US2224820A - Electrolytic fused bath - Google Patents

Description

Dec. 10, 1940. R. E. HULSE ELECTROLYTIC FUSED BATH Filed Nov. 29, 1937 IN VENTOR.

ROBERT E. HULSE ATTORNEY:

Patented Dec, 10, 1940.

UNITED i AT;E

azzatszo v ELECTROLYTIC Fusnn BATH a v p I Robert vEdwin Hulse, Niagara Falls; N. Y., asslgnor to E. I. du Pont de Nemours & 'Gompany,.Wil-

mington,Del.,acorporatlonofpelaware Application November 29, 1937, Serial No.17'l,012

5 Claims.

added for the purpose of decreasing the melting point to a temperature below the boiling point of sodium, so that the metal may be removed from the electrolytic cell in liquid state. For example, a suitable electrolyte for making sodium by this method consists of a mixture of sodium chloride with calcium chloride which has a melting point well below the boiling point of sodium. A difliculty with this electrolyte is that a considerable amount of calcium is liberated at the cathode together with the sodium, with the result that the sodium discharged from the cell is contaminated with considerable amounts of calcium metal.

At the high temperatures existing in the-electrolytic cell, the calcium liberated is ordinarily completely dissolved in the sodium. However, as the sodium calcium alloy is progressively cooled as it fiows'from the electrolysis zone to the receiver located outside of the cell, the solubility of the calcium is decreased by the decreasing temperature andthere is a tendency for calcium to precipitate and collect on the walls of the outlet and eventually to obstruct the opening through which the sodium must pass. To maintain continuous operation it is necessary to mechanically break up such deposits of calciumor by other means prevent it from obstructing the sodium outlet. The sodium eventually removed from the cell also contains considerable amounts of calcium and must be subjected to a purification process to remove the calcium. 7 I

Further, the removal of calcium from-the cell with the product depletes the concentration of calcium saltin the electrolyte and it is necessary to add calcium chloride intermittently or continuously to the electrolyte in order to maintain the desired electrolyte composition.

An object of the present invention is to provide an improved method for the'manufacture of alkali metalb'y the electrolysis-of a fused mixture of an alkali metal halide and an'alkalineearth metalhali'de, for'exanlple, the production of sodium by electrolysis of a fused mixture of )sodium chloride and calcium "chloride; A furtherlobject.

is to prevent'or decrease calcium or other metal 110 contamination of sodium produced by fused salt electrolysis, Another object isto maintain a constant. electrolyte composition in fused salt electrolysis to produce alkali metal, while adding only alkali metal salt to the electrolyte. Other objects will be apparent from the following de- 5 scription of my invention.

The appended, drawing is a vertical cross section of one type of electrolytic cell in which the present invention may be carried out.

The above stated objects may be attained in 10 accordance with the present invention by electrolyzing a fused salt mixture containing alkali metal halide and alkaline earth metalhalide to form a liquid alloy at the cathode and thereafter contacting said liquid alloy with a solid alkali metal salt at a temperature above the melting point of the alloy. I prefer to use a solid salt which has a melting point above the temperature of the electrolyte and to contact the alloy coming from the cathode with the solid salt at some point beneath the surface of the electrolyte. A convenient method consists in locating a body of granular salt or a bed of lumps of the solid salt in the cell in such manner that the alloy liberated at the cathode will riseupwardly through the bed of solid salt and thence pass on out of the cell or in some other manner to cause the alloy to flow through or over the molten salt at a point beneath the surface of the electrolyte. In this manner the solid salt and the alloy contacted therewith are maintained at the elevated temperature necessary to cause the reaction involvedto occur rapidly and the alkaline earth metal does not tend to precipitate outfrom the alloy so as to obstruct the passageways in the apparatus. I have discovered that under such conditions the alkaline earth metal reacts with the solid alkali metal salt to form a liquid mixture of the corresponding alkaline earth salt and the alkali metal salt, which may be caused to flow back into the electrolyte. Forexample, when the sodium calcium alloy is'contacted with solid sodium chloride at a temperature of, for example, 500---'I00 C'., the calcium'reacts with the sodium chloride to form sodium and calcium chloride according to the following equation:

" "Ihe append ed drawing illustrates 0135 method of practiciii'gfmy invention byfelectrolysis' offla V fused mixture of sodium chloride and calcium chloride. The electrolytic cell shown in the drawing consists of a steel electrolyte container i which is lined with a refractory brick 2 and has centrally located therein, anode I and annular cathode I. The anode 3 which extends up through the bottom of the cell is commonly made of graphite. The cathode 4, commonly made of steel, is cylindrical in shape and surrounds the anode so as to leave a small annular space between. Two or more side arms I project from the cathode out through the side walls of the cell for the purpose of making electrical connections. A cylindrical diaphragm i. which may be made of wire gauze, is suspended between the anode and cathode to prevent products formed at the two electrodes from mixing. Suspended above the cathode and also serving to support diaphragm 6 is an annular inverted trough member I which serves to collect the liquid metal which rises from the cathode. A conical gas collector 8 is supported over the anode 3, in such manner as to collect gases arising from the anode. The conical gas collector 8 is surmounted by gas collecting dome 9, which extends above the surface of the electrolyte. The gases collected in dome 9 are led off through outlet pipe Hi. The collectors I .and 8 and other parts are supported in the cell by conventional means not shown in the drawing. For the purpose of removing the liquid metal which collects in the trough shaped collector I from the cell, a vertical pipe ll, termed riser pipe, is connected at one point with the collecting trough I so that a metal collected in this in a cell of the type just described, I provide near the bottom of riser pipe II a grid or foraminous plate i5 which supports a bed of granules or lumps of solid alkali metal salt iii. The top of the riser pipe I! is provided with a removable cover I! held in place by bolts [8. The solid salt may be charged into the riser pipe I i through the opening covered by cover l1. 7

In practicing my invention I may operate the cell illustrated by the drawing and just described by electrolyzing, for example, a fused'mixture of sodium chloride and calcium chloride at a temperature of 500-700 C. and charge lumps of sodium chloride into riser pipe H. Since this temperature is well below 800 0., the melting point of sodium chloride, the lumps oi solid salt charged into the lower portion of riser l6 and resting on the grid I 5 do not melt. Preferably, the grid 15 is located suiiiciently high so that the solid salt is completely surrounded by the rising column of metal and out of contact with the electrolyte. However, it desired, the solid salt may be'in contact with the electrolyte. In such case I prefer to use large lumps of salt, e. g. about 1 inch in diameter or larger, to decrease the rate of solution of the salt in the electrolyte. Under such conditions the salt will undergo little or no solution by the electrolyte.

As the electrolysis proceeds. chlorine gas formed at the anode rises through the collector 8 and dome 0 and out through pipe to. Sodium calcium alloy formed at the cathode rises into the collector trough I and thence rises upward inriserpip'e Inpassingthroughiiserpipe ii, the alloy slowly passes through grid II and the bed of solid salt It, whereat the calcium in the alloy reacts with the solid salt to form calcium chloride. In this manner substantially pure sodium containing little or no calcium flows out into the collector It. The calcium chloride, as formed, dissolves or combines with the solid salt to form a salt mixture which will melt at the existing temperature and the resulting liquid salt mixture descends into the electrolyte.

Various modifications of the'above described method will beapparent to those skilled in the art of electrolysis of fused salts. The method of contacting the sodium calcium alloy with the solid sodium salt naturally will vary depending upon thetype of electrolysis cell utilized. In the various types of electrolytic cells suitable for carrying out this electrolysis, various means are provided for collecting the molten metal and leading it out of the cell. My invention may be adapted to the various types of cells by locating a body of the solid sodium salt at a convenient point in the passageway through which the metal is led from the cell. Preferably the solid salt is located close to or beneath the surface of the electrolyte so that the reaction will occur at about the electrolyte temperature. I also prefer to keep the solid salt out of contact with the fused salt electrolyte, for example, by maintaining it in a column of salt-free liquid metal rising from the cathode. If the solid salt comes in contact with the electrolyte it will slowly dissolve, requiring a more frequent replenishment but the desired metal purification is not adversely aflected. Satisfactory operation can be had with the solid salt in contact with the electrolyte. especially if large sodium salts other than the halide used in the electrolyte is that their continued use introduces a foreign anion into the bath which tends to contaminate the anode product. Also the-continued use of such salt will in time materially change the composition of the electrolyte which, in general, would be disadvantageous.

The temperature at which the alloy produced in the cell is contacted with the solid salt may vary, depending on the nature of the metal and salts involved in the operation. For example, when a. sodium-calcium alloy produced by electrolyzing a mixture of sodium and calcium chloride is contacted with solid sodium chloride, the temperature of the alloy in contact with the salt is preferably maintained at 500-700 C. In this temperature range, there is little or no tendency for the sodium to volatilize and the calcium chloride-sodium chloride mixture formed will be sufriciently fluid to flow back into the electrolyte. However, lower temperatures may be used if desired, e. g. the reaction will occur at temperatures as low as 400 C. The disadvantage of such lower temperature is that the calcium chloride will remain solid and in time will render the bed of solid salt inactive. The solid salt then must be removed and replaced with fresh salt. Also, satisfactory results may be obtained in the temperature range '700-800 0., provided the melting point of the solid salt is not reached, 1. e. the temperature is kept below 800. However, there is ordinarily no advantage in exceeding the temperature of about 700 C. and as the temperature approaches 800 0., the tendency for the sodium to vaporize is increased. Obviously, the suitable operating temperatures will vary for other alkali metals and salts, depending on the melting and boiling points of the alkali and alkaline earth metals and their alloys, the melting points of the alkali and alkaline earth metal salts and the temperature at which the electrolysis should be operated.

An advantage of the present invention is that it prevents the precipitation and deposition of calcium crystals in the alkali metal outlet of the fused salt cell. This results in important economy of operation, since the labor involved in frequent mechanical removal of the calcium is eliminated. A further advantage is that little or no calcium appears in the metal removed from the receiver of the cell, thus greatly decreasing or eliminating the task of purifying the product. A further important advantage is that the electrolyte composition is maintained substantially constant, since the alkaline earth metal component is not removed with the outgoing alkali metal. Previously, it has been necessary to add the alkaline earth metal salt to the cell at frequent intervals, to maintain constant bath composition. In electrolyzing a mixture of alkali and alkaline earth metal salts in accordance with my invention, it is only necessary to add the alkali metal salt to maintain the desired bath level in'the cell and the alkaline earth metal salt need be added only at infrequent intervals in small amounts solely to compensate for mechanical losses of electrolyte.

In the foregoing description and in the appended claims, I use the term "alkaline earth metal" to designate an element of the group: calcium, barium and strontium.

I claim:

1. The process comprising electrolyzing a fused mixture of sodium chloride and calcium chloride at an electrolyte temperature of 500-700 0., flowing the resulting liquid sodium-calcium alloy directly from the cathode, into contact with solid sodium chloride at a temperature of 500-700 C. and flowing the resulting purified liquid metal from the electrolytic cell.

2. In an electrolytic cell adapted for electrolysis of a fused salt mixture, collecting means for collecting liquid metal formed at the cathode, an upwardly extending conduit adapted to lead liquid metal from said collecting means to a point outside the cell and a means for holding a solid salt within said conduit out of contact with the electrolyte at a point below the surface of said electrolyte, said means for holding solid salt being adapted to permit liquid metal to flow therethrough.

3. In an electrolytic cell adapted for electrolysis of a fused salt mixture, collecting means for collecting liquid metal formed at the cathode, an upwardly extending conduit adapted to lead liquid metal from said collecting means to a point outside the cell and a foraminous support adapted to support a solid salt located within said conduit .out of contact with the electrolyte, at a point beneath the surface of the electrolyte.

4. The process comprising electrolyzing a fused mixture comprising essentially alkali and alkaline earth metal halides, collecting a liquid alloy of alkali and alkaline earth metals at the cathode, flowing said liquid alloy from the cathode directly into contact with the surface of solid alkali metal halide at a temperature which is below the melting point of said solid halide but sufliciently high to prevent substantial precipitation of alkaline earth metal from said alloy, so as to cause reaction between said solid halide and alkaline earth metal in said alloy to form the corresponding alkaline earth metal halide and flowing the resulting purified liquid alkali metal from the electrolytic cell.

5. The process comprising electrolyzing a fused mixture comprising essentially sodium and calcium chlorides, collecting liquid sodium-calcium alloy at the cathode, flowing said liquid alloy from the cathode directly into contact with the surface of solid sodium chloride at a temperature of 500 .to 100 0., so as to cause reaction between said solid sodium chloride and the calcium in said alloy to form calcium chloride and flowing the resulting purified liquid sodium from the electrolytic cell.

ROBERT. EDWIN HULSE.

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