Initial Scaling on Metal Surfaces at Binary Geothermal Plant of Obama Hot Spring in Japan

Abstract

Microstructures of scales adhered on steel pipes for a transportation pipe of geothermal fluid and a plate for a heat exchanger collected from a geothermal plant in Obama town, Unzen city, Nagasaki, Japan were analyzed by SEM-EDS and XRD. There were three locations where scales were sampled: steel pipe for a transportation of geothermal fluid before aeration and decarboxylation, steel pipe after them, and titanium plate for heat exchanger after them. By observation from cross section, the mineral phases of the main scales, that of the initial scales, and the nucleation sites of the initial scale in their scales were investigated. Calcium carbonate was the mineral phase of the main scale adhered on a steel pipe for transportation of geothermal fluid before aeration and decarboxylation. Magnesium silicate was the mineral phase of the main scale on the other locations. The mineral phases of the initial scales at all locations were magnesium silicate. Corrosion product seems to be preferred site for scaling as steel pipes because corrosion product of iron covered with magnesium silicate. On the other hand, the nucleation site of magnesium silicate adhered on a titanium plate could not identified. The XRD patterns for their initial scales exhibited the layered magnesium silicate with low crystallinity. Their patterns were not corresponding to those of typical layered magnesium silicates (e.g. talc and sepiolite). It is possible that scaling at system utilizing the geothermal fluid at Obama town becomes slow by preventing precipitation of layered magnesium silicate which is the mineral phase of the initial scale.