Answer

If the ore or the gangue can be attracted by the magnetic field, then the ore can be concentrated by the process of magnetic separation. Among the ores mentioned in table 6.1, the ores of iron such as haematite $\left(\mathrm{Fe_2} \mathrm{O_3}\right)$, magnetite $\left(\mathrm{Fe_3} \mathrm{O_4}\right)$, siderite $\left(\mathrm{FeCO_3}\right)$, and iron pyrites $\left(\mathrm{FeS_2}\right)$ can be separated by the process of magnetic separation.

Answer

In the extraction of aluminium, the significance of leaching is to concentrate pure alumina $\left(\mathrm{Al_2} \mathrm{O_3}\right)$ from bauxite ore. Bauxite usually contains silica, iron oxide, and titanium oxide as impurities. In the process of leaching, alumina is concentrated by digesting the powdered ore with a concentrated solution of $\mathrm{NaOH}$ at $473-523 \mathrm{~K}$ and $35-36$ bar. Under these conditions, alumina $\left(\mathrm{Al_2} \mathrm{O_3}\right)$ dissolves as sodium meta-aluminate and silica $\left(\mathrm{SiO_2}\right)$ dissolves as sodium silicate leaving the impurities behind.

$$ \mathrm{Al_2} \mathrm{O _{3(s)}}+2 \mathrm{NaOH _(a q)}+3 \mathrm{H _2} \mathrm{O _(0)} \xrightarrow[35-36 \mathrm{bar}]{473-525 \mathrm{~K}} 2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH}) _{4}\right] _{(\mathrm{aq})} $$

$\mathrm{ \underset{\text{Alumina}}{Al_2O_{3(s)}} + 2 NaOH_{(aq)} +3 H_2O_{(l)} \xrightarrow[{35-36 bar}]{473-525 K} } $ $ \underset{\text{Sodium aluminate}}{ \mathrm{2 Na[ Al(OH)_4 ] {aq}}}$

$\underset{\text{Silica}}{\mathrm{SiO_2(i)}}+2 \mathrm{NaOH_{(a q)}} \xrightarrow[{35-36 \text { bar }}] {473-523 \mathrm{~K}} \mathrm{NaSiO_{3(a q)}}+\mathrm{H_2} \mathrm{O_{(i)}}$

The impurities are then filtered and the solution is neutralized by passing $\mathrm{CO_2}$ gas. In this process, hydrated $\mathrm{Al_2} \mathrm{O_3}$ gets precipitated and sodium silicate remains in the solution. Precipitation is induced by seeding the solution with freshly prepared samples of hydrated $\mathrm{Al_2} \mathrm{O_3}$.

$2 \mathrm{Na}\left[\mathrm{Al}(\mathrm{OH})_{4}\right] {(\mathrm{aq})}+\mathrm{CO_2(\mathrm{~g})} \longrightarrow \underset{\text{Hydrated alumina}}{\mathrm{Al_2O_3 \cdot xH_2O {(s)}}} + 2 \mathrm{NaHCO_3(\mathrm{aq})}$

Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina $\left(\mathrm{Al_2} \mathrm{O_3}\right)$.

$$ \mathrm{Al_2} \mathrm{O_3} x \mathrm{H_2} \mathrm{O_(s)} \xrightarrow{1470 \mathrm{~K}} \mathrm{Al_2} \mathrm{O_3(s)}+x \mathrm{H_2} \mathrm{O_(g)} $$