Ind. Eng. Chem. Res. 1996, 35, 1369-1377
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SEPARATIONS Kinetic Analysis of the Simultaneous Nondispersive Extraction and Back-Extraction of Chromium(VI) Inmaculada Ortiz,* Berta Gala´ n, and Angel Irabien Departamento Quı´mica, Escuela Te´ cnica Superior de Ingenieros Industriales y Telecomunicacio´ n, Universidad de Cantabria, Avenida los Castros s/n, 39005 Santander, Spain
This work has been focused on the study of the viability and kinetics of Cr(VI) removal from industrial waste waters and simultaneous concentration for its reuse in electroplating processes, working with liquid-liquid systems in two hollow-fiber (HF) modules, and using Aliquat 336 as an organic carrier. For the kinetic analysis of the non-steady-state system the macroscopic mass balances of the chromium in the fluid phases into HF modules were developed and solved simultaneously with the mass balances in the homogenization stirred tanks. An optimization procedure to calculate the mass transport parameters was developed, leading to the value Km) 1.92 × 10-8 m/s (membrane mass-transport coefficient). In the extraction process a kinetic masstransfer parameter depending on the experimental conditions was obtained. Introduction The treatment of industrial waste water containing Cr(VI) is one of the major concerns nowadays due to the high toxicity of chromium’s ionic form and the prevalence of the chromium in a wide variety of industrial processes (Cheremisinoff and Cheremisinoff, 1993). The use of chromium in metal finishing industries is the major generator of chromium effluents (Conner, 1990). Chromate conversion coating is mainly applied as a posttreatment of zinc electrodeposit steel, being also applied to many other metals such as cadmium, aluminum, magnesium, copper, etc. The concentration of chromium compounds commonly used in the chromate dips (Na2CrO4, Na2Cr2O7, H2CrO4) varies between 0.56 g/L for clear bright baths and 25 g/L for olive-drab dips (Geduld, 1988). In addition to these basic builders, electroplating chromate dips may contain sodium chloride, sodium acetate, acetic acid, etc., for specialized effects. The electroplating process is immediately followed by a series of rinse steps to purify the metal from undesirable compounds. The great volume of water used during the rinsing processes generates the main problem to metal-finishing industries (Walker et al., 1990). Concerning the concentration of chromium in these effluents, only a wide generalization can be made because of the great variability of these types of industries and types of baths. As a general consideration, it can be said that rinse waters present a concentration between 50 and 500 mg/L of chromium(VI), being necessary to treat them before discharge to natural streams (Conner, 1990). Although standard technologies employed for the treatment of these effluents, such as reduction and precipitation, ion exchange or reverse osmosis, reduce the chromium concentration in the waste effluents to a low level, these techniques create an additional sludge and/or are costly (Lankford, 1990); thus, the research * Author to whom correspondence should be addressed. e-mail:
[email protected].
0888-5885/96/2635-1369$12.00/0
and development of cleaner technologies that would allow removal and concentration of the chromium compounds for their reuse in the coating processes presents high interest. Solvent extraction processes using hollow-fiber (HF) modules have been used for the removal and/or concentration of different metallic elements, anions, organic compounds, etc. (Prasad and Sirkar, 1989, 1990; Seibert et al., 1993; Hutter et al., 1994). A nondispersive extraction technique overcomes most of the conventional liquid extraction shortcomings: backmixing, emulsion generation, flooding limitations on independent phase flow rate variations, and requirements of density differences. Apart from that, nondispersive contactors present several advantages, namely, (i) a very large interfacial area without direct mixing of the aqueous and organic phases, (ii) capability of treating dilute solutions, (iii) reduction of the solvent losses, and (iv) reduction in the equipment volume and space. In these types of contactors, aqueous and organic solutions flow continuously, one through the lumen of the fibers and the other through the shell side; both phases get into contact through the pores of the fiber wall. D’Elia et al. (1986) and Dahuron and Cussler (1988), among others, have demonstrated that phase entrainment can be avoided by applying a differential static pressure in one of the phases. To carry out both processes simultaneously, two different configurations can be used: (i) contained liquid membranes (Sengupta et al., 1988; Pakala et al., 1993); (ii) two different modules, one for the extraction process (EX) and the second one for the back-extraction (BEX), flowing the organic solution in a closed cycled between both modules (Prasad et al., 1990; Basu and Sirkar, 1992). Different extractants have been researched for the removal of Cr(VI); e.g., Yun et al. (1993) reported preliminary results using LIX 84; however, the application of Aliquat 336 as an organic carrier for the separation and concentration of Cr(VI) has been intensively investigated (Alonso et al., 1993a,b, Salazar et al., 1992a,b); moreover, the complexity of the description © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
Table 1. Hollow-Fiber Membrane Module Characteristics fiber type internal diameter wall thickness number of fibers nominal porosity shell material potting material shell inner diameter shell length effective mass-transfer length effective mass-transfer area
X-10/polypropylene 240 µm 30 µm 2100 30% polypropylene Epoxy resin 25 mm 200 mm 160 mm 0.23 m2
of the extraction behavior of the quaternary ammonium bases has also been widely mentioned in the literature (Cerna´ et al., 1993; Mizelli and Bart, 1994; Gala´n et al., 1994). During the extraction process, the chromate anions are extracted from the aqueous phase into the organic phase by the formation of a quaternary salt of chromium. The chemical reaction taking part at the porous interface of the organic and feed aqueous solutions is
CrO42- + 2(R4N)X h 2X- + (R4N)2CrO4
(1)
During the back-extraction process, (R4N)2CrO4 is contacted with an aqueous solution containing the BEX agent. This removes the chromate from the organic phase and regenerates the extractant for recycling:
(R4N)2CrO4 + 2X- h 2(R4N)X + CrO42-
(2)
This work has been focused on the analysis of the viability and kinetics of the Cr(VI) concentration up to the required levels in order to be reused in chromate dips working with two HF modules where the EX and BEX steps are carried out simultaneously. A methodology of a wide range of applicability has been used for the kinetic analysis; starting with the integration of the macroscopic mass balances to the chromium compound in the fluid phases flowing through the HF modules together with mass balances of the mixing stirred tanks, an optimization procedure has been developed in order to determine the mass transport parameters of the EX and BEX processes. Experimental Method and Procedure Aliquat 336 (Fluka), a commercial mixture of trialkylmethylammonium chlorides (trialkyl ) C8-C10, mainly,
capryl), was used as the extractant, and kerosene (Petronor, S.A.) was used as the solvent. In order to avoid the segregation of a third (second organic) phase, the addition of a modifier (a high molecular weight alcohol) was necessary. A total of 30% (v/v) of isodecanol (Exxon Chemicals) was added into the organic phase. The nondispersive experimental system has two gear pumps capable of flowing up to 1 L/min for the organic and BEX phases, powered by a variable-speed dc motor, and a peristaltic pump for the feed aqueous phase capable of flows up to 1 L/h. The pressures of the aqueous phases were maintained 3 psi higher than the pressure of the organic phase, ensuring that no displacement of the organic phase from the pores of the hollow fiber wall took place. Characteristics of the hollow-fiber membrane modules are given in Table 1, and a schematic diagram of the setup for both configurations is shown in Figure 1. Both EX and BEX operations were carried out by working in a recirculating mode and with the fluid phases flowing cocurrently. Samples were taken out at different times from the aqueous reservoirs, and the chromium concentration was analyzed after preparation. The Cr(VI) concentration in the aqueous phases was measured in a Perkin-Elmer 1100 B atomic absorption spectrophotometer. Once the concentration in the feed solution was low (
