The performance of aluminium electrolysis in cryolite based electrolytes containing LiF, KF and MgF2 in a laboratory-scale cell
Abstract
The electrolyte composition plays a very important role in aluminium electrolysis.
Higher current efficiency and lower energy consumption can be obtained through
modifying the composition of the electrolyte. The important developing trend towards
aluminium reduction cells may require higher current efficiency because high current
efficiency can bring huge economic and environmental benefits. It is an effective and
direct method to raise current efficiency through adding some additives to improve
chemical and physical properties of the electrolyte.
In this study electrolysis was performed in order to study the effect of electrolyte
composition on current efficiency and cathode process for aluminium production in a
laboratory scale cell. Firstly, this study was focused on the sodium cryolite based
electrolyte because it has been used for many years in aluminium production. Some
common additives such as LiF and MgF2 were added to the electrolyte attempting to
achieve higher current efficiency. On the other hand, using potassium cryolite as the
basis for low temperature electrolytes is regarded as a promising option for aluminium
production. It can lower the electrolysis temperature which may be an effective way to
increase the current efficiency and to reduce energy consumption. Low-temperature
aluminium electrolysis was also performed in order to investigate the effect of
potassium cryolite on current efficiency.
Chapter 1 is the introduction of this study. The thesis contains a literature review on the
behaviour of additives in different electrolytes used in aluminium electrolysis in chapter
2. The experiment details of this study are given in chapter 3.
In chapter 4 and 5, results from experiments where some additives such as LiF, KF and
MgF2 were added to the traditional electrolyte. The effects of additions to the electrolyte
on current efficiency were investigated during aluminium electrolysis using electrolytes based on the molten NaF−AlF3−Al2O3−CaF2 system at 980. The electrolyte was
saturated with respect to alumina. Standard experimental conditions were Na3AlF6-
Al2O3 (sat) with excess AlF3 corresponding to a molar ratio of NaF/AlF3 of 2.5 and 5 wt%
CaF2 and a constant current density of 0.85 A/cm2. The duration of each electrolysis
experiment was 4 h. The effects of KF and LiF on the cathode process were also studied
using the same conditions. The alkali metals concentrations including K, Na and Li in
deposited aluminium were determined as a function of the cathodic current density by
analyzing metal samples.
The results showed that the current efficiency was found to vary with the lithium
fluoride and potassium fluoride concentration. Additions of LiF up to 5 wt% were found
to improve the current efficiency. Additions of more than 1 wt% of KF were found to
reduce the current efficiency significantly. Blending KF and LiF didn’t raise current
efficiency compared with single addition of LiF. MgF2 has a positive effect on current
efficiency. However, the combination of LiF and MgF2 wasn’t found to raise current
efficiency significantly as expected. Metal analysis showed that the content of lithium in
aluminium increases with increasing amounts of LiF, and additions of KF have a
smaller effect on the contents of sodium and potassium in aluminium.
In chapter 6, results are presented for the electrolyte based on the molten KF-AlF3-NaF
system for low-temperature aluminium electrolysis. The cryolite ratio of the molar
concentrations of the alkali fluoride and the aluminum fluoride KR = (NKF+NNaF)/NAlF3,
was always retained constant, equal to 1.3 or 1.5. The cryolite-based melts contained 5
and 10 mass% NaF. The temperature was 750°C for KR=1.3 and 800°C for KR=1.5.
The contents of sodium and potassium in aluminium produced from the KF-NaF-AlF3
melts were determined as a function of the cathodic current density.
It was found that the current efficiency for aluminium deposition decreased with
increasing content of NaF. The concentration of potassium in aluminium increased with
increasing cathodic current density. The concentration of sodium in aluminium
increased slightly with increasing cathodic current density.
In chapter 7, experimental results are presented for the effects of operating parameters,
such as additions, cryolite ratios and operating temperature, on current efficiency in
electrolytes for low-temperature aluminium electrolysis. Standard experimental
conditions were KF-AlF3-Al2O3 (5%) corresponding to a molar ratio of KF/AlF3 of 1.3
at 750°C and a constant current density of 0.85 A/cm2.
The results showed correlations between current efficiency and operating parameters.
The CE increases with decreasing temperature. Loss in current efficiency was caused
mainly by limitations of the anodic reaction with decreasing KR. Lithium fluoride up to
2 wt% affects the CE positively, and the CE decreases with increasing concentration of
LiF. With increasing the cathodic current density, short circuiting and anode effect
caused losses in current efficiency.