Molten salt metal refining

The principal use of AIF. is as a makeup ingredient in the molten cryoflte, Na.. AIF AI2O2, bath used in aluminum reduction cells in the HaH-Haroult process and in the electrolytic process for refining of aluminum metal in the Hoopes cell. A typical composition of the molten salt bath is 80—85%... 

Sla.g ReHning. Unwanted constituents can be removed by transfer into a slag phase. Slag refining is also used for operations in which the Hquid metal is maintained in contact with a slag or a molten salt. This second immiscible Hquid is usually more oxidizing than the metallic phase and selective oxidation of the impurities renders them soluble in the slag or molten salt. Impurities that are less easily oxidized remain in the Hquid metal. 

In this process, uranium metal is electrodeposited at the cathode, while plutonium and other transuranium elements remain in the molten salt as trichlorides. Plutonium is reduced in a second step at a metallic cathode to produce Cd—Pu intermetallics. The refined plutonium and uranium metals can then be refabricated into metallic fuel (137). 

The melt is heated by passing a large elecuical cunent between two electrodes, one of which is tire metal rod to be refined, and the otlrer is the liquid metal pool standing in a water-cooled copper hearth, which collects the metal drops as tlrey fall tluough the molten electrolyte. This pool tlrerefore freezes at the bottom, forming the ingot. Under optimum chcumstances tire product billet takes the form of a cylindrical solid separated from the molten salt by... 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

The quality of the refined metal, and the current efficiency strongly depend on the soluble vanadium in the bath and the quality of the anode feed. As the amount of vanadium in the anode decreases, the current efficiency and the purity of the refined product also decrease. A laboratory preparation of the metal with a purity of better than 99.5%, containing low levels of nitrogen (30-50 ppm) and of oxygen (400-1000 ppm) has been possible. The purity obtainable with potassium chloride-lithium chloride-vanadium dichloride and with sodium chloride-calcium chloride-vanadium dichloride mixtures is better than that obtainable with other molten salt mixtures. The major impurities are iron and chromium. Aluminum also gets dissolved in the melt due to chemical and electrochemical reactions but its concentrations in the electrolyte and in the final product have been found to be quite low. The average current efficiency of the process is about 70%, with a metal recovery of 80 to 85%. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Magnesium chloride and excess magnesium are removed by distillation at reduced pressure. Pure zirconium may be prepared by several methods that include iodide decomposition process, zone refining, and electron beam melting. Also, Zr metal may be electrorefined in a molten salt bath of potassium zirconium fluoride, K2ZrFe... 

Magnesium electrochemistry in molten salts media is especially important for the mass production of the pure metal. Magnesium is frequently produced and refined by electrolysis of high-temperature, molten eutectic mixtures of anhydrous MgCl2 with various salts, such as KC1, CaCl2 etc. High-temperature molten salts baths, with similar compositions, with the addition of borate are also studied for the production of MgB2. 

The recovery of aluminum metal is divided into two steps, i. e., the production of pure alumina (Bayer Process) and its molten salt electrolysis. Raw aluminum obtained by reduction electrolysis already has a high purity (99.5-99.7%). Refining methods for raw aluminum to obtain higher purities include the segregation process (99.94-99.99% Al) and three-layer electrolysis (99.99-99.998% Al) [142, 236]. Besides these, processes are available whereby the aluminum is anodically dissolved in an organic electrolyte and then cathodically deposited [37, 118, 217, 221]. The dissolution as well as the deposition process contribute to the electrolytic refining of aluminum. 

Scrap aluminum, earlier used for a new product, must be re-melted and refined. This technology consists of melting aluminum under a molten salt mixture in order to prevent oxidation and to enhance the coalescence and recovery of the molten metal. In this process, the interfacial tension between aluminum and the salt mixture plays a significant role in terms of both metal recovery and dross de-wetting. Since scrap metal always has an oxide layer, it is required, by either mechanical or chemical means, to break this layer to allow the metallic droplets to coalesce. 

In our natural environment, metals are most stable in an oxidized state, e.g., Fe203 or AI2O3. As a consequence, one step of metal ore refining is the reduction of the metal oxide to its zero oxidation state. An electrochemical reduction process, where the electrolysis medium is a molten salt, is preferred for very electropositive metals (e.g., aluminum, sodium, lithium, and magnesium) and for metal refining where the chemical route suffers from environmental problems. 

Many processes used to produce or refine metals are based on electrolysis. Collectively these processes are referred to as electrometallurgy. Electrometallurgical procedures can be broadly differentiated according to whether they involve electrolysis of a molten salt or of an aqueous solution. 

The principal metal refined in a molten salt medium is aluminium. Something approaching 2% of the total aluminium produced is refined by a process based on the principle illustrated in Fig. 4.7. The density of the impure aluminium is increased by the addition of copper (25—30%) and that of a cryolite melt by the addition of barium fluoride so that three distinct layers, pure aluminium, melt and aluminium/copper, are formed in the cell. On electrolysis the aluminium is transferred from the anode of impure aluminium to the top layer while the major impurities (i) Na, Mg, Ca and Sr are oxidized from the anode pool to the melt but do not reduce at the cathode and therefore accumulate in the melt, and (ii) Fe, Si, Mn, Zn (and Cu) are oxidized less readily than aluminium and hence remain in the anode pool. The aluminium obtained is very pure, being in the range 99.99—99.999%. 

Molten salt processes for refining metals as diverse as lead, beryllium, magnesium, molybdenum and uranium have been described but are probably never used commercially. 

Plutonium metal is often purified by electrolytic refining the plutonium sample is immersed in a molten chloride salt under an inert atmosphere, comprising the anode in an electrolytic cell. Liquid Pu metal is collected on the surface of a tungsten cathode and drips off into a collector. Transition-metal contaminants remain in the residue of the anode, and rare earths and other actinides concentrate in the molten salt. The yield of purified Pu metal can be as high as 97%. Zone melting is also used to purify metallic plutonium the plutonium is fabricated into a bar along which a high-temperature zone is passed. As the melt zone is moved... 

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