Electrochemical properties of phosphorus compounds in fluoride melts

Abstract

In the present work the behaviour of phosphorus species in fluoride-based melts was investigated. The motivation for the study was based on the deleterious effect of phosphorus on the aluminium production process. It is “known” that for each 100 ppm phosphorus (P) in the bath the current efficiency is reduced by 1%. In addition, the properties of cast alloys are influenced by the phosphorus content.

The thesis contains a literature review on the chemistry of phosphorus with focus on its behaviour in molten salts.

The solubility of aluminium orthophosphate in cryolite melt was studied. Part of the phase diagram of the system Na3AlF6-AlPO4 was investigated. The eutectic point was determined to be at (57.21 ± 0.02) mole% AlPO4 and (695.9 ± 1.0) ºC. It was found that the orthophosphate dissociates partly into a metaphosphate species and an oxide ion in pure cryolite melts:

PO 43- <=> PO3 – + O2-

Electrochemical measurements were performed in the systems Na3AlF6- AlPO4, Na3AlF6-Al2O3-AlPO4, Na3AlF6-Al2O3-Na3PO4 and NaF-Na3PO4.

By means of cyclic voltammetry, chronoamperometry, chronopotentiometry and square wave voltammetry the behaviour of the P(V) species were studied. The most suitable electrode materials were found to be tungsten and copper. With a tungsten working electrode the number of electrons transferred when reducing P(V) in an alumina-saturated cryolite melt was found to be two. The diffusion coefficient for the PO4 3- added as Na3PO4 at 1000 ºC was calculated to be (9.2 ± 1.0)·10-6 cm2/s with cyclic voltammetry and (6.8 ± 0.7)·10-6 cm2/s with chronopotentiometry. When adding AlPO4 the diffusion coefficients of the P(V) species at 1000 ºC seem to vary from 1.2·10-5 to 2.1·10-5 cm2/s for cyclic voltammetry, while for chronopotentiometry and chronoamperometry the values were between 1.9·10-5 and 2.5·10-5 cm2/s. The reduced phosphorus species seemed to adhere on the tungsten working electrode.

In the pure cryolite system the reduction of P(V) on a copper electrode at 1020 ºC seemed to involve three electrons. However, the number of electrons transferred when using the tungsten electrode at 1020 ºC was found to be 2 as for the alumina-saturated system. For both electrode materials the system under study was found to be soluble-soluble. The diffusion coefficients for the P(V) species determined with the copper electrode was found to be between 1.6·10-5 and 1.8·10-5 cm2/s. In the case of the tungsten working electrode, they were found to be about 1.9·10-5 to 2.2·10-5 cm2/s.

At low contents of aluminium orthophosphate in pure molten cryolite, two peak couples were observed. Two possible mechanisms were proposed. One of them included reduction of P(V) to P(III) and further to P(I), while in the other the reduction of two different P(V) species (PO4 3- and PO3 -) to P(III) species was suggested.

Since the deposition of aluminium might hide further reduction steps of phosphorus, electrochemical studies in the pure sodium fluoride system with addition of Na3PO4 were performed at 1002 ºC and 1005 ºC with a tungsten working electrode. The investigation indicated that the orthophosphate ion dissociates into an oxide ion and a phosphorus containing species. The number of electrons transferred when reducing the phosphorus compounds was determined to be 2. The oxide ion seemed to oxidize the tungsten working electrode forming tungsten oxide compounds. The number of electrons transferred in this reaction was found to be 1. The diffusion coefficient of the O2- species was determined to be between and 3.2·10-5 to 7.4·10-5 cm2/s.