The short answer
Powder factor is the weight of explosive used to break a unit of rock. Divide the kilograms of explosive in a hole by burden x spacing x bench height to get kilograms per cubic metre. Quarry bench blasting typically runs from about 0.25 kg/m3 in soft rock to 0.6 kg/m3 or more in hard, massive rock. Many Indian quarries quote the inverse instead: tonnes of rock broken per kilogram.
Powder factor is the first number anybody asks about a blast and one of the least informative on its own. It tells you how much explosive was used. It does not tell you where in the rock it was.
01The formula
- Hole length including 0.9 m sub-drill
- 8.9 m
- Less stemming
- 2.5 m
- Charged length
- 6.4 m
- ANFO in a 110 mm hole at 0.85 g/cc
- 8.1 kg per metre
- Charge per hole
- 52 kg
- Rock per hole: 3.0 x 3.5 x 8.0
- 84 m3
- Powder factor
- 0.62 kg/m3
At a rock density of 2.7 tonnes per cubic metre that is 0.23 kg per tonne. Turned upside down it is 1.6 cubic metres per kilogram, or 4.4 tonnes per kilogram. All four describe the same blast.
02Two conventions, opposite directions
Textbooks and most specifications give powder factor as kilograms per cubic metre, where a higher number means more explosive. On many Indian quarry floors it is spoken of as tonnes per kilogram, where a higher number means less explosive. A manager who says the powder factor has gone up may mean the blast was heavier or lighter. Ask for the unit before agreeing that it is good news.
03Typical values
| Rock | kg per m3 | kg per tonne | Tonnes per kg |
|---|---|---|---|
| Soft: weathered sandstone, shale, soft limestone | 0.15 to 0.30 | 0.06 to 0.13 | 8 to 16 |
| Medium: sound limestone, sandstone, softer basalt | 0.30 to 0.45 | 0.11 to 0.17 | 6 to 9 |
| Hard: granite, dense basalt, quartzite | 0.45 to 0.70 | 0.17 to 0.26 | 4 to 6 |
For production benches feeding a crusher. Massive unjointed rock sits at the top of its range, closely jointed rock at the bottom.
04What the figure hides
Two blasts with the same powder factor can produce very different muckpiles. Take 0.4 kg per cubic metre delivered through 165 mm holes on a 5 by 6 m pattern, and the same 0.4 delivered through 76 mm holes on a 2.3 by 2.8 m pattern. The second puts a charge within two metres of every piece of rock. The first leaves blocks nearly four metres from the nearest explosive. The fragmentation will not be alike.
Distribution up the hole matters in the same way. A long stemming column means the top third of the bench has no explosive in it, whatever the average says. This is the usual source of the slabs described in oversize at the hopper.
05The false economy
Explosive is a visible cost with an invoice every week, so it attracts attention. Opening the pattern a little cuts that invoice and the drilling bill at the same time. The saving is real and it is immediate.
The cost arrives later and under other headings. Coarser rock digs more slowly, so loading cost per tonne rises. More of it needs a breaker. The primary crusher works harder on every piece and passes fewer tonnes an hour, and its liners wear sooner. Each of those is larger per tonne than the explosive saved, and none of them is on the blasting sheet.
The reverse error exists as well. Too much explosive throws rock, damages the wall behind the shot and turns good stone into excess fines that nobody ordered.
06Finding the right figure for a face
- Start from the middle of the range for the rock type, with the pattern set from the hole diameter.
- Record for each blast: powder factor, breaker hours on the muckpile, excavator loading rate, and crusher tonnes per hour on that rock.
- Change one thing at a time, by a small step.
- Stop when the total cost per tonne at the stockpile stops falling. That figure, not the lowest powder factor, is the one to hold.
It takes a firm that sees both ends to do this, which is the practical argument for giving drilling, blasting and crushing to one contractor. The pattern side is in reading a drilling pattern.