Waste treatment of metal plating solutions

Abstract

An efficient and economical process for the waste treatment of spent metal plating solutions, primarily spent electroless nickel solutions, to significantly reduce the metal content therein, so as to permit direct discharge to municipal water systems without violation of the law. The waste treatment process is a sequential two-step chemical precipitation process, whereby the dissolved metal content is precipitated first by sodium borohydride reduction, followed by sodium dimethyldithiocarbamate precipitation, with an intermediate filtration step and pH adjustment step interposed there between. The amount of metal-bearing sludge produced is minimal. Furthermore, the bulk of the metals in the sludge may be recovered and reused, resulting in significant reduction in hazardous waste disposal costs.

What is claimed is: 
     
       1. A process for the waste treatment of a metal-bearing waste solution to reduce the dissolved metal content prior to discharge, comprising the sequential steps of contacting the solution with a reducing agent for a sufficient time to precipitate a substantial portion of said dissolved metal content by chemical reduction, followed by contacting said solution with a precipitating agent for a sufficient time to precipitate substantially all of the remaining dissolved heavy metal content by organosulfur precipitation, then separating the precipitated metal compounds from the treated solution, and finally discharging the treated solution into a waste stream, wherein prior to organosulfur precipitation an intermediate separation is performed to remove substantially all of the reduction precipitate from said solution. 
     
     
       2. The process of claim 1, wherein said reducing agent comprises sodium borohydride. 
     
     
       3. The process of claim 1, wherein said precipitating agent comprises sodium dimethyldithiocarbamate. 
     
     
       4. The process of claim 1, wherein said metal-bearing waste solution comprises one or more solutions including at least a spent electroless nickel plating solution. 
     
     
       5. A process for the waste treatment of a spent metal plating solution to reduce the dissolved metal content to dischargeable levels, comprising sequentially contacting said solution with a borohydride reducing agent for a sufficient time to precipitate a substantial portion of the dissolved metal content in said solution, intermediately separating the reduced metal precipitate from said solution, followed by contacting said solution with a thiocarbamate precipitating agent for a sufficient time to precipitate substantially all of the remaining dissolved metal content in said solution, separating the precipitated metal thiocarbamates from said solution, and finally discharging said solution directly into a waste stream. 
     
     
       6. The process of claim 5, wherein said borohydride reducing agent comprises sodium borohydride. 
     
     
       7. The process of claim 5, wherein said thiocarbamate precipitating agent comprises sodium dimethyldithiocarbamate. 
     
     
       8. The process of claim 5, wherein said process is run in a batchwise manner. 
     
     
       9. The process of claim 5, wherein said process is run in a continuous manner. 
     
     
       10. The process of claim 5, wherein said metal plating solution comprises a spent electroless nickel plating solution. 
     
     
       11. The process of claim 5, wherein the metal contained in said reduced metal precipitate is recoverable. 
     
     
       12. The process of claim 5, wherein the pH of said solution is adjusted to between about 4 and about 11 prior to borohydride reduction. 
     
     
       13. The process of claim 5, wherein the pH of said solution is adjusted to between about 5 and about 8 prior to thiocarbamate precipitation. 
     
     
       14. The process of claim 5, wherein said metal thiocarbamate precipitates are contacted with a filtering aid selected from the group consisting of coagulants, flocculants, and diatomaceous earth, to facilitate subsequent separation. 
     
     
       15. The process of claim 5, wherein prior to borohydride reduction, sodium metabisulfite and lime are added to said solution to facilitate reduction and intermediate separation. 
     
     
       16. The process of claim 5, wherein said borohydride reducing agent and said thiocarbamate precipitating agent are each provided in excess of stoichiometric amount based on the dissolved metal content of said solution. 
     
     
       17. The process of claim 10, wherein the discharged solution contains less than 3 ppm dissolved nickel content. 
     
     
       18. The process of claim 10, wherein the discharged solution contains less than 0.5 ppm dissolved nickel content. 
     
     
       19. A process for the waste treatment of a spent electroless nickel plating solution to reduce the dissolved nickel content to dischargeable levels, comprising adjusting the pH of said solution to between about 7 and 11, contacting said solution with excess sodium borohydride reducing agent for a sufficient time to precipitate a substantial portion of the dissolved nickel content in said solution, intermediately separating the reduced nickel precipitate from said solution, pH adjusting said solution to between about 5 and 8, followed by contacting said solution with excess sodium dimethyldithiocarbamate precipitating agent for a sufficient time to precipitate substantially all of the remaining dissolved nickel content in said solution, contacting said solution with a filtering aid selected from the group consisting of coagulants, flocculants, and diatomaceous earth, separating the precipitated metal thiocarbamates from said solution, and finally discharging said solution directly into a waste stream. 
     
     
       20. The process of claim 19, wherein prior to borohydride reduction, an oxidizer scavenging agent and lime are added to said solution to facilitate reduction and intermediate separation.