dc.contributor.author |
Patterson, Edwin Campbell |
en |
dc.date.accessioned |
2010-02-24T03:16:36Z |
en |
dc.date.available |
2010-02-24T03:16:36Z |
en |
dc.date.issued |
2002 |
en |
dc.identifier.citation |
Thesis (PhD--Chemical and Materials Engineering)--University of Auckland, 2002. |
en |
dc.identifier.uri |
http://hdl.handle.net/2292/5662 |
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dc.description.abstract |
Modern aluminium electrolysis cells generate between 15 to 40 kg fluoride per tonne of
aluminium fluoride produced. This represent a large material recycle load, of which
over 99% of this fluoride content is returned to the cell via a dry scrubbing system.
Most past research has concentrated on increasing the efficiency of this end scrubbing
system, neglecting the actual cause of the fluoride emissions. Lower emissions would
reduce this loading, resulting in smaller scrubbing systems, and ultimately lower capital
costs. Fluoride emissions also contribute to the changes in heat balance in the cell. The
fluoride evolved represents a material loss that requires to be replaced with AlF3. The
addition of this species is a major variable in cell heat balance instability [1]. Overall a
better understanding of the contributors to fluoride generation in an aluminium cell
would benefit the operation and economics of an aluminium smelter.
Past studies [2-6] have identified most of the important emission contributors to
hydrogen fluoride (HF) generation. Two sources were shown to be significant. The
most studied source was primary HF generation. This is HF generation from
electrolytic reactions between constituents of the fluoride based electrolyte and water
from the fed alumina and hydrogen in the anode. Overlooked in some studies were
generation reactions outside the electrolyte - defined in this study as secondary HF
generation. This source was found to occur mainly from hydrolysis reactions with the
generated particulate fluorides. However the results and the relative contributions from
these studies are not strictly applicable to the current generation scenario. The present
smelting practices and technology, dry scrubbing technology and raw material
specifications differ substantially from those used during the period of each past
investigation. The present study was designed to identify and quantify emission sources
for the current smelter technology and practices. This used controlled laboratory based
studies of the individual generation sources complemented by in plant studies analysing
the HF emission from production aluminium electrolysis cells.
It was shown that the main contributor to primary HF generation is the addition of
water to the electrolyte from the feed alumina. This results in the cyclic short term
variations in HF emission, which correlate to the rate of alumina addition to the cell.
Laboratory and industrial studies show that only a fraction of the added water reacts.
The water reacted is likely the structural water of the alumina. The adsorbed water is
thought to be flashed off before addition to the bath. Depending on feeding technology
and crust integrity, between 10 to 50% of this water can react. This produces 7 to 14 kgF/tonneAl. This makes it the most significant emission component. The remainder of
this structural water is either entrained in the anode gases or forms part of the
electrolyte dissolved water content.
Dissolved water generation is the second most significant primary generation
contributor, and third most significant emission component. Dissolved water is in
equilibrium with the alumina content of the bath. It represents a constant emission
source. Depending on feeding technology and cell design, the emission can vary
between 3 to 10 kgF/tonneAl.
The final primary HF emission contributor results from electrolytic generation of the
hydrogen content of the anode. Laboratory studies found the emission to have a
reaction efficiency of approximately 10%. This results in a small emission, of 2 – 5
kg/tonneAl. The remainder of the hydrogen content is expected to be entrained in the
anode gases, as the generation of this CO/CO2 mixture is an order of magnitude greater
than the HF generation reaction.
Secondary generation of hydrogen fluoride is also a significant HF emission source. In
an industrial cell this results from mainly thermal hydrolysis of the particulate fluoride
emissions at the crust–air interface. Laboratory studies have shown that other identified
secondary emission sources are unlikely due to the ambient conditions and feeding
practices in a modern prebake aluminium electrolysis cell. Hence previously proposed
generation from hydrolysis of the particulates in the ducts and desorption of the surface
fluoride from the fed secondary alumina, have been found to be insignificant compared
to the main HF generation sources.
The thermal hydrolysis emission contributor is the only significant secondary generation
emission component. Industrial measurements show that it varies with ambient
humidity and crust integrity, two parameters which vary constantly in a modern prebake
anode cell. Measurements show that the emission from this source is responsible for 2
to 8 kgF/tonneAl. This makes it the second highest HF emission component. Control
of the crust condition and feeder hole states reduces this component significantly.
Hence in a modern prebake aluminium cell, the most significant operational factors
affecting emission are related mainly to secondary generation. Industrial measurements
show that the long term variations in an emission result from changes in the ambient
humidity and the cells crust cover. Control of the crust integrity is thus paramount in reducing such variations. This relates both to normal operation and batch operations.
Reduction of all other sources of emission are a material composition problem. Simply
reducing the water content (LOI(300) and LOI(1000)) and reducing the hydrogen
content of the anodes will reduce the emission. However material considerations affect
other aspects of cell operation and hence these factors are not as simple to adjust. |
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dc.publisher |
ResearchSpace@Auckland |
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dc.relation.ispartof |
PhD Thesis - University of Auckland |
en |
dc.relation.isreferencedby |
UoA1016286 |
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dc.rights |
Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. |
en |
dc.rights.uri |
https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm |
en |
dc.title |
Fluoride Emissions from Aluminium Electrolysis Cells |
en |
dc.type |
Thesis |
en |
thesis.degree.discipline |
Chemicals and Materials Engineering |
en |
thesis.degree.grantor |
The University of Auckland |
en |
thesis.degree.level |
Doctoral |
en |
thesis.degree.name |
PhD |
en |
dc.date.updated |
2010-02-24T03:16:37Z |
en |
dc.rights.holder |
Copyright: The author |
en |
pubs.local.anzsrc |
0912 - Materials Engineering |
en |
pubs.org-id |
Faculty of Engineering |
en |
dc.identifier.wikidata |
Q112857962 |
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