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3.2 Composition of the Electrolyte

The composition of the cryolite electrolyte is described in several ways:

3.3 Systems NaCry - AlF₃ and NaF - AlF₃

The next figure shows the phase diagram sodium fluoride (NaF) - aluminum fluoride (AlF₃). In this figure the concentrations are given in mole percent (100·mole fractions):

3.3 Phase Diagram NaF - AlF3. The concentration of the components sodium fluoride (NaF) and aluminum fluoride (AlF3) are drawn in mole percent. The red area indicates the concentration range of modern electrolysis cells namely namley 25 - 32 mole percent (0 - 15 weight percent).

Lines separate the temperature - composition diagram into regions where different phases are in equilibrium. You find the coexistent phases in Table 3.1.

Table 3.1: Phases of the System NaF - AlF₃ in Figure 3.3

Heating the solid phases of (8, e.g. solid cryolite and solid sodium fluoride) and (11, e.g. solid chiolite and solid aluminum fluoride) to the Eutectic Temperatures (888 °C and 695 °C) they melt either to pure liquid (5) or to a mixture of solid and liquid ( 6, e.g. solid cryolite and liquid, 7, e.g. solid sodium fluoride and liquid and 9, e.g. solid chiolite and liquid, 12, e.g. solid aluminum fluoride and melt). When heating (9, e.g. solid chiolite and liquid, 10, e.g. solid cryolite and chiolite) respectively cooling (4, e.g. solid cryolite and liquid) over the Peritectic Temperature (734 °C) chiolite decomposed into cryolite and aluminum fluoride or chiolite is formed by the reaction of cryolite with aluminum fluoride.
The reaction which happens at the peritectic temperature is written as:

For practical purposes one use only part of the NaF - AlF₃ phase diagram (compare with Figure 3.3)

Figure 3.4 Transformation Phase Diagram. Part of the phase diagram of the system NaF ‑ AlF3 (Figure 3.3) is used and transformed to the phase diagram of the system NaCry ‑ AlF3 (Na3AlF6 ‑ AlF3, see Figure 3.5).

and transforms it to the phase diagram of the system NaCry - AlF₃ (Na₃AlF₆ - AlF₃) by using weight percents:

Figure 3.5 Phase Diagram NaCry-AlF3 (Na3AlF6 ‑ AlF3). Contrary to Figure 3.3 the units of the abscissa are weight percent of AlF3.

The coexistent phases are given in Table 3.2.

Table 3.2: Phases of the System NaCry-AlF₃ (Na₃AlF₆ ‑ AlF₃) in Figure 3.5

Suppose a NaCry - AlF₃ melt of 10 wt % AlF₃ (90 % Na₃AlF₆) at 1000 °C is cooled down (Figure 3.6 A). When the temperature of the melt reaches the Liquidus Temperature (Figure 3.6 B) pure solid sodium cryolite is precipitating.

Figure 3.6 Cooling a Cryolite-Aluminum Fluoride (NaCry ‑ AlF3) Melt.

Continuing this process e.g. cooling the melt cryolite solidifies constantly and the composition of the melt follows the liquidus line e.g. the AlF content of the melt increases (Figure 3.6 C). When the temperature reaches the peritectic value the whole melt solidifies and the Peritectic Reaction (Equation 3.1) takes place (Figure 3.4 D).
These processes are essential when bath is solidifying or melting like the formation and dissolution of side ledge and top crust. Normally the content of cryolite side ledge and top crust (more basic e.g. less acidic) is increased compared to the composition of the liquid bath.

AlWeb and AlPrg use the NaCry-AlF₃ System i. e. sodium cryolite (Na₃AlF₆) with additions as Standard System where the compositions are given in weight percent (wt %):

Often the concentrations of AlF₃ and NaF are called Excess Aluminum Fluoride or Excess Sodium Fluoride concentration. This specification is not necessary since the standard system is defined as a multicomponent system with cryolite as one component and the additions as the other components.
To convert composition from mole fractions into weight fractions (and vice versa) please use Table 3.3. These relations may be easily extended to systems of more components.

Table 3.3: Conversion Mole Fractions ↔ Weight Fractions.

The molten salt system aluminum fluoride (AlF₃) - sodium fluoride (NaF) can be considered as a mixture of the Lewis acid AlF₃ and the Lewis base NaF. A Lewis Acid is defined as an electron pair acceptor and a Lewis Base as an electron pair donor. The acid - base pair AlF₃ - NaF is forming the neutral compound cryolite (Na₃AlF₆). To take into consideration the acidity of the molten electrolyte other values than concentrations are used to describe the composition.

the Cryolite Ratio:

	(3.3)

and the Bath Ratio:

	(3.4)

writing:

the following relation holds:

	(3.5)

When other Lewis acids (MgF₂) or Lewis bases (LiF) are present in the electrolyte some authors modify the bath or cryolite ratio to the Equivalent Weight Ratio (EWR), Tabereaux [Lit.]:

(3.6)

and Haupin [Lit.]:

	(3.7)

To convert the concentrations of both systems one can write for c_AlF3 and c_NaF the sodium and aluminum fluoride concentration in the in the NaF ‑ AlF₃ system and c_AlF3, c_NaF in the NaCry ‑ AlF₃ system:

(3.8)

with

	(3.9)

Table 3.4 contains relations to convert bath ratio values into the aluminum fluoride and sodium fluoride concentrations of the standard system and the sodium fluoride - aluminum fluoride (NaF - AlF₃ - additions) system.

Table 3.4: Conversion Concentration, Bath Ratio of Different Systems.

A negative sodium fluoride concentration (for instance c_NaF = -7.5 wt%) indicates the amount of sodium fluoride to neutralize the excess aluminum fluoride. Similarly for a basic electrolyte the (excess) sodium fluoride concentration is positive (c_NaF > 0) and the aluminum fluoride content negative (c_AlF3 < 0).