SOME RESULTS OF STUDIES ON MOISTURE-RESISTANT INERT DUST

Z. M. Guseva

The State of the Question of the Moisture Resistance of Inert Dust

The moisture resistance of inert dust is of great importance for coal mines. Ordinary inert dust made from limestone is characterized by considerable hygroscopicity; as a result, it quickly cakes and loses its basic property of remaining for a long time in a suspended state. This leads to the need to replace the dust in barriers frequently—almost every month. According to practical data from a large mine, the consumption of inert dust for shale barriers reaches 800–1000 tons per year. The cost of transporting the dust and the labor for filling the barriers is very considerable—almost 30 rubles per 1 ton.

In order to reduce expenditures, moisture-resistant inert dust is used in mines in the USA, England [1], and Poland [6]. To impart moisture resistance to the dust, paraffin and stearin are used in amounts of 0.25–0.30% of the total weight of the dust.

Preparation and Study of the Properties of Moisture-Resistant Inert Dust

Moisture-resistant inert dust was obtained by the author under laboratory conditions by joint grinding in ball mills of limestone with hydrophobizing (water-repellent) additives.

In 1955, moisture-resistant inert dust was obtained by treating limestone (with an insignificant quartz content) with mylonaphth during grinding in a ball mill.

At the beginning of 1956, moisture-resistant inert dust was obtained:

a) by treating limestone (with an insignificant quartz content) with rosin;

b) by treating limestone with rosin and oleic acid (with a considerable quartz content).

In the process of joint grinding of limestone with hydrophobizing additives, an adsorption film forms on the limestone particles, which prevents wetting [2].

Moisture-resistant inert dust obtained in 1955 by treating limestone with mylonaphth in an amount of 0.2% of the weight of the limestone was previously studied for moisture absorption [3].

The dust samples during the indicated period of time were in an atmo-

...sphere with 90% humidity and absorbed only 0.4% water, calculated from the initial weight of the dust.

More detailed studies were carried out by the author in 1956. Experiments to determine the degree of moistening of moisture-resistant and ordinary (for comparison) inert dust in the laboratory were conducted along the following lines: 1) moistening of the dust as a function of time was determined; 2) moistening was determined and caking of the dust as a function of time was observed on the shelf of a barrier; 3) moistening of the dust by layers as a function of time was determined.

To study the degree of moistening as a function of time, ordinary (untreated) and moisture-resistant dust with a fineness passing completely through sieve No. 75, and with an initial moisture content of: moisture-resistant—0.14%, ordinary—0.89%, was poured into glass test tubes, 1 g in each. The test tubes with weighed portions were placed in a desiccator above water at a distance of 5 mm from it. The desiccator was hermetically sealed; the relative humidity in it was maintained at 100%. Every month over the course of six months the test tubes were weighed and the increase in moisture was determined as a function of time.

Table 1

Moistening of inert dust as a function of time

Fig. 1. Dynamics of dust moistening as a function of time

The results of the studies characterizing the moistening of ordinary and moisture-resistant dust as a function of time are given in Table 1 and presented in the form of curves (Fig. 1).

The nature of the moisture-absorption curves shows that moistening of inert dust takes place over a prolonged period of time. Moisture-resistant dust becomes moistened uniformly over time; no caking is observed, and the dust remains free-flowing. Ordinary dust increased its moisture content by 2.13% over the course of a month. Subsequently its moistening proceeds more uniformly. This is explained by the fact that, up to the moment of caking, the dust more rapidly absorbs moisture from the surrounding air; then it cakes and becomes compacted, and as a result its absorption of moisture proceeds more slowly.

Over 6 months the moisture-resistant dust absorbed 1.86% moisture, while ordinary dust absorbed 6.71% over the same time. Consequently, the moisture-resistant dust became moistened almost 4 times less than the ordinary dust.

The lesser moistening of the moisture-resistant dust is explained by the fact that its particles have a monomolecular coating that hinders wetting, although this coating is not continuous: it contains microcracks [4].

The degree of moistening of ordinary and moisture-resistant dust on the shelf of the barrier was determined by weighing samples on an analytical balance before drying and after drying of the sample, while the nature of caking was determined by external appearance. For this purpose inert dust was poured onto a model shelf of a barrier 250 mm wide and 40 mm long. The height of the filling was 86 mm, and the natural angle of repose of the dust on the shelf was 35°. The fineness of the dust met the requirement for industrial use, i.e., it passed completely through sieve No. 12, and 50% through sieve No. 75. The indicated shelf with dust was placed in a special chamber in which a relative humidity of 80–85% was maintained. To determine the moistening of the dust over 6 months, samples were taken each month. The selected samples of ordinary and moisture-resistant dust were dried for two hours in a drying cabinet at \(t = 100\text{–}105^\circ\). The percentage of moisture was determined from the difference in weights before drying and after drying of the sample. The results of the investigations are presented in Fig. 2. Analysis of these investigations shows—

Fig. 2. Moistening of dust on the shelf of a barrier as a function of time

—that the moisture-resistant dust becomes moistened uniformly over the course of 6 months, whereas the ordinary dust does so rapidly during the first month; thereafter its moistening proceeds more uniformly. Over 6 months the moisture-resistant dust absorbed 4 times less moisture than the ordinary dust; moreover, observations of the dust on the shelf of the barrier at a relative air humidity of 80–85% clearly confirm that the absolute moistening of both ordinary and moisture-resistant dust occurs to a lesser degree than the moistening of dust at a relative air humidity close to 100%.

Observations carried out on the dust on the shelf of the barrier confirmed an increase in the caking of the dust with the passage of time. Ordinary

dust, after 1.5 months, caked and compacted. After 6 months the moisture-resistant dust had not caked and remained free-flowing and suitable for further use on the barrier shelf. Only after 12 months was the formation of small lumps of dust observed on its surface. This is explained by the fact that moisture-resistant dust has a monomolecular film which, being located between the dust particles, prevents them from sticking together.

In order to determine the moistening of the dust by layers, observations were made of it in test tubes, which were placed in a desiccator over water. The thickness of the layer in the test tubes was taken to be from 10 to 60 mm. After

Table 2

Moistening of inert dust in two layers as a function of time

Table 3

Moistening of inert dust by layers as a function of time

Continuation of Table 3

Each month over the course of half a year, samples were taken from the test tubes by layers. The moistening of the dust in a layer was determined from the difference in the weights of the weighed portions before and after drying. The results of these observations, given in Tables 2 and 3 and presented in Figs. 3 and 4, show that the moistening of dust in layers depends on time.

Fig. 3. Moistening of dust in layers as a function of time

Moisture-resistant dust is moistened uniformly over the entire period. Ordinary dust absorbs moisture very rapidly in the first month; thereafter its moistening proceeds more slowly. All layers of moisture-resistant dust are moistened almost uniformly; the outer layers of ordinary dust are moistened more. If the upper layer of ordinary dust became moistened by 8.4% in 6 months, the lower layer did so only by 3%. This is explained by the fact that ordinary dust is hygroscopic and in the first month rapidly absorbs moisture, and as a result a compacted layer is formed on the outer layers, which to some extent impedes the penetration of moisture into the deeper layers. However, the inner layers also become sufficiently moistened, since the dust in them cakes and becomes unfit for use.

The author carried out tests of moisture-resistant dust as a means of neutralizing the explosibility of coal dust. In the portable MakNII apparatus [5] for studying the explosibility of coal dust,

ignition was carried out of a mixture of coal dust and moisture-resistant inert dust (obtained by treatment with mylonaphthene, rosin, and oleic acid). As a result it was established that a mixture consisting of 75% moisture-resistant inert dust and 25% coal dust (the coal dust was taken with a volatile-matter content of 35%—the Boltyrevsky seam of the S. M. Kirov mine) does not explode.

Fig. 4. Moistening of dust by layers after 6 months

Conclusions

1. Moisture-resistant inert dust, owing to the formation on its particles of a monomolecular film, is characterized by reduced hygroscopicity.

2. Moistening of moisture-resistant inert dust, in comparison with ordinary dust, occurs 4–10 times less.

3. Despite the moistening of moisture-resistant inert dust, it remains free-flowing for a long time and does not cake, owing to the film, which prevents the dust particles from sticking together.

4. The replacement intervals for moisture-resistant inert dust on shale barriers in workings with a relative humidity of 90–95% can be increased to 4–6 months.

5. In workings far removed from the working face, with a relative humidity of 80–85%, the replacement interval may be increased.

6. The neutralizing action of moisture-resistant inert dust is not reduced by the introduction into it of organic additives. Therefore its standards may be taken the same as for ordinary inert dust.

REFERENCES

1. Bulletin of MakNII, No. 16, 1947.

2. Rebinder, P. A. Research in the Field of Surface Phenomena. ONTI, 1936.

3. Instruction for Testing Inert Dust. MakNII, 1956.

4. Khigerovich, M. I., Gorchakov, G. I., and Deibovich, Kh. M. Hydrophobic Cement and Hydrophobic-Plasticizing Additives in Concretes and Mortars. Promstroiizdat, 1953.

5. Kravets, V. I. Inert Dusts and Shale-Dusting of Mine Workings. State Technical Publishing House of Ukraine, 1951.

6. Pył kamienny wodoodporny do opylania wyrobisk, “Przegląd gorny,” No. 3, 1953.