US3472746A - Electrolytic system for production of alkali metals - Google Patents

Electrolytic system for production of alkali metals Download PDF

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US3472746A
US3472746A US621580A US3472746DA US3472746A US 3472746 A US3472746 A US 3472746A US 621580 A US621580 A US 621580A US 3472746D A US3472746D A US 3472746DA US 3472746 A US3472746 A US 3472746A
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sodium
amalgam
cell
molten
electrolyte
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US621580A
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Laszlo A Heredy
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Boeing North American Inc
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North American Rockwell Corp
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/02—Electrolytic production, recovery or refining of metals by electrolysis of melts of alkali or alkaline earth metals

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  • the alkali metalamalgam cell may also be utilized as a subcell of a unitary electrolytic cell for producing an alkali metal starting with electrolysis of an aqueous solution of an alkali-metal salt.
  • Exemplary and preferred is the recovery of sodium from its amalgam in an electrolytic cell wherein the molten electrolyte is fusible below 160 C. and is a ternary salt mixture of sodium amide, sodium hydroxide, and sodium iodide, the amalgam being produced preferably by electrolysis of an aqueous sodium chloride solution using a mercury or amalgam cathode.
  • This invention relates to an improved electrolytic process for recovery of an alkali metal from its amalgam. More particularly, the present invention relates to a low-temperature electrolytic process for the recovery of sodium, potassium, and mixtures thereof from their amalgams.
  • the Gilbert process shown in US. 2,148,404, proposes producing an alkali metal amalgam by electrolysis of an equeous solution of an alkali-metal salt in the presence of a mercury (or amalgam) cathode, and then recovering the alkali metal electrolytically from the so-produced amalgam by using an anhydrous fused mixture of alkali metal hydroxide and alkali metal halide as electrolyte.
  • a mercury (or amalgam) cathode for the production of sodium from sodium amalgam, Gilbert proposes as molten electrolyte a sodium hydroxide-sodium iodide eutectic (M.P. 225 C.) at a cell operating temperature of 230-250 C.
  • German Patent 862,519 a molten salt electrolyte that isa ternary eutectic of NaOH-NaI-NaBr is proposed for electrolytic recovery of sodium from a sodium amalgam.
  • This electrolyte has a melting point of 215 C. and serves to lower the operating temperature of the electrolytic cell by about 10 degrees compared with Gilberts process.
  • a ternary mixed salt consisting of NaOH-NaI-NaCN M.P. 195 C.
  • molten electrolyte in a sodium amalgam electrolytic cell for operation at a temperature of 210- 230 C. While the two ternary mixtures described above result in operation at a slightly lower temperature, the mercury loss and contamination of the produced sodium by mercury are still significant.
  • cell operation above 200 C. requires use of expensive corrosionresistant materials.
  • an electrolytic process for the recovery of an alkali metal selected from sodium, potassium, and mixtures thereof from its molten amalgam by using in the electrolytic cell as molten electrolyte a fused salt mixture melting below 180 C. and containing as essential component the amide of the alkali metal.
  • the process is particularly suitable for the recovery of sodium from sodium amalgam using as molten electrolyte fusible below C. a ternary mixed salt having a composition, in mole percent, of 30-78 sodium amide, 20-60 sodium hydroxide, and l20 sodium iodide.
  • the particularly preferred composition of the foregoing ternary mixed salt is the eutectic mixture, M.P.
  • a preferred molten electrolyte is a lowmelting binary mixture of sodium amide and potassium amide.
  • a unitary high-efficiency electrolytic cell for the electrolytic recovery of an alkali metal from its amalgam prior produced by the electrolysis of an aqueous chloride solution has not been feasible because of the high temperature required for amalgam electrolysis when using a high-melting molten salt electrolyte compared with the lower temperature used for the aqueous electrolysis.
  • the present invention permits operation of a unitary cell at a temperature as low as about 135 C. under optimum conditions for sodium production and as low as about 100 C. under optimum conditions for the production of sodium-potassium mixtures, an operating temperature about 10 degrees above the melting point of the fused salt electrolyte being preferred.
  • FIG. 1 is a diagrammatic representation of the cell reactions that occur in a preferred embodiment of an electrolytic cell of this invention.
  • FIG. 2 is a schematic representation, partly in crosssection, of a preferred embodiment of a unitary electrolytic cell of this invention.
  • the process provided by the present invention is particularly applicable to the recovery of sodium, potassium and sodium-potassium mixtures from their molten amalgams.
  • a low melting molten potassium salt e.g., a ternary mixture containing, in mole percent (m/o), 30-50 potassium amide, 20-35 potassium hydroxide, and 30-50 potassium iodide, is preferred for use as electrolyte.
  • a particularly preferred potassium salt mixture contained 36i2 m/o KNH 27:2 m/o KOH, and 37:2 m/o KI and had a melting point of 171 C.
  • suitable low melting electrolytes that may be used are the NaNH -KNH binary eutectic (M.P. 93 C.), shown by C. A. Krause and E. G. Cuy in J. Am. Chem. Soc. 45, 712-715 (1923), and a mixed salt composition of the amide, hydroxide and iodide of sodium and potassium.
  • FIG. 1 there is shown an idealized representation of the reactions which occur in a dual cell system during electrolysis.
  • a concentrated aqueous sodium chloride solution is shown as supplied continuously to an electrolyte compartment 1 through an opening 2, which is connected to a sodium chloride storage tank, not shown.
  • Dilute sodium chloride solution is removed from compartment 1 through an opening 3.
  • the sodium ions gain electrons at the sodium chloride-mercury interface 4 to form sodium amalgam in a compartment 5 by combination with the mercury or amalgam present in this compartment.
  • chlorine ions lose electrons to form chlorine gas in anode compartment 7.
  • the chlorine gas may be removed from anode compartment 7 by way of an opening 8.
  • some hydrogen may be formed by self-discharge at the interface 4 of the amalgam electrode and the aqueous sodium chloride electrolyte. This hydrogen is removed from the cell through opening 3 and collected.
  • the sodium in the sodium amalgam loses electrons to form sodium ions. The net result of these reactions then may be considered as the electrochemical transfer of sodium ions from compartment 1 to compartment 10 by way of amalgam compartments 5 and 5'.
  • the sodium ions migrate through the molten electrolyte in compartment 10, and at an interface 11 between the molten electrolyte compartment 10 and cathode compartment 12 gain electrons to discharge molten sodium in the cathode compartment.
  • This molten sodium may be conveniently removed by way of an opening 13.
  • the reactions which occur in cells I and II correspond to those which occur in a unitary cell containing two equivalent subcells.
  • the amalgam serves as the cathode of this first cell.
  • the amalgam serves as the anode of the cell.
  • the amalgam in compartments 5 and 5, which circulates in countercurrent fiow thrpugh openings 14 and 14 in heat exchanger 15 acts as a bipolar electrode, serving simultaneously as the cathode of the aqueous electrolyte first cell and the anode of the molten salt electrolyte second cell.
  • the two cells When the two cells are operated at the same temperature, they can be united to form a unitary cell having a single amalgam compartment. Heat exchanger 15 is then eliminated.
  • FIG. 2 is shown a cross-sectional view of a preferred embodiment of a compact unitary electrolytic cell, in accordance with this invention, in which the two subcells of the electrolytic cell are housed together in the same unit in isothermal relation with one another, the entire electrolytic cell being operated at substantially the same temperature below C.
  • This unitary arrangement of the subcells employing an immobilized horizontal amalgam interelectrode eliminates any requirement for circulating amalgam between two separate cells with associated conduits, pumps, and heat exchangers.
  • Such an arrangement employing an immobilized amalgam intermediate electrode is a simpler and more efiicient way for producing sodium starting with an aqueous sodium chloride solution.
  • the electrolytic cell 20 may be considered as consisting of two subcells, a first aqueous subcell and a second molten electrolyte subcell, although the separate identities of the two subcells are merged under conditions of actual operation.
  • a concentrated sodium chloride solution is fed into a cell compartment 21 by way of a conduit 22; a dilute sodium chloride solution leaves the cell compartment by way of a conduit 22a, and is conveniently stored in a storage tank therefor, not shown.
  • a porous plastic separator 23 serves to prevent mixing of chlorine with hydrogen, since the latter may be evolved in small amounts by self-discharge at the amalgam surface of screen 28. Evolved hydrogen would leave the cell through conduit 22a.
  • substantially pure chlorine gas is evolved in a compartment 24 in contact with an anode electrode 25, which preferably consists of a suitable graphite electrode.
  • anode electrode 25 which preferably consists of a suitable graphite electrode.
  • the chlorine gas may conveniently be removed by way of an opening 26 in outer wall 27 of the anode compartment, made of a suitable corrosion-resistant material, e.g., stainless steel.
  • sodium ions in the sodium chloride solution gain electrons and deposit at the amalgam electrode consisting of a metal screen or porous metal 28 wetted by an immobilized horizontal layer of dilute amalgam 29, contained between screen 28 and a porous ceramic matrix 30.
  • the molten salt electrolyte is conveniently retained in the pores of matrix 30, which is preferably of a ceramic material but may also be made of a plastic material. Alternatively, the ceramic matrix layer containing the molten salt electrolyte may be replaced by a high polymer ion-exchange membrane.
  • the cell is operated at as low as temperature as feasible, since the minimizes mercury loss, contamination of sodium by mercury, and permits use of less eX- pensive lightweight materials of construction that are able to withstand corrosive attack at lower temperatures.
  • the cell operating temperature will be kept as close as feasible to the melting temperature of the fused salt electrolyte, ordinarily the cell will be operated at a temperature about 5 to degrees above that of the melting point of the electrolyte.
  • a cell operating temperature as low as about 130 C. may be obtained by utilizing as the low melting molten sodium salt electrolyte the eutectic composition of the ternary salt mixture NaNH -NaOH-NaI, which has a melting point of about 127 C.
  • the sodium ions gain electrons to form molten sodium, which is collected in cathode compartment 31 and conveniently drained therefrom by way of an opening 32 and stored in a sodium storage tank, not shown.
  • the outer wall 33 of the sodium cathode compartment 31 is made of a suitable nonreactive conductive material, such as stainless steel, which is insulated from the amalgam interelectrode by suitable O-rings 34 made of a material such as polypropylene, hard rubber, or ceramic. Insulating O-rings are also used to electrically insulate the amalgam interelectrode from the anode compartment.
  • Example 1 As illustrative of the practice of this invention utilizing two cells maintained at different temperatures, an aqueous sodium chloride solution having a concentration of 350 grams/liter is electrolyzed at a temperature of 50 C. in a first cell having a sodium amalgam cathode and a graphite anode. The two electrodes are separated by a porous plastic membrane.
  • the cathode is a vertical steel plate down which flows a continuous stream of dilute sodium amalgam.
  • the sodium concentration of the amalgam entering the cell is 0.5 atom percent; its concentration on leaving the cell is 3.1 atom percent.
  • An average current density of 250 ma./cm. is used during the electrolysis.
  • the cell voltage is 4.25 v. Chlorine is evolved at the anode.
  • the sodium amalgam is fed to a second electrolytic cell made of dense alumina and equipped with nickel electrode terminals.
  • the cell is charged with a sodium salt electrolyte consisting of 52 mole percent NaNH 38 mole percent NaOH and 10 mole percent NaI.
  • the cell is then heated to 130 C.
  • sodium amalgam containing 3.1 atom percent sodium is introduced to the bottom of the cell, followed by addition of a small amount of sodium which floats on the top of the molten electrolyte.
  • the amalgam and sodium layers are in contact with the anode and cathode electrode terminals, respectively.
  • the thickness of the molten salt electrolyte is 0.5 cm.
  • a current density of 200 ma./cm. is used during the electrolysis.
  • the operating cell voltage is about 1.2 v.
  • the operating cell voltage is about 1.35 v. and the sodium content of the amalgam is about 1.0
  • Example 2 Sodium amalgam containing 3.1 atom percent sodium is prepared by the same method as shown in Example 1. Sodium is recovered from the amalgam in an electrolysis cell made of polypropylene, equipped with nickel electrode terminals. The composition of molten salt electrolyte is the same as in Example 1, but it is contained in the pores of an alumina disk of 45% porosity which separates the anode and cathode compartments. The thickness of the porous alumina disk is 0.2 cm. The anode compartment is filled with dilute sodium amalgam containing about 3 atom percent sodium, and the cathode compartment is filled with molten sodium.
  • the dilute sodium amalgam is continuously supplied to the anode compartment and a depleted amalgam of 0.5 atom percent sodium content is continuously removed from the anode compartment.
  • Sodium is deposited in the cathode compartment.
  • the electrolysis is carried out at a temperature of 130 C. At a current density of ma./cm. the operating cell voltage is 1.3 v. The coulombic efficiency of sodium recovery is greater than 98%.
  • the sodium product is essentially free of mercury contamination.
  • Example 3 A unitary electrolytic cell with a structure as shown in FIG. 2 and equipped with feed and receiver storage tanks for aqueous sodium chloride solution and a third tank for sodium storage is used for the electrolysis.
  • the cell is operated at a temperature of C. and under an inert gas pressure of 30 p.s.i.g.
  • An aqueous sodium chloride solution of 350 grams/liter concentration is used as feed.
  • the molten salt electrolyte has the same composition as in Example 2 and is contained in a porous alumina disk of 0.2 cm. thickness.
  • the electrolysis is carried out at a current density of 50 ma./cm. at an operating cell voltage of 5.2 v.
  • the chloride produced at the anode and any evolved hydrogen are vented from the cell at the cell operating pressure.
  • Sodium essentially free of mercury contamination is collected in the sodium storage tank.
  • the cell in which the aqueous salt solution is electrolyzed is operated at a temperature between 20 and 100 C.
  • the cell for the recovery of the alkali metal from its amalgam is operated at a higher temperature determined by the melting point of the molten salt electrolyte utilized.
  • a process for the recovery of an alkali metal selected from sodium, potassium and mixtures thereof from a molten amalgam of said selected alkali metal comprising employing said amalgam as anode in an electrolytic cell which contains as molten electrolyte a fused salt mixture having a melting point below 180 C. and containing as essential component the amide of said selected alkali metal.
  • said alkali metal is sodium and said fused salt mixture is a ternary mixed salt fusible below 160 C. and containing, in mole percent, 30-78 sodiu-m amide, 20-60 sodium hydroxide, and 1-20 sodium iodide.
  • a process according to claim 2 wherein said fused salt mixture contains, in mole percent, 52:2 sodium amide, 38:2 sodium hydroxide, and :2 sodium iodide.
  • fused salt mixture contains, in mole percent, 36:2 potassium amide, 27:2 potassium hydroxide and 37:2 potassium iodide.
  • alkali metal consists of a mixture of sodium and potassium and the molten electrolyte is a binary mixture of sodium amide and potassium amide.
  • a process for the production of sodium-potassium metal from an aqueous solution of sodium and potassium chlorides utilizing a unitary electrolytic cell containing first and second subcells, the electrolytic cell being operated a a temperature below C., comprising (a) maintaining in said first subcell an aqueous solution of sodium and potassium chlorides in contact with an anode and a sodium-potassium amalgam as cathode,

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US621580A 1967-03-08 1967-03-08 Electrolytic system for production of alkali metals Expired - Lifetime US3472746A (en)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
RU2250933C2 (ru) * 1998-12-22 2005-04-27 Басф Акциенгезельшафт Электролитическая ячейка и способ получения щелочного металла из амальгамы щелочного металла и объединенный способ получения хлора и щелочного металла из хлорида щелочного металла
RU2252981C2 (ru) * 1999-06-11 2005-05-27 Басф Акциенгезелльшафт Электролитический элемент для получения щелочного металла
WO2019236907A1 (en) * 2018-06-08 2019-12-12 Lawrence Livermore National Security, Llc Molten electrolyte dual-phase membranes for intermediate temperature fuel cells

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2075150A (en) * 1932-11-07 1937-03-30 Justin F Wait Process for the producing of metals and utilization thereof
US2150289A (en) * 1933-10-31 1939-03-14 Moltkehansen Ivar Juel Method for the electrolytic production of alkali metals

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2075150A (en) * 1932-11-07 1937-03-30 Justin F Wait Process for the producing of metals and utilization thereof
US2150289A (en) * 1933-10-31 1939-03-14 Moltkehansen Ivar Juel Method for the electrolytic production of alkali metals

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
RU2250933C2 (ru) * 1998-12-22 2005-04-27 Басф Акциенгезельшафт Электролитическая ячейка и способ получения щелочного металла из амальгамы щелочного металла и объединенный способ получения хлора и щелочного металла из хлорида щелочного металла
RU2252981C2 (ru) * 1999-06-11 2005-05-27 Басф Акциенгезелльшафт Электролитический элемент для получения щелочного металла
WO2019236907A1 (en) * 2018-06-08 2019-12-12 Lawrence Livermore National Security, Llc Molten electrolyte dual-phase membranes for intermediate temperature fuel cells
US12308485B2 (en) 2018-06-08 2025-05-20 Lawrence Livermore National Security, Llc Molten electrolyte dual-phase membranes for intermediate temperature fuel cells

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BE711914A (enrdf_load_html_response) 1968-07-15
DE1608230A1 (de) 1970-12-03

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