Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/36—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in mercury cathode cells
  • C25B1/42—Decomposition of amalgams
  • C25B1/44—Decomposition of amalgams with the aid of catalysts

Definitions

• This invention relates to improvements in amalgam decomposers which are used in conjunction with mercury cathode chloralkali cells. More particularly this invention relates to improvements in the electrode material used to discharge the amalgam in such decomposers and provides means for efiicient decomposition of the amalgam.
• Horizontal mercury cells usually consist of an enclosed, elongated trough which slopes slightly toward one end.
• the cathode is a flowing layer of mercury which is introduced at the higher end of the cell and flows along the bottom of the cell toward the lower end.
• the anodes are generally composedvof rectangular blocks of graphite suspended from conductive lead-ins so that the bottom of the graphite anode is spaced a short distance above the flowing mercury cathode.
• An aqueous electrolytic solution for example, a brine of sodium chloride is fed to the upper end of the cell, covering the anodes and flowing concurrently with the mercury.
• the impressed electric current passing through the electrolytic solution between the anodes and the mercury cathode liberates chlorine at the anodes and sodium is dissolved in the mer cur as an amalgam.
• the sodium amalgam flows from the lower end of the cell to a decomposer where it is contacted with water to form sodium hydroxide, hydrogen and mercury.
• the mercury is recycled to the cell for reuse as cathode.
• aqueous electrolytes may be used particularly brines of alkali metal halides, for example, potassium chloride and lithium chloride and also sodium sulfate.
• anodes than graphite are also well known for use in mercury cells including particularly titanium anodes at least partially coated with a thin layer of platinum metal.
• titanium are alloys consisting essentially of titanium which are also suitable for the fabrication of anodes.
• platinum metal includes an element of the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum or alloys of two or more of these metals or oxides thereof.
• the decomposition reaction between the amalgam and water does not proceed readily due to the high overvoltage of hydrogen on a mercury surface. Discharge of hydrogen gas must be facilitated by contact of the amalgam and water with a discharge electrode material, commonly graphite.
• the decomposer is usually packed with particles of graphite for this purpose.
• hydroxylic liquids can be substituted for water including lower aliphatic alcohols containing one to four carbon atoms and aqueous solutions, for example, of caustic soda.
• Crushed graphite usually from broken anodes, has the advantage of low cost, since the broken anodes are otherwise discarded.
• graphite balls about 15 mm. in diameter have recently become readily available. They have the advantage of uniformly close packing which reduces the mercury inventory residing in these amalgampacked decomposers. The balls roll readily when it is necessary to remove them from the decomposer and repack it.
• graphite balls are washed with water, dried, and flame-sprayed first with a ferrous metal and then with nickel to apply over a portion of their surface a tightly adherent dual coating of underlying ferrous metal and an overlying coating of nonferrous, amalgam-resistant metal.
• the graphite balls used according to this invention are suitably about 5 to 500 millimeters in diameter, although diameters of 10 to 25 millimeters are preferred. It is advantageous, due to hydrogen evolution in the decomposer, to pack smaller balls at the bottom and larger balls at the top of the decomposer. These sizes tend to remain segregated in use and they increase the hydrogen escape velocity without carry over.
• Suitable ferrous metals include iron, low carbon steel and other iron-rich alloys but ferrous alloys containing manganese or other elements which dissolve in aqueous caustic and contaminate it are preferably avoided.
• Particularly effective non-ferrous, amalgam-resistant metals suitable for the overlying metal are nickel and cobalt of Group VIII of the Periodic System and chromium, molybdnum and tungsten of Group VI of the Periodic System.
• beryllium, germanium, antimony, francium, tantalum, titanium, uranium, vanadium and zirconium which are included among those metals which form satisfactory amalgam-resistant coatings.
• Suitable flame-spraying devices for use according to the present invention include metallizing spray guns of the wire type, powder type or plasma type but particularly satisfactory results have been obtained using spray guns of the wire type.
• the 'balls are suitably held under a wire screen or by other means which prevent rolling and both metals are sprayed on one-half of each ball to the desired thickness, leaving the graphite uncovered on one side.
• the bond is poor and in use the metal flakes off and the activity ing to the present invention is that the inventory of mercury in the decomposed is greatly reduced.
• the average mercury inventory within the packing is about 500 pounds.
• Example II shows, when the packing is replaced with 15 mm. graphite balls, the inventory of mercury is reduced by 324 pounds or about 65 percent.
• a still further advantage of the packing of the present invention is that the vastly greater activity of the novel packing permits the use of much smaller decomposers with the larger cells, still obtaining efficient denuding.
• an E-510 decomposer packed with the coated graphite balls of the present invention is adequate for use with the E-8l2 cell having twice the capacity of the iE-5 10 cell.
• the saving in mercury inventory in the decomposer over the decomposer filled with crushed graphite and usually used with the E-812 cell is about 600 pounds or 80 percent. In a plant having 100 cells, the dollar value of the investment in mercury inventory is reduced by use of the present invention by about 400,000.
• the E-510 cell earlier called E-l 1
• E-l 1 is fully described in the art; see, for example, Electrochemical Technology, Vol. 1, pp. 71-76 (1963).
• the E-8l2 cell is similar, except that the anode area and capacity of the cell is approximately doubled. In addition, the cross-sectional area of the decomposer is approximately doubled.
• Tests No. 1, 2 and 3 were placed the graphite balls in a vessel attached to a gas burette and containing 0.31 percent sodium amalgam covered with about 2" of percent aqueous caustic soda at 90-95 C. and measuring the evolved hydrogen at atmospheric pressure and temperature.
• the metallized graphite balls of Test No. 3 had over 700 times the activity of the unmetallized balls, although the manganese in the caustic was undesirable.
• the iron-graphite bond is E A [I tightly adherent to the washed and dried graphite and the metal-to-metal bond is also strong.
• the resulting dual coated graphite balls have long life, high conductivity and excellent decomposing activity in denuding amalgams resulting in high temperatures, high rates of flow of amalgam and of decomposing liquid without flaking of the ,metal coating, attrition or loss of activity.
• the coated graphite balls of this invention are appropriately used, if
• the residence times of the amalgam in the decomposer is materially reduced and the throughput of amalgam and decomposing liquid is materially increased.
• a commercial E-510 cell had been operated using a decomposer having a diameter of 36" and filled to a depth of 2 feet with A1" to /2" crushed graphite. When the graphite was replaced by 15 mm. graphite balls coated according to this invention, the residence time of the amalgam in the decomposer was reduced from 28 to 36 seconds to less than 4 seconds, greatly increasing the throughput but still efiec tively removing the sodium.
• Example III Tests using the procedure of Example II showed that amalgams containing 0.45 percent sodium were satisfactorily denuded at mercury flow rates up to 3500 pounds per minute which is more than the flow rate in a commercial E-812 cell of about 2200 pounds per minute.
• the E-8l2 cell is about 6' x 48 and operates at about 250,000 amperes. It has a decomposer with a bed of crushed graphite about 42" in diameter and 24" deep.
• E-SlO decomposer was packed with dual metallized 15 mm. graphite balls as in Example II and it replaced the larger decomposer filled with crushed graphite as normally used with the E-812 cell.
• the saving in mercury inventory in the decomposer was 600 pounds per E812 cell, having a value of $4464.
• Graphite balls as claimed in claim 1 having a diameter of from 5 to 50 millimeters.
• Graphite balls as claimed in claim 1 in which said overlying coating is selected from the group consisting of cobalt, nickel, chromium, molybdenum and tungsten.
• alkali metal amalgam is sodium amalgam containing from 0.01 to 0.7 percent by weight of sodium.

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• Chemical & Material Sciences (AREA)
• Inorganic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Carbon And Carbon Compounds (AREA)
• Electrolytic Production Of Metals (AREA)

Applications Claiming Priority (1)

Publications (1)

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Family Applications (1)

Country Status (5)

Cited By (6)

Families Citing this family (1)

• 1969
  • 1969-11-21 US US878927A patent/US3595614A/en not_active Expired - Lifetime
• 1970
  • 1970-11-17 GB GB54637/70A patent/GB1272496A/en not_active Expired
  • 1970-11-20 DE DE19702057161 patent/DE2057161A1/de active Pending
  • 1970-11-20 FR FR7041876A patent/FR2069818A5/fr not_active Expired
  • 1970-11-21 JP JP45103234A patent/JPS4949120B1/ja active Pending

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