Section Nine
INDUSTRIAL WASTES-C. METAL PROCESSING
WASTES
80    SELECTIVE PRECIPITATION OF MIXED METAL
HYDROXIDES
Clyde S. Brooks, President
Recycle Metals
Glastonbury, Connecticut 06033
INTRODUCTION
Selective precipitation is examined here as a technically feasible approach for improving the efficiency for recovering non-ferrous metals from industrial waste effluents in the presence of contaminants such as aluminum, chromium or iron. Precipitation provides probably the lowest cost alternative for the non-selective removal of metals from industrial waste waters to meet EPA standards for
effluents. Consideration of the pH dependence of hydroxide precipitation of mixed metal systems
such as copper or nickel in the presence of aluminum, chromium or iron would seem to provide an
opportunity to conduct staged hydroxide precipitation of the contaminants, aluminum, chromium or
iron below about pH 5-6 delaying precipitation of the copper or nickel to a pH above 6. Unfortunately freshly precipitated aluminum, chromium and iron hydroxides provide considerable capacity
for adsorption/ion exchange leading to mixed coprecipitates of the trivalent contaminants and the
divalent copper or nickel frustrating efficient separations. Recent experimentation with iron precipitated in the presence of selected anionic surfactants in a moderate pH range 3.5-4.5 permitted
efficient separation from copper or nickel which remained in solution pending a secondary separation
process.'
The formation of mixed precipitates called jarosites occurs by hydroxide reaction with NaOH of
KOH from sulfuric acid solutions of mixed metals.2,3 A typical composition for a jarosite might be
Na(FeCr)3(OH)6(S04)2 with inclusion of anions such as phosphates or chromates. Investigations have
shown a preferential ordering of metal cations coprecipitated in jarosite system is of the order of
Fe3+ >>Cu2+>Zn2+ >Co2+ >Ni2+ >Cd2+. Advantage has been taken of the collection action of
freshly precipitated aluminum or ferric hydroxide to remove by adsorption colloid flotation low
concentrations of metal cations such as Cd2+, Cu2+, Cr(III), Ni2+ and Zn2+ in waste effluents in the
presence of anionic flotation surfactants such as sodium lauryl sulfate.4'5 Co-precipitation of ferrous
and ferric cations to form ferrites with soluble metal cations such as Cd2+, Cu2 + , Cr(III), Ni2+ or
Pb2+ has been applied successfully to removal of metal contaminants from waste effluents.6 More
recent EPA funded investigations7 have evaluated co-precipitation effects for iron and other heavy
metals such as Cr(III), Cu, Ni, Zn and Pb in efforts to develop metal recovery processes from
industrial waste streams.
The approach in the present investigation is quite different than in any of the studies cited above. In
this investigation the objective has been to take advantage of the competitive adsorption of anionic
surfactants in a moderately acid pH regime on freshly precipitated ferric hydroxide to effect separations from copper or nickel in solution. Furthermore, an evaluation of an extension of this approach
to separations of copper or nickel form aluminum or chromium (III) hydroxides has been
undertaken.
In this study, model experiments have been conducted in which ferric iron in solution with copper
or nickel plus selected anionic surfactants, primarily carboxylic acid types, is adjusted in pH with
NaOH to precipitate the iron as ferric hydroxide leaving the copper or nickel in solution. Screening
tests were conducted with a series of anionic surfactants, lauric acid, oleic acid, ascorbic acid, acetic
acid, citric acid, and ethylene diamine tetra acetic acid (EDTA) to establish the anionic providing the
most effective competition with nickel for adsorption/ion exchange on freshly precipitated ferric
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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