Quantificação mineralógica de bentonitas via DRX usando um método combinado Rietveld-Le Bail-Padrão Interno

Abstract

In the bentonite industry ore quality control has been traditionally carried out using the swelling parameter, denominating those that swell as naturally sodium bentonites and those that do not as naturally non-sodium bentonites. However, the properties of the bentonites can vary due the quantity and cationic type of montmorillonite. These variations cannot always be predicted, because there is no consolidated and practical method for clays mineralogical quantification. A quantification via PXRD using the conventional Rietveld method via X-ray diffractometry is only reliable when all the crystalline structures of the mineral phases are known. This is not the case for bentonites, since the montmorillonite shows turbostratic disorder that is not considered in the available structural models, making it the mineralogical quantification a major challenge. Thus, in this work, was generated a calibrated hkl phase model for a so-called Formosa Mgmontmorillonite applying the combined Rietveld-Le Bail-Internal Standard method developed by Paz et al. (2018). The montmorillonite was obtained via centrifugation according to the Stokes law by particle size separation of the clay fraction (< 2 μm) from the Formosa bentonite. The sample was characterized by XRD, XRF, FTIR, SEM, MS and PSD, finding a low concentration of other mineral phases (impurities). According to the XRF and MS analysis, it is classified as a beidellitic montmorillonite with octahedral charge of >50 %. Quantitative results were satisfactory using the hkl model for montmorillonite-fluorite binary mixtures with montmorillonite content >50 % (statistical indices χ2 and RBragg < 5). The method was reproducible for three replicates of the montmorillonite-fluorite binary mixture (80-20%). The greater precision and reproducibility of the results, shows that the calibrated hkl phase model can be used satisfactorily for the mineralogical quantification of bentonites (60-80% montmorillonite content). These results showed that the combined method is a convenient and fast procedure, essential requirements for an industrial application, allowing deal with phases that have partial crystallographic information and handling severe diffractometric effects of structural disorder, such as turbostratic disorder, typical of clay minerals.