Abstract:
A method and apparatus are described for removing a chemical product species produced by electrochemical action at an electrode or by a chemical reaction which occurs at the surface of a solid catalyst. The method involves providing a flow of a host solution generally parallel to a surface at which a chemical reaction occurs. The flow of the host solution causes the products from the chemical reaction to be entrained in a thin boundary layer which moves along the surface. These products can be removed from a cell by withdrawing the boundary layer from the cell as it reaches the trailing edge of the surface. The products may additionally be removed at intermediate points along the surface to prevent the host solution from becoming depleted. Various apparatus for practising this method are also described. The apparatus and method are capable of removing a desired product species in a more highly concentrated form than is possible with commonly available current techniques and apparatus. Furthermore, the method and apparatus avoid the necessity of using a diaphragm or membrane to divide an electrochemical cell, This reduces the expense and increases the reliability and durability of the apparatus as compared to conventional prior art divided electrochemical cells.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of application No. PCT/CA99/00680 which designates the United States of America and is entitled METHOD AND APPARATUS FOR RECOVERING A REACTION PRODUCT PRODUCED AT A SURFACE which is, in turn, a continuation of U.S. patent application Ser. No. 09/123,199 filed Jul., 27, 1998. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to methods and apparatus for recovering reaction products from surfaces. The invention may be applied to recovering a reaction product formed at the surface of an electrode in an electrochemical cell. The invention may also be applied to recovering a product of a catalysed chemical reaction from the surface of a catalyst. The invention has particular application in collecting reaction products generated at the anodes or cathodes of electrochemical cells used for electrical refining and electrowinning of metals. Another aspect of the invention relates to electrochemical cells adapted to be operated according to the methods of the invention.  
         BACKGROUND  
         [0003]    In some industrial chemical processes, chemical reaction products are produced at electrodes which are immersed in a bath of electrolyte as part of an electrochemical cell. It is often desirable to remove such reaction products before they can intermingle with other species in the electrolyte. The reaction products may be desirable in their own right or it may be necessary to remove them from the cell to prevent them from interfering with other chemical processes taking place within the cell.  
           [0004]    For example, some processes for producing copper metal involve leaching copper sulphide ores with an acidic solution containing an oxidizing agent such as ferric ions (Fe 3+ ). The copper dissolves leaving behind elemental sulfur by the reaction:  
           Cu 2 S+4Fe 3+ −2Cu 2+ +4Fe 2+ +S 0   (1)  
           [0005]    Copper metal is then recovered from the resulting copper-containing solution by an electrowinning step. The electrowinning step takes place in an electrochemical cell containing a cathode, an anode and an electrolyte containing the cupric (Cu 2+ ) ions. When electrical current is passed through the cell from the cathode to the anode, copper metal is plated onto the cathode by a process involving the reaction:  
           Cu 2+ +2e − −Cu  (2)  
           [0006]    At the anode, ferrous (Fe 2+ ) ions are converted to ferric ions by the reaction:  
           Fe 2+ −Fe 3− +e −   (3)  
           [0007]    If the ferric ions so created are allowed to co-mingle with the electrolyte then they may ultimately return to the cathode where they will be converted back to ferrous (Fe 2+ ) ions. This reduces the efficiency with which copper metal is plated onto the cathode. The inventors have recognized that this also results in a loss of opportunity to use the ferric ions in other parts of the overall process.  
           [0008]    There are various means currently being used to prevent species produced at an electrode in an electrochemical cell from interfering with electrochemical processes taking place at the other electrode. One approach is to use a divided cell. A divided cell has a barrier between the electrodes. The barrier is permeable to charge carriers but has a low permeability to the species in question, The barrier nay comprise a suitable diaphragm or membrane. A disadvantage of divided cells is that the membrane introduces electrical resistance which increases power consumption. The membrane also adds both capital and maintenance expense, can degrade over time and may become plugged.  
           [0009]    In some cases a reaction product produced at an electrode in an electrochemical cell is a desired product which should be recovered. The prior art provides various ways to recover such reaction products. In general, these methods all make it necessary to withdraw a substantial amount of electrolyte, together with the reaction product which is being collected. For example, in cells where the desired reaction product is much less dense than the electrolyte then the desired reaction product may be obtained by withdrawing the top portion of the electrolyte. In some prior art devices, the electrodes are themselves porous, thereby allowing the reaction products produced at the electrodes, together with some electrolyte, to be withdrawn through the electrodes. Porous electrodes are susceptible to becoming plugged with small particles of sediment or the like which are often present in electrochemical cells. In general, all of these processes allow the desired species to mix with a substantial quantity of electrolyte as it is being removed. Therefore, the concentration of the desired reaction product removed from the cell is relatively low. Subsequent concentration steps are often required.  
           [0010]    Another example of a situation where a desired species is created at a surface is a process which uses a solid catalyst to promote a reaction involving species in the fluid to produce a reaction product at the surface of the catalyst.  
           [0011]    There exist systems for water hydrolysis which have a pair of very closely spaced electrodes. Water flows between the electrodes. The close spacing of the electrodes forces laminar flow of the water between the electrodes. Acidic water can be drawn off through a slit. Such systems could not be effectively used in industrial settings where the very narrow space between the electrodes would be susceptible to becoming plugged with debris. Examples of such systems are disclosed in Patent Abstracts of Japan No. JP-A-08168767 and U.S. Pat. No. 5,534,120.  
           [0012]    There is a continuing need for methods and apparatus capable of efficiently collecting dissolved reaction products produced at a surface in an electrochemical cell or a catalyst. There is a particular need for such apparatus and methods which can be practised on an industrial scale with a minimum of maintenance.  
         SUMMARY OF THE INVENTION  
         [0013]    This invention creates a condition in which an electrolyte, or other fluid, flows in a thin boundary layer across a surface at which a reaction product is generated. The reaction product enters the fluid flowing in the boundary layer and is carried by the flowing fluid to a collection zone. Fluid containing the reaction product is removed at the collection zone. Various flow conditions in which the boundary layer does not mix significantly with the bulk of fluid adjacent the surface may be used to practise the invention.  
           [0014]    A specific embodiment of the invention provides a method for recovering a reaction product at a surface. The method comprises the steps of: providing a surface immersed in a fluid; causing the fluid to flow past the surface so as to cause fluid in a boundary layer region adjacent the surface to flow across the surface from an upstream side of the surface to a downstream side of the surface without mixing substantially with fluid from outside of the boundary layer region; allowing a reaction product to form at the surface; allowing the fluid flowing in the boundary layer region to carry the reaction product across the surface toward a downstream collection zone; and, at the collection zone, collecting fluid flowing in the boundary layer region together with the reaction product being carried by the fluid flowing in the boundary layer region. By applying this method the collected reaction product can be obtained in a form which is much more concentrated than would be the case it the reaction product was simply allowed to mix with the fluid.  
           [0015]    In the method of the invention the surface may be a surface of a catalyst where the reaction product is formed by a chemical reaction catalysed by the catalyst. In the alternative, the surface could be a surface of an electrode where the reaction product is formed by an electrochemical reaction. In one exemplary preferred embodiment the reaction product comprises ferric ions produced at an electrode surface by oxidation of ferrous ions.  
           [0016]    The step of collecting the fluid flowing in the boundary layer region preferably comprises drawing fluid flowing in the boundary layer region into an elongated aperture extending transversely to the direction of fluid flow as the fluid flowing in the boundary layer region arrives at the collection zone. Most preferably the elongated aperture comprises an elongated slot extending transversely to a direction of the boundary layer flow.  
           [0017]    Another aspect of the invention provides a method for recovering a reaction product at a surface. The method comprises the steps of: providing a surface immersed in a fluid and fluid driving means for causing the fluid to flow past the surface; operating the fluid driving means to cause the fluid to flow past the surface so as to cause fluid in a boundary layer region adjacent the surface to flow across the surface from an upstream side of the surface to a downstream side of the surface without mixing substantially with fluid outside of the boundary layer region; allowing a reaction product to form at the surface; allowing the fluid flowing in the boundary layer region to carry the reaction product across the surface toward a fluid removal means in a downstream collection zone; and, at the collection zone, operating the fluid removal means to withdraw fluid flowing in the boundary layer together with the reaction product.  
           [0018]    Yet another aspect of the invention provides an electrochemical cell comprising: a container holding a liquid electrolyte; first and second electrodes in the container in contact with the electrolyte; electrical conductors for connecting the first and second electrodes respectively to first and second poles of an electrical power supply; fluid driving means for causing electrolyte to flow past a surface of the first electrode with a velocity sufficient to cause the electrolyte in a boundary layer adjacent the surface to flow across the first electrode from an upstream side to a downstream side without mixing significantly with electrolyte from outside the boundary layer; and, fluid removal means at the downstream side of the surface for withdrawing fluid containing a reaction product from the boundary layer.  
           [0019]    The fluid removal means preferably comprises an elongated parallel-sided slot oriented generally transversely to a direction of fluid flow in the boundary layer. The slot most preferably has a width in the range of about 1 millimeter to about 1.5 millimeters. An opening according to preferred embodiments of the invention is much more resistant to becoming plugged with debris than a prior art porous electrode.  
           [0020]    A further aspect of the invention comprises an electrode assembly for use in an electrochemical cell. The electrode assembly comprises an electrically conductive surface; a first elongated opening extending along an edge portion of the surface; a plenum in fluid communication with the first elongated opening; and, a port connected to the plenum. When the electrode assembly is immersed in an electrolyte, fluid can be drawn into the first elongated opening by applying suction to the port. In some applications it is preferable to provide one or more additional elongated openings in fluid connection with the plenum. The additional elongated openings extend generally parallel to the first elongated opening and are spaced apart on the electrically conductive surface. Withdrawing fluid through the additional openings can help to stabilize flow in a boundary layer adjacent the surface. The additional openings also permit removal of depleted fluid at various points across the electrically conductive surface . 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    In drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way:  
         [0022]    [0022]FIG. 1 is a partially schematic, partially cut away, isometric view through a simple prior art divided electrochemical cell;  
         [0023]    [0023]FIG. 2 is a sectional view through a prior art divided cell of the type which permits a reaction product to be retrieved from an upper portion of the cell;  
         [0024]    [0024]FIG. 3 is a cross-section of a prior art divided cell wherein desired reaction products may be removed by drawing them through porous electrodes;  
         [0025]    [0025]FIG. 4 is a partially schematic, partially cut-away, isometric view showing a simplified electrochemical cell according to the invention;  
         [0026]    [0026]FIG. 5 is a fragmentary sectional view showing fluid flow in a region adjacent an upstream end of one of the electrodes in the electrochemical cell of FIG. 4;  
         [0027]    [0027]FIGS. 6A, 6B and  6 C show various means for removing a reaction product species from a downstream collection zone in the electrochemical cell of FIG. 4;  
         [0028]    [0028]FIG. 7 is a plot of maximum attainable average current density as a function of distance across an anode in a copper electrowinning cell;  
         [0029]    [0029]FIGS. 8A and 8B are respectively perspective and sectional views of an electrode according to the invention having a plurality of slots for removing fluid from a boundary layer flowing adjacent a surface of the electrode; and,  
         [0030]    [0030]FIG. 9 is a flow chart illustrating steps in the method of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0031]    1. Prior Art  
         [0032]    [0032]FIG. 1 shows a partially schematic view of a prior art divided electrochemical cell  10 . Electrochemical cell  10  comprises a container  12  containing an electrolyte  14 . Container  12  is divided into two volumes  16  and  18  by a diaphragm  20 . A conductive cathode plate  22  is immersed in the electrolyte in region  16  and a conductive anode plate  24  is immersed in the electrolyte in region  18 . Cathode plate  22  and anode plate  24  are connected respectively to negative and positive terminals of a suitable power supply (not shown) . When an electrical current is passed between the cathode and the anode through the electrolyte, chemical reactions occur at the surfaces of the cathode and anode. These chemical reactions release product species into the electrolyte contained within regions  16  and  18  respectively.  
         [0033]    [0033]FIG. 2 is a schematic view showing one way to recover the reaction products produced at anode  24  and cathode  22 . In the apparatus of FIG. 2, electrolyte is withdrawn from regions  16  and  18  through conduits  26  and  28 . Fresh electrolyte is introduced into regions  16  and  18  through conduits  29  in an amount sufficient to replenish the electrolyte removed through conduits  26  and  28 . A problem with the apparatus of FIG. 2 is that the product species are withdrawn together with a relatively large amount of electrolyte such that the concentration of product species in the fluid withdrawn through conduits  26  and  28  is relatively low. Subsequent processing steps may be necessary to concentrate the desired species in the effluent removed through conduits  26  and  28 .  
         [0034]    [0034]FIG. 3 shows another approach which is sometimes used to retrieve product species generated at the cathode and/or the electrode of an electrochemical cell. In the cell  10 A of FIG. 3, cathode  22 A and anode  24 A are porous. Each electrode is backed by a plenum chamber  32 ,  33 . Product species may be removed by applying suction through conduits  26 A and  28 A which are respectively connected to plenum chanter  32  of cathode  22 A and plenum chamber  33  of anode  24 A respectively. A conduit  29  is provided to replenish the electrolyte in each of chambers  16  and  18  as electrolyte is withdrawn through conduits  26 A and  28 A. Cell  10 A may also suffer from the disadvantage that the product species removed through conduits  26 A and  28 A is mixed with a relatively large amount of electrolyte and therefore subsequent processing steps are often required to increase the concentration of the product species in the removed fluid. If the pores in the electrodes are reduced in size to reduce the amount of electrolyte withdrawn from regions  16  and  13  with the product species then the porous electrodes  22 A, and  24 A become prone to plugging.  
         [0035]    A disadvantage of divided electrochemical cells generally is that the diaphragm or membrane which divides the cell may be specific to the particular reaction being carried out in the cell. Therefore, such cells are not flexible in their use. Furthermore, the diaphragm or membrane may degrade over time or become plugged. When this happens power is dissipated at the membrane.  
       DETAILED DESCRIPTION  
       [0036]    2. This Invention  
         [0037]    This invention creates a condition in which an electrolyte, or other fluid, flows in a thin boundary layer across a surface at which a reaction product is generated. The reaction product is carried by the flowing fluid in the boundary layer to a collection zone. Fluid containing the reaction product is removed at the collection zone. Various flow conditions in which the boundary layer does not mix significantly with the bulk of fluid adjacent the surface may be used to practise the invention. The invention may be practised at a surface which is separated from other surfaces and in conditions where there is turbulence in the bulk flow of the fluid in which the surface is situated.  
         [0038]    The method of the invention begins by providing a surface immersed in a fluid (step  90 , FIG. 9). The surface may, for example be the surface of an electrode in an electrochemical cell.  
         [0039]    [0039]FIG. 4 shows a electrochemical cell  40  according to this invention. Electrochemical cell  40  has a single chamber  42  containing electrolyte  14 . An anode  43  and a cathode  44  are provided within chamber  42 . Anode  43  is connected to the positive terminal of a suitable power supply  45  and cathode  44  is connected to the negative terminal of power supply  45 .  
         [0040]    Electrolyte  14  is circulated through chamber  42  by a fluid driving means, which typically comprises a pump  47 , a gang of pumps, or one or more suitable eductors, so that the bulk  50  of electrolyte  14  between anode  43  and cathode  44  can be made to flow past and generally parallel to the surface  48  of cathode  44  with a velocity V, as indicated by arrows  51  (FIG. 51. The fluid driving means may be any practical device capable of generating flow in electrolyte  14 .  
         [0041]    Electrolyte  14  will have a near zero velocity immediately adjacent surface  48 . In a thin hydraulic boundary layer  52  adjacent surface  48  there is a significant gradient to the velocity of electrolyte  14 . The outer edge of hydraulic boundary layer  52  may be defined as the surface on which electrolyte  14  has a velocity 90% of the velocity of the bulk of electrolyte  14  (i.e. 0.9×V). Those skilled in the art will realize that a boundary layer similar to boundary layer  52  will be formed adjacent the surface of anode  43 . The step of causing fluid, such as electrolyte  14  to flow past surface  48  so as to cause fluid in a boundary layer region  52  to flow along surface  48  is identified as step  92  in FIG. 9.  
         [0042]    The method continues by allowing a reaction product to form at surface  48  (step  94 ) . In an electrochemical cell  40 , step  94  involves passing an electrical current between anode  43  and cathode  44  so that a reaction product is generated in an electrochemical reaction at surface  48 .  
         [0043]    This invention deliberately creates flow conditions wherein a reaction product generated at surface  48  and released into electrolyte  14  will become entrained in hydraulic boundary layer  52 . Boundary layer  52  flows across surface  48  front an upstream side  54  to a downstream side  56  without mixing significantly with the bulk  50  of electrolyte  14 . As a result, product species which are formed at or near surface  48  of cathode  44  will be carried along in the hydraulic boundary layer  52  toward the downstream side  56  of cathode  44  (step  96 ). The product species are carried to one or more locations where the product species may be collected (step  98 ).  
         [0044]    While this example has described the collection of product species at a cathode (product species produced at a cathode may be termed a catholyte), the method of the invention may also be used to collect product species generated at anode  43  (such product species may be called anolytes) or species generated at the surface of a catalyst.  
         [0045]    The fluid flow in hydraulic boundary later  52  should be kept from mixing with the bulk fluid flowing past surface  48 . Preferably the fluid flow in boundary layer  52  is kept laminar. Some turbulence may exist in the bulk of the fluid as long as the flowing boundary layer  52  is not thereby caused to become mixed significantly with the bulk of the fluid. The invention could possibly also be practised in a flow regime wherein a turbulent boundary layer exists between surface  48  and the bulk of electrolyte  14  as long as there is not significant mixing between the turbulent boundary layer and the bulk of the electrolyte  14 . It will be appreciated that there are many geometrical arrangements of fluid flows and surfaces which will result in fluid flowing across the surfaces in a hydraulic boundary layer without mixing significantly with the bulk of the fluid. In an electrochemical cell, the cell should preferably be shaped such that the electrolyte flows symmetrically between the electrodes and closely similar laminar flow patterns are maintained in the boundary layers adjacent each of the electrodes. It is not necessary for the electrodes to be very closely spaced. The electrodes may be spaced far enough apart that a fully developed laminar flow pattern does not exist in the region between the electrodes. Indeed, in an electrochemical cell it is often necessary for the electrodes to be separated by significant distances to permit accumulation of electrodeposited species and to prevent debris from building up between the electrodes to the point that the proper operation of the electrochemical cell is interfered with.  
         [0046]    Depending upon the velocity V of flow in region  50 , the boundary layer adjacent surface  48  may be quite thin. In typical electrochemical cell applications V will be in the range of about 0.02 m/s to about 0.3 m/s. With an aqueous electrolyte  14 , if surface  48  has a width on the order of 1 meter and velocity V is on the order of 10 cm·s −1 , then hydraulic boundary layer  52  will be no more than about 1.5 cm to 2 cm in thickness. The thickness of hydraulic boundary layer  52  has been exaggerated in the drawings for clarity. Preferably surface  48  is spaced apart from other adjacent surfaces and objects by a distance significantly greater than (for example, at least 3 times) the thickness of hydraulic boundary layer  52 .  
         [0047]    A diffusion boundary layer  59  can be defined as the zone immediately adjacent surface  48  in which the reaction product has significant concentration. The outer limit of diffusion boundary layer  59  may be defined as a surface on which the concentration of the reaction product is a small percentage (e.g. 5% or 1%) of its maximum value. In general diffusion boundary layer  59  will be significantly thinner than hydraulic boundary layer  52 . It is only necessary to remove the electrolyte from diffusion boundary layer  59  to recover almost all of the reaction product. Diffusion boundary layer  59  is typically not greater than about 2 millimeters in thickness. In the drawings the thickness of diffusion boundary layer  59  has been exaggerated for clarity.  
         [0048]    By extracting a portion of boundary layer  52  which contains diffusion boundary layer  59  the concentration of the desired reaction product in the removed fluid will be significantly greater than it would be if the reaction product were allowed to mix with the bulk of electrolyte  14  and the mixed electrolyte  14  was removed from cell  40 .  
         [0049]    The thickness of diffusion boundary layer  59  is determined by the rate at which reaction product migrates away from surface  48 . The reaction product may migrate from surface  48  toward the bulk of electrolyte  14  by one or more of several mechanisms. For example, some of the reaction product will diffuse into the bulk of electrolyte  14  by diffusion arising from the difference in concentration of the desired species between hydraulic boundary layer  52  and the remainder of electrolyte  14 . Turbulent mixing within the flowing electrolyte can also give rise to migration of species from hydraulic boundary layer  52  into the electrolyte as a whole. To avoid this, laminar flow conditions should be maintained adjacent surface  48 , as described above. Diffusion as a result of electrical gradients may also occur. If the rate of flow of the bulk of electrolyte  14  between cathode  44  and anode  43  is sufficiently high, all of the diffusion effects will be small compared to the main flow of electrolyte across surface  48 , the thickness of layer  59  will be small, and very little reaction product will escape from hydraulic boundary layer  52  into the bulk of electrolyte  14 .  
         [0050]    Cell  40  includes fluid withdrawal means, indicated generally by  60 , for collecting and withdrawing from the cell electrolyte  14  flowing in hydraulic boundary layer  52  which contains the desired reaction species. In general, fluid withdrawal means  60  should be constructed so that it does not disrupt the boundary layer flow of electrolyte  14  over surface  48 .  
         [0051]    Fluid withdrawal means  60  may comprise, for example, an elongated slot  61  along the downstream edge  56  of the surface  48  of an electrode (e.g. cathode  44 ) as shown in FIG. 6A. Suction is maintained in a plenum  62  behind slot  61  by any suitable means, such as a pump  82  connected to a plenum  62  by a suitable conduit  64 . The point at which conduit  64  connects to plenum  62  may be called a suction port. The suction draws electrolyte  14  from hydraulic boundary layer  52  into plenum  62  from where it can be removed from the cell. Slot  61  may be quite narrow. For example, slot  61  may comprise a parallel-edged gap having edges on the order of 1 mm to 1.5 mm apart. It can be appreciated that slot  61  has dimensions much larger than the dimensions of pores in prior art porous electrodes. Slot  61  is therefore much less prone to plugging with debris than a typical porous electrode. The edges of slot  61  are preferably even with one another to avoid creating turbulence within boundary layer  52 . Plenum  62  and slot  61  are configured so that there is a significant pressure drop between the inside and outside of slot  61 .  
         [0052]    Suitable baffles or the like may be placed behind slot  61  to maintain smooth flow patterns of fluid into slot  61  and to limit the amount of fluid drawn into slot  61 . If slot  61  draws in much more fluid than is necessary then the concentration of the reaction product in the removed fluid will be reduced. In an extreme case the flow of fluid into slot  61  could interfere with the flow of fluid in boundary layer  52 . Whether or not baffles are necessary and the design of such baffles are design details which are dependent upon the particular configuration of the apparatus in question and its conditions of operation in ways that will be apparent to those skilled in the art.  
         [0053]    It is not necessary to remove the entire hydraulic boundary layer  52  from cell  40 . In general, if laminar conditions are maintained in hydraulic boundary layer  52  then the reaction product species will lie in the layer  59  of hydraulic boundary layer  52  closest to surface  48 . If diffusion boundary layer  59  does not mix significantly with the rest of boundary layer  52  then is only necessary to remove the portion of boundary layer  52  containing layer  59  to collect most of the reaction product species.  
         [0054]    It can be appreciated that substantially all of the reaction product produced at surface  48  may be removed from cell  40  without withdrawing a large amount of electrolyte  14  from cell  40 . The diffusion boundary layer  59  has a very small volume. Again, while the foregoing example describes collection of a reaction product at a cathode, the same methods could be used to collect a reaction product formed at a surface of an anode or a surface of a catalyst.  
         [0055]    Those skilled in the art will realize that there are many possible arrangements for fluid removal means  60  which may be used to withdraw electrolyte  14  containing a reaction product from flowing hydraulic boundary layer  52  in its portion near the downstream edge  56  of surface  48 . For example, FIG. 6B shows a fluid removal means SO comprising a plenum  62 A in fluid connection with a pair of slots  66 , and  68 . Slot  66  penetrates surface  48  slightly upstream from slot  68 .  
         [0056]    [0056]FIG. 6C shows yet another construction for a fluid removal means  60  in which hydraulic boundary layer  52  flows over a streamlined bump  70 . Fluid containing a reaction product is removed through slot  72  into plenum  62 B from where it is removed. The interior of plenum  62 B is maintained at reduced pressure so that fluid is drawn into plenum  62 B.  
         [0057]    It can be appreciated that reactant species in boundary layer  52  will be depleted as those reactant species are consumed in the reactions which take place on surface  48 . This can happen, for example, if the methods and apparatus of this invention are used in the process for electrowinning copper matte described in U.S. Pat. No. 5,622,615. That process results in copper metal being plated onto a cathode. At the same time, ferrous ions are oxidized to ferric ions at the anode. It is desirable to use the methods of this invention to remove a concentrated stream of ferric ions at the anode. One possible set of process conditions is set out in Table I.  
                           TABLE I                                       Fe 2+  concentration in    35 grams/liter           electrolyte           flow velocity past anode    10 cm/s           average anode current   500 Amperes/m 2             density    1 m × 1 m           anode dimensions           gap between anode and    3 cm           cathode           anolyte withdrawal rate   110 liters/hour                      
 
         [0058]    As the electrolyte in boundary layer  52  is swept along the anode the concentration of Fe 2+  ions in boundary layer  52  fails off as Fe 2+  ions are oxidized to Fe 3+  ions. Eventually the concentration of Fe 2+ + ions will be reduced to the point that the desired current density cannot be maintained in downstream portions of the anode. FIG. 7 shows a plot of the maximum obtainable average current density as a function of position along an anode in the direction of flow of boundary layer  52 . The curve in FIG. 7 was generated from simulated data based upon the process conditions of Table I. It can be appreciated from FIG. 7 that after the boundary layer has flowed along the anode for a distance of about 0.15 meters, the concentration of Fe 2+  ions is depleted to the point that the desired average current density of 500 Amperes/m 2  can no longer be maintained. This problem can be reduced or eliminated in apparatus according to the invention by providing a fluid removal means  60  which removes fluid at several spaced apart locations across the surface of an electrode.  
         [0059]    [0059]FIGS. 8A and 8B show one electrode (or a catalyst) having a surface  148  at which a reaction product is formed. In the embodiment of FIGS. 8A and 8B, the fluid removal means comprise a plurality of parallel slots  72  spaced apart or surface  148 . Each slot  72  opens into a plenum chamber  162  located behind surface  148 . The pressure within plenum chamber  148  is reduced by a pump  32  or some other suitable suction means so that fluid from layer  59  in boundary layer  52  is drawn into slots  72  as boundary layer  52  flows over slots  72 . Slots  72  may all be connected to the suction means by a shared plenum  262 , as shown, or may be connected to individual plenums. Each slot  72  stops at a depth below the level on the electrode which will be at the electrolyte surface when the electrode is in use. This prevents air from being sucked into slots  72 .  
         [0060]    If slots  72  are spaced closely enough together then depleted electrolyte is removed before it becomes so depleted that the performance of the electrode becomes adversely affected. The removal of depleted electrolyte at each location brings fresh electrolyte into the region  59  immediately adjacent surface  148 .  
         [0061]    A further advantage of the apparatus of FIG. 8A and 8B is that removing fluid from boundary layer  52  can stabilize boundary layer  52 . If the flow in boundary layer  52  is such that a transition to turbulent flow conditions in the boundary layer night occur then such a transition may be prevented by withdrawing sufficient amounts of fluid through slots  72 , thus stabilizing the laminar flow regime in boundary layer  52 . This further reduces the possibility that significant amounts of product species can escape into the bulk of electrolyte  14  before being withdrawn into slots  72 .  
         [0062]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, While it is generally not be necessary to do so, it is possible to practice the method of invention in a divided cell by maintaining a sufficient flow of electrolyte past the surface of the electrodes in a divided cell and then removing the anolyte or catholyte from the trailing edge portion of the anode or cathode.  
         [0063]    While several embodiments of fluid removal means  60  which are described above include elongated slots, a series of appropriately located openings, such as round holes, or square holes could be substituted for one or more slots without departing from the broad scope of the invention. The holes should preferably each have diameters large enough that they are not susceptible to becoming plugged.  
         [0064]    Furthermore, while the invention has been described above in respect of an electrochemical cell, the invention could be used as well to collect the product of a chemical reaction which occurs at the surface of a solid catalyst.  
         [0065]    While the invention has been described, for simplicity of illustration, with reference to flat surfaces  48  the invention is not limited to planar surfaces. Boundary layer flows capable of transporting a reaction product across a surface  48  to a collection zone without allowing the reaction product to diffuse significantly into the bulk of the surrounding fluid can be created with many surface geometries. For example, surface  48  could be the cylindrical surface inside a tube, one of a series of concentric surfaces, etc,  
         [0066]    While the direction of boundary layer flow has been illustrated as being horizontal across vertical surfaces, the boundary layer flow may he in some other direction. The surfaces need not be vertically oriented although in industrial electrochemical cells it is generally convenient to use planar rectangular vertically oriented electrodes. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.