Formation of alternating layered Ba-Sr-sulfate and Pb-sulfide scaling in the geothermal plant of Soultz-sous-Forêts
Nitschke, Fabian; Scheiber, Julia; Kramar, Utz; Neumann, Thomas
published: Feb 1, 2014
ArtNo. ESP154019102003, Price: 29.00 €
Scaling formation in surface installations of geothermal power plants can substantially affect power production by impairing heat transfer and reducing pipe diameters. In addition, the mineral deposits can incorporate naturally occurring radioactive nuclides into the crystal lattice during precipitation and have to be regarded as a potential hazard to health and environment. A profound understanding of formation mechanisms should facilitate the prevention of scaling in the future. Therefore fluid samples and scalings from the geothermal power plant at Soultz-sous-Forêts were investigated in detail. The fluid shows a total salinity (TDS) of 92 g/l and can be classified as Na-(Ca)-Cl-type. Considerations of the saturation state reveal a slight oversaturation with respect to barite (BaSO4) and celestine (SrSO4). X-Ray diffraction measurements together with scanning electron microscopic observation reveal that the scalings consist of barite-celestine solid solution ((Ba, Sr)SO4) interlayered with very fine layers of galena (PbS). The mineralogical composition was confirmed by X-ray fluorescence analysis showing a bulk composition of Ba (31.7?34.6 %), Sr (10.8?12.1 %), Pb (6.2?12.4 %) and S (13.1?14.5 %) for the sulfates, and Pb (66.6 %) and S (11.7 %) for the sulfidic part of the scalings. Other metals/metalloids like Sb (5.6 %), Cu (4.2 %), As (2.3 %) and Fe (2.0 %) were found to be present in minor amounts in the sulfides. Sulfur isotope studies show strong fractionation between the sulfate (?34S =+15 ?) and sulfide (?34S = ?12 ?) phases. This indicates that bacterial sulfate reduction occurs, initiating sulfide precipitation from sulfate-rich fluids. The layered structure of the scalings can be correlated well with the operation state of the plant. Accordingly, sulfate layers precipitate under regular operation conditions, whereas sulfides were formed during start and shut-off phases of the plant.