# Removing aluminum oxide from impure silica

We have an assay for a mine. Our goal is to have only silica. Is there a cost effective way to extract the contaminants? The product is a fine powder called diatomaceous earth, or fossil shell flour. Our goal is to use the product to help poor pacific islanders increase their silica uptake and assist in heavy metal detox. Many members of the pacific island nations suffer from heavy metal poisoning. The composition of the powder is:

• > 84.7% $\ce{SiO2}$
• < 4% $\ce{Al2O3}$
• < 2% $\ce{Fe2O3}$
• < 0.33% $\ce{CaO}$
• < 0.36% $\ce{MgO}$
• < 0.5% "other"

We aren't concerned with the calcium, iron or magnesium oxides, just the aluminum oxide.

• Is there a simple process to reduce the aluminum oxide levels?

• Also, what is the easiest process to convert the silica to orthosilicic acid for increased bio-availability?

You may try the famous Bayer process, which is used for extracting alumina form bauxite. When treated at $$160°$$C and high pressure with $$\ce{NaOH}$$, a mixture like yours dissolves $$\ce{SiO2}$$ and $$\ce{Al2O3}$$, and lets the other compounds insoluble. This is a first purification. $$\ce{SiO2 + 2 NaOH -> Na2SiO3 + H2O}$$ $$\ce{Al2O3 + 2 NaOH + 3 H2O -> 2 Na[Al(OH)4]}$$Later on, bubbling $$\ce{CO2}$$ in the solution makes $$\ce{Al(OH)3}$$ to precipitate. $$\ce{Na[Al(OH)4] + CO2 -> Al(OH)3(s) + NaHCO3}$$ Of course $$\ce{NaHCO3}$$ is immediately transformed into $$\ce{Na2CO3}$$ by $$\ce{NaOH}$$. $$\ce{Al(OH)3}$$ can be eliminated by filtration. And the final solution contains only sodium silicate, apart from the excess of $$\ce{NaOH}$$. Silica can then be obtained pure by adding enough $$\ce{HCl}$$ to destroy $$\ce{Na2SiO3}$$ and the NaOH in excess.$$\ce{Na2SiO3 + 2 HCl + H2O -> Si(OH)4 + 2 NaCl}$$ It is advisable to highly dilute the $$\ce{NaOH + Na2SiO3}$$ solution before adding $$\ce{HCl}$$. Because the precipitate of $$\ce{Si(OH)4}$$ is extremely voluminous, and fills up the entire solution.
Treat it with concentrated $\ce{HCl}$, it will react with the aluminium oxide and turn it into aluminium chloride. You can separate it from the silica powder by a simple filtration.
$$\ce{Al2O3 + 6HCl -> 2AlCl3 + 3H2O}$$