Shelf life stability of diatomites

Abstract

Diatomites or diatomaceous earths are biogenic sediments mainly consisting of porous siliceous algae skeletons. Some diatomites are industrially processed and used for different applications. For some applications and sometimes for the authorization of products for new applications it is important to prove the ‘shelf life stability’ which means that the properties of the product should not change upon storage. Freshly prepared silica gel is known to alter upon aging e.g. with respect to the specific surface area and porosity, respectively. This can be relevant for diatoms as well since recent skeleton consist of XRD amorphous silica which – upon aging – crystallizes, finally forming microcrystalline silica. The age of most of the diatomites being mined is higher than 1 million years. Accordingly, one could expect that storage for some years does not alter the diatomite properties. However, it is at least conceivable that prolonged open storage (exposure to air) can lead to the adsorption of molecules from air which in turn would block reactive adsorption sites. The aim of this study is to assess the shelf-life-stability of diatomite and perlite based on long term tests. Therefore, two different diatomite samples and a perlite sample (XRD amorphous volcanic alumosilicate glass) were stored at different conditions for 1 year. All materials were investigated with respect to structural aspects as well as adsorption capacity. Using the common mineralogical methods X-ray diffraction (XRD) and infrared spectroscopy no structural changes could be observed. In contrast, the determination of the amount of soda soluble silica turned out to be a valuable tool for the identification of structural changes. Using this method a slightly higher reactivity of the perlite surface after storage at 60 °C in water was found which possibly can be explained by the beginning devitrification process. At the same conditions, the diatomite sample which mostly consists of XRD amorphous silica showed a slight decrease of the amount of soda soluble silica which was interpreted as the beginning (re)crystallization process. The diatomite sample containing swellable clay minerals lost some water adsorption capacity upon extensive drying. Systematic changes of material properties after 1 year storage could only be observed under exaggerated conditions (120 °C or storage in water at 60 °C). It is concluded that the investigated materials are stable under common storage conditions.