Optics: measuring and testing – By dispersed light spectroscopy – With raman type light scattering
Reexamination Certificate
2001-07-18
2003-10-28
Juba, John (Department: 2872)
Optics: measuring and testing
By dispersed light spectroscopy
With raman type light scattering
C250S339070, C205S372000
Reexamination Certificate
active
06639667
ABSTRACT:
The present invention relates to a method and an apparatus for the analysis of chemical constituents in an electrolysis cell for production of metal. In particular, the invention relates to continuous analysis of the oxide concentration and the cryolite ratio in electrolysis cells for production of aluminum working in accordance with the Hall-Héroult process.
In the Hall-Héroult process, aluminum is produced by electrolytic reduction of alumina (Al
2
O
3
) dissolved in a bath based on molten cryolite (Na
3
AlF
6
). The metal is formed at the molten aluminum cathode, and oxygen is discharged at the carbon anode. Increasing knowledge of the processes involved, has enabled modifications to be made on the bath compositions. Some fluorides in small and limited amounts are added to the bath in order to lower the bath temperature, and increase the efficiency of the electrolytic process. Aluminum fluoride (AlF
3
) is the most common additive, and commonly cells are operated with an excess aluminum fluoride content relative to cryolite. In addition, the bath may contain a certain amount of calcium fluoride (CaF
2
), which mainly originates from the calcium oxide (CaO) impurity in the alumina feed. In some cases, calcium fluoride is voluntary added to the bath as well.
To operate the cells in a manner that is effective with respect to several criteria such as dissolution of alumina, energy consumption, current. efficiency, sludge formation, compensation of additives as a result of evaporation losses and losses when draining metal, environmental regulations, etc., it is of great importance that parameters such as the cryolite ratio CR (the molar ratio of NaF and AlF
3
), bath ratio (the mass ratio of NaF and AlF
3
), excess AlF
3
(the mass % AlF
3
in excess of the Na
3
AlF
6
composition) and the A oxide concentration can be determined in a precise and continuous manner.
Up to present, numerous suggestions have been made as to possible structural species in cryolite-alumina melts as such. In this work, Raman spectroscopic measurements have been performed to analyse the bath composition. Such measurements are based upon the fact that the different species emit light at a characteristic wavelength, and the technique is commonly used to indicate the presence of different species in laboratory work. One example of such work performed is published in the paper: “Reinvestigation of Molten Fluoraluminate Raman Spectra: The Question of the Existence of AlF
5
2−
Ions, B. Gilbert and T. Materne, Applied Spectroscopy, Volume 44, Number 2, 1990”. In addition, the paper discloses a laboratory equipment for spectroscopic analysis of melt samples.
Further, one commonly used method for indicating the alumina concentration in an electrolysis cell is based upon. cell voltage measurements related to the electrical conductivity of the electrolyte. Other methods involve taking samples of the bath for the analysis of the composition in a laboratory.
However, present methods are either not very precise or not continuous and need further evaluation to fit the steadily increasing demands, of the electrolysers for optimising their processes.
In accordance with the present invention, it is now possible to determine both the alumina (oxide) concentration and the cryolite ratio in a precise and continuous manner. in an electrolysis cell under its production of metal. The invention involves the use of Raman spectroscopy where spectrums of light emitted from the melt/bath are compared to reference spectrums generated from samples of known compositions. This “fingerprint” recognition. method has proved to be very accurate, and the apparatus together with the proposed method make it possible to perform the analysis in an continuous manner.
REFERENCES:
patent: 4802761 (1989-02-01), Bowen
patent: 5412465 (1995-05-01), Baylor
patent: 5822072 (1998-10-01), Dai
B. Gilbert, et al., “Reivestigation of molten fluoroaluminate Raman spectra: the question of the existence of AIF5 ions”, Applied Spectroscopy, 44(2), 1990, pp. 299-304.*
Applied Spectroscopy, vol. 44, No. 2, 1990, B. Gilbert et al., “Reinvestigation of Molten Fluoroaluminate Raman Spectra: The Question of the Existence of A1F 5 2—Ions” pp. 299-304.
Foosnæs Trygve
Gilbert Bernard
Huglen Reidar
Boutsikaris Leo
Norsk Hydro ASA
Wenderoth , Lind & Ponack, L.L.P.
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