Characterization of Precipitated Calcium Carbonate (PCC) compounds on the basis of powder X-ray diffraction data

  • place:Prof. Dr. D. Stüben
  • person in charge:Dr. Katayoon Mohseni

Motivation

Summary

Precipitated calcium carbonate (PCC) and ground calcium carbonate (GCC) compounds are used in various industrial applications especially in paper-making industry. In recent years, numerous commercial PCC and GCC products have emerged with a variety of physical chemical properties. These properties are a consequence of the phase composition, crystal size, crystal morphology, formation of aggregates. Based on this background, an efficient analytical method is desired, to identify PCC and GCC products on a routine basis. The aim of this Ph.D. is to characterize PCC and GCC compounds on the basis of powder XRD measurements.

In this Ph.D. thesis, 22 commercially available PCC and GCC were studied: Pre800, Pre320, 360V, 160V, Fb25, Pre720, Pre100, Fb230, PFS210, PRP, GCCX, GCC90, PRF120, PS, PCLS, PCCSG, PCC1, Pre600, PSU1, PSU2, PSU3, PSU4. The studied PCC and GCC samples contain only calcite and aragonite. No additional crystalline phases such as vaterite or other impurities were identified within the limits of error of powder XRD. Also, there is no indication of an amorphous phase. Calcite and aragonite occur in various proportions. The phase quantification of the studied samples clusters into two groups, as determined by the external standard and Rietveld method. One group consists of almost pure calcite (>96%). The other group consists of aragonite as the dominant mineral (>80%) and calcite (<20%) as lesser mineral.

During industrial precipitation, structural defects such as dislocations of unit cells are introduced into calcite / aragonite crystals. As a consequence, these crystals are built up of structural domains. Coherent diffraction does not occur over the entire crystal but is limited to the domains - also called crystallites. Furthermore, structure defects result in deformation of the crystal lattice (strain). The defect density depends on the actual precipitation reaction and can be used to characterize PCC samples (assuming that precipitation conditions are different for different PCC samples).

Crystallite size as well as strain affects the width of diffraction peaks. In calcite FWHM  (104) lies in the range of 0.198-0.560 and for aragonite  FWHM (040) in the range of 0.210-0.606, calculated by the Rietveld method. The crystallite size and strain values were calculated by Williamson-Hall plots. In addition to the crystallite size, powder XRD data contain information about the crystal morphology, which can also be used to characterize PCC and GCC samples. In principle, non-spherical particles tend to orient preferentially according to the morphology, and result in intensity variations of diffraction peaks. The peak intensity as derived from the peak area of XRD lines (calcite and aragonite) is related to the preferred orientation effect. The description of aggregates morphology (shape) is carried out by the calculation of the intensity ratio of the strongest diffraction line of calcite (104) to the intensity of the consecutive diffraction peaks (116), (012), (202), (113), (018) and (122). There is a significant difference between these values, which indicates that the non-spherical sample orients preferentially in different ways. The same method was considered for aragonite in terms of the intensity ratio of the strongest diffraction line (040), to the intensity of (111), (202), (022), (113), (102) XRD lines. In this step, the sample preparation method is very important and should be reproducible. It should be mentioned that, this method is not a straightforward analytical technique but the approach seems to provide reproducible results.

In this Ph.D. thesis, the characterization of the certain kinds of PCC and GCC samples in terms of identification, quantification, crystallite size and strain determination and aggregates morphology description was investigated by means of powder XRD. The XRD results were verified by complimentary analytical techniques, these were SEM, FT-Raman spectrometers.