THE MASSACHUSETTS TOXICS USE REDUCTION INSTITUTE
Danaher Tool Group, Springfield, MA
SummaryAs part of their commitment to reducing environmental impact, improving occupational safety, and reducing operational costs, Danaher Tool Group has implemented the toxics use reduction (TUR) technique of in- process recycling of nitric acid using diffusion dialysis. This process has reduced their annual use of nitric acid from approximately 15,000 Ibs to 6,900 Ibs.
BackgroundDanaher Tool Group is a manufacturer of steel ratchets for a variety of distributors who demand excellent performance, visual quality, and corrosion resistance in the tools. After the tools are forged and machined, the main steel components receive a nickel and chrome plate in order to ensure that the standards for corrosion resistance and aesthetic quality are met.
The electroplating process entails securing the tools to a cathodic (negatively charged) rack which is submerged in a series of two anodic (positively charged) metal baths. The first of these is a nickel plating bath. The cathodic potential passes into the secured tools which then attract nickel cations to the surface of the tool from the bath. The rack is then submerged in the chrome bath where chrome cations are similarly attracted to the surface of the tool. Most of the rack's surface area is covered with an insulating plastic coating so that the racks themselves don't attract the nickel and chrome ions, but the tip of the rack is left exposed so that electric current can pass through the tools. After five uses, the tips of the holding racks become plated to the point where they no longer transfer their cathodic potential to the secured tools.
In order to remove the accumulated chrome and nickel plating, the racks are submerged off-line in a 400 gallon stripping tank of 75% by volume nitric acid (HN03). Over a period of time, this nitric bath becomes saturated with chrome and dissolved nickel rendering the solution ineffective for farther stripping. An increase in cosmetic defects in tools was traced to poorly stripped racks. Efforts to decrease cosmetic rejection rates through use of a cleaner, more effective, stripping bath, led to increased frequency of bath disposal. Historically, the contents of the saturated stripping bath were shipped off-site for disposal as hazardous waste. Danaher realized that extension of the acid bath's service life would be a more cost-effective and environmentally friendly way to resolve these quality issues.
Toxics use reduction planning at Danaher resulted in the installation of a diffusion dialysis system in May of 1994. The diffusion dialysis equipment allows for the reuse of the stripping acid through in-process recycling, reducing annual use of nitric acid.
Arid Recycling Using Diffusion DialysisOnce five plating cycles have been completed using a given rack, the rack is dipped into anitric stripping tank and then two rinse tanks. The nitric acid stripping solution dissolves the nickel from the exposed rack tip causing the chrome to flake off. In order to reuse the nitric acid, the chrome flake and the dissolved nickel must be separated from the bath. The stripping solution is routed into an adjacent 10 micron filtration reservoir where the chrome flakes are removed, and then flows into the diffilsion dialysis unit for removal of the dissolved nickel. To accommodate the chrome filtration system a larger stripping tank and thus a larger ventilation system were installed.
The dialysis unit used by Danaher is Model No. AJ-20 manufactured by Pure Cycle Environmental Technologies, Inc of Palmer, MA. This unit uses a membrane separation process which consists of a series of anionic copolymer membranes positioned to separate counterflowing spent acid and clean water streams at a flow rate of 12 gallon/day. The concentration gradient between the acidic stripping solution and the water drives the hydrogen ions (H*) and the nitrate ions (HNO3) through the membranes into the water. The metal ions are unable to pass through the membrane due to their comparatively large size and slow movement. The resulting acid solution on the clean water side of the membrane exhibits 80% to 90% of the original acidity with only 10% of the nickel. This 6.0 to 7.0 N (Normal) acid solution is piped directly back into the 7.0 to 9.0 N stripping tank. The remaining spent solution containing the bulk of the metals is sent to wastewater treatment. The efficiency of the recovery process is reduced if the bath becomes saturated with contaminant so it is recommended to recycle the bath on a constant basis. Approximately 12 gallons/week of make-up acid is added to the stripping tank to compensate for the reduction in normality of the recycled solution and losses due to drag-out.
Toxics Use Reduction Assessment
TransferabilityDiffusion dialysis units can be used to recover almost any strong acid, such as those used for stripping, cleaning or plating operations. Units are available which handle flow rates from less than 5 gallons/day up to 1000 gallons/day. The range of sizes and the efficiencies of the unit makes it appropriate for a wide variety of applications.