Treating Small Batches

Ask an Expert From: Products Finishing, , from Hixson Inc.

Posted on: 11/1/2003

Question: I am seeking a simple treatment system for the elimination of approximately 10 gallons of plating rinse water per day coming from a copper pyrophosphate bath, a nickel sulfamate bath and a gold cyanide bath.

Question:

I am seeking a simple treatment system for the elimination of approximately 10 gallons of plating rinse water per day coming from a copper pyrophosphate bath, a nickel sulfamate bath and a gold cyanide bath. I’m expecting very dilute rinse water, approximately 250 ppm of chemicals. I had hoped to just use a small evaporator, but now I’m not so sure. I’ve never done waste treat before, and I don’t know whether I’ll need a cyanide destruct system. What are the local, state and federal regulations? Where should I even start? M.L.

Answer:

For so little wastewater generated, the potential environmental regulatory impact of your discharge could be enormous. To answer your question fully is outside of the scope of this column, but let me give you some guidance.
First, pretreating your wastewater in several very small batch treatment tanks for the oxidation of cyanide and the removal of heavy metals and then discharging to sewer would be relatively easy, but the permitting, monitoring and analytical costs are huge since the wastewater is regulated by EPA’s Metal Finishing pretreatment standards (40CFR433).

Second, because of gold’s very high value and that the bath typically operates at an elevated temperature (140-160F), all of the dragout from the gold cyanide bath can be recovered by several dragout, counterflow, recovery rinse tanks located after the bath. Here, distilled or deionized water flows into the last rinse tank at the same rate as evaporation from the gold plating tank, and then flows from tank to tank to plating tank in the opposite direction of work flow. If the quality of the rinse water in the last rinse tank is not good enough, add another recovery rinse tank. By doing so, you will eliminate the need to treat for cyanide in your rinse water. Also, if you were to mix these rinse waters, you would run the risk of generating deadly cyanide gas, and the cyanide would combine with the nickel, forming an extremely stable nickel cyanide compound that vigorously resists treatment.

Third, evaporation is likely your best option. You may want to consider this concept. Combine your copper and nickel wastewaters into a batch tank where you will check its pH. If the pH is less than 2, add some alkali or caustic in order to raise the pH, preferably above 7. If the pH is greater than 12.5, add some acid to reduce the pH, preferably below 9. The purpose of this pH adjustment is to keep you from “treating” a hazardous waste under EPA regulations; elementary pH adjustment is an exception. After checking pH, pump the wastewater to your evaporator tank, which may need a heater; because of the very small volume, an atmospheric evaporator is likely the most cost effective technology. The wastewater would recirculate through the evaporator and tank, becoming more concentrated. At some point though, the wastewater will become so concentrated that you will need to pump it out of the tank into a plastic, DOT-approved drum for off-site disposal.

While copper and nickel are not U. S. EPA hazardous waste metals, some states do regulate them as such. In addition, as the waste is concentrated, other trace heavy metals could rise in concentrations above maximum regulatory levels or the waste could become more acidic (pH < 2) or caustic (pH > 12.5) and become a corrosive hazardous waste. Because of the copper and nickel concentration, the waste may be a candidate for recycling. At this point, we recommend that you contact your state hazardous waste agency to give you more specific guidance. Every state that we have worked in has a small business ombudsman that can give assistance without threat of enforcement action.

One last thought. I hope your very small plating operation is worth all of this hassle. Good luck.

 



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