Detection of alkali emissions from alumina refining processes

In this work, the feasibility of using pH based indicators as a microfluidic based online alkali detection method was evaluated. The indicators were tested towards sodium hydroxide solution through a flow cell followed by the measurement of their colour changes via UV-visible spectroscopy. Among the tested indicators, cresol Red and neutral red showed the largest usable sensitivity range towards solutions containing alkaline compounds. In order to test if the method would be suitable for operating in an alumina refinery atmosphere, each indicator was tested against weak bases and other species such as Na2CO3, NaHCO3, MgNO3 and NH3. Under such situations, neutral red was found to be useful for sensing low NaOH concentrations (i.e. in the range of 10 to 50 μM in liquid, which is equivalent to 0.2 to 2 mg/m3 in air) and thus may be able to indicate when the concentration is approaching the alkali threshold (2 mg/m3). Similarly, cresol Red was also identified as a potential candidate to differentiate between low and higher threshold limits of equivalent NaOH concentrations (0.2 to 2 mg/m3) as it exhibited low susceptibility towards NaCl, NH3 and NaHCO3 contaminants.