HUMIDITY AND AMMONIA INTERFERENCE STUDY FOR MODIFIED AND NON-MODIFIED GOLD BASED MERCURY VAPOUR SENSORS FOR ALUMINA REFINERIES

Techniques for monitoring of gaseous mercury emissions from alumina refineries are essential for evaluating the efficiency of implemented mercury removal processes. This work reports on the medium to long-term stability of non-modified gold sensitive layers, and the sensitivity improvements for a modified film for mercury (Hg) vapour sensing. For sensing purposes, it is important to obtain a high sensitivity when using Quartz Crystal Microbalances (QCMs) in the presence of common interferent gas species found in alumina production processes. Increasing the active surface area of the QCM electrode by electro-deposition is thought to reduce the required exposure time of the sensor towards mercury vapour, thus increasing the ability and rate of the QCMs’ regeneration.

The long-term stability of the tested sensors is shown to be within 5% for dry and humid conditions over any sequential 2 to 4 day period towards Hg concentrations of 1.87 and 5.7mg/m3. The results indicated that a response magnitude degradation of ~40% occurred over the 45 day test period. The effect on response magnitude of the sensors was investigated in the presence of ammonia (1180 – 1770 mg/m³) and humidity (4.2 – 10.4 mg/m³) interferents. Interference caused by 1770 mg/m3 of ammonia degraded the response of the non-modified QCMs, leading to the sensors reporting a 10 to 20% lower concentration for the tested Hg concentrations. The electro-deposited sensitive layer was observed to have much higher affinity towards Hg than the non-modified. A response magnitude increase of 67% was obtained for a Hg concentration of 4.31mg/m³.