In Baeyer's process the insoluble red residue left after leaching bauxite with hot concentrated NaOH consists mainly of

Alumina dissolves as sodium aluminate, whereas the basic iron(III) oxide (and $\mathrm{TiO}{}_2$) do not dissolve and remain as red mud. Silica reacts with hot concentrated alkali to form soluble silicate, and $\mathrm{Al}(\mathrm{OH}){}_3$ is precipitated later from the filtrate.