LONG-TERM STUDY OF NANO-ENGINEERED MERCURY VAPOUR SENSOR TESTED UNDER SIMULATED ALUMINA REFINERY CONDITIONS

Mercury (Hg) emission reduction targets set by industry and regulators have spurred attempts to develop real-time monitoring technologies for evaluating the efficiency of Hg removal processes in alumina refineries. This work reports on the long-term stability of newly developed quartz crystal microbalance (QCM) based Hg vapour sensors which have been tested in simulated streams containing common interfering gas species found in alumina refinery stack gas. The developed sensors employ highly orientated nano-engineered gold surfaces formed via a novel electrodeposition process which significantly increase the sensitivity, selectivity and repeatability of the sensor towards Hg vapour over a non-modified (Au control) QCM. The modified QCM sensors were observed to have ~3 times higher sensitivity than the Au control QCM towards Hg concentrations between 1 and 10 mg/m3 in the presence of humidity, ammonia, acetaldehyde, acetone, dimethyl disulfide and methyl ethyl ketone when operated at 89°C. These conditions were chosen to represent the typical conditions of digestion and evaporation exhaust streams within the Bayer process. The coefficient of variance (CoV) of the modified and Au control based QCMs were found to be 2.8% and 7.8% respectively following 10-days of continuous operation. The modified QCM was found to have Hg detection limit of 30.72 µg/m3, which is ~50% better than the Au control QCM. Furthermore, the modified sensors were observed to have significantly larger dynamic range which resulted in better accuracy and performance over the non-modified Au control sensor. The promising results of the developed sensor indicate their high potential for use as a cheap alternative to Hg monitoring techniques currently employed by the alumina industry which generally involve long sampling and offsite analysis procedures.