Powder Contact Angle Research Articles

Contact angle studies of particulate sulphide minerals

The flotation separation of sulphide minerals is achieved by controlling particle hydrophobicity, which is often described in terms of the liquid-vapour contact angle determined from wettability studies on single surfaces. These may not be representative of the true hydrophobicity of mineral particles in a pulp and there is a requirement to determine “particle contact angles”. We report an experimental approach, based on equilibrium capillary pressure measurements across a packed bed of particles, which reliably and routinely determines the powder contact angle of mineral particles in the size range 10 to 200 pm. The influence of particle size and surface treatment on the powder contact angle and flotation response of galena particles are reported. The natural hydrophobic nature of galena was shown to depend on the hydrodynamic conditions during particle conditioning. The high shear environment experienced during cyclosizing resulted in galena particles of greater hydrophobicity than those prepared by sieving. A mechanism is proposed to explain the relationship between hydrodynamic conditions, surface chemistry and hydrophobicity; this may give insight into the role of high intensity conditioning and sonication in sulphide mineral flotation. The correlation between collector coverage, particle wettability and flotation recovery was also investigated. Ethyl xanthate at surface coverages in excess of 0.1 equivalent monolayers significantly increased the particle contact angle and particle floatability. The influence of other reagents on galena particle contact angles is discussed.

The extent of the errors associated with contact angles obtained using liquid penetration experiments

Assessment of values for the contact angles of powders is important in many pharmaceutical processes, and the use of surface energy data estimated from contact angle determinations is gaining in popularity. Liquid penetration experiments offer one approach to obtaining an estimate of wettability, and its use continues despite practical and theoretical criticisms. This paper examines the sources and magnitude of errors that are associated with the estimate of wettability that is derived by use of the Washburn model and equation, for model particles (polymer coated glass spheres). It is necessary to ascribe one liquid as perfectly wetting and to compare the behaviour of the test liquid (for which the finite wettability is to be determined) to that of this defined liquid. The following conclusions were made: (1) the major source of error is in the measurement of penetration rates through the powder bed, with surface tension and viscosity data being far more reproducible; (2) the choice of the perfectly wetting liquid is not too critical if the test liquid wets reasonably well, but is far more important if the test liquid has a low contact angle; (3) the error in cos θ, when expressed as a percentage of the mean, was in the range of 2–6%, this being the accuracy of the experiment; (4) if the results were transformed to θ (rather than cos θ) then the error no longer reflected the accuracy of the experiment, but rather the magnitude of the contact angle, as equal changes in cos θ will result in unequal changes in θ (low contact angles showing more apparent error than large), thus it is more appropriate to quote accuracy in the cos θ domain for liquid penetration results, rather than converting to θ.

Contact angle, adsorption and wettability — a review with respect to powders

The significance of wettability, polarity and surface energy of powders with respect to their usage is discussed in terms of spreading coefficients. Indications as to how surface energies can be used to predict interactions are given. The techniques for measuring contact angles of powders have been reviewed critically. Vapour adsorption is a critical stage in the wetting process and a good deal of information can be obtained from its study. The application of the techniques of adsorption microcalorimetry, a vacuum microbalance and dielectric spectroscopy are outlined. By use of dielectric spectra and the thermodynamic parameters of adsorption, it is possible to attempt a description of the mechanism of the wetting process. The use of thermodynamic functions of immersion is also discussed. The role of changes in molecular structure and of different forms of physical treatment (i.e., milling) in controlling the wetting of powders is considered and pointers to further work are given.