ON THE GRAIN-SIZE COMPOSITION AND TECHNICAL VALUE OF FRESHLY DEPOSITED MINE ROCKS ON THE WASTE HEAPS OF THE PROKOPYEVSK DEPOSIT

A. S. BOGMA

(Presented by Prof. I. V. Gebler, Doctor of Technical Sciences, and Docent N. K. Beloglazov)

In underground coal mining, tens of thousands of cubic meters of waste rock are brought to the surface every day. The beneficiation of coals is likewise accompanied by the separation of enormous quantities of waste crushed rock—the tailings of coal beneficiation. Fresh, unburned mine waste rock and coal-beneficiation tailings, being wastes of the coal industry, are almost nowhere used and are transported to waste-rock dumps—slag heaps.

At the Stalin Mine alone in the city of Prokopyevsk (Kuzbass), with daily production of 5.5–6.0 thousand tons of coal (1955), up to 1.5 thousand tons of rock were transported to the slag heaps, i.e., on average 18–25% of total gross output.

As a rule, freshly deposited rock from mines is mixed with coal waste, combustible shales, and other admixtures of organic origin. Coal and admixtures, under the action of atmospheric conditions on the rock, oxidize and ignite spontaneously, thereby contributing in varying degrees to the burning of the entire mass of rock.

Although the uninterrupted operation of a mine inevitably requires that the rock brought to the surface be sent to the slag heaps, the organization and further growth of slag heaps is disadvantageous for mines both from the economic standpoint and from the standpoint of safety engineering, for the following reasons:

1) the dumping of rock onto slag heaps greatly complicates the mine’s surface facilities, cluttering the mine yards;

2) harmful gases released during the burning of the rock, and dust, are constantly carried by the wind from the slag heaps, worsening the living conditions of nearby residential buildings;

3) during the period of their existence, slag heaps do not remain unchanged; for many reasons (gravity, atmospheric agents, burning of the rock) they become deformed, which is outwardly expressed in collapses and ejections (explosions);

4) transporting rock to the slag heaps is the narrowest point and requires, especially in winter, a considerable number of service personnel, and in bad weather may even disrupt the operation of the entire mine;

5) moving the trusses for the tippler, repairing and clearing the tracks, and other operations connected with stockpiling the rock require continuous supervision; moreover, these operations have to be carried out under unfavorable conditions;

6) coal that reaches the waste heaps together with the rock is irretrievably lost.

Attempts to utilize rock dumps have proceeded in two directions. The first of these provides for leaving the rock in the mines and using it as stowing material. Although experiments carried out at a number of mines have yielded positive results, the widespread adoption of this method is hindered by the lumpiness of the rock and by the absence of special means of transport for placing it as fill [1].

The second direction in the utilization of rock dumps is the use of materials from burnt-out waste heaps for construction purposes.

Studies carried out [2] have shown that in most cases the rock consists of clay shales. As a result of firing, kaolinite, which forms the basis of clay shales, is dehydrated and acquires hydraulic activity, i.e., it can enter into compounds with calcium oxide with the formation of amorphous hydrosilicates of various compositions.

The work of a number of researchers (G. N. Sivertsev, L. S. Gubler, and others) and of a number of research organizations (the All-Union Scientific Research Coal Institute—VUGI—and the All-Union Institute for the Organization and Mechanization of Mine Construction—VNIOMShS) in the field of the use of burnt rocks, as well as experience in their industrial application (Donbass) as silicate raw material, has now yielded many valuable substitutes for scarce construction materials, including cement. Although burnt rocks are additional resources for obtaining construction materials, the actual use of burnt rocks in construction is still small, mainly because of their low frost resistance, owing to their great heterogeneity. It has been established that, depending not only on the nature and type of the rock and on the presence of carbonaceous and sulfur-bearing substances, but also on the method of formation of the waste heap itself and especially on the firing process, materials with very different physicomechanical properties are obtained.

Despite the high temperatures (up to 1200°C) that develop during the burning of the rocks of mine waste heaps, the degree of their firing is far from uniform, owing to the uneven intensity of burning in different places of the burning layer [2].

Consequently, along with well-fired rock, the waste heaps contain a certain amount of poorly fired and unfired rock, inclusions of organic origin, and fines (dust).

All the impurities listed, as well as unquenched particles of calcium and magnesium oxides formed during the firing of rock from limestones and dolomites, not only reduce the strength of products made on the basis of burnt rocks, but especially reduce their frost resistance and weather resistance.

Thus, the shortcomings noted entail many additional costs and require strict control over the purity of separation of unburnt and weakly fired particles when using burnt rocks.

The way out of the situation that has arisen is evidently not the transport of freshly extracted rock from the mine to the waste heaps, but its artificial firing in special furnaces.

As a result, the probability of more uniform firing of the particles increases considerably in comparison with the natural firing of rock in waste heaps, and the firing process itself can be regulated. A rather important point is that the combustible mass contained in the rocks, in an amount sufficient to ensure self-firing, will be used much more rationally and completely.

The first experiments in this direction on the use of freshly deposited rock in brick production proved an indisputable economic and technical advantage \[3, 4\].

A more confident solution of this problem can be achieved only with proper testing of specific freshly deposited mine rocks. The study of mine rocks in freshly deposited form in Kuzbass has been carried out hardly at all.

In the present work, rocks in fresh fill at the Stalin mine of the Prokopyevsk mine field were investigated.

Samples of rocks in the fresh fill were taken by staff members of the Department of Coal Preparation and Safety Engineering and Mine Ventilation of the Tomsk Polytechnic Institute, with the participation of representatives of the mine, in 1954 on the southern waste heap of the Stalin mine \[5\].

Sampling was carried out in the face part of the waste heap along the lines of movement of the fresh rock fill directed by chutes 30–40 m long. Samples were taken from the top every 20 m along the generatrix of the waste heap to the upper boundary of the haulage track and weighed 22–32 kg each.

Of the 15 samples taken in 1954, three were collected in the direction of the axis of the waste heap and were significantly oxidized. The remaining 12 samples in the fresh fill were subjected to screening into the following classes: \(+25\) mm; \(-25+13\) mm; \(-13+6\) mm; \(-6+3\) mm and \(-3+0\) mm. The \(+25\) mm class was divided into visible coal and rock. The average results of the particle-size composition for the northern and southern slopes of the waste heap are given in Table 1.

Table 1

Results of particle-size analysis of samples of mine rocks in fresh fill

To obtain an idea of the composition and technical value of the rocks in the fresh fill, the samples were analyzed for moisture content, yield of total sulfur, carbon dioxide, yield of volatile matter, and calorific value. The analyses were carried out in accordance with established GOST standards; the results of the analyses performed are given in Table 2. The results of technical analysis of 72 samples showed the following.

1. The ash content in the mine rocks noticeably increases from the fine classes to the coarse ones. The increase in ash content of the coarse classes (\(+25\) mm), in comparison with the ash content (on a dry basis) of the \(-3+0\) mm class, for all samples amounts to 43.19–49.78%.

2. The content of total sulfur (on a dry basis) in the rocks does not exceed 0.40%. The maximum sulfur content in the fine classes is 0.17–

Table 2

Results of the technical analysis of samples of mine rocks in a fresh embankment

0.36%; with increasing particle size the sulfur content decreases noticeably, and the minimum amount is found in the \(+25\) mm class (0.01–0.17%).

1. The carbon dioxide content in mine rocks is insignificant and amounts to 0.19–1.47%, which indicates a low carbonate content in the rocks.

2. The yield of volatile substances (\(V^a\)), amounting to 11.88–15.53% for the \(-3+0\) mm class, decreases noticeably with increasing particle size and is 6.05–8.91% for the \(+25\) mm classes.

3. The calorific value by bomb calorimeter is 4823–5143 kcal/kg for the 0–3 mm class and decreases noticeably with increasing particle size. The minimum calorific value is observed in the \(+25\) mm classes and ranges from 522 to 1462 kcal/kg.

Conclusions

1. Along with rock, a noticeable amount of coal is sent to the waste heap. In the \(+25\) mm class the average coal content (by weight) ranges from 4.0 to 6%, and in individual cases reaches 13%.

2. In most cases the predominant classes in freshly deposited rocks on waste heaps are the fine classes (from 0 to 13 mm), whose yield averages 60–65%.

3. In view of its relatively high calorific value and large content of combustible matter, freshly deposited mine rock (classes finer than 13 mm) is not inferior to average technical fuel; therefore it is inadvisable to use it as stowing material.

4. The most effective way to utilize mine rock is evidently to use it for construction purposes, making use of the large reserve of thermal energy contained in freshly deposited mine rocks not only for firing these rocks, but also for firing limestone.

Literature

1. Lukyanov P. F. Use of rock from mine workings for stowing and underground crushing installations. Kuzbass Publishing House, 1951.

2. Grigoryev G. V. On the use of rocks from mine waste heaps as construction material. Ugletekhizdat, 1949.

3. Khristenko L. Experience in the use of mine rocks for the manufacture of construction materials. Ugol, No. 4, 1951.

4. Knigina G. N. Wastes of the Kuzbass coal industry as raw material for brick production. Ugol, No. 3, 1953.

5. Leonov P. A. Report on the work “Development of Safe Methods for the Operation of Rock Waste Heaps of the Mines of the Kuzbassugol Combine,” Part II, Tomsk, 1955.