Experimental Study of Stability of Amorphous Mg-SiO₂ Scale up to 300°C

Abstract

Experimental study of amorphous Mg-SiO₂ scale was done to investigate the stability and chemical composition from 100 to 300°C in pH range from 3 to 7. MgCl₂ fluid (23 mg/kg in mixed water) and Na₂SiO₃ fluid (1500 mg/kg SiO₂) were mixed and sealed in quartz glass tube after adjusting each pH. The tube was packed into a SUS reaction vessel and heated up to 300°C for 3 days. The product was filtered out through 0.22μm filter and was analyzed by XRD and SEM-EDX for the chemical composition and mineral identification. The reaction product is in amorphous state and chemical composition is mainly composed of SiO₂ and a minor amount of Mg. The concentrations of Mg in fluids after reaction decreased with increasing pH at all temperature conditions. The ion activity product (Mg²⁺) (H₃SiO₄^-) (OH^-) of the amorphous Mg-SiO₂ scale was calculated thermodynamically and is decreased with increasing temperature up to 300°C. This tendency is almost accordant with the previous results reported in Icelandic geothermal fluids. However, the solubility of product calculated in this study is lower than that in Iceland (0.02 Mol/kg NaCl) and is depend on the salinity (0.05 and 0.5 Mol/kg NaCl in this study), that is, the solubility of product is higher in lower salinity.