Analysis of Silica Activity Index from Geothermal Silica Scaling

Abstract

Abstract Silica scaling causes problems in geothermal powerplants piping system such as clogging and reduction of pipe diameter. A suspended bulk sample was taken from Dieng powerplant Indonesia, with more than 80 % silica content. Before further utilization, the properties of the silica powder should be well understood to ensure for an example the correct usage of the application or another example choosing the efficent procedure in clogging prevention. The measure of the activity of silica powder can be expressed in the term of silica activity index (SAI). This index is the level of strength for the silica reactivity, it also indicated the increasing amount of amorphous phase relative to the amount of the crystal phase of silica. This study aims to determine SAI in variation with temperature of silica sample taken from suspended powder from geothermal brine. Experimental treatment on SAI determination consists of samples without preheating and with preheating at 400 °C, 500 °C, 650 °C, and 900 °C. The index was then calculated from the ratio of dissolved silica in the solution of 1 and 2 M 100 ml NaOH for 10 minutes. The experimental results show that the maximum value of SAI is 79% in the sample without heating at dissolving temperature 99 °C. The minimum value of SAI was 8% in samples with 900 °C preheated, treated with the same solution at dissolving temperature 80 °C. The XRD characterization result indicated that the sample without heating has an amorphous structure and tends to crystalize versus increasing heating temperature. At 900°C heating, the sample has a cristobalite and tridymite structure. The XRF analysis in the soluble fractions showed that the silica content in samples without heating is 0.52 wt% and the concentration also decreasing versus increasing temperature to 0.05 wt% for samples with 900°C heating. Hence, the silica activity index is influenced by sample crystallization during treatment with a strong base and heated dissolution.