United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/128 Mar. 1986
Project Summary
Metal Value Recovery from
Metal Hydroxide Sludges
L. G. Twidwell
A two-year study of the potential for
metal values from metal finishing hy-
droxide sludges was completed. The
objectives of the study were to:
• Develop flowsheets to separate and
recover metal values from metal fin-
ishing hydroxide sludge materials and
operate the flowsheets on a labora-
tory scale (Phase I),
• Develop a test assembly of unit
operations to accomplish the sepa-
ration of metal values on a pilot scale
of 75-100 pounds of sludge per day,
• Verify that the pilot-scale unit opera-
tions accomplish appropriate separa-
tions, and
• Delineate potential operational prob-
lems.
The Phase I research successfully
accomplished the stated objectives.
Flowsheets were designed and tested
on a laboratory scale prior to pilot-scale
testing. The flowsheets consist of:
sulfuric acid leaching, iron removal by
jarosite precipitation (high iron bearing
solutions) or iron solvent extraction (low
iron bearing solutions); copper removal
by solvent extraction and copper re-
covery by either electrowinning or
copper sulfate crystallization; zinc re-
moval by solvent extraction and zinc
recovery as zinc sulfate by crystalliza-
tion; chromium oxidation and subse-
quent recovery by lead chromate
precipitation; nickel removal by sulfide
precipitation or nickel sulfate crystalliza-
tion, and final solution cleanup of low
concentration residual ions by ion ex-
change.
Full-scale tests were performed to
ascertain the applicability of each unit
operation. Continuous tests were per-
formed to investigate solvent reagent
degradation; develop mass balances,
and delineate operational problems. The
successful application of metallurgical
unit operations to multicomponent
complex mixed metal sludges demon-
strated that treatment of such material
is possible and economical.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research 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
In recent years, increased emphasis
has been placed on preventing the intro-
duction of heavy metal containing indus-
trial wastewaters into publicly owned
treatment works and the environment.
Legislation has established regulatory
authority for controlling the discharge of
heavy metals into the environment and
also has mandated resource recovery
whenever economically feasible. Many of
the newer treatment and control tech-
nologies can remove metals from waste-
water, i.e., a sludge, concentrate, or
regenerate form is created and is, in most
cases, disposed of in a landfill. Metals are
recoverable, but are not recovered sig-
nificantly because of a lack of proven
technologies.
Process wastewaters from the metal
finishing and electroplating industry con-
tain cyanides and heavy metals. These
wastewaters have a detrimental effect on
the environment if discharged directly.
Such discharges are regulated by Federal,
state, county, or city ordinances, and
require installation of treatment facilities.
One of the treatment technologies pres-
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ently in use is oxidation (or reduction),
neutralization and precipitation, which
destroys cyanide and removes heavy
metals as a hydroxide sludge. Traditional-
ly, hydroxide sludge has been disposed of
in hazardous landfill sites.
Disposal in landfills has certain inher-
ent disadvantages:
1. Perpetual maintenance of the dis-
posal site is required,
2. Dilution of metal content by mixing
with other types of waste materials,
thus making recovery at a later date
more difficult, and
3. Permanent loss of nonrenewable
metals.
Recovery of heavy metals from metal
finishing sludges will alleviate or reduce
the disposal problem and provide for
conservation of energy and metal re-
sources. In the full report, the present
study outlines a technical methodology to
treat metal bearing sludges by hydro-
metallurgical techniques.
The treatment of hydroxide sludges for
metal value recovery will produce several
beneficial results, i.e., economic benefits
from the metal values recovered will help
offset the cost of recovery/treatment;
nonrenewable resource metals will be
recycled for use by society; and signif-
icantly less hazardous material will be
disposed of in landfills.
Purpose and Objectives
The purpose of the present study was to
investigate at an advanced laboratory
scale the potential for application of well-
established hydrometallurgical tech-
niques to a mixed metal sludge. The
design, development, fabrication, acquisi-
tion, assembly, and testwork for such a
treatment system was conducted at the
Montana Tech Foundation Mineral Re-
search Center in Butte, Montana (Phase
I). Further testwork (Phase II) was con-
ducted at the Montana College of M ineral
Science and Technology, Butte, Montana,
and at a waste generating site in Camaril-
lo, California.
The objectives of the study were to:
• Develop flowsheets to separate and
recover metal values from metal fin-
ishing hydroxide sludge materials,
• Develop a test assembly of unit opera-
tions to accomplish the separation of
metal values on a scale of 75-100
pounds of sludge per day, and
• Confirm that the large scale unit
operations can accomplish appropriate
separations and establish the metal
recovery efficiency for each unit opera-
tion; delineate process and materials
handling problems when treating com-
plex mixed metal sludge materials.
Study Methodology
The experimental study was conducted
in two phases. The first-phase study
objectives were to develop preliminary
flowsheets for the separation and recov-
ery of metal values from mixed metal
sludge materials; to perform laboratory
studies to test the applicability of the
preliminary flowsheets; to develop a test
assembly of unit operations capable of
treating 75-100 pounds of sludge per day;
and to conduct preliminary testwork in
the test assembly to delineate conditions
for successful operation and/or to note
potential operational problems.
Phase I was conducted by the Montana
Tech Foundation at its Minerals Research
Center in Butte, Montana. The technical
supervision of the project was performed
by Dr. L. G.Twidwell, Montana Enviromet.
A team of extractive metallurgists was
assembled as an advisory and review
group to propose, discuss, and select
potential flowsheets for laboratory test-
ing and development. The team included:
Dr. L. G. Twidwell, Director of Montana
Enviromet and Professor of Metallurgical
Engineering at the Montana College of
Mineral Science and Technology, whose
expertise is pyrometallurgy and hydro-
metallurgy; Dr. D. Robinson, consultant
for DREMCO Corporation, whose exper-
tise is electrometallurgy and solvent
extraction processes; Dr. T. J. O'Keefe,
Professor of Metallurgical Engineering at
the University of Missouri at Rolla, whose
expertise is electrometallurgy; Dr. W.
Opie, President for Research and Devel-
opment, AMAX Corporation, whose ex-
pertise is extractive metallurgical tech-
niques applied to secondary materials;
Mr. A. Mehta, a consultant for Phoenix
Metals, whose expertise is environmental
concerns and waste processing technol-
ogy as related to the electroplating
industry; and Mr. J. Downey, a private
consultant whose expertise is develop-
ment of pilot-scale studies.
The recommended flowsheets formu-
lated by the advisory and review team
were investigated at a bench-scale level
to ascertain applicability or nonapplicabil-
ity of potential unit operations. A flow-
sheet was developed that had several
feasible alternative unit operations Lab-
oratory demonstrations were performed
for each unit operation and each alternate
unit operation. A test assembly was then
developed to treat 75-100 pounds of
sludge material per day. Preliminary
testwork was conducted on a pilot scale
during Phase I of the project.
The second-phase objectives of the
study were to investigate potential alter-
nate unit operations identified in Phase I;
further test the assembly developed in
Phase I; develop long-term continuous
test data for the unit operations; and
delineate potential process and materials
handling problems.
Conclusions—Large-Scale Test
Program
• A sulfuric acid leach operation is
effective and efficient in redissolving
metal values. The dissolution is rapid
and without control problems. Condi-
tions can be specified to achieve
greater than ninety percent extraction
of all contained metals; between eighty
and ninety percent of the starting
sludge mass is taken into solution.
• Iron is removed from solution by a
jarosite precipitation process. This
process allows iron to be removed
from an acidic, pH 1.5-2.5, solution as
a crystalline compoundthat is a readily
filterable solid product. Elevated tem-
peratures, 88°-92°C, and chemical pH
control are required for the precipita-
tion to be accomplished in a relatively
short period of time, six to eight hours.
Mechanical control of the system is
not a problem. Over two hundred liters
of solution can be treated in an eight-
hour shift. Solid—liquid separation is
readily accomplished by simple set-
tling; pumping most of the cleared
solution from the settling vessel, and
filtering the remaining slurry using a
filter press for cake consolidation and
washing.
• An alternate iron removal process
applicable to low iron bearing solu-
tions (
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appreciable extraction of any other
metal specie. The recovery of copper
by solvent extraction requires only
three stages of extraction and two
stages of strip. Five cells will accom-
modate the treatment of over 200
liters of leach solution per day (eight
hours). Large-scale continuous test-
work has been performed for periods
up to seven hours. Mechanical control
of the solution flowrate and interface
levels is easily achieved and does not
require constant attention.
• Zinc is effectively and selectively re-
moved from a zinc-chromium-nickel
bearing solution by solvent extraction.
Iron, aluminum, and calcium are par-
tially coextracted with zinc. The iron
concentration is normally relatively
low because of the previous jarosite
precipitation process. The solvent ex-
traction process provides a means of
removing the residual iron from the
leach solution. Subsequently, the iron
can be separated from the zinc by a
selective stripping process.
• Chromium removal is accomplished by
first oxidizing the chromium with
chlorine gas; electrochemically or po-
tentially with S02-02 gas mixtures,
then precipitating the dichromate ion
as lead chromate. Oxidation was
shown to be effective in laboratory-
scale test reactors. Large-scale oxida-
tion test-work using chlorine and an
electrochemical reactor has been per-
formed successfully. A recycle system
for stripping the oxidized chromium
from the leach solution has been
operated successfully: the solution is
exposed to lead sulfate in an agitated
reactor; lead chromate precipitates;
the lead chromate product is crystal-
line and dense and settles rapidly; the
solution, essentially free of lead chro-
mate solid, is pumped from the solids
for further treatment for nickel re-
moval; the lead chromate is redissolved
in sulfuric acid to form a concentrated
chromic acid solution and lead sulfate;
the lead sulfate solid is separated from
the chromic acid and is recycled to the
lead chromate precipitation reactor.
• Nickel can be removed by sulfide
precipitation. The reaction is rapid and
near quantitative. The pH is maintained
in the range4-5 so hydrogen sulfide is
not released. The solid product is
readily filterable. Quantitative removal
of nickel is not necessary because
practically all the final solution can be
recycled to the leach-jarosite precipi-
tation unit operation. Therefore, the
addition of a deficiency of sulfide (less
than the stoichiometric requirement
for complete nickel removal) is desir-
able so that all of the added sulfide
ions are consumed. Then, when the
solution is recycled to the acid leach
step, hydrogen sulfide gas will not be
formed.
Economics of Recovery
First-order cost estimates indicate that
a 50-ton-per-day recovery plant with a
more or less typical plating plant sludge
would yield a return on investment of
41 % ± 12%. This estimate is based on the
very preliminary equipment costs devel-
oped by the investigators. Table 1 illus-
trates the method used to calculate ROI,
taking an overall tax rate of 50%.
Table 1. Process Cost: First Order Estimate
Cost($)
Factored Capital Annual/zed Capital Operation Cost
Cost Estimate Cost Per Year
Total Cost
Per Year
Product
Value
3.868.800
1.071,900
1.362.200
2.434.100
5.643.400
ROI = (•
5.643,400-2.434.100
3,868.800
•) (.51 (100) = 41% + 12%
L G. Twidwell is with the Montana College of Mineral Science and Technology,
Butte, MT 59701.
S. Garry Howell is the EPA Project Officer (see below).
The complete report, entitled "Metal Value Recovery from Metal Hydroxide
Sludges," (Order No. PB 86-157 294/A S; Cost: $40.95, 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 Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
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