Abstract:
Disclosed is an electrolytic cell for removal of material from a solution. The cell comprises a cavity for receiving the solution, a rotatable electrode located within the cavity, a counter-electrode in spaced relation to the rotatable electrode, and an ultrasonic generator coupled to said cavity for directing ultrasonic energy toward the rotatable electrode to displace solid material extracted from the solution by an electrochemical reaction. The rotatable electrode may form a cathode, and the counter-electrode may form an anode, wherein metal in the solution is deposited on the cathode as a metal powder, such that the ultrasonic energy displaces the metal powder from the cathode. Alternatively, the rotatable electrode may form an anode, and the counter-electrode may form a cathode, wherein organic waste in the solution is deposited on the anode, such that the ultrasonic energy removes the deposited organic waste from the anode.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit under 35 USC 119(e) of U.S. provisional application No. 60/502,950, filed Sep. 16, 2003, the contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an electrochemical method and apparatus for treating solutions either by electrowinning or electrooxidization.  
       BACKGROUND OF THE INVENTION  
       [0003]     The concept of recovering materials from solutions by electrolysis is not new. Many industries, such as plating processes, mining processes and metal finishing, produce waste product of solutions containing ions of metals, and recovery of this metal is both environmentally and economically beneficial. Waste from solutions with unrecovered metal increases the amount of sludge disposed on land-fields. Many systems of metal recovery currently use a mechanical device, such as a blade, to remove deposited material from an electrode. The use of a mechanical device has the disadvantages of increased wear, risk of breakage, and tendency to slow the treating process down.  
         [0004]     U.S. Pat. No. 4,028,199 describes a method for extracting powder from solution using a rotatable electrode. The powder is removed from the electrode with a mechanical scraper. The applicants have found that such a system does not work satisfactorily in practice because after the first removal the scraper tends to leave a hard film on the electrode, which cannot be easily removed.  
       SUMMARY OF THE INVENTION  
       [0005]     A method and apparatus for the recovery of metal from either an aqueous or non-aqueous solution, and facile separation of electrochemically deposited metal from an underlying cathode is disclosed. Also disclosed is a method for the destruction of organic contaminants from either an aqueous or non-aqueous solution. Both rely on the formation of a powder and utilize ultrasonic energy to remove the material deposited in powder form on a rotatable electrode from the electrode, which permits efficient removal of the material. It is important that the conditions be such that the material be deposited in powder form. The applicants have found that it is not feasible to dislodge the material if it becomes deposited as a solid film.  
         [0006]     Thus, according to one aspect, the invention provides an electrolytic cell for removal of material from a solution. The cell comprises a cavity for receiving the solution, a rotatable electrode located within the cavity, a counter-electrode in spaced relation to the rotatable electrode, and an ultrasonic generator coupled to said cavity for directing ultrasonic energy toward the rotatable electrode to displace solid material extracted from the solution as a powder by an electrochemical reaction.  
         [0007]     In one embodiment, the rotatable electrode forms a cathode, and the counter-electrode forms an anode, wherein metal in the solution is deposited on the cathode as a metal powder, such that the ultrasonic energy displaces the metal powder from the cathode. In another embodiment, the rotatable electrode forms an anode, and the counter-electrode forms a cathode, wherein organic waste in the solution is deposited on the anode, such that the ultrasonic energy removes the deposited organic waste from the anode. It should be understood that the term “ultrasonic” embraces sound vibrations capable of causing a cavitation effect sufficient to dislodge the power from the electrode whether strictly beyond the audible range or not. A suitable range is 16 to 40 KHz, with 25 KHz being preferred.  
         [0008]     The ultrasonic generator may comprise an oscillator for producing alternating-current energy, and a transducer coupled to the cavity for converting the alternating-current energy to mechanical vibrations. There may be two transducers coupled to the cavity at 180 degrees across the cavity.  
         [0009]     The rotatable electrode may be in the shape of a disk. The disk may be formed of a generally flat sheet of flexible material with an electrically-conductive surface provided on one major surface thereof. The counter-electrode may be a rod coaxial within the cell. Further, the cell may be funnel-shaped.  
         [0010]     The electrolytic cell may further comprise a collecting bin for collecting the material removed from the rotatable electrode, such as powdered metal from the cathode, or organic waste from the anode.  
         [0011]     The electrolytic cell may be equipped with a device that has the property of breaking the liquid rise effect caused by the rotation movement of the rotatable electrode. Such device, referred to as a “meniscus-breaker”, is required when the tangential speed (U) of the electrode is higher than 1 m/sec. The order of magnitude of the rising (R) of the liquid level above its nominal value (liquid level when U=0) is given by the following relationship: 
 
 R=U   2 /4  g  
 
 where g is the acceleration due to gravity. 
 
         [0012]     The geometry and dimensions of the meniscus-breaker are determined from the following consideration: a) evolution of hydrogen, oxygen and other possible gases produced at the electrodes during the electrolysis process, b) liquid section above the meniscus-breaker that has to go down through the center hole of the device for being treated, c) presence of solid particles within that liquid (e.g. metallic powder). Because of these considerations, the bottom section of the meniscus-breaker must have a conical or pyramidal shape while the upper section must have an inverted similar shape on top of the bottom section, given an overall hour-glass shape. The angle present within both sections must be such that the bottom section allows the gases to exit upwardly toward the center hole of the meniscus-breaker (where the shaft of the rotatable electrode goes through) while the upper section allows the liquid charged with the solid particles (mostly metallic) to go back into the cell by gravity.  
         [0013]     To be efficient, the meniscus-breaker should be located below the nominal level of the of the liquid into the cell. Its rim (or borders) must closely touched the inner wall of the cell in such a manner that no liquid can go between the wall and the meniscus-breaker.  
         [0014]     The electrolytic cell may further comprise a collecting bin for collecting the material removed from the rotatable electrode, such as powdered metal from the cathode, or organic waste from the anode.  
         [0015]     In another aspect, the invention provides a method for electrowinning metals comprising the steps of passing a solution containing a metal through an electrolytic cell having an anode and cathode, simultaneously applying a direct current to the solution between the anode and the cathode, so as to deposit the metal on the cathode as a metal powder, rotating the cathode during deposition, and directing ultrasonic energy toward the cathode in order to remove the powdered metal therefrom. The ultrasonic energy may be directed toward the cathode at intervals.  
         [0016]     The electrolytic cell can be equipped with an hollow rotatable electrode that can be refrigerated; thus, it can be used for the electroextraction of a metal such as gallium that has a low melting point, that is present into a high temperature electrolyte such as those found in the aluminum extraction industry (bauxite processes). In such application, the gallium powder that is being produced when the rotatable electrode is polarized cathodically is recovered without using ultrasonic energy, but rather by removing the cathode from the high temperature electrolyte and dipping it into another liquid, preferably water, at a temperature above the melting temperature of gallium Hence, the powder melts down from the cathode and can be easily recovered under a solid metallic deposit when the liquid is cooled down below the melting point of gallium. The methods, equipments and chemicals used to refrigerate the hollow rotatable electrode are numerous: any combination of method, equipment and chemical that allows the refrigeration of the surface of the electrode may be applied.  
         [0017]     In another aspect, the invention provides a method for oxidizing organic compounds comprising the steps of passing a solution containing organic compounds through an electrolytic cell having an anode and cathode, simultaneously applying a direct current to the solution between the anode and the cathode, so as to oxidize the organic compounds at the anode, rotating the anode during oxidation, and directing ultrasonic energy toward the anode in order to clean its surface therefrom. The ultrasonic energy may be directed toward the cathode at intervals.  
         [0018]     There are many advantages in using this invention. First, the invention allows the recovery of metals from diluted electrolytes, more specifically from solutions where the total metal concentration ranges from 0 to 3000 ppm, preferably between 20 to 500 ppm. The invention also provides economical recovery of metals from such solutions because metals are obtained in a form of a powder which can be removed from a rotatable electrode without using a mechanical device, such as a blade, to remove it. The powdery metal deposit is easily removed from the rotatable electrode with the use of ultrasonic energy. Then the removed powder metal can be recovered using an appropriate filtration system. This separation is further simplified when the rotatable electrode is cathodic.  
         [0019]     Third, the use of ultrasonic energy to remove deposit from the rotatable electrode cleans this latter at the same time, hence, almost eliminating the use of mineral acids or other toxic chemicals to condition its surface for further use. Fourth, the ultrasonic device of the invention can also be used to clean the surface of the rotatable electrode from organic fouling when the rotatable electrode is polarized as an anode for electrooxidation, thus avoiding complex methods for cleaning its conductive surface. However, the equipment is designed such that the rotatable electrode can be raised. Thus can be inspected, cleaned or repaired at will.  
         [0020]     The invention allows one also to selectively purify concentrated electrolytes from undesired low concentration metallic contaminants present into them. Furthermore, the invention can also be used to destroy organic contaminants present in low concentration in inorganic or organic conductive electrolytes, by electrooxidation. The desired electrochemical reaction is achieved depending upon the induced polarity of the rotatable electrode.  
         [0021]     The invention is more adapted to recover metals from plating processes and mining processes, but can be applied to other types of industries such as metal finishing. The recovery of metals lowers the amount of generated waste when the apparatus is installed up-stream a wastewater system, thus, reducing the amount of sludge to dispose on land-fields.  
         [0022]     Other aspects and advantages of embodiments of the invention will be readily apparent to those ordinarily skilled in the art upon a review of the following description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Embodiments of the invention will now be described in conjunction with the accompanying drawings, wherein:  
         [0024]      FIG. 1  is a high-level illustration of an electrolytic cell in accordance with the teachings of this invention;  
         [0025]      FIG. 2  is a schematic cross-section of the cell cavity of the electrolytic cell of  FIG. 1 ;  
         [0026]      FIG. 3  illustrates the electrolytic cell of  FIG. 1  in an industrial application. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     This invention will now be described in detail with respect to certain specific representative embodiments thereof, the materials, apparatus and process steps being understood as examples that are intended to be illustrative only. In particular, the invention is not intended to be limited to the methods, materials, conditions, process parameters, apparatus and the like specifically recited herein.  
         [0028]     The apparatus provided by the invention may be used either for electrowinning metals or oxidizing organic compounds. The operation of the apparatus is selectively charged by changing the polarization of a rotatable electrode, as is described below.  
         [0029]     Referring to  FIG. 1 , an electrolytic cell  10  has a cell housing  12 . The cell housing  12  defines a cell cavity  14 . The form of the housing  12  of the electrolytic cell  10  is not restricted, and may be composed of any suitable material so long as the housing is electrically insulated from the electrodes. Generally, the housing is cylindrical, although other shapes are possible. In this embodiment, it is shown to be funnel-shaped.  
         [0030]     A rectifier  16  provides the necessary current and voltage required between the anode and the cathode to produce the powdery deposit when the rotatable electrode is polarized cathodically or to oxidize organic contaminants when the rotatable electrode is polarized anodically. The current is supplied to the electrodes by electrical busbars  26 ,  28 . At least two electrodes, namely a cathode and an anode, are connected to the cathode and anode busbars  26 ,  28 , respectively. The rotatable electrode can be polarized as the cathode or as the anode. The rotatable electrode can also be called the working electrode, and the static electrode is called the counter-electrode.  
         [0031]     The housing  12  includes an inlet port  18  and flow passage  20  for feeding the solution to be treated from a storage tank (not shown) to the cell  10 , and an outlet port  22  for removal of the solution, both being effected by a pump  24 . When the powder is being deposited as a result of the electrochemical reaction the solution will be depleted of metal or organic contaminant. The depleted solution is passed through a tank  32  containing filter  52  to a wastewater facility. In the case of a solution containing copper, it is found that even during the deposition stage some powder becomes dislodged and is entrained with the depleted solution to the filter  52 .  
         [0032]     Periodically the current is switched off and ultrasonic energy is applied to the electrode to dislodge the powder. Typically the current may be stopped for anywhere from one to four minutes every 24-36 hours. Typically during the dislodgement phase the speed of rotation of the rotatable electrode is reduced by 25%.  
         [0033]     When the powder is being dislodged from the electrode by the application of ultrasonic energy, the dislodged powder is entrained in the liquid flowing through the outlet and subsequently passed through the filter  52  for removal. Since the liquid flowing through the cell in this phase is not depleted, the resulting liquid, after flowing through the tank  32 , is switched to a buffer tank (not showns) rather than the wastewater facility. The liquid in the buffer tank can be subsequently returned to the cell for further processing during a subsequent deposition stage.  
         [0034]     The cell  10 , according to principles of the invention, also includes an ultrasound generator having an oscillator  30  and ultrasound transducers  31  for directing ultrasonic energy at the rotatable electrode during the powder removal phase.  
         [0035]     Referring to  FIG. 2 , the rotatable electrode  40  is a distinct discrete component separate from the housing mounted on a drive shaft  60 . In  FIG. 2 , the rotatable electrode  40  is shown as being drum-shaped with a concentric fixed cylindrical counter electrode  42 . The precise form of the rotatable electrode  40  depends on the metal or organic contaminant to be recovered. For example, the rotatable electrode  40  may be frustoconical, with its larger radius end uppermost, that is towards the circular upper opening of the electrolytic cell. Or, the rotatable electrode may be in the shape of a V with an opening at the bottom and a wide opening at the top. The rotatable electrode may comprise two face plates and a spacer member therebetween. Other shapes are also possible, such as a tooth shape cylinder, disk or cylinder with grooves, multi-disk, hollow ellipsoidal shape, etc. In any shape, the rotatable electrode is an integral structure comprising a carrier sheet with a conductive element at least on one side, which is conveniently of metal.  
         [0036]     The rotatable electrode  40  may be a hollow disk. Such a shape has a simple mechanical construction. The rotatable electrode may be formed from a generally flat sheet of flexible material with an electrically conductive surface on one major face thereof, and an electrically non-conductive surface on a portion of the other major face thereof, and securing means to enable the sheet to be folded and secured in place. Suitable conductive materials include stainless steel, titanium and its alloy aluminum, or any other conductive material. There is no limitation concerning the tangential speed of the rotatable electrode as far as the equipment is built for high speed and as required by the process being considered.  
         [0037]     The counter-electrode  42  is also situated within the cell cavity  14 . The material of the counter-electrode  42  is not limited in any particular way and may be selected from any material typically used in the art. Usable materials may include stainless steel, platinized titanium, lead or graphite, among others.  
         [0038]     Regardless of the target application or operation of the apparatus, the working electrode is the one that rotates. The rotatable electrode is the electrode where the target reaction occurs. The rotatable electrode can therefore be polarized cathodically or anodically.  
         [0039]     The shape of each of the electrodes  40 ,  42  and electrolytic cell  10  should all correspond with one another. For example, if the anode is in the shape of a rod, its axis will coincide with the axis of the electrolytic cell, where this is also in cylindrical form. The cell may be tubular wherein if the cathode is cylindrical, it surrounds the cylindrical anode. Alternatively, the cell may be box-shaped and divided into a cathode compartment and an anode compartment by a diaphragm. In the illustrated embodiment, the cell is cylindrical, and the anode and cathode are both cylindrical and in spaced relationship to one another.  
         [0040]     The source  16  of direct electrical current is connected between the anode and cathode via leads  26 ,  28  to allow current to flow. When the rotatable electrode is polarized cathodically, metal ions in solution in the cavity migrate toward the cathode where the metal is deposited. Therefore, the cathode rotates to improve the mass transport and reduce the thickness of the diffusion layer. The cathode is rotated by means such as one rotating shaft which may be made of the same metal as the cathode, through which the electric current is fed and which rotate in two bearings formed in walls of the cell. Rotation of the cathode can be achieved by means of an electric motor (not shown) through a speed controller (not shown). Although the rotatable electrode  40  is shown to rotate clockwise, the direction of rotation may also be counter-clockwise.  
         [0041]     When more than one rotatable electrode  40  is used to treat a certain volume of solution, they can be connected in parallel or in series in order to achieve the desired contamination level of the solution to be treated. Each rotatable electrode  40  can operate under similar or different operation modes.  
         [0042]      FIG. 3  illustrates the electrolytic cell of  FIG. 1  in an industrial application. Solution from storage tank  50  is pumped into the cell  10  for processing by means of pump  54 . The cell is fitted with an ultrasonic level detector that controls the operation of pumps  24 ,  54  to maintain the liquid in the cell at the desired level.  
         [0043]     The liquid flowing out of the base of the cell  10  flows into the tank  32  with the filter  52  for removing power entrained in the liquid exiting the cell  10 .  
         [0044]     The filter  52  can include filter bags arranged such that the liquid flows through their walls and deposits the powder within the bags for subsequent removal. Any suitable filter technology can be employed for this purpose.  
         [0045]     The busbar  26  is electrically connected to the rotatable electrode  40  by means of a brush connector  62  in contact with the shaft  60 .  
         [0046]     The shaft  60  is driven in rotation by a motor  64  and pulley system  66 . The shaft rotates in bearings  68 .  
         [0047]     The electrolytic cell may preferably be equipped with a device  27  referred to as a “meniscus breaker that eliminates the meniscus rising effect that occurs when the tangential speed is higher than about 1 m/sec. The device  27  has a “Chinese hat” shape, that is it is in the form of a disk with a central aperture  27   a , the disk having upper and lower surfaces  27   b  tapering inwardly toward the central aperture  27   a . This device prevents the meniscus from rising up the cell while permitting gases formed within the cell to escape.  
         [0048]     In operation, a cathode and anode are put into the cell  10 . The inlet port is connected to the storage tank holding solution to treat, and the solution is pumped via a pump from the tank into the cell cavity to fill the cavity and close the circuit between the cathode and anode. The vast majority of expected applications are in aqueous media, but in certain cases it could be in non-aqueous solutions or electrolytes (e.g. ethanol, benzoic acid, etc.). Preferably, enough solution is pumped into the cavity to completely submerge both the cathode and the anode. The solution is suitably pumped into the cell cavity. For electrolysis, the total metal concentration of the solution is from 0 to 3000 ppm (mg/L), preferably between 20 to 500 ppm (mg/L).  
         [0049]     The cell can be supplied with any form of electric current, such as direct current, alternating current, pulsed, periodic reverse pulse, etc. The anode and cathode of the electrolytic cell are connected to a rectifier which controls the application of electrical power to the anode and cathode.  
         [0050]     The apparatus of this invention can be used to produce metal powders when the rotatable electrode is cathodically polarized. Powders may include metals or alloys in pure forms or metallic hydroxides or oxides. The definition of a powder shall be broad (grain size, shape, metal ceramic, metal, alloys etc.). The formation of a powder, instead of a compact film of metal or alloy, allows the use of ultrasounds to remove to metal from the cathode (as is described below).  
         [0051]     Deposition of metal powder is accomplished by the rigid control of process parameters. The parameters to be controlled include: voltage, current density (pushed toward the limiting current) at the cathode, plating time, cathode rotation speed, electrolytic conditions through proper adjustments of pH, composition, temperature, conductivity, viscosity, concentration, and other parameters to ensure the metal precipitates on the cathode (being reduced) as a powder. The voltage and current are selected by fixing the current level across the electrodes at an optimum level for the range of concentrations found in a particular application. This current level has been determined by experimentation, and is present depending upon the particular use of the apparatus when the apparatus is installed. For instance, to produce zinc powder from an electrolyte that contains only 100 ppm of this metal, a disk of a diameter 0.5 meter (two times its width) will turn at 175 rpm with a current density of 60 mA/cm 2 . If the metal concentration is different, electrowinning conditions will be different as well. If the sought metal is copper instead of zinc, present at the same concentration, speed of rotation and applied current will also be different. Electrowinning conditions are determined on a case by case basis.  
         [0052]     As noted the metal powder produced at the cathode may be removed periodically by switching off the current and applying ultrasonic energy. The metal deposit removal period may vary from one electrolyte to another. Preferably, the deposit does not exceed 10% of the distance between anode and cathode. For example, the preferred gap between electrodes is 2 cm, thus, a 0.2 cm thick deposit will be removed by using the ultrasonic device. Powder removal conditions can vary from one case to another. For instance, powder can be removed as per determined numbers of coulombs or thickness, depending upon powder properties and electrolyte composition.  
         [0053]     The ultrasonic generator  30  supplies an alternating-current energy at an excitation frequency in an ultrasonic range, for example, from 16 kHz to 40 kHz, 25 kHz being preferred. The ultrasonic electrical energy is converted into ultrasonic mechanical vibrations at a frequency corresponding to the excitation frequency. The mechanical vibrations produced by the transducers  31  are applied directly toward the cathode to cause cavitation at the surface of the cathode. This effect causes the metal powder to be removed from the electrode surface. For example, to remove a zinc powder deposit from the rotatable electrode, 2 to 4 minutes of 20% intense ultrasounds at 25 kHz every 24 hours of deposition is sufficient to loosen the powder from the rotatable electrode. The loosened power deposit is subsequently collected by the filter  52 .  
         [0054]     In one embodiment, two ultrasonic transducers  31  placed at 180 degrees from one another are installed (as seen in  FIG. 1 ). The width of the facing plate of the vibrators is half its height, this latter being equal to the height of the rotatable electrode. There are no limitations as to the quantity, location and dimensions of the ultrasonic vibrators inside the apparatus, as long as they face the working electrode (rotatable electrode) and do not shield the electrical field of the counter-electrodes.  
         [0055]     The metal becomes deposited as discrete particles at the cathode and is collected at the bottom of the cell, which is preferably conical or shaped as a funnel having a practical solid angle from 20 to 75 degrees, 45 degrees being preferred, or as a loosely adherent deposit which may be lifted from the cell and washed off the cathode. The metal powder accumulated at the bottom of the cavity can be removed periodically or continuously through the bottom outlet on removal of a plug or through a valve. A collecting bin is located at the bottom of the cell, and collects the powdered metal removed from the cathode. The powder can be collected either by recovering metals from industrial process waters (plating shops, smelters, mining, etc.) and by producing a specific powder from a defined electrolyte. Electrolyte composition can be such that metal powder can be made of a pure metal or alloys.  
         [0056]     The apparatus of this invention may also be used to oxidize organic compounds when the rotatable electrode is anodically polarized. The rotatable electrode is capable of destroying organic contaminants from organic or inorganic electrolytes. If fouling of the rotatable electrode occurs during such application, ultrasonic cleaning is performed using the ultrasonic generators. For example, phenol or creosols can be electrooxidized from 1500 ppb (μg/L) down to 20 ppb (μg/L) using a rotatable electrode and cathode made of stainless steel. The nature of the organic compounds to be destroyed, its concentration, and the material to use as electrodes such as anode and cathode are not limited. The rotatable electrode is most efficient in destroying organic compounds found in low concentrations in organic or aqueous solutions.  
         [0057]     For return of the solution once treated either by electrowinning or electroxidation, the outlet port is connected to the original tank in a closed loop fashion or to another tank for further use or disposal of the solution. When the rotatable electrode works in a way that the treated solution meets disposal rules and regulations (or concentrations required by a specific process EX: 3000 ppm to 1000 ppm of zinc for the chromate bath), the treated solution may go directly into the sewer. Otherwise, the treated solution may be connected to a conventional wastewater system (or returned to the process). The flow rate of the liquid being treated is such that the volume of the liquid that enters the inlet is the same than the one that comes out of the outlet.  
         [0058]     It can be seen that the cell can be employed repeatedly with the same anode and cathode.  
         [0059]     The method of the present invention may be illustrated in the following examples. These examples are provided for further illustrating the present invention, but are in no way to be taken as limiting.  
       EXAMPLE 1  
       [0060]     A solution containing 100 ppm of zinc from a zinc chloride plating solution is reduced to 15 ppm in two steps: the first step uses a current density of 80 mA/cm 2  at a rotatable electrode whose tangential speed is 3.5-4.5 m/sec with a treatment time 1.33 times the flow rate; and the second uses half the current density of the first step but twice the time of treatment.  
       EXAMPLE 2  
       [0061]     A solution containing 200 ppm of copper from an acid copper plating solution is reduced to 20 ppm with a current density of 60 mA/cm 2  at a rotatable electrode with a tangential speed of 3.0-4.0 m/sec with a treatment time equal to 1.25 times the flow rate.  
       EXAMPLE 3  
       [0062]     A solution containing 200 ppm of nickel from an acid nickel sulfamate plating solution is reduced to 30 ppm with a current density of 27 mA/cm 2  at a rotatable electrode with a tangential speed of 2.5-3.5 m/sec with a treatment time equal to 1.50 times the flow rate.  
       EXAMPLE 4  
       [0063]     A solution containing 200 ppm of tin from an acid tin chloride plating solution is reduced to 30 ppm with a current density of 40 mA/cm 2  at a rotatable electrode with a tangential speed of 3.0-3.5 m/sec with a treatment time equal to 1.15 times the flow rate.  
         [0064]     Numerous modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.