The ore which can be attracted by the magnetic field can be concentrated by the process of magnetic separation. Among the ores mentioned in table 6.1, the ores of iron such as haematite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃), and iron pyrites (FeS₂) can be separated by the process of magnetic separation. 

Leaching is significant as it helps in removing the impurities like SiO₂, Fe₂O₃ etc. from the bauxite ore to get concentrated pure alumina (Al₂O₃)

The change in Gibbs energy is related to the equilibrium constant, K as
 
$∆\mathrm{G} = -\mathrm{RT} \mathrm{lnK}$

At room temperature, all reactants and products of the given reaction are in the solid state. As a result, equilibrium does not exist between the reactants and the products. Hence, the reaction does not take place at room temperature.Certain amount of activation energy is essential even for such reactions which are thermodynamically feasible. 

Yes, below 1350°C Mg can reduce Al₂O₃ and above 1350°C. Al can reduce MgO. This can be inferred from ΔG° Vs T plots.




Fig.  Gibbs energy (ΔG°) V T plots for formation of some oxides.