Nanoparticle exposure during laser irradiation of ceramic tiles in an industrial setting

Abstract

Nanoparticle formation and emission mechanisms were assessed at pilot plant scale during irradiation of commercial ceramic tiles using a state-of-the-art continuous laser furnace. Laser irradiation of ceramic tiles is a novel technique for tile sintering developed in the framework of LIFE project LASERFIRING (www.laserfiring.eu), with numerous advantages regarding the sintering process (speed, temperature), durability and coating layer of the tiles. As a result, the implementation of higher powered CO2 lasers for commercial industrial processes is currently being assessed under LIFE project CERAMGLASS (www.ceramglass.eu). However, nanoparticle emissions during laser irradiation of tiles have so far never been evaluated. The aim of this work was to characterise nanoparticle formation and emission mechanisms, as well as their impact on exposure, during this industrial process. To this end, nanoparticle number, size distribution, mass concentration, surface area and size-resolved chemical composition were monitored using two sets of instruments (TSI water-CPC, TSI NanoScan, Grimm particle counter 1108, DiscMini, and PCIS impactors) located simultaneously at the emission source and the worker breathing zone. The sintering of six different types of commercial tiles was assessed, undergoing maximum temperatures of 1200°C. The tiles consisted of porcelain and red clays, and in each case the tiles were raw, frit coated and frit and decoration coated. Results evidenced that nanoparticle formation takes place during tile sintering through SO2-induced nucleation processes, and that primary nanoparticles are also emitted during laser irradiation of the tiles. As a result, nanoparticle concentrations reached a maximum of 9.7*105 #/cm3 (mean size =18 nm) at the emission source over the 2-hour periods during which each of the tiles underwent the thermal treatment. For all tiles, concentrations at the emission source were in the range of 105 #/cm3, with mean sizes ranging between 8 and 18 nm. When emissions were transported toward the area where the workers were exposed, and at breathing zone level, particle number concentrations decreased by approximately one order of magnitude (maximum mean N concentration = 1.1*105 #/cm3) and particle diameter increased from 18 to 26 nm, thus remaining at extremely high concentrations with very low and potentially toxic particle sizes. In terms of mass, an increase was detected from the emission source towards the breathing zone, as expected due to particle ageing. The chemical characterisation of the nanoparticles collected on filter samples displayed high contents of toxic metals (mainly Zn, Pb, Cr and As), especially in the PM0.25 particle size fraction (the lowest fraction collected by the impactors). Results evidenced the need for protective actions and equipment in ceramic industries using laser irradiation during sintering.