Controlling silica scale formation is important because silica scale problems in geothermal power plants affect the stable operation. Currently, many geothermal power plants use a pH adjustment method in which sulfuric acid is added to geothermal water to retard silica scale formation. However, pH adjustment cannot be an ultimate solution for the prevention of silica scaling and in power plants where not only silica scale, but also calcium sulfate scale is occurring Therefore, other methods are have been demanded, and the use of scale inhibitors has attracted attention. Although there are many commercially available silica scale inhibitors, their effectiveness has not been fully understood because no method has been established to evaluate their effectiveness quickly and properly. In our previous study, we introduced our research on the development of immersing metal plates (test pieces) by a batch test, which we originally developed as a method for evaluating silica scale inhibitors. In a preliminary study using that method, we found that Tiron(C₆H₄S₂O₈Na₂)) acts as a effective silica scale inhibitor, forming a stable 1:3 silicate-Tiron complex in aqueous solution between pH 6~10 (Bai et al., 2011). In this study, we compared the effect of the amount of Tiron added to several geothermal waters on the silica scale prevention. We evaluate the Tiron as a silica scale inhibitor by the surface observation of the test piece using SEM-EDX and the quantification of Si using LA-ICP-MS analysis. The following three types of geothermal water with different properties were used in this study: (1) pH 7.3, high salinity, and high polymerization rate. (2) pH 8.5, low salinity, and low polymerization rate and (3) pH 8.8, high silicic acid concentration, and high polymerization rate. As a result of the batch experiment using Tiron, the inhibitory effect was particularly significant in the case of (1), where approximately 99% inhibition was observed. When no chemicals were added to the geothermal water (1), polymerization of silicic acid progressed, and much precipitation was observed in the container used for the batch test. However, when a certain amount of Tiron was added, the amount of precipitation was significantly reduced, and there was a remarkable difference in the appearance of the surface of the test piece. In the case of (2), the Tiron inhibitory effect was also observed, and the Si amount adsorbed) was reduced by more than 50%. On the other hand, in the case of (3), although the inhibitory effect was observed, the amount of inhibition was not as obvious as in the case of (1) and (2). This may be related to the depolymerization effect of Tiron of polysilicic acid due to the faster polymerization rate and higher poly-silicic acid concentration in (3). In all cases, the amount of Si deposited on the surface of the test piece was reduced as a result of the batch test with the addition of Tiron, suggesting Tiron can be a useful silica scale inhibitor. However, as its effectiveness varies depending on the conditions of the geothermal water, further factor analysis on the effectiveness is needed.

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