Abstract:
The leaching and precipitation of noble metals when circulating an electrolyte through a vertical cylindrical electrolytic cell comprising a fixed granular catalyst bed and a three-dimensional cathode filled with activated carbon granules are performed in the same step. Because the electrochemical leaching process and the electrochemical sorption process are performed simultaneously, the consumption of electric energy is reduced and the use of equipment becomes easy. An apparatus for extracting noble metals from inorganic granular waste catalysts comprises a vertical type electrolytic cell, conduit lines, an electrolyte circulating pump, a unit for automatically maintaining the required acidity of the electrolyte being circulated, a filter for filtering activated carbon particles from the electrolyte, control valves, and stop valves. The electrolytic cell comprises a heat exchanger for heating the electrolyte being circulated, an insoluble anode and a three-dimensional cathode filled with activated carbon granules.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to electrochemical hydrometallurgy for reducing noble metal waste, and more particularly to a method and apparatus for extracting noble metals from inorganic granular waste catalysts. 
         [0003]    2. Description of Related Art 
         [0004]    A method for extracting noble metals from inorganic granular waste catalysts means a method comprising: electrochemically leaching noble metals in an electrolytic cell; precipitating the noble metals in a cathode; and then separating the noble metals from the cathode. 
         [0005]    In a prior method for dissolving and extracting noble metals from waste catalysts [Prior-Art Document 1: U.S. Pat. No. 4,775,452, 1988, “Process for dissolution and recovery of noble metals”], leaching is carried out in the anode chamber of a horizontal type electrolytic cell. The horizontal type electrolytic cell comprises a fluorine resin-based anion exchange membrane that separates the electrolytic cell into two chambers, anode and cathode chambers. The bottom of the anode chamber comprises a diffusion lattice. In the first step of extracting noble metals, a granular waste catalyst fixed bed is introduced into the anode chamber, and an electrolyte is circulated upward through the diffusion lattice. As the electrolyte, hydrochloric acid, nitric acid, sulfuric acid or an acidic compound is used. Preferably, a 5-35% hydrochloric acid is used. Herein, the anode and cathode membranes are positioned along the side of the electrolytic cell in parallel with the flow direction of the electrolyte. 
         [0006]    The porous anode of stable size is made of titanium coated with noble metal oxide. The cathode is made of titanium. The electrolytic cell is 85 mm in length, 115-250 mm in width and 200-1000 mm in depth. In the second step after leaching noble metals, the electrolyte is 6-50-fold diluted and the noble metals are precipitated, whereby the noble metals are separated to activated carbon granules present in a fluidized state in the cathode space of a second electrolytic cell including a cationic membrane. 
         [0007]    The disadvantage of this extraction method is that the efficiency for extracting noble metals decreases as the distance between the anode and the cathode increases. This is because hydrochloric oxide moves upward in parallel with the anode membrane by the electrolyte flow and its concentration decreases as it goes away from the surface of the anode membrane toward the cathode. For this reason, the leaching of novel metals is mainly performed in the anode bed close to waste catalysts. 
         [0008]    Because the electrolyte is pumped once through the electrolytic cell, a large amount of solution flows out, such that additional equipment is required, thus increasing economic losses. 
         [0009]    The apparatus that is used to realize the extraction method according to Prior-Art Document 1 is energy-intensive, has low efficiency in extracting noble metals and requires the use of high-concentration (5-35%) acid (mainly hydrochloric acid). 
         [0010]    A prior art for extracting noble metals from inorganic waste catalysts, sludge, ore concentrates and other metals [Prior-Art Document 2: Russia Patent No. 21199646, 1997, “Method for extracting noble metals and apparatus for carrying out the same”] has a characteristic in that the leaching of noble metals and the precipitation of a filled cathode during the circulation of an electrolyte through a fixed filter bed or fluidized bed of leached particles are carried out simultaneously in the same step. 
         [0011]    The extraction of noble metals is carried out simultaneously through an electrolyte cell including a leaching block and a filled cathode. A 10-25% sodium chloride aqueous solution containing a required amount of hydrochloric acid and alkali is used as an electrolyte. Herein, noble metals are deposited on the filled cathode. The leaching block comprises one or several reactors which are provided with conventional units for introducing and discharging a leaching material. The leaching block includes an electrolyte cell provided with a pH-measuring chamber and an automatic discharge control unit. 
         [0012]    After noble metals have been deposited, the filled cathode is separated from the electrolytic cell and sent to a recycling process. For metal extraction, the filled cathode is incinerated. Metal extraction may also be performed without separating the cathode from the electrolytic cell. In this case, noble metals are dissolved by passing an electric current of opposite polarity through the cathode, thus obtaining a high-concentration chloride solution. 
         [0013]    The disadvantages of the method according to Prior-Art Document 2 are that the leaching process is complicated and the functional technical blocks are separated from each other to make the design of the apparatus difficult. 
         [0014]    The prior art for extracting noble metals from inorganic waste catalysts, ore concentrates and other metals [Prior-Art Document 3: Russia Patent No. 21989477, Sep. 12, 2000, “Method for extracting noble metals”] is technically closest to the present invention and comprises carrying out leaching in an electrolyte, circulating the electrolyte along a closed circuit through a filling material, precipitating metals in an electrolytic cell, and then separating noble metals from a cathode according to a conventional method, wherein the metals treated in the filled form are placed in the space between the electrodes of the electrolytic cell. The electrochemical leaching of noble metals can be activated by previously developing the polarity reversal of the electrodes. For this purpose, the electrodes are changed into a large-capacity multipolar electrode which allows the anode dissolution of metals regardless of the amount of material. Meanwhile, by inhibiting the formation of a brown cloud in the cathode, hydrated anionic chloride compounds of noble metals which are formed in a process of leaching the filling material are prevented from being burned out destroyed by a fire with the cathode, and the electrolyte is circulated from the anode to the cathode at a rate suitable for such conditions. Herein, acidic water containing 0.3-4.0% hydrochloric acid is used as the electrolyte. 
         [0015]    In order to study the efficiency of said noble metal extraction method and examine the disadvantages thereof, the present inventors constructed an electrolytic cell ( FIG. 1 ) corresponding to the description of Prior-Art Document [3]. As described in Prior-Art Document [3], the electrolytic cell has a horizontal structure, the effective cross-sectional area of the electrolytic cell is 1600 cm 2  (40 cm×40 cm), and the length of the filling material is 100 cm. The filling material in the space between the electrodes is fixed with a dielectric lattice. The parameters of the experiment are consistent with those described in Prior-Art Document [3]. 
         [0016]    According to a study conducted by the present inventors using said prototype, the influence of polarity reversal on the rate and depth of leaching was insignificant. The leaching time increased by the time during which the polarity was developed. Also, noble metals were not formed as a compact foil on the surface of the titanium cathode and were precipitated in the form of niello which was easily separated from the cathode surface by rising hydrogen bubbles. Hydrogen bubbles separated from the surface of the cathode membrane rose to the surface of the electrolyte and formed convection current. As a result, the noble metal niello in a fluidized-bed state was placed in the cathode space of the electrolytic cell. Such conditions make the noble metal niello returning to the filling material of the waste catalyst through the lattice holes. In addition, the noble metal niello moves to the anode space of the electrolytic cell by the electrolyte flow being circulated. The filling material sample was analyzed after conducting the experiment, and as a result, it could be seen that the leaching of noble metals at the bottom of the filler was incomplete. This is because the rate of circulation of the electrolyte from the anode to the cathode is not constant along the cross section of the electrolytic cell. The electrolyte circulation rate is slower in the lower portion of the electrolytic cell than in the upper portion. This can be clearly explained because the waste catalyst particles of the lower portion of the electrolytic cell are under the pressure of the particles of the upper portion. This reduces the size of the free space in which the electrolyte circulation between the particles of the lower portion of the electrolytic cell occurs. Such conditions impose limitations on increasing the depth of the electrolytic cell in order to use the electrolytic cell in industrial applications. In addition, the area of the electrolytic cell in which the electrolyte evaporates is large. If the above-described process is carried out at 70° C., anode hydrochloric acid oxide actively evaporates, and thus additional means for reducing negative effects on the environment are required. Also, the acidity of the solution decreases, because hydrochloric acid is used to partially dissolve catalysts in the electrolytic process. It was found that, when the acidity (pH) of the solution was more than 1, the rate of leaching significantly decreased. 
         [0017]    In order to maintain acidity at a constant level, it is required to periodically discharge the electrolyte from the electrolytic cell and to supplement hydrochloric acid to a required concentration. 
       PRIOR-ART DOCUMENTS 
       [0000]    
       
         1. U.S. Pat. No. 4,775,452, 1988, “Process for dissolution and recovery of noble metals” 
         2. RU Patent No. 2119964, 1997, Method for extracting noble metals and apparatus for carrying out the same” 
         3. RU Patent No. 21989477, Sep. 12, 2000, “Method for extracting noble metals” 
       
     
       BRIEF SUMMARY OF THE INVENTION 
       [0021]    It is an object of the present invention to develop a an effective method for extracting noble metals from granular waste catalysts by leaching and construct an apparatus which is easily used to realize this method. 
         [0022]    This object is accomplished by the inventive method for extracting noble metals from inorganic granular waste catalysts and other materials, which includes leaching noble metals in the space between the electrodes of a vertical electrolytic cell. The leaching is performed by an electrolyte which circulates upward from the anode to the cathode along a closed circuit. The precipitation of noble metals is performed in a three-dimensional cathode filled with activated carbon granules. Unlike the prototype, a hydrochloric acid having an acidity (pH) of 1 is used as the electrolyte and contains 0.1-5% aluminum chloride (AlCl 3 ). The leaching of noble metals and the precipitation thereof in the three-dimensional filled cathode are performed simultaneously in the same step. The noble metals are separated from the cathode by incinerating the activated carbon or dissolving the precipitated metals in the anode. 
         [0023]    The electrolytic cell according to the present invention allows waste catalysts to be electrolyzed in a granular form without being powdered. The present invention can greatly improve the yield of extraction of platinum-group metals from metal compound-supported granular catalysts to extract almost all the amount of the metals, reduces electricity consumption and extraction time, and has improved ecological compatibility. Also, the present invention has improved working efficiency, because it can minimize the amount of liquid waste to be recycled and allows a large amount of waste catalysts to be introduced and leached. In addition, the reliability and electrical safety of the electrolytic cell can be increased and the repair and maintenance of the electrolytic cell is simple and convenient. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0024]      FIG. 1  is a cross-sectional view of an electrolytic cell according to the prior art. 
           [0025]      FIG. 2  is a cross-sectional view of an apparatus for extracting noble metals according to the present invention. 
           [0026]      FIG. 3  is a cross-sectional view of a vertical electrolytic cell according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In the present invention, an apparatus ( FIG. 2 ) for extracting noble metals from inorganic granular catalysts and other materials has a vertical flow electrolytic cell  1  including an insoluble anode  3  and a three-dimensional filled cathode  4 . Charging of the vertical flow electrolytic cell is performed using a charging block  18 . The anode and cathode spaces are connected with conduit lines. An electrolyte is circulated by a pump  6  which operates at a predetermined speed which is controlled by a flow meter  7 . In order to prevent activated carbon powder from penetrating from the three-dimensional filled cathode into the anode space, a filter-press  19  is placed in a circulation line. The acidity of a solution in the circulation line is measured by a pH meter  21  and maintained at a constant level by an automatic hydrochloric acid discharge controller  24 . The apparatus also includes stop valves  8 ,  9 ,  10 ,  11 ,  12  and  13 . 
         [0028]    The apparatus for extracting noble metals operates in the following manner. 
         [0029]    The vertical flow electrolytic cell is filled with granular waste catalysts from which organic mixtures have been removed. Novel metals contained in the catalysts in an amount of 0.05-5% should be in a regenerated (metal) state. Corks (valves)  10  and  13  are opened, valves  8 ,  11  and  12  are closed, and the automatic discharge controller  24  is off, and in this state, an electrolyte consisting of a hydrochloric acid solution having a pH of 1 and 0.1-5% aluminum chloride (AlCl 3 ) is fed into the electrolytic cell through an inlet  16 . The electrolyte is fed along a high-speed electrolyte pumping line  15 . After feeding the electrolyte into the apparatus, the electrolyte is heated by a tube heater  25 . As the electrolyte reaches a predetermined temperature, the valve  10  is closed and the valve  12  is opened. At this time, the electrolyte circulates through the flow meter  7  at a predetermined speed. The charging block  18  is used to set the current value of the electrolytic cell. Hydrochloric acid of amount required to maintain the acidity of the electrolyte at a pH of 1 is discharged by the automatic charge controller  24  to the space in the front of the anode of the vertical electrolytic cell. Conditions set for performing this process can be maintained using a conventional automatic control system. After a sufficient amount of extracted noble metals have been precipitated in the three-dimensional filled cathode  4 , the cathode is disassembled from the vertical electrolytic cell and incinerated. In the case in which the precipitated noble metals are dissolved in the anode, the process is stopped, the electrolyte is poured out of the electrolytic cell, and the filled cathode is detached and washed with hot water. After washing, the cathode is placed in a tube containing a titanium electrode, the tube is filled with hydrochloric acid or nitric acid, and then anode polarity is applied to the three-dimensional carbon electrode supported with noble metals. In the process in which the polarity is changed, the metals deposited on the activated carbon granules are gradually dissolved. 
         [0030]      FIG. 3  shows a cross-sectional view of the electrolytic cell according to the present invention. 
         [0031]    The vertical flow electrolytic cell comprises a vertical cylindrical body  101  of a three-dimensional multipolar electrode including regenerated catalyst granules and additionally comprises a distributor  103  for distributing electrolyte flow, wherein the distributor is provided with an electric heater  104  for maintaining a predetermined solution temperature. Herein, the circulation direction of the electrolyte flow facing upward has the same axis as the direction of the electromagnetic field in the space of the electrolytic cell. 
         [0032]    As noble metals are leached from a three-dimensional multipolar electrode chamber  108 , chlorine formed in a horizontally placed anode  106  is distributed throughout filled granular waste catalysts of dielectric metal oxide nature by the upward flow of the electrolyte. A right-angle outlet  110  is placed on the lower side of the cylindrical body structure of the electrolytic cell, whereby the granular catalysts can be discharged in a simple and rapid manner after the metal leaching process. 
         [0033]    The lower end of the outlet  110  is located on the same plane as a protecting/supporting dielectric lattice  109  placed on the anode  106  of the multipolar electrode chamber, whereby labor can be minimized and the granular catalysts can be completely discharged. 
         [0034]    A corrosion-resistant dielectric supporting lattice  105 , which has mechanical rigidity and is placed between the electrolyte flow distributor  103  and the cylindrical body  101 , acts as a barrier for the filled granular catalysts, thereby preventing the granular catalysts of the multipolar electrode of the electrolytic cell (space between the electrodes) from penetrating (flowing out) from the vertical cylindrical body into the conical electrolyte flow distributor  103  (space in the front of the anode). 
         [0035]    The anode  106 , which is horizontally disposed and made of a titanium lattice, distributes the total flux density of an oxidizer, which is formed in the anode, evenly throughout the multipolar electrode. A protective film for the titanium anode, which is made of iridium dioxide (IrO 2 ), prevents either anodic oxidation (formation of a dielectric layer of titanium dioxide (TiO 2 )) caused by oxygen-containing acid anions or electrochemical corrosion upon oxidation caused by oxygen-free acid anions. 
         [0036]    The protecting/supporting dielectric lattice  109  is placed between the titanium lattice of the anode and the regenerated granular catalyst (three-dimensional multipolar electrode) and made of a material (Teflon) having corrosion resistance, heat resistance and mechanical rigidity. It prevents the coating of the anode made of iridium dioxide (IrO 2 ) from being mechanically destroyed by an abrasive material for the granular catalysts. 
         [0037]    A diaphragm (made of polypropylene)  114  that separates the cathode of the electrolytic cell from the three-dimensional multipolar electrode chamber minimizes the precipitation of materials such as aluminum oxide on the cathode surface, such that dissolved metals are more completely removed from the electrolytic cell by the electrolyte flow. 
         [0038]    A pair of dielectric supports  113  that are horizontally placed between the anode chamber of the electrolytic cell and the three-dimensional multipolar electrode including the regenerated granular catalysts fixes the interval between the anode and the cathode, allows an electromagnetic field to be distributed evenly in the three-dimensional multipolar electrode, and maintains the anode chamber in the upper portion of the cylindrical space of the electrolytic cell. 
         [0039]    Because an electric current is applied to the horizontally placed anode  106  through metal bars  107  that perforate the multipolar electrode chamber, the sealing of the electrolytic cell is guaranteed and the electrical safety and convenience of use of the electrolytic cell are improved. 
         [0040]    The center of the conical flow distributor  103  is provided with an inlet  117 , such that the leached electrolyte is supplied directly to a heat source. The upward thermal convention of the electrolyte flow forms a thermal cushion in a space close to the anode in a state in which the flow rate is not high, thereby preventing the cold electrolyte from penetrating into the cylindrical chamber  108  of the three-dimensional multipolar electrode including the regenerated granular catalysts. 
         [0041]    An overflow outlet  118  placed in the upper portion of the cylindrical cathode  111  of the vertical flow electrolytic cell discharges a noble metal salt solution and determines the maximum amount of the electrolyte in the electrolytic cell to prevent the electrolyte from overflowing. 
         [0042]    An insulation material  119  surrounding the cylindrical and conical portions of the electrolytic cell minimizes heat loss and reduces energy consumption when carrying out the electrochemical leaching process. 
         [0043]    An electrolytic cell lid  120  having a temperature lower than the vapor temperature of the acidic electrolyte allows vapor to be condensed on the inner surface thereof. This reduces electrolyte loss and heat loss and increases the environmental safety of the electrochemical leaching process. 
         [0044]    An outlet  121  placed at the electrolytic cell lid  120  removes hydrogen formed in the cathode and prevents hydrogen from being accumulated in the body of the electrolytic cell not filled with the electrolyte, thereby improving the operational stability of the electrolytic cell. 
         [0045]    The electrolytic cell comprises a cylindrical body  101 , which is placed on a support  102  and connected to the conical flow distributor  103  (space in the front of the anode). The conical flow distributor  103  is provided with an electric hater  104 . The cylindrical body is divided from a corrosion-resistant dielectric supporting lattice  105  having mechanical rigidity. On the supporting lattice  105  is placed the anode  106  made of a titanium lattice that is protective-coated with iridium dioxide (IrO 2 ). An electric current is applied to the anode through the metal bars  107  that perforate the multipolar electrode chamber  108 . On the anode  106  is placed the supporting/supporting dielectric lattice  109  made of a material (e.g., Teflon) having corrosion resistance, heat resistance and mechanical rigidity. The lower portion of the cylindrical multipolar electrode chamber structure of the electrolytic cell is provided with an outlet  110  for discharging the granular catalysts, and the lower end of the outlet  110  is placed on the same plane as the protecting/supporting dielectric lattice  109  on the anode. The cathode space block  111  placed in the upper cylindrical portion of the vertical flow electrolytic cell is placed on a pair of dielectric supports that are horizontally placed between the cathode chamber of the electrolytic cell and the three-dimensional porous electrode including the regenerated granular catalysts. The cathode body is made of a cylindrical dielectric material. The bottom of the cylindrical body consists of a porous bottom  113  on which a porous diaphragm  114  is placed. On the diaphragm is provided a titanium cathode  115  to which an electric current is supplied through metal bars  116 . The electrolytic cell includes an inlet  117  for introducing a leaching electrolyte, an outlet  118  for discharging a noble metal salt solution, and a thin dielectric lid  120  including an outlet  121  for discharging gas. 
       EXAMPLE 
     Example 1 
     Example of Operation of Vertical Electrolytic Cell 
       [0046]    In order to leach a noble metal-containing inorganic (metal oxide) dielectric granular waste catalyst (e.g., a 0.02-0.03% palladium-alumina catalyst), the catalyst is introduced through the top of the cylindrical portion  101  of the electrolytic cell. Before the catalyst is introduced, the cathode compartment  111  is dissembled from the electrolytic cell. A leaching electrolyte (e.g., 3% HCl aqueous solution) is introduced into the conical flow distributor  103  through the lower inlet  117 , and the inside of the distributor is heated to a predetermined temperature by the electric heater  104 . The heated electrolyte laminar flow passes through the dielectric supporting lattice cell  105 , is oxidized in the horizontal anode lattice  106 , and passes through the porous protecting/supporting lattice  109  to the three-dimensional porous electrode including the regenerated granular catalyst. The noble metal is leached from the granules into the electrolyte solution in the form of a salt during the process in which the oxidized electrolyte solution passes through the granular catalyst bed. This leaching process occurs when overvoltage is significantly decreases as a result of a decrease in electric current density, because the working area of the three-dimensional multipolar electrode is large. After the noble metal salt solution has been discharged from the granular waste catalyst bed, it is discharged from the vertical flow electrolytic cell body through the overflow outlet  118 . The cathode space is filled with the electrolyte through the porous diaphragm when the electrolytic cell is first filled with the electrolyte. The diaphragm controls the movement of the noble metal ions to the cathode space, thereby reducing the amount of noble metal ions that precipitate in the cathode. The electrolyte that evaporates is condensed on the cold wall of the thin lid  120  of the electrolytic cell, and hydrogen that is separated from the cathode is removed through the outlet  121  from the space of the cylindrical portion of the electrolytic cell, which is not filled with the electrolyte. After completion of the leaching process, the electrolyte is discharged through the lower outlet  118 , and the granular catalyst is discharged through the outlet  110 . 
         [0047]    After carrying out the above Example, the granular catalyst was examined. As a result, it was found that the amount of platinum-group metal remaining on the granular catalyst after subjected to electrochemical leaching was not more than 1 ppm in the lower portion of the electrolytic cell and 1-10 ppm in the upper portion.