POLLUTION PREVENTION ALTERNATIVES TO CYANIDE - BASED SOLUTIONS IN ELECTROPLATING ---------------------------------------------------- Braum Intertec Environment, Inc. Mendota Heights, Minnesota, U.S.A. Zinc, copper, cadmium, silver, gold brass and nickel are commonly plated using cyanide solutions. Although cyanide solutions are extremely toxic, their use has become wide-spread due to their intrinsic cleaning ability and effectiveness in keeping metals in solutions during the plating process. Motivation for eliminating cyanide solutions stem from cyanide's toxicity, potential liability public distrust, increasing regulation and rising waste treatment and disposal costs. Table below lists alternative solutions for the various cyanide plating solutions, along with their advantages and disadvantages. Zinc and to a certain extent, copper cyanide solutions are the most commonly replaced. Alternatives for cyanide silver, cadmium, nickel and gold are currently limited in application. Rinsing is vital in the plating process because it ends the chemical reactions that occur as part of the process and prevents cross-con- tamination of the subsequent plating tanks. Poor rinsing can cause staining, spotting, blistering or peeling of the coating on the plated parts. The purpose of alternative rinsing practices is to control dragout of the various plating solutions from the baths and to minimize water use. Proper design, operation and maintenance are vital to the success of the various alternative rinsing methods. Methods to control dragout include: - Decreasing withdrawal are of the parts from the plating baths. - Increasing drip time over the solution tanks. - Racking parts to avoid cupping solution within the part cavities. - Shaking, vibrating or passing the parts through an air knife. - Angling drain boards between the tanks. - Using wetting agents to decrease surface tension in the rank. - Increasing batch temperatures. - Flow restrictors on flowing rinses. - Flow control valves on each tank. - Agitation to assure adequate rinsing and homogeneity within the rinse tank. - Conductivity controllers. - Dead rinses. - Counter-current rinsing systems. - Fog or spray rinsing. - Reactive rinses which allow reuse in compatible rinsing systems. - Purified or softened water. Most platers are aware of the existence of cyanide plating alternatives and have done at least some research into the alternatives. The reasons most often provided for investigating the alternatives include increasing environmental regulations, higher waste management and disposal costs, and process cost savings. The primary barrier appears to be the high capital costs associated with converting to non-cyanide solutions, followed by the lack of customer acceptance of the alternatives. Most platers have done some modification of their rinsing systems. Most zinc electroplaters have performed pilot testing on non-cyanide solutions, or at least partially replaced their zinc cyanide plating. Most copper cyanide platers have investigated alternative solutions, while a few have run pilot testing on or converted to non-cyanide solutions. Brass, cadmium, gold, nickel and silver platers surveyed felt that there were few, if any, viable alternatives to cyanide solutions for their processes. Cyanide Eletroplating Solution Alternatives Summary of options ----------------------------------------------------------------------- Alterntive advantage Limitation Application solution potential ----------------------------------------------------------------------- Alkaline Good coverage Loss of intrinsic Promising for plating Zinc in low-current cleaning ability under 0.5 mils. Firms density areas of cyanide using solutions must solution must compensate for loss of intrinic cleaning ability of cyanide and control postblistering problems Bright Harder to plate deposits on cast iron and carbonitried steel. Throwing Generally requires power similar additional filtration. to cyanide solutions Use of existing tanks Allows for gradual phase-out of cyanide solutions Chemical costs similar to cyanide solutions Acid Faster deposition Loss of intrinsic promising for firms zinc speed than cleaning ability willing to provide alkaline zinc of cyanide. the necessary solutions. modifications and investments in their lines. Yield bright Corrosive nature deposits the of solutions may level surface require modifications irregularities. to plating equipment. Plate readily on Higher maintenance cast iron and costs. carbonitrided steel Less prone to Additional cooling and post-blistering filtration equipment than alkaline zinc may be necessary. solutions. Less sensitive to Cannot be gradually make-up water than phased in. alkaline zinc solutions. Better able to Poor throwing power in accept chromate low-current density areas. sealers than alkaline zinc solutions. Make-up water may require iron removal. Acid Superior leveling Corrosivity of solution Promising; has sulfate and brightness. is hard on plating used since copper equipment. 1950s and accepted in a wide variety of plating applications. Pretreatment is Hard to recover relatively easy dragout. and inexpensive. Make-up costs are Poor macro-throwing inexpernsive. power. High plating Solution may attack densities are base metal (strike possible. coatings may be necessary) High line speeds Additional cooling are possible equipment may be necessary. Only bright copper Acid-resistant works well on ventilation system may plastic. be necessary. Pyroph- Excellent throwing High inital solution Promising; ophate power. costs. provided the loss of the intrinsic cleaning ability of cyanide is compensated for and production speed cab be lowered to compensate for the longer plating time required. Does not attack May require longer base metal or plating times. plating equipment Dragout recovery Steel and zinc parts is possible. require copper cyanide strike. Pretreatment is May contain significant relatively easy. amounts of amonia that may pose pretreatment problems. Excellent Life of solution is subsequent plating limited. adhesion. Anode bage are not needed. High deposition of metals. Alkaline Works well on Additional cleaning Less promising- copper steel, brass, white and process controls more difficult metal, zinc die may be necessary. and expensive cast and zincated necessary. aluminum surfaces. Good throwing power. Good coverage capability. Copper Can accommodate May be more expensive Less promising- fluborate higher line to operate and more difficult speeds. difficult to control. and expensive to operate More soluble than sulfuric acid. Electr- Eliminates need Plating process much Limited oless for a copper more complex. application- nickel strike on zinc tested as an parts. alternative to copper/nickel plating on zinc die casts. Improved coverage capability. Improved corrosion protection of zinc substrates. Lower reject rates. Ammonium Bath generates Not promising silver ammonium hydroxide, due to worker which poses an health and exposure concern safety concerns. for line operators. Limited information is availabe in solution. Halide Very stable and Light-sensitive Limited silver easy to operate. solution. appilication since solution is fairly unstable. Initial cost high for electronic and decorative applications. Solution is toxic. Methane- Yield fine-grained Not yet developed on a Only tested on a sulfona- structured commercial scale. laboratory tepotas- deposits similar scale. No tests sium to cyanide in commercial iodide solutions. setting have silver been performed Amino Readiness of Not promising. -or thio- thiosulfate ions to be At one time the complex oxidized. solution was silver widely marketed, but has since been withdrawn. No free Developed Limited test cyanide specifically for application. silver electronics Developed for industry. high speed electronics plating. Good contact properties. less susceptible to tarnishing. Silver can be precipitated as AgCN and reused. Neutral pH and no free cyanide allows for free rinsing. Cadmium Limited Not promising. chloride information is Cadmium plating available on likely to be solution. phased out due to the toxicity of cadmium. Cadmium Can produce Not promising. deposits up to Cadmium plating 0.02 inches with likely to be good adhesion and phased out due density properties. to the toxicity of cadmium. Cadmium Limited Not promising. fluoborate information is Cadmium plating available on likely to be solution. phased out due to the toxicity of cadmium. Cadmium Limited Not promising. perchlorate information is Cadmium plating available on likely to be solution. phased out due to the toxicity of cadmium. Gold Excellent throwing Solutions are less More research sulfite power. stable. therefore is required for require more electronic monitoring and application control. Can plate on complex parts. Performs well on slide -wear applications. Cobalt Works well on slide Deposits are More research hardened -wear applications. brittle and is required for (no free thermal shock electronic cyanide) may cause application. gold cracking. Limited information is available on solution. ----------------------------------------------------------------------- SOURCE : MINNESOTA OFFICE OF WASTE MANAGEMENT DATE : 18TH AUGUST, 1999