Trent Ernst, Editor
There’s been some confusion about the difference between types of mining. For many, underground mining is underground mining, but there are vast differences between hard rock mining (zinc, iron copper, etc), which in turn has a number of variations, and coal mining, which breaks down into two main types of mining techniques: room and pillar and longwall.
There are over 100 mines in the United States that use the Longwall technique, which was introduced into the US in the 1950s. While there are over 1300 coal mines in the US in total, longwall mines produce nearly half the total output of coal.
Longwall mining was also introduced to Canada at around the same time, with one of the first Longwall Mines in Canada being the 4-Star mine in Nova Scotia, which used the first hydraulically powered roof supports in 1966.
However, there are currently only two underground coal mines in Canada, both of which use the room and pillar technique. One of those, Grande Cache Coal, used longwall mining up until about 1996. However, since then, there have been no longwall mines in Canada.
According to Gary Gould, vice-resident of Hillsborough Resources, the company that operates one of those mines on Vancouver Island, the two techniques are vastly different. “Imagine if you will the downtown of a city,” says Gould, of the room and pillar method. “You have roads running between blocks of apartment buildings. The buildings are the pillars, which hold the roof up, while the roads are the rooms that are being mined.”
Gould says that once all the rooms have been mined, they then start removing the pillars from the far end, allowing the roof to collapse behind the miners. “It only takes about four or five months to train somebody to a point where they can operate the equipment for normal mining. But when you’re removing the pillars behind you, you want someone with four or five year’s worth of experience.”
Retreat mining, as this stage of the room and pillar process is known, is one of the most dangerous types of mining, accounting for fully one quarter of all coal mining deaths in North America, even though it represents only about ten percent of coal mining activity. Pillar removal must be done very precisely, and the collapse must be controlled lest the concussion of the falling roof blast all the miners out of the mine. Many mines don’t even bother with retreat mining. As a result, most room and pillar mines only recover about 50% of the coal in a seam.
Longwall mining, on the other hand, could be considered a continual form of retreat mining, as the roof of the mine is constantly collapsing behind the mining activity. Because they do not have to leave any pillars behind (except along the access corridors), the recovery rate is significantly higher than room and pillar, usually between 80 and 90 percent.
Tim Joseph is a professor in the University of Alberta School of Mining. He says that one of the reasons that longwall mining isn’t used in Alberta and BC is because the geological stresses in this region are difficult. “For the underground coal mines back in Cape Breton, the mines were a lot like the mines back in the UK. But the closer you get to the mountains, the more horizontal stress you have. If you are going to do a longwall mine in that area, you need a very good geotechnical engineer.”
Most of the longwall mines in the US are located on the east coast.
In longwall mining, the coal seam is divided into long panels. These panels are typically a few hundred metres across and a few kilometers long. These panels typically fall off a central shaft (or main gate) where crews, equipment and fresh air travel.
Gate roads are drilled to the back of the panels for access, and the miners work from the far end of the panel back towards the main access shaft. Rather than leave pillars for support, hydraulic roof supports are put in that run the entire length of the panel. A cutting head runs along the face of the seam, knocking the coal down onto a conveyor belt.
With each cut (or series of cuts, depending on the thickness of the seam), the entire longwall unit—shearer, conveyor and hydraulic roof supports move forward. As the unit moves forward, the roof behind the unit is collapsed, filling in the open space.
Because the roof is constantly collapsing behind the miner, it is important that everything is functioning properly. Rather than rely on humans to move the hydraulics forward, the system is highly automated.
While companies do what they can to mitigate risk, and Huiyong Holdings, the lead partner in HD Mining, has never had a fatality at any of their nine mines, the technique is inherently dangerous. The roof is constantly collapsing behind the advancing sheerer. In 2010, 25 miners died in West Virginia, in the Upper Big Branch mine. It is the deadliest mine accident in North America in the last 25 years. And it was a longwall mine.