US2248137A - Electrolysis of alkali metal brines - Google Patents

Description

Patented July 8, 1941 UNITED STATES PATENT OFFICE 2,248,137 ELECTROLYSIS OF ALKALI METAL BRINES Maurice C. Taylor and William C. Gardiner, Niagara Falls, N. Y., assignors to The Mathieson Alkali Works, Inc., New York, N. Y., a corporation of Virginia No Drawing. Application December 17, 1937,

Serial No. 180,330

' 10 Claims.

ing alkali metal hydroxide.

One of the outstanding difficulties encountered Q in the operation of the mercury cell process for the electrolysis of alkalimetal brines'has been the deposition of molecular hydrogen together with the alkali metal on the surface of the mercury cathode. Such 'a deposition results in a substantial loss in current efficiency and also creates a serious explosion hazard. An apparent relation between the amount of hydrogen deposited and the amount of impurities present in the brine has led to the belief that such excessive discharge of hydrogen gas on the mercury cathode is in large measure due to impurities present. Purification of the brine has, as a consequence, been proposed and has been practiced, but generally neither the extent nor the direction of previous purifications have been economically effective. Ameasurefof purification has been obtained by the practice of regularly discarding a portionof the brine and replacing the discarded portion with fresh brine thus to eliminate accumulated impurities, but this practice of course is wholly ineifective with respect to impurities in such concentrations as they appear in the fresh brine. t

We have found that this deposition of hydrogen is peculiarly related to .two impurities, namely iron and magnesium, and that this deposition of hydrogen can be controlled and reduced to a minimum, well within practicable working limits, by proper control of the concentration of the brine with respect to these two impurities, and that if the concentrations of these two impurities are so controlled the other impurities ordinarily present do not have any marked effect on'this hydrogen deposition. For

examp1e,"ca1cium, one of themost frequent im-' purities in alkali metal brine solutions, has no eifect on hydrogen deposition in concentrations .up toabout 2 grams per liter, expressed as 09.0, and in concentrations up to 5 grams per liter, expressed as CaO, the eifect isv very slight.

These values expressed as calcium ion concentration are approximately 1.5 grams per liter and 3.6 grams per liter, respectively We thus overcome this difliculty with respect to hydrogen deposition by a selective and peculiarly effective purification. This particular purification, although quite extreme, becomes economically effective because it is selective. Our invention further includes particularly advantageous methods for effecting this particular purification.

We have found that hydrogen deposition is to all practical purposes eliminated if the iron prescut in the solution is kept below a value of about 00001 gram per liter and if the magnesium is kept below a value of about 0.006 gram per liter.

:It is one object of the present invention, therefore, toreduce to a minimum the discharge'of hydrogen on the cathode in an alkali-chlorine cell of the mercury type, by providing a simple and eflicient means whereby the concentration of iron and magnesium in the solution being electrolyzed is kept below certain minimum values.

It is another object of the present invention to reduce anode destruction by means of a related control of the brine composition with re-- spect to calcium, a material hitherto regarded as an objectionable impurity and one of those to which purifications previously proposed and practiced have been directed, as will more fully appear hereinafter.

Our invention contemplates the improvement in the electrolysis of alkali metal brines involving the-deposition of the alkali metal from the brine at a mercury cathode in an electrolytic cell which comprises circulating the brine through the cell while maintaining in the brine in the cell calcium'in substantial'proportions but not exceeding about 3.6 grams per liter, iron in a concentration not exceeding about 0.0001 gram per liter and magnesium in a con'cen'tration not exceeding about 0.006 gram per liter. In our preferred practice, the brine solution discharged from the cell is dechlorinated and the resulting iron-containing precipitate is separated from the brine prior to returning it to the cell. Thus, the brine discharged from the cell may be retained under a reduced vapor pressure with respect to chlorine for a time 'sufficient to remove dissolved but unreacted chlorine gas therefrom.- To accomplish the same result, inert gas may he passed through the brine discharged from the the dissolved but unreacted chlorine gas.

In the usual practice of operating mercury cells for the production of chlorine and an'alkali metal hydroxide, a saturated or nearly saturated brine solution is supplied continuously to one of brine back to the cell.

the known types of cell. At the same time chlorine gas is recovered from the anode, and the partially electrolyzed brine which is saturated with chlorine is continuously withdrawn. This is then resaturated with the alkali metal salt and returned to the cell. In addition to the minor proportions of iron which may be present in the salt; varying quantities may be acquired from the vessels and various other pieces of equipment with which the brine comes into contact. We have discovered that by dechlorinating .this brine solution prior to returning it to the cell, ,the iron present is rendered insoluble, probably in the form of hydrated oxides, and may be filtered oil. The discharged solution may be rendered substantially free of chlorine either by subjecting it to a vacuum or by bubbling an inert gas such as air or nitrogen through it. By this expedient the iron present in the solution is reduced to below about 0.0001 gram per liter. In the event that the alkali metal salt used is very low in magnesium, the filtrate may then be resaturated and directly returnedto the cell for further electrolysis. If, however, the brine solution contains magnesium in excess of about 0.006 gram per liter, the quantity in excess of this amount should be removed. This is accomplished according to our invention" by rendering the solution alkaline folowing the dechlorination step.- The preferred method is to add alkali hydroxide to the solution in an amount appropriate to bring the solution to a pH value within the range 9.4 to 10.4.. This alkalinity is suflicient to precipitate the magnesium as the hydroxide but is not high enough to precipitate any calcium which may be present with the exception' of that corresponding to any small amount of carbon dioxide'p-resent in the brine forming calcium carbonate. This alkalinity also precipitates iron, probably as hydrated oxides.

While this partially spent brine solution may then be directly'supplied to .the cell for further electrolysis, after removal of precipitated material by filtration, in the most advantageous cyclic operation it is first resaturatedwith alkali metal chloride'.' The solution should be maintained at a pH of from 9.4 to 10.4 until after the resaturation step at which time itis filtered in order to remove'theprecipitated material. The filtered brine is then supplied to the cell with or without neutralizing the relatively small amount of alkali present.

In addition to being saturated with chlorine,

the brine solution discharged from an alkalichlorine cell of the mercury cathode type contains varying amounts of hypochlorous acid and sodium chlorate produced by various side reactions during electrolysis. For the most efficient cyclic operation the concentration of these substances should be reduced before recycling the It is a further object of our invention to provide means whereby this may be done in conjunction with the treatment abovedescribed for the limitation of iron and magnesium concentrations.

present instance the chlorine is removed: as formed, the following reaction occurs:

As stated above, chlorate ion is also formed during the electrolysis. If this is not diminished, the concentration will-gradually build up to a point where the solubility of the alkali chloride The chlorate may be decomposed according to i the following reaction:

NaClOa+ 6HCl:NaCl+.3Cl2+3HzO An excess of acid above that indicated in the equation hastens this reaction. We have. found that for the efiicient reduction and control of.

the chlorate ion concentration it is necessary to maintain a constant relatively small chlorate ion concentration in the brine throughout the cycle.

It is further necessary for the successful removal of chlorate to remove the chlorine resulting from the above reaction as it is formed. For example,

it was found that in a typical plant operation carried on in accordance with the present invention chlorate was forming at the rate of 0.0504 gram NaClOa per liter per minute. The brine temperature was 70 C. It-was found that ifsix equivalents of HCl or' 0.104 gram were added per liter of discharged brine per minute and .airbubbled through the brine to remove the chlorine as formed, then the chlorate content of the brine remained stationary at 2.0 grams NaclOz-p'er liter,

9. concentration well-be1ow that at which the referred-to disadvantages due to "the presence of chlorate are significant. 1

Itis thus evident that according to the present invention the iron concentration may be reduced to below the specified limit, the hypochlorous aciddestroyed and the chlorate concentration reduced to a small constant value by the combined, ex-

pedients of continuously dechlorinating, adding acid in appropriate amounts, and subsequently adding alkali in appropriate amounts. In the cyclicoperation, if magnesium is to be-removed, the solution is brou'ght'to a pH of 9.4 to 10.4 after the completion of' the chlorine removal and the remainder of the cycle is carried out as described above.

We have found that the dechlorination of the I is added to the solution in an amount equivalent to the hypochlorous acid present. If as in the It will be understood that it is not always necessary to treat'the entire amountof brine solution as described above. A portion of the discharged brine only may be separated and treated and the treated and untreated recombined before returning to the cell. The only requisite is that the recombined portions, i, e., the'solution actually fed to the cell, shall meet the above-setf0rth specifications with respect to iron and magnesium.

Another difliculty frequently encountered in the operation of alkali-chlorine cells of the mercury cathode type is the destruction of the graphite anode. One of the factors contributing to this is the presence of increasing amounts of sulfate ion in the brine during-electrolysis. The alkali metal salt used to saturate the brine solution contains varying proportions of other salts,

such for example as ,CaSOr, Na2SO4 and lime. The brine can increase in calcium concentration up to the saturation point with respect to CaSO4. We havedetermined that the solubility of'CaSOi in saturated sodium chloride brine is approxi-- mately 4.67 grams per liter at C. for example.

We have further determined the solubility prod-' not of 09.804 in such brine and found it to be 1.18 10 The amount of calcium and sulfate per liter.

in the brine will therefore be limited, at 65 C.

for example, by the equation:

[Ca] [S04] =1.18

6.33 grams per liter (equivalent to 9.4 grams Na2SO4). Further, if the calcium ion concentration were 2.15 grams per liter (3.08 grams CaO), the sulfate ion could not then exceed 2 grams per liter, (3 grams Na2SO4) According to previous practice the calcium ion has been removed from the brine solution. vOur discovery that calcium ion may be tolerated in the solution in amounts not exceeding 3.6 grams per liter and preferably not above 1.5 grams per liter without substantial ill effects therefore permits a simple and effective method of keeping the sulfate ion just stated.

We claim:

1. In the electrolysis of alkali metal brines involving the deposition of the alkali metal from the brine at a mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell while maintaining in the brine in the cell calcium in substantial proportions but not exceeding about 3.6 grams per liter, iron in a. concentration not exceeding about 0.0001 gram per liter and magnesium in a concentration not exceeding about 0.006 gram per liter.

2. In the electrolysis of alkali metal brines involving the deposition of the alkali metal from the brine at a mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell in contact with the cathode and maintaining in the brine in the cell a substantial proportion of calcium but not to exceed about 3.6 grams per liter, a magnesium content not to exceed 0.006 gram per liter and an iron concentration not exceeding about 0.0001 gram per liter, dechlorinating the brine solution discharged from the cell and separating from the brine the resulting iron-containing precipitate prior to returning the brine to the cell.

3.. In the electrolysis of alkali metal brines involving the deposition of the alkali metal from the brine at a. mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell and maintaining in the brine in the cell a substantial proportion of calcium but not exceeding about 3.6 grams per liter and a magnesium content not to exceed 0.006 gram per liter and also maintaining an iron content in the brine in the cell below about 0.001 gram per liter by maintaining the brine discharged from the cell under a reduced vapor pressure with respect to chlorine for a. time sufficient to remove dissolved but unreacted chlorine gas therefrom and separating the resulting ironcontaining precipitate prior to returning the brine solution to the cell.

4. In the electrolysis of alkali metal brines involving the deposition of the alkali metal from the brine at a mercury cathode in an electrolytic ing the brine through the cell while maintaining in the brine in the cell a substantial content of calcium but not to exceed about 3.6 grams per liter, a magnesium content below about 0.006 gram per liter and an iron content not exceeding 0.0001 gram per liter, passing inert gas through the brine discharged from the cell for a time suiiicient to substantially remove dissolved but unreacted chlorine gas from the brine and separating the resulting iron-containing precipitate prior to returning said brine solution to the cell.

5. In the electrolysis of alkali brines involving 'the deposition of the alkali metal at a mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell and maintaining in the brine in the cell a substantial proportion of calcium but not to exceed 3.6 grams per liter, a. concentration of magnesium below about 0.006 gram per liter and an iron concentration not exceeding about 0.0001 gram per liter by dechlorinating the brine discharged from the cell, separating from the brine theresulting iron-containing precipitate and thereafter resaturating the separated brine solution with an alkali metal chloride prior to returning the brine to the cell.

6. In the electrolysis of alkali metal brines involving the deposition of alkali metal from the brine at a. mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell and maintaining in the brine in the cell a substantial concentration of calcium but not to exceed about 3.6 grams per liter, a magnesium concentration below about 0.006 gram'per liter and an iron concentration not to exceed 0.0001 gram per liter, dechlorinating the brine discharged from the cell, separating the resulting iron-containing precipitate from the brine, adding to the brine discharged from the cell caustic alkali in amount suflicient to produce in, the brine an alkalinity equal to a pH within the range 9.4 to 10.4, removing the magnesium containing precipitate and returning the brine solution to the cell.

7. In the electrolysis of alkali metal chloride brines involving the deposition of the alkali metal from the brine at a mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell during electrolysis and maintaining a low concentration of hydrochlorite ion and chlorate ion in the brine in the cell by removing chlorine from the brine discharged from the cell while simultaneously adding to said brine hydrochloric acid during'a portion of the dechlorination period and prior to returning the brine to the cell, the brine thus subjected to electrolysis in the cell having maintained therein a substantial concentration of cell, the improvement which comprises circulatper liter and a concentration of, magnesium below about 0.006 gram per liter, reducing hypochlorite ion concentration and chlorate ion concentration in the brine discharged from the cell through removal of chlorine from said discharged turning the brine thus treated to the cell.

9. In. the electrolysis of alkali metal chloride brines involving deposition of alkali metal at a mercury cathode in an electrolytic cell, the improvement which comprises circulating the brine through the cell, maintaining in the brin in the cell a relatively high concentration of calcium but not to exceed about 3.6 grams per liter, a

concentration of iron below about 0.0001 gram per liter and a concentration of magnesium below about 0.006 gram per liter, the concentrations of magnesium and iron being maintained by rev moving chlorine from brine discharged from the cell while simultaneously adding hydrochloric acid to the brine before dechlorination is completed, thereafter adding caustic alkali to the discharged brine in a quantity sufficient to produce an alkalinity equivalent to a pH ranging from 9.4

to 10.4, removing the resulting precipitated material and returning the brine to the cell.

10. In the electrolysis of alkali metal chloride brines involving the deposition of alkali metal at a mercury cathode in an electrolytic cell, the

improvement which comprises circulating the brine through the cell, maintaining in the brine in the cell a relatively high concentration of calcium but not to exceed 3.6 grams per liter, a concentration of iron below about 0.0001 gram per liter and a concentration'o'f magnesium below about 0.006 gram per liter, the concentration of iron and magnesium in the brine in the cell being maintained by reducing hypochlorite concentration and chlorate ion concentration in the brine discharged from the cell by passing airtherethrough and while simultaneously adding hydrochloric acid, the quantity of hydrochloric acid added being at least as great as the sum of the molar equivalent of hypochlorite present in the brine plus six times the molar equivalent of chlorate present therein, thereafter adding to the brine caustic alkali in amounts sufficient to produce alkalinity ranging from a pH of 9.4 to 10.4 in the brine, resaturating the brine with alkali metal chloride, removing resulting precipitated material from the brine and returning it to the cell.

MAURICE C. TAYLOR.

WILLIAM C. GARDINER.

- C TIFICATEZOF CORRECTiON v w Patent No. 2,2L ,'157'. July 1.9m; 1

MURICE-d. TAYLOR, ET AL, It is hereby cer't ifie dl that eiro'r ppers ifi th s prin fsed sfiecification of the above numbered pater t I'GQUli-Ililg c orr'ectionas'follows: Page fli fifst cqlmnn, line 61L, claim 5, for"'O OO1 read O.OOO 1- same page, second 3 column; line 51, claim 7, for "hydrd'chlorlte" r ead 'hy'pochloi ie that the said Letters Pat'nt sh oii ld-be re ad wit h thiscom 'eg ati on@ihereifi that the game 'xha ay' confo nn tofthe regard-of the 'ca e i-nfthe P gi fieht OffiZC G- Signed and seal e1; thi 5th'day 6f 1iugdst; A11 I 19L 1. I

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