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A bench-scale study on chromium recovery from tanning wastewater
~ Pergamon
Wal. ScL Ttch. Vol. 31, No.9, pp. 73-81,199'. Copyrigbt C 1995 IAWQ Printed in Oreat Britain. All rigbll reserved.
0273-1223(95)00408-4
0:m-I223195 $9'50 + O{)()
A BENCH-SCALE STUDY ON CHROMIUM
RECOVERY FROM TANNING WASTEWATER
Thongchai Panswad, Orathai Chavalparit, Yaowanud Sucharittham and Sasithom Charoenwisedsin Department 0/ Environmental Engineering, Chulalongkom University,
Bangkok 10330, Thailand
ABSTRACT In order to recover chromium, wastewaters from the chrome-tanning process in two Thai tanneries were coagulated with magnesium oxide or sodium carbonate at various dosages. Different types of polymer were also tried to improve the settleability of the chromium sludge which, in tum. was redissolved by sulfuric acid. The produced chromium III sulfate (Cr2(SO~Y. solution was sufficiently concenttated for direct reuse in the tanning step. The settling pattern of the chromium sludge was also observed and optimum sedimentation periods ranging from 1 to 17 bours were suggested. Under no tanning-additive (a cbemical to improve cbromium sorption onto bides) circumstances, the discharged wastewater cootained 5-11 g CrfJfI and the most promising coagulant was MgO at a dose of twice the stoichiometric value and an optimum pH of 7-8. The resulting chromium precipitate or sludge was quite dense (21 % solids) and possessed a good settling velocity (234 mm/b), making the decantation process very easy, whereas the corresponding acid requirement for the dissolution process was 1.5 times the stoichiometric value. On the other band, in the tanning-additive situation, the chromium concenttation in the drain was reduced to 3-6 g CrfJ-JI. Magnesium oxide was shown to be also the best coagulant but the dose was two times higher than that of the first circumstance, ie., four times that of stoichiometric requirement.
KEYWORDS Chromium recovery; dissolution;' precipitation; tannery wastewater. INTRODUCTION Chromium (Ill) sulphate has been widely used as a tanning agent in the leather industry (Macchi et al., 1991). InteractIon between chromium (Cr3+) and collagen leads leather to stabilise and to possess a shrinkage-temperature property of over 100°C. In chrome tanning, only 70-80% of chromium applied is taken up by the leather, and the rest is discharged as waste, inducing an important source of chromium contamination. The biological sludge from the tannery waste treatment was reported to have 11 high content of chromium and can cause accumulation of the metal in the soil (Dreiss, 1988; Lo and Chen, 1990). The chromium when accumulated in plants and animals at high levels can generate serious diseases (Richard and Bourg 1991). Taking into account the large volume of wastewater containing very high chromium concentrations and the difficulties in finding proper disposal sites for the sludge produced, many countries have turned to "clean" technologies, including chromium recovery and reuse. Chromium recovery means 73
74
T. PANSWAD et al.
separating chromium from the spent liquour by precipitation, then redissolving the chromium sludge for reuse. The primary investigations showed that chromium (III) could be precipitated by all common basic compounds, but the settling characteristics of the precipitate formed by the reaction of magnesium oxide were superior due to its minimum sludge volume (Bogaerts, 1992; Wood, 1992; Boat,1988; Langerwerf, 197H). Leather production in Thailand has shown an extremely high growth rate, especially during the last two years (Meyhoefer, 1992). In 1990, 140,000 metric tons of hides (mostly bovine) were tanned in approximately 150 tanneries in the country (Banjongpru, 1992). More than 80% of the tanneries used a chrome - tanning process (Porst, 1991). Annually, 1050 metric tons of Cr203 were used and, out of this amount, S6 metric tons of Cr203 were discharged as wastewaters into streams. It was calculated that the tanning industry discharged the wastewater with a total annual volume of 2.73 million m3 and the total annual loss of chromium was equal to 29.7 million baht or approx US $ 1.1 million (Banjongpru, 1992; Panswad and Chavalparit, 1992). EXPERIMENTAL This study was carried out with wastewaters discharged from two chrome tanning processes, namely, with and without additives, commercially known as Feliderm CS. Samples from both sources were collected and screened prior to the physical and chemical analysis which were done before and after experimental tests for chromium hydroxide precipitation using different alkalis, i.e., MgO, heated MgO (40 to 50°C) and Na2C03 at various dosages, (see Fig. I). Three different types of polymer were also used in an attempt for improvement in flocculation and· sedimentation. A precalculated quantity of precipitation reagents (see equations below) were added to the liquor and stirred for two hours, after which the pH measurement, sedimentation pattern, sedimentation time, sludge volume and chromium content in the supernatant were taken or studied. After the settling process, the decanted supernatant was taken for laboratory analysis to determine the Cr precipitation (or removal) efficiency. The sludge was redissolved by using cone H2S04, A portion of the sludge was subsequently dewatered by vacuum filtration using a conventional suction flask and the dewatered cake was redissolved with 1+4 H2S04 solution. Continuous and at-initial-stage mixing as well as raising of the temperature were also investigated in order to determine the effect of the degree of mixing and temperature increase on the recovery process. The recovery efficiency as well as preliminary cost evaluation were subsequently investigated. . Chemical reactions :Cr2(S04h + 3MgO +3H20 ~ 3MgS04 + 2Cr(OH}J the by-weight stoichiomatic value for Cr : MgO is 1 : 1.16.
(I)
Cr2(S04}J+3Na2C03 + 3H20 • 2Cr(OH}J+3Na2S04+3C02 the by-weight stoichiomatic value for Cr : Na2C03 is I : 3.06.
(2)
2Cr(OH)3 + 3H2S04 • Cr2(S04» + 6H20 the by weight stoichiomatic value for Cr : H2S04 is 1 : 2.83.
(3)
RESULTS AND DISCUSSION Characteristics of waste chromium liQuor The average pH of the two wastewaters, i.e., with and without additive, was 3.56 and 2.93, respectively, with corresponding chromium concentrations of 1,540 and 3.070 mglI, (see Table I). The redox value of the noadditive wastewater was higher than that of its counterpart, probably due to its higher Cr concentration.
7S
Chromium recovery from tanning wastewater
Tanning without additive
Tanning with additive
l Sampling
l Screening
l Lab Analysis
l
Precipitation Tests with MgO, Heated MgO and Na2C03 and 3 types of polymer
l
Sedimentation Study
! Decantation t
~
l
Supernatant
Sludge
.'
Lab Analysis
l
Vacuum Filtration
% Precipitation Efficiency StudX % Cr Recovery Study
l
~
Cake
Preliminary Cost Analysis FigUre 1. Experimental now chart.
In addition, the corresponding tanning process required less NaCI as weUas ammonia and consequently, discharged less of those salts. It is widely known that the final pH of the basic chromium liquor as well as the presence of other reagents from the tanning process could affect the subsequent precipitation process.
T. PANSWAD tl at.
76
Table 1. Chemical characteristics of waste chrome liquor Chrome tanning wastewater Parameter
pH Conductivity, mS/em Redox, mv Total Solids, mgll Dissolved Solids, mgll Suspended Solids, mg/\ Chloride, mgll Ammonia Nitrogen, mgll COD, mgll Chromium (soluble), mgll Chromium (total), mg/\
without additive
With additive
(n=9)
(n=8)
2.93 63.6 342 86,850 85,570 .1,280 24,900 450 4,030 3,070 4,130
3.56 91.5 247 131,670 129,170 2,510 45,280 810
5,500 1,540 2,170
Optimum conditions for precipitatjon Qfchromjum From the laboratory bench-scale experiments with non-additive wastewater, it was found that the precipitation step using MgO at the dose of twice the stoichiometric value of the chromium in the waste liquor (or MgO 2.3 gig Cr) and a settling time of I hour was optimal. The chromium content of 2,740 mgIJ in raw waste was reduced to 48 mgll in the supernatant, resulting in 98% removal efficiency, (see Fig. 2 and Table 2). With the optimum reaction pH of 7.3, the produced sludge was very dense and possessed a high settling velocity (approx. 234 mm/h). Polymers were found not to improve the flocculation and sedimentation perfonnance. Moreover, results obtained from the tests of precipitation with heated MgO were found to be even less satisfactory, (see Fig. 3).
=
Using the NaZCO] at dosage of twice the theoretical value (6.1 g of NaZCO] were needed for I g of Cr) it was apparent that a similar outcome, 98% removal efficiency, might result, (see Table 2). The chromium hydroxide precipitate in this case however settled very slowly. After being allowed to settle overnight, the sludge volume was found to be 540 mllI and the supernatant contained 46 mg Crll with a relatively low pH of 6.3. The NazC03 treatment was therefore less attractive due to the further necessity of a dewatering unit. Besides, the settling process took such a long time (at least 17 hours), that the precipitate then became aged, resulting in a growing insolubility of the substance due to the transfonnation of hydroxide to mixed-oxides complexes (Rajamani etat., 1992; Wood, 1992). Regarding the tests with wastewaters from with-additive tanning, it was apparent that the MgO dose had to be two times higher than that of the non-additive case (Le. 4.6 g MgO/g Cr), (see Table 2). After treatment, 97% of the chromium could be precipitated, resulting in 34 mg Crll in the supernatant from the initial concentration of 1,120 mgIJ. The settled volume of sludge at 60 min was about 250 mllI, (see Fig.S). An anionic polymer in this case proved able to enhance the settling property; the optimum settling time could be reduced from 1 hour to 10 min. With NazC03 treatment for the same wastewater, a similar conclusion could be drawn, i,e.• 96% removal efficiency and a high volume of sludge produced (300 mUll. The results of precipitation of chromium by various dosages of chemicals are shown in Table 2 and Figs. 5-7.
77
Chromium recovery from tanning waslewater
Cr • 2,742 mg/l, Sedimentation TIme 1 h.
: . r-;::-=-----'----=-....;...--------=-=~280:;::::=2::(80)-, m
~
m
m
200 180 180
140
120
100 80
133.17 13l1.411 14lI.O:S
I~!_----=~~:::::_~!J:!.--~~~-~~.--,~~
13.17
80
40 20
~ llO."
I 20
SO.1I1
112."
ttl.' EJ 22.4
O'---'-----''---''---'_-''_--I._-'"_--'-_-'-_........_ - ' - _......._ - l 1.4
S
1.11
1.'
Cr Cone. (mgll)
Figure 2. Results of precipitation of chromium by MgO for "non-additive" wastewater.
Optimum conditions for dissolution Dissolution of chromium by sulfuric acid depended on mixing time and types of precipitate, namely sludge or dewatered cake. The required dissolution time for chromium cake under no-mixing (i.e., mixing at only the initial stage) condition was 90 min whereas under other conditions the corresponding time was only 60 min. The recovered Cr2S04 solution under the scenario of non-additive waste chrome liquor was more concentrated than that from with-additive wastewater under the same conditions because it needed less H2S04 for the dissolution. For the same reason, the chromium concentration in the solution obtained from the dissolution of the dewatered cake was higher than in that from the chrome sludge. The lower than 3.5 pH was found to be very effective for dissolution, the recovery efficiency was as high as 85-98% and the raised temperature did not have any significant effect on the dissolution. The optimum doses of sulfuric acid for the dissolution of chromium oxide are summarised in Table 3. The acid requirement varied from 2.5-10.2 gig Cr depending on types of wastewater, precipitation reagents and methods of dewatering. The recovered Cr~04 solution had 13,380 and 31,820-46,220 mgll concentrations for the acid treatment of sludge and cake, respectively. Cr .2.742 mgll, Sedimentation Time 1 h.
320r---------~--=-~--------__:~--_.,
E 220 200
180
------~22~11-_-2~~---~
6VV
()
lllO 140 120
100 80
llO 40 20
0'--''--'--''---1.--'"---'--........- - ' - -.......-----'--....--' 1.4
B
u
Cr Cone. (mglll
1.1
1.7 ~
1.1
SV 1 h. (mIl
U
2
2.1
MgO (X)
2.2
U
-frCOST (bMllm3WW.1
2.4
e
2.lI
2.1
2.7
COST (bahllkG.CI)
Figure 3. Resutts of precipitation of chromium by bealed MgO for "Don-additive" wastewater.
78
T. PANSWAD et al.
700
Cr • 2.742 mgn. Sedimentation Tlme1D h. ,..----------=----"--=-------------:--.,
IlOO IlOO
400
400
300 200
100
'".04
143.12
131.11
'8U7
~:::::::::::,_:::::::::::_~~-:-~i~.-.:::e;.=---6:---~eo.lIt
o L.Q~~::::;:=::::==~~~~~~~~::=:~L:=~4~.tU
. 1.4
a
1.5
Cr Cone. (mgll)
1.11
1.7
~
1.1
1.8
2
2.'
2.2
Na2C03 (X)
2.3
2.4
2.11
-6: COST (bahlfm3WW.)
SV 19 h. (ml)
2.11
e
2.7
2.'
2.t
COST (bahllkg.Cr)
Figure 4. Results of precipitation'of chromium by NaZC03 for "non-additive" wastewater.
In chrome tanning process, the chrome, which contains about 26% Cr20J. is usually utilised at the rate of 510% with regard to pelt weight This amounts to 8.9-17.8 gil chrome liquor and the optimum pH for tanning is 3.8. The recovered chromium III sulphate solution, which was dissolved from the cakes obtained from the
MgO and Na2C03 treatment of both types of wastewaters, contained chromium concentrations of 23.6-46.2 gil and 23.6-31.8 gil, respectively. These solutions could be directly reused as a tanning liquor by dilution to meet the required concentration and pH. The chromium solution recovered from the sludge, obtained from the MgO treatment and contained chromium concentrations of 9.2-13.4 gil, could also be directly reused. But the recovered Cr solution from the sludge obtained from Na:zC0J was too dilute and needed further addition of fresh chrome before reusing as an effective tanning agent Table 2. Optimum chemical doses for precipitation of chromium Precipitation Supernatant Sludge Chemical Raw waste Predp _ _ _ Reagent cost Precip. Sedimen Final Sludge %Cr Additive dose time pH volume Chrome Precip. pH Cr gig Cr (hr) (ppm Cr) (ppm) (mill) (ppm Cr) (bahtlkgCr)
NO NO NO
YES YES
2.7 2.7 2.8 3.5 3.5
2.3 2740 MgO 2740 heated Mg02.3 2740 Na2C03 6.1 1120 MgO 4.6 1120 Na2C03 9.1
Note: US $ 1... Baht 25.S (approx).
1 1 19 1 17
7.4 7.3 6.3 8.6 7.8
48 63 46 34 44
220 225 540 250 300
6630 5817 3800 1710 '1630
98.2 97.7 98.3 96.7 96.0
40.7' 40.7 43.5 ·81.4 65.0
79
Chromium recovery from tanning wastewater
700 100
500 400 300
200
45.0
110.0
100 0
B
14.0
'1:----(}
10.0 71.20
2
Cr Cone. (mg/l)
u ~
3
4
3.11
MgO (X)
-ts COST (bIIhl/m3WW.) B
SV1 h.(ml)
U
COST (bIIhtlkg.CI)
Figure S. Results of precipitation of chromium by MgO for "additive" wastewater. BOO 700
700
100
500 400
300 200
104.0
100
0
B
711.0 .
102.041
7 .0
.0
2
S
U
4
, .- MgO (X) Cr Cone. (mg/l)
~
ft
SV1 h.(ml)
COST (bIIhl/m3WW.)
B
U
COST (bIIhtlkg.Cr)
Figure 6. Results of precipitation of chromium by heated MgO for "additive" wastewater. 700 r - - - -_ _.cr...!-1J.Ul1nIlilW~ImII1J1I.o1LIlmU1~
...,
BOO 500
400 300
200
14.'7
100
101.12
117.00
e
U
4
133,37
~ '91.u=1'16,
B.2::!..S
e!,:.!..,__-..!1·eJ
4.11
II
o LS:Ji:C===::Ea::!!!===--EjJ.§. 3
121.2S
.1
11.11
Na2C03 (X) Cr Cone. (mg/l)
~
SV17h.(ml)
-ts COST (bahtlm3WW.)
~
COST (bahtlkg.Cr)
Figure 7. Results of precipitation of chromium by Na2;C03 for "additive" wastewater.
T. PANSWAD et al.
80
Table 3. Optimum dose of sulfuric acid for dissolution of chromium
Additive Precip. reagent
Sludge pH or Cake
Cr H2 SO 4 (mg/l) (gig Cr) pH (mglg lll )
NO
sludge cake sludge cake sludge cake sludge cake
15210 54· 5041 37· ' 11400 29· 4090 35-
MgO
NO
Na2C03
YES
MgO
YES
Na2C03
9 9.1 8 8.1 8.9 8.7 8.6 8.5
%
Solution
Sludge
Note: US $ 1 = Baht 25.5 (approx).
4.2 4.0 4.5 2.5 10.2 8.5 5.7 4.5
2.9 3.5 2.6 2.5 2.8 2.3 1.7 1.1
ppmCr
Recovery
13380 46220 4160 31820 9200 23590 4000 23590
92.9 .98
84.8 89.8 92.6 90.4 96.2 85.5
Chemical cost (Baht/kgCr) 21 20 25 14 53 37 25 17
- = test for sludge cake
COST ANALYSIS Cost analysis in this study was preliminary and only covered the costs of chemicals, i.e. precipitation reagents and sulfuric acid. The cost of the precipitation process was apparently more expensive than that of the dissolution process, so the overall treatment cost depend more on types and doses of the precipitation reagents. The treatment cost for various conditions of chromium recovery, based on the commercial basic chrome sulfate containing 26% Cr203 at 25 bahtlkg or equivalent to 140 bahtlkg Cr, is shown in Table 4. It is evident that the chrome recovery cost for the "no-additive" circumstance was lower than for its counterpart. In. the first case, the cost of recovered chromium is 62-78 bahtlkg whereas the cost of fresh chromium salt was about 140 bahtlkg. That is, recovered chromium was only 50% of the cost of fresh chromium salt. On the other hand, in the case of "with-additive" tanning, the recovery cost was normally higher (97-143 bahtlkg Cr), making the process less promising. . Table 4. Treatment costs of various conditions Additive Precip. Content reagent NO
YES
Precip. Sludge H2S04 conc. or (gig Cr) Cake (gig Cr)
MgO
2.3
Na2C03
6.1
MgO
4.6
Na2C03
9.1
Sludge Cake Sludge Cake Sludge Cake Sludge Cake
Note: US $ 1 = Baht 25.5 (approx).
4.2 4.0 4.5 2.5 10.2 8.5 5.7 4.5
Treatment cost % Recovery _ _--,.--_ (Bahtlm3) (Baht/kg Cr) 92.9 98 84.8 89.8 92.6 90.4 96.2 85.5
190 186 203 173 204 191 144 135
67 62 78 63 143 137 97 103
Chromium recovery from tanning wastewater
81
CONCLUSIONS It was concluded from this study that the best approach to chromium recovery was through the precipitation process, using MgO as the precipitant for the "non-additive" tanning wastewater and subsequent redissolution of the chrome sludge by cone. H 2S04, The corresponding overall cost was 67 bahtlkg Cr or about 190 bahtlm 3 of tanning wastewater. The chromium hydroxide sludge, optimally precipitated at an MgO dose of 2.3 g per g of chromium in raw waste, possessed the high settling velocity of 234 mm/h and could produce a compact sludge of high density in only 1 hour . The supernatant could then be easily separated from the sludge through simple decantaion. After the acid treatment of the obtained sludge, the chromium sulfate solution could be recovered and directly reused in the tanning process with a recovery efficiency as high a.~ 93%. The other alternative using Na2C03 as the precipitant was less attractive due to its slow-settling sludge (17 hours required) and very high sludge volume (540 mVl), resulting in the utmost necessity of a dewatering machine which, in tum, cost more and created more operation and maintenance problems.
ACKNOWLEDGEMENT The writers are indebted to Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH for financial support of this work. We are grateful to Miss B. Lewsrivilai and Mr. B. Meyhoefer for valuable suggestions.
REFERENCES Banjongpru, P. (1992). Potenlial of PollUJion from TaMing Industry in Thaiklnd. Master's thesis, Cbulalongkom University, Bangkok, Thailand. Boal, D.A. (1988). Large Scale Chrome Recovery from Chrome Wash Liquors. J. Am. Leather Chern. Assoc., 83(1). 17·23. Bogaerts, K&H. (1989). Design of Proposed Wastewater Treatment Pklnt for the Ceylon Leather Products Corporation. Sri· Lanka; Environmental Engineers Consultant Dreiss, SJ. (1986), Chromium Migration through Sludge.Treated Soils. Ground Wat.. 14. Langerwerf. J.S.A.(l978). Recovery and Reuse ofTrivalent Chromium. Proc. Congr. Leather Ind.. l, 251· 261. Lo, K.S.L. and Chen, Y.H. (1990). Extracting Heavy Metals from Municipal and Industrial Sludge. The Science of The Total Environment. 90, 99·116. Macchi. G., Pagano. M., Pettine, M., SanlOri, M. and Trivanti, G. (199\). A Bencb Swdy on Chromium Recovery from Tannery Sludge. Wat. Res., 25(8). 1019-1026. Meyhoefer. B. (1992). Treaunent of Wastewater in the Leather Tanning Industry. Seminar on The Profllabilily of Clean Technology in the Leather Tanning Industries. 20-21 OclOber 1992. Thailand. Panswad. T. and Chavalparit, O. (1992). Report on Tannery Wastewater Survey in Thailand. Chulalongkom University, Bangkok. Thailand. Porsl, J. (1991). Expert Report on Waste in the TaMing Industry. Bangkok • Deutsche Gesellschaft fur Technische Zusammenarbeit (Gm GmbH. Rajamani, S.• Gupta, S.N.. Mitra, F.B., Scbapman, J.E.and Hatman, G. (1992). Chrome Recovery and Reuse in India. Waler Environment and Technology, January, 6O-{j3. Riclkvd, F.C. and Bourg. A.CM. (\991). Aqeous Geochemistry of Chromium. Wat Res., 15. 807· 816. Wood, B. (1992). Oean Technology Options in \he Lea\her Tanning Industry, Seminar on The Profitability of Clean Technology in the Leather Tanning Industries, 20-21 OclOber 1992, Thailand.
Wal. ScL Ttch. Vol. 31, No.9, pp. 73-81,199'. Copyrigbt C 1995 IAWQ Printed in Oreat Britain. All rigbll reserved.
0273-1223(95)00408-4
0:m-I223195 $9'50 + O{)()
A BENCH-SCALE STUDY ON CHROMIUM
RECOVERY FROM TANNING WASTEWATER
Thongchai Panswad, Orathai Chavalparit, Yaowanud Sucharittham and Sasithom Charoenwisedsin Department 0/ Environmental Engineering, Chulalongkom University,
Bangkok 10330, Thailand
ABSTRACT In order to recover chromium, wastewaters from the chrome-tanning process in two Thai tanneries were coagulated with magnesium oxide or sodium carbonate at various dosages. Different types of polymer were also tried to improve the settleability of the chromium sludge which, in tum. was redissolved by sulfuric acid. The produced chromium III sulfate (Cr2(SO~Y. solution was sufficiently concenttated for direct reuse in the tanning step. The settling pattern of the chromium sludge was also observed and optimum sedimentation periods ranging from 1 to 17 bours were suggested. Under no tanning-additive (a cbemical to improve cbromium sorption onto bides) circumstances, the discharged wastewater cootained 5-11 g CrfJfI and the most promising coagulant was MgO at a dose of twice the stoichiometric value and an optimum pH of 7-8. The resulting chromium precipitate or sludge was quite dense (21 % solids) and possessed a good settling velocity (234 mm/b), making the decantation process very easy, whereas the corresponding acid requirement for the dissolution process was 1.5 times the stoichiometric value. On the other band, in the tanning-additive situation, the chromium concenttation in the drain was reduced to 3-6 g CrfJ-JI. Magnesium oxide was shown to be also the best coagulant but the dose was two times higher than that of the first circumstance, ie., four times that of stoichiometric requirement.
KEYWORDS Chromium recovery; dissolution;' precipitation; tannery wastewater. INTRODUCTION Chromium (Ill) sulphate has been widely used as a tanning agent in the leather industry (Macchi et al., 1991). InteractIon between chromium (Cr3+) and collagen leads leather to stabilise and to possess a shrinkage-temperature property of over 100°C. In chrome tanning, only 70-80% of chromium applied is taken up by the leather, and the rest is discharged as waste, inducing an important source of chromium contamination. The biological sludge from the tannery waste treatment was reported to have 11 high content of chromium and can cause accumulation of the metal in the soil (Dreiss, 1988; Lo and Chen, 1990). The chromium when accumulated in plants and animals at high levels can generate serious diseases (Richard and Bourg 1991). Taking into account the large volume of wastewater containing very high chromium concentrations and the difficulties in finding proper disposal sites for the sludge produced, many countries have turned to "clean" technologies, including chromium recovery and reuse. Chromium recovery means 73
74
T. PANSWAD et al.
separating chromium from the spent liquour by precipitation, then redissolving the chromium sludge for reuse. The primary investigations showed that chromium (III) could be precipitated by all common basic compounds, but the settling characteristics of the precipitate formed by the reaction of magnesium oxide were superior due to its minimum sludge volume (Bogaerts, 1992; Wood, 1992; Boat,1988; Langerwerf, 197H). Leather production in Thailand has shown an extremely high growth rate, especially during the last two years (Meyhoefer, 1992). In 1990, 140,000 metric tons of hides (mostly bovine) were tanned in approximately 150 tanneries in the country (Banjongpru, 1992). More than 80% of the tanneries used a chrome - tanning process (Porst, 1991). Annually, 1050 metric tons of Cr203 were used and, out of this amount, S6 metric tons of Cr203 were discharged as wastewaters into streams. It was calculated that the tanning industry discharged the wastewater with a total annual volume of 2.73 million m3 and the total annual loss of chromium was equal to 29.7 million baht or approx US $ 1.1 million (Banjongpru, 1992; Panswad and Chavalparit, 1992). EXPERIMENTAL This study was carried out with wastewaters discharged from two chrome tanning processes, namely, with and without additives, commercially known as Feliderm CS. Samples from both sources were collected and screened prior to the physical and chemical analysis which were done before and after experimental tests for chromium hydroxide precipitation using different alkalis, i.e., MgO, heated MgO (40 to 50°C) and Na2C03 at various dosages, (see Fig. I). Three different types of polymer were also used in an attempt for improvement in flocculation and· sedimentation. A precalculated quantity of precipitation reagents (see equations below) were added to the liquor and stirred for two hours, after which the pH measurement, sedimentation pattern, sedimentation time, sludge volume and chromium content in the supernatant were taken or studied. After the settling process, the decanted supernatant was taken for laboratory analysis to determine the Cr precipitation (or removal) efficiency. The sludge was redissolved by using cone H2S04, A portion of the sludge was subsequently dewatered by vacuum filtration using a conventional suction flask and the dewatered cake was redissolved with 1+4 H2S04 solution. Continuous and at-initial-stage mixing as well as raising of the temperature were also investigated in order to determine the effect of the degree of mixing and temperature increase on the recovery process. The recovery efficiency as well as preliminary cost evaluation were subsequently investigated. . Chemical reactions :Cr2(S04h + 3MgO +3H20 ~ 3MgS04 + 2Cr(OH}J the by-weight stoichiomatic value for Cr : MgO is 1 : 1.16.
(I)
Cr2(S04}J+3Na2C03 + 3H20 • 2Cr(OH}J+3Na2S04+3C02 the by-weight stoichiomatic value for Cr : Na2C03 is I : 3.06.
(2)
2Cr(OH)3 + 3H2S04 • Cr2(S04» + 6H20 the by weight stoichiomatic value for Cr : H2S04 is 1 : 2.83.
(3)
RESULTS AND DISCUSSION Characteristics of waste chromium liQuor The average pH of the two wastewaters, i.e., with and without additive, was 3.56 and 2.93, respectively, with corresponding chromium concentrations of 1,540 and 3.070 mglI, (see Table I). The redox value of the noadditive wastewater was higher than that of its counterpart, probably due to its higher Cr concentration.
7S
Chromium recovery from tanning wastewater
Tanning without additive
Tanning with additive
l Sampling
l Screening
l Lab Analysis
l
Precipitation Tests with MgO, Heated MgO and Na2C03 and 3 types of polymer
l
Sedimentation Study
! Decantation t
~
l
Supernatant
Sludge
.'
Lab Analysis
l
Vacuum Filtration
% Precipitation Efficiency StudX % Cr Recovery Study
l
~
Cake
Preliminary Cost Analysis FigUre 1. Experimental now chart.
In addition, the corresponding tanning process required less NaCI as weUas ammonia and consequently, discharged less of those salts. It is widely known that the final pH of the basic chromium liquor as well as the presence of other reagents from the tanning process could affect the subsequent precipitation process.
T. PANSWAD tl at.
76
Table 1. Chemical characteristics of waste chrome liquor Chrome tanning wastewater Parameter
pH Conductivity, mS/em Redox, mv Total Solids, mgll Dissolved Solids, mgll Suspended Solids, mg/\ Chloride, mgll Ammonia Nitrogen, mgll COD, mgll Chromium (soluble), mgll Chromium (total), mg/\
without additive
With additive
(n=9)
(n=8)
2.93 63.6 342 86,850 85,570 .1,280 24,900 450 4,030 3,070 4,130
3.56 91.5 247 131,670 129,170 2,510 45,280 810
5,500 1,540 2,170
Optimum conditions for precipitatjon Qfchromjum From the laboratory bench-scale experiments with non-additive wastewater, it was found that the precipitation step using MgO at the dose of twice the stoichiometric value of the chromium in the waste liquor (or MgO 2.3 gig Cr) and a settling time of I hour was optimal. The chromium content of 2,740 mgIJ in raw waste was reduced to 48 mgll in the supernatant, resulting in 98% removal efficiency, (see Fig. 2 and Table 2). With the optimum reaction pH of 7.3, the produced sludge was very dense and possessed a high settling velocity (approx. 234 mm/h). Polymers were found not to improve the flocculation and sedimentation perfonnance. Moreover, results obtained from the tests of precipitation with heated MgO were found to be even less satisfactory, (see Fig. 3).
=
Using the NaZCO] at dosage of twice the theoretical value (6.1 g of NaZCO] were needed for I g of Cr) it was apparent that a similar outcome, 98% removal efficiency, might result, (see Table 2). The chromium hydroxide precipitate in this case however settled very slowly. After being allowed to settle overnight, the sludge volume was found to be 540 mllI and the supernatant contained 46 mg Crll with a relatively low pH of 6.3. The NazC03 treatment was therefore less attractive due to the further necessity of a dewatering unit. Besides, the settling process took such a long time (at least 17 hours), that the precipitate then became aged, resulting in a growing insolubility of the substance due to the transfonnation of hydroxide to mixed-oxides complexes (Rajamani etat., 1992; Wood, 1992). Regarding the tests with wastewaters from with-additive tanning, it was apparent that the MgO dose had to be two times higher than that of the non-additive case (Le. 4.6 g MgO/g Cr), (see Table 2). After treatment, 97% of the chromium could be precipitated, resulting in 34 mg Crll in the supernatant from the initial concentration of 1,120 mgIJ. The settled volume of sludge at 60 min was about 250 mllI, (see Fig.S). An anionic polymer in this case proved able to enhance the settling property; the optimum settling time could be reduced from 1 hour to 10 min. With NazC03 treatment for the same wastewater, a similar conclusion could be drawn, i,e.• 96% removal efficiency and a high volume of sludge produced (300 mUll. The results of precipitation of chromium by various dosages of chemicals are shown in Table 2 and Figs. 5-7.
77
Chromium recovery from tanning waslewater
Cr • 2,742 mg/l, Sedimentation TIme 1 h.
: . r-;::-=-----'----=-....;...--------=-=~280:;::::=2::(80)-, m
~
m
m
200 180 180
140
120
100 80
133.17 13l1.411 14lI.O:S
I~!_----=~~:::::_~!J:!.--~~~-~~.--,~~
13.17
80
40 20
~ llO."
I 20
SO.1I1
112."
ttl.' EJ 22.4
O'---'-----''---''---'_-''_--I._-'"_--'-_-'-_........_ - ' - _......._ - l 1.4
S
1.11
1.'
Cr Cone. (mgll)
Figure 2. Results of precipitation of chromium by MgO for "non-additive" wastewater.
Optimum conditions for dissolution Dissolution of chromium by sulfuric acid depended on mixing time and types of precipitate, namely sludge or dewatered cake. The required dissolution time for chromium cake under no-mixing (i.e., mixing at only the initial stage) condition was 90 min whereas under other conditions the corresponding time was only 60 min. The recovered Cr2S04 solution under the scenario of non-additive waste chrome liquor was more concentrated than that from with-additive wastewater under the same conditions because it needed less H2S04 for the dissolution. For the same reason, the chromium concentration in the solution obtained from the dissolution of the dewatered cake was higher than in that from the chrome sludge. The lower than 3.5 pH was found to be very effective for dissolution, the recovery efficiency was as high as 85-98% and the raised temperature did not have any significant effect on the dissolution. The optimum doses of sulfuric acid for the dissolution of chromium oxide are summarised in Table 3. The acid requirement varied from 2.5-10.2 gig Cr depending on types of wastewater, precipitation reagents and methods of dewatering. The recovered Cr~04 solution had 13,380 and 31,820-46,220 mgll concentrations for the acid treatment of sludge and cake, respectively. Cr .2.742 mgll, Sedimentation Time 1 h.
320r---------~--=-~--------__:~--_.,
E 220 200
180
------~22~11-_-2~~---~
6VV
()
lllO 140 120
100 80
llO 40 20
0'--''--'--''---1.--'"---'--........- - ' - -.......-----'--....--' 1.4
B
u
Cr Cone. (mglll
1.1
1.7 ~
1.1
SV 1 h. (mIl
U
2
2.1
MgO (X)
2.2
U
-frCOST (bMllm3WW.1
2.4
e
2.lI
2.1
2.7
COST (bahllkG.CI)
Figure 3. Resutts of precipitation of chromium by bealed MgO for "Don-additive" wastewater.
78
T. PANSWAD et al.
700
Cr • 2.742 mgn. Sedimentation Tlme1D h. ,..----------=----"--=-------------:--.,
IlOO IlOO
400
400
300 200
100
'".04
143.12
131.11
'8U7
~:::::::::::,_:::::::::::_~~-:-~i~.-.:::e;.=---6:---~eo.lIt
o L.Q~~::::;:=::::==~~~~~~~~::=:~L:=~4~.tU
. 1.4
a
1.5
Cr Cone. (mgll)
1.11
1.7
~
1.1
1.8
2
2.'
2.2
Na2C03 (X)
2.3
2.4
2.11
-6: COST (bahlfm3WW.)
SV 19 h. (ml)
2.11
e
2.7
2.'
2.t
COST (bahllkg.Cr)
Figure 4. Results of precipitation'of chromium by NaZC03 for "non-additive" wastewater.
In chrome tanning process, the chrome, which contains about 26% Cr20J. is usually utilised at the rate of 510% with regard to pelt weight This amounts to 8.9-17.8 gil chrome liquor and the optimum pH for tanning is 3.8. The recovered chromium III sulphate solution, which was dissolved from the cakes obtained from the
MgO and Na2C03 treatment of both types of wastewaters, contained chromium concentrations of 23.6-46.2 gil and 23.6-31.8 gil, respectively. These solutions could be directly reused as a tanning liquor by dilution to meet the required concentration and pH. The chromium solution recovered from the sludge, obtained from the MgO treatment and contained chromium concentrations of 9.2-13.4 gil, could also be directly reused. But the recovered Cr solution from the sludge obtained from Na:zC0J was too dilute and needed further addition of fresh chrome before reusing as an effective tanning agent Table 2. Optimum chemical doses for precipitation of chromium Precipitation Supernatant Sludge Chemical Raw waste Predp _ _ _ Reagent cost Precip. Sedimen Final Sludge %Cr Additive dose time pH volume Chrome Precip. pH Cr gig Cr (hr) (ppm Cr) (ppm) (mill) (ppm Cr) (bahtlkgCr)
NO NO NO
YES YES
2.7 2.7 2.8 3.5 3.5
2.3 2740 MgO 2740 heated Mg02.3 2740 Na2C03 6.1 1120 MgO 4.6 1120 Na2C03 9.1
Note: US $ 1... Baht 25.S (approx).
1 1 19 1 17
7.4 7.3 6.3 8.6 7.8
48 63 46 34 44
220 225 540 250 300
6630 5817 3800 1710 '1630
98.2 97.7 98.3 96.7 96.0
40.7' 40.7 43.5 ·81.4 65.0
79
Chromium recovery from tanning wastewater
700 100
500 400 300
200
45.0
110.0
100 0
B
14.0
'1:----(}
10.0 71.20
2
Cr Cone. (mg/l)
u ~
3
4
3.11
MgO (X)
-ts COST (bIIhl/m3WW.) B
SV1 h.(ml)
U
COST (bIIhtlkg.CI)
Figure S. Results of precipitation of chromium by MgO for "additive" wastewater. BOO 700
700
100
500 400
300 200
104.0
100
0
B
711.0 .
102.041
7 .0
.0
2
S
U
4
, .- MgO (X) Cr Cone. (mg/l)
~
ft
SV1 h.(ml)
COST (bIIhl/m3WW.)
B
U
COST (bIIhtlkg.Cr)
Figure 6. Results of precipitation of chromium by heated MgO for "additive" wastewater. 700 r - - - -_ _.cr...!-1J.Ul1nIlilW~ImII1J1I.o1LIlmU1~
...,
BOO 500
400 300
200
14.'7
100
101.12
117.00
e
U
4
133,37
~ '91.u=1'16,
B.2::!..S
e!,:.!..,__-..!1·eJ
4.11
II
o LS:Ji:C===::Ea::!!!===--EjJ.§. 3
121.2S
.1
11.11
Na2C03 (X) Cr Cone. (mg/l)
~
SV17h.(ml)
-ts COST (bahtlm3WW.)
~
COST (bahtlkg.Cr)
Figure 7. Results of precipitation of chromium by Na2;C03 for "additive" wastewater.
T. PANSWAD et al.
80
Table 3. Optimum dose of sulfuric acid for dissolution of chromium
Additive Precip. reagent
Sludge pH or Cake
Cr H2 SO 4 (mg/l) (gig Cr) pH (mglg lll )
NO
sludge cake sludge cake sludge cake sludge cake
15210 54· 5041 37· ' 11400 29· 4090 35-
MgO
NO
Na2C03
YES
MgO
YES
Na2C03
9 9.1 8 8.1 8.9 8.7 8.6 8.5
%
Solution
Sludge
Note: US $ 1 = Baht 25.5 (approx).
4.2 4.0 4.5 2.5 10.2 8.5 5.7 4.5
2.9 3.5 2.6 2.5 2.8 2.3 1.7 1.1
ppmCr
Recovery
13380 46220 4160 31820 9200 23590 4000 23590
92.9 .98
84.8 89.8 92.6 90.4 96.2 85.5
Chemical cost (Baht/kgCr) 21 20 25 14 53 37 25 17
- = test for sludge cake
COST ANALYSIS Cost analysis in this study was preliminary and only covered the costs of chemicals, i.e. precipitation reagents and sulfuric acid. The cost of the precipitation process was apparently more expensive than that of the dissolution process, so the overall treatment cost depend more on types and doses of the precipitation reagents. The treatment cost for various conditions of chromium recovery, based on the commercial basic chrome sulfate containing 26% Cr203 at 25 bahtlkg or equivalent to 140 bahtlkg Cr, is shown in Table 4. It is evident that the chrome recovery cost for the "no-additive" circumstance was lower than for its counterpart. In. the first case, the cost of recovered chromium is 62-78 bahtlkg whereas the cost of fresh chromium salt was about 140 bahtlkg. That is, recovered chromium was only 50% of the cost of fresh chromium salt. On the other hand, in the case of "with-additive" tanning, the recovery cost was normally higher (97-143 bahtlkg Cr), making the process less promising. . Table 4. Treatment costs of various conditions Additive Precip. Content reagent NO
YES
Precip. Sludge H2S04 conc. or (gig Cr) Cake (gig Cr)
MgO
2.3
Na2C03
6.1
MgO
4.6
Na2C03
9.1
Sludge Cake Sludge Cake Sludge Cake Sludge Cake
Note: US $ 1 = Baht 25.5 (approx).
4.2 4.0 4.5 2.5 10.2 8.5 5.7 4.5
Treatment cost % Recovery _ _--,.--_ (Bahtlm3) (Baht/kg Cr) 92.9 98 84.8 89.8 92.6 90.4 96.2 85.5
190 186 203 173 204 191 144 135
67 62 78 63 143 137 97 103
Chromium recovery from tanning wastewater
81
CONCLUSIONS It was concluded from this study that the best approach to chromium recovery was through the precipitation process, using MgO as the precipitant for the "non-additive" tanning wastewater and subsequent redissolution of the chrome sludge by cone. H 2S04, The corresponding overall cost was 67 bahtlkg Cr or about 190 bahtlm 3 of tanning wastewater. The chromium hydroxide sludge, optimally precipitated at an MgO dose of 2.3 g per g of chromium in raw waste, possessed the high settling velocity of 234 mm/h and could produce a compact sludge of high density in only 1 hour . The supernatant could then be easily separated from the sludge through simple decantaion. After the acid treatment of the obtained sludge, the chromium sulfate solution could be recovered and directly reused in the tanning process with a recovery efficiency as high a.~ 93%. The other alternative using Na2C03 as the precipitant was less attractive due to its slow-settling sludge (17 hours required) and very high sludge volume (540 mVl), resulting in the utmost necessity of a dewatering machine which, in tum, cost more and created more operation and maintenance problems.
ACKNOWLEDGEMENT The writers are indebted to Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH for financial support of this work. We are grateful to Miss B. Lewsrivilai and Mr. B. Meyhoefer for valuable suggestions.
REFERENCES Banjongpru, P. (1992). Potenlial of PollUJion from TaMing Industry in Thaiklnd. Master's thesis, Cbulalongkom University, Bangkok, Thailand. Boal, D.A. (1988). Large Scale Chrome Recovery from Chrome Wash Liquors. J. Am. Leather Chern. Assoc., 83(1). 17·23. Bogaerts, K&H. (1989). Design of Proposed Wastewater Treatment Pklnt for the Ceylon Leather Products Corporation. Sri· Lanka; Environmental Engineers Consultant Dreiss, SJ. (1986), Chromium Migration through Sludge.Treated Soils. Ground Wat.. 14. Langerwerf. J.S.A.(l978). Recovery and Reuse ofTrivalent Chromium. Proc. Congr. Leather Ind.. l, 251· 261. Lo, K.S.L. and Chen, Y.H. (1990). Extracting Heavy Metals from Municipal and Industrial Sludge. The Science of The Total Environment. 90, 99·116. Macchi. G., Pagano. M., Pettine, M., SanlOri, M. and Trivanti, G. (199\). A Bencb Swdy on Chromium Recovery from Tannery Sludge. Wat. Res., 25(8). 1019-1026. Meyhoefer. B. (1992). Treaunent of Wastewater in the Leather Tanning Industry. Seminar on The Profllabilily of Clean Technology in the Leather Tanning Industries. 20-21 OclOber 1992. Thailand. Panswad. T. and Chavalparit, O. (1992). Report on Tannery Wastewater Survey in Thailand. Chulalongkom University, Bangkok. Thailand. Porsl, J. (1991). Expert Report on Waste in the TaMing Industry. Bangkok • Deutsche Gesellschaft fur Technische Zusammenarbeit (Gm GmbH. Rajamani, S.• Gupta, S.N.. Mitra, F.B., Scbapman, J.E.and Hatman, G. (1992). Chrome Recovery and Reuse in India. Waler Environment and Technology, January, 6O-{j3. Riclkvd, F.C. and Bourg. A.CM. (\991). Aqeous Geochemistry of Chromium. Wat Res., 15. 807· 816. Wood, B. (1992). Oean Technology Options in \he Lea\her Tanning Industry, Seminar on The Profitability of Clean Technology in the Leather Tanning Industries, 20-21 OclOber 1992, Thailand.