Page 389 - Advanced Mine Ventilation

358 Advanced Mine Ventilation

The intake airways are separated from return airways by “stoppings.” It is prefer-
able to mine the full thickness of the coal seam leaving no coal in the roof or floor
to minimize the risk of spontaneous combustion.

21.3.1.1 Roadways and Pillar Size

As far as possible (consistent with ground support requirements) the roadways should
be reasonably wide and pillars should be large with few crosscuts. Most heating in pil-
lars is because it is too small and air can leak through it. Small airways require higher
pressure to supply the mine ventilation air and exacerbate air leakage through the
pillar. Crosscuts at regular intervals are needed but they should be preferably 300
feet apart. The stoppings in the crosscut used to separate intake air from return should
be substantial and leakproof.

21.3.1.2 Direction of Mains
These main roadways are usually closest to the mine fan and are exposed to higher
pressure differential. Besides making the pillars large, it is desirable that they are

driven at 45 to the “face” cleat to minimize spalling and fracturing. Loose coals in
the roadways are potential sources of spontaneous combustion.

21.3.1.3 Roof Supports in the Airway
All development entry roofs are reenforced to make them strong. Timber or concrete
blocks for roof support should be avoided because they increase the pressure differen-
tial. The preferred roof support is a long roofbolt.

21.3.1.4 Premining Infusion of Coal With Silicate Gel to Reduce
Permeability
Considering the fact that most fires owing to spontaneous combustion start in coal
pillars, it would be desirable to infuse the coal prior to mining with a silicate gel
that makes it impermeable. In a typical three-heading development, the central entry
is horizontally drilled to 500 feet on weekends. Two other horizontal boreholes are
dilled at an angle to cover the headings on either side. A mixture of sodium silicate
gel and sodium bicarbonate (as discussed in methane control section earlier) is pumped
into the coal to be mined. Typically the amount of gel is 1%e1.5% of the volume of
coal 500 feet ahead of the injection point. The technique was very helpful in preventing
the reoccurrence of spontaneous heatings in mines overseas.

21.3.2 Secondary Extraction
There are many ways for secondary extraction of coal but two most commonly used
techniques are as follows:
1. Pillar extraction by continuous miners.
2. Retreat longwall mining.