Electrolyte properties

The electrolyte used to dissolve the alumina has several properties that change according to its chemical composition. Discussing on the various electrolyte properties we will see that the same changes in bath composition will improve some properties while worsening some others. As a result, there is not an optimum bath composition, but it is always fundamental to look for an optimal bath composition, which will be the best compromise between the various electrolyte properties.

The main electrolyte properties will be reviewed with a brief discussion on the impact on the process.

Liquidus Temperature

The electrolyte liquidus temperature, i.e. the temperature at which the electrolyte starts to freeze, depends on its composition and plays a very important role on many aspect of the process, including the heat balance and alumina dissolution.

Generally, we can say that every additive lowers the liquidus temperature. In particular, the following table gives only qualitative informations about the effect of the various additives on the liquidus temperature:

Increased Bath Component

Qualitative Changes in Liquidus Temp

CaF2

AlF3

↘, not linearly

LiF

MgF2

NaCl

NaF

↘, not linearly

Al2O3

Temp

-

 

Alumina solubility

The electrolyte has the ability to dissolve in it the alumina, but up to a certain percentage. Above this percentage, the alumina added to the bath will be deposited into the bottom of the pot, disturbing the current flow and generating metal pad motion with decreased current efficiency as a result.

The alumina saturation concentration depends on the electrolyte composition, but generally we can say that every additive (except for KF, rarely used) lower the alumina solubility. Also the electrolyte temperature has an effect on the saturation concentration.

Again, the following table will give only qualitative information on the impact of the various additives on the alumina solubility:

 

Increased Bath Component

Qualitative Changes in Al2O3 Solubility

CaF2

AlF3

LiF

MgF2

NaCl

NaF

↗↘, first up, then down

Al2O3

-

Temp

 

The following chart shows quantitatively the effect of the temperature on the alumina solubility, with a given bath composition:

Alumina Saturation vs Bath Temperature Chart

Electrical Conductivity

The electrical conductivity of the bath has a direct impact on the pot voltage. In fact, working with a constant ACD, if the conductivity of the bath increases the pot voltage will decrease, or if we want to keep constant the pot voltage, an increase in the bath conductivity will increase the ACD.

Qualitatively, the effect of the various bath additives on electrical conductivity is:

Increased Bath Component

Qualitative Changes in Electrical Conductivity

CaF2

AlF3

LiF

MgF2

NaCl

NaF

Al2O3

Temp

Density

The electrolyte floats above the metal pad because between bath and molten aluminum there is a density difference. The bath is “lighter” than the molten aluminum, so it floats above the molten aluminum like the oil above the water.

To achieve a good current efficiency it is fundamental that the two liquids, the bath and the aluminum, are clearly separated, and this is achieved with a electrolyte composition that gives the maximum difference in the densities between the bath and the aluminum pad.

The density of the molten aluminum is around 2.38 gr/cm3 while the bath density ranges between 2.07 and 2.15 gr/cm3. Again, the following table gives qualitative informations on the effect of the various bath additives:

Increased Bath Component

Qualitative Changes in Bath Density

CaF2

AlF3

LiF

MgF2

NaCl

NaF

↗↘, first up, then down

Al2O3

Temp

 

Viscosity

The bath viscosity affects the hydrodynamic processes of a pot, where the electrolyte has a very complex pattern of movements due to gas bubble release, metal pad movement, etc…

Generally speaking, an increase in the bath viscosity will decrease the diffusion rate of the aluminum from the metal pad to the bath, hence increasing the current efficiency.

The qualitative viscosity behavior is shown in the following table as a function of the bath additives:

Increased Bath Component

Qualitative Changes in Bath Viscosity

CaF2

↘↗, first down, then up

AlF3

LiF

MgF2

NaCl

NaF

↗↘, first up, then down

Al2O3

↗, not linearly

Temp

 

Metal Solubility

The main cause of the loss in current efficiency is the so-called back reaction, where dissolved aluminum in the bath reacts with dissolved CO2 producing alumina, CO and releasing heat according to the reaction:

Decreasing the amount of dissolved aluminum into the bath is one of the mean to increase the current efficiency. This can be done taking into account that the saturation concentration for the dissolved aluminum in the bath is affected by the electrolyte composition and its temperature.

The following table contains the qualitative effect of the bath additives on the metal solubility:

Increased Bath Component

Qualitative Changes in Metal Solubility

CaF2

AlF3

LiF

MgF2

NaCl

NaF

Al2O3

Temp

 

Surface Tension

The surface tension of the bath affects the dimensions of the CO2 bubbles. So, having a high surface tension implies to have bubbles with reduced dimensions and hence making more difficult the diffusion of the CO2 into the bath, with an increase in current efficiency.

The following table contains the qualitative effect of the bath additives on the bath surface tension:

Increased Bath Component

Qualitative Changes in Bath Surface Tension

CaF2

AlF3

LiF

MgF2

NaCl

NaF

Al2O3

↘, not linearly

Temp

 

Vapor Pressure

The bath vapour pression is a measure of the bath volatility, hence affecting the amount of gas evaporated from the molten bath during electrolysis.

Again, the following table summarizes the quantitative effect of the bath additives to the vapour pressure:

Increased Bath Component

Qualitative Changes in Bath Vapor Pressure

CaF2

AlF3

LiF

MgF2

NaCl

?

NaF

Al2O3

Temp