粒子の凝集に対する水分の影響 (粉体(特集))

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In order to investigate the effect of water on the cohesion of powder, the experiments were made with a system composed of powder, water and non polar liquid in which a very small amount of water is soluble. Benzen was chosen as such liquid, and spherical glass powders with four kinds of particle sizes (12.4μ∼144μ) were used. In these systems, the adsorption weight of water, sedimentation volumes and angles of repose were measured.In a typical adsorption isotherm, the adsorption weight reached a certain value (point A), and was kept on at this value until another point (B) is reached beyond which it began to increase with a steep slope. In the range of lower concentration of water in benzene than that at B, the sedimentation volume and the angle of repose did not vary with water concentration for the powder of large particle size, and they increased slowly for the smaller one. But when the adsorbed water content approached that at B, the sedimentation volumes and the angles of repose increased sharply, and then decreased with much larger content of water. Representing the adsorption isotherms in the form of adsorption weight per unit surface area of particles, the isotherm curves roughly coincided in the range between A and B. The adsorption weight in this form was 20∼30×10-8g/cm2 and corresponded to the thickness of 20∼30Å for the adsorbed layer.On the basis of the above results, it was considered that the water was uniformly adsorbed on the surfaces of particles first, and then was condensed at the contact points between the particles and formed the water bridges. The adsorbed water layer made the surface of particle more hydrophylic and these particles became easy to flocculate each other in such a non polar medium as benzene. The cohesiveness due to the above mechanism was considered to have been influensed by the particle size.On the other hand, it is widely accepted theoretically that the attractive force due to the liquid bridge between the two spherical particles decreases with increase in the volume of the liquid. These sedimentation volumes were plotted in terms of the mean vloume of bridge water at a contact point, which were calculated from the experimental data of sedimentation volume and adsorption of water. It is found that the curves show the same tendency as the theoretical one for the attractive force versus the volume of the bridge water.
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粒子の凝集に対する水分の影響 (粉体(特集))

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