AU466752B2 - Method of recovering a rare earth phospher - Google Patents

Description

2 9,8 3 1

Method of recovering a rare earth phosphor.

The invention relates to a method of recovering

a rare earth phosphor from a mixture which comprises this

phosphor and at least one sulphide and/or selenide of zinc

and/or cadmium. Furthermore, the invention relates to a

rare earth phosphor obtained by such a method, and to a

cathode-ray tube which comprises such a rare earth phosphor.

Cathode-ray tubes for the display of coloured

images, particularly coloured television images, generally

have a luminescent picture screen which may be excited by

electrons originating from one or more electron guns present

in the tube. Such a luminescent screen generally has a large

number of phosphor groups which are arranged on the surface

of the screen in accordance with a certain pattern. Each

phosphor group is built up from a number of, for example,

three phosphor elements in the form of lines or dots which

upon excitation by electrons emit light of a given colour,

for example, green, blue and red, respectively.

Different methods are known for the manufacture

of these colour screens. In a method which is frequently

used the starting material is an aqueous suspension which

comprises a certain phosphor to be provided on the screen

and a photosensitive material, for example, polyvinyl alhocol

and ammonium bichromate. A quantity of this suspension is

provided on the screen whereafter a large part of the suspen-

sion (the suspension excess) is removed again. The material

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which is left on the screen has the shape of a thin layer having certain dried and subsequently exposed by radiation arranged in wavelengths through an apertured mask which is in the mask corre- the vicinity of the screen. The apertures formed. Under the influ- spond to the phosphor elements to be is polyme- ence of the radiation the photosensitive material so that at these rized in the irradiated areas of the screen water. Poly- areas the polyvinyl alcohol becomes insoluble in areas of the merisation is not effected in the non-exposed from screen, so that the material can readily be removed (so-called deve- these areas by means of rinsing with water remains on the lopment) and the desired phosphor pattern to be screen. This method is repeated for the phosphors a different further provided while every time exposing from

direction, so that the desired pattern of phosphor groups is

formed.

In the above-described method only a small portion the aid of a of the original phosphor material provided with residues suspension is left on the screen. The manufacturing removed from t.he formed consisting of the suspension excess have screen and the development residues removed by rinsing

a considerable value especially when a rare earth phosphor the is concerned. These rare earth phosphors, for example, oxides, known red luminescent europium cr samarium-activated are general.- vanadates or oxysulphides of a rare earth metal screen. As a result ly provided as the last material on the of pro- the manufacturing residues formed during the process

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viding the rare earth phosphor not only contain polyvinyl

alcohol and chromium compounds, but also phosphor particles

originating from previously provided phosphor elements as

impurities. These previously provided phosphors generally

consist of sulphides and/or selenides of zinc and/or cadmi-

um, for example, blue luminescent silver-activated zinc

sulphide and green luminescent silver-activated zinc cadmi-

um sulphide.

Also when using methods other than the one des-

cribed above for the manufacture of colour screens manufac-

turing residues are often obtained which contain the costly

rare earth phosphors. The solid substances present in the

manufacturing residues are generally first separated from

the liquid, for example, by spinning or filtration, whereby

a wet mixture is obtained which may be optionally dried. For

a further effective use of the rare earth phosphors present

in the mixture it is necessary to remove the impurities, par-

ticularly the traces of other phosphors, which influence the

emission colours, as much as possible from these residues.

A method of recovering a rare earth phosphor from screen

manufacturing residues is known, for example, from U.S.

Patent Specification 3,474,040. In this known method the

manufacturing residues are treated with a heated mineral a--id for example, HNO HCI or H 2 S0 4 The sulphides and/or sele-

nides present in the residues dissolve in the acid used and

are subsequently removed by rinsing. A drawback of the known

method is that a portion of the rare earth phosphor is also

4 466.752

29,831I.

In the case of a often dissolved and is consequently lost. cannot be used luminescent rare earth oxide the known method dissolves at all because the rare earth oxide substantially oxysulphides of in the acids used. Also the luminescent respect so that rare earths present difficulties in this

considerable losses may occur. to provide The object of the present invention is phosphors from an improved method of recovering rare earth of the known manufacturing residues in which the drawbacks it is possible method are largely obviated and with which oxides. to recover also luminescent rare earth recover- According to the invention a method of which contains this ing a rare earth phosphor from a mixture selenide of zinc phosphor and at least one sulphide and/or the mixture is mixed and/or cadmium is characterized in that with a saltoxydizingiat an increased temperature and is

at a temperature of subsequently heated for at least 1 hour thus obtained is wast- at least 3000 C, and that the product

ed with water.

In a method according to the invention the sul-

mixture are oxydized phides and/or selenides present in the

at an increased temperature to form the oxides of zinc and/'

and/or selenium oxidoQ. or cadmium and volatile sulphur oxides are formed whose During oxydation further reaction products salt to be used and nature is determined by the oxydizing a method according which are generally soluble in water. In by washing with to the invention they are completely removed

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water. The mixture obtained from screen manufacturing resi-

dues may be dried in advance in a method according to the

invention, but it is alternatively possible to start from a

wet mixture which is mixed with the oxydizing salt.

In many cases it is desirable to remove the oxides

of zinc and/or cadmium formed during the heat treatment from

the reaction product. Therefore a method according to the

invention is preferred in which the product obtained during

the heat treatment is rinsed with a diluted acid before it

is washed with water. The oxides of zinc and cadmium complete-

ly dissolve in these diluted acids, so that they are separa-

ted from the rare earth phosphor.

A method according to the invention has the advan-

tage that it is more selective than the known method, that

is to say, the rare earth phosphor is attacked to a slighter

extent so that a larger yield of the desired rare earth

phosphor is possible. Furthermore, an important advantage

is that a method according to the invention is very suitable

notably for the recovery of rare earth oxides for which the

known method cannot be used.

The temperature at which the heat treatment is to

take place is dependent on the temperature at which the oxy-

dizing salt to be used starts to decompose and is generally

to be at least 3000 C. The heating period may be chosen within

very wide limits, on the understanding that when using a

certain oxydizing salt this period can be chosen to be shorter

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as the temperature is chosen to be higher. It has been found

that generally the heat treatment is to take place at least

V hour. The heating temperature and period are also dependent

on the quantity of contaminating sulphides and/or selenides

and on the resistance of the sulphides and/or selenides,

which resistance is also determined by the crystal shape and

the crystal size.

All salts decomposing while forming oxygen at the

reaction temperature may be used as oxydizing salts. However,

nitrates and/or halogenates, for example, the nitrates, chlo-

rates or bromates of alkali metals or of alkaline earth me-

tals are preferred. These salts are cheap and are available

in a pure form. As an Example of a chlorate, which may be

used advantageously, potassium chlorate may be mentioned

which decomposes at already relatively low temperatures (from

approximately 3000 C) in which 1 mole of KCIO gives rise to

1 mole of in water soluble KC1 and 3 atoms of oxygen. As an

Example of a nitrate, potassium nitrate may be mentioned for

which, however, higher temperatures are required (from apprc-

ximately 4000 KNOJ decomposes at these temperatures wbiJe

forming KNO 2 soluble in water and oxygen.

In a method according to the invention the oxyd'z-

ing salt is preferably used in a quantity which is larger

than is required in accordance with'the stoichiometry for

the oxydation of the sulphide and/or selenide present, so

that an intimate mixture of the phosphor mixture and the

salts, which are in a molten state at the reaction temperature,

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is obtained. A full reaction of the zinc and/or cadmium sul-

phides and/or selenides is then ensured.

In one preferred embodiment of a. method accord-

ing to the invention the heat treatment is performed for T

to 3 hours at a temperature of between 400 and 8000 C. In

fact, it has been found that for the oxydizing salts to be

used optimum results are obtained at these values of the

heating period and temperature.

A method according to the invention may be advan-

tageously used for recovering luminescent oxides and vana-

dates of rare earth metals. Many of the red luminescent

phosphors which are most generally used in cathode-ray tubes

for the display of coloured images belong to this group of

phosphors.

It is to be noted that the polyvinyl alcohol pre-

sent in the phosphor residues is completely oxydized to form

gaseous compounds such as carbon dioxide and water vapour

and that traces of chromium compounds which are often still

present are oxydized to form chromates which are soluble in

water. Furthermore, it is to be noted that phosphor mixtures

originating from screen failures and from cathode-ray tubes

rejected during the manufacture of the tubes may alternative-

ly be treated by a method according to the invention.

In order that the invention may be readily carried

into effect, it will now be described by way of several

Examples.

Example 1.

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gms of a phosphor mixture consisting of red

luminescent GdVO 4 -Eu, green luminescent (Zn,Cd)S and blue

luminescent ZnS which mixture contained 2.0 by weight of

ZnS and 0.01 by weight of CdS were mixed with 3 gms of

NaNO3 and subsequently heated for 1 hour at a temperature

of 7000 C.

Subsequently to cooling, the product obtained

was rinsed in 200 mls of aqueous 10% acetic acid solution

for 20 minutes at room temperature. The phosphor was then

washed electrolyte-free with water and dried.

The GdVO 4 -Eu obtained contained less than 0.005

by weight of sulphides and had a luminance upon electron

excitation which was equal to that of the original pure

vanadate. The quantity of gadolinium vanadate obtained was

98.5 by weight calculated on the original quantity.

Example 2.

In a manner analogous to that in Example 1, 15 gmin

of a phosphor mixture consisting of Y 0 3-Eu and approximate-

ly 2% by weight of ZnS were treated in which, however, 2 gms

of KBrO3 were used as an oxydizing salt and in which the

heat treatment was performed for 1.5 hours at 6000 C.

The Y2 0 3-Eu obtained contained less than 0.01

by weight of sulphides and bad a luminance upon electron

excitation which was equal to that of the original pure oxide.

The quantity obtained was in this case 99.2 by weight.

Example 3. In a manner analogous to that in Example 1,

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gms of a phosphor mixture consisting of Y202S-Eu and appro-

ximately 2% by weight of ZnS were treated in which, however,

gms of KCIO were used as an oxydizing salt and in which

the heat treatment was performed for 2 hours at 450° C.

The Y202S-Eu obtained contained less than 0.005

by weight of sulphides and had a luminance upon electron

excitation which was 95 of that of the original yttrium

oxysulphide. The quantity obtained was in this case 90.5

by weight.

10- 466.752

Claims (1)

1. 29,8 3 1 n/.

   THE CLAIMS DEFINING TIt INVENTION ArU AS FOLLOWS:

   I. A method of recovering a rare earth phosphor

   from a mixture which contains said phosphor and at least

   one sulphide and/or selenide of zinc and/or cadmium,

   characterized in that the mixture is mixed with a salt/

   oxydizing at'an increased temperature and is subsequently

   heated for at least V hour at a temperature of at least

   300° C, and that the product thus obtained is washed with

   water.

   2. A method as claimed in Claim 1, characterized

   in that the product obtained is rinsed with a diluted acid

   before it is washed with water.

   3. A method as claimed in Claim 1 or 2, characteriz-

   ed in that a nitrate is used as an oxydizing salt.

   4. A method as claimed in Claim 1 or 2, charact-

   erized in that a halogenate is used as an oxydizinG salt.

   A method as claimed in Claim 1, 2, 3 or 4, cha-

   racterized in that the oxydizing salt is used in a quanti-

   ty which is larger than is required in accordance with the

   stoichiometry for the oxydation of the sulphide and/or

   seienide present.

   6. A method as claimed in any of the preceding

   Claims, characterized in that the heat treatment is perform-

   ed for to 3 hours at a temperature of between 400 and

   800° C.

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   7. A method as claimed in any of the preceding

   Claims in which the rare earth phosphor is a member from

   the group of the oxides and vanadates.

   8. A method of recover ng a rare earth phosphor,

   substantially as herein describe

   9. A rare earth phosphor obtained by a method as

   claimed in any of the preceding Claims.

   A cathode-ray tube for the display of coloured

   images provided with a rare earth phosphor as claimed in

   Claim 9.

   Dated this seventh day of June 1971

   N.V. PHILIPS' GLOEILAMPENFABRIEKEN

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