Question

Explain Bayer's method for obtaining alumina from bauxite with equations?

Answer 1

In bauxite, in addition to aluminium oxide $\left(A{l}_2{O}_3\right)$ impurities like iron oxide $\left(F{e}_2{O}_3\right)$ and sand $\left(Si{O}_2\right)$ are also present. On refining bauxite by Bayers method, pure aluminium oxide is obtained which is also called alumina. In this method, bauxite powder is taken and concentrated ($45$%) and sodium hydroxide solution is added and then heated in a closed vessel at $433$ K temperature and $5$ to $6$ bars of pressure, for $6$ to $8$ hours, so that aluminium oxide present in bauxite is converted into sodium aluminate which is soluble in water.
$A{l}_2{O}_3\left(s\right)+2NaOH\left(aq\right)\to 2NaAl{O}_2\left(aq\right)+H_{2}O\left(l\right)$
Iron oxide does not dissolve in sodium hydroxide. Hence, it can be removed by filtration. Silica forms sodium silicate which is soluble in water. This sodium aluminate and sodium silicate are there in the filtrate. Excess of water is added to it and continuously stirred so that hydrolysis of sodium aluminate takes place and precipitates of aluminium hydroxide are obtained and the sodium silicate remains as an impurity in the solution. Some precipitates of aluminium hydroxide are added from outside in order to make the reaction simple and fast.
$NaAl{O}_2\left(aq\right)+2H_{2}O\left(l\right)Al\left(OH{)}_3\right(s)+NaOH(aq)$
The precipitates are washed repeatedly with water, dried and on heating at $1473$ K temperature pure aluminium oxide (alumina) is obtained.
$2Al\left(OH{)}_3\right(s\left)\stackrel{}{\to }\right[1473K\left]A{l}_2{O}_3\right(s)+3H_{2}O(g)$
Thus, about $99.5$ % pure alumina is obtained by Bayers method.