Investigating the cross-interference effects of alumina refinery process gas species on a SAW based mercury vapor sensor

Abstract

In this study, the effect of some common alumina refinery process gas species on the response of a surface acoustic wave (SAW) based elemental mercury (Hg0) vapor microsensor was investigated. The developed sensor was based on a SAW delay line, fabricated on a ST-cut quartz substrate with nickel (Ni) interdigitated transducer (IDT) electrodes and gold (Au) film sensitive layer. Ni was chosen as IDT material due to its immiscibility in mercury and thus allowing the sensor to operate for a longer time, as required by the industries. The sensor was exposed to mercury vapor concentrations between 24 and 365ppbv without/with the presence of ammonia, acetaldehyde, ethyl mercaptan, dimethyl disulphide, methyl ethyl ketone and humidity. The detection limit of the sensor toward Hg0 vapor was calculated to be 1.4ppbv and 4.0ppbv at operating temperatures of 35 and 75°C, respectively. The coefficient of variance of the sensor response magnitude was found to be within ±1.86 and ±5.73% at 75°C when exposed to repeated pulses of 365ppbv of Hg0 vapor without and with the presence of interfering gas species, respectively. This indicates high precision of sensor while measuring low concentrations of Hg0 vapor. Results showed that when any one of the interfering gas species was exposed along with Hg0 vapor, the response magnitude of the sensor deviated up- to 7% (93% selectivity) at an operating temperature of 75°C. Furthermore, the sensor showed a selectivity of 87% when Hg0 vapor was tested along with a mixture of all interfering gases. There was insignificant cross-sensitivity effect when the humidity content was increased within the range of 15,300 to 30,600ppmv (equivalent to 50 to 100% relative humidity at room temperature). The overall results indicate that the developed sensor is able to detect Hg0 vapor selectively even in simulated industrial conditions and can be a potentially cheaper and more reliable alternative to the current Hg0 monitoring methods used in alumina refineries.