XX-th ARS SEPARATORIA Szklarska Poręba, Poland 2005

Transcription

1 THE EFFECT OF DIFFERENT FACTORS ON EXTRACTION OF CHROMIUM(III) FROM SPENT TANNING LIQUORS WITH ALIQUAT 336 Barbara WIONCZYK a), Wiesław APOSTOLUK b) and Witold A. CHAREWICZ b) a) Institute of Leather Industry, Zgierska 73, Łódź, Poland; b) Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroc³aw University of Technology, Wybrzeże Wyspiańskiego 27, 5-37 Wrocław, Poland; INTRODUCTION Salts of chromium(iii) are the most widely applied tanning agent in the leather industry because they allow to produce leather of very high quality. However, chrome tanning produces solid and liquid wastes containing chromium(iii). Large quantities of chromium(iii) remain in spent tanning liquors and represent an environmental hazard when are to be discharged. Different methods could be used to remove of chromium(iii) from tannery effluents, namely: liquid-liquid extraction [1,2], ion exchange [3], partial freezing [4], membrane technologies [5,6,7], absorption on kaolinite [8]. Our previous studies [9,1] on extraction of chromium(iii) from model solutions with quaternary ammonium compounds gave solid grounds for application of this method in separation of Cr(III) from spent tanning liquors. Therefore, we now present studies on extraction of chromium(iii) with trioctylmethylammonium salts from real spent tanning liquors. EXPERIMENTAL The spent tanning liquors (bath 1, bath 2) of composition given in Table 1 were obtained from experimental chrome tanning process. Extraction of chromium(iii) was carried out with Aliquat 336 (Merck-Schuchardt) containing 9% of trioctymethylammonium chloride (R 4 NCl) as well trioctymethylammonium hydroxide (R 4 NOH) (86%), and trioctymethylammonium sulphate {(R 4 N) 2 SO 4 } (96%) both obtained by conversion from R 4 NCl. They were used as.5 M solutions in n-heptane (POCh) modified with 1% of 1-decanol (Merck-Schuchardt). Distilled water and reagent grade NaOH (POCh) were used in all extractions. Temperature (25 ±.5 C) was kept constant in all experiments, except for studies on the effect of temperature. Concentration of chromium(iii) in the aqueous 66

2 solutions was determined spectrophotometrically with 1,5-diphenylcarbazide after oxidation Cr(III) to Cr(VI). RESULTS AND DISCUSSION We have already proved [11] that extraction of chromium(iii) with Aliquat 336 from alkaline model solutions depends on initial ionic strength as well as on concentration and kind of electrolytes present in feed aqueous phases. Therefore, the best conditions (initial ionic strength, concentration of electrolytes and of NaOH in feed solutions) of optimal extraction of Cr(III) from tested spent chrome baths were Table 1. Chemical composition of spent tanning liquors (bath 1 and bath 2) Component Bath 1 Bath 2 Chromium(III), (mol/dm 3 ).74.9 Sulphates, (mol/dm 3 ) Chlorides, (mol/dm 3 ) Total nitrogen, (mol/dm 3 ) Ammonium nitrogen, (mol/dm 3 ) Initial ionic strength (I ), (mol/dm 3 ) ph selected on the base of their composition (Table 1) and of the following correlations for model solutions [11]: I log Kex = ( 6, 13 ±. 3) ( ± 128. ) ( 179. ±, 48) I ( ±. 76) c 1 + I (1) I log Kex = ( ±. 3) ( ± 125. ) ( 12. ±. 46) I ( ± 17. ) c 1 + I NaCl K2SO4 (2) were K ex denotes conditional extraction constant of Cr(III); I stands for initial ionic strength while c NaCl and c K denote initial concentration of NaCl and K 2SO4 2SO 4 in the aqueous phase, respectively. Predicted optimal conditions of chromium(iii) extraction were verified experimentally. Obtained results reinforced this prediction. Since the ionic strength of real spent chrome liquors (Table 1) is very high, their 2-times dilution is needed to decrease the negative effects of ionic strength and of electrolytes concentration in 67

3 order to reach efficient extraction of chromium(iii) with Aliquat 336. Moreover, the spent baths must be alkalized prior extraction with mole of sodium hydroxide per 1 dm 3 of feed solution, so that, tetrahydroxochromate(iii) anions could be formed. Under established conditions, the experimental investigation was performed on the effect of phase contact time, of the volume phase ratio, and of temperature on the yield of extraction of chromium(iii) from bath 1 and 2 with Aliquat 336. At volume phase ratio equal to 1, the yield of extraction of Cr(III) slightly increased with prolongation of phase contact time from 15 to 5 minutes only from bath 1 and was equal to 97.9% and 99,7%, respectively. Extraction of Cr(III) from bath 2 was practically complete and its yield exceeded 99.9% after 15 minutes. Moreover, results proved that Cr(III) was almost completely extracted from tested baths at aqueous to organic phases volume ratio within the range from 1 to 5. Over this volume phase ratio, the yield of Cr(III) extraction decreased by 13-27% depending on the composition of bath used. So that, the large dilution (2-times) of spent tanning liquors needed for efficient extraction of Cr(III) can be reduced in part by applying higher phase s volume ratio. The effect of temperature on extraction of Cr(III) with Aliquat 336 was examined for 1-times diluted spent baths at temperature varying from 25 to 4 C. The results showed that temperature positively affects the extraction of Cr(III). After 15 minutes and at constant volume phase ratio equal to 1, an increase in temperature by only 5 C gave the increase in the yield of extraction by over a dozen percent. However, this positive effect can be limited in part by formation of precipitate of Cr(OH) 3 in the aqueous phase since heating favours the hydrolysis of tetrahydroxochromate(iii) anions. This phenomenon was observed during extraction of Cr(III) from 1-times diluted bath 1. Experiments made at higher volume phase ratio (5) and at temperature 4 C showed that Cr(III) was extracted quantitatively from 1-times diluted bath 2 and its concentration in the organic phase was almost equal to initial concentration of extractant in this phase. This is connected with large capacity of organic phase containing Aliquat 336 [1]. Therefore, one can conclude that elevated temperature compensates in part the negative effect of ionic strength and of concentration of electrolytes present in feed liquors. Moreover, it allows for reduction of dilution of spent chrome liquors required prior to an efficient extraction of Cr(III) at temperature 25 C. The effect of anions of quaternary ammonium salts (R 4 NCl, R 4 NOH, (R 4 N) 2 SO 4 ) on extraction of Cr(III) were examined for 1-times diluted bath 1 and 2, alkalized with.1 mole of NaOH per 1 dm 3 of feed solution at temperature 25 C and at volume phase ratio equal to 1. These experiments revealed that trioctymethylammonium sulphate and hydroxide are much better extractants of chromium(iii) than trioctylmethylammonium chloride because the percent of extraction of Cr(III) from tested bath with R 4 NOH and (R 4 N) 2 SO 4 always exceeded 9 while with R 4 NCl was equal only to 47.9 for bath 1 and 87.4 for bath 2 under the same conditions. 68

4 CONCLUSIONS Optimal conditions for efficient extraction of Cr(III) from real spent tanning liquors with Aliquat 336 can be designed basing on composition of a bath and from our empirical dependencies established for model solutions [11]. Extraction of Cr(III) depends on initial ionic strength and on concentration of electrolytes in a spent chrome bath. Their negative effect can be compensate in part by suitable dilution of liquors prior extraction and/or by small increase in temperature of extraction. High dilution of spent liquors needed to reach an efficient extraction can be reduced by using higher volume phase ratios in extraction. Moreover, with R 4 NOH and/or (R 4 N) 2 SO 4 extractants one can compensate in part both high dilution of baths and negative effect of chlorides on extraction of chromium(iii) without elevation of temperature. ACKNOWLEDGEMENT The work has been sponsored within the grant No T9B of Ministry of Science and Information Technology (Poland). REFERENCES 1. Pandey, B.D., Cote, G., Bauer, D., Extraction of chromium(iii) from spent tanning baths. Hydrometallurgy 4, Schügerl, K., Segelken, G., Gudorf, M., Extractive recovery of chromium(iii) from effluents of fur finishing industry. Proc. ISEC 96, University of Melbourne, Australia, pp O Dwyer, T.F., Hodnett, B.K., Recovery of chromium from tannery effluents using a redox adsorption approach. J. Chem. Tech. Biotechnol. 62, Turtoi, D., Untea, I., Zainescu, G., Tudorache, E.L., Orbeci, C.R., Belcu, M., 24. Chromium(III) separation from tannery wastewaters by partial freezing. J. Soc. Leather Technol. Chem. 88(4), Fabiani, C., Ruscio, F., Spadoni, M., Pizzichini, M., Chromium(III) salts recovery process from tannery wastewaters. Desalination 18, Cassano, A., Drioli, E., Molinari, R., Bertolutti, C., Quality improvement of recycled chromium in the tanning operation by membrane processes. Desalination 18, Aloy, M., Vulliermet, B., Membrane technologies for the treatment of tannery residual floats. J. Soc. Leather Technol. Chem. 82(4),

5 8. Tavani, E.L., Volzone, C., Adsorption of chromium(iii) from a tanning wastewater on kaolinite. J. Soc. Leather Technol. Chem. 81(4), Wionczyk, B., Apostoluk, W., 24. Solvent extraction of chromium(iii) from alkaline media with quaternary ammonium compounds. Part I. Hydrometallurgy 72, Wionczyk, B., Apostoluk, W., 24. Solvent extraction of chromium(iii) from alkaline media with quaternary ammonium compounds. Part II. Hydrometallurgy 72, Wionczyk, B., Apostoluk, W., 25. Equilibria of extraction of chromium(iii) from alkaline solutions with trioctylmethylammonium chloride (Aliquat 336). Hydrometallurgy (in press). 7