KINETIC EQUATIONS AND TYPE CURVES FOR PREDICTING THE PRECIPITATION OF AMORPHOUS PREDICTING THE PRECIPITATION OF AMORPHOUS SILICA FROM GEOTHERMAL BRINES

Abstract

We have experimentally studied the kinetics of amorphous silica precipitation from aqueous solution up to 100 deg. C and containing up to 1M NaCl. Empirical equations for the rate of molecular deposition on surfaces as a function of temperature, dissolved silica concentration, pi and salinity are presented. Empirical type curves which depict the decrease in dissolved silica concentration through homogeneous nucleation of colloidal silica are also presented. Two practical examples related to geothermal practice are given.

Introduction

It appears that under most conditions silica precipitates as relatively pure amorphous silica. precipitates as relatively pure amorphous silica. This conclusion is supported by geothermal field experience, laboratory research and theoretical considerations. The scope of our study was restricted accordingly.

The process of amorphous silica precipitation from supersaturated bulk aqueous phase consists of the following steps:

  1. Growth of polymeric silicic acid complexes to critical nucleus size.

  2. Nucleation of an amorphous silica phase (from here on simply AS) in the form of colloidal particles.

  3. Growth of the supercritical AS particles by further molecular deposition of silicic acid on their surfaces.

  4. Coagulation or flocculation of colloidal particles to give a gel.

  5. Cementation of the particles in the gel by chemical bonding and further deposition of silica between the particles.

  6. Rarely, growth of a secondary phase in the interstices between the S particles. Such secondary deposition of FeS and of calcite has been reported, but is uncommon.

The above process occurs when the concentration of dissolved silica is high enough for homogeneous nucleation to occur at a significant rate. Very roughly, this requires a saturation ratio (the ratio of concentration-to the equilibrium solubility) of two or greater. If this condition is met, massive precipitation occurs. This is the case with the precipitation occurs. This is the case with the residual (flashed) brine at Niland, Cerro Prieto and Wairakei, and dealing with the consequences presents the greatest technical problems encountered at these sites.

If the concentration of dissolved silica is too low for massive homogeneous nucleation to occur, relatively slow molecular deposition upon solid surfaces becomes the major precipitation mechanism. The product of this process (essentially step 3 of the product of this process (essentially step 3 of the above scheme alone) is a dense vitreous silica.

The goal of this study has been to generate sufficient experimental data and theoretical analysis concerning steps 1) to 3) to enable their phenomemology and kinetics to be quantitatively predicted and phenomemology and kinetics to be quantitatively predicted and interpreted over most of the range of practical concern.

The results presented in this paper are of a preliminary nature and subject to expansion and preliminary nature and subject to expansion and revision. We do, however, consider them to be adequate for most practical applications as far as they go.

Values of constants not defined in the text are given in the Nomenclature section.

The Homogeneous Nucleation of Colloidal Amorphous Silica

The voluminous gal-like deposits encountered at Cerro Prieto, Wairakei, and Niland consist of flocculated colloidal amorphous silica.

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