Occupational exposure and analysis of microcrystalline cristobalite in mullite operations

Three analytical methods currently used for crystalline free silica determination in occupational exposure samples were evaluated for their applicability to ceramic materials containing synthetic mullite. X-ray powder diffraction is the only method that can be used with sufficient precision and potentially adequate accuracy for occupational exposure monitoring. Personal respirable dust exposure samples were collected in two foundries. The results of exposure evaluations in Plant A showed overexposure to the dust, particularly in the shakeout area. The cristobalite concentration in the respirable dust exceeded that in the original material for this particular area. This may be related to the preheating of molds to more than 1100 degrees C for the pouring of stainless steel castings, causing conversion of some of the colloidal silica binder to cristobalite, and related to high vibration fracture of the material during removal of castings from molds. In Plant B, quartz was sometimes present along with cristobalite in personal samples. Respirable dust levels exceeded the Occupational Safety and Health Administration's (OSHA) permissible exposure limit (PEL) and the National Institute for Occupational Safety and Health's (NIOSH) recommended time-weighted average (TWA) for crystalline free silica, indicating a need for better engineering controls to reduce dust levels. The inadequacy of reference standards currently available for cristobalite analysis in these types of materials is cited. The need for more toxicological research is emphasized.