8.5 Throwback Method
311
6. Calculation of time needed to dilute gases (t 2 ) (Eq. 8.24):
t 2 =
V
Q
ln
C i
C f
(8.24)
7. Calculation of total ventilation time (t t ): t t = t 1 + t 2
Exercise 8.5 A 5 × 5 m gallery, whose current length is 600 m is being excavated by
drilling and blasting. The advance in it takes place at a rate of 3.5 m per cycle, in a
rock of density 2.6 t m
−3 . The specific consumption factor for ANFO is 2.05 kg m
−3
of blasted rock, the ventilation airflow rate at the face is 25 m
3 s
−1 . Calculate:
(a) Tonnes of rock blasted per cycle.
(b) Mass of ANFO used.
(c) Fumes throwback length.
(d) The time it takes the gases to reach the main gallery.
(e) Face fumes clearance time.
(f) Total ventilation time.
Note: Suppose you generate 0.0026
m
3 NO 2
kg of ANFO
Solution
(a) Calculation of the mass of rock blasted:
5 · 5 · 3.5 = 87.5 m
3
87.5 m
3
· 2.6 t m
−3
= 227.5 t per blast
(b) Calculation of the mass of ANFO:
87.5 m
3
· 2.05 kg ANFO
m 3 blasted rock
= 179.38 kg of ANFO
(c) Fume throwback length:
L i =
K M
F a D
√
A
=
25 000 · 179.38 kg
3.5 m · 2 600
kg
m 3 ·
√
5 m · 5 m
= 98.56 m
(d) To solve this problem the following parameters have to be calculated first:
Throwback volume = 98.56 m · 5 m · 5 m = 2464 m
3
Volume of gases immediately after blasting (V NO 2 ):
V NO 2 = 0.0026
m
3 NO 2
kg ANFO
179.38 kg ANFO = 0.46638 m
3 NO 2
311
6. Calculation of time needed to dilute gases (t 2 ) (Eq. 8.24):
t 2 =
V
Q
ln
C i
C f
(8.24)
7. Calculation of total ventilation time (t t ): t t = t 1 + t 2
Exercise 8.5 A 5 × 5 m gallery, whose current length is 600 m is being excavated by
drilling and blasting. The advance in it takes place at a rate of 3.5 m per cycle, in a
rock of density 2.6 t m
−3 . The specific consumption factor for ANFO is 2.05 kg m
−3
of blasted rock, the ventilation airflow rate at the face is 25 m
3 s
−1 . Calculate:
(a) Tonnes of rock blasted per cycle.
(b) Mass of ANFO used.
(c) Fumes throwback length.
(d) The time it takes the gases to reach the main gallery.
(e) Face fumes clearance time.
(f) Total ventilation time.
Note: Suppose you generate 0.0026
m
3 NO 2
kg of ANFO
Solution
(a) Calculation of the mass of rock blasted:
5 · 5 · 3.5 = 87.5 m
3
87.5 m
3
· 2.6 t m
−3
= 227.5 t per blast
(b) Calculation of the mass of ANFO:
87.5 m
3
· 2.05 kg ANFO
m 3 blasted rock
= 179.38 kg of ANFO
(c) Fume throwback length:
L i =
K M
F a D
√
A
=
25 000 · 179.38 kg
3.5 m · 2 600
kg
m 3 ·
√
5 m · 5 m
= 98.56 m
(d) To solve this problem the following parameters have to be calculated first:
Throwback volume = 98.56 m · 5 m · 5 m = 2464 m
3
Volume of gases immediately after blasting (V NO 2 ):
V NO 2 = 0.0026
m
3 NO 2
kg ANFO
179.38 kg ANFO = 0.46638 m
3 NO 2