PROCESS AND APPARATUS FOR RECOVERY OF PRECIOUS METAL COMPOUND

Abstract: A process and an apparatus for recovering a precious metal compound, in which the precious metal compound is separated from impurity metals by use of a reverse osmotic membrane (6), and the separated precious metal compound is recovered in a concentrated state through electrodialysis (9) employing anion-exchange and cation-­ exchange membranes, whereby the precious metal compound in a process liquid can be recovered efficiently. When the recovered precious metal compound is added in the as-is form to precious metal plating solution, it is possible to control the plating solution.

Description: This invention relates to a process and an apparatus for recovering a precious metal compound. More particularly, the invention relates to a process and an apparatus for recovering efficiently a precious metal compound by use of a reverse osmotic membrane and anion-exchange and cation-exchange membranes, and to a process and an apparatus for controlling a precious metal plating solution employing the recovery process. BACKGROUND OF THE INVENTION Rare precious metals such as gold, silver, palladium and rhodium have been playing an increasingly important roll in the modern technology, and there has been a keen demand for wasteless and effective utilization and consumption of the precious metals. In practice, however, various process liquids in precious metal plating processes, such as waste liquids of precious metal plating solutions, wash liquids and stripping solutions, which contain precious metal compounds, have not been so effectively utilized. Where the precious metal is "gold", for instance, there is known a method of recovering gold from a gold-­ containing process liquid by electolyzing the liquid to deposite gold on a cathode (see Japanese Patent Application Laid-Open No. 55-l64045 (l980)). According to the method, however, gold is recovered in the form of metal (metallic gold), which needs to be converted through complicated and troublesome reprocessing into gold potassium cyanide or the like for reuse thereof in plating or the like. There is also known a method of recovering gold contained in a process liquid by adsorbing gold onto an ion-exchange resin or activated carbon. In this method, however, it is not easy, after adsorption of gold on the ion-exchange resin or the like, to recover gold through dissolution thereof; therefore, the thus adsorbed gold has hitherto been recovered by incinerating the ion-­ exchange resin or the like. Accordingly, the method requires incineration of the expensive ion-exchange resin each time of recovery of gold, leading to cost problems, and also has the problem that gold is recovered in its metallic form which cannot be easily reused, as in the above-mentioned method. BRIEF SUMMARY OF THE INVENTION An object of this invention is to provide a process and an apparatus for recovering a precious metal compound, by which a precious metal compound can be recovered, solely and in the as-is form, from a process liquid containing the precious metal compound. Another object of this invention is to provide a process and an apparatus for controlling a precious metal plating solution by adding the recovered precious metal compound to the plating solution. The above and other objects, features and advantages of this invention will become more apparent from the following description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE l is a schematic representation of an apparatus for recovery of a precious metal compound according to Embodiments l to 4 of this invention; FIGURE 2 is a schematic representation of an apparatus for recovery of a precious metal according to Embodiment 5 and 6 of this invention; and FIGURE 3 is a schematic representation of a cyan treating tank according to Embodiment 7, in the state of being disposed on the upstream side of and connected to a reverse osmotic membrane device. DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, a process liquid in a precious metal plating process, such as a waste liquid of the plating solution, a wash liquid and a stripping solution, contains large amounts of organic acids, impurity metals and the like in addition to the precious metal compound. Therefore, it is necessary first to separate the precious metal compound from the process liquid (liquid to be treated). Paying attention to the fact that a reverse osmotic membrane has permselectivity to precious metal compounds, the present inventors first osmosed the process liquid by a reverse osmotic membrane to thereby separate the precious metal compound solely from the liquid. Next, for reuse of the thus separated precious metal compound, it is necessary to concentrate the compound while maintaining the form of the compound. Thus, according to this invention, the separated precious metal compound is concentrated by electrodialysis employing anion-exchange and cation-exchange membranes, before recovery thereof. Use of the ion-exhange membranes does not involve the adsorption-and-exchange process as occurring in the case of using an ion-exchange resin, but, instead, involves permeation of ions as they are; namely, cations permeate the cation-exchange membrane, while anions permeate the anion-exchange membrane. As a result, the precious metal compound can be recovered as a concentrate which retains the as-is form of the compound. In addition, the concentrate obtained by the electrodialysis can be dried by evaporation to recover the precious metal compound in a crystalline or powdery form. Furthermore, because the precious metal compound can be recovered in its as-is form, regardless of whether it is recovered as a concentrate or in a crystalline or powdery form, the recovered precious metal compound can be added directly to a precious metal plating solution to control the plating solution. Embodiment l One embodiment of this invention will now be explained below while referring to the drawing. FIGURE l illustrates the device for recovery and reuse of gold potassium cyanide, as the "precious metal compound". In the figure, a plating tank l is filled with a gold plating solution which contains gold potassium cyanide. A recovery tank 2 contains l00 ℓ of a wash liquid 3 for washing a plated body, as the "process liquid". After plating, the plated body is washed with the wash liquid 3 in the recovery tank 2. A pump 4 and a pre-filter 5 for filtering out solid impurities are provided. A reverse osmotic membrane device 6 comprises a cellulose acetate based reverse osmotic membrane 6a, and the wash liquid 3 is fed into the device 6 under a predetermined pressure by the pump 4. Because the osmotic quantity across the reverse osmotic membrane device 6 is not so large in view of the properties of the reverse osmotic membrane 6a, of the wash liquid fed under pressure to the device 6, a major portion which does not permeate the membrane 6a is returned into the recovery tank 2 via a circulation line 7. An intermediate tank 8 is provided for momentarily storing l0 ℓ of the liquid 3a osmosed through the reverse osmotic membrane device. An electrodialysis device 9 comprises dilution chambers l0 and concentration chambers ll formed by partitioning the interior of the device 9 alternately by anion-exchange membranes A and cation-exchange membranes K. Into the dilution chambers l0 is circulated the wash liquid 3 (osmosed liquid 3a) as a dilute liquid from the intermediate tank 8 by a pump l2, whereas into the concentration chambers ll is circulated 3.5 ℓ of a dilute solution of potassium hydroxide and potassium cyanide as a concentrated liquid l3 from a concentration tank l5 by a pump l4. On one end side of the alternately arranged dilution and concentration chambers l0 and ll is provided a cathode l6, and on the other end side is provided an anode l7. The cathode l6 and the anode l7 are disposed respectively in electrode chambers l8 and l9, into which a dilute solution of potassium hydroxide as an electrolyte 20 is circulated via a pump (not shown). The membrane facing the anode l7 is constituted of the cation-exchange membrane K to prevent leakage of the cyanogold complex ion. The concentrated liquid l3 in the concentration tank l5 can be fed appropriately into the plating tank l according to the concentration of gold in the plating solution in the tank l, by a pump 2l provided as a "feeding device". A drying device 22 is provided such that the concentrated liquid l3 taken out of the concentration tank l5 is subjected to vacuum distillation to crystallize gold potassium cyanide, which is washed with cold water and is dried in a hot air stream to give powdery gold potassium cyanide of high purity. An ion-exchange resin tower 23 is provided for the final recovery of the remaining low-concentration gold potassium cyanide when the concentration of gold potassium cyanide in the intermediate tank 8 has become extremely low with the progress of the concentration by the electrodialysis device 9. The operations of the apparatus will now be described in detail below. Analysis of the wash liquid 3 as the process liquid in the recovery tank 2 revealed that the liquid 3 contained 500 mg/ℓ of gold in the form of gold potassium cyanide, l5 g/ℓ of potassium citrate, 25 mg/ℓ of iron, 25 mg/ℓ of nickel and l0 mgℓ of copper. The wash liquid 3 was circulated to the reverse osmotic membrane 6a under a pressure of 28 kg/cm² by the pump 4 to cause the liquid 3 to gradually permeate the membrane 6a. When the osmosed liquid 3a having just permeated the membrane 6a was analyzed, neither impurity metals such as iron, copper and nickel nor potassium citrate or the like could be detected in the osmosed liquid 3a, and only l gℓ of gold potassium cyanide could be detected in the liquid. This shows that the cellulose acetate based reverse osmotic membrane 6a has permselectivity to gold potassium cyanide. The liquid 3a thus osmosed through the membrane 3a was then collected into the intermediate tank 8, from which the liquid 3a was circulated to the dilution chambers l0 of the electrodialysis device 9 at a rate of 6/ℓ,min by the pump l2, upon each collection of l0 ℓ into the tank 8. Meanwhile, the concentrated liquid l3 was circulated from the concentration tank l5 into the concentration chambers ll of the electrodialysis device 9 by the pump l4, and the electrolytes 20 in the electrode chambers l8 and l9 were circulated respectively. While circulating the osmosed liquid 3a and the concentrated liquid l3 respectively, electolysis was carried out between the cathode l6 and the anode l7 at a current density of 0.2 A/dm² for 40 min, and after replacement of the osmosed liquid 3a, the same operation was repeated four times. In the osmosed liquid 3a, gold potassium cyanide is dissociated to be potassium ions and cyanogold complex ions, and when an electric current is passed through the liquid, the potassium ions in the dilution chamber l0 permeate the cation-exchange membrane K into the concentration chamber ll, whereas the cyanogold complex ions permeate the anion-exchnage membrane A on the opposite side into the concentration chamber ll. Therefore, both the potassium ions and the cyanogold complex ions constituting gold potassium cyanide move successively from the dilution chambers l0 into the concentration chambers ll, resulting in that gold potassium cyanide in the osmosed liquid 3a is steadily concentrated into the concentrated liquid l3. Thus, although the gold concentration in the osmosed liquid 3a in the intermediate tank 8 before concentration was only l g/ℓ, the contentrated liquid l3 in the concentration tank l5 was found, by analysis, to contain gold in a high concentration of ll g/ℓ in the form of gold potassium cyanide. The above test results are summarized in the following table. The concentration of gold potassium cyanide in the osmosed liquid 3a can be detected from the applied voltage and the electric conductivity, whereby the end point of ion exchange can be known. Gold potassium cyanide can be supplied and the gold plating solution can be controlled, by sequentially returning the concentrated liquid l3 into the plating tank l by the pump 2l. Further, a gold plating solution can be newly prepared by adding predetermined additives to the concentrated liquid l3 itself. Moreover, the concentrated liquid l3 may be dried by the drying device 22 to give powdery gold potassium cyanide, which may be added to the plating tank l or may be used to prepare newly a plating solution. Embodiment 2 A waste liquid from gold palting, in place of the wash liquid 3, was treated directly as the process liquid while using the same apparatus and conditions as in Embodiment l, except that electrolysis was conducted only two times. The test results are summerized in the following table. Because the process liquid was the waste liquid from plating, the concentrations of components in the process liquid were high, and a little potassium citrate and the like remained in the concentrated liquid, but the concentrations of the residual impurities were so low that there occurred no problem. Embodiment 3 Tests were carried out by changing the kind of the gold plating solution, that is, by using as the process liquid a wash liquid in non-cyan-based gold plating instead of the cyan-based gold plating solution. Gold was contained in the plating solution in the form of gold sodium sulfite. As the concentrated liquid in the electrodialysis device 9, a dilute aqueous solution of sodium hydroxide and sodium sulfite was used. The other conditions were the same as in the preceding embodiment. The test results are summarized in the following table. The components such as sodium sulfite were found to be entirely removed, on osmosis through the reverse osmotic membrane 6a. Embodiment 4 Tests were carried out in relation to silver plating, by using a wash liquid of a silver plating process as the process liquid. Silver was contained in the plating solution in the form of potassium silver cyanide. As the concentrated liquid, a dilute aqueous solution of potassium hydroxide and potassium cyanide was used. The test results are summarized in the following table. Citric acid and the like components were found to be entirely removed, on osmosis through the reverse osmotic membrane 6a. Embodiment 5 Tests were carried out in relation to rhodium plating, by using as the process liquid a wash liquid in a rhodium plating process. Rhodium was contained in the plating solution in the form of rhodium sulfate. As the concentrated liquid, a dilute aqueous solution of sulfuric acid was used. Due to the relationship between ions, the arrangement of the anion-exchange and cation-exchange membranes in the electordialysis device in the case of rhodium plating is reverse to that in the above-mentioned cases, as shown in FIGURE 2. The test results are summarized in the following table. Sulfuric acid and the like components were found to be entirely removed, on osmosis through the reverse osmotic membrane 6a. Embodiment 6 Tests were carried out in relation to palladium plating, by using as the process liquid a wash liquid in a palladium plating process. Palladium was contained in the plating solution in the form of diamino-palladium dinitrite. As the concentrated liquid, a dilute aqueous solution of aqueous ammonia and sodium nitrite was used. The arrangement of ion-exchange membranes as shown in FIGURE 2 was used for palladium plating, as in the case of rhodium palting. The test results are summarized in the following table. Sodium nitrite and the like components were found to be entirely removed, on osmosis through the reverse osmotic membrane 6a. Almost the same test results can be obtained also where palladium is contained in a palladium plating solution in the form of palladousamine chloride. As in the above embodiments, the "precious metal compound" is not limited to gold potassium cyanide but may be gold sodium sulfite, potassium silver cyanide, rhodium sulfate, palladousamine chloride, diamino-palladium nitrite or the like. Namely, the precious metal compound can be recovered solely and in its as-is form, not accompanied by various components or other metals which are contained in the plating solution, and the recovered precious metal compound can be sequentially added to the plating tank l. Also, the recovered precious metal compound can be converted into a crystalline or powdery form by the drying device. Further, the process liquid is not limited to the waste liquid from plating or the wash liquid but may be any other liquid that contains a precious metal compound, such as a stripping solution for platings. Moreover, although a cellulose acetate based reverse osmotic membrane 6a was mentioned by way of example in the above embodiments, other types of reverse osmotic membrane can also be used. Embodiment 7 In this embodiment, the process liquid to be treated in the reverse osmotic membrane device 6 shown in FIGURE l is a concentrated cyan solution, namely, a waste liquid of a gold potassium cyanide based plating solution or a stripping solution which contains a large amount of free cyan. The waste liquid from plating is subjected to a pretreatment to decompose sufficiently the free cyan, thereby enhancing the efficiency of separation of impurity metals by the reverse osmotic membrane device 6. The pretreatment is carried out using a cyan treating tank 24 shown in FIGURE 3. In the cyan treating tank 24, the process liquid (waste liquid from plating) 25 and a cathode chamber 26 are separated from each other by a cation-exchange membrane 27 so that the process liquid 25 does not make contact with a cathode 28 and, therefore, gold is not deposited as metal from the process liquid 28. Numeral 29 denotes an insoluble anode such as a carbon electrode and a ferrite electrode, numeral 30 an anode chamber, and numeral 3l an electrolyte such as an aqueous potassium hydroxide solution. When electrolysis is conducted under the condition where the waste liquid from plating as the process liquid 25 is added to the anode chamber 30 of the cyan treating tank 24 whereas the aqueous potassium hydroxide solution is added to the cathode chamber 26, the free cyan undergoes oxidative decomposition on the anode 29. However, gold is not deposited as metallic gold but remains dissolved as ions. After the free cyan only is sufficiently decomposed in the cyan treating tank 24, the process liquid 25 is fed to the reverse osmotic membrane device 6 shown in FIGURE l to separate gold potassium cyanide and impurity metals contained in the process liquid 25 from each other, and the osmosed liquid (aqeuos solution of gold potassium cyanide) 3a from the device 6 is subjected to electrodialysis in an electrodialysis device 9. According to this embodiment, the efficiency in separating the impurity metals in the reverse osmotic membrane device 6 was high even where the waste liquid from plating was treated as the process liquid. Furthermore, since gold was not in the metallic state, the electrodialysis in the electrodialysis device 9 could also be performed efficiently, as in Embodiment l. Embodiment 8 A waste liquid from plating was selected as the process liquid, as in the preceding embodiment. After a chelating agent is added to the waste liquid to form complex salts with metals other than gold potassium cyanide, the resultant liquid was treated sequentially by the reverse osmotic membrane device 6 and the electrodialysis device 9 shown in FIGURE l in the same manner as in Embodiemnt l. If the waste liquid pretreated for decomposing or neutralizing the free cyan contains considerable amounts of impurity metals as cyano-complexes, the impurity metals cannot be easily separated by the reverse osmotic membrane device 6. On the other hand, when the impurity metals are present in other forms of metal ions than cyano-complexes, it is possible to separate the impurity metals efficiently by the reverse osmotic membrane device 6. In view of this point, a chelating agent having a chelate formation constant for the impurity metals higher than that of cyan was added to the waste liquid to convert the impurity metals from the cyano-­ complexes thereof into complexes thereof with the chelating agent. In this embodiment, the impurity metals in the waste liquid from plating (concentrated cyan solution) were preliminarily converted from cyano-complexes into other form of complexes by the addition of the chelating agent and, accordingly, it was possible to separate efficiently gold potassium cyanide from the impurity metals by the reverse osmotic membrane device 6. Then, the thus separated gold potassium cyanide was subjected to electrodialysis in the electrodialysis device 9, whereby it was possible to recover gold potassium cyanide in a concentrated state, as in Embodiment l. Incidentally, when the pretreatment described in Embodiment 8 is carried out after the pretreatment described in Embodiment 7, it is possible to separate and concentrate gold potassium cyanide (precious metal compound) more securely. The process and the apparatus for recovery of a precious metal compound according to this invention, as has been described hereinabove, make it possible to recover the precious metal compound, solely and in the as-is form of the compound, either as a concentrated liquid or in a crystalline or powdery form, from a process liquid containing a variety of components and other metals. Furthermore, the process and the apparatus are profitable on an economical basis because the ion-­ exchange membranes used therein can be used for a long period of time without needing regeneration, unlike ion-­ exchange resins. In addition, according to the process and apparatus for controlling a precious meatl plating solution as has been described above, it is possible to use the recovered precious metal compound directly for controlling the plating solution, which is more profitable, on an economical basis, than the recovery of the precious metal compound through conversion of the compound into a metallic form.

Claims: 1. A process for recovering a precious metal compound, comprising separating a precious metal compound contained in a process liquid by a reverse osmotic membrane, concentrating said separated precious metal compound by electrodialysis empolying anion-exchange and cation-exchange membranes, and recovering said precious metal compound in the as-is form of the concentrated liquid obtained by said electrodialysis. 2. A process for recovering a precious metal compound, comprising separating a precious metal compound contained in a process liquid by a reverse osmotic membrane, concentrating said separated precious metal compound by electrodialysis empolying anion-exchange and cation-exchange membranes, and subjecting said electrodialyzed concentrated liquid to drying by evaporation to recover said precious metal compound in a crystalline or powdery form. 3. A process according to claim l or 2, wherein said reverse osmotic membrane is a cellulose acetate based reverse osmotic membrane. 4. A process according to claim l, wherein said precious metal compound is gold potassium cyanide. 5. A process according to claim 2, wherein said precious metal compound is gold potassium cyanide. 6. A process according to claim l, wherein said precious metal compound is gold potassium cyanide and said reverse osmotic membrane is a cellulose acetate based reverse osmotic membrane. 7. A process for controlling a precious metal plating solution by use of the process according to claim l, comprising separating a precious metal compound contained in a process liquid by a reverse osmotic membrane, recovering said separated precious metal compound after concentrating said separated precious metal compound by electrodialysis employing anion-exchange and cation-­ exchange membranes, and sequentially adding said recovered previous metal compound to a precious metal plating solution. 8. A process according to claim 7, wherein said precious metal compound is gold potassium cyanide and said precious metal plating solution is a gold plating solution. 9. A process according to claim l or 2, wherein a chelating agent is added to said process liquid to thereby convert impurity metals from cyano-complexes thereof into complexes thereof with said chelating agent, and thereafter said precious metal compound in said process liquid is separated from said impurity metals by said reverse osmotic membrane. l0. A process according to claim l or 2, wherein said process liquid is a concentrated cyan solution containing a large amount of free cyan, said concentrated cyan solution is pretreated in a cyan treating tank comprising an anode chamber and a cathode chamber separated from each other by a cation-exchange membrane so that said concentrated cyan solution placed in said anode chamber does not make direct contact with said cathode, thereby causing oxidative decomposition of said free cyan on said anode, and thereafter said precious metal compound contained in said process liquid is separated from impurity metals by said reverse osmotic membrane. 11. An apparatus for recovering a precious metal compound, comprising: a reverse osmotic membrane device for selective permeation of only a precious metal compound contained in a process liquid, and an electodialysis device employing anion-exchange and cation-exchange membranes for concentrating the osmosed liquid obtained by said reverse osmotic membrane device. 12. An apparatus for recovering a precious metal compound, comprising: a reverse osmotic membrane device for selective permeation of only a precious metal compound contained in a process liquid, an electodialysis device employing anion-exchange and cation-exchange membranes for concentrating the osmosed liquid obtained by said reverse osmotic membrane device, and a drying device for subjecting said concentrated liquid obtained by said electrodialysis device to drying by evaporation to recover said precious metal compound in a crystalline or powdery form. 13. An apparatus according to claim ll or l2, wherein the reverse osmotic membrane is a cellulose acetate based reverse osmotic membrane. 14. An apparatus for controlling a precious metal plating solution, comprising: a reverse osmotic membrane device for selective permeation of only a precious metal compound contained in a process liquid, an electrodialysis device employing anion-exchange and cation-exchange membranes for concentrating the osmosed liquid obtained by said reverse osmotic membrane device, and a feeding device for sequentially adding said concentrated liquid obtained by said electrodialysis device to a precious metal plating liquid. 15. An apparatus according to claim ll or l2, which further comprises a cyan treating tank comprising an anode chamber and a cathode chamber separated from each other by a cation-exhcange membrane, said cyan treating tank destined to perform electrolysis under the condition where a concentrated cyan solution containing a large amount of free cyan is added to said anode chamber as said process liquid and an electrolyte is added to said cathode chamber so that only said free cyan is decomposed by said electrodialysis, and said cyan treating device disposed on the upstream side of and connected to said reverse osmotic membrane device. 16. An apparatus according to claim l5, wherein the reverse osmotic membrane of said reverse osmotic membrane device is based on cellulose acetate. 17. An apparatus according to claim l4, which further comprises a cyan treating tank comprising an anode chamber and a cathode chamber separated from each other by a cation-exchange membrane, said cyan treating tank destined to perform electrolysis under the condition where a concentrated cyan solution containing a large amount of free cyan is added to said anode chamber as said process liquid and an electrolyte is added to said cathode chamber so that only said free cyan is decomposed by said electrodialysis, said cyan treating device disposed on the upstream side of and connected to said reverse osmotic membrane device.

Inventors: SUGITA, NORIYUKI
Applicants: ELECTROPLATING ENGINEERS OF JAPAN LIMITED
Application year: 1987
Main classification: B01D61
IPC codes: C02F1, C25D21