US20140178272A1

US20140178272A1 - Heap-leach pad injection system and method

Info

Publication number: US20140178272A1
Application number: US14/106,592
Authority: US
Inventors: David Decker
Original assignee: Desert Research Institute DRI
Current assignee: Desert Research Institute DRI
Priority date: 2012-12-14
Filing date: 2013-12-13
Publication date: 2014-06-26

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.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

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

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

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.
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.
There are additional features and advantages of the subject matter described herein. They will become apparent as this specification proceeds.
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

Various embodiments are shown and described in connection with the following drawings in which:

FIG. 1 is a schematic diagram illustrating a prior art system and method for applying leachate to a heap leach pad.

FIG. 2 is a schematic diagram illustrating leachate flow in the system and method of FIG. 1.

FIG. 3 is a schematic diagram illustrating a system and method for injecting steam into a heap leach pad.

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.

FIG. 5 is a schematic diagram illustrating fluid flow, such as leachate flow, after applying the system and method of FIG. 4.

DETAILED DESCRIPTION

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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° C., such as at least 200° C., 300° C., 400° C., 500° C., or 600° C.
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.
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.
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.
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.
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.
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.
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.
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.

Claims

What is claimed is:

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.