Elemental mercury emitted as vapour from some aluminium refineries operating the Bayer process is recognized by many international governments as a health risk and research worldwide is focused on solutions for mercury adsorption and safe disposal.
Research published in Chemical Engineering, studied SIACs and compared them for mercury absorption effectiveness within Bayer gas emissions which are unique in composition and highly complex in their chemistry.
By products of the Bayer process include non-condensable gases like volatile organic carbons, water vapour, ammonia and elemental mercury in vapour form.
Lead author, Mr Mark Mullett who completed research while at Alcoa said that these conditions are challenging for mercury adsorption because VOCs and water vapour can compete for absorption sites on the activated carbon’s surface.
Research found SIACs in general stored up to 10 times more mercury than non impregnated (virgin) activated carbons because the mercury reacts with sulphur forming mercury sulphide. The adsorption process is proportional to the amount of sulphur that is loaded onto the carbon.
Mr Mullett said that “We were able to demonstrate that in the presence of water and VOCs we were still able to get the stoichiometric mercury loading onto the carbon. Four different brands of SIACs, in extruded and granular forms, were compared in laboratory conditions that mimicked a non-condensable gas stream. Further research concluded that submitting the SIACs to higher temperatures than the recommended (80oC) yielded better mercury uptake because higher temperature reduces the adsorption affinity for VOCs and water vapour. Higher temperatures also caused a redistribution of sulphur within carbon’s pores, which aided mercury removal.”
He said that “Since this work was completed about six years ago, SIAC has become an industry standard for mercury removal from gaseous emissions. The Pica Selexorb HG’ SIAC was recommended for a pilot trial which later went ahead.”