Silica Precipitation from Analcime Dissolution.

Abstract

Pure monosilicic acid solutions mimic the precipitation of silica dissolved from analcime in HCl. Thus, mineral dissolution and silica precipitation are decoupled and pure monosilicic acid solutions were used to study the precipitation of silicon dissolved from minerals.

Monosilicic acid solutions were used to study silicon precipitation using UV-Vis and DLS. UV-Vis results showed that under very acidic conditions monosilicic acid disappears rapidly from solution and follows second-order disappearance kinetics. Then, from DLS, it is shown that within the first few minutes, monosilicic acid polymerized to form primary particles ~5nm in diameter, which then flocculate, with mean floc diameter increasing exponentially with time. Both the monomer disappearance and flocculation rate increase with increasing HCl concentration.

The flocculation was computer-simulated using a Smoluchowski equation modified for a geometric population balance with a reaction-limited aggregation (RLA) kernel. DLS-measured mean silica particle size agreed well with the simulated mean particle sizes.

Salt experiments indicate that 1M salt added to HCl can increase monosilicic acid disappearance rate and silica particle growth rate in the order: AlCl3 > CaCl2 > MgCl2 > NaCl > CsCl > no salt. UV-Vis results showed that monosilicic acid disappears rapidly and follows a third-order kinetic rate law in contrast to second-order when no salt is present. DLS results showed that silica particle size also increases exponentially with time when salt is present. It was also found that specific reaction rate constants: monosilicic acid disappearance rate constant, particle growth rate constant and computer-simulation aggregation rate constant, increase exponentially for all salts as a function of solution ionic strength.

Finally, initial dissolution rate of analcime in the strong acids HCl, HBr, and HNO3 are very similar and follow a Michaelis-Menten mechanism. At proton concentrations less than 0.055M, initial dissolution rate in citric acid was faster than initial dissolution rate in the strong acids.

The observed plateau in AAS/ICP-MS silicon concentration during analcime dissolution exists because of a recondensation reaction between silanol groups within dissolving analcime. The recondensation rate varies in strong acids corresponding to the order: H2SO4 > HI > HBr > HCl > HNO3