CROSS-REFERECE OF RELATED APPLICATIONS

This application is an international application which claims the benefit of U.S.

Provisional Patent Application No. 61/917,250 filed on 17 December 2013.

FIELD OF THE INVENTION

This invention relates to systems and methods for leaching metals from ores such as metal sulfide ores and concentrates thereof, and more particularly to systems and methods for separating gangue minerals during or prior to primary leaching of metal values from sulfide ores and concentrates and after primary flotation. The invention further relates to the leaching of copper-containing, zinc-containing and/or molybdenum-containing sulfide concentrates produced from a typical primary flotation process.

BACKGROUND OF THE INVENTION

This invention relates to the hydrometallurgical processing of sulfide ores for metals recovery. Chalcopyrite (CuFeS ₂ ) is the primary copper-containing mineral found in the majority of the copper sulfide ores of commercial interest. Other copper-containing ore minerals of commercial interest include chalcocite (Cu ₂ S), bornite (Cu ₅ FeS ₄ ), covellite (CuS), digenite (Cu ₂ S), enargite (Cu ₃ AsS ₄ ), tennantite (Cu ₁₂ As ₄ S ₁₃ ), and tetrahedrite (Cui ₂ Sb ₄ S ₁₃ ). Copper sulfide ores, aside from containing a variety of copper-containing minerals, will typically also contain a wide variety of gangue minerals including, but not limited to, silicates, pyrite (FeS ₂ ), pyrrhotite (FeS), and arsenopyrite (FeAsS).

In the processing of metal sulfide ores, flotation is commonly and successfully used to effect separations of metal values (i.e., metalliferous minerals) from gangue minerals found in comminuted ore. Unlike acidic leaching, flotation is generally performed under high pH (e.g., greater than 10) to suppress the recovery of gangue minerals. However, certain types of gangue/mineral separations are difficult to achieve - a non-limiting example is the separation of primary copper sulfides from pyrite. Therefore, it is fairly common for materials containing a target mineral/metal value to be floated and removed from collection launders in the flotation circuit, along with large amounts of lesser or non- valued gangue.

When large amounts of gangue material may make it into the flotation launder with the desired copper-bearing sulfide minerals - and ultimately, into the final concentrate used in a leach process, it can disrupt or negatively impact downstream leach kinetics and negatively impact solvent extraction processes by generating excessive interfacial cruds. For example, more catalyst may be consumed due to the additional surface area and/or larger volume of non- producing solids being processed. When acid leaching at much lower pH levels (e.g., between 1 and 3), mineral particles in these flotation concentrates (or flotation tailings) retain enough residual flotation reagent to cause severe frothing in leach reactors. This frothing causes difficulties in controlling solution redox potentials within the leach reactors and requires excessive reactor heights to contain the froth. In addition, the gangue minerals reduce the efficiency of post-flotation processing, such as ultra-fine grinding, and occupy valuable space within the leach reactors.

Separation of individual metal sulfides from each other can be a challenge, and the separation of copper-bearing sulfide minerals from pyrite, pyrrhotite, arsenopyrite, quartz, layered silicates, and/or other gangue by selective flotation remains a technical problem. For efficiency's sake, it would be desirable to separately recover copper, zinc, and/or molybdenum by hydrometallurgical processes, such as leaching, in the most efficient manner, without the negative effects from large amounts of gangue minerals, like pyrite, being present in the feed to a leaching circuit.

To date, there is no known prior art which combines redox control, a low pH

environment, acid leaching, and designed flotation, in order to selectively separate pyrite, pyrrhotite, arsenopyrite and/or similar gangue minerals from the rest of a target metal concentrate (e.g., copper concentrate) during preliminary leaching.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to control redox potential in at least a first reaction vessel of a leaching process, or a conditioning tank wherein leaching and flotation may occur simultaneously and advantageously;

It is another object of the present invention to provide a system and method for removing flotation reagents used in upstream flotation processes from value metal-bearing particles, in order to render said value metal-bearing particles non-floatable or less floatable during leaching. It is yet another object of the present invention to separate, encourage the flotation of, and/or purposefully recover impurities like pyrite gangue during initial leaching.

Moreover, it is an object of the present invention to promote the stripping of flotation reagents from value metal-bearing minerals in a concentrate, while simultaneously floating a gangue fraction of the concentrate and leaching non-floated value metal-bearing minerals.

It is yet even another object of the present invention to generate additional revenue by recovering and isolating pyrite gangue from a ferric-catalyzed leaching process, which can then be sold to a roasting operation for use as a fuel in a roaster.

A further object of the present invention is to reduce the amount of unnecessary solids (by volume or by weight) which are processed by a leach circuit.

A further object of the present invention is to reduce the amount of acid-generating waste, such as pyrite, that must be stored and monitored indefinitely in waste impoundments.

Additionally, it is an object of the present invention to provide a system and method which allows a metal-leaching process to take place at desired higher solids densities.

These and other objects of the present invention will be apparent from the description herein. Although every object of the invention is believed to be attained by at least one embodiment of the invention, there is not necessarily any one embodiment of the invention that achieves all of the objects of the invention.

SUMMARY OF THE INVENTION A met od of leaching a metal sulfide ore is disclosed. The method may comprise the steps of providing slurry having metal sulfide ore particles therein; subjecting the slurry to a flotation step at a pH level above nine to obtain a first concentrate; subjecting the first concentrate to an acidic leach solution; subjecting the first concentrate to a flotation step at a pH level below five to obtain a second concentrate; and, chemically leaching a metal into the acidic leach solution. In some embodiments, the flotation step which takes place at a pH level above nine may be accomplished in a different vessel than the flotation step which takes place at a pH level below five. In some embodiments, the flotation step at a pH level above nine may be accomplished at a different temperature than the flotation at a pH level below five. In some embodiments, the flotation in step at a pH level above nine may be accomplished at a different pressure than the flotation at a pH level below five. In some embodiments, the flotation in step at a pH level above nine may be accomplished at a different redox potential than the flotation at a pH level below five, in some embodiments, the step of subjecting the first concentrate to a flotation step at a pH level below five to obtain a second concentrate may occur after the step of subjecting the slurry to a flotation step at a pH level above nine to obtain a first concentrate. In some embodiments, the step of subjecting the first concentrate to a flotation step at a pH level below five to obtain a second concentrate may occur prior to the step of chemically leaching a metal, into the acidic leach solution. In some embodiments, the flotatio step at a pH level below five may occur simultaneously with the step of chemically leaching a metal into the acidic leach solution, In some embodiments, the flotation step at a pH level below five may occur in a flotation leach reactor having an agitator, gas sparging system, and a launder. In some embodiments, the flotation step at a pH level belo five may occur in a flotation-leach reactor having means for introducing a gas or reagent. In some embodiments, the step of subjecting the first concentrate to a flotation step at a pH level below five to obtain a second concentrate may involve separating a first product containing a gangue material from the first concentrate to form the second concentrate, in some embodiments, the first product containing a gangue material may be obtained from froth generated during the chemical leaching step. In some embodiments, the method may further comprise the step of fine-grinding the second concentrate prior to introducing the second concentrate into a leach circuit. In some embodiments, the method may further comprise the step of applying steam directly or indirectly to the first concentrate. In some embodiments, the method may comprise the step of forming a cake from the first concentrate with filtration means. In some embodiments, steam may be applied to the cake. In some embodiments, the cake may be re-pulped with a low pH solution in a re-pulp tank.

A system for leaching a metal-sulfide ore is also disclosed. The system may comprise means for providing slurry having metal sulfide ore particles therein, means for subjecting the slurry to a flotation step at a pH level above nine to obtain a. first concentrate, means (e.g., a flotation leach reactor) for subjecting the first concentrate to an acidic leach solution, and means for simultaneously subjecting the first concentrate to a flotation step at a pH level below five to obtain a second concentrate and chemically leaching a metal into the acidic leach solution, In some embodiments, the means for subjecting the slurry to a flotation step at a pH level above nine may be separate from the means for subjecting the first concentrate to a flotation step at a pH level below five. In some embodiments, the means for subjecting the first concentrate to a flotation step at a pH level below five may be configured to operate at or otherwise be held at a different temperature than the means for subjecting the slurry to a flotation step at a pH level above nine. In some embodiments, means for heating and/or controlling temperature of the means for subjecting the first concentrate to a flotation step at a pH below five. In some embodiments, the means for subjecting the first concentrate to a flotation step at a pH level below five may be configured to operate at or otherwise be held at a different pressure than the means for subjecting the slurry to a flotation step at a pH level above nine. In some

embodiments, the means for subjecting the first concentrate to a flotation step at a pH level below five is configured to operate at or otherwise be held at a different redox potential than the means for subjecting the slurry to a flotation step at a pH level above nine. In some

embodiments, the means for subjecting the first concentrate to a flotation step at a pH below five may comprise means for controlling redox potential which includes adjusting and maintaining redox potential. In some embodiments, the means for subjecting the first concentrate to a flotation step at a pH level below five comprises an agitator and a launder. In some

embodiments, the means for subjecting the first concentrate to a flotation step at a pH level below five may comprise means for introducing a gas or reagent. In some embodiments, the means for subjecting the first concentrate to a flotation step at a pH level below five may be configured to produce a first product containing a gangue material and the second concentrate from the first concentrate. In some embodiments, the first product containing a gangue material may be obtained from froth. In some embodiments, the system may further comprise a fine- grinding mill configured to receive the second concentrate prior to solvent extraction. In some embodiments, the system may comprise means for applying steam directly or indirectly to the first concentrate. In some embodiments, the system may comprise filtration means for forming a cake from the first concentrate. In some embodiments, the means for applying steam may be configured to apply steam to the cake. In some embodiments, the system may comprise a re- pulp tank which is configured for re-pulping the cake in a low pH solution. BRIEF DESCRIPTION OF THE DRAWINGS

Present preferred embodiments of systems having a combination flotation/leach devices are shown in the accompanying drawings and certain present preferred methods of using the same are also illustrated therein. It should be appreciated that like reference numbers used in the drawings may identify like components. In the figures,

FIG. 1 is a process flowsheet showing a combined flotation/leaching step after primary flotation according to some embodiments.

FIG. 2 is a process flowsheet showing a combined flotation/leaching step after primary flotation according to some embodiments.

FIG. 3 is a process flowsheet showing a more detailed depiction of certain downstream portions of the flowsheets shown in FIGS. 2 and 3.

In the following, the invention will be described in more detail with reference to drawings in conjunction with exemplary embodiments.

DETAILED DESCRIPTION OF PRESENT PREFERRED EMBODIMENTS

Froth produced in a metal leach reaction may be rich in or contain amounts of gangue minerals. In some circumstances, it would be advantageous to actively and purposely separate and/or remove the gangue from other portions of a final concentrate 15 produced from a conventional primary flotation circuit 2. This may be done by collecting and/or separating froth 48 that accumulates in a flotation-leach reactor 40 during leaching of the concentrate 15 in an acidic or low pH solution 27 as will be described hereinafter. In doing so, portions of the concentrate 15 may be further concentrated into a more highly-concentrated concentrate 49 by separating out and/or removing a substantial quantity of gangue 48. Such separations may be done in a "batch" manner, but are preferably performed continuously. According to certain aspects of the invention, a flotation-leach reactor 40 comprising an agitator 44 and launder 46 may be provided and utilized to facilitate the post-flotation preliminary leaching and gangue separations. The reactor 40 may be purposely built in accordance with special design, it may be provided as a leach reactor (e.g., modified to remove froth), or it may be provided as a flotation cell or (whether modified to hold pressure or low pH, or left unmodified).

By removing gangue minerals via a flotation-leach reactor 40, metal value leaching may progress more efficiently, and at higher solids density in downstream leach reactors 91, 95, 97 of a conventional leach reactor circuit 90. One or more flotation leach reactors 40 may be provided between traditional primary flotation 2 and primary leaching 90 steps for provisional leaching and simultaneous separation of gangue by flotation. Flotation leach reactors 40 may be open to the atmosphere or covered/sealed so as to control gas composition and/or consumption.

Flotation leach reactors 40 discussed herein may be heated by heat transfer lines or direct steam injection. Flotation-leach reactors 40 discussed herein may be temperature-adjustable or configured to be held at a flotation-leach temperature which is different (e.g., greater) than the temperature of an upstream conventional flotation step 2. Flotation reagents 45 may be used within one or more flotation leach reactors 40. The reagents 45 may be of the same type or may differ from flotation reagents 1 added to feed 3 in the primary flotation circuit 2. The reagents 45 may be added to a flotation leach reactor 40 to promote the flotation of certain gangue minerals during preliminary leaching. Redox potential within a flotation leach reactor 40 may be controlled by controlling the solution composition (such as the concentration and ratio of Fe(II)/Fe(III)), the sparging of air, the sparging of oxygen-enriched air, the sparging of pure oxygen, or alternatively and/or sparging an inert gas such as nitrogen.. By controlling redox potential in a preliminary reaction vessel 40 prior to traditional leaching 90, the flotation reagents 1 used in the upstream primary flotation process 2 may be removed from value metal-bearing particles in the first concentrate 15. Such reagent-stripping measures render the value metal- bearing particles non-floatable. However, impurities in the first concentrate 15, such as pyrite and pyrrhotite, remain naturally floatable. Accordingly, such impurities may be selectively removed from the first concentrate 15 by capturing froth within the flotation leach reactor 40, thereby forming a second concentrate 49. Acid leaching conditions may be employed, adjusted, maintained, or otherwise managed to promote the stripping of the flotation reagents 1 from the value metal-bearing minerals within the first concentrate 15, while simultaneously allowing the flotation of gangue fractions within the first concentrate 15.

For minerals like chalcopyrite, chalcocite, and covellite, ferric-assisted/catalyzed leaching in sulfuric acid may alone be effective in stripping flotation reagents 1 added during primary flotation 2. Gangue, (in particular, pyrite), which is recovered in launder 60 of the flotation leach reactor 40 during preliminary acid leaching, may be sold as a first product 50, such as a fuel for a roasting operation. If amounts of the first product 50 contain precious metals (such as gold or silver), the precious metals in the first product 50 may be recovered from one or more cyanidation processes used to treat precious metal ores and/or concentrates thereof.

According to some embodiments, a value metal-containing first concentrate 15 is treated by a multi-stage leaching process 90 wherein simultaneous flotation of gangue minerals and oxidative leaching of the value metal-containing minerals takes place simultaneously in the first or otherwise earliest stage of the multi-stage leaching process 90. Selective flotation reagents 47 may also be used to enhance the selective removal of problem gangue minerals like quartz and layered silicates. The selective flotation reagents 47 may be added during the first steps of, or just prior to leaching. Flotation of the first concentrate 15 may occur under leaching conditions to separate the other gangue fraction 48 from the rest of the first concentrate 15 to form a second concentrate 49 having higher content of value mineral(s).

Turning, now, to FIG. 1, a flotation leaching circuit 100 is shown. The flotation leaching circuit 100 may comprise a typical flotation circuit 2 which receives feed 3 and an amount of flotation reagent 1. The typical flotation circuit 2 may comprise one or more rougher cells 5a-c, one or more cleaner cells 5d, and one or more scavenger cells 5e-f. For example, as shown, the typical flotation circuit 2 may comprise slurry delivery means 4 such as a high pH slurrying device or slurry container which feeds a first rougher cell 5a. A first rougher cell 5a may deliver first rougher tails 6a to a second rougher cell 5b and first rougher froth 8a to a cleaner cell 5d. A second rougher cell 5b may deliver second rougher tails 6b to a third rougher cell 5c and second rougher froth 8b to the cleaner cell 5d. A third rougher cell 5c may deliver third rougher tails 6c to a first scavenger cell 5e and third rougher froth 8c to a cleaner cell 5d. Cleaner froth 8d from the cleaner cell 5d may be steam-stripped and/or advance to a filtration step as will be described hereinafter. Cleaner tails 6d from the cleaner cell 5d may be reground and delivered as return/recycle feed 12 to the slurry delivery means 4. A first scavenger cell 5e may send first scavenger tails 6e to a second scavenger cell 5f and first scavenger froth 8e to the return/recycle feed 12. Second scavenger tails 6f from the second scavenger cell 5f may be moved to a tailings 9 disposal. Second scavenger froth 8f may also be delivered as return/recycle feed 12 to the slurry delivery means 4. Regrinding of the first 8e and second 8f scavenger froth may be accomplished with an optional regrind device 10.

The cleaner froth 8d may be sent to an optional holding tank 14 which feeds a filter 16, such as a belt filter, with first concentrate 15. Steam delivery means 18 such as a pressurized water tank having a heating element and nozzle may be provided locally to the filter 16. Steam delivery means 18 may provide steam 19 in various amounts, flow rates, temperatures, pressures, and at one or more locations along filter 16 to steam- strip residual high pH flotation reagents from the first concentrate 15. The filter 16 may be configured to produce a steam- stripped, value mineral-laden cake 21, which is dewatered of filtrate 23 containing residual high pH solutions, flotation reagents, and/or residual water from steam 19. The cleaned cake 21 may be added to a re-pulp tank 30 and reconstituted with a low pH solution 27 to form a re-pulped slurry 32. The low pH solution 27 may be stored in a low pH solution tank 25. In some instances, re-pulping may occur in a flotation leach reactor 40.

Re-pulped slurry 32 leaving the re-pulp tank 30 may be sent to one or more flotation leach reactors 40 each having a launder 46 and an agitator 44. In the particular embodiment shown, only one flotation leach reactor 40 is use. In some embodiments, flotation leach reactor 40 may be maintained at a pH level below five, for example, between about 0.1 and 4, and preferably under 3. In more preferred embodiments, the leach reaction within the flotation leach reactor is a ferric-catalyzed leach reaction. In some embodiments, flotation leach reactor 40 may be maintained at a temperature below the boiling point of the slurry for example, between approximately 20°C and 100°C and preferably between approximately 60°C and 80°C. One or more gases 41, such as air, oxygen enriched air, pure oxygen, and/or inert gas or gasses, such as nitrogen, oxygen depleted air, carbon dioxide may be added to a flotation leach reactor 40 at pressures between atmospheric and pressures equal to or above the hydrostatic pressure within the flotation-leach reactor, via gas delivery means 43 such as one or more gas-holding tanks. In some embodiments, a flotation-leach reactor 40 may be open to the atmosphere, and in some embodiments, a flotation-leach reactor 40 may be enclosed or otherwise sealed to prevent unnecessary escape or consumption of gas and facilitate gas recycle back to the flotation-leach reactor or recycle to downstream leach reactor(s). If more than one flotation leach reactor 40 is used (e.g., in series), then one or more of the operating conditions described herein may be similar or dissimilar between reactors 40.

One or more reagents 45 commonly used in the reverse flotation of gangue minerals may be added to the flotation leach reactor 40, e.g., via one or more reagent tanks 47. By virtue of the low pH conditions, pyrite, pyrrhotite, arsenopyrite, , and other gangue such as may be floated and caught in launder 46 during leaching. Overflow froth 48 from the reactor 40 may form a first product 50 which may be sent to a smelter to recover target minerals contained therein (e.g., copper, gold, silver). Overflow froth 48 may also be sent to a grinding circuit (not shown) for further downstream processing, or prepared for tailings disposal. With a majority of the gangue being removed from the first concentrate 15, low gangue-containing underflow (i.e., second concentrate 49) may be sent to a fine grinding mill 60. Effluent finely-ground slurry 63 exiting the fine grinding mill may then be sent to a leach reactor circuit 90 and then to an SX/EW circuit 70. Leaching of second concentrate 49 within the leach reactor circuit 90 may efficiently take place without negative side-effects of residual float reagents 1 on solids and/or negative side- effects of surplus gangue. A second product 72 obtained from the SX/EW circuit 70 may comprise more highly-concentrated target minerals (e.g., >95% cathode copper obtained through electrowinning 78) and may be sent to a second product storage tank 80. In some instances, first product 50 and second product 72 may be combined and sent to the same smelter together. In some instances, first product 50 and second product 72 may be sent to the same smelter independently. In some instances, first product 50 and second product 72 may be sent to different smelters in different product streams altogether. Turning, now, to FIG. 2, fine-grinding may take place prior to flotation-leaching in flotation leach reactor 40. In the instance shown, a fine-grinding mill 60 may receive re-pulped slurry 32 leaving a re-pulp tank 30. Residual flotation reagent 1 remaining with first concentrate 15 particles in the re -pulped slurry 32 may be removed during fine-grinding of said particles. Subsequently, flotation leaching may take place in the flotation reactor 40. Effluent from the flotation reactor 40 may form a second concentrate 49 having a lower gangue content than the first concentrate 15. The second concentrate 49 may be reground again in another fine-grinding mill 60 (not shown), or sent downstream to a leach reactor circuit 90 and ultimately to a SX/EW plant 70 in a manner similar to FIG. 1.

As shown in FIG. 3, a leach reactor circuit 90 may comprise one or more leach reactors 91, 95, 97. For instance, a first leach reactor 91, a second leach reactor 95, and a third leach reactor 97 may be provided in a leach reactor circuit 90. Any one or more of the leach reactors 91, 95, 97 may comprise means 43 for introducing a gas 41, or means 47 for introducing a leach catalyst or reagent 45. The leach reactors 91, 95, 97 may be of the conventional type having agitation means as shown. The exact number, configuration, and/or plumbing of leach reactors 91, 95, 97 in the circuit 90 may change depending on particle sizes within the second concentrate 49, desired residence time, and nature or method of leaching. In some embodiments (not shown), the leach circuit 90 may comprise one or more counter-current recycle/feedback streams.

All references disclosed herein are specifically incorporated by reference thereto. While preferred embodiments of this invention have been described, it should be understood that changes and modifications can be made therein in accordance with ordinary skill in the art without departing from the invention in its broader aspects as defined in the following claims. For instance, while not shown, flotation leach reactors described herein may also contain one or more froth-removal mechanisms (e.g., a movable rake device or air blow nozzle) in order to aid in the transfer of the froth to the froth collection launder by sweeping the froth outward to the launder 46. Examples of such froth removal mechanisms may be found in any one or more of US Patent No, 6,926,154, US Patent No. 5,174,973, US Patent No. 4,347,126, US Patent No. 4,913,805, US Patent No. 4,514,291, US Patent No. 5,660,718, US Patent No. 4,659,458, US Patent No. 2,416,066, European Patent No. EP1032472, English Patent No. GB537538, and English Patent No. GB215622, without limitation, which are hereby incorporated herein by reference. Moreover, flotation leaching processes described herein are also applicable to zinc and molybdenum containing sulfides, as well as copper-containing sulfides. In yet other embodiments, re-pulp tank 30 may be used to float pyrite or other gangue materials at a low pH and may be provided with a launder or froth removal mechanism. Alternatively, re-pulp tank 30 may be completely replaced or combined with one or more flotation leach reactors 40 described herein. In such instances, cake 21 and low pH solution 27 may be added directly or indirectly to the one or more flotation leach reactors 40.

Accordingly, while certain present preferred embodiments of a method and process for flotation leaching copper sulfide minerals have been shown and described above, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced. It is also to be understood that although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of these teachings, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the described invention. Accordingly, it is to be understood that the description herein is proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.

REFERENCE NUMERALS IDENTIFIERS

1 Reagent

2 Flotation circuit

3 Slurry of metal sulfide ore particles

4 Slurry container

5a-5c Rougher cells

5d Cleaner cell

5e-5f Scavenger cells

6a-6c Rougher tails

6d Cleaner tails

6e-6f Scavenger tails

7 Feed slurry

8a-8c Rougher froth

8d Cleaner froth (first concentrate)

8e-8f Scavenger froth

9 Tailings

10 Optional regrind device

12 Return/recycle feed

14 Optional concentrate tank

15 First concentrate

16 Belt filter

18 Water tank/heater

19 Steam

21 Cake

23 Filtrate

25 Low pH solution tank

27 Low pH solution

30 Re -pulp tank

32 Re -pulped slurry

40 Flotation leach reactor

41 Gas

43 Gas tank

44 Agitator

45 Reagent

46 Launder

47 Reagent tank

48 Overflow/

49 Underflow/second concentrate

50 First product

60 Fine grinding mill

63 Ground slurry

70 Downstream SX/EW circuit

72 Second product (e.g., cathode copper)

74 Mixer settlers

76 Pregnant liquor/electrolyte

78 Electrowinning tankhouse

80 Second product storage tank

90 Leach reactor circuit 91 First leach reactor

93 First leach reactor effluent

95 Second leach reactor

96 Second leach reactor effluent 97 Third leach reactor

100 Flotation leaching circuit/method