CHROMIUM IN TANNERY EFFLUENT AND ITS RECOVERY
Akash Deep, S.N. Tandon*, A.R. Khwaja
(Department of Chemistry, University of Roorkee, Roorkee, UP,247667, INDIA)
*Corresponding author, Email : email@example.com
The paper embodies studies on the recovery of pure chromium from spent chrome liquor(SCL) of tanneries employing solvent extraction - electrowinning (SX-EW). A toluene solution of bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) has been used to separate Cr(III) from Fe(III), Co(II), Ni(II) and Cu(II) which are present as impurities in SCL. All these impurities were extracted in the organic phase leaving pure Cr(III) in the aqueous layer. The extractant has been regenerated by washing it with 6M H2SO4 followed by water. Before subjecting SCL to solvent extraction MgO precipitation step has been introduced to concentrate Cr(III) to around 7g/l. The chromium deposit from the trivalent bath is around 99% pure. The cathode efficiency is around 44%. Effects of various parameters on the extraction and deposition have been investigated.
The leather industry has shown a phenomenal growth during the past few decades, particularly in developing countries like India. The tanning activity is vital for this industry. and is one of the largest polluters among the various activities. The chrome tanning is more popular than the vegetable tanning and thus the effluents emerging out of the factory are invariably loaded with trivalent chromium. Sludges formed as a result of chrome recovery process or basic tanning activities have been found to contain an elevated concentration of chromium1. The release of chromium is not only damaging the environment but also causing a substantial financial loss. It is estimated that in India alone almost ten million US dollars worth of chromium is wasted every year. A number of technologies to recover chromium from such effluents have been proposed but none seems to be attractive to put it to actual practice2-4. The present project is an effort in this direction whereby pure chromium is recovered from chrome tanning effluent. It is estimated that the cost involved in the recovery of chromium will be met by the higher cost of pure metal. Needless to say, the acceptability of the proposed procedure will also minimize the ongoing damage to the environment.
Solvent extraction is a well known technique for the separation of the dissolved metals and has been employed for the purification of Cr(III) of spent chrome liquor which contains Fe(III) as the major impurity with traces of Mn(II), Co(II), Ni(II) and Cu(II). Electrowinning of chromium from Cr(VI) baths has been practised since long. The tanning industry uses basic Cr(III) salt and thus the electrodeposition of chromium from such an effluent will need an additional oxidation step thereby adding to the cost of the process. Also the toxicity, low cathodic efficiency and high concentration requirements of Cr(VI) discourage the use of hexavalent chromium baths. In the recent years, there has been a growing interest in developing electrodeposition procedure employing Cr(III) solution5-10 but the development of an efficient and convenient electrodeposition procedure for commercial use still remains the need of the hour.
In the present paper experiments have been performed at the laboratory scale to recover Cr(III) from SCL using solvent extraction-electrowinning. Bis(2,4,4-trimethylpentyl)dithiophosphinic acid [Cyanex 301], a known commercial extractant11-18, has been employed for the decontamination of Cr(III) solution from Fe(III), Co(II), Ni(II) and Cu(II). The presence of Mn(II) does not affect the cathodic deposition of pure chromium from a Cr(III) bath19. Since the concentration of Cr(III) in SCL is much lower than that required for electrodeposition the metal is concentrated at the initial stage by MgO precipitation. After concentration and subsequent purification by solvent extraction the solution is subjected to electrodeposition. It is known that the alkali and alkaline earths metals do no codeposit with chromium in Cr(III) baths20. Semibright nodular deposits of around 99% purity have been obtained with a cathode efficiency » 44%.
1. Methodology :
To study the extraction behaviour of the relevant metal ions in Cyanex 301 sulphates of Cr(III), Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) used were analytical grade material from E.Merck/BDH, India. MgO used for the concentration was laboratory grade material of S.D. Fine Chemicals, India. All other chemicals and solvents were Analytical/Synthetic Grade materials from E.Merck/Qualigens, India.
Spent Chrome Liquor was obtained from a large size tannery at Kanpur, (UP), INDIA.
A well type NaI(Tl) scintillation counter (ECIL, INDIA) was used for the measurement of 51Cr activity for the distribution studies. The studies on the distribution of other metal ions and analysis of metals containing waste were carried out using a ICP-AES (Plasma Labtum, Australia) or AAS (Perkin Elmer, USA).
The electrolytic cell used for the deposition studies was a rectangular pyrex glass trough (250 ml.) in which a G-4 (porous disk 1.2 mm thick) pyrex glass sintered crucible (25 ml.) was fixed to one of its sides. This sintered glass crucible formed the anodic compartment and was filled with 0.10 M H2SO4. The anode was a silver cylinder (surface area = 20 cm2). The remaining part of the trough containing purified SCL forms the cathodic compartment; the cathode being a rotating (50 - 100 rotations per minutes) circular copper disc of surface area 3.14 cm2. A 20 volt DC supply provided with a digital voltmeter and an ammeter was used as the source of power.
2. Procedure :
(a) Sample Pre-treatment
A 250 ml. aliquot of SCL was treated with 1% (w/w) slurry of MgO until a pH 7-8 was attained. The chromium containing precipitate was recovered by decanting the supernatant. The precipitation cycle was repeated 5 times to concentrate Cr(III) to the required level of concentration. The precipitate was finally dissolved in 10 ml. 6M H2SO4 and made up to a known volume.
(b) Distribution Studies
To study the distribution behaviour of the metal ions equal volumes of aqueous and organic phases (Cyanex 301 in an appropriate diluent) were shaken at room temperature (25 ± 30 C) for five minutes. The two phases were separated and suitable aliquot of each phase was analyzed by ICP-AES/AAS or assayed for radioactivity.
(c) Deposition Studies
The electrodeposition parameters were optimized by using a chromium sulphate solution maintained at a pH of the sulphuric acid filled in the anodic compartment. Appropriate DC potential was applied through the electrodes to attain the required current density. After deposition the material of the cathode was carefully scrapped, washed with water and methanol and dried.
The different results reported are a minimum of three runs. Blank determinations were carried out wherever necessary.
RESULTS AND DISCUSSIONS
1. Extraction Studies :
(a) Extraction Behaviour
The extraction behaviour of Cr(III) alongwith Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) from 1.0 x 10-3 to 6.0 M H2SO4 in 0.10 M toluene solution of Cyanex 301 is given in the fig.1. The extraction of the Cr(III) and Mn(II) is negligible (<3%) over the entire investigated range and not depicted in the plot. Cu(II) is almost quantitatively extracted (>95%) over the entire range of acid molarity while Fe(III), Co(II) and Ni(II) show a decreasing extraction with the increasing acid molarity. The above behaviour has been utilized for the separation of Cr(III). By carrying out a double extraction at 5.0 x 10-3 H2SO4 Fe(III), Co(II), Ni(II) and Cu(II) were extracted in the organic phase leaving Cr(III) in the aqueous layer .
(b) Diluent Effect
The diluent effect was investigated by studying the extraction of Fe(III) in 0.10M Cyanex 301 solution at 0.01M H2SO4. It was observed that toluene shows lower extraction (»80%) than n-hexane (»88%), petroleum fraction of hexane (»87%) and kerosene 160 - 2000C (»91%). But all further studies were carried out with toluene as a diluent because of faster phase separations in this diluent. It is two to three minutes with toluene compared to approximately one and a half hour with other diluents.
The loaded extractant after the separation of Cr(III) contains Fe(III)/ Co(II)/ Ni(II) and Cu(II) as impurities. The metal ions from organic phase were removed by using following stripping agents
(i) Fe(III) and Co(II) stripped by 2(volumes) x 6.0 M H2SO4
(ii) Ni(II) stripped by 2(volumes) x 5.0% NH4Cl in NH3
(iii) Cu(II) stripped by 2(volumes) x concentrated HNO3
Fe(III) is a major impurity in SCL therefore in the actual Cr(III) recovery process the organic phase was washed only with 6.0 M H2SO4 to remove it.
(d) Recycling Capacity and Hydrolytic Stability of the Extractant
Experiments were conducted on successive extraction-stripping cycles for 1.0 x 10-3 M Fe(III) from 5.0 x 10-2 M H2SO4 employing two volumes of 0.10 M toluene solution of Cyanex 301. The organic phase after stripping Fe(III) with 6.0M H2SO4 was regenerated by washing it with water until the washings were neutral. The recovery of each step was calculated from the amount of Fe(III) that is extracted in the organic phase in that particular cycle. The results show practically insignificant change in the efficiency of the regenerated extractant thus establishing its utility for commercial use. Hydrolytic stability of Cyanex 301 against the prolonged contacts with H2SO4 is well documented21.
2. Deposition Studies :
(a) Effect of Cr(III) Concentration on the Deposition Rate
Experiments were conducted to observe the effect of Cr(III) concentration on the deposition rate. Results of a total run of one hour revealed that a minimum of 5g/L of Cr(III) is required to achieve a cathode efficiency of around 43%. Beyond this level of Cr(III) concentration the deposition rate becomes constant and does not alter with the increasing Cr(III) concentration.
(b) Effect of Current Density on the Deposition Rate
The results of the effect of current density on the deposition rate revealed that an increase in the current density results in higher deposition rates. However, an average current density of 125-175 mA cm-2 was maintained for deposition. A higher current density than this leads to blackish deposits with the evolution of gas at the anode.
(c) Effect of Additive on Deposition Rate
It was observed that the addition of sodium sulphate to the catholyte improves the current density. The results indicate that a minimum ratio of 7 : 1 (w/w) of chromium to the additive should be maintained.
3. Recovery of Chromium from Purified SCL :
After precipitation the concentrated Cr(III) solution was subjected to solvent extraction employing 0.10M toluene solution of Cyanex 301. This procedure gives Cr(III) solutions of around 99% purity with respect to the transition metal ion impurities. The pH of this solution containing appropriate amount of Na2SO4 was adjusted to 1.4 and it was fed to the electrolytic cell as catholyte while the anolyte was H2SO4 solution (pH 1.4). During the electrolysis the concentration of Cr(III) and H+ ions in the bath was maintained. The deposited chromium metal shows a purity of around 99%.
The present study conducted at the bench scale offers a method for obtaining pure chromium from chromium laden waste of tanneries. The procedure employs precipitation, solvent extraction and electrowinning steps. It may not be difficult to scale up all the three steps. It is expected that by following the cited procedure the loss of a large amount of chromium can be avoided. Moreover, the environment pollution due to chromium can be minimized. However, the efficacy of the method at a large scale is still to be tested.
Thanks are due to Cytec Industries, Canada for providing Cyanex 301 as a gift. Financial support of All India Council of Technical Education, New Delhi, India is gratefully acknowledged.
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