INFLUENCE OF SURFACE FILMS OF HUMIC SUBSTANCES ON THE AGGREGATE PROPERTIES OF QUARTZ SAND

L. F. Karnovich, V. M. Vityugin, S. A. Babenko

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

The processes of mechanical dewatering and pelletizing of natural silicate materials are extremely sensitive to changes in the physicochemical properties of the surface of the grains of these materials.

Quartz sands from many deposits, under natural conditions, are covered with a film of humic substances. The presence of humic films will undoubtedly affect the processes of technological processing of quartz sands, especially at the first stages of processing technology (dewatering, pelletizing). It is therefore of interest to study the nature of the interaction of water with the surface of the grains of such materials.

In the present communication, the influence of films of humic substances on the aggregate properties of quartz sand is analyzed. A sample of Tugan quartz sand containing 0.97% humic substances was used as the object of study.

In terms of granulometric composition, the sand is fine-grained. The yield of the \(+0.2\) mm fraction is 2.48%, of the \(-0.2+0.1\) mm fraction—39.5%, of the \(-0.1+0.05\) mm fraction—50.44%, of the \(-0.05\) mm fraction—5.74%, and of clay—1.84%.

The chemical analysis of the sand is represented by the following oxides:
\(\mathrm{SiO_2}\)—96.88%, \(\mathrm{TiO_2}\)—0.70%, \(\mathrm{Al_2O_3}\)—4.43%, \(\mathrm{Fe_2O_3}\)—0.5%, \(\mathrm{CaO}\)—0.61%, \(\mathrm{MgO}\)—0.39%.

For a comparative assessment of the influence of films of humic substances, part of the sample was thoroughly washed with a 1% solution of caustic soda until it was completely clarified, and then with distilled water until the alkaline medium disappeared. The natural unwashed sand was soaked in water and held for 1 month in order to eliminate hysteresis phenomena during tests of surface properties. The aggregate properties of quartz sand were assessed by determining the maximum molecular moisture capacity by the method of A. F. Lebedev [1], studying the kinetics of saturation of a sand column with water [2], and measuring the density of aggregates obtained by the “drop” method [3], improved in the laboratory of the Tomsk Polytechnic Institute.

The results of measurements by the above-mentioned methods are presented in Table 1 and in Fig. 1.

The film of humic substances promotes considerable hydrophilization, the most complete wetting of microroughnesses and irregularities of the surface of sand grains; by expanding the sphere of action of molecular forces with respect to water, it increases the amount of bound water. The action-

Table 1

Aggregate properties of sand

Consequently, the maximum molecular moisture capacity of quartz sand in the presence of films of humic substances is twice as high as that of washed sand. It is known that the aggregability of powdery hydrophilic materials is determined by their moisture capacity. At the same time, when the maximum molecular moisture capacity of a material approaches in magnitude the moisture content of the aggregate, the maximum strength of these aggregates is attained [4]. Thus, an increase in the maximum molecular moisture-capacity index indicates an improvement in the aggregability of quartz sand with a film of humic substances.

Fig. 1. Dependence of the height of water rise on time: б — initial sand containing a film of humic substances; а — sand treated with a 1% NaOH solution.

The rate of movement of water in a layer of hydrophilic powdery material is inversely dependent on the degree of hydrophilicity. From Fig. 1 it is evident that the presence of films of humic substances on quartz sand markedly reduces the rate of wetting of the sand layer. Thus, to wet a layer of the original sand 5 cm high in the column requires twice as much time as for washed sand, and for a layer 8 cm high—three times as much. Complete saturation of the sand column 10 cm high is recorded, respectively, at 26 min and 4 min 20 sec from the beginning of the experiment. The decrease in the wetting rate of quartz sand in the presence of films of humic substances is explained both by an increase in the forces of molecular interaction between water and the sand with a film of humic substances and, to some extent, by a decrease in pore size as a result of an increase in the thickness of the layer of bound water on the surface of the grains.

As was to be expected, the increase in the forces of molecular interaction between water and sand with a film of humic substances leads

toward an increase in the density of the aggregates. In this case, the relative increase in the density of aggregates forming spontaneously upon contact of water with sand bearing a film of humic substances was 8.66%.

Analyzing the results of the above studies as a whole, one may conclude that the presence of films of humic substances in quartz sands promotes their aggregation and will have a positive effect during sand granulation. Dewatering of sands containing inclusions of humic substances, on the contrary, will be impeded.

REFERENCES

1. A. M. Vasil'ev. Fundamentals of Modern Methods and Techniques for Laboratory Determination of the Physical Properties of Soils. Gosstroiizdat, Moscow, 1953.

2. V. I. Korotich. On the rate of water propagation in a layer of finely ground iron ores and concentrates. Engineering-Physical Journal, No. 7, 1961.

3. A. M. Parfenov. Agglomeration of Iron Ores. Metallurgizdat, Moscow, 1954.

4. L. A. Bernshtein, M. B. Frenkel'. Granulation of Cement Raw Mixes by Dry and Wet Methods of Preparation. Gosstroiizdat, Moscow, 1959.