Page 403 - Advanced Mine Ventilation

372 Advanced Mine Ventilation

Figure 22.8 A suggested design for a polycrystalline carbide bit.
minimize broken bits, which are very liable to cause frictional ignitions. Bit tip is
generally made of tungsten carbide. Because experiments show that the hot steel
body ignites methane air mixture more easily than the carbide itself, larger carbide
tip bits became popular. Even the grade of carbide was changed to optimize the design.
Tungsten carbide containing 15% cobalt was strong but too soft and wore out quickly,
but carbide containing 8% cobalt was wear resistant and brittle. Sandvik made some
dual property bits that had an outer body of hard carbide and an inner body of tougher
carbides, but it did not reduce the frictional ignitions. Other attempts to cover normal
carbide with an outer layer of ceramic (e.g., Syalon) or cubic boron nitride were also
not successful. Some ceramic material, in fact, increased the ignition frequency when
cutting sandstone. The future improvements in bit design will probably result by

1. incorporating polycrystalline diamond tips or
2. directly depositing polycrystalline diamond on tungsten carbide tips
Polycrystalline diamonds have been used on rotary drill bits with excellent results.
The life of a rotary carbide bit improved 100e200-fold when all carbide tips were
covered with a very thin (1 mm) wafer of polycrystalline diamond. A suitable design
for a continuous miner bit or a longwall shearer bit is yet to come, but a suggested
design is shown in Fig. 22.8. Deposition of polycrystalline diamond directly on
carbide tips is feasible, but the present cost is prohibitively high.

22.5 Summary and Conclusions

Frictional ignition frequency in US coal mines is on the decline, but it still is a
cause of concern because it has a potential to cause a mine explosion leading
to a large-scale mine fire. Prevention of frictional ignition is based on the
following:
1. Coal seam degasification to reduce original gas content of the coal seam by 50e80%.
Horizontal boreholes drilled for degasification can also be used for water infusions.
2. Using ventilation quantities that will not only dilute methane to statutory limits but also
prevent gas layering near the roof or floor.