Abstract:
The invention relates to a method for the treatment of work pieces with a palladium colloid solution by bringing the work pieces into contact with the colloid solution, palladium being recovered after the use of the colloid solution. This is achieved by separating palladium colloid particles from the colloid liquid by means of a molecular filter. With this method it is easy to continuously and largely completely separate palladium from the spent processing solutions without investing large amounts of chemicals, energy and time. The spent processing solution may especially be worked up after separation of the part of the solution containing palladium such that palladium can be recovered completely and be reused for further processing.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for the treatment of work pieces with a palladium colloid solution, preferably to a method for the treatment with a hydrochloric palladium colloid solution being stabilized with tin.  
         BACKGROUND OF THE INVENTION  
         [0002]    For the electroplating of work pieces the surfaces thereof must first be treated to render them electrically conductive if the work pieces have electrically nonconducting surfaces. For this purpose the work pieces are dipped into a solution containing colloidal palladium. The palladium particles thereupon adsorbed on the surfaces serve as activators for initiating electroless metal deposition, which results in the production of an electrically conductive layer on the surface of the work pieces. This conductive layer may afterwards be electroplated with any metal. This process may for example be applied to the production of printed circuit boards and of sanitary appliances, metallized parts for automotive industry and of furniture mountings, especially for chromium plating of plastics parts.  
           [0003]    If work pieces provided with electrically nonconducting surfaces are treated and thereafter are removed from the activating solution, the solution partly adheres to the surfaces of the pieces. This adhering solution is normally rinsed off with water.  
           [0004]    If conventional procedures are applied for activating work pieces, a solution containing colloidal palladium with a palladium concentration of from 50 to 100 mg/l palladium is typically employed. In general therefore about 5 mg palladium are adsorbed to plastics parts having a geometrical surface area of 1 square meter. This amount is required for activating the plastics surfaces. If the work pieces to be treated are removed from the respective treatment station, however, about 0.2 liter activating solution are left on each square meter of the surface of the work pieces, this remaining solution being dragged out. For this reason from about 10 to about 20 mg palladium are lost per square meter of surface area of the work pieces by drag out from the activating solution, rinsing-off of the work pieces&#39; surfaces and by transferring this liquid to the waste water treatment.  
           [0005]    Palladium containing activating solutions are also used for direct plating on electrically nonconducting surfaces without using electroless plating methods. In this case the adsorbed layer containing palladium is converted into an electrically conducting layer, which may be used to directly electrodeposit metal on an activated nonconducting surface. For this purpose a higher concentration of the palladium colloid particles of about 200 mg palladium per liter solution is required.  
           [0006]    Drag out of palladium from the activating solution amounts to about 50 mg/m 2  if conventional direct plating procedures are applied. By making use of appropriate measures, for example by preceding adsorption of polyelectrolytic compounds on the nonconducting surfaces, adsorption of the palladium particles may be enhanced considerably to about 50 mg/m 2  surface area of the work pieces. All the same still about 50% of the employed palladium is lost due to drag out. Only 50% are still available for electroplating of the work piece surfaces.  
           [0007]    For example it is known to recover palladium from processing solutions.  
           [0008]    In U.S. Pat. No. 4,078,918 a process for recovering palladium from various materials is disclosed, the materials containing dissolved or non-dissolved palladium. The materials are first treated with an oxidizing agent in order to destroy organic components if necessary and are then treated with ammonium hydroxide to form ammine complexes. The palladium complexes being formed are then reduced by means of ascorbic acid, thereby precipitating palladium from the processing solution, which may finally be separated by filtration.  
           [0009]    Further a process for recovering palladium from solutions containing colloidal Pd/SnCl 2  is disclosed in “Reclamation of Palladium from Colloidal Seeder Solutions” in  Chem. Abstr.,  1990:462908 HCAPLUS, these solutions usually serving as a pretreatment agent in an electroless plating process. In this procedure the solution is aerated for 24 hours, thereby coagulating palladium from the solution. The precipitate formed is then separated, dried and processed further.  
           [0010]    A method for precipitating palladium from a solution by adding metallic tin to this solution at a temperature of 90° C. is described in “Recovery of Palladium and Tin Dichloride from Waste Water Solutions of Colloidal Palladium in Tin Dichloride” in  Chem. Abstr.,  1985:580341 HCAPLUS.  
           [0011]    A further process for recovering palladium from spent catalytic colloidal palladium baths is disclosed in U.S. Pat. No. 4,435,258, these baths being used for activating nonconductive surfaces for subsequent electroless metallization. The activating solutions are worked up by dissolving colloidal palladium by adding an oxidizing agent to the solution, for example hydrogen peroxide, thereby forming a true solution, subsequently heating the solution to destroy residual hydrogen peroxide and afterwards precipitating palladium at a cathode by electrodepositing palladium.  
           [0012]    Finally a process for recovering palladium from Pd/SnCl 2  solutions is described in “Recovery of the Colloidal Palladium Content of Exhausted Activating Solutions Used for the Current-Free Metal Coating of Resin Surfaces” in  Chem. Abstr.,  1976:481575 HCAPLUS. In this case palladium is precipitated by adding concentrated nitric acid to the solution and separated by filtrating.  
           [0013]    The known methods for treating the work pieces with a palladium colloid containing solution are complicated and expensive.  
           [0014]    The problem of the present invention therefore consists in avoiding the disadvantages of the known methods and especially in finding a method to treat work pieces with a palladium colloid solution which may be carried out easily. Only small amounts of additional chemicals shall be required for performing the method. Moreover the method shall be carried out with low expense of energy and time.  
         SUMMARY OF THE INVENTION  
         [0015]    The method according to the invention serves to treat work pieces with a palladium colloid solution by bringing the work pieces into contact with the colloid solution.  
           [0016]    For continuous operation of the method, after treatment of the work pieces with the palladium colloid solution, palladium is recovered from said solution by a method comprising  
           [0017]    bringing the the palladium colloid solution into contact with a molecular (membrane) filter and  
           [0018]    separating palladium colloid particles contained in the palladium colloid solution from said solution by means of the molecular filter.  
           [0019]    Preferably the invention relates to the treatment of work pieces with a hydrochloric palladium colloid solution being stabilized with tin.  
           [0020]    It is possible by carrying out the method according to the invention under continuous operation to largely separate palladium, especially palladium colloid particles, from spent palladium processing solutions with low expenditure of chemicals, energy and time. It is especially possible to work up the spent processing solutions after separating the palladium containing part of the solution from the spent solution in order to recover and reuse palladium completely.  
           [0021]    As compared to the method for recovering palladium from colloidal Pd/SnCl 2  solutions, as descibed in  Chem. Abstr.,  1990:462908 HCAPLUS, the method according the present invention advantageously leads to complete separation of palladium between the concentrate part and the permeate part of the liquid. In contrast with this new method the known method suffers from the fact that a considerable part of palladium is oxidized to bivalent soluble palladium during the precipitation method. Therefore palladium cannot be separated entirely from the solution by filtration. For this reason palladium is lost partly due to the recovery method.  
           [0022]    Furthermore in contrast to the method described in  Chem. Abstr.,  1985:580341 HCAPLUS, the method according to the present invention advantageously does not require considerable expenditure of additional chemicals, such as for example of metallic tin, as well as of additional energy and time for heating the colloid solution as is the case if the known method would be carried out.  
           [0023]    The method according to the invention also has the additional advantage to be a single-stage recovery method. This is in contrast to the method as described in U.S. Pat. No.4,435,258. Therefore the method according to the invention is very easy to perform. Furthermore palladium can be removed essentially completely from palladium containing solutions, whereas, if the method as described in U.S. Pat. No. 4,435,258 is used, only a very low current yield may be achieved when the palladium concentration is low which is normally the case after prolonged electrolysis time. Therefore complete removal of palladium is very complicated oder not at all possible if this known method is used.  
           [0024]    Furthermore in contrast to the method as described in  Chem. Abstr.,  1976:481575 HCAPLUS, the method according to the invention is especially suited for continuous operation. The method described in this prior art document moreover does not manage without additional chemicals.  
           [0025]    Colloidal palladium activating solutions contain palladium particles which are surrounded by a protective coating (protective colloid). Experiments using high-resolution electron microscopy (HREM) and atomic force microscopy (AFM) have shown that the palladium particles have a diameter of at least 2.5 nm. Surprisingly the size distribution is extremely narrow: It has proven that no considerable deviations from the mean particle size of 2.5 nm occurs in solutions containing colloidal palladium that are usually used to activate nonconductor surfaces.  
           [0026]    Normally colloid solutions are acidic and very often contain hydrochloric acid. Further they often contain chloride ions and if necessary bivalent and tetravalent tin or organic polymeric stabilizers and reducing agents. With the exception of polymers, which are used in very small amounts, all other components contained in these solutions are ionic. It is guessed that these ionic components are considerably smaller in size than the palladium particles.  
           [0027]    For this reason it was surprising that palladium particles can be removed selectively and completely from these colloid solutions with appropriate molecular filters having different porosity though tin at the same time is present at a high concentration (typically more than 70-fold of the palladium concentration) and moreover tin compounds are known to form colloid solutions being difficult to filtrate. It has also been known that tin compounds are predominantly present in the palladium particles so that selective separation of the palladium particles from tin was not to be expected at all with this method.  
         DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
         [0028]    For performing ultrafiltration or nanofiltration various types of membranes made of various materials are known that can be used for carrying out the present invention. For example molecular filters made of polysulfones, especially polyethersulfones (PES), perfluorinated polymers and ceramics made advantageously be applied. For the selection of the type to be used for carrying out the present invention it has proven that it depends only on a sufficient stability of the membrane material towards the liquid to be treated, for example the activating solution which may contain up to 15 wt.-% hydrochloric acid. Surprisingly it has further been found out that the porosity of the molecular filter is not critical at all for the performance of separation of the palladium particles from the liquid if a molecular filter is chosen with an exclusion pore size of from 200 Dalton to 10.000 Dalton, since in this case the palladium particles completely remain in the concentrate liquid.  
           [0029]    In order to separate the palladium colloid particles molecular filters are used that preferably have an exclusion pore size of from 200 Dalton to 10.000 Dalton.  
           [0030]    If a molecular filter with an exclusion pore size of less than 200 Dalton is used, palladium is still able to pass through the filter membrane, but in this case a considerable amount of tin compounds is held back by the membrane.  
           [0031]    Therefore under these conditions separation efficiency of palladium particles versus tin compounds is very low. If molecular filters are used that have an exclusion pore size of considerably more than 10.000 Dalton, the palladium particles pass through the membrane. Therefore in this case the palladium particles can no longer be separated from the residual liquid and its components, namely the tin compounds. The range of from 200 Dalton to 10.000 Dalton therefore is an optimum with respect to the selectivity of separation of palladium particles from the other components of the solution to be treated, namely the tin compounds. An exclusion pore size of at least 500 Dalton is especially preferred. Most advantageous has proven an exclusion pore size of at least 2.000 Dalton. These further lower limits of the range represent preferred embodiments of the present invention, presenting an even better selectivity of separation between palladium particles and tin compounds under the circumstances mentioned.  
           [0032]    The molecular filters are preferably made of a material, selected from the group comprising polysulfones, especially polyethersulfones (PES), perfluorinated polymers, for example polytetrafluorethylene (for example TEFLON®, trade name of DuPont de Nemours), and ceramics. These materials are sufficiently chemically resistant towards the strongly acidic solutions containing hydrochloric acid.  
           [0033]    Observations and investigations made in favor of the present invention have led to the conclusion that it is possible to recover palladium particles from liquids, especially from rinsing liquid s, by means of molecular filters. For this purpose the following method steps are carried out:  
           [0034]    a. The work pieces are brought into contact with the colloid solution for activating the work pieces.  
           [0035]    b. After the activating treatment has been carried out the colloid solution adhering to the surfaces of the work pieces is removed from the surfaces with a rinsing liquid.  
           [0036]    c. The rinsing liquid is pressurized und thus led through the molecular filter, the liquid being led through the molecular filter being a permeate liquid and the liquid not being led through the molecular filter being a concentrate liquid.  
           [0037]    d. Preferably a palladium colloid solution is produced by using the concentrate liquid and adding suitable replenishment agents to the concentrate liquid in appropriate amounts.  
           [0038]    After activating treatment has been carried out the work pieces, preferably made from nonconducting material, are rinsed with a rinsing liquid in an appropriate device. Rinsing is preferably performed by spraying in order to minimize the volume of rinsing liquid. Prior to separating the colloid particles from the liquid by means of the molecular filter hydrochloric acid can be added to the solution even during spraying the rinsing liquid to the work pieces. If a sufficient amount of hydrochloric acid is added to the liquid, the tin compounds contained in the liquid do not hydrolyze so that cloudiness does not occur and precipitates are not formed due to these compounds. Subsequently the rinsing liquid is pressed through the selective molecular filter membrane by means of a pressurized pump, the membrane holding back the palladium particles und letting pass the rinsing liquid, especially rinsing water, and all other components contained in the rinsing liquid. The permeate liquid can then be led to waste water treatment.  
           [0039]    The palladium being held back and present as a homogeneous metal dispersion concentrate can be used to produce an activating solution. According to the composition of the concentrate tin(II) or tin(IV) salts and hydrochloric acid in higher or lower amounts are to be added to the concentrate liquid. In another alternative the palladium held back may be dissolved und be used as a solution, for example a palladium chloride solution, in order to produce an activating solution or alternatively to use this solution for any other purpose. 
       
    
    
       [0040]    For clarification of the method according to the present invention reference is made to FIG. 1, which shows a schematic drawing of an apparatus, that may be used for carrying out the ultrafiltration or nanofiltration of a palladium colloid solution. 
     
    
       [0041]    After treatment in the palladium colloid solution has been cariied out the work pieces (not shown) are transferred to a spraying container  1  in which the work pieces are held vertically and are rinsed by spraying rinsing water to the pieces. Spraying is carrier out by using spray nozzles  2  which are located are the lateral side walls of the rinsing spraying container  1 . The water being sprayed to the surfaces of the work pieces wet the surfaces of the work pieces so that the colloid solution is rinsed off the surfaces. For this purpose used rinsing liquid Z from a further rinsing station is used, in which the work pieces being rinsed in this spraying container  1  will be rinsed again with rinsing liquid. The spent rinsing liquid is led to the spray nozzles  2  via a pipeline  4  by means of a pump  3 . The liquid will only be sprayed into the spraying container  1  if work pieces are present in this container  1 . Regulation of the rinsing liquid is performed by means of a valve  5  which only allows the liquid to pass to the container  1  is work pieces are to be treated.  
         [0042]    The rinsing liquid running down at the surfaces of the work pieces and containing colloid solution due to the rinsing treatment accumulate at the bottom of the rinsing container  1 . This liquid is removed from the rinsing container  1  via a pipeline  6 .  
         [0043]    Concentrated hydrochloric acid contained in the container  7  is admixed via a further pipeline  8  to the rinsing liquid coming out from the container  1 , thereby lowering the pH of the rinsing liquid. Due to this lowering cloudiness does not occur and precipitates do not form in the rinsing liquid though tin compounds are present in the liquid.  
         [0044]    The rinsing liquid made acidic with hydrochloric acid is afterwards led to the molecular filter unit  10  via a pump  9 . A filter membrane is arranged inside the molecular filter  10 . The liquid present in the region in front of the filter membrane in the molecular filter is pumped in a circuit (not shown). Therefore the colloid particles are permanently in motion in the region in front of the filter membrane, so that the pores of the membrane may not be clogged (cross-flow).  
         [0045]    The part of the liquid that has passed through the filter membrane represents the permeate liquid P. The part of the liquid that has not passed through the filter membrane represents the concentrate liquid K. This part K is intermittently or continuously removed from the filtration unit  10 .  
         [0046]    In the following examples are given to more clearly describe the present invention:  
       EXAMPLE 1  
       [0047]    By means of cross-flow technique a solution containing 200 mg/l colloidal palladium, 330 ml/l hydrochloric acid (37 wt.-%) and 34 g/l tin (as a mixture of tin(II)-chloride and tin(IV)-chloride) was pressed through a filter membrane made from polyvinylidenfluorides (PVDF) with an exclusion pore size of 6.000 Dalton at a pressure of 10 bar (Δ10 6 Pa). The permeate liquid that had passed through the filter membrane was colorless and clear without any cloudiness. The concentrate liquid remained black due to the colloidal palladium particles. It was found out that the flux through the filter membrane diminished after a few minutes because the colloid had entered the membrane.  
       EXAMPLE 2  
       [0048]    The experiment of example 1 was repeated by using a membrane made of polysulfone (PES). This membrane had an exclusion pore size of 1000 Dalton. The flux through this membrane was less than that experienced from example 1 though the same pressure was applied (10 bar Δ10 6 Pa). However a decrease of flux during the experiment, which lasted 1 hour, could not be detected.  
       EXAMPLE 3  
       [0049]    Since a larger pore size would allow a higher flux, the experiment was repeated with a membrane with an exclusion pore size of 55.000 Dalton. In this case the filtrate (permeate) liquid was also black. This pointed at the fact that colloidal metal could pass the membrane.  
       EXAMPLES 4-6  
       [0050]    Example 1 was repeated with three further membranes which were made from PVDF. The exclusion pore size of these three membranes were 250 Dalton, 400 Dalton and 6.000 Dalton, respectively.  
         [0051]    After execution of the separation of the palladium particles from the liquid the concentrations of palladium, tin(II), tin(IV) and hydrochloric acid were determined and the total tin content was calculated from the tin(II) and tin(IV) concentrations. The results are given in table 1. Moreover in this table the ratios of the tin concentration in the concentrate liquid to the tin concentration in the permeate liquid as well as the respective ratio for the hydrochloric acid concentations are given.  
         [0052]    The data presented in table 1 indicate that the total amount of palladium remains in the concentrate liquid irrespective of the exclusion pore size of the membranes, whereas no palladium could be detected in the permeate liquid. Furthermore the ratios for the tin concentrations and for the hydrochloric acid concentrations drop as the exclusion pore size increases. This points at the fact that these substances arrive easier into the permeate liquid and remain to a smaller extent in the concentrate liquid as the exclusion pore size increases. By using a membrane with an exclusion pore size of 6.000 Dalton the concentrate liquid therefore contained the lowest amount of tin and hydrochloric acid.  
                                                                                                                                                   TABLE 1                                       Size Exclusion Limit:   Size Exclusion Limit:   Size Exclusion Limit:           250 Dalton   400 Dalton   6,000 Dalton            Original Concentration   Permeate   Concentrate   Ratio   Permeate   Concentrate   Ratio   Permeate   Concentrate   Ratio                    Palladium   198,5   mg/l   0   mg/l   190,0   mg/l       0   mg/l   398,5   mg/l       0   mg/l   229,0   mg/l           Tin (II)   27,0   g/l   12,7   g/l   24,1   g/l       22,3   g/l   27,4   g/l       23,4   g/l   10,6   g/l           Tin (IV)   7,0   g/l   7,1   g/l   5,5   g/l       10,9   g/l   9,3   g/l       7,3   g/l   12,5   g/l           Tin, tot.   34,0   g/l   19,8   g/l   29,6   g/l   1,50   33,2   g/l   36,7   g/l   1,11   30,7   g/l   23,1   g/l   0,75       Hydro-   332,2   ml/l   207,0   ml/l   324,0   ml/l   1,57   313,3   ml/l   334,0   ml/l   1,07   318,0   ml/l   302,0   ml/l   0,95       chloric       Acid                  
 
         [0053]    Reference Numerals:  
         [0054]    [0054] 1  rinsing container  
         [0055]    [0055] 2  spray nozzles  
         [0056]    [0056] 3  pump  
         [0057]    [0057] 4  pipeline  
         [0058]    [0058] 5  valve  
         [0059]    [0059] 6  pipeline  
         [0060]    [0060] 7  container for hydrochloric acid  
         [0061]    [0061] 8  pipeline  
         [0062]    [0062] 9  pump  
         [0063]    [0063] 10  membrane filtration unit  
         [0064]    Z rinsing liquid  
         [0065]    p permeate liquid  
         [0066]    K concentrate liquid