2,784,061 United States Patent Office Patented Mar. 5, 1957 2 the preparation of the oxides of sodium. The first, direct oxidation of the metal in air or oxygen produces both the 2,784,061 oxide and peroxide, but production of the superoxide PROCESS FOR PRE PARNG ALKAL METAL requires prolonged oxidation by means of gaseous oxygen OXDES AND PEROXDES at high temperatures and pressures. The objection to such procedures is the requirement of metallic sodium George L. Cuaningham, Cleveland Heights, Ohio, assignor which must be produced by an expensive electrolysis. A to Horizons incorporated second proposed method, the dehydration of sodium hy No Drawing. Application December 6, 1954, droxide has not been found to be feasible. A third sug Serial No. 475,843 10 gested method which is disclosed in U. S. Patent 1,961,160, is based on the distillation of dilute sodium amalgams to 3 Claims. (CI. 23-184) produce pure sodium which is then oxidized by means of pure oxygen. it has been found virtually impossible to ob This invention relates to a new and useful process for 5 tain pure sodium by this process because of several opera the preparation of metallic oxides. More particularly it tional obstacles inherent in the patented process. The relates to a process whereby metallic oxides, peroxides method involves the distillation of relatively large amounts and superoxides may be inexpensively prepared in the of metallic mercury per unit of sodium produced. The form of pure compounds. separation is rendered difficult because the vapor pressure Many processes have been suggested for the prepara 20 of mercury decreases in a very marked fashion with rela tion of oxides of the various metals. Generally speaking, tively slight increase in the concentration of the sodium. one common approach involves the direct oxidation of the Thus, in very concentrated amalgams, the vapor pressure metal. An obvious disadvantage of methods based on of mercury is very low and when the temperature is direct oxidation of the metal is that to be commercially raised the vapor pressure of the Sodium approaches that attractive, the metal itself must be available at a reason 25 of the mercury which renders the removal of me ury able price. Another suggested approach is the decompo alone difficult and almost impossible. Accordingly, sition of oxygen containing compounds such as carbonates, sodium and the other alkali metals made in this fashion nitrates, Sulphates, phosphates, oxalates, tartrates and the are contaminated with considerable amounts of mercury, like, either with or without the presence of reducing agents and the method has not found a wide acceptance. such as carbon. Such methods are of limited application, 30 Since the aforementioned patented process discloses and have not been found suitable for the direct prepara that when an amalgam is distilled, two liquid layers are tion of the oxides of most metals. Still another approach obtained, and the published phase diagrams for the alkali involves electrolytic production of the metals followed by metal-mercury systems show no regions of concentrations oxidation of the metal. Still another procedure which has or temperatures where two liquid layers would exist, the 35 inability of the process to produce pure alkali metal is beenamalgams. suggested is the oxidation of relatively dilute apparently inherent therein. One object of my invention is to produce the oxides, I have found a method whereby most oxidic derivatives peroxides or superoxides of various metals by a simple of the alkali metals can be prepared at relatively low process not Subject to the disadvantages present in the temperatures in a relatively simple manner. According prior art. to the method I have invented, a pool of alkali metal 40 amalgam is formed in any convenient manner, in which A more specific object of my invention is to produce the the alkali metal concentration is chosen after consulting oxides, peroxides and Superoxides of some of the alkali the alkali metal-mercury system diagram, so as to be metals by a process which does not employ the alkali higher than the concentration at which a high melting metal, in the form of a pure metal, perse. solid product exists. in the case of sodium, for example, These and other objects are accomplished by a process 45 the Na-Hg system published in Mellor's Treatise on In in which an amalgam is prepared containing the metal organic and Theoretical Chemistry indicates that at about whose oxidic derivative is to be produced and the amal 5% sodium, a compound melting at about 353 C. exists, gam is then oxidized under carefully controlled conditions corresponding to NaHg2. The phase diagrams for the . by direct contact with oxygen or an oxygen containing gas. other alkali metals exhibit a similar configuration, the in a preferred embodiment of my invention a relatively 50 maximum-dystectic-point being shown at about 50 atomic dilute amalgam is obtained by electrolysis of a compound percent for lithium (LiHg), 25 atomic percent for potas of the metal whose oxidic derivative is sought and this sium (KHg3), 33 atomic percent for cesium (CSHg2) and dilute amalgam is added to a reservoir of a more concen 14 atomic percent for rubidium (RbHg6). The pool of trated amalgam which is then oxidized and from which amalgam of the desired concentration is prepared in any the oxide derivative is readily removable. 55 convenient manner. One convenient method is to plunge While the method of my invention is applicable gen pieces of alkali metal, held in an inverted cup, under the erally to the formation of most, although not all, of the surface of a body of mercury, in which the pieces will oxidic derivatives of the metals, it has been found to be readily dissolve. Another method by which the pool of particularly useful for the preparation of oxidic com amalgam may be prepared is by the carefully controlled pounds of the alkali metals. Since the preparation of 60 distillation of dilute amalgams. Whatever the manner sodium oxide, sodium peroxide and sodium superoxide in which it is formed, in accordance with my invention, may be taken to be representative of the practice of my there is fed into the prepared pool of amalgam, either invention, the process will be hereinafter specifically de continuously or intermittently, dilute amalgam formed in scribed with reference to these three compounds, it being a manner well known in the art, and the pool is subjected understood that except for incidental variations in the 65 to a controlled oxidation to effect the formation of the specific process conditions (e. g., higher temperatures, desired alkali metal oxidic product, the rate of oxidation different concentrations, etc.), this description applies with . being controlled so that the amount of alkali metal re equal force and effect to the preparation of oxides, perox acted is approximately equal to the amount of alkali metal ides and superoxides of the other alkali metals generally furnished to the pool in the form of a dilute amalgam. insofar as they are amenable to the described treatments. 70 Sodium amalgam can be produced by any suitable At least three distinct methods have been proposed for method. For example, sodium amalgam is presently 2,784,061 usra 4. made commercially by the electrolysis of a substantially still, by any suitable type of heat exchanger. In this way. saturated solution of sodium chloride at about 90 C., the heat of vaporization of the mercury may be largely using a graphite or platinum ancde and a mercury cathode. recovered. Chlorine and sodium amalgam formed at the anode and The still in which the pool of combined amalgams is cathode respectively, are recovered. In accordance with being distilled also serves as the vessel in which the sodium present practice such a ceil operated at about 4 volts (in or other alkalimetal is oxidized. The oxidation is effected comparison with about 7 volts in a fused sodium chloride by means of a suitable oxidizing agent, preferably air or cell) will have a current efficiency of about 95%. Cor oxygen, which must be carefully added to avoid the oxi rosion of the apparatus is negligible. The cell operates dation of the mercury. Mercury can be distilled at tem with brines which may be relatively impure, and over 95% peratures of about 350° C. in the presence of air without of the sodium chloride in the brine can be converted into any appreciable oxidation of the mercury. In the pres chlorine and Sodium amalgam. Other cells, e. g. a cell ence of even trace amounts of sodium or other alkali employing aqueous sodium hydroxide could be electro metals, mercury under otherwise similar conditions is lyzed whereby oxygen as well as the desired amalgam rapidly oxidized. could be obtained. 5 I have found that in spite of the aforesaid difficulties, The sodium amalgam produced in such cells usialiy the alkali metal in the amalgam may be controllably oxi contains about 0.1% sodium (by weight). At slightly dized to form the oxide, peroxide or, in the case of potas lower efficiencies, amalgams with up to about 0.5% (by sium, cesium or rubidium, the superoxide, by carefully weight) of sodium can be produced. The production of controlling the factors influencing the reaction, particu amalgams with more than 1% sodium is not generally 20 larly the temperature, oxygen partial pressure and alkali practiced because the efficiency of the cell is materially metal concentration. By proper control of these vari reduced. Furthermore, the formation of a product which ables, oxidation of the mercury may be prevented or any is solid at the cell temperature places an upper limit on mercury which may be oxidized, may be decomposed the amount of sodium which the amalgam can contain, as as rapidly as it forms. may be readily seen from the sodium-mercury phase 25 The relative ease with which sodium reacts with oxy diagram. gen prevents the system from approaching a condition It has been proposed to form the oxides of the alkali such that mercury would be in equilibrium with oxygen metals by oxidation of the dilute amalgam. With very and an oxide of mercury, but in the event that the oxide dilute amalgams such as those above described, separa did form, it would be readily perceived because of its ration of the relatively minute amounts of sodium oxide 30 red-yellow color, it would only be necessary to lower the or peroxide from the large amount of mercury has been oxygen pressure, or raise the temperature, or otherwise found to be difficult. Furthermore, the presence of the alter the operating conditions, so that the reaction excess mercury tends to decompose some of the sodium 2HgOa22Hg--O2 proceeds to the right instead of the left. oxide or peroxide. Reaction rates are slow. When In practice the oxygen admitted to the system is caused amalgams having the desired concentration of alkali metal 35 to enter the mercury still at the surface of the heated are oxidized in accordance with my invention, that is, amalgam, or at a point slightly below the Surface. The with simultaneous addition of a dilute amalgam, these heat of formation of the oxidic derivative of sodium, or difficulties are avoided. From the sodium-mercury phase other metal, is thus effectively recovered at a point where diagram it will be seen that at about 5% sodium (by it assists in the distillation of the mercury without incur weight) the amalgam has a maximum melting point of 40 ring any tendency to spatter or bump in the amalgam. approximately 353 C. At this point a solid having the Since the specific gravities of the various oxidic deriva composition NaHg2 is the solid phase. Accordingly the tives are all considerably lower than the specific gravity dilute amalgam is added to an amalgam of such con of the amalgams, the oxidic derivatives will float on the centration that the composite always possesses a con surface, from which they can be readily removed by centration somewhat greater than 5% sodium, e.g. more mechanical means, such as a skimming tool. The fol than about 7% sodium by weight and preferably greater lowing example will serve to further illustrate the prac than about 10% sodium whereby the formation of undue tice of my invention under one set of operating condi amounts of any solid phase and the difficulties this en tions. Obviously those skilled in the art will appreciate tails are avoided. Thus, when the sodium amalgam is that to produce other-oxygen derivatives of other metals, concentrated in accordance with my invention, the possi 50 the operating conditions may be suitably modified, as bility of forming a thickamalgam containing solid NaHg2 necessary, without departing from the intended scope is minimized. of my invention. In accordance with my invention a dilute sodium Example amalgam is prepared by electrolysis in a manner well 55 Apod of sodium amalgam containing approximately known in the art. It may be produced in an electrolytic 10% sodium by weight (49.2 mol percent Na) was pre cell such as that described in the aforesaid Moulton Pat pared by feeding lumps of metallic sodium into mercury. ent 1961,160, into which the electrolyte is fed through To this pool, 11,500 parts of dilute sodium amalgam an interrupter to avoid short circuiting the cell and from previously prepared in a conventional electrolytic cell which the amalgam is withdrawn, through another inter 60 employing a mercury cathode was continuously added. rupter. A pool of more concentrated sodium amalgam The dilute amalgam contained about 0.4% sodium, by is also prepared, with a sodium content of preferably weight. A vacuum of about 50 millimeters of mercury about 10% by weight, although in some instances, it may and a temperature of about 450°C. was maintained in be found advantageous to employ amalgams with a higher the still. While approximately 11,450 parts by weight concentration of sodium. The prepared amalgam is . of mercury were distilled off and recovered, 78 parts of charged into a mercury still, provided with means to ad sodium-peroxide were produced which were Scraped of mit the dilute amalgam either continuously or intermit the pool and recovered substantially free from any con tently. One particularly suitable arrangement is to shower the dilute amalgam into the vapor space in the still from tamination. a point near the top of the still. The still is maintained In the foregoing operation it should be noted that the under reduced pressure by means of a vacuum pump. stray currents are to be avoided in the electrolytic pro The same pump is advantageously employed to compress duction of the dilute amalgam. This may be accom the mercury vapor drawn off from the still, which will plished by employing an intermittent feed for the Sodium condense at elevated temperatures dependent upon the chloride brine and an intermittent discharge for the brine pressure. The condensed mercury may be employed to leaving the cell as described in Moulton Patent 1961,160. heat the dilute amalgam prior to its admission into the ' The mercury fed into the still in the form of a dilute 2,784,081 5 6 amalgam and the mercury returned to the electrolytic cell amalgams to produce the desired oxidic compound and after distillation and condensation of same is handled in recovering the oxidic compound. like manner, i. e. as semicontinuous streams, enabling the 2. The process of claim 1 in which the dilute amalgam entire operation to be conducted continuously. is continuously introduced into the composite liquid It will be observed that except for the metal employed 5 amalgam. when the initial pool of amalgam is prepared, the fore 3. A process for preparing sodium peroxide which going process does not require the use of the expensive comprises: preparing a relatively dilute sodium amalgam alkali metal as such, but instead converts readily pre containing up to 0.5% by weight of sodium; forming a pared and inexpensive dilute amalgams into products of pool of a concentrated liquid amalgam of sodium, con high purity for which commercial uses are widespread. 10 taining more than 7% sodium by weight; introducing the I claim: dilute liquid amalgam into the said pool of concentrated 1. A process for preparing oxidic compounds from the amalgam to form a composite liquid amalgam in which group consisting of monoxides and peroxides of an alkali the relative amounts of dilute and concentrated amalgam metal which comprises: preparing a relatively dilute are proportioned to insure that the resulting concentrtaion amalgam of said metal, having a content of less than 0.5% 5 of the sodium is greater than the concentration corre by weight of said metal; forming a pool of a concentrated sponding to NaHg2; maintaining the concentration of the liquid amalgam of a metal from the group consisting of sodium in the mixture of amalgams greater than the con alkali metal amalgams in which the concentration of the centration corresponding to NaHg2 by distilling mercury said metal is substantially greater than the concentration from the liquid pool at a rate not substantially in excess corresponding to the said metal-mercury compound with of the rate at which mercury is introduced into the pool a maximum melting point; forming a composite liquid as dilute amalgam, oxidizing the mixture of amalgams amalgam wherein the relative amounts of dilute liquid to produce sodium peroxide and recovering the sodium amalgam and concentrated liquid amalgam are chosen peroxide. to insure that the resulting concentration of the said metal in the composite liquid amalgam is substantially greater References Cited in the file of this patent than the concentration corresponding to the metal-mer cury compound with a maximum melting point by intro UNITED STATES PATENTS ducing the dilute liquid amalgam into the said pool of 1961,160 Moulton ------June 5, 1934 concentrated amalgam; maintaining the concentration of 2,158,523 Fleiderer ------May 16, 1939 the metal in the composite liquid amalgam greater than 30 2,648,596 Schechter ------Aug. 11, 1953 the concentration corresponding to the metal-mercury compound with a maximum melting point by distilling OTHER REFERENCES mercury from the liquid pool at a rate not substantially J. W. Mellor: "A Comprehensive Treatise on In in excess of the rate at which mercury is introduced into organic and Theoretical Chem...,' vol. 4, 1923 ed., pp. 35 1010-1018 inclusive and pp. 1030-1034, Longmans, the pool as dilute amalgam; oxidizing the mixture of Green & Co., New York.