1994
Ind. Eng. Chem. Res. 2003, 42, 1994-1997
Study of Microemulsified Systems Applied to Mineral Flotation Afonso A. Dantas Neto,* Tereza N. Castro Dantas, M. Carlenise P. A. Moura, Eduardo L. Barros Neto, and Lindemberg J. N. Duarte Programa de Po´ s-Graduac¸ a˜ o em Engenharia Quı´mica, Universidade Federal do Rio Grande do Norte, Avenida Senador Salgado Filho s/n, Campus Universita´ rio, Lagoa Nova, 59.072-970 Natal/RN, Brazil
An investigation of calcite and fluorite microflotation using microemulsified systems is reported. Several reagents that are commonly used in mineral froth flotation, such as sodium silicate, quebracho, and pH modifiers, were tested. The main objective of this research was to simplify the conventional flotation process by the use of microemulsions. This application is based on their capacity to contain variable amounts of water, oil, and surface-active material in a single phase and their ability to decrease interfacial tension, a fact that favors their application in separation processes. The microflotation assays were accomplished in a modified Hallimond tube under specific operating conditions. The results obtained are pioneer and demonstrated the viability of the use of microemulsions in mineral flotation, leading to an improvement of the conventional process. Introduction Froth flotation is a physicochemical property-based separation process used in mineral treatment and mining to concentrate the metal-bearing mineral in an ore. During the process many reagents are added to the mineral suspension (pulp), according to their function in the flotation system. The most important are pH modifiers, collectors, depressants, and frothers.1-5 The collectors are chemical substances used to increase the hydrophobicity of the surface of the mineral species onto which they are adsorbed. The frother is a substance that, when dissolved in water, imparts to it the ability to form a stable froth. The depressants are the agents used to inhibit the fixation of the collector on a surface on which it could normally adhere. In flotation, ionic surfactants (collectors) are used for the selective hydrophobization of mineral surfaces and the nonionics are used for froth formation, except for naturally hydrophobic materials.6 Microemulsions are systems composed of water, an organic solvent, a surfactant, and occasionally an alcohol as a cosurfactant. Schulman et al. (1959)7 and Winsor (1968)8 observed that certain mixtures of oil, water, surfactant, and a cosurfactant yielded clear visually homogeneous systems even without a substantial input of mechanical energy. These systems are formed from minute droplets of one liquid in the other and differ from macroemulsions because of their long-term stability, which, in fact, results from a thermodynamic stability.9,10 These systems can be used as a homogeneous solvent or associated with an immiscible aqueous or organic phase.11 In some of our previous works,12-16 microemulsions have been used to extract metal ions from an aqueous phase using liquid-liquid and adsorption processes. This work attempts to simplify the flotation process using microemulsified systems. Among the properties of these systems, one can mention their high solubilization power, their low interfacial tension vs water, and * To whom correspondence should be addressed. Fax: (55)(84)215-3827. E-mail:
[email protected].
hence their important capacity to gather in one phase great amounts of water, oil, and active matter, which guarantees the use of all flotation reagents at the same time, reducing the complexity of the conventional process. This study is a preliminary investigation which used only pure crystalline species, calcite and fluorite, as model systems to optimize the flotation using microemulsions. Materials and Methods Chemicals. The chemicals used during the experiments were of analytical grade, except for coconut oil (regional production), and were used without further purification. The water used in all experiments and solutions was distilled (Tecnal TE-078 distiller). Mineral Species. The calcite (the most stable polymorph of CaCO3 at room temperature and under normal pressure) and fluorite (CaF2) used in this work were selected crystals obtained from Universidade Federal do Rio Grande do Norte, Natal/RN, Brazil. The samples were dried and ground using a porcelain mortar and pestle. After grinding, the samples were sieved to produce particles in the narrow size range of -100 to +200 mesh. After sieving, the samples were washed with distilled water to remove fine particles and dried at 80 °C for 4 h, prior to use in the microflotation tests. Microemulsion. To determine the microemulsion region in a pseudoternary diagram, the methodology described by Bellocq and Roux (1987)17 was followed. The microemulsion used to perform the experiments of calcite and fluorite microflotation consisted of a 93 wt % aqueous phase, a 0.5 wt % oil phase (coconut oil), 2.17 wt % surfactant (saponified coconut oil), and 4.34 wt % cosurfactant (n-butyl alcohol-99% Pro-analysis). Depressant Agents. The depressants used were sodium silicate (SS; 5.0 × 10-4 M) and quebracho (8.8 × 10-5 M) solutions. These solutions were added to the microemulsion aqueous phase to elucidate their influence on the behavior of the system. Quebracho is an evergreen tree whose wood is valuable and whose name is due to its hardness. It contains six alkaloids: aspidospermine, aspidospermatine, aspidosamine, quebra-
10.1021/ie0205952 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/27/2003
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 1995 Table 1. Operation Conditions Used in the Microflotation Experiments with a Hallimond Tube
Figure 1. Schematic diagram of the experimental setup: (1) magnetic stirrer; (2) Hallimond tube; (3) compressor; (4) collector; (5) pH meter; (6) oven; (7) analytical balance; (8) Hallimond tube support; (9) collector support; (10) magnetic stirring bar; (11) porous plate.
chine, hypoquebrachine, and quebrachamine. The quebrachine is the most active. pH Modifiers. The pH modifiers were solutions of NaOH (0.5 M) and H2SO4 (0.5 M) and were added to the system prior to the microflotation process. Experimental Methods and Conditions, Apparatuses, and Calculations. The microflotation experi-
parameter
operating conditions
conditioning time flotation time air flow rate microemulsion volume granule size
5 min 6 min 60 mL/min 100 mL -100 to +200 mesh
ments were performed in a 200-mL modified Hallimond tube, conceived originally by Hallimond (1944)18 and modified by Fuerstenau et al. (1957),19 at an air flow rate of 60 mL/min to ensure an efficient operation of the apparatus (Figure 1). In the essays, calcite and fluorite were used separately with the microemulsions prepared with or without addition of the depressants and/or pH modifiers. The pH of the pulp, mineral sample (1.5 g) plus a microemulsion (V ) 70 mL), was adjusted to the required value and then conditioned for 5 min under constant stirring. After this period, air was bubbled through the porous plate located in the base of the Hallimond tube, for 6 min at a constant aeration rate, measured with a flowmeter. During the flotation, the level was regulated with a microemulsion (V ) 30 mL). Table 1 summarizes the operating conditions used in all of the experiments. The products of froth and sediment were dried in an oven at 100 °C. The weights of the flotation products were used for the
Figure 2. Pseudoternary diagrams for the studied systems composed of the aqueous phase, coconut oil, saponified coconut oil, and n-butyl alcohol (cosurfactant/surfactant ratio ) 2; pH 10; T ) 27 °C).
1996
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003
Table 2. Microemulsion Aqueous-Phase Composition system
aqueous phase
a b c d
distilled water distilled water + SS distilled water + quebracho distilled water + SS + quebracho
calculation of mineral recovery (eq 1), where MFLOT is the mass floated and MFED is the initial mineral mass.
% flotability )
MFLOT × 100 MFED
(1)
Results and Discussion Pseudoternary Diagrams. The aim of this empirical study is to analyze the changing behavior of the microemulsion region when flotation reagents are added to the aqueous phase, with the objective of verifying whether it is possible to gather together in a single phase all of the necessary reagents for the flotation tests. Figure 2 presents the pseudoternary diagrams for the system: aqueous phase, coconut oil, saponified coconut oil, and n-butyl alcohol. The aqueous-phase composition is shown in Table 2. Analysis of the pseudoternary diagrams indicates that, at the concentrations used, the addition of SS (Figure 2b), quebracho (Figure 2c), and SS + quebracho (Figure 2d) to the aqueous phase does not exert a significant influence on the surface area of the microemulsion region (Figure 2a). Consequently, it can be concluded that these additives can be added to the system without damage to its stability. Effect of the Microemulsion pH on the Microflotation Process. The microemulsion used in performing the calcite and fluorite microflotation experiments has been described above (aqueous phase: distilled water). Its composition was chosen to favor the contact between surfactant and mineral species and to promote the hydrophobicity of the latter. Another feature that should be cited is that the chosen microemulsion is located in the water-rich region, consuming, consequently, small amounts of surfactant, cosurfactant, and oil phase. The microemulsion pH was varied, covering a range of 8-12. Figure 3 illustrates the results obtained. Figure 3 shows that the highest flotability levels of calcite and fluorite were achieved at pH 10 (microemulsion natural pH), reaching recoveries of above 93%. A significant decrease in flotability was observed at higher pH values, probably due to an electrostatic repulsion effect between the collector agent (SCO) and calcite and fluorite particles. On the other hand, it is essential to mention that the addition of the pH-modifying agents does not impair the microemulsion stability. Effect of SS on the Microflotation Process. The microemulsions used to perform these experiments had the same composition as the previous one, but the aqueous phase was composed of distilled water and SS (5.0 × 10-4 M). The microemulsion pH was varied, covering a range of 8-12. Figure 4 shows that the inhibition action of SS becomes sharper above pH 10 and up to pH 12. Under these operating conditions, the action of SS, as a depressant of the calcite and fluorite minerals in the microflotation process using the microemulsions, was satisfactory.
Figure 3. Calcite and fluorite microflotation as a function of the microemulsion pH (flotation time ) 6 min; air flow rate ) 60 mL/ min; mineral sample ) 1.5 g; temperature ) 27 °C).
Figure 4. Calcite and fluorite microflotation as a function of the microemulsion pH in the presence of SS (flotation time ) 6 min; air flow rate ) 60 mL/min; mineral sample ) 1.5 g; temperature ) 27 °C).
Effect of Quebracho in the Microflotation Process. In these experiments, the microemulsion aqueous phase was composed of distilled water and quebracho (8.0 × 10-5 M). The microemulsion pH was varied, covering a range of 8-12. It may be observed from Figure 5 that the flotability behavior as a function of microemulsion pH (with quebracho) is similar to that obtained with SS, with the sharpest depression at pH 12. It is evident from these experiments that SS and quebracho are both good calcite and fluorite flotation inhibitors but the high flotability efficiency presented by the microemulsions may have prevailed over the inhibition effect of the depressant agents. It must be emphasized that the best results were obtained with the microemulsion at natural pH (pH 10), which may be explained by the possibility that the pH modifiers
Ind. Eng. Chem. Res., Vol. 42, No. 9, 2003 1997
Literature Cited
Figure 5. Calcite and fluorite microflotation as a function of the microemulsion pH in the presence of quebracho (flotation time ) 6 min; air flow rate ) 60 mL/min; mineral sample ) 1.5 g; temperature ) 27 °C).
(NaOH and H2SO4) affect the effectiveness of the collector (SCO). Another important observation is that when the depressant agents are added to the microemulsion, they do not cause any perceptible damage to its stability. Conclusions The accomplishment of this research allows one to conclude that microemulsions can be used with great efficiency in the calcite and fluorite microflotation process. The construction of the pseudoternary diagrams demonstrates that the microemulsion region is not affected by the addition of SS, quebracho, or SS + quebracho to the aqueous phase. By analysis of the pH effect of a water-rich microemulsion on calcite and fluorite flotability, it has been verified that the best results, 95% recovery for calcite and 93% for fluorite, were obtained at pH 10 (the microemulsion natural pH). These results open the way to the use of microemulsions in mineral microflotation processes, with potential applications that make the process simpler and more efficient. Acknowledgment The authors acknowledge a financial contribution from CNPQ.
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Received for review August 4, 2002 Revised manuscript received January 2, 2003 Accepted February 17, 2003 IE0205952
