Assessment of the Aggregate State of Coal Cake by Gas Permeability

V. A. Babenko, V. M. Vityugin

(Presented by the seminar of the Department of General Chemical Technology, TPI)

Flotation concentrate (cake), obtained after dewatering on vacuum filters, consists of mud-like agglomerated lumps of coal particles, as a rule smaller than 1 mm. Such cake is poorly transported, adheres to parts of mechanisms and even to the vertical walls of chutes and bunkers, hangs up in them, and emerges unevenly from the chutes of vacuum filters, disrupting the metering of coal entering the drying units. Such overmoistened cake is poorly loosened by the feeder; as a result, cake lumps larger than 13–20 mm in tube dryers fall through, which leads to disruption of the drying regime, a decrease in its productivity, and an increase in dust formation in the cyclones. Thus, studies of the operation of tube dryers at the Chertinskaya preparation plant showed that when drying concentrate containing 15–20% cake, the moisture of the coal entering the tube dryer fluctuated from 9 to 22%, while the moisture of the dried product ranged from 1 to 10%; dust emissions into the atmosphere in this case amounted respectively to from 12 to 0.3 g/m³. The dust content of the air at the transfer points of dry coal, under normal operation of the ventilation installations, reached 50 mg/m³ during overdrying. However, operating practice also showed that in individual shifts, when the cake moisture was up to 24%, it had an optimal aggregate structure, as a result of which the coal-drying process was significantly improved and stabilized.

The present work is devoted to the study of the aggregate properties of flotation concentrate.

Of the various forms of moisture that determine the aggregation of wet concentrate under preparation and drying conditions, capillary moisture is of primary importance.

Molecularly bound water, coating cake grains with films, has only a slight effect on the cohesion of this material, since it is a weakly compacted loose mass of increased porosity. The number of contacts between adjacent grains is too small, and the distance between grains at the points of contact is too large, for molecular forces to determine cohesion to any extent. Therefore, flotation concentrate saturated with water to its maximum molecular moisture capacity (MMC) almost does not lose its free-flowing properties; aggregation of grains in such a material concerns only the finest fraction. At MMC, only microaggregates can exist. Microaggregates do not reduce the overall flowability of the material but, on the contrary, improve its mechanical composition by equalizing the grain size.

Aggregation of medium and large coal particles begins only after the material has been moistened beyond MMB, due to capillary forces arising in water collars at the points of contact of grains. As moisture increases, the number of collars grows and, correspondingly, the cohesion of the material increases; as a result, a polyaggregate structure is formed, created by large grains and microaggregates of dust particles.

Such a structure is characterized by high porosity of the material, with considerable intra-aggregate and slight inter-aggregate cohesion. Under external mechanical action on such a structure, it readily breaks down into microaggregates and large grains.

With a further increase in the moisture content of the cake, which corresponds to the transition of moisture from the junctional (pendular) state to the capillary state, the polyaggregate structure is disrupted. First the microaggregates break down, and then, after complete capillary saturation, the microaggregates also disappear. The bond between grains under capillary saturation decreases, since the capillary forces of the liquid bridges vanish, while molecular forces are of little significance because of the great thickness of the water layers between the grains.

The overall cohesion of lumps of capillary-moistened cake is determined by the magnitude of the capillary vacuum arising inside the lumps due to meniscus films at the outer ends of the through capillaries of the lump. The magnitude of this adhesion is small. Because of the low compaction of the cake, the capillary dimensions are comparatively large, and the magnitude of the capillary vacuum is inversely proportional to the capillary diameter. On the whole, the material is characterized by weak but continuous cohesion. Disintegration of such material is extremely difficult because of its comparatively uniform cohesion. As practice has shown, the greatest difficulties in coal preparation and drying are associated precisely with this structure of flotation concentrate.

Thus, the most favorable moisture content for feeding dryers is one that creates a polyaggregate structure. In this case, microaggregates of dust-like particles will not be lost through entrainment, while macroaggregates will ensure easy disintegration of the material under the mechanical action of loading devices.

Polyaggregateness is determined by the combined effect of a large number of factors: granulometric composition, the natural surface, moisture content, etc., the exact evaluation of which is difficult. The final state of such a system is manifested in the degree of porosity, and, consequently, it can be assessed by gas permeability.

The study of gas permeability was carried out for flotation concentrate from the Chertinskaya preparation plant.

Granulometric composition of the flotation concentrate:

Natural moisture content — 25—29%.

Ash content on a dry basis — $A^c = 11,1\%$.

Maximum molecular moisture capacity — 9,36%.

Capillary moisture capacity — 29,4%.

Gas permeability was determined by the method adopted for molding sands [1], by passing air at a temperature of 15—20°C through a standard specimen of the material under study. It is expressed

it as a number without indicating the unit of measurement. To determine gas permeability, the standard apparatus of the Uman Mechanical Plant was used.

The dependence of the gas permeability of the flotation concentrate on its moisture content and on different compacting loads is shown in Fig. 1. The principal compacting load is experienced by the flotation concentrate in the bunkers before it is fed into the dryer tubes. For the conditions of the Chertinskaya plant, the compacting load, calculated by the method proposed by N. V. Vasil’ev and V. A. Olevskii [2], is \(0.13\ \mathrm{kg/m^2}\).

As can be seen from the figure, the gas-permeability value is inversely proportional to the compacting load. Depending on moisture content, gas permeability has a maximum in the range from 22 to 24% under all compacting loads and, for our conditions, is 800 conventional units (curve 2, Fig. 1).

Such a dependence of gas permeability on moisture content is explained as follows: a dry flotation concentrate, having the particle-size composition indicated above, is compacted well and allows gas to pass through poorly. As moisture content increases, the aggregate composition of the concentrate begins to change. Individual coal particles aggregate with the aid of water, as a result of which intra-aggregate and interaggregate pores are formed, sharply increasing the total porosity. Then, as the pores are filled with water, the gas permeability of the material decreases and reaches zero at the moment of complete capillary saturation of the concentrate with water, \(W = 29\text{–}30\%\).

Fig. 1. Dependence of the gas permeability of coal cake on its moisture content. Compacting loads, \(\mathrm{kg/cm^2}\): 1—0.0255; 2—0.130; 3—0.230.

Thus, to obtain a flotation concentrate with the greatest gas permeability, it is necessary to bring its moisture content to the value at which maximum gas permeability is attained.

LITERATURE

1. A. A. Tumanskii. Molding Sands. Mashgiz Publishing House, Moscow, 1956.
2. N. V. Vasil’ev, V. A. Olevskii. Transport Devices and Storage Facilities of Concentrating Plants. Ugletekhizdat, 1954.