Precipitation of silica from alkali solution

April 29th, 2009

Previously, i talked about forming potassium silicate through alkali fusion at moderate temperature to refine silica from clay. The original source of that process used hydrochloric gas in argon for the neutralization process. Where do we get HCl in an ecological way? how do we handle it with our awkward backyard techniques?

Some other acid would be better if it could work. Especially acids from the flue gasses of combustion/pyrolysis. The salt of this acid with potassium and/or sodium must be soluble in water (to wash from the silica precipitate), and reasonably regnerable (to get the alkali and acid back). HCl is not easily regenerated (lots of electricity in electrolysis or >2000C for thermal decomposition, though some charged membrane type process does exist)

here is what i think about using carbonic acid.

Carbonic acid is actually a stong acid (high pKa) but appears weak because of the low soluability of carbon dioxide in water. At low temperatures and high pressures, a low pH solution can be obtained (atleast pH 2.7 for 800 psi in an ice water bath, calculated before i had this blog to keep track of things like this). Also, potassium and sodium carbonate are soluable in water, meaning they could be washed from the silicic acid precipitate.

two questions arise:

Although a low pH can be obtained, how much alkali solution can actually be neutralized before the water is saturated with carbonate ions?

How can the potassium carbonate be regenerated?

regeneration is possible by thermal decomposition, though it is difficult (how difficult is hard for me to find. it will decompose at reasonable temp, but it may require excessive pulling of the co2 off with a vaccum pump ). The kraft process in paper pulping mixes calcium oxide with the alkali carbonate and precipitates calcium carbonate. CaCo3 (limestone) is regularly reduced in lime kilns to regenerate CaO, giving a CO2 pressure of 1 atm at 800C.

thus, i think Carbonic acid may be workable assuming it can neutralize enough alkali to give a good yield in one pass. Otherwise, large volumes of dilute alkali-silicate solution will have to be used, meaning either lots of water pumped to 800 psi into the vessel, or a large vessel for a batch process.