| An investigation on the effect of different fluoride additions on the vaporization of molten cryolite has been carried out by the use of three experimental methods. The total vapour pressures were determined by the boiling point method. Identification of the different gas secies arising from the systems under study was attempted by using mass spectrometry and X-ray analysis of the condensed gas.; The results of the total vapour pressure measurements above the NaF-AlF(,3) system up to 40 mol% AlF(,3), confirm the previous literature data. NaAlF(,4) is the main gaseous species above this system, and its vapour pressure increases with increasing additions of AlF(,3).; A relationship has been found between the cationic radius of the fluoride added and the vapour pressure of liquid cryolite. At a constant NaF/AlF(,3) ratio (in this case at the cryolite composition), the vaporization from the melt due to alkali-fluoride additions, increases in the sequence LiF, KF, RbF, CsF.; When additions of alkaline-earth fluorides are made to cryolite, the following sequence from lower to higher vapour pressures is observed: BaF(,2)-SrF(,2), CaF(,2), MgF(,2), BeF(,2).; With the exception of mixtures of cryolite with KF, RbF or CsF, the vapour species arising from all the binary melts studied in this work, consists mainly of NaAlF(,4)(g). KAlF(,4)(g)-KF(g), RbF(g) and CsF(g) are the main vapour species above cryolite with the corresponding addition. Thus, the vapour pressure of NaAlF(,4)(g) above the melts investigated, was found to increase in the order CsF, RbF, KF, NaF-BaF(,2)-SrF(,2), LiF, CaF(,2), MgF(,2), BeF(,2), AlF(,3).; In Chapter VIII, technical implications of interest for the aluminium industry are considered. The vapour pressure data obtained during the present work provides a better basis than previous literature data, as far as regards the estimation of fluoride emissions due to vaporization from Hall Heroult electrolytes with different fluoride additions. |
PDF Full Text Request