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Characterization of scaling material obtained from the geothermal power plant of the Balmatt site, Mol
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Geothermal energy is a renewable energy source by which heat is extracted from the subsurface. Due to changes in pressure and temperature and chemical reactions when the geothermal fluid is pumped through the wells and surface installations, scales can be formed. These scales can accumulate in different locations of the geothermal system and/or parts of the surface installation and cause clogging of the wells, pumps, pipes, heat exchangers and/or filter units. Consequently, the efficiency of the geothermal system can decrease. To obtain insight into the formation of scales in the surface installation of the Balmatt geothermal site, heat exchanger and filters scaling material was sampled after a circulation time of 240 h and 46 h respectively. This material was chemically, mineralogically and radiologically characterized. The scaling material found in the heat exchanger consists mainly of galena (PbS). Minor amounts of challacolloite (KPb2Cl5) and laurionite (PbClOH) were also observed. Additionally, in the filter scaling material carbonates such as witherite (BaCO3), siderite (FeCO3) and possibly rhodocrosite (MnCO3) are present as well, together with an amorphous phase and elemental copper. Salt minerals such as sylvite (KCl), halite (NaCl) and challacolloite (KPb2Cl5) and iron oxides such as hematite (Fe2O3) were also observed in the scaling material. However, these minerals are thought to be formed during drying of the scalings after sampling. Together with these minerals, radionuclides such as 226Ra, 210Pb and 210Po are observed. 226Ra seems to predominantly co-precipitated with and/or sorb on witherite and hematite, while 210Pb and 210Po are rather accumulated in galena and laurionite. Equilibrium calculations, using the PhreeqC software were performed to gain insight into the geochemical processes. The calculations indicate that galena is one of the main oversaturated minerals. Precipitation of native copper and carbonates are also observed and supported by the equilibrium calculations. The formation of these precipitates is probably a result of electrochemical corrosion of carbon steel and/or corrosion by dissolved CO2.
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