EP0872686A1 - Candoluminescent material, method of preparation and use as hard incandescent mantle for public lighting - Google Patents

Abstract

A novel incandescent material consists of an oxide mixture containing (in mol%) 50-70% ZrO2, 5-8% Y2O3, 13-20% MgO, 4-10% Li2O, 0.2-0.6% CeO2, 0-25% K2O and 0-1% WO3. Also claimed is preparation of an incandescent material by impregnation of a combustible woven fabric with a solution having pH 0.6-1 and containing zirconium, yttrium, lithium, magnesium and cerium salts in concentrations corresponding to a molar composition of 50-70% ZrO2, 5-8% Y2O3, 13-20% MgO, 4-10% Li2O and 0.2-0.6% CeO2, followed by combustion to eliminate the fabric and convert the salts to oxides.

Description

Technical area

The invention relates to a material non-radioactive candoluminescent usable for making candoluminescent hard sleeves public gas lighting.

The terms "candoluminescent hard sleeve" used here are intended to define sleeves lighting with characteristics and conditions of use different from those of the sleeves recreational lighting, for example for camping and for the garden, which are referred to below as sleeves flexible.

State of the art

Since the work of Carl Auer von Welsbach in 1893, candoluminescent sleeves were made lighting on a similar basic principle, i.e. by impregnation of a combustible fabric using of a solution containing a mineral forming, after combustion of the tissue, a solid state network giving in the flame of a gas an intense phenomenon of candoluminescence.

In the case of flexible sleeves, these are marketed in the form of impregnated fabrics and it is during their first use that the combustion of the tissue produces a solid state network thus acquiring the final shape and texture solid resistant to mechanical and thermal shock.

They are used for leisure lighting (camping, garden) by being mounted on a bottle of butane. Their weight in the calcined state is 60 to 90 mg. Their conditions of employment correspond to pressure gas of 1 bar, and a butane flow of 31 g / h in bottles. The mechanical stresses to which they are subject, are mainly vibrations and shocks due to transport in a caravan for example. Their mechanical strength must be at least 2 minutes at vibrations of 50 Hz. Their minimum illumination is 37 lux.

In the case of hard lighting sleeves public, these are marketed as hard sintered corresponding to the final form, ie for example of bulb preformed by an initial burning of so as to obtain a ceramic structure and this after installation on a suitable fastening system according to the structure on which the hard sleeves will have be installed (gas burners for urban lighting, for example).

Hard sleeves also have larger dimensions and weight (130 to 160 mg) that flexible leisure lighting sleeves (60 to 90 mg).

Their conditions of employment are different because they run on town gas with pressures of very lower gases (28 mbar). They also must have superior mechanical strength, i.e. withstand 500 cycles on a cam machine and a lifespan of around 200 to 400 h. Their lighting must also be at least 37 Lux.

The target markets are different, that of hard sleeves represents static use since mainly concerning public lighting in and the use of distribution channels gas at 28 mbar.

The appearance and packaging are different. The final aspect is characterized in one case by the necessity for the user to carry out sintering by pre-burning after mounting on the bottle butane whereas, in the case of the hard sleeve, the sintering is carried out in the factory after mounting on a ceramic ring that will allow setting up for use in the lighting position. The sleeves flexible cannot be used for application public lighting, the need to carry out the sintering and the method of fixing by knotting excludes them of this type of use.

Hard sleeves are specifically reserved one of the uses at fixed lighting points the main ones being public lighting. Sleeves this type equips street lamps in Berlin, Strasbourg, Sarlat and Grenoble. These floor lamps which are supplied with low-capacity natural gas heaters use 5 to 6 sleeves distributed over a crown, and they operate under gas pressure natural weak, around 2800 La, while the sleeves flexible use butane or propane gas under a higher pressure around 0.1 MPa.

Generally, the mineral used for the impregnation of the combustible tissue was based on thorium oxide ThO ₂ . However, the use and production of sleeves based on thorium oxide have several drawbacks because thorium is a naturally radioactive element, α-emitter, which has a period of 1.4.10 ¹⁰ years and gives by fixation various radioactive isotopes which are short-lived α, β or γ emitters including a radioactive gas thoron 220, leading to lead 208.

• lors de la phase de stockage des solutions thoriées destinées à l'imprégnation du tissu combustible,
• lors de l'imprégnation du tissu avec exposition et risque d'inhalation par les opérateurs,
• lors de la confection manuelle des manchons, et
• lors du changement et de la destruction des manchons dont les cendres ou imbrûlés sont dispersés.

Radioactive emissions occur:

• during the storage phase of thoriated solutions intended for impregnating the combustible tissue,
• during the impregnation of the fabric with exposure and risk of inhalation by operators,
• when making the sleeves manually, and
• when changing and destroying the sleeves in which the ashes or unburnt materials are dispersed.

Also, for a few years, the legislations implemented restrict the use of this product radioactive.

In recent years, we have therefore sought candoluminescent materials for the realization of light sleeves, which do not contain thorium.

Document FR-A-2 551 178 describes sleeves flexible glowing mechanical strength and improved durability, made from oxide yttrium to which cerium oxide is added and optionally magnesium oxide or oxide aluminum to inhibit crystal growth.

US-A-5,124,286 also describes glowing flexible sleeves with no thorium, which are obtained from a mixture of oxide zirconium, yttrium oxide, erbium oxide and cerium oxide.

Document FR-A-2,518,218 describes materials candoluminescent for flexible sleeves based on oxide zirconium and calcium oxide, containing optionally aluminum oxide and or oxide of magnesium and one or more oxides chosen from the oxides of iron, manganese, praseodymium and / or cerium. However, these compositions have a certain brittleness due to the presence of magnesium.

Document FR-A-2 560 604 describes a material candoluminescent for flexible sleeves having a higher brightness than the previous material, in reason for replacing calcium oxide with a mixture of yttrium and magnesium oxides. We obtain thus average luminosities ranging from 35 to 45 lux.

Flexible sleeves made from such materials have satisfactory characteristics, but when you want to use these materials for the making hard sleeves, the sleeves obtained do not do not have lighting characteristics (at minus 37 lux), impact resistance and duration of sufficient lives.

Indeed, the initial illumination of a sleeve hard using the candoluminescent material of FR-A-2 560 604 is only 27 lux.

• au poids plus important (double) du réseau cristallin d'oxydes après frittage du manchon dur (une masse plus importante nécessite une énergie calorique plus importante pour l'obtention de la même énergie lumineuse),
• à la pression du gaz utilisé, le débit et la capacité calorique du gaz de ville utilisé pour l'alimentation de manchons durs sont inférieurs à ceux du butane utilisé pour les manchons souples, et
• à la nécessité d'une lumière blanche. Par rapport aux manchons souples, les caractéristiques de blancheur de la lumière ont plus d'importance du fait du montage de plusieurs manchons dans un même bec de gaz, l'objectif étant la similitude avec une ampoule électrique.

This decrease in lighting performance is due to:

• the greater (double) weight of the crystal lattice of oxides after sintering the hard sleeve (a larger mass requires greater caloric energy to obtain the same light energy),
• at the pressure of the gas used, the flow rate and the calorific capacity of the town gas used to supply hard sleeves are lower than those of butane used for flexible sleeves, and
• to the need for white light. Compared to flexible sleeves, the characteristics of whiteness of light are more important due to the mounting of several sleeves in the same gas nozzle, the objective being the similarity with an electric bulb.

Furthermore, it is important for hard sleeves to have maximum life (for an illumination greater than 37 lux). Limits according to users range from 200 to 400 hours for an illumination of 37 lux with thorium sleeves.

Research has also been continued with a view to to obtain a composition of candoluminescent material suitable for making hard lighting sleeves audience with lighting characteristics, mechanical strength and satisfactory service life.

Statement of the invention

The object of the present invention is precisely a candoluminescent material usable for the making hard sleeves with these characteristics.

• 50 à 70 % d'oxyde de zirconium, ZrO₂,
• 5 à 8 % d'oxyde d'yttrium Y₂O₃,
• 13 à 20 % d'oxyde de magnésium MgO,
• 4 à 10 % d'oxyde de lithium Li₂O, et
• 0,2 à 0,6 % d'oxyde de cérium CeO₂,

According to the invention, the candoluminescent material is formed from a mixture of oxides comprising in moles:

• 50 to 70% zirconium oxide, ZrO ₂ ,
• 5 to 8% of yttrium oxide Y ₂ O ₃ ,
• 13 to 20% of magnesium oxide MgO,
• 4 to 10% of lithium oxide Li ₂ O, and
• 0.2 to 0.6% of cerium oxide CeO ₂ ,

According to an alternative embodiment of the invention, the candoluminescent material comprises in in addition to potassium oxide and possibly tungsten oxide.

1)

• 50 à 70 % d'oxyde de zirconium, ZrO₂,
• 5 à 8 % d'oxyde d'yttrium Y₂O₃,
• 13 à 20 % d'oxyde de magnésium MgO,
• 4 à 10 % d'oxyde de lithium Li₂O,
• 0,2 à 0,6 % d'oxyde de cérium CeO₂, et
• 0 à 25 % de potassium K₂O; et

2)

• 50 à 70 % d'oxyde de zirconium, ZrO₂,
• 5 à 8 % d'oxyde d'yttrium Y₂O₃,
• 13 à 20 % d'oxyde de magnésium MgO,
• 4 à 10 % d'oxyde de lithium Li₂O,
• 0,2 à 0,6 % d'oxyde de cérium CeO₂,
• 0 à 25 % d'oxyde de potassium K₂O, et
• 0 à 1 % d'oxyde de tungstène WO₃,

Such materials correspond respectively to the following compositions in moles:

1)

• 50 to 70% zirconium oxide, ZrO ₂ ,
• 5 to 8% of yttrium oxide Y ₂ O ₃ ,
• 13 to 20% of magnesium oxide MgO,
• 4 to 10% of lithium oxide Li ₂ O,
• 0.2 to 0.6% of cerium oxide CeO ₂ , and
• 0 to 25% potassium K ₂ O; and

2)

• 50 to 70% zirconium oxide, ZrO ₂ ,
• 5 to 8% of yttrium oxide Y ₂ O ₃ ,
• 13 to 20% of magnesium oxide MgO,
• 4 to 10% of lithium oxide Li ₂ O,
• 0.2 to 0.6% of cerium oxide CeO ₂ ,
• 0 to 25% potassium oxide K ₂ O, and
• 0 to 1% of tungsten oxide WO ₃ ,

• la candoluminescence du zirconium,
• la stabilisation de la zircone par l'yttrium qui améliore la résistance du réseau cristallin,
• la lumière plus blanche due à l'ajout d'oxyde de magnésium, et
• la longévité par l'ajout de tungstène.

This composition of candoluminescent material which differs in particular from the composition of the candoluminescent material described in FR-A-2 560 604 by a lower content of zirconium oxide, and by the addition of lithium oxide and optionally tungsten and potassium oxide, allows the production of hard candoluminescent sleeves with very superior initial lighting characteristics and a longer service life. The improved properties of this material are due in particular to the following properties of the oxides constituting it:

• candoluminescence of zirconium,
• stabilization of zirconia by yttrium which improves the resistance of the crystal lattice,
• whiter light due to the addition of magnesium oxide, and
• longevity by adding tungsten.

In addition, the addition of lithium and potassium allows to use the emission properties of these oxides in the visible range 300 to 8000 Å, mainly in blue and purple, to get a whiter light.

• 38 à 60 % de Zr,
• 8 à 14 % de Y,
• 9 à 18 % de Mg,
• 6 à 11 % de Li,
• 0,2 à 0,4 % de Ce.
• 0 à 36 % de K, et
• 0 à 0,7 % de W.

The composition of the candoluminescent material of the invention corresponds to the following molar percentages of metals:

• 38 to 60% of Zr,
• 8 to 14% of Y,
• 9 to 18% Mg,
• 6 to 11% of Li,
• 0.2 to 0.4% of Ce.
• 0 to 36% K, and
• 0 to 0.7% of W.

Hard sleeves prepared from this composition are very interesting because they meet with minimum lighting requirements of 37 lux, good impact resistance, light whiteness and lifetime.

The invention also relates to a method for preparing a candoluminescent material having the above composition.

- a) préparer une solution de sels de zirconium, d'yttrium, de lithium, de magnésium et de cérium, et ajuster son pH à une valeur de 0,6 à 1, les concentrations en sels de ladite solution étant telles qu'elles correspondent à un mélange d'oxydes ayant la composition molaire suivante :

• 50 à 70 % de ZrO₂,
• 5 à 8 % de Y₂O₃,
• 13 à 20 % de MgO,
• 4 à 10 % de Li₂O, et
• 0,2 à 0,6 % de CeO₂ ;

b) imprégner un tissu de filaments combustibles de ladite solution ; et

c) soumettre le tissu imprégné à une combustion pour éliminer le tissu et convertir les sels en oxydes.

This process includes the following steps:

a) prepare a solution of zirconium, yttrium, lithium, magnesium and cerium salts, and adjust its pH to a value of 0.6 to 1, the salt concentrations of said solution being such that they correspond to a mixture of oxides having the following molar composition:

• 50 to 70% of ZrO ₂ ,
• 5 to 8% of Y ₂ O ₃ ,
• 13 to 20% MgO,
• 4 to 10% of Li ₂ O, and
• 0.2 to 0.6% of CeO ₂ ;

b) impregnating a fabric of combustible filaments with said solution; and

c) subjecting the impregnated tissue to combustion to remove the tissue and convert the salts to oxides.

For the preparation of the solution in step a), different zirconium salts can be used, yttrium, lithium, magnesium and cerium, for example of chlorides or nitrates.

Generally, nitrates are used because that they are more soluble.

The total concentration of oxides in the solution is chosen so as to obtain a good hold sleeve mechanics. Preferably, it is 1.3 to 1.5 mol / l.

Generally the pH of the solution obtained at starting from soluble salts, especially nitrates, is very low, for example of the order of -0.5.

It is therefore necessary to adjust its pH to the desired value, in the range from 0.6 to 1, especially 0.8.

Several techniques can be used to make this adjustment, this neutralization acidity.

According to a first embodiment of the process of the invention, this can be carried out by means of ammonium carbonate. In this case, the pH of the solution is adjusted in step a) by adding ammonium carbonate or bicarbonate. The neutralization reaction is as follows:

In step c) of the process of the invention, the ammonium nitrate produced during neutralization will be decomposed into N ₂ O and H ₂ O and no additional element will be introduced.

According to a second embodiment of the process of the invention, the pH of the solution in step a) is adjusted by neutralization using potassium carbonate or bicarbonate. This corresponds to the following reaction: 2HNO ₃ + K ₂ CO ₃ → H ₂ CO ₃ + 2KNO ₃

In step c) of the process of the invention, the potassium nitrate will be converted to oxide what potassium, which is interesting because the addition of potassium is favorable for improving illuminance with a whiter light. This addition leads generally to be introduced into the mixture of oxides 20 to 25% by mol of potassium oxide.

According to a variant of this second embodiment of the process of the invention, the start of neutralization is carried out by the addition of a solution of tungstic acid (soluble in strong alkalis, potassium hydroxide for example), and the pH adjustment is completed by means of potassium carbonate or bicarbonate. The tungstic acid is then transformed into tungsten oxide WO ₃ and it is thus possible to improve both the white light illumination (addition of K ₂ O) and the lifetime of the sleeves (addition of WO ₃ ).

In this case, the concentration of tungstic acid in the KOH solution is such that it corresponds to the introduction of 0.2 to 1 mol% of WO ₃ into the mixture of oxides.

According to a third embodiment of the process of the invention, the pH of the solution in step a) is adjusted by treatment on an anion exchange resin in OH ^- form.

In the case where the salts are nitrates, this adjustment corresponds to the following reaction: R-OH + HNO ₃ → R-NO ₃ + H ₂ O where R represents the resin.

The resin in nitrate form RNO ₃ is then subjected to a regeneration cycle by KOH, which corresponds to the following scheme: R-NO ₃ + KOH → R-OH + KNO ₃

After preparation and adjustment of the pH of the solution, we use this to impregnate a textile fuel formed from combustible filaments.

The combustible textile used is viscose but could be a cotton-based textile, rayon, cellulose acetate or others natural or synthetic filaments such as those which are generally used for the realization of hard candoluminescent sleeves.

The textile can have the form of a canvas, gauze, tulle, veil etc. Preferably, it has a thread fineness of 300 DEN, a width flat from 6.5 to 7 cm and a weight of 14 g / m.

After the impregnation treatment, the fabric is put in the form of a sleeve and put in place on a fixing system adapted to the type of lamp on which it should be installed.

For this purpose, we take a length of fabric impregnated and placed on a heating mandrel widening of the woven tube, the woven tube is cut, widened to have a tablecloth. We put it on a guide support, then fix it on a ring ceramic fixing by means of a ceramic wire. We collect the blank from the sleeve thus fixed and we calcine by heating with butane.

After combustion, a hard sleeve is obtained which can be marketed and used for lighting public.

Other features and benefits of the invention will appear better on reading the following examples given of course as illustrative and not limiting, with reference to the drawing Annex.

Brief description of the drawing

The attached figure is a diagram representing the illumination as a function of time of two sleeves according to the invention.

Example 1

• 600 grammes de solution de nitrate de zirconium (NO₃)₃, 6H₂O) de concentration 20 % de ZrO₂,
• 82,8 grammes de nitrate d'yttrium (Y₂(NO₃)₃, 6H₂O) contenant 29,45 % de Y₂O₃,
• 71,4 grammes de nitrate de magnésium contenant 15,25 % de MgO,
• 2,4 grammes de nitrate de cérium (Ce(NO₃6H₂O) contenant 39,5 % de CeO₂,
• 6,6 grammes de nitrate de lithium (LiNO₃) contenant 43,23 % de Li₂O.

In this example, a candoluminescent sleeve is prepared from 1 kg of solution having a total molar concentration of oxides of 1.4437 mol / l and a pH of +0.8. For this purpose, we weigh:

• 600 grams of zirconium nitrate solution (NO ₃ ) ₃ , 6H ₂ O) with a 20% concentration of ZrO ₂ ,
• 82.8 grams of yttrium nitrate (Y ₂ (NO ₃ ) ₃ , 6H ₂ O) containing 29.45% of Y ₂ O ₃ ,
• 71.4 grams of magnesium nitrate containing 15.25% MgO,
• 2.4 grams of cerium nitrate (Ce (NO ₃ 6H ₂ O) containing 39.5% CeO ₂ ,
• 6.6 grams of lithium nitrate (LiNO ₃ ) containing 43.23% Li ₂ O.

This corresponds to the following concentrations: Mole oxide Mole Metal % oxide % metal ZrO ₂ 0.974 0.974 67.5 59.45 Y ₂ O ₃ 0.108 0.216 7.5 13.18 MgO 0.270 0.270 18.7 16.48 CeO ₂ 0.005 0.005 0.3 0.3 Li ₂ O 0.0867 0.1734 6 10.6 Total 1.4437 1.6389

The pH of the mixture obtained is -0.7.

The pH of the solution is adjusted to a stable value of 0.8 by adding 65 g of ammonium bicarbonate (NH ₄ ) ₂ CO ₃ , 6H ₂ O in small quantities and stirring so as to avoid even exceeding local the critical pH value of 1.0. Indeed, at pH = 1, the zirconium hydroxide begins to precipitate.

We then complete the solution to one kilogram with demineralized water.

We then impregnate a fabric of radiate with this solution, then we make a hard sleeve from the impregnated fabric and we submit it to combustion and sintering to obtain the hard sleeve suitable for use. We then determine the illumination of the sleeve, using a burner standard carrying the sleeve which emits at the end of a tube whose inner wall is blackened, towards a photocell connected to a luxmeter.

This gives an illumination of 50 to 53 lux with a yellowish light and a lifespan of about 25 hours.

Example 2

In this example, we follow the same mode as in Example 1 to prepare the impregnation solution, but we adjust the pH of this solution using an anionic resin, the resin Lewattit MP62 in OH form, adding it in small quantities to the solution with stirring. The amount required to adjust the pH, i.e. bring it from -0.5 to 0.8, is 500 g. We keep stirring for 30 to 45 minutes. We then complete the 1 kg solution with demineralized water.

Then prepare the hard sleeve candoluminescent in the same way as in Example 1, and we see that these lighting characteristics are similar to that of Example 1 while the network strength and integrity characteristics are improved. In this case, the solution presents a better stability and therefore efficiency of use most important.

Example 3.

In this example, the same procedure is followed as in Example 1, but the pH of the solution is adjusted by adding potassium carbonate. The quantity of potassium carbonate required to bring the pH of the solution from -0.75 to 0.8 is 60 g, ie 0.433 mol of K ₂ O. The carbonate is added in small quantities with stirring. When the pH value = 0.8 is obtained, the solution is made up to one kilogram with demineralized water.

The concentrations of oxides and metals in the solution, expressed in molar%, are as follows: Oxide % Oxide % metal ZrO ₂ 51.9 38.9 Y ₂ O ₃ 5.75 8.62 MgO 14.38 10.78 CeO ₂ 0.27 0.2 Li ₂ O 4.6 6.92 K ₂ O 23.1 34.58

We then use the solution manufacture of the candoluminescent hard sleeve. This one is carried out as in Example 1.

The illumination obtained with the sleeve is 60 to 65 lux with a whiter light than previously. This result is due to the presence of potassium (23.1% by mol of K ₂ O).

Example 4

In this example, the same procedure is followed as in Example 1, but the pH is corrected by means of a tungstic acid solution and then it is adjusted with potassium carbonate. For this purpose, 3.4 g of tungstic acid are dissolved in 100 g of 30% KOH solution with stirring (0.0136 mol of WO ₃ and 0.26 mol of K ₂ O) at a temperature of 60 ° C.

The total concentration will be 1.8767 moles of oxides or 2.5044 metal moles.

This solution is then added to the solution prepared by mixing the various nitrates as in Example 1, which brings its pH from -0.7 to about -0.2. To then obtain the desired pH of 0.8, about 25 g of potassium carbonate (0.188 mol of K ₂ O) is added while stirring. The solution is then made up to one kilogram by addition of demineralized water.

We then make a hard sleeve candoluminescent as in Example 1.

The solid solution after sintering has the composition:
1.4437 moles of (Zr, Y, Mg, Ce) + 0.0136 moles of WO3 + (0.26 + 0.188 moles of K2O) or 1.9053 moles of oxides representing 2.548 moles of metals.
Expressed in molar% in the solution, we obtain: Oxide % Oxide % Metal ZrO ₂ 51.12 38.22 Y ₂ O ₃ 5.67 8.48 MgO 14.17 10.6 CeO ₂ 0.26 0.2 Li ₂ O 4.55 6.8 WO ₃ 0.71 0.53 K ₂ O 23, 52 35.17

The illumination obtained with this sleeve is 70 to 85 lux with white light and a duration of life multiplied by 4 compared to Example 3.

An improvement in the illumination is therefore obtained further due to the presence of K ₂ O (23.5% by mol).

We then test the lighting properties sleeves produced in Examples 3 and 4, in function of time in hours.

The results obtained are given in the figure 1 which represents the evolution of the illumination in function of time.

In this figure, curve 1 relates to Example 4, and curve 2 relates to Example 3.

The dotted line represents minimum illumination (37 lux) required for lighting urban audience.

It can thus be seen that the presence of tungsten and potassium is favorable not only at the level of the illumination but also of the lifetime since this is 135 hours for the sleeve with W of example 4 instead of 30 hours in the case of tungsten-free sleeve of Example 3.

By way of comparison, it can be noted that the characteristics of initial illumination of a hard sleeve prepared from the candoluminescent material of FR-A-2 560 604 is only 27 lux.

The invention therefore makes it possible to achieve surprisingly better results.

Claims (8)

Candoluminescent material formed from a mixture of oxides comprising in moles: 50 to 70% zirconium oxide, ZrO ₂ , 5 to 8% of yttrium oxide Y ₂ O ₃ , 13 to 20% of magnesium oxide MgO, 4 to 10% of lithium oxide Li ₂ O, 0.2 to 0.6% of cerium oxide CeO ₂ , and 0 to 25% potassium oxide K ₂ O ,. 0 to 1% of tungsten oxide WO ₃ . Process for the preparation of a candoluminescent material, comprising the following steps: a) prepare a solution of zirconium, yttrium, lithium, magnesium and cerium salts, and adjust its pH to a value of 0.6 to 1, the salt concentrations of said solution being such that they correspond to a mixture of oxides having the following molar composition: 50 to 70% of ZrO ₂ , 5 to 8% of Y ₂ O ₃ , 13 to 20% MgO, 4 to 10% of Li ₂ O, and 0.2 to 0.6% of CeO ₂ ; b) impregnating a fabric of combustible filaments with said solution; and c) subjecting the impregnated tissue to combustion to remove the tissue and convert the salts to oxides. Method according to claim 2, in which salts are nitrates. Method according to any one of claims 2 and 3, in which the pH of the solution in step a) by adding carbonate or ammonium bicarbonate. Method according to any one of claims 2 and 3, in which the pH of the solution in step a) by adding carbonate or potassium bicarbonate. Process according to Claim 5, in which a potassium hydroxide solution containing tungstic acid, the concentration of tungstic acid of which is equivalent to the introduction of 0.2 to 1%, is first added to the solution. of WO ₃ in the mixture of oxides Process according to either of Claims 2 and 3, in which the pH of the solution is adjusted in step a) by treatment on an anion exchange resin in the OH ^- form. Use of material according to claim 1 in a hard sleeve of public lighting gas.

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