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Ind. Eng. Chem. Res. 2005, 44, 6784-6788
SEPARATIONS New Surfactant for Gallium and Aluminum Extraction by Microemulsion T. N. Castro Dantas,*,† M. H. de Lucena Neto,‡ A. A. Dantas Neto,‡ M. C. P. Alencar Moura,‡ and E. L. Barros Neto‡ Departamento de Quı´mica and of Engenharia Quı´mica, Universidade Federal do Rio Grande do Norte, Campus Universita´ rio, 59072-970 Natal (RN), Brazil
A new surfactant, 12-(N,N-diethylamino)-9,10-dihydroxyoctadecanoate, synthesized from castor oil, was used in microemulsified systems to separate gallium and aluminum from a synthetic Bayer liquor. The extraction/re-extraction process by microemulsion consists of two stages. In the first one, the metal ion, present in the aqueous phase, is extracted by the microemulsion phase. In a second step, the re-extraction process occurs: the microemulsion phase, rich in metal, is acidified, and the metal is released to a new aqueous phase that contains higher metal concentration. The knowledge of these systems, as well as of its properties, requires a previous study based on pseudoternary phase diagrams, which were constructed with the purpose of delimitating Winsor’s regions. The experiments were carried out by varying the cosurfactant/ surfactant (C/S) ratio from 4 to 60, seeking to use the smallest amount of surfactant. The surfactant saturation study was made using the best C/S ratio, where metal concentrations (Ga and Al) were varied in the synthetic Bayer liquor. The microemulsion extraction points (18) were chosen inside the Winsor II region. In re-extraction experiments, hydrochloric acid, in different concentrations, was used, and it was observed that gallium and aluminum can be reextracted in a selective way. Ga and Al extraction/re-extraction processes by microemulsion showed to be effective, reaching extraction percentiles of 99.63% for gallium and 98.89% for aluminum. 1. Introduction The Bayer process is a way of performing alumina extraction from bauxite. Essential steps in the Bayer process involve grinding the bauxite and then dissolving or digesting the gibbsite in a hot caustic soda solution. The insoluble bauxite residue, called red mud, is separated from the solution by allowing it to settle out, leaving the liquor, which is rich in alumina. Gallium follows the aluminum in its bauxite ores, whose content is from 20 to 80 ppm. In the Bayer process, about 70% of the gallium is leached from the bauxite and follows the aluminum into the caustic soda solution, with the remaining 30% being disposed of with the red mud.1,2 Gallium is found most commonly in association with zinc, germanium, and aluminum. The toxicity of metallic gallium or gallium salts is very low. The application of gallium that has received the most attention is the production of semiconducting compounds and analogintegrated circuits. The most important compounds of gallium are the ones made with antimony, arsenic, or phosphorus. Nowadays, gallium arsenide (GaAs) is undoubtedly the most used. This compound is used in * To whom correspondence should be addressed. Tel. and Fax: +55 (84) 215. 3827. E-mail:
[email protected]. † Departamento de Quı´mica. ‡ Departamento de Engenharia Quı´mica.
the production of several electronic parts such as diodes and transistors. Other GaAs applications are microwave generation and also in several sensors to measure the temperature, light, or magnetic field.2,3 Solvent extraction is the facile method for metal ion removal. To extract Ga in a selective way, in relation to Al, the most used extracting agent is Kelex-100.1,4,5 Castro Dantas et al.1 achieved 99.9% for Al and 100% for Ga. Puvvada et al.4,5 using Kelex-100 and a concentrated Indian Bayer process liquor recovered 90% of Ga at 14 vol %, and when this surfactant was used in a microemulsified system, they recovered 98% of Ga at 5.0 vol %. The search for new extracting agents is the aim of this research, where microemulsion systems with a new surfactant [12-(N,N-diethylamino)-9,10-dihydroxyoctadecanoate] (NNDDO) were employed. A microemulsion is defined as a system formed by the dispersion of microdroplets of two immiscible liquids, stabilized by an interfacial membrane formed by surfactant and cosurfactant. They are thermodynamically stable, homogeneous, and optically isotropic solutions.6-10 Surfactants are molecules that form oriented monolayers at interfaces and show surface activity, lowering the interfacial tension of the medium in which they are dissolved. In some usage, surfactants are defined as molecules capable of associating to form micelles. A
10.1021/ie030393l CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6785
cosurfactant is a molecule added to a system to enhance the effectiveness of another surfactant.11 The knowledge of the characteristics of microemulsified systems, as well as of its properties, requires previous studies based on pseudoternary phase diagrams. These diagrams were obtained using a mixture of surfactant, cosurfactant, an aqueous phase, and an oil phase. To study the extraction/re-extraction process behavior, some experiments were accomplished according to an experimental matrix inside the Winsor II region. The results showed that Ga and Al can be re-extracted in a selective way. 2. Experimental Section 2.1. Chemicals. Chemicals used during the experiments were analytical grade, except castor oil (regional production) and kerosene (Petrobra´s; chemical composition of paraffinic hydrocarbons, min 70%; of aromatic hydrocarbons, max 20%; of olefinic hydrocarbons, max 5%; density,
