The project encompassed development and incorporation of a waste treatment process for alkaline soak cleaners used in our aluminum chemical finishing processes. The chelating agents in our cleaners were tying up the heavy metals (in particular chromium, the major contaminant) in our waste chemical streams. This prevented complete precipitation of the chromium for proper filtration removal in our waste treatment facility.
The old method of disposal was to have the cleaning solutions either drummed up or placed in a tank truck for off-site disposal. This was not desirable due to trucking/disposal costs and potential liabilities. Another method of disposing of the solution was suggested by the manufacturer of the cleaning solution. This method involved breaking the chelating effect, thereby rendering the solution treatable through our existing waste treatment facility. The chelating effect was broken by adding calcium oxide (lime), mixing the treated solution for one hour, and finally adding the mixed solution to the waste treatment pH adjust tank at a controlled rate for treatment. While the calcium oxide (lime) treatment process was an alternative, there was the cost of additional process equipment (mixing tank, mixer, metering pump, etc.) and time involved. Another alternative investigated was reducing the rate at which the cleaner was metered into the waste treatment process. While this did reduce the level of chromium in the effluent, the levels exhibited with practical cleaner flow rates still did not provide chrome levels below 1 ppm (total chrome), as desired. One final study was initiated. This involved the possible use of nucleation agents to improve the precipitation process. Some of the possible nucleating agents were coprecipitated metals such as copper, nickel, zinc, iron, calcium, aluminum or cationic polymers. Looking for a naturally occurring source of one of the above metals, the concept for possible use of aluminum, which builds up in our alkaline etch solution was researched. A 50/50 mix of the waste soak cleaners and alkaline etch solutions, when added at low percentages by volume of reduced chrome solution, provided satisfactory chrome levels in the waste treatment process effluent. This was, in part, due to the dissolved aluminum in the alkaline etch acting as a nucleating agent for more effective precipitation of heavy metals, while reducing the effects of the chelating agents in the cleaners.
This use of waste alkaline etch solutions provided an innovative means for improved in-house treatment of alkaline soak cleaners, thereby eliminating the costs and liabilities of having to transport these waste solutions (some 1600 gallons/year) off-site for disposal.
Details of Reductions
Additional Information :
This process saved some $2300/year in trucking/disposal costs per tank, while being a process which was accomplished without the cost of additional equipment and processing time.