USING CAPILLARY POTENTIAL TO ASSESS THE AGGLOMERABILITY OF DISPERSED MATERIALS

V. M. VITYUGIN, V. V. IVANOV

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

For the first time, thermodynamic potential was applied to characterize the energetic state of the soil–water system. According to modern concepts, thermodynamic potential is composed of a number of partial potentials, one of which is the capillary potential, representing the work (calculated per 1 g of pure water) that must be expended to overcome capillary forces. The magnitude of the capillary potential depends on the structure of the layer and the nature of the material particles. These same characteristics of dispersed materials determine their agglomerability.

To determine the capillary potential, a Seker capillarimeter modified by Dolgov was used. The moistened material under study was placed in a cartridge on the surface of a finely porous membrane. After the free gravitational water had drained, a vacuum was created in the system, the magnitude of which was changed successively every 100 mm Hg from 0 to 700 mm.

The following materials, widely used in agglomeration processes, were selected for the study:

a) magnetite concentrate with a particle size of 0.1–0 mm and a total Fe content \(F_{\text{tot}}\) of 67.60%;

b) Tashlinskii sand with a particle size of 0.5–0 mm and a \(\mathrm{SiO_2}\) content of 99.04%;

c) limestone ground to a particle size of 0.25–0 mm and a \(\mathrm{CaO}\) content of 53.54%;

d) return fines after firing of fluxed iron-ore pellets, ground to 0.1–0 mm;

e) marshalite (pulverized quartz) with a particle size of 0.5–0 mm and a \(\mathrm{SiO_2}\) content of 93.70%.

The results of the determinations are shown in Fig. 1 as dependences of residual moisture on the magnitude of the vacuum.

From the curves it is possible to calculate the values of the capillary potentials \(K_{\text{p}}\), using the formula:

\[ K_{\text{p}}=\frac{\Delta P \cdot 100 \cdot \gamma_{\text{v}}}{\Delta W \cdot \gamma_{\text{m}}}, \]

where \(\Delta P\) is the magnitude of the vacuum, mm Hg;
\(\Delta W\) is the amount of water removed, %;
\(\gamma_{\text{m}}, \gamma_{\text{v}}\) are the densities of the dispersed material and water, g/cm\(^3\).

As can be seen from Fig. 1, the magnitude of the capillary potential depends not only on the properties of the dispersed material, but also on the level of rarefaction. This is especially noticeable for low rarefaction values. At rarefactions above 100 mm Hg, the dependences \(W_{\text{res}}=\varphi(\Delta P)\) are practically rectilinear. This is evidently explained by the different properties and ratios of the forms of capillary water (pendular, funicular, and the capillary state proper).

To assess the contribution of the capillary potential to the agglomerability of dispersed materials, the maximum values of molecular \((W_{\text{m.m.v.}})\) and capillary \((W_{\text{m.k.v.}})\) moisture contents were determined in parallel, and the agglomerability indices were calculated by the known method [2]. The results of the determinations and calculations are summarized in Table 1. The absence of a complete correlation between the agglomerability index and the magnitudes of the capillary potentials is evidently due to the different structural state of the dispersed materials, as estimated experimentally by the first and second methods.

Fig. 1. Dependence of the residual moisture content in a layer of dispersed materials on the magnitude of vacuum

Fig. 2. Sizes and distribution of pores in a layer of moist dispersed materials

This is confirmed by the fact that the values of the capillary potential at \(\Delta P = 100\) mm Hg correlate to a greater extent with the agglomerability indices.

Table 1

Agglomerability and capillary potentials of dispersed materials

Of considerably greater importance is the determination of the capillary potential for assessing the pore size in a layer of moist dispersed material. The determination is based on the fact that if a negative pressure \(H\), expressed in cm, is applied to a layer of moist dispersed material,

of a water column, then this vacuum will cause water to be sucked out of all pores whose diameter \(d\) satisfies the inequality

\[ d \geq \frac{0.30}{H}. \tag{2} \]

By measuring the volume of water sucked out at a given vacuum, one can determine the volume of capillaries of any size and calculate their ratio in the bed. The results of such measurements and calculations are shown in Fig. 2.

Knowledge of the pore sizes in a bed of moist dispersed material makes it possible to calculate the magnitude of the capillary forces binding the particles and thereby to predict the mechanical strength values of moist granules.

Thus, determination of the capillary potential can provide sufficiently valuable information for a preliminary assessment of the agglomerability of dispersed materials.

REFERENCES

1. A. A. Rode. Fundamentals of the Theory of Soil Moisture. Gidrometeoizdat, Leningrad, 1965.
2. V. M. Vityugin, A. S. Bogma. Assessment of the agglomerability of dispersed materials. Izv. VUZov, “Ferrous Metallurgy,” No. 4, 1969.