CONTROLLING COAL EDGE LOSS
The extent of coal loss from mining operations varies between 5 to 25 percent of in-situ coal. Much of this loss occurs during blasting. Coal edge movement (especially during free face blasting), dislodging coal into spoil are a frequent occurrence:
Significant coal edge movement to lowall.
Coal edge movement and formation of trenches.
A carefully designed and implemented blast have successfully addressed edge loss problem. Locating front rows of holes, assessing rock property immediately above and below the coal seam, managing stand-off distances from the roof of coal, multiple point of initiation etc. have drastically resulted into minimising coal edge losses:
Intact coal edge with insignificant movement.
Intact coal edge, showing no movement to the lowall.
Intact coal edge.
Pre-blast model, showing rolls.
Post-blast exposed rolls, showing no coal damage.
Fault identified prior to blasting - coal was therefore saved from damage.
Minimise blast induced damage
Formation of trenches are a well-known phenomenon. General observations and extensive work in this area suggest that the following factors contribute to or associated with, coal damage and loss:
- Geological disturbances e.g. faults, folds and intrusions. Identifying these and designing blast incorporating these irregularities have resulted in minimising coal damage and loss.
- Coal damage usually results when the material layers overlying or underlying the coal seam are weaker than the coal seam. One of the worst situations appears to be when the layers comprise saturated clay, shale or mudstone.
- The location of the control row for the throw blast. Generally, the coal edge moves along the control rows, which is followed by trenches. For example, if a row-by-row throw blast timing is used parallel to the strike, the coal edge moves and trenches are formed parallel to strike. Likewise, if a “V” timing design is used then trenches are often formed at an angle to the strike direction following the orientation of the “V”.
- Blast blocks with a lot of water where energy can be transmitted between the explosive decks thereby losing control on energy release. It is also observed that water can “lubricate” the coal seam by reducing cohesion between the coal and overburden.
- Improperly designed or inaccurately implemented stand-off distances from the coal. This is a crucial factor as explosive placed too close to the coal inevitably damages the coal.
- Blast timing - certain timing designs have been observed to result in increased coal movement.
- Primer location and explosive type may have an impact on the extent of damage.
If the images below look familiar at your minesite, then BOS can assist you to minimise coal damage and loss and thereby maximise coal recovery.
Examples of coal damage / loss:
Coal roof damage.
Blast-induced trenches, resulting in significant coal loss.
Recovering high-value, thin coal seams.
Carefully locating geological disturbances to maximise coal recovery.
Maximising coal recovery
It is vitally important to identify recoverable coal seams in order to maximise its recovery. Due to production pressures mining of multiple coal seams are not always attempted as it becomes too onerous and costly. To assist you in maximising recoveries we can support you with the following:
- Identify recoverable coal seams which can be relatively easily done by passive gamma logging or from Aquila (or other rock recognition) data.
- Blast to protect these coal seams and avoid hard dig.
- Recover thin coal seams normally wasted.
- Dilution control - leave a skin of cover on the roof of the coal. This not only increases wash plant yield but also reduces other resource usage e.g. water, electricity etc. It has also been observed that cleaner ROM is lower in ash and moisture percentages.
- Protect from spontaneous combustion. It has been an issue during summer months. Again, a skin of rock layer can be left on the roof of coal (as a cover) to avoid direct exposure and thereby minimise oxidation. This layer can be ripped and removed just in time to meet the demand of product mix/quality.