U.S. patent application number 12/811285 was filed with the patent office on 2010-11-11 for method for metal recovery and leaching agent recycle in agitation leach plants.
This patent application is currently assigned to COGNIS IP MANAGEMENT GMBH. Invention is credited to Andrew Nisbett.
Application Number | 20100282025 12/811285 |
Document ID | / |
Family ID | 40377569 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100282025 |
Kind Code |
A1 |
Nisbett; Andrew |
November 11, 2010 |
Method For Metal Recovery and Leaching Agent Recycle in Agitation
Leach Plants
Abstract
This invention is directed to an improved process for metal
recovery from ore using agitation leaching comprising dividing
leaching of the crushed and mined ore into at least two sequential
leaching-solids-liquid separation-solvent extraction sub-circuits,
with no significant dilution during solids-liquid separation in
these units, and the raffinate from solvent extraction being
recycled back to the leaching, with the underflow pulp from the
second liquid-solids separator being sent to a final solid-liquid
separator, with water washing, from which the washed solids are
sent to disposal and the clarified aqueous wash solution is sent to
a final solvent extraction, with some or all of the metal-depleted
aqueous raffinate from this final solvent extraction being
optionally neutralized, and/or being circulated back to the third
solid-liquid separation as wash solution and/or to recovery of
other metals and/or to disposal to maximize metal recovery and
maintain water balance.
Inventors: |
Nisbett; Andrew; (Tuscon,
AZ) |
Correspondence
Address: |
FOX ROTHSCHILD LLP
997 Lenox Drive, Bldg. #3
Lawrenceville
NJ
08648
US
|
Assignee: |
; COGNIS IP MANAGEMENT GMBH
Duesseldorf
DE
|
Family ID: |
40377569 |
Appl. No.: |
12/811285 |
Filed: |
December 20, 2008 |
PCT Filed: |
December 20, 2008 |
PCT NO: |
PCT/EP2008/010979 |
371 Date: |
June 30, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61009719 |
Dec 31, 2007 |
|
|
|
Current U.S.
Class: |
75/743 |
Current CPC
Class: |
C22B 15/0084 20130101;
Y02P 10/236 20151101; Y02P 10/234 20151101; Y02P 10/20 20151101;
C22B 15/0065 20130101; C22B 3/0005 20130101; C22B 15/0071
20130101 |
Class at
Publication: |
75/743 |
International
Class: |
C22B 5/00 20060101
C22B005/00; C22B 15/00 20060101 C22B015/00 |
Claims
1. A process for recovering desired metal values from crushed and
milled ore solids comprising the steps of: (a) mixing a first
aqueous leach solution with the crushed and milled ore solids in a
first agitated tank leach unit, whereby at least a portion of the
desired metal values in the ore solids is dissolved into the first
aqueous leach solution to obtain a first aqueous leach pulp
comprising a mixture of leached solids and first aqueous leach
solution; (b) subjecting the first aqueous leach pulp to a first
solids-liquid separation, without significant water dilution, to
provide a first clarified aqueous leach solution and a second
aqueous leach pulp, wherein the second aqueous leach pulp comprises
leached solids at a percent solids level that is greater than that
in the first aqueous leach pulp; (c) subjecting the first clarified
aqueous leach solution to a first solvent extraction, whereby at
least a portion of the desired metal values are extracted into a
first organic phase comprising one or more extraction reagents
specific for the desired metal, and a first aqueous raffinate,
depleted of desired metal values, is obtained; (d) mixing a second
aqueous leach solution with the second aqueous leach pulp in a
final agitated tank leach unit, whereby at least a portion of the
desired metal values formerly in the second aqueous leach pulp is
dissolved into the second aqueous leach solution to obtain a third
aqueous leach pulp, wherein the third aqueous leach pulp comprises
a mixture of twice-leached solids and a second aqueous leach
solution, rich in desired metal values; (e) subjecting the third
aqueous leach pulp to a second solids-liquid separation, without
significant water dilution, to provide a second clarified aqueous
leach solution and a fourth aqueous leach pulp, wherein the fourth
aqueous leach pulp comprises leached solids at a percent solids
level that is greater than that in the third aqueous leach pulp;
(f) subjecting the second clarified aqueous leach solution to a
second solvent extraction, whereby at least a portion of the
desired metal values are extracted into a second organic phase
comprising one or more extraction reagent(s) specific for the
desired metal, and a second aqueous raffinate, depleted of desired
metal values, is obtained; (g) subjecting the fourth aqueous leach
pulp to a third solids-liquid separation, with significant dilution
via an aqueous stream, to provide a third clarified aqueous leach
solution and a fifth aqueous pulp, wherein the concentration of
desired metal values in the third clarified aqueous leach solution
is less than the concentration of desired metal values in the
second clarified aqueous leach solution, and the fifth aqueous pulp
comprises a mixture of leached solids and aqueous leach solution;
and (h) subjecting the third clarified aqueous leach solution to a
third solvent extraction whereby at least a portion of the desired
metal values are extracted into a third organic phase comprising
one or more extraction reagents(s) specific for the desired metal,
and a third aqueous raffinate, depleted of desired metal values, is
obtained.
2. The process according to claim 1, wherein the providing of a
second aqueous leach pulp in step (b) further comprises an
intermediate leaching step, wherein a first intermediate aqueous
leach pulp obtained from the first solid-liquid separation is mixed
with an intermediate aqueous leach solution in an intermediate
agitated tank leach unit to obtain a second intermediate aqueous
leach pulp, subjecting the second intermediate aqueous leach pulp
to an intermediate solid-liquid separation to obtain the second
aqueous leach pulp and an intermediate clarified aqueous leach
solution, wherein the intermediate clarified aqueous leach solution
is subjected to an intermediate solvent extraction to obtain an
intermediate aqueous raffinate.
3. The process according to claim 1, wherein the desired metal is
selected from the group consisting of copper, zinc, nickel and
cobalt.
4. The process according to claim 1, wherein the first aqueous
leach solution and the second aqueous leach solution comprise
sulphuric acid.
5. The process according to claim 1, wherein the first aqueous
leach solution and the second aqueous leach solution comprise
ammonia.
6-9. (canceled)
10. The process according to claim 1, wherein the third
solid-liquid separation comprises counter-current decantation.
11. The process according to claim 1, wherein the concentration of
the desired metal in the first clarified aqueous leach solution is
at least 30% greater than the concentration of the desired metal in
the second clarified aqueous leach solution.
12. The process according to claim 1, wherein the concentration of
the desired metal in the first clarified aqueous leach solution is
at least 50% greater than the concentration of the desired metal in
the second clarified aqueous leach solution.
13. The process according to claim 1, wherein the concentration of
the desired metal in the first clarified aqueous leach solution is
at least 70% greater than the concentration of the desired metal in
the second clarified aqueous leach solution.
14. The process according to claim 1, wherein the concentration of
the metal in the first clarified aqueous leach solution is at least
100% greater than the concentration of the desired metal in the
second clarified aqueous leach solution.
15. The process according to claim 1, wherein the first aqueous
leach solution comprises at least a portion of the first aqueous
raffinate.
16. The process according to claim 15, wherein the first aqueous
leach solution further comprises fresh leaching agent.
17. The process according to claim 16, wherein the second aqueous
leach solution comprises at least a portion of the second aqueous
raffinate.
18. The process according to claim 17, wherein the second aqueous
leach solution further comprises fresh leaching agent.
19. The process according to claim 18, wherein the first aqueous
leach solution, second aqueous leach solution, and fresh leaching
agent comprise sulphuric acid.
20. The process according to claim 18, wherein the first aqueous
leach solution and second aqueous leach solution comprise ammonia,
and the fresh leaching agent comprises gaseous ammonia and/or
ammonium hydroxide.
21. The process according to claim 1, wherein the third aqueous
raffinate is neutralized and further processed according to a step
selected from the group consisting of being circulated back to the
third solids-liquid separation, being sent to disposal, being
treated to recover one or more other metal values, and combinations
of two or more of these.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the design and operation of
the leaching and solvent extraction steps in a metal recovery plant
for recovering desired metal values from mined ores containing such
metal values possibly comingled with other metal values.
BACKGROUND OF THE INVENTION
[0002] To obtain metals (e.g., copper, nickel, cobalt, zinc,
uranium, and the like) in a pure, useful form, these metals must be
removed and recovered from the ores in which they are found through
a series of physical, hydrometallurgical and/or chemical steps.
[0003] Conventionally, the mined ore, containing a greater or
lesser amount of the desired metal value, in addition to possibly
one or more other more-or-less desirable metal values and a large
amount of gangue and other more-or-less complicating minerals, is
leached with an aqueous acidic (commonly sulphuric acid) or basic
(commonly ammonium hydroxide) solution. This leaching is
accomplished by either distributing the leaching agent over a pile
or bed of mostly dry ore solids in dump leaching, heap leaching or
vat leaching, wherein these ores are either leached as mined, or
they may be crushed, but not ground or milled, to a size that gives
higher metal recovery and/or faster metal recovery, or, as in
agitation leaching, by mixing the leaching agent with an aqueous
slurry of crushed and milled ore solids in one or more stirred
tanks in an attempt to ensure optimal distribution of the leaching
solution throughout the ore solids.
[0004] In heap, dump or vat leaching, dry ore is placed in a
pile/bed leach system, or, where ore is agglomerated with moisture
prior to being placed in a bed/heap, with only a small amount of
added water. With these methods, there is often significant
evaporation of any water from the pile/bed, and, so as not to
depend solely on such evaporation to keep the ore dry, most plants
using pile/bed leach systems employ at least one, and often
several, large ponds in which to hold water that may accumulate in
a short event, such as a heavy rain. Thus, there is no need to
bleed water on a continual basis from a heap/dump/vat leach
system.
[0005] By comparison, in a plant employing agitation leaching,
crushed ore that is to be agitation-leached is generally ground or
wet-milled to a desired size distribution for achieving an
acceptable metal recovery in leaching, with the resulting ore
solids being added to the agitation leach unit(s) as aqueous
slurry. Thus, in agitation leaching, a considerable amount of water
is normally brought into the leaching system with the ore. This
water must eventually leave or be removed from the system in order
to maintain a water balance and it does so, mainly and continually,
with the leached solids in the tailings or by intermittent bleeds
from the circuit. Any desired metal or other valuable metals in the
water leaving with the leached solids is lost (called the "soluble
metal loss"). In addition, any leaching agent in this water is also
lost and often has to be neutralized prior to the final disposal of
the leached solids.
[0006] Selection of the type of leaching to be employed is based on
several factors including the grade of the ore, the clay content of
the ore, the hardness of the ore and the way the ore responds to
the various leaching methods. A dump or heap leach system is
generally much less costly in both capital (equipment) costs and
operating (energy) expense, and is therefore selected for use with
lower grade ores, where costs are critical, or with higher grade
ores that respond well to heap leaching, permitting a high metal
recovery. Agitation leaching, on the other hand, provides for a
faster and more complete recovery of the desired metal(s), is
easier to control, and often gives higher recovery of secondary
valuable metals, such as cobalt, but it is also more expensive due
to the capital cost of additional equipment, such as mills, leach
tanks and clarifiers, and has a higher operating cost because of,
for example, the energy required to mill the ore and the chemicals
needed for the solids-liquid separation.
[0007] Following the leaching step in a circuit employing agitation
leaching (such circuits being the focus of this invention), the
resulting mix of aqueous leachate, now containing a high proportion
of the desired metal values, as well as leached ore solids from
which the desired metal values have been dissolved, is then
normally sent to a solids-liquid separation process, such as by
counter-current decantation ("CCD"), with washing, or by
filtration, also with washing. Following this solids-liquid
separation process, the clarified or partially-clarified aqueous
phase is sent to one or more units in a solvent extraction process
for transfer of the metal values from the aqueous leachate into an
organic phase comprising one or more extraction reagents.
[0008] In that solvent extraction process, the particular desired
metal value is extracted from the leach solution containing that
metal value into an organic phase by one or more extraction
reagents specific for that desired metal, which reagent(s) is/are
dissolved in an organic phase that comprises the extraction
reagent(s), optionally with one or more equilibrium modifiers,
kinetic additive(s) and/or other compounds, in a water-insoluble,
water-immiscible organic solvent. During such extraction, hydrogen
ions are released from the organic phase into the aqueous phase,
now largely depleted of the desired metal values, as represented by
the equation below for extraction when copper is the desired metal,
sulphuric acid is the leaching agent, and where "RH" represents the
copper-specific extraction reagent(s):
2RH+CuSO.sub.4.revreaction.R.sub.2Cu+H.sub.2SO.sub.4
[0009] In the extraction of 1 ton of copper, 1.54 tons of sulphuric
acid (useful for further leaching of copper when the leach
solution, depleted of copper values, is returned to the leaching
unit(s)) is regenerated in the leach solution from which the copper
was extracted. Thus, the greater the amount of copper extracted
from an aqueous solution, the higher the concentration of sulphuric
acid generated in that solution. In general, more of the leached
copper can be extracted when the concentration of copper in the
leach solution is higher, thus, the higher the copper concentration
in the leach solution to be treated by solvent extraction, the
greater is the potential to return more sulphuric acid back to the
leaching unit. Other metals, such as Zn, Ni and Co, also show this
behavior, depending on the leach solution and extraction reagent(s)
employed.
[0010] Following the extraction, the metal-rich organic phase
containing one or more complexes of the desired metal with the
extraction reagent(s) is then possibly washed to reduce the level
of undesired iron and/or other undesirable species, and stripped of
its desired metal content with a stripping agent, such as a
relatively concentrated acid solution (normally sulphuric acid)
that breaks apart the complex(es), freeing the desired metal into
the aqueous "pregnant stripping solution". That metal is then
finally captured in a pure form from the desired metal-rich
pregnant stripping solution, by electrodeposition in an
electrowinning stage, or by one or more alternative metal recovery
processes.
[0011] The great quantities of solids and the large volumes of
leaching and stripping agents, extraction reagents, organic
solvents and purified water involved in large-scale mining and
metal recovery operations mandate efforts to use these resources
most efficiently, both from a purely economic perspective, and in
consideration of the potential environmental impact of accidental
discharges and intentional disposal of no-longer-useful substances.
Increased recycling of expensive agents and reagents, and the
reduction of losses resulting both from the disposal of
metal-depleted tailings slurry still containing some desired metal
and other metal values dissolved in the water in the tailings
slurry and from bleeds of the aqueous phase in order to maintain
the overall water balance and/or adjust/correct levels of
undesirable metals or acid, as well as from other conservation
measures, have become critical to the successful economic and
environmentally-responsible operation of mining and metal recovery
operations.
[0012] A particular issue addressed by this invention is the high
cost both of replacing leaching agents lost or bled from the
leaching-solvent extraction-electrowinning circuits and of
purchasing substances used to neutralize excess leaching agents
prior to further metal recovery activities and/or disposal of spent
leaching and/or washing solutions containing these leaching agents.
Another issue addressed is the need to recover a higher percentage
of the desired metal values from the leached ores and thereby
reduce the amount of valuable metal that is ultimately lost from
the circuit in bleed streams or to tailings disposal, resulting in
the loss of significant revenue that could be realized by the
operator.
[0013] US 2005/0031512 A1 (Kordosky et al) showed that good metal
extraction may be achieved while also significantly improving the
recovery of the leaching agents by proposing a "splitcircuit"
arrangement of leachate solution flows that does not require as
much fresh leaching agent to be added to supplement the recycling
of the leachate solution back to the leaching stage. This method
does not follow the conventional practice of washing, and thereby
diluting, the entire solution flow from the one or more agitation
leach units during the solids-liquid separation stage following
leaching. Such washing is intended to minimize the loss of metal
values with the disposal of the metal-depleted tailings slurry, but
it also reduces the concentration of the desired metal in the
clarified leach solution exiting solids-liquid separation and
thereby reduces the leaching agent concentration which can build in
this solution as the desired metal is extracted. Since only a
portion of this leach solution, now depleted of desired metal
values, but increased in leaching agent concentration, is recycled
back to leaching, less leaching agent is recycled back to leaching
than would be if the leaching solution had not been diluted.
[0014] Instead, the split circuit design involves subjecting the
first leached pulp from the leach unit(s), comprising a mixture of
metal-depleted leached solids and an aqueous leach solution
containing dissolved salts of the desired metal, leaching agent,
water, and possibly other metal values, to a first solids-liquid
separation, without significant dilution. The solids pulp from that
separation is then sent to a second solids-liquid separation, with
significant washing/dilution, with the clarified metal-rich aqueous
leach solutions from each solids-liquid being separation circulated
to separate solvent extraction units. The solids, as aqueous
slurry, from the second solids-liquid separation are then sent to
disposal, with the metal-depleted raffinate from the solvent
extraction unit(s) following the first solids-liquid separation,
without dilution, being recycled as leach solution, possibly
supplemented with additional fresh leaching agent, to one or more
of the leach unit(s). The raffinate, depleted of desired metal
values, exiting the solvent extraction unit(s) following the second
solids-liquid separation is neutralized, as necessary, and/or
circulated to one or more additional units to possibly recover
other metal values that may also have been present in the original
ore in sufficient quantities, and/or recycled back to the second
solids-liquid separation as wash solution, with the possibility
that some of the neutralized solution may be bled to disposal in
order to maintain a water balance.
[0015] Nevertheless, mineral industry is still interested in
improved processes for the leaching of ores allowing significant
reductions in leaching agent replacements and neutralizations, as
well as further significant increases in the recovery of the
desired metal from the original ore. The problem underlying the
present invention has been to serve these needs.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The present invention is directed to a process for leaching
desired metal values from crushed and milled ore solids and
extracting those values into organic phases for further recovery
efforts, in order to eventually obtain the desired metal in a
usable form. This process initially comprises leaching crushed and
milled ore solids with an acidic or basic leaching solution in one
or more initial/"first" agitation leach units, to dissolve a
significant portion of the desired metal values from the crushed
and milled ore solids into an aqueous phase. The slurry of
partially-leached solids with aqueous leach solution (called a
"leach pulp") resulting from the initial leaching unit(s) proceeds
to a first solids-liquid separation and clarification to produce
two products, a first undiluted aqueous leach solution, rich in
desired metal values, and a second leach pulp.
[0017] The first undiluted aqueous leach solution is then
circulated, without significant dilution, to one or more "first"
solvent extraction units for extracting the desired metal values
from the aqueous solution into an organic phase. From the first
solvent extraction unit(s), the aqueous solution ("raffinate"),
depleted of desired metal values, is recycled as leaching solution,
possibly augmented by fresh leaching agent and/or recycled
raffinate from one or more other solvent extractions later in the
process/circuit, back to the initial leach unit(s).
[0018] After the first solid-liquid separation and clarification,
the second leach pulp is sent, without significant dilution, to one
or more "final" agitation leach units for additional leaching, in
order to dissolve another significant portion of the desired metal
values remaining in the pulp. The leach pulp resulting from the
"final" leach unit(s) is subjected to a second solids-liquid
separation and clarification to produce two products, a second
undiluted aqueous leach solution, rich in desired metal values, and
a third leach pulp. The second undiluted aqueous leach solution is
circulated, without significant dilution, to one or more "second"
solvent extraction units for extracting the desired metal values
from the aqueous solution into an organic phase, with the aqueous
solution ("raffinate"), depleted of desired metal values, exiting
the second solvent extraction unit(s) being recycled as leaching
solution, possibly augmented by fresh leaching agent and/or
recycled raffinate from one or more other solvent extractions
earlier and/or later in the process/circuit, back to the final
agitation leach unit(s).
[0019] After the second solids-liquid separation and clarification,
the third leach pulp, now largely depleted of desired metal values
after two leachings, is sent to a last solids-liquid separation,
with water washing and significant dilution for the first time,
from which the washed solids slurry is sent to disposal, and the
clarified aqueous wash solution is sent to one or more final
solvent extraction units. From the final solvent extraction
unit(s), the aqueous solution (raffinate), depleted of desired
metal values, is optionally neutralized and then either possibly
sent to one of more units to recover any other valuable metal
values also present in the original ore, or recycled back to the
last solid-liquid separation unit(s) as wash solution, or it may be
split, with some portion sent to recovery of other metal values,
and some portion sent to recycle back to the last solids-liquid
separation unit(s) as wash solution, and perhaps even some to final
disposal.
[0020] In some cases, depending on the grade of ore and the
leaching characteristics of that ore, there may be one or more
additional "intermediate" sub-circuits of leaching/solids-liquid
separation/solvent extraction steps similar to the initial or first
leaching/solids-liquid separation/solvent extraction sub-circuit,
inserted between the first and "final" leaching/solids-liquid
separation/solvent extraction sub-circuit.
[0021] By following this process and changing the current design
and operation of the leaching and solvent extraction processes of
current metal-recovery plants accordingly, the amount of leaching
agent recycled to leach may be significantly increased, and both
the amount of leached metal that is lost to tailings disposal and
the quantities of additional fresh leaching agent purchased and the
quantities of chemicals that must be expended to neutralize excess
leaching agent in the circuits may be significantly decreased.
[0022] Surprisingly it has been observed that It surprisingly has
now been found that further significant reductions in leaching
agent replacements and neutralizations, as well as further
significant increases in the recovery of the desired metal from the
original ore, may be realized over the "split circuit" arrangement
by dividing the duty of leaching the desired metal values from the
original crushed and milled ore solids among two or more agitation
leach units in series. Each of these units then leaches desired
metal values from the same ore solids, with each unit being
followed by its own solids-liquid separator, without significant
dilution, then its own solvent extraction unit(s), prior to a final
solids-liquid separation, with washing, and a final solvent
extraction on the clarified solution exiting the final
solids-liquid separation, with washing, to try to recover any final
amounts of valuable metal. The metal-depleted aqueous solution
exiting the final solvent extraction unit is neutralized, as
necessary, and/or circulated to one or more additional units to
possibly recover other metal values that may also be present in the
original ore, prior to disposal and/or recycle back to the final
solid-liquid separation, with washing, as wash solution. The
metal-depleted aqueous slurry of the leached solids exiting the
final solids-liquid separation, with washing, is then sent to final
disposal, which, in most cases, includes neutralization. With this
new circuit design, raffinates from each solvent extraction unit,
except the final solvent extraction unit, may be totally recycled
to one or more of the preceding agitation leach unit(s), and, in
doing so, much more leaching agent is recycled to leaching and,
therefore, much less leaching agent is lost to final disposal as
compared to the conventional and "split circuit" flow sheets. In
addition, the desired metal lost to final disposal in the leached
and washed solids is minimized when compared to either the
conventional or "split circuit" flow sheets.
[0023] In particular, it has been found that by not depending on a
single initial leach unit to dissolve all or mostly all of the
desired metal values from the crushed and milled ore solids at one
time, and breaking the leaching function into two or more units in
a series or sequential arrangement, with accompanying solids-liquid
separators, without dilution, and solvent extraction units, as
described below, the amount of desired metal that may be recovered
and the amount of leaching agent that may be recycled may be
substantially increased. According to this invention, the crushed
and milled ore solids are subjected to a sequence of leach units,
each leach unit dissolving a portion of the desired metal values
from, effectively, the same crushed ore solids (the original
crushed and milled ore solids in the first agitation leach unit and
progressively-more-leached solids pulps in subsequent agitation
leach units in the series) with each such leaching unit being
followed by its own solids-liquid separation without significant
dilution, then one or more solvent extraction units to extract the
desired metal value from the aqueous leach solution, rich in
desired metal values, coming from the respective solids-liquid
separation unit(s). In addition, all, or almost all, of the aqueous
raffinates regenerated by the solvent extractions are recycled back
to either their respective leach units, or recycled among two or
more of the previous or following leach units in the circuit, for
additional leaching, prior to a final solids-liquid separation,
with washing. That final separation is applied to the crushed and
milled leached solids exiting the last solids-liquid separation
without significant dilution, and is followed by a final solvent
extraction on the clarified leach solution exiting the final
solids-liquid separation.
[0024] To understand the significant benefits of the present
invention over the conventional standard agitation leaching-solvent
extraction operation, as well as over the "Split Circuit" agitation
leaching-solvent extraction process, and not depend on any
particular theory, involves closely comparing the flow diagrams and
accompanying mass balances in FIGS. 1 through 3, with copper as the
desired metal and sulphuric acid as the leaching agent, as later
explained in the Examples.
[0025] For example, in any copper agitation leach-solvent
extraction recovery process, all the sulphuric acid recycled back
to leaching may be used to leach more copper, while all the acid
taken to neutralization or contained in the tailings is lost, and,
therefore, cannot be used to leach more copper. The more acid that
can be recycled, the less acid that needs to be purchased, and the
less the amount of acid that must be neutralized and/or that would
be lost to disposal.
[0026] In each flow sheet, copper recovery from leaching is set at
a realistic 90% and copper recovery from solvent extraction is also
assumed to be a realistic 90%, even though copper recovery in an
agitation leaching process can be up to nearly 100% in some cases
and copper recovery across a copper solvent extraction unit can be
more than 90% in some cases.
[0027] In one aspect, the instant invention provides a process for
recovering metal values from crushed and milled ore solids
comprising desired metal values that may be comingled with one or
more other metal values, which process comprises: [0028] (a) mixing
a first aqueous leach solution with a body of the crushed and
milled ore solids in a first agitated tank leach unit in order to
dissolve at least a significant portion of the desired metal values
formerly in the ore solids into the first aqueous leach solution
and to obtain a first aqueous leach pulp, which pulp comprises a
mixture of leached solids and first aqueous leach solution, rich in
the desired metal values; [0029] (b) subjecting the first aqueous
leach pulp to a first solids-liquid separation, without significant
water dilution, to provide a first clarified aqueous leach solution
and a second aqueous leach pulp, which pulp comprises leached
solids at a percent solids level that is greater than that in the
first aqueous leach pulp; [0030] (c) sending the second aqueous
leach pulp to a final agitated tank leach unit, and circulating the
first clarified aqueous leach solution to a first solvent
extraction, wherein, in such solvent extraction, at least a
significant portion of the desired metal values are extracted into
a first organic phase by one or more extraction reagent(s) specific
for the desired metal, which extraction reagent(s) is/are dissolved
in an organic formulation that comprises such extraction
reagent(s), optionally with one or more equilibrium modifiers,
kinetic additives and/or other compounds in a water-insoluble,
water-immiscible organic solvent, creating a first organic phase,
rich in the desired metal as one or more desired metal-extraction
reagent(s) complexes, that is sent to further metal recovery
processes, and a first aqueous raffinate, depleted of desired metal
values, up to all of which raffinate may be recycled/circulated
back to the first agitated tank leach unit as at least a part of
the first aqueous leach solution, which solution may be
supplemented by fresh leaching agent and/or one or more other
raffinates from later in the process; [0031] (d) mixing a second
aqueous leach solution with the second aqueous leach pulp in the
final agitated tank leach unit in order to dissolve another portion
of the desired metal values formerly in the partially leached
crushed and milled ore solids (now comprising the second aqueous
leach pulp) into the second aqueous leach solution and to obtain a
third aqueous leach pulp, which pulp comprises a mixture of
twice-leached solids and a second aqueous leach solution, rich in
desired metal values; [0032] (e) subjecting the third aqueous leach
pulp to a second solids-liquid separation, without significant
water dilution, to provide a second clarified aqueous leach
solution and a fourth aqueous leach pulp, which pulp comprises
leached solids at a percent solids level that is greater than that
in the third aqueous leach pulp; [0033] (f) sending the fourth
aqueous leach pulp to a third solids-liquid separation, and
circulating the second clarified aqueous leach solution to a second
solvent extraction, wherein, in such solvent extraction, at least a
significant portion of the desired metal values are extracted into
a second organic phase by one or more extraction reagent(s)
specific for the desired metal, which extraction reagent(s) is/are
dissolved in an organic formulation that comprises such extraction
reagent(s), optionally with one or more equilibrium modifiers,
kinetic additives and/or other compounds in a water-insoluble,
water-immiscible organic solvent, creating a second organic phase,
rich in the desired metal as one or more desired metal-extraction
reagent(s) complexes, that is sent to further metal recovery
processes, and a second aqueous raffinate, depleted of desired
metal values, up to all of which raffinate may be
recycled/circulated back to the final agitated tank leach unit as
at least a part of the second aqueous leach solution, which
solution may be supplemented by fresh leaching agent and/or one or
more other raffinates from earlier or later in the process; [0034]
(g) subjecting the fourth aqueous leach pulp to a third
solids-liquid separation, with significant dilution via an aqueous
stream, in order to obtain a third clarified aqueous leach
solution, wherein the concentration of desired metal values in the
third clarified aqueous leach solution is less than the
concentration of desired metal values in the second clarified
aqueous leach solution, and a fifth aqueous pulp, which pulp
comprises a mixture of leached solids and aqueous leach solution;
and [0035] (h) sending the fifth aqueous pulp to disposal and
circulating the third clarified aqueous leach solution to a third
solvent extraction unit, wherein, in such solvent extraction, a
third organic phase of water-insoluble, water-immiscible organic
solvent formulation comprising one or more extraction reagents
extract at least a portion of the desired metal values from the
third clarified aqueous leach solution creating a third organic
phase, rich in the desired metal as one or more desired
metal-extraction reagent(s) complex(es), that is sent to further
metal recovery processes, and a third aqueous raffinate, depleted
of desired metal values, that is optionally neutralized and
circulated back to the third solids-liquid separation as at least a
part of the aqueous washing solution to recover at least a portion
of any remaining desired metal values from the fifth aqueous pulp,
or is optionally neutralized and sent to disposal, or is optionally
neutralized and treated to recover one or more other metal values,
if present in sufficient amounts, that may be present in the mined
ore solids, or is optionally neutralized with portions circulating
back to the third solid-liquid separation and/or to further metal
recovery and/or to disposal.
[0036] For purposes of clarity, in each instance in this process,
when reference is made to a single "unit", it should be understood
that such "unit" may actually be several units in parallel or in
series. Specifically, each leaching "unit" may consist of several
agitated leaching tanks in parallel or in series, and each solvent
extraction "unit" may consist of a single stages or a multiple
number of stages, either extraction only or extraction and
stripping in a typical arrangement, such as solvent extraction
units or stages in parallel or series. It is also possible that all
of the solvent extraction units are actually just different stages
in a single solvent extraction plant. Generally, the solvent
extraction process is highly flexible and the particular
arrangement of solvent extraction units or stages for any given
leach solution is done in order to optimize recovery of the desired
metal and to optimize regeneration of the leaching agent for
recycle.
[0037] In the present inventive process, the leached-solids pulp
from each solids-liquid separator, prior to the final one that does
involve a final washing/dilution, becomes the leachable body of the
following leach unit (the leach unit next in the series/sequence).
It should also be understood that each solids-liquid separation,
with or without dilution/washing, may be conducted in any manner
capable of separating solids from liquids; the method of such
separations is not critical. For example, solids may be separated
from liquids by methods including, but not limited to, decantation
and/or filtration. In the final solids-liquid separation, with
significant washing/dilution, according to the invention,
counter-current decantation is preferred, but is not mandatory.
[0038] The term, "significant dilution" or "significant
washing/dilution", when used in the process in accordance with the
instant invention, refers to the addition of a measurable amount of
water or other aqueous solution. Dilution of any of the clarified
leach solutions prior to circulation of them to solvent extraction
could cause a build-up of the volume of aqueous phase in one of the
loops, and as such, would be undesirable and could decrease
leaching agent recovery. Significant dilution of such aqueous leach
solution is only used in the instant process in the final
solids-liquid separation as part of the final solids--liquid
separation wash process to try to recover the last vestiges of the
desired metal values from the pulp prior to disposal of the
metal-depleted ore solids.
[0039] Additionally, the solvent extractions in accordance with the
processes of the present invention may also be carried out in any
known manner, wherein aqueous leach solution is contacted with an
organic phase containing an extraction reagent, specific to the
desired metal. For example, these solvent extractions may be
carried out using mixer--settler solvent extraction units, wherein
the organic phase and the aqueous leach solution are vigorously
intermixed in a mixer, and the resulting dispersion of organic and
aqueous is then passed to a settler where the two phases settle,
and from which there exits a clear organic phase and a clear
aqueous phase.
[0040] Also, the "further metal recovery processes" to which the
organic phases, rich in the desired metal values, may be subjected
might comprise additional metal extraction followed by washing with
a solution designed to remove undesirable species prior to
contacting the organic phase, rich in desired metal values, with a
suitable stripping agent that breaks apart the desired
metal-extraction reagent complex and allows passage of the desired
metal into an aqueous phase containing the desired metal in a
concentrated and purified state from which final metal recovery
takes place by electrowinning, or one or more other final metal
recovery methods. With certain metals is may also be possible to
recover the desired metal directly from the organic phase, rich in
desired metal values, even though this is not a common
technique.
[0041] And all solutions, phases, raffinates, and pulps may be
conveyed within the circuits of the process by pipes or any other
natural or man-made conduit.
[0042] The process according to the instant invention may be
practiced in a new plant designed specifically for the instant
invention, or it may be practiced in an existing plant by
reconfiguring existing equipment, and pulp and solution flows,
without necessarily adding a great deal of handling and/or process
equipment.
[0043] In a preferred application of the process according to the
instant invention, a majority of the desired metal values in the
mined ore is intended to be leached from the crushed and milled ore
solids in an initial leach unit, at least a majority of the desired
metal values remaining in the solids pulp from the solids-liquid
separator following the initial leach unit is leached in the next
leach unit, and, in embodiments of the instant process comprising
more than two leach units, the number of such units being limited
by the economics of diminishing returns, the desired metal values
in the mined ore are leached in sequential leach units
progressively from a majority in the initial leach unit in the
total circuit, a majority of the metal values remaining being
leached in the next leach unit, and so on, until the last
reasonably-recoverable amount of the remaining desired metal values
from the original crushed and milled ore solids are leached by the
final leach unit in the process/circuit.
[0044] For example, in a preferred application of the instant
process, which comprises leaching with two leach units, 60 to 75%
of the desired metal values in the original ore might be preferably
leached in the first leach unit, and the remaining 25 to 40% of
such desired metal values would then be leached in the second leach
unit. In a preferred application of the instant process, which
comprises leaching the desired metal values with three leach units,
45 to 55% of such desired metal values in the original ore might be
preferably leached in the first leach unit, 25 to 35% of such
desired metal values might be preferably leached in the second
leach unit, and the remaining 10-30% of such desired metal values
might preferably be leached in the third leach unit. In a
particularly preferred aspect of the process according to the
instant invention, at least a majority of the original or remaining
desired metal values from the ore solids or pulp, as the case may
be, is removed in the initial and each successive leach
unit-solvent extraction unit combination, in order to maximize the
regeneration of the leaching agent during the process and thereby
maximize the recycling of such leaching agent to the leaching
process. This sequential leaching practice generally results in the
concentration of desired metal in the first clarified aqueous leach
solution being at least 30% greater than the concentration of the
desired metal in the second clarified aqueous leach solution,
preferably this difference is at least 50%, more preferably this
difference is at least 70%, and still more preferably this
difference is 100%.
[0045] In another embodiment of the instant invention, one or more
intermediate agitation tank leach units may be inserted after the
first solid-liquid separation and before the final agitated tank
leach unit in step (c), such intermediate agitated tank leach unit
sending an aqueous leach pulp, resulting from an aqueous leach
solution being distributed through an aqueous leach pulp coming
from the first solids-liquid separation, to an intermediate
solids-liquid separation, from which an intermediate aqueous leach
pulp is sent to the final agitated tank leach unit, and an
intermediate clarified aqueous leach solution is circulated to an
intermediate solvent extraction, from which an intermediate aqueous
raffinate up to all of which raffinate may be recycled/circulated
back to the intermediate agitated tank leach unit as at least a
part of the aqueous leach solution for such leach unit, which
solution may be supplemented by fresh leaching agent, and an
intermediate desired metal-rich organic phase, rich in desired
metal values, is sent to further metal recovery processes. It being
readily understood by the skilled practitioner that the number of
additional such sub-circuits would be determined by economic
practicality, i.e., the capital and operating cost of the
sub-circuit will be measured against the diminishing returns that
may be realized in further recovery of desired metal and reduction
in leaching agent and neutralization substances.
[0046] For purposes of illustration, one such additional
sub-circuit may comprise a leach unit, labelled an "intermediate
agitation leach unit", a solids-liquid separator labelled an
"intermediate solids-liquid separation", following this agitation
leach unit, and a solvent extraction following the intermediate
solids-liquid separation, this solvent extraction labelled an
"intermediate solvent extraction". Such an intermediate agitation
leach unit might be inserted after the first solids-liquid
separation and before the final agitated tank leach unit in step
(c) in the process described above, such intermediate agitation
leach unit sending an aqueous leach pulp, resulting from an aqueous
leach solution being distributed through an aqueous leach pulp
coming from the first solids-liquid separation, to an intermediate
solids-liquid separation, from which an intermediate aqueous leach
pulp is sent to the next agitation leach unit, and an intermediate
clarified aqueous leach solution is circulated to an intermediate
solvent extraction, from which exits an intermediate aqueous
raffinate, up to all of which raffinate may be recycled/circulated
back to the intermediate leach unit, or an earlier or later
agitation leach unit, as at least a part of the aqueous leach
solution for such leach unit, which solution may be supplemented by
fresh leaching agent, and an intermediate organic phase, rich in
the desired metal as a desired metal-extraction reagent(s)
complex(es), is sent to further metal recovery processing.
[0047] The process according to the instant invention may be used
in any metal recovery operation which employs an aqueous agitation
leaching operation, where the leaching agent is regenerated in the
solvent extraction process, and essentially with any leaching agent
that is water-miscible, capable of leaching the desired metal from
the mined ore into the desired metal leaching solution. Such
leaching agents include, but are not limited to acids, including
sulphuric acid, hydrochloric acid, nitric acid, organic acids, and
combinations of two or more thereof, and basic substances,
including gaseous ammonia and ammonium hydroxide. In certain
preferred embodiments of the present invention, the leaching agent
is sulphuric acid, resulting in each aqueous leach solution, i.e.,
the first aqueous leach solution, the second aqueous leach
solution, the third aqueous solution, any intermediate aqueous
leach solution, and so on, as well as each raffinate, i.e., the
first aqueous raffinate, the second aqueous raffinate, the third
aqueous raffinate, and so on, being sulphuric acid solutions. In
other certain preferred embodiments of the instant invention, the
preferred leaching agent is gaseous ammonia or ammonium hydroxide,
resulting in each of the leach solutions, i.e., the first aqueous
leach solution, the second aqueous leach solution, the third
aqueous solution, any intermediate aqueous leach solution, and so
on, as well as each raffinate, i.e., the first aqueous raffinate,
the second aqueous raffinate, the third aqueous raffinate, and so
on, being ammonia/ammonium hydroxide solutions.
[0048] The process of the invention is preferably used in the
leaching and solvent extraction of desired metals that occur
naturally as oxide and/or sulphide ores, preferably in the leaching
and solvent extraction of divalent metals, such as copper, zinc,
nickel and cobalt, and including, for example, transition metals.
In a preferred embodiment of the invention, the desired metal is
copper, and, particularly, when the desired metal is copper, the
preferred leaching agent is sulphuric acid. In another preferred
embodiment of the invention, the desired metal is copper, and the
preferred leaching agent is gaseous ammonia or ammonium hydroxide.
In still another preferred embodiment, the desired metal is zinc,
and particularly, when the desired metal is zinc, the leaching
agent is sulphuric acid or gaseous ammonia or ammonium hydroxide.
In yet another preferred embodiment of the invention, the desired
metal is nickel, and, particularly, when the desired metal is
nickel, the preferred leaching agent is sulphuric acid or gaseous
ammonia or ammonium hydroxide. In another preferred embodiment of
the invention, the desired metal is cobalt and the preferred
leaching agent is sulphuric acid.
[0049] The aqueous raffinate from each solvent extraction process
is generally recycled back to the leach unit from which the
clarified aqueous leach solution that was circulated to that vessel
originated most recently in order to leach more desired metal from
the crushed and milled ore solids or a subsequent leach pulp.
However, a portion of the first aqueous raffinate, a portion of the
second aqueous raffinate, a portion of the third aqueous raffinate,
a portion of any intermediate aqueous raffinate(s), or a mixture of
two or more thereof, may be circulated to any of the leach units
and/or the third solids-liquid separator in the process according
to the present invention if needed to maintain a water balance or
to more efficiently distribute leaching agent.
[0050] The first aqueous raffinate produced in accordance with the
processes of the present invention will generally have a leaching
agent concentration which is greater than the concentration of
leaching agent present in the second aqueous raffinate, the second
aqueous raffinate produced in accordance with the processes of the
present invention will generally have a leaching agent
concentration which is greater than the concentration of leaching
agent present in the third aqueous raffinate, and so on, with the
aqueous raffinate from a solvent extraction component of any
additional "intermediate" sub-circuit in accordance with the
processes of the present invention generally having a leaching
agent concentration which is greater than the concentration of
leaching agent present in the aqueous raffinate from the solvent
extraction vessel next following in the instant process. In
preferred embodiments of the present invention, the first aqueous
raffinate will have a leaching agent concentration which is at
least 10% greater than the concentration of leaching agent present
in the second aqueous raffinate, while in increasingly more
preferred embodiments of the present invention, the first aqueous
raffinate will have a leaching agent concentration which is at
least 20% greater, preferably at least 50% greater, more preferably
at least 75% greater and most preferably 100% greater. Such
differentials of concentrations of leaching agents in second
aqueous raffinates over the concentration of leaching agent in the
third aqueous raffinate are similar to those between the first and
second aqueous raffinates. The concentration of leaching agent in
any intermediate aqueous raffinate over the concentration of
leaching agent is the aqueous raffinate from the next-following
solvent extraction vessel in the present process are also similar
to those between the first and second aqueous raffinates. The
aqueous stream for diluting the fourth aqueous leach pulp (step
(g)) is normally the raffinate from the final solvent extraction
process, optionally with neutralization or optionally following
metal recovery of a second valuable metal value, but it may
comprise fresh water introduced into the process and/or a portion
of other aqueous process streams to maintain a water balance. Where
the leaching agent comprises an acid, any of the aqueous
raffinate(s) may be at least partly neutralized (e.g., to any pH up
to about 8) with any basic substance (e.g., lime when the leaching
agent is sulphuric acid) prior to its use for diluting the fourth
aqueous leach pulp in the third solid-liquid separation.
[0051] It will be appreciated by those skilled in the art that
changes could be made to the embodiments described above without
departing from the broad inventive concept thereof. It is
understood, therefore, that this invention is not limited to the
particular embodiments disclosed, but it is intended to cover
modifications within the spirit and scope of the present invention
as defined by the description of the process herein and the Claims
attached hereto.
[0052] A computer simulation, based on mass balance principles and
using iterative Excel spreadsheets, was run to compare the
economics of the conventional leaching and solvent extraction
circuits in widespread use (illustrated in FIG. 1), against the
economics of the split circuit configuration of the leaching and
solvent extraction stages currently in use in some plants
(illustrated in FIG. 2), and against the economics of the simplest
configuration of the instant invention (illustrated in FIG. 3). In
this non-limiting simulation, copper was used as the desired metal
to be recovered, sulphuric acid was used as the leaching agent, and
all numbers expressing quantities or concentrations are to be
understood as approximations (to be understood, where not already
present, as modified by "about"), not representations, for
comparison purposes only, and affected by the ore grade, the water
content of the crushed ore solids, the metal recovery achieved in
leaching, the desired pulp density in leaching, desired wash ratios
and thickener flow densities achievable in a CCD solid-liquid
separation, the response of the leached solids to solid-liquid
separation, the total flow of leach solution to be treated, the
design of the solvent extraction process, and other parameters
determined by the plant operators. The simulation is intended to be
illustrative of the instant invention's advantages, but should not
be interpreted to limit the scope of the current invention in any
way. For purposes of the simulation, the following conditions were
used:
TABLE-US-00001 TABLE 1 Case Study Basis Ore treated (tons per day)
8,700 Ore grade (% Cu) 3.5 Ore Specific Gravity 2.8 Pre-leach
thickener U/F (% solids) 55 % Solids in Leach 22 % Recovery in
Leach 90 All CCD thickener U/F (% solids) 50 Number of CCD stages 6
Wash ratio in CCD 2:1 Copper recovery in each SX unit (%) 90
[0053] The economic benefits of the Sequential Circuit flow sheet
relative to both the Split Circuit flow sheet and the conventional
flow sheet are detailed in Table 2.
BRIEF DESCRIPTION OF THE FIGURES
[0054] FIG. 1 is a flow diagram, containing pertinent components of
the mass balance for the circuit, representing the flows in a
standard conventional agitation leaching-solvent extraction flow
sheet, wherein all of the aqueous leach solution is treated in the
same manner.
[0055] FIG. 2 is a flow diagram, with pertinent mass balance
values, representing a "Split Circuit" flow sheet, wherein an
aqueous leach solution is divided into two portions--one portion
without significant dilution and the other with water
dilution--prior to being subjected to solvent extraction.
[0056] FIG. 3 is a flow diagram, with pertinent mass balance
values, representing an embodiment of the "Sequential Circuit" flow
sheet according to the present invention.
[0057] Optional units/operations are shown in dashed lines, with a
cobalt recovery unit representing one or more units for recovering
other metal values that may be present in sufficient quantities in
the incoming ore.
EXAMPLES
Comparative Example A
[0058] Comparative Example A is based on FIG. 1, which depicts a
process flow diagram of a standard conventional copper agitation
leaching and solvent extraction circuit, with pertinent mass
balance numbers included for aqueous flows, copper concentrations
and acid concentrations.
[0059] The leach pulp exiting the leach unit/train ("LEACH"),
consisting of about 1224 cubic meters/hour of aqueous leach
solution, comprising 9.92 g/l of copper and 2.0 g/l of sulphuric
acid, and about 362.5 tonnes/hour of crushed and milled ore less
the mass leached, is mixed/washed, in a counter-current decantation
("CCD"), with about 622 cubic meters/hour of recycled aqueous
raffinate from the copper solvent extraction unit/train ("SX1").
For modelling purposes, the 622 cubic meters/hour of raffinate
containing 0.80 g/l Cu was assumed to be neutralized to contain 2
g/l sulphuric acid before addition to the CCD circuit as wash
solution, thus diluting the copper concentration of the aqueous
leach solution exiting the CCD circuit from about 9.92 g/l copper
to about 8.05 g/l copper prior to this solution being fed to the
solvent extraction. An aqueous leach solution obtained from the CCD
of about 1535 cubic meters/hour, comprising 8.05 g/l copper and 2.0
g/l sulphuric acid, is circulated to SX1, and an aqueous raffinate,
comprising 0.80 g/l copper and 13.2 g/l sulphuric acid, exiting
from the SX1, is split, with about 913 cubic meters/hour being
recycled back to the leaching operation, where the acid is used to
dissolve more copper, and about 622 cubic meters/hour that is
recycled to neutralization and then to the CCD. The about 622 cubic
meters/hour of raffinate, which is recycled to the CCD operation,
is used to wash the leach solution from the leached solids, in
order to minimize soluble metal losses in the aqueous phase portion
of the leached pulp that is eventually disposed to tailings. A
small portion of fresh water may be added to the overall leach/wash
system or a small portion of aqueous solution may be bled from the
overall leach/wash system to maintain a water balance.
[0060] In Comparative Example A, 913 cubic meters per hour of
raffinate containing 13.2 g/l sulphuric acid would be returned to
leaching, about 622 cubic meters per hour of raffinate containing
13.2 g/l sulphuric acid is neutralized to 2.0 g/l sulphuric acid
prior to recycle back to the CCD circuit, and about 311 cubic
meters per hour of aqueous solution containing 0.91 g/l copper is
lost in tailings.
Comparative Example B
[0061] Comparative Example B is based on FIG. 2, which depicts a
process flow diagram of a "Split Circuit" copper leaching and
solvent extraction system, with pertinent mass balance numbers
included for aqueous flows, copper concentrations and acid
concentrations. The leach pulp exiting the leach unit/train
("LEACH"), consisting of about 1224 cubic meters/hour of aqueous
leach solution, comprising 9.92 g/l of copper and 2.0 g/l of
sulphuric acid, and about 362.5 tonnes/hour of crushed and milled
ore, less the mass leached, is passed to an initial solids-liquid
separation (S/L), comprising a clarifier using decantation. Then
about 913 cubic meters/hour of this solution, containing about
10.07 g/l Cu and about 2.0 g/l sulphuric acid, is taken directly to
solvent extraction (SX 1), where the copper is extracted and
sulphuric acid is regenerated. SX 1 will reasonably produce a
raffinate containing about 1.01 g/l copper and about 15.95 g/l
sulphuric acid, which solution is then recycled back to leaching.
The leach pulp exiting the initial solid-liquid separation, which
contains about 311 cubic meters/hour of leach solution, is taken to
a counter-current decantation wash circuit (CCD) where it is mixed
with about 622 cubic meters/hour of raffinate from SX 2 that has
been, optionally, partially neutralized to 2.0 g/l sulphuric acid.
About 622 cubic meters/hour of leach solution from the CCD circuit,
comprising 5.21 g/l copper and 2.0 g/l sulphuric acid, is taken to
SX 2 to give a raffinate containing 0.52 g/l copper and 9.2 g/l
sulphuric acid. A small portion of fresh water may be added to the
overall leach/wash system or a small portion of aqueous solution
may be bled from the overall leach/wash system to maintain a water
balance.
[0062] In Comparative Example B, 913 cubic meters/hour of raffinate
containing 15.95 g/l sulphuric acid is returned to leaching, about
622 cubic meters/hour of raffinate from SX 2 containing 9.2 g/l
acid is neutralized to 2.0 g/l sulphuric acid and about 311 cubic
meters per hour of aqueous solution containing 0.67 g/L copper is
lost to Tails.
[0063] The amount of acid in any aqueous stream at a particular
time is the stream flow at that time multiplied by the acid
concentration in the stream. A simple calculation shows that for
this particular case about 2.51 more metric tons of acid/hour, or
about 60.3 more metric tons of acid/day, is recycled to leaching
using the "Split Circuit" flow sheet of Comparative Example B over
the standard conventional flow sheet of Comparative Example A. Acid
costs vary widely from, currently, about US$60/ton to above
US$250/ton depending on the location. For low-cost acid, the
savings in acid using the "Split Circuit" flow sheet instead of the
conventional standard flow sheet would be about US$3618/day, while
for high-cost acid, the savings in acid would be about
US$15,075/day or greater.
[0064] A second simple calculation shows that for these Comparative
Examples, about 2.49 less metric tonnes of acid per hour, or about
59.8 less metric tonnes acid per day, are neutralized using the
"Split Circuit" of Comparative Example B over the standard
conventional circuit of Comparative Example A. This is well within
rounding error, since the greater amount of acid recycled to
leaching using the "Split Circuit" flow sheet of Comparative
Example B, over the conventional circuit flow sheet of Comparative
Example A, should equal the lesser amount of acid that is
neutralized using the "Split Circuit" flow sheet of Comparative
Example B compared to the acid that is neutralized using the
conventional circuit flow sheet of Comparative Example A.
[0065] Savings in neutralization can vary widely, depending on the
cost of the neutralizing agent (typically lime or limestone), the
capital required to build a larger neutralization plant and the
cost to dispose of the greater amount of gypsum formed.
[0066] The amount of copper in any aqueous stream at a particular
time is the stream flow at that time multiplied by the copper
concentration in the stream. A third simple calculation shows that
the total copper recovered using the "Split Circuit" flow sheet of
Comparative Example B is greater than the total copper recovered
using the standard conventional flow sheet of Comparative Example A
by 74.64 kilograms/hour or about 1.79 tonnes/day. At a copper price
of, currently, US$2.50/pound, this additional copper has a value of
US$9,866/day or about US$3.55M annually.
Example 1
[0067] Example 1 illustrating the present invention is based on
FIG. 3, which depicts a process flow diagram of a simple example of
a copper leaching and solvent extraction process according to the
instant invention (denominated "Sequential Circuit"), with
pertinent mass balance numbers included for aqueous flows, copper
concentrations and acid concentrations. In this Example 1, a leach
pulp consisting of about 1224 cubic meters/hour of aqueous leach
solution, comprising 7.56 g/l of copper and 2.0 g/l of sulphuric
acid, and about 362.5 tonnes/hour of crushed and milled ore less
the mass leached, flows directly without dilution from this first
leach unit (Leach 1), where about 75% of the copper from the
crushed, mined ore has been dissolved into an aqueous acidic leach
solution, to a solid-liquid separator (S/L1). From separator S/L1
about 913 cubic meters/hour of an aqueous leach solution,
comprising 7.56 g/l copper and 2.0 g/l sulphuric acid, is
circulated to a solvent extraction unit/train (SX1), and a pulp,
consisting of about 311 cubic meters/hour of leach solution and
about 362.5 tonnes/hour of partially leached crushed and milled ore
less the small mass of ore leached in Leach 1, is sent to a second
leach unit. The entire 913 cubic meters/hour of metal-depleted
aqueous leach solution, comprising 12.5 g/l sulphuric acid and 0.77
g/l copper, exiting SX1 is recycled to LEACH 1 where the acid
contained in this first raffinate is used to leach copper from more
fresh crushed and milled ore solids.
[0068] The remaining amount of copper in the partially leached
crushed and milled ore solids exiting Leach 1, (25% of the original
amount in the crushed and milled ore solids entering Leach 1) is
then dissolved from the solids in LEACH 2, from which 1224 cubic
meters/hour of aqueous leach solution, comprising 4.6 g/l copper
and 2.0 g/l sulphuric acid, and about 362.5 tonnes/hour of solids
less the total mass leached, is sent to another solid-liquid
separator (S/L2), again, without dilution. About 913 g/l of a
second aqueous leach solution, comprising about 4.6 g/l copper and
2.0 g/l sulphuric acid emerges from S/L2 and is circulated to a
second solvent extraction unit (SX2), and a pulp, consisting of
about 331 cubic meters/hour of leach solution and about 362.5
tonnes/hour of almost totally leached solids less the mass leached,
is sent to a third solid-liquid separator (CCD) where the pulp is
diluted and washed The entire 913 cubic meters/hour of
metal-depleted aqueous leach solution, comprising 8.4 g/l sulphuric
acid and 0.46 g/l copper, are recycled from SX2 to LEACH 2 where
the acid contained in this second raffinate is used to leach copper
from the partially leached crushed and milled ore solids entering
Leach 2 from S/L1.
[0069] Exiting the CCD wash process is a pulp, consisting of 311
cubic meters/hour of aqueous solution, comprising 0.31 g/l copper
and 2 g/l sulphuric acid, and 362.5 tonnes/hour of almost totally
leached solids, which is sent to tails and 622 cubic meters/hour of
a third aqueous leach solution, comprising 2.4 g/l copper and 2.0
g/l sulphuric acid, which is sent to a final solvent extraction
unit (SX3). From SX3, 622 cubic meters/hour of metal depleted
aqueous leach solution, comprising 0.24 g/l copper and 5.3 g/l
sulphuric acid, emerge as an aqueous raffinate for possible
neutralization ("Neut") of excess acid before possible recovery of
additional metal ("Co"), and recycle to the CCD as wash solution or
supplemental wash solution.
[0070] In Example 1, about 913 cubic meters/hour of raffinate
containing 12.5 g/l acid and about 913 cubic meters/hour of
raffinate containing 8.4 g/l acid are returned to Leach 1 and 2,
respectively. About 622 cubic meters/hour raffinate from SX 3
containing 5.3 g/l acid is neutralized to 2 g/l acid and about 311
cubic meters/hour of aqueous solution containing 0.31 g/l copper is
lost to tailings
Example 3
Economic Benefit Calculation
TABLE-US-00002 [0071] TABLE 2 Economic Benefits of the Sequential
Circuit and Split Circuit Flowsheets over the Conventional
Flowsheet in a One Year Period of Time Conven- Split Sequential
tional Circuit Circuit Operating days per year 360 360 360
Neutralization Cost 200 200 200 ($/ton acid) Cu price ($/lb) 2.50
2.50 2.50 Acid to neutralization 167.2 107.5 49.7 (MT/day)
Neutralization cost 12.04 7.74 3.58 ($ million/yr) Benefit ($
million/yr) 4.30 (A) 8.46 (A) Cu soluble loss (ton/day) 6.80 5.00
2.31 Revenue loss ($ million/yr) 13.45 9.90 4.57 Benefit ($
million/yr) 3.55 (B) 8.88 (B) Total benefit ($ million/yr) 7.85 (A
+ B) 17.34 (A + B)
[0072] Calculation of the financial savings the invention offers
when compared to the conventional flow sheet and the split circuit
flow sheet are based on three advantages of the invention: the
total greater amount of leaching agent recycled to Leach 1 and
Leach 2, the decrease in acid neutralized and the decrease in the
concentration of the desired metal in the aqueous solution leaving
the circuit to the tailings with the washed leach pulp exiting the
CCD wash circuit (called the "copper soluble loss").
[0073] In Table 2, the neutralization cost/year is calculated by
multiplying the acid neutralized/day by the cost of neutralization
times 360 days/year. The acid neutralized/day is the flow of the
respective steam being neutralized multiplied by the g/l acid
neutralized, for example, the acid neutralized/day for the
conventional circuit is [622 cubic meters/hour times (13.2 g/l
acid-2 g/l acid) times 24=167.2]. The cost of neutralization,
assumed to be US$200/tonne acid, is the total cost of the acid plus
the cost of the base needed to neutralize the acid plus a small
operating cost. A neutralization cost of US$200 tonne acid is used
for this example and such cost is reasonable for neutralization and
well within the range of today's costs for neutralization. The
benefit of the acid savings on an annual basis for the Sequential
Circuit flow sheet according to the instant invention over the
Split Circuit flow sheet and over the conventional standard flow
sheet is calculated from the difference in the neutralization cost
for each of the three flow sheets.
[0074] Also in Table 2, the benefit on an annual basis associated
with the lower soluble copper loss offered by the by the Sequential
Circuit flow sheet according to this instant invention over the
"Split Circuit" flow sheet and over the conventional standard flow
sheet is determined from the differences in the economic value of
the soluble copper lost on an annual basis (the concentration of
the copper in the respective streams exiting the CCD wash circuit
to Tails times the flow calculated on an annual basis times the
copper price) for each of the three flow sheets.
[0075] From Table 2 it can be seen that the use of the Sequential
Circuit flow sheet according to the instant invention offers an
annual savings of US$17.34 million over the conventional
leaching-solvent extraction flow sheet and a savings of US$9.49
million over the use of the Split Circuit flow sheet (which Split
Circuit flow sheet has shown an annual savings US$7.85 million over
the conventional flow sheet).
[0076] Clearly, the use of the process according to the present
invention would result in much more leaching agent (in these
Examples, acid) being returned to additional leaching than would be
recycled with the conventional standard flow sheet or with the
split circuit flow sheet. Also clearly the use of the process
according to the instant invention would result in more copper
being produced, and less copper lost as soluble copper, when
compared to the conventional standard flow sheet and the split
circuit flow sheet.
[0077] Since the amount of ore and copper content of the ore being
treated is the same for the Conventional Circuit (Comparative
Example A), the Split Circuit (Comparative Example B) and the
Sequential Circuit according to this invention (Example 1), a
direct and valid comparison can be made for the amount of acid
neutralized and the soluble copper loss using each flow sheet.
[0078] The values calculated in Table 2 are both realistic and
reasonable considering that, in December, 2007; the price of acid
varies between US$60/tonne to over 250/tonne, depending on location
and logistics, with most acid prices well above the low figure of
US$60/tonne. Also at the present time the price of copper is about
US$3.00/pound.
[0079] No matter what values are used to calculate the annual total
benefit in US$, some benefit in less acid neutralized and more
copper produced is always present for the Sequential Circuit flow
sheet according to this instant invention over the Split Circuit
flow sheet and the conventional standard flow sheet.
[0080] In addition to the benefits of more acid recycled to
leaching, less acid being neutralized and less copper being lost as
soluble copper in the pulp exiting the CCD process, a fourth
advantage of the Sequential Circuit flow sheet pertains to the
leaching efficiency. When the leaching process is divided into a
first leach, where a majority of the copper is leached, and a
second leach, where the remainder of the copper is leached, the
copper concentration in the aqueous phase in contact with the
almost-totally-leached, crushed and milled ore solids in the last
leach unit is considerably lower (4.60 g/l Cu) than when all the
leaching is done in one leaching unit/train (10.07 g/l Cu in the
case of the split circuit flow sheet and 9.92 g/l Cu in the case of
the standard conventional flow sheet). A lower copper concentration
in the leach solution in contact with the final leached solids
should allow a very slightly higher overall leach recovery because
the diffusion of leached copper from the pores in the ore particles
is faster. In addition, the acid to leach can be better controlled
and thus made more efficient.
[0081] A fifth benefit may occur in those cases where the acid in
the stream being recycled to the CCD wash process does not need to
be neutralized, but the bleed of this stream from which a component
of value in the bleed is recovered, for example cobalt, must be
neutralized prior to cobalt recovery. Neutralization with a soluble
base, such as caustic or ammonia, is very expensive, thus the lower
the acid content of the bleed stream, the lower the amount of
expensive base needed for neutralization. Furthermore, the use of a
solution of caustic for neutralization adds water to the bleed
stream, thereby diluting the valuable cobalt stream. Alternatively,
neutralization can take place with lime or limestone, which is a
less costly base. In this case, a lesser amount of acid in the
bleed stream requires less lime or limestone for neutralization,
and in the process, a lesser amount of gypsum precipitate, that
must be removed from the system, is produced. A lesser amount of
gypsum allows the use of smaller equipment for this particular
solid-liquid separation. Since, when finely-divided solids
separated from a liquid, the solids will always contain some of the
liquid, a lesser amount of gypsum will contain a lower volume of
the neutralized bleed stream that contains the valuable second
component, for example, cobalt. Thus, the ultimate recovery of the
secondary valuable component in the bleed stream is higher when
using the process according to the invention.
* * * * *