© Ronald Frommann



Machines that have been "made in Germany" still dominate the global coal mining industry today. But how does largely automated mining work at depths of 1,200 meters?

By Hans Wille

Pictures © Roland FrommannThe first impression of the world underground is dark gray. The anthracite-colored coal dust blocks out all other colors here. Two minutes ago, the three-level conveyor cage left the world of colors and light and slid downwards into the 1,200-meter-deep shaft 10 of the Prosper-Haniel mine. Only a gentle pressure in the ears indicates the brisk movement speed of 40 km/hour. After leaving the elevator, we enter a seemingly black-and-white underground world. As our eyes slowly adapt to the dim neon light, we recognize some pale colors in spite of the dust - red cable looms, blue switch cabinets and yellow hard hats.

The material terminal in shaft 10 is otherwise similar to a subway station, with domed ceilings and walls and railway tracks that disappear into the depth of narrow tunnels. However, here the rails are suspended from the ceiling. The unfortified soil composed of earth, gravel and coal dust is equally surprising.

Everything is suspended from the ceiling

"The mountain is alive, so we must suspend everything," says Holger Stellmacher, who works for the public relations department of the Prosper-Haniel mine and guides visitors through the mine. "The rock is active and expands towards the weakest point of resistance." The kilometer-long tunnels are shored up by yielding arch supports, consisting of two profiled steel elements, each bent to form a quarter circle, which rest on the floor, cover the side walls and overlap under the ceiling. The yielding arches slide over each other when the mountain pressure increases. When the pressure increases further, the yielding arches transfer the pressure into the ground, which then swells upwards. "The convergence may actually amount to 2 cm a day, depending on the surrounding strata," explains Stellmacher.

Everything is therefore suspended from the walls and the ceiling - the tools, an incredible number of chains that are used for various tasks, the explosion-proof electrical cabinets and all supply lines such as water pipes, electrical cables and thick, flexible plastic piping to provide fresh air. Even the diesel trolleys are suspended. They are overhead monorail conveyors that transport humans and material on the seventh level of the Prosper-Haniel mine, which was put into operation in 2011. "This is the very last level that has been opened at this depth in Germany," explains Stellmacher. "We are therefore using the latest mining technology."

Traveling by overhead monorail conveyor

The ride starts, and the diesel trolley rattles and moves along at a leisurely 10 km/hour like an old tramway. This is why we were given foam ear plugs and why all visitors are encouraged to take along several pairs of them. During the 3.5 km ride to the mining level, passengers are made to sit behind one another in a single row. The dead-straight tunnels with a semi-circular profile are slightly short of 7 meters wide. Everything is arranged in a linear direction and nothing extends towards the sides. The hanging monorail is therefore hardly wider than the back of a miner.

As soon as the rail has moved along the first 90-degree bend, the temperature increases considerable - in spite of the ventilation that provides a permanent flow of fresh air towards the mining level where the miners work. A gigantic fan in the extraction shaft continuously sucks the waste air upwards. Located above the surface, this machine extracts up to 26,000 cubic meters of air per minute out of the mine workings. This is easily ten times the amount actually needed by the miners. It generates a strong suction in the tunnels that continuously pulls fresh air downwards through the intake shaft near the elevator.

Neon tubes dimly illuminate the tunnel. Beams are suspended under the ceiling at regular intervals and two water-filled, easily breakable trays are placed on each beam. "This is still the best way to control a firedamp explosion," explains Stellmacher. Methane escaping from the seam and the surrounding strata mixes with air to form an explosive mixture that could be ignited by a spark and then create a pressure wave in the narrow shaft.

Miners therefore make extreme efforts to avoid any source of ignition. No smartphones, no watches with batteries - and the only tobacco is taken as snuff. This is rather a common habit, however.

If a firedamp explosion should nevertheless occur, the pressure wave will rupture the trays and the falling water will suffocate the expanding fire before it can shoot through the shafts. The mine gas is omnipresent and can apparently even be heard on occasion when it escapes the rock with a scraping sound.

Communication via access points

Measuring devices are installed in strategic places and manual measuring devices are attached to the belts of many mining foremen to monitor the gas concentration at any time. The mining foremen also wear explosion-proof PDA devices. "In this way, we communicate with our colleagues on the surface by transmitting data via the access points," says Stellmacher. The control stations for the mining operations and the safety center, which monitor the entire operation, are located above ground. All data is collected here, including on air flow, temperature, production volumes and the current location of the miners and diesel trolleys, as the PDA devices continuously communicate with the access points.

After a few minutes, the train stops in front of a bend. Two miners climb in and one gets out. Two bends and a quarter of an hour later, we drive downhill - in as far as this is the appropriate term when you are inside a mountain. The air gets significantly warmer and more humid. It feels almost tropical. The density of the dust particles dancing in the dim light is increasing. Yellow markers at the top and bottom of the walls are visible in spite of the lack of color. They show the thickness of the seam that is waiting to be mined. We must be close to the mining level.

We walk the last few meters, past the massive armored chain conveyor system that feeds the raw coal into a crusher and then along the conveyor system. At the end of our path, we climb over the long, narrow machine system of extraction and conveying technology and the roof support. Then we are awarded with a breathtaking view of the seam. The 1.4 meter layer of finely grained, pure coal glistens in a beautiful anthracite color. The energy that it contains is almost palpable. The freshly mined edge of the seam feels surprisingly soft and silky. By now, our fingers are covered with a gray film similar to the fine crumbs from a pencil sharpener.

300 million year old plant remains

The notion that all this consists of plant remains that covered large parts of the earth 300 million years ago is simply overwhelming. Today, the seam extends in the form of a broad, wavy ribbon under the Netherlands and the North Sea up to England, diving more deeply into the earth in this direction. However, there is little time for philosophical thought - the coal plow is currently standing still.

This is the ideal opportunity to climb directly into the coal face, the actual mining area, which is 284 meters long and arranged at a right angle to the two parallel tunnels that extend into the seam. They have been cut in advance by high-powered road headers.

We crouch under the shield, a massive C-shaped protective device that is open towards the seam. Closely spaced on the floor are the approximately 1.5-meter-wide feet, which resemble the extra-wide forks of a forklift. At their rear, a back wall of the same width makes use of hydraulic force to press the actual shields upwards in order to support the ceiling, on which 1,200 meters of mountain rest. The shields are approximately 6.5 meters long, extend up to the seam and bridge the armored face chain conveyor on the floor, which transports the broken coal away from the coal face like a conveyor belt.

Behind the back wall of the shield, where the seam has already been mined, is a hollow space called the caved area. This is where the mountain moves down and closes off the hollow behind the shield structure. Rumbling noises can be heard from there on occasion. The seam surface is mined using the retreat method in order to leave this highly dangerous area behind. Two parallel tunnels were originally cut up to the end of the planned mining level and connected at the end by a rise heading. Since that time, the coal plow has been cutting through the seam area using the retreat mining method. Thus far, 900 meters of mining level 572 have been mined, which is one of the last mining levels being mined in Germany.

"Over there is the coal plow." Stellmacher points at several gray cast-iron molded parts that seem to cling to the seam in a rather meaningless way, protruding through a slope of small and large lumps of coal. A layperson cannot discern anything here. "Be careful!" the mining foreman calls to Martin Schweiger. "We are moving the plow a few meters forwards." Once it has been assured that all people are wearing dust masks, ear plugs and protective goggles, the plow moves at walking speed and without noticeable effort out of the gravel and into the tunnel. Now we can see the 50 martial chisels that have eaten their way through the seam from top to bottom. They remind me of the metal-toothed villain Jaws from the James Bond movies.

Within two minutes, the plow zooms along the 284-meter coal face, making lots of unpleasant noise and breaking a 5-cm-thick coal layer off the seam. When the plow returns from the darkness at the end of the coal face, it has already gnawed 40 tons of coal out of the mountain. It feels as if at least a ton of it is in the air in the form of fine dust.

No job for softies

The freshly opened seam exudes heat - at this depth at approximately 42 °C. Sweat is pouring in buckets, in spite of the permanent cooling effect of the air flow and the coal face cooling machines. Now we realize why Holger Stellmacher had strongly recommended that we put at least two bottles of water into the inside pockets of our jackets. Dust and sweat blend into a mixture that blackens each face in no time, particularly when the air comes from the front. Only the eyes and teeth sparkle brightly. "This is the worst job in the entire mine," Stellmacher screams against the noise of the plow. "You have to be really fit to do it."

After approximately 10 plow drives, the gap between the seam front and shield has grown to about half a meter. Even more unprotected coal face area would be dangerous. The mining foreman stops the plow and activates the fully automated control of the shield extender. Every second shield releases its hydraulic pressure with a loud hiss and the shield moves away from the ceiling. The elements move approximately half a meter towards the seam before the shields again press against the unfortified ceiling of the coal face that had been supported by the seam a few minutes ago. During this process, the shields have pushed the armored face chain conveyor along the entire width of the coal face. It is now again close to the plow.

For a moment, the shield looks like the rail of a rack-and-pinion railway, one part in front and the other at the back. But the elements left behind are already moving forwards and close up the gap. The chain conveyor system onto which the armored face chain conveyor drops the coal during the operation also moves back by half a meter.The entire longwall face equipment has now automatically advanced with scraping and crunching sounds. Unlike in the tunnels, the machines here are not suspended, but are standing on the floor, which is hard to see under the circumstances. The floor is extremely uneven, and large amounts of coal and gravel are lying everywhere. It is indeed surprising for a layperson that the coal face equipment should nevertheless still be usable. High tech in gray.

While the plow continues to scrape, the miners close up the new access to the caved area in the drift wall, which was opened by the advancing shields. They wedge massive wooden pillars between floor and ceiling. Those will later be equipped with shuttering and poured with concrete to form roadside packing. The caved area must always be properly closed off, so that dropping rocks cannot roll into the tunnel. This way, it is also ensured that the air flows through the coal face area and not towards the caved area.

Manufacturers in Germany can look to the future with optimism

It is now time to find our way back through the long tunnels to the shaft, as the elevator only transports people once every hour. The rest of the time it transports only material - but not the coal. The black gold is moved along a separate path.

The coal moves from the armored face conveyor past a junction frame and into the chain conveyor system, through the crusher and past a transfer point onto a 1-meter-wide conveyor in the tunnel, which is suspended from the ceiling and runs in parallel to the overhead monorail. The coal accompanies the diesel trolley for several kilometers. A miner dives onto the conveyor, using a dedicated climbing-on point that is marked by a green lamp. Lying on the coal, he moves through the long tunnel for a few minutes until a red lamp tells him to climb off the conveyor.

Downstream, the conveyor soon turns towards the right and disappears in the gray darkness of a tunnel. Its black load will be dropped into the central bunker after approximately 3 kilometers. After another 15 kilometers of conveyor system, a conveyor that moves at an incline inside the mountain transports the coal mined at all three mining levels to the surface for processing - a total of 11,000 tons of raw coal per day.

At level 572, another 1,100 meters of seam are still waiting to be mined and brought to the surface. This will be done at the end of the coming year, when the very last layer will be mined in Germany.

However, this will not mark the end for manufacturers of mining machinery in Germany. They may have to do without their oldest market - but they can look to the future with optimism.

Further Information

VDMA Mining   |   VDMAimpulse 06-2017: "Closure of mines - loss to the supplier industry?"

Klaus Stöckmann, VDMA Mining.

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