Gas Permeability of Wet Fine-Grained Materials as a Characteristic of Their Aggregate State
Abstract Generated abstract
This paper examines gas permeability as an indirect, rapid indicator of the aggregate state and flowability of moist fine-grained coal materials. Using a standard apparatus, the authors measured air permeability of coal flotation concentrate and 0 to 1 mm run-of-mine coal fractions at varying moisture contents and compaction loads. The results show a strong dependence of permeability on moisture, with sharp changes between maximum molecular moisture capacity and capillary moisture capacity, interpreted in terms of particle spacing, aggregation, and water-filled capillaries. Comparison with plant observations suggests that minimum gas permeability corresponds to maximum cohesion, supporting the use of a one-minute permeability test for express monitoring before drying.
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GAS PERMEABILITY OF MOIST FINE-GRAINED MATERIALS AS A CHARACTERISTIC OF THEIR AGGREGATE STATE
V. M. Biryulin, O. A. Fuks
(Presented by the scientific seminar of the Department of General Chemical Technology)
The aggregate state of a fine-grained material, its flowability, is determined by its moisture capacity. This is the principal characteristic determining the state of the material. However, flowability can be characterized not only by moisture capacity, but also by other parameters, such as gas permeability, angle of repose, throughput capacity, or the rate of discharge from a bunker. These are indirect ways of determining the aggregate state of a material, but they are related to moisture capacity and make it possible to characterize the material under study more fully and more rapidly.
The gas permeability of a material depends on a whole series of factors, namely: on the size and shape of the grains, on the uniformity of the grain composition, on the degree of compaction, and also on the moisture content of the material being studied. For one and the same material with a definite grain composition and at a constant degree of compaction, gas permeability will depend only on its moisture content.
Thus, the aggregate state of moist carbonaceous fine-grained materials can be characterized by the magnitude of their gas permeability.
Gas permeability is determined by forcing air at a temperature of 15–20° through a standard sample of the material being tested, and is expressed as a dimensionless number. To determine gas permeability, a standard instrument manufactured by the Uman Mechanical Plant is used.
We determined the gas permeability of coal flotation concentrate and of a fraction of run-of-mine coal with grain sizes of 0–1 mm at different moisture contents of these materials, and also under different compacting loads. The results obtained are given in Figs. 1 and 2.
It is evident from the figures that gas permeability depends on the moisture content of the material and changes especially sharply in the interval from the maximum molecular moisture capacity (MMM) to the capillary moisture capacity (CMC) of the corresponding coal material.
The complex dependence of gas permeability on moisture content can be explained by the fact that dry flotation concentrate with particle sizes of 0–1 mm constitutes a dense mass in which all pores between the coarser particles are filled with fine dust; therefore such a material is impermeable to gas. With an increase in moisture content above the maximum molecular moisture capacity (MMM), the volume of the coal mass increases, and the distance between individual grains grows.
Fig. 1. Dependence of the gas permeability of flotation concentrate on moisture content.
Axes: gas permeability; moisture content, %.
Curve labels: \(P_1 = 0.23\ \mathrm{kg/cm^2}\); \(P_2 = 0.48\ \mathrm{kg/cm^2}\); \(P_3 = 0.74\ \mathrm{kg/cm^2}\).
Fig. 2. Dependence of the gas permeability of run-of-mine coal on moisture content.
Axes: gas permeability; moisture content, %.
Curve labels: \(P_1 = 0.23\ \mathrm{kg/cm^2}\); \(P_2 = 0.48\ \mathrm{kg/cm^2}\); \(P_3 = 0.74\ \mathrm{kg/cm^2}\).
and with a further increase in moisture content the fine-grained material begins to aggregate, its particles become larger, and, correspondingly, the distance between particles increases; this is what causes the sharp increase in gas permeability.
At high moisture content, close to the value of capillary moisture capacity (CMC), the moisture films covering individual grain particles merge with one another, fill with moisture all capillaries and the spaces between grains, and this leads to a sharp reduction in gas permeability.
Comparison of the gas-permeability indices with practical data on the aggregate state of fine-grained coal materials at the Chertinskaya preparation plant shows that the greatest cohesion of these materials corresponds to the minimum value of gas permeability. Thus, it becomes possible, by means of the gas-permeability index, quickly and simply to characterize moist materials before drying.
The drying of moist fine-grained coal materials is extremely sensitive to their aggregate state, which changes indeterminately over time. At the same time, with a change in cohesion or flowability, a corresponding adjustment of the dosing and loading equipment is required, and consequently constant express monitoring of the aggregate state is necessary. Evaluation of the aggregate state by gas permeability requires one minute.