Question

Use equations to show how acid-base properties are used to separate ${\mathbf{\text{Fe}}}_{\mathbf{\text{2}}}{\mathbf{\text{O}}}_{\mathbf{\text{3}}}$and${\mathbf{\text{TiO}}}_{\mathbf{\text{2}}}{\mathbf{\text{from Al}}}_{\mathbf{\text{2}}}{\mathbf{\text{O}}}_{\mathbf{\text{3}}}$ in the Bayer process.

Short answer

Since ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{\text{and TiO}}_{\text{2}}$are not soluble in basic conditions, they are eliminated as a ‘red mud’ from ${\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}$.

The equation is

$\begin{array}{rcl}{\text{SiO}}_2\left(s\right)+2\text{NaOH}\left(aq\right)+2{\text{H}}_2\text{O}\left(\text{l}\right)& \to & {\text{Na}}_2\text{Si}(\text{OH}{)}_6\left(aq\right){\text{Al}}_2{\text{O}}_3\left(s\right)+2\text{NaOH}\left(aq\right)\\ +3{\text{H}}_2\text{O}\left(l\right)& \to & 2\text{NaAl}(\text{OH}{)}_4\left(aq\right)\\ & & \end{array}$

Step-by-step solution

Concept Introduction

Carl Josef Bayer invented the Bayer process, which is the most used commercial method of processing bauxite to generate alumina (aluminium oxide). Bauxite, the most important aluminium ore, contains only $\mathbf{\text{30-60\%}}$aluminium oxide (${\mathbf{\text{Al}}}_{\mathbf{\text{2}}}{\mathbf{\text{O}}}_{\mathbf{\text{3}}}$), with the rest consisting of silica, various iron oxides, and titanium dioxide. Before it can be processed into aluminium metal, the aluminium oxide must be purified.

Information Provided

The Bayer method is used to separate${\text{Al}}_{\text{2}}$ and${\text{O}}_{\text{3}}$ from bauxite. The original plan is to remove the basic oxides${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{\text{and TiO}}_{\text{2}}$from the bauxite by pre-treating it in boiling$\text{30 NaOH}$.

The acidic$\left({\text{SiO}}_{\text{2}}\right)$and amphoteric$\left({\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}\right)$oxides would dissolve like,

$$\begin{gathered} {\text{SiO}}_2\left(s\right)+2\text{NaOH}\left(aq\right)+2{\text{H}}_2\text{O}\left(\text{l}\right)\to {\text{Na}}_2\text{Si}(\text{OH}{)}_6\left(aq\right){\text{Al}}_2{\text{O}}_3\left(s\right)+2\text{NaOH}\left(aq\right) \\ +3{\text{H}}_2\text{O}\left(l\right)\to 2\text{NaAl}(\text{OH}{)}_4\left(aq\right) \end{gathered}$$

in the base.

On the other hand, the basic oxides do not dissolve and are insoluble in the mixture:

${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}\text{+ NaOH(aq)}\to \text{no reaction.}$

Thus, the${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{\text{and TiO}}_{\text{2}}$ canbeeasilyfilteredoutasa'red mud'from the${\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}$ utilising the acid base characteristics.