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FUSED • Commercially Important Today • Challenging Field for Research for Tomorrow DR. RICHARD B. ELLIS, Southern Research Institute, Birmingham, Ala. High temperature is the watchword in today's chemical industry. Because of their great stability at elevated temperatures, fused salts are becoming increasingly important industrially. Other reasons for the expanding use of fused salts: their low vapor pressure, low viscosity, good electrical conductivity, and their ability to dissolve many different materials. The term "fused salt," as used here, refers to the molten state of primarily ionic compounds—sodium chloride, for example. Molten glasses and slags, which have high viscosity and low electrical conductivity, belong in another category. Some fused salt processes have been used metallurgically for years—in aluminum and magnesium production, in heat treating baths, in descaling metals, and in other ways. The • growth of atomic energy and rocket propulsion has increased the demand for new 96
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structural materials and has rapidly expanded the list of new metallurgical processes. Many of these involve fused salts. Aluminum and Other Major Metals Commercially, the most important use of fused salts is in making alkali and alkaline earth metals. Aluminum, magnesium, sodium, and related metals are being made in huge quantities today by fused salt electrolysis. Fused salt was a basic ingredient in making the 1.95 million tons of primary aluminum turned out by the U.S. last year. Output expected in 1960: at least 2.1 million tons. Invasion of aluminum into the automobile and container fields has accounted for much of its expansion in the last few years. Use of aluminum in construction continues to grow amazingly in both volume and variety.
Aluminum is made by electrolyzing alumina (aluminum oxide) dissolved in molten-cryolite (sodium aluminum fluoride) at slightly below 1000° C. This process was developed by Charles M. Hall in the U.S. in 1886 and independently in the same year by Paul L. T. Heroult in France. Since then, great improvements have been made in the equipment and techniques for producing aluminum. The electrolytic cells now carry more than 100,000 amperes—over twice as much as they did in 1950. Sodium is also turned out in large quantities using fused salts. Last year's U.S. production of sodium totaled about 112,000 tons. Almost the same amount is expected to be made this year. About 9 5 % of this sodium is produced in Downs cells, patented in the U.S. by J. C. Downs in 1924. In these cells, sodium is made by electrolyzing a fused, purified mixture of
sodium chloride and calcium chloride (the latter used to reduce the electrolyte's melting point to about 580° C ) . Most of the nation's sodium goes into making tetraethyllead. But new uses are continually being developed, particularly as a chemical reducing agent. Magnesium likewise ranks among the important metals. Its output, however, is much smaller than that of either aluminum or sodium—about 31,000 tons in 1959. Expected production in 1960: about 40,000 tons. Biggest outlets for magnesium are in alloys for aircraft and in other structural uses. Most magnesium today is made by electrolyzing. a fused mixture of magnesium chloride, calcium chloride, and sodium chloride at about 700° to 750° C. (the Dow process). Less Common Metals Fused salts are also finding important use in producing the less common metals, such as titanium, zirco-
FROM SALTS. An important metal made from fused salts, sodium is produced at this U.S. Industrial Chemicals plant from mixture of sodium and calcium chlorides
nium, niobium, and tantalum. Capable of withstanding high temperatures, highly corrosive conditions, and intense radiation, these metals are especially valuable in atomic reactors. They are also being used in supersonic
aircraft, missiles, and space vehicles because of their great strength despite their low weight. To produce these metals, fused salts are usually reduced by sodium, calcium, or magnesium, which are themselves made by electrolyzing fused salts. In the Kroll process, for example, titanium and zirconium are made by reducing their molten halides with magnesium metal. The various technical difficulties of this process, however, have spurred the search for other routes. Many patents, for example, describe methods for reducing fused salts of titanium, zirconium, and other metals electrolytically. These processes divide into two groups. In one, a compound of the desired metal is added to the electrolyte. In the other, the impure metal itself is used as the anode. Even though they are compounds, the carbides, nitrides, and borides of titanium, zirconium, niobium, and tantalum conduct electricity well enough for them to be used as a consumable anode (rather than being dissolved in the electrolyte). The laboratories at Norton Co. have produced high quality titanium by electrolyzing an anode of titanium carbide in a fused salt. Titanium carbide is an at-
ATOMIC PROJECT. Much research on fused salts is being carried out at atomic labs. Here, J. R. Shugart of Oak Ridge National Laboratory manipulates remotecontrol devices at molten salt reactor mock-up, while he views the equipment on television screens
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... fused salts have advantages over solid fuels in atomic reactors fused salt solution acts as the primary coolant and fuel. High Purity Uranium
REMOVING SCALE. Fused salts are useful in descaling metals where ordinary acids fail. At this International Nickel plant, fused salt bath cleans various alloy parts
tractive raw material for making titanium because its cost, per pound of metal, is only about one third that of titanium tetrachloride. Electrolysis of fused salts is enabling many rare earth metals to step out of the laboratory and into commercial use. For example, the Bureau of Mines in 1958 began producing high purity cerium in pound quantities by electrolyzing fused cerium oxyfluoride. Valuable in Atomic Field Fused salts are aiding in the development of atomic power. Scientists have long recognized that an atomic reactor containing a circulating liquid fuel, such as a mixture of molten salts, could have many advantages over a reactor using a solid fuel: • The high fabricating cost of solid fuels is avoided. • Undesirable fission products can be removed from the circulating fluid continuously. • An excess of reactive material is unnecessary since make-up fuel can be added at will. • Nuclear radiation cannot • damage molten fluoride fuels structurally, as it does most solid fuel elements. The intense radiation inside a reactor core can cause solid fuel elements to warp, crack, and blister. Because the fuel rod must be removed from the core 98
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before the element ruptures, the percentage of solid fuel actually burned is limited, thus reducing the efiiciency of fuel use. Although nuclear reactors do make use of water-containing systems, fused salts have the advantage of permitting the reactor to be operated at lower pressures and higher temperatures. And molten salts can act as the fuel solvent or coolant without troublesome gas mixtures being formed —a problem with reactors that are cooled or moderated with water. Compared to systems containing molten metals, fused salts dissolve a higher percentage of materials that are either fertile (can be made fissionable by neutron bombardment) or are already fissionable. Especially important to equipment designers, construction materials are now available that are corroded only very negligibly by molten salt fuels—less than 0.0005 inch per year. In 1954, Oak Ridge National Laboratory demonstrated in its aircraft reactor experiment (ARE) that an aircraft atomic reactor can be successfully fueled with a solution of uranium tetrafluoride in a molten mixture of sodium fluoride and zirconium fluoride. In another fused salt project, Oak Ridge is working on a molten salt reactor (MSR) containing lithium fluoride, beryllium fluoride, thorium fluoride, and uranium fluoride. The
Fused salts have found other uses in the atomic program. In the early days of the Manhattan Project, research on atomic fission was hampered by lack of high purity uranium metal in larger than gram quantities. Dr. John W. Mar den and co-workers at Westinghouse tackled the problem and were soon making pounds of uranium by electrolyzing fused potassium uranium fluoride. In the spring of 1942, making 10 kilograms of nuclear-grade metal was considered a major accomplishment. Yet by the end of 1943, Westinghouse had turned out no less than 65 tons of the pure metal. Fused salt electrolysis is still finding application as a means of producing uranium metal. General Electrie's Knolls Atomic Power Laboratory has been testing a method comparable to the Hall-Heroult process for aluminum. Uranium oxide is fed into a cell containing a molten mixture of barium fluoride, magnesium fluoride, and uranium fluoride. The molten uranium metal is collected in a cathode pool at the bottom of the cell. In its pilot plant, Horizons, Inc., is operating a 7500-ampere cell for making thorium metal semicontinuously by electrolyzing molten sodium thorium chloride. The cell, completely enclosed, produces the metal under an inert atmosphere. Thorium is important in the atomic energy program because, when bombarded with neutrons, it is converted to fissionable uranium* 233. Atomics International is using a molten mixture of sodium chloride, potassium chloride, and zinc chloride to make electrorefined thorium. The process is designed to recover fissionable and fertile material from spent atomic fuel. With the spent-fuel alloy acting as the anode, the cell's operating voltage is adjusted to deposit thorium at the cathode. The elements less positive than thorium settle out as an insoluble anode sludge. The more positive elements remain in solution. Fused salts are also being put to work separating uranium from spent fuel by the fluoride volatility process. In this method being studied at Oal
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