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Патент USA US2585465

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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.
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