Abstract: Lime is used widely in the Bayer process, which plays an important role in various applications. In particular, it can minimize soda loss in red mud. The conventional and lime Bayer processes using diasporic bauxite (w(Al2O3/w(SiO2):5.36) were contrastively investigated. The effects of lime addition and pre-desilication on the composition of red mud were mainly studied. The results showed that the x(Al2O3/x(SiO2) in red mud increases linearly but the x(Na2O)/x(SiO2) decreases obviously with lime charge and that the main constituent of red mud in lime Bayer and conventional processes are 3CaO · Al2O3 · xSiO2 · nH2O and Na2O · Al2O3 · xSiO2 · nH2O by which a big amount of soda will be lost, respectively. After pre-desilicating for 4 h, the x(Al2O3)/x(SiO2) and x(Na2O)/x(SiO2) in red mud decrease from 1.53 to 1.43 and 0.28 to 0.24, respectively.