Patent Application
20110298270
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.
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.
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.
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.
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.
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.
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.
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.
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.
The systems and methodologies disclosed herein may be appreciated with reference to the first particular, non-limiting embodiment of a system depicted in
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.
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.
Various materials may be used in the implementation of a system 101 of the type depicted in
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.
Various materials may be used in the implementation of a system 201 of the type depicted in
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° 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.
Unlike the embodiment of
Moreover, unlike the embodiment of
Compared to a typical brine-based system of the type depicted in
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.
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.
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
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.
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
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.
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.
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.
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.
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.
In some variations of the embodiment depicted in
In another variation of the embodiment depicted in
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
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
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.
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.
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.
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.
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
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.
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.
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.
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
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 (H2SiF6). 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.
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.
Various other materials or catalysts may be utilized to aid or enhance the leaching process. These include, without limitation, the use of SO2 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.
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.
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.
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.
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.
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 CO2 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.
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.
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.
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.
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.
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.
