On a recent visit to Australia, Jose-Luis Fuentes-Cantillana, director of Spain’s mining research group, Association for Research and Industrial Development of Natural Resources, spoke to International Longwall News about safety research and coal mining in Spain.
Spain’s coal output is currently about 15 million tones per annum, produced from a range of small-scale mining operations, producing at rates of 200,000-500,000tpa, including open-cast and underground operations. No bord and pillar mining is undertaken.
AITEMIN services the mining industry as well as construction and environmental industries. It conducts research into mining safety but training is a priority as Spain confronts the same issues that are changing the face of mining in the US and Australia.
These include loss of experience as people retire and the consequent erosion of operational knowledge. As Fuentes-Cantillana points out, so much of what experienced mine operators know is not written down or recorded.
AITEMIN trains electricians, ventilation engineers and people that handle explosives – a critical aspect of Spain’s unique approach to harvesting its steeply dipping coal deposits.
Spain’s coal deposits are characterized by variety: from steeply dipping seams (dipping by up to 70º) to narrow seams. There are roughly 20 underground mines using either conventional short/longwall mining or sub-level caving methods.
Flat seams and seams with a gradient of up to 30º are mined by conventional longwall or shortwall methods.
During the past few decades, sublevel caving methods have become widely used in Spain, as they have proved to be an effective and safe system for the exploitation of steep and thick coal seams. Practically all seams in the country with an inclination over 60º and thicker than 1.5m are now mined by this method.
Mining is carried out by excavating an incline in the floor strata, from which cross-cuts are made to the seam with a vertical separation of 15m to 30m. Single-entry horizontal roadways, or sublevels, are then driven into the seam in both directions, generally 100m to 200m. Retreat mining is carried out, caving the coal pillar between sublevels with explosives.
Coal is normally extracted by the incline with armoured chain conveyors until it reaches the incline, where it falls by gravity to the lower level.
“This mining system requires a reduced amount of manpower, and has safer working conditions that other alternative methods for these seams, as men are always located under a well supported roof,” Fuentes said.
“However, this method implies also a number of difficulties, for instance the complex ventilation and environmental control system required, and the continuous need for moving equipment between sublevels as the mining process advances.”
This type of mining is associated with high-risk explosions in the caved area, caused by blasting, even if safety explosives are used. Methane explosions in sublevel coal mining operations caused Spain’s two major disasters of the past 10 years.
Difficulties in controlling the caving process result in production drillholes coming in contact with the gases accumulated in the caved area, which can cause a methane explosion at the moment of blasting.
To improve safety, blasting operations are now conventionally carried out remotely, with all personnel located outside the mine or at a very large distance from the active zones. This measure eliminates the human risks, but there is still a risk of damaging equipment, or causing larger explosions.
In some cases high-pressure gas blasting has been used instead of explosives but this approach is limited to seams with soft coal.
Another option under investigation is the development of suppression barriers that can be placed at the end of the sublevel, and that would be activated some seconds before blasting. This would serve to mitigate the potential explosions, or confine them into the caving area.
Some of Spain’s mines have high sulphur content – some are over 6%. At one operation – since closed – the working face had to be sealed over weekends when the face was stopped to prevent spontaneous combustion related to the SO2.
Another unique operation is a mine working a 70m thick, steeply dipping seam. Rather than using sub-level caving, the deposit is mined in slices using shortwalls with sub-level caving on the goaf side.
Fuentes said a research project is testing a new DBT-designed longwall roof support system for a steeply dipping (60º), thick seam.
The main challenge is not side pressure bearing on the shields but finding a way to correct the roof support height given the downward drag of gravity on the shields.
The steeply dipping longwall mines use no face conveyor as the coal simply drops to the lowest point thanks to gravity and is collected by a conveyor. The shearer is moved along the face with ropes.
As Fuentes remarked, very different from what people in the US would be used to seeing.