Optimal Treatment Conditions for Changing Characteristics of Naturally Occurring Asbestos

Abstract

Naturally occurring asbestos (NOA) refers to asbestos found as a natural component of rocks and soils. The purpose of this study was to determine effective methods for changing the characteristics of NOA. The study used chrysotile-containing soils and rocks. Unprocessed chrysotile-containing soils and rocks were treated using two methods: (1) heat treatment at 700–1100˚C for 1–3 hours, or (2) combination treatment of 100–250˚C and 0.3, 0.5, or 1.0 M oxalic acid for 1–3 hours. A Polarized light microscope (PLM) was used for the qualitative analysis of both untreated and treated samples, referring to the National Institute of Occupational Safety and Health (NIOSH) Manual of Analytical Methods (NMAM) Method No. 9002. To confirm judgement, morphological and chemical changes in pre- and post-treated samples were examined using a field emission scanning electron microscope (FE-SEM) with an energy-dispersion spectrometer (EDS). PLM analyses revealed gradual changes in optical properties according to increased temperature, acid concentration, and time. Sign of elongation, extinction, and dispersion staining characteristics of NOA changed under conditions of 1000 ˚C on heat treatment and 1 M oxalic acid at 250˚C for 1 hour on combination treatment. SEM/EDS analyses revealed morphological and chemical changes under conditions of 1 M oxalic acid at 250˚C for 1 hour. The morphology of the asbestos changed into the angulated form rather than the twisty, long and winding form. The elemental composition of combination treated samples showed low levels of magnesium and high levels of silicon compared to untreated samples. The optimal treatment condition was 1 M oxalic acid at 250˚C temperature for 1 hour. Combination treatment resulted in changes in NOA characteristics at lower temperatures than did heat treatment alone.