Minimising reversion, using seawater and magnesium chloride, caused by the dissolution of tricalcium aluminate hexahydrate

The increase in pH and aluminium concentration after the neutralisation of bauxite refinery residues is commonly known as reversion. This investigation reports the extent of reversion in synthetic supernatant liquor and possible methods to reduce reversion. This work is based on bauxite refinery residues produced from alumina refineries, where reversion is a real life situation in neutralised refinery residues. Tricalcium aluminate hexahydrate, a common phase in bauxite refinery residues, has been found to cause reversion. It has been established that reductions in both pH and aluminium from the seawater neutralisation process are due to the formation of ‘Bayer’ hydrotalcite Mg7Al2(OH)18(CO32-,SO42-)·xH2O. This is the primary mechanism involved in the removal of aluminium from solution. Increasing the volume of seawater used for the neutralisation process minimises the extent of reversion for both synthetic supernatant liquor and red mud slurry. The addition of MgCl2·6H2O also showed a reduction in reversion and confirmed that the decrease in aluminium and hydroxyl ions is due to the formation of Bayer hydrotalcite and not simply a dilution effect.

This investigation reports the extent of reversion in synthetic supernatant liquor and possible methods to reduce reversion. It has been established that reductions in both pH and aluminium from the seawater neutralisation process are due to the formation of ‘Bayer’ hydrotalcite Mg7Al2(OH)18(CO32-,SO42-)·xH2O. This is the primary mechanism involved in the removal of aluminium from solution. The concentration of: (a) aluminium and (b) magnesium for SWN synthetic SNL containing 0.10 M TCA with additional MgCl2·6H2O added to seawater reduces reversion.Figure optionsDownload high-quality image (71 K)Download as PowerPoint slideResearch highlights
► This work is based on bauxite refinery residues produced from alumina refineries, where reversion is a real life situation in neutralised refinery residues.
► This investigation looks at reducing reversion in bauxite refinery residues, and therefore has direct industrial applications.
► The mechanism for reversion has been determined and discussed.
► This investigation has established a number of ways of reducing reversion and thus rendering the residue more environmentally friendly.