Q.How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.

The undissolved impurities (including much of the associated silica/iron oxide residue) are filtered off as red mud. $CO_2$ gas is then passed through the clear filtrate to precipitate hydrated alumina, leaving soluble sodium silicate (if any silica dissolved) in the mother liquor:

$2Na[Al(OH)_4](aq) + CO_2(g) \rightarrow Al_2O_3.xH_2O(s)\downarrow + 2NaHCO_3(aq)$ …

Step 1 – The chemical basis of the separation

Alumina ($Al_2O_3$) is amphoteric — it dissolves readily in hot concentrated NaOH. This is the key property exploited to separate it from the silica and other impurities typically present in bauxite ore.

Step 2 – Digestion (leaching) step

Crushed bauxite is digested with a concentrated NaOH solution at about 473–523 K under a pressure of 35–36 bar. The alumina dissolves as soluble sodium aluminate:

Most impurities (iron oxide and the bulk undissolved silicate residue) do not dissolve under these conditions and are removed by filtration as an insoluble residue ("red mud").

Step 3 – Precipitation of pure alumina

The clear filtrate, containing sodium aluminate, is treated by passing carbon dioxide gas through it (and seeding with a little freshly precipitated $Al(OH)_3$ to help crystallisation). This neutralises/hydrolyses the aluminate, precipitating pure hydrated alumina, while any dissolved silicate stays behind in solution as soluble sodium silicate and is not precipitated:

Step 4 – Calcination …

Exploit alumina's amphoteric character: hot concentrated NaOH dissolves $Al_2O_3$ selectively as sodium aluminate; precipitate pure hydrated alumina back out …

• Forgetting the second step entirely (precipitation with $CO_2$) and stopping at 'dissolve in NaOH', which only separates it from insoluble impurities, not from the soluble aluminate form. …