Wednesday, 12 December 2012

Sulphur impregnated activated carbons scrub mercury in alumina refining

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scrubbersBy-products of the Bayer process include non-condensable gases like volatile organic carbons (VOCs), water vapour, ammonia and elemental mercury in vapour form. Image: VascoRECENTLY published research has given insight into the uses and performance of sulphur impregnated activated carbons (SIAC) in mercury adsorption.

Elemental mercury—emitted as vapour from some aluminium refineries operating the Bayer process—is recognised 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 (VOCs), water vapour, ammonia and elemental mercury in vapour form.

Lead author, Mark Mullett, who completed research while at Alcoa, says 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.

“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,” Mr Mullett says. 

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.

“Since this work was completed about six years ago, SIAC has become an industry standard for mercury removal from gaseous emissions,” Mr Mullett says.

The ‘Pica Selexorb HG’ SIAC was recommended for a pilot trial which later went ahead.

An Alcoa spokesperson says pilot scale testing highlighted limitations to the full scale implementation of this technology, namely fire hazard associated with the high organic and high hydrogen content in the gas stream being passed through the SIAC; and the environmental issue associated with disposal of the mercury contaminated carbon.

Mercury in liquor chemistry modification technology (studied in parallel) was later implemented in Alcoa’s Wagerup and Pinjarra refineries.

“Briefly we have developed a way of introducing a sulphur based additive into the process to inhibit mercury emission to air and instead keep it in the liquor stream until it reaches an emission exit point fitted with our condenser technology,” the spokesperson says. 

Mercury is collected and sent to a licensed waste management facility where, since it is predominately in elemental (liquid) form it is typically purified and recycled.

Low concentrations of mercury are naturally found in bauxite—the mineral used to make aluminium.

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