Q. Our company has a wastewater treatment plant for three different processes: hex chrome plating, zinc plating and stainless steel polishing. Once wastewater is received in an equalization tank, it is pumped to a chrome-reduction tank. Next, zinc, nickel and trivalent chromium are precipitated with calcium hydroxide (lime). We then add a metal precipitator and flocculant before the clarifier in order to achieve compliance with our limits to the municipal sewer. This process generates a lot of sludge, however, the metal content does not qualify for recycling, so we need to send it to a landfill. Is there a process or commercial equipment to electrodeposit these metals ( Zn, Ni, Cr, Fe) and remove the heavy metals from the wastewater? H.E.
A. Electrowinning is the technology used to recover metals, however, based on your processes it is highly unlikely that this technology will be cost-effective. There are a few ideas that you may want to explore in order to reduce the amount of sludge generated, however.
First, I would work with your chemical supplier to replace calcium hydroxide with sodium hydroxide. The calcium in calcium hydroxide creates much of the sludge as it precipitates out other ions such as sulfates and phosphates. None of our metal finishing clients use calcium hydroxide for pH adjustment. Some are able to meet very stringent limits with sodium hydroxide plus other chemistries, including cationic polymers, polyaluminum chlorides and organic sulfide metal scavengers.
Second, based upon your description, it appears that all of your wastewater goes through chrome reduction, which typically operates at about a pH of 2.0–2.5. This means that all of your wastewater is acidified with sulfuric acid, increasing the amount of calcium hydroxide needed for pH adjustment and increasing the amount of sulfuric acid used. Both the additional amount of calcium hydroxide and the additional precipitated calcium sulfate (sulfate from the sulfuric acid) contribute to the excessive amount of sludge and the dilution of metals. Explore sending only your hex-chrome-containing rinse waters and wastes through chrome reduction.
Reducing the amount of sludge and the “junk” in your sludge will increase its value for recycling. There are firms that will “recycle” metal hydroxide sludges from metal finishing and electroplating operations no matter what their metal content, it is just a matter of the cost.