Abstract:
The present invention relates to a metal recovery reactor and a metal recovery system. The metal recovery device according to the present invention comprises an electrolytic cell which receives a solution containing metal ions from the outside, and which reduces and precipitates the metal ions of the solution on the surface of a cathode when the solution is supplied to a reaction space formed between an anode and the cathode surrounding the anode. The cathode comprises a main cathode and an auxiliary cathode positioned inside the main cathode and capable of being detached and attached from the main cathode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a reactor for recovering metal and a system for recovering metal that can rapidly metal fast using an electrolyzer. 
         [0003]    2. Related Art 
         [0004]    Valuable metals are usually contained in wastewater, plating wastewater, or washing water produced in the electronic industry including a semiconductor manufacturing process. In particular, a considerable amount of precious metals are contained in wastewater or washing water produced in industrial processes using those precious metals, so there is a need for recovering and recycling them. 
         [0005]    Precious metals in wastewater or washing water are generally recovered by an ion exchange method, an activated carbon method, and electrowinning, and the liquid after recovering is neutralized and then thrown out or purified and then reused. 
         [0006]    The electrowinning is a method of performing electric reduction on a water solution or an extracting solution containing precious metals into an electrolyte and then extracting desired precious metals on a cathode. The electrowinning can obtain high-purity metals at a time without undergoing a crude metal and can reuse a solvent for extracting because it is recycled in accordance with electrolysis. 
         [0007]    However, despite those advantages, the electrowinning is easy to be applied when the concentration of metal ions in a water solution is high, and when the concentration is low, metal ions slowly move to the surface of a cathode, so the recovery rate decreases. 
       DOCUMENTS OF RELATED ART 
     Patent Document 
       [0008]    Korean Patent Application Publication No. 2012-0138912 (published on Dec. 27, 2012) 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a reactor for recovering metal and a system for recovering metal that can rapidly recover metal using an electrolyzer. 
         [0010]    In an aspect, a reactor for recovering metal is provided. The reactor includes an electrolyzer that receives a water solution containing metal ions from the outside and reduces and extracts metal ions in a water solution on the surface of cathodes, when the water solution is supplied into a reaction space between an anode and the cathodes surrounding the anode, in which the cathodes include a main cathode and a sub-cathode disposed inside the main cathode and separable from the main cathode. 
         [0011]    The reduction and extraction of the metal ions may occur on the inner side of the sub-cathode. 
         [0012]    The main cathode may have a ring shape and the sub-cathode may have a plate shape and may be wound inside the main cathode. 
         [0013]    The sub-cathode may be made of a material that is dissolved by acid that does not dissolve metal to be recovered. 
         [0014]    The sub-cathode may be in close contact with the main cathode and may substantially fully cover the inner side of the main cathode. 
         [0015]    The anode may be formed in a bar shape and may have a plurality of grooves on the outer side. 
         [0016]    The anode may be a hollow part with both ends open and the side of the anode may not be open. 
         [0017]    The ratio of the surface area of surface area/cathode of the anode in the reaction space may be larger than 1. 
         [0018]    In another aspect, a system for recovering metal is provided. The system includes: a reservoir that keeps a water solution containing metal ions; and an electrolyzer that receives a water solution containing metal ions from the outside and reduces and extracts metal ions in a water solution on the surface of cathodes, when the water solution is supplied into a reaction space between an anode and the cathodes surrounding the anode, in which the cathodes include a main cathode and a sub-cathode disposed inside the main cathode and separable from the main cathode. 
         [0019]    The sub-cathode may substantially fully cover the inner side of the main cathode in close contact with the main cathode, and the reduction and extraction of the metal ions may occur on the inner side of the sub-cathode. 
         [0020]    The sub-cathode may be made of a material that is dissolved by acid that does not dissolve metal to be recovered. 
         [0021]    The anode may be formed in a bar shape and may have a plurality of grooves on the outer side. 
         [0022]    The anode may be a hollow part with both ends open and the side of the anode may not be open. 
         [0023]    The ratio of the surface area of surface area/cathode of the anode in the reaction space may be larger than 1. 
         [0024]    The system may further include a solid-liquid separator that receives a water solution discharged from the electrolyzer and separates metal particles. 
         [0025]    The system may further include: an assistant tank that is disposed between the electrolyzer and the solid-liquid separator; and a controller that reduces a water solution supplied to the electrolyzer when the level of the assistant tank is a first level or more, and that reduces a water solution supplied to the solid-liquid separator when the level of the assistant tank is a second level, which is smaller than the first level, or less. 
         [0026]    According to the present invention, there are provided a reactor for recovering metal and a system for recovering metal that can rapidly recover metal using an electrolyzer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a diagram illustrating the configuration of a system for recovering metal according to a first embodiment of the present invention. 
           [0028]      FIG. 2  is a diagram illustrating a control architecture of the system for recovering metal according to the first embodiment of the present invention. 
           [0029]      FIG. 3  is a cross-sectional view of an electrolyzer according to the first embodiment of the present invention. 
           [0030]      FIG. 4  is a schematic exploded perspective view of the electrolyzer according to the first embodiment of the present invention. 
           [0031]      FIG. 5  is a view showing the shape of an anode of the electrolyzer according to the first embodiment of the present invention. 
           [0032]      FIG. 6  is a view showing the configuration of a cathode according to the first embodiment of the present invention. 
           [0033]      FIG. 7  is a view showing assembling of the cathode according to the first embodiment of the present invention. 
           [0034]      FIG. 8  is a perspective view showing an assembly of the electrolyzer according to the first embodiment of the present invention. 
           [0035]      FIG. 9  is a cross-sectional view taken along line IX-IX of  FIG. 8 . 
           [0036]      FIG. 10  is a flowchart illustrating an operation method according to the level of an assistant tank in the system for recovering metal according to the first embodiment of the present invention. 
           [0037]      FIG. 11  is a flowchart illustrating an operation method for washing a solid-liquid separator in the system for recovering metal according to the first embodiment of the present invention. 
           [0038]      FIGS. 12 and 13  are graphs showing a recovery behavior according to an anode/cathode area ratio. 
           [0039]      FIG. 14  is a view showing the configuration of a cathode according to a second embodiment of the present invention. 
           [0040]      FIG. 15  is a view showing the configuration of a cathode according to a third embodiment of the present invention. 
           [0041]      FIG. 16  is a view showing the configuration of a cathode according to a fourth embodiment of the present invention. 
           [0042]      FIGS. 17 and 18  are graphs showing recovery behaviors according to the materials of an anode. 
           [0043]      FIG. 19  is a graph showing recovery behaviors according to applied currents. 
           [0044]      FIG. 20  is a graph showing recovery behaviors according to flow rates. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0045]    Hereinafter, a reactor for recovering metal and a system for recovering metal according to the present invention will be described with reference to accompanying drawings. 
         [0046]      FIG. 1  is a diagram illustrating the configuration of a system for recovering metal according to a first embodiment of the present invention and  FIG. 2  is a diagram illustrating a control architecture of the system for recovering metal according to the first embodiment of the present invention. 
         [0047]    The system for recovering metal includes an electrolyzer  100  (reactor for recovering metal), an assistant tank  200 , a solid-liquid separator  300 , and a reservoir  400 . There are provided pumps  501  and  502  and valves  601 ,  602 ,  603 , and  604  for transporting and blocking a water solution containing metal ions and/or metal particles to be recovered (hereafter, referred to as ‘water solution’). Further, there are provided a level measurer  210  for measuring the level of the assistant tank  200  and a timer  800  for measuring the operation time of the solid-liquid separator  300 , and the system includes a controller  700  that controls operation of the pumps  501  and  502  and the valves  601 ,  602 ,  603 , and  603  on the basis of signals inputted from the level measurer  210  and the timer  800 . 
         [0048]    The electrolyzer  100  receives a water solution from the reservoir and takes (recovers) metal from the water solution using cyclone electrowinning method. The electrolyzer  100  will be described in detail again. 
         [0049]    The assistant tank  200  receives the electrowon water solution from the electrolyzer  100 . The assistant tank functions as a buffer between the electrolyzer  100  and the solid-liquid separator  300  and solves problems with operational stability that may be caused by a flow rate difference between the first pump  501  and the second pump  502 . The assistant tank  200  has a level sensor  210  and the level sensor  210  senses whether the level of the assistant tank  200  is within an appropriate range, over an upper limit, or under a lower limit. The level sensor  210  may be achieved using various ways, such as using the entire weight or pressure of the assistant tank  200 . 
         [0050]    The solid-liquid separator  300  separates granular metal from a water solution. The granular metal may be produced, when metal electrowon by the electrolyzer  100  grows and divided. The solid-liquid separator  300 , though not limited, may include a filter capable of separating particles. 
         [0051]    The water solution with metal particles separated by the solid-liquid separator  300  is sent back into the reservoir  400 . 
         [0052]    A water solution containing metals to be recovered which is supplied from plating process etc. and a water solution with metals recovered through the electrolyzer  100  and the solid-liquid separator  300  are mixed in the reservoir  400 . In another embodiment, the water solution passing through the electrolyzer  100  and the solid-liquid separator  300  may be treated through additional equipment/process without being mixed with the water solution supplied from plating process etc. 
         [0053]    The system for recovering metal further includes a washing unit capable of washing the solid-liquid separator. The washing unit is composed of a washing water supplier, valves  603  and  604 , a washing water discharger, and a washing water line. 
         [0054]    The electrolyzer  100  according to the first embodiment of the present invention is described in detail with reference to  FIGS. 3 to 9 . 
         [0055]      FIG. 3  is a cross-sectional view of an electrolyzer according to the first embodiment of the present invention,  FIG. 4  is a schematic exploded perspective view of the electrolyzer according to the first embodiment of the present invention,  FIG. 5  is a view showing the shape of an anode of the electrolyzer according to the first embodiment of the present invention,  FIG. 6  is a view showing the configuration of a cathode according to the first embodiment of the present invention,  FIG. 7  is a view showing assembling of the cathode according to the first embodiment of the present invention,  FIG. 8  is a perspective view showing an assembly of the electrolyzer according to the first embodiment of the present invention, and  FIG. 9  is a cross-sectional view taken along line IX-IX of  FIG. 8 . 
         [0056]    Referring to  FIGS. 3 to 9 , the electrolyzer  100  according to the present invention includes an electrolytic cell  10 , cathodes  20  and  22 , and an anode  30 . 
         [0057]    The electrolytic cell  10  is a part for providing a space for an electrowinning process to be described below. In the embodiment, the electrolytic cell  10  is formed in a cyclone shape and has a body  11  and a conical part  15 . 
         [0058]    In the embodiment, the body  11  is formed in a cylindrical shape and has a uniform diameter from the top to the bottom. An inlet  12  is formed at a side of the body  11 , through the inner side and the outer side so that a water solution to be described below can flow inside. An inlet port  13  guiding a water solution to the inlet  12  is connected to the inlet  12 . Further, a connection hole  14  is formed at a side of the body  11  so that a wire for supplying power to the cathodes  20  and  22  to be described below can be inserted. 
         [0059]    In the embodiment, the conical part  15  extends from the bottom of the body  11  and has a diameter gradually decreasing as it goes down, so it has an entirely conical shape. An outlet  16  for discharging the water solution in the body  11  is formed at the bottom of the conical part  15 . Further, an outlet port  17  for discharging a water solution to the outside is connected to the outlet  16 . 
         [0060]    Further, a sealing cap  18  for opening/closing the internal space of the body  11  is provided. That is, female threads are formed around the inner side of the upper portion of the body  11  and male threads are formed around the outer side of the sealing cap  18 , so the sealing cap  18  is thread-fastened to the body  11 . An O-ring  18   a  is disposed between the sealing cap  18  and the body  11  and ensures sealing. 
         [0061]    An insertion hole  18   b  is formed through the top and the bottom of the sealing cap  18  and the anode  30  having a bar shape to be described below is inserted in the insertion hole  18   b . The O-ring  18   c  is disposed to surround the insertion hole  18   b , so it prevents unsealing between the anode  30  and the insertion hole  18   b , which will be described below. A pressing cap  19  is thread-fastened to the upper portion of the sealing cap  18  to increase sealing by pressing the O-ring  18   c  to the top of the sealing cap  18 . A through-hole  19   c  is formed also at the center of the pressing cap  19 , so the anode  30  can be fitted therein. 
         [0062]    The structure of the cathodes according to an embodiment of the present invention is described. 
         [0063]    The cathodes  20  and  22  have an overall cylindrical shape and are fitted inside the body  11 . In the embodiment, the cathodes  20  and  22  are formed in an overall cylindrical shape having a uniform diameter from the top to the bottom. 
         [0064]    The cathodes  20  and  22  include a main cathode  20  and a sub-cathode  22 . The main cathode  20  has a cylindrical shape. The sub-cathode  22  has a plate shape and bends inside the main cathode  20  in assembly. Accordingly, in the embodiment, the main cathode  20  and the sub-cathode  22  are not physically combined and can be separated at any time, if necessary. 
         [0065]    An inlet  21  of the main cathode  20  is formed at a position corresponding to the inlet  12  of the body  11  and communicates with the inlet  12  of the body  11 . A sub-inlet  23  corresponding to the inlet  21  of the main cathode  20  is formed also at the sub-cathode  22 . A water solution containing metal ions flows into the cathodes  20  and  22  through the inlet  12 , the inlet  21 , and the sub-inlet  23 . 
         [0066]    In the embodiment, a water solution is required to flow into the cathodes  20  and  22  and generate a turbulent flow, and for this purpose, the flow direction of the water solution flowing into the cathodes  20  and  22  is required to substantially be the direction of a tangent line of the cylindrical cathode. That is, assuming that the cylindrical cathode is a circle, it should be flow inside in the tangential direction at the edge of the circle. The water solution can generate a turbulent flow while rotating along the inner side of the cathodes  20  and  22 , only when it flows inside in the tangential direction. 
         [0067]    For example, when the water solution flows inside radially toward the center of the cathode, turbulence is not generated in the electrolyte cell  10 , so a desired effect cannot be obtained. 
         [0068]    The main cathode  20  is electrically connected with a power through the connection hole  14  of the body  11 . The main cathode  20  and the sub-cathode  22  are electrically connected in close contact with each other and the sub-cathode  22  is connected to the power through the main cathode  20 . 
         [0069]    The sub-cathode  22  substantially fully covers the inner side of the main cathode  20 , in close contact with the main cathode  20 . Accordingly, reduction and extraction of metal ions concentrate on the inner side of the sub-cathode  22 . Little or substantially no reduction and extraction of metal ions may occur on the inner side of the main cathode  20 . Further, metal ions are little reduced and extracted on the outer side of the sub-cathode  22  too. 
         [0070]    It is possible to prevent unnecessary reduction and extraction by coating the inner side of the main cathode  20  and the outer side of the sub-cathode  22 , where metal ions are little reduced and extracted, with Teflon. 
         [0071]    As an electrowinning process is performed, metal to be recovered is extracted on the inner side of the sub-cathode  22 . After the process, the sub-cathode  22  is easily separated from the main cathode  20  and a post process for separating metal to be recovered such as gold from the sub-cathode  22  is performed. When metal that is dissolved in acid is used for the sub-cathode  22 , precious metals such as gold or platinum are not dissolved, but only the sub-cathode  22  is dissolved in an acid solution, so precious metals can be easily separated from the cathode. The sub-cathode  22  may be made of, for example, iron, zinc, tin, nickel, or copper. 
         [0072]    The main cathode  20  may be made of a material different from the sub-cathode  22 , for example, stainless steel or titanium. 
         [0073]    As described above, since the sub-cathode  22  is not physically combined with the main cathode  20 , it can be easily inserted inside the main cathode  20  and separated after processes. Accordingly, the metal on the surface can be recovered by separating only the sub-cathode  22  after processes. It is possible to start a new process by inserting only a new sub-cathode  22  with the main cathode  20  remaining. Further, since metal is little extracted on the main cathode  20 , work such as washing is easy. 
         [0074]    On the other hand, when the extraction amount of metal to be recovered increases, the extracted metal can be separated in particles and the separated metal particles are separated by the solid-liquid separator  300 . Further, metal having a feature of dendritic growth is easily separated from a cathode and separated by the solid-liquid separator  300 . 
         [0075]    The anode  30  is formed in a long bar shape and inserted in the electrolyte cell  10  through the through-hole  19   c  of the pressing cap  19  and the insertion hole  19   b  of the sealing cap  18 . The top of the anode  30  is electrically connected with the power. 
         [0076]    The anode  30  is a hollow part, so the inside of the electrolytic cell  10  communicates with the outside through the hollow portion of the anode  30 . A water solution in the electrolytic cell  10  falls to the conical part  15  and then, some of the water solution is discharged outside through the outlet  16  at the bottom of the conical part and the other is discharge outside through the anode  30 . 
         [0077]    A plurality of grooves  32  is formed on the outer side of the anode  30 . The grooves  32  circumferentially formed with regular intervals on the anode  30  and have the same width ‘d’ and gap ‘c’. The grooves  32  increase the surface area of the anode  30 . The grooves  32  can be formed at a lower cost than through-holes. Forming the grooves  32  is for easily increasing the surface area of the anode  30  in comparison with forming through-holes. Increasing the surface area of the anode  30  by forming the grooves  32  influences a recovery efficiency, which will be described below. 
         [0078]    The surface area of the anode  30  can be adjusted by changing the width ‘d’, gap ‘c’, and depth ‘y’ of the grooves  32 . 
         [0079]    The grooves  32  can be changed in various arrangements and shapes. In another embodiment the grooves  32  may have different widths ‘d’ and may be formed with irregular intervals. Further, the grooves  32  may be formed in the longitudinal direction of the anode or may be formed in the shape of lattices. The cross-section of the grooves  32  may be variously changed such as in a trapezoid or a semicircle, not a rectangle as in the embodiment. 
         [0080]    In the embodiment, the anode  30  may be made of titanium, in which the strength is increases by coating the titanium with an iridium oxide. The anode formed by coating titanium with an iridium oxide stably remains in a strong acid solution or a strong alkali solution without be dissolved. Further, the anode  30  may be made of stainless steel, in which the stainless steel may be coated with platinum. 
         [0081]    The electrowinning process generally requires high decomposition voltage, and when overvoltage is applied with graphite used as an anode, the surface of the graphite anode weakens and it is worn by high-speed liquid in many cases. However, as in the embodiment, when an electrode formed by coating titanium with an iridium oxide or an electrode formed by coating stainless steel with platinum is used, it is not worn even at high overvoltage and a high flow speed due to its own mechanical strength and maintained the original shape, so stability is high. 
         [0082]    The reason that the electrolyzer  100  according to the present invention can effectively recover metal even at low concentration of metal ions has been explained in detail in Korean Patent Application Publication No. 2012-0138921 invented by the inventors of present application. 
         [0083]    A method of recovering metal using the system for recovering metal described above is described hereafter. 
         [0084]    A water solution in the reservoir  400  is supplied to the electrolyzer  100  by the first pump  501 . In detail, it is supplied to the electrolyzer  100  through the inlet  12  of the electrolyzer  100 . Power is connected to the cathodes  20  and  22  and the anode  30  of the electrolyzer  100 . 
         [0085]    The water solution is sent into the electrolyzer  100  at an inflow speed of 2˜20 m/sec. When it is sent at a speed less than 2 m/sec, it cannot generate a turbulent flow in the cathodes, so desired result cannot be achieved, and when the speed is larger than 10 m/sec, it is not economical. 
         [0086]    The water solution flows inside in the tangential direction of the cathodes  20  and  22  and moves down while rotating along the inner side of the cathodes  20  and  22 , in which some of the water solution is discharged out of the conical part  15  through the outlet  16  and some flows into the hollow portion of the cathode  30  and is discharged up to the outside. The water solution flowing inside in the tangential direction of the electrolytic cell having a cyclone shape is discharged outside through the anode while generating a rising current at the lower portion inside the electrolytic cell. 
         [0087]    The anode  30  and the cathodes  20  and  22  are electrically connected by the water solution in the electrolytic cell, and metal ions such as gold, silver, and platinum is reduced by electrons from the cathodes and extracted in a solid state on the sub-cathode  22 . 
         [0088]    Metal can be effectively recovered through electrowinning in the related art, generally, when 3 g/L or more metal ions are in a water solution, but in the present invention, electrowinning is possible even at a concentration of metal ions of 0.3 g/L or less and this is because the movement speed of metal ions is high due to the cyclone type electrolytic cell. 
         [0089]    The water solution generates a turbulent flow in the electrolytic cell and the generation of a turbulent flow can be found even from the relationship between a dimensionless Reynolds number (Re) showing a flow speed and a dimensionless Sherwood number (Sh) showing mass transfer. 
         [0090]    Generation of a turbulent flow is based on the inherent geometrical features of a cyclone. In the turbulent flow, mass transfer of metal ions rapidly increases. That is, a diffusion layer that is the distance of diffusion of metal ions becomes thin, so the distance that the metal ions diffuse to the surface of a cathode relatively decreases, and accordingly, the reaction speed increases. Further, particularly, random fluctuation of metal ions that is an inherent feature of a turbulent is generated, so the metal ions are suddenly moved to the surface of a cathode and accordingly mass transfer is rapidly increased. 
         [0091]    After the electrolysis, the water solution discharged through the outlet  16  of the electrolytic cell  100  and the anode  30  is supplied to the assistant tank  200 . The assistant tank  200  functions as a buffer between the electrolyzer  100  and the solid-liquid separator  300 . That is, it removes stability in processes that may be caused by a difference between the flow rate through the pump  501  supplying a water solution to the electrolyzer  100  and the flow rate through the pump  502  supplying a water solution to the solid-liquid separator  300  from the electrolyzer  100 . 
         [0092]    The water solution in the assistant tank  200  is supplied to the solid-liquid separator  300  by the second pump  502 . Metal particles are separated from the water solution in the solid-liquid separator  300  so that only the liquid is supplied to the reservoir  400 . 
         [0093]    The metal electrodeposited on the assistant cathode  22  in the electrolyzer  100  and the metal separated by the solid-liquid separator  300  are recovered after the operation continues for a predetermined time and then the process is stopped, and then the operation is started again. 
         [0094]    In the process recovering metal described above, continuous operation is stably made by the assistant tank  200 , so economic value is very increased. Further, metal that is easy to separate from the cathodes  20  and  22  is effectively recovered by the solid-liquid separator  300  and continuous operation is stably performed. Further, it is possible to effectively process a water solution having two or more components with different recovery features simultaneously using the electrolyzer  100  and the solid-liquid separator  300 . 
         [0095]    Operation when there is a problem with the level of the assistant tank  200  and when the solid-liquid separator  300  is washed, which is different from the process in the normal state described above, is described hereafter. 
         [0096]    The case when there is a problem with the level of the assistant tank  200  is described first with reference to  FIG. 10 . 
         [0097]    Even in a normal operation (S 100 ), the flow rate of the assistant tank  200  is the changed by the difference between the flow rates through the pumps  501  and  502 . When the flow rate to the electrolyzer  100  is larger than the flow rate to the solid-liquid separator  300 , the level of the assistant tank  200  is continuously decreased, and in the opposite case, the level of the assistant tank  200  is continuously increased. When the decreased level and the increased level become a predetermined level or more, the assistant tank  200  cannot appropriately function as a buffer. 
         [0098]    The controller  700  receives a level value from the level sensor  210  of the assistant tank  200  and determines whether the level is or not between a high level and a low level ( 5110 ). 
         [0099]    When the level value is very low, under the low level, the controller  700  stops the second pump  502  supplying a water solution to the solid-liquid separator  300  (S 120 ). Accordingly, the level of the assistant tank  200  increases. After a predetermined time passes, the controller  700  checks again the level, and when the level is between the high level and the low level, it performs the normal operation by operating the second pump  502  (S 140 ). 
         [0100]    In another embodiment, the controller  700  can restart the pump  502 , when the level of the assistant tank  200  becomes a predetermined level between the low level and the high level (for example, 50%, 60%, and 70%) after the second pump  502  is stopped. Further, it may be possible to reduce the work flow rate without stopping the second pump  502 . 
         [0101]    When the level value is very high, over the high level, the controller  700  stops the first pump  501  supplying a water solution to the electrolyzer  100  (S 130 ). Accordingly, the level of the assistant tank  200  decreases. After a predetermined time passes, the controller  700  checks again the level, and when the level is between the high level and the low level, it performs the normal operation by operating the first pump  501  (S 140 ). 
         [0102]    In another embodiment, the controller  700  can restart the first pump  501 , when the level of the assistant tank  200  becomes a predetermined level between the low level and the high level (for example, 30%, 40%, and 50%) after the first pump  501  is stopped. Further, it may be possible to reduce the flow rate without stopping the first pump  501 . 
         [0103]    In another embodiment, when the level of the assistant tank  200  is low, the controller  600  can increase of the flow rate through the first pump  501  and decrease the flow rate through the second pump  502 , and then the level of the assistant tank  200  is high, the controller can decrease the flow rate through the pump  501  and increase the flow rate through the pump  502 . Further, this adjustment may be always performed so that the level of the assistant tank  200  is a predetermined level (for example, 40%, 50%, and 60%). 
         [0104]    As the level of the assistant tank  200  is controlled, as described above, the assistant tank  200  can keep stably functioning as a buffer, so the reliability of the continuous process is improved. 
         [0105]    Next, the operation when the solid-liquid separator  300  is washed is described with reference to  FIG. 11 . 
         [0106]    During the normal operation, when the controller  600  determines that it is time to wash, washing is started. The controller  600  can determine the start of washing at each predetermined operation time on the basis of time information received from the timer  800 . 
         [0107]    In another embodiment, the controller  600  can determine the start of washing on the basis of the pressure of the solid-liquid separator  300  (washing is started when the pressure becomes a predetermined level or more), and the metal concentration in a water solution may be considered in determining the start of washing (washing is started earlier when the metal concentration is high). 
         [0108]    When start of washing is determined, first, the first pump  501  for supplying a water solution to the electrolyzer  100  and the first valve  601  at the outlet of the assistant tank  200  are turned off (S 210 ). Next, the second pump  502  for supplying a water solution to the solid-liquid separator  300  and the second valve  602  at the outlet of the solid-liquid separator  300  are turned off (S 220 ). Accordingly, the flow of a water solution is removed in the electrolyzer  100  and the solid-liquid separator  300 . 
         [0109]    Next, the washing unit is started. In detail, the third valve  603  connected to the washing water supplier, the second pump  502  connected to the solid-liquid separator  300 , and the fourth valve  604  connected to the washing water discharger are turned on (S 230 ). Accordingly, a washing process in which washing water is supplied from the washing water supplier to the solid-liquid separator  300 , washes the solid-liquid separator  300 , and then discharge to the washing water discharger is performed (S 240 ). 
         [0110]    When the washing is finished, the washing water supply is stopped by turning off the third valve  603 , and the second pump  502  and the fourth valve  604  are also turned off (S 250 ). Accordingly, the washing water supplier and the washing water discharger are separated from the solid-liquid separator  300  and the operation of the washing unit is stopped. 
         [0111]    After the washing process described above is finished, the normal state operation (S 260 ) is performed. 
         [0112]    The system for recovering metal described above may be changed in various ways. In particular, a plurality of electrolyzers  100  and/or the solid-liquid separators  300  may be provided to achieve stable operation and continuous operation. 
         [0113]    When the electrolyzers  100  are provided in parallel and metal electrodeposited by any one of the electrolyzers  100  is recovered, a continuous process can be maintained by another electrolyzer  100 . 
         [0114]    When the solid-liquid separators  300  are provided in parallel and any one of the solid-liquid separators  300  is washed or metal is recovered from a filter, the continuous process can be maintained by another solid-liquid separator  300 . 
         [0115]    The recovery behavior of recovering metal depends on the area ratio of anode/cathode. 
         [0116]    A recovery behavior according to an area ratio of anode/cathode is described with reference to  FIGS. 12 and 13 . 
         [0117]    In order to observe the recovery behavior according to the area of an anode, the area ratio of anode/cathode was changed to 0.42, 0.55, 0.67, 0.79, 0.93, and 1.02 by changing the area of the anode. The material of the anode was SUS 304, the flow rate was fixed to 7.7 M/s (145 LPM), and the total applied current was 51.3 A, twice the electrorefining reference current density (550 A/e). 
         [0118]      FIG. 12  shows a behavior when Au is recovered. The concentration of remaining gold linearly decreased to about 50 ppm, but thereafter, the reduction largely decreased, so the recovery efficiency is like decreasing while the earlier recovery efficiency is high and the remaining concentration of the gold lowers. When the area ratio of anode/cathode was smaller than 1.0, the remaining concentration of the gold after about 10 minutes passed was 140˜160 ppm, whereas when the area ratio was larger than 1, the concentration was 107.6 ppm. Accordingly, it was found that the area ratio of anode/cathode was larger than 1.0, the earlier recovery ratio was excellent. When the area ration of anode/cathode is larger than 1 even after about 22 minutes passed, a recovery behavior that the remaining concentration of the gold was 28.7 ppm, which was more excellent than 48 to 70 ppm in other cases. However, after 45 minutes passed, the remaining concentration of the gold was 5.1 ppm to 9.1 ppm, so the difference of the recovery ratio was largely decreased. 
         [0119]      FIG. 13  shows a recovery behavior when the area ratio of anode/cathode was 0.93 and 1.02 and the process time was increased up to 180 minutes. Similar to the result shown in  FIG. 12 , when the area ratio was larger than 1, the earlier recovery ratio was excellent, but it almost converged after 45 minutes. Further, when 180 minutes was reached, the remaining concentration of the gold decreased to 1.3 ppm for the area ratio of 1.02 and to 3.3 ppm for the area ratio of 0.93. 
         [0120]    From this result, it could be found that the area ratio of anode/cathode is preferably larger than 1 to increase the earlier recovery ratio. In detail, the area ratio of anode/cathode may be 1 to 1.5 or 1 to 1.2 
         [0121]    The configuration of a cathode according to a second embodiment is described in detail with reference to  FIG. 14 . 
         [0122]    In the second embodiment, a cathode connection hole  21   a  corresponding to the connection hole  14  of the body  11  is formed at the main cathode  20 . The sub-cathode  22  can be connected directly to a power through the connection hole  14  and the cathode connection hole  21   a.    
         [0123]    The configuration of a cathode according to a third embodiment is described in detail with reference to  FIG. 15 . 
         [0124]    In the third embodiment, the sub-cathode  22  is formed in a cylindrical shape. Accordingly, it can be quickly inserted into the main cathode  20 . When metal is recovered after a process, the sub-cathode  22  may be cut into a plate shape, if necessary. 
         [0125]    The configuration of a cathode according to a fourth embodiment is described in detail with reference to  FIG. 16 . 
         [0126]    In the fourth embodiment, projections  24  are formed on the surface of the sub-cathode  12  that is brought in contact with the main cathode  20 . The sub-cathode  22  can be more surely electrically connected to the main cathode  20  by the projections  24 . The projections  24  may be changed into various shapes and arrangements, for example, into the shape of a line or a lattice. 
         [0127]    A recovery behaviors according to the materials of an anode are described with reference to  FIGS. 17 and 18 .  FIG. 17  shows recovery behaviors when an anode coated with platinum and an SUS anode are used. Tests were performed under the condition that the area ratio of anode/cathode was 1.02 and the flow rate and the applied current were the same as those described with reference to  FIGS. 12 and 13 . The remaining concentration of gold converged to 1.3 ppm to 1.8 ppm at the two kinds of anodes.  FIG. 18  is an enlarged graph of the earlier stage for observing the earlier recovery behavior. 
         [0128]    The anode coated with platinum looks light have a slightly larger recover ratio at the earlier stage of the test, but it was found that there is little difference from the earlier stage in the recovery behavior. However, the surfaces of the anodes observed after the test were considerably different. That is, as for the SUS anode, there was considerable corrosion on the surface, so it is expected to have an adverse influence on the purity of the recovered gold. As for the anode coated with platinum, elution was maximally suppressed and the purity of the gold maintained almost at 100%. 
         [0129]    Recovery behaviors according to applied currents are described with reference to  FIG. 19 . 
         [0130]    Total currents of 38.5 A, 51.3 A, and 76.9 A were applied with respect to the area of an anode, by selecting 1.5 times, two times, and three times the electrorefining reference current density. In the tests, when a current of 76.9 A was applied, resistant heat was generated too much at the joint of the anode and the cathode, so a portion of a hydrocyclone was melted. Accordingly, the test for the current was stopped and recover behaviors under the other conditions were shown in  FIG. 19 . After 20 minutes passed, the remaining concentration of the gold was 26.4 ppm when the current was 51.3 A, and when the current is low at 38.5 A, the remaining concentration was 34.0 ppm, lower than that for the current of 51.3 A, but then, the convergence concentration was almost similar. That is, in the test for 180 minutes, the remaining concentrations were 1.5 ppm and 1.7 ppm, in which there is little difference. 
         [0131]    Considering only the recovery ratio of the gold, it is important to increase the current density, but considering the entire energy consumption efficiency, it is considered that it is preferable to perform recovering at a high speed under applied current of 35 A to 45 A. 
         [0132]      FIG. 20  is a graph showing recovery behaviors according to flow rates. 
         [0133]    Recovery behaviors at flow rates of 5.3 m/s (100 LPM) and 7.7 m/s (145 LPM) were observed. Similar to the tests with different current densities, in this case, the recovery ratio behaviors were different only at the earlier stages of the tests. That is, the remaining concentration of gold at the flow rate of 7.7 m/s was 26.4 ppm and 4.1 ppm, respectively, after 22 minutes and 45 minutes passed, and was 45.4 ppm and 6.3 ppm at 5.3 m/s. After 180 minutes passed, the remaining concentration was 1.5 ppm and 1.6 ppm, so it can be found that as the time passes, the recovery ratio behaviors become similar and converge to the same value. 
         [0134]    The recovery ratio behavior according to a change in applied current and the recovery ratio behavior according to a change in flow rate show similar tendencies, but when the earlier recovery ratio is important, it is considered that it is more effective to increase the flow rate rather than the applied current. 
         [0135]    Although the present invention has been described with reference to the exemplary embodiments illustrated in the drawings, those are only examples and may be changed and modified into other equivalent exemplary embodiments from the present invention by those skilled in the art. Therefore, the actual protection range of the present invention should be determined only by the accompanying claims.