Abstract:
An apparatus for separating a mixture of two liquids of different densities which liquids are substantially insoluble in one another includes a hollow permeable body having a recess for receiving a first fluid which can flow from the recess through the permeable body to an exterior of the permeable body. A housing surrounds and is spaced from the exterior of the permeable body. The housing has an inlet for a second fluid and an outlet for a mixture of the first and second fluid. A baffle or baffles are provided in the space between the exterior of the permeable body and the housing, and to define a mixing channel in space between the exterior of the permeable body and the housing so that the second fluid can enter the housing inlet and flow through the mixing channel to the outlet, while picking up fluid on the exterior of the permeable body.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates solvent extraction and stripping systems, i.e. methods and apparatus. The invention has particular utility in connection with the processing of solutions containing metals, such as, for example, the production of copper by solvent extraction of copper containing solutions followed by electrowinning of a rich copper electrolyte obtained by stripping copper from a copper containing solvent, and will be described in connection with such utility, although other utilities are contemplated. 
       DESCRIPTION OF RELATED ART 
       [0002]    In many industries it often is useful to disperse a first fluid of one type in a second fluid of a different type. For example, carbon dioxide gas is dispersed (dissolved) in water to form carbonated water. In another example, liquid-liquid extraction (where two mutually insoluble liquids contact each other) is used to produce pharmaceuticals and other chemicals, treat water, process food, and recover metals from ore. For example, the mining industry has used liquid-liquid ion exchange for many years to recover metal from aqueous leaching solutions, such as described in U.S. Pat. No. 5,196,095 to Sudderth et al, and U.S. Pat. No. 4,683,310 to Dalton et al. Liquid-liquid ion exchange has also been used for years to recover dissolved copper from etching solutions, such as described in U.S. Pat. No. 3,705,061 to King, in accordance with a recovery process such as that disclosed in U.S. Pat. No. 5,466,375 to Galik. Each of the four patents mentioned above are incorporated herein by reference. 
         [0000]    The four classes of extractants typically employed in the industry are: 
       1. Chelation Extractants 
     2. Ion Pair Extractants 
     3. Neutral or Solvating Extractants 
     4. Organic Acid Extractants 
       [0003]    These extractants are compared in Tables 1 to 4 below: 
         [0000]    
       
         
               
             
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Chelation Extractants 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 EXTRACTANT CLASS 
               
               
                 CHELATING AGENTS 
               
               
                 FORMULA OR STRUCTURE 
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 EXTRACTION CHEMISTRY 
               
               
                 2RH (org)  + MSO 4              R 2 M (org)  + H 2 SO 4   
               
               
                 or 
               
               
                 2RH (org)  + M(NH 3 ) 4   +2               R 2 M (org)  + 2NH 3  + 2NH 4   +   
               
               
                 MODIFIERS 
               
               
                 ALCOHOLS, PHENOLS, ESTERS (TXIB) 
               
               
                 KETOXIMES/ALDOXIME MIXTURES 
               
               
                 SPECIAL FEATURES 
               
               
                 Main commercial extractants for copper 
               
               
                 Operate on hydrogen ion cycle. Stripping is reverse of extraction 
               
               
                 Function with acid and ammoniacal leach solutions 
               
               
                 More selective than other extractant classes 
               
               
                 Kinetically slower than ion pair extractants 
               
               
                 Have good physical properties in terms of phase separation, low  
               
               
                 aqueous solubility, chemical stability 
               
               
                 Relatively expensive to manufacture 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 ION-PAIR EXTRACTANTS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 EXTRACTANT CLASS 
                 ION-PAIR EXTRACTANTS 
               
             
          
           
               
                 FORMULA OR STRUCTURE 
               
             
          
           
               
                 Quaternary Amines 
                 R 3 R′N + Cl −   
               
               
                 Primary Amines 
                 RNH 2   
               
               
                 Secondary Amines 
                 R 2 NH 
               
               
                 Tertiary Amines 
                 R 3 N 
               
               
                 Trialkyl Guanidines 
                   
               
             
          
           
               
                 EXTRACTION CHEMISTRY - 
               
               
                 QUARTERNARY AND TERTIARY AMINES 
               
               
                 R 3 R′N +   (org)  + M −             R 3 R′N +  M −   (org)   
               
               
                 R 3 N (org)             R 3 NH +   (org)  Protonation 
               
               
                 R 3 NH +             R 3 NH + M −   (org)  + H +   
               
             
          
           
               
                 MODIFIERS 
                 ISODECANOL OR TRIDECANOL, 
               
               
                   
                 AROMATIC DILUENT 
               
             
          
           
               
                 SPECIAL FEATURES 
               
               
                 Commercial extractants for uranium, thorium, vanadium, gold, cobalt and 
               
               
                 other metals 
               
               
                 Modifiers promote solubility of the extractant - metal complex in the 
               
               
                 diluent 
               
               
                 Kinetics, both extraction and stripping are fast 
               
               
                 Extraction is usually of a metal anion complex such as Au(CN) 2   −  or 
               
               
                 UO 2 (SO 4 ) 3   4−   
               
               
                 Selectivity is not high. Other anions can compete with the metal 
               
               
                 being extracted 
               
               
                 Stripping can be ion exchange for all types or deprotonation for 
               
               
                 primary, secondary and tertiary amines and trialkylguanidines 
               
               
                 Tertiary amines and trialkylguanidines are more selective than 
               
               
                 primary and secondary amines 
               
               
                 For all except quaternary amines, extraction must be at pH below 
               
               
                 the pk b  of the extractant 
               
               
                 Selectivity can be pH dependent 
               
               
                 Primary, secondary and tertiary amines are relatively simple to produce 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
             
               
               
               
             
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 NEUTRAL OR SOLVATING EXTRACTANTS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 EXTRACTANT CLASS 
                 NEUTRAL OR SOLVATING 
               
               
                   
                   
                 EXTRACTANTS 
               
             
          
           
               
                 FORMULA OR STRUCTURE 
               
             
          
           
               
                   
                 Tri Octyl Phosphine 
                 R 3 P = O R = CH 3  and 
               
               
                   
                 Oxide (TOPO) 
                 (CH 3 ) 2 CHCH 2   
               
               
                   
                 Tri Butyl Phosphate (TBP) 
                 (RO) 3 PO 
               
               
                   
                 Ketones (MIBK) 
                 R 2 CO 
               
               
                   
                 Alcohols 
                 ROH 
               
             
          
           
               
                   
                 EXTRACTION CHEMISTRY 
               
               
                   
                 Extraction is by adduct formation 
               
               
                   
                 2(RO) 3 PO (org)  + UO 2 (NO 3 ) 2             UO 2 (NO 3 ) 2  2(RO) 3 PO (org)   
               
               
                   
                 Stripping is with concentrated HNO 3   
               
               
                   
                 SPECIAL FEATURES 
               
               
                   
                 TBP is used extensively in nuclear fuel reprocessing 
               
               
                   
                 Kinetically fast 
               
               
                   
                 Extract neutral metal complexes 
               
               
                   
                 Selectivity is low 
               
               
                   
                 Organometallic complex must be organic soluble 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 ORGANIC ACID EXTRACTANTS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 EXTRACTANT CLASS 
                 ORGANIC ACID EXTRACTANTS 
               
             
          
           
               
                 FORMULA OR STRUCTURE 
               
             
          
           
               
                 Phosphinic Acids 
                 R 3  P(O)OH 
               
               
                 Sulphonic Acids 
                 R SO 2 OH 
               
               
                 Carboxlic Acids 
                 R 3 C—COOH, Versatic Acid 
               
               
                 Phosphoric Acids 
                 (C 4 H 9 CH(C 2 H 5 )CH 2 O) 2  POOH, 
               
               
                   
                 D2EHPA 
               
             
          
           
               
                 EXTRACTION CHEMISTRY 
               
               
                 4RH (org) + Zn +2             R 2 Zn 2RH (org)  + 2H +   
               
               
                 SPECIAL FEATURES 
               
               
                 Phosphinic acids are widely used for cobalt extraction 
               
               
                 Versatic acids can be used for Cu and Ni extraction 
               
               
                 D2EHPA extracts a wide range of metals 
               
               
                 Operate on a hydrogen ion cycle but do not display hydrogen ion 
               
               
                 stoichiometry. 
               
               
                 Often between 1 and 2 moles of extractant are required for each mole of 
               
               
                 hydrogen produced during extraction. This is because adduct 
               
               
                 formation is also involved in the extraction 
               
               
                 Selectivity is poor and careful pH control may be required to 
               
               
                 achieve reasonable selectivity 
               
               
                   
               
             
          
         
       
     
         [0004]    By way of example, in the recovery of copper, copper ore, typically a copper oxide ore or other copper source, is formed into a particulate mass, a so-called “heap” and a leaching solution trickled over and through the heap to dissolve copper in the ore, forming a copper containing solution. The leachant typically is a weak solution of sulfuric acid and usually is obtained as a recycle stream from a downstream process step such as a raffinate stream from an organic copper extraction step. The copper-rich weakly acidic aqueous solution typically is referred to as a pregnant leach solution (PLS) and is mixed with an organic solvent in a mixer. The organic solvent, which is substantially immiscible in the aqueous solution, extracts copper from the pregnant leach solution to form what is commonly termed a loaded organic stream. The mixture comprising the pregnant leach solution and organic extractant are then transferred to a settler tank where the organic phase and aqueous phases are allowed to separate to form an upper, copper loaded organic phase and a lower, copper depleted acidic aqueous phase called a “raffinate phase”. 
         [0005]    The lower aqueous raffinate phase is removed from the settler tank and typically is recycled and used as a leachant to leach more copper from the ore in the heap. The loaded organic phase is transferred to a second mixer and mixed with lean electrolyte which is obtained from a downstream electrowinning cell. The mixture is transferred to a second settler and the organic and aqueous phases allowed to separate. The lean electrolyte which typically is a highly acidic sulfuric acid stream extracts the copper from the loaded organic phase and forms a rich copper electrolyte aqueous phase. The rich copper electrolyte aqueous phase is fed to an electrowinning cell to form the copper product. Depleted electrolyte from the electrowinning cell termed “lean electrolyte” is recycled and the stream mixed with the loaded organic phase in the mixer and settler to extract more copper from the loaded organic phase. In the mixer/settler operations the loaded organic phase which is now largely depleted of the extracted copper typically is termed the “stripped organic phase” and this phase typically is recycled to the first mixer to contact and extract more copper from new pregnant leach solution. 
         [0006]    In a typical prior art copper solvent extraction-electrowinning process, there are a number of process streams which are either recycled or used in subsequent steps of the process. Because of the nature of the mixer and settler operations and the physical and chemical characteristics of the process streams, these process streams typically contain entrained liquids which are detrimental to subsequent steps or recycle steps or which may be lost in the process causing a significant replacement cost and/or environmental problem. For example, when the raffinate stream is recycled and used to leach more copper ore, entrained organic solvent, typically kerosene, will be lost in the leaching operation adding a make-up expense to the process economics and also creating environmental and other process problems including safety hazards. Similarly, entrained water in the loaded organic phase decreases the efficiency of stripping copper from the loaded organic phase in the stripping step of the process and adds to the load on the mixer and stripper. This entrained water also transfers undesirable contaminates from the leach solution to the stripping solution or electrolyte. 
         [0007]    In U.S. Pat. No. 4,874,534 an improved method of separating organic solvents from aqueous process streams is disclosed in connection with a copper solvent extraction/electrowinning process. Aqueous solutions such as a raffinate process stream having droplets of an organic solvent entrained therein are fed into the upper part of a vertically extending vessel having air bubbles rising therein from an air inlet near the bottom of the vessel. The entrained organic solvent in the raffinate is collected and removed from the top of the vessel and recycled in the process. 
         [0008]    U.S. Pat. Nos. 4,269,676, 4,272,492 and 5,176,802 show typical solvent extraction processes for recovering copper from copper sulfide ores, waste products from the pyrometallurgical processing of copper ores and copper containing acidic chloride solutions. 
         [0009]    See also U.S. Pat. No. 5,849,172 in which is disclosed a method of removing entrained liquids from liquid process streams obtained in a copper solvent extraction electrowinning process wherein a copper containing pregnant leach solution is mixed with an immiscible organic extractant to extract the copper from the pregnant leach solution to form an aqueous raffinate process stream and a copper loaded organic process stream. The loaded organic stream then is mixed with an aqueous acidic stream, to extract copper from the loaded organic stream to produce a stripped organic extractant process stream and a copper rich aqueous electrolyte process stream which aqueous electrolyte process stream is fed into an electrowinning cell and copper produced by electrowinning, the method comprising feeding one or more of the liquid process streams to one or more cyclones to remove entrained water or entrained organic extractant from the stream before the stream is used further in the process. 
         [0010]    The above prior art processes and other prior art processes are somewhat inefficient, due to inefficient mass transfer, necessitating large volumes of organic extractant/solvent during the extraction process, and use of open tanks with long residence time resulting in large amounts of organic extractant/solvent being lost to the atmosphere. The loss of organic extractant/solvent is both costly, and creates environmental and safety hazards. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention overcomes the aforesaid and other problems of the prior art, by providing, in one aspect, is a unique micro dispersion device, which significantly improves mass transfer and thus stage efficiency. The micro dispersion device of the present invention can create different dispersed phase droplet sizes and thus an optimum droplet size can be determined for a specific application. Optimum droplet sizes are determined to maximize mass transfer as well as maximizing phase separation efficiency. A disadvantage of prior art liquid-liquid ion exchange systems for dispersing one liquid in another is that air bubbles are entrained during the dispersing step. The entrained air interferes with material transfer and with coalescensce of the dispersed liquid, and delays separation of the two liquids. Our micro dispersion device avoids the formation of air bubbles as one liquid is dispersed in the other, thus providing a more efficient recovery of the desired material. Our micro dispersion device also allows the dispersion of a specific phase independent of phase volume or flow ratio. Thus, in a system where the organic to aqueous (O/A) ratio is for example, 4:1, the organic phase can be dispersed using our micro dispersion device without having to recycle the aqueous phase. Our micro dispersion device produces a more uniform distribution of droplet sizes within the continuous phase compared to heretofore existing phase mixing devices available. 
         [0012]    The micro dispenser device controls which solution phase is dispersed and which solution phase is continuous. Conventional prior art mixers do not control the dispersed or continuous phase but rather the solution flow to the mixer controls this parameter. It follows that regardless of which solution has the overall larger volume, the micro dispenser device of the present invention does not require the larger solution volume (or a recycle on the lower solution volume) to ensure a continuous phase for the mixer. This provides the operator with more operational flexibility and avoids the costs of recycling a lower volume solution which needs to be the continuous phase. 
         [0013]    A further advantage associated with the control of the dispersed phase through the use of the micro dispenser device is that entrainment can be reduced. Entrainments, as general principle, in the dispersed phase is much less than entrainment in the continuous phase. 
         [0014]    Additionally, the micro dispenser device of the present invention ensures the optimum droplet size is created for the dispersed phase which leads to improved mass transfer and improved stage efficiency. 
         [0015]    The micro dispenser device of the present invention also ensures a uniform droplet size which will improve phase separation. 
         [0016]    The micro dispenser device of the present invention is a closed system, which does not introduce air into the system. This reduces the formation of a third (air) phase into the settler or centrifuge and thus improves phase separation. Additionally, the reduction or elimination of air in the system will reduce the degradation of organic as well as lessening organic/solvent losses. 
         [0017]    In another aspect we provide a unique centrifuge separator, which allows high phase separation efficiency due to the high ‘g’ force created in a rotating bowl. The centrifuge separator of the present invention has a lower cost compared to other centrifugal extractors and separators. The economical design and manufacturing techniques allows the use of a centrifugal separator in, for example, hydrometallurgical solvent extractors operations. Currently available centrifugal contactors and separators are too costly and are uneconomical to use in most solvent extractors operations. Our centrifuge separator offers the advantages over other currently available process equipment such as low hold-up, low residence time, and low organic/solvent inventory. The rotating bowl of our centrifuge separator imparts the liquid in a practically rigid body rotation. The inner surface of the rotating liquid has almost a vertical shape because of the high ‘g’ force. The dispersion entering at the bottom gets separated as it moves upward. The rate of separation depends upon the drop size distribution, their settling velocities under the centrifugal action (rΩ 2 )—where r is the radius of the centrifuge bowl and Ω is the rotational speed, plus densities, viscosities and coalescing behavior of the two phases. For complete separation, adequate height needs to be provided for a given level of centrifugal action—(rΩ 2 ). The outlet ports at the top are positioned in such a way that only very clean light and heavy phases exit the unit after separation. 
         [0018]    The design of our centrifuge separator is unique. These design techniques eliminate or significantly reduce the shortcomings found in existing centrifugal based systems. Our centrifuge separator (with or without our micro dispenser device) is designed so the entering flow does not contain, or contains very little, entrained air. Feed pipes and the separation chamber are occupied essentially 100% by liquid. This avoids air entrainment, which increases phase separation times. This also eliminates a third phase in the centrifuge chamber—thus increasing the volume occupied by the liquids and hence increasing residence times compared to existing centrifuge designs of the same size. 
         [0019]    Moreover, our centrifuge separator, which is significantly less expensive to make. This makes the use of a centrifuge separator much more economically viable. 
         [0020]    Furthermore, the rotational speed of our centrifugal separator is independent of the mixing step. In conventional centrifugal based extraction systems, the mixer is operated at the same speed as the separating bowl. Thus the mixing operation is dependent on the separation RPM and vice versa. Mixing and separation are two discrete operations and should be independent. Our centrifugal separator, is totally separate from the mixing operation. 
         [0021]    Finally, unlike conventional settlers, in our system separation of the phases occurs within a closed system. A closed system greatly reduces, if not eliminates VOC emissions as well as organic/solvent losses due to evaporation. 
         [0022]    Working together, our micro dispersion device and our centrifuge separator significantly improve mass transfer and stage efficiency, while at the same time, improve separation and reduce entrainment losses all at a lower cost than conventional solvent extraction systems and existing centrifuge systems. Moreover, our solvent extraction system is an essentially closed solvent extraction system, which minimizes organic solvent loss from evaporation. By way of example, as applied to production of copper by solvent extraction, in accordance with present invention, we provide a solvent extraction process in which an aqueous based leach solution is mixed with an immiscible organic solvent that is passed through a permeable or porous body, forming micro dispersed droplets of the organic solution in a continuous aqueous phase solution. The resulting dispersion is then passed through a centrifugal separator. 
         [0023]    More particularly, in one aspect the present invention provides a micro dispersion apparatus for mixing of two liquids of different densities which liquids are substantially insoluble in one another, said apparatus comprising a hollow permeable body having a recess for receiving a first fluid which can flow from the recess through the permeable body to an exterior of the permeable body; a housing surrounding and spaced from the exterior of the permeable body, said housing having an inlet for a second fluid and an outlet for a mixture of the first and second fluid; and a baffle or baffles in the space between the exterior of the permeable body and the housing, the baffle or baffles being spaced to define a mixing channel in the space between the exterior of the permeable body and the housing so that the second fluid can enter the housing inlet and flow through the mixing channel to the outlet, while picking up fluid on the exterior of the permeable body. 
         [0024]    In one embodiment, wherein the mixing channel is substantially in the shape of a helix, or the baffle is formed of a series of elongated segments formed end-to-end. 
         [0025]    In one embodiment, the permeable body has pores in the range of 0.2 to 400 microns, preferably 20 to 200 microns, more preferably 60 to 100 microns. 
         [0026]    The micro dispersion device is similar to the device described in our prior U.S. Published Application US 2003/0029795 A1. However, unlike the micro dispersion device described in our prior U.S. Published Application US 2003/0029795 A1, the permeable body is filed, at least in part, with loosely packed finely divided media or fits. 
         [0027]    The present invention also provides an apparatus for separating and recovery of a metal such as copper from a metal-containing source such as a copper-containing source by a solvent extraction/electrowinning process, comprising a mixing device for mixing a metal-containing aqueous solution and an immiscible organic extractant, wherein the mixing device comprises apparatus as above described, the apparatus further comprising a centrifugal separator, and a mixing conduit connecting the mixing device with a centrifugal separator. 
         [0028]    In one embodiment, the mixing conduit is sized and shaped to provide a travel or residence time between the mixing device and the centrifugal separator of 5-120 seconds, preferably 20-60 seconds, more preferably 35-45 seconds. 
         [0029]    In one embodiment, the centrifugal separator separates the mixture into two streams, a heavy phase and a light phase, the heavy phase comprising primarily an aqueous phase containing copper, said apparatus further comprising a conduit carrying the heavy phase to an electrowinning stage. 
         [0030]    In another embodiment, the apparatus further comprises a conduit carrying the light phase conduit to the mixing device. 
         [0031]    The invention also provides a method for separating a mixture of a first and a second fluid of different densities, which fluids are substantially insoluble in one another, said method comprising providing an apparatus as above described, flowing the first fluid from an interior of the permeable body to an exterior of the permeable body; and contacting the first fluid on the exterior of the permeable body with the second fluid. 
         [0032]    In one embodiment, the permeable body has pores in the range of 0.2 to 400 microns, preferably 20 to 200 microns, more preferably 60 to 100 microns. 
         [0033]    In another embodiment of the method the permeable body is filled, at least in part, with loosely packed finely divided media or fits, and including the step of flowing the first fluid through the media or fit. 
         [0034]    In one aspect of the invention, as applied specifically to processing of copper sulfate, the aqueous based solution compromises an aqueous acid solution, while the organic solution comprises an organic solvent, such as kerosene. 
         [0035]    The present invention also provides a method for separating and recovery of metals such as copper from a metal-containing source by a solvent extraction electrowinning process, comprising providing a copper-containing aqueous solution and an immiscible organic extractant to a mixing device, wherein the mixing device comprises an apparatus as above described; whereupon the organic extractant is dispersed in the copper-containing aqueous solution and extracts metal from the aqueous solution, passing the resulting dispersion through a mixing conduit to a centrifugal separator, and separating the organic extractant containing metal from the aqueous solution. 
         [0036]    In one embodiment, the mixing conduit is sized and shaped to provide a travel or residence time between the mixing device and the centrifugal separator of 5-120 seconds, preferably 20-60 seconds, more preferably 35-45 seconds. 
         [0037]    In another embodiment of the method, the centrifugal separator separates the mixture into two streams, a heavy phase and a light phase, the heavy phase comprising primarily an aqueous phase containing copper; including the steps of passing the heavy phase to an electrowinning stage. 
         [0038]    Yet another embodiment of the method includes the step of returning the light phase, at least in part, to the mixing device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Further features and advantages of the present invention will be seen from the following detailed description, taken in conjunction with the accompanying drawings, wherein like numerals depict like parts, and wherein: 
           [0040]      FIG. 1  is flow diagram of a system for the hydrometallurgical production of a metal such as copper in accordance with one aspect the present invention; 
           [0041]      FIG. 1A  is a side elevational view, in cross section, showing details of an apparatus for dispersing an organic fluid such as kerosene into an aqueous-based solution in accordance with another aspect of the present invention; 
           [0042]      FIG. 2  is a side elevational view, in cross section, of a centrifugal contactor-separator employed in accordance with the present invention; 
           [0043]      FIG. 2A  is a cross-sectional view of the rotating cylinder portion of the centrifugal contractor-separator of  FIG. 2 ; and 
           [0044]      FIG. 3  is a flow diagram, similar to  FIG. 1  of an alternative system for the hydrometallurgical production of a metal such as copper in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]    The invention will now be described in connection with the production of copper from a copper-containing ore. Referring first to  FIGS. 1 and 1A , a copper containing pregnant leach solution (PLS)  10  from a copper oxide ore heap  12 , is fed to a first mixing vessel  14 A where the pregnant leach solution is mixed with an organic liquid extractant such as a hydroxyl oxime ion exchanger in kerosene supplied from tank  16 . Referring in particular to  FIG. 1A , mixing vessel  14 A comprises an elongate cylindrical housing  18  having an inlet  20  at one end, and an outlet  22  at the other end. A permeable body  24  in the shape of a cylindrical tube is coaxially disposed within the cylindrical housing  18 . However, housing  18  and permeable body  24  need not be cylindrical—they may be square, or rectangular or have other geometric shapes in cross-section. The permeable body  24  is connected to the housing inlet  20  at one end  26 , and a disc  28  closes the end of permeable body  24  adjacent the housing outlet  22 . 
         [0046]    The outer wall of permeable body  24  is spaced from the interior wall of housing  18 . A helical baffle  30  is located within the annular space between the outer wall of permeable body  24  and the inner wall of housing  18 . Baffle  30  may be a continuous elongated helical strip or formed as a series of segments. Mixing vessel  14 A also has a lateral inlet  32  adjacent the inlet  20  end. 
         [0047]    Permeable body  24  can be made of permeable or porous metal, and is filled with loosely packed finely divided media or frits such as powdered metal particles or ceramic particles. Various permeable and porous metals are available commercially from a variety of vendors including Mott Metallurgical Corporation of Farmington, Conn. The permeable or porous metal used in this invention preferably has substantially uniform pore sizes, or at least most of the pores are within an acceptable range for the intended purpose, and typically are in the range of 0.2 to 400 microns, preferably 20 to 200 microns, more particularly 60 to 100 microns. The porous media or fits should be inert to the liquids being handled. For example, the media or frits can be made of particles of ceramic, or stainless steel, Nickel 200, Monel® 400, Inconel® 600, Hastelloy® C276, Alloy 20, gold, platinum, silver, and titanium. As will be described below, the media or frits, by their nature, cause the droplets of the organic solvent to finally divide, dispersing fine droplets on the outer surface of the permeable body  24 , where they are picked up by the PLS. 
         [0048]    In use, the organic liquid extractant in kerosene is introduced through inlet  20  into the interior of permeable body  24 . PLS is introduced into the interior of mixing vessel  14 A through lateral inlet  32 , into the space between the outer wall of permeable body  24  and the inner wall of mixing vessel  14 A. The organic liquid extractant is forced through the permeable body  24  and emerges from the permeable body in the form of a fine organic liquid extractant droplets where the droplets are picked up by the flowing PLS, forming a dispersion of kerosene droplets in the PLS. The PLS preferably is flowed under turbulent conditions so that the droplets of the organic liquid extractant are quickly dispersed before having an opportunity to coalesce. The organic liquid extractant, which is substantially immiscible with the aqueous based PLS solution, extracts copper from the pregnant leach solution, and emerges from the mixing vessel  14 A via outlet  22 . 
         [0049]    Alternatively, the PLS may be introduced into the interior of permeable body  24 , and the organic liquid extractant introduced into the interior of the mixing vessel  14 A through lateral inlet  32 , into the space between the outer wall of permeable body  24  and the inner wall of mixing vessel  14 A. In such case, the PLS is forced through the permeable body  24 , and emerges from the permeable body in the form of fine droplets which are picked up by the flowing organic liquid extractant, forming a dispersion of aqueous droplets in the kerosene. 
         [0050]    The solution emerging from outlet  22  is passed via conduit  34 A to an extract stage centrifugal separator  100 A as will be described in detail below. Conduit  34 A includes inline baffles shown in phantom as  98 A for maintaining the fluid in a mixed condition. Conduit  34 A is sized and shaped relative to the flow of fluid from mixing vessel  14 A to provide a travel or residence time sufficient to permit substantial mass transfer of copper in the aqueous solution to the organic liquid extractant. Ordinarily, a residence time of 5-120 seconds, preferably 20-60 seconds, more preferably 35-45 seconds, is sufficient before the fluid is introduced into a centrifugal separator  100 A. Alternatively, one or more loops may be included in the conduit  34 A, or the cross sectional size of the conduit  34 A increased so that the flow from mixing vessel  14 A is controlled to within the target residence time of 5-120 seconds. 
         [0051]    As will be described below, centrifuge separator  100 A creates two exit streams-a light phase (organic) and a heavy phase (aqueous raffinate). The aqueous raffinate is recycled to the leach heap to dissolve more copper. The organic phase exiting the centrifuge  100 A is transferred to another mixing vessel  14 B, similar to mixing vessel  14 A, where it is mixed with lean electrolyte from the electrowinning stage  60  as will be discussed below. As before, the organic liquid extractant is forced through the permeable body  24  contained in mixing vessel  14 B, and emerges from the permeable body in the form of a fine organic liquid extractant droplets where the droplets are picked up by the flowing electrolyte, forming a dispersion of kerosene droplets in the electrolyte. As before, the electrolyte preferably is flowed under turbulent conditions so that the droplets of the organic liquid extractant are quickly dispersed before having an opportunity to coalesce. The electrolyte or stripping solution, which is substantially immiscible with the organic liquid extractant removes (strips) copper from the organic liquid extractant and emerges from the mixing vessel  14 B, where it is passed via conduit  34 B which also contains inline baffles shown in phantom as  98 B, similar to conduit  34 A, for maintaining the fluid in a mixed condition. As before, conduit  34 B is sized and shaped relative to the flow of the fluid from mixing vessel  14 A to provide a travel or residence time sufficient to permit substantial mass transfer of copper in the aqueous solution to the organic liquid extractant. The fluid then passed to a second strip stage centrifuge  100 B which is similar in construction to centrifuge  100 A as will be described in detail below, and in which a light organic phase is partially stripped of copper and returned to tank  16 , and a rich copper electrolyte phase is passed to an electrowinning cell  60 . 
         [0052]    Referring in particular to  FIG. 2 , there is shown centrifugal separator  100 A. However, centrifugal separator  100 B is essentially the same. Centrifugal separator  100 A, which is similar to the centrifugal separator described in our prior U.S. Pat. No. 6,440,054, comprises a rotatable cylinder  102  in the shape of a vertical right cylinder contained in a housing  104  having vertical side wall  106  bottom wall  108  and top wall  109 . A vertical drive shaft  112  is suspended at the upper end of housing  104  by an upper thrust bearing. Centrifugal separator  100 A has an inlet  116  for input of an organic/aqueous mixed phase, i.e., from conduit  34 A. The solution enters the central opening (orifice)  140  of the rotating cylinder  102 . The dispersion entering central orifice  140 , gets deflected towards the outside wall of the cylinder by a horizontal deflecting baffle  142  provided close to the entrance. Referring also to  FIG. 2A , unlike the centrifugal separator described in our aforesaid U.S. Pat. No. 6,440,054, the mixing box at the bottom of the centrifugal separator is eliminated, and the upper end of rotating cylinder  102  provided with a plurality of vertical baffles  146  which create several chambers ranging from 4 to 8. In a preferred embodiment, we create four (4) chambers. The rotating cylinder  102  imparts to the liquid a practically rigid body rotation. The inner surface of the rotating liquid has almost a vertical shape because of high ‘g’ except a small parabolic portion adjacent the bottom. The dispersion entering at the bottom region  148  gets separated as it moves upwards. The rate of separation depends upon the droplet size distribution, their settling velocities under the centrifugal action (rΩ2), where r is the radius of the bowl/chamber and Ω is the rotation speed), densities, viscosities and coalescing behavior of the two phases. For complete separation, adequate height needs to be provided for a given level of (rΩ2). Inside the bowl/chamber the solution is separated into two phases—a light phase which is discharged through the ports  144  and exits the unit through the top port  110 , and a heavy phase which is discharged through outlet ports  156  and leaves the unit through outlet  158 . The heavy phase outlet ports  156  have variable positions which are selected and changed according to the relative flow rates of the heavy and light phases and the relative volumes of each phase within the centrifugal separator  100 A. 
         [0053]      FIG. 3  shows an alternative embodiment of the invention. The  FIG. 3  embodiment includes two mixing vessels  14 A 1  and  14 A 2 , and  14 B 1  and  14 B 2  for both the extract stage as well as the strip stage connected through valving and conduits  10 A,  10 B,  34 A,  34 B, so that one mixing vessel may remain in service, while the other mixing vessel is taken off line for maintenance or cleaning. 
         [0054]    The present invention provides various advantages over prior art processes. For one, the system is closed. Thus, loss of organic solvent, i.e. due to evaporation is avoided. Also, by passing the organic phase through finely divided media or frits, and a permeable body before the organic phase is mixed with the PLS, a micro dispersion of the organic phase is formed in the PLS. Thus, less organic solvent is needed in the overall process. Also, higher throughput may be achieved with smaller equipment overall, thus adding to equipment savings, as well as operational savings. 
         [0055]    Also, from studies and tests we found that entrainment of the organic phase in the aqueous is generated in the mixing step and not influenced by the separator. The quantity of entrainment is substantially effected by air ingestion. Using the hollow permeable body mixing apparatus as above described greatly reduces the possibility of air entrainment in the liquid and thus improves separation in the downstream separator (any separator for that matter). 
         [0056]    Also, if air is excluded from the dispersion in the mixer, then organic-in-aqueous entrainment is minimized and aqueous-in-organic entrainment essentially reduced essentially to undetectable levels. Thus, our mixing apparatus as above described allows for a reduction, if not essentially elimination of air entrainment in the liquid thus reducing entrainment of one phase in the other phase. Conventional prior art mixing devices cannot achieve this since by design conventional mixing systems are exposed to the atmosphere and draw air into the liquid. 
         [0057]    Various changes may be made in the above invention without the departing from the spirit and scope thereof. While the recovery of copper by electrowinning (post solvent extraction) has been described in the above working example, recovery of other metals, or other post-solvent extraction steps are possible. By way of example, Uranium may be recovered as ‘yellow cake’ (approximately 80% U3O8) by precipitation and calcining of ammonium uranyl sulfate. Also, some recovery systems employ crystallization to create a nickel sulfate or copper sulfate crystal. And, while electrowinning creates 99.99+ pure copper at the cathode, other metals, for example, zinc, are also electrowon. And some operations also use spray drying technology to create a metal salt dust. Still other changes including recovery of other metals including, but not limited to zinc, nickel, cobalt and uranium, using appropriate extractants, e.g. as above described, are possible.