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|Title: ||The investigation of scale formation in the Bayer process|
|Authors: ||Armstrong, Jennifer A.|
|Issue Date: ||1999|
|Publisher: ||© Jennifer Ann Armstrong|
|Abstract: ||This thesis reports an investigation of the formation of zeolite scales in the industrial
extraction of alumina from bauxite ore. These materials have been characterised by
powder x-ray and neutron diffraction, infra-red spectroscopy, scanning electron
microscopy and thermogravimetric analysis.
Samples of zeolite scale were removed from both low temperature (c.a. 135°C) and
high temperature (c.a. 220°C) Bayer plants and analysed. At low temperature the
scale crystallises predominantly as natrodavyne with minority phases of sodalite and
cancrinite whilst at high temperature the cancrinite phase is exclusively formed.
Although a variety of different anions are present in both the scale and the liquor in
small amounts (e.g sulphate, hydroxide, vanadate, and oxalate) the majority anions,
with apparently controlling influence, are the carbonate and hydroxide anions.
Laboratory experiments using the representative anions in Bayer liquor (hydroxide,
carbonate, oxalate, sulphate and chloride) as a function of temperature (90° - 225°C)
and anion concentration (10-3
- lO-IM) were performed using hydrothermal methods.
Sulphate formed cancrinite at all temperatures and concentrations and sodalite was
consistently formed for both oxalate and chloride anions. However, both temperature
and concentration were critical in the case of carbonate where sodalite and
intermediate phase dominated at low temperatures and concentrations but at high
temperature and concentration, the cancrinite structure was exclusively formed.
Experiments with sodium hydroxide liquors of different concentrations and standing
times have shown that the cancrinite structure only forms after standing, where carbon
dioxide is readily absorbed from the air, indicating a pure hydroxycancrinite is
unlikely to form. In the absence of standing time, hydroxysodalite is formed at all
temperatures and all concentrations.
The addition of concentrations of lime used extensively to remove carbonate from
Bayer liquor between 0.013 M and O.13M was also investigated. At high
concentrations the addition oflime results in no zeolite scale being formed.|
|Description: ||A Master's Thesis. Submitted in partial fulfilment of the requirements for the award of Master of Philosophy of Loughborough University.|
|Appears in Collections:||MPhil Theses (Chemistry)|
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