Silicosis Hazard of Rocks and Mine Dust in Kuzbass

Kh. A. Baev

The problem of silicosis is undoubtedly one of the most important and at the same time difficult scientific and practical problems in protecting the health of workers at mining enterprises. In a number of basins, silicosis is observed among workers in the faces of coal mines. According to medical data, the driving of rock workings and the construction of new mines in the coal districts of the Donbass, the Urals, and others are accompanied by a considerable danger of silicosis. This is explained by the fact that the faces pass through rocks containing free silicon dioxide.

The concept of silicosis as an occupational disease has its roots in deep antiquity. New technical methods of underground mining, in particular the introduction of pneumatic drilling, under the conditions of the capitalist system caused mass disease among miners. Thus, for example, among the miners of South Wales alone there were, in 1947, 10,500 persons ill with silicosis. The number of miners in the Ruhr suffering from silicosis, identified during a medical examination in 1949, exceeded 20,000 persons. Among miners examined for the first time, those ill with silicosis at various stages amounted to 56.6%; among those examined repeatedly, the number of the ill rose to 87.15%. Recently in the Ruhr more than 1,000 persons have died annually from silicosis, which is five times greater than the annual number killed by all kinds of occupational injuries [4].

Long neglect of questions of dust control, as well as hunger and poverty, intensified as a result of the Americans’ management in this largest hard-coal basin of Western Europe, created especially favorable conditions for the mass development of silicosis. Mortality from silicosis is also increasing in the USA, England, and France [1].

In the Soviet Union, thanks to the unremitting care of the Communist Party and the Soviet Government for the health and welfare of the Soviet people, significant successes have been achieved in preventing and reducing the incidence of silicosis.

On the basis of Order of the Minister No. 132 of February 23, 1952, work was begun on the study of the silicosis hazard of rocks of coal basins throughout the country. By the same decree, in 1953, work was begun on the investigation of the silicosis hazard of the rocks of the Kuznetsk Basin.

According to Directive Instruction of the Ministry No. D-176 of February 23, 1953, rocks are considered silicosis-hazardous if they

contain more than 10% free silicon dioxide. In this connection it was recommended to be guided by the “Temporary Instruction,” compiled by the late Academician D. S. Belyankin and approved by the commission of the USSR Academy of Sciences on combating silicosis, according to which the content of free silicon dioxide was to be determined by the petrographic method in thin sections under a microscope, or in powder by the immersion method.

However, as work experience has shown, the study of the content of free silicon dioxide in thin sections cannot be applied to the investigation of all rocks; for the finest-grained varieties the method proved to be altogether unacceptable. Therefore, in the petrographic laboratory of the trust “Kuzbassuglegeologiya” a number of research works were carried out to develop a more general method for determining the content of free silicon dioxide.

On the basis of the principle of the average chemical composition of aluminosilicates in sedimentary rocks, we developed a procedure for recalculating chemical analyses into mineralogical composition and, in particular, derived the following formula for determining the content of free silicon dioxide:

\[ Q = \mathrm{SiO}_2 - 1.5\,\mathrm{Al}_2\mathrm{O}_3 - \mathrm{R}_2\mathrm{O}, \tag{1} \]

where
\(Q\) is the content of free silicon dioxide in the rock,
\(\mathrm{SiO}_2;\ \mathrm{Al}_2\mathrm{O}_3\) are the oxides according to chemical analysis,
\(\mathrm{R}_2\mathrm{O}\) are the alkalis \((\mathrm{Na}_2\mathrm{O}\ \text{and}\ \mathrm{K}_2\mathrm{O})\).

Study of the content of free silicon dioxide using the calculation formula, as well as the petrographic method (in coarse-grained varieties), showed that all rocks of Kuzbass, with the exception of siderite interbeds and strongly carbonaceous argillite, which occur rarely in the sequence, contain from 25 to 50% chemically free silica and, according to the established standard, are silicosis-hazardous.

A definite dependence is observed between the content of free silicon dioxide and the granulometric composition of the rock—the content of free silica decreases from sandstones to argillites. This dependence is illustrated in Figure 1, where along the abscissa are plotted the median particle sizes of the rocks according to the decimal scheme on a logarithmic scale, and along the ordinate the percentage contents of free silicon dioxide.

The curve of average contents obeys a certain power function and is expressed by the following empirical formula:

\[ Q = 29.36 + 47.9 \cdot 3.73^{\lg Md}, \tag{2} \]

where
\(Q\) is the content of free silica,
\(Md\) is the median particle size of the rock.

The curve and its formula express a natural law of distribution of detrital quartz and all its modifications in different rock varieties during the process of sedimentation. Curves of this type, expressing average statistical (probable) dependences, are known in the literature as distribution or dispersion curves.

At present we have studied a large number (about 6000) of samples, for which curves of the same type have been constructed for various districts. They differ very little from the one given above and are expressed by analogous formulas.

Right vertical axis: Content of free silicon dioxide

Bottom scale, mm: 0.00025; 0.0005; 0.001; 0.002; 0.004; 0.008; 0.016; 0.032; 0.064; 0.128; 0.256; 0.512; 1.0 mm

Rock-size categories along the bottom:

• Argillites: fine, medium, coarse
• Siltstones: fine, small, medium, coarse
• Sandstones: fine, small, medium, coarse

Circled sample counts shown in the graph: 15, 11, 10, 26, 24, 53, 24, 25, 9.

Fig. 1. Content of free silicon dioxide in rocks of the Balakhon Series of the Prokopyevsk district (the figures in circles indicate the number of samples studied for each rock).

The deviations of the content of free silicon dioxide from the mean value are not especially large. As can be seen from the graph, about 75% of the values (points) lie within ±6%. It should be noted that the scatter of the points from the mean curve is to a considerable extent due to errors in determining the content of free silica, which may also reach as much as 6% absolute. Therefore additional studies are required for an accurate determination of the coefficient of variation and, consequently, of the homogeneity of the mineralogical composition of the rocks investigated in Kuzbass.

As further studies of silicosis hazard have shown, the content of free silicon dioxide in rock cannot serve as the sole factor in the question of silicosis morbidity. It is well known that silicosis and other fibrotic lung diseases arise primarily from the presence of quartz particles in mine dust. N. A. Vigdorchik indicates the following factors favoring the development of silicosis [2, p. 106].

“1. The concentration of quartz dust in the air of the workplace: silicosis develops the more often, the earlier, and the more severely, the greater the amount of inhaled quartz (emphasis ours).

1. The dispersion of the dust, the average sizes of the dust particles. The increase in the hazard of silicosis with a decrease in the caliber of dust particles is a generally known fact, repeatedly proved, and at present causing no doubts.

2. At one and the same concentration of quartz dust and with identical dispersion, it is of importance whether it enters the lungs in pure form or accompanied by impurities.”

As mentioned above, the silicosis hazard of rocks is established according to the content of free silicon dioxide, after which the silicosis hazard of faces and workings is established. Such an approach to determining the silicosis hazard of drivage workings is not entirely correct—the content of free silicon dioxide in the rock does not correspond to the same content of quartz in the dust. This can be shown by the following simple example: when drilling sandstones composed of grains larger than 0.1 mm, with the formation of dust of 0.001 mm size, almost all mineral grains turn into monomineral dust particles; drilling argillites composed of grains smaller than 0.001 mm, with the formation of dust of the same size, will not yield monomineral dust particles—each dust particle will consist of several (two or more) minerals. Such particles will essentially be microfragments of argillite.

Dust consisting entirely of microfragments of rock we call rock dust; dust consisting of monomineral particles, mineral dust.

We conceive the process of formation of these varieties of dust as follows: the content of mineral dust in a face will be the higher (and that of rock dust the lower), the relatively coarser the mineral grains composing the rock from which this dust was formed. Conversely, as the grain of the rock decreases to the size of the dust and smaller, drilling will produce from it predominantly rock dust, i.e., polymineral dust. The scheme of formation of the two mentioned types of dust is clearly presented in Figure 2.

We have theoretically derived formulas for determining the content of quartz dust with different mechanical compositions of the rock and the dust.

For dust homogeneous in size (or of average size) and for the mechanical composition of the rock, the following formula has been proposed:

\[ P = Q \left[ \alpha_1 \left( 1 - \frac{d}{D_1} \right) + \alpha_2 \left( 1 - \frac{d}{D_2} \right) + \ldots + \alpha_m \left( 1 - \frac{d}{D_m} \right) \right], \tag{3} \]

where: \(P\) is the quartz content in mine dust, in percent,

\(Q\) is the content of free silicon dioxide in the rock, in percent,

\(\alpha_1\) is the content of fractions of the mechanical composition of the rock, in fractions of unity,

\(d\) is the size of dust particles in \(\mu\),

\(D_1\) is the size of the rock grains of the corresponding fraction, in \(\mu\).

Fig. 2. Schematic representation of the formation of monomineral and polymineral dust particles depending on the granulometric composition of the rock. (\(D\) is the diameter of mineral grains, \(d\) is the diameter of the resulting airborne dust particle.)

For calculation by this formula it is necessary to know the mechanical composition of the rock. Determination of the mechanical composition of rocks was carried out by petrographic study in thin sections according to the method developed by us and set forth in the report of the petrographic laboratory of the Kuzbassuglegeologiya trust. For rocks of the Balakhon suite of the Prokopyevsk–Kiselevsk district, the mechanical composition of the average varieties (according to the study of 272 samples) is given in Table 1.

Using the data obtained, with the aid of formula 3 we calculated the contents of quartz dust for various rock varieties and dust sizes. The resulting dependence of the quartz-dust content on the mechanical composition of the rocks and on dust size is illustrated in Figure 3. Along the abscissa axis is plotted the classification of sedimentary rocks; along the ordinate axis, the percentage contents of free silicon dioxide in the rocks and of the quartz dust formed from these rocks.

As can be seen from the graph, the quartz content in mine dust in ob-

Table 1

Mechanical composition of rocks of the Balakhon Series in the Prokopyevsk–Kiselevsk district

Fig. 3. Dependence of the quartz-dust content on the mechanical composition of the rock and the fineness of the dust.

Visible graph labels:

• Vertical axis: Content of free silicon dioxide, %
• Horizontal axis: particle size, mm
• Size fractions indicated along the horizontal axis: clay suspensions; medium and coarse fractions; fine dusty; medium dusty; coarse dusty; fine sandy; medium sandy.
• I — distribution curve of free silicon dioxide in rocks.
• II — curve of quartz content in mine dust.

than much less (depending on the grain size of the rock and dust particles) than the content of free silicon dioxide in the rock.

For experimental confirmation of the dependence obtained, the following special investigations were carried out. Drill cuttings were selected from six varieties of rocks of the Balakhon series of the Prokopyevsk–Kiselevsk district during the drilling of shot holes from the faces of the Stalin and 3–3-bis mines of the “Stalinugol” trust. The latter were divided into eight fractions of different sizes (0.25–0.1; 0.1–0.05; 0.05–0.025; 0.025–0.01; 0.01–0.005; 0.005–0.001 and less than 0.001 mm). In all, 48 fractions were thus obtained, which were studied by the immersion method for the content of monomineral quartz. The determinations were carried out by the staff members of the petrographic laboratory, N. N. Ruzanova, with consultation from Associate Professor Yu. P. Kazanskii of the Tomsk Polytechnic Institute.

In parallel with this, monomineral quartz was determined by the calculation method according to formula 3, for comparison with the data obtained by means of the immersion investigation. The determination of the content of free silicon dioxide in the rocks selected together with the drill cuttings was carried out according to formula 1.

Chemical analysis of the rocks was carried out in the chemical laboratory of the “Kuzbassuglegeologiya” trust. Along with this, chemical analyses of the dust fractions were made and the contents of free silicon dioxide in them were calculated. These data are given in Table 2 and in Figure 4.

As is evident from the table and Figure 4, the content of free silicon dioxide in the dust fractions first increases and then decreases. This is of special interest. It may be noted that the content of free silicon dioxide in the fractions and in the rock is the same in the case when the drill cuttings are coarser than the grains composing the rock. This is especially clearly seen in argillite. The maximum enrichment occurs at the moment when the dust fraction and the average grain size of the rock coincide, after which gradual depletion occurs.

This phenomenon should be explained by the fact that quartz, as the strongest mineral, is less subject to crushing during drilling, as a result of which the coarse fractions are enriched with quartz, while the fine fractions are depleted of it. The average content in all fractions remains constant, corresponding to its content in the rock. The enrichment process begins at the moment when the crushing of individual mineral grains begins; crushing to the state of fragments, of course, does not change the distribution of quartz among the fractions.

This circumstance provided grounds for refining the basic formula for calculating the content of quartz dust, since the theoretical derivation of the formula was based on the assumption that all grains of the rock have the same strength and that crushing occurs to an equal degree in the same way.

Allowance for the factor of different crushability of grains in the drilling process is theoretically difficult to implement.

To determine the content of quartz dust with account taken of the latter factor, instead of the content of free silicon dioxide in formula (3), one should substitute the value of the content of free silicon dioxide in the fraction.

\[ P = q \left[ \alpha_1 \left( 1 - \frac{d}{D_1} \right) + \alpha_2 \left( 1 - \frac{d}{D_2} \right) + \ldots + \alpha_m \left( 1 - \frac{d}{D_m} \right) \right], \tag{4} \]

where \(q\) is the content of free silicon dioxide in the fraction of the corresponding size.

Table 2

Content of free silicon dioxide in fractions of drill cuttings, % by weight

![Graph showing free silicon dioxide content versus dust fraction particle size for various rocks. The vertical axis is “Content of free silicon dioxide in percent,” and the horizontal axis is “Particle size (logarithmic scale),” in mm. Curves are labeled 1-п, 3-п, 5-п, 6-п, 7-п, and 9-п.]

Fig. 4. Content of free silicon dioxide as a function of the coarseness of dust fractions in various rocks.

For calculation by this formula, we determined the mechanical composition of rock samples in thin sections under the microscope (these data are given in Table 3).

Table 3

Mechanical composition of rock samples

For the final calculation by formula (4), it is also necessary to take into account the content in the rock of rock fragments having a microcrystalline structure and containing free silicon dioxide—these will mainly be effusive rock fragments. The latter are usually the largest fragmental grains of the rock, which are readily counted in the thin section. Table 4 gives the mechanical composition of these rocks without fragments of effusives.

Table 4

Mechanical composition of rock samples without fragments of effusives

Siltstones and argillites contain almost no fragments of rocks, and therefore they were not counted.

According to the data of Tables 2, 4 and formula (4), the content of quartz dust was calculated for various fractions of the indicated rocks. In Table 5,

data are presented on the content of quartz dust, determined by the immersion method (in the numerator), in comparison with data calculated by the computational method.

As can be seen from Table 5, the results obtained by different methods show an identically consistent dependence on the variety of rock and on the size of the dust fractions. The discrepancies in some determinations are rather large. Here one must take into account the error of each method of investigation.

The immersion method gives relatively good results in the study of individual and pure mineral particles.

In order to compare the errors of each method for studying fractions of drilling dust, we shall indicate several factors affecting the accuracy of the investigation.

1. Fractions consisting of mono- and polymineral particles are subjected to study by the immersion method. Among polymineral particles, particles are often observed that consist of only two or three minerals cemented together by the cement of the rock. We agreed to assign to the group of quartz dust particles consisting of more than half quartz, since such particles may still be considered silicotic-hazardous; particles consisting of less than half quartz, and all polymineral particles, were assigned to the group of rock dust. The precise assignment of dust particles to one group or another is greatly complicated by the fact that the particles are contaminated by products of feldspar decomposition (sericite, kaolinite, etc.).

2. To count quartz particles by the Becke line, immersion liquids are used with refractive indices corresponding to the mean refractive indices of quartz, \(N_o = 1.54445\) and \(N_e = 1.55354\) (7). Counting is first carried out in an immersion liquid with refractive index 1.544, after which the liquid should be replaced by one with refractive index 1.553. In practice this is a difficult operation, especially for fine fractions. Therefore the quartz count was made in two preparations with immersion liquids having refractive indices of 1.544 and 1.553, which, of course, is reflected to some extent in the accuracy of the count.

3. The accuracy of immersion investigation decreases with decreasing fraction size. In practice it is considered possible to study well fractions down to 0.01 mm. Finer fractions are very difficult to study.

4. In immersion investigation, subjective errors occur.

The accuracy of the computational determination of the quartz-dust content is affected by errors in the mechanical analysis of the rock. To obtain some idea of the magnitude of these errors, we present the results of calculations by the formula:

\[ \lambda = \alpha_1\left(1 - \frac{d}{D_1}\right) + \alpha_2\left(1 - \frac{d}{D_2}\right) + \ldots + \alpha_m\left(1 - \frac{d}{D_m}\right). \tag{5} \]

Formula 5 is part of formula (3), with the same notation. \(\lambda\) is called by us the mineralogical-mechanical coefficient and expresses the dependence of the mineral-dust content on the mechanical composition of the rock.

Using the data on the mechanical composition of the rocks in Table 1 and formula (5), the data of Table 6 were calculated for different dust sizes.

Table 3

Mineralogical-mechanical coefficients of dust as applied to the Balakhon Series of the Prokopyevsk–Kiselevsk district

Let us suppose that, in studying the mechanical composition of a rock, the maximum error is made, i.e., instead of the mechanical composition of some variety, the mechanical composition of a neighboring variety is obtained. What difference in the results of determining quartz dust is obtained in this case? Examining the mineralogical–mechanical coefficients of any two neighboring varieties of rock, we note that the difference between them is not especially large and depends on the rock variety and the size of the dust. On average we shall take this discrepancy as equal to 20% relative. Then the difference in determining quartz dust will be less than 10%, since the rocks of Kuzbass contain less than 50% free silicon dioxide.

The error of the calculated determination is due to errors in the chemical analyses and in the calculations, according to formula (1), of free silicon dioxide. The maximum magnitude of these errors, as established by special studies, is about 10% relative. The accuracy of the calculated determination is affected by errors in the calculation formulas themselves.

The total error of the study is equal to the algebraic sum of the partial errors. Therefore, when studying individual samples, large errors are possible, since there is a probability that errors of the same sign will be summed. In mass analysis, virtually all random errors compensate one another.

In order to draw a conclusion regarding the preference of the methods (immersion and calculation) of investigation, the quantitative material is insufficient. In view of the fact that the calculation method makes it possible to obtain data for a wider range of dust fractions, and also yields more regular graphical constructions, we use the data of the calculated determinations. On the basis of these data the graph shown in Figure 5 was constructed (the data for fine sandstone No. p-1 were not used, as deviating in three points from the regularities common to the other samples).

The curves for the content of quartz dust are, in the main, subject to the same relationship as in Figure 3, but they are additionally affected by the different crushability of the rock grains in the drilling process. In connection with this, the silicosis hazard of the dust is reduced to an even greater degree, especially for the fine fractions. Airborne dust consists mainly of fractions smaller than 10 (0.01 mm), and, as is evident from the graph, these fractions (0.01—0.005; 0.005—0.001 and less than 0.001 mm) contain the smallest amount of quartz dust.

In light of the data obtained, the relatively low incidence of silicosis in coal deposits becomes understandable, since their rocks contain an insignificant amount of free silicon dioxide, in comparison with the incidence of silicosis at ore enterprises, where free silicon dioxide in the rocks occurs in the form of thick veinlets of quartz. When such rocks are drilled, almost all the quartz is converted into monomineral quartz dust.

Mine dust from sedimentary rocks is less hazardous with respect to silicosis also because, along with quartz dust, it contains a considerable amount (depending on the rock variety and the size of the dust) of rock dust. Any admixture to quartz dust, as N. A. Vigdorchik notes, reduces the silicosis hazard of quartz dust.

Questions concerning the influence of admixtures and the dispersion of dust have been considered by us in a special article; therefore we shall not dwell on these questions here.

Fig. 5.

Fig. 5. Content of quartz dust as a function of the mechanical composition of the rocks and the coarseness of the dust fractions.

I — distribution curve of free silicon dioxide in the rocks; II — curves of the content of quartz dust.

Conclusions

1. The method for determining the content of free silicon dioxide by means of chemical analyses and formula (1) makes it possible to study rocks of any lithological structure, from sandstones to argillites, which is a major advantage over petrographic, X-ray, and thermometric methods of investigation. According to special studies, the error of the method does not exceed 10% relative.

2. Investigation of the rocks of Kuzbass by this method has shown that they contain from 25 to 50% free silicon dioxide and, according to the standard adopted by the Ministry of the Coal Industry, are silicosis-hazardous.

3. Investigation of mine dust from rocks shows that it contains less free silicon dioxide and considerably less quartz dust. Argillites form dust containing less than 10% quartz dust, and according to the standard they may be assigned to rocks that are not silicosis-hazardous.

4. The method of determining silicosis hazard by the standard for the content of free silicon dioxide in the rock is incorrect, and by the content of quartz in the dust is imprecise. Silicosis hazard should be determined by permissible concentrations under a specified standard for the quartz content in mine dust.

Literature

1. Academy of Sciences of the USSR. The Fight against Silicosis. Collection of articles. Publishing House of the Academy of Sciences of the USSR, 1953.

2. Vigdorchik N. A. The Doctrine of Silicosis. Medgiz, 1954.

3. Preobrazhenskii I. A., Sarkisyan S. G. Minerals of Sedimentary Rocks. Gostoptekhizdat, 1954.

4. Karl Schulte. Aus der Arbeit der Leutschen Kohlenbergbauleitung auf dem Gebiet der Staub— und Silikosebekämpfund. Glückauf, No. 41—42, 967—975.

Trust “Kuzbassuglegeologiya”