Journal of Scientific & Industrial et Research SIVAKUMAR al: ULTRASOUND ASSISTED Cr–RECOVERY PROCESS IN LEATHER INDUSTRY Vol. 66, July 2007, pp.545-549
545
Use of ultrasound in chrome recovery process in leather industry V Sivakumar1*, V Ravi Verma1, G Swaminathan1 and P G Rao2 1
Chemical Engineering Division, Central Leather Research Institute, Adyar, Chennai 600 020 2 Regional Research Laboratory, Jorhat 785 006 Received 10 July 2006; revised 13 February 2007; accepted 15 February 2007
In this study, the influence of ultrasound on the stoichiometric use of precipitating agent magnesium oxide, (MgO) as an alkali in the recovery of chrome from the spent tanning liquors has been studied. The use of MgO in chrome recovery assists settling characteristics but suffers from low solubility and reactivity. A significant improvement (3-fold) is observed in settling rate of chromic hydroxide precipitate with the use of pre-sonicated MgO as compared to control (no sonication). A considerable decrease of Cr in the supernatant liquor after precipitation suggests better recovery due to better dispersion and particle-size reduction of MgO in water due to sonication. No appreciable improvement with sonicated sodium carbonate solution prior to chrome recovery may be due to sodium carbonate itself is highly water soluble in comparison to MgO. Keywords: Chrome recovery, Leather, Sonochemistry, Tanning, Ultrasound IPC Code: C14C3/00
Introduction Chrome tanning, carried out using commercially available basic chromium sulfate (BCS) salts, exhibits only 55-70% uptake of the chromium used for tanning using conventional tanning procedures. In India, tanning salts, equivalent to approx. 4000 tons of chromium, are being wasted with an annual consumption of about 40,000 tons of BCS salt1. The international specifications for the permissible level of chromium in the industrial wastewaters stipulating 0.3-2.0 ppm in the spent chrome tanning solutions has become one of the most serious streams with regard to pollution due to leather processing. Therefore, there is a pressing need to improve the efficiency of the end-of-pipe treatments by way of augmentation techniques such as the use of power ultrasound. Ultrasound (frequency, 20-100 kHz) is termed as power ultrasound2 and commonly employed for enhancing physical processes such as cleaning, emulsification, crystallisation, extraction etc., and for accelerating/performing chemical reactions3. Earlier studies4-10 have focussed on the application of power ultrasound in various unit operations in leather *Author for correspondence E-mail:
[email protected]
processing for effective diffusion rate of chemicals through skin/leather matrix, improving the process efficiency, quality of leather and for the environmental benefits. Application of Ultrasound in Chrome Recovery Process
Ultrasound aids effective precipitation and agglomeration in separation science and technology11,12. Use of MgO in chrome recovery13 provides better settling characteristics compared to other alkalies [NaOH, Na2CO3, Ca(OH)2]. Influence of ultrasound is proposed to overcome the disadvantages with MgO. This study presents application of pre-sonicated MgO to BCS solution to accelerate the settling rate of chromic hydroxide precipitate and hence chrome recovery. Materials and Methods Ultrasonic cleaner (4 l), generating maximum available output power of 150 W at 33 kHz frequency, as quoted by Roop Telsonic-ultrasonics, India, was used in presence of ultrasound5,7. Preparation of Synthetic BCS Spent Liquor
Spent chrome tanning liquor from the tanneries generally has chrome concentration of up to 0.4% (w/v basis). Therefore, BCS, Cr(OH)SO4 spent liquor was
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prepared synthetically by dissolving suitable quantity of BCS (in such a way to get 0.4% Cr with the basis of 1.2692 g of BCS contains 0.4 g of Cr). Commercial grade BCS was purchased from Ayes Dyes & Chemicals Ltd., India. MgO powder (98% purity) and Na2CO3 AR has been purchased from S.D. Fine-Chem Ltd., India. Distilled water was used for experiments and doubly distilled water for analysis.
experiment. Use of higher amount of MgO (0.5 g) than stoichiometric quantity has also been studied as a control process. Three different samples (each containing 1.2692 g of BCS in 50 ml water in 100 ml cylinder) were prepared by adding MgO as follows: Control 1, 0.32 g; Ultrasound (MgO suspension sonicated for 15 min), 0.32 g; and Control 2, 0.5g. Use of Sodium Carbonate as Alkali
Stoichiometric Use of MgO
Two solutions (each containing 1.2692 g BCS in 50 ml water) and another two suspensions (each containing 0.32 g MgO in 50 ml water) were prepared. One of the MgO suspensions was sonicated for 15 min in ultrasonic cleaner. Control MgO suspension was prepared by simple stirring using glass rod. Sonicated MgO and control MgO samples were then simultaneously added to 100 ml measuring jars containing BCS solution labeled as ‘control’ and ‘ultrasound’ respectively. Settling rate of the chromic hydroxide precipitate in measuring jar was the total precipitate volume (ml) that has settled for every 5 min in both samples. Initial settled volume (100%) decreases with time of settling. Settling of chromic hydroxide precipitate is visible through the measuring jar as phase separation from the supernatant liquor.
Synthetic BCS solution was prepared to get 0.4% Cr (w/v basis). Then, according to stoichiometry, 1 g of BCS requiring 0.6424 g of Na2CO3 (AR reagent) for precipitation as Cr(OH)3. 50 ml of Na2CO3 solution was sonicated for 15 min and then added to the chrome solution for observing the settling behaviour as compared to that of control. Results and Discussion Analysis of Chrome Content in Supernatant Solution of Cr(OH)3 Precipitate
Samples were taken from supernatant solution of Cr(OH)3 precipitate for 15, 30, 60, 120 and 180 min from both the sonicated and control experiments. Samples were made up to a known volume and then oxidized by heating in the presence of H2O2 and NaOH as, r H202
Stoichiometry
Reaction involved in precipitation of chromium using MgO from spent chrome solutions is,
Cr(OH)SO4
Cr(OH)SO4 + MgO + H2O Cr(OH)3 + MgSO4
(Cr3+) Green
...(1)
Settling rate of chromic hydroxide from aqueous medium would depend on the Stoke’s law,
2r 2 ( d 1 − d 2 ) g V = 9η
...(2)
where V, sedimentation velocity; r, particle radius; (d1−d2), density difference; and η, viscosity of medium. According to stoichiometry of the reaction in Eq. (1), at 165 g of BCS requiring 40 g of MgO for Cr(OH)3 formation, 1 g of BCS will require 0.242 g of MgO. So, 1.2692 g of BCS containing 0.4 g of Cr will require 0.307 g of MgO to precipitate as Cr(OH)3. At 98% purity, 0.32 g of MgO has been used for the
2-
CrO4 (Na2CrO4) 2NaOH (Cr6+ as chromate) Yellow
The samples were suitably diluted, made up to know volumes and then analysed for determining the amount of chrome using UV 2101 PC Scanning UV2Visible Spectrophotometer. CrO4 has an Absorbance maximum (λmax) of 373 nm, at which absorbance values were determined in the spectrophotometer for all the samples. Beer-Lamberts law is used to calculate the concentration of a chemical from absorbance values as A=ξ×C×t where ‘A’ is absorbance value, ‘ ξ ’ is extinction coefficient of particular chemical substance, ‘C’ is concentration of chemical and ‘t’ is cell width used in spectrophotometer.
SIVAKUMAR et al: ULTRASOUND ASSISTED Cr–RECOVERY PROCESS IN LEATHER INDUSTRY
547
100
90
Ultrasound (0.32 g MgO)
Settled volume of Cr- hydroxide precipitate, %
Control (0.32 g MgO) 80
70
60
50
40
30
20
10
0 0
30
60
90 120 Time, min
150
180
210
Fig. 1—Influence of ultrasound on Cr(OH)3 precipitate settling rate using stoichiometric use of MgO, 0.32 g
For CrO 4 2-, with extinction coefficient at 4830 cm 2 /mole, and cell width (t) used in the spectrophotometer at 1 cm, absorbance value (A) has been determined for the samples as Chrome concentration (C) = [(A / 4830) × dilution factor] mol/l = [(A / 4830) × dilution factor × 52] g/l where 52 is the atomic weight of chromium. Sample calculation (for the sample taken from control for 15 min) C = (0.8471 / 4830) × 50 × 52 g/l = 0.456 g/l × 1000 = 456 ppm where 0.8471 is the absorbance value for the sample and 50 is the dilution factor. Stoichiometric Amount of MgO
Settling rate of chromic hydroxide precipitate of sonicated sample has been observed faster than that
of control sample for the entire period of 3 h (Fig. 1). There is 50% settling due to ultrasound as compared to only 70% settling for the control process for 25 min time; while, control process takes 3 h for 50% settling. Therefore, ultrasonication could help in reducing ~86% settling time or ~3-fold increase in the settling rate. Settling rate of chromic hydroxide precipitate of sonicated sample (stoichiometric amount) is even better than the settling rate of precipitate for control sample with MgO higher than the stoichiometry (Fig. 2). The possible reasons for the faster settling rate of chromic hydroxide precipitate when the sonicated MgO suspension is added to BCS solution may be due to: i) Better dispersion of MgO in water due to sonication (better dispersion of sonicated MgO suspension as compared to control has been visually observed); ii) Better dispersion helps in increasing reactivity of MgO with BCS solution, thereby, leading to a better precipitation of chromic hydroxide and a higher recovery of chrome; and iii) Reduction in the particle size of MgO sample leading to an increase in its surface area. However, these reasons are under investigation for confirmation.
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J SCI IND RES VOL 66 JULY 2007
100 With ultrasound (0.32 g MgO) Without US (0.32 g MgO) Without US (0.5 g MgO)
Settled volume of Cr- hydroxide precipitate, %
90
80
70
60
50
40 0
5
10
15 20 Time, min
25
30
35
Fig. 2—Influence of ultrasound on Cr(OH)3 precipitate settling rate in comparison with higher use of MgO, 0.5 g
Table 1 - Amount of chromium present in supernatant liquor of chrome precipitate Time min
Cr in supernatant liquor, ppm
Reduction in chrome due to sonication, %
Ultrasound
Control
15
149.30
456.00
67.26
30
23.30
28.63
18.62
60
20.68
24.34
15.04
120
15.36
15.96
3.76
180
3
8.29
63.80
Cr- Content in Supernatant Liquor
Analysis indicates that for 15 min sample, Cr concentration is as follows: control, 456; and ultrasound, 49.3 ppm (Table 1). This means 67.26% reduction in Cr concentration due to sonication of MgO suspension alone and adding it to the BCS solution. At the end of 3 h, Cr concentration in sonicated sample is 3 ppm, which is nearly equal to the BIS specification for the permissible limit of 2 ppm of chrome in industrial wastewaters14. There is a 63.8% reduction in the chrome
content in supernatant due to sonication of MgO suspension for 3 h. Sonication of Sodium Carbonate Solution
There was no perceptible improvement in settling rate of the precipitate when sonicated Na2CO3 solution was added to BCS solution as compared to the control. Solubility of Na2CO3 in water (7.1 g per 100 ml of water) is sufficiently high and forms a true solution in water; sonication is not expected to cause any further increase in its solubility. So there was no improvement in settling rate of the precipitate when sonicated Na2CO3 solution is added to BCS solution. Conclusions MgO has been preferred as a suitable precipitating agent in the chrome recovery process due to its compact and better settling characteristics. However, it has certain disadvantages like low solubility in water and reactivity. Sonication of MgO (stoichiometric amount) suspension prior to the addition to the spent chrome liquor has been found to be beneficial even compared with higher stoichiometric use in the control process, thereby, helping in judicial use of MgO
SIVAKUMAR et al: ULTRASOUND ASSISTED Cr–RECOVERY PROCESS IN LEATHER INDUSTRY
in the Cr recovery process. There is a significant improvement (3-fold) in the settling rate of chromic hydroxide precipitate formed due to the pre-sonication of MgO, may be attributed to better dispersion and greater surface area of MgO available for the reaction. There is also a significant reduction in the chrome content found in the supernatant solution due to sonication. Pre-sonication of sodium carbonate in the chrome recovery has no appreciable effect due to its better soluble nature in water. In conclusion, the use of ultrasound in chrome recovery process has significant positive influence and thereby increasing the efficiency of the chrome recovery process for cleaner environment. Acknowledgements Authors thank Dr T Ramasami, Secretary, DST, Govt of India for encouragement and Prof A B Mandal,
Acting Director, CLRI, Chennai for motivation. Author (VS) is grateful to CSIR, New Delhi, India for support. Authors also thank Dr N R Rajagopal, BITS, Pilani for motivation, Prof R Kumar and Prof K S Gandhi, IISc, Bangalore, for valuable suggestions, and Dr P D Tyagi, Editor, Journal of Scientific & Industrial Research, NISCAIR, New Delhi, for valuable suggestions in preparation of the manuscript. References 1
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Raghava Rao J, Venba R, Unni Nair B C, Suthanthararajan R, Rajamani S & Ramasami T, Chromium Reuse, Proc LERIG (CLRI, Chennai) 1995, 37-56. Mason T J, The Uses of Ultrasound in Chemistry (Royal Society of Chemistry, Cambridge) 1990, 3.
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