An experiment on monomeric and polymeric silica precipitation rates from supersaturated solutions

Abstract

Experimental solutions initially supersaturated with amorphous silica were allowed to polymerize while being pumped through a column packed with silica gel. The concentrations of silica monomers and polymers in the solutions were monitored at the inlet and outlet of the column. Results of experimental runs conducted at 50°C with pure water and saline solutions (0.1 M, 0.2 M, 0.3 M and 0.4 M NaCl) showed shorter induction time and faster polymerization with increasing salinity, consistent with theory and past studies. In all runs, monomeric silica dropped to saturation for amorphous silica in the column whereas the concentration of polymerized silica changed little. A solution of 0.3 M NaCl at 90°C also showed a lesser tendency for polymeric silica than monomeric silica to precipitate on the surface of the silica gel in the packed column. However, higher temperature and salinity allowed polymers to grow large enough to deposit in the source tank. In the column, some of the polymeric silica was lost either by precipitation or filtration from solution in the void spaces of the column. The experiment demonstrated that amorphous silica scaling from geothermal brine can be mitigated by allowing dissolved silica in excess of amorphous silica solubility to polymerize, provided that factors affecting polymerization rate and polymer deposition rate are known.