Mechanisms of novel method for removing cristobalite from montmorillonite through exfoliation of 2D montmorillonite

The preparation of high purity montmorillonite (Mt) is of great significance for its high-value application. However, the difficulty in removing fine cristobalite from Mt hinders the preparation of high purity Mt. In this work, a novel method for removing cristobalite from Mt is studied through exfoliating Mt to two-dimensional nanosheets, which is proposed based on the difference of the structure and hydration expansion property between the two minerals. The XRD pattern of the purified sample shows that the cristobalite in Mt has been removed completely. Meanwhile, the mechanisms of the separation of Mt and cristobalite were investigated from the differences of particle size and hydration shell. The results show that the decrease of Mt particle size provides the possibility for the separation of cristobalite and Mt, but it is not the main factor. The densities of hydrated Mt and cristobalite before and after exfoliation were calculated by molecular dynamics simulation. After considering the hydration shell, the density of the initial Mt is 2.22 g/cm3, while the density of the exfoliated MtNS is 1.52 g/cm3 because of its two-dimensional structure. In addition, the difficulty level of the separation changes from greatly difficult to easy after considering the hydration shell, which indicates that the hydration shell is the main factor affecting the separation of Mt and cristobalite. This work explains the reasons why this new method can satisfactorily remove cristobalite from Mt and will guide the preparation and application of high purity Mt.