US3265490A - Production of alkali metals from alkali amalgam - Google Patents

Description

United States Patent 3,265,490 PRODUCTION OF ALKALI METALS FROM ALKALI AMALGAM Shiro Yoshizawa and Nobuatsu Watanabe, Sakyo-ku, Ky-

oto, and Tsukiro Morimoto, Masamichi Miura, and Yashuhiro Yamada, Toyama, Japan, assignors to Tekkosha Co., Ltd., Tokyo, Japan, a corporation of Japan Filed Apr. 9, 1963, Ser. No. 271,707

4 Claims. (Cl. 75-66) This invention relates .to a process, and to an electrolytic bath for use in said process, for the production of alkali metals by an amalgam electrolysis, and also to a process for the purification of the thus obtained alkali metals.

As metallic sodium has desirable physical properties and is a strong reducing agent, it is finding increasing use for various purposes. The melting point of metallic sodium is low MiP. 98 0.), its electric conductivity (s.r. 4.98x- Qcm, at 25 C.) and its thermal conductivity (0.170 caL/(cm (sec.) C./cm.) at 400 C.) are extremely good and its specific gravity (0.892 g./cc. at 250 C.) and its viscosity (0.381 c.p. at 250 C.) are low. Therefore, metallic sodium is very suitable for use as a heat .transfer medium and, as it can be used at high temperatures, it could find even greater use in many chemical apparatuses and heat exchangers if it were made available at a sufliciently low price. Also, although sodium is a very strong reducing agent, major uses of metallic sodium as a reducing agent have been limited because it is relatively expensive. Therefore, if its cost can be reduced, its use for .this purpose will be increased.

The Castner process and the Downs process are commonly used for the production of metallic sodium. In the Castner process, metallic sodium is produced by the fused-salt electrolysis of anhydrous sodium hydroxide. The current elficiency of the process is usually about 40 percent so that process is not economical. In the Downs process, fused sodium chloride is electrolyzed directly and there are many disadvantages due to the high temeratures which must 'be employed.

This invention provides a process and an apparatus for the electrolytic production of alkali metals, and also provides a process for the purification of the metals, whereby alkali metal of high purity may be produced relatively easily and economically.

According to this invention, alkali metals are produced from alkali amalgams in an electrolytic cell. The cell consists of a first metallic plate serving as the anode and another metallic plate which serves as the cathode. The anode plate can be a fiat plate disposed horizontally or slightly inclined, or it can be conically shaped. The anode plate is rotated slowly and the alkali metal amalgam from a chlorine cell is flowed onto the center of the anode plate and it flows to the edge of the anode plate by centrifugal force. A fused salt is used as the electrolyte in the cell.

According to this invention, the amalgan is flowed forcibly onto the anode plate in such fashion that the rate of flow can be adjusted as desired. It is an advantage of the invention that the process can be operated at a low voltage. Also, this invention is advantageous in that the metal deposited at the cathode is not contami- 3,265,490 Patented August 9, 1966 nated by the amalgam as it is in the case with verticaltype cells. Therefore, the mercury content of the deposited metal is relatively low. The current efficiency is very high because the deposited metal floats quickly to the surface of the electrolyte, and the operating temperature is lower. Also, the anode plate is always covered with the amalgam and thus, the anode plate is not oxidized. Therefore, the anode plate can be made of iron instead of nickel.

In addition to the advantages and merits mentioned above, the cost of the apparatus is lower than with other procedures, the operation can be carried out easily and smoothly, and the current efficiency is remarkably high, so that alkali metals of high purity can be produced at a markedly reduced cost.

The invention is diagrammatically illustrated by way of example, in the attached drawing, in which:

FIGURE 1 is a central sectional view of an electrolytic cell used for producing alkali metal according to this invention;

FIGURE 2 is the equilibrium diagram near the eutectic point of the three components of the electrolyte used in this invention consisting of sodium hydroxide, sodium iodide and sodium cyanide; and

FIGURE 3 is the equilibrium diagram near the eutectic point of a conventional three component electrolyte consisting of sodium hydroxide, sodium iodide and sodium bromide.

Referring to FIGURE 1, the cell comprises a horizontal, iron disc 1 which is supported for rotation by a hollow shaft 2 which extends downwardly from the lower surface of said disc at the center thereof.

The shaft 2 can be rotated at any suitable speed by means of any conventional drive system, such as a motor and gear box arrangement (not shown). In this example, the upper surface of the rotary iron disc 1 is horizontal and flat, but it may be conical or inclined. 'Further, the upper surface of disc 1 is not necessarily flat, and it may be provided with suitable grooves, projections, teeth or agitating means. Further, it is not necessarily of disc-form. Thus, the apparatus can be modified suitably within the scope of this invention.

The central opening 3 in the rotary shaft 2 serves as an amalgam transport pipe through which the amalgam is supplied from a suitable source of supply, such as a mercury cell, to the upper surface of the rotary iron disc 1. However, the amalgan supplying conduit may be placed at other places in the apparatus so that the amalgam can be supplied to the upper surface of the disc 1 from above or from the side of said disc. As shown in FIGURE 1, the amalgam is flowed directly onto the surface of the iron disc, but it may be flowed into a circular recess or an annular weir on the upper surface of or at the central portion of the iron disc and then the amalgam may overflow onto the upper surface of said disc.

An amalgam discharging pipe 5 is connected to the bottom wall 14 of the cell adjacent the periphery of the disc 1. The other end of the pipe 5 is connected to an amalgam level controlling chamber (not shown) placed outside the electrolytic cell. Any suitable type of level control apparatus can be used for controlling the level of the amalgam in the cell. For example, an annular, dilute amalgam reservoir can be placed at the periphery of the bottom plate 4 of the electrolytic cell, the peripheral region of the rotary iron disc 1 be bent down, and the bent region is immersed in the dilute amalgam reservoir, whereby the region is sealed by the amalgam to prevent leakage and the amalgam is discharged at a suitable place from the amalgan reservoir. Heating jackets 6 are placed at the bottom and around the sides of the cell so that the operating temperature of the cell can be maintained at a definite temperature by a heating medium, or a hot gas, or an electric heater in the jacket.

The stationary cathode plate 7 is of the same shape as the rotary iron disc 1 and is placed thereabove with its lower surface face to face with, and spaced vertically from, the upper surface of the rotary iron disc 1. The cathode plate 7 is supported on a cover 8 by supports 8a. The plate 7 may be made of a suitable metal, such as iron. The cathode plate 7 is shown in FIGURE 1 as being a relatively thick plate, but it may be thin or screenlike.

A hydrogen inlet pipe 9 extends through the cover 8 of the cell, and the end of said pipe is placed between the cathode plate 7 and the anode plate 1 adjacent the periphery thereof. Hydrogen is used to prevent the oxidation of deposited sodium (Na O-j- /zH Na-l-NaoH). Only one hydrogen inlet pipe is illustrated in FIGURE 1, but two or more such pipes can be used according to the capacity of the cell. A scraper device 13 is positioned between the rotary anode plate 1 and the cathode plate 7 and it is rotated by a rotary shaft 14 which extends through the cell cover 8 and the central opening in the cathode plate 7. The scraper device 13 is near the cathode plate 7 and the deposited metal on the cathode plate 7 is quickly scraped off by the blades thereon so that the formation of sodium oxide is minimized.

The cell is filled to a level above the upper surface of cathode plate 7 with an electrolyte. Since the electrolyte is brought into contact with hydrogen gas by the agitation effected by the scraper device 13 and the disc 1, the viscosity of the electrolyte does not increase, the metal can be deposited smoothly and, thus, an increase of the electrolytic voltage and a reduction of the current efiiciency can be prevented so that the electrolysis can be continued at a high efiiciency for a long time.

The level of the electrolyte is usually maintained above the upper surface of the cathode plate 7. When the cathode 7 is of disc-form, slots 10 or many perforations are provided in it through which the metal rises up to the upper surface of the electrolyte by its bouyancy. The metal on the surface of the bath is withdrawn from the cell through a metal discharging pipe 11 which extends through the wall of the cell near the surface of the bath. A hydrogen discharging pipe 12 is welded to the cover 8 of the cell.

It will be understood that various conventional devices, such as electrical connections to the anode plate 1 and cathode plate 7, various bearing and seal units, etc., will be provided but these have been omitted from the drawing because they are conventional.

The electrolyte bath composition according to this invention contributes effectively to enable metallic sodium of high purity to be produced easily and economically. In conventional processes, a fused-salt bath of a NaOH-NaI-NaBr mixed-salt system has been used. Electrolysis using this mixed-salt system, however, must be carried out at a high temperature, e.g., 230250 C. Further, since the amalgam supplied has a relatively large vapor pressure of mercury at such high temperatures, the deposited metal contains a small amount of mercury corresponding to the vapor pressure of mercury. Therefore, in order to reduce the mercury content in the deposited metal, it is necessary to reduce the mercury vapor pressure of the amalgam by lowering the operating temperature as much as possible. Also, the resolution and diffusion of the deposited metal into the bath can be lowered by reducing the operating temperature so that the current efliciency can be higher. However, as the eutectic point of the NaOH-NaI-NaBr bath is 215 C., it is practically impossible to operate systems employing such a bath at a temperature lower than 230 C.

According to our knowledge and experiments, bath compositions which are acceptable for the purposes of our invention, are very limited. For example, since in a fused-salt bath containing different kinds of cations, such as potassium and calcium, electrolytic deposition of all the cations occurs, only sodium cations can be used in the bath of our invention to produce sodium metal. Moreover, although a salt, such as NaNO has a comparatively low melting point (M.P. 308 C.), it reacts violently with deposited metallic sodium. Also, a fusedsalt having a low specific gravity can not be used because it disturbs the floating of the deposited metal. Also, sodium salts of a low electric conductivity and high viscosity are not suitable.

We have discovered that a novel fused mixed-salt bath consisting of anhydrous salts of NaOH, NaI, and NaCN in most suitable for use as the bath for the production of sodium metal from an amalgam. The melting point and the viscosity of such a bath are low and its electric conductivity and density are high, so that it is particularly advantageous for the production of the alkali metal from the amalgam. Also, such a bath is not corrosive to the cell.

The equilibrium diagram of a NaOH, NaI and NaCN mixed-salt bath is shown in FIGURE 2. We have found it possible to use a bath having 4070% by weight sodium hydroxide, less than 50 weight percent sodium iodide and less than 40 weight percent sodium cyanide, all of such percentages being in such proportions as to be encompassed within the 230 C. isotherm of FIGURE 2 of the accompanying drawing. The operating temperature can be remarkably lowered compared with the conventional electrolytic bath employing a NaOH-NaI-NaBr system. Thus, the operation can be carried out at from 210 to 230 C., which is 20 C. lower than the temperature used with the conventional electrolytic bath. Therefore, in accordance with this invention, the current efficiency will be higher and the mercury content in the deposited metal will be lower.

Further, as is clearly understood by comparing the equilibrium diagram of the conventional NaOH-NaI-NaBr system shown in FIGURE 3 with the equilibrium diagram of our system shown in FIGURE 2, the electrolytic bath of this invention has the feature that the region enclosed by a given isotherm is several times larger than that in the corresponding isotherm in the conventional system. The sodium hydroxide component in the 220 C. isotherm ranges from 43 to 65 weight percent in the diagram of FIGURE 2, and, on the other hand, it ranges from 47 to 51 weight percent in the diagram of FIGURE 3. The sodium hydroxide concentration gradually increases during the electrolysis due to the water in amalgam. Even if the composition ratio of the electrolytic bath according to our invention changes through a considerably broader range than is the case with the conventional electrolytic bath, the bath temperature still can be maintained lower. This is another advantage of the bath of the present invention.

Since, in an amalgam electrolysis, the deposited metal reacts with the sodium hydroxide in the bath to form sod um oxide, which is then converted with hydrogen into sodium hydroxide, the bath composition changes during the operation. The bath of this invention has, however, a strong resistance to this composition change.

Thus, the NaOH-NaI-NaCN electrolytic bath of the present invention is most suitable as the electrolytic bath to be used for the production of alkali metal from an amalgam. If desired, small amounts of other sodium salts, such as NaCl, NaBr and the like, may be added to the mixed-salt bath of the NaOH-NaI-NaCN three component system.

However, even with the improved amalgam electrolysis process of this invention, the contamination of the deposited metal with mercury is not avoidable and it is desirable to remove as much of the mercury from the deposited metal as possible. In accordance with the purification feature of this invention, this can be carried out effectively and at a low cost, which makes a very important contribution from an industrial viewpoint.

In the purification processes hitherto reported, the deposited sodium contaminated by mercury has been purified with metallic calcium at about 380 C. whereby mercury is removed as a calcium amalgam. By this process the mercury content in the sodium metal can be reduced to about 0.01 percent. However, expensive metallic calcium is needed in an amount almost equal to that of the mercury in the deposited sodium.

We have found that mercury can be removed from the deposited metal by using metallic magnesium instead of metallic calcium. For example, when the contaminated metal containing mercury (the mercury content is about 1 percent) is passed through an apparatus packed with metallic magnesium, in an amount almost the same as that of the mercury contained in the sodium, at 350 to 400 C., up to 99.9 percent of the mercury can be removed easily from the metal.

According to the purification process of this invention, metallic magnesium is added into the molten deposited metal at a temperature higher than 400 C. and is stirred, and then the mixed metal is allowed to settle at 100200 C. for one or two hours, and then the pure sodium metal is separated from the other metals. In the purification process, a small amount of other metals may be used together with magnesium, or magnesium alloy may be used. By this purification process, metallic sodium having a mercury content lower than 0.01 percent and usually of 0.002 to 0.005 percent can be obtained.

Example I An electrolysis was carried out using a horizontal type cell with a rotary anode disc as shown in FIGURE 1 (the diameter of the rotary iron disc was 140 cm., its speed of rotation r.p.m., and the speed of rotation of the scraper blade 10 r.p.m.) and using an electrolytic bath having a composition of 47 weight percent of NaOH, 36 weight percent of NaI and 17 weight percent of NaCN. The sodium concentration of the supplied sodium amalgam was 0.2 to 0.3 percent, the feeding rate of the amalgam was 45 kg./min., the cell voltage was 2.0 to 2.3 volts, the anode current density was 39 to 52 amp/cm. (the electric current was from 6,000 to 7,900 amp), and the electrolysis temperature was 210 to 230 C. After 50 hours, 282 kg. of metallic sodium having a mercury content of 0.7 percent was obtained with a current efficiency of 92 percent.

Example 11 To 1 kg. of the metallic sodium containing 0.7 percent of mercury obtained as stated in Example I 7 g. of metallic magnesium was added and the mixture was heated at 450 C. with agitation in a reactor for one hour. The mixture was then cooled to l20200 C., allowed to be settled and then metallic sodium was separated. The mercury content of the metallic sodium was 0.002 percent.

As mentioned above in detail, as the amalgam is pumped or otherwise forcibly flowed into the cell the supplied amount of the amalgam can be increased remarkably whereby the electrolysis can be operated at a high current density. When the pure metallic sodium is produced from an amalgam by using the process of the invention in an industrial-scale cell (of a capacity of more than 30,000 amp.), it should be expected that the current efliciency of 96 to 98 percent can be obtained.

What is claimed is:

1. An electrolytic bath for the electrolytic production of alkali metal, comprising a three-component, fused, mixed salt consisting essentially of sodium hydroxide, sodium iodide and sodium cyanide, the sodium hydroxide being present in an amount of 4070% by weight, the sodium iodide being present in an amount of less than 50% by weight and the sodium cyanide being present in an amount of less than 40% by weight, all of such percentages being in such proportions as to be encompassed within the 230 C. isotherm of FIGURE 2 of the accompanying drawing.

2. A process for producing alkali metals by an amalgam electrolysis, using an electrolytic cell comprising an anode plate having a substantially horizontal upper surface and a cathode disposed above and spaced from said upper surface, which process comprises:

continuously flowing a fluid alkali metal amalgam onto said upper surface to forma moving layer thereon; maintaining a fused, mixed salt, electrolytic bath in said cell at a level at least as high as said cathode; electrically energizing said cathode and said layer so that said layer functions as an anode whereby the alkali metal is deposited on said cathode; flowing hydrogen gas into the space between said anode plate and said cathode so that said hydrogen gas contacts the electrolyte and hinders an increase in the viscosity of the electrolyte; and collecting the alkali metal deposited on said cathode. 3. A process for producing alkali metals by an amalgam electrolysis, using an electrolytic cell comprising an anode plate having a substantially horizontal upper surface and a cathode disposed above and spaced from said upper surface, which process comprises:

continuously flowing a fluid alkali metal amalgam onto said upper surface to form a moving layer thereon;

maintaining a fused, mixed salt, electrolytic bath in said cell at a level at least as high as said cathode, said bath consisting essentially of sodium hydroxide, sodium iodide and sodium cyanide, the sodium hydroxide being present in an amount of 4070% by weight, the sodium iodide being present in an amount of less than 50% by weight and the sodium cyanide being present in an amount of less than 40% by weight, all of such percentages being in such proportions as to be encompassed within the 230 C. isotherm of FIGURE 2 of the accompanying drawing;

electrically energizing said cathode and said layer so that said layer functions as an anode whereby the alkali metal is deposited on said cathode; and collecting the alkali metal deposited on said cathode. 4. A process for producing alkali metals by an amalgam electrolysis, using an electrolytic cell comprising an anode plate having a substantially horizontal upper surface and a cathode disposed above and spaced from said upper surface, which process comprises:

continuously flowing a fluid alkali metal amalgam onto said upper surface to form a moving layer thereon;

maintaining a fused, mixed salt, electrolytic bath in said cell at a level at least as high as said cathode, said bath consisting essentially of sodium hydroxide, sodium iodide and sodium cyanide, the sodium hydroxide being present in an amount of 4070% by weight, the sodium iodide being present in an amount of less than 50% by weight and the sodium cyanide being present in an amount of less than 40% by weight, all of such percentages being in such proportions as to be encompassed within the 230 C. isotherm of FIGURE 2 of the accompanying drawing;

electrically energizing said cathode and said layer so that said layer functions as an anode and the alkali metal is deposited on said cathode;

collecting the alkali metal deposited on said cathode;

melting the thus obtained alkali metal;

2,054,316 9/1936 2,124,564 7/1938 Gilbert 75-63 2,234,967 3/ 1941 Gilbert 20468 mixing metallic magnesium into the molten alkali metal 2,745,552 at a temperature in excess of between 400 C.; 2,916,425 cooling the solution to a temperature of between 100- 200 C.; allowing the mixture to settle; and 5 635,747 then separating the purified alkali metal. 1 0,980

References Cited by the Examiner UNITED STATES PATENTS 8 5/ 1956 Bruggeman 75-66 12/1959 Fujioka 204-68 FOREIGN PATENTS 1/ 1962 Canada.

1904 Great Britain.

1914 Great Britain.

DAVID L. RECK, Primary Examiner.

Gilbert 75 66 10 BENJAMIN HENKIN, Examiner.

H. w. CUMMINGS, H. WiTARRING,

Assistant Examiners.

Claims (1)

1. 4. A PROCESS FOR PRODUCING ALKALI METALS BY AN AMALGAM ELECTROLYSIS, USING AN ELECTROLYTIC CELL COMPRISING AN ANODE PLATE HAVING A SUBSTANTIALLY HORIZONTAL UPPER SURFACE AND A CATHODE DISPOSED ABOVE AND SPACED FROM SAID UPPER SURFACE, WHICH PROCESS COMPRISES: CONTINUOUSLY FLOWING A FLUID ALKALI METAL AMALGAM ONTO SAID UPPER SURFACE TO FORM A MOVING LAYER THEREON; MAINTAINING A FUSED, MIXED SALT, ELECTROLYTIC BATH IN SAID CELL AT A LEVEL AT LEAST AS HIGH AS SAID CATHODE, SAID BATH CONSISTING ESSENTIALLY OF SODIUM HYDROXIDE, SODIUM IODIDE AND SODIUM CYANIDE, THE SODIUM HYDROXIDE BEING PRESENT IN AN AMOUNT OF 40-70% BY WEIGHT, THE SODIUM IODIDE BEING PRESENT IN AN AMOUNT OF LESS THAN 50% BY WEIGHT AND THE SODIUM CYANIDE BEING PRESENT IN AN AMOUNT OF LESS THAN 40% BY WEIGHT, ALL OF SUCH PERCENTAGES BEING IN SUCH PROPORTIONS AS TO BE ENCOMPASSED WITHIN THE 230*C. ISOTHERM OF FIGURE 2 OF THE ACCOMPANYING DRAWING; ELECTRICALLY ENERGIZING SAID CATHODE AND SAID LAYER SO THAT SAID LAYER FUNCTIONS AS AN ANODE AND THE ALKALI METAL IS DEPOSITED ON SAID CATHODE; COLLECTING THE ALKALI METAL DEPOSITED ON SAID CATHODE; MELTING THE THUS OBTAINED ALKALI METAL; MIXING METALLIC MAGNESIUM INTO THE MOLTEN ALKALI METAL AT A TEMPERATURE IN EXCESS OF BETWEEN 400*C.; COOLING THE SOLUTION TO A TEMPERATURE OF BETWEEN 100200*C.; ALLOWING THE MIXTURE TO SETTLE; AND THEN SEPARATING THE PURIFIED ALKALI METAL.

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