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
• • 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
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes

Definitions

• the present invention relates to a process for reducing losses of mercury in the alkali metal chloride electrolysis according to the amalgamation process.
• Chlorine and alkali metal hydroxide solutions are produced to an increasing extent by electrolysis of the corresponding chloride solutions according to the amalgamation process.
• the electrolysis plants generally use an electrolyte cycle, the necessaryy increase in the concentration of alkali metal chloride being brought about at least in a partial stream of the electrolyte coming from the electrolysis cells by the addition of a salt con taining alkali metal chloride to the electrolyte which has become poor in alkali metal chloride.
• the electrolyte absorbs, in addition to the alkali metal chloride, also soluble and insoluble foreign matter during this dissolving process.
• the foreign matter must be prevented from penetrating into the electrolysis cells and must therefore be removed from the electrolyte. This is generally achieved by precipitation in an alkaline medium and subsequent alkaline fitration.
• the electrolyte solution which flows off from the electrolysis cells and is called thin brine always contains mercury, generally from 1 to 20 milligrams Hg per liter of thin brine.
• the electrolyte which is now called pure brine contains only a fraction of the initial content of mercury.
• the loss of mercury in the electrolyte cycle can be exactly calculated from the diiference between the mercury content of the thin brine and that of the pure brine. Most of the lost mercury is present in the filter residue of the alkaline "ice filtration. Since an economic process for the recovery of mercury from these filter residues has not so far been known, the said filter residues are rejected in the form in which they are obtained. This does not only lead to looses of mercury, but also creates an industrial-hygienic problem.
• Processes have also been proposed for reducing the losses of mercury in the electrolyte cycle.
• the mercury is first removed from the electrolyte and then returned to the electrolysis cells, optionally after a work-up. It has been proposed, for example, to precipitate the mercury from the electrolyte in the form of the sulfide thereof or to adsorb it on a strong basic anion exchange resin.
• the cathodic separation of the mercury from the electrolyte by the application of an electromotive force has also been proposed.
• these processes are so complicated that they are not suitable for use in large-scale alkali metal chloride electrolysis plants.
• the present invention provides a process for reducing losses of mercury in alkali metal chloride electrolysis plants operated according to the amalgamation process with electrolyte cycle and alkaline filtration of the electrolyte, which comprises adding such an amount of hypochlorite to the electrolyte before subjecting it to the alkaline filtration that it still contains hypochlorite ions after the filtration.
• the process of the invention enables the mercury contained in the electrolyte coming from the electrolysis cells to be maintained completely or almost completely in the electrolyte cycle and thus to be returned to the electrolysis cells in a simple manner.
• the losses of mercury in the electrolyte cycle are within the range of from 0 to at most 10% by weight of the amount that would be lost without the application of the process of the invention, and the filter residue obtained in the electrolyte filtration which is consequently free from or contains only traces of mercury can be rejected without hesitation as regards industrial hygiene.
• hypochlorite the pure brine leaving the filtration should contain at least about 1 gram 010- per cubic meter.
• concentration of hypochlorite is therefore advantageously adjusted before the filtration in a manner such that the concentration is not only reliably prevented from dropping below this minimum value but that a somewhat larger amount of hyprochlorite, advantageously 1 to 30 grams, and preferably 3 to 10 grams, of Cl0 per cubic meter is always present in the pure brine leaving the filtration. It is also possible to use considerably higher concentrations of hypochlorite, but this does not ofier any advantages.
• the concentration of hypochlorite to be adjusted de pends on numerous factors, for example, the residence time between adjustment and filtration, the temperature of the electrolyte, the quality of the salt used for saturating the electrolyte, and the materials with which the electrolyte comes into contact.
• a hypochlorite concentration within the range of from 10 to grams C10 per cublic meter brine is necessary.
• the temperatures in the electrolyte cycle ' which, for example for sodium chloride or potassium chloride electrolyses, are generally within the range of from 40 to 90 C. are of no importance for the balance of mercury in the process of the invention.
• the required hypochlorite concentration is adjusted. It may be adjusted before, during or after raising the concentration of alkali metal chloride in the brine. As the hypochlorite undergoes a pronounced decomposition in the hot electrolyte solution and the hypochlorite-containing electrolyte is somewhat more corrosive than electrolyte free from hypochlorite, the required hypochlorite concentration is advantageously adjusted, as regards time and space, only shortly before the alkaline filtration, preferably for example shortly before or simultaneously with the alkaline precipitation of the impurities contained in the electrolyte.
• the required hypochlorite concentration may be adjusted in several chemical ways.
• a finished hypochlorite solution is added. It is advantageous to use the alkali metal hypochlorite solution with the alkali metal of which the hydroxide solution is produced by the electrolysis.
• a sodium hypochlorite solution is therefore preferably used.
• Such solutions designated bleaching liquors are available at a low price in alkali metal chloride electrolysis plants or are obtained as undesirable by-products in the absorption of chlorine-containing waste gases.
• the use of hypochlorite solutions has the advantage that exactly measuredout quantities can be used with only little expenditure.
• the required hypochlorite concentration may also be adjusted by the reaction of chlorine with hydroxyl ions according to the following scheme in the electrolyte itself. It is not necessary in this case that the alkalization agent, which in a sodium chloride electrolysis is advantageously sodium hydroxide solution, be added in the same place of the electrolyte stream as the chlorine.
• the alkalization agent which in a sodium chloride electrolysis is advantageously sodium hydroxide solution
• chlorine there may be used pure gaseous or pure liquefied chlorine as well as waste chlorine gas diluted with air. Especially the latter is preferably used for industrial electrolysis plants because such mixtures of chlorine and air are generally obtained as waste gases, for example, in the dechlorination of the thin brine and in other places.
• the required hypochlorite concentration may be adjusted by leaving such an amount of chlorine in the electrolyte when dechlorinating the acid thin brine that the necessary amount of hypochlorite is formed in the subsequent alkalization.
• it is necessary because of the decomposition of hypochlorite in the hot electrolyte solution already mentioned above, to adjust the electrolyte solution to a considerably higher initial concentration of hypochlorite, which in turn increases the corroding property of the electrolyte. This fact must already be taken into consideration in process stages of the electrolyte cycle for which correspondingly corrosion-resistant materials have not generally been used, for example the salt dissolution plant.
• EXAMPLE Through a sodium chloride electrolysis plant operated according to the amalgamation process, 500 cubic meters electrolyte solution were passed per hour in uniform flow.
• the thin brine flowing off from the cells at a temperature of about 80 C. contained, in addition to small amounts of sulfate, chlorate and chlorine, also an average of 275 kilograms sodium chloride and 3.85 grams mercury per cubic meter.
• the brine was adjusted to a pH of about 2 by the addition of hydrochloric acid and freed from the major part of the dissolved chlorine in a vacuum dechlorination plant. The residual chlorine was subsequently expelled by blowing in air.
• the brine was then adjusted to a pH of 9.5 by adding sodium hydroxide solution and pumped into a salt dissolution plant where the concentration was increased to an average of 310 kilograms NaCl per cubic meter by contacting with natural rock salt.
• the brine was then passed through so-called precipitation vessels where the alkaline earth ions and the sulfate were converted to a large extent into a diflicultly soluble precipitate by the addition of soda and barium carbonate.
• the brine was filtered, whereby the salts and substances causing turbidity accumulated as a filter cake on filter cloths of synthetic fabric.
• the filtered pure brine was returned to the electrolysis cells via bulfer vessels.
• the pure brine contained an average of only 2.05 grams mercury per cubic meter. Consequently an average of 1.80 grams mercury had been lost per cubic meter of brine.
• the loss of mercury was 87 grams per ton of chlorine produced. In a fully continuous operation about 7.88 tons mercury per year were lost in this manner.
• the lost mercury was almost completely contained in the filter cake of the alkaline filtration.
• the filter cake which had been dried at C. contained, on an average, 1.4 kilograms mercury per ton of dry substance.
• the brine was first adjusted to a hypochlorite content of about 30 grams C10 per cubic meter.
• the hypochlorite content of the filtered pure brine was within the range of from 3 to 10 grams ClO- per cubic meter and the average content of mercury was 3.78 grams per cubic meter.
• the loss of mercury was therefore on an average only 70 milligrams per cubic meter brine, that is it had been reduced to about 4% of the original value.
• the specific loss of mercury in the electrolyte cycle was only 3.4 grams per ton of chlorine produced and the annual loss arising from this source had been reduced to about 0.3 ton mercury. No mercury could be traced in the filter cake of the alkaline filtration and the filter residues were consequently harmless from the viewpoint of industrial hygiene.
• a process for electrolysis of alkali metal chloride brine in electrolytic flowing mercury cathodic cells which comprises (a) introducing a substantially saturated aqueous solution of an alkali metal chloride brine into the flowing mercury cathodic cells (b) electrolyzing said brine in said cells to produce chlorine and alkali metal amalgam, thereby depleting the alkali metal content in the brine (c) removing the thus depleted brine from the cells,
• brine contains mercury impurities picked up from the flowing mercury cathodes (d) adding caustic alkali to the brine (e) resaturating the brine with alkali metal chloride (f) adding to the resaturated brine soda and barium carbonate to precipitate the alkaline earth ions and the sulfate ions, and then filtering the brine, and
• the brine still contains at least one gram of hypochlorite ions per cubic meter after the filtration, whereupon the filtered and resaturated brine is fed back into the electrolytic cells.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)

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• 1968
  • 1968-03-22 DE DE19681767026 patent/DE1767026B1/de not_active Withdrawn
• 1969
  • 1969-01-10 SE SE00280/69A patent/SE337582B/xx unknown
  • 1969-02-11 BR BR206301/69A patent/BR6906301D0/pt unknown
  • 1969-02-24 FI FI690572A patent/FI49285C/fi active
  • 1969-03-12 NL NL6903819.A patent/NL161203C/xx not_active IP Right Cessation
  • 1969-03-17 GB GB03869/69A patent/GB1207772A/en not_active Expired
  • 1969-03-20 AT AT276669A patent/AT281873B/de not_active IP Right Cessation
  • 1969-03-24 BE BE730335D patent/BE730335A/xx unknown
  • 1969-03-24 FR FR6908550A patent/FR2004577A1/fr not_active Withdrawn
• 1971
  • 1971-09-20 US US00182200A patent/US3801480A/en not_active Expired - Lifetime

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