EFFECT OF SODIUM HUMATE SOLUTIONS ON THE BOND STRENGTH OF WETTED MARSHALITE PARTICLES

A. F. KARPOVICH, V. M. VITYUTIN

(Submitted by the scientific seminar of the Department of General Chemical Technology)

Effective control of the physicomechanical and technological parameters of a granulated charge is ensured by knowledge of the characteristics of its individual components. Plasticity is imparted to the mixture by water and various binders. The finest particles of the material, interacting with water and forming a finely dispersed heterogeneous system, are bound by molecular and capillary forces to one another and to the dipolar molecules of water. As a result, the system possesses definite plastic and strength characteristics. At the initial moment of granulation, the moistened charge has some properties of a loose body, since the bonding forces between the elements of the mixture unite them into separate conglomerates that are weakly connected with one another. The size and shape of these conglomerates depend on the properties of the material, as well as on the size of the droplets of liquid supplied for granulation, and on its physicochemical properties: viscosity, surface tension, molecular structure, and degree of polarity. In the process of compacting the granules, the adhesion of particles to one another must be overcome, and the smaller it is, the more mobile the mixture. The mobility (flowability) of the mixture can be determined by the magnitude of the shear resistance, and the latter is composed of the forces of adhesion and friction of individual particles with one another. The adhesion of particles to one another is ensured by the adsorption properties of the liquid films. The friction of particles against one another (internal friction) increases as the external pressure rises.

For the studies, sodium humate solutions with concentrations of 0.1%, 1.17%, 2.03%, and 3.51% were taken; their density at 20° C is respectively 0.9972, 0.9997, 1.0096, and 1.0099 g/cm³; the viscosity increases respectively from 1.01564 to 1.3118 cP, and the surface tension decreases from 72.9 to 58.9 erg/cm².

The plasticity or mobility of the mixture during granulation is an essential factor in creating a strong structure. The shear strength was determined on the simplest apparatus used in soil science for characterizing the shear of soils. For the investigation, marshalite from the Bolotovskoye deposit was selected; its principal fraction—0.05 mm—amounts to 77.2% of the initial material. For comparison, shear strength was determined with water and with sodium humates. The moisture content of the material, the concentration of humates, and the degree of compaction (normal load on the specimen) were varied. The results of determining the shear strength at an external pressure on the specimen of 0.102 kg/cm² are shown in Fig. 1. The character of the course of the strength curves

Fig. 1. Dependence of shear strength on the concentration of humate solutions. External pressure on the specimen: 0.102 kg/cm². Curves 1, 2, 3, and 4 correspond respectively to moisture contents of 10, 15, 20, and 25%.

At a pressure on the specimen of 0.204 kg/cm² and in its absence, the curves are analogous to that shown in Fig. 1. The difference consists only in the quantitative expression of the shear strength.

In the course of granulation the material is subjected to dynamic actions. The influence of surfactant additives on the compactability of marshalite under dynamic actions was studied using a Copra drop hammer as the compacting apparatus. Marshalite was wetted to the prescribed moisture content, thoroughly mixed, and poured into a mold. The charge was compacted in a mold with a cross-sectional area of 19.625 cm² in a layer 12 cm thick. Compaction was carried out with a weight of 6.370 kg, freely falling a specified number of times from a height of 5 cm. After compaction, the marshalite briquette was removed from the mold, its height was measured, and its density was determined.

The results are summarized in Table 1.

Table 1

Results of compacting marshalite using sodium humates

When determining the shear strength of loose materials, an increase in the external pressure on the specimen entails an increase in shear strength. In the present case, the latter increases both with water and with sodium humates (Fig. 1). Wetting marshalite with weak humate solutions (concentrations of 0.1% and 0.90%) contributes to a certain increase in shear strength. This is explained by the greater cohesion of particles wetted with humates as a result of their somewhat increased viscosity in comparison with water. At concentrations above 0.90%, the shear strength decreases, since at this content the plasticizing, lubricating action of the humates is already manifested. The mixture becomes more mobile and is more readily compacted, which, evidently, will occur most effectively at a humate concentration of 2.03%, since the shear strength reaches its minimum value (Fig. 1). A further increase in the humate content (3.51%) promotes an increase in shear strength. The results of compacting marshalite on a Copra apparatus (Table 1), although indirectly, indicate an increase in the mobility of the mixture wetted with humates. Moreover, the maximum density is achieved with humate solutions at a concentration of 1.17%. In this case the mechanical action on the particles is greater than in the shear-strength determination experiments; therefore, the plasticizing effect of the humates is manifested at a lower concentration. The use of solutions of higher concentration causes loosening of the wetted mixture.

Conclusions

1. The cohesion of marshalite particles wetted with humates increases up to a certain content of them in the mixture; above this content their lubricating, plasticizing action is manifested.

2. When high pressures are applied, the plasticizing action of sodium humates is manifested at a lower content of them.