40
2 Environmental Conditions in the Mine
Table 2.8 Relative specific
density of different gases. The
specific density data are based
on standard conditions
Gas Molecular weight (g mol -1 ) Specific gravity (relative
density)
H 2
2
2/29 = 0.07
CH 4 16
16/29 = 0.55
NH 3 17
17/29 = 0.59
CO
28
28/29 = 0.97
Air
29
1
NO
30
30/29 = 1.03
H 2 S 35
35/29 = 1.21
CO 2 44
44/29 = 1.53
NO 2 46
46/29 = 1.59
SO 2 64
64/29 = 2.26
yet have taken place, and so some stratification may be observed, although this will
disappear over time. Graham’s law states that the diffusion rate of a gas is inversely
proportional to its density. Accordingly, less dense gases will have a higher diffusion
rate and therefore, tend less to stratify (Table 2.8).
Thus, gases tend to appear in galleries and stopes in a similar way to that shown
in Fig. 2.1. According to the graph, the light gas will move to occupy the upper part
H2S
CO 2
NO 2
SO 2
H 2
CH 4
NH 3
O 2
CO
NO
Fig. 2.1 Strict theoretical stratification of gases in a mining gallery. Figure does not take into
account either the molecular movement or the effects of forced ventilation