Monitoring and possible reduction of HF in stack flue gases from ceramic tiles

Abstract

HF (hydrogen fluoride) emission control is one of the critical environmental parameters in the firing of ceramic materials at peak temperatures higher than 1000 °C. In this study, in-stack concentrations of HF were monitored on-line with laser-based equipment during the fast firing of ceramic tiles in continuous industrial roller hearth kilns under standard kiln operating conditions. Three different ceramic tile compositions: porous red-body wall tiles, red-body stoneware tiles, and porcelain tiles were fired in industrial kilns customarily used to manufacture these types of tiles, modifying the heating rate and tile dry bulk density. The in-stack concentrations of HF can be explained on the basis of tile HF adsorption and emission processes in the kiln preheating and firing zones, analysed in previous studies. The methodology used in this study has significant advantages for industrial studies compared with previous methodologies based primarily on laboratory-scale studies and analysis of solid material. The main advantages are: (a) rapid response, allowing abrupt changes in the HF emission to be studied; and (b) direct measurement of the in-stack concentration of HF, which is the most common parameter used as a legal limit for industrial emission control. The study shows that tile HF adsorption in the preheating zone is very important. It also shows that HF stack emissions can vary significantly in continuous kilns depending on whether glazed or unglazed tiles are produced, or important changes occur in production (such as gaps in the kiln feed). The results indicate, furthermore, that HF stack emissions do not decrease significantly when realistic changes are made in industrial operating conditions if these industrial kilns are run at peak temperatures above 1100 °C. The most important reduction in in-stack concentrations of HF is observed when glazed ceramic tiles are fired, so further research in this field can be made in order to reduce HF emissions.