The influence of the mine dust mineral composition on the characteristics of thermal destruction processes

The article describes the results of comprehensive laboratory tests of mine dust samples taken from active mine faces in the Kuznetsk coal basin (Kemerovo region, Russia). The purpose of the experiments is to study the characteristics of thermal destruction processes in mine dust depending on its mineral composition. Based on the results of electron microscopy, the mineral composition of the dust samples under study was determined, and the quantitative relationships of the mineral phases composing the sample were assessed. Using thermogravimetric analysis the lower limits of thermal destruction intervals were established for samples of mine dust with different compositions and percentages of mineral inclusions. The practical significance of the results obtained is substantiated.


Introduction
Changes in the conditions of underground coal mining, associated with the emergence of complicating mining and geological factors, as well as with the intensification of mining operations due to the use of modern high-performance equipment, are accompanied by an increase in various natural and manmade dynamic manifestations that negatively affect production safety [1,2].One of these hazards, characteristic of coal mining enterprises, is thermal processes that arise under certain conditions in mine dust and lead to fires in mine workings and goafs [3][4][5].Since mine dust in mine workings includes both coal particles and particles of host rocks, the risk of a thermal reaction is also determined by its mineral composition, or rather the percentage of coal and mineral inclusions.
This article presents the results of comprehensive laboratory tests demonstrating the need to take into account the influence of the mineral composition of mine dust on the nature of thermal processes occurring in it when developing measures to improve the level of industrial safety of coal enterprises.

Research methods and results
Mine dust samples intended for laboratory testing were collected using standard methods at the operating mines of JSC SUEK -Kuzbass.The numbering of samples and the location of their collection (name of the mine and coal seam) are given in Table 1.
The laboratory tests included analytical scanning electron microscopy and thermogravimetric analysis.

Electron microscopy
The experiments of the first stage of the research were carried out to identify the presence of mineral inclusions and estimate their percentage content in the studied samples of mine dust.The composition of rock minerals was studied on an analytical scanning electron microscope Leo1420 VP with an INCA 350 microanalysis system.A comparative assessment of the content of host rock components in samples was carried out using ASEM data based on quantitative analysis of images in back-scattered electrons using the threshold segmentation method using the Gwyddion modular software package [6].
For each sample, 5 areas measuring 408 µm by 206 µm were analyzed.An example of the result of scanning one of the visual fields of sample No. 1 is presented in the form of a photograph in Figure 1a.
Figure 1b shows a photo after segmentation with discriminated host rock mineral inclusions.Using the analytical scanning electron microscopy research, data on the mineral composition of the samples was also obtained (microanalysis with the INCA 350 system).An example of the results of such analysis for sample No. 1 is shown in Figure 2. Table 2 shows the electron microscopy data.It displays the predominant minerals in the studied samples, as well as the value of their percentage content averaged over five measurements.

Thermogravimetric analysis of dust samples
To identify the characteristics of thermal destruction processes in the studied samples of mine dust, the method of thermogravimetric analysis was used [7][8][9][10].The result of thermogravimetric analysis is TG -curves (thermograms) -the dependence of the mass m (or change in mass) of the test sample on temperature or time.Analysis of the so-called DTG curves, which are derivatives of thermograms (rate of mass change), makes it possible to establish the point in time or temperature corresponding to the maximum rate of change in the mass of the sample.
To carry out thermogravimetric studies, dust samples of fraction <50 µm were prepared from each sample, the mineral composition of which was previously determined using analytical scanning electron microscopy.Data on the mass of manufactured samples are presented in Table 3 (the designation of the samples coincides with the previously entered sample numbering).
Table 3. Characteristics of mine dust samples for thermogravimetric analysis.
Sample number 1 2 3 4 5 Sample mass, g 0.23 0.30 0.23 0.29 0.30 Testing of mine dust samples was carried out on a Leco TGA-701 thermogravimetric analyzer in the temperature range from 24 to 900°C.The samples were heated at a constant rate of temperature change without blocking the access of oxygen.The heating rate in the experiments was taken equal to 3°C/min.The specified value was assigned taking into account the analysis of the results of previously performed laboratory tests [11], in which the dependence of the results of TG analysis on the heating rate was studied.The thermograms obtained during the experiments are presented in Figure 3, and their derivatives (DTG curves) in Figure 4.A quantitative assessment of the change in the mass upon heating was made for each sample based on the dependences shown in Figure 3.The obtained values are presented in Table 4. Analyzing the DTG curves, it can be established that their character in the positive region of the coordinate field changes significantly at fairly clearly defined points of a sharp increase in the rate of change of mass, indicated in the graphs of Figure 4 as Tign.Certain Tign values corresponding to the beginning of the thermal destruction interval of dust particles are also presented in Table 4.

Conclusion
The thermal properties of mine dust undergo significant variations when depending of dust composition.Thus, analysis of the data presented in Table 4 shows that sample of mine dust No.  2), it can be noted that with an increase in the content of host rock mineral inclusions in a mine dust sample, the loss of its mass during thermal destruction processes decreases.The results of assessing the lower boundaries of thermal destruction intervals (Tign) for the tested samples of mine dust showed a significant dependence of this parameter on the composition of mineral inclusions.It has been shown that in samples of mine dust containing illite (or illite and calcite), the values of the beginning of the thermal destruction interval Tign lie in the region of higher temperatures, compared with similar values determined for samples containing only calcite.The demonstrated differences in the thermal test data obtained during the described experiments, due to the specific content of mineral inclusions in them, indicate the need to take these features into account when developing measures to improve the level of industrial safety of coal enterprises.

Figure 1 .
Figure 1.a) Image in back-scattered electrons of a fragment of sample No. 1, b) Image of a same fragment after segmentation.

Figure 4 .
Figure 4. Example of DTG curves for two samples No. 2 and No. 4.

3 (
Mine named after V.D. Yalevsky, seam 50) lost 87.3% of its original mass as a result of thermal destruction.Next, in descending order of the values of the indicator under consideration, are dust samples No. 2, No. 1, No. 4 and No. 5. Comparing these data with the results of electron microscopy (Table

Table 1 .
Numbering of samples and places of their collection.

Table 2 .
Characteristics of mine dust samples for thermogravimetric analysis.

Table 4 .
Results of thermogravimetric analysis.