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3.5 Liquidus Temperature

The liquidus temperature (TL) is the temperature at which solid material is precipitating when a liquid is cooled. The bath temperature (TB) is the temperature of the liquid electrolyte in a electrolysis cell that is measured during normal pot operation. The difference between bath temperature and liquidus temperature is called SuperheatT).

Superheat Power: Meaning of Symbols

(3.5.1)

The electrolyte is a multicomponent system of cryolite (Na3AlF6) with additions of aluminum fluoride (AlF3), calcium fluoride (CaF2), lithium fluoride (LiF), magnesium fluoride (MgF2) and potassium fluoride (KF). In the literature several relations of the cryolite liquidus temperature for this multicomponent system are found.

Remark:You should be aware that NOT all equations take care of all the possible components of the electrolytic bath. Only the relations of Kolås and Solheim, for instance, use the potassium fluoride content. The lithium fluoride concentration is lacking in the relations of Peterson and Lee and magnesium fluoride in the equations of Røstum, Peterson and Lee. If a concentration term is lacking the calculated liquidus temperature is obviously not influenced by the corresponding concentration.
AlWeb does NOT check the limits of the input values as demanded by some authors. Therefore the calcution results may be misleading.

Alweb and AlPrg apply the following relations:

S. Yuezhong (2013) [Lit.]

Equation Liquidus Temperature Yuezhong Liquidus Temperature: Meaning of Symbols

(3.5.2)

S. Kolås (2007) [Lit.]

Equation Liquidus Temperature Kolås Liquidus Temperature: Meaning of Symbols

(3.5.3)

A. Solheim (1995) [Lit. A, B]

Equation Liquidus Temperature Solheim Liquidus Temperature: Meaning of Symbols

(3.5.4)

A. Røstum (1990) [Lit.]

Equation Liquidus Temperature Røstum Liquidus Temperature: Meaning of Symbols

(3.5.5)

R. D. Peterson (1987) [Lit.]

Equation Liquidus Temperature Peterson Liquidus Temperature: Meaning of Symbols

(3.5.6)

G. L. Bullard (1986) [Lit.]

Equation Liquidus Temperature Bullard Liquidus Temperature: Meaning of Symbols

(3.5.7)

S. S. Lee (1984) [Lit.]

Equation Liquidus Temperature Lee Liquidus Temperature: Meaning of Symbols

(3.5.8)

E. W. Dewing (1974) [Lit.]

Equation Liquidus Temperature Dewing Liquidus Temperature: Meaning of Symbols

(3.5.9)

The expression of Dewing uses the concentration of lithium cryolite (Li3AlF6). Lithium cryolite is formed according to the following reaction equation from lithium fluoride and aluminum fluoride:

3LiF + AlF3 = Li3AlF6

(3.5.10)

To use Dewing's equation the concentration of lithium cryolite must be calculated if the lithium fluoride concentration is given and the input aluminum fluoride concentration must correspondingly be corrected.

Equation Correction Li3AlF6 Correction Li3AlF6: Meaning of Symbols

(3.5.11)

Liquidus Enigma

If one calculates the liquidus temperature of the electrolyte using its chemical analysis and one of the shown relations, one finds that some cells operate successfully for extended periods (weeks) at temperatures below the calculated liquidus. Haupin [Lit.] has analyzed this so called Liquidus Enigma. He offers the following explanations:

(1) impurities not determined in the industrial bath analysis lower the true liquidus,
(2) an analytical bias of bath composition may exist, 
(3) a bias in how bath temperature is measured may exist,
(4) bath may remain supersaturated with alumina for extended period of time,
(5) industrial cells may be operating with suspended precipitate.

Haupin considers explanation 1, 3 and 5 as most likely.

Tarcy et al. [Lit.] have analyzed the analytical methods to determine the bath composition especially concerning the aluminum oxide concentration. They have shown that the extraction method which is normally used to determine the alumina content of bath samples delivers systematically too low values. The observation of cells operating below the calculated liquidus temperature is, to a large extend, accounted for a systematic alumina measurement error. Some cases of liquidus enigma may also be due to impurities or measurement errors especially of the bath temperature.

Recently Moxnes et al. [Lit.] looked again at the problem of the liquidus enigma. The superheat calculated from measured bath temperatures and bath sample analyses using Solheim's equation often show negative values, although measurements made with the superheat-sensor from Heraeus Electro-Nite (Lit. 2001, 1997, 1996) always show positive values. Simultaneously, bath samples were taken for aliminum carbide (Al4C3) analysis, XRF (x-ray fluorescence), XRD (x-ray diffraction) and ICP (inductively coupled plasma) spectrometer analysis (see Teledyne Instruments, for instance), as well as alumina (Al2O3) analysis by LECO and thermal analysis. It turned out that Al4C3 content in the bath was very low. By comparing all the measured temperatures, the Heraeus probe seemed to give a somewhat high superheat. The key factor, however, appeared to be the precision of the bath analysis. Furthermore, it is possible that some of the trace elements in the bath will reduce the liquidus temperature more in an acidic bath than in pure cryolite.