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82 E&MJ; • JUNE 2016 www.e-mj.com OPERATING STRATEGIES Lignite can be successfully extracted from open-cast operations only if the ar- ea's ground water level is lowered below the coal seam. This has a strong infl uence on the region's water balance. Depending on the chemical nature of the overburden, the ground water that often rises again to a higher level after the cessation of min- ing in the pit may become acidic due to pyrite weathering. Germany's Federal Mining Act (BBergG) stipulates that oper- ators carry out rehabilitation measures on the surfaces areas of former mines. This also applies to the lakes they produce. Thus, ground water remediation pro- jects seek to balance-out the ground wa- ter defi cit and neutralize the contents of any ensuing pit lake so that fl ora and fau- na may be established. Lime is usually used in this process, but large quantities are generally needed to treat an entire body of water because only a low level of blending can be achieved using conven- tional methods. To overcome this obsta- cle, a German fi rm has developed what it calls the Underwater Nozzle Pipelines (UNP) method as an alternative: lime mixed with lake water is pumped from the nozzles of pipes fl oating in the lake, using the lake's natural circulation pat- tern to thoroughly blend and distribute the suspension evenly throughout. The method was successfully demonstrated in a pilot project at Lake Scheibe, in the Lu- satia region of eastern Germany. The lake contains approximately 110 million m 3 of water. Before primary neutralization was achieved, it had an acid concentration of 381 million moles. The lignite mining region of Lusatia alone has 36 pit lakes with an aggregate expanse of 146.8 million m 2 and a wa- ter volume of 2.287 billion m 3 . Their pH values vary from 7.5 to an acidic 3, which in itself precludes the emergence of a functioning ecosystem. This prompt- ed GMB GmbH, a mining services pro- vider, to begin development of the UNP method in 2010, in partnership with Fels-Werke GmbH, Institut für Wasser- wirtschaft, Ökologie und Siedlungsbau IWSÖ GmbH, the Kemmer/Harbauer GmbH Group and Linde Gas. The pro- ject group used the laboratories of the Brandenburg University of Technology Cottbus-Senftenberg (BTU). "GMB GmbH applied to carry out both a pilot and a demonstration pro- ject—initial neutralization and CO 2 buff- ering," said Eckhard Scholz, head of the Engineering Division at Lausitzer und Mitteldeutsche Bergbau-Verwal- tungsgesellschaft mbH (LMBV). The LMBV is responsible for recultivation of the region's lignite mining area. Among other measures, it uses pilot and demon- stration projects to develop innovative technical solutions for water treatment, from their technical inception to a scale suitable for actual remediation measures. A fi xed plant was built on the lake shore in early 2012 for the fi rst stage of the project—initial neutralization. The plant's purpose was to blend lake water with lime, on-site. Six pairs of nozzles were attached at 20-m intervals to a fl oat- ing pipe located about 50 cm beneath the water surface during this stage. A 4% suspension consisting of 15,200 mt of slaked lime and 440,000 m 3 of lake water was pumped into the lake at a ve- locity of 7.6 m/s and a fl ow rate of 260 m 3 /h, six days per week over a period of 16 weeks. "The UNP method exploits the phases of lake circulation to ensure ideal blend- ing with the lake water," explained Dr. Mi- chael Strzodka, head of the engineering offi ce at GMB GmbH. "The lime is intro- duced to the epilimnion (upper layer of a stratifi ed lake). But the waters in the lake possess different densities, which ensure that the lime is distributed throughout Making the Lake Do the Work An innovative method to neutralize acidic water in fl ooded open-cast mines uses natural circulation to accomplish the task, with low cost and high effi ciency This treatment plant on the shore of a lake in a German lignite mining district was built in two phases to carry out a successful lime-injection experiment to neutralize the acidic lake water.