EPA
TECHNOLOGY
TRANSFER
-0/7
RECOVERY OF
SPENT
SUIRJRIC ACID
FROM
STEEL PICKLING
OPERATIONS
PREPARED BY
U.S. ENVIRONMENTAL
PROTECTION AGENCY
ENVIRONMENTAL
RESEARCH
INFORMATION
CENTER
-------
EPA
TECHNOLOGY
TRANSFER
EPA-625/2-78-017
RECOVERY OF
SPENT
SULKJRICACID
FROM
STEEL PICKLING
OPERATIONS
PREPARED BY
U.S. ENVIRONMENTAL
PROTECTION AGENCY
ENVIRONMENTAL
RESEARCH
INFORMATION
CENTER
-------
Acid recovery tower (left), acid mist filter (center), fresh add storage tank (right)
-------
[I
Approximately 100 million tons of steel are pro-
duced every year in the United States. A large portion
of this steel is initially produced in some intermediate
form, such as sheet, rod, or ingot, and then sent to
other manufacturers for further processing into final
steel products. During the manufacturing process as
well as during the storing and shipping of the inter-
mediate forms, the steel develops a surface coating of
oxidized iron which must be removed before the
steel can be processed further.
The removal of the surface scale through a
chemical reaction of the scale and steel with sulfuric
(H2SO4) or hydrochloric (HCI) acid is called pickling.
Sulfuric and hydrochloric acids are the major acids
used in the pickling process, although other acids
may be used. In the pickling process the steel is
immersed in an acid bath which dissolves the scale
and some of the iron. After a required period of
Figure 1. Handling
Spent Pickle Liquor
Acid
Makeup
time, the steel is removed from the acid bath and the
residual acid is rinsed from the steel. As the iron
content in the pickling acid bath increases, the pickl-
ing efficiency is reduced and fresh acid must be
added to the bath to maintain an efficient reaction
rate. Eventually, the iron content becomes excessive
for effective pickling, and the "spent pickle liquor"
must be replaced with fresh acid.
It is estimated that about one-half of the steel
produced in the United States is cleaned by the pickl-
ing process. The disposal of this spent pickle liquor
and the associated rinse waters creates a pollution
problem for the manufacturer. The sources of the
wastewaters and the types of treatment technologies
are shown in Figure 1.
Pickling Acid Streams
Acid
Returned
Spent Pickle Liquor
I
Contract
Hauling
Deep Well
Injection ;
By-P(-pduct
Disposal:
Treatment Possibilities
Type
of Acid
Acid Neutralization Recovery
HCI
HNO3
H2S04
Mixtures
Contaminated
Rinse Water
Neutrali-
zation,
Chemical
Precipi-
tation
•Waterway
Solids
Separation
or
Lagoon
Sludge
Disposal
-------
The large primary manufacturers of steel products
generally use hydrochloric acid in the pickling
process. Sulfuric acid is used by the smaller, secondary
steel finishers. .Although the amount of steel pickled
by hydrochloric acid is approximately twice the
amount pickled by sulfuric acid, the number of facili-
ties using sulfuric acid for batch pickling far exceeds
those using hydrochloric acid.
The nation-wide control of this source of pollution
is complicated by the large number of manufacturers
using relatively small amounts of sulfuric acid. The
spent sulfuric acid pickle liquor, totaling about 375
million gallons per year, is highly concentrated, con-
taining up to 10 percent dissolved iron and up to 10
percent unreacted sulfuric acid. The rinse water used
to remove the residual acid also adds to the volume
of contaminated wastewater. The following are four
major methods that are used to control pollution
caused by sulfuric acid pickling.
• Acid Recovery — The dissolved iron salts are
removed from the spent acid, which then can
be reused.
• Neutralization — A base (usually lime) is reacted
with the spent acid, which is then discharged.
« Contract Disposal — For a fee, a contractor takes
the spent pickle liquor from the manufacturer and
finds a method for disposal.
• Deep Well Injection — The pickle liquor is
injected deep into the earth, where it is contained
between impervious layers of rock.
This report is intended to provide small manufac-
turers using sulfuric acid pickling with the technical
and economic information necessary to select the
treatment technologies best suited for their facilities.
Because deep well injection has only limited applica-
tion, to these plants, it is not discussed in this report.
Wire rod entering pickling operation
-------
Basic Principles of Operation
There are several different commercially applied
processes for recovery of sulfuric acid from spent
pickle liquor. All processes, however, rely upon the
basic principles of crystallization of iron salts (mainly
ferrous sulfate) from the spent liquor and the addition
of enough fresh sulfuric acid to return the pickling
solution to its original acid strength. These com-
mercial acid recovery systems allow the free sulfuric
acid remaining in the spent pickling solution to be
reused. However, they do not regenerate sulfuric acid
from the iron salts obtained in the reaction with the
steel.1 The processes differ in the methods used to
crystallize the ferrous sulfate.
^Although there are processes under development which do
regenerate sulfuric acid from these iron salts, the economics for
commercial application have not yet been proven.
The solubility of iron salts in pickle liquor is such
that crystallization can be induced by cooling or by
evaporating water from the solution. Commercial
processes are available which use either of the above
techniques, as well as combinations of the two. In
addition, both batch and continuous processes are
available. The selection of the best process for a
specific pickling operation depends upon several
factors, which will be discussed in Section 4.
Figure 2 illustrates the basic principles of a batch-
wise cooling process, a system widely used by smaller
facilities in the United States. In this illustration, the
acid recovery unit is integrated into a wire produc-
tion facility.
Figure 2. Acid Recovery System
Make-up
Sulfuric
Acid
Exhaust Gases to Atmosphere
Demisting Filter
Fan
LJ H( ^> /-Fumes Rinse fc
Water
Pickling!
v Tank I Spent Water
Overflow -
Rinse
Water 1
Steam
Condensate
Dip and Spray
Rinse Tank
-------
When the dissolved iron and the sulfuric acid
content of the pickling solution reach a predetermined
level, it is pumped to the batch crystallizer. The
pickling tank is then immediately recharged with
recovered acid from a previous batch, and pickling
resumes.
The crystal yield can be enhanced by adding fresh
sulfuric acid to the crystallizer at this time. Since the
make-up acid is at a high concentration, the solubility
of the ferrous salt is reduced, and a higher yield is
obtained. The disadvantage is that heat of dilution
must be removed by the refrigeration unit.
In the crystallizer, over a period of 8 to 16 hours,
the temperature of the spent liquor is slowly reduced
to a temperature between 35° to 50°F. During this
cooling cycle ferrous sulfate heptahydrate
(FeSO4»7H2O) is crystallized from the solution.
The ferrous sulfate crystal slurry is transferred
into a crystal collection chamber which retains the
crystals but allows the pickle liquor to pass through to
an acid recovery tank. The retained ferrous sulfate
heptahydrate crystals are then washed with small
amounts of fresh water to remove free acid. The water
adhering to the crystals is partially removed by
drawing air through the crystal bed. The resulting
ferrous sulfate crystals are removed for disposal, or,
preferably, marketed. The recovered acid is preheated
using steam, and is ready to be returned to the pickle
liquor tank.
The net effect of the acid recovery system is to
recover the unreacted sulfuric acid for reuse, instead
of discharging it as a waste product. Additionally, the
ferrous sulfate crystals are recovered in a form which
usually can be marketed.1
In this batch system, cooling is provided to the
spent liquor by circulating chilled water through
cooling coils. Teflon® heat exchange coils often are
1 Ferrous sulfate is used as a coagulant in municipal
waste-treatment plants, as a fertilizer additive, animal
feed additive, pigment production, and as a raw material
for iron oxide in magnetic tapes.
used as a cooling surface, since coating and corrosion
of metallic cooling coils can be a problem.
Other processes differ from the one described
only in the methods used in the crystallization step
and in the separation of the formed crystals from the
recovered acid. One continuous process uses a
vacuum crystallizer. By decreasing the pressure in the
vessel to 28 in. Hg. vacuum, the pickle liquor is
cooled and a portion of the water is vaporized. The
Ferrous sulfate heptahydrate crystal collection box
-------
crystals are discharged from the crystallizer body to a
centrifuge which separates and washes the crystals
from the liquor. The liquor is returned to the pickling
tank and the crystals are sold or stored.
Adapting Acid Recovery to the Pickling Process
If acid recovery is to be used to minimize or even
eliminate the discharge of wastewater in an existing
pickling facility, normally it will be necessary to
reduce the amount of water being added to the
pickling system. If this is not done, the efficiency of
the recovery system is impaired. Ideally, in a zero
discharge condition, the water leaving the system
through free moisture and the water of hydration of
the crystals combined with the evaporation occurring
during the pickling process will equal the water
being added.
There are three major sources of added water in
the pickling process: (1) large amounts of water often
are used for rinsing the pickled steel; (2) the common
technique of using live steam injection as a means of
heating and agitating the pickling solution adds water
as condensed steam to the pickle liquor; and (3) some
water is produced by the reaction of the iron oxide
scale with sulfuric acid.
The most common method for reducing the rinse
volume is through counter-current contacting of the
work and the rinse water. One of the least expensive
techniques to do this for batch pickling is to use the
dip and spray system shown in Figure 2.
After dipping in the rinse vessel, a spray of fresh
water is applied to remove the slightly contaminated
rinse water remaining on the steel. The slightly
contaminated spray water then falls into the rinse
tank. The more concentrated water from the rinse
1 This increased evaporation does require some
additional steam.
tank eventually is transferred to the pickling tank,
where evaporation further reduces the water volume.
To eliminate the dilution of the pickle liquor by
condensate, the live steam injection for heating the
pickling solution is replaced with a steam heat ex-
changer, which prevents steam from contacting the
pickle liquor.
The use of direct steam injection as a method for
heating and agitating the pickling bath developed
because of the corrosiveness of the pickling solution
and its tendency to coat heat transfer surfaces.
However, the dilution effect of the steam condensing
in the pickle liquor made recovery of the acid very
difficult. Teflon® coils, which prevent the heating
steam from contacting the pickle liquor, can be used
as heat exchangers. Air injection can then be used for
agitation, which results in improved pickling rates.
This air also maintains the negative water balance for
the plant, since water evaporation from the pickling
tank is improved by the air flow.1
Some wire mills have achieved zero discharge of
wastewater from the pickling plants by installing an
acid recovery system and by modifying the pickling
line to include staged rinsing, indirect heating and air
injection agitation to reduce water usage. Because
zero discharge will require additional investment for
the modifications and an exact balancing of water
rates, each plant must examine the operating costs
and economics for its situation. This is discussed
further in Section 4.
-------
Neutralization
Neutralization has been the traditional method
for treating acid streams, and it has been used for
dilute pickle1 liquors. There are many possible neutral-
izing agents, such as caustic soda, soda ash, hydrated
lime, and quicklime. Lime in a slurry form is used
most frequently. A system similar to that shown in
Figure 3 is needed for plants which use quicklime.
Neutralization with lime requires the preparation
of the lime slurry by "slaking" quicklime. The follow-
ing chemical reaction occurs:
CaO + H2O
"quicklime"
Ca(OH)2
"hydrated lime"
The calcium hydroxide is then mixed with the pickle
liquor. The lime slurry is fed at an excess of 15 to 35
percent above the theoretical rate to assure complete
neutralization.
Neutralization of spent pickle liquor requires two
reactions. The free sulfuric acid is neutralized by the
calcium hydroxide:
H2SO4 + Ca(OH)2 *- CaSC>4 + 2H2O
and the ferrous sulfate is changed to ferrous hydroxide
by the calcium hydroxide:
FeSO4 + Ca(OH)2 »• CaSO4+ Fe(OH)2
Thus, the lime requirements are greater than those for
neutralizing only the acid.
After the acid is neutralized, the precipitated
calcium sulfate and iron hydroxide must be
separated from the liquid.
Separation of the finely divided solids from the
liquid is difficult, since ferrous hydroxide does not
settle easily. Therefore, obtaining an overflow of suf-
ficient clarity to discharge to a stream often is a
problem.1 If the sludge is to be transported a long
distance for disposal, the additional water may be
removed by vacuum filtration. In many cases, the
neutralized stream is pumped directly into a lagoon
which has a large holdup time (several days), which
allows the separation of the liquid and solids.
1 If the plant is discharging to a municipal system, this
may not be critical.
Combined Neutralization and Oxidation
To overcome the difficulty in separating the
ferrous hydroxide, a modified neutralization technique
which includes neutralization of the spent pickle
liquor plus oxidation of the ferrous hydroxide to
magnetite (Fe3
-------
Figure 3. Complete Neutralization System
1
LimJ!
Holding
Tan(c
i
1
i
i
1
I
'1
Lime
Slaker
Lime
Storage
To Onsite Disposal
or Hauling
-------
The use of an acid recovery system offers the
manufacturer a savings in acid costs and greatly
reduces his pollution control problems. Neutralization
not only does not recover the acid in the pickle
liquor, but it requires the manufacturer to purchase
lime for its neutralization. On the other hand, the
original investment for a neutralization system can be
less than that for acid recovery under certain condi-
tions, and the installation of neutralization technology
normally does not require many modifications in the
manufacturing area. Contract disposal of spent pickle
liquor offers minimization of investment at the
possible price of high operating costs. The relative
advantages of each of these factors must be evaluated
before a wise decision can be made for any specific
facility. This section contains information which will
allow the manufacturer to evaluate the performance
and economics of these options under the operating
conditions at his plant. To arrive at this data, the
manufacturer needs to know only the percentage of
iron and free acid in the pickle liquor when it is
considered "spent," and the percentage of steel loss
in pickling.
If the pickling iron loss is not known, it can be
estimated by averaging the iron and sulfuric acid
composition in the spent pickle liquor over a period
of time and using the following formula:
Iron =
Tons of 100% H2SO4 used x 100
Loss Tons of
Steel V/CTC . % Free H2SC*4 in spent liquor\
Pir-H XI1./3 + — 1
ed/yr % Fe in spent liquor /
This formula neglects the iron lost in the rinse water.
Purchased Materials
The acid recovery system requires no lime and
reduces acid requirements. If the wastewater flow is
reduced so that all the pickling solution and rinse
waters pass through the acid recovery system (i.e., no
free acid is lost in the rinse water), the makeup
sulfuric acid requirements can be estimated from
Figure 4 if one knows the amount of iron loss through
pickling. The amount of ferrous sulfate crystals
produced also can be estimated from this figure.
For example, at an average iron loss during pickl-
ing of 0.5 percent, 18 pounds of sulfuric acid will be
required per ton of steel. From this same figure it can
be determined that 50 pounds of ferrous sulfate
heptahydrate crystals will be produced per ton of
steel pickled.
The sulfuric acid required to pickle a ton of steel,
when hauling or neutralization is used, is a function
of the free sulfuric acid and the iron remaining in the
spent pickle liquor, as well as the percent of iron loss
during pickling. The sulfuric acid makeup require-
ments can- be determined from Figure 5. The quick-
lime required for neutralization also can be
determined from this figure. Figure 5 was prepared
on the basis of 1 percent iron loss during pickling.
The required acid or lime at any other pickling loss
can be calculated by multiplying the value at 1
percent loss by the ratio of the actual iron loss to 1
percent. For example, if a plant considering neutrali-
zation is currently dumping its pickle liquor when
free acid is at 6 percent and the iron content is at
8 percent, Figure 5 shows that 50 pounds of sulfuric
acid makeup will be required per ton of steel pickled,
and 38 pounds of lime will be required for neutraliza-
tion if the iron loss is 1 percent. If the iron loss is 0.5
percent, then 25 pounds of 100 percent sulfuric acid
and 19 pounds of lime are needed per ton of steel.
Similarly, the calcium and iron hydroxide sludge
produced by the neutralization process can be
determined from Figure 6.
Figure 7 shows the amount of pickle liquor
produced as a function of iron loss and pickle liquor
composition.
Using these figures, Table 1 was constructed for a
100,000 TPY wire plant under the stated conditions.
Economics
Variabilities in operating conditions and in the >
availability of land among pickling facilities must be
considered in evaluating the economics of each
process. In this section the major factors which may
affect the economics are discussed, and detailed costs
are presented for a typical case so that each manu-
facturer may evaluate the best choice for his particular
situation.
-------
ter-
Figure 4.
Suifuric Acid Makeup Requirements for Pickling
and Ferrous Sulfate Crystals
Produced with Acid Recovery
ferrous sulphate
crystals produced
Read left for sulfuric acid i
-------
Figure 5.
Sulfuric Acid and Lime Requirements for
Neutralization of Pickle Liquor and Rinse
Water for 1% Iron Loss
Read Right
for Quicklime Requirements
To obtain Ibs. of hydrated lime
multiply x 1.32
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Weight, % Free Sulfuric Acid in Spent Pickle Liquor
1 o'
§
Q. ' ~
1 « ".
.£ 250'
100-
-C 50-
O
...... to
Figure 6.
Sludge Production from Neutralization of Spent Pickle Liquor for 1% Iron Loss
NOTE: ; , , ":; ':
this graph gives dry solids. To correct for water contained in the
sludge, multiply by the following factors: % Solids Factor
70"" 1.43
50 2.00
40 2.50
30 3.33
20 5.00
10 10.00
The expected % solids for various solid/liquid separating
devices can be obtained from laboratory settling tests
by vendors. A 20% solids value might be used as a basis
for initial estimating. This value must be confirmed
before reaching a final, decision.
Iron Composition '
10% Fe-H-
3 4 5 6 7 8 9 10 11 ••"':.: 12 13
Weight, % Free Sulfuric Acid in Spent Pickle Liquor
14
15
^J
-------
Figure 7.
Spent Pickle Liquor vblume/Ton Steel Pickled
for Hauling arid Neutralization
Rinse Water is Not
Density @ 10 jb/gal
Curves apply for all
levels in spent liq
% Dissolved Iroti in Spent Liquor
-------
Table!
Performance Comparison
Acid Recovery vs. Neutralization
for a Plant Pickling 100,000 Tons of Steel Per Year
«nMi
Case I
0.5% Iron Loss
Case II
1.0% Iron Loss
Acid
Recovery
Neutralization
Acid
Recovery
Neutralization
Total Iron Loss
(tons/year)
Sulfuric Acid Makeup
(tons/year)
,"il!"', 'I' II- "I1 '' J1 i: 'II ' 1 .'>,'' '. '
Quicklime Required
(tons CaO/year)
Spent Liquor Composition (weight %)
500
900
Fe
Unreacted H2SO4
t_ Plant Wastes and By-products
Crystalline FeSC>4. 7H2O
(10% Moist.) (tons/year)
Sludge at 20% Average Solids
(tons/year)
Wastewater*
(gallons/day)
8
8
2870
500
1400
1075
8
8
15700
95600
1000
1800
5740
0
'Assumes 250 gallons of rinse water per ton of steel, and 250 operating days per year.
1000
2800
2150
, 3.1300..
92000
-------
Investment for Acid Recovery
The investment required for installing an acid
recovery system falls into three distinct categories:
1. The acid recovery equipment.
2. Modifications to existing pickling and rinse
tanks to reduce water flow—these changes bring the
water balance to the point where zero discharge is
achieved.
3. The installation of an improved air exhaust and
demisting system—this system greatly improves the
operating environment around the pickling line, and
it is optional for each plant.
It is important to note that part of the investment
required for acid recovery results in a direct improve-
ment of the pickling operation. Staged rinsing, for
example, reduces the water usage of the plant. Air
agitation may improve pickling rates. The air exhaust
system vastly improves the plant environment by
controlling acid fumes and, therefore, can be at least
partially chargeable as an air pollution control
investment.
The investment for acid recovery, as shown in
Figure 8, assumes an iron and sulfuric acid concentra-
tion in the spent pickle liquor of about 8 percent.
Neutralization investment costs are a strong function
of the spent pickle liquor composition and volume
because these values set the size of the lime slaking
system as well as that of the neutralization vessels.
The amount of rinse water also is a critical value and
directly affects the size of the neutralization vessels.
Unlike the investment shown for acid recovery, the
neutralization costs do not include any investment for
modifications of the rinse system. Therefore, the
installed investment for neutralization is shown for
two rinse water rates. The installed investment in-
cludes equipment for neutralization of the pickle
liquor and rinse water and provides for a clarifier and
thickener to concentrate the neutralized sludge. It is
assumed that the concentrated solids leave the
thickener containing about 20 percent solids. Costs
for further solids concentration equipment, such as
vacuum filters or centrifuges, are not included. No
in-plant modifications or land costs are included.
The neutralization investment costs were derived
from a recently completed comprehensive study by
EPA on the costs required to maintain the pH of
strong acid wastes between six and nine almost all of
the time.
Because of the minimum practical size for lime
slaking and the necessity for a basic control system
regardless of size, the investment costs for a con-
tinuous system do not decrease significantly for very
small plants. Plants with small flows should investigate
batch neutralization, which may decrease the invest-
ment requirements.
The investment for contract hauling is too de-
pendent upon local conditions for inclusion in this
report. However, depending upon the contractor's
schedule, some storage and pumping capacity will be
required. Neutralization facilities for rinse water also
will be required.
Operating Costs
Operating costs are a function of the following
factors:
• Raw material and utility consumption;
• Iron loss to the pickling solution;
« Spent pickle liquor composition;
• Volume of rinse waters.
The annual operating costs for acid recovery and
neutralization are presented in Figure 9. Figure 10
shows the operating costs per ton of steel pickled.
Table 2 lists all of the major assumptions related to
Figure 9.
The acid recovery operating costs bracket the
most likely conditions. In one case, sulfuric acid is
valued at $25/ton (including freight) and a net loss of
$10/ton is taken for ferrous sulfate crystals. In the
second case, sulfuric acid is valued at $50/ton and a
$5/ton credit is taken for ferrous sulfate crystals.
Neutralization costs are very sensitive to the
method of sludge disposal and are shown for two
conditions:
1. Sludge is disposed of at $3.50/ton. This would
allow about a 5-mile haul for sludge containing 20
percent solids.
2. Sludge is disposed of on-site at a negligible cost.
-------
I
1,200 '
800 —
Figure 8.
Investment for Handling Spent
Pickle Liquor at 1% Iron Loss
£ 500
Acid Recovery with Air
Exhaust System
Acid Recovery without
Air Exhaust System
Neutralization with Rinse Water
to Pickle Liquor Ratio = 10
Neutralization with Rinse Water
to Pickle Liquor Ratio = 2
NOTE:
These investment figures are based upon a 1% iron loss and a
composition of 8% iron and S% sulfuric acid in the spent pickle
liquor. This figure can be used with somewhat less accuracy
for plants which have other percentage iron losses by using the
lower scale.
50,000
500
100,000 150,000 200,000
Plant Capacity (tons of steel/year)
1,000 1,500 2,000
Iron Loss (tons of steel/year)
250,000
2,500
Figure 9.
Annual Operating Costs for
Handling Spent Pickle Liquor
100,000 150,000 200,000
Pickling Capacity (tons/year)
250,000
Contract disposal @ $0.14/gallon
Neutralization With 20%::splids;
Sludge Hauled @ $3,50/tprv
Contract disposal @ $0.07/gallon
Neutralization with ortrsite
disposal of sludge
Acid recovery with sulfuric acid
@ $25/ton and a $107ton
disposal cost for ferrous
.sulfate crystals
Acid recovery with sulfuric acid
@ $50/ton and a $5/ton credit
for ferrous sulfate
300,000
-------
™
Economic Cost Basis Pjckling Operation with Spent Liquor Treatment
IOPERATING LABOR
fc'Pickling Volume to 100,000
fflL tons/year—Operators
Acid Recovery Neutralization Hauling^ ^
[Man hours/day) (Man hours/day) (Man hours/Hay)
4
gTPickling Volume"ab6ve 100,000
Jjtonjs/year--Operators
—Foremen
".:; i -
13.8
16
"f.6"
8
' T
6
4
0
8
0
^ IIUJTJES/T9N STEEL PICKLED*
|^^?£i~csS:^-"~ t-*~ ^
Water
(gal/ton)
Electricity
(kW-h/ton)
Recove'ry—Zero Discharge
@ 0.5% Fe Loss
ife-- @ 1_.0% Fe Loss
|- @ 1.5% Fe Loss
.— . .
^Neutralization,
^Contract Hauling
6^-
JCOST BASIS
If? "Operating Labor
p~ Foremen
l*: Steam
Water
_
Electricity
^
Quicklime
Solids Shipping Costs
Neutralized Sludge Hauling Fee
Pjckle Liquor Hauling Fee
pH Adjustment
Sewer Fees
Maintenance t t ,
General Plant Overhead
Depreciation
Taxes and Insurance
Maintenance Labor
$12,500 per man year
$15,000 per man year
$2750/1000 Ibs.
$0.30/1000 gal.
$0.02/kW-h
"$5b/ton
CjaO, $32/ton
$5/ton for crystals
$3.50/ton
$0.14/gal.
$*2."50/1000 gal. of rinse water
$6.40/1000" gaL
6% of investment
1.25 (wages and salaries + maintenance labor)
10% of investment
0.5% of investment
@ 37°/'o of maintenance cost
"Utility rates are Incremental values for pollution control systems Acid recovery case gives credit to water usage because modifi-
cations reduce rinse water to 25 gal/ton steel pickled. Zero discharge for acid recovery increases steam rates to achieve water balance.
Without zero discharge, there would be no steam rate increas^ for plant
-------
For comparison, costs for contract disposal under
specific conditions also are shown. More specific costs
for contract disposal should be determined by using
the amount of pickle liquor shown in Figure 7 in
combination with a quote from a contract disposal
firm.
It is important to note that it is prohibitively
expensive to handle rinse water by contract hauling,
and a method for eliminating these wastes still must
be found if contract disposal is used. These wastes
can be handled by municipal systems after pH adjust-
ment to a range of six to nine. The cost of placing the
rinse waters into the proper condition for acceptance
by a municipality has been taken at $2.50/1,000
gallons, and a municipal charge of $0.40/1000 gallons
has been included in Figure 9. This would allow only
very simple in-line neutralization, with minimal
solids/liquid separation. Depending upon the strength
of the rinse waters, this may be acceptable for some
municipal systems. Each manufacturer must check
this for his site.
The more optimistic case for contract disposal of
pickle liquor $0.07/gallon would allow about a 25-
mile haul if there were no other disposal costs. This
would be realistic for the cases where a municipal
system would be using the spent pickle liquor as a
coagulant. In the past, contract haulers had more
latitude for disposal of spent pickle liquor than at
present. The less optimistic ($0.14/gallon) case
presumes there is some charge for disposal after
hauling. Contract disposal costs are specific to each
site and the manufacturer must independently evalu-
ate this option.
It should be mentioned that the use of a contract
hauler places the manufacturer in the position of
complete dependence on the contractor for con-
tinued operation. The disposal contract should be
carefully written.
Table 3 gives a complete economic analysis of
all three options for one mill pickling 100,000 TRY of
steel. Rinse water disposal requirements were taken
at 250 gallons per ton of steel for neutralization .and
contract hauling. For acid recovery, complete
recovery of rinse water and zero discharge were
assumed.
Figure 10.
Operating Costs per Ton of Steel Pickled
vs. Thousand Tons of Steel Pickled per Year
50
100 150 200
Thousand Tons of Steel Pickled per Year
250
Contract Hauling @
$0.14/gal Hauled
Neutralization @$3.50/
ton of Sludge Hauled
Contract Hauling
@ $0.07/gal Hauled
Neutralization with
On-site Disposal
Acid Recovery @ $257
ton H2SC>4 arid $10 Net
Loss FteSO4 . 7H2O
4,cid Recovery @ $507
ton HaSO4 and $5 Net
Credit FeSO4 . 7H2O
300
-------
/
BASIS:
Item
Investment Salaries
& Wages
f Operators
Foremen
Utilities
Steam
Process Water
Electricity
Raw Materials
H2SO4
CaO
Shipping & Hauling Costs
Crystals
Sludge
Pickle Liquor
Ma/ntenance
Genera/ Plant Overhead
Waste Water Costs
Sewer Fees
pH Adjustment
Taxes and Insurance
Depreciation
By-Product Credit
Total Annual Costs
Rinse Water included in process
T
Comparison c
Plant Pickling 1
Basis
Figure
$12,500/man year
$15,500/man year
$2.50/1000 Ibs.
$0.30/1000 gallons
$0.02/kW-h
$50.00/ton
$32.00/ton
$5.00/ton FeSO4 . 7H2
$3.50/ton
$0.14/gallon
6% of investment
1.25 (wages & salaries H
maintenance labor)
$0.40/1000 gallons
$2.50/1000 gallon rinse
0.5% of investment
10% of investment
ible3
f Economics for a
00,000 TPY of Steel
1% iron loss;
8% dissolved
thousands of
spent pickle liquor
iron, 8% H2SO4 all
l
composition
figures in
dollars per year (1976 base) ,
Acid Recovery Neutralization
630.0
12.5
1.5
35.4
(8.1)
19.0
(50.0)
0
D 28.8
0
0
37.8
35.0
0
water 0
3.2
63.0
$10.00/ton FeSO4 • 7HJ2O (52.0)
water for acid recovery pi
*Maihtenance labor at 37% of maintenance cost.
V
126.1
mts at 25 gal/ton pickled
-
770.0
12.5
3.8
4.2
—
4.0
—
68.7
0
110.0
0
46.2
41.7
9.2
—
3.9
77.0
0
381.2
steel.
Contract
Hauling
0
6.2
0
—
—
—
—
0
0
0
350.0
—
7.8
10.0
62.5
0
0
0
436.5
•J
-------
Since the acid recovery process is the only treat-
ment unit which produces a by-product for sale or
disposal, the operating expenses will vary with the
costs or credits associated with that product—ferrous
sulfate heptahydrate.
Most existing acid recovery plants now receive a
net credit of $8 to $15 per ton of heptahydrate. The
widespread use of acid recovery may change this
situation. If complete drying and bagging facilities
were installed, the crystal value would increase to $35
to $45 per ton, shipped in 50-pound bags. The
economics usually are not favorable for installing
these extra facilities since the crystal volume is low
and drying and marketing expenses are high. Shipping
costs for the crystals can be high. Therefore, the net
return or loss is a function of the market and the
location of the customers.
The effect of changes in the net value of the by-
product credit on the total operating cost is tabulated
in Table 4.
Because of the varying unit costs and operating
conditions, the operating costs for each individual
plant may differ greatly from those presented here.
By using the correction factors and costs in this
report, each manufacturer can compare his costs with
those listed, and make the appropriate revisions.
The use of the data in this report to prepare Table
3 is illustrated under the "Example Calculation"
(Table 5).
Although each company must evaluate the
economics for its specific plant, some general com-
ments can be made:
« At pickling capacities above 75,000 TRY, the acid
recovery process appears to be the most economical
choice unless a special disposal situation is present,
or conditions are such that the existing pickling
line cannot be modified to minimize the rinse
water produced.
• For capacities below 50,000 TRY, special site condi-
tions will determine the most economical choice.
• Neutralization appears to be economical only for
special cases where ample land is available to
impound the neutralized pickle liquor and rinse ;
water, and inexpensive sulfuric acid is available for
pickling.
• A most important action which can be taken to
reduce investment and operating costs for handling
spent pickle liquor is to reduce the iron loss from ;
overpickling and by using steel with minimum
scale. This reduces the spent pickle liquor volume
to be handled, and applies to acid recovery,
neutralization, and contract disposal.
,,"-*"
f
t
LHB
1
1
Table 4
— \
. "~'-x. >
The Effects of Net Value of Crystal By-Product (FeSO4 • 7H2O) from ;
Acid Recovery on Total Operating Expense/Ton of Steel Pickled
% Iron Loss
Net Value of Crystal
$10/ton Nlet Value
$5/ton Net Value
$0/ton Net Value
$10/ton Net Loss
- '•• '' " '•'••
Net Value = Crystal Credit — Snipping Costs
Shipping costs calculated on wet basis assuming
Crystal credit calculated on dry basis as FeSO4 •
0.5
Increase (+
in Operating
-0.25
-0.13
0
+0.25
10% moisture
7H2O
1-Q , 1-5 ,
or Decrease (-)
Costs (Dollars/ton)
-0-50 -0.75 ,
-0.25 --0.38 *
o o :
.+0,50, +0.75 '
J
I
-------
~" ' Calculations Derivation for Operating Costs,
Based on Examples for Table 3
Acid Recovery
from, Figure 8
Man-days from Table 2
142 Ib/ton steel from Table 2*
Neutralization
Contract
Hauling
From Figure 8
Man-days from Table 2
17 Ib/ton steel
" from Table 2
gfr_ErQcesa,Waterrt ,"27,0,gal/ton steel* ,
"~ Reduced consumption and it is
treajed as a credit. See Table 21
9.5 kW-h/ton steel from Table 2f
Reduced consumption, and it
is treated as_a credit. Objtam _j
acid consumption from Figures'
4 and^S. Tjie difference
J(56"^36 = 20lb/tori) S£credTted
at current price
None
100 Ib/ton steel dry basis from1
JEigure 4. At 10% free moisture |
crystal rate = 111 Ib/ton steel
(100 * 0.9). Shipping costs at [
, prevailing rate.
',,. Sludge Hauling None
None charged to
pollution control system
2.6 kW-h/ton steel
from Table 2
None chargeable
42.9 Ib/ton steel from
figure 5
None
Pickle Liquor
j Hauling
^ J|j^i^L-» : ^r~^T-
11. Maintenance
General T'lant
None
At 6% of investment
At 1.25 (wages + maintenance
labor). Maintenance labor at
.- „ 37% of maintenance cost
626 Ib/ton steel
(wet basis) from Figure 6
None
At 6% of investment
At 1.25 (wages +
maintenance labor).
Maintenance labor at 37%
of maintenance cost
No investment required
Man-days from Table 2
None required
None charged to
pollution control system
None chargeable
None chargeable
None
None
None
25 gal/ton of steel from
Figure 7. Obtain fees
from .contractor
At no charge
At no charge
- _/ "Numbers for 1.0% Iron Loss
~ T
-------
€
I
i.aDie jj n-ononuea;
Calculations Derivation for Operating Costs,
t
Based on Examples for Table 3
"• "
._
1 13.
*
^14.
15.
s.
tie.
i '
L
1 '"• ••• '• ' • '•"" ?.>•!;. \
Acid Recovery
Sewer Fees None
pH Adjustment None
(Rinse Water)
Taxes and At 0.5% of investment
Insurance
By-product Sales
(FeSO4»7H2O) From Figure 4, 111 Ib/ton steel.
Credited at sales price
"' • , «"" •
Neutralization
Rinse water at 250 'gal/ton
steel + lime slurry (Fig. 5)
+ pickle liquor (Fig. 7)
less wet sludge (Fig. 6)
No charge, this is included
in neutralization
At 0.5% of investment
None
: , "»
Contract ^
Hauling
Rinse water at 250 gal/ton
steel
i
Rinse Water at 250 gal/ton
steel
At no charge j
!
None "I
j
//
Wire rod before pickling (left), wire rod after pickling and lime coating (right)
-------
Although the on-site energy requirement for acid
recovery systems aimed at zero discharge usually is
higher than the requirement for neutralization
(depending upon the amount of pickle liquor evapo-
ration required), a comparison of the ultimate energy1
requirements of the two processes indicates that acid
recovery and neutralization energy requirements are
about equal at 0.75 percent iron loss and favorable to
acid recovery at higher iron losses.
The lime needed for neutralization is manu-
factured by the calcination of limestone:
CaCOs + HeatJ5=rtCaO + CC>2
limestone quicklime
The theoretical heat input is approximately 3 mil-
lion Btu per ton of quicklime (CaO). In actual plant
practice, this heat requirement approaches 10 million
Btu. Lime rates are, therefore, a very significant
portion of the ultimate energy requirements.
Although plants using lime do not directly pro-
vide the energy for calcination, eventually they do
pay the cost. The cost of lime has risen from $18 per
ton in 1972 to $32 per ton in 1976. There is no doubt
that future energy cost increases also will be reflected
in the price of lime.
Since the amount of pickle liquor used per ton of
steel is a function of the iron loss, the ultimate energy
consumption for both neutralization and acid recovery
also is a function of the same variable. Figure 11 shows
the ultimate energy requirements as a function of
iron loss for each ton of steel pickled.
TRY plant.
Since sulfuric acid is produced by an exothermic
reaction, savings in sulfuric acid by acid recovery do
not result in large ultimate energy savings.
Figure 11 shows that as the iron losses approach 0.7;>
percent, the ultimate energy requirements for neutral-
ization and acid recovery are equal at 220,000 Btu/ton
of steel. As the iron losses increase, acid recovery is
favored. Although the zero discharge acid recovery
1 The "ultimate" energy requirement considers the
energy to manufacture the required raw materials.
system requires significant energy for evaporation and
refrigeration, neutralization uses large volumes of
lime produced from energy intensive processes.
Approximately 10 million tons of steel are pickled
with sulfuric acid annually and iron losses are usually
1 percent. At this volume, the installation of acid
recovery systems achieving zero discharge would
increase the national energy usage by 2.4 trillion Btu
or the equivalent of 400,000 barrels of fuel oil. The
energy usage for neutralization would be about 25
percent higher. This increase in energy usage for acid
recovery systems does not take into account the
energy savings associated with improving capacities or
operation of the plant when operating a closed loop
acid recovery system. A decrease in average iron loss
per ton of steel would also greatly reduce the energy
requirements.
Discharge of ferrous sulfate crystals to storage building
-------
500
Figure 11.
Ultimate Energy Requirements
Simple Neutralization vs. Acid Recovery
Spent Liquor Composition at 8% Fe and 8% H2SO4
Basis: ,..'•.
Steam — 1QOO Btu/lb
Power — .10,5.00 Btu/kW-h!
Lime CaO — 9 x 106 Btu/ton of lime
Neutralization Utilities —
16.8 I bs. steam/ton steel
2.0 kW-hAon ' -\
Acid Recovery Utilities —
141.6 Ibs. of steam/ton
5.9 kW-h/ton @. 0.5% loss
9.5 kW-h/ton @ 1.0% loss :
13.1 kW-h/ton @ 1,5% loss
i.o
Iron Loss %
-------
Pickling tank (foreground), acid mist exhaust system (background)
-------
Dip and spray rinse tank with spray nozzles in operation
-------
Lime coating tank and exhaust system
-------
The Industrial Environmental Research Laboratory
in Research Triangle Park, North Carolina, is respon-
sible for control technology in the ferrous metallurgical
industry. For environmental information on steel
pickling operations and other EPA sponsored pro-
grams regarding ferrous metallurgical processes, write:
Metallurgical Process Branch
Industrial Processes Division
Industrial Environmental Research Laboratory
Environmental Protection Agency
Research Triangle Park, N.C. 27711
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