Determination of free silica in dust particles: effect of particle size for the X-ray diffraction and phosphoric acid methods.

The X-ray diffraction method and the phosphoric acid method are widely used to determine the fraction of free silica (mainly quartz and other silica polymorphs) in respirable dust sampled in working environments in Japan. In this study, we clarified the size effect of quartz dust for the X-ray diffraction method and the phosphoric acid method using size controlled quartz samples. The quartz samples were classified into 6 fractions with different size ranges: 1 microm and smaller, 1 to 3 microm, 3 to 5 microm, 5 to 7 microm, 7 to 10 microm and 10 microm and larger. Both of the determination methods were affected by the particle size, and especially particles smaller than 3 microm fairly dissolved in hot phosphoric acid and reduced X-ray diffraction intensity remarkably. If the content of these fine particles in the standard quartz sample is lower than that of the test samples, the fraction of free silica may be underestimated by these methods. For this reason, the standard quartz sample should have a representative size distribution of the field samples. The dust samples containing quartz were collected at a foundry and dissolved by phosphoric acid to remove non-quartz materials. The size fractions of dissolved samples were 50% for 5-10 microm, 25% for 3-5 microm, 20% for 1-3 microm and 5% for 1 microm and smaller. As the size distribution is similar to the present standard sample widely used in Japan, we concluded that the standard sample is suitable for these determination methods.