US3817847A - Reconcentrating spent alkali metal chloride solution for recycling toelectrolysis cell - Google Patents

Reconcentrating spent alkali metal chloride solution for recycling toelectrolysis cell Download PDF

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Publication number
US3817847A
US3817847A US00254274A US25427472A US3817847A US 3817847 A US3817847 A US 3817847A US 00254274 A US00254274 A US 00254274A US 25427472 A US25427472 A US 25427472A US 3817847 A US3817847 A US 3817847A
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Prior art keywords
alkali metal
metal chloride
brine
concentration
cell
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US00254274A
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English (en)
Inventor
H Guth
E Zirngiebl
R Eberlein
H Wiechers
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Bayer AG
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Bayer AG
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    • 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

  • Evaporation is preferably effected by multiple-stage flashing, the concentration of spent solution ranging from about 200 to 280 grams per liter and recycle solution having a concentration of at least about 310 grams per liter.
  • the preferred alkali metal chloride is sodium chloride.
  • This invention relates to a process for reconcentrating impoverished electrolysis brines by concentration through evaporation, preferably by flash evaporation.
  • the difference is based on the fact that, in the diaphragm cell, the brine entering the anode compartment is guided through a diaphragm into the cathode compartment, resulting in the electrolytic formation of an approximately 11% NaOH solution which also contains all the unreacted sodium chloride.
  • this solution known in the art as the cell liquor," is worked up by evaporation into solid sodium chloride and a 50% sodium hydroxide solution.
  • the mercury process it is only that proportion of alkali metal chloride which is required for current conversion that is removed from the inflowing brine.
  • concentration of the alkali metal chloride in the infiowing brine is made as high as possible, in order to obtain as high a level of conductivity as possible and to prevent the hydrogen from being evolved, for example instead of the sodium, at the mercury cathode.
  • the present invention relates to a process for reconcentrating a dilute aqueous alkali metal chloride solution up to an alkali metal chloride content in the range of from 200 g./liter up to the saturation concentration for use in the electrolysis of alkali metal chlorides by the amalgam process, distinguished by the fact that the dilute, alkali metal chloride solution issuing from the cells of an alkali chloride electrolysis installation is adjusted to a pH value of from 4 to 10, after which this solution is subjected to a single-stage or multiple-stage evaporation, preferably flash evaporation, and the losses of the alkali metal chloride during electrolysis and of water during evaporation are compensated by the addition of brine with a concentration in the range of about 200 g./l. of alkali metal chloride up to the saturation concentration.
  • the advantage of the process according to the invention is that there is no need for the conventional form of brine purification in which all the brine in the system has to be passed through a purification stage.
  • the quantity of brine to be purified for example by precipitation and filtration, only amounts to substantially to to 20% of the original quantity because it is only necessary to treat that proportion of brine which corresponds to the output of chlorine and alkali liquor.
  • the process according to the invention is carried out by adjusting the brine issuing from the cells, at a temperature of from about 70 to 90 C., to the pH value of the brine entering the cells and then subjecting it to evaporation, preferably flash evaporation. Evapoartion of a corresponding quantity of water results in the formation of a brine which is substantially saturated with alkali metal chloride and which has a temperature of about 55 to C.
  • the reconcentrated brine can be delivered to the cells without any further purification. Purified and optionally preheated fresh brine is added before re-entry into the cell in order to compensate for the losses of brine which occur during electrolysis through the separation of chlorine and alkali metal.
  • adjustment of the pH value of the brine to a value of from about 4 to 10 is preferably carried out before evaporation so that the brine can be returned to the cells immediately after it has been reconcentrated. Adjustment of the pH value can also be carried out at any other stage in the process should this be necessary under the particular process conditions.
  • Evaporation which takes place either in a single-stage or in several stages can be carried out in any way, although flash evaporation is preferably used for the purposes of the process according to the invention.
  • flash evaporation is preferably used for the purposes of the process according to the invention.
  • Of particular use to the process according to the invention is the fact that considerable quantities of heat are generated at what are today normal current loads of from about 8 to 10 ka. per m3 of cathode surface, this being reflected in an increase in temperature of the brine between the cell inlet and the cell outlet. This increase in temperature can be used for evaporating the water and considerably reduces the amount of outside heat which has to be supplied for evaporation.
  • flash evaporation which is a method known per se (cf. for example Ullmann, Encyklopadie der techn. Chemie, vol. 18, p. 463 (1967)), is carried out by exposing the brine coming from the cells to a reduced pressure in suitable vessels, for example flash vessels. Some of the water evaporates, so that there is an increase in the alkali chloride concentration of the brine. The pressures prevailing in the flash vessels during flash evaporation amount to between about 100 and 700 Torr.
  • 1, 4, 8, 9 and 10 are pumps, 2 and 5 are flash chambers, 3 and 6 are condensers, 7 is a heat exchanger, 11 is a vacuum pump, 12 is an inlet pipe and 13 is an outlet pipe.
  • the brine undergoes evaporation in stages, accompanied by cooling from 85 C. to 65 C.
  • the vapors formed are deposited on the condensers (3).
  • the salt solution cooled by expansion is then pumped by means of the pump (8) through the condensers (6) built into the flash chambers (5) where it is reheated to 105 C.
  • the brine is then heated by external heat to such a high temperature (approximately 115 C.) that it is able to cool in stages during expansion to around 70 C. in the flash chambers (5).
  • the condensate accumulating on the condensers (6) and (3) is collected and run off via the pumps (4) and (9) together with the fresh steam condensate.
  • water is evaporated in such a quantity that the required increase in concentration of from 280 to 315 g. of NaCl per liter is obtained.
  • the concentrated brine is returned to the brine circuit via the pump (10).
  • the vacuum pump (11) provides for the corresponding vacuum in the flash chambers.
  • the cooling water for the condensers (3) enters via inlet pipe (12) and leaves via outlet pipe (13).
  • the process according to the invention is suitable for all kinds of installations used for the electrolysis of alkali metal chlorides by the amalgam process. Although it is preferably intended for application in the electrolysis of NaCl, it is by no means limited to this particular application.
  • EXAMPLE 1 Conventional procedure An installation for the electrolysis of alkali metal chlorides by the mercury process with a daily output of around 5 5 0 tons of Cl: has a brine circuit with a throughput of substantially 1200 mfi/hr. The salt consumption amounts to substantially 950 tons of NaCl per day.
  • the brine enters the cells with a temperature of around 70 to 75 C. and leaves them with a temperature of around 85 C.
  • the brine issuing from the cells with a concentration of around 285 g./liter of NaCl is pumped via a dechlorination stage into a saturation stage and thence to a purification stage.
  • the brine purification stage requires a filter surface of around 2000 m.
  • the purification stage only has to handle around 120 mF/hr. of brine and its filter surface can be between 200 and 400 m. depending on whether or not it is preceded by a thickener.
  • the improvement which comprises carrying the flash evaporation of the spent solution to about the saturation concentration of the alkali metal chloride, purifying substantially saturated fresh alkali metal cholride solution and adding it to the flash evaporated solution in an amount suflicient to compensate for the loss of alkali metal chloride during electrolysis and of water during evaporation, and forwarding the combined solution to the electrolysis cell.

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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)
  • Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
US00254274A 1971-05-28 1972-05-17 Reconcentrating spent alkali metal chloride solution for recycling toelectrolysis cell Expired - Lifetime US3817847A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DE19712126516 DE2126516A1 (de) 1971-05-28 1971-05-28 Verfahren zum Aufkonzentrieren von Elektrolysesole bei der Alkalichlorid-Elektrolyse

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US3817847A true US3817847A (en) 1974-06-18

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US00254274A Expired - Lifetime US3817847A (en) 1971-05-28 1972-05-17 Reconcentrating spent alkali metal chloride solution for recycling toelectrolysis cell

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US (1) US3817847A (enrdf_load_stackoverflow)
BE (1) BE784036A (enrdf_load_stackoverflow)
CA (1) CA1001987A (enrdf_load_stackoverflow)
DE (1) DE2126516A1 (enrdf_load_stackoverflow)
FR (1) FR2139991B1 (enrdf_load_stackoverflow)
GB (1) GB1375656A (enrdf_load_stackoverflow)
NL (1) NL7207067A (enrdf_load_stackoverflow)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2741985C3 (de) * 1977-09-17 1981-10-15 Hoechst Ag, 6000 Frankfurt Verfahren zur Verminderung von Quecksilberverlusten bei der Alkalichlorid-Elektrolyse

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Publication number Publication date
BE784036A (fr) 1972-11-27
DE2126516B2 (enrdf_load_stackoverflow) 1974-04-11
FR2139991B1 (enrdf_load_stackoverflow) 1978-06-02
NL7207067A (enrdf_load_stackoverflow) 1972-11-30
GB1375656A (enrdf_load_stackoverflow) 1974-11-27
DE2126516A1 (de) 1972-12-07
FR2139991A1 (enrdf_load_stackoverflow) 1973-01-12
CA1001987A (en) 1976-12-21

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