- Email: [email protected]
Rinse water regeneration in stainless steel pickling
Desalination 211 (2007) 64–71
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 Tel. +34 (91) 5538900; Fax +34 (91) 5347425; email: [email protected]
Received 17 November 2005; revised 2 March 2006; accepted 2 March 2006
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
*Corresponding author. Presented at the 9th Environmental Science and Technology Symposium, September 1–3, 2005, Rhodes, Greece. Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean, Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.591
B. Schmidt et al. / Desalination 211 (2007) 64–71
65
1. Introduction
2. Experimental
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.
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.
Table 1 Analysis of rinse water samples
Turbidity, FTU Suspended solids, mg/L pH Conductivity, µS/cm Fe, mg/L Cr, mg/L Ni, mg/L NO3– , mg/L F–, mg/L
S1
S2
34 28 2.8 1229 130 29 23 331 149
96 79 2.9 988 95 16 22 250 115
Table 2 Characteristics of Amberlite IRA67 resin
Polymer matrix Functional group Ionic form Exchange capacity, meq/mL (wet) Operating temperature, °C Effective size, mm Swelling, %
Acrylic–DVB Tertiary amine Free base >1.6 35 (maximum) >0.36 30
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B. Schmidt et al. / Desalination 211 (2007) 64–71
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 prefiltration 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 Table 3 Reduction of solids content in pre-filtration experiments Reduction of solids content (%) 15 µm filtera 10 µm filtera 5 µm filtera 0.3 µm microfilterb 0.1 µm microfilterb
18 25 43 99.9 99.9
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 4.4×10–3 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)
a
Glass fiber, bHydrophilic polycarbonate
For further studies on the reverse osmosis membrane performance, different runs with vari-
Table 4 Main parameters of membranes tested in reverse osmosis experiments Membrane
Material
Salt rejection
pH range
T (ºC)
Maximum pressure (bar)
1 2 3 4 5 6 7
polyacryl polyacryl not specified not specified not specified not specified polyacryl
98.5%(NaCl) 96%(MgSO4) 90%(NaCl) 95%(NaCl) 75%(NaCl) 95%(MgSO4) 99%(NaCl)
1–11 1–11 0–14 2–10 2–10 1–12 2–10
50 50 70 40 40 50 40
60 40 40 40 40 60 40
B. Schmidt et al. / Desalination 211 (2007) 64–71
ous 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 25°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 additionally, 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
67
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 (Fig. 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
68
B. Schmidt et al. / Desalination 211 (2007) 64–71
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].
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 (Fig. 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, Fig. 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.
B. Schmidt et al. / Desalination 211 (2007) 64–71
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Fig. 2. Rinse water regeneration concept (alternative 1).
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 7210-PR/301 (01.E/06). References [1] C. Frías, C. Negro, A. Formoso, A. De Jong, M. Pars, J. Kemppanien and F. Mancia, Novel process to recover by-products from the pickling baths of
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Fig. 3. Rinse water regeneration concept (alternative 2, with ion exchange).
stainless steel, Project funded by the European Community under the Industrial and Materials Technologies Programme (Brite-Euram III), Project No. BE3501, Contract No. BRPR-CT 97-0407, 1997–2000. [2] B. Nymen and T. Koivunen, The Outokumpu process for pickling acid recovery, in: Iron Control in Hydrometallurgy, J. Dutrizac and A.J. Monhemius, eds., The Metallurgical Society of CIM-Ellis
Horwood Ltd., Chichester, 1986, pp. 520–536. [3] C.J. Brown and M. Sheedy, The Fluorex process for regeneration of nitric/hydrofluoric stainless steel pickling liquors, in: Iron Control and Disposal, J.E. Dutrizac and G.B. Harris, eds., The CIMP, Montreal, 1996, pp. 457–469. [4] S.E. Lunner, Possible methods for complete recovery of acids and metals from mixed acid pickling of
B. Schmidt et al. / Desalination 211 (2007) 64–71
[5]
[6] [7]
[8]
[9]
[10]
stainless steel, in: Recycling and Waste Treatment in Mineral and Metal Processing: Technical and Economical Aspects, Vol. 1, B. Björkman, C. Samuelsson and J.O. Wikström, eds., Lulea, 2002, pp. 529–539. R. Wolters, B. Schmidt, R. Levonmaa, Th. Schneiker, J. Kaplin, A. Lopez-Delgado, F.A. Lopez and F.J. Alguacil, Eco-efficient technology for recovering acids and metals from rinse water in stainless steel pickling, Final Report of Contract 7210-PR/ 301 funded by the European Commission-Research Fund for Steel, 2004, and references therein. K.Scott, Handbook of Industrial Membranes. Elsevier Advanced Technology, Kidlington, 1997. M. Regel, A.M. Sastre and J. Szymanowski, Recovery of zinc(II) from HCl spent pickling solutions by solvent extraction, Environ. Sci. Technol., 35 (2001) 630–635. F.J. Alguacil and M.A. Villegas, Liquid membranes and the treatment of metal-bearing wastewaters, Rev. Metal. MADRID, 38 (2002) 45–55. M.A. Lobo-Recio, F.J. Alguacil and A. Lopez-Delgado, Co-extraction and selective stripping or iron(III), HNO3 and HF from stainless steel rinse waters, AIChE J., 50 (2004) 1150–1155. T. Melin and R. Rautenbach, MembranverfahrenGrundlagen der Modul-und Anlagenauslegung.
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Springer Verlag, Berlin, 2004. [11] C.K. Gupta and T.K. Mukherjee, Hydrometallurgy in Extraction Processes. Vol. II, CRC Press, Boca Raton, 1990. [12] R. Wolters, B. Schmidt, F.J. Alguacil and R. Biwer, Process Water Treatment with Excess Heat. ECSC Steel Publications, Brussels, 2002. [13] F.J. Alguacil, The removal of toxic metals from liquid effluents by ion exchange resins. Part III: Copper(II)/sulphate/amberlite 200, Rev. Metal. MADRID, 39 (2003) 205–209. [14] R.S. Juang, S.H. Lin and T.Y. Wang, Removal of metal ions from the complexed solutions in fixed bed using a strong- acid ion exchange resin, Chemosphere, 53 (2003) 1221–1228. [15] M.A. Lobo-Recio, A. López-Delgado and F.J. Alguacil, The application of ion exchange to the treatment of stainless steel rinse waters, in: Global Symposium on Recycling, Waste Treatment and Clean Technology, Vol. II, I. Gaballah, B. Mishra, R. Solozabal and M. Tanaka, eds., TMS and INASMET, San Sebastian, 2004, pp. 1097–1105. [16] F.J. Alguacil, M. Alonso and L.J. Lozano, Chromium (III) recovery from waste acid solution by ion exchange processing using Amberlite IR-120 resin: batch and continuous ion exchange modelling, Chemosphere, 57 (2004) 789–793.
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 Tel. +34 (91) 5538900; Fax +34 (91) 5347425; email: [email protected]
Received 17 November 2005; revised 2 March 2006; accepted 2 March 2006
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
*Corresponding author. Presented at the 9th Environmental Science and Technology Symposium, September 1–3, 2005, Rhodes, Greece. Organized by the Global NEST organization and prepared with the editorial help of the University of Aegean, Mytilene, Greece and the University of Salerno, Fisciano (SA), Italy. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.591
B. Schmidt et al. / Desalination 211 (2007) 64–71
65
1. Introduction
2. Experimental
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.
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.
Table 1 Analysis of rinse water samples
Turbidity, FTU Suspended solids, mg/L pH Conductivity, µS/cm Fe, mg/L Cr, mg/L Ni, mg/L NO3– , mg/L F–, mg/L
S1
S2
34 28 2.8 1229 130 29 23 331 149
96 79 2.9 988 95 16 22 250 115
Table 2 Characteristics of Amberlite IRA67 resin
Polymer matrix Functional group Ionic form Exchange capacity, meq/mL (wet) Operating temperature, °C Effective size, mm Swelling, %
Acrylic–DVB Tertiary amine Free base >1.6 35 (maximum) >0.36 30
66
B. Schmidt et al. / Desalination 211 (2007) 64–71
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 prefiltration 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 Table 3 Reduction of solids content in pre-filtration experiments Reduction of solids content (%) 15 µm filtera 10 µm filtera 5 µm filtera 0.3 µm microfilterb 0.1 µm microfilterb
18 25 43 99.9 99.9
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 4.4×10–3 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)
a
Glass fiber, bHydrophilic polycarbonate
For further studies on the reverse osmosis membrane performance, different runs with vari-
Table 4 Main parameters of membranes tested in reverse osmosis experiments Membrane
Material
Salt rejection
pH range
T (ºC)
Maximum pressure (bar)
1 2 3 4 5 6 7
polyacryl polyacryl not specified not specified not specified not specified polyacryl
98.5%(NaCl) 96%(MgSO4) 90%(NaCl) 95%(NaCl) 75%(NaCl) 95%(MgSO4) 99%(NaCl)
1–11 1–11 0–14 2–10 2–10 1–12 2–10
50 50 70 40 40 50 40
60 40 40 40 40 60 40
B. Schmidt et al. / Desalination 211 (2007) 64–71
ous 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 25°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 additionally, 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
67
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 (Fig. 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
68
B. Schmidt et al. / Desalination 211 (2007) 64–71
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].
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 (Fig. 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, Fig. 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.
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Fig. 2. Rinse water regeneration concept (alternative 1).
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 7210-PR/301 (01.E/06). References [1] C. Frías, C. Negro, A. Formoso, A. De Jong, M. Pars, J. Kemppanien and F. Mancia, Novel process to recover by-products from the pickling baths of
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Fig. 3. Rinse water regeneration concept (alternative 2, with ion exchange).
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