U.S. patent application number 12/795459 was filed with the patent office on 2011-12-08 for in situ ore leaching using freeze barriers.
This patent application is currently assigned to EMC Metals Corporation. Invention is credited to Willem P.C. Duyvesteyn.
Application Number | 20110298270 12/795459 |
Document ID | / |
Family ID | 45063896 |
Filed Date | 2011-12-08 |
United States Patent
Application |
20110298270 |
Kind Code |
A1 |
Duyvesteyn; Willem P.C. |
December 8, 2011 |
IN SITU ORE LEACHING USING FREEZE BARRIERS
Abstract
A method is provided herein for performing a leaching process on
a substrate (113). In accordance with the method, a freeze wall
(115) is created which isolates a portion (117) of a substrate, and
the isolated portion of the substrate is subjected to a leaching
process.
Inventors: |
Duyvesteyn; Willem P.C.;
(Reno, NV) |
Assignee: |
EMC Metals Corporation
|
Family ID: |
45063896 |
Appl. No.: |
12/795459 |
Filed: |
June 7, 2010 |
Current U.S.
Class: |
299/5 ; 166/57;
75/401 |
Current CPC
Class: |
Y02P 10/234 20151101;
C22B 3/02 20130101; C22B 3/18 20130101; C22B 23/0415 20130101; Y02P
10/20 20151101; E21B 43/28 20130101; C22B 3/04 20130101 |
Class at
Publication: |
299/5 ; 75/401;
166/57 |
International
Class: |
E21B 43/28 20060101
E21B043/28; C22B 11/00 20060101 C22B011/00; E21B 36/00 20060101
E21B036/00; C22B 19/00 20060101 C22B019/00; C22B 23/00 20060101
C22B023/00; C22B 60/00 20060101 C22B060/00; C22B 1/00 20060101
C22B001/00; C22B 15/00 20060101 C22B015/00 |
Claims
1. A method for performing a leaching process on a substrate,
comprising: creating a freeze wall which isolates a portion of the
substrate; and subjecting the isolated portion of the substrate to
the leaching process.
2. The method of claim 1, wherein creating a freeze barrier
comprises: creating a plurality of holes in the substrate; and
circulating a working fluid through said plurality of holes such
that said working fluid absorbs heat from the substrate.
3. The method of claim 2, further comprising: extracting the heat
from the working fluid by passing the working fluid through a heat
exchanger.
4. The method of claim 3, further comprising: recirculating the
working fluid through said plurality of holes after the working
fluid is passed through the heat exchanger.
5. The method of claim 2, further comprising: after the working
fluid absorbs heat from the substrate, venting the heated working
fluid to the atmosphere.
6. The method of claim 5, wherein the working fluid comprises a
material selected from the group consisting of nitrogen and carbon
dioxide.
7. The method of claim 5, wherein the working fluid undergoes a
phase transition from a liquid to a gas when it absorbs heat from
the substrate.
8. The method of claim 2, wherein the working fluid is circulated
through the plurality of holes in series.
9. The method of claim 2, wherein the working fluid is circulated
through the plurality of holes in parallel.
10. The method of claim 2, wherein the working fluid is a
brine.
11. The method of claim 10, wherein the working fluid comprises
calcium chloride.
12. The method of claim 2, wherein the working fluid is selected
from the group consisting of nitrogen, ammonia and carbon
dioxide.
13. The method of claim 12 wherein, after the freeze wall is
created, maintaining the freeze wall with a device selected from
the group consisting of thermosyphons and heat pipes.
14. The method of claim 1, wherein the freeze wall consists of at
least one essentially vertical wall and at least one essentially
horizontal wall.
15. The method of claim 1, further comprising: subjecting the
isolated portion of the substrate to a remediation process after
the leaching process; and removing a portion of the freeze wall
after the remediation process.
16. The method of claim 15, wherein the remediation process
comprises treating the isolated portion of the substrate with an
alkaline solution.
17. The method of claim 1, further comprising: subjecting the
isolated portion of the substrate to a remediation process after
the leaching process; and removing the entire freeze wall after the
remediation process.
18. A method for performing a leaching process on a substrate,
comprising: performing the method of claim 1 on a first portion of
a substrate; and performing the method of claim 1 on a second
portion of the substrate which is adjacent to the first portion of
the substrate; wherein a portion of the freeze wall used to isolate
the first portion of the substrate is reused to isolate the second
portion of the substrate.
19. The method of claim 1, wherein the leaching process comprises
treating the isolated portion with a metal leaching solution.
20. The method of claim 19, wherein the metal is selected from the
group consisting of nickel, gold, copper, uranium and zinc.
21. The method of claim 19, wherein the metal is nickel.
22. The method of claim 1, wherein the leaching process comprises
treating the isolated portion with an acidic leaching solution.
23. The method of claim 1, wherein the leaching process comprises
treating the isolated portion with a biological leaching
solution.
24. The method of claim 1, wherein the leaching process comprises
treating the isolated portion with a bacterial leaching
solution.
25. The method of claim 1, wherein said freeze wall
hydrodynamically isolates a portion of the substrate.
26. A method for extracting nickel from a porous substrate
containing a nickel bearing ore, comprising: creating a freeze wall
in the substrate such that a portion of the substrate is
hydrodynamically isolated from the rest of the substrate; and
treating the isolated portion of the substrate with an acidic
leaching solution which dissolves a portion of the nickel content
in the substrate.
27. A mining construct, comprising: an ore bearing substrate; a
freeze wall which hydrodynamically isolates a portion of the
substrate; and a leaching solution disposed in the isolated portion
of the substrate.
28. The mining construct of claim 27, further comprising: a heat
exchange system adapted to maintain said freeze wall.
29. The mining construct of claim 28, wherein said heat exchange
system comprises a thermosyphon.
30. The mining construct of claim 28, wherein said heat exchange
system comprises a heat pipe.
31. The mining construct of claim 28, wherein said heat exchange
system comprises a plurality of downpipes and a plurality of freeze
pipes.
32. The mining construct of claim 31, wherein said heat exchange
system comprises a working fluid, and wherein said working fluid is
a brine.
33. The mining construct of claim 32, wherein said brine comprises
calcium chloride.
34. The mining construct of claim 31, wherein said heat exchange
system comprises a working fluid, and wherein said working fluid is
selected from the group consisting of nitrogen, ammonia and carbon
dioxide.
35. The mining construct of claim 31, wherein said heat exchange
system comprises a working fluid, and wherein the working fluid
undergoes a phase transition from a liquid to a gas when it absorbs
heat from the substrate.
36. The mining construct of claim 31, wherein the working fluid is
circulated through the plurality of downpipes in series.
37. The mining construct of claim 31, wherein the working fluid is
circulated through the plurality of downpipes in parallel.
38. The mining construct of claim 27, wherein the freeze wall
consists of at least one essentially vertical wall and at least one
essentially horizontal wall.
39. The mining construct of claim 27, wherein the leaching solution
is a metal leaching solution.
40. The mining construct of claim 39, wherein the metal is selected
from the group consisting of nickel, gold, copper, uranium and
zinc.
41. The mining construct of claim 39, wherein the metal is
nickel.
42. The mining construct of claim 27, wherein the leaching solution
is an acidic leaching solution.
43. The mining construct of claim 27, wherein the leaching solution
is a biological leaching solution.
44. The mining construct of claim 43, wherein the leaching solution
is a bacterial leaching solution.
Description
FIELD OF THE DISCLOSURE
[0001] The present disclosure relates generally to methods for
leaching ores, and more particularly to systems and methods which
utilize freeze walls to isolate a portion of an ore bearing
substrate for a subsequent leaching process.
BACKGROUND OF THE DISCLOSURE
[0002] Various methods have been developed in the art to extract
desirable minerals, such as nickel, copper, zinc or uranium, from
ore bearing substrates. In the past, such minerals were frequently
recovered through conventional mining operations involving
drill-and-blast, open-cut or underground mining techniques.
[0003] More recently, in-situ leaching (ISL), also known as in-situ
recovery (ISR) or solution mining, has emerged as a viable
technique for recovering certain types of minerals from ore bearing
substrates which are conducive to the use of this technique. In a
typical implementation, a series of holes are drilled into the ore
bearing substrate. These holes are then used to inject a leaching
solution into the ore formation, and to extract the pregnant
leaching solution (that is, the leaching solution containing
dissolved minerals) from the substrate. In some cases, explosive or
hydraulic fracturing may be used to create open pathways in the ore
bearing substrate to allow the leaching solution to penetrate it
more effectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is an illustration of a first embodiment of a system
for establishing a freeze wall in an ore bearing substrate for
subsequent leaching operations in accordance with the teachings
herein.
[0005] FIG. 2 is an illustration showing some of the details of a
heat exchange system which may be utilized in the system of FIG.
1.
[0006] FIG. 3 is an illustration of a second embodiment of a system
for establishing a freeze wall in an ore bearing substrate for
subsequent leaching operations in accordance with the teachings
herein.
[0007] FIG. 4 is an illustration of a first embodiment of a
thermosyphon which may be used to create or maintain a freeze wall
in the systems and methodologies disclosed herein.
[0008] FIG. 5 is an illustration of an embodiment of a heat pipe
which may be used to create or maintain a freeze wall in the
systems and methodologies disclosed herein.
[0009] FIG. 6 is an illustration of a second embodiment of a
thermosyphon which may be used to create or maintain a freeze wall
in the systems and methodologies disclosed herein.
[0010] FIG. 7 is an illustration of the use of thermosyphons in
creating a horizontal freeze wall.
SUMMARY OF THE DISCLOSURE
[0011] In one aspect, a method for performing a leaching process on
a substrate is provided. In accordance with the method, a freeze
wall is created which isolates a portion of a substrate, and the
isolated portion of the substrate is subjected to a leaching
process.
[0012] In another aspect, a method is provided for extracting
nickel from a porous substrate containing a nickel bearing ore. The
method comprises (a) creating a freeze wall in the substrate such
that a portion of the substrate is hydrodynamically isolated from
the rest of the substrate; and (b) treating the isolated portion of
the substrate with an acidic leaching solution which dissolves a
portion of the nickel content in the substrate.
[0013] In yet another aspect, a mining construct is provided which
comprises (a) an ore bearing substrate; (b) a freeze wall which
hydrodynamically isolates a portion of the substrate; and (c) a
leaching solution disposed in the isolated portion of the
substrate.
DETAILED DESCRIPTION
[0014] While in-situ leaching (ISL) has many desirable features and
avoids many of the costs and safety concerns associated with more
conventional mining techniques, it also has certain notable
drawbacks. For example, in-situ leaching typically requires the use
of acidic, caustic or ammoniacal leaching solutions. Such solutions
may result in a change in the pH of groundwater in the vicinity of
the ore bearing substrate, and may also contaminate the ground
water with heavy metals and other potentially toxic materials that
have been mobilized by the leaching solution. In addition, the
efficacy of the technique may be adversely affected by rainfall and
other such events which promote movement in, and contamination of,
the local water table. There is thus a need in the art for an ore
extraction technique that overcomes the foregoing infirmities.
[0015] It has now been found that these infirmities may be overcome
by constructing a (preferably removable) barrier around a portion
of a substrate which is to be treated with an ore leaching
solution. The substrate may be, for example, a section of a metal
or mineral bearing ore body which extends through barren bed rock.
The barrier is preferably a freeze wall which is deployed such that
the portion of the substrate to be treated with the leaching
solution is hydrodynamically isolated from the rest of the
substrate.
[0016] This approach ensures that, during the subsequent leaching
process, no ground water can penetrate into, and no leaching
solution can escape from, the isolated region. Consequently,
various attributes of the leaching process, such as the pH of the
leaching solution in situ, may be more precisely controlled, thus
allowing for more optimal ore extraction. Moreover, contamination
of surrounding aquifers by the leaching solution and extracted
metals is prevented. In addition, this approach provides for
greater economy in the use of leaching solutions than is possible
with conventional leaching approaches, since it provides an
effective means for the recovery and/or recycling of leaching
solutions. Similarly, this approach permits the economical use of
more expensive leaching solutions (which may be more effective in
some applications) than is possible with conventional leaching
methodologies, since the increased cost of the leaching solution is
at least partially offset by the ability to recover and recycle
it.
[0017] The systems and methodologies disclosed herein may be
appreciated with reference to the first particular, non-limiting
embodiment of a system depicted in FIG. 1 for isolating a portion
of an ore bearing substrate for subsequent leaching operations in
accordance with the teachings herein. The system 101 depicted
therein comprises a supply 103 of a heat transfer medium which, in
the particular embodiment depicted, is preferably a suitable brine
such as an aqueous solution of calcium chloride (CaCl.sub.2). The
supply 103 is in fluidic communication with a plurality of down
pipes 105 by way of a series of outlets 107, and is in fluidic
communication with a plurality of freeze pipes 109 by way of the
down pipes 105 and a series of inlets 111.
[0018] The freeze pipes 109 are disposed at appropriate intervals
(preferably within the range of about 4 to about 16 feet apart) in
an ore bearing substrate 113 which is to be subjected to subsequent
leaching operations, and each down pipe 105 extends through the
center of, and is in open fluidic communication with, an associated
freeze pipe 109. During operation of the system 101, cold brine is
circulated from the supply 103 to the freeze pipes 109 by way of
outlets 107 and down pipes 105, where it removes heat from the
substrate 113. Over time (typically about 6 to about 16 weeks),
this results in the formation of a freeze wall 115 between adjacent
freeze pipes 109, thus hydrodynamically isolating a portion 117 of
the ore bearing substrate for subsequent leaching operations.
Thereafter, it is typically necessary to circulated brine only as
needed to maintain a desired temperature profile in the substrate
113 so that the freeze walls 115 can continue to be effective.
[0019] FIG. 2 illustrates some of the details of a heat exchange
unit 119 of the system of FIG. 1. The heat exchange unit 119 is
typically a component of the supply 103. As seen therein, the heat
exchange unit 119 comprises a coolant loop 121, a refrigerant loop
123 and a cooling water loop 125. The coolant loop 121 includes a
brine pump 127 which draws warm brine from the freeze pipes 109,
circulates it through a heat exchanger 129 to extract the heat from
it, and pumps cold brine into the down pipes 105.
[0020] The heat exchanger 129 transfers heat from the coolant loop
121 to the refrigeration loop 123 which, in addition to the heat
exchanger 129, further comprises a compressor 131, a condenser 133
and an expansion valve 135. The condenser 133 transfers heat from
the refrigeration loop 123 to the cooling water loop 125 which, in
addition to the condenser 133, also comprises a water cooling unit
137.
[0021] Various materials may be used in the implementation of a
system 101 of the type depicted in FIGS. 1-2. In a preferred
embodiment, however, the freeze pipes 109 are steel pipes which are
preferably closed on one end and have a diameter of about 3 inches
to about 5 inches, and the down pipes 105 are preferably open-ended
polyethylene pipes that are typically about 2 inches to about 2.5
inches in diameter. The heat transfer medium is preferably a
calcium chloride (CaCl.sub.2) brine.
[0022] FIG. 3 depicts a second particular, non-limiting embodiment
of a system for isolating a portion of an ore bearing substrate for
subsequent leaching operations in accordance with the teachings
herein. The system 201 depicted therein comprises a supply 203 of a
heat transfer medium. In the particular embodiment depicted, the
heat transfer medium is preferably nitrogen and is more preferably
liquid nitrogen (L-N.sub.2), though other gases or liquefied gases,
such as, for example, carbon dioxide and ammonia, may also be used,
it being understood that the heat transfer medium may undergo a
phase change within the system 201. The supply 203 is in fluidic
communication with at least one down pipe 205 by way of at least
one outlet 207.
[0023] The freeze pipes 209 are disposed at appropriate intervals
in an ore bearing substrate 213 which is to be subjected to
subsequent leaching operations, and each down pipe 205 extends
through the center of, and is in open fluidic communication with,
an associated freeze pipe 209. During operation of the system 201,
cold nitrogen is circulated from the supply 203 to the freeze pipes
209 by way of outlet 207 and down pipes 205, where it removes heat
from the substrate 213. Over time, this results in the formation of
a freeze wall 215 between adjacent freeze pipes 209, thus
hydrodynamically isolating a portion 217 of the ore bearing
substrate for subsequent leaching operations.
[0024] Various materials may be used in the implementation of a
system 201 of the type depicted in FIG. 3. In a preferred
embodiment, however, the freeze pipes 209 are steel or copper pipes
which are preferably closed on one end and have a diameter of about
2 inches, and the down pipes 205 are preferably open-ended copper
pipes that are typically about 0.5 inches to about 0.75 inches in
diameter.
[0025] Liquid nitrogen is preferably fed into the system 201
through an insulated surface manifold which typically consists of
copper pipes and quick-connect cryogenic hoses. The liquid nitrogen
typically begins to vaporize in the annulus between the down pipe
205 and the freeze pipe 209 at a temperature of about 196.degree.
C., and absorbs heat from the substrate 213 as it travels upward
through the freeze pipe 209. The temperature of the exhaust at the
vent 251 is preferably monitored with temperature sensors, and the
amount of liquid nitrogen fed into the system 201 is preferably
controlled by a cryogenic two-way solenoid valve. The solenoid
valve is preferably controlled in response to the temperatures
recorded by the temperature sensors at the exhaust vent 251, with
the valve typically being opened and closed in accordance with
predetermined temperature limits.
[0026] Unlike the embodiment of FIGS. 1-2 in which the down pipes
105 and the freeze pipes 109 are arranged in parallel, in the
embodiment of FIG. 3, the down pipes 205 and the freeze pipes 209
are arranged in series. In other words, the outlet 211 of each
freeze pipe 209 in the system (excluding the last freeze pipe 209
in the system, whose outlet is in fluidic communication with the
vent 251) is in fluidic communication with the downpipe 205 of the
next downstream freeze pipe 209.
[0027] Moreover, unlike the embodiment of FIGS. 1-2 in which the
heat transfer media is recycled and cooled, in the embodiment of
FIG. 3, the warm liquid nitrogen (which will typically be in
gaseous form after traversing the circuit of down pipes 205 and
freeze pipes 209) may be safely vented to the atmosphere via vent
251, although embodiments are also possible (conceptually similar
to the system 119 depicted in FIG. 2) in which the warm nitrogen
may be recycled and cooled for further use. In embodiments in which
the warm nitrogen is vented to the atmosphere, the supply 203 may
be periodically recharged as, for example, from a tank of liquid
nitrogen (which may be a stand-alone tank or a tank mounted to a
truck or railcar) or from a nearby gas liquification and separation
plant.
[0028] Compared to a typical brine-based system of the type
depicted in FIGS. 1-2 which is capable of establishing a freeze
wall in a matter of weeks, a liquid nitrogen-based system of the
type disclosed in FIG. 3 is typically capable of establishing a
freeze wall much faster (typically in days), though the daily
operating cost of the liquid nitrogen-based system may be greater
in some implementations. The system 201 depicted in FIG. 3 is
typically capable of achieving average soil temperatures in the
freeze wall 215 within the range of -20.degree. C. to -40.degree.
C.
[0029] Various other types of active cooling systems may be used to
create or maintain freeze walls in the systems and methodologies
described herein. These include, without limitation, active cooling
systems which utilizes liquid ammonia, and which operate in a
manner analogous to the cooling systems utilized in common domestic
refrigerator/freezer appliances.
[0030] In some embodiments of the systems and methodologies
described herein, separate systems may be used to create and
maintain freeze walls. For example, in some embodiments, a liquid
nitrogen-based system may be used to initially establish a freeze
wall, after which a brine-based system may be utilized to maintain
the freeze wall. In such embodiments, the same down pipes and
freeze pipes may be used for both the liquid nitrogen-based system
and the brine-based system, although variations are also possible
in which, for example, only the freeze pipes are used for both
systems, or in which separate down pipes or freeze pipes are used
for both systems.
[0031] In some embodiments of the systems and methodologies
described herein, the freeze walls may be created or maintained
with passive devices rather than active devices. One particular,
non-limiting embodiment of such a passive device is the
thermosyphon 301 depicted in FIG. 4. The thermosyphon 301 comprises
a first heat absorbing portion 303 which is buried in the portion
of the substrate where the freeze wall is to be created or
maintained (this may be, for example, within a well bore or freeze
pipe of one of the systems described above), and which is in open
communication with one or more heat radiating portions 305 that are
exposed to the atmosphere. The interior of the thermosyphon
contains a suitable working gas, such as CO.sub.2 or ammonia, which
absorbs heat from the ground via the heat absorbing portion 303 and
which releases heat to the atmosphere via heat radiating portions
305. The thermosyphon 301 will continue to operate so long as the
ambient air is colder than the ground, or when local wind
conditions or condensation can provide evaporative cooling.
[0032] Preferably, the conditions within the thermosyphon are such
that the working gas condenses into a liquid as it releases heat in
the heat radiating portions 305, and the condensed liquid flows by
gravity down into the heat absorbing portion 303 where it absorbs
heat, undergoes a phase transition into a gas, and begins the cycle
all over again. However, thermosyphons may also be constructed for
use in the systems and methodologies described herein which operate
solely by convection, or by both convection and phase change.
[0033] In some embodiments of the systems and methodologies
described herein, the freeze walls may also be created or
maintained with heat pipes. One particular, non-limiting embodiment
of a heat pipe 401 is depicted in FIG. 5. As with the thermosyphon
301 depicted in FIG. 4, the heat pipe 401 is a passive device. The
heat pipe 401 comprises a casing 403, a wick 405 and a vapor cavity
407 which contains a working fluid. Heat pipes of the type depicted
may be made in a variety of shapes and sizes and may, for example,
be cylindrical or planar.
[0034] In operation, the working fluid absorbs heat (as a liquid)
at a first end 409 of the heat pipe 401 which is disposed in a
relatively high temperature environment (e.g., the ground) and
evaporates. The working fluid (now a gas) then migrates along the
length of the vapor cavity 407 until it comes into contact with a
second end 411 of the heat pipe 401 which is disposed in a
relatively low temperature environment (e.g., the ambient air),
where it undergoes condensation and releases the absorbed heat. The
condensed fluid then travels back to the first end 409 through the
wick via the capillary effect, where the process repeats itself. As
with the thermosyphon 301 depicted in FIG. 4, the heat pipe 401
will only absorb heat from the ground when the ambient air is
cooler than the ground, but is less dependent on gravity for its
operation.
[0035] FIG. 6 illustrates a second particular, non-limiting
embodiment of a thermosyphon 501 which may be utilized to create or
maintain a freeze wall in the systems and methodologies disclosed
herein. The thermosyphon 501 depicted therein is a hybrid system in
that it is capable of operating in both an active mode and a
passive mode.
[0036] In the particular embodiment depicted, the thermosyphon 501
comprises a freeze pipe 503 which is shown imbedded vertically
within an ore bearing substrate 504. The freeze pipe 503 is sealed
and contains a circulating working fluid. The thermosyphon 501 has
an evaporator section 507 for contacting the soil, and a condenser
section 509 which is in fluidic communication with the evaporator
section 507. The condenser section 509 is located above the surface
505 of the substrate 504 and is exposed to the ambient environment.
The condenser section 509 includes a finned portion 511 which has a
plurality of heat transfer fins protruding therefrom. The finned
portion 511 is in thermal communication with the ambient
environment and enhances heat exchange between the working fluid
within the freeze pipe 503 and the ambient environment.
[0037] In operation, the working fluid removes heat from the
substrate 504 by evaporation. The resulting vapors move upwardly in
the freeze pipe 503 to the condenser section 509, where the working
fluid releases heat through condensation. The condensed working
fluid then flows downwardly against interior walls of the freeze
pipe 503 until it reaches the evaporator section 507, thereby
completing the refrigeration cycle. The cycle proceeds as long as
the temperature of the ambient air above the surface 505 of the
substrate 504 is cooler than the temperature of the substrate
504.
[0038] In some applications, the thermosyphon 501 will be required
to work in situations in which the temperature of the ambient air
is not always lower than the temperature of the substrate 504. To
accommodate such situations, a supplementary refrigeration system
515 is provided for removing heat from the substrate 504 during
those times. The mechanical refrigeration system 515 is coupled to
the thermosyphon 501 by means of a heat exchanger 517 and
interconnecting inlet 519 and outlet 521 lines. In the particular
embodiment depicted, the heat exchanger is in the form of a heat
exchange coil 523 which is wound about the exterior surface of the
freeze pipe 503. The heat exchange coil 523 may be welded to the
exterior surface of the freeze pipe 503 or may be attached to it by
other suitable means as are known to the art. In this way, working
fluid which is evaporated in the evaporation section 507 condenses
in the vicinity of the heat exchanger 517 to thereby enable the
continuing operation of the refrigeration cycle.
[0039] A temperature sensor 525 may be provided which is in thermal
communication with the finned portion 511 of the condenser section
509. The temperature sensor 525 provides a suitable signal on line
527 to the mechanical refrigeration system 515 when the temperature
of the condenser section 509 exceeds a predetermined threshold
value, thus deactivating the mechanical refrigeration system 515.
Alternatively, a temperature sensor 529 may be provided in the
evaporator section 507 for detecting the temperature thereof such
that, when the temperature of the condenser section 511 exceeds the
temperature of the evaporator section 507, the mechanical
refrigeration system 521 is activated.
[0040] In some variations of the embodiment depicted in FIG. 6, the
heat exchange coil 523 may be replaced with a hollow, annular shell
which extends around a portion of the freeze pipe 503, which is in
open communication with inlet 519 and outlet 521 lines, and which
is in thermal communication with the freeze pipe 503. The overall
operation of such variations, however, is similar to the operation
of the embodiment depicted in FIG. 6.
[0041] In another variation of the embodiment depicted in FIG. 6
which is illustrated by phantom lines, a horizontal evaporator
portion 531 is provided. In the particular arrangement illustrated,
the horizontal evaporator portion 531 is in fluidic communication
with the condenser section 509. The horizontal evaporator portion
531 may be employed to remove heat closer to the surface 505 and
over a wider surface area. A condensate return pipe 533 may be
provided for directing condensate from the condenser section 509 to
a point near the distal end 535 of the horizontal evaporator
portion 531.
[0042] In some applications, the ore to be leached resides in a
formation disposed on top of relatively impermeable bed rock, and
hence only vertical isolation of the substrate is necessary. In
other applications, however, particularly where the underlying
substrate is relatively permeable or porous (this situation is
frequently encountered with many nickel-based ores), one or more
freeze walls may also be created in the substrate in a lateral or
horizontal direction. One particular, non-limiting embodiment of a
system for achieving such a freeze wall is depicted in FIG. 7.
[0043] In the system 601 depicted therein, an installation trench
603 is provided in the substrate 605. The installation trench 603
is slightly deeper than the desired depth of the horizontal freeze
wall. A plurality of bore holes are formed in the substrate, and a
series of freeze pipes 607 are inserted into the bore holes.
Preferably, the freeze pipes 607 are disposed in a generally
parallel orientation to the surface, though in some applications,
they may instead be disposed at an angle to the surface. Each
freeze pipe 607 terminates in a thermosyphon 609 of the type
depicted in FIG. 6, it being understood that any of the types of
heat exchange systems described herein may be utilized in place of,
or in addition to, such thermosyphons.
[0044] In operation, the cold heat transfer fluid circulates
through the freeze pipe 607, where it absorbs heat from the
substrate. The warm heat transfer fluid then circulates through the
thermosyphon 609 where the heat is rejected to the atmosphere,
after which the cool heat transfer fluid is circulated back to the
freeze pipe 607.
[0045] The freeze pipes used in the systems and methodologies
disclosed herein may be disposed in the substrate in any suitable
arrangement to isolate a portion of the substrate for subsequent
leaching. Typically, the isolated portion of the substrate (the
so-called "bath tub") will contain sufficient ore to be processed
in situ for a period of between 1 month and three years, with the
optimum time frame being determined by the chemistry and geology of
the ore bearing substrate, its location, and other such
factors.
[0046] In many applications, it will be preferable to dispose the
freeze pipes on the perimeter of a rectangle, hexagon, or other
polygon, since this allows the substrate to be readily divided into
well-defined portions of equal volume that require the same or
similar processing conditions. Moreover, after ore extraction has
been completed in one section, the freeze pipes may be reused to
form freeze walls in adjacent sections. In some embodiments, a
freeze wall which has been established between first and second
adjacent volumes may be maintained over the time interval when
leaching is completed in the first volume and leaching commences on
the second volume. Preferably, the freeze pipes are positioned
about 2 feet to about 20 feet apart, and more preferably about 4 to
about 16 feet apart. Most preferably, the freeze pipes are
positioned about 6 feet to about 10 feet apart on the perimeter of
a rectangle.
[0047] The dimensions of the freeze wall may vary from one
application to another. Preferably, however, the length of the
freeze wall perimeter is within the range of about 500 feet to
about 3000 feet, more preferably within the range of about 700 feet
to about 2500 feet, even more preferably within the range of about
1400 feet to about 2200 feet, and most preferably about 2100 feet.
The freeze wall thickness is preferably within the range of about 2
to about 20 feet, more preferably within the range of about 4 to
about 12 feet, even more preferably within the range of about 6 to
about 10 feet, and most preferably about 8 feet. The freeze wall
height will depend on the depth of the ore body that might be
covered under a layer of overburden. Typically, if the ratio of
overburden to ore zone thickness is larger than from about 5 to
about 10, conventional mining becomes cost prohibitive, since the
cost of mining the waste will exceed the value differential between
the ore mined and the operating costs.
[0048] As noted above, various systems and methodologies may be
used in accordance with the teachings herein to create or maintain
a freeze wall, and in some applications, it may be desirable to use
combinations of these systems and methodologies. By way of example,
a freeze wall may be initially construed through the use of one or
more active systems such as, for example, the systems depicted in
FIGS. 1-3, after which one or more passive systems, such as those
depicted in FIGS. 4 and 5, or a hybrid system, such as the systems
disclosed in FIGS. 6-7, may be used to maintain the freeze wall.
Alternatively, one or more active systems and/or one or more hybrid
systems may be used to create and/or maintain a freeze wall in the
systems and methodologies described herein.
[0049] After the desired freeze walls are established, leaching
processes may be performed on the isolated portion of the
substrate. The leaching solutions utilized in the systems and
methodologies described herein may have various compositions, with
the preferred composition being determined in part by the target
minerals which are the principle object of the mining operation.
Preferably, these leaching solutions contain one or more acids,
which may include one or more inorganic acids and/or one or more
organic acids. Possible inorganic acids include sulfuric acid,
hydrochloric acid and nitric acid. Possible organic acids include
malic acid, citric acid, oxalic acid, acetic acid, glycolic acid
and formic acid. The use of some of the aforementioned organic
acids may be particularly desirable in certain applications, due to
the ability of some of these acids to enhance metal recovery by
acting as chelating agents for certain metals.
[0050] The systems and methodologies disclosed herein may also make
effective use of biological leaching processes and agents. The use
of such agents is rendered safer in these systems and methodologies
because the biological agents are sequestered from the remainder of
the substrate, and hence do not contaminate underground aquifers.
Moreover, such biological agents can be effectively killed as part
of a subsequent remediation process.
[0051] The choice of biological agent will vary depending, for
example, on the recovery conditions and the target ores. However,
by way of example, mixed consortia of mesophilic acidophiles may be
utilized in the extraction of nickel and cobalt. Similarly,
bacteria from the genus thiobacillus may be effectively utilized to
leach a variety of metal sulfide ores, and fungi such as
aspergillus niger, penicilium sp, and the like may be utilized for
oxidic ores.
[0052] In some applications of the systems and methodologies
described herein, the kinetics of the leaching reaction may be
enhanced by heating the portion of the substrate to which the
leaching solution is to be applied. Preferably, this is
accomplished through the use of hot fluids, and more preferably
through the use of steam or other gases, though in some
applications, hot liquids may be used instead. In some embodiments,
such hot fluids may be applied through the downpipes of systems
analogous to those disclosed in FIGS. 1-3 which, instead of being
adapted to apply a cool fluid to the substrate, are instead adapted
to apply a warm or hot fluid to the substrate. In other
embodiments, the substrate may be heated through resistive heating,
or by exothermic chemical or biological reactions. For many
applications, the portion of the substrate to which the leaching
solution is to be applied is heated to a temperature of not greater
than 80.degree. C., preferably within the range of about 20.degree.
C. to about 80.degree. C., more preferably within the range of
about 30.degree. C. to about 60.degree. C., and most preferably
within the range of 40.degree. C. to 50.degree. C.
[0053] In some applications of the systems and methodologies
described herein, the kinetics of the leaching reaction may also be
enhanced through the use of one or more catalysts. One such
catalyst system comprises fluoride ions, which are typically
introduced into the leaching solution as aqueous HF or as
fluosilicic acid (H.sub.2SiF.sub.6). The use of such a catalyst may
enhance the ability of the leaching solution to dissolve various
metal oxides and silicate minerals. The use of such a catalyst is
challenging in typical leaching operations, because HF is a contact
poison that can readily penetrate human skin and can be harmful or
fatal in relatively small doses. The use of HF can also contaminate
underground water reservoirs. Moreover, in order for HF-containing
leaching solutions to be effective, it is frequently necessary to
maintain the fluoride ion concentration within a particular range.
This task can be significantly complicated by the movement of
groundwater into or out of the portion of ore bearing substrate
being subjected to leaching operations. The foregoing concerns are
especially significant with certain ore bearing substrates, such as
those containing common nickel ores, which are frequently highly
porous.
[0054] By contrast, in a bathtub of the type described herein, the
freeze walls surrounding a volume of material to be subjected to a
leaching process may be used to provide an effective barrier to the
movement of groundwater into or out of the bathtub, and hence, the
amount of water in the bathtub remains static. Moreover, this
barrier allows the fluoride content within the bathtub to be
reduced to safe levels, either through normal reaction with the
substrate or by subsequent treatment or remediation, before the
area within the bathtub is brought back into contact with the water
table.
[0055] Various other materials or catalysts may be utilized to aid
or enhance the leaching process. These include, without limitation,
the use of SO.sub.2 in either gaseous or aqueous form, and the use
of air or oxygen as a possible oxidant to maintain the leach
solution at a redox potential that is optimal for metal
recovery.
[0056] An additional advantage of the use of freeze walls in
accordance with the teachings herein is that they may significantly
facilitate in situ remediation of the substrate after it has been
treated with the leaching solution. In particular, after ore
extraction is complete, it will typically be necessary to return
the treated substrate to an environmentally friendly condition.
Since the leaching solutions typically utilized in ore extraction
are highly acidic, it is usually desirable to treat the substrate
with a suitable base or neutralizing agent, such as milk of lime
(an aqueous suspension of calcium hydroxide particles), in order to
return the substrate to a more neutral pH (that is, close to 7).
Since the freeze walls form a barrier to the movement of
groundwater into or out of the bathtub, the true pH of the treated
substrate may be more accurately ascertained, the amount of
neutralizing agent required may be more accurately determined, and
the remediation of the treated substrate may be more effectively
and efficiently implemented.
[0057] Another example of a catalyst system which may be used to
enhance the kinetics of the leaching reaction in the systems and
methodologies described herein is a catalyst system comprising a
chloride ion source, a nitride ion source and an ammonium ion
source. The chloride ion source is selected from the group
consisting of ammonium chloride, hydrogen chloride, lithium
chloride, potassium chloride, and sodium chloride, the nitrate ion
source is selected from the group consisting of ammonium nitrate,
nitric acid, lithium nitrate, potassium nitrate and sodium nitrate,
and the ammonium ion source is selected from the group consisting
of ammonium sulphate, ammonium sulfite, ammonium fluoride and
ammonium bifluoride. In some embodiments, the ammonium ions may
also be provided in part by a member selected from the group
consisting of ammonium chloride, ammonium nitrate and mixtures
thereof, an anionic hydrophile selected from the group consisting
of sodium dodecylatedoxydibenzene disulfonate, sodium lauryl
sulphate, sodium N-alkylcarboxy sulfosuccinate, sodium
alkylsulfosuccinate, polyalkanolamine-fatty acid condensate, sodium
alkylbiphenyl sulfonate, sodium alkylnaphthalene sulfonate and
sodium dodecylbenzene sulfonate.
[0058] Various physical techniques may also be utilized in the
systems and methodologies disclosed herein to enhance the recovery
of ore from the bath tub. Such techniques include cyclic steam
stimulation (also known as the "huff and puff" method). This
technique includes an injection stage, a soaking stage and a
production stage. In a typical implementation, in the injection
stage, steam is injected into one or more well bores for a certain
amount of time until the surrounding ore bearing substrate has
reached a target temperature. This may be, for example, a
temperature which is optimal or conducive to a subsequent leaching
process. In the subsequent soaking stage, the substrate is allowed
to "soak" in the steam, typically for no more than a few days. In
the production stage, pregnant leaching solution is recovered from
the production wells. The cycle is then repeated a desired number
of times.
[0059] Another technique which may be utilized in the systems and
methodologies disclosed herein to enhance the recovery of ore from
the bath tub is steam flooding (also known as "steam drive"). In a
typical implementation of this method, a plurality of wells are
formed in the ore-bearing substrate. Some of these wells are
utilized as steam injection wells, and other wells are utilized as
production wells. In this process, as in cyclic steam stimulation,
steam is utilized to enhance the recovery of ore from the bath tub.
In addition, however, steam is further utilized to drive pregnant
leaching solution or aqueous solutions of dissolved minerals
towards the production wells.
[0060] While the use of cyclic steam stimulation and steam flooding
have been explicitly discussed, it will be appreciated that
variations of these methods, and various analogous methods, may be
utilized to enhance the recovery of ores in the systems and
methodologies described herein. For example, in some applications,
water flooding, air flooding and/or CO.sub.2 flooding may be
utilized in place of, or in addition to, these methods. Moreover,
when cyclic steam stimulation or steam flooding are utilized,
various additives may be used in the steam to further facilitate
ore recovery.
[0061] Various minerals and ores may be recovered using the systems
and methodologies disclosed herein. These include, without
limitation, minerals and ores containing cobalt, nickel, zinc,
copper, uranium, cadmium, tin, thorium, silver, gold, trona (an ore
containing trisodium hydrogen dicarbonate dihydrate), iron, cobalt,
magnesium, aluminum, manganese, and various cyanides, oxides
(especially zinc oxides), mixed oxides, and sulfides. Of these, the
use of these systems and methodologies in recovering minerals and
ores containing nickel (such as nickel laterites, and
nickel-enriched clays such as nontronite and saprolite), is
especially advantageous, since nickel ores frequently occur in
porous substrates where contamination of local groundwater is
especially problematic.
[0062] Various means as are known to the art may be utilized to
effect the leaching of ores in the systems and methodologies
disclosed herein. Preferably, this is accomplished by providing a
number of injection wells and at least one production well in the
portion of the substrate isolated by the freeze walls. Each
injection well will typically comprise a well bore through which
the leaching solution may be introduced into the substrate.
Similarly, each production well will typically comprise a well bore
through which the pregnant leaching solution may be recovered from
the substrate. In a typical implementation, there will be between 1
and 10 injection wells per each recovery well, preferably between 2
and 8 injection wells per each recovery well, and most preferably
between 2 and 6 injection wells per each recovery well. Of course,
it will be appreciated that the ratio of injection wells to
recovery wells may depend on such factors as the permeability of
the ore-bearing substrate, the nature of the ore, the chemistry of
the leaching solution, and other such factors.
[0063] After the pregnant leaching solution is recovered from a
production well, it may be subjected to various processes as are
known to the art to extract target metals and other desirable
substances therefrom. By way of example, the pregnant leaching
solution may be subjected to ion exchange, solvent extraction,
precipitation, crystallization, electrowinning, sublimation,
distillation, and/or filtration. The spent solution may then be
refortified as, for example, through pH adjustment or the
replenishment of one or more of its components, and may be
re-injected into the bathtub. In some embodiments, the extraction,
recovery and/or re-injection processes may be partially or fully
automated, and may continue until the recovery of desirable
materials drops below some predetermined threshold value.
[0064] In some applications, various methods may be utilized,
either before or after construction of the freeze wall, to enhance
the permeability of the substrate to the leaching solution. These
include, without limitation, explosive, pneumatic or hydrolytic
fracturing techniques, the use of bore holes, and other such
methods as are known to the art.
[0065] The above description of the present invention is
illustrative, and is not intended to be limiting. It will thus be
appreciated that various additions, substitutions and modifications
may be made to the above described embodiments without departing
from the scope of the present invention. Accordingly, the scope of
the present invention should be construed in reference to the
appended claims.
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