United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-88/019 Mar. 1988
Project Summary
Metal Value Recovery from
Metal Hydroxide
Sludges:Removal of Iron and
Recovery of Chromium
L G. Twidwell and D. R. Dahnke
This experimental study was
conducted in three phases. The third
phase results are summarized in this
report. The first phase study
objectives were to develop
preliminary flowsheets for the
separation and recovery 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.
The second phase objectives
were: to investigate potential
alternate unit operations identified in
Phase I; to further test the assembly
developed in Phase I; to develop
long-term continuous test data for
the unit operations; and to delineate
potential process and materials
handling problems.
The results of the Phase I and
Phase II studies are reported in EPA
600/ 2-85/128 "Metal Value Recovery
from Metal Hydroxide Sludges,"
March 1985 (PB86 157294/AS).
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
The objectives of this Phase III
investigation were: to develop flowsheets
to separate and recover metal values
from electroplating sludge materials; to
collect bench scale experimental data to
verify the feasibility of the proposed
flowsheets; to demonstrate the
effectiveness of the separation
techniques on a large-scale (75-100
pounds of sludge per day); and to
develop a first order economic evaluation
of the proposed flowsheets for an
exemplary centralized treatment facility.
The emphasis of the project was directed
toward investigating the application of
phosphate precipitation as a means of
selectively separating iron and chromium
from divalent cation species.
These objectives have been
accomplished. Flowsheets and
alternatives are discussed in the body of
the main report. The developed
flowsheets have been verified to be
feasible by laboratory test work and
selective metal value separations have
been shown to be possible, e.g., iron and
chromium can be separated from divalent
metals such as zinc, nickel, and
cadmium. Large-scale test work has
also verified that effective separations are
feasible and practical, and an economic
evaluation has been performed showing
that an excellent return on investment is
possible.
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Background
In recent years, increased emphasis
has been placed on preventing the
introduction of heavy metal-containing
industrial wastewaters into publicly
owned treatment works and the
environment. Legislation has established
regulatory authority for controlling the
discharge of heavy metals into the
environment. It also has mandated
resource recovery whenever eco-
nomically feasible. Many treatment and
control technologies have come into
existence to remove metals from these
wastewaters, but a sludge, concentrate,
or regenerate form is created and, in
most cases, disposed in a landfill. Metals
are recoverable, but are not recovered
significantly because of a lack of proven,
cost-effective technologies.
Process wastewaters from the metal
finishing and electroplating industry
contain cyanides and heavy metals.
These wastewaters have a detrimental
effect on the environment if directly
discharged. Such discharges are
regulated by Federal, State, County
and/or City ordinances, which may
require installation of pretreatment works.
Some of the treatment technologies
presently in use involve chemical
oxidation (or reduction), neutralization,
and precipitation to destroy cyanide and
remove heavy metals as hydroxide
sludges. These sludges have traditionally
been disposed in hazardous landfill sites.
Should heavy metals be recovered from
metal finishing sludges, the alleviation of
disposal problems can provide for
conservation of energy and metal
resources. This study outlines a technical
method that offers a procedure for
treating metal bearing sludges via
hydrometallurgical techniques. The
treatment of hydroxide sludges for metal
value recovery will produce several
beneficial results: economic benefits
from the metal recovered will help offset
the cost of recovery/treatment; non-
renewable resources will be recycled for
use by society; and there will be
significantly less hazardous material to
be disposed.
Iron and Chromium Removal
It has been demonstrated that
conventional hydrometallurgical unit
operations can be applied to mixed metal
hydroxide sludge materials. Effective and
selective separations of metal values
from complex mixed metal solutions
have been accomplished, e.g.,
separation of Fe, Cu, Zn, Cr, and Ni.
However, two unit operations are high
cost energy processes, i.e., removal of
iron and oxidation with subsequent
recovery of chromium.
The present study was initiated to
investigate an alternative and potentially
more cost-effective way for removing
iron and recovering chromium from
mixed metal solutions. Bench-scale test
work by Dahnke has shown that trivalent
cations can very effectively be stripped
from solutions in preference to divalent
cations under conditions of low pH and
room temperature by phosphate
precipitation. The successful application
of simple precipitation of iron and
chromium from mixed solutions could
mean elimination of the two high cost
unit operations:
1. Iron removal by jarosite precipitation;
2. Chromium recovery by oxidation to
chromate.
The substitution of relatively simple
precipitation processes for the above
more complex processes should
significantly increase the cost-ef-
fectiveness of the overall metal value
recovery sequence.
Results and Conclusions
An extremely large data base has
been generated during the course of the
present study for both the bench-scale
and the large-scale test work. The
bench-scale study results support the
following conclusions:
1. Trivalent cations can be effectively
and selectively separated from
divalent and monovalent solution
species.
2. Ferric iron concentrations can be
lowered to a few mg/liter in acidic
solutions.
3. Ferric phosphate precipitation is
rapid and selective over trivalent
chromium and divalent cations under
room temperature conditions.
4. Ferric phosphate precipitates as
small spherites showing excellent
filtering characteristics.
5. Ferric phosphate precipitates in a
similar manner from acidic solution
under essentially the same
experimental conditions regardless
of the solvent matrix, e.g., from
sulfate, chloride, nitrate or
ammonium solutions.
6. Ferric phosphate can be converted
to ferric hydroxide with the
regeneration of the phosphate
reagent by a caustic leach.
7. Chromium phosphate requires a
precipitation incubation time of
several hours at room temperature
but is very rapid at elevated
temperatures, therefore, the
difference in the room temperatu
precipitation kinetics for iron ar
chromium provides a means f
separating these two trivale
cations.
8. Filterability of chromium phospha
(small spherites) precipitated from i
elevated temperature solution
about the same as the filter ability
ferric phosphate.
9. Chromium phosphate can t
converted to marketable products I
a soda ash roast process producir
high market value metal chromati
or chromic acid.
The objective of the large-scale te
work was to demonstrate on a significa
quantity of actual sludge material th
selective separations cou
accomplished. The large-scale test wo
confirmed the bench scale result
Important conclusions that have result*
include:
1. Sulfuric acid leaching was ve
effective in redissolving the mete
from electroplating sludge. In tl
case of the electromachining sludg
however, the leach residue contaim
most of the niobium and titanium ar
therefore provided a valuab
recoverable residue.
2. The weight of solids that must I
disposed, including the leach residi
and converted ferric phosphate (
ferric hydroxide), was less than tl
weight of the starting sludge. Bo
the leach residue and ferr
phosphate solids pass the El
TLCP test can normally be dispos<
in non-hazardous disposal sites.
3. The metal products that a
recovered are of sufficient purity
serve as feedstock for commerc
uses or for conversion to other mo
marketable products.
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L G. Twidwell and D. R. Dahnke are with Montana College of Mineral Science
and Technology, Butte, MT 59701.
John F. Martin is the EPA Project Officer (see below)
The complete report, entitled "Metal Value Recovery from Metal Hydroxide
Sludges: Removal of Iron and Recovery of Chromium," (Order No. PB 88-
176 0781 AS; Cost: $25.95, 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
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/S2-88/019
0001961 HUE* GN
LIBRA?* REGIOH V
US EP*
60604
•ff U.S GOVERNMENT PRINTING OFFICE. 1988—548-013/870c
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