Amorphous silicas are important raw materials used in a wide range of applications. Owing to the large number of possible applications of these substances at diverse workplaces and their effects, in some respects similar to silicosis, occupational exposure limits have been introduced for amorphous silicas. In order to verify compliance with the occupational exposure limits, methods for the analysis of amorphous silicas in workplace dusts have been developed both at the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) and at the US National Institute for Occupational Safety and Health (NIOSH). In the analysis method developed at the IFA, the amorphous silicas were analysed by infrared spectroscopy. The method developed by NIOSH employs indirect X-ray diffraction of amorphous silicas in the form of cristobalite; this process exploits the fact that amorphous silicas are transformed into cristobalite by annealing. The aim of the project was to compare the analysis methods used in Germany and the USA by means of selected parameters. The intention was to identify differences between the two methods and determine whether these lead to differences in evaluation during comparison of existing measurement results.
The aim of the studies was to determine whether amorphous silicas of different origin convert reproducibly to cristobalite and form comparable quantities of it. The studies were also to determine the effect of the annealing process upon the cristobalite fractions already present, and the effect of fluxes (such as sodium fractions) and a range of attendant materials occurring in dusts at workplaces upon the transformation processes. For this purpose, annealing tests were carried out at 1,100 °C, 1,500 °C and 1,650 °C for different annealing durations (2, 6, 24, 48 and 72 hours). Further tests were carried out with the addition of flux in the form of sodium carbonate in a range of quantities. The influence of diverse attendant materials was examined by way of example on three such materials frequently encountered at workplaces (kaolin, feldspar and calcium carbonate). The samples obtained from the annealing tests were subsequently analysed by both infrared spectroscopy and X-ray diffraction.
Complete conversion of amorphous silica to cristobalite was not attained with use of the parameters specified by NIOSH. It had been shown that various materials tended to leave the amorphous state and undergo rising transformation to cristobalite only at an annealing temperature of 1,650 °C or higher and a significantly longer duration of annealing (at least 24 hours).
The results of the studies showed the transformation of amorphous silicas to cristobalite to be dependent on several different factors, not all of which are controllable. At lower temperatures in the order of 1,100 °C, the genesis of tridymite must be taken into account. This competes with the formation of cristobalite and prevents complete transformation of amorphous phases to it. The various attendant materials in atmospheric samples must also be taken into account, as these significantly impair crystallization to cristobalite. Depending upon the type, composition and treatment of amorphous silicas, cristobalite is also formed in different proportions, which would generally lead to lower results during determination of exposure to amorphous silicas.
The studies also showed that thermal treatment of samples of amorphous silicas leads to variants of cristobalite being formed which cannot be quantified reliably by means of the cristobalite standard normally used. This resulted in cristobalite components of up to around 120% by mass being determined for some samples in which amorphous phases had transformed almost completely into cristobalite. A need therefore exists for the existing methods for cristobalite analysis to be developed further.
Ultimately, it was shown that amorphous silicas differing in their properties also vary in the course of their conversion to cristobalite. Quantifying the amorphous silicas in a generic and reproducible way by means of the method recommended by NIOSH thus presents issues. Exposure data obtained by this method are not comparable with data obtained in Germany. The results can generally be assumed to be lower.
-cross sectoral-Type of hazard:
analytical methods, dust, fibers, particlesDescription, key words:
amorphous silicas, analytical methods, cristobalite, X-ray diffractometry, infrared spectroscopy