The Effect of Colloidal Silica on Silica Scaling from Geothermal Fluid

Abstract

Silica scaling experiments were undertaken at well in the Ohaaki geothermal field to determine the effect of colloidal silica on the deposition rate. Fresh geothermal fluid containing only monomeric silica was in known proportions with aged geothermal fluid containing high concentrations of colloidal silica. The fluid was passed through mm long 9 I.D.stainless steel tubes at 1 The total concentration of silica was approximately 730 and the temperature of the mixed fluid ranged between 50 and 75?C. Scaling was found to increase to a maximum on reducing the fraction of monomeric silica to about 0.8 of the total silica concentration. At a fraction of 0.65 the weight of Scale deposited was approximately equal to fresh unpolymerized fluid (butof differentmorphology)while scalingwas negligible at monomeric silica fractions of 0.5 and below. Surprisingly the scale deposited at monomeric silica fraction of 0.8 and below was soft and porous. At the a fraction of 0.85 the scale was compact but vitreous. Vitreous silica was only deposited from fresh unpolymerized fluid.