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Chromium(III) salts recovery process from tannery wastewaters
DESALINATION ELSEVIER
Desalination
108 (1996)
183-191
Chromium(II1) salts recovery process from tannery wastewaters C. Fabianil*, F. Rusciol, M. Spadonil,
and M. Pizzichini2
lEnvironment Department, and 21nnovative Technologies Department, B.N.E.A. CR-Casaccia, Via Anguillarese 301, 00060 Roma, Italy. Tel.: +39-6-304&l; Fax.: +39-6-30484203 Received
27 July 1996; accepted 2 August 1996
Abstract
Chromium(II1) salts are the most widely used chemicals for tanning processes [ 11, but only 60% of the total chromium salt reacts with the hides. Therefore, about 40% of the chromium amount remains in the solid and liquid wastes (especially spent tanning solutions). The presence of chromium(111) and salts in the sludges of both the wastewater biological treatment plants or the chemical plants for recycling spent tanning liquors, represents an inconvenience for the safe reuse of these sludges and a cost forming factor for their disposal. Among the several proposed methods for chromium recovery [l-6], lime or sodium hydroxide precipitation and filterpressing of the chromimn hydroxide is the usual way to recover chromium salts from spent process solutions or from leaching solutions of residues of hides. However, usually the chromium salts quality must be improved
for their reuse in tanning processes 171.The integration of membranes in the treatment process for chromium recovery reduces the environmental impact, favours the reuse of both the protein residue and the biological plant sludges, reduces the consumption of chemicals and decreases the costs of waste disposal. Keywords: Chromium recovery; Tanning processes; Microfiltration; Ultrafiltration 1.
Introduction
More than one hundred different chemicals (350,000 ton/y of inorganic and heavy metal salts, soaps, tensioactives, oils, waxes, solvents, dyes, etc) used in tanning processes are found in process wastes and wastewaters. Chromium(II1) salts and sulfur compounds are the main pollutants released in tannery wastewaters and in the atmosphere. The Presented at the Second Annual Meetin of the Euro ean Desalination Society (EDS) on Desa 7’mation an dpthe Environment, Genoa, Italy, October 20-23, 1996. *Corresponding
author.
001 l-9164/97/$17.00 Copyright PZZ SO0 1 l-9 164(97)00026-X
function of chromium salts in tanning processes is to form, through complexation with the polypeptide collagen components of leather, a protective layer which prevents the penetration of water in the leather pores avoiding putrefaction. In Italy [3], second world producer of leather goods (more than 2,500 production units, 25,000 employees and 600,000 ton/y of treated hides), tannery wastewaters amount reaches the relevant level of 40,000,OOO m3/y even if process improvements in recent years have reduced the technical needs of process water: from 100 liters of water per kg of leather to 12-13 liters per kilogram. Waste-
e 1997 Elsevier Science B.V. All rights reserved
C. Fabiani et al. /Desalination
184
water treatment plants produce more than 700,000 ton/y of sludges and residues. Treatment and disposal costs amount to about 840,000 Mlitiy (560 millions of US $). dimension of this The impressive environmental problem pushes to transfer in tanneries any available technological improvement to reduce the environmental impact and recover and recycle water and the main chemicals. As far as the chromium salts is concerned, the tanning process consumes only 60% of the chromium of the tanning bath and the possibility of recover the residual metal represents a main goal in process improvement. Plants devoted to chromium salts (mainly chromium sulphate) recovery from the exhausted tanning baths are based on the cycle reported in Fig. 1. According to this cycle, the Consorzio Recupero Cromo (CRC)‘s plant, where the experimental study was performed, daily treats 450-700 m3 of
108 (1996) 183-191
waste solutions collected from 172 tanneries of the district of Santa Croce sull’Arno (Pisa) in Tuscani. In the referred plant chromium solutions, with a chromium content (5-6 g/l), and flocculated or pressed chromium sludges (12-20 g/l or loo-140 g/l metal content, respectively) are produced. This corresponds to a daily average of 1.12 ton of metal (as CrzOs). The chromium(II1) is recovered as basic sulphate (8-20 m3/d) with 31-33% of alkalinity which corresponds to an average of 21 ton/d of recovered product. However, the low chromium concentration and the presence of organic molecules (especially proteins,) makes the recovered solution not fully acceptable for a good quality result in the tanning process [3]. An improvement of the chromium recovery cycle by means of membranes processes will be presented and the environmental and economic benefits will be discussed.
2. Experimental Chromium solutions. 199 ton/d
waste
The mean composition of waste solution to be treated for chromium recovery is reported in Table 1.
I
Table 1 Composition recovery
i--I Settling
of the waste solution
Species
used for chromium
Concentration
Screening
Neutralization Cr2(S04)3
Fig. 1. Chromium
recovery
process
scheme.
basic
PH TSS Oils, fats COD N-Kijeldal N-NH3 Chloride Sulphate Cr(II1) Other metals (cations) Fe(m) Al(W Mn(IU Ca(W Mg(IU
3.7-4.2 0.7-2.9 0.3 8.6 1.07-1.11 0.6 11-16 22-23 3.6
0.02-0.05 0.1-0.4 0.003 0.4-0.8 0.4-0.8
(g/l)
C. Fabiani et al. /Desalination
Table 2 Tested membranes for the polishing waste chromium solutions
The usual process for chromium recovery is based on a classical precipitation/filtration/ solubilization scheme (Fig. 1). The recovered solution shows acceptable tanning power but the quality of tanned leather is not so good because of the low concentration of chromium and of the presence of organic compounds (mainly proteins from the leather material), oils and fats. This fact makes it necessary to polish the recovered solutions by means of adsorption on bentonites or infusorial earths which produce an additional waste to be sent to landfilling after reducing water content by means of filterpressing. To simplify the recovery scheme, reduce the use of chemicals and cut the disposal costs, the hypothesis is made that a preliminary polishing step based on microfiltration (MF) and/or ultrafiltration (UF) a chromium solution could produce (membrane permeate stream) clean of organic contaminants and more suitable for reuse in tanning baths. Experiments were performed according to the loop reported in Fig. 2 with feed solutions received from the equalization step of the cycle reported in the scheme of Fig. 1. The feed solution was pretreated with a filter unit made of tissue candles after or without a settling step. The solution is then sent through a microfiltration unit (ceramic membrane)
Recovered -
solution
Filter
I
c______-___--_~--__---J
membrane
concentrates
Fig. 2. Experimental layout for testing membranes for chromium recovery.
UF and MF
of
Membranes
Commercial
features
Ultrafiltration Separem MOPSL4040U006 Spiral wound Polysulfone
MWCO 60 KDalton 6 m2; Press. max. 10 bar Temp. max. 50°C pH 2-11; Water perm. 58 l/h.mz.bar
Ultrafiltration Osmonics 411 TA Spiral wound Polyvinylidenfluoride
MWCO 15-25 KDalton 3 m2; Press. max. 3.8 bar Temp. max. 45°C pH 2-11; Water perm. 60 l/h.m2.bar
Microfiltration SEPRA Osmonics MEMRRALOX Tubular Ceramic
0.2 m2 Pores 10 nm Pressure 4 bar Water perm. 250 l/h.m2.bar
3. Results and discussion
I
Cr sulphate
and recovery
which permeate is ultrafiltrated in a spiralwound ultrafiltration module. Membranes taken into account had the commercial features as reported in Table 2.
~~~~_~~~_~_~__~~~~~~~__, I
185
108 (1996) 183-191
The effectiveness of the settling and prefiltration steps are reported in Table 3 (Fig. 3). The solution turbidity (NTU) when the slurry or the solution layer is recycled through the filter several times strongly decreases. While for the slurry, with a suspended solid content as high as 23,960 mg/l, the lowest NTU value can be achieved at expenses of an increase in the applied pressure from 0.7 to 2 bar, in the case of settled solution the initial filtering pressure remains constant and the filtrated solution NTU value still remains cl. The slurry content of suspended solid is reduced to 1,060 mg/l (95.6%) while the coupling of settling and filtration reduces this value to 212 mg/l (99.1%) being the settling step the more effective mean to remove suspended
C. Fabiani et al. /Desalination
186
Table 3 Pretreatment of the chromium waste solution and slurry from the equalization tank. Concentrations are in mg/l, percent are removed quantities with reference to initial concentrations (slurry).
Treatm.
TSS
Slurry Filtrat.
23,690 1,060 (95.6%)
5,253 31,500 4,337 7,880 (17.4%) (75%)
Slurry
23,690
5,253
Settling Filtrat.
Cr
(98.6%) 342 212 (99.1%)
COD
N-prot.
411 161 (65%)
31,500
4,077 (22.4%) 3,615 (31.2%)
477
6,700 (78.7%) 5,800 (81.2%)
Oils, fats
4,268 151 (96.4%) 4,268
(55.6%) 212 137 (71.3%)
(9?5%) 5 (99.8%)
4am-
NTU Settling
plus
filtration
IWJ :
\ Filtration 0
I(mln) 30
Fig. 3. Feed solution turbidity filtration and settling.
(10
reduction
00
by means
of
solids (98%). Similarly, oils and fats are reduced by simple filtration by 96% (from 4,268 mg/l to 152 mg/l) while settling followed by filtration removes these substances completely (99.9%; 5 mg/l in the final solution). It is worth to note that part of the COD, chromium and N-protein are also removed, both by simple slurry filtration (COD = -75%; Cr = -17.4%; N-prot. = -65%) or
IO8 (1996) 183-191
combined settling and filtration (COD = -81.2%; Cr = -31.2%; N-prot. = -71.3%). The data of chromium show that part of the metal is present in the slurry linked to the organic suspended particles. The other components of the waste solution like salts or N-NH4 are insensitive to the considered pretreatments. Following the settling and filtration pretreatment of the wastewater feed solution, a set of experiments were performed based on microfiltration and ultrafiltration (on the microfiltration permeate). 3.1, Ultrafiltration Ultrafiltration tests were performed both directly on the slurry and on the pre-treated waste solution (settled and filtrated slurry). The MOPSL Separem four inches module was chosen with an initial water permeability of 58 l/m2.h.bar. The permeate fluxes under a full recirculation operating mode, at a recirculating flow rate of 3 m3/h and 1-3 bar of increasing applied pressure, decrease at 15 l/m2.h, no matter the settled or unsettled waste solutions is processed (Fig. 4, initial part of curves). Under the concentration operation mode fluxes still decrease as the volume concentration ratio increases (final VCR = 2.2). A plateau is reached around 8-9 l/m2.h. The flux reduction is due to a persistent deposition of material on the membrane surface as shown by the decreased membrane water permeability after washing with permeate solution, water and tensioactive solutions (Ultrasil 1% from Henkel) (Fig. 5). The final rejections for the different dispersed or dissolved species are reported in Table 4. The quality of the recovered chromium solution (permeate) is comparable for both feeds. Total rejections for organic nitrogen (75%), oils and suspended particles (more than 99%) allow to recover chromium in a clean solution. On the contrary, according to the low chromium total rejection, a convenient metal recovery can be expected. As for the N-NH4, Cl- and S042- are not retained by the membrane (rejection 2-3%).
C. Fabiani et al. /Desalination
108 (1996) 183-191
187
3.2. Microfiltration plus ultrafiltration The microfiltration treatment with the ceramic Membralox membrane, which characteristics are reported in Table 2, was tested on a Sepra microfiltration unit at 830°C and a feed recirculation rate in the 3-10 m3/h range according to a feed-and-bleed operation mode. The initial 310 1 volume was reduced at 40 1 (VCR = 7.74) at a constant pressure of 4 bar during about 13 hours of discontinuous operation. The permeate flux, from the initial 195 l/m2.h value corresponding to the pure waster permeability of the module, reduces to 70 l/m2.h. This value remains almost constant in the last six hours of the treatment (Fig. 6). In Table 5, initial and final compositions of the concentrate and permeate liquid streams are reported. The observed increase (Fig. 7) of the membrane rejection towards the soluble and insolubles species (chromium, COD, nitrogen containing species and TSS) depends on the dynamically formed deposits on the ceramic membrane surface. The microfiltration mass balances which can be calculated from the reported data shows a chromium salt recovery (permeate) corresponding to the 70.4% of the content of the treated waste solution. On the contrary, 64% of the initial organic nitrogen and 93% of the suspended solids appear in the permeate solution.
Fig. 4. UF permeate flux for (a) unsettled and (b) settled waste solution MOPSL membrane. Recirculating feed flow rate 3 m3/h; applied pressure l-3 bar; final VCR 2.2 (a) and 3.8 (b).
0
t
Fig. 5. Water permeability brane.
2
3
of clean and washed
4
mem-
To improve the performance of the ultrafiltration step the hypothesis was made that a more extended pretreatment with a microfiltration unit could produce a more acceptable ultrafiltration permeate flux and a higher volume yield still improving the quality of the recovered chromium solution.
m
I ”
I
(mln)
I
0
200
Fig. 6. Microfiltration
&a
4aa
permeate
flux.
&a
C. Fabiani et al. /Desalination
188
108 (1996) 183-191
Table 4 Rejections by ultrafiltration of chromium waste slurry solution with Separem pressure 2 bar; average temperature 35°C. (Concentrations are in mg/l) Cr Feed
Pretreated solution R% Retentate Permeate R% (final)
Table 5 Feed, retentate
5,253 4,337 17.5 5,012 3,720 30.5
and permeate Cr
Feed Feed pretreated Retentate Permeate R% (initial) R% (final)
3,369 3,278 5,823 2,651 19.4 54.5
COD 31,500 7,880 73.0 9,500 3,886 33.0
composition
following
N-prot.
TSS 23,960 1,060 55.1 600,995 150 99.6
the microfiltration
COD 7,500 7,450 11,800 6,930 4.0 41.3
MOPSL modules.
TSS
609 579 4.9 587 565 3.7
(Concentrations
N-prot.
180 130 600 40 61.5 93.3
270 483 174 7.4 64.0
Table 6 Composition of the recovered chromium solution (UF permeate) and concentrated produced by microfiltration and ultrafiltration coupling. (Concentrations are in mg/l) Cr Feed MF permeate R% UF retentate UF permeate R%
3,369 2.65 1 54.5 3,200 2,310 27.0
COD
TSS
7,500 6,930 41.5 7,180 4,850 32.4
180 40 93.3 100 25 70.0
Microfiltrated chromium solution were then ultrafiltrated with the Osmonics module. The ultrafiltrated fluxes (Fig. 8) are higher than those previously obtained with MOPSL membranes even if the MWCO of the Osmonics membrane is as low as 15-25 KDalton. These results are due to the high rejection of TSS, oils and greases obtained
N-prot. 308 174 64.0 215 173 20.0
4,268 152 96.4 49 4 91.8
are in mgil)
N-NH4 308 444 514 400 12.2 22.2
waste
rate 3 m3/h;
Oils, fats
N-NH4
477 167 64.9 239 117 51.0
treatment.
Recycling
solution
N-NH4 476 400 22.0 437 446 0
Oils, fats 49 36 47 18 47.2 61.7
(UF retentate)
Oils, fats 49 18 93.3 25 2 92.0
with the microfiltration treatment of the feed solution, The rejections shown in Table 6 and the composition of the permeate solution show that still a 70% recovery of dissolved chromium is obtained in a solution with low oils and suspended organic nitrogen, particles.
C. Fabiani et al. /Desalination
Fig. 7. Rejection
of MF membrane.
Wn)
Fig. 8. MF+UF permeate
flux
Chromium solution, still too diluted for tanning purposes, is however clean enough to be used with the addition of new chromium salts as a make-up of new tanning baths or alternatively treated for concentration up to a level suitable for direct use in tanning baths WI. 3.3. The proposed process scheme Previous results can be summarized with the process scheme shown in Fig. 9. Mass balances and recovery are calculated from the discussed data with reference with a
108 (1996) 183-191
189
chromium recovery plant that treats 520 m3/d of exhausted bath with the classical precipitation/dissolution method of Fig. 1. According to the data reported in Fig. 9, 63% of the feed volume is recovered as water in the chromium solution with a concentration of free metal available for tanning purposes of 2.3 g/l. This corresponds to a recovery of 28% of the metal initially present in the wastewaters) that is 758.8 kg of metal per day. This recovery yield refers to the initial content of chromium in the wastewaters but if the calculation is performed on the chromium available in solution after the settling and filtration pre-treatment the chromium recovered by coupling micro- and ultrafiltration is about 42%. In fact, the chromium presents in wastewaters is not completely free but partially bounded or complexed to organic matter or suspended particles as can be deduced from the strong reduction in metal content (-48%) after the mechanical pre-treatment and the microfiltration steps. This retained chromium is collected as sludges from settler and filtration units and as microfiltration concentrate. These process residues must recycled in the equalization tank or treated for disposal. Organic species content is strongly reduced (170 mg/l) while suspended particles and oils or greases are practically absent. Economic evaluations cannot be made at this point of the research because data on long lasting tests are lacking. The only reasonable anticipation can be drawn from the chromium recovery, the savings of chemicals and the reduction of the produced sludges and hence on the landfilling costs. Chemicals for chromium precipitation (about 2 kg per kg of metal [6] (which means daily 1,500 kg) and costs for sludges (daily about 3,000 kg) disposal are consequently reduced. 4. Conclusions The actual management of the chromium exhauted baths for metal recovery and reuse
C. Fabiani et al. /Desalination
190
vo
108 (1996) 183-191
= s2om
TSS Cr COD N-prot 011s
23,690 5253 3 1,500 477 4268
mg/L mq/L m$L mg/L mg/L
EQUALIZATION w
TANK
PRE-TREATMENT -
FEED
TSS EbD N-trot OtlS
2rZmglL 3369 mg/L 7500 mg/L 270 mg/L 56 mg/L
ULTRAFILTRATION
VS = 328.5U TSS 30 mg/L 0 2331 mg/L 4850 mg/L COD N-prot 173 mg/L OllS 2 mg/L
V2
= 437
TSS Cr COD N-prot 011s
5 m?/Q
40 mg/L 2651 mg/L 6930 mg/L I74 mg/L 18 mg/L
in the tanning process allows to reach a high recovery yield of chromium. However, the quality of the salt is depleted because of the presence of organic matter. Moreover, the recovery process produces a lot of sludges especially in the flocculation and finishing steps. The use of chemicals for pH control, precipitation and acid dissolution of chromium hydroxide also contribute to the process costs. The proposed process on the contrary allows a recovery of chromium in solution at a level of 28% while the presence of organic matter is low suggesting the possibility of a direct concentration of the obtained solution or its use to make up new tanning bath with addition of chromium salt. These results could be achieved with direct ultrafiltration of the waste solution at expenses of the permeate flow which is very low because of the membrane clogging due the presence of
Fig. 9. Mass balance coupled microfiltration process scheme.
of the proposed and ultrafiltration
suspended solids, oils and fats. Low permeate flow means large membrane area and high investment costs. The coupling of a microfiltration pretreatment of the waste solution with ultrafiltration produces a clean chromium salt (permeate) solution and allows at the same time to work with fluxes four-five time higher reducing the membrane area needed for ultrafiltration. According to the proposed process, a reduction of the sludges produced is expected which means a saving in chromium recovery process costs.
Acknowledgements The experimental work was supported by the Italian Ministry of Research and University in the frame of the national program for chemical research (Murst-PNC).
C. Fabiani et al. /Desalination
References [l] [2] [3]
[4] [5]
T.F. O’Dwyer and B.K. Hodnett, J. Chem. Tech. Biotechnol. 62 (1995) 30-37. G. Macchi, M. Pagano, M. Pettine, M. Santori, and G. Tiiavanti, Water Res. 25 (1991) 1019-1026. R. Leyva-Ramos, L. Fuentes-Rubio, R.M. GuerreroCoronado, and J. Mendoza-Barron, J. Chem. Tech. Biotechnol. 62 (1995) 64-67. K.K. Panday, Gur Prasad, and V.N. Singh, J. Chem. Tech. Biotechnol. 34A (1984) 367-374. SK. Thampy, P.K. Narayanan, D.K. Chauhan, J.J. Trivedi, and V.K. Indusekhar, Sep. Sci. Tech. 30 (1995) 3715-3722.
IO8 (1996) 183-l 91
191
[6] S.E. Jorgensen, Industrial waste water management, Elsevier, Amsterdam, 1979. [7] Consorzio Recupero Cromo (CREC-Pisa) Plant: private communication. [8] Imprese e unita locali. Industria, Italian National Institute of Statistics, ISTAT, Rome, 1996. [9] A. Cassano, E. Drioli, R. Molinari, and C. Bertolutti, Quality improvement of recycled chromium in the tanning operation by membrane processes, Desalination 108 (1996) 193-203.
Desalination
108 (1996)
183-191
Chromium(II1) salts recovery process from tannery wastewaters C. Fabianil*, F. Rusciol, M. Spadonil,
and M. Pizzichini2
lEnvironment Department, and 21nnovative Technologies Department, B.N.E.A. CR-Casaccia, Via Anguillarese 301, 00060 Roma, Italy. Tel.: +39-6-304&l; Fax.: +39-6-30484203 Received
27 July 1996; accepted 2 August 1996
Abstract
Chromium(II1) salts are the most widely used chemicals for tanning processes [ 11, but only 60% of the total chromium salt reacts with the hides. Therefore, about 40% of the chromium amount remains in the solid and liquid wastes (especially spent tanning solutions). The presence of chromium(111) and salts in the sludges of both the wastewater biological treatment plants or the chemical plants for recycling spent tanning liquors, represents an inconvenience for the safe reuse of these sludges and a cost forming factor for their disposal. Among the several proposed methods for chromium recovery [l-6], lime or sodium hydroxide precipitation and filterpressing of the chromimn hydroxide is the usual way to recover chromium salts from spent process solutions or from leaching solutions of residues of hides. However, usually the chromium salts quality must be improved
for their reuse in tanning processes 171.The integration of membranes in the treatment process for chromium recovery reduces the environmental impact, favours the reuse of both the protein residue and the biological plant sludges, reduces the consumption of chemicals and decreases the costs of waste disposal. Keywords: Chromium recovery; Tanning processes; Microfiltration; Ultrafiltration 1.
Introduction
More than one hundred different chemicals (350,000 ton/y of inorganic and heavy metal salts, soaps, tensioactives, oils, waxes, solvents, dyes, etc) used in tanning processes are found in process wastes and wastewaters. Chromium(II1) salts and sulfur compounds are the main pollutants released in tannery wastewaters and in the atmosphere. The Presented at the Second Annual Meetin of the Euro ean Desalination Society (EDS) on Desa 7’mation an dpthe Environment, Genoa, Italy, October 20-23, 1996. *Corresponding
author.
001 l-9164/97/$17.00 Copyright PZZ SO0 1 l-9 164(97)00026-X
function of chromium salts in tanning processes is to form, through complexation with the polypeptide collagen components of leather, a protective layer which prevents the penetration of water in the leather pores avoiding putrefaction. In Italy [3], second world producer of leather goods (more than 2,500 production units, 25,000 employees and 600,000 ton/y of treated hides), tannery wastewaters amount reaches the relevant level of 40,000,OOO m3/y even if process improvements in recent years have reduced the technical needs of process water: from 100 liters of water per kg of leather to 12-13 liters per kilogram. Waste-
e 1997 Elsevier Science B.V. All rights reserved
C. Fabiani et al. /Desalination
184
water treatment plants produce more than 700,000 ton/y of sludges and residues. Treatment and disposal costs amount to about 840,000 Mlitiy (560 millions of US $). dimension of this The impressive environmental problem pushes to transfer in tanneries any available technological improvement to reduce the environmental impact and recover and recycle water and the main chemicals. As far as the chromium salts is concerned, the tanning process consumes only 60% of the chromium of the tanning bath and the possibility of recover the residual metal represents a main goal in process improvement. Plants devoted to chromium salts (mainly chromium sulphate) recovery from the exhausted tanning baths are based on the cycle reported in Fig. 1. According to this cycle, the Consorzio Recupero Cromo (CRC)‘s plant, where the experimental study was performed, daily treats 450-700 m3 of
108 (1996) 183-191
waste solutions collected from 172 tanneries of the district of Santa Croce sull’Arno (Pisa) in Tuscani. In the referred plant chromium solutions, with a chromium content (5-6 g/l), and flocculated or pressed chromium sludges (12-20 g/l or loo-140 g/l metal content, respectively) are produced. This corresponds to a daily average of 1.12 ton of metal (as CrzOs). The chromium(II1) is recovered as basic sulphate (8-20 m3/d) with 31-33% of alkalinity which corresponds to an average of 21 ton/d of recovered product. However, the low chromium concentration and the presence of organic molecules (especially proteins,) makes the recovered solution not fully acceptable for a good quality result in the tanning process [3]. An improvement of the chromium recovery cycle by means of membranes processes will be presented and the environmental and economic benefits will be discussed.
2. Experimental Chromium solutions. 199 ton/d
waste
The mean composition of waste solution to be treated for chromium recovery is reported in Table 1.
I
Table 1 Composition recovery
i--I Settling
of the waste solution
Species
used for chromium
Concentration
Screening
Neutralization Cr2(S04)3
Fig. 1. Chromium
recovery
process
scheme.
basic
PH TSS Oils, fats COD N-Kijeldal N-NH3 Chloride Sulphate Cr(II1) Other metals (cations) Fe(m) Al(W Mn(IU Ca(W Mg(IU
3.7-4.2 0.7-2.9 0.3 8.6 1.07-1.11 0.6 11-16 22-23 3.6
0.02-0.05 0.1-0.4 0.003 0.4-0.8 0.4-0.8
(g/l)
C. Fabiani et al. /Desalination
Table 2 Tested membranes for the polishing waste chromium solutions
The usual process for chromium recovery is based on a classical precipitation/filtration/ solubilization scheme (Fig. 1). The recovered solution shows acceptable tanning power but the quality of tanned leather is not so good because of the low concentration of chromium and of the presence of organic compounds (mainly proteins from the leather material), oils and fats. This fact makes it necessary to polish the recovered solutions by means of adsorption on bentonites or infusorial earths which produce an additional waste to be sent to landfilling after reducing water content by means of filterpressing. To simplify the recovery scheme, reduce the use of chemicals and cut the disposal costs, the hypothesis is made that a preliminary polishing step based on microfiltration (MF) and/or ultrafiltration (UF) a chromium solution could produce (membrane permeate stream) clean of organic contaminants and more suitable for reuse in tanning baths. Experiments were performed according to the loop reported in Fig. 2 with feed solutions received from the equalization step of the cycle reported in the scheme of Fig. 1. The feed solution was pretreated with a filter unit made of tissue candles after or without a settling step. The solution is then sent through a microfiltration unit (ceramic membrane)
Recovered -
solution
Filter
I
c______-___--_~--__---J
membrane
concentrates
Fig. 2. Experimental layout for testing membranes for chromium recovery.
UF and MF
of
Membranes
Commercial
features
Ultrafiltration Separem MOPSL4040U006 Spiral wound Polysulfone
MWCO 60 KDalton 6 m2; Press. max. 10 bar Temp. max. 50°C pH 2-11; Water perm. 58 l/h.mz.bar
Ultrafiltration Osmonics 411 TA Spiral wound Polyvinylidenfluoride
MWCO 15-25 KDalton 3 m2; Press. max. 3.8 bar Temp. max. 45°C pH 2-11; Water perm. 60 l/h.m2.bar
Microfiltration SEPRA Osmonics MEMRRALOX Tubular Ceramic
0.2 m2 Pores 10 nm Pressure 4 bar Water perm. 250 l/h.m2.bar
3. Results and discussion
I
Cr sulphate
and recovery
which permeate is ultrafiltrated in a spiralwound ultrafiltration module. Membranes taken into account had the commercial features as reported in Table 2.
~~~~_~~~_~_~__~~~~~~~__, I
185
108 (1996) 183-191
The effectiveness of the settling and prefiltration steps are reported in Table 3 (Fig. 3). The solution turbidity (NTU) when the slurry or the solution layer is recycled through the filter several times strongly decreases. While for the slurry, with a suspended solid content as high as 23,960 mg/l, the lowest NTU value can be achieved at expenses of an increase in the applied pressure from 0.7 to 2 bar, in the case of settled solution the initial filtering pressure remains constant and the filtrated solution NTU value still remains cl. The slurry content of suspended solid is reduced to 1,060 mg/l (95.6%) while the coupling of settling and filtration reduces this value to 212 mg/l (99.1%) being the settling step the more effective mean to remove suspended
C. Fabiani et al. /Desalination
186
Table 3 Pretreatment of the chromium waste solution and slurry from the equalization tank. Concentrations are in mg/l, percent are removed quantities with reference to initial concentrations (slurry).
Treatm.
TSS
Slurry Filtrat.
23,690 1,060 (95.6%)
5,253 31,500 4,337 7,880 (17.4%) (75%)
Slurry
23,690
5,253
Settling Filtrat.
Cr
(98.6%) 342 212 (99.1%)
COD
N-prot.
411 161 (65%)
31,500
4,077 (22.4%) 3,615 (31.2%)
477
6,700 (78.7%) 5,800 (81.2%)
Oils, fats
4,268 151 (96.4%) 4,268
(55.6%) 212 137 (71.3%)
(9?5%) 5 (99.8%)
4am-
NTU Settling
plus
filtration
IWJ :
\ Filtration 0
I(mln) 30
Fig. 3. Feed solution turbidity filtration and settling.
(10
reduction
00
by means
of
solids (98%). Similarly, oils and fats are reduced by simple filtration by 96% (from 4,268 mg/l to 152 mg/l) while settling followed by filtration removes these substances completely (99.9%; 5 mg/l in the final solution). It is worth to note that part of the COD, chromium and N-protein are also removed, both by simple slurry filtration (COD = -75%; Cr = -17.4%; N-prot. = -65%) or
IO8 (1996) 183-191
combined settling and filtration (COD = -81.2%; Cr = -31.2%; N-prot. = -71.3%). The data of chromium show that part of the metal is present in the slurry linked to the organic suspended particles. The other components of the waste solution like salts or N-NH4 are insensitive to the considered pretreatments. Following the settling and filtration pretreatment of the wastewater feed solution, a set of experiments were performed based on microfiltration and ultrafiltration (on the microfiltration permeate). 3.1, Ultrafiltration Ultrafiltration tests were performed both directly on the slurry and on the pre-treated waste solution (settled and filtrated slurry). The MOPSL Separem four inches module was chosen with an initial water permeability of 58 l/m2.h.bar. The permeate fluxes under a full recirculation operating mode, at a recirculating flow rate of 3 m3/h and 1-3 bar of increasing applied pressure, decrease at 15 l/m2.h, no matter the settled or unsettled waste solutions is processed (Fig. 4, initial part of curves). Under the concentration operation mode fluxes still decrease as the volume concentration ratio increases (final VCR = 2.2). A plateau is reached around 8-9 l/m2.h. The flux reduction is due to a persistent deposition of material on the membrane surface as shown by the decreased membrane water permeability after washing with permeate solution, water and tensioactive solutions (Ultrasil 1% from Henkel) (Fig. 5). The final rejections for the different dispersed or dissolved species are reported in Table 4. The quality of the recovered chromium solution (permeate) is comparable for both feeds. Total rejections for organic nitrogen (75%), oils and suspended particles (more than 99%) allow to recover chromium in a clean solution. On the contrary, according to the low chromium total rejection, a convenient metal recovery can be expected. As for the N-NH4, Cl- and S042- are not retained by the membrane (rejection 2-3%).
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108 (1996) 183-191
187
3.2. Microfiltration plus ultrafiltration The microfiltration treatment with the ceramic Membralox membrane, which characteristics are reported in Table 2, was tested on a Sepra microfiltration unit at 830°C and a feed recirculation rate in the 3-10 m3/h range according to a feed-and-bleed operation mode. The initial 310 1 volume was reduced at 40 1 (VCR = 7.74) at a constant pressure of 4 bar during about 13 hours of discontinuous operation. The permeate flux, from the initial 195 l/m2.h value corresponding to the pure waster permeability of the module, reduces to 70 l/m2.h. This value remains almost constant in the last six hours of the treatment (Fig. 6). In Table 5, initial and final compositions of the concentrate and permeate liquid streams are reported. The observed increase (Fig. 7) of the membrane rejection towards the soluble and insolubles species (chromium, COD, nitrogen containing species and TSS) depends on the dynamically formed deposits on the ceramic membrane surface. The microfiltration mass balances which can be calculated from the reported data shows a chromium salt recovery (permeate) corresponding to the 70.4% of the content of the treated waste solution. On the contrary, 64% of the initial organic nitrogen and 93% of the suspended solids appear in the permeate solution.
Fig. 4. UF permeate flux for (a) unsettled and (b) settled waste solution MOPSL membrane. Recirculating feed flow rate 3 m3/h; applied pressure l-3 bar; final VCR 2.2 (a) and 3.8 (b).
0
t
Fig. 5. Water permeability brane.
2
3
of clean and washed
4
mem-
To improve the performance of the ultrafiltration step the hypothesis was made that a more extended pretreatment with a microfiltration unit could produce a more acceptable ultrafiltration permeate flux and a higher volume yield still improving the quality of the recovered chromium solution.
m
I ”
I
(mln)
I
0
200
Fig. 6. Microfiltration
&a
4aa
permeate
flux.
&a
C. Fabiani et al. /Desalination
188
108 (1996) 183-191
Table 4 Rejections by ultrafiltration of chromium waste slurry solution with Separem pressure 2 bar; average temperature 35°C. (Concentrations are in mg/l) Cr Feed
Pretreated solution R% Retentate Permeate R% (final)
Table 5 Feed, retentate
5,253 4,337 17.5 5,012 3,720 30.5
and permeate Cr
Feed Feed pretreated Retentate Permeate R% (initial) R% (final)
3,369 3,278 5,823 2,651 19.4 54.5
COD 31,500 7,880 73.0 9,500 3,886 33.0
composition
following
N-prot.
TSS 23,960 1,060 55.1 600,995 150 99.6
the microfiltration
COD 7,500 7,450 11,800 6,930 4.0 41.3
MOPSL modules.
TSS
609 579 4.9 587 565 3.7
(Concentrations
N-prot.
180 130 600 40 61.5 93.3
270 483 174 7.4 64.0
Table 6 Composition of the recovered chromium solution (UF permeate) and concentrated produced by microfiltration and ultrafiltration coupling. (Concentrations are in mg/l) Cr Feed MF permeate R% UF retentate UF permeate R%
3,369 2.65 1 54.5 3,200 2,310 27.0
COD
TSS
7,500 6,930 41.5 7,180 4,850 32.4
180 40 93.3 100 25 70.0
Microfiltrated chromium solution were then ultrafiltrated with the Osmonics module. The ultrafiltrated fluxes (Fig. 8) are higher than those previously obtained with MOPSL membranes even if the MWCO of the Osmonics membrane is as low as 15-25 KDalton. These results are due to the high rejection of TSS, oils and greases obtained
N-prot. 308 174 64.0 215 173 20.0
4,268 152 96.4 49 4 91.8
are in mgil)
N-NH4 308 444 514 400 12.2 22.2
waste
rate 3 m3/h;
Oils, fats
N-NH4
477 167 64.9 239 117 51.0
treatment.
Recycling
solution
N-NH4 476 400 22.0 437 446 0
Oils, fats 49 36 47 18 47.2 61.7
(UF retentate)
Oils, fats 49 18 93.3 25 2 92.0
with the microfiltration treatment of the feed solution, The rejections shown in Table 6 and the composition of the permeate solution show that still a 70% recovery of dissolved chromium is obtained in a solution with low oils and suspended organic nitrogen, particles.
C. Fabiani et al. /Desalination
Fig. 7. Rejection
of MF membrane.
Wn)
Fig. 8. MF+UF permeate
flux
Chromium solution, still too diluted for tanning purposes, is however clean enough to be used with the addition of new chromium salts as a make-up of new tanning baths or alternatively treated for concentration up to a level suitable for direct use in tanning baths WI. 3.3. The proposed process scheme Previous results can be summarized with the process scheme shown in Fig. 9. Mass balances and recovery are calculated from the discussed data with reference with a
108 (1996) 183-191
189
chromium recovery plant that treats 520 m3/d of exhausted bath with the classical precipitation/dissolution method of Fig. 1. According to the data reported in Fig. 9, 63% of the feed volume is recovered as water in the chromium solution with a concentration of free metal available for tanning purposes of 2.3 g/l. This corresponds to a recovery of 28% of the metal initially present in the wastewaters) that is 758.8 kg of metal per day. This recovery yield refers to the initial content of chromium in the wastewaters but if the calculation is performed on the chromium available in solution after the settling and filtration pre-treatment the chromium recovered by coupling micro- and ultrafiltration is about 42%. In fact, the chromium presents in wastewaters is not completely free but partially bounded or complexed to organic matter or suspended particles as can be deduced from the strong reduction in metal content (-48%) after the mechanical pre-treatment and the microfiltration steps. This retained chromium is collected as sludges from settler and filtration units and as microfiltration concentrate. These process residues must recycled in the equalization tank or treated for disposal. Organic species content is strongly reduced (170 mg/l) while suspended particles and oils or greases are practically absent. Economic evaluations cannot be made at this point of the research because data on long lasting tests are lacking. The only reasonable anticipation can be drawn from the chromium recovery, the savings of chemicals and the reduction of the produced sludges and hence on the landfilling costs. Chemicals for chromium precipitation (about 2 kg per kg of metal [6] (which means daily 1,500 kg) and costs for sludges (daily about 3,000 kg) disposal are consequently reduced. 4. Conclusions The actual management of the chromium exhauted baths for metal recovery and reuse
C. Fabiani et al. /Desalination
190
vo
108 (1996) 183-191
= s2om
TSS Cr COD N-prot 011s
23,690 5253 3 1,500 477 4268
mg/L mq/L m$L mg/L mg/L
EQUALIZATION w
TANK
PRE-TREATMENT -
FEED
TSS EbD N-trot OtlS
2rZmglL 3369 mg/L 7500 mg/L 270 mg/L 56 mg/L
ULTRAFILTRATION
VS = 328.5U TSS 30 mg/L 0 2331 mg/L 4850 mg/L COD N-prot 173 mg/L OllS 2 mg/L
V2
= 437
TSS Cr COD N-prot 011s
5 m?/Q
40 mg/L 2651 mg/L 6930 mg/L I74 mg/L 18 mg/L
in the tanning process allows to reach a high recovery yield of chromium. However, the quality of the salt is depleted because of the presence of organic matter. Moreover, the recovery process produces a lot of sludges especially in the flocculation and finishing steps. The use of chemicals for pH control, precipitation and acid dissolution of chromium hydroxide also contribute to the process costs. The proposed process on the contrary allows a recovery of chromium in solution at a level of 28% while the presence of organic matter is low suggesting the possibility of a direct concentration of the obtained solution or its use to make up new tanning bath with addition of chromium salt. These results could be achieved with direct ultrafiltration of the waste solution at expenses of the permeate flow which is very low because of the membrane clogging due the presence of
Fig. 9. Mass balance coupled microfiltration process scheme.
of the proposed and ultrafiltration
suspended solids, oils and fats. Low permeate flow means large membrane area and high investment costs. The coupling of a microfiltration pretreatment of the waste solution with ultrafiltration produces a clean chromium salt (permeate) solution and allows at the same time to work with fluxes four-five time higher reducing the membrane area needed for ultrafiltration. According to the proposed process, a reduction of the sludges produced is expected which means a saving in chromium recovery process costs.
Acknowledgements The experimental work was supported by the Italian Ministry of Research and University in the frame of the national program for chemical research (Murst-PNC).
C. Fabiani et al. /Desalination
References [l] [2] [3]
[4] [5]
T.F. O’Dwyer and B.K. Hodnett, J. Chem. Tech. Biotechnol. 62 (1995) 30-37. G. Macchi, M. Pagano, M. Pettine, M. Santori, and G. Tiiavanti, Water Res. 25 (1991) 1019-1026. R. Leyva-Ramos, L. Fuentes-Rubio, R.M. GuerreroCoronado, and J. Mendoza-Barron, J. Chem. Tech. Biotechnol. 62 (1995) 64-67. K.K. Panday, Gur Prasad, and V.N. Singh, J. Chem. Tech. Biotechnol. 34A (1984) 367-374. SK. Thampy, P.K. Narayanan, D.K. Chauhan, J.J. Trivedi, and V.K. Indusekhar, Sep. Sci. Tech. 30 (1995) 3715-3722.
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191
[6] S.E. Jorgensen, Industrial waste water management, Elsevier, Amsterdam, 1979. [7] Consorzio Recupero Cromo (CREC-Pisa) Plant: private communication. [8] Imprese e unita locali. Industria, Italian National Institute of Statistics, ISTAT, Rome, 1996. [9] A. Cassano, E. Drioli, R. Molinari, and C. Bertolutti, Quality improvement of recycled chromium in the tanning operation by membrane processes, Desalination 108 (1996) 193-203.