containing solution; applying a countercurrent decantation
wash step, to form an underflow stream of washed oxidized
concentrate slurry and an overflow stream of a first
acid-containing solution; and applying a filter and wash step, to
form the first washed slurry/solid and a filtrate stream of a first
acid-containing solution.
7. The process of claim 1, wherein the first solid/liquid
separation and wash step comprises: applying a thickening step;
applying a countercurrent decantation wash step; and applying a
filter and wash step.
8. The process of claim 1, wherein at least one of the
acid-containing solutions from the first solid/liquid separation
and wash step is recycled to the acidulation step and/or to the
pressure oxidation step.
9. The process of claim 1, wherein in the reductive leach step, the
first washed slurry/solid is reacted with sulfur dioxide and at
least one of sulfuric acid and water.
10. The process of claim 1, wherein in the reductive leach step,
the first washed slurry/solid and sulfur dioxide are reacted
together for from about 1 hour to about 10 hours, at a temperature
of from about 50.degree. C. to about 150.degree. C., at a total
pressure of from about 1 psig to about 150 psig, and at a partial
pressure of sulfur dioxide from about 1 psi to about 50 psi.
11. The process of claim 10, wherein in the reductive leach step,
the first washed slurry/solid and sulfur dioxide are reacted
together for from about 3 hours to about 6 hours, at a temperature
of from about 70.degree. C. to about 100.degree. C., at a total
pressure of from about 1 psig to about 30 psig, and a partial
pressure of sulfur dioxide of from about 1 psi to about 30 psi.
12. The process of claim 10, wherein in the reductive leach step, a
catalyst of copper sulfate is added to enhance the reaction
rate.
13. The process of claim 1, wherein the second solid/liquid
separation and wash step, comprises one or more of: a thickening
step, to form an underflow stream of thickened leached concentrate
slurry and an overflow stream of a second acid-containing solution;
a countercurrent decantation wash step, to form an underflow stream
of washed thickened slurry/solid and an overflow stream of a second
acid-containing solution; and a filter and wash step, to form the
second washed slurry/solid and an filtrate stream of a second
acid-containing solution.
14. The process of claim 1, wherein the second solid/liquid
separation and wash step comprises: a thickening step; a
countercurrent decantation wash step; and a filter and wash
step.
15. The process of claim 1, wherein the at least one second
acid-containing solutions from the second solid/liquid separation
and wash step is recycled to the acidulation step and/or to the
pressure oxidation step.
16. The process of claim 1, wherein before the pressure oxidation
step, the silver-bearing material undergoes mechanical regrinding
in a regrinding step.
17. The process of claim 1, wherein following the second
solid/liquid separation and wash step, the second washed
slurry/solid is subjected to a surface cleaning step to form the
free-milling silver-bearing material, wherein the surface cleaning
step is selected from treatment with an oxidizing agent, treatment
by extended aeration, and treatment by regrinding.
18. The process of claim 17, wherein the oxidizing agent is
selected from a group consisting of hydrogen peroxide, ozone, pure
oxygen, and hypochlorite.
19. The process of claim 1, further comprising a silver extraction
step, wherein the free-milling silver-bearing material is leach
treated with a lixiviant to extract the silver.
20. The process of claim 17, wherein the lixiviant is selected from
a group consisting of sodium cyanide, calcium thiosulfate, ammonium
thiosulfate, thiourea, and glycine.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to methods for recovering
silver from silver-bearing material. More particularly, the present
invention relates to a process for recovering silver from gold
concentrate and other silver-bearing metal ore concentrates from
mining operations.
BACKGROUND OF THE INVENTION
[0002] In silver-bearing mineral ores, silver is typically found
together with other metals (such as gold copper, lead, zinc, etc.).
Various silver extraction techniques can be applied to
silver-bearing mineral ore/concentrate, depending on what the main
metal in the ore/concentrate is (e.g. whether the major metal is
gold, copper, zinc or lead).
[0003] Gold and silver are sometimes present together in mined
ore--sometimes alloyed together (as electrum). They are also
sometimes found in the form of a solid solution within sulfide
minerals, such as iron sulfide (pyrite--FeS.sub.2), iron arsenic
sulfide (arsenopyrite--FeAsS), lead sulfide (galena--PbS) and zinc
sulfide (sphalerite--ZnS). These ores typically contain
sub-microscopic gold and/or silver that are encapsulated within a
crystal matrix of the sulfide mineral. These ores are naturally
resistant to recovery/extraction of the gold and silver using
conventional cyanide leaching processes (cyanidation), and are
often referred to as "refractory ores". These refractory ores
conventionally have to undergo pre-treatment in order to break down
and oxidise the metal sulfide mineral matrix so that the subsequent
cyanidation process will be effective in recovering the silver or
gold. In the case of gold concentrate, such pre-treatment
conventionally involves a pressure oxidation process, which is used
to prepare the concentrate for subsequent conventional metal
extraction processes such as cyanidation. (Roasting and bacterial
leaching are also possible steps for this pre-treatment process,
but, for various reasons, are generally less popular than pressure
oxidation).
[0004] Although not specifically discussed herein, it is
conventional practice in gold mining operations when dealing with
refractory ores, that the mined ore will also undergo froth
flotation (or other concentration or separation techniques) to
first concentrate the target metal content, as one part of the
pre-treatment process, before putting the concentrate through the
pressure oxidation process. As used herein, "concentrate" can
generally refer to the flotation concentrate, the gravity
concentrate, or the ore after oxidation, dissolution, leaching or
any pre-treatment, or any solid product from combinations of the
foregoing pre-treatments.
[0005] The above-mentioned pressure oxidation process is typically
performed in an autoclave at high pressure and temperature, where
high-purity oxygen is mixed with a slurry of the refractory ore or
concentrate. In this pressure oxidation reaction, the original
sulfidic mineral is oxidised and broken down, releasing the trapped
target metal (which in the case of gold concentrate, is gold). In
gold mining/extraction, pressure oxidation produces a high gold
recovery--normally 10% higher than when roasting is used
instead.
[0006] In the case of gold ore concentrate, in which the gold is
associated predominately with pyrite (FeS.sub.2) or arsenopyrite
(FeAsS), the sulfide minerals are oxidised in the pressure
oxidation step to form sulfuric acid, soluble compounds such as
ferric sulfate, and solid compounds such as hematite, basic ferric
sulfate, jarosite, scorodite or basic iron arsenate sulfate. The
iron sulfate-based solid compounds are generally undesirable, since
they can potentially release acid and heavy metals into the
environment, which presents an environmental challenge. They can
also make subsequent precious metal recovery more difficult due to
their acidic nature when being subjected to alkaline conditions.
During pressure oxidation, conventionally, arsenic in the
ore/concentrate is converted to solid scorodite inside the
autoclave, allowing it to be easily disposed of due to its alleged
stability in natural environment. This is an advantage over other
processes such as roasting where arsenic is released as toxic gases
which must be fully captured or scrubbed. During pressure
oxidation, basic ferric arsenate sulfate may also form in addition
to scorodite; however, basic ferric arsenate sulfate is typically
less stable than scorodite.
[0007] A disadvantage of conventional pressure oxidation, however,
is that any silver in the feed material will often react to form
silver jarosite, AgFe.sub.3(OH).sub.6(SO.sub.4).sub.2 inside the
autoclave under pressure oxidation conditions, which makes it
difficult and expensive to recover the silver using conventional
processes. This is because silver jarosite does not dissolve in
cyanide solution, which is conventionally used to leach out the
silver and gold content. Thus, in conventional processing of gold
concentrate which also happens to contain silver, where a
conventional pressure oxidation step is employed as part of that
processing (typically in the case of refractory ores or
concentrates) and cyanidation is used to extract the gold, much of
the silver content cannot be easily recovered using such
cyanidation (because it forms silver jarosite, which does not
readily dissolve in the cyanide leaching solution). Therefore, in
conventional gold extraction operations, much of the silver content
is either lost or requires further processing such as using lime
boil treatment (discussed below) to break down silver jarosite
before cyanidation is applied (thus adding to the processing cost).
In ores/concentrates where the silver content is significant, this
can mean that a significant amount of the silver value of such ore
is not realized.
[0008] One known commercial application that is utilised following
a pressure oxidation step to break down silver jarosite before
recovering the silver from cyanidation, is lime boil treatment.
This treatment involves treating the silver jarosite-containing
material (slurry) with strongly alkaline lime slurry at elevated
temperature near the boiling point of water under atmospheric
pressure for several hours. The general reaction for this lime boil
treatment is as follows:
2AgFe.sub.3(SO.sub.4).sub.2(OH).sub.6+4Ca(OH).sub.2+H.sub.2O.fwdarw.6FeO-
OH+Ag.sub.2O+4CaSO.sub.4.2H.sub.2O
[0009] This reaction serves to decompose the silver jarosite, thus
releasing the silver so that it is amenable to cyanide leach. (The
iron associated with jarosite may be converted to goethite FeOOH
(as shown), or ferric hydroxide Fe(OH).sub.3, or a mixture of these
two). However, this lime boil treatment has a number of downsides:
firstly it results in a significant increase in the slurry
viscosity; secondly, any arsenic in the solid is made less stable
(which is generally not preferred). Further, the lime boil
treatment consumes a large amount of lime, which is very costly.
Further, the product slurry after lime boil may have a pH which is
too high for conventional cyanidation. Also, gold grade and silver
grade are diluted due to the formation of gypsum
(CaSO.sub.4.2H.sub.2O) and goethite (FeOOH) in the solid. As such,
lime boil treatment is not fully satisfactory and usually not
justifiable in many circumstances.
[0010] Accordingly, it is contemplated that it would be
advantageous to provide an alternative process for enhancement of
silver recovery from silver-bearing material, such as gold
concentrate or gold concentrate material from pressure oxidation
operations. Furthermore, there are advantages in being able to
provide a process which, in addition to recovering the silver
content, can also produce a significant mass reduction in the
concentrate material. The solid mass reduction from the concentrate
can lead to processing efficiencies, since less material needs to
be transported and less material itself has to be further processed
downstream.
[0011] We shall describe the present invention as a process for
recovering silver in the context of processing of gold concentrate
from gold mining operations. However, it should be understood that
the disclosed process may also be applied to the processing of any
silver-bearing metal concentrate (e.g. gold/silver/copper
concentrate, gold/silver concentrate) or other silver-bearing
materials. The disclosed invention may also be considered as a
process for recovering silver from the products of pressure
oxidation or from silver jarosite.
BRIEF SUMMARY OF THE INVENTION
[0012] Disclosed herein is a process for recovering silver from
silver-bearing gold concentrate or other silver-bearing material,
including from the solid that is derived from pressure oxidation
operations.
[0013] The disclosed process comprises several basic steps (such as
regrinding, acidulation, pressure oxidation, solid/liquid
separation and washing and reductive leaching, etc.) that may
sometimes be included among some of the processing steps in which
mined gold ore may be processed/concentrated into a more
concentrated form. The input for the subject process is
silver-bearing gold concentrate (which generally speaking is
gold/silver concentrate that has been extracted from a mine, and
which may or may not have undergone some pre-treatment in the form
of froth flotation or gravity concentration, but which generally
has not yet been subject to hydrometallurgical, biological or
pyrometallurgical processing).
[0014] The disclosed process comprises: (i) optionally regrinding
the input silver-bearing material; (ii) optionally treating the
reground silver-bearing material in an acidulation step; (iii)
applying a pressure oxidation step; (iv) applying a post pressure
oxidation conditioning step; (v) applying a first solid/liquid
separation and wash step; (vi) applying a reductive leach step to
the underflow (solid) from the first solid/liquid separation and
wash step; (vii) applying a second solid/liquid separation and wash
step; and (viii) applying an optional surface cleaning step, to
produce a free-milling silver-bearing material, which is amenable
to conventional cyanidation to recover the silver therefrom.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Embodiments of the present invention are described below
with reference to the accompanying drawings in which:
[0016] a. FIG. 1 is a simplified flowchart illustrating the process
in accordance with one aspect
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention now will be described more fully
hereinafter with reference to the accompanying drawing(s), which
form a part hereof, and which show, by way of illustration,
exemplary embodiments by which the invention may be practiced. The
invention may, however, be embodied in many different forms and
should not be construed as limited to the embodiments set forth
herein; rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the
scope of the invention to those skilled in the art. The following
detailed description is, therefore, not to be taken in a limiting
sense.
[0018] Referring to FIG. 1, this is a simplified flowchart setting
out an exemplary method and process 10 for recovering silver from
silver-bearing gold concentrate. The disclosed process comprises:
(i) optionally regrinding the input silver-bearing gold
concentrate; (ii) optionally treating the reground silver-bearing
gold concentrate in an acidulation step; (iii) applying a pressure
oxidation step; (iv) applying a post pressure oxidation
conditioning step; (v) applying a first solid/liquid separation and
wash step; (vi) applying a reductive leach step to the underflow
(solid) from the first solid/liquid separation and wash step; (vii)
applying a second solid/liquid separation and wash step; and (viii)
optionally applying a surface cleaning step, to produce a
free-milling silver-bearing material.
[0019] The initial input for the subject process is silver-bearing
gold concentrate or other silver-bearing material 12 (which we
shall generally refer to as "silver-bearing gold concentrate").
Preferably, the input silver-bearing gold concentrate has already
undergone some pre-processing or concentration before it arrives at
this input stage, in which case it is generally in the form of an
aqueous slurry or moist filter cake. For example, froth flotation
techniques (and/or other applicable gravity concentration
techniques) may have been applied to the gold ore to concentrate
the gold (particularly in the case of gold ore where the gold is
closely associated with sulfide minerals such as pyrite,
arsenopyrite, pyrrhotite, chalcopyrite, sphalerite and galena. It
should be understood, however, that the subject process could also
be applied where freshly mined silver-bearing gold ore is the
initial input.
[0020] Where appropriate, the input silver-bearing gold concentrate
12 undergoes mechanical regrinding (step 14), to form a reground
gold concentrate 15. The extent of such regrinding required, and
whether it is necessary at all, will depend on the state of the
input silver-bearing gold concentrate (for example, it may already
have undergone some degree of grinding during the above mentioned
pre-processing, in which case, little or no further regrinding may
be necessary). The main purpose of this regrinding step is simply
to break down the silver-bearing gold concentrate particles into
smaller pieces, so that it has a greater surface area, in order to
enhance reaction rate and reaction completeness in the downstream
processing steps.
[0021] The silver-bearing gold concentrate 12 or the reground gold
concentrate 15, as the case may be, (either of which is in the form
of an aqueous slurry) is then treated with acid in an acidulation
step 16 to form an acidulated concentrate slurry 17. The acid used
is preferably either concentrated sulfuric acid or an aqueous
sulfuric acid solution. The acid generally serves to react with and
break down the carbonates in the gold concentrate to form carbon
dioxide (CO.sub.2), which can then be removed in the form of
CO.sub.2 gas. The acidulation step 16 can generally be carried out
in one or multiple stirred tanks at ambient pressure for one or a
few hours.
[0022] One of the objectives, of using acid and dissolved metals
(such as dissolved ferric sulfate) during the acidulation step 16,
is to make the solid compounds formed during pressure oxidation 18
unstable, so that mass reduction of the solid phase and
solubilization of arsenic can be maximized in the ensuing pressure
oxidation step 18 and subsequent post pressure oxidation
conditioning step 22. This objective is counterintuitive to what is
typically desired in the mining industry as a whole when utilizing
comparable pressure oxidation steps; in those situations, the
general objective is to form the most stable solid compounds such
as hematite and scorodite during pressure oxidation (particularly
when processing arsenic-bearing materials). The purpose of the
acidulation step 16 in the disclosed process, besides adding extra
acid for the subsequent pressure oxidation step 18, is to maximise
the instability of the solid compounds, and to dissolve and keep as
much iron, sulfur and arsenic in solution as possible.
[0023] For greater operational efficiency and flexibility, it is
contemplated that, as shown in FIG. 1, acid-containing solutions
from other parts of the process (i.e. the acid-containing solutions
from downstream steps such as 30, 31, 32, 46, 47 and/or 48) may be
recycled for this acidulation step 16. Where appropriate, cooling
may be applied to these warm/hot acid-containing solutions before
recycling to minimize heat input when the downstream pressure
oxidation is already surplus in heat generation. The extent of
recycling of the acid-containing solutions streams 30, 31 and 32 to
the acidulation step 16 can be as needed, and can vary from 0% to
nearly 100%. These acid-containing solutions may also contain,
besides the acid itself, dissolved iron (e.g. in the form of
ferrous and/or ferric), dissolved arsenic (e.g. in the form of
arsenite and/or arsenate), and dissolved sulfate salts, etc. For
example, the acid-containing solution can either be substantially
free of solids (such as the overflow stream 32 from the thickening
step (step 24) described below or it can contain solids, (such as
the oxidized concentrate slurry 19 from the autoclave discharge or
the underflow stream of thickened oxidized concentrate slurry 25,
etc.). Besides free acid (sulfuric acid), acid can also come from
the hydrolysis of dissolved metals such as ferric iron. The
contained solids in the recycled stream tend to serve as seeding
material which is helpful to the reaction rates, to the stability
of the solid compounds and to the mitigation of scale formation
inside the autoclave.
[0024] The acidulated concentrate slurry 17 is then fed into a
pressure oxidation vessel or autoclave. Oxygen and, if required for
temperature control, water are added (stream 20) to the autoclave.
In a preferred embodiment, and as shown in FIG. 1, the
acid-containing overflow streams from downstream steps (30, 31, 32,
46, 47, 48), as needed, may also be recycled and added to the
autoclave. Where appropriate, cooling may be applied to these
warm/hot acid-containing solutions before recycling to minimize
heat input when the pressure oxidation is already surplus in heat
generation. The contents of the autoclave are reacted together to
oxidize the sulfide minerals therein under high pressure and
temperature conditions (sometimes referred to herein as oxidizing
pressure and oxidizing temperature, respectively) to form an
oxidized concentrate slurry 19. During this pressure oxidation step
18, as the sulfides are oxidized, the silver in the feed material
is initially liberated, but is subsequently precipitated, mostly as
silver jarosite [AgFe.sub.3(SO.sub.4).sub.2(OH).sub.6] or is
incorporated into other jarositic species, all of which are
refractory to cyanide leach.
[0025] The reaction conditions for this pressure oxidation step 18
are preferably: from about 190 to about 240.degree. C.; from about
200 to about 600 psig total pressure; from about 15 to about 250
psi oxygen partial pressure; from about 30 to about 120 minutes
retention time. More preferably, the reaction conditions for the
pressure oxidation step are: from about 220 to about 230.degree.
C.; from about 430 to about 530 psig total pressure; from about 25
to about 100 psi oxygen partial pressure; and from about 60 to
about 90 minutes retention time.
[0026] During pressure oxidation (step 18), sulfide minerals are
oxidized to various compounds. For instance, sphalerite (ZnS) is
oxidized to zinc sulfate (ZnSO.sub.4) in solution. Chalcopyrite
(CuFeS.sub.2) is oxidized to copper sulfate (CuSO.sub.4) and
sulfuric acid (H.sub.2SO.sub.4) in solution, and iron compounds
including hematite (Fe.sub.2O.sub.3), jarosite
[MFe.sub.3(OH).sub.6(SO.sub.4).sub.2] and/or basic ferric sulfate
(FeOHSO.sub.4) in the solid, plus the formation of dissolved
ferrous iron sulfate (FeSO.sub.4) and dissolved ferric iron sulfate
[Fe.sub.2(SO.sub.4).sub.3] in solution. Galena (PbS) is oxidized to
form insoluble lead sulfate (PbSO.sub.4) and/or insoluble lead
jarosite (plumbojarosite)
[Pb.sub.0.5Fe.sub.3(OH).sub.6(SO.sub.4).sub.2]. Depending on pH and
temperature conditions, oxidation of pyrite (FeS.sub.2) can lead to
formation of hematite (Fe.sub.2O.sub.3), jarosite
[MFe.sub.3(OH).sub.6(SO.sub.4).sub.2] and/or basic ferric sulfate
(FeOHSO.sub.4) in the solid, plus the formation of sulfuric acid
and dissolved ferrous iron sulfate (FeSO.sub.4) and dissolved
ferric iron sulfate (Fe.sub.2(SO.sub.4).sub.3) in solution. For the
maximum solid weight loss, the formation of iron sulfates in
solution, and of labile basic ferric sulfate (FeOHSO.sub.4) as the
preferred precipitated form of iron in the solids, should be
facilitated during pressure oxidation.
[0027] Oxidation of arsenopyrite (FeAsS) during pressure oxidation
can result in a series of solid compounds in addition to the
formation of sulfuric acid and soluble trivalent arsenic and
pentavalent arsenic in solution. The solid arsenic compounds can be
the simple scorodite FeAsO.sub.4.2H.sub.2O or more complicated
basic iron arsenate sulfate
Fe.sub.x(OH).sub.y(SO.sub.4).sub.z(AsO.sub.4).sub.m.nH.sub.2O. For
the maximum solid weight loss and maximum re-dissolution of iron,
sulfur and arsenic solid compounds, the formation of basic iron
arsenate sulfate
Fe.sub.x(OH).sub.y(SO.sub.4).sub.z(AsO.sub.4).sub.m.nH.sub.2O
should be facilitated during pressure oxidation.
[0028] As shown in the preferred embodiment of FIG. 1, it is
contemplated that the subject process should preferably include a
separate acidulation step 16 (for the reasons previously
described), before the slurry is subject to the subsequent pressure
oxidation step 18. However, it is to be understood that acidulation
may to some extent occur as part of or in combination with the
pressure oxidation step (given that the pressure oxidation step
generally takes place under acidic condition); as such the separate
acidulation step 16 may be regarded as optional.
[0029] The oxidized concentrate slurry 19 is then subjected to a
post pressure oxidation conditioning step 22, wherein the oxidized
concentrate slurry 19 is discharged from the autoclave and
maintained for several hours at a temperature in the range of from
about 50 to about 100.degree. C. (the upper limit of the range
being at or near the boiling point of water) and at ambient
pressure, to form a conditioned slurry 21; most preferably, the
temperature at which the oxidized concentrate slurry 19 is
maintained at is about 95.degree. C. As the oxidized concentrate
slurry 19 is discharged from the autoclave, there is a reduction in
pressure and temperature. In general, most of the unstable solid
compounds containing iron, arsenic, sulfate and/or hydroxyl will be
dissolved in this step. This step is needed to reduce operating
cost in the downstream reductive leach step 36.
[0030] The conditioned slurry 21 is then subjected to a first
solid/liquid separation and wash step 23. Where appropriate,
cooling may be applied to the hot conditioned slurry 21 before this
first solid/liquid separation and wash step 23. This step comprises
at least one of several conventional techniques for facilitating
the separation of a slurry into solids and a solution, and for
washing of the resultant solids. The first solid/liquid separation
and wash step 23 can include at least one or a combination of: a
thickening step; a counter current decantation (CCD) step; and a
filter and wash step. Optionally, re-pulping of the filter cake may
be included as well between filtrations to further improve wash
efficiency.
[0031] In a preferred embodiment, as shown in FIG. 1, the first
solid/liquid separation and wash step 23 comprises: a thickening
step 24; a CCD wash step 26; and a filter and wash step 28. In the
thickening step 24, the conditioned slurry 21 is thickened to form
a thickened oxidized concentrate slurry 25 and an overflow stream
of acid-containing solution 32 comprising a solution of acid and
dissolved metal sulfates/arsenates. The thickening step 24 improves
the wash efficiency of the subsequent CCD wash step (step 26) and
also recovers a portion of relatively concentrated solution of acid
and dissolved metal sulfates/arsenates for recycling to the
acidulation step (step 16) and/or the pressure oxidation step (step
18). In the CCD wash step 26, wash water is applied to the
thickened oxidized concentrate slurry 25 to form an underflow
stream 27 of washed oxidized concentrate slurry and an overflow
stream of acid-containing solution 30 comprising a solution of acid
and dissolved metal sulfates/arsenates. The CCD wash step 26
involves the removal of acid and dissolved metal sulfates/arsenates
in multiple thickeners by applying clean water wash. The amount of
dissolved metal sulfates/arsenates in the underflow stream 27 of
washed oxidized concentrate slurry (as well as in the first washed
slurry/solid 29) should be preferably kept to a minimum, because
any dissolved ferric iron (Fe.sup.3+) and dissolved pentavalent
arsenate (As.sup.5+) will consume sulfur dioxide during the
subsequent reductive leach step (step 36). The filter and wash step
28 can be one or more of a number of conventional filtration and
washing techniques for filtering and washing slurry/solids. In the
filter and wash step (step 28), the underflow stream 27 of the
washed oxidized concentrate slurry undergoes filtration and/or wash
to further reduce the amount of sulfuric acid and dissolved metal
sulfates/arsenates. Optionally, re-pulping of the filter cake may
be included as well between filtrations to further improve wash
efficiency. The filtrate stream 31 from the filter and wash step 28
is acid-containing solution.
[0032] In the above embodiment, the first solid/liquid separation
and wash step 23 forms an underflow (solid) stream 29 of first
washed slurry/solid and overflow/filtrate streams (30, 31 and 32)
of an acid-containing solution (which also contains dissolved metal
sulfates/arsenates). In the preferred embodiment shown in FIG. 1,
the first washed slurry/solid 29 stream corresponds to the
underflow (or filter cake) stream from the filter and wash step
28.
[0033] There are a number of other variations for the solid/liquid
separation and wash step 23. These can include, for example, the
following options:
[0034] Option #1--"CCD wash" only;
[0035] Option #2--"Filter and wash" only;
[0036] Option #3--"CCD wash"+"Filter and wash";
[0037] Option #4--"Thickening"+"CCD wash"; or
[0038] Option #5--"Thickening"+"Filter and wash".
[0039] The different options and the appropriateness of using such
given the circumstances will generally be understood by a person
skilled in the art. For instance, when the solid content in the
conditioned slurry 21 is high, the thickening step 24 may be
unnecessary and so can be omitted. Multiple stages of filter and
wash may be considered. Also, re-pulping of the filter cake may be
included between filtrations to further improve wash
efficiency.
[0040] The overflow stream 32 from the thickening step 24; the
overflow stream 30 from the CCD wash step 26; and the filtrate
stream 31 from the filter and wash step 28; all of which consist
mostly of process water with acid and dissolved metal
sulfates/arsenates in solution, can be recycled, as needed, to the
acidulation step 16 and/or to the pressure oxidation step 18, to
provide the increased acidity to make the solid iron, sulfur and
arsenic compounds less stable and to serve as a water source
therefor for temperature control. Where appropriate, cooling may be
applied to these warm/hot acid-containing solutions before
recycling to minimize heat input when the pressure oxidation is
already surplus in heat generation. The surplus of the
overflow/filtrate streams (32, 30 and 31) may undergo a further
water treatment step 34, where acid is neutralized and any
dissolved metal sulfates/arsenates may be precipitated or
recovered, if desired, using any commercially available
conventional processes. For example, it may be desirable that
dissolved copper be recovered from these overflow/filtrate streams.
Again, the treated water from the water treatment step 34 may also
be recycled to the acidulation step 16, the pressure oxidation step
18, the CCD wash step 26, the filter and wash step 28, and other
steps related to the process including flotation and grinding,
and/or as wash water or dilution water in various steps in order to
serve as a source of water therefor. Again, where appropriate,
cooling may be applied to the treated process water before
recycling to allow proper heat balance.
[0041] The first washed slurry/solid 29 from the first solid/liquid
separation and wash step 23 is then subjected to a reductive leach
step 36. Sulfuric acid, sulfur dioxide and water (and optionally, a
catalyst of copper, such as copper sulfate) are added (stream 35)
to the first washed slurry/solid 29 in a sealed reactor, and
reacted with the first washed slurry/solid 29 under moderately
elevated temperatures and total pressure (see below for suitable
reaction conditions) to reduce ferric iron (Fe.sup.3+) and
pentavalent arsenic (As.sup.5+) in the solid to form soluble
ferrous iron (Fe.sup.2+) (such as ferrous sulfate FeSO.sub.4) and
soluble trivalent arsenic (As.sup.3+)(such as arsenous acid,
H.sub.3AsO.sub.3) in solution, respectively. Any sulfates, which
are associated with ferric iron (Fe.sup.3+) and pentavalent arsenic
(As.sup.5+) in the solid, will be dissolved into solution as well
after ferric iron (Fe.sup.3+) and pentavalent arsenic (As.sup.5+)
in the solid are reduced to ferrous iron (Fe.sup.2+) and trivalent
arsenic (As.sup.3+). In general, hematite Fe.sub.2O.sub.3, jarosite
MFe.sub.3(OH).sub.6(SO.sub.4).sub.2 (including silver jarosite),
residual basic ferric sulfate FeOHSO.sub.4, scorodite
FeAsO.sub.4.2H.sub.2O, and any basic iron arsenate sulfate
Fe.sub.x(OH).sub.y(SO.sub.4).sub.z(AsO.sub.4).sub.m.nH.sub.2O, etc,
will all be broken down. The added sulfur dioxide is oxidized to
sulfate or sulfuric acid during this reductive leach step 36.
[0042] Assuming arsenic in the solid is present as basic iron
arsenate sulfate (which for example may be expressed as
Fe.sub.6(SO.sub.4).sub.3(AsO.sub.4).sub.2(OH).sub.6.nH.sub.2O), the
reductive reaction may be expressed as:
Fe.sub.6(OH).sub.6(SO.sub.4).sub.3(AsO.sub.4).sub.2.nH.sub.2O+5SO.sub.2+-
2H.sub.2O.fwdarw.6FeSO.sub.4+2H.sub.3AsO.sub.3+2H.sub.2SO.sub.4+nH.sub.2O
(1)
[0043] The reductive reactions for scorodite FeAsO.sub.4.2H.sub.2O
may be expressed in the following:
2FeAsO.sub.4.2H.sub.2O+3SO.sub.2.fwdarw.2FeSO.sub.4+2H.sub.3AsO.sub.3+H.-
sub.2SO.sub.4 (2)
[0044] The reductive reactions for hematite Fe.sub.2O.sub.3, silver
jarosite AgFe.sub.3(OH).sub.6(SO.sub.4).sub.2 and basic ferric
sulfate FeOHSO.sub.4 may be expressed in the following:
Fe.sub.2O.sub.3+SO.sub.2+H.sub.2SO.sub.4.fwdarw.2FeSO.sub.4+H.sub.2O
(3)
2AgFe.sub.3(OH).sub.6(SO.sub.4).sub.2+3SO.sub.2.fwdarw.6FeSO.sub.4+H.sub-
.2SO.sub.4+Ag.sub.2O+5H.sub.2O (4)
2FeOHSO.sub.4+SO.sub.2.fwdarw.2FeSO.sub.4+H.sub.2SO.sub.4 (5)
[0045] (Note: Besides the formation of silver oxide Ag.sub.2O in
(4), other insoluble silver compounds may form as well during
reductive leach).
[0046] Suitable reaction conditions for this reductive leach step
36 are: a temperature of from about 50 to about 150.degree. C.;
total pressure of from about 5 to about 150 psig, a sulfur dioxide
partial pressure of from about 1 to about 100 psi; and reaction
time of from about 1 to about 10 hours. Preferably, the reaction
conditions for this reductive leach step 36 are: a temperature of
from about 50 to about 100.degree. C.; total pressure of from about
5 to about 50 psig, a sulfur dioxide partial pressure of from about
1 to about 30 psi; and reaction time of about 6 hours. In this
reductive leach step 36, over about 80% of solid arsenic compounds
in the first washed slurry/solid 29 can be dissolved into solution.
In addition to the dissolution of solid arsenic compounds, the
majority of other arsenic-free compounds such as hematite
(Fe.sub.2O.sub.3) and silver jarosite
(AgFe.sub.3(OH).sub.6(SO.sub.4).sub.2), etc., in the solid, are
also dissolved during this reductive leach step 36.
[0047] As mentioned previously, it is generally preferable to keep
the amount of dissolved metal sulfates/arsenates in the underflow
stream 27 of washed oxidized concentrate slurry, and in the first
washed slurry/solid 29 to a minimum, since any dissolved ferric
iron (Fe.sup.3+) and dissolved pentavalent arsenate (As.sup.5+)
will consume sulfur dioxide (thus adding to the cost and
potentially adding to the processing time/requirements).
Preferably, the pulp density for reductive leach is chosen such
that the dissolved iron and arsenic in the solution is always below
the solubility limits of ferrous sulfate (FeSO.sub.4) and arsenic
trioxide (As.sub.2O.sub.3) in solution at the operating temperature
or even at ambient temperature.
[0048] This reductive leach step 36 may form a vent gas 38 (which
may comprise sulfur dioxide that is vented to a sulfur dioxide
scrubber) and a leached concentrate slurry 37. Venting of the
off-gas from the reductive leach step 36 will be necessary when
less than 100% pure sulfur dioxide is added to the reactor(s). The
sulfur dioxide bearing vent gas 38 will also occur when the slurry
is discharged out of the reactor(s), and the amount of vent gas may
be reduced when cooling or purging with an inert gas such as
nitrogen is provided to the slurry before discharge. To minimize or
eliminate venting when either 100% pure sulfur dioxide or less than
100% pure sulfur dioxide is added to the reactor(s), the added
sulfur dioxide must be adequately sheared and dispersed into the
solution/slurry, and the sulfur dioxide in the gas phase must be
sufficiently re-entrained into the solution/slurry through
appropriately designed reactor(s), baffles, spargers and agitators.
The solid in the leached concentrate slurry 37 contains primarily
silicates, other inert gangues, and a very low level of arsenic. If
lead is present, lead sulfate may be expected in the solid. Small
amounts of other compounds, such as gypsum, may be present as well.
Silver (any gold and any platinum group elements like platinum and
palladium) remains in the solid without loss during the reductive
leach step 36.
[0049] The leached concentrate slurry 37, which generally contains
ferrous sulfate (FeSO.sub.4), trivalent arsenic
(H.sub.3AsO.sub.3and other arsenite compounds) and acid
(H.sub.2SO.sub.4), etc., in the solution, is then treated in a
second solid/liquid separation and wash step 39. Where appropriate,
cooling may be applied to the hot leached concentrate slurry 37
before this second solid/liquid separation and wash step 39. This
second solid/liquid separation and wash step 39 also comprises at
least one of several conventional techniques for facilitating the
separation of a slurry into solids and a solution and for washing
of the resultant solids (as described above in relation to the
first liquid/solid separation and wash step 23). As shown in the
preferred embodiment of FIG. 1, the second solid/liquid separation
and wash step 39 can comprise: a thickening step 40; a counter
current decantation (CCD) wash step 42; and a filter and wash step
44.
[0050] In the thickening step 40, the leached concentrate slurry 37
is thickened to form a thickened leached concentrate slurry 41 and
an overflow stream of acid-containing solution 46. The thickening
step 40 improves the wash efficiency of the subsequent CCD wash
step (step 42) and also recovers a portion of the acid in solution,
dissolved arsenic and dissolved metal sulfates for recycling to the
acidulation step (step 16) and/or the pressure oxidation step (step
18). In the CCD wash step 42, wash water is applied to the
thickened leached concentrate slurry 41 to form an underflow stream
43 of washed thickened slurry/solid and an overflow stream of
acid-containing solution 47. The CCD wash step 42 involves the
removal of acid, dissolved arsenic and dissolved metal sulfates in
multiple thickeners by applying clean water wash. In the filter and
wash step (step 44), the underflow stream 43 of washed thickened
slurry/solid can undergo filtration and/or wash to further reduce
the amount of acid, dissolved arsenic and dissolved metal sulfates;
the filtrate stream 48 from the filter and wash step 44 is
acid-containing solution.
[0051] The overflow streams 46 and 47 contain primarily ferrous
sulfate FeSO.sub.4, trivalent arsenic (H.sub.3AsO.sub.3and other
arsenite compounds), sulfuric acid H.sub.2SO.sub.4, and a small
amount of dissolved sulfur dioxide SO.sub.2 (or sulfurous acid
H.sub.2SO.sub.3).
[0052] The washed thickened slurry/solid 43 is then subject to a
filter and wash step 44, which comprises at least one of a number
of conventional filtering and washing techniques for washing the
washed thickened slurry/solid 43, to form an acid-containing
solution 48 and a second washed slurry/solid 45. Multiple stages of
filter and wash may be considered. Also, re-pulping of the filter
cake may be included between filtrations to further improve wash
efficiency.
[0053] As mentioned above, and as shown in FIG. 1, some or all of
the acid-containing solutions 46, 47 and 48 from the second
solid/liquid separation and wash step 39 following the reductive
leach step 36 are preferably recycled to the acidulation step 16
and/or to the pressure oxidation step 18, generally to help oxidize
trivalent arsenic (As.sup.3+) to pentavalent arsenic (As.sup.5+)
and oxidize ferrous iron (Fe.sup.2+) to ferric iron (Fe.sup.3+),
because it is desirable to have pentavalent arsenic and ferric iron
when the process water is treated for better stability of the
resultant solid precipitate. Where appropriate, cooling may be
applied to these acid-containing solutions before recycling to
allow proper heat balance in the downstream. Alternatively, despite
being less desirable in terms of capital and operating costs, these
trivalent arsenic and ferrous iron bearing acid-containing
solutions 46, 47 and 48 can be oxidized and treated separately.
Another alternative is to partially or nearly completely oxidize
these trivalent arsenic and ferrous iron bearing acid-containing
solution streams 46, 47 and 48 in a separate circuit and then
recycle them to the acidulation step 16 and/or to the pressure
oxidation step 18, to avoid any undesirable effects which may occur
during acidulation step 16 and/or pressure oxidation step 18 when a
large amount of trivalent arsenic (As.sup.3+) and ferrous iron
(Fe.sup.2+) is present in the solution/slurry
[0054] The possible variations as described above for the first
solid/liquid separation and wash step 23, are also applicable for
the second solid/liquid separation and wash step 39, including the
following options:
[0055] Option #1--"CCD wash" only;
[0056] Option #2--"Filter and wash" only;
[0057] Option #3--"CCD wash"+"Filter and wash";
[0058] Option #4--"Thickening"+"CCD wash"; or
[0059] Option #5--"Thickening"+"Filter and wash".
[0060] The different options and the appropriateness of using such
given the circumstances will generally be understood by a person
skilled in the art. Multiple stages of filter and wash may be
considered. Also, re-pulping of the filter cake may be included
between filtrations to further improve wash efficiency.
[0061] The second washed slurry/solid 45 may then be subjected to
an optional surface cleaning step 50. This surface cleaning step 50
may not be required, for example, where the product second washed
slurry/solid 45 is to be sold to a smelter for further processing.
However, where the product second washed slurry/solid is to be
hydrometallurgically processed further (such as leaching using
sodium cyanide solution), this surface cleaning step 50 will be
necessary to enhance the silver leach rate and increase silver
recovery.
[0062] One suitable possibility for the surface cleaning step 50
involves treating the second washed slurry/solid 45 with hydrogen
peroxide (or another oxidizing agent like ozone or hypochlorite),
at ambient temperature or slightly elevated temperature, after the
second washed slurry/solid 45 is re-pulped or diluted. When
hydrogen peroxide is used, acidic conditions are generally
preferred in order to ensure the stability of hydrogen peroxide.
Another effective surface cleaning method may be achieved through
regrinding of the second washed slurry/solid 45 prior to silver
leaching.
[0063] The product of the foregoing is a free-milling
silver-bearing material 52. In comparison with the input
silver-bearing material, this free-milling silver-bearing material
52 will have shown a substantial reduction in mass in terms of the
amount of solid material involved. The free-milling silver-bearing
material 52 can undergo conventional silver extraction (step 54)
including, for example, leach treatment with cyanide or other
lixiviants (such as thiosulfate, thiocyanate, chloride, bromide,
hypochlorite, thiourea, glycine, etc. (including, more
specifically, sodium cyanide, calcium thiosulfate, ammonium
thiosulfate)).
EXAMPLE
[0064] Silver-bearing gold concentrate was processed in accordance
with the disclosed process. Samples were collected from certain of
the individual steps in the process, and assays were conducted.
When silver-bearing gold concentrate was processed through the
conventional process of cyanide leaching following pressure
oxidation, the silver recovery was less than 5%. However, when
silver-bearing gold concentrate of similar quality was processed in
accordance with the disclosed process (with the additional
reductive leach step), the silver recovery from cyanide leach
following the reductive leach step (step 52) was increased to over
88%.
[0065] Furthermore, there was a substantial reduction in mass of
the solid material, which also led to a significant increase in the
silver grade. Silver grade in the solid was increased from 20 grams
per tonne in the initial input stream 12 to about 175 grams per
tonne in the final product stream 52 after reductive leach.
* * * * *