U.S. patent application number 14/106592 was filed with the patent office on 2014-06-26 for heap-leach pad injection system and method.
This patent application is currently assigned to Board of Regents of the Nevada System of Higher Education, on behalf of the Desert Research Insti.. The applicant listed for this patent is David Decker. Invention is credited to David Decker.
Application Number | 20140178272 14/106592 |
Document ID | / |
Family ID | 50974882 |
Filed Date | 2014-06-26 |
United States Patent
Application |
20140178272 |
Kind Code |
A1 |
Decker; David |
June 26, 2014 |
HEAP-LEACH PAD INJECTION SYSTEM AND METHOD
Abstract
In one embodiment, the present disclosure provides an extraction
method. A conduit is formed in a heap-leach pad. The heap-leach pad
includes a plurality of poorly perfused areas. The conduit is in
fluid communication with poorly perfused areas of the heap-leach
pad. A fluid that includes steam is injected into the conduit. The
fluid travels through the heap-leach pad and condenses in poorly
perfused areas of the heap-leach pad. An extraction solution is
applied to the heap-leach pad. Condensation of steam in the poorly
perfused areas of the heap-leach pad provides new flow pathways for
the extraction solution.
Inventors: |
Decker; David; (Reno,
NV) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Decker; David |
Reno |
NV |
US |
|
|
Assignee: |
Board of Regents of the Nevada
System of Higher Education, on behalf of the Desert Research
Insti.
Reno
NV
|
Family ID: |
50974882 |
Appl. No.: |
14/106592 |
Filed: |
December 13, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61737439 |
Dec 14, 2012 |
|
|
|
Current U.S.
Class: |
423/1 ;
423/512.1; 423/658.5 |
Current CPC
Class: |
Y02P 10/234 20151101;
C22B 3/04 20130101; Y02P 10/20 20151101 |
Class at
Publication: |
423/1 ;
423/658.5; 423/512.1 |
International
Class: |
C22B 7/00 20060101
C22B007/00 |
Claims
1. An extraction method comprising: in a heap-leach pad comprising
a plurality of poorly perfused areas, forming a conduit in a
heap-leach pad, the conduit in fluid communication with the poorly
perfused areas of the heap-leach pad; injecting a fluid comprising
steam into the conduit, the fluid contacting, and the steam
condensing in, the poorly perfused areas of the heap-leach pad; and
applying an extraction solution to the heap-leach pad; wherein
condensation of steam in the poorly perfused areas of the
heap-leach pad provides new flow pathways for the extraction
solution.
2. The method of claim 1, wherein the extraction solution comprises
cyanide.
3. The method of claim 1, wherein the extraction solution comprises
an acid, a base, or other chemical or biological component that
facilitates release of metals or chemical compounds from rock.
4. The method of claim 1, wherein the fluid comprises heated
steam.
5. The method of claim 1, wherein forming a conduit in a heap-leach
pad comprises forming a plurality of conduits in the heap-leach
pad.
6. The method of claim 5, wherein forming a plurality of conduits
in a heap-leach pad comprises drilling boreholes in the heap-leach
pad.
11. The method of claim 5, wherein the conduits are formed in a
grid.
12. The method of claim 5, wherein the conduits are formed in an
irregular pattern.
13. The method of claim 1, wherein forming a conduit in the
heap-leach pad comprises drilling a borehole in the heap-leach
pad.
14. The method of claim 1, wherein the poorly perfused areas
comprise dry areas.
15. The method of claim 1, wherein the method is carried out in the
absence of an extraction well to remove injected fluid.
16. A mine closure or remediation method comprising: in a
heap-leach, forming a conduit in a heap-leach pad, the heap-leach
pad comprising dissolvable or oxidizable minerals; and injecting a
fluid comprising steam into the conduit; wherein the steam contacts
the minerals and enhances oxidation or dissolution of the
minerals.
17. The method of claim 16, wherein the minerals comprise sulfides
and the steam enhances sulfide oxidation.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of, and incorporates by
reference, U.S. Provisional Patent Application Ser. No. 61/737,439,
filed Dec. 14, 2012.
TECHNICAL FIELD
[0002] The present disclosure relates to a method for improving
mining operations. In particular examples, the method includes
injecting a heated fluid into a heap-leach pad.
SUMMARY
[0003] In one embodiment, the present disclosure provides a method
for extracting materials, such as valuable metals, from a
heap-leach pad. A plurality of conduits, such as wells, are formed
in the heap-leach pad. In a particular example, the plurality of
conduits are drilled into the heap-leach pad. A heated fluid is
injected into the plurality of conduits, such as through a steam
conduit, such as a pipe. In a particular example, the heated fluid
includes steam. In a specific implementation of this example, the
steam is transported into at least a portion of poorly perfused
rock, such as substantially dry rock, condenses, and forms a liquid
water film. The film can establish new flow paths in the heap-leach
pad. An extraction solution is then applied to the heap-leach pad.
When passing through the boreholes and/or new flow paths, the
extraction fluid can produce enhanced material recovery from the
heap-leach pad. In a particular example, the extraction solution
includes cyanide.
[0004] In another embodiment, the present disclosure provides a
method of heap-leach pad and/or waste-rock dump closure. A
plurality of conduits, such as wells, are formed in the heap-leach
pad. In a particular example, the plurality of conduits are drilled
into the heap-leach pad. A heated fluid is injected into the
plurality of conduits, such as through a pipe. In a particular
example, the heated fluid includes steam. This method can provide
advantages, such as enhanced removal of environmentally sensitive
extraction fluids, metals, or chemical compounds from the
heap-leach pad or waste-rock dump.
[0005] There are additional features and advantages of the subject
matter described herein. They will become apparent as this
specification proceeds.
[0006] In this regard, it is to be understood that this is a brief
summary of varying aspects of the subject matter described herein.
The various features described in this section and below for
various embodiments may be used in combination or separately. Any
particular embodiment need not provide all features noted above,
nor solve all problems or address all issues in the prior art noted
above. Additional features of the present disclosure are described
in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Various embodiments are shown and described in connection
with the following drawings in which:
[0008] FIG. 1 is a schematic diagram illustrating a prior art
system and method for applying leachate to a heap leach pad.
[0009] FIG. 2 is a schematic diagram illustrating leachate flow in
the system and method of FIG. 1.
[0010] FIG. 3 is a schematic diagram illustrating a system and
method for injecting steam into a heap leach pad.
[0011] FIG. 4 is a schematic diagram illustrating a system and
method for increasing perfusion in poorly perfused areas of a heap
leach pad using steam injection.
[0012] FIG. 5 is a schematic diagram illustrating fluid flow, such
as leachate flow, after applying the system and method of FIG.
4.
DETAILED DESCRIPTION
[0013] Unless otherwise explained, all technical and scientific
terms used herein have the same meaning as commonly understood by
one of ordinary skill in the art to which this disclosure belongs.
In case of conflict, the present specification, including
explanations of terms, will control. The singular terms "a," "an,"
and "the" include plural referents unless context clearly indicates
otherwise. Similarly, the word "or" is intended to include "and"
unless the context clearly indicates otherwise. The term
"comprising" means "including;" hence, "comprising A or B" means
including A or B, as well as A and B together. Although methods and
materials similar or equivalent to those described herein can be
used in the practice or testing of the present disclosure, suitable
methods and materials are described herein. The disclosed
materials, methods, and examples are illustrative only and not
intended to be limiting.
[0014] While heap-leach mining methods are an efficient means of
processing vast volumes of low-grade ore, heap-leach operations
typically recover only a fraction of the assayed metal mass.
Recovery percentages of 60 to 70% are typical, meaning that 30 to
40% of the assayed metal content remains in the heap following the
cessation of mining. Gold heap-leach operations can suffer from the
continual degradation and loss of cyanide from the leach fluid, and
by an inability to effectively transport leach fluid to all parts
of the heap volume. During operation of a typical heap-leach
process, cyanide is continually replenished to replace that lost
during leaching. Loss of circulation in the heap is a result of two
primary constraints: irrigation methods and heap-leach pad rock
structure.
[0015] From an engineering cost standpoint, it can be impractical
to irrigate the entire heap surface at once. As a consequence, heap
operators often irrigate the surface of the heap in a staggered
"patch-work" pattern where specific areas of the heap are
alternately leached for periods of days-weeks. The heap-leach pad
is constructed as end-dumped lifts of rock transported directly
from the pit. The resultant rock volume typically includes material
that is highly variable in particle size (micron to meter sized
particles) and is both vertically and horizontally stratified.
[0016] The confluence of irrigation/leaching practices and
heap-leach pad structure can contribute to the occurrence of large
volumes of heap-leach pad rock volume that are never contacted by
leach fluid, and are thus poorly perfused. This is typically a
result of the establishment of preferential fluid transport
pathways, or conduits through the rock volume, resulting in large
volumes of rock in the interstitial spaces that are poorly perfused
or dry. These dry volumes are then isolated from the mining
process, and the metal contained within is not released, thereby
contributing to overall heap-leach recovery efficiency loss.
[0017] A key goal to improving heap recovery efficiency in this
patent disclosure is to establish leachate flow in the "dry" or
"poorly perfused" zones of the heap, or zones otherwise isolated
from contact by the leachate fluid. Changing surface irrigation
techniques or patterns may have a limited effect--leach fluid will
transport in high conductivity channels established by prior
irrigation occurrences. These channels typically exhibit higher
(more saturated) matric potential with concurrently higher
hydraulic conductivity. In contrast, the "dry zones" exhibit much
lower (dryer) matric potential with orders of magnitude lower
hydraulic conductivity. These zones are both dry (no reaction with
leach agent) and will tend to remain so because of the inability of
water to move in them.
[0018] The disclosed system and method increases the hydraulic
conductivity of the heap, and therefore the leachability of
heap-leach pad "dry zones". Boreholes are drilled into the surface
of the heap-leach pad to a depth suitable to deliver fluid into the
heap-leach pad. In one example, the holes are drilled to about 75%
of total rock height at the drill location. The boreholes are
drilled at a density suitable to deliver a desired amount of fluid
to the heap-leach pad. In some examples, the boreholes are drilled
on a regular grid. In other examples, the boreholes may be drilled
in irregular locations. In some implementations, borehole depth and
spacing is dependent on the method of construction and ore
characteristics of the heap-leach pad.
[0019] Heated fluid is injected into the boreholes. In one example,
the heated fluid includes or consists of steam, such as high
temperature steam. The injected fluid travels into the heap rock
volume. As a heated fluid, the injectate may be comparatively
unaffected by gravitational gradients or fluid matric potential
conditions. Heat from the fluid is transferred from the heated
fluid to the rock. Fluid vapor, such as water vapor, will
subsequently condense, resulting in a liquid film on the rock. In
the "dry zones" of the heap, this will be the first time that a
water film has been established since the initiation of mining.
[0020] Without intending to be limited by theory, this water film
is expected to be drawn downward by the action of gravity, thereby
establishing new fluid transport pathways within the formerly dry
volumes of the heap. Following the injection of steam, leachate
fluid is applied at the heap surface, such as using known methods,
thereby allowing the admission of leachate, such as cyanide, into
the previously dry rock volume, and the removal of metal within,
thereby increasing the overall recovery efficiency of the heap
operation.
[0021] In addition to increasing overall metal recovery or metal
recovery efficiency, the disclosed method can provide additional
advantages. For example, the disclosed method can aid in meeting
the environmental requirements associated with closure or
remediation of heap-leach pads or similar environments, such as
tailings piles. When used for mine closure or remediation, the
leachate application step can be omitted or replaced with other
steps to aid in mine closure or remediation.
[0022] Cyanide chemical stability is reduced under high
temperatures. Thus, heated fluids, such as steam, can assist in
degrading residual cyanide concentration in the rock volume. Heated
fluids, such as steam, can also enhance overall pad stability by
increasing the dissolution rate of primary minerals that
subsequently form secondary mineral precipitates following the
cessation of heated fluid. These secondary precipitates act to
geochemically cement the porous media fabric, enhancing overall
rock strength and slope stability.
[0023] Fluid injection, such as steam injection, can increase the
sulfide oxidation rate of reducing zones in the rock volume,
thereby releasing acidic byproducts quickly and efficiently. This
approach may also be useful to mitigate the effects of sulfide
oxidation in unlined waste rock dumps.
[0024] FIG. 1 illustrates prior art heap leach pad environment 100.
The environment 100 includes a heap leach pad 105 having a heap toe
110. The heap leach pad 105 includes rock sections 115 that are
either dryer or wetter than the surrounding rock. A plurality of
leachate delivery mechanisms 120, such as sprinklers or drippers,
are located on the surface of the heap leach pad 105. A liner 125,
typically made from polyethylene or other moisture impervious
material, is placed under the heap leach pad 105 over the
underlying surface 130. The underlying surface 130 is typically
sloped downwardly toward the heap toe 110. The liner 125 overlaps a
lined drainage swale 135 located adjacent the heap toe 110.
[0025] In practice, leaching solution is placed atop the heap leach
pad 105 using leachate delivery mechanisms 120. The leaching
solution travels through the leach pad 105 in flow paths 210. The
flow paths typically follow preferential flow patterns.
Preferential flow patterns may develop, for example, due to areas
of dry or wet zones 115 in the heap leach pad 105. These rock
sections 115 may result from having non-uniform rock grain sizes in
the heap leach pad 105. The nonuniformity by itself can lead to
preferential flow patterns. However, the situation can be
compounded as rock material is added to the heap leach pad 105,
with larger rock tending to fall towards the bottom of a lift in a
heap leach pad 105, with finer material stabilizing towards the top
of the lift. The leachate contacts the liner 125 and flows to the
swale 135, where it is carried off to be further treated, such as
to removal metals of interest.
[0026] FIG. 3 illustrates a system 300 for injecting steam into a
well. The system 300, includes a steam generator 310, such as a
mobile steam generator. The steam generator 310 could be other than
a mobile stream generator, such as being associated with a more
permanent structure.
[0027] The steam generator 310 is coupled to a conduit 315 in
communication with a well 320. The well 320 includes a plurality of
apertures 330, such as slits, slots, screens, or meshes. Steam
enters the well 320 at the exit 325 of the steam conduit 315 and
enters the surrounding rock through well apertures 330. A well seal
335 is positioned in the well 320, above the apertures.
[0028] Each well 320 is formed to a depth sufficient to provide a
desired degree of flow enhancement to the heap leach pad 105. In
some cases the well 320 extends substantially the depth of the heap
leach pad 105, with apertures 330 formed periodically along the
length of the well 320. In other cases, the well 320 is drilled to
a shallower depth, particularly if dry, or other poorly perfused,
areas 115 are known to be at shallower depths. Although shown
injected through a well 320, steam can be introduced into the heap
pad through other methods, such as using an auger that injects
steam while passing through soil or rock.
[0029] FIG. 4 illustrates a system and method 400 for using steam
to create new flow channels in areas of dry or wet zones of rock
115. The system and method 400 show two wells 320 penetrating the
heap leach pad 105. Although two wells are shown, the disclosed
system and method can be carried out by one well or using more than
two wells. Wells can also be drilled/injected with steam
sequentially or in parallel. The wells 320 are in communication
with steam generating apparatus (not shown), which may include the
steam generator 310 (FIG. 3). The wells 320 have apertures 330,
such as slits, slots, screens, or meshes. Steam 410 flows through
the apertures 330 into the heap leach pad 105, including the dry
and wet zones of rock 115.
[0030] The steam may be selected to have a desired steam quality
(amount of liquid water). Typically, having higher quality steam
(having closer to 100% steam and 0% liquid water) is advantageous,
as it can facilitate permeation into rock spaces prior to
condensing. Similarly, steam temperature can also be selected to
achieve desired operational goals. That is, hotter steam, including
superheated steam, may penetrate further from the well 320 prior to
condensing. In specific examples, the steam is superheated steam,
with a steam quality of 1 (100% of water in the vapor phase) and a
temperature of at least 100.degree. C., such as at least
200.degree. C., 300.degree. C., 400.degree. C., 500.degree. C., or
600.degree. C.
[0031] The steam is typically injected at a pressure and rate
sufficient to provide a desired degree of permeation into the rock
surrounding the wells 320. If a greater degree of penetration is
required, the pressure/steam injection rate is typically increased.
The operational conditions are interdependent, in at least some
implementations. So, for example, use of higher quality steam can
use a lower injection pressure in order to achieve an equivalent
level of penetration. Similarly, deeper injections typically
require higher steam pressures than shallower injections.
[0032] In a specific example, the steam is injected at a pressure
of at least about 5 pounds per square inch, such as at least about
10, 20, 30, 40, or 60 pounds per square inch. In further examples,
steam is applied at a rate of at least about 10 kg/hr, such as at
least about 25 kg/hr, 50 kg/hr, 75/kg/hr, 125 kh/hr, 200 kg/hr, or
250 kg/hr.
[0033] Steam is applied for a period of time sufficient to achieve
a desired level of perfusion enhancement in the heap leach pad 105.
In some implementations, steam is applied at least until water from
the condensed steam is observed in the swale 135. Steam application
can continue past this point, if desired, in order to help ensure
that poorly perfused rock 115 has had sufficient contact with the
steam to form new flow paths. Steam can be applied for shorter
durations if desired, such as terminating prior to condensed steam
being observed in the swale. In some cases, it can be beneficial to
monitor the flow of water due to condensed steam in the swale 135
compared with the steam injection rate. An increased flow in the
swale may indicate the formation of new flow channels in the heap
leach pad 105 due to steam action. Cessation of steam injection may
be indicated if the swale flow reaches or approaches a steady
state. In specific examples, steam is applied for at least about 1
hour, such as at least about 5, 10, 15, 50, 100, or 125 hours.
[0034] In some cases, an extraction well (not shown), such as a
well to which a vacuum is applied or which is in communication with
a pump, can be used to assist fluid removal, including in cases
where there is not an existing drainage mechanism or to augment
existing drainage. In other cases, such in the case of a heal-leach
pad, the method and system can be used without an extraction
well.
[0035] FIG. 5 illustrates a system and method 500 showing travel of
leachate 510 through the heap leach pad 105 after steam injection
according to an embodiment of the present disclosure (such as shown
in, and described with respect to FIG. 3). The leachate 125 is
introduced to the leach hap pad 105 by the leachate delivery
mechanisms 120. The leachate 510 travels in flow paths. Due to the
action of the steam, the leachate 510 now travels through rock
areas 115, which were previously areas of rock dryer or wetter than
surrounding rock.
[0036] The leachate 510 travels to the bottom of the leach heap pad
105 and contacts the liner 125. The leachate 510 then flows down
the liner 125 to the swale 135.
[0037] The methods and systems presented above, including FIGS. 1-5
and the accompanying discussions, generally apply to surface
remediation, including cyanide decomposition, sulfide oxidation,
and primary mineral dissolution. In such systems and methods,
leachate delivery mechanisms (FIG. 1, 120), are typically not
needed. In addition, in the figures, rock masses 115 would
represent areas of cyanide, sulfide, or primary minerals, rather
than wet or dry rock areas. The other steam injection parameters
(including well depth, steam quality, steam pressure, and duration
of steam application) would apply analogously to remediation
methods.
[0038] It is to be understood that the above discussion provides a
detailed description of various embodiments. The above descriptions
will enable those skilled in the art to make many departures from
the particular examples described above to provide apparatuses
constructed in accordance with the present disclosure. The
embodiments are illustrative, and not intended to limit the scope
of the present disclosure. The scope of the present disclosure is
rather to be determined by the scope of the claims as issued and
equivalents thereto.
* * * * *