Effect of Sodium Humates on the Caking Properties of Marshalite

A. F. Karpovich, V. M. Vityugin

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

The process of granule formation during the caking of moistened dust-like materials is determined by a large complex of factors, depending both on the properties of the raw material being caked and on technological parameters. The most important factor in the process is the strength of the structure of the raw granules. Regardless of the mechanism of bonding of grains in granules, the strength of the structure of the latter may be approximately estimated by the following analytical Rumpf equation [1]:

\[ \sigma = \frac{1 - \varepsilon}{\varepsilon}\cdot \frac{3.1}{\pi d^2}\,H, \]

where
\(\sigma\) — strength of the granule structure, kg/cm\(^2\);
\(\varepsilon\) — porosity of the granules in fractions of unity;
\(d\) — diameter of the material grains, cm;
\(H\) — force of bonding of grains in contact, kg.

It follows from the formula that the porosity value is of greatest importance for the strength of granules. Thus, when porosity is decreased by 10% in the interval from 0.4 to 0.3, the strength of granules increases by 55%. Consequently, the compactability of a fine-grained material is a most important characteristic. The degree of compaction can be significantly increased by introducing plasticizers into the material being granulated. One such plasticizer may be an aqueous solution of sodium humate. The introduction of sodium humate into the slurry of a fine-grained material promotes the formation of a coagulation structure characterized by a high degree of deformation under the action of mechanical treatment.

Experimental Part

Marshalite from the Bolotovskoe deposit was used as the object of investigation. The chemical and granulometric compositions of the material are given below.

Chemical composition:

p.p.p. — 0.74%,
SiO\(_2\) — 93.94%,
Al\(_2\)O\(_3\) — 3.01%
Fe\(_2\)O\(_3\) — 0.24%,
CaO — 0.77%,
MgO — 1.63%.

Granulometric composition:

Fraction \(+0.25\) mm — 2.22%,
Fraction \(-0.25 + 0.05\) mm — 5.78%,
Fraction \(-0.05 + 0.01\) mm — 77.2%,
Fraction \(-0.001\) mm — 0.88%,
Fraction \(-0.01 + 0.001\) mm — 3.52%,

Sodium humates were obtained from peat from the Vasugan deposit by treating it at room temperature with a 1% solution of caustic soda. For the work, solutions were prepared with humate concentrations of 0.1%, 1.17%, 2.03%, 3.51%, and 6.42%. Granulation of marshalite was carried out on a laboratory pan granulator with a diameter of 300 mm and a rim height of 50 mm. The inclination angle of the pan was 45°, and the rotation speed was 30 rpm.

Initially, the effect of humate concentration on the density and strength of raw and dried granules was established. The moisture content of the material was 15%, with two thirds of the liquid introduced into the dry material, and one third supplied dropwise during the granulation process. The granulation time was 3 min. From the total mass of granules obtained, the \(-10 + 7\) mm fraction was tested for density and strength. The granules were dried in a drying cabinet for 2 hours at a temperature of \(+105^\circ\mathrm{C}\).

The averaged experimental results are summarized in Table 1.

Table 1

Effect of sodium humates on the mechanical properties of marshalite granules

The introduction of sodium humate into marshalite, as can be seen from Table 1, noticeably plasticizes the mass being granulated and promotes better compactability. At the same time, the strength of the granules increases. The strength of wet granules increases by a relatively small amount. The reason for this lies mainly in the weak cohesion of the coagulation structures. Dried granules, when humates are added, sharply increase their strength. Here the binding properties of humates are manifested, bonding marshalite grains at the points of contact. Obviously, the use of sodium humates may be important as a factor that strengthens chiefly the structure of granules during drying.

The comparatively small increase in the strength of raw granules upon the introduction of humates is significant as a factor that can accelerate the granulation process. To study this effect, a second series of experiments on marshalite granulation was carried out. Marshalite was moistened with water or with a two-percent solution of sodium humate up to

moisture content of 20%. The granulation time was varied from 2 to 10 min. The granulation effect was evaluated from the size distribution of the granules, which is given in Table 2.

As can be seen from Table 2, the introduction of sodium humate sharply intensifies the process of formation and growth of granules. The high plasticizing action of sodium humate solutions is due, on the one hand, to the colloidal properties of humates and, on the other hand, to the high

Table 2

Effect of sodium humates on the rate of pelletizing of marshalite

uniformity of distribution in the material being granulated. This makes it possible to conclude that sodium humates should be used for granulating a wide range of fine-grained materials. For example, it is advisable to use sodium humate solutions instead of bentonite when granulating fine-grained iron-ore concentrates. The advantage of such a substitution lies in the possibility of sharply reducing the consumption of plasticizer while simultaneously improving the quality of the granules. The difficulty of uniformly mixing moist iron-ore concentrates with finely ground bentonite leads to its overconsumption by several times. It has been established experimentally that, as a result of nonuniform mixing, the amount of bentonite added, instead of 0.3%, in practice reaches 1–2%. The equivalent addition of sodium humate will obviously be 0.1–0.2%.

REFERENCES

1. Rumpf. Chemistry and Chemical Technology (Inorganic Chemistry), No. 1, 1959.