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Vo1. 1, No. 2Adsorption of Surfactant Dispersed Nanometer Magnetite137Figure 5. The adsorption of nonionic MR-45 (polyoxyethylene-4 lauryl ether) on quartz,calcite, kaolinite, and pyrite at various pHs. The adsorption of MR-45 on calcite and kaolinite are quite different when the magneticreagent dosage is increased. As shown in Figure 6, kaolinite had a linear relationship between the increase of magnetic susceptibility and the increase of magnetic reagent dosage at pH 7. Calcite, however, had reached a saturation of the magnetic enhancement at a 0.005 g/g reagent dosage. Further increase of magnetic reagent dosage had no effect on the magnetic susceptibility. The strength of hydrogen bonding on the two minerals may not be significantly different. Surface area and van der Waals force may have assisted the continuous adsorption of MR-45 on kaolinite at high reagent dosages.Figure 6. Magnetic enhancement of calcite and kaolinite at various MR-45 dosages.
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J. Y. Hwang Vo1. 1, No. 2 138 The pH is only one of the factors affecting the adsorption. It functions by changing thesurface charge and surface species on fine particles and the solubility and stability of surfactants. Some salts dissolved in water have similar functions. They can activate or depress the adsorption through modifying the surface properties of fine particles and are usually called activators or depressants. Figure 7 shows an example of the activation of quartz for the adsorption of MR-30.Quartz did not adsorb MR-30, which was explained in Figure 1. However, quartz may adsorb calcium on its surface to change its surface properties. The calcium species on a particle surfacecan then facilitate the adsorption of sodium oleate, which is the outer layer surfactant of MR-30. A series of experiments was conducted to examine this type of adsorption. About 300 ppmcalcium chloride was added to a quartz slurry before conditioning with MR-30 following the standard test procedures. The results, presented in Figure 7, show that magnetic enhancement can indeed be achieved with this approach. The stronger adsorption of MR-30 at high pH was related to the stronger adsorption of calcium. At pH 3, sodium oleate may have been destabilized. Figure 7. Calcium chloride for the activation of quartz on MR-30 adsorption. Sodium tripolyphosphate (STPP) is a common depressant for calcite. Adsorption of theanions of this salt on calcite will shield the calcium atoms on calcite and yield a strong negativecharge on the calcite surface. The adsorption of sodium oleate on calcite can then be depressed. Figure 8 demonstrates the depressing effect for the adsorption of MR-30 on calcite. By adding 300 ppm STPP to a calcite slurry before conditioning with MR-30, the magnetic enhancement of calcite decreased significantly. With the increase of pH, the depressing effect increased becauseof the stronger dissociation of STPP.