EPA
TECHNOLOGY
TRANS
REQOJNG
ZINC IN
VISCOSE
RAYON
PLANTS BY
TWO STAGE
PRECIPITATION
U.S.
ENVIRONMENTAL
PROTEC~~
.. flAL
DEMONSTRATDN
GRANT
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EPA
TECHNOLOGY
TRANSFK
RECYOJNG
ZINC IN
VISCOSE RAYON
PLANTS BY
TWO STAGE
PRECIPITATION
ENTAL
., __ .„ . .\lr\L
DEMONSTRATION
GRANT
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*».
Densator Reactor
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Over 50 million Ibs.
of zinc sulfate are
used annually in the
United States for the
manufacture of
approximately one billion
pounds of viscose rayon.
Zinc is used as a
regeneration retardant in
the acid spinning bath.
Since it is not consumed
in any of the viscose
reactions, these 50
million pounds of zinc
represent process losses,
through dragout by the
filaments to the
subsequent wash streams,
filter backwashing,
splashes, leaks and the
washing of equipment.
The effects of zinc
as a pollutant are
well documented.
Concentrations as low as
1.0 ppm have been shown
to be harmful to fish.
In addition, there is some
evidence indicating that
zinc has a synergistic
property when associated
with copper.
Although it has been
known that zinc can be
precipitated from the acid
waste streams by the use
of lime, the resultant
sludge has been of low
zinc assay, contaminated
with other compounds,
and with very poor
settling characteristics. In
commercial operations,
the sludge presented a
disposal problem and
recovery of zinc suitable
for recycle was impossible.
In this EPA
demonstration grant with
the American Enka
Company, a process for
precipitating a dense
sludge of high zinc assay
was proven. The zinc in
the sludge was recovered
and recycled to the rayon
manufacturing plant.
This recycling of zinc was
shown to have no ill
effects on the rayon yarn.
There are 10 viscose
rayon manufacturing
plants in the United
States, all of which are
believed to use zinc sulfate
in their spinning bath.
This process greatly
enhances the economics
of removing this source of
zinc pollution, allowing
neutralization of the acid
stream and recovery of
the zinc of a good profit
for industrial yarns and at
a moderate cost for
textile yarns.
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American Enka Waste Treatment Facilities
The key to this zinc
recovery process is a
two-stage precipitation
with the second
precipitation taking place
under careful pH control,
using sodium hydroxide,
in contact with a
circulating slurry of zinc
hydroxide crystals. All of
the zinc precipitates in
the second step, most
of the impurities in
the first.
Referring to the flow
sheet, Figure 1, the
elements of the process
are as follows:
Acid and alkaline waste
streams are collected in a
neutralization tank (T-l).
Here sufficient lime is
added to raise the pH
to 6.0. At this point, no
zinc hydroxide will
precipitate but a portion
of the iron, calcium
sulfate, and other
impurities will form a
light precipitate. With a
coagulant aid, the
mixture is sent to a
clarifier (C-l) where a
clear overflow containing
the dissolved zinc
is obtained.
This clear overflow is
contacted in a reactor
(R-l) with a circulating
stream of previously
precipitated sludge
containing zinc hydroxide.
The pH is raised
subsequently to 9.5 —10.0
with sodium hydroxide.
The bulk of the zinc
precipitates onto the
existing crystals in the
circulating slurry. At
steady state conditions,
a withdrawal of the
circulating slurry stream
is made equivalent to the
zinc being added. This
dense sludge is then
settled (T-2). The settled
sludge of 4-7% zinc assay
is converted back to zinc
sulfate with sulfuric acid
(T-3) and sent back to
the spinning bath. If
desired, the sludge can
be filtered or centrifuged
to 18% solids before
dissolving with acid.
The zinc content of the
overflow water from the
densator-reactor is set by
the pH-solubility
relationship of zinc in
water and results in a zinc
content of 0.5 — 1 ppm
atpH = 10. Once
the precipitated zinc is
removed from the
wastewater, the pH can
be readjusted to a
lower value.
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Typical Influent
vs. the
Treated Effluent
at American Enka's
North Carolina Plant
(Basically Textile Yarn
Nominally 50 Million Ibs. Per Year)
Ibs./lOOO Ibs. yarn
unless otherwise noted
IN
OUT
Flow
pH
Zn
H2SO4
Na2SO4
CaSO4
MgSO4
BOD
COD
131,000 (1500 GPM)
1.5-3.0
12.5 (95 ppm)
184 (1400 ppm)
288 (2200 ppm)
13.8 (105 ppm)
3.1 (24 ppm)
7.0 (53 ppm)
131,000 (1500 GPM)
8.0 (after final
pH adjustment)
0.13(1 ppm)
320 (2440 ppm)
256 (1940 ppm)
13.8 (105 ppm)
3.1 (24 ppm)
7.0 (53 ppm)
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The conventional
technique for removing
zinc from the spinning
acid waste stream has
been direct lime
precipitation to a pH of
about 10, with no zinc
recovery. The economics
of this approach are
compared to the
American Enka Zinc
Recycle Process. In order
to protect proprietary
spinning information
relating to acid/zinc
ratios for a given product
mix of yarns, it has been
necessary to choose a
hypothetical product mix
and acid ratio which,
although reasonable for
economic evaluation,
does not correspond to
an actual production
situation of the American
Enka Plant.
The economics of
recovery are a very
strong function of the
amount of zinc used in
the preparation of the
yarn and the ratio of acid
to zinc in the spinning
bath. In manufacturing
industrial yarns and tire
cords, it is common to
use 4.5 — 7.5 pounds of
zinc per 100 pounds
of yarn. This high
concentration of zinc
makes recovery extremely
attractive. Textile yarns
use less zinc and although
recovery is still the most
economic solution, it
offers less of a return.
These two cases are
presented as extremes,
with many plants falling
between the two values.
For the 50 MM Ibs./yr
plant considered, the use
of two stage precipitation
combined with zinc
recycle offers a savings
of $383.000 over
neutralization for a plant
producing industrial yarns
and a savings of $68,000
for textile yarns. Many
plants produce a mix of
the two and results
would be between these
values. The costs
associated with the more
extensive sludge
handling and storage in
neutralization and
precipitation only
aren't included.
The cost of installing
the complete
neutralization and zinc
recycle system would
Lime Slaking System
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50MM/LBS./YR.
OF
TEXTILE YARN
(Acid/Zn SO4 = 5.5)
Ibs./Zn
100 Ibs. yarn = 1.5
Total Investment
(including engineering)
Operating Costs (all costs in $/yr.)
I Wages
II Electric Power @ 8 mils
III Maintenance (3% on Invest./yr.)
IV Laboratory
V General Plant Overhead
VI Raw Materials
Lime ($20/ton)
NaOH ($40/ton)
Polyelectrolyte
Total Materials
Total Operating Cost
Fixed Charges
Depreciation @ 15 yrs.
Taxes, Insurance
Total Fixed Charges
Zinc Credit (@ 15.5/lb.)
Conventional Lime
Neutralization
Precipitation
$425,000
Two Stage
Precipitation
With Zinc
Recycle
$625,000
5,000 10,000
7,000 9,300
12,800 18,800
All on-site, included in wages
8,000 10,000
72,900
72,900
105,700
28,300
2,000
30,300
0
64,800
20,800
5.500
91,100
139,200
41,600
3,000
44,600
[116,000]
Net Yearly Cost Total
136,000
67,800
Net Yearly Advantage
(Recycle over
Conventional Neutralization)
68,200
Pretax Return on Differential Investment $ 68.200 = 34.1%
$200,000
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50 MM IBS./ YR.
OF
INDUSTRIAL YARN
(Acid/Zn SO4 - 2.0)
Ibs. Zn
100 Ibs. yarn - 6.0
Total Investment
{including engineering)
Operating Costs (all costs in $/yr.)
I Wages J/2, 1 man @ $10,000/yr.
II Electric Power @ 8 mils
HI Maintenance {3% on Invest/yr.)
IV Laboratory
V General Plant Overhead
VI Raw Materials
Lime ($20/ton)
NaOH ($40/ton)
Polyelectrolyte
Total Materials
Total Operating Cost
Fixed Charges
Depreciation @ 15 yrs.
Taxes, Insurance
Total Fixed Charges
Zinc Credit (@ 15.5/Ib.)
Conventional Lime
Neutralization
Precipitation
$425,000
Two Stage
Precipitation
With Zinc
Recycle
$625,000
5,000
7,000
12,800
All on-site, included in wages
8,000
123,500
123,500
156,300
28,300
2.000
30,300
0
10,000
9,300
18,800
10,000
94,000
76,000
5.500
175,500
223,600
41.600
3.000
44.600
[465.0001
Net Yearly Cost Total
186,000
Net Yearly Advantage
(Recycle over
Conventional Neutralization)
Pretax Return on Differential Investment
383,600
383.600
200,000
192%
Pretax Return on Total Zn Pollution $197,000 = 31.5%
Control Facility $625,000
[197,000] Credit
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Sludge Settling and Sludge Dissolving Tanks
have negligible economic
impact on the rayon
industry, running from
$.40/100 Ibs. profit to a
$.13/100 Ibs. cost
compared to selling
prices of $30-35/100 Ibs.
of staple, $70-80/100 Ibs.
of tire yarn, and
$100-150/100 Ibs. of
filament. Zinc oxide
manufacturers face the
loss of the bulk of a
50 million Ib./year
market as this product is
reused rather than
wasted. At the present
rate of consumption, the
known world zinc
reserves are estimated to
last 23 years. Allowing
for an estimated yearly
increase in consumption
of 2.9%, these reserves
would last only 18 years.
Thus, the recycling of
zinc in the rayon industry
is desirable to extend
the domestic supply.
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This technology, with
only small modifications
to conform with local
plant conditions, could
have immediate
application in
any viscose rayon plant
with soluble zinc in the
plant waste stream. The
techniques of initially
precipitating the
impurities which would
prohibit zinc recycle as
well as the use of a
sludge recirculation
process to obtain a
dense sludge are excellent
examples of good process
engineering being applied
to a waste problem.
In a broader sense this
technology could have
application to any waste
stream containing
soluble zinc in a form
which can be precipitated
by lime or caustic
addition. The possibility
of recycling the
Pump Room
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precipitated zinc would
depend upon the nature
of the process considered
and may require further
work. Examples of other
areas with zinc containing
wastes are ground-wood
pulp, metal plating, zinc
refining, and recirculating
water systems. To our
knowledge, no R&D
activity to demonstrate
the economics and
effectiveness of this
technology in these
applications is in evidence
to date.
For further
information:
Detailed information
on this project,
including equipment
list, is available from
the Superintendent of
Documents as
EPA Report 12090 ESG
"Zinc Precipitation
and Recovery from
Viscose Rayon
Wastewater".
Or write:
Technology Transfer
Environmental
Protection Agency
Washington, D.C.
20460
* U. S. GOVERNMENT PRINTING OFFICE : 1974 O - 534- 948
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