Opencast mining and technology

Before an opencast mine

Lignite must be extracted in opencast mines, i.e. in an open pit. Underground extraction, as is familiar from the mines in the Ruhr region, is not possible in the Rhineland due to the loose layers of earth above the lignite seams. There, mining has to start by stripping the complete surface, before it can work its way into the depth; although temporary, this nevertheless amounts to serious interference with the natural and cultivated landscape. Besides excavating farmland and wooded areas and relocating roads and waters, we also have to resettle entire townships.

Groundwater lowering

Interference by mining operations goes even deeper: to ensure safe mining, we must lower the groundwater table to below the deepest point in the opencast mine. For this purpose, some 600 million cubic metres of water must be removed from more than 1,000 wells every year. RWE Power limits the consequences of such drainage for Nature to the absolute minimum. This is done by removing only as much water as is essential and by percolating treated sump water into the soil of wetlands and by discharging it into streams. This allows us to stabilize today‘s groundwater level in wetlands worthy of protection located in river meadows or hollows and to maintain the ecological structure, value and qualities.

Outside these areas, lowering the groundwater has no effect on vegetation anyhow. The loess stores the water like a sponge, and the plants survive on precipitation. This is evidenced not only by the healthy old forests along the rim of the opencast mines, but also by the landscapes in the drainage area that are intact both ecologically and in their water economy. Drinking and industrial water, too, for people and companies is ensured in the long term. Technical authorities and RWE Power have a clear picture of the region‘s water balance. The measuring data of the groundwater wells are radioed to central offices and continuously evaluated by computer together with the water levels of the control gauge – an effort that would be hardly feasible without such assistance in view of the many millions of readings.

Mining and materials handling technology

In the opencast mines, the bucket wheel excavators start by stripping the topsoil, the fertile loess clay, and then dig the so-called overburden: clay, gravel and sand. The biggest excavators are 96 metres high, 225 metres long and weigh some 13,500 tons – and are operated by four to five men.

Conveyor belts or trains deliver the coal to the power plants and upgrading operations in the mining area; there, it is used to generate electricity or further processed to make solid fuels and filter coke. Loess and overburden, too, are transported by belt or rail. They are distributed in those sections of an opencast mine where the coal has already been extracted: there, spreaders stack the material. Immediately afterwards, recultivation starts, i.e. designing and shaping the new landscape.

Process optimization

With the development of the Hambach opencast mine in 1978, mining technology reached today‘s dimensions: the first bucket wheel excavators with a daily output of 240,000 bank cubic metres of overburden and coal were commissioned. Opencast mining technology reached its maximum here, but not its optimum: ever since, engineers and technicians have been steadily improving the extraction process. In doing so, they are pursuing two main goals: first, with the lowest possible outlays for personnel, maintenance and energy, they wish to achieve the highest productivity in the core process, viz. “mass transportation of coal and overburden“. Second, they wish to systematically and continuously monitor and ensure the quality of the coal on its entire route from deposit to power plant.

Still, the casual visitor will notice nothing of these initiatives in the bucket wheel excavators. This precision work is largely a matter of electronics, although it also has a deep impact on the organization of an opencast mine.

Optimizing extraction – that does not mean maximum output at any cost: that would wear out the equipment too quickly, for example, and unacceptably increase the risk of disruptions and, hence, loss of output. What matters in fact is that the entire extraction system of excavator plus conveyor belt plus spreader operates at a continuous high level, as sparingly as possible and, hence, glitch-free. This requires ultra-modern process and control technology. And that means, for example: being able to dose the speed of the bucket wheel in such a way that the conveyor belts, depending on their state, have to carry enough, though not too much coal and overburden; to monitor coal quality using a deposit file, GPS and online measuring equipment, so that, at the end of the transport route, each power plant boiler receives the coal type tailored to its needs; and to reduce the slip that is a drain on energy and material when the conveyor belts are started up. Raising productivity also means, e.g., perhaps keeping a slightly battered spreadergoing at just 80% of its target performance until its repair fits optimally into the standstill plan.

This development has two main consequences: those in charge of opencast-mine planning, production and maintenance need more information about current operating states than in the past. This calls for a lot of electronics for measuring, data transmission and evaluations. In addition, RWE Power has aligned the organization of its opencast mines to its operating equipment and processes, and no longer to the occupations and functions of the staff involved. Excavator operatives, for example, work together with fitters and electricians in charge of excavator maintenance in one department. This simplifies collaboration between specialists, shortens decision-taking routes and strengthens a team‘s own sense of responsibility. The hierarchies in the new organization, too, are much flatter than they used to be.