Implication of calcium hydroxide in the seawater neutralisation of bauxite refinery liquors

Palmer, Sara J., Smith, Matthew, & Frost, Ray L. (2012) Implication of calcium hydroxide in the seawater neutralisation of bauxite refinery liquors. In 9th International Alumina Quality Workshop, 18-22 March, 2012, Perth, WA.

Abstract

Tricalcium aluminate, hydrocalumite and residual lime have been identified as reversion contributing compounds after the seawater neutralisation of bauxite refinery residues. The formation of these compounds during the neutralisation process is dependent on the concentration of residual lime, pH and aluminate concentrations in the residue slurry. Therefore, the effect of calcium hydroxide (CaOH2) in bauxite refinery liquors was analysed and the degree of reversion monitored. This investigation found that the dissolution of tricalcium aluminate, hydrocalumite and CaOH2 caused reversion and continued to increase the pH of the neutralised residue until a state of equilibrium was reached at a solution pH of 10.5. The dissolution mechanism for each compound has been described and used to demonstrate the implications that this has on reversion in seawater neutralised Bayer liquor. This investigation describes the limiting factors for the dissolution and formation of these trigger compounds as well as confirming the formation of Bayer hydrotalcite (mixture of Mg6Al2(OH)16(CO32-,SO42-)•xH2O and Mg8Al2(OH)12(CO32-,SO42-)•xH2O) as the primary mechanism for reducing reversion during the neutralisation process. This knowledge then allowed for a simple but effective method (addition of magnesium chloride or increased seawater to Bayer liquor ratio) to be devised to reduce reversion occurring after the neutralisation of Bayer liquors. Both methods utilise the formation of Bayer hydrotalcite to permanently (stable in neutralised residue) remove hydroxyl (OH-) and aluminate (Al(OH)4-) ions from solution.

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ID Code: 72155
Item Type: Conference Paper
Refereed: Yes
Keywords: red mud, bauxite residue, seawater neutralisation, reversion, calcium hydroxide
Subjects: Australian and New Zealand Standard Research Classification > CHEMICAL SCIENCE (030000) > INORGANIC CHEMISTRY (030200)
Divisions: Current > Schools > School of Chemistry, Physics & Mechanical Engineering
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Copyright Owner: Copyright 2012 [please consult the author]
Deposited On: 27 May 2014 23:08
Last Modified: 13 Jun 2014 05:00

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