IL24699A - Electrolytic apparatus and process for removing trace metals - Google Patents

Description

Patents Form Wo. 3 PATENTS AND DESIGNS ORDINANCE SPECIFICATION "ELECTROLYTIC APPARATUS AND PROCESS FOB REMOVING TRACE ETALSt( "/DAD ma s? jnon »Θ9 η©,·? κ |pnn» WE, MONSANTO COMPANY of 800 North Lindbergh Boulevard, St. Louis, Missouri, United States of America, a corporation organized and existing under the laws of the State of Delaware, United States of America, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement This invention relates to a process and apparatus for purifying salt solutions and is more particu-larly related to a process and apparatus for removing traces of metals from aqueous salt solutions electrolytically.

It has been demonstrated recently that certain organic materials may be dimerized electrolytically. In general, electrolytic dimerization of a desired organic compound may be performed in solution in a cathode compartment of an electrolytic cell having an anode and cathode compartment separated by an ion-permeable membrane. The anode and cathode compartments of such an electrolytic cell contain an anode and cathode, respectively, and upon anode the application of direct electric current to the reetnoef©- and cathode, dimerization of the organic compound takes place in good yield in the liquid catholyte solution. The catholyte solution generally comprises an aqueous solution of a salt which increases the solubility of the organic compound and its produced dlmer in water.

One such electrohydrodime ization of com-mercial significance is the production of adiponitrile, an organic intermediate of known commercial importance, from acrylonitrile, and in long-term continuous operation of a process for the electrohydrodimerization of acrylonitrile, recovery of product and unconverted acrylonitrile from the catholyte solution with recirculation of this aqueous salt solution electrolyte is necessary to make the process economically and commercially feasible. During the long-term continuous operation of such an electrohydrodimerization process, it has been found that the unwanted by-products Increases. Efforts to prevent this by the removal of deposited organic solids within the electrolytic cell, filtration of the circulated catholyte and selection of optimum operating conditions have not in maintaining been successful ffce-flWHL-afce-ift product yields at initial high levels and by-product formation at initial low levels.

After much work it has been determined that trace residual quantities of extraneous metals such as copper, nickel, silver^ and other metals plate or deposit on the cathode surface in the electrolytic cell* deposition,. and this cathode fouling by the (¾e»pe>e¾-S4»g of metals on the cathode surface was found to correspond with the un-desired lowering of product yield and increasing formation of unwanted by-products.

An object of this invention is to provide a process and apparatus for preventing cathode fouling in an electrohydrodimerization process.

Another object of this invention is to provide a process and apparatus for removing trace quantities of metals from aqueous electrolytes used in electrohydrodimerization reaction processes.

A further object of this invention is to provide a process and apparatus for removing trace quantities of metals from liquid solutions.

These and other objects of this invention will become apparent from the following description of the process and apparatus are described and defined with fpa¥¾*e¾ie¾-^in the claims.

The objects of this invention are accomplished removal of trace quantities of metal from liquid solutions. •¾3 The accompanying drawing, Pig. 1 is a sectional elevation of an electrolytic apparatus in accordance with this invention. The illustrated embodiment of the invention comprises tank 10 having bottom 13 and top 11+ connected with each other by adjustable spacer bolts 15 which are adjusted and supported by nuts 16 and 17. Gasket l8 is provided at the top end and bottom end of tank 10 between top li+ and tank 10 and bottom 13 and tank 10 for hydraulic seal. Tank 10 may be constructed of glass or a noncontaminating metal such as stainless steel and it is clear to those skilled in the art that other supporting arrangements for the top and bottom or other unitary configurations for tank 10, top li+, and bottom 13 are clearly within the scope of this invention. When tank 10 and its supporting parts are constructed of a noncontaminating metal such as stainless steel, the metal parts may be electrically connected to the cathode voltage to eliminate the possibility of corrosion of the tank equipment and the subsequent contamination of the aqueous salt solution being treated therein.

Bottom 13 is provided with liquid inlet 11 and top li+ is provided with liquid outlet 12. Although the direction of flow of liquid through tank 10 is not critical, design of tank 10 and liquid inlet and outlet 11 and 12, respectively, should be such as to provide substantially maximum residence time and uniform distribution of the liquid to be treated within tank 10. As is well known in the art, suitable means may be provided within tank 10 so that substantially all the volume may be utilized for liquid.

Anode 19 is located vertically substantially water^ within the center of tank 10 and surrounded by f¾y¾Fw.b5re»H3r permeable spacer 20. The anode material has little effect upon the operation of the cell and materials well known in the art for anodes such as carbon and platinum plated titanium as well as many others may be used with great success for the anode. A platinum plated anode has an advantage over a carbon anode of not causing minor contamination of an aqueous salt solution being treated with 1 sur- rounds [bydrau k¾^3rly permeable spacer 20 and substantially completely fills the remaining portion of tank 10. Hy- Water draalioal-ly permeable spacer 20 may be constructed or formed from any noncontaminating plastic or metal material or screening such as polyethylene, polypropylene, or stainless steel and others, and the quantity and the size of openings 26 in spacer 20 should be such as to permit maximum flow of liquid therethrough and the passage of no particulate cathode 21. The diameter of spacer 20 is not critical and in general should be maintained at a minimum dependent upon the diameter of anode 19. Spacer 20 should have an inside diameter sufficient to provide an annular space between anode 19 and the inside of spacer 20 large enough to prevent electrical shorting between the anode and particulate cathode and to permit the escape of generated gas, if any, as is well known in the art.

Particulate cathode 21 rests upon and is in i) support screen 2i prevents cathode 21 from entering inlet 11 and insulates the cathode therefrom-. Any suitable metal having a high hydrogen over-voltage may be used for particulate cathode 21 and cathode connector plate 22 . In one embodiment designed for the removal of trace quantities of silver and other metal ions which may contaminate an aqueous electrolyte salt solution, lead shot may be used as particulate cathode 21 and lead plate may be used as cathode connector plate 22 . Other metals possessing similar hydrogen over-voltages such as zinc, cadmium, and others may be used, either directly or as a plating material, for both particulate cathode 21 and cathode connector 22 . It is clear to those skilled in the art that the type of metal suitable for particulate cathode and cathode connector also depends upon the pH of the aqueous salt solution being contacted within the apparatus. The size and shape of particulate cathode 21 is not critical; however, whenever possible, configurations should be chosen to provide substantially maximum cathode surface area within a given volume tank while permitting substantially maximum liquid flow at low pressure drop through the tank.

Insulating gasket 23 may be provided to insulate cathode connector plate 22 from bottom 13 , if desired, and saturated calomel electrode 25 may be provided through top lij. within tank 10 to measure the electrical potential between the liquid within tank 10 and the cathode for reference and current control, if desired. If saturated calomel electrode is provided, care should be taken so as to have saturated calomel electrode but in a liquid space above particulate cathode.

In operation, an aqueous salt solution having trace quantities of metal impurity or impurities in a concentration up to 100 parts per million and higher concentrations may be treated in accordance with the apparatus and process of this invention by pumping the metal containing aqueous solution up through particulate - cathode, by means of inlet 11 and outlet 12 in the embodiment shown in Pig. 1, at a volumetric rate selected to permit a desired residence time, and direct electric current is passed through the apparatus, by means of anode 19 and cathode connector plate 22 in the embodiment shown, to maintain a desired cathodic potential on particulate cathode 21.

Aqueous salt solutions which may be purified in accordance with the process and apparatus of this invention comprise aqueous solutions of organic and inorganic salts which have cations that are not deposited at a potential less negative than the impurity metal ion. Aque- ous salt solutions suitable as feed stock for this process and apparatus include aqueous salt solutions of alkali metals, alkaline earths, organic quaternary ammonium salts, and organic amine salts. Particular aqueous organic salt solutions which have been purified successfully in accordance with the apparatus and process of this invention include tetramethy1ammonium toluene sulfonate solution, tetraethylammonium ethyl sulfate solution, tetra- ethylammonium benzene sulfonate solution, and many others.

The concentration of the salt in aqueous The upper limit of the concentration is set by the solubility of the salt in water and by the increased viscosity of a high concentration aqueous solution which may lower the rate of diffusion of the reducible ion. The lower limit of the concentration of a salt in aqueous solution is set by the conductivity of the aqueous salt solution which must be sufficient to support a plating current ■without producing an impractical voltage drop across the anode and particulate cathode of an apparatus.

In a preferred application concerning the electrohydrodimerization of acrylonitrile to adiponitrile wherein an aqueous solution of a quaternary alkyl ammonium alkyl sulfate or sulfonate such as tetramethylammonium toluene sulfonate or tetraethylammonium ethyl sulfate, may be used as catholyte, a salt solution having a concentration of 1+0- 80 may be purified in accordance with this invention. This concentration is most desirable because operation of the apparatus and process of this invention with the aqueous solution in this salt concen-tration range will not require additional concentration adjustment of the aqueous salt solution after purification and prior to its use as catholyte in an electrohydrodimerization cell.

The pH of a salt solution to be purified may be both greater than and less than 7 depending upon the metal ion to be removed and the materials of construction of the apparatus. In a preferred embodiment for the purification of an aqueous salt solution for use as a catholyte in an electrohydrodimerization process, the H of an aqueous salt solution to be purified Irelow 7 is to reduce the cathode voltage required for the discharge may prevent the most effective deposition of metal impurity ions on particulate cathode surfaces. When trace quantities of silver, copper, iron, and other easily deposited metals are to be removed from an aqueous salt solution, the pH of the solution may be lower than when the removal of trace quantities of more difficultly deposited metals such as lead, cadmium, nickel, and— etfw is necessary.

The geometry of the apparatus of this invention is of importance to obtain practical rates of deposition of trace metal impurities present in aqueous salt solutions at parts per million concentration levels. The ratio of the particulate cathode surface area to salt solution volume should be maintained at a practical maximum. Apparatus having a ratio of particulate cathode surface area to available salt solution volume of 10 to 500, with a preferred range of 30 to 100 square centimeters of cathode surface per cubic centimeter of aqueous salt solution volume within the apparatus have been found to be practicable, and in a preferred embodiment, a ratio of 38 square centimeters of particulate cathode surface area per cubic centimeter of aqueous salt solution volume may be obtained using number 8 lead shot which are spherical in shape and have a diameter of 0. β9 inches as particulate cathode.

In a single anode type apparatus such as is disclosed in the embodiment of Fig. 1, the annular space between the anode surface and the inside of the surface to a particulate cathode surface, is important because of non-uniform current density produced by this type of particulate cathode apparatus. This annular distance may be up to 2.5 inches, and it is preferred that the annular distance be from 1.0 inch to 2.0 inches. In a preferred embod excellent rates silver and copper from an aqueous organic salt solution, and apparatus with a 1.5 inches annular space may be utilized for large scale commercial equipment.

It is clear that apparatus having two or more anodes arranged vertically in a spaced relationship with each other is within the scope of this invention. In multiple anode apparatus the geometric arrangement of the anodes should be such as to provide substan ially multiples of the apparatus shown and described in Fig. 1. In such an embodiment, the distance between any two anodes should not exceed substantially twice the annular distance described above for a single anode type apparatus so that the distance between any given particulate cathode surface and an anode will not exceed an annular distance of 2.5 inches .

It is clear to those skilled in the art that annular distances greater than that specified above for both single and multiple anode apparatus will not make the apparatus inoperable per se but may permit the existence of areas in the annular space, which is in excess of that specified, which are substantially dead electrically. These substantially dead areas will be have the deleterious effect of reducing the efficiency of the process and apparatus for the removal of trace metallic impurities.

In a continuous process, efficiency of the removal of trace quantities of metal may be changed by the residence time and the temperature of the solution being treated. Residence time is defined as the liquid volume of the apparatus surrounding particulate catalyst divided by the flow rate of an aqueous solution to the apparatus. In general, the greater the residence time and the higher the temperature of the aqueous salt solution being purified the greater the efficiency of the removal of trace quantities of metal a given voltage and amperages for an apparatus. It has been found that a residence time of 10 seconds is adequate; however, a residence time of at least 30 seconds is preferred for m&sb treatment of most aqueous salt solutions. It is clear, also, that care should be taken to operate at a temperature below the boiling point of water and the vaporizing point of any of the components of the aqueous solution being purified.

In general, direct electrical current to the anode and cathode of an apparatus should be sufficient to supply adequate voltage to plate or deposit unwanted metals on particulate cathode but not high enough to cause appreciable electrolysis of the water of the aqueous solution with the attending liberation of gaseous hydrogen at particulate cathode surfaces. It has been found that with^, cathode voltages of -0.5 volts to -1.5 voltsfiw reference removal of trace quantities of unwanted metals in apparatus having particulate cathode surface area to salt solution volume of 10 to 500 square centimeters per cubic centimeter of liquid and an annular spacing up to 2.5 inches.

The effects of temperatures and residence times on the efficiency of silver removal from an aqueous salt solution in accordance with the process and apparatus of this invention are shown in Example I.

EXAMPLE I An aqueous solution consisting of 35 parts by weight water and 65 parts by weight tetrameth l-ammonium toluene sulfonate was prepared, and to this solution, sufficient silver was added to obtain a silver concentration of 10 parts per million. The apparatus of the embodiment shown in Pig. 1 having an annular space of 1.5 inches and particulate cathode of No. 8 lead shot having a diameter of .09 inch in a bed depth of 10 inches was used. The apparatus was operated at a cathode voltage of - 1.2 volts to a saturated calomel electrode Five ( 5 ) samples of the aqueous solution of tetramethyl-ammonium toluene sulfonate were treated on a continuous basis at varying temperatures and varying residence times, and the silver concentration in the effluent from the apparatus for each of the samples was measured. Table 1 shows results for samples.

Table 1 Residence Time Temp, of of Sample, $ Silver Sample Sample, °C Minutes Removed 1 1+0 0.5 6 2 1+0 1.0 68$ The removal of trace quantities of copper from aqueous salt solutions in accordance with the process of this invention is demonstrated by Example II.

EXAMPLE II A 6 % aqueous solution of tetramethyl-ammonium toluene sulfonate was prepared containing 11 parts per million copper. The apparatus of the embodiment shown in Pig. 1 having an annular space of 1.5 inches and using Wo. 8 lead shot having a diameter of 0.09 inches in a bed depth of 10 inches as particulate cathode was used to purify the solution. The prepared aqueous solution was passed through the apparatus at a liquid temperature at of 25°C. c&r a volumetric rate sufficient to provide a residence time of 10 minutes within the apparatus while a cathode voltage of -1.2 volts, in reference to a saturated calomel electrode, was maintained upon the apparatus. Analysis of the effluent treated salt showed a copper concentration therein of 2 parts per million.

EXAMPLE III The apparatus of Examples I and II was used on a continuous basis for controlling the level of silver in an aqueous catholyte solution being circulated continuously through an electrolytic cell operated for the electrohydrodlmerization of acrylonitrile to adiponitrile .

The catholyte solution was an aqueous solution of tetra-methylammonium toluene sulfonate in a concentration of 65%t by weight. The apparatus was operated to provide a temperature of 25°C for the aqueous salt solution within the apparatus and the cathode voltage was maintained at Plow to the apparatus was controlled to provide a residence time of 10 minutes therein, and samples of feed salt solution to the apparatus and effluent from the apparatus were taken daily for 8 days and analyzed for silver present therein. Table 2 below reports the results of the analysis for each of the samples in parts per million of silver.

Table 2 Silver Present in Parts per Million Sample Feed Effluent 1 0.5 Less than 0.1 2 0.5 Less than 0.1 3 0.3 Less than 0.1 k 0.9 0.2 5 0.3 Less than 0.1 6 0.9 0.1 0.6 Less than 0.1 0.1+ Less than 0.1 The advantages of the electrolytic process and apparatus of this invention for the removal of trace metal impurities from aqueous salt solutions are many.

The apparatus described is capable of producing practical rates of deposition of trace metallic impurities at rates which are not obtainable with previously known electrolytic apparatus. Aqueous salt solutions having concentrations of silver or other metals in a range of 0.5 to 1 part per million or less may be obtained rapidly and economically from the effluent of the apparatus. The electrolytic process and apparatus of this invention for controlling of trace quantities of metals permit/ a markedly increased service life of cathodes in electrohy-drodimerization processes. Further, the apparatus and process of this invention permit the purification of commercial economical and efficient means.

As many apparently widely different embodiments of this invention may be made without depart ing from the spirit and scope thereof, it is to be under stood that we do not limit ourselves to the specified embodiments thereof except as defined in the appended claims.

Claims (1)

1. 2i*699/2 1 · A process for removal of trace quantities of metals from an aqueous solution containing said trace quantities, characterized by subjecting said aqueous solution to direot electric current for a period of at least ten seconds in the presence of a particulate cathode at a cathode voltage of **0·5 volts to -1 ·5 volts with re erence to a saturated calomel electrode and at least one anode* 2· The process of claim 1, characterized in that said process is continuous and said particulate cathode has from 10 to 500 square centimeters of surface area for each cubic eentimeter of said solution surrounding said particulate cathode* 3. The process Of claim 2, characterized in that said aqueous solution is a salt solution. k» The process of claim 3» characterized in that said aqueous solution containing said trace quantities is subjected to direct electric current at a temperature below the "boiling point of said solution, 5· The process of any of claims 2- , characterized in that substantially all particulate cathode surface area is within a distance of 2#5 inches from the surface of said at least one anode. 6. An electrolytic apparatus for use in the process of any of claims 1-5» characterized by comprising a containing means having a particulate cathode and at least one anode, means for supplying direct electric current to said particulate cathode and said at least 24699/2 one anode, said particulate cathode having sufficient surface area to provide from 10 o 500 square centi** meters of said surface area for each cubic centimeter of void space surrounding said particulate cathode within said containing means, and substantially all said surface area being within a distance of 2·$ inches from the surface of said; at least one anode. 7. The apparatus of claim 6# characterized in that said containing means has a liquid inle and liquid outlet, and said one anode is insulated from said particulate oathode. 8. The apparatus of claim 7» characterized in that said particulate Oathode is lead shot· 9· A process for the removal of trace quantities of metals f 6m anaqueous solution containing said trace quantities substantially as described in the herein Examples. 10· An electrolytic apparatus for use in the process claimed in any one of claims i-5 and 9 substantially as described and illustrated in the accompanying drawing.