US2393686A - Electrolytic production of magnesium - Google Patents

Description

Jan. 29, 1946. B. G. HUNT ET A1. 2,393,685

ELECTROLYTIC PRODUCTION OF MAGNESIUM Filed Feb. 6, 1942 INVENTORS ATTORNEYS a number of aspects, and must therefore be kept Patent-rl 29, 194e 2,393,

UNITED STATES PATENT oFrlcE ELECTROLYTIC PRODUGTION F MAGNESIUM Basil G. Hunt, Tram-British Columbia, Canada, and Robert B. MacMullln, Niagara Falls, N. Y.,

assigner-s, by mesne assignments, to The Mathieson Alkali Works (Inc.) a corporation of Virxinia Application February 6, 1942, Serial No. 429,730

4 claims. (c1. zot-1o) Our invention relates to improvements in the lowed by rapid deterioration of the refractory. electrolysis of chloride fusions heavier than mag- Also, to the extent that the refractory becomes nesium and comprising magnesium chloride to conducting in this manner, magnesium tends to produce magnesium metal and chlorine. Our be liberated between the baille and the anode and invention includes improvements in the electhus the original purpose of the baille is defeated. trolysis of such fused electrolytes and in the con- The resulting problem with respect to refractories struction of cells for carrying out the electrolysis. is one of the more serious problems involved in Magnesium and chlorine are liberated by the the construction of cells for this electrolysis. electrolysis of such fused electrolytes comprising Contrary to the View generally accepted, the

magnesium chloride. Hitherto it has been rel0 prompt separation of liberated magnesium and" garded as essential to effect a prompt separation of liberated chlorine and magnesium within the region of electrolysis in order to avoid recombinaliberated chlorine and the liberated metal can be tion of these elements. Such recombination maintained as a composite mixture, both present would of course destroy the work intended to be but separately dispersed in the fused electrolyte, done by the electrolysis to the extent that it ocfor suiiicient time to permit their removal from curred, with corresponding losses in efficiency in the region of electrolysis as such a mixture for subsequent separation and recovery of the chloat a minimum. In general, this separation of the rine and the magnesium without substantial reliberated chlorine and magnesium has been efcombination. Perhaps the fused electrolyte, as fected by interposing a refractory baille or curtain an external phase surrounding the separately between the cathode and anode in the cell in the dispersed liquid particles or globules of magupper part of the region of electrolysis to trap nesium and gaseous particles of chlorine, is sufthe magnesium liberated at the cathode on one ficient chemical insulation, while the dispersion side and to permit the chlorine to escape on the particularly of the chlorine, is maintained, to other side, between the baille and the anode at inhibit recombination. But whatever the explawhich the chlorine is liberated. Cells embodying nation may be, the fact that substantial recombithis accepted principle of design suffer a number nation does not occur for some time in such comof disadvantages, but prior to our invention no posite mixtures of liberated magnesium and other way of effecting satisfactory recoveries of 80 chlorine in the fused electrolyte is more importhe magnesium and chlorine liberated by the electant. In these composite mixtures, the dispertrolyss has been devised. sion with respect to chlorine is, usually, fairly The nature of the problem can be illustrated uniform and intimate but, as we will explain, the by summary reference to some of the difficulties dispersion with respect to magnesium may be involved in the construction and operation of this quite irregular and the Size 0f the particles 0r general type of cell. The interposition of a. bale globules may vary widely. We have applied this between the anode and the cathode necessarily discovery in our process and apparatus, our presincreases the distance between them with a. conent invention, to overcome the disadvantages of sequent increase in the voltage required for operthe general type of cell previously used by reation and corresponding increase in power conmoving the magnesium and chlorine from the sumption per unit of metal produced. This inregion of electrolysis as a composite mixture in crease in the distance between the anode and the the fused electrolyte without attempting any sepcathode also increases the size of the cell required, aration of the magnesium and chlorine within per unit of capacity, which not only increases the region of electrolysis. the cost of cell construction but also involves a. The electrolytic cell of our invention comprises cumulative power loss following from the ina cell chamber, a superposed chlorine collection creased heat losses from a larger cell. 'I'he rechamber communicating with the electrolysis fractory used in the construction of the baille or chamber through an aperture in the floor of the curtain must withstand the action of the chloride chlorine collection chamber, a cathode surface in fusion and the hot liberated chlorine and must 5" the electrolysis chamber arranged beneath and maintain its mechanical and electrical properties around that aperture confining to that aperture notwithstanding such exposure. Where the the upward now of electrolyte and of chlorine baille or curtain is suspended, as in conventional and magnesium liberated by electrolysis from the construction, extreme strength, extreme resistelectrolysis chamber to the collection chamber, an .ance to breakage and extreme resistance to 5 anode depending through the chlorine collection erosion are required of the refractory. Further, chamber and the aperture into the electrolysis since the baille or curtain lies in part between chamber and co-operating with the cathode surthe cathode and the anode. electrolysis tends to faces to form an annular electrolysis channel, a take place through the refractory with consequent magnesium collection chamber laterally disposed impregnation of the refractory with metal folwith respect to the electrolysis channel and the chlorine within the region of electrolysis is not essential under all conditions of operation. The

chlorine collection chamber, a passage between the lower parts of the electrolysis channel and the magnesium collection chamber, and a. portA opening from beneath the normal electrolyte level in the chlorine collection chamber into the magnesium collection chamber. In operation, the magnesium and chlorine vare removed from the region of electrolysis as a composite mixture in the fused electrolyte and the chlorine and magnesium are separated from that dispersion outside of the region of electrolysis. Circulation of the fused electrolyte from the electrolysis channel successively through the chlorine collection chamber and the magnesium collection chamber and back to the electrolysis channel is maintained by the gas lift action of the chlorine liberated at the anode assisted by the thermosiphonic action resulting from the liberation of heat in the region of electrolysis. No separation ci liberated magnesium and chlorine is attempted within the region of electrolysis and consequently no refractory baiile or curtain between the anode and the cathode is required. As the fused electrolyte rises through the aperture, in the free space around the anode, from the electrolysis channel into the chlorine collection chamber, it spreads radially over the floor of the chlorine collection chamber and, with the resulting reduction in velocity, a liquid level being maintained in the chlorine collection chamber, gaseous chlorine separates from the electrolyte. The magnesium present is carried, by the circulating electrolyte, on into the magnesium collection chamber where it separates The discharge of liberated magnesium from the electrolysis channel may be fairly regular but in some cases, apparently with electrolytes of high purity, the liberated magnesium tends to wet the cathode and thus to accumulate in mas. es there which are carried oi'f by the circulating electrolyte as relatively large globules at more or less irregular intervals. We thus provide a special process and apparatus for carrying out this electrolysis which has a number of important advantages.

'I'he cell, and the operation of the cell, illustrated in the accompanying drawing, will exemplify our invention. In the drawing, Fig. 1 is an elevation in section of a cell embodying our invention, Fig- 2 is a plan in section on line 2-2 of Fig. 1, Fig. 3 is a plan in section on line 3 3 of Fig. 1 and Fig. 4 is a plan in section on line 4-4 of Fig. 1. The accompanying drawing is substanti-ally to scale of a cell in which the anode illustrated is about 4 inches in diameter.

The cell illustrated in Figs. 1 to 4, inclusive, comprises an electrolysis chamber 1, a chlorine collection chamber 8 and a magnesium collection chamber 9. The lower part of the magnesium collection chamber communicates with the lower part of the electrolysis chamber through the passage I0. The electrolysis chamber and the magnesium collection chamber are with advantage constructed of steel plate as illustrated. The chlorine collection chamber 8, lined with refractories as illustrated, is superposed upon the electrolysis chamber from which it is separated by a steel plate II, an annular baille I5 and the refractories forming the floor of the chlorine collection chamber. The anode I2, a conventional graphite rod. extends through the chlorine collection chamber and an aperture I8 in the floor of the chlorine collection chamber 8 into the electrolysis chamber 1. The cathode surface comprises the inner surface of the metal walls of the electrolysis chamber indicated at I3 and I4,

'action at its inner edge.

and the inner surface of the annular baille I5 to be described. The aperture I8 is restricted in area as compared with the horizontal area of the electrolysis chamber, as defined by the cathode surface, to accelerate the flow oi fused electrolyte through this aperture into the chlorine collection chamber thus to assist in the maintenance of the dispersion of magnesium and chlorine in the composite mixture formed in the electrolysis chamber. The refractories forming the floor of the chlorine collection chamber 8 are not suspended but are supported from below by the plate and the annular baille I5, advantageously made of steel and welded to the walls of the electrolysis chamber. These refractories overhang the inner edge of the baille I5, which, as illustrated, extends but a limited distance toward the anode to avoid excessively energetic cathodic Chlorine separated in the chamber 8 is recovered through connection I6. The port I I in the wall of the chlorine col.. lection chamber forms a passageway that connects the lower part of the chlorine collection chamber with the magnesium collection chamber. This passageway opens into the chlorine collection chamber from beneath the normal electrolyte level therein, thereby serving as a liquid outlet while the lower edge of the top of this passageway, being positioned below the normal electrolyte level, forms a gas seal with the electrolyte. The opening in the roof of the chlorine collection chamber 8 through which the anode I2 enters is sealed with iire clay 24 or other appropriate compound.

In operation, the cell illustrated in Figs. 1 to 4 is charged with fused electrolyte to a level submerging the port I'I, and current is passed be tween the anode and the cathode to liberate magnesium and chlorine in the electrolysis chamber 1. Circulation of the electrolyte is maintained as previously described, the electrolyte from the electrolysis chamber containing the liberated magnesium and chlorine as a composite mixture rising through the aperture I8 around the anode into the chlorine collection chamber 8. Anhydrous magnesium chloride is introduced through the chamber 8 as required to maintain the electrolyte level. As the electrolyte leaves the aperture I8, it spreads radially over the floor of the chlorine collection chamber and gaseous chlorine separates from the electrolyte therein as previously described. The level in the magnesium col lection chamber 9 being lower than that in the chlorine collection chamber 8, due to the circulation of the electrolyte, the electrolyte and the magnesium it carries are rapidly discharged from the chlorine collection chamber into the magnesium collection chamber through the port I1. 'I'here the rate of iiow of the circulating elec- 00 trolvte is reduced and the magnesium, separating the anode and the cathode under an impressed potential of from 6 to 14 volts depending upon the temperature to be maintained in the fusion, these temperatures ranging from about 670-780 C. We estimated the rate o1' flow of the dispersion rising through the aperture above the electrolysis chamber to be of the order of one or more feet per second. Additional anhydrous magnesium chloride was supplied as required to maintain the level of the electrolyte by introduction into the magnesium collection chamber. Recoveries of magnesium metal at rates exceeding 1/r to pound per hour were secured withv the current eiiiciencies exceeding 70%80%.

In the cell illustrated, the refractories forming the floor of the chlorine collection chamber, and in contact with the fused electrolyte, are supported from below, materially reducing the mechanical burdens imposed upon the refractories and requiring, in construction of the cell, only simple refractory shapes, In the cell illustrated there is no refractory within the region of electrolysis between the anode and the cathode surface and there is no refractory within this region closer to the anode than is the cathode curface. Electrolytic degradation of refr-actories is thus avoided.

In referring to chloride fusions, we intend to include generally salt fusions, comprising magnesium chloride heavier than magnesium and liberating magnesium and chlorine when subjected to electrolysis. Such fusions may contain chlorides other than magnesium and may contain salts other than chlorides. The various additions made to control properties of the fusion such as density, viscosity and melting point are generally useful in our process and apparatus as in conventional practice.

While the cell illustrated, and more particularly described, embodies but a single anode, our invention is equally useful in unitary cells embodying a plurality of anodes in either a single electrolysis chamber or a corresponding plurality of electrolysis chambers.

The Withdrawal of the liberated magnesium and chlorine from the electrolysis chamber as a composite mixture in fused electrolyte, characteristic of our invention, is applied in our process and apparatus to secure a number of important advantages. The problems presented with respect to refractories, in the construction `and operation of such electrolytic cells, are either avoided or substantially ameliorated. Since anode and cathode surfaces can be brought into proximity impossible in cells of conventional construction, the cells can be made smaller perunit of capacity thus to secure several advantages.

The smaller size of the cells, coupled with their simplicity, reduces construction costs, less space is required and less copper is required in busbars to deliver the current for electrolysis to the cells. Smaller cells for a given metal producing capacity also reduce heat losses and consequently power consumption. The proximity of the anode and cathode surfaces further reduces the voltage required to effect the electrolysis, by reducing the resistance of the fused electrolyte between the anode and cathode surfaces, which makes possible important power savings. 'I'he rapidcirculation of electrolyte within the cell promotes the maintenance of uniform temperatures throughout the cell and thus promotes ease and uniformity of operation. The liberated magnesium can be collected in a relatively deep pool, promoting the recovery of metal of high purity. The cell is well adapted to the production and recovery of chlorine in high concentration.

We claim:

l. In the electrolysis of chloride fusions heavier than magnesium in a dlaphrasmless cell to produce magnesium and chlorine, the improvement which comprises flowing the electrolyte through the electrolysis zone at a rate sufficient to remove essentially all of the magnesium and chlorine from the region of electrolysis as a composite mixture in the fused electrolyte and separating in the order named the chlorine and the magnesium from that mixture outside of the region of electrolysis.

2. In the electrolysis of chloride fusions heavier than magnesium in a diaphragmless cell to produce magnesium and chlorine, the improvement which comprises flowing the electrolyte through the electrolysis zone at a rate sufcient to remove essentially all of the magnesium and chlorine from the region of electrolysis as a composite mixture in the'fused electrolyte, successively separating in the order named the chlorine and the magnesium from that mixture outside of the region of electrolysis and returning the fused electrolyte to the region of electrolysis.

3. In the electrolysis of chloride fusions heavier than magnesium in a diaphragmless cell to produce magnesium and chlorine, the improvement which comprises circulating the fused electrolyte upwardly from the region of electrolysis to and through a chlorine collection chamber, thence laterally to and through a magnesium collection chamber and thence back to the region of electrolysis, removing the magnesium and chlorine from the region of electrolysis as a composite mixture in the fused electrolyte, maintaining an electrolyte level in the chlorine collection chamber whereby gaseous chlorine is separated from the electrolyte therein,y maintaining a substantially lower electrolyte level in the magnesium collection chamber whereby the electrolyte and magnesium carried by the electrolyte are rapidly and continuously discharged from the chlorine collection chamber to the magnesium collection chamber, reducing the rate of iiow of the electrolyte in the magnesium collection chamber whereby floating molten magnesium is separated from the electrolyte therein, and returning the fused electrolyte from the magnesium collection chamberito the region of electrolysis.

4. In a diaphragmless cell for the electrolysis -of chloride fusions heavier than magnesium to produce magnesium and chlorine, the combination comprising a generally vertical electrolysis channel defined by confining walls comprising A separated anode and cathode surfaces, a superposed chlorine collection chamber, said electrolysis channel vcommunicating at its upper end with the chlorine collection chamber only, whereby all electrolyte, magnesium and chlorine discharged from the upper end of the electrolysis channel will be required to enter the chlorine collection chamber, means forming a metal collecting well laterally disposed with respect to the electrolysis channel and to the chlorine collection chamberl the walls of the chlorine collection chamber being shaped to provide a passageway extending between the chlorine collection chamber and the metal collecting well with the lower edge of the top of said passageway being positioned below the normal electrolyte level forming a gas seal with the electrolyte, and means forming a passage extending between the lower part of the metal collecting well and the lower part of the electrolysis channel. y

BASIL G. HUNT. RQBERT B. MACM'ULLIN.

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