Experimentation proved valuable for Rightway Fasteners. It saved money on sludge disposal costs simply by changing the coagulant in its zinc waste treatment...
In 1992, Indiana Metal Coatings was established in response to the coating needs of Japanese automakers. The company provided dip/spin coatings on brake rotors and other automotive parts. Later, after the company changed its name to Rightway Fasteners, Inc. (RFI), it added fastener manufacturing capacity and spray coatings. Spraying the coating provided for thicker films and greater control of film builds.
In 1998, RFI added alkaline non-cyanide zinc plating and chromating to its services. the company installed an automated barrel plating system, capable of handling a variety of fastener sizes. Because of the chromate conversion coating process, RFI’s wastewater discharge was covered by the metal finishing categorical standard, 40 CFR 433. Like many metal finishers, RFI was also concerned about the metal products and machinery (MP&M) standard proposed in 2001.
With the plating line, RFI installed a conventional hydroxide precipitation wastewater treatment system. The original wastewater treatment process used calcium chloride as the coagulant. “Calcium chloride caused tremendous scale (calcium hydroxide) problems in some of our treatment tanks,” commented Doug Logan, senior safety and environmental specialist.
To counter the scale problem, RFI switched to ferric chloride, which had an additional benefit of low cost. “Ferric chloride worked well until the plating department changed its cleaning chemistry in the last electrocleaning step,” noted Mr. Logan. It started using an additive with an amine compound that proved to be a strong chelator and upset the wastewater treatment process.
Once the chelator problem had been identified, Mr. Logan began experimenting with the ferric chloride dose. He found that to treat the wastewater effectively, the coagulant dose had to be increased. “As I experimented with the treatment process, I found that we consistently needed a fairly high dose of ferric chloride to achieve the results we were looking for,” said Mr. Logan. “Around that time, a chemical supplier suggested a coagulant blend of calcium chloride with a cationic polymer. We switched to that after I did extensive testing.” Testing at RFI, for example, showed that the new coagulant worked best when added to the wastewater stream at a pH of 5, rather than 4 as suggested by the supplier.
RFI saw an immediate reduction in its sludge volume once it started using the new coagulant blend. The increased ferric chloride dose needed to treat the amine compound effectively doubled the iron concentration in the wastewater influent. Calculations suggested that eliminating the ferric chloride coagulant would reduce the sludge volume by 40%. The actual result of the change was a 37% reduction in sludge volume that saved RFI over $2,000 in the first full year.
“Apparently, calcium has a higher affinity for chelators, not necessarily like the amine compound from the electrocleaner that we do not use anymore, but things like gluconates. Essentially, calcium displaces the metals we are trying to remove: – zinc, copper and chromium – from the chelator” explained Mr. Logan. The target metals are now removed effectively by the hydroxide precipitation process.
In the same set of experiments that established operating parameters for the new coagulant, RFI evaluated two anionic flocculation polymers. With the new coagulant, the preferred polymer and minor operational changes to its sand filter, RFI now finds low metal concentrations in the treated effluent. Average effluent concentrations now are: chromium <0.03 mg/liter, copper <0.05 mg/liter, and zinc <0.04 mg/liter.
RFI’s improvements to its wastewater treatment process earned the company the 2003 Industrial Plant Award from the Indiana Industrial Operators Association, a professional organization for wastewater treatment operators.
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