US2562169A - Method of purifying alkali metal hydroxides - Google Patents

Description

umrso STATE destroy chlorates.

' sulfite material. I pound used may be employed in an amount equivalentto the stoichiometric amount necessary to Patented July 31, 1951 METHOD OF PURIFYING; ALKALI. METAL v I .HYDROXIDES Chester CiBrumbaugh, Painesville, Ohio, ass'ignor to Diamond Alkali Company,. Gleveland, hi0,,

a corporation of Delaware NoDrawing. Application September 17, 1-948,

Serial No.4'9',858

.1- This invention relates to chemical manufacture and more particularly relates'to a novel and eiiicient method of preparing alkali metal hydroxides of high purity, particularly with respect to lrraterials of oxidizing. character, such asalkali -.i netal chlorates' This invention is especially adapted to the production of pure caustic soda.

Large volumes of caustic soda which must meet stringent specifications,especially as to content electrolytic caustic soda from chlorates. Consequently, it has long been the desire of electrolyticv caustic manufacturers to acquire a suitable method of chlorate removal and many attempts have been made without success to devise such a method. The prior art methods have either been prohibitively expensive or have not resulted in a suitablechlorate-free caustic product for rayon;

uses. Numerous of these prior efforts have been directed to the use of various forms of iron to Such methods are unsuccessful, however, in that there is either toomuch chlorate or too much iron in the final product. The other methods of chlorate removal which have been tried include the extraction of chlorate from caustic soda, extraction of caustic soda from chlorate, inhibition of the oxidative'effect of chlorate, and the like, all of which were too expensive for commercial use or were insufficiently effective.

. giTh'epresent. invention contemplates-the substantially complete removal of chlorates from alkali meta-l. hydroxides, more particularly from caustic soda solutions or from anhydrous caustic soda, either as an independent step in the manufacture of caustic soda or preferably andmore conveniently inthe course of obtaining caustic soda in high concentration aqueous solution or in' anhydrous condition. -In'g'enera1, the objects oi lthe invention-areaccomplished by treating "caustic soda solutions containing an undesirable quantity of chlorate ions with alkali metal sulfites, the alkali metal ion being suitably that of the alkali metalhydroxide being treated with In general, the sulfite comdestroy the known amount of chlorate ion present in the material, the amount-of chlorate ion having'been previously-ascertained by analytical "methods. It has been found desirable to use somewhat of an excess of the stoichiometric amount, such as a 20% excess, but those skilled in the art will appreciate that once the amount 6 Claims. (01. 23.--184') 2 of" chlorate present. in the alkali-metal hydroxide, such as caustic soda, being treated has'bee'n determined, the specific amount of sulfite material in excess. of the stoichiometric amount necessary to reduce the chlorate will become somewhat a matter. of. choice.

Asnoted above, it is contemplated that the present invention maybe practiced with alkali.

metal hydroxide solutions and especially caustic soda solutions of more or less concentration, it being, preferred to treat solutions ofelectrolytic caustic soda of 50% or greater concentration and particularly sinoelthose solutions have especially high concentrations. of chlorate-relative to chemically obtained'caustic soda, the present invention has its particular, applicability in the removal of chlorate from electrolyticcaustic soda, which is obtained by electrolysis of sodium chloride solutions. I Y

The alkali metal hydroxides, more particularly caustic soda, may suitably be treated with the sulfite compound, in the case of causticsoda, more suitably sodium sulfite, preferably at somewhat elevated temperatures, such as temperatures from C. to the meltingtemperature "of anhydrous caustic: soda, it being preferre'din general to incorporate the. sodium sulfite as, an incident of concentration procedure, whereby higher temperatures willv be realized, Moreover, the sodium sulfite may, if desired, be added'asa solid salt or. may be added as a saturated aqueous solution or in dilute. aqueous solution, depending upon the point of the. sodium hydroxide manufacture in which the addition is made.

The. present invention has the particular advantage. that it reduces, the sodium chlorate to a tolerable limit without adding any other difiiculties, or in the case of the use of sodium sulfite to caustic soda, any foreign ions to the caustic'soda, particularly foreign metallic ions which are espeoially undesirable in this connection. A minor increase in the concentration of sulfate ion in the ultimate alkali metal will, of course, be experienced in the practice of this invention. However, higher concentrations'of sulfate can much more readily be tolerated, and what amounts in efiect to. an exchange of chlorate for. sulfate, is therefore a desirable. improvement in the purity of the alkali metal hydroxide.

The invention is particularly applicable in the reduction of the caustic soda to relatively anhydrous condition by heat exchange evaporator systems 30f Water removal and more particularly by" for uses-for whichheretofore electrolytic caustic particularly was not available.

As pointed out above, it is in general preferred to employ an amount of sodiumsulfite equal to a slight excess of the stoiochiometric amount of sodium chlorate found in the caustic soda product. Thus, a typical amount of sodium sulfite to be used per ton of anhydrous caustic soda obtained will be based on sodium chlorate content, which is typical in the particular electrolytic operation referred to. An example of such operation is a caustic soda solution obtained elec trolytically containing 0.166% of sodium chlorate, for which purpose it has been found useful to employ 14.1 pounds of sodium sulfite'per ton of anhydrous caustic soda, whereby the chlorate content has been reduced to about 0.002%, a tolerable amount of this material for practically all industrial purposes.

The present invention is applicable to all of the alkali metal hydroxides, including lithium, potassium, sodium, rubidium, cesium, and the like. However, it will be appreciated inview of the commercial importance of the product that it has its greatest significance as applied to sodium hydroxide or caustic soda. Moreover, other sulfites than sodium sulfite, such as lithium, potassium, rubidium, and cesium, may be employed, it being particularly desirable to employ the sulfite which has a common ion with the hydroxide being treated. In accordance therewith, it will be appreciated that sodium sulfite will be preferred for treating caustic soda.

In order to inform those skilled in the art of the manner in which the present invention may preferably be practiced but in no sense to be taken as limiting upon the scope of the invention described herein, the following examples are set EXAMPLE I Electrolytic caustic soda solution previously concentrated to a concentration of about 50% in caustic soda is passed through a falling film type evaporator, followed by a separator in which the products of the evaporator are impinged upon the wall of the separator, the entire system be ing maintained at a wall temperature in excess of the melting temperature of anhydrous caustic soda and as is more particularly described in the above referred to application of R. E. Hulme. Sodium sulfite aqueous solution in a ratio of 14.1 pounds of sodium sulfite per ton of caustic product from the evaporator is added to the feed of the evaporation system. The content of chlorate in the entering caustic soda solution based on anhydrous caustic is 0.166%. The content of chlorate in the material issuing from the evaporator is of the order of 0.002%.

EXAMPLE. II

The procedure above was followed, varying the amount of sodium sulfite added to the feed with the following results:

Addition of sodium sulfite to electrolytic feed While there have been described various embodiments of the invention, the methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in sub stantially the same or equivalent manner, it being intended to cover the invention broadly in what ever form its principle may be utilized.

What is claimed is:

1. The method of purifying an alkali metal hydroxide with respect to alkali metal chlorate, which comprises contacting a mixture of said hydroxide and said chlorate with an alkali metal sulfite at an elevated temperature.

2. The method as claimed in claim 1 in which the mixture of alkali metal hydroxide and alkali metal chlorate is contacted with sodium sulfite at an elevated temperature.

3. The method as claimed in claim 1 in which the amount of sulfite added to the alkali metal hydroxide solution is at least equal to the stoichiometric equivalent of chlorate present therein.

4. The method of removing sodium chlorate from caustic soda solutions which includes the step of contacting a caustic soda solution containing such chlorate with sodium sulfite at an elevated temperature.

5. The method of purifying sodium chlorate contaminated caustic soda with respect to said sodium chlorate which includes the step of contacting caustic soda solution of more than 50% concentration with an amount of sodium sulfite at least equal to the stoichiometric equivalent of sodium chlorate present with such caustic soda.

6. The method of dehydrating sodium chlorate contaminated caustic soda while purifying said caustic soda with respect to said sodium chlorate which includes the steps of heating such caustic soda until all of the water associated therewith has been removed, and contacting said caustic soda during the anhydrizing process with sodium sulfite.

CHESTER C. BRUMBAUGH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,982,241 Aydelotte Nov. 27, 1934 2,142,670 Eichelberger Jan. 3, 1939 2,207,595 Muskat July 9, 1940 2,258,545 Davies Oct. 7, 1941 2,403,789 Cummins July 9, 1946 2,404,453 Osborne July 23, 1946 2,415,798 Pye et al Feb. 11, 1947 OTHER REFERENCES McPherson and Hendersons General Chemistry, pp. 514-515, 3rd ed., Ginn and Co.,' N. Y.

Handbook of Chemistry and Physics, 28th ed., pp. 460-461, 464-465, Chem. Rubber Pub. Co., Cleveland, Ohio.

Hackhs Chemical Dictionary, 2nd ed., p. 860, P. Blackistons Son and Co., Inc., Philadelphia.

Claims (1)

1. THE METHOD OF PURIFYING AN ALKALI METAL HYDROXIDE WITH RESPECT TO ALKALI METAL CHLORATE, WHICH COMPRISES CONTACTING A MIXTURE OF SAID HYDROXIDE AND SAID CHLORATE WITH AN ALKALI METAL SULFITE AT AN ELEVATED TEMPERATURE.