Study of the Aggregation Properties of Flotation Sludges Before Drying
Abstract Generated abstract
This study examines the aggregate properties of coal flotation concentrate before tube drying, where high moisture content produces a sticky cake that dries unevenly and is difficult to distribute in the dryer. The authors relate flowability and gas permeability to maximum molecular moisture capacity, particle size, moisture content, and additives, using rolling angle on an inclined metal plane as an experimental measure of flowability. The results show that rolling angle rises sharply above about 18 to 20 percent moisture, while adding ordinary dried material would require impractically large returns to improve handling. Returning hot cyclone dust of the minus 0.1 mm fraction together with 3 to 5 percent calcium oxide is proposed as a practical way to increase moisture capacity and flowability, facilitating tube dryer operation.
Full Text
INVESTIGATION OF THE AGGREGATE PROPERTIES OF FLOTATION SLUDGES BEFORE DRYING
O. A. FUKS, V. M. VITOGIN
(Presented at the scientific final conference of the Faculty of Chemical Technology)
At coal-preparation plants, where the flotation method is used for the beneficiation of fine classes, the sludge of the flotation concentrate is fed to vacuum filters. The cake removed from the vacuum filters goes for drying in tube dryers. Drying this material in tube dryers is a difficult and complex process, since the incoming cake is a sticky, dense mass with a moisture content of 25–30%, equal to the capillary moisture capacity of this material. Such a state of the material makes it difficult to distribute it uniformly over the cross section of the dryer tube, which leads to nonuniform drying, as a result of which either an underdried or an overdried product is obtained.
To normalize the operation of tube dryers and determine a rational drying regime, it was necessary to study the aggregate properties of flotation sludges and establish the possibility of changing them in order to increase flowability and gas permeability.
The main characteristic of the aggregate state of fine-grained materials is moisture capacity. The maximum molecular moisture capacity, characterizing the internal or bound moisture in coals, determines the amount of moisture adsorbed on the internal surface of the pores of coal particles. This value corresponds to the maximum moisture content at which the material retains flowability [1]. At a moisture content equal to the maximum molecular moisture capacity, the material does not freeze, since moisture bound by adsorption forces differs in its physical properties from moisture retained by capillary forces [2].
Thus, the task of increasing the flowability and gas permeability of a raw fine-grained material, in our case a flotation coal concentrate, is reduced to increasing its maximum molecular moisture capacity. It is known that the moisture capacity of a material depends on its granulometric composition and increases with decreasing particle size.
Table 1
Dependence of the moisture capacity of coal flotation concentrate on particle size
| Particle size, mm | Maximum molecular moisture capacity, % |
|---|---|
| 0–0.1 | 17.5 |
| 0–0.25 | 11.4 |
| +0.25–0.5 | 9.62 |
| +0.5–1.0 | 9.02 |
| 0+2.0 | 7.3 |
The addition of dry coal dust from cyclones to the flotation concentrate going to drying should increase the flowability of this concentrate, first, through a simple reduction in moisture content and, second, through an increase in the maximum molecular moisture capacity of the resulting mixture.
The moisture capacity of a fine-grained material also depends on various additives introduced into it, such as CaO, CaCl₂, etc.
We evaluated the flowability of coal flotation concentrate of different moisture contents by the value of the rolling angle of the material under study on an inclined metal plane.
As a result of experimental studies it was established that the rolling angle increases with increasing moisture content of the material. The value of the rolling angle increases especially sharply when the moisture content rises from 18 to 20%. Flotation concentrate with a moisture content above 20% did not roll at all and fell under the action of gravity from the vertically positioned plane.
Fig. 1. Dependence of the rolling angle of coal flotation concentrate on its moisture content
The addition of various amounts of dried material (with \(W^{r} = 4\%\)) correspondingly reduced the moisture content, decreased the rolling angle of the material, and increased its gas permeability. However, to reduce the moisture content of the initial flotation concentrate from 30 to 18%, when its rolling angle is sharply decreased, it is necessary to introduce 85% dried material, which is clearly impractical.
The use, as a return, of dry dust of the class \(-0.1\) mm in an amount equal to the yield of this class, and an additional addition of calcium oxide, will increase the moisture capacity. In addition, the fine drying feed will be added in the hot state, which will undoubtedly increase flowability by reducing the surface tension of the water in the raw sludge.
The experimental results confirm the assumption given above and show that flotation concentrate with a moisture content of 30%, with the addition of 30% dry dust of the class \(-0.1\) mm at a temperature of 90° and 3% CaO (calculated on the total dry mass), formed a fairly free-flowing mass with maxi-
with a low molecular moisture capacity, equal to 11.1%. The angle of repose of such a mixture was 81°.
Thus, in order to increase flowability and facilitate the drying of coal flotation concentrate in tube dryers, it is necessary to return all hot dust from the cyclones and additionally introduce 3–5% calcium oxide.
LITERATURE
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A. A. Agroskin et al. Increase in the bulk weight of coal by microadditives of hydrocarbon liquids. Academy of Sciences of the USSR, 1947.
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S. G. Aronov, L. L. Nesterenko. Chemistry of Solid Fossil Fuels. Publishing House of Kharkov State University, 1960.