APPARATUS AND METHOD FOR REMOVING DISSOLVED METALS FROM SOLUTIONS CONTAINING SAME

Abstract: An apparatus comprising a container (12) having a perforated bottom portion (10) and, arranged on and above a retaining member (14) in the container, a plurality of absorbent particles containing a metal or metal compound capable of catalyzing the deposition of the dissolved metal from the solution. The solution is first passed through dispersing or distributing means (18), which can be integral part of the container, prior to passing downward through the container for contact with the absorbent particles.

Description: APPARATUS AND METHOD FOR REMOVING DISSOLVED METALS FROM SOLUTIONS CONTAINING SAME BACKGROUND OF THE INVENTION The present invention relates to the treatment of solutions containing dissolved metals and, more particularly, to the treatment of metal plating solutions to remove dissolved metal therefrom. Solutions for plating metal are well known in the art and are used in both electrolytic processes and electroless plating processes. In either case, the solutions necessarily contain a source of the metal plating ions along with various other materials required for the particular plating operation (e.g., complexing and reducing agents for electroless plating; brighteners, grain refiners, surfactants for electrolytic processes). As plating solutions are used in plating processes, certain components thereof necessarily are consumed and depleted, thereby decreasing the efficiency of the plating solution. It is well known in the art to extend the useful life of operating plating solutions by periodically replenishing the solution with formulations containing the consumed -1— components (e.g., the source of metal ions). However, the plating solution, evenwith replenishing, eventually has to be replaced by a new solution. Moreover, addition of replenishing formulations adds to the volume of the plating bath, requiring decanting and disposal of excess solution. Still further, rinse waters used to wash plated parts eventually build up metal values and must be disposed of. As a consequence of these facts, plating processes -•-generate a significant quantity of waste streams (used collectively herein to refer to spent plating solutions, rinse waters and other like streams) which require disposal. These waste streams generally cannot be directly discharged into effluent streams, however, since they can contain appreciable quantities of dissolved metals and their discharge would contravene both Federal disposal regulations and the disposal regulations of many States. In addition to the foregoing, it would be of economic interest to be able to recover the metal values dissolved in these waste streams. Systems for removal of dissolved metals from solutions encountered in the plating industry are known. In one such system used in the electroless copper plating industry, the copper-containing solution in question is pumped into a tank where its pH is adjusted, reducing agent added if necessary, and a so-called "seeder", such as ferrous sulfate, also added. This treatment promotes the plating out or deposition of copper metal from the solution. The remaining solution is then pumped from the tank for disposal. Batch processes of this type, however, are time-consuming and equipment- and labor- intensive, and can result in plating out of the metal onto the walls of the tank, from which it is difficult to remove. Another system suggested for use in the electroless plating industry is described in U.S. Patent No. 4,260,493 to Kretas, et al. and in published European Patent Application No. 0059 350 (published September 8, 1982). In this system, the dissolved metal content of a solution is reduced by contact of the solution with a seeder comprised of a particulate, porous filter aid material having absorbed thereon a material catalytic to the deposition of the dissolved metal. The catalytic material is, for example, palladium or other noble metal, gold or silver. In this process, the waste solution being treated must function as an electroless plating solution and, thus, generally requires addition of reducing agent and pH adjustment. After removal of dissolved metal, the waste solution is then further treated (by contact with halogen) to remove complexing agents therefrom. In the earlier-noted published European Patent Application, there is further disclosed a module for conducting that portion of the process involving deposition of dissolved metals from the waste solution. In the module, the filter aid particles on which the catalytic metal is absorbed are arranged as a bed in the module for receiving a solution which has been pH-adjusted and to which reducing agent has been added (to cause the solution to function as an electroless plating solution) . A standpipe also is arranged in the module for removing solution which has contacted the seeder particles and from which metal values have been removed. The standpipe is arranged so as to remove solution from the bottom of the module, which solution is then moved up through the standpipe for exit at the top of the module. Difficulties with the foregoing system include the need for pH adjustment and reducing agent addition to the waste solution prior to treatment with the seeder particles. Also, in accordance with the teachings of these patents, the preferred catalytic material is palladium, a very expensive material, and the palladium/filter aid material is provided with yet a further coating of, e.g., copper. Thus, the overall materials of use are quite expensive and the module is overly cumbersome and heavy. In addition, the proposed modular unit for effecting contact of solution with the seeder particles is overly complicated and expensive to construct, and does not easily lend itself to easy removal and replacement when the seeder particles become saturated with deposited metal. SUMMARY OF THE INVENTION It is an object of the present invention to provide a modular apparatus for removing dissolved metals from solutions such as waste solutions encountered in the plating industry. Another object of the invention is to provide an apparatus of the type described above which can be produced economically and which lends itself to operation in combination with a number of similar such apparatus and to easy replacement when required. A further object of the invention is to provide an apparatus of the type described which can be readily and easily employed in conjunction with metal plating systems and processes of a variety of constructions and arrangements. Yet another object of the invention is to provide a process for removing dissolved metals from solutions, and particularly for removing dissolved copper from a waste copper plating solution or copper rinse water. These and other objects are achieved through the provision of a modular apparatus containing a plurality of closely-packed absorbent particles which themselves contain (impregnated therein and/or absorbed thereon) a triggering metal or metal compound which catalyzes the deposition of the dissolved metal from the solution onto the absorbent particles. The module comprises a container having a perforate planar bottom portion and upstanding side walls extending from the periphery thereof. Within the container, adjacent and substantially parallel to the bottom portion, is a perforate planar retaining member above which the impregnated absorbent particles are arranged, the size of the particles and the size of the openings in the retaining member being such as to substantially prevent the impregnated particles from passing therethrough. In operation, a solution containing dissolved metal is first passed through a dispersing (distributing) mechanism, which preferably is constructed as an integral part of the modular apparatus, and then passed downwardly through the closely-packed absorbent particles and, finally, out of the module through the perforate bottom portion of the container. From this point, if any additional metal removal is required, the solution can be directly passed downwardly through another modular apparatus of the same type described above, arranged immediately below the first apparatus, the perforate bottom portion of the first apparatus acting as the distributor for the solution before it contacts the bed of particles in the second apparatus. Among ■ the important advantages of the present invention is the fact that the modular apparatus can be easily and economically constructed of inexpensive materials and does not require piping to remove solution from the module after it has passed through the bed of closely-packed absorbent particles. In addition, by reason of the passage of the solution through a distributor or dispersing means prior to contact with the bed of absorbent particles, the bed is more efficiently used and undesired channeling and/or other flow irregularities are greatly minimized. Other advantages of the present invention include the ease with which the modular apparatus can be shipped to an ultimate user; stored; readied for use; employed with any number and variety of processes where solutions containing dissolved metals are encountered; discarded or taken off strea when the absorbent particles become saturated with the deposited metal; and utilized in series or in parallel with other similar modular units, thereby enabling a decrease in the size of each individual unit and facilitating shipping, handling and other operations. In preferred embodiments of the invention, the solution containing dissolved metals is a waste or spent solution generated in a plating process of the electroless type, such as a spent plating bath, excess decant from a replenished plating bath and rinse waters used to wash plated parts. A particularly preferred use for the present invention is in the treatment of waste or spent solutions generated in an electroless copper plating process. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of one form of modular apparatus according to the invention, shown in cut-away view to permit viewing of the interior contents and elements. FIG. 2 is. an illustration of a combination of two modular units according to the invention, each shown in cut-away view to permit viewing of the interior contents and elements. DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, a preferred single modular apparatus comprises an enclosed container having a bottom portion 10 and upstanding side walls 12. The side walls 12 are affixed to and contiguous with the outer periphery of bottom portion 10 and may, if desired, also extend some small, predetermined distance below the plane of the bottom portion. Generally, bottom portion 10 will be of circular or substantially circular shape but, of course, can be of any desired shape so as to provide for a square, rectangular, cylindrical, etc. container area. If desired, side walls 12 can be outwardly tapered from the bottom portion 10, an arrangement which facilitates stacking one module on top of another. Bottom portion 10 and side walls 12 may be constructed of any material which possesses structural rigidity and which is inert to the solution containing dissolved metals as well as to any by-products of any reactions occurring in the container. Preferred materials of construction are plastics such as polypropylene. The bottom portion 10 is of perforate construction, the perforation size and frequency not being critical so long as the solution passed through the container can exit through bottom portion 10 without undue restriction or hold-up. Arranged in the bottom portion of the container, substantially parallel to bottom portion 10, is a planar retaining means 14 which is of perforate or foraminous construction. This retaining means 14 generally will be shaped so as to be in contact about its periphery with side walls 12 and, if desired, may even directly rest upon bottom portion 10. Retaining means 14 may be constructed of any material offering a suitable degree of rigidity and inertness to the solutions, materials and compounds with which it will be in contact, and may, for example, be constructed of plastic or metal. Retaining means 14 is, as noted, perforate or foraminous. The size of the openings in the retaining means (essentially, its permeability) is such as to permit passage therethrough of solution passed through the container while excluding from passage the impregnated absorbent particles 16 which rest upon and extend upward from the plane of retaining means 14 in a closely-packed arrangement. Impregnated absorbent particles 16 are comprised of inert porous absorbent material having a large contact surface area containing (impregnated within the pores thereof and/or deposited on the surface thereof) a metal or metal compound capable of triggering the decomposition of the solution in question to cause deposition therefrom, onto the particles, of the particular dissolved metal sought to be removed from the solution. The porous absorbent material preferably is carbon but may also consist of any other suitable porous material. The triggering metal or metal compound will, of course, vary depending upon the particular solution and dissolved metal in question. Simply by way of example, for electroless copper plating solutions, the triggering material may be ferrous chloride, ferric chloride, ferrous sulfate, cuprous chloride, cupric chloride, copper metal itself, cupric oxide or cuprous oxide. The concentration of triggering metal in any particular absorbent particle is not critical per se, so long as a sufficient quantity of such triggering material is present throughout the closely-packed bed of absorbent material to catalyze the required degree of decomposition of the solution containing the dissolved metal. The impreganted absorbent particles 16 are, as noted, in closely-packed arrangement above retaining means 14 so as to insure adequate contact of solution therewith as it passes downwardly through the apparatus. Also arranged within the enclosed container in the preferred embodiment of the invention, above the level of absorbent particles, is a substantially planar dispering means 18 which is of perforate or foraminous, porous construction. Again, this element may be constructed of any suitably rigid, inert material inherently perforate or in which perforations can be made. Dispersing means 18 generally will be substantially contiguous about its periphery with side walls 12 so as to insure that solution must pass therethrough before contacting the absorbent particle bed. Dispersing means 18 serves to distribute solution across a substantial portion of the upper surface of the absorbent particle bed so that the bed is efficiently utilized and so as to minimize flow distribution irregularities such as channeling. Included in the construction of FIG.1 is a perforated planar cover element 20 spaced apart from and parallel to dispersing means 18. Cover element 20 serves to assist in the •* * dispersion/distribution of solution flow through the apparatus, and also serves as a covering element to facilitate shipping and handling, prevent inadvertent spills, etc. It is preferred in this regard that the perforation size and permeability of the dispersing means 18 be such as to resist passage therethrough of impregnated absorbent particles 16. In construction and operation, impregnated absorbent particles are prepared, for example, by soaking the absorbent particles in an aqueous solution or suspension of the triggering material, and other materials which may be required to assist solubility and impregnation, for a predetermined period of time. By way of example, particles for use in removing copper from a copper-containing electroless plating waste stream have been prepared by dissolving 4.2 pounds of ferrous sulfate in 2.5 gallons of hot water (acidified with H-SO. to aid solubility). The dissolved ferrous sulfate is then added to 7 pounds of WHT . particulate carbon and cold water is added to just cover the surface of the carbon. After mixing, the mixture is permitted to sit for about 12 hours. The mixture is then added to a pre-constructed cylindrical modular apparatus using a 1/4-inch thick polypropylene filter as the retaining means (500 μ holes, evenly spaced) which rests on the bottom portion of the container (also polypropylene, 1/4-inch holes, randomly spaced; side walls * also made of polypropylene; volume of container about 5 gallons). The mixture in the apparatus is allowed to drain for about one hour until no further liquid drains from the holes in the bottom portion. A perforate polypropylene filter of the same size and perforation as the retaining member is then placed on top of the carbon particles to act as a dispersing member. About 2 gallons of a 10% * solution of caustic (50% rayon grade) is then poured through the dispersing member and allowed to drain for about one hour. 5 A top perforated cover (polypropylene) is then placed over the container. In operation of the process of removing a dissolved metal from a solution, the impregnated absorbent particle bed may be pre-treated (e.g., as noted above with caustic) to 10 provide conditions (e.g., of pH) either required for the catalyzed deposition of dissolved metal or which enhances the effectiveness and speed of such deposition. The solution containing dissolved metal itself may be treated in any appropriate manner for this purpose, but such is not generally 15 preferred since it diminishes the advantage of ease of use of the modular apparatus. As earlier noted, the means for dispersing the solution prior to contact with the impregnated absorbent particles preferably is an integral part of the modular 20 apparatus, but need not be so if desired. A distinct advantage of the apparatus of the invention is the ability to utilize two or more such units in series, in a stacked or nested arrangement. In this manner, a highly efficient removal of dissolved metal from the solution can be 25 effected without the need for a long residence time in any one individual unit and without need for making individual units very large, thereby facilitating individual handling. shipping, etc. In addition, a serial, stacked arrangement facilitates removal and replacement of units which have become inefficient due to saturation of the particles with the depositing metal. Thus, for example, in a stacked arrangement of three units, the uppermost unit, which receives the most concentrated dissolved metal-containing solution, will become saturated in time and can simply be removed from the stack while a fresh (or regenerated) unit is then added to the bottom of the stack, operation then continuing with solution being fed to the original second or middle unit. A particularly preferred stacked arrangement is illustrated (for two units) in FIG. 2. As will be noted, this arrangement permits elimination of dispersing means integral with individual units, since perforated retaining means 14 and bottom portion 10 for a particular unit can serve as the means for dispersing flow prior to contact with the impregnated particles 16 in a lower unit. In this arrangement, the units should be constructed and stacked in a manner which does not permit a module to rest direclty on the bed of impregnated particles in the module below it. Of course, if desired, one or more of the individual units may contain an integral dispersing means. After a solution containing dissolved metal has passed through one or more of the modular units it generally can be discarded into effluent streams directly since its dissolved metal content is greatly reduced (e.g. less than 5 ppm) . However, the solution may be treated in any other manner if further purification or recovery of other materials therefrom is desired. The impregnated absorbent particles can, if desired, be treated to remove deposited metal therefrom or can be sold as is as scrap. The foregoing constitutes a description of particular features of the invention intended to illustrate, rather than limit, the breadth of the invention, the scope of which is defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A modular apparatus for removing dissolved metals from a solution, comprising a container having a perforate planar bottom portion and upstanding side walls extending above the plane of said bottom portion and affixed along the periphery of said bottom portion; perforated planar retaining means arranged within said container adjacent to and sub¬ stantially parallel to said bottom portion, the periphery of said -planar retaining means being substantially contiguous with said upstanding side walls; perforated planar dispersing means for receiving said solution containing dissolved metals, said dispersing means arranged within said container above the plane of said retaining means and substantially parallel thereto, the periphery of said planar dispersing means being substantially contiguous with said upstanding side walls, said perforated planar retaining means, per¬ forated planar dispersing means and said side walls defining a chamber within said container; and, arranged in said chamber, a plurality of absorbent particles having absorbed thereon a metal or metallic compound capable of catalyzing the deposition of said dissolved metal from said solution onto said absorbent particles, the particle size of said absorbent particles and the size of perforations in said retaining means and said dispersing means being such as to substantially confine said absorbent particles within said chamber during shipment and use. 2. The modular apparatus according to Claim 1 wherein said dispersing means comprises a combination of two spaced-apart, substantially parallel perforated planar elements. 3. The modular apparatus according to Claim 1 wherein said upstanding side walls are outwardly tapered to a degree sufficient to permit a second modular apparatus of similar construction to be nested upon said first modular apparatus. 4. The modular apparatus according to Claim 1 wherein said -dissolved metal is copper; wherein said absorbent particles are finely-divided carbon particles; and wherein said catalyzing metal or metallic compound comprises a ferrous or ferric salt. 5. An apparatus for removing dissolved metals from a solution, comprising in combination: • a first container having a perforate planar bottom portion and upstanding side walls extending above the plane of said bottom portion and affixed along the periphery of said bottom portion; perforated planar retaining means arranged within said container adjacent to and s.ub.stantially parallel to said bottom portion, the periphery of said planar retaining means being substantially contiguous with said upstanding side walls; and, arranged within said container, above the plane of said planar retaining means, a plurality of absorbent particles having absorbed thereon and/or impregnated therein a metal or metallic compound capable of catalyzing the deposition of said dissolved metal from said solution onto said absorbent particles, the particle size of said absorbent particles and the size of the perforations in said retaining means being such as to retain said absorbent particles within said container; and • a second container having all the elements of said first container.^nd vertically stacked upon said first container such that the perforate planar bottom portion of said second container lies in contact with or within said first container above the level of absorbent particles in said first container, whereby said perforated planar retaining means and said perforate bottom portion of said second container act to disperse said solution containing said dissolved metals prior to contact of said solution with absorbent particles contained in said first container. 6. A method for removing dissolved metals from a solution, comprising the steps of: a) providing a container having a perforate planar bottom portion and upstanding side walls extending abov§ the plane of said bottom portion and affixed along the periphery of said bottom portion; perforated planar retaining means arranged within said container adjacent to and substantially parallel to said bottom portion, the periphery of said planar retaining means being substantially contiguous with said upstanding side walls; and, arranged within said container, above the plane of said planar retaining means, a plurality of absorbent particles having absorbed thereon and/or impregnated therein a metal or metallic compound capable of catalyzing the deposition of said dissolved metal from said solution onto said absorbent particles, the particle size of said absorbent particles and the size of the performations in said retaining means being such as to retain said absorbent particles within said container; b) passing said solution containing dissolved metals through a dispersing means and then downwardly through said container for contact with the absorbent particles therein, at conditions at which said the deposition of said dissolved metal from said solution is effected; and c) thereafter recovering the solution fromwhich said dissolved metal has been removed. 7. The method according to Claim 6 wherein said dispersing means is an integral element of said container. 8. The method according to Claim 6 wherein said dispersing means comprises the perforated bottom portion of a second container, having all the elements of the said container, and through which said solution passes prior to passing downwardly into the said container. 9. The method according to Claim 6 wherein said dissolved metal comprises copper; wherein said absorbent particles comprise finely-divided carbon particles; and wherein said catalyzing metal or metallic compound comprises a ferrous or ferric salt. 10. The method according to Claim 9 wherein said solution comprises a spent copper plating solution.

Inventors: ARDZIJAUSKAS ROBERT, TOLLER WILLIAM H, ARDZIJAUSKAS, ROBERT, TOLLER, WILLIAM, H.
Applicants: MACDERMID INC, MACDERMID, INCORPORATED
Application year: 1985
Main classification: B01D15
IPC codes: B01D15, C02F1, C23C18, C25D21