United States
Environmental Protection
Agency
Risk Reduction Engineering
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-91/041 Sept. 1991
iSrEPA Project Summary
Recovery of Metals from Sludges
and Wastewaters
This report presents information on
the state-of-the-art of metals recovery
technologies to assist in Identifying
waste-management options for metal-
bearing sludges and wastewaters that
may be regulated under the Resource
Conservation and Recovery Act (RCRA).
Only a few of the technologies ad-
dressed In this report (e.g., electrowin-
ning, high-temperature metals recovery
[HTMR]) are directly applicable to the
recovery of metals from wastes; other
technologies treat the wastes to a physi-
cal form that may be amenable to even-
tual metals recovery.
Wastewaters can be treated effec-
tively by several methods. Precipitation
processes have been widely used to
remove arsenic, cadmium, chromium
(+3), copper, iron, manganese, nickel,
lead, and zinc from metal-bearing waste-
waters. For economic reasons, electro-
winning is a commercial technology that
has normally been restricted to the treat-
ment of wastewaters containing noble
metals such as gold and silver.
After appropriate pretreatment, slud-
ges can be effectively treated by HTMR
processes. These processes allow for
the direct recovery of metals from slud-
ges. The economic feasibility depends
on the amount of sludges treated and
the amount of metals contained In the
sludges. Membrane separation pro-
cesses such as microfiltratlon (MF) and
ultra filtration (UF) can be used in com-
bination with chemical treatment for the
physical separation of metal sludges.
Leaching may be used to extract cad-
mium, chromium, copper, lead, nickel,
and zinc directly from sludges by using
various process trains.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
Section 3004 of the Resource Conser-
vation and Recovery Act (RCRA), as
amended by the Hazardous and Solid
Waste Amendments of 1984, prohibits plac-
ing untreated RCRA-regulated hazardous
wastes in or on the land. Waste manage-
ment options are needed to help recyclers
comply with these regulations. In the full
report, summarized here, we address the
following processes that are amenable for
recovery of metals from hazardous wastes:
chemical precipitation, electrolytic recov-
ery, HTMR, membrane separation, leach-
ing, ion exchange and evaporation. For
each of these technologies, the following
parameters are summarized: (1) design
specifications of applicable processes, (2)
waste characteristics affecting perfor-
mance, (3) pretreatment/posttreatment re-
quirements, (4) available performance data,
and (5) availability of the technology and
feasibility for treating various hazardous
waste categories.
Waste Chacterizatlon
This report covers nine major metal-
waste-producing industries:
Printed on Recycled Paper
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(1) metal coatings; (2) smelting and
refining of nonferrous metals; (3) paint, ink,
and associated products; (4) petroleum
refining; (5) iron and steel manufacturing;
(6) photographic industry; (7) leather tan-
ning; (8) wood preserving; and (9) battery
manufacturing. Waste streams from each
of these industries have unique character-
istics; however, the wastes also contain
common metals, such as aluminum (Al),
arsenic (As), cadmium (Cd), chromium (Cr),
copper (Cu), lead (Pb), nickel (Ni), silver
(Ag), and zinc (Zn).
Table 1 presents the number of metal-
waste generators (as of 1983) by Standard
Industrial Classification (SIC) code for the
major industry categories discussed in the
report.
Table 2 indicates the amount and num-
ber of generators of metal-bearing wastes
by D (wastes which are hazardous be-
cause they exhibit a particular hazardous
characteristic), F (wastes from non-spe-
cific sources), and K (wastes from specific
sources) EPA hazardous waste codes. Until
very recently, only about half of the indus-
tries that generate metal-bearing wastes
recovered the metals from wastewaters
and sludges.
Table 3 presents brief descriptions of
the hazardous wastes generated from the
major industry categories included in the
report.
Metals Recovery Technologies
Chemical Precipitation
Precipitation of metal-laden wastewa-
ters involves adding chemicals to alter the
physical state of the dissolved or sus-
pended metals and to facilitate their re-
moval through sedimentation. These
precipitates may then be processed fur-
ther for metals recovery. Chemicals used
to effect precipitation include: caustic soda,
lime, ferrous and sodium sulfide, soda ash,
sodium borohydrkJe, and sodium phos-
phate. Some wastewater constituents, e.g.,
hexavalent chromium, cannot be effectively
precipitated without first chemically reduc-
ing the metal to a more favorable form for
precipitation. Reducing agents typically
used by industry include sulfur dioxide,
sodium bisulfite, sodium metabisulfite, and
ferrous sutlate. Coagulation chemicals may
be needed to enhance settling times of the
precipitated metal particles. Examples of
coagulants currently used by industry in-
clude lime, alum, and synthetic polyelec-
trotytes.
Chemical precipitation is commonly
used to treat metal-bearing wastewaters
from electroplating, pigment manufacture,
Table 1.
Number of Major Metal-Waste Generators, by SIC Code, in 1983
SIC
Code No.
3471
2851
3479
3714
2819
3341
3400
9711
3721
3900
3356
2893
3312
3321
4911
2869
2821
3662
3679
3711
3545
SIC Description
Plating and surface finishing
Paints and allied products
Metal coating and allied products
Motor vehicle parts and accessories
Industrial inorganic chemicals
Metals, nonferrous, secondary
Fabricated metal products
National security
Motors and generators
Miscellaneous manufacturing industries
Metal, nonferrous, rolling, drawing
Printing ink
Blast furnaces, steel mills
Foundries, gray iron
Electric services
Industrial organic chemicals
Plastics material
Radio and TV communication equipment
Electronic components
Motor vehicle bodies
Machine tool accessories
No. of
Facilities
4,287
2,145
2,902
4,151
2,183
876
55,380
393
966
32,867
384
609
1,229
1,229
2,614
1,160
1,529
4,656
5,392
1,040
3,432
Tablo 2. Nationwide Metal-Waste-Generation Data by Waste Group
D Wastes
F Wastes
K Wastes
Waste
Volume,
10*gal/yr
3685
3920
219
Percent
of
Total
Metals
46.9
49.9
2.8
Number of
Generators
3860
2091
402
the photographic industry, leather tanning,
wood preserving, the electronics industry,
battery manufacture, and nonferrous metal
production. Approximately 75% of all elec-
troplating facilities use precipitation in the
treatment of their wastewaters. The pro-
cess is several decades old, and chemical
feed reagents are being improved to yield
better metal removals from the aqueous
phase.
Specific waste characteristics that af-
fect the performance of chemical precipita-
tion systems include (1) the concentration
and type of metals, (2) the concentration of
total dissolved solids, (3) the concentration
of complexing agents, and (4) the concen-
tration of oil and grease.
Pretreatment of wastewaters before
metals precipitation can involve segrega-
tion, removal of large solids, flow equaliza-
tion, cyanide destruction (if applicable),
chrome reduction, oil separation, neutral-
ization, and/or waste treatment of the indi-
vidual process streams.
Sand filtration is a common post-pre-
cipitation/sedimentation effluent treatment
technique. If concentrations in the effluent
do not meet discharge standards, addi-
tional metal treatment technologies (e.g.,
ion exchange, reverse osmosis) may be
needed.
Electrolytic Recovery
Electrolytic processes are used exten-
sively to recover metals from industrial
wastewaters. The electrolytic cell is the
basic device used in electrolytic recovery
operations. The cell consists of an anode
and a cathode immersed in an electrolyte.
When current is applied, dissolved metals
in the electrolyte are reduced and depos-
ited on the cathode. Because the metal(s)
removed from solution can be reused, the
technology, termed "electrowinning," is con-
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Table 3. Metal-Bearing Hazardous Wastes From Major Industry Categories
EPA Hazardous Listed
Waste No. Hazardous Waste Description Constituent(s)
F006
F007
F008
F009
F019
K002
K003
K004
K005
K006
Wastewater treatment sludges from electro-
plating operations except the following:
(1) sulfuric acid anodizing of aluminum;
(2) tin plating on carbon steel; (3) zinc
plating (segregated basis) on carbon steel;
(4) aluminum or zinc-aluminum plating on
carbon steel; (5) cleaning/stripping asso-
ciated with tin, zinc, and aluminum plating
on carbon steel; and (6) chemical etching and
milling of aluminum.
Spent cyanide plating bath solutions from
electroplating operations.
Plating sludges from the bottom of plating
baths from electroplating operations where
cyanides are used in the process.
Spent stripping and cleaning bath solutions
from electroplating operations where cya-
nides are used in the process.
Wastewater treatment sludges from the chemi-
cal conversion coating of aluminum.
Wastewater treatment sludge from the pro-
duction of chrome yellow and orange pigments.
Wastewater treatment sludge from the pro-
duction ofmolybdate orange pigments.
Wastewater treatment sludge from the pro-
duction of zinc yellow pigments.
Wastewater treatment sludge from the pro-
duction of chrome green pigments.
Wastewater treatment sludge from the pro-
duction of chrome oxide green pigments
Cadmium, hexa-
valent chromium,
nickel, cyanide
(complexed)
Cyanide/salts
Cyanide/salts
Cyanide/salts
Cadmium,
hexavalent
chromium,
cyanide
(complexed)
Hexavalent
chromium,
lead
Hexavalent
chromium, lead
Hexavalent
chromium
Hexavalent
chromium, lead
Hexavalent
chromium
K007
K008
K048
K049
K050
K051
K052
K060
K061
K062
K064
(anhydrous and hydrated).
Wastewater treatment sludge from the pro-
duction of iron blue pigments.
Oven residue from the production of chrome
oxide green pigments.
Dissolved air flotation (DAF) float from the
petroleum refining industry.
Slop oil emulsion solids from the petroleum
refining industry.
Heat exchanger bundle-cleaning sludge from
the petroleum refining industry.
API separator sludge from the petroleum
refining industry.
Tank bottoms (leaded) from the petroleum
refining industry.
Ammonia still lime sludge from coking
operations.
Emission control dust/sludge from the primary
production of steel in electric furnaces.
Spent pickle liquor generated by steel-
finishing operations of facilities
within the iron and steel industry.
Acid plant blowdown slurry/sludge resulting
from the thickening of blowdown slurry from
primary copper production.
Cyanide
(complex),
hexavalent
chromium
Hexavalent
chromium
Hexavalent
chromium, lead
Hexavalent
chromium, lead
Hexavalent
chromium
Hexavalent
chromium, lead
Lead
Arsenic
Hexavalent
chromium,
lead, cadmium
Hexavalent
chromium, lead
Lead, cadmium
(continued on p.4)
sidered a recovery process. If a membrane
is used between the cathode and the an-
ode for the selective transport of some
ions, the process is called electrodialysis.
Electrowinning is most effective for recov-
ery of noble metals such as gold and sil-
ver. These metals have high electrode
potentials and are easily reduced and de-
posited on the cathode. Metals such as
cadmium, copper, chromium, lead, tin, and
zinc can be removed, but a greater amount
of current is required. Electrowinning is
very effective for plating solutions used in
printed circuit boards; these contain che-
lated metals that are difficult to remove by
other means.
Electrowinning of metals is a particu-
larly attractive process because it com-
pletely eliminates the generation of a
metal-bearing sludge. Its applicability, how-
ever, is limited to waste streams contain-
ing metals in solution such as cadmium,
copper, chromium, gold, lead, silver, tin, or
zinc. For dilute solutions, electrowinning
can be difficult because of the low mass-
transfer rates; however, mass transfer rates
can be enhanced both by agitating the
solution and by increasing the effective
surface area of the cathode.
The principal area of application of elec-
trodialysis is the recovery of metals from
electroplating bath rinse waters.
In many cases, the wastewater must
be filtered before it is fed through the elec-
trolytic reactor. Adjustment of pH is a nec-
essary pretreatment measure because the
waste pH affects metal speciation.
Metal recoveries of up to 98% from
plating rinse waters have been demon-
strated with the use of high-surface area
(HSA) electrodes.
Several vendors are currently manu-
facturing electrodialysis systems for treat-
ment of wastes from gold, chromium, silver,
and zinc cyanide plating operations and
from nickel plating operations. Other suc-
cessful electrodialysis applications include
recovery of metals from tin and trivalent
chromium baths and the recovery of chro-
mic acid and sulfuric acid from spent brass
etchants. Electrowinning and electrodialy-
sis systems have both been used exten-
sively in industrial applications.
High-Temperature Metals Recovery
(HTMR)
Several types of HTMR processes are
currently available or under development
for the recovery of metals from sludges
generated either directly by industrial pro-
cesses or from the treatment of industrial
wastewaters. These HTMR processes may
involve plasma-based or high-temperature
fluid-wall reactor systems (which use elec-
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Table 3. (Continued)
EPA Hazardous
Waste No.
Hazardous Waste Description
(continued on p.4)
Listed
Constituents)
K065
K066
K069
K086
K090
K100
D004
D006
D007
D008
D009
D011
Surface impoundment solids contained in and
dredged from surface impoundments at primary
lead smelting facilities.
Sludge from treatment of process wastewater
and/or acid plant btowdown from primary zinc
production.
Emission control dust/sludge from secondary
lead smelting.
Solvent washes and sludges, caustic washes,
and sludges from cleaning tubs and equipment
used in the formulation of ink from pigments,
driers, soaps, and stabilizers containing
chromium and lead.
Emission control dust or sludge firom ferro
chromium silicon production.
Waste leaching solution from add leaching
of emission control dust/sludge from
secondary lead smelting.
Characteristic waste based on concentrations.
Characteristic waste based on concentrations.
Characteristic waste based on concentrations.
Characteristic waste based on concentrations.
Characteristic waste based on concentrations.
Characteristic waste based on concentrations.
Lead, cadmium
Lead, cadmium
Hexavalent
chromium,
lead,
cadmium
Lead,
hexavalent
chromium
Chromium
Hexavalent
chromium, lead,
cadmium
Arsenic
Cadmium
Chromium
Lead
Mercury
Silver
trterty as the energy source) or coal/natu-
ral- gas-based technologies.
HTMR processes are applicable only
for the processing of sludges, not for waste-
waters. One significant advantage of the
HTMR processes is that other toxic con-
stituents in the wastes, such as complexed
cyanides/organics, would also be destroyed
at the high temperatures (>1100°C) pre-
vailing in the furnaces.
Important waste characteristics affect-
ing the performance of HTMR processes
include: (1) concentrations of undesirable
volatile metals, (2) boiling points of the
metal constituents, and (3) thermal con-
ductivity of the waste. Pretreatment re-
quirements for HTMR processes vary with
the type of process. This may include op-
erations such as drying of feed sludges or
pelletizing with special additives. The crude
metallic oxides produced in certain HTMR
processes must be further treated for sepa-
ration and recovery of metals. Gases from
the high temperature furnaces must be
treated before atmospheric release.
The INMETCO Plant in Ellwood City,
PA (which utilizes a rotary hearth/electric
furnace) and the Horsehead Waelz Kiln in
Palmerton, PA, have processed hazard-
ous wastes (sludges) under an Interim Per-
mit status. The INMETCO Plant has
processed the following waste codes: F006,
K061, K062, D006, D007, and D008. The
Horsehead Waelz kiln has processed F006,
F019, K061, D006, and D008.
Horsehead has two operating Waelz
plants in the United States—one at
Palmerton, PA, and one at Calumet City,
IL. The Palmerton plant has three Waelz
kilns with a total capacity of 270,000 tons/
y r; the Calumet plant has one kiln with a
capacity of 80,000 tons/yr. The INMETCO
plant is capable of treating 50,000 tons of
wastes per year. Both of the Horsehead
Waelz plants as well as the INMETCO
plant are operated primarily to treat steel-
making electric arc furnace dust (K061);
however, as previously mentioned, they
are capable of treating other sludges. A
third Horsehead Waelz plant with a capac-
ity of 60,000 tons/yr is planned for
Rockwood, TN.
Membrane Separation
The commercially available membrane
processes for removal of metals from in-
dustrial wastewaters are microfiltration, ul-
trafiltration, reverse osmosis, and
electrodialysis. Microfiltration (MF) and ul-
trafiltration (UF) are used in combination
with chemical treatment for the physical
separation of metal sludges. Reverse os-
mosis (RO) and electrodialysis (ED) are
used to recover plating compounds from
rinse water and to enable reuse of rinse
waters.
MF and UF membranes cannot be ap-
plied directly to recover metals present as
dissolved solids in wastewaters. UF, how-
ever, can be used as a pretreatment
method for RO units to avoid fouling of the
RO membranes.
When applied to heavy metal wastes
with appropriate pretreatment chemistry,
the metal content of the effluent can be
extremely low. Each application requires
treatability studies to integrate the chemi-
cal pretreatment and the MFAJF mem-
brane system.
If oil and grease are present, addi-
tional pretreatment of the waste stream
will be required. Well-run precipitation/
UF and MF systems can achieve metal
removals greater than 99%. Over 150
full-scale industrial systems, ranging in
size up to 400 gal/min, have been in-
stalled in the electroplating, printed cir-
cuit board manufacturing, battery
manufacturing, and photographic process-
ing industries.
RO systems consist of several mod-
ules connected in series, or parallel, or a
combination of both. The application of RO
to the treatment of metal-containing wastes
is limited by the pH range in which the
membrane can operate. Cellulose acetate
membranes cannot be used on waste
streams where the pH is much above 7.
The amide or polysutfone membranes, how-
ever, have a pH range of 1 to 12. Collodial
matter, low-solubility salts, and dissolved
organics can seriously inhibit the effective-
ness of RO. Pretreatment steps such as
pH adjustment, carbon adsorption, chemi-
cal precipitation, or filtration are therefore
recommended to ensure extended service
life of RO systems. Systems are being
used commercially to recover brass,
hexavalent chromium, copper, nickel, and
zinc from metal-finishing solutions.
Leaching
Leaching is a process in which a solid
material is contacted with a liquid solvent
for selectively dissolving some components
of the solid into the liquid phase. Leaching
can sometimes be used to extract various
metals from sludges. The goals of this
process are: (1) to dissolve the metals in a
liquid phase to produce a solution that can
be reused directly in a process or from
which the metal can be recovered by other
techniques, such as electrowinning; and
(2) to produce a secondary sludge that is
nonhazardous or from which additional
metals can be reclaimed by other pro-
cesses. Several leaching agents can po-
tentially be used, including sulfuric acid,
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ferric sutfate, ammonia or ammonium car-
bonate, hydrochloric acid, sulfur dioxide,
ferric chloride, nitric acid, or a caustic solu-
tion. Selection of a suitable solvent and
unit process depends on the chemical state
and physical environment of the metals.
Sludges that contain only one metal
often can be sent directly to a refiner for
reclamation; however, in some operations
(e.g., electroplating), all metals are precipi-
tated from solution in the same wastewa-
ter treatment plant, usually as hydroxides.
A process train with numerous unit opera-
tions, therefore, is necessary to separate
each metal. Complete recovery of the met-
als typically includes electrowinning of the
leachate.
At the Recontek waste recycling facility
in Newman, IL, zinc-bearing solutions are
leached with alkaline solutions, whereas
non-zinc sludges are treated with acidic
solutions. Zinc-bearing sludges are di-
gested at approximately 80*C whh sodium
hydroxide for a sufficient period of time,
cooled, and filtered. The filtrate is pro-
cessed in a zinc cementation tank to pre-
cipitate metals more electronegative than
zinc (e.g., lead, cadmium) and then pumped
to a zinc electrowinning system. The non-
zinc sludge waste from the digester (pri-
marily copper and nickel) is digested with
suffuric acid and filtered to produce a resi-
due containing precious metals (e.g., gold,
silver). The filtrate is then sent to the cop-
per electrowinning system for production
of copper cathodes.
Ion Exchange
Ion exchange is a treatment technol-
ogy applicable to (1) metals in wastewa-
ters where the metals are present as soluble
ionic species (e.g., Cr3 and CnV); (2)
nonmetallic anions such as halides, sul-
fates, nitrates, and cyanides; and (3) wa-
ter-soluble, ionic organic compounds
including (a) acids such as carboxylics,
sulfonics, and some phenols, at a pH suffi-
ciently alkaline to yield ionic species, (b)
amines, when the solution acidity is suffi-
ciently acid to form the corresponding acid
salt, and (c) quaternary amines and aklyl-
sulfates.
Ion exchange is a reversible chemical
reaction in which an ion from solution is
substituted for a similarly charged ion at-
tached to an immobile solid particle. The
use of this process is practical only on
wastewaters and sludge leachates. In con-
ventional ion exchange, metal ions from
dilute wastewater solutions are exchanged
for ions electrostatically held on the sur-
face of the exchange medium. Ion ex-
change systems have proven to be effective
in the removal of barium, cadmium, chro-
mium (VI), copper, lead, mercury, nickel,
selenium, silver, uranium, and zinc.
Evaporation
Evaporation is a simplified recovery sys-
tem for the separation of substances based
on volatility differences. Although the tech-
nology is established, recent advancements
have made mechanical evaporation a more
viable cost-efficient method for metals re-
covery. The four basic types of evapora-
tors used in the electroplating industry today
are rising-film, flash, submerged-tube, and
atmospheric.
Conclusions
Only a few of the technologies ad-
dressed in this report (e.g., electrowinning,
HTMR) are directly applicable to recover-
ing metals from wastes; other technologies
treat the wastes to a physical form that
may be amenable to eventual metals re-
covery.
Commercial waste recycling facilities
render services that are important if metals
are to be recovered as opposed to being
treated and disposed. Technologies used
at the metals recovery facilities include
chemical precipitation, leaching, electro-
winning, and evaporation.
Current information on metals recovery
technologies show that combinations of
technologies may often be required to re-
cover metals from wastewaters and slud-
ges. Additional studies are needed to
determine the specific combinations of
methods that will most effectively recover
metals from different types of wastes.
The full report was submitted in fulfill-
ment of EPA Contract No. 68-03-3413,
Work Assignment 2-63, by IT Corporation,
Cincinnati, OH (formerly PEI Associates,
Inc.), under the sponsorship of the U.S.
Environmental Protection Agency.
6l).S. GOVERNMENT PRINTING OFFICE: 1991 - S4H-OU/40080
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This Project Summary was prepared by the staff of IT Corporation, Cincinnati, OH
45246.
Ronald J. Turner is the EPA Work Assignment Manager (see below).
The complete report, entitled "Recovery of Metals from Sludges and Wastewaters,"
(Order No. PB91-220384/AS; Cost: $23.00, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Work Assignment Manager can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/041
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