PROCESS AND APPARATUS FOR PRODUCING METALS AT POROUS HYDROPHOBIC CATALYTIC BARRIERS

Abstract: Process and apparatus for recovery of hydrogen-reduced metals from aqueous salt solutions at a porous hydrophobic catalytic barrier (2). Previously, the prior art provided a diffusion barrier between a hydrogen anode and a cathode to prevent metal deposition on the hydrogen catalyst anode surface. This disclosure provides direct metal production at the catalytic barrier (2) by feeding hydrogen (5) at one surface of the barrier and recovering metal from a second solution contacting surface of the barrier (9). Metal reduction and deposition occur in the absence of an external electric circuit.

Description: PROCESS AND APPARATUS FOR PRODUCING METALS AT POROUS HYDROPHOBIC CATALYTIC BARRIERS The present invention relates to the hydrogen reduction , at porous hydrophobic catalytic, barriers of hydrogen-reducible ions in solution , and more particularly , though not exclusively, to the recovery of metals from aqueous solutions of such metal ions at ordinary temperatures . . The term "hydrogen-reducible ion " ' , as used in this specification and ■ the appended claims , means an ion produc¬ ing a positive potential - when it is reduced in aqueous solu¬ tion in the electrochemical reaction in which hydrogen gas goes to hydrogen ion, - i. e. H_ -+ 2H + 2e ; that is , the reduced ion has an oxidation potential below that of hydrogen . Recoverable metal ions yielding "hydrogen-reduced metals " in this reaction , as this term is used herein , include such metals as copper , silver , gold , the platinum metals and the like; whereas "partially hydrogen-reducible ions" , include the ferric , mercuric, ' permanganate and similar ions which are reαυced in solution from a higher valence state to a lower valence state without normally being reduced to the metal . OMPI Finally, the term "ordinary temperature"-, as used in the sp ification and claims, is intended to connote ambient tempera tures and above; but in any event, temperatures below about the boiling point of water. Underlying the invention in one of its important aspects is the discovery of an unexpected behavior of poro hydrophobic catalytic barriers, void of external electrical connections, when directly contacted on opposite surfaces by hydrogen gas and, for example, an aqueous solution of a hydrogen-reducible metal ion. Considering, for example, t interesting case of a copper ion solution, such as copper sulfate, the art is replete with different types of electr deposition techniques for extracting the copper metal from the solution, generally as a cathodic deposition. In, for example, U.S. Patent No. 3,793,165 of common assignee " herewith, a method is described for electr depositing copper and the like from an aqueous copper salt solution by means of a hydrogen anode. Care is taken to prevent physical contact of the copper salt solution and the anode, however, to avoid deposition ' of copper on said anode, instead of the desired deposition of massive electr plated copper on the cathode, as otherwise it was expected that the process would become self-arresting because the hydrogen anode becomes rapidly covered by the metal, part of which is chemically reduced at the anode, thus requirin O -3- re oval of the anode and cleaning before reuse. (Col. 1, lines 37-42) . Surprisingly, we have now found that upon contact¬ ing, at ordinary temperatures, one face of an appropriate porous hydrophobic catalytic barrier with an aqueous solu¬ tion of a hydrogen-reducible metal ion, such as the before- mentioned copper sulfate solution, and applying hydrogen to the other face of said barrier, the reduction and deposition of copper occurs in the absence of any external electrical circuit. It lias further been found that the buildup of copper on the solution-contacted face is not arrested by the initial layer of copper, but continues to any desired level of removal of said copper from said solution. While above-described with reference to the exemplary illustration of copper metal, this technique has also been found to be useful for the reduction of a partially hydrogen-reducible ion, such as, for example, the reduction of the ferric to the ferrous ion or the mercuric to the mercurous ion, as later described, with recovery of the reduced ion either as a deposition on the barrier as in the mercurous case, or in solution, as in the ferrous reduction. In all cases, however, the reduction is carried out by supply¬ ing hydrogen to one ' face of the barrier and the ion solu¬ tion to the other face of the barrier. The apparatus does not require, and hence excludes, the usual circuitry of -_J - an electrochemical cell (including two separate electrodes and an external electrical path) , and the apparatus is the fore sometimes referred to hereinafter as "circuitry-free" An object of the invention, accordingl} 7 , is to provide a new and improved process and apparatus for the recovery of hydrogen-reduced metals and ions, employing hydrogen reduction at hydrophobic catalytic barriers. A further object is to provide such a novel proc and apparatus that is particularly useful for the recovery of metals from aqueous solutions thereof. Still another object is to provide for the impro recovery of metals by depositing the same upon such barri from a dilute aqueous solution thereof and to enable the subsequent removal of the deposited metal by well-known techniques. 7*JI additional object is to provide an improved process whereby the metal deposited on the barrier may als be dissolved from he barrier by catalytic or electrochem¬ ical oxidation in the presence of a small volume of electr lyte to produce a concentrated aqueous solution thereof which can then be subjected to metal electrowinning or electrorefining and the like. Other arid further objects will be explained here ¬ inafter and are more particularly delineated in the appended claims. In summary, however, from one of its broader aspects, the invention embraces a process for recovering hydrogen-reduced metals or ions from an aqueous solution of an ionized salt thereof, that comprises, providing a porous hydrophobic catalytic barrier, contacting one surface of the barrier with said solution, and applying hydrogen gas to said other surface, thereby to produce the hydrogen-reduce metals or ions at said one surface. Preferred and best mode embodiments and details are later presented. The invention will now be described with refer¬ ence to the accompanying drawings, Fig.1 of which is a schematic longitudinal section of a preferred apparatus for carrying out the process of the invention; Fig. 2 is a similar view of a modification; and Fig. 3 is a graph demonstrating the rate of metal deposition attainable with the invention in specific example involving copper. In general, suitable barriers for the purpose of this invention are porous to hydrogen and at the same time O PI -6- hydrophobic to the agueouε solutions so that the liguid a gas phases remain separate while permitting the hydrogen ionization reaction to take place on the catalyst of the barrier. The barrier prevents intermingling of hydrogen and solution, thereby allowing separate control of flow rates and easy confinement of hydrogen gas, at or near at pheric pressure, for good hydrogen utilization ( for examp by recycling the hydrogen) . Any catalyst for the hydrogen ionization reacti is suitable,but platinum catalysts are preferred because their corrosion resistance and durability. In particular the platinum catalysts having a particle size substantial , o o between 15 A and 25 A when deposited on finely divided hi surface area carbon carriers, (herein referred to as 15- o . A pla in m-on-carbon catalysts) as described, for example in U.S. Patents Nos. 3,992,331, 4,044,193 and ,059,541 common assignee herewith, have been found to be especiall effective in amounts between0.05 and0.5 g/sq. ft. of barrier, in catalyzing the hydrogen ionization reaction a ordinary temperatures. -7- The hydrogen-ionizing catalyst is preferably deposited, together with a hydrophobic agent such as a fluorinated hydrocarbon polymer (herein referred to as Teflon on an electrically-conducting porous substrate such as a metal screen, a porous carbon, and, especially a carbon yarn cloth as described in our copending application Serial No. 074,470 , comprising, for example, PANEX-type carbon cloth of the Stackpole Fibers Company. The resulting barrier has the structure of a typical gas diffusion ( hydrogen) elec¬ trode ; even though, in accordance with this invention, the hydrogen reduction thereon is carried out in the absence of an external electrical circuit. It is often advantageous to use such a hydrogen electrode in the case of, for example, copper recovery because the copper deposited on the barrier ' can then be removed from the barrier by an electrolytic process, such as in a copper refining cell in which the copper-covered barrier is used as the anode. The recovery of silver from, for example, a silver nitrate solution can benefit similarly from the use of barriers of such carbon- based substrates. However, electrically insulating porous substrates, such as a porous ceramic, a glass cloth, or a porous glass fiber mat, are also suitable ' substrates for the illustrative -8- platinu -on-carbon-Teflon mixture (or other catalyst hydro phobic mixture) . niformly deposited thereon, since the red tion of the present invention, as before stated, takes pla in the absence of external circuitry. When, for example, copper is deposited on such a barrier from a dilute coppe sulfate solution, it can then be removed from the barrier by contacting the coppered face with a small volume of a solution of sulfuric acid and contacting the gas face wit air or oxygen, whereby the. oxidation and subsequent dissol tion of the copper into the acid is catalyzed again at or inary temperatures, by the platinum catalyst of the barri The copper removed from the barrier into the small volume of solution has now been concentrated many fold over the original dilute solution, thereby constituting a suitable for conventional electrowinning of copper or the method o above-mentioned U.S. Patent No. 3,793,165. This techniqu is, of course, also applicable to the above-described hyd gen electrode type barrier. The removal of, for example, copper by anodic redissolution or by oxidation, can be carried out by keep the barrier in a fixed position and alternating (1) the flows of dilute and concentrated solutions and (2) the fl of hydrogen and D.C. current or air/oxygen, respectively. -9- When, however, a flexible cloth-based barrier is used, the barrier may conveniently be continuously moved from the dilute solution with hydrogen behind the barrier, to effect copper deposition on said barrier, to, for example, a tank containing concentrated solution to effect copper removal therefrom, as above explained. The metal deposited on the barrier, if the barrier substrate is combustible as in the case of the before-mention carbon cloth , may be removed by incinerating the same. Scraping or mechanical stripping may also be useful in some instances. The recovery of platinum metals, especially of platinum and palladium, which are excellent catalysts for the EL ■ * ■ 2H + 2e reaction, is advantageously carried out on a barrier which bears a platinum-on-carbon- Teflon mixture or a palladium-on-carbon-Te lon mixture, respectively. There is then built up on said barrier, by the hydrogen reduction of this invention, a deposit of platinum or pallad¬ ium, respectively, in amount far in excess of the initial amount present on said barrier. In view of the high values of platinum and palladium, by comparison with the cost of carbon and Teflon, simple incineration of th e barrier yields directly the recovered platinum or palladium, respectively, as the residue. In the case of platinum, especially, incineration is indued the preferred method of recovery from the barrier because of the outstanding resis ance of platinum to redissolution by chemical and electro¬ chemical oxidation at ordinary temperatures. In the absence of agitation of the aqueous solu¬ tion } the rate at which the hydrogen-reduced metal, for example, copper, is deposited on the barrier is diffusion- controlled. In the case of dilute solutions of metal, for example, copper sulfate solutions containing between abou 0.1 and about 5 grams/liter copper, it is usually importan to agitate the solution or otherwise provide substantiall turbulent flow of solution onto the barrier, to overcome the limitation of the low diffusion rate of a stagnant dilute solution. Thus, in the case of metal recovery fro dilute solutions,the apparatus is advantageously provided with means to agitate the solution as by directing the fl of solution onto said barrier under rapid flowing conditi A typical circuitry-free apparatus suitable for the purpose of this invention comprises the barrier assem a cross section of which is schematically shown in Fig. 1. A gas plenum 1 is bounded on one side by, for example, a flat sheet of plastic 3, such as of Lucite„and on the oth side by the flat hydrophobic catalytic barrier 2 of the -11- invention , with the two sides of the plenum being kept apart by gaskets 4 as of rubber, plastic or the like. The gaskets are provided with gas inlet and outlet ports 5 and 5 ' , res¬ pectively , with hydrogen applied to the right-hand face of the barrier 2 through the inlet 5. A gasket 6 holds the hydrophobic barrier 2 in place , with the assembly being clamp together by means , for example , of clamp 7. V7hile flexible gasket 4 serves to make the plenum 1 gas-tight, the gasket 6 can be of any material , flexible or stiff , enabling the holding of the hydrophobic barrier 2 in place. A pump 8 feeds the metal or other ion solution through piping 8 * to a distributor head 9 , the latter being designed to permit even (and , when turbulent flow is desired , rapid ) flow of solution onto the preferably total left-hand face of the barrier on which the metal , for example , is to be deposited. Fig . 2 shows , in cross section , an alternate assembly for flowing the aqueous solution onto the barrier 2 under controlled and, when desired , rapid flow rates. Here the barrier 2 with its gasket 6 constitutes one side (the right side) of a solution plenum 10 bounded on the other side by a plastic sheet 11. The solution plenum 10 is provided with a solution inlet 12 and an outlet 12 ' , with the solution . being circulated through the plenum 10 at turbulent flow rates when desired, by means of the pump 8. Other shapes than flat barriers are also suita By way of example, a porous carbon or ceramic tube or pi can be made into a porous hydrophobic catalytic barrier coating its outside with the hydrogen-ionizing catalyst- mixture. "When a hydrogen-reducible metal ion-bearing so tion is flowed over the outside of such a tube or pipe b and hydrogen gas is fed to the inside thereof, the metal is deposited on the outside of the tube or pipe. Other flow and barrier configurations will also occur to those skilled in the art. One case of special importance that is particu useful with the invention is the recovery of copper from dilute copper-bearing leach solutions such as are obtain by sulfuric acid leaching of low grade mine and waste du materials. Such solutions contain less than about two g per liter of copper and also small amounts of the order 3 grams or less of total iron per liter, the iron being present as ferric and ferrous ions in the approximate ra of, for example, about 1:2. Copper is now commonly rec from such solutions by cementation; i.e. by reduction o Cu and Fe with scrap iron, which recovery is costl and causes accumulation of iron in dumps. We have reduced Fe in dilute ferric sulfate solution; and we have recovered copper from dilute copper ++ sulfate solution and from dilute solution containing Cu , +++ and Fe by the above-described hydrogen reduction process and apparatus, such copper-bearing solutions containing as little as 0.2-0.3 grams per liter (g/1) of copper, as shown in the following Examples 1 and 3, illustrating a preferred method of recovering hydrogen-reduced metal ' s in accordance with the invention. Example 1 A two— lenum apparatus.consisting of the gas plenum shown schematically in Fig. 1 and the solution plenum shown schematically in Fig. 2 was provided with a porous hydro- phobic catalytic barrier having an exposed area 2" by 2" in size, said barrier having been prepared by the following procedure. A platinum-on-carbon sample was prepared substan¬ tially in accordance with Example 1, col. 9 of U.S. Patent No. 4,044,193, the pH being adjusted to 3 during the prep¬ aration. The air-dried material, containing 9.9% by weight -14- of platinum on Vulcan XC-72 carbon, which carbon has a ε ace area of approximately 200 2/g, was compounded with by weight of wet-proofing fluorinated hydrocarbon, herei referred to as Teflon, whereby a typical catalytic carbo Teflon mixture was formed. The compounding may advantag use the technique described in U.S. Patent No. 4,166,143 the present assignee. In this example, 1.0 gram of the platinu -on-Vulcan carbon was suspended in 60 ml of dist water containing 1.4 g/1 of lanthanum sulfate. The susp sion was ultrasonically dispersed and 11.75 ml of the aqu colloidal Teflon dispersion described in col. 1 lines 35 of U.S. Patent No. 4,166,143, containing 85 g/1, was add and the stirring was continued for 5 minutes, whereby th Teflon was completely flocced, forming the uniform catal carbon-Teflon mixture. The floc-containing liquid s pension was then filtered, leaving, on the filter, the m ture in a form of a paste suitable for coating the subst The coating procedure consisted in applying 0.38 guts of paste to 9 sq. inches of the above-described carbon clot PANEX PWB-3, the paste being spread uniformly on the sur and into the open pores of the cloth. The coated fabric then heated to 340°C for about 20 minutes. The resultin electrode-type structure had a platinum loading of 0.32 of electrode area, the platinum being in the form of pa predominantly in the 15-25 Angstrom range. Hydrogen was fed to the gas plenum at the rate of about 50 ml/min. under a pressure of 5 inches of water above atmospheric. 500 ml of copper sulfate solution containing initially 0.296 g/1 copper and having an initial pH of 1.95 was recirculated at a flow rate of about 2500 ml/min. through the liquid plenum 10, with the stream of solution directed across the left-hand face of the barrier 2. The solution was mainta ' ined at substantially constant room temperature of about 25 C. One ml aliquotε of the recirculating solu¬ tion were taken at successive time intervals and the copper concentration in the aliquots was determined by atomic absorption. The rate of copper removal, from this solution is shown in Table 1. Table 1 Time Grams/liter Cu +'+ ^ in Feed 0.296 5 mins. 0.267 15 mins. 0.250 30 mins. 0.215 " 60 mins. 0.122 90 mins. 0.074 120 mins. 0.049 150 mins. 0.028 -16- The cumulative rate of copper deposition (corre ponding to the data of Table 1) is shown in Fig. 3, curve The barrier war, weighed initially and after com pletion of the test, and it was found that about 0.135 gr of copper had been deposited, in substantial agreement wi the experimentally determined loss of copper in the solut Deposition of a layer of copper was observed within 15 seconds after the flows of solution and hydroge had started. This initial deposit of copper on the cata¬ lytic platinum did not hinder the continued reduction and deposition of copper thereon, as is evidenced by the fact that the reaction was nearly brought to completion (i.e. to 95S. copper removal) at the end of 3% hours. This finding was indeed surprising since metall copper is not a catalyst, at ordinary temperatures, for t reaction H_ → 2H + 2e and the initial copper layer would have been expected to arrest the reaction after covering catalytic platinum of the barrier. One conceivable expla tion for this unexpected behavior is the existence of wha might be termed a shorted electrochemical fuel cell coupl of a hydrogen-on-platinum anode (right side of barrier 2) a metallic copper cathode (left side of barrier 2) wetted copper ion-containing electrolyte. This effectively shor couple continues to function so long as the barrier is we by the electrolyte containing some dissolved copper ions. Whatever the explanation, however, it is observed in practice that the reduction and the resulting build up of copper con¬ tinues up to nearly completion, if desired. This phenomenon, of course, makes the method of the invention most useful in the recovery of copper and like metals. The techniques before-described have been used to remove the deposited copper, as later more fully described. Example 2 The method of Example 1, using the apparatus and barrier thereof, were repeated except that the recirculat¬ ing solution was a ferric sulfate solution containing 3.00 gm/1 of Fe . Here the Fe + + was reduced to the Fe ion. Table 2 shows the concentrations of Fe produced by this reduction as a function of time. The reduction is associated with the formation of hydrogen ions, the pH column of Table 2 showing the increasing acidity of the solution accompanying the increase in Fe concentration.- OfPI - -18- In this experiment, the ferrous ion produced b the reduction remained in solution, in contrast to the deposited copper of Example 1. Table 2 3 g/1 Fe Time (mins.) g/1 Fe pH 0 .00 1.9 15 .21 1.89 45 .688 1.75 75 1.03 1.71 105 1.30 1.65 135 1.52 1.59 Example 3 The method of Example 1, using the apparatus barrier thereof, was repeated except that the sulfate s ution contained 0.30 gm/1 of copper and 3.00 gm/1 of fe ion. The appearance of a film of copper was again obse after 12 seconds on the left-hand solution face of the rier 2. Thee rraate ooff F - e = + — ++ - to - F - e + + + + reduction is shown ±n Table 3, and the corresponding rate of copper deposition ±β shown in Fig. 3f curve 2 _ Table 3 +++ 3.0 g/1 Fe plus 0.9 g/1 Cu ++ Time (mins.) g/1 Fe ++ pH 0 .00 1.90 5 0.134 1.90 15 0.335 1.85 30 0.668 1.80 60 1.27 1.68 90 1.69 1.60 120 2.27 1.55 It is seen that the presence of Fe slowed copper deposition initially, but faster copper deposition occurred after substantial reduction of Fe -20- The following examples illustrate additional variations of the hydrogen reduction process of the inv tion involving a typical electrically insulating barrie substrate and typical different ionization catalysts, well as hydrogen-reduced metals other than copper. Example 4 The method of Example 1 was repeated with the apparatus thereof, using a glass-cloth based barrier 2 having a glass cloth substrate 12 mils thick and having about 19 yarns per inch in each of the warp and fill di tions, and provided with a catalyzed and hydrophobic coa ing by the method of Example 1. The barrier was thus s stantially the same as the barrier of Example 1 with res to platinum particle size, platinum loading and Teflon loading. The copper deposition rate obtained on this barrier using the copper sulfate solution of Example 1, shown in curve 3 of Fig. 3. It is seen that the coppe reduction was catalyzed on this barrier, but that the c responding rate is distinctly slower than that attained the carbon cloth-based barrier of Example 1 (Fig- 3, cur -21- E>:ample 5 The method of Example 1 was again repeated with the apparatus thereof, except that a conventional platinum- o on-carbon catalyst was substituted for the 15-25 A platinum- on-carbon catalyst of Example 1. The conventional catalyst was prepared ' by the known technique of impregnating a sampl of Vulcan XC-72 carbon with a solution of chloroplatinic aci followed by evaporation, drying and hydrogen reduction, the ingredients being selected in amounts to produce a platinum on-carbon containing 10% of Pt by weight. The barrier of this example was -then prepared by the method described in Example 1 using this platinum-on-carbon and loading the barrier with .34 mg Pt/cm , i.e. about the same as that of Example 1. The copper deposition rate on this barrier usin the solution of Example 1, is depicted in curve 4 of the graph of Fig. 3. It is seen that the copper ion reduction is catalyzed by this conventional platinum catalyst but o that it .is less effective than the preferred 15 - 25 A platinum-on-carbon catalyst, when both are used in substan¬ tially equal amounts. Examp3 α G The method of Example 1 was further repeated except that a palladium-on-carbon catalyst was substituted o for the 15 - 25 A platinum catalyst of Example 1. The palladium-on-carbon catalyst was prepared by impregnating a sample of Vulcan XC-72 carbon with an aqueous palladium sol, followed by evaporation, drying and hydrogen reduction the ingredients being selected in amounts to produce a palladium-on-carbσn containing 10% palladium. The palladiu sol was prepared by solvent extraction, with an organic a i of an aqueous palladium nitrate solution. The barrier 2 of this example was then prepared by the method " described in Example 1, using this palladium-on-carbon catalyst and 2 loading the barrier with 0.25 mg Pd/cm ; i.e. nearly the amount, by weight, of the platinum of Example 1. The coppe deposition rate on the barrier using the solution of Exampl 1 is depicted in Fig. 3, curve 5. It is seen that the copper ion reduction is catalyzed by the palladium catalyst but that it is significantly less effective than the platin on-carbon catalysts, when all are used in amounts of the same order. (Note that more gram-atoms of Pd per square inch were on the barrier than gram-atoms of Pt per square inch) . O Example 7 The method of Example 1, using the apparatus and barrier thereof was repeated except that 2 liters of a silver nitrate solution containing 1.5 g/1 Ag was recircu¬ lated. Build-up of silver was extremely rapid. A loosely adhering silver deposit of 0.53 grams was obtained on the left face of the barrier in 15 minutes. Example The method of Example 1 was repeated except that 2 liters of a chloroplatinic acid solution containing 1.5 g/1 Pt was recirculated. .About 600 mg of firmly adher ¬ ing platinum was deposited on the barrier in two hours. The 2" x 2" barrier had an initial platinum loading of about 10 mg as the 15 - 25 A platinum-on-carbon catalyst. Example 9 The method of Example 1 was repeated except that 2 liters of a mercuric chloride solution containing 1. 5 g/1 Hg ■ >'-+ was recirculated. About 140 mg of firmly adhering mercurous chloride , Hg Cl„ ,was deposited on the barrier in two hours . A control experiment using the barrier of Exam except that no platinum or other hydrogen ionizing catal was used thereon and using the method and solution of Example 1, resulted in no copper deposition. In general, as before discussed, metal deposit on the barrier adheres thereto and needs therefore to be removed therefrom to be put into useable form. With a s ficiently thick deposit on the barrier, such as the silv deposit of Example 7, mechanical stripping is one such t nique which is especially useful when the original silve ion-containing solution is not contaminated by other hyd reducible metal ions. As previously described, however, is often desireable to redissolve the metal from the bar into a concentrated solution suitable for either electro winning or electrorefining. In one instance, for exampl the area bearing the copper deposit obtained in Example above was contacted with about 5 ml of 1.5 molar sul uri acid on its coppered face and air was supplied to the ga plenum. Dissolution of the copper from the barrier int this small volume of acid occurred quite readily , - B thereby producing an acid copper sul fate solution contain¬ ing a copper concentration of about 27 g/1 of copper . This procedure coupled with the hydrogen reduction of Example 1 thus resulted in a nearly 100 fold concentration of copper. The concentrated solution is suitable for electrowinn ing in conventional cells as well as in the cell described in U . S. Patent No . 3 , 793 , 165 of common assignee . In an alternate procedure , the coppered barrier of Example 1 was used in a conventional electroref ining cell as the anode . Here the copper deposit was rediεsolved into the conventional electrorefining electrolyte, a substant¬ ially equivalent amount of copper being electrodeposited as massive cathodic copper . Typical experimental conditions for such an electrode refining operation are , for example , shown in Table 27 , pages 150-151 of Electrochemical Engin¬ eering , by C . L. Man tell , 4 th Edition , McGraw-Hill Book Com¬ pany, New York , 1960 . Whereas the redissolution of copper from the barrier by means of air or oxygen as illustrated above is applicable to barriers having either a conducting or a non¬ conducting substrate , the stripping of the copper from the barrier by electrorefining requires that the barrier co priε an electrically-conducting substrate that is the barrier having the structure of a hydrogen anode . -26- In the case of recovering platinum, or like- n metal capable of catalyzing the H_ • + 2H + 2e reaction, above-described incineration technigue iε the preferre method of removing platinum or the like from the barri For example, the barrier of Example 8 with its platinu depoεit was incinerated at about 800 C leaving a resid of platinum weighing 0.5812 grams. The above examples are merely illustrative o preferred modes of carrying out the invention. Clearl ■ many variations are possible including the recovery of gold, palladium and other hydrogen-reduced metals from aqueous solution thereof, use of different hydrogen io izing catalysts, such as, for example, tungsten carbid for the H- →- 2H _r + 2e reaction and operation of the pro in the range of temperatures above freezing but below ing of the aqueous solution. Further, the method is b means limited to dilute solutions of the hydrogen-redu metals, though here the process is especially advantag These and other variations which will occur to those s in the art are considered to fall within the scope of invention as defined in the appended claims.

Claims: 1. 7. proccεc for recovering hydrogen-reduced metals or ions from an aqueous solution of an ionized salt thereof, that comprises, providing a porous hydrophobic cata ¬ lytic barrier, contacting one surface of the barrier with said solution, and applying hydrogen gas to one surfac ' of the barrier thereby to produce the hydrogen-reduced metals or ions at said one surface. 2. A process as claimed in claim 1 and in which the solu ¬ tion is an ionized salt of said .metal and the hydrogen- reduced metal is deposited on th said one surface of the barrier. 3. A process as claimed in claim 2 and in which the said metal is selected from the group consisting of copper, silver, gold and the platinum metals. 4. A process as claimed in claim 2 and in which said metal is copper and said solution contains copper ions in amount less than about 5 grams/liter of the solution. 5. A process as claimed in claim 2 and in which the depos ¬ ited metal is removed from the barrier by mechanical stripping. 6. A process as claimed in claim 2 and in which the depos¬ ited metal is removed from the barrier by dissolving the deposited metal to produce a concentrated solution of the metal and electrowinning the metal therefrom. 7. A process as claimed in claim 2 and in which the barrie is an electrode and the deposited metal is removed from said barrier by redissolving the deposited metal into an electrorefining electrolyte and electroplated there¬ from using the barrier as an anode. 8. A process as claimed in claim 2 and in which the depos ¬ ited metal is removed from said barrier by incineratin the same. 9. I. process as claimed in claim 2 and in which the depos ited metal is removed from said barrier by contacting the deposited metal with an electrolyte solution and the said other barrier surface with air or oxygen. 10. The process of claim 1 wherein said solution comprises a partially hydrogen-reducible ion. 11. The process of claim 10 wherein said partially hydroge reducible ion is selected from the group consisting of ferric, mercuric and permanganate ions. -29- 12. The process 'of claim 1 wherein said catalytic barrier is formed by uniformly depositing on and adhering to a porous substrate a substantially uniform mixture of particles of a wet-proofing agent and catalytic noble metal particles. 13. The process of claim 12 wherein said noble metal is platinum and said wet-proofing agent is Teflon. 14. The process of claim 12 wherein said platinum is present in amount between 0.04 and 1.0 gra /sg. ft. of said barrier. 15. The process of claim 12 wherein said porous substrate is electrically conducting. 16. The process of claim 15 and in which the barrier is circui ry-free. 17. The process of claim 15 wherein said substrate is sel¬ ected from the group of porous carbon, carbon cloth, porous metal and metal screen. 18. The process of claim 12 wherein said substrate is of insulating material. -30- 1 . The process of claim 1 wherein said aqueous solution contacts said barrier under turbulent flow conditions. 20. The process of claim 1 wherein said hydrogen reduction is effected at ordinary temperature. 21. The process of claim 1 wherein said metal is copper, said copper having been depoεited from a first solutio containing a concentration of copper ionε of less than 5 grams/liter of solution, and wherein the said other surface of the barrier is contacted by air or oxygen a the deposited copper on said one surface of the barrie is contacted by an electrolyte acid solution in amount to yield a second redissolved copper ion solution con¬ taining at least five times the copper ion concentra¬ tion of said first solution. 22. The process of claim 21 and including the additional step of subjecting said second copper ion solution to electrowinning. 23. Circuitry-free apparatus for recovering a hydrogen- reduced metal having, in combination, a porouε hydro¬ phobic catalytic barrier, means for supplying hydrogen to one face of said barrier, and means for supplying a solution of an ionized salt of said metal to a face of said barrier. -31- 24. The apparatus of claim 23 wherein said barrier comprises a porous substrate containing an adhered mixture of particles of a wet-proofing agent and catalytic noble metal particles. 25. The apparatus of claim 24 wherein said noble metal is platinum and said wet-proofing agent is Teflon. 26. The apparatus of claim 25 wherein said platinum is present in amount between 0.04 and 1.0 gram/sg. ft. of said barrier. 27. The apparatus of claim 24 wherein said substrate is electrically conducting. 28. The apparatus- of claim 27 wherein said substrate is selected from the group of porous carbon; carbon cloth, porous metal and metal screen. 29. The apparatuε of claim 24 wherein said substrate is of insulating material. 30. The apparatus of claim 23 wherein said means for supply¬ ing said solution comprises turbulence-promoting means. ■ 32- 31. -A process for recovering hydrogen-reduced metals or from an aqueous solution of an ionized salt thereof comprises, providing in contact with the solution a hydrophobic catalytic barrier, and flowing hydrogen upon the catalytic barrier to reduce metals or ions said solution at surface portions of the catalytic contacting the solution.

Inventors: ALLEN R, JUDA W, LINDSTROM R
Applicants: PROTOTECH CO
Application year: 1980
Main classification: C25C1
IPC codes: C22B3, C22B5, C22B11, C22B15, C22B43, C22B47, C22C11, C23F1, C25C1