Amorphous aluminous silica scales: Thermochemistry for better scale prediction models

BRUCE. W. MOUNTAIN JULIA. K. BJÖRKE
GNS Science Wairakei Research Centre Taupo New Zealand b.mountain@gns.cri.nz

TERRY. M. SEWARD
Victoria University Wellington New Zealand j.bjorke@gns.cri.nz terry.seward@vuw.ac.nz

Scaling is a common problem in geothermal power plants. In particular it occurs from brines where minerals become saturated and precipitate. To prevent scaling geochemists use thermodynamic modelling to apply treatment methods such as adjusting temperature or pH. Amorphous aluminous silica scaling has been reported in many geothermal power plants such as in New Zealand Philippines Salton Sea Japan and Iceland [1]. Currently the sole study on the solubility of amorphous aluminous silica was carried out by Gallup 1998 [2]. These were simple batch experiments in which there was no control on pH or Al concentration. A general reaction can be written for the precipitation of this phase as:

xH4SiO4 + yAl(OH)4- + yH+ = xSiO2.yAlO1.5 + (2x + 2.5y)H2O.

Unlike amorphous silica precipitation this reaction is pH and Al concentration dependent over a wide region of pH. Unfortunately there have been no well-controlled experiments to determine thermodynamic data for this reaction. Preliminary calculations using the data of Gallup 1998 [2] were used to estimate the solubility of amorphous aluminous silica and compare it with the solubility of pure silica. These show that scaling caused by amorphous aluminous silica can occur at temperatures up to 50oC higher than for pure silica based on the chemistry of a cooling flashed Ohaaki brine.

Four samples of amorphous aluminous silica scale were collected from Wairakei and Ohaaki geothermal power plants. XRD XRF and SEM analyses were carried out on the samples and to verify that Al was not present as a distinct Al phase from the amorphous silica. Results from the XRD analyses showed amorphous material with quartz peaks in all samples. XRF results show concentrations of SiO2 ranging from 73.89-76.70% Al2O3 from 8.38-10.46% K2O from 2.13-3.05% Na2O from 1.72-2.15% CaO from 1.08-2.13% and MgO from 0.06-1.99%. Other oxides were less than 1% of the total sample. XRD results and EDX mapping by SEM verified that Al is present within the silica and is not in a distinct phase. Al concentration is zoned and correlates with increases in alkali concentration. Experiments are planned to investigate the solubility of amorphous aluminous silica in the temperature range 100 - 350°C. These will be conducted at saturated vapor pressure and with varying pH using a continuous flow-through system and batch reactors.

[1] Gallup (1997) Geothermics 26 483-499. [2] Gallup (1998) Geothermics 27 485-501.