Abstract
The polymerization of dissolved silica in aqueous solutions up to 100 degrees C and containing up to 1 M NaCl has been studied experimentally, and theoretically. In this paper, the results of this work are presented in a form suitable for practical use in interpreting and predicting the chemistry of silica in geothermal brines. Empirical equations for calculating the rate of molecular deposition of silica on surfaces as a function of silica concentration. temperature, pH. and salinity are presented. Theoretically calculated type curves that depict the decrease of dissolved silica concentration by homogeneous nucleation and particle growth are presented, along with the procedures for using them to predict the course of this process under different conditions.
Introduction
Usually, silica precipitates from geothermal brines as colloidal amorphous silica (AS). The process of AS precipitation consists of the following steps.
1. Random growth of silica polymers past critical nucleus size. Above this size, the polymers become colloidal AS particles that are large enough to grow spontaneously and without interruption. This process is called homogeneous nucleation.
2. Growth of the supercritical AS particles by further chemical deposition of silicic acid on their surfaces.
3. Coagulation or flocculation of the colloidal particles to give a floc-like precipitate or gel.
4. Cementation of the coagulated particles by chemical bonding and further deposition of silica between them to form silica scale and other solid deposits.
The preceding sequence of processes occurs when the concentration of dissolved silica is high enough for homogeneous nucleation to occur at a significant rate. Very roughly, this requires supersaturation by a factor of 2.5 or more. If this condition is met, rapid polymerization occurs, and massive precipitation or scale deposition may follow. This is the case with the brine at Niland (CA). Cerro Prieto (Mexico), and Wairakei (New Zealand). after it has been flashed down to atmospheric pressure. The voluminous floc-like silica deposits encountered in these areas consist of colloidal AS that has been flocculated by the salts in the brine. The crumbly gray and white scales associated with this material are cemented aggregates of colloidal silica.
If the concentration of dissolved silica is too low for rapid homogeneous nucleation to occur, relatively slow heterogeneous nucleation and the deposition of dissolved silica directly on solid surfaces become the dominant ploymerization processes. The product of the latter process (essentially Step 2 of the preceding sequence alone) is a dense vitreous silica. At higher temperatures, this process may produce scale at a significant rate.
This paper has two purposes: to summarize succinctly and quantitatively what we have learned in our kinetic studies of silica polymerization and to demonstrate by example how our results may be applied to studying practical problems in geothermal energy utilization. Because it is a summary, actual experimental data and most details of derivation have been omitted, they may be found elsewhere. Because some of the material in this paper is condensed from an earlier paper, it is partly of a review nature. It is an updated version of an earlier article.
Studies of the actual formation of silica scale and the removal of colloidal silica from geothermal brines have been reported elsewhere.
Molecular Deposition on Solid Surfaces
By molecular deposition we mean the formation of compact, nonporous AS deposits by chemical bonding of dissolved silica directly onto solid surfaces. This is also the mechanism by which colloidal silica particles grow once nucleated.
SPEJ
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