Crystallisation rate behaviour of aluminium hydroxide as a function of solution concentrations
Date
2003
Authors
Li, H.
Addai Mensah, J.
Thomas, J.C.
Gerson, A.R.
Editors
Crepeau, P.N.
Advisors
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Conference paper
Citation
Light Metals, 2003 / Crepeau, P.N. (ed./s), pp.65-72
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Light Metals Symposium held at the 132nd TMS Annual Meeting (2 Mar 2003 - 6 Mar 2003 : San Diego, California, USA)
Abstract
Al-containing particle growth rates and induction times have been measured using dynamic light scattering and UV-vis absorption measurements. Several supersaturated caustic aluminate solution series have been analysed: 1. constant 5.00 M NaOH with Al concentration ranging from 2.40 to 3.60 M; 2. constant 0.82 M Al solutions with NaOH concentration ranging from 1.00 to 6.00 M; 3. constant 4.12 M Al solutions with NaOH concentration ranging from 5.00 to 7.00 M; 4. constant [NaOH]/[Al] = 1.22 solutions with NaOH concentration ranging from 1.00 to 6.00 M. For the first three solution series and for the fourth solution series above 3.00 M NaOH the induction time decreased and particle growth rates increased with increasing supersaturation, as expected. Below 3.00 M NaOH, for the [NaOH]/[Al] = 1.22 solution series the induction time increases and particle growth rate decreases upon increasing supersaturation, i.e. decreasing solution concentration. This change in crystallisation behaviour is linked to a change in solution speciation at 2.0-2.5 M NaOH [1,2] from monomer/dimer speciation dominated by Al(OH) <inf>4</inf> <sup>-</sup> at higher concentrations to polycondensed species of high solution phase contrast at low NaOH concentrations, most probably Keggin ions. It has been concluded that [NaOH], rather than [Al], controls the speciation in the freshly prepared solutions. The lowest NaOH concentration at which Al(OH) <inf>4</inf> <sup>-</sup> will dominate the solution speciation is approximately 3 M NaOH and this gives rise to shortest induction time and the most rapid particle growth rate. Above this solution concentration the induction time increases and the particle growth rate slows.
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