56    POLYMER FILTRATION: A NEW TECHNOLOGY
FOR SELECTIVE METALS RECOVERY
Barbara F. Smith, Thomas W. Robison, Michael E. Cournoyer, Kennard Wilson,
Nancy N. Sauer, and Man T. Lu
A new technology is under development to selectively recover regulated metal ions from electroplating rinse waters. The electroplating metal ions are recovered in a concentrated form with
the appropriate counter ions ready for return to the original electroplating bath. The technology
is based on the use of specially designed water-soluble polymers that selectively bind with the
metal ions in the rinse bath. The polymers have such a large molecular weight that they can be
physically separated using available ultrafiltration technology. The advantages of this technology
are high metal selectivity with no sludge formation, rapid processing, low energy, low capital
costs, and small size. We have tested and demonstrated the recovery of zinc and nickel (using a
new alloy electroplating bath designed to replace cadmium) from rinse waters. The metal-ion
concentrate was returned to the original electroplating bath. Impurity metals such as iron and
copper were removed from the zinc/nickel concentrate and were not returned to the electroplating bath. Test panels were electroplated as a baseline and compared with new test panels electroplated after the addition of the recovered zinc and nickel. No adverse effects on the bath integrity
were observed. The rinse water was depleted of the electroplating metals to less than 0.1 ppm
zinc and nickel, and the water was satisfactory for discharge to the sewer system.
INTRODUCTION
Closed-loop surface finishing systems and zero-discharge goals require different approaches to
wastewater handling. Traditional precipitation technologies or even improving current precipitation technology to meet metal ion discharge limits is no longer acceptable.' New selective metal-
ion recovery systems are required to recover metal value, to reduce cost and liability of toxic
sludge formation and burial, and to reduce reagent, water, and sewer costs.
The concept of smaller point source treatment units in conjunction with plating line segregation becomes important because less energy is expended than in treating metal-ion mixtures
which occur with end-of-pipe treatment systems. By focusing on the point source, the chemistry
of metal separations can be optimized for the particular plating line, thereby optimizing the removal and recovery of the metal ions of interest. In contrast, the shotgun approach of precipitation tries to find one set of conditions that is appropriate for precipitation of most metal ions in a
combined waste stream. What normally occurs is that the optimum conditions for one suite of
metals is not optimum for others, and the metal discharge concentrations are variable and often
times exceed EPA limits. The added problem of soluble chelators that occur in electroplating
waste waters often interferes with the precipitation process, adding to the variability of metal
concentration in the discharge waters.
The newer approaches involve multiple or tandem technologies to reach the near-zero discharge requirements. Some of the proposed approaches are proposed are (1) ion exchange with
electro-recovery followed by evaporation, (2) evaporation with electro-recovery, or (3) reverse
osmosis with electro-recovery. Other technologies that can be put together include carbon treatment, recrystallization, electrodialysis. and chelating ion exchange. The tandem-technologies
chosen depend upon the ultimate goal; for example, if water is to be recovered, if the metal value
is to be recovered, if sludge is is to be eliminated, if discharge limits are to be met, or all of the
above.
52nd Purdue Industrial Waste Conference Proceedings. 1997, Ann Arbor Press, Chelsea. Michigan 48118. Printed in
U.S.A.
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