In an example of a Plating Operation, an open-cycle heat pump was installed in a plant that
produces copper foil for the printed circuit board industry. The foil is
manufactured by electrodepositing copper onto a rotating drum, peeling off the thin
copper foil, then plating with additional copper and zinc. The rinse waters from
these two final plating processes, containing copper sulfate and zinc sulfate
respectively, are concentrated 8 times in a reverse osmosis (RO) system, with the
product (pure water) being recycled as rinse water.
The concentrate (RO reject) stream was formerly sent to the plant waste treatment facility for removal of copper, zinc, and sulfates before discharge. The copper and zinc were removed by precipitation with lime, which also removed much of the sulfate. However, sufficient sulfate remained in the wastewater such that additional treatment with expensive barium acetate was required.
The lime sludge, rich in copper and zinc, was sold to a reclaimer of plating solutions for a fraction of the original value. The barium sulfate sludge had no market value.
The plant installed an MVR evaporator to further concentrate the RO reject stream. The distillate
from the evaporator is recycled as process water, and the concentrate is recycled
to the plating baths.
An MVR evaporator is at its best when evaporating large quantities of water from initially dilute solutions that don't become too viscous or result in a high boiling-point elevation. These commonly occur where plating or dye materials can be recycled for reuse using evaporation.
In the plant, the MVR evaporator uses external heat energy only on startup. In operation, the evaporated vapor is compressed and used as the heating steam to the evaporator. Thus, the heat of vaporization is recovered. This method is similar to multiple-effect evaporation, except that the vapor is compressed for use in the same evaporator instead of being used in additional effects (evaporators).
During normal operation, the evaporator operates at 150°F and at a vacuum of about 24 inches Hg. A concentration of 10:1 can be achieved at flow rates as high as 17 gallons/minute and at energy consumption as low as 40 Btu/lb of evaporated water.
Shipped pre-assembled, piped, and skid-mounted with all instrumentation, control panel, and wiring, this packaged concept reduced evaporator installation cost and interface engineering.
Plating rinses are now a closed-loop, or "zero discharge," system. Rinse water is concentrated for recycle directly back to the plating bath, and the pure water from the RO and evaporator concentrators is reused in the process. Copper sulfate and zinc sulfate, worth $120,000 year, are now returned to the process rather than being sold as sludge. The load on the plant waste treatment facility has been reduced; with only 10 percent of the original sludge volume now being processed. This has reduced treatment chemical costs and disposal costs of the barium sulfate sludge.
Providing a full recycle of this waste stream would not have been practicable with just RO or just MVR evaporation, but used together, the system has been economically and environmentally successful.
