What is Molten Salt Technology?

    Molten salts have been used in many industries as a high temperature heat transfer medium. The 'highest profile' use of molten salts in this regard is the Solar Power Tower near Dagget, California (excuse the pun). It uses a Sodium Nitrite/Nitrate mixture to absorb and store the sun's heat from the focus of many mirrors in the desert upon a central tower. The heat from the salt is then transfered via a heat exchanger to produce steam to drive a conventional steam turbine and generator to produce electricity from the sun for Southern California.

    Molten Salt (Pyroprocessing) of Non-Ferrous Metals

    Molten salts are also used to produce most non-ferrous metals (non-iron like metals; e.g., aluminum, titanium, etc.). The most notable and oldest use was the previously mentioned production of aluminum via electrolytic decomposition of alumina (aluminum oxide; Al₂O₃). A non-electrical method of using molten salts to produce metals is used to produce titanium. Titanium oxide from various ores is reacted with chlorine and carbon (usually in the form of petroleum coke) to form titanium tetrachloride (TiCl₄₎, which is a salt of titanium. This salt is melted and boiled off so as to distill and purify the TiCl₄. It can then be contacted with sodium (produced via the previously mentioned molten salt Down's Cell) or magnesium metal in the Hunter or Kroll process (respectively) to produce titanium sponge and sodium chloride (NaCl) or magnesium chloride (MgCl₂). The titanium sponge is a raw material in the fabrication of finished titanium metal products.

     Molten Salt Salt Electrolytics - Fuel Cells

    Molten salts are also used in Fuel Cells. While there are many different type currently being researched, the usual characteristics is to employ a mixture of various salt carbonates (e.g., Na₂CO₃, sodium carbonate & other carbonates of lithium, potassium, etc.) as the electrolyte of a battery called a fuel cell. The advantage of this type of fuel cell is its ability to use carbon containing fuels (methanol, gasoline, etc.) directly in the production of electricity. The disadvantage is these molten salts corrode metal very easily. Lifetimes of the electrodes is still a problem area.

    Molten salts are currently used commercially to strip metal clean of impurities. This simple process utilizes the high temperature and catalytic and oxidative properties of Sodium Nitrite/Nitrate (NaNO₂/NaNO₃) salts. For example, Whirlpool (the appliance manufacturer) uses a molten salt bath of sodium nitrite/nitrate salts to clean the paint off of its appliances that failed quality control checks. The molten salt completely cleans the metal of all paints by thermally decomposing and oxidizing the paint into carbon dioxide (CO₂) & water (H₂O) vapors. The stripped metal appliance can then be repainted.
    Molten Salt Oxidation (MSO) - Coal Gasification

    Molten salts have also been studied since the early 1900s to gasify coal in a process called Molten Salt Oxidation (MSO). The molten salt used is usually sodium carbonate heated above its melting point of 851° C ( 1564° F) to around 900° - 1000° C. At this temperature the red hot salt functions as a catalyst, fluid reacting bed, and heat transfer medium; all in one! The coal is flash pyrolyzed such that no tars or oils are produced. Steam is usually injected too so that the combination of coal's thermally decomposed higher organic molecules along with catalytically assisted carbon-steam reactions (i.e., C + H₂O = CO + H₂) produces mainly carbon monoxide (CO) and hydrogen (H₂) gases at atmospheric pressures. At higher pressures, there will be significant methane (CH₄) and higher hydrocarbons produced. Carbon monoxide (CO) and hydrogen (H₂) can be used directly as a fuel gas or as a synthesis gas to produce virtually any organic material. The most common use of synthesis gas however, is to produce Methanol (methyl alcohol - CH₃OH) which can also be used as a fuel, and is used in race cars, but is usually a raw material for the production of various organics (octane boosters, gasoline additives, plastics, chemicals, and drugs). Given the renewed interest in the so called "Hydrogen Economy", whereby hydrogen is used as a "carrier fuel" to provide energy portability and transmission, it is likely MSO will play a significant role in the production of hydrogen fuel for the Hydrogen Economy via the "water shift" reaction where the Synthesis gas (the mixture of CO and H₂ gases decribed above) is converted into nearly pure H₂ gas by the following, catylized reaction:

CO + H₂O = CO₂ + H₂

    The CO₂ can then be fairly easily removed via various reactions, as it is a mild acidic gas which can be combined with various alkaline substances [e.g., CaCO₃ (s) + H₂O + CO₂ = Ca(H2CO₃)₂ (aq)]. Generally, the more alkaline the substance the faster and more complete the reaction (absorbtion) of the CO₂. Thus, sodium hydroxide (i.e., "Drano", or Lye - NaOH) would work much better at absorbing the carbon dioxide. Another method to remove the CO₂ from the H₂ gas is via liquification (using pressure and lower temperatures) of the carbon dioxide gas which liquifies much easier than the hydrogen gas. The liquified CO₂ could then be resold for various industrial or food processes.
    Molten salt gasification of coal has also been proposed for wastes, to include garbage (Municipal Solid Wastes - MSW). 
    Molten Salt Oxidation (MSO) - Chemical Weapon
  In the mid-1950s Rockwell, Inc. conducted extensive tests of molten salts for the purpose of destroying chemical weapons. This was also called Molten Salt Oxidation (MSO --> good description at Lawrence Livermore National Lab's Upadhye's MSO description), and was a spin-off of the earlier coal gasification studies. Similar to the advantages of using molten sodium carbonate to gasify coal, MSO has the additional advantage of having large amounts of sodium in close proximity to the decomposing chemical weapons molecules. This is significant because chemical weapons usually contain large amounts of fluorine, sulfur and/or chlorine, all of which can form radicals which may cause the production of carcinogens such as dioxins. The long residence times of the chemical weapons in a molten salt bath, as compared to incineration, combined with the presence of large amounts of sodium allows the chlorine, sulfur, and fluorine radicals plenty of time to form stable, and safe, sodium compounds such as sodium sulfate (a laundry soap and food additive), sodium chloride (table salt), and sodium fluoride (an anti-cavity toothpaste ingredient). Although there were no significant technical obstacles to employing MSO for chemical weapons' destruction, widespread Molten Salt ignorance and inertia prevented its deployment.
    Molten Salt Reactors (MSRs) - a type of Nuclear Reactor
    The most interesting application of molten salt technology was the development of the Molten Salt (Nuclear) Reactor (MSR). Originally developed to power a deep penetration bomber for targets in the Soviet Union during the early Cold War (1946 - 1962)4, it is a remarkable, yet virtually unknown reactor. Part of the problem was the limited geographical experience of the MSR as both operating MSRs were built only at Oak Ridge National Laboratory (ORNL), near Knoxville, Tennessee, USA.
    The first MSR was the 1954, 100-hour operation of the Aircraft Reactor Experiment (ARE) at ORNL5. Its sole purpose was to demonstate the then unheard of notion of operating a reactor at red heat (~750° C; ~1,550° F) with a molten fuel and coolant consisting of melted fluoride salts (sodium fluoride, NaF; zirconium fluoride, ZrF ₄; and UF₄ [enriched in ²³⁵U])6. The second MSR was a civilian power plant prototype, the Molten Salt Reactor Experiment (MSRE)7. Hugely successful, it was ignored by the US Atomic Energy Commission (US AEC), which had decided to favor the Liquid Metal Fast Breeder Reactor (LMFBR). The Director of ORNL, Dr. Alvin Weinberg, pushed for the MSR, but was fired for his efforts 8.
The notable features of this reactor are
    Meltdown proof
    Does not produce weapons grade plutonium
    Has inherent non-proliferation features
    Thousands of years of energy
    Simplified fuel cycle (no fuel elements nor reprocessing required)
  Its wastes are simpler and less toxic than current nuclear wastes
  Only hundreds of years of storage versus thousands for the current wastes
  Can completely destroy military plutonium
  Can burn the existing wastes (spent fuel)!
  Higher thermal efficiencies (operates at a "Red Heat"; ~700° C [1,260° F])