The Effect Of Adsorbed Vapours On Grinding
THE EFFECT OF ADSORBED VAPOURS ON GRINDING
ABILITY OF HARD COAL
B. Buczek, L. Czepirski
University of Mining and Metallurgy, Faculty of Fuels and Energy,
al. Mickiewicza 30, 30-059 Cracow, Poland
The effect of adsorbed methanol, ammonia and benzene on grinding efficiency of hard coals of different rank was studied. Methanol and ammonia show the largest effect, whereas the presence of benzene decreases the grinding efficiency for all types of coal.
The phenomenon of the impact of adsorption of vapours and gases on the mechanical properties of solids has been known. Rebinder [1], as the first one, as demonstrated that the adsorbing substances affect the strength and strain of materials. Recently, an increase in the interest for the operations of drilling, breaking up and grinding of rocks and mineral raw materials [2, 3] as well as hard coals [4, 5] in the presence of various substances in order to raise their efficiency has been observed.
Experimental
The impact of vapours and gases on the grinding process of hard coals in a ball mill was investigated. The properties of different rank coals are presented in Table 1.
Table 1. Technical and densimetry characteristics of hard coals investigated
Coal
Wa [%]
Aa [%]
Vdaf [%]
rHe [g/cm3]
rHg [g/cm3]
Vp [g/cm3]
A
2.4
6.7
34.0
1.44
1.31
0.069
B
1.0
4.3
29.3
1.37
1.35
0.011
C
0.6
6.1
11.8
1.43
1.37
0.030
Prior to the grinding, air-dry samples of coal of size grade 1.2 – 2.0 mm were saturated with methanol, benzene and gaseous ammonia up to the equilibrium state. A coal sample saturated with a determined adsorbate was loaded into the ball mill and then ground for the period of 5 min. After the process, the sample was subjected to sieve analysis. The mean fineness degree
h = dp/dk was used as the measure of the efficiency of the grinding process. The results of the measurements and calculations are summarised in Table 2.
Table 2. Degree of fineness of coal as a function adsorbate type
Adsorbate
Mean fineness degree, h
A
B
C
Without
2,00
2,46
2,50
Methanol
2,46
4,44
4,71
Ammonia
3,14
3,33
4,32
Benzene
1,93
2,16
1,86
Results and Discussion
Considering the grindability of coals A, B and C that had been saturated with no adsorbate, it can be said that low-rank-coal (A) undergoes disintegration with the highest difficulty and high-rank coal (C) with the least difficulty, whereas coking coal (B) shows medium grindability. Methanol vapours strongly influence all the types of coal and the grindability increases with the rising rank of coals. Ammonia affects most strongly the coal C, almost at the same degree as methanol. The impact of ammonia on to the coals A and B is alike and the values of h are close to each other. The adsorbed benzene vapours make the grinding more difficult. For all the types of coal, the fineness degree is lower for the saturated samples than for the coals that were not subjected to saturation.
Conclusions
(i) The sorption of some polar substances of molecules of the size comparable to the size of micropores can increase the grindability and the ability to break up the coals. (ii) The interpretation of the phenomena occurring during the grinding of coal saturated with an adsorbate could be most easily carried out on the basis of the polymer model of hard coal structure. (iii). The lowering of the strength of coal caused by the sorption of vapours and gases can be used in industrial practice to increase the efficiency of mechanical operations.
References
1. P. A. Rebinder, Zeitschrift Phys., 72, 191 (1931).
2. J. Zheng, C.C. Harris, P. Somasundaran, Powder Tech, 91, 173 (1997).
3. H. El-Shall, R. Haecker, et al., Mining Eng., 52, 48 (2000).
4. L. Besra, R. B. Rao, Fuel Sci.&Techn. Int., 14, 1405 (1996).
5. G. Atesok, M. S. Celik, Fuel, 79, 1509 (2000).
Related articles::