RINSE WATER REGENERATION IN STAINLESS STEEL PICKLING
Burkhard Schmidta, Ralf Woltersa, Jyri Kaplinb, Thorsten Schneikerb, Maria de los
Angeles Lobo-Recioc, Felix Lópezc, Aurora López-Delgadoc, Francisco José
Alguacilc*
a
BFI, Sohnstrasse 65, 40237 Düsseldorf, Germany
b
Outokumpu Stainless AB, Sweden,
c
Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8,
Ciudad Universitaria, 28040 Madrid, Spain. E-mail: fjalgua@cenim.csic.es
*
Corresponding author
Abstract
Since stainless steel must be rinsed with water after pickling operation with mixtures of
nitric and hydrofluoric acids, the present investigation was undertaken to define a major
step towards the close loop circulation of water in steel plant. This can be done by a new
procedure for removing also nitrates from rinse water in such a way that clean water can
be recycled back to rinsing use. The concentrated part of used rinse water will be fed to
pickling acid regeneration plant, where nitric and hydrofluoric acids can be separate and
recycled back to the pickling use.
The tests were carried out mainly in one of Outokumpu´s production plants in Sweden.
Membrane technique was used for rinse water treatment. A filtering system based on
reverse osmosis was developed to separate the acids and metals from the rinsing water.
A pre-filtering method necessary for reverse osmosis was tested. Since the composition
of permeate is not applicable for direct reusage in the rinsing zone, post-treatment
methods were examined. Amberlite IRA67 ion exchange resin was tested for treatment of
reverse osmosis filtrate (pH adjustment). For the recovery of the mixed acids, the
concentrate of reverse osmosis was further treated with electrodialysis.
On the basis of these results, a regeneration concept was developed, in order to close
the rinse water loop. The proposed rinse water recycling concept reduces the overall
usage of water.
Keywords: Stainless steel; Pickling; Rinse water; Regeneration; Membranes; Ion
exchange
1. Introduction
Very commonly, pickling liquors for stainless steel is today mixed acid, which is a mixture
of nitric and hydrofluoric acids. After pickling, stainless steel surface must be rinsed in
order to clean and get rid of all acids on the steel surface. Water is used to remove
residuals of these acids.
Exhausted rinse waters from pickled stainless steel contain mainly Fe3+, Cr3+ and Ni2+
ions and nitric and hydrofluoric acids, though its composition may change from one to
another plant, typical averaging composition being ~1 g/L Fe3+, ~0.14 g/L Cr3+, ~0.07 g/L
Ni2+, ~2 g/L HNO3 and ~1 g/L HF [1]. These rinse waters are usually neutralized with lime
previously to their discharge, being the recovery of metals and/or acids less generally
made [1-5]. However, more stringent environmental legislation, especially in most
industrialized countries, claims for a decreasing in the discharge of nitrates and other
pollutant agents contained in industrial effluents (Directive 96/61/CE of the European
Union Council). Different technologies, specially useful for diluted solutions, such as
liquid-liquid extraction, membranes (microfiltration, nanofiltration, reverse osmosis, etc.)
and ion exchange are being developed with the aim of recovering both metals and acids
[6-11].
The objective of the present investigation was to take a major step towards the close loop
circulation of water in steel plants. This can be done by a new method for removing also
nitrates from rinse water in such a way than clean water can be recycled back to rinsing
use. The concentrated part of used rinse water will be fed to the pickling acid
regeneration plant, where both acids can be separate and back-recycled to the pickling
line. The new concept is widely applicable in all steel industry and it is characterized by
the use of membrane and ion exchange technologies.
The new concept will help to produce stainless steel with reduced pollution of
environment and without increasing production costs. It will make stainless steel
production more economical and increase its competitiveness.
2. Experimental
To achieve the objectives, an operational test unit for continuous rinse water treatment
was designed, constructed and installed in the operating pickling and regeneration plant
[5]. Design was based on membrane and ion exchange technologies previously
developed [12,13] and is now further developed by systematic laboratory and field tests.
Rinse water samples of various annealing lines from an Outokumpu plant in Sweden
were characterized and observed over a representative period of time. Studies were
carried out in order to characterize the rinse water with respect to its composition: nitrate,
fluoride, metal content, pH, and suspended solids (Table 1).
The characteristics of Amberlite IRA67 resin are given in Table 2.
3. Results and discussion
3.1. Pre-filtering of rinse water
Rinse water is contaminated with foreign particles like cinder. Thus, the determination of
solid content showed high values (see Table 1). For the pressure driven process reverse
osmosis, the abrasion potential of the rinse water is not acceptable. Therefore, it is
necessary to separate the particles from rinse water in an economical way, i.e.
sedimentation, belt filtration or microfiltration.
Using microfiltration, the best results for pre-filtration were achieved. The results correlate
with the particle size distribution and the pore size of the used filters (Table 3).
3.2. Selecting the membranes for reverse osmosis
Then, a membrane technology was used for rinse water treatment. A filtering system
based on reverse osmosis was developed to separate the acids and metals from the
rinsing water.
First, a membrane screening was carried out to find the best membranes in a laboratory
scale basis, and using rinse water from the operational annealing line (see Table 1) as
feed solution. The main parameters of the tested membranes are given in Table 4. After
experimentation, the tests showed that it is possible to get 95% rinse water back to the
process, whereas the metals are concentrated for every membrane, though the quality of
the permeate was best for membrane 1. Moreover, membrane 1 was the only membrane
which concentrates also the free acid content. After further continuous tests (typically 8
days), the membrane permeate flux averaging 21 L/m2 h for a membrane area of 4.4x10-3
m2.
The best suitable membrane (membrane 1) was used for demonstrating the regeneration
concept in operational trials. Main parts of the regeneration concept are the above
mentioned microfiltration unit with pore size of 0.2 μm and a reverse osmosis as
concentration unit. The operational trials were conducted over a period of several months
at a production line of an Outokumpu plant in Sweden.
3.3. Reverse osmosis tests (Outokumpu plant)
For further studies on the reverse osmosis membrane performance, different runs with
various range of concentrations were carried out. Results are summarized as follows:
i) Development of permeate and concentrate flux during a long period (minimum 24 h) of
concentrate production results in that the permeate flux increased with temperature and
pressure. Using experimental conditions of 22 bar and 25º C, the permeate flux
decreased with increasing conductivity of concentrate (e.g. >75 L/h at 15 mS/cm against
55 L/h at 55 mS/cm).
ii) There is a concentration of acids, main part of the nitrates were found in the
concentrate (14 L), only 22% remained in the permeate (86 L). Nitrate was concentrated
from 3.5 g/L up to 19.5 g/L.
iii) The metal content in the permeate of reverse osmosis operation is low, the content of
chromium and nickel is less than 0.01 g/L and the iron rejection of the membrane is more
than 99%. The average content of fluorides in permeate is near 0.5 g/L.
Membrane performance is excellent, since laboratory results after 14 months of contact
with mixed acids shown >99% iron and >95% nitrates rejection, values that are in
complete accordance with results obtained using a new unused membrane.
3.4. Nanofiltration tests
Furthermore and aditionally, nanofiltration membrane performance was studied at
different experimental variables (concentration, pressure and temperature):
i) The permeate flux increased with increasing temperature and pressure. The higher the
concentration factor, the lower the permeate flux (e.g., at 40 bar, 260 L/h for a
concentration factor of 2 against 120 L/h for a concentration factor of 12.5).
ii) The nitrate balance for the nanofiltration membrane showed that due to the higher
permeability of these membranes, a higher flux of nitrates can be observed. Thus, most
of the nitrates (70%) are passing the membrane, only the residual nitrates are
concentrated.
iii) Metal recovery is high. The metal content (iron, nickel and chromium) in the permeate
is lower than 0.1 g/L. Average content of fluorides in permeate is of near 0.5 g/L.
Because of the higher retention of nitrates, metals and fluorides, the use of reverse
osmosis membranes is preferred in the following concept. Basically, the tested reverse
osmosis membrane showed nearly constant high permeate flux over time (averaging 60
L/h during more than 250 h production of permeate). No defects of active and supporting
layer were found with respect to the contact of acids and metals. Scaling on the surface
of the membrane was not identified.
3.5. Ion exchange resin tests
Ion-exchange technology offers an attractive alternative for the treatment of dilute
solutions due to its possibilities in the managing of great volumes of solutions (normally
found in wastewaters) with a medium-low content in toxic solutes [14,15], even
sometimes ion exchange is preferred to liquid-liquid extraction due to ease of operation
and absence of organic impurities.
Since the composition of permeate is not applicable for direct re-usage in the rinsing
zone, and considering the above, post-treatment methods were examined, thus,
Amberlite IRA67 medium basic (tertiary amine) ion exchange resin was tested for the
treatment of reverse osmosis filtrate in order to separate the acids from the water stream.
Various operational parameters, typical of the ion exchange technology, were
investigated in order to get information about the resin performance prior to its
implementation in a continuous mode operation (Figure 1), which was similarly performed
as described in the literature [16]. This study showed the feasibility of the ion exchange
technology to remove acids from rinse water. Ion exchange is able to close the loop of
the water circuit in the steel work plant.
3.6. Electrodialysis tests
For the recovery of the mixed acids, the concentrate of reverse osmosis was treated with
electrodialysis. Thus, the nitrate concentration in the incoming diluate (waste acid) is
reduced down to a dischargeable level and at the same time the product is concentrated
to a reusable acid. The recycling rates for nitric acid are very high, however, hydrofluoric
acid can only be recycled to a some extent. Typical results of these tests showed a final
conductivity in the recovered acid of at least of 630 mS/cm, whereas the nitrate content in
the resulting waste water can be reduced to near 0.14 mol/L.
4. CONCLUSIONS
On the basis of the results obtained, a modular regeneration concept was developed in
order to close the rinsing water loop. The process (Figure 2) includes different treatment
steps. Particles are firstly removed from the pickling rinsing sections by microfiltration. For
rinsing section 2, the microfiltration is installed directly after the bath, concentrate is fed to
the neutralization; the permeate is divided into two streams, one is fed to the rinsing
section tank 1 and the other back to the rinsing section tank 2. Because of the new
concept, only the tank of the rinsing section 2 is filled with fresh water. Thus, the total
amount of fresh water decreases. Water from the rinse water section tank 1 is treated by
a second microfiltration step to separate particles. The concentrate is fed to the
neutralization and the permeate to the reverse osmosis plant. A pure permeate and a
concentrate with high acid content is produced in the plant. The concentrate is fed to the
electrodialysis plant in order to recover the mixed acids. The permeate of the reverse
osmosis is neutralized to fit the pH value, and then a sedimentation and sand filter follow
as post-treatments steps to minimize solid content. After treatment, the permeate of
reverse osmosis is clean enough for the use in the rinsing section 1, reducing the fresh
water consumption of the rinse water process.
Alternatively, Figure 3 shows the regeneration concept with inclusion of the ion exchange
treatment step. The rest concentration of nitrates in the reverse osmosis permeate could
be reduced at near 90%, and near 80% for fluoride. Ion exchange and the addition of
25% fresh water would lead to water quality acceptable for most rinsing applications.
The main advantages of the proposed concept can be summarized as:
i) 60% reduction of nitrate discharge through rinse water, hence nitric acid can be
recycled.
ii) 75% reduction of water consumption for rinsing purposes.
iii) 25% reduction of calcium hydroxide for rinse water neutralisation.
iv) 20% reduction of metal hydroxide sludge formed during rinse water neutralisation.
The proposed rinse water recycling concept reduces the overall usage of water and
nitrate discharge, cost savings are dependent on the production line and on special
pollution fees of some countries, though it is expected that the new concept may be
widely applicable in all steel industry.
Acknowledgements
Authors wished to thank to the Research Fund for Coal and Steel for contract 7210PR/301 (01.E/06).
References
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Table 1
Analysis of rinse water samples
Turbidity, Suspended
FTU
pH
solids,
Conductivity,
Fe,
Cr,
Ni,
NO3-
F-,
μS/cm
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
S1
34
28
2.8
1229
130
29
23
331
149
S2
96
79
2.9
988
95
16
22
250
115
Table 2
Characteristics of Amberlite resin
Table 3
Reduction of solids content in pre-filtration experiments
15 µm filtera
Reduction of
10 µm filtera
18%
solids content
aGlass
fiber, bHydrophilic polycarbonate
25%
5 µm filtera
43%
0.3 µm
0.1 µm
microfilterb
microfilterb
99.9%
99.9%
Table 4
Main parameters of membranes tested in reverse osmosis experiments
Membrane
Material
Salt rejection
pH range
T, ºC
Maximum
pressure, bar
1
polyacryl
98,5%(NaCl)
1-11
50
60
2
polyacryl
96%(MgSO4)
1-11
50
40
3
no specified
90%(NaCl)
0-14
70
40
4
no specified
95%(NaCl)
2-10
40
40
5
no specified
75%(NaCl)
2-10
40
40
6
no specified
95%(MgSO4)
1-12
50
60
7
polyacryl
99%(NaCl)
2-10
40
40
Fig 1. Experimental (unfilled circles, triangles and squares) proton adsorption
breakthrough curves of Amberlite IRA67 for different flow rates. Theoretical curves (solid
lines) calculates as in reference [16]
Fig 2. Rinse water regeneration concept (alternative 1)
Fig 3. Rinse water regeneration concept (alternative 2, with ion exchange)
Fig 1
Fig 2
Fig 3