Double Shroud Delivery of Silica Precursor for Reducing Hexavalent Chromium in Welding Fume
The welding process yields a high concentration of nanoparticles loaded with hexavalent chromium (Cr6+), a known human carcinogen. Previous studies have demonstrated that using tetramethylsilane (TMS) as a shielding gas additive can significantly reduce the Cr6+ concentration in welding fume particles. In this study, a novel insulated double shroud torch (IDST) was developed to further improve the reduction of airborne Cr6+ concentration by separating the flows of the primary shielding gas and the TMS carrier gas. Welding fumes were collected from a welding chamber in the laboratory and from a fixed location near the welding arc in a welding facility. The Cr6+ content was analyzed with ion chromatography and X-ray photoelectron spectroscopy (XPS). Results from the chamber sampling demonstrated that the addition of 3.2∼5.1% of TMS carrier gas to the primary shielding gas resulted in more than a 90% reduction of airborne Cr6+ under all shielding gas flow rates. The XPS result confirmed complete elimination of Cr6+inside the amorphous silica shell. Adding 100∼1000 ppm of nitric oxide or carbon monoxide to the shielding gas could also reduce Cr6+ concentrations up to 57% and 35%, respectively; however, these reducing agents created potential hazards from the release of unreacted agents. Results of the field test showed that the addition of 1.6% of TMS carrier gas to the primary shielding gas reduced Cr6+ concentration to the limitation of detection (1.1 μg/m3). In a worst-case scenario, if TMS vapor leaked into the environment without decomposition and ventilation, the estimated TMS concentration in the condition of field sampling would be a maximum 5.7 ppm, still well below its flammability limit (1%). Based on a previously developed cost model, the use of TMS increases the general cost by 3.8%. No visual deterioration of weld quality caused by TMS was found, although further mechanical testing is necessary.