Patented Apr. 10, 1945 2,373,257 UNITED STATES PATENT OFFICE 2,373.25 PROCESS FOR PRECIPIANG ALEAL METAL DEROKOE S. Irving E. Muskat, Akron, Ohio, assignor to Pitts burgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvarais, No Drawing. Application March 4, 1942, era No. 433,359 5 Claims. (C. 23-302) This invention relates to a method of concen a small amount. of ammonia added to a solution trating an aqueous alkali metal hydroxide and of sodium hydroxide containing 40 to 50 percent is particularly related to the production of con of more of NaOH causes precipitation of solid centrated sodium hydroxide from an aqueous SO hydroxide which when separated from mother lution thereof. Sodium hydroxide as COInner 5 liquor possesses a concentration of 6U to 'u per cially produced, is prepared as a relatively dilute cent sodium hydroxide. By adding further solution and, in general, concentrated sodium hy mounts of ammonia, further quantitles of caus droxide is prepared by evaporation of Such solu tic are precipitated and the caustic may be more tions. The evaporation is conducted in metallic concentrated. Upon treatment of such a solution receptacles and, in general, because of the cor at a higher temperature, for example, 35 to 60° C., rosive nature of these solutions, it is found that or above, with ammonia, the mixture separates the solutions are objectionably contaminated with into two liquor layers, the lower of which con metallic impurities during the evaporation. tains a relatively high concentration of sodium Prior to the present invention, it has been hydroxide, a portion of water and a relatively known that concentrated sodium hydroxide could 6 Small amount of ammonia, while the upper layer be secured by cooling a more dilute solution in contains the major portion of the ammonua, a order to crystallize out of Solution a hydrate which portion of the Water, and a minor quantity of is more concentrated as to sodium hydroxide and caustic. Upon addition of a still further quantity recovering the hydrate. This process ordinarily of annonia, a solid hydroxide precipitates in the requires extensive cooling which is expensive. 20 presence of the two liquid layers. When a further Moreover, solutions of sodium hydroxide are ex 9tlantity of ammonia, is added, the two separate tremely viscous at temperatures at Which the liquid phases cease to exist and the solid hyr hydrate thereof normally crystallizes, and in con droxide remains in equilibrium with a single liquid sequence, crystals precipitating therefron remain phase. This hydroxide when recovered contains suspended therein and do not settle out of solute 25 69 percent or often in excess of 85 to 90 ercent tion. Due to the high viscosity of such solutions, of Sodium hydroxide. . a recovery of Crystals is extremely difficult and Thus, when about 0.1 to 1.1 parts of ammonia thus, the concentration of caustic by crystallizare are nixed with one part of a 50 percent solution tion of hydrates thereof has not achieved success. of sodium hydroxide in a suitable closed container In United States Patent No. 1961,590, granted 3. at 60 C, two liquid phases are formed which Sep to MacMullin, it is stated that Sodium hydroxide arate in the container as an upper and lower monohydrate can be precipitated from aqueous layer. Upon adding more ammonia, however, the solutions containing upwards of 20 percent of so two liquid phases merge and solid caustic exists ( dium hydroxide by treatment of the solution with in equilibrium with a liquid phase Containing annonia. However, this patent points out that 35 annonia, water, and caustic. This solid is of this process is objectionable because of the substantially high concentration, generally hav amount of ammonia required. ing a concentration of at least 69 percent, or In accordance with my invention, I have found above, and if sufficient ammonia is present, the that by treatment of aqueous solutions of alkali Solid may be substantially anhydrous caustic. metal hydroxide preferably containing at least There is a marked difference between the 40 percent, preferably not less than about 50 per anoint of annonia required to cause separation cent sodium hydroxide, only small amounts of of a pair of liquid phases or to cause precipitation ammonia are required to cause precipitation of a of Solid hydroxide from a solution Containing 50 solid hydrate and that it is possible to precipitate percent of more of sodium hydroxide and that hydrated hydroxide which is substantially higher required to cause precipitation from a solution in concentration (generally containing in excess containing only 20 to 25 percent sodium hy of 60 percent and often about 70 percent by weight droxide. of sodium hydroxide) than the solution under The following table illustrates the differences in going treatment. By treating such solutions and 50 the amounts, at a temperature of 25 to 300 C. recovering the solid hydroxide which is precipi r-ro-w------tated, Concentrated sodium hydroxide may be re covered in a simple manner with only a small Saif3 per SE Conc. of NaOH solin, percent go NES', quantity of ammonia and without contamination SEO so of the caustic. By use of a larger quantity of 55 iteys it crys annonia, a product substantially higher in con tallization "E Centration containing 75 to 100 percent NaOH: -Hr-sa-e- 16.7------e------5 9, 2 raay be obtained. 88: a .3 6, 6 The process may be regulated by controlling the 6. 0.80 2.7 temperature and the amount of ammonia present 60 5. 0.33 O8. in the system. At a temperature of about 25° C., 0.04 0.09 2 2,878,257 It will be noted that the amount of ammonia was recovered by filtration and was found to. required is dependent upon the concentration of contain about 90 per cent sodium hydroxide, the solution and also upon the amount of hy Although the process is particularly adapted to droxide to be precipitated from solution and the the use of anhydrous liquid ammonia, mixtures of temperature of the solution during crystalliza water and annonia, which contain sufficient an tion. Generally speaking, in treating solutions monia (in general about 65% NH3) to cause containing 40 percent or more of sodium hydrox precipitation of solid hydroxide may be used, if ide, it is found desirable to precipitate up to 50 desired. While this process has been described percent or more of the sodium hydroxide and for the most part with relation to sodium hy for most purposes, up to about two or three parts droxide and its hydrates, the process is equally of ammonia is used per part of Sodium hydroxide applicable to the preparation of any of the hy in solution. drates of other alkali metal hydroxide, such as In accordance with the present invention, I those of lithium or potassium hydroxide. have found that caustic may be effectively con Although the present invention has been de centrated by first treating the caustic with suf scribed in connection with the specific details of ficient ammonia to cause separation of two liq certain embodiments thereof, it is not intended uid phases, causing the two phases to merge and that such details shall be regarded as limitations precipitating solid sodium or other alkali metal upon the Scope of the invention, except insofar hydroxide. These phases may be caused to merge as included in the accompanying claims. by addition of further ammonia or by cooling This application is a continuation-in-part of the mixture to a suitable temperature, for exam application Serial No. 325,789, filed March 25, ple, below 35° C. to cause the liquid phases to 1940, now Patent No. 2,285,300, issued June 2, merge and formation of solid hydroxide of high 1942. concentration in equilibrium with a single liquid claim: phase. 1. A method which comprises treating aqueous This method is particularly desirable, involving Sodium hydroxide with a material of the group the step of first forming two liquid phases fol consisting of liquid ammonia and mixtures of lowed by the step of causing the two liquid phases liquid ammonia and water, the amount of am to merge, either by cooling or by addition of monia, being sufficient to insure the existence of a ammonia, in treating solutions of 40 or 50 per 30 liquid phase containing a major portion of the cent since in treating solutions of such high ammonia and a second liquid phase contain concentration with ammonia, it is found that ing a substantial portion of the hydroxide, add if a large amount of ammonia is added initially, ing more ammonia in an amount sufficient to caustic often precipitates in large lumps which cause the two liquid phases to merge and sepa when removed contain so much mother liquor 35 rating Solid hydroxide which is precipitated in the that little or no concentration is secured. In mixture from the ammonia liquor. accordance with the present method, it is pos 2. A method which comprises treating aqueous sible to eliminate this difficulty. w alkali metal hydroxide with a material of the . The process may be conducted batchwise or group consisting of liquid ammonia and mixtures continuously in any suitable type of closed appa 40 of liquid annonia and water, the amount of am ratus. Suitable apparatus and methods of treat monia being sufficient to insure the existence of ment have been described in my application a liquid phase containing a major portion of the Serial No. 179,334, filed December 11, 1937, now ammonia and a second liquid phase containing Patent No. 2,196,594. When substantial amounts a Substantial portion of the hydroxide, adding of sodium chloride are present in the solution, 45 more ammonia in an amount Sufficient to cause a purified hydroxide is prepared by the present the two liquid phases to merge and separating process. This method has been described and Solid hydroxide which is precipitated in the mix claimed in my application Serial No. 179,336, ture from the ammonia liquor. filed December 11, 1937. The present invention, 3. A method of concentrating aqueous alkali however, is directed mainly to the concentration 50 metal hydroxide which comprises treating said of sodium hydroxide in various solutions, includ hydroxide with a material of the group consisting ing solutions which may contain little or no of liquid ammonia and mixtures of liquid am sodium chloride. monia and water, the amount of ammonia being The following examples are illustrative: sufficient to insure the existence of a liquid phase Eacample I 55 containing a major portion of the ammonia and - a second liquid phase containing a substantial 0.15 part by weight of anhydrous liquid an portion of the hydroxide and cooling the mixture monia was mixed with one part by weight of a to a temperature at which the liquid phases merge solution containing 50 percent sodium hydroxide and Solid hydroxide precipitates and separating at a temperature of 45 C. The mixture was 60 the solid from the ammonia. then cooled to 25° C. whereupon solid hydroxide 4. The process of claim 3 wherein the solution precipitated and the two liquid phases merged. contains at least 40 percent NaOH. Upon recovery of the solid it was found to con 5. A method which comprises treating aqueous tain 68 percent NaOH. (, . alkali metal hydroxide with a material of the 65 group consisting of liquid ammonia and mixtures Eacample II . of liquid ammonia and water, the amount of am One part by weight of anhydrous liquid an monia being sufficient to cause the formation of a monia, was mixed with one part by weight of a liquid phase containing a major portion of the solution containing 50 percent sodium hydroxide ammonia and a Second liquid phase containing a at a temperature of 70° C. . The mixture was 70 substantial portion of the alkali hydroxide, add agitated to prevent separation of the two liquid ing more ammonia while simultaneously cooling phases thus formed into layers. Thereafter, 19 until the two phases merge into a single liquid parts of additional ammonia was added and the phase, and recovering the Solid alkali hydroxide two liquid phases merged leaving solid hydroxide which is precipitated. and a single liquid phase. The solid hydroxide 75 IRVNG E, MUSKA.