SYMPOSIUM
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Recent Advances in the Chemistry of Fused Salts
Technological advances are constantly opening up new areas for experimental research. The interaction works both ways, many new tools and techniques being t.he product of st,imulat,ed research interest, whet,her aimed at practical or intellect,ual goals. Snch reciprocat,ion has played an import,ant part in the rapidly accelerat,inggrowth of act,ivity in t,he field of endeavor t,raditionally known as fused salts. "Traditionally" to everyone, and eurrent,ly t,o all those not actively working with them. For some unexplained reason, the popular designation now used by the initiated has shifted t o molten salts. It remains to be seen whether the situation has stabilized; recent inroads have been made by yet another label (probably of British origin): i m i c melts. The latter term has the advantage of incorporating fused hydroxides into an area from which they were previously excluded in name only. Molten oxides also fit into this classification, as logically they should. The study of molten oxides by fused-salt chemists has been relatively meagre, however, due to technological reasons rather than any (known) fundamental difference in character. Those oxides that form ionic melts require temperatures in the neighborhood of 2000°C, while most salts and hydroxides of interest can be studied a t less than 1000°. Fused silicates and borates, which contain highly polymerized anionic species, could he excluded by expanding the designation to simple i m i c melts. Even the majority of workers who have left the study of silicates and borates t o a relatively small group of devotees (which includes slag chemists, glass chemists, and ccrarnicists) would probably refuse to go along with this one. After the maze of experimental headaches and perplexing results encountered in the most "routine" projects, few of us are inclined t o think of any fused salt as simple. Besides the stimulus of experimental feasibility, it is possible to single out several specific goals that have contributed to current interest in fused salts. These objectives can be classified under the headings practical and theoretical as follows: Practical. To produce titanium and other potentially useful but expensive metals by electrolysis of melts; t o improve ancient electrolytic processes used in sodium and aluminum production; t o develop commercially successful fuel cells with low internal resistance and highly efficient electrode reactions; to realize liquid-fuel nuclear reactors capable of operat,ing at high temperatures. Theoretical. To approach the theory of concentrated 56
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Journol of Chemicol Education
electrolytic solutions by starting with pure fused salts; to extend knowledge and understanding of "coordination chemistry"; to study the nature and mechanisms of ionic reactions in the absence of water or similar solventas; t,o provide a focal point for currently dcveloping t,heories of the liquid state wit,h conceptual simplirit,~comparable to the liquified rare gases. The intensified pursuit of t,hese and other ohject,ivrs has turned out a wealth of previously unavailable information in the past few years. While this has hcen summarized to some extent in monographs based on fused-salt symposia and in review articles dealing with various aspects of the field, much of it has not been presented in a form easily digested by those not diligently keeping up with the field. To help remedy this situation was the primary intent of the Division of Chemical Education's Symposium on "Recent Advances in the Chemistry of Fused Salts" held a t the St. Louis Meeting of the ACS in March, 1961. The symposium lasted one day, with a morning and an afternoon session. Each session was scheduled to consist of three 50-min contributions, each followed by a 10-min discussion period. Contributors were all active research workers in the field. Each paper covered a rather broad area, not necessarily confined t o the immediate interests of the speaker. It was hoped in this way t o survey the field fairly completely, while dealing also with specific problems of particular interest. The morning session was somewhat more general in this respect. The attempt was made to characterize molten salts as a class under the headings "Reaction Mechanisms," "Electrochemistry," and Y"ysica1 Properties and Structure." These papers appear in the following pages, the authors being F. R. Duke, R. W. Laity, and G. J. Janz, respectively. It will be seen that the distribution was not really even, the burden of each reviewer being somewhat greater than that of his predecessor. Papers in the afternoon session dealt with more specific topics. The first one scheduled, "Spectroscopy
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situation. The relative positioning of the ions implies some degree of short range order but, as shown by X-ray and neutron diffraction dat,a, little long-range order exists. This "structure of fused salts" portrait also has caught some local density fluctuztion. For a more complete diaoussion, the article hg Professor Janz (p. 65) should he oonwlted.
of Fused Salts," by D. M. Gruen of Argonne National Laboratory, could not he given due to sudden illness of the author. Some of the scheduled time was used to continue the discussion of electrochemistry. M. A. Bredig of Oak Ridge National Laboratory then reviewed recent studies on a fascinating type of system that has no counterpart a t ordinary temperatures: the solution of a metal in a molten salt. He distinguished two types of "Metal-Salt Solutions." The first is analogous to subhalide formation under more familiar conditions, such as the reaction of mercury metal with HgC12to give HgzCI1,the latter being formed homogeneously in the liquid state. No simple explanation is available for the second type, which occurs, for example, when sodium metal dissolves in NaCI. Dr. Bredig reviewed some of the current hypotheses, none of which has received general acceptance. The subject of "Molten-Salt Nuclear Reactors" was discussed by H. G. MacPherson of Oak Ridge National Laboratory in the final offering on the program. The emphasis here was technological; a formidable array of specific problems in design, operation, and control was
shown to have been handled by a variety of ingenious engineering techniques. Possible advantages of such reactors for "breeding" purposes were also discussed. Of the practical objectives listed above this particular application of fused-salt research seems to have been the subject of the most intensive efforts. Since none of the papers on the afternoon program appears in this issue, interested readers are referred to the extended abstracts which appear in Abstracts of Papers, 159th Meeting, ACS, pp. 3F-7F (1961). The writer applauds the objectives of this type of meeting under the auspices of the Division of Chemical Education and feels that the efforts of the participants were more than adequately rewarded. Whether or not the information we imparted gets into the curriculum or the classroom in the immediate future, I suspect we made a few converts to a stimulating and challenging field of research. R. W. Laity, Chairman of the Symposium Princeton Universiv, Princeton,
Volume 39, Number 2, February 1962
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