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~ 2,585,461 Patented Feb. 12, 1952 UNITED STATES PATENT OFFICE 2,585,461 METHOD OF MAKING FLUORESCENT ZINC OXIDE Benjamin L. Hirsch, Ottawa Hills, Ohio No Drawing. Application January 4, 1946, Serial No. 639,153 11 Claims. (Cl. 23-—148) 2 The invention relates to ?uorescent material, particularly pigments, and the preparation thereof. . Ordinary zinc oxide may exhibit a very faint ?uorescence under ultra-violet light. As pointed out by Eisenbrand and Siewert in Archiv der Pharmazie 1934, vol. 272, pages 440-451, the ?uo rescence exhibited by ordinary zinc oxide is gen ‘ vapor lamp, having a wave-length of 2537 Ang strom units. It has particular sensitivity to a wave-length of 3650 Angstrom units. It ?uo resces with a brilliant white light which has a yellowish-green cast and therefore is “soft” or pleasing to the eyes. It does not appreciably darken or lose its ?uorescence upon long exposure to ultra-violet radiation and high humidity. erally so faint that it can scarcely be detected. The ?uorescence of this material reaches its In the same publication, Eisenbrand ‘and Sie 10 'inaximum brilliance instantly upon exposure of Wert show that the ?uorescence of zinc oxide can the material to the exciting radiation, and is cut be increased by heating it in an atmosphere of 401T very sharply upon cessation of the exciting hydrogen or an atmosphere of hydrogen and radiation. The material thus is suitable for the water vapor. Similar results are reported by The production of television screens as well as coat British Thomson-Houston Company in British 15 ings for?uorescent lamps. The quick “cuto?” Patent No. 558,213 and by Beutel and Kutzelnigg in Monatsheft fiir Chemie, 1932, vol. 61, pages 69 makes the material highly advantageous for mili tary use in the production of ?uorescent dials and to 86. ' charts. Heating zinc oxide in an atmosphere contain An oxide of zinc in ?uorescent form embody ing hydrogen, as described in these publications, 20 ing the invention, except for the fact that it is gives extremely variable results. The intensity ?uorescent, has the same properties as ordinary of the ?uorescence of a product thus prepared zinc oxide, 1. e., it is a snowy white material very is very rarely more than about half that of the useful as a pigment, has high “hiding power” lowest-intensity ?uorescent pigments that are in and tinting power because of its ?neness and is‘ commercial use. As pointed out by Eisenbrand 25 an excellent re?ector of day light or other light. and Siewert, the particle size of zinc oxide in An oxide of zinc in ?uorescent form embody creases rapidly during the heating in an atmos ing the invention may be obtained in various par phere containing hydrogen. This increase in lticle sizes ranging down to' particles so ?ne as particle size seriously impairs the value of the zinc oxide as a pigment, and substantial reduc tion of the particle size by grinding impairs the ?uorescence. Eisenbrand and Siewert recom mend that appreciable ?uorescence be taken as an indication of coarseness of any zinc oxide on to be barely visible under an ordinary microscope. 30 The material of extremely ?ne particle size is of the type commonly called an “amorphous pow der” but it may consist of minute crystals. Although grinding thus is not necessary to pro duce ?neness in a material embodying the inven the ground that they found ‘appreciable ?uores 35 tion, other materials such as pigmentsmay be cence to be unobtainable without great increase ground with a material embodying the invention in particle size. l for the purpose of producing an intimate mix The principal object of the invention is the ture. preparation of an oxide of zinc in a state in which A ?uorescent pigment embodying the invention it has greatly improved and uniformly high ?uo 40 may be incorporated like the best zinc oxide pig 'rescence and very ?ne particle size. More speci?c ments in paints, enamels, lacquers, printing inks, objects and advantages are apparent from the plastics, ceramics, textiles and the like. In order description, which merely discloses and‘ illlus to produce various colors of ?uorescent light or’ trates the invention and which is not intended re?ected visible light, the present pigment may to impose limitations upon the claims. , Properties and uses of the product An oxide of zinc prepared in accordance with be mixed with various other ?uorescent or non ?uorescent pigments. For example, a zinc sul phide that ?uoresces blue or bluish white can be mixedwith a ?uorescent oxide of zinc in .various the invention ?uoresces with a brightness ap proportions to give a white or variously tinted proximating that of the best inorganic ?uores 50 ?uorescence. A ?uorescent oxide of zinc may be cent pigments in commercial use, upon exposure to cathode ray, near ultra-violet radiation having a Wave-length of about 3,000 to about 4,000 Ang; strom units, and shorter wave-length ultra-violet mixed with such non?uorescent pigments as whitelead, lithopone, titanium dioxide and dia tomaceous earth.‘ The present pigment does not react with lead or iron pigments, and the effect radiation as low as the radiation of a mercury 55. of mixing it with other pigments is merely to 2,585,461‘ ' 3 ment may be modi?ed by using it as a base in luminated by “black light.” Markings made by chalk containing the present pigment have added visibility when illuminated by daylight. the preparation of color lakes, as by adsorption of dyes on the surface of the ?uorescent oxide The present pigment may be used to give added brilliance to motion picture screens, lighting re— dilute it so as to reduce the intensity of the light produced by its ?uorescence. The present pig of zinc. ' When used in ?uorescent paper, ?uorescent ?ectors, shoe whiting, paints for exterior deco rating, golf balls, tennis balls and the like. The pigment may also be used in under coats for paints, ?uorescent printing inks or ?uorescent phosphorescent paints. ' powder for dusting on surfaces, the present pig When used in photographic paper or ?lm, the ment may be illuminated by “black light” or day 10 present pigment provides a background of im light to produce many useful effects. Activation. of a ?uorescent pigment embodying the invention proved re?ective, power and affords a means of increasing the speed and contrast of the emul by means of “black light” may be used to pro? sion. vide selective illumination of markings without Useof the present pigment as an inside or out any accompanying glare in the case of ?uorescent 15 automobile instrument panel dials and knobs, ?uorescent paving bricks or other ?uorescent tramc markings, railroad signal markings, model ing clay and night targets for archery and other games. The present pigment may also be used to produce “invisible" markings such as laundry markings, which can. beobserved only. under “black light” (near ultra-violet radiation). The present pigment may be used in the manu side coating for incandescent lamp bulbs im proves‘ the quality of light emitted by converting ultra-violet radiation to visible light and gives the light a pleasing greenish cast. The present pigmentv has been found to be highly advantageous for use in cosmetics. The human skin has. a natural ?uorescence. that..is masked by face powders and other cosmetics which tend to produce a dull appearance. It facture of paper or decalcomanias or may be 25 has been found that a powder base, cream, used in coating, sizing or impregnating paper. Because the present pigment may be made much ?ner than other ?uorescent pigments, luminosity may be obtained by the use of smaller amounts powder or other cosmetic containing the present pigment gives the appearance of a natural glow, because of the ?uorescence of the pigment. A cosmetic containing the present pigment provides in a matrix such as paper sothatthestrength 30 an attractive glowing color similar tov the. glowing of the paperris not impaired asit is when it is necessary to employ larger amounts of other coarser pigments. Paper so prepared may be color of a healthy skin, whereas, ordinary cos metics produce only a ?at or non?uorescent color. The present pigment is believed to be the first ?uorescent pigment sufficiently inert and “black light,” but such paper has the advantage 35 ?ne to be suitable for general use incosmetics. used in applications requiring illumination by of improved brilliance when illuminated by ordi Although ?uorescent pigments heretofore known nary daylight. A photograph printed on such paper has a pleasing glow or re?ection caused by the generation of ?uorescent light, which makes the details of the photograph stand out with ex are sensitive to theultra-violet radiation present in daylight, they would be too dangerous for. use in cosmetics because of their toxiccharacter. ceptional clarity. Such paper is useful for pho tographs and drawings from which photoengrav Nature of the product The preferred product embodying the invention ings or blueprints are made. Photographs and drawings printed on ordinary paper may be is an oxide of zinc that when subjectedto ultra? violet radiation having a wave length of 253'? dusted with the present pigment to bring out the 45 angstrom units ?uoresces with an intensity sub details of the photograph or drawing in a similar stantially as great as phosphorescent zinc sul manner. An analogous effect may be produced phide. For example, one oxide of zinc embodying by suspending the present pigment in offset solu the invention has been found to ?uoresce under tions which are sprayed upon freshly printed such radiation with an intensity identical with matter. 50 that of New Jersey Zinc Company’s phospho Ordinarypaper printed with an ink containing rescent zinc sulphide 2330, which is a standard the present ?uorescent pigment may be illumi grade, the radiation being produced by a quartz nated by “black light” and photographed topro mercury arc lamp, and the intensity of the ?uo duce “direct positives” in photoengraving and rescence being measured with a Macbeth i1lumi~ similar applications. Finger prints made with 55 nometer. Under radiation. of the same wave length, an oxide of zinc embodying the invention such an ink are likewise useful for the production ?uoresces with an intensity not less than about of “direct positives” by photographing. ‘The one-fourth that of-synthetic willemite. (Under present pigment may also be used to make “carbon paper” for producing copies that can be illumination by ultra-violet light of 2537 A; wave length, theinstant zinc oxide ?uoresces with a read by “black light.” relative intensity of 100; zinc sul?de .2330 with a In order to utilize the ultra-violet radiation relative intensity of 100 and synthetic willemite present in daylight and to provide more efficient lighting, a pigment embodying the invention may with a relative intensity of‘ 370, measured with a be used in coatings for Venetian blinds and for Macbeth illuminometer.) " interior decorating and as a pigment for awnings, 65 The same sample of ?uorescent zinc oxide em-s bodying the invention, under radiation having a wall paper, linoleum, oilcloth, plaster and con wave length of 3650 angstrom units, has been crete. found. to ?uoresce with about 4.0.per cent of the Because of its ability to absorb ultra-voilet intensity of the. same standard grade of ‘P151051 light, transforming it into visible light instead, of heat, the present pigment may be used for 70 phorescent zinc sulphide, the radiation being fur nished by a quartz monochrometer. At both of. protective purposes in rubber compositions and these wave lengths the sample of ?uorescent zinc in paints, and also in coatings for preventing overheating of gasoline storage tanks. oxide was found to ?uoresce with an intensity more than twice that of New‘ JerseyZinc Com-1 The present pigment may be usedto produce pencils or chalk for making markings to be il~ 75 pany’s ?uorescent .zinc sulphide _No.. 2265'. and 2,585,461 about 70 per cent greater than that of New Jersey preferred atmosphere containing a reducing gas’ 2110 Company’s ?uorescent zinc sulphide No. is an atmosphere consisting of 28% carbon mone 35. Other samples of ?uorescent zinc oxide em bodying the invention have been found to ?uo resce with even greater intensity. oxide, 3.5% carbon dioxide, 66% nitrogen and 2.5% hydrogen by volume, although an atmos phere of pure carbon monoxide may be used. When such comparison is made, the zinc oxide being tested preferably is sifted through a ?ne An oxide of zinc embodying the invention is not known to have any chemical property dif screen into the container in which it is to be ferent from that of ordinary zinc oxide. The only tested, so as to keep it in a ?uffy condition. physical property by which an oxide of zinc em 10 The type of French process zinc oxide that bodying the invention may be distinguished from may be thus identi?ed as a proper starting ma other zinc oxides is the much greater intensity terial is relatively rare among commercially ‘ of ?uorescence of an oxide of zinc embodying the available French process zinc oxides. However, invention. any French process zinc oxide may be converted, The ?uorescence of a zinc oxide embodying the 15 by a very simple step into a proper starting ma invention may be due to the presence of isolated terial for use in preparing a ?uorescent product zinc atoms or isolated molecules of a suboxide of in accordance with the invention. Such step con zinc in the crystal lattice. However, as in the sists in exposing a French process zinc oxide to case of other phosphores, any foreign atoms or an atmosphere that contains sulphur dioxide and molecules that may be the source of the ?uores 20 is substantially free from gases that react with cence are so few in number that they cannot be the zinc oxide otherwise than by reduction. The identi?ed by any chemical or physical means. concentration of sulphur dioxide in such atmos An oxide of zinc embodying the invention con phere may vary from a trace to 100%. sists in the highly ?uorescent product of the ex The temperature at which the zinc oxide is posure at temperatures of incandescence, to an 25 exposed to such atmosphere may be any tempera atmosphere that contains a gas capable of re ture from room temperature to temperatures at ducing zinc oxide and is substantially free from which the zinc oxide is incandescent. It is pref gases that react otherwise with the zinc oxide, of erable to carry out this step at room tempera a French process zinc oxide that under said con ture for the ‘sake of convenience and also because ditions agglomerates no more than it would under 30 the particle size of zinc oxide gradually increases similar conditions in an atmosphere of sulphur at high temperatures. ’ dioxide, preferably a French process zinc oxide .The time required for such step is simply a time that contains a trace of sulphur dioxide. long enough to impart to the zinc oxide the A preferred oxide of zinc embodying the in properties which are described above as necessary vention consists in the highly ?uorescent prod 35 to make the zinc oxide a proper starting material uct of the exposure of a French process zinc oxide for the preparation of the ?uorescent product. at temperatures of incandescence to an atmos The time required varies with the concentration phere that contains sulphur dioxide and a gas of sulphur dioxide in the atmosphere employed, capable of reducing zinc oxide and is substantially which as stated above may vary from a trace to free from gases that' react otherwise with the 40 100%. zinc oxide. Such step preferably is carried out by exposing A ?uorescent product embodying the invention the French process zinc oxide to an atmosphere may be made as ?ne as ordinary ?uorescent zinc of substantially pure sulphur dioxide at room sulphide pigments and much ?ner than phos temperature. Under these conditions an exposure phorescent zinc sulphide pigments. The par 45 of about one minute is su?icient if the zinc oxide ticles of an oxide of zinc embodying the invention contains the normal moisture content of about may have an average diameter from about two to .075% by weight. If the zinc oxide contains less about three microns and a surface area from than about .075% moisture, it is desirable to add about 0.4 to about 0.5 square meter per gram, as moisture thereto in order to shorten the time for determined by nitrogen permeability. 50 which it is necessary to expose the zinc oxide to Requirements of starting material an atmosphere containing sulphur dioxide. In the rare cases in which a commercial French process zinc oxide is found to be a proper starting The present invention is based upon the discov material for use in the preparation of a ?uores~ ery that an oxide of zinc having greatly improved and uniformly high ?uorescence may be prepared 55 cent product in accordance with the invention, the zinc oxide may have been exposed to sulphur by using a particular type of zinc oxide as the dioxide in one of the ?nal stages of its manufac starting material. More speci?cally, the starting material used in the practice of the present in vention is a French process zinc oxide (i. e., a ture. Such a zinc oxide may show a trace of sulphur dioxide when subjected to the starch zinc oxide made by burning zinc metal) that may 60 iodine test. A U. S. P. grade French process zinc oxide that contains a trace of sulphur dioxide is very rare because it is di?icult to produce a guish it from other French process zinc oxides. French process zinc oxide of acceptable purity A French process zinc oxide suitable for use as the when sulphur .dioxide is present in one of the starting material for the preparation of a ?uo rescent product in accordance with the invention 65 ?nal stages of its manufacture. The presence of a trace of sulphur dioxide in a French process may be distinguished by the fact that when main zinc oxide is a reliable indication that the zinc tained at temperatures of incandescence, in any oxide is a proper starting material for use in the atmosphere that contains a gas capable of re~ preparation of a ?uorescent product in accord ducing zinc oxide and is substantially free from gases that react otherwise with the zinc oxide, 70 ance with the invention. The starting material for use in the preparation agglomerates no more than it would under simi of a ?uorescent product in accordance with the lar conditions in an atmosphere of sulphur di oxide. For making such comparison, the pre invention, which must have the properties here be identi?ed by certain properties which distin ferred temperature is 900° 0., the preferred time inbefore described, is preferably U. S. P. grade. at such temperature is one half hour and the 76 A C. P. grade. reagent grade or spectroscopically 2,535.;481 pure gradeof zinc.oxide.cambeusedbut ordinarie ly, ‘does. lot. give . an improvement. in. results that would compensate for its. greater’ cost. Preparation Qf-the ?uorescent product A; fluorescent product is prepared in accord: The atmosphere herein; referred: to:.iS1the..-at-. mosphere: with which the; zinc oxide at :tempera: tures of incandescence is actually in contact: Such atmosphere may contain oxygen, but. it would not ordinarily contain an excess; of oxygen described, at temperatures of incandescence in over that requiredto oxidize all of the carbon" monoxide and hydrogen present, because in that case the atmosphere-in contact with the 111C311‘: descent zinc oxide would notbe likely to'contain contact with an atmosphere that contains a, gas ' anycarbon monoxide Orv-hydrogen. The presence capable of reducing zinc oxide, and is substan tially free from. gases that react otherwise with the,‘ zinc. oxide, untilwa??uorescent productcan; be. obtained upon cooling. of an excessof oxygen inan atmosphere in con? ance with the, invention by maintaining; a: start: ing material having-the properties hereinbetore, tact-with which zinc .oxide is attemperaturesof incandescence ordinarily prevents the. formation of a ?uorescent product. » . Thetime for whichthezinc~ oxide mustbe In the commercial production of zinc?oxide as maintained at temperatures; of, incandescence: in; practiced heretofore a- largeexcess of air isusecl: contactvwith _ such’ an atmosphere varies; with-the to expedite the oxidationofthezine at .the high speci?c temperature used. The temperaturesoi, temperatures employed, and under. such condi‘: incandescence of zinc oxidenare well known, and tions the: presence of. a gaszcapable of. reducing.» the preferred temperatures of incandescence for 2.0v zinc oxide is precluded. use in the practice of the invention are tempera The concentration of the gas capable of _re-._ tures from about 600° C. toabout 1300_°- C. At tern: ducing zinc oxide inthe atmosphere used-in-the. peratures less than about 650° C. prolongedcon practice of the invention may be from a. trace tact with an atmosphere containing a gas capable to 100%. The suf?ciency of a trace of such'a'gas. of reducing zinc oxidemay be necessary beforea. 25 is understandable in View of the fact that any ?uorescent product can be obtained upon cooling. reaction productof such a. gasin the final .?uor Heating at the higher temperature,gradually in escent product is present in such a minuteamount creases the particle sizeoff the;producll. It is sel-. that it cannot be identi?ed by any chemical or: dom necessary to maintainthe zinc-oxide at tem— physical means. The concentration of : the. gas peratures of incandescenceior more than an hour 30 capable of reducing zinc oxide. in the atmosphere. and sometimes not for more thanabout?fteen to is not critical so long as-the zinc oxide. is main thirty minutes,v A preferred temperature range tained in contact with such atmosphere at tem is fromhabout 8000 C. to about 1000° C. At still peratures of incandescence until a ?uorescent higher temperatures the, increase in theparticle product can be obtained uponcooling. size-of the zinc oxide is more rapid; At tempera 35 Of course it is desirable notito lose any sub-, tures greater than about 1300° C. zinc'oxideis lost by volatilization, stantial amount of the zinc. oxide by reducing itito. metallic zinc. Metallic zinc thusproduced ordinarily is lostbyvolatilization and therefore trolled at will and variesdirectly Wlthi thetem does not. materially contaminate. the product. perature and time of heating. A very dense prod 40 Reduction of. the oxide to metalllic zinc. usually uct suitable for use in a dental cement may be can be prevented by cessation of the heating-as. made if the heating is at 1300° C.‘ for eight to soon as fluorescent product can beobtained upon ten hours._ Quick cooling assists in keeping the cooling. Reduction of. the zinc oxide to metallic particle size to a minimum. zinc also. can be prevented by the presence of a Thermal shock such as that. produced; by sufficient proportion of carbon dioxide. or water quenching the hot product in water may, also, be vapor in the atmosphere used. used to produce. ?ne particle size. The preparation of an oxideof'zinc in fluores Cooling to produce a‘ fluorescent product may cent form can be- carried. out in an atmosphere be carried out in an atmosphere containing a gas consisting of carbon dioxide containing only a, capable of reducing zinc oxide, but there is-no minute concentration of carbon monoxide. Such. loss of luminosity upon quick cooling. inmair or an atmosphere actually is capable of oxidizing quenching inwater. If the hot product is allowed zinc to zinc oxide. The atmosphere used may to'fall into water thewater may be permitted" to consist of pure, carbon monoxide, but, the pres boil, and a stream of cool air may be passed. enceof carbon dioxide, formed by the reduction through the water if desired. Other» pigments 55 of minute quantities of the'zinc oxide, is not pre such as White lead, lithopone, titaniumndioxideor cluded even when the atmosphere supplied com, .diatomaceous earth may be in suspension inv the sists of pure carbon monoxide. water,» with or without a wetting agent, or may An atmosphere consisting of purecarbon mon be added subsequently to the water order to oxide or pure hydrogen, or’ a mixture of carbon produce a dispersion of the ?uorescentzincfoxide 60 monoxide and hydrogen in any proportion what with the other pigments. The water-‘may be re-: soever gives satisfactory results when brought in moved from theproduct by decantation or ?l to contact with the zinc oxide in the praoticeof The particle size, of the product may be‘ con tration followed by drying. the invention. An atmosphere consistingof hyThe atmosphere in contact with which the drogen and water. vapor gives even better results oxideof zinc is maintainedat temperatures oi 65 than the use of pure hydrogen, andthe water incandescence in the practice of the invention is vapor in such an atmosphere may be present in an- atmosphere containing: a} gas capable-of reduc! any concentration from a trace upto saturation. ing zinc oxide such as carbon monoxide or hydro. An atmosphere containing carbon monoxide gen, but is not necessarily a reducing atmosphere. may be obtained in any convenient manner, for It has been found that such'atmosphere-may con 70 example by the incomplete combustion or crack tain an excess of carbon dioxide or water vapor ing of a carbonaceous material such as carbon, so that it, is an atmosphere capable-goiroxidizing carbon monoxide, methane, ethane, propane,.bu zinc, so long as it contains at least a, trace-of tane, benzene, oil,_producer gas, water gas,.,coal, carbon monoxide, hydrogen or other gas capable of, reducing zinc oxide. coke, wood, or cellulosic material... Anatmose 75 phere containing; carbon monoxidemay also. be 2,586,461 produced by contacting air or carbon dioxide with nance of the zinc‘ oxide at temperatures of in carbon at an elevated temperature or by reducing carbon dioxide in any desired manner. An at mosphere that gives good results is one consist phere permits a highly ?uorescent product em bodying the invention to be obtained upon cool- - candescence in contact with the same atmos ing substantially of about 35% carbon monoxide and 65% nitrogen obtained by passing air in con tact with hot charcoal. An excellent product may be produced by maintaining zinc oxide in con mg. Nitrogen and other inert gases may be present in the atmosphere used, and the atmosphere may mixing up to about one per cent by weight of such a sulphur-containing material with zinc oxide from which a fluorescent product is to be The sulphur dioxide present may be introduced into the atmosphere from sulphur-containing fuel, or any sulphur-containing material, such as tact with such an atmosphere at about 900° C. for free sulphur, a sulphide, or _a sulphite, which upon about 25 minutes and then cooling rapidly in the 10 heating provides sulphur dioxide. ‘The sulphur same atmosphere or in air. dioxide in the atmosphere may be provided by contain both carbon monoxide and hydrogen, either with or without carbon dioxide and water 15 produced. “Zinc oxide containing sulphur di vapor. . oxide,” as used herein, includes zinc oxide con The atmosphere supplied to the chamber con taining sulphur dioxide 'in the form of zinc sul taining the incandescent zinc ovide may contain phite. reducing gases ather than carbon monoxide and The sulphur dioxide may also be formed as hydrogen, such as methane. If methane is used, 20 the result of the reduction of a sulphate by car- ' however, care should be taken to avoid serious bon or another reducing agent. _ contamination of the product with carbon formed An important advantage of the present process by decomposition of the methane. Contamina is that it is not necessary to add any activating tion of the zinc oxide with free carbon’ may cause substance to the zinc oxide in order to obtain a excessive formation and loss of metallic zinc. 25 fluorescent product. . Such contamination may be avoided by introduc Furnaces or crucibles in which the zinc oxide is ing just enough air or oxygen to oxidize the car heated may be made of a wide variety of mate bon or to prevent its deposition. _ rials, such as nickel, porcelain, graphite, Alun The atmosphere that is supplied to the cham dum, quartz, clay or even iron. ' ' ber containing the incandescent zinc oxide may 30 A convenient method of preparation consists contain any other gas capable of reducing zinc in passing a French process zinc oxide that has oxide, such as ethane, propane, butane or any other hydrocarbon, or a carbohydrate. Such the properties necessary to make it a proper reduction. starting material through a rotary furnace at a gases, like methane, tend to decompose at the op temperature of incandescence while maintaining erating temperature, and the introduction of a 35 in the furnace an atmosphere containing a gas Slight amount of air, oxygen, or water vapor capable of reducingr zinc oxide. Another method usually is required to prevent serious contamina-' consists in dropping such a zinc oxide through tion of the product with carbon. Because of the a tall vertical mu?ie containing such an atmos decomposition of such gases the atmosphere ac phere, in which the zinc oxide becomes heated to tually in contact with the zinc oxide in each 40 incandescence. In either case the product pref case may contain hydrogen or carbon monoxide. erably is cooled in the air or in water. . ' Nitrogen and other inert gases may be present An atmosphere containing carbon monoxide in any proportion in the atmosphere used. Of may be provided in a chamber in which the zinc course any atmosphere to which the zinc-oxide oxide is 'maintained at temperatures of incan is exposed should be substantially free from gases 45 descence either by introducing into the chamber that react with the zinc oxide otherwise than by a stream of gas containing carbon monoxide. such The term “substantially free from as producer gas or water gas, or by maintaining gases that react with the zinc oxide otherwise than by reduction” is used herein to mean that in the chamber solid carbonaceous material which reacts with oxygen or carbon dioxide in the concentration of such gases is low enough so I the chamber to produce carbon monoxide. A very simple and‘ advantageous ‘method em that under the conditions of operation used the zinc oxide is not substantially contaminated by reaction products of such gases with the zinc ox ide, such as a chloride or other zinc compound. In summary, the atmosphere to which the zinc oxide is exposed at temperatures of incandescence in the practice of the invention may have any bodying the invention for preparing an oxide of zinc in a highly ?uorescent state constitutes a simple modi?cation of an ordinary French process by which zinc oxide is produced from zinc snelter. In such a modi?ed French process embodying the invention, the zinc oxide is ini composition whatsoever so long as it contains at tially produced in the form of a cloud in an at least a trace of a gas capable of reducing zinc mosphere that contains at least a trace oflsul oxideand is substantially free from gases that 60 phur dioxide, and a gas capable of reducing zinc react otherwise with the zinc oxide. oxide, and the zinc oxide, as initially‘ formed, is If such atmosphere contains at least a trace at temperatures of incandescence in contact with of sulphur dioxide, the zinc oxide that is ex such atmosphere so that a ?uorescent product posed to such atmosphere in the practice of the is obtained upon cooling. invention may be any French process zinc oxide When a French process zinc oxide is brought whatsoever, because the mere presence of a trace into contact with an atmosphere containing sul or more of sulphur dioxide in the atmosphere phur dioxide and a gas capable of vreducing zinc converts the French process zinc oxide into a oxide, it may be necessary to hold the zinc oxide proper starting material for the preparation of a at temperatures of incandescence in contact with ?uorescent product in accordance with the inven 70 such atmosphere for ?fteen minutes or longer tion. Since the French process zinc oxide is in order to complete the conversion of the zinc properly conditioned or converted into a proper oxide to a highly ?uorescent product. However, starting material by the presence of sulphur di in a modi?ed French process embodying the in oxide in the atmosphere containing a gas capable vention, the zinc oxide is actually formed in such of reducing zinc oxide, mere continued mainte an'atmosphere, and it is in such intimate contact 2,535,461 11 with vthe atmosphere as it is formed that the production of a‘highly ?uorescent product is' sub stantially instantaneous. Such'a zinc oxide may be considered to be highly ?uorescent at the in stant when it is formed. The gas capable of‘reducing zinc oxide that is present in the atmosphere used in a modi?ed French process embodying the invention may consist of carbon monoxide or hydrogen result ing ‘from the reaction of zinc metal with carbon 10 dioxide or water vapor to form the zinc oxide. 12 C. or above, ‘for example at 800° C. for thirty minutes, and cooled, it no longer shows the pres ence of sulphur dioxide, but it is vstill a proper starting material. A proper'starting material, prepared as described in the present example, when subjected to the comparative heating test described in Example 1, agglomerates substantial ly no more in the charcoal gas atmosphere than in the sulphur dioxide atmosphere. Example 3 However, it is preferably that the atmosphere About 100 grams of a French process zinc oxide used contain carbon monoxide or hydrogen in excess vof that formed in the production of the zinc oxide, although the excess should not be that has been found, by the tests described in Example 1, not to be a proper starting material for the preparation of a highly ?uorescent prod— great enough to interfere substantially with the initial formation of the zinc oxide by oxidation uct is held in an electric furnace in an open por? celain crucible at 800 to 1000° C. for one-half hour in a slow current of gas consisting of 95% of zinc metal. ‘In a modi?ed French process embodying the inventionzinc vapor may be produced by vaporiz ing spelter. The resulting'vapor is then burned by reacting the zinc vapor with ‘carbon ‘dioxide or- water vapor in an atmosphere containing sul— 'p'hur dioxide, ‘a vvfluorescent product being -pro— duced as hereinbefore described. Example 1 Two 50 gram samples of a ‘French process U. S. P., C. P. or reagent grade zinc oxide are hydrogen and 5% sulphurdioxide. Under these conditions the sulphur dioxide ?rst converts the zinc oxide into aproper starting material and the hydrogen ‘then converts’the material into .a product ‘that when :cooled is highly ?uorescent. At .the end of the period of oneehalf hour the crucible with its contents is removed and allowed to cool in the air to obtain thehighly ?uorescent product. In the foregoing procedure the time and temperature may be varied within wide limits by passing air through a bed of charcoal at about 600° C. The samples are then allowed to cool and examined. If the sample heated in the char depending upon the size of the batch, the nature of the zinc oxide and the design of the furnace. The concentration of sulphur dioxide that gives the best results varies with the speci?c nature of the zinc oxide used. The hydrogen may be re placed in whole or in part by carbon monoxide or by charcoal gas. If in the foregoing procedure the sulphur dioxide is omitted from the atmos phere, the ?uorescence of the resulting product is reduced at least v50 to 80% and is not sufficient coal gas atmosphere shows ‘no substantial ag for commercial use. sifted into'two quartz dishes. One dish is held at 900° C..for one-half hour in an atmosphere of sulphur dioxide, and the other dish is held at 900° C. for one-half hour in an atmosphere con sisting of 28% carbon monoxide, 3.5% ‘carbon di oxide, 66% nitrogen and 2.5% hydrogen, obtained glomeration into dense lumps when compared with the other sample, the zinc oxide sampled is a proper starting material for the preparation of a highly ?uorescent product. A French process U. S. P., C. P. or reagent grade zinc oxide rarely is shown‘by this test to be a proper starting mate rial. If, as is even more rarely the case, a, French Example 4 Av procedure is carried out which is the same as that of ‘Example 3 except that 0.3 gram of zinc sulphide is added to the starting material and the sulphur dioxide is omitted from the gas in troduced into the furnace. At the furnace tem perature the zinc sulphide produces sulphur di process zinc oxide is found to contain sulphur di oxide within the charge so that the ?nal' product oxide by the starch-iodine test, it may be consid is similar to that obtained in Example 3. ered ‘a proper starting ‘material and the forego ing comparative heating test is unnecessary. A 50 Example 5 spectroscopically pure grade of French process zinc oxide, if it is not a proper starting material, About 10 grams of a French process zinc oxide agglomerates into hard, sandy granules rather than dense lumps when heated in a charcoal gas atmosphere in the foregoing comparative heating test. Example 2 If, ,as is ordinarily the case, a French ‘process zinc oxide is shown, by the tests in Example 1, not to bea proper starting material, .it is treated as follows: Its moisture content is determined by heating a small sample to constant weight at 110° C. If the moisture content is less than about 075% by weight, thezinc oxide is steamed or moisture is added to bring the moisture content up to that value, inorder to accelerate the action of sulphur dioxide in the next step. The zinc oxide is sifted through .a ?nescreen to'?uff it up that has been identi?ed as a proper starting ma terial by a test described in Example 1 or has been prepared as described in Example 2 is placed in a small electric arc furnace provided with .car bon electrodes. The furnace is provided with a vent just large enough to relieve any pressure caused by generation of gases during the heating operation. The zinc oxide is heated for several minutes under the arc. The atmosphere in the furnace contains carbon monoxide formed from the carbon electrodes. The zinc oxide, upon'be ing cooled in the air, is highly ?uorescent. Example 6 About 10 grams of a French process zinc oxide that has been identified as aproper starting ma terial by a test described in Example 1 or has and then a 50 gram batch is agitated for one min~ ute in a 250 ml. ?ask containing sulphur dioxide 70 been prepared as described in Example 2 is placed in an open porcelain crucible and is held at tem-' at atmospheric pressure. The resulting product peratures from 800 to 1000° C. in an electric fur is shown to contain sulphur dioxide by the starch nace for about 15 minutes while a slow stream .of iodine test, and is a proper starting material for pure carbon monoxide or hydrogen is passed the preparation of a highly ?uorescent product. If it is then held at a temperature of about 500° 75 through the furnace. ‘Upon being cooled in the 2,685,461 14 air the resulting product is highly ?uorescent and of very ?ne particle size. thereby maintaining a substantially constant zinc oxide particle size during the process. Example 7 8. A method of imparting ?uorescence to a French process zinc oxide, that comprises the steps of (1) contacting the zinc oxide with an atmosphere containing sulfur dioxide and (2) then contacting the zinc oxide, in an incandes cent state, with a gaseous reducing atmosphere for imparting ?uorescence thereto, step (1) caus About 10 grams of C. P. zinc showing traces of sulphur on analysis is placed in a combustion tube and held at about 1000° C. While a stream of Water vapor is passed slowly through the tube. The zinc vaporizes and reacts with the water vapor to form incandescent zinc oxide and hydro 10 ing the zinc oxide to maintin a substantially con gen. Since the sulphur in the zinc is converted stant particle size during step (2) . to sulphur dioxide, the atmosphere in-contact 9. A method of producing a ?uorescent zinc with which the zinc oxide is formed in an incan oxide, that comprises exposing a sulfur dioxide descent state contains sulphur dioxide as well as containing French process zinc oxide pigment, The zinc oxide formed under these 15 in an incandescent state, to .a gaseous reducing conditions, when cooled, is highly ?uorescent. atmosphere, the sulfur dioxide causing the main The water vapor introduced into the tube may tenance of a pigment particle size during the contain hydrogen in a concentration insufficient process. to prevent the oxidation of the zinc to zinc oxide. 10. A method of preparing an oxide of zinc in 20 a highly ?uorescent state that comprises (1) in Example 8 hydrogen. About 20 grams of C. P. zinc is placed in a clay boat and is held at 800 to 1000° C. in an electric furnace while there is passed through the fur nace a gas consisting of 11% carbon monoxide, 25 14% carbon dioxide, 74% nitrogen and 1.0% hy drogen. A small amount of sulphur dioxide is bled into the furnace with such gas. The zinc vaporizes and the zinc vapor oxidizes to zinc oxide, When removed from the furnace and cooled in troducing a French process zinc oxide into a heating zone and heating the zinc oxide to in oandescence, (2) introducing into the heating zone a gaseous reducing atmosphere, so as to impart ?uorescence to the incandescent zinc oxide, and (3) bringing the zinc oxide in contact with a sulfur dioxide-containing atmosphere prior to imparting ?uorescence thereto so as to maintain a substantially constant zinc oxide par~ ticle size during the imparting of ?uorescence thereto. the air, the product is highly ?uorescent, Various embodiments of the invention may be devised to meet various requirements. Having described the invention, I claim: 1. An improvement in the process of producing 11. A method as claimed in claim 10 wherein the introduction of the zinc oxide into the heat ing zone and the heating to incandescence are accomplished simultaneously by oxidation of zinc ?uorescent zinc oxide by exposing incandescent 35 vapor. French process zinc oxide to a gaseous reducing BENJAMIN L. HIRSCH. atmosphere, characterized by the step of contacts ing the zinc oxide with an atmosphere contain REFERENCES CITED ing sulfur dioxide within a time ranging from 40 The following references are of record in the a time prior to and up to and including the time ?le of this patent: when the zinc oxide reaches a state of incan UNITED STATES PATENTS descence, thereby maintaining a substantially constant zinc oxide particle size during the ex Number Name Date posure of the incandescent zinc oxide to a gaseous 715,238 Bartlett ___________ __ Dec. 9, 1902 reducing atmosphere. 2. A process as claimed in claim 1 in which the gaseous reducing atmosphere contains carbon monoxide [and carbon dioxide. ' 3. A process as claimed in claim 1 in which the gaseous reducing atmosphere contains hy drogen. 4. A process as claimed in claim 1 in which 1,339,544 Breyer et al. ____ __ May 11, 1920 1,523,417 1,574,317 1,647,279 Grine ___________ __ Jan. 20, 1925 Utley et a1. ______ __ Feb. 23, 1926 De Saulles _______ __ Nov. 1, 1927 1,670,169 Singmaster ______ __ May 15, 1928 1,781,702 1,941,569 2,141,228 2,408,475 Pierce __________ __ Nov. 18, McCraven ________ __ Jan. 2, Singh ___________ __ Dec. 27, Nickle ____________ __ Oct. 1, the zinc oxide is produced in situ by oxidation reaction of zinc with an oxygen donor gas present in the reducing atmosphere, said oxygen donor 55 Number gas being present in an amount insu?icient to oxidize the reducing gases present. 558,213 5. A process as claimed in claim 4 in which the oxygen donor gas is steam. mosphere that contains sulfur dioxide, thereby maintaining a pigment particle size during the process. 7. A method of imparting ?uorescence to a Date Great Britain ____ __ Dec. 24, 1943 OTHER REFERENCES 6. A method of preparing fluorescent French 60 process zinc oxide pigment, that comprises ex— posing French process zinc oxide pigment, in an incandescent state and prior to substantial ag glomeration thereof, to a gaseous reducing at FOREIGN PATENTS Country 1930 1934 1938 1946 Beutel and Kutzelnigg, “Monatshefte Fur Chemie,” Band 61, pages 69-86 (1932). Eisenbrandt and Siewert, “Archives Phar macy,” vol. 272, pages 440-451 (1934). Mellor, “Modern Inorganic Chemistry,” 1939 65 edition, page 451. Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” vol. 4, pages 513, 514 (1923). . Leverenz, “Phosphors Versus the Periodic Sys French process zinc oxide, that comprises heat ing the zinc oxide to incandescence in a gaseous 70 tem of Elements,” Proceedings of the Institute of reducing atmosphere that contains sulfur dioxide, Radio Engineers, May 1944, pages 256-263.