I am trying to precipitate Silicon / Silica from an acidic solution at ambient temperature, but I'm having no luck. The Si in solution was first extracted from a solid amorphous silica sample using a 0.2M NaOH solution (1:400 soil / solution ratio) at 85 degrees Celsius to give a Si solution concentration of 760 - 900 mg Si/L (27 - 32 mM). I then took a 75mL aliquot (of the cooled solution) and whilst the sample was being agitated I acidified it (by titration) using approximately 20mL of 2M HCl to give a resulting pH of around 1.1. Given the Si concentration appears to exceed the solubility of amorphous Si I was expecting it to precipitate into a white solid, but after 3 days of agitation the solution still remains clear and no precipitate has formed (no sol or gel either has formed).
What am I missing and what should I do to make the Si precipitate from the acidic solution?