PRODUCT CONTAINING CHROMIUM 3 OXIDE FOR GLASS FURNACE

Abstract

A glass furnace including an additive-containing product including an additive selected from: phosphorus compounds other than glasses and vitroceramics, tungsten compounds other than glasses and vitroceramics, molybdenum compounds other than glasses and vitroceramics, iron in the form of metal, aluminum in the form of metal, silicon in the form of metal, and their mixtures, silicon carbide, boron carbide, silicon nitride, boron nitride, glasses including elemental phosphorus and/or iron and/or tungsten and/or molybdenum, vitroceramics including elemental phosphorus and/or iron and/or tungsten and/or molybdenum, and their mixtures, and having the following chemical analysis, exclusively of the additive, as a percentage by weight on the basis of the oxides: Cr.sub.2O.sub.32%, and Cr.sub.2O.sub.3+Al.sub.2O.sub.3+CaO+ZrO.sub.2+MgO+Fe.sub.2O.sub.3+SiO.sub.2+TiO.sub.290%, and Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO60%, the content by weight of additive being in the range 0.01% to 6%.

Claims

1. A glass furnace comprising an additive-containing product comprising, at the surface and/or in the core, an additive selected from: phosphorus compounds other than glasses and vitroceramics, tungsten compounds other than glasses and vitroceramics, molybdenum compounds other than glasses and vitroceramics, iron in the form of the metal, aluminum in the form of the metal, silicon in the form of the metal, and their mixtures, silicon carbide, boron carbide, silicon nitride, boron nitride, glasses comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, vitroceramics comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, and their mixtures, and having the following chemical analysis, exclusively of the additive, as a percentage by weight on the basis of the oxides: Cr.sub.2O.sub.32%, and Cr.sub.2O.sub.3+Al.sub.2O.sub.3+CaO+ZrO.sub.2+MgO+Fe.sub.2O.sub.3+SiO.sub.2+TiO.sub.290%, and Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO60%, the content by weight of additive being in the range 0.01% to 6% on the basis of the additive-containing product.

2. The glass furnace as claimed in claim 1, wherein the additive-containing product is in the form of a block, in particular a vessel block or a bottom block, a glass feeder channel, or a consumable part for a glass feeder channel, in particular a spout, an orifice ring, a sleeve, a plunger, a stirrer or a rotor.

3. The glass furnace as claimed in claim 1, wherein the additive-containing product is disposed in a zone of the furnace in which it is likely to come into contact with molten glass and/or comprises at least one region defining a surface which is not intended to come into contact with molten glass and which comprises additive.

4. The glass furnace as claimed in claim 1, wherein the additive-containing product has a Cr.sub.2O.sub.3 content of 9%, exclusively of the additive, as percentages by weight on the basis of the oxides.

5. The glass furnace as claimed in claim 1, wherein the additive-containing product has, exclusively of the additive, as percentages by weight on the basis of the oxides: a content of Cr.sub.2O.sub.3 of more than 15% and less than 98%; and/or a content of CaO of more than 0.1% and less than 3%; and/or a content of Cr.sub.2O.sub.3+Al.sub.2O.sub.3 of more than 55%; and/or a content of SiO.sub.2 of more than 0.5% and less than 12%; and/or a content of ZrO.sub.2 of more than 1% and less than 19%; and/or a content of MgO of less than 20%; and/or a content of Fe.sub.2O.sub.3 of less than 30% and/or a content of constituents other than Cr.sub.2O.sub.3, Al.sub.2O.sub.3, CaO, ZrO.sub.2, MgO, Fe.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2 of less than 5%.

6. The glass furnace as claimed in claim 1, wherein the additive-containing product has, exclusively of the additive, as percentages by weight on the basis of the oxides: a content of Cr.sub.2O.sub.3 of more than 30%; and/or a content of CaO of more than 0.3% and less than 1.5%; and/or a content of Cr.sub.2O.sub.3+Al.sub.2O.sub.3 of more than 80%; and/or a content of SiO.sub.2 of more than 1% and less than 8%; and/or a content of ZrO.sub.2 of more than 3% and less than 15%; and/or a content of MgO of less than 5%; and/or a content of Fe.sub.2O.sub.3 of less than 5%; and/or a content of constituents other than Cr.sub.2O.sub.3, Al.sub.2O.sub.3, CaO, ZrO.sub.2, MgO, Fe.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2 of less than 3%.

7. The glass furnace as claimed in claim 1, wherein the additive-containing product is in the form of a hardened concrete or a sintered concrete.

8. A method for the manufacture of a glass furnace as claimed in claim 1, said manufacture of the additive-containing product comprising the following steps in succession: A) preparing a feedstock comprising a particulate mixture and water; B) shaping said feedstock in a manner such as to form a preform; C) optionally, sintering said preform in a manner such as to obtain a sintered product, a precursor of said additive being added to the feedstock and/or applied to the surface of the preform and/or applied to the surface of the sintered product.

9. The method as claimed in claim 8, wherein the additive precursor is selected from phosphorus compounds other than glasses and vitroceramics, tungsten compounds other than glasses and vitroceramics, molybdenum compounds other than glasses and vitroceramics, iron in the form of the metal, aluminum in the form of the metal, silicon in the form of the metal and their mixtures, silicon carbide, boron carbide, silicon nitride, boron nitride, glasses comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, vitroceramics comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, and their mixtures.

10. The method as claimed in claim 9, wherein said additive precursor is selected from FePO.sub.4, MgPO.sub.4, ZnPO.sub.4, CuPO.sub.4, H.sub.3PO.sub.4, WO.sub.3, WC, MoO.sub.3, Si, Al, Fe, SiAl, FeSi, SiC, B.sub.4C, Si.sub.3N.sub.4, a glass comprising iron, and their mixtures.

11. The method as claimed in claim 10, wherein said additive precursor is selected from FePO.sub.4, MgPO.sub.4, H.sub.3PO.sub.4, WO.sub.3, MoO.sub.3, SiAl, FeSi, SiC, a glass comprising iron, preferably a glass comprising an iron content, expressed in the form of Fe.sub.2O.sub.3 in the range 1% to 15%, preferably in the range 4% to 15%, and their mixtures.

12. The method as claimed in claim 8, wherein the content by weight of additive precursor, on the basis of the weight of the particulate mixture exclusively of the shaping agent, is more than 0.1% and less than 6%.

13. The method as claimed in claim 8, in which the content by weight of additive precursor, on the basis of the weight of the particulate mixture exclusively of the shaping agent, is more than 0.5% and less than 3%, or in which the additive precursor is deposited onto the surface of the preform, the content by weight of additive precursor being in the range 0.01% and 5% on the basis of the weight of the preform after depositing the additive precursor, or in which the additive precursor is deposited onto the surface of the sintered product, the content by weight of additive precursor being in the range 0.01% to 5% on the basis of the weight of the sintered product after depositing the additive precursor.

14. The method as claimed in claim 8, wherein the feedstock comprises a granulate with a median circularity of more than 0.87 and/or a matrix fraction constituted by particles with a size of less than or equal to 50 m which does not contain a hydraulic binder.

15. Use of an additive for a glass furnace product as claimed in claim 1, for limiting the quantity of chromium 6 generated by said product.

16. Use of an additive for a glass furnace product of an additive precursor in a method as claimed in claim 8, for limiting the quantity of chromium 6 generated by said product.

Description

DETAILED DESCRIPTION

[0143] The detailed description of step A) below relates to a concrete, but the invention encompasses any product comprising chromium 3 oxide.

[0144] In step A), the feedstock for manufacturing a concrete is constituted by mixing a particulate mixture in accordance with the invention and water, in order to obtain, at the end of step B) and/or C), an additive-containing product in accordance with the invention.

[0145] In addition to the particulate mixture and water, it may also contain a liquid shaping agent.

[0146] Particulate Mixture

[0147] The manufacture of a particulate mixture conventionally results from a mixture of powdered starting materials with compositions and granulometric distribution that are adapted to the desired additive-containing product.

[0148] For a concrete, the particulate mixture preferably comprises, as a percentage by weight, from 0.9% to 8%, preferably from 2% to 6% of particles of a hydraulic cement. The hydraulic cement may be a high alumina cement or a mixture of different cements such as CA25 or CA14 cements from Almatis. Preferably again, the hydraulic cement contains alumina and calcium aluminates as the principal constituents (constituents with the highest contents).

[0149] Preferably, the particulate mixture has a total content of Cr.sub.2O.sub.3+Al.sub.2O.sub.3 of more than 47%, preferably more than 51%, preferably more than 56%, preferably more than 60%, preferably more than 70%, preferably more than 75%, or even more than 80%, or even more than 85%, or even more than 89%, as a percentage by weight.

[0150] In one embodiment, the particulate mixture comprises a total content of Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO of more than 80%, preferably more than 85%, preferably more than 90%, preferably more than 95%, as a percentage by weight.

[0151] In one embodiment, the particulate mixture comprises a content of Cr.sub.2O.sub.3 of more than 13%, more than 17%, more than 21%, more than 26%, more than 30%, more than 35%, and/or less than 71%, less than 66%, or less than 62%, less than 50%, as a percentage by weight. In one embodiment, the content of Cr.sub.2O.sub.3 is more than 48%, or even more than 52%.

[0152] In one embodiment, the particulate mixture comprises a content of Cr.sub.2O.sub.3 of more than 3%, preferably more than 4%, preferably more than 5% and preferably less than 15%, preferably less than 12%, preferably less than 9%, as a percentage by weight.

[0153] In one embodiment, the particulate mixture comprises a content of Al.sub.2O.sub.3, preferably in the form of alumina, of more than 2.5%, more than 4.5%, more than 9%, more than 13%, more than 17%, more than 21% and/or less than 95%, less than 90%, less than 85%, less than 80%, less than 76%, less than 71%, less than 66%, less than 62%, less than 57%, less than 52%, or even less than 33%, as a percentage by weight. In one embodiment, the particulate mixture comprises a content of Al.sub.2O.sub.3 of more than 33%, of more than 35%, or even of more than 39%. In one embodiment, the particulate mixture comprises a content of Al.sub.2O.sub.3 of more than 70%, preferably of more than 75%, preferably of more than 80%, or even of more than 85%, or even of more than 90%.

[0154] The content of SiO.sub.2, preferably as silica, of the particulate mixture may be more than 0.4%, more than 0.9%, and/or less than 11.5%, or less than 7.5%, as a percentage by weight.

[0155] The content of ZrO.sub.2, preferably in the form of zirconia, of the particulate mixture may be less than 18%, less than 14.5%, and/or more than 0.9%, or more than 2.6%, as a percentage by weight.

[0156] The content of constituents other than Cr.sub.2O.sub.3, Al.sub.2O.sub.3, CaO, ZrO.sub.2, MgO, Fe.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2 of the particulate mixture is less than 20%, preferably less than 15%, preferably less than 12%, preferably less than 8%, or even less than 5%, as a percentage by weight.

[0157] In one embodiment, the content of MgO of the particulate mixture is less than 19%, preferably less than 14%, preferably less than 10%, preferably less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%, or even less than 0.1%, as a percentage by weight.

[0158] In one embodiment, the content of MgO of the particulate mixture is more than 5%, preferably more than 7%, preferably more than 10% and less than 15%, as a percentage by weight.

[0159] In one embodiment, the content of Fe.sub.2O.sub.3 of the particulate mixture is less than 5%, preferably less than 3%, preferably less than 1%, preferably less than 0.5%, as a percentage by weight.

[0160] In one embodiment, the content of Fe.sub.2O.sub.3 of the particulate mixture is less than 30% and more than 1%, preferably more than 3%, as a percentage by weight.

[0161] In a preferred embodiment, exclusively of the additive precursor, the particulate mixture has, as a percentage by weight on the basis of the oxides: [0162] a content of MgO of less than 20%, preferably less than 19%, preferably less than 15%, preferably less than 14%, preferably less than 10%, preferably less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%, or even less than 0.1%; and [0163] a content of Fe.sub.2O.sub.3 of less than 5%, preferably less than 3%, preferably less than 1%, preferably less than 0.5%.

[0164] The content of TiO.sub.2 of the particulate mixture may be more than 0.3%, more than 0.5%, more than 0.7%, more than 1%, and/or less than 5%, less than 4.5%, less than 4%, less than 3.5%, less than 3%, as a percentage by weight. In one embodiment, the content of TiO.sub.2 of the particulate mixture is less than 0.2%.

[0165] In one embodiment, the content of CaO of the particulate mixture is more than 0.2%, preferably more than 0.3%, preferably more than 0.4% and/or less than 2.4%, preferably less than 1.9%, preferably less than 1.4%, preferably less than 1%, preferably less than 0.8%, as a percentage by weight.

[0166] In one embodiment, the content of CaO of the particulate mixture is less than 0.5%, preferably less than 0.3%, as a percentage by weight.

[0167] Preferably, the total Cr.sub.2O.sub.3, Al.sub.2O.sub.3, ZrO.sub.2, SiO.sub.2, CaO and TiO.sub.2 content in the particulate mixture is more than 77%, more than 83%, more than 87%, more than 90%, or more than 93%, as a percentage by weight.

[0168] Preferably, the constituents other than the oxides represent less than 14%, preferably less than 10%, preferably less than 8%, preferably less than 5% of the weight of the particulate mixture.

[0169] The granulometric distribution is not limiting. In particular, it may be adapted to the bulk density of the product that is to be obtained.

[0170] The particulate mixture for a concrete comprises a matrix fraction and a granulate.

[0171] Matrix Fraction

[0172] The particulate mixture preferably comprises more than 10%, more than 15%, more than 20%, or even more than 25%, and/or less than 40%, or even less than 35%, or even less than 30% of matrix particles, as a percentage by weight.

[0173] The median size of the matrix fraction may be less than 30 m, less than 25 m, less than 15 m, less than 10 m, or even less than 7 m.

[0174] Preferably, at least 90% by weight of the matrix particles have a size of less than 40 m, preferably less than 30 m, preferably less than 20 m, or even less than 10 m.

[0175] Preferably, the matrix fraction has a chemical composition such that, as percentages by weight and for a total of 100%: [0176] Cr.sub.2O.sub.3+Al.sub.2O.sub.3+ZrO.sub.2+MgO+Fe.sub.2O.sub.3+SiO.sub.2+TiO.sub.2+CaO82%, preferably Cr.sub.2O.sub.3+Al.sub.2O.sub.3+ZrO.sub.2+MgO+Fe.sub.2O.sub.3+SiO.sub.2+TiO.sub.2+CaO87%, and [0177] Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO45%, and [0178] preferably, Cr.sub.2O.sub.36%, and [0179] preferably, 15%SiO.sub.20.1%.

[0180] Preferably, the composition of the matrix fraction is such that: [0181] the total Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO content is more than 60%, preferably more than 65%, preferably more than 70%, preferably more than 80%, or even more than 85%, as a percentage by weight; and/or [0182] the content of SiO.sub.2 is less than 12%, preferably less than 10%, preferably less than 8%, preferably less than 6%, preferably less than 5%, or even less than 4%, or even less than 3%; and/or [0183] the content of MgO is less than 20%, preferably less than 15%, preferably less than 10%, preferably less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%, as a percentage by weight; and/or [0184] in one embodiment, the content of Fe.sub.2O.sub.3 is less than 5%, preferably less than 3%, preferably less than 1%, preferably less than 0.5%, as a percentage by weight; and/or [0185] in one embodiment, the content of Fe.sub.2O.sub.3 is less than 30% and more than 1%, preferably more than 3%, as a percentage by weight; and/or [0186] the content of TiO.sub.2 is less than 7%, or even less than 4%, or even less than 3%, or even less than 2%; and/or [0187] the complement to Cr.sub.2O.sub.3, Al.sub.2O.sub.3, CaO, ZrO.sub.2, MgO, Fe.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2 preferably represents less than 8%, preferably less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3%.

[0188] In one embodiment, the composition of the matrix fraction is such that Cr.sub.2O.sub.3+Al.sub.2O.sub.3>73%, Cr.sub.2O.sub.3+Al.sub.2O.sub.3>80%, or in fact Cr.sub.2O.sub.3+Al.sub.2O.sub.3>90%.

[0189] In one embodiment, the composition of the matrix fraction is such that Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO>75%, Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO>80%, or in fact Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO>90%.

[0190] In one embodiment, the composition of the matrix fraction is such that Al.sub.2O.sub.3+MgO>75%, Al.sub.2O.sub.3+MgO>80%, or in fact Al.sub.2O.sub.3+MgO>90%.

[0191] In one embodiment, the composition of the matrix fraction is such that the content of TiO.sub.2 is less than 0.2%.

[0192] In one embodiment, the composition of the matrix fraction is such that the content of Al.sub.2O.sub.3 is more than 4%, more than 5%, more than 7.5%, more than 10%, more than 15%, and/or less than 70%, less than 65%, less than 60%, less than 50%.

[0193] The matrix fraction preferably comprises particles of eskolaite on the one hand and, on the other hand, particles of alumina and/or particles of zirconia and/or particles of titanium oxide and/or particles of silica and/or particles of cement and/or particles of additive. Preferably, the matrix fraction comprises particles of eskolaite on the one hand and, on the other hand, of alumina and/or of zirconia and/or of titanium oxide and/or of cement and/or particles of additive.

[0194] In one embodiment, the particulate mixture does not contain particles of zirconia, in particular matrix particles of zirconia.

[0195] In one embodiment, the matrix fraction preferably comprises particles of alumina on the one hand and, preferably, on the other hand, particles of eskolaite and/or particles of magnesia and/or particles of additive.

[0196] Granulate

[0197] The particulate mixture preferably comprises less than 90%, preferably less than 85%, preferably less than 80%, of grains, as a percentage by weight.

[0198] Preferably, at least 90% by weight of the grains have a size of more than 100 m, preferably more than 200 m, preferably more than 300 m, preferably more than 400 m.

[0199] Preferably again, more than 80%, preferably more than 90%, preferably more than 95%, preferably more than 99% by weight of the grains of granulate have a size of more than 200 m, preferably more than 300 m, preferably more than 400 m, or even more than 0.5 mm and/or less than 10 mm, preferably less than 5 mm.

[0200] Preferably again, the particulate mixture contains at least 10% of grains with a size of more than 2 mm, as a percentage by weight.

[0201] In one embodiment, more than 90%, more than 95% of the weight of the particulate mixture is constituted by sintered particles.

[0202] Preferably, the granulate has a bulk density of more than 85% of the theoretical density, preferably more than 88%, preferably more than 90%, preferably more than 91%, preferably more than 92% of the theoretical density, or even more than 93%, or even more than 94%, or even more than 95%, or even more than 96% of the theoretical density.

[0203] Preferably, the granulate has an open porosity of less than 10%, preferably less than 6%, preferably less than 5%, preferably less than 3%, preferably less than 2%, preferably less than 1%, or even less than 0.7%, or even less than 0.6%.

[0204] Preferably, the granulate has a median circularity of more than 0.87, preferably more than 0.88, preferably more than 0.90, preferably more than 0.91. Advantageously, the resistance to thermal shocks and the corrosion resistance, in particular in an application in which the product is brought into contact with molten glass, are improved.

[0205] The granules are particles having a circularity of 0.8 or more. Preferably, the granules are agglomerated particles, in particular sintered particles. The agglomeration may also be obtained by means of a binder, for example a polymeric binder, in particular by atomization or spray drying and/or by using a granulator or pelletizing equipment.

[0206] In a particular embodiment, at least 80%, preferably at least 90%, preferably at least 95%, preferably at least 99%, or even substantially 100% by number of the grains are granules.

[0207] The granulate is preferably constituted by particles of additives and by particles comprising Cr.sub.2O.sub.3 on the one hand and, on the other hand, comprising Al.sub.2O.sub.3 and/or ZrO.sub.2 and/or MgO and/or Fe.sub.2O.sub.3 and/or TiO.sub.2 and/or SiO.sub.2. Preferably, the granulate is constituted by particles comprising Cr.sub.2O.sub.3 on the one hand and, on the other hand, comprising Al.sub.2O.sub.3 and/or ZrO.sub.2 and/or TiO.sub.2 and/or SiO.sub.2.

[0208] In one embodiment, the granulate is constituted by particles of additive and by particles comprising Al.sub.2O.sub.3 and/or by particles comprising Cr.sub.2O.sub.3 and/or by particles comprising MgO and/or by particles comprising a mixture of at least two oxides selected from Al.sub.2O.sub.3, Cr.sub.2O.sub.3 and MgO. In one embodiment, the granulate is constituted by particles comprising Al.sub.2O.sub.3 and Cr.sub.2O.sub.3 on the one hand, and by particles comprising Al.sub.2O.sub.3 and/or by particles comprising MgO on the other hand. Preferably, the granulate has a chemical composition such that, as percentages by weight and for a total of 100%: [0209] Cr.sub.2O.sub.3+Al.sub.2O.sub.3+ZrO.sub.2+MgO+Fe.sub.2O.sub.3+SiO.sub.2+TiO.sub.290%, preferably Cr.sub.2O.sub.3+Al.sub.2O.sub.3+ZrO.sub.2+MgO+Fe.sub.2O.sub.3+SiO.sub.2+TiO.sub.2295%, and [0210] Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO60%, and [0211] preferably Cr.sub.2O.sub.39%, and [0212] preferably 20%SiO.sub.20.5%.

[0213] Preferably, the composition of the granulate is such that [0214] the total Cr.sub.2O.sub.3+Al.sub.2O.sub.3+MgO content is more than 65%, preferably more than 70%, preferably more than 80%, or even more than 90%, or even more than 92%, or even more than 94%, as a percentage by weight; and/or [0215] the content of SiO.sub.2 is less than 16%, preferably below 13%, preferably less than 10%, preferably less than 8%, preferably less than 6%, preferably less than 5%, or even less than 4%, or even less than 3% (advantageously, the densification is improved thereby, without in any way reducing the corrosion resistance); and/or [0216] in one embodiment, the content of MgO is less than 20%, preferably less than 15%, preferably less than 10%, preferably less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1%, preferably less than 0.5%, as a percentage by weight; and/or [0217] in one embodiment, the content of MgO is more than 1%, preferably more than 3% and less than 20%, preferably less than 10%; and/or [0218] in one embodiment, the content of MgO is less than 1%, preferably less than 0.8%; and/or [0219] in one embodiment, the content of Fe.sub.2O.sub.3 is less than 5%, preferably less than 3%, preferably less than 1%, preferably less than 0.5%, as a percentage by weight on the basis of the oxides; and/or [0220] in one embodiment, the content of Fe.sub.2O.sub.3 is less than 30% and more than 1%, preferably more than 3%, as a percentage by weight; and/or [0221] in one embodiment, the content of TiO.sub.2 is more than 0.5%, or even more than 0.7%, and/or less than 4%, preferably less than 3%, less than 2.2%, or even less than 2%; and/or [0222] the complement to Cr.sub.2O.sub.3, Al.sub.2O.sub.3, CaO, ZrO.sub.2, MgO, Fe.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2 preferably represents less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1%.

[0223] In certain embodiments, the composition of the granulate is such that Cr.sub.2O.sub.3+Al.sub.2O.sub.3>80%, Cr.sub.2O.sub.3+Al.sub.2O.sub.3>90%, or in fact Cr.sub.2O.sub.3+Al.sub.2O.sub.3>95%.

[0224] Preferably again, the sum of the contents of the oxides in the grains, preferably the granules of the granulate, represents more than 90%, more than 95%, or even substantially 100% of the weight of said grains or granules.

[0225] Shaping Agent

[0226] The particulate mixture may contain at least 0.1% and/or less than 6% by weight of particles of a shaping agent, as a percentage by weight on the basis of the particulate mixture.

[0227] The optional shaping agent may be introduced in the liquid form in equivalent quantities.

[0228] The shaping agent may in particular be selected from the group constituted by: [0229] clays; [0230] plasticizers, such as polyethylene glycol (or PEG) or polyvinyl alcohol (or PVA); [0231] binders including temporary organic binders such as resins, lignosulfonates, carboxymethylcellulose or dextrin; [0232] deflocculating agents such as alkali metal polyacrylates, or polycarboxylates; and [0233] mixtures of these agents.

[0234] Preferably, the shaping agent is selected from the group constituted by deflocculating agents, clays, lignosulfonates, PVA and their mixtures.

[0235] Additive and Additive Precursor

[0236] The median size of the powdered additive or, more generally, of the powdered additive precursor, in the particulate mixture is preferably less than 150 m, preferably less than 100 m, preferably less than 80 m, preferably less than 60 m, preferably less than 50 m, preferably less than 40 m, preferably less than 30 m, or even less than 20 m.

[0237] Preferably, the content of additive or, more generally, of additive precursor in the particulate mixture, on the basis of the weight of the particulate mixture exclusively of the shaping agent, is more than 0.1%, preferably more than 0.2%, preferably more than 0.3%, preferably more than 0.4%, preferably more than 0.5%, and preferably less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3%.

[0238] Preferably, the content of the additive or, more generally, of the additive precursor in the particulate mixture is adjusted in a manner such that the quantity of additive in the additive-containing product is more than 0.3%, on the basis of the weight of the additive-containing product.

[0239] In a first preferred embodiment, the quantity of additive or, more generally, of additive precursor in the particulate mixture, is adjusted in a manner such that the quantity of additive in the additive-containing product (preform or sintered product) is more than 0.1%, preferably more than 0.2%, preferably more than 0.3%, preferably more than 0.4%, preferably more than 0.5%, and less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3%, on the basis of the weight of the additive-containing product. Preferably in this embodiment, the additive or, more generally, the additive precursor, is selected from phosphorus compounds other than glasses and vitroceramics, tungsten compounds other than glasses and vitroceramics, molybdenum compounds other than glasses and vitroceramics, glasses comprising elemental iron, boron nitride and their mixtures. Preferably the additive or, more generally, the additive precursor is selected from phosphorus compounds other than glasses and vitroceramics, tungsten oxides, molybdenum oxides, boron nitride and their mixtures; preferably, the additive or, more generally, the additive precursor, is selected from FePO.sub.4, MgPO.sub.4, ZnPO.sub.4, CuPO.sub.4, phosphoric acid, tungsten oxides, molybdenum oxides and their mixtures, preferably from FePO.sub.4, MgPO.sub.4, phosphoric acid and their mixtures. Preferably, the additive or, more generally, the additive precursor, is selected from FePO.sub.4, MgPO.sub.4 and their mixtures.

[0240] This first embodiment is particularly suitable when the additive-containing product is intended to be subjected to a temperature in the range 100 C. to 400 C.

[0241] In a second preferred embodiment, the quantity of additive or, more generally, the quantity of additive precursor incorporated into the particulate mixture, is adjusted in a manner such that the additive content in the additive-containing product is more than 0.1%, preferably more than 0.2%, preferably more than 0.3%, preferably more than 0.4%, preferably more than 0.5%, and less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 3%, on the basis of the weight of the additive-containing product. Preferably, in this embodiment, the additive or, more generally, the additive precursor, is selected from phosphorus compounds other than glasses and vitroceramics, tungsten compounds other than glasses and vitroceramics, molybdenum compounds other than glasses and vitroceramics, iron in the form of the metal, aluminum in the form of the metal, silicon in the form of the metal and their mixtures, silicon carbide, boron carbide, silicon nitride, boron nitride, glasses comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, vitroceramics comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, and their mixtures. Preferably in this embodiment, the additive or, more generally, the additive precursor, is selected from phosphorus compounds other than glasses and vitroceramics, tungsten oxides, molybdenum oxides, and their mixtures, preferably from FePO.sub.4, MgPO.sub.4, ZnPO.sub.4, CuPO.sub.4, phosphoric acid, tungsten oxides, molybdenum oxides, and their mixtures. Preferably the additive or, more generally, the additive precursor, is selected from FePO.sub.4, MgPO.sub.4, phosphoric acid and their mixtures. Preferably, the additive or, more generally, the additive precursor, is selected from FePO.sub.4, MgPO.sub.4 and their mixtures.

[0242] This second embodiment is particularly suitable when the additive-containing product is intended to be subjected to a temperature in the range 500 C. to 1200 C.

[0243] The introduction of the additive or, more generally, of the additive precursor in step A), can advantageously be used to obtain a substantially homogeneous distribution of the additive. Preferably, the mixing time is determined to have this effect.

[0244] The particles of additive or, more generally, of additive precursor are accounted for, as a function of their size, in the granulate or the matrix fraction.

[0245] The particulate mixture may be supplied in a ready-to-use form. For a concrete in particular, it merely suffices to mix it with water in order to prepare the feedstock.

[0246] Water

[0247] The quantity of water is a function of step B).

[0248] In the case of casting, an addition of a quantity of water in the range 3% to 7% as a percentage by weight on the basis of the particulate mixture, optionally containing additive, is preferred.

[0249] In contrast to the feedstock for the manufacture of a concrete, the feedstock for the manufacture of a ramming mass does not include a hydraulic binder, and thus is not activated by any moisture. However, it may comprise a chemical and/or ceramic and/or organic binder. The means for activation are determined as a consequence.

[0250] In step B), any of the conventional methods used for the manufacture of preforms, in particular manufactured from a hardened concrete, may be envisaged.

[0251] The feedstock may in particular be shaped in situ, and so the preform is placed in its service position.

[0252] In particular for a ramming mass, shaping may conventionally result from a vibration or ramming operation. The preform obtained thus has a low mechanical strength, and thus is preferably manufactured in situ. Conventionally, after unmolding, the preform holds together, but does not have the physical integrity which would allow it to be transported, for example.

[0253] In one embodiment, the additive or, more generally, the additive precursor, is applied to the surface of the preform. Any technique which is known for depositing a composition onto a block may be used, in particular deposition using a trowel or brush, or wet or dry spraying, such as a glaze, so that a thin or thick film is formed.

[0254] Preferably, the additive or, more generally, the additive precursor, is mixed with a liquid, for example water and/or an oil, before depositing it onto the surface. The quantity of liquid can vary and is a function of the granulometry of the additive or, more generally, of the additive precursor, in order to key it well onto the surface.

[0255] In a third preferred embodiment, the quantity of additive or, more generally, of additive precursor, deposited in step B) onto the preform or in step C) onto the sintered product, is adjusted in a manner such that the additive content in the additive-containing product is more than 0.01%, preferably more than 0.015%, preferably more than 0.02%, and less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1.5%, preferably less than 1%, on the basis of the weight of the additive-containing product. Preferably in this embodiment, the additive or, more generally, the additive precursor, is selected from phosphorus compounds other than glasses and vitroceramics, tungsten compounds other than glasses and vitroceramics, molybdenum compounds other than glasses and vitroceramics, iron in the form of the metal, aluminum in the form of the metal, silicon in the form of the metal and their mixtures, silicon carbide, boron carbide, silicon nitride, boron nitride, glasses comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, vitroceramics comprising elemental phosphorus and/or iron and/or tungsten and/or molybdenum, and their mixtures. Preferably, the additive or, more generally, the additive precursor, is selected from phosphorus compounds other than glasses and vitroceramics, tungsten oxides, molybdenum oxides, glasses comprising elemental iron and their mixtures, preferably selected from FePO.sub.4, MgPO.sub.4, ZnPO.sub.4, CuPO.sub.4, phosphoric acid, tungsten oxides, molybdenum oxides, boron nitride, glasses comprising elemental iron and their mixtures. Preferably, the additive or, more generally, the additive precursor, is selected from FePO.sub.4, MgPO.sub.4, phosphoric acid, tungsten oxides, molybdenum oxides, glasses comprising elemental iron, preferably glasses comprising an iron content, expressed in the form of Fe.sub.2O.sub.3, in the range 1% to 15%, preferably in the range 4% to 15%, and their mixtures, preferably from FePO.sub.4, MgPO.sub.4, tungsten oxides, molybdenum oxides, glasses comprising elemental iron, preferably glasses comprising an iron content, expressed in the form of Fe.sub.2O.sub.3, in the range 1% to 15%, preferably in the range 4% to 15%, and their mixtures.

[0256] This third embodiment is particularly suitable when the product is intended to be subjected to a temperature in the range 100 C. to 1000 C., or even a temperature in the range 100 C. to 850 C., and a reduction in the quantity of chromium 6 over at least one of the faces of the product is desired.

[0257] In one embodiment, a bonding agent is mixed with the additive or, more generally, with the additive precursor, to promote its deposition onto the surface of the product. The bonding additives may be selected from clays, plasticizers, celluloses and their mixtures, polyvinyl alcohols or PVA, polyethylene glycols or PEG.

[0258] In step C), which is optional, the conditions for sintering, and in particular the temperature for sintering, depend on the composition of the particulate mixture. Normally, a sintering temperature in the range 1400 C. to 1700 C., preferably in the range 1450 C. to 1650 C., preferably in the range 1500 C. to 1600 C., is highly suitable. The sintering may be carried out in situ, i.e. after the preform has been shaped or placed in its service position.

[0259] At the end of step C), a sintered product in accordance with the invention is obtained, in particular a sintered concrete or a sintered ramming mass.

[0260] In one embodiment, the additive or, more generally, the additive precursor, is applied to the surface of the sintered product. The techniques described for applying the additive in step B) are applicable.

EXAMPLES

[0261] In order to manufacture the Cr.sub.2O.sub.3, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 particles used in the products of Examples 1 to 4 and 7 to 12, the following starting materials were used: [0262] chromium oxide Cr.sub.2O.sub.3 pigment with a purity of more than 95%, having a specific surface area equal to 4 m.sup.2/g and a median size of 0.7 m; [0263] alumina, Al.sub.2O.sub.3, with a purity of more than 99%, having a specific surface area equal to 7 m.sup.2/g and a median size of 0.6 m; [0264] fumed silica, with a purity of more than 92%; and [0265] titanium oxide, in the form of rutile, with a purity of more than 93% and having a median size of 1.5 m.

[0266] These starting materials were measured out and mixed in a manner such as to obtain a mixture of oxides having the following chemical composition:

TABLE-US-00001 TABLE 1 Chemical analysis Cr.sub.2O.sub.3 (%) 40.0 Al.sub.2O.sub.3 (%) 48.0 SiO.sub.2 (%) 3.2 TiO.sub.2 (%) 1.70 ZrO.sub.2 (%) 6.00 Others (%) 1.10

[0267] For each example, 3000 g of oxide mixture, 350 g of water and 150 g of polyvinyl alcohol (PVA) were introduced into an Eirich RV02 mixer.

[0268] This was then all mixed for 1 minute with a head rotating at 300 rpm and a pan adjusted to 43 rpm in order to obtain a homogeneous mixture. The rotation rate of the head was then increased to 1050 rpm, and a supplemental quantity of 900 grams of oxide mixture was then added steadily over one minute. The rotation was maintained for 2 minutes after introduction of the supplemental quantity was complete. The particles were then discharged, dried in air for 24 h at 110 C. before being sintered at 1550 C. with a constant temperature stage of 3 hours, in air, with a temperature ramp-up rate and a temperature ramp-down rate of 50 C./h. After sintering, the particles had an open porosity equal to 1.05% and a median circularity of more than 0.85. They were then sieved and three granulometric fractions were retained: 0-0.5 mm, 0.5-2 mm, and 2-5 mm.

[0269] The hardened concretes of Examples 2 and 3, and 7 to 12 were then manufactured in accordance with steps A) and B) described above.

[0270] In step A), the following starting materials were then mixed with the Cr.sub.2O.sub.3, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 particles: [0271] chromium oxide Cr.sub.2O.sub.3 pigment with a purity of more than 95%, having a specific surface area equal to 4 m.sup.2/g and a median size of 0.7 m, [0272] alumina, Al.sub.2O.sub.3, with a purity of more than 99%, having a specific surface area equal to 7 m.sup.2/g, and a median size of 0.6 m, [0273] a high alumina cement, CA25R from Almatis.

[0274] The contents by weight of the different starting materials are summarized in Table 2 below:

TABLE-US-00002 TABLE 2 Cr.sub.2O.sub.3, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 particles, 2-5 mm 28.5% Cr.sub.2O.sub.3, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 particles, 0.5-2 mm 26.5% Cr.sub.2O.sub.3, Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2 particles, 0-0.5 mm 23.5% Chromium oxide pigment 15.5% Alumina 5% High alumina cement CA25R 1%

[0275] A modified polycarboxylate ether was then added in a quantity equal to 0.17% of the weight of said mixture of starting materials.

[0276] An additive precursor was then added, in accordance with the invention, in a manner such as to obtain a ready-to-use mixture. The nature and the quantity of the additive precursor are summarized in Table 3 below:

TABLE-US-00003 TABLE 3 Quantity of additive Nature of precursor, as a percentage additive on the basis of the weight of precursor the ready-to-use mixture Example 2 Tungsten oxide, WO.sub.3 0.5% Example 3 Iron phosphate, FePO.sub.4 .sup.1% Example 7 Tungsten oxide, WO.sub.3 .sup.1% Example 8 Silicon carbide, SiC 0.5% Example 9 Iron phosphate, FePO.sub.4 0.5% Example 10 Alloy of aluminum and 0.5% silicon, AlSi Example 11 Glass comprising iron 0.5% Example 12 Glass comprising iron .sup.1%

[0277] The tungsten oxide used had a purity of more than 99% and a median size equal to 35 m.

[0278] The iron phosphate used was the iron phosphate E53-98 marketed by Budenheim.

[0279] The silicon carbide used was a Sika Unikiln FCP07 powder marketed by Saint-Gobain Silicon Carbide.

[0280] The alloy of aluminum and silicon had a content by weight of silicon equal to 12.3%, a content of elements other than silicon and aluminum of less than 1.5%, and a median size equal to 40 m.

[0281] The glass powder comprising iron had a median size equal to 13 m, and the following chemical analysis: SiO.sub.2=56.1%, Fe.sub.2O.sub.3=9%, Al.sub.2O.sub.3=17.4% Na.sub.2O=2.4%, K.sub.2O=1.7%, CaO=7.9%, MgO=3.8%, TiO.sub.2=1.2%, others=0.5%. 4.5% of water, as a percentage by weight on the basis of the weight of the ready-to-use mixture, was added in order to produce the feedstock. The mixing time was 12 minutes.

[0282] In step B), the feedstock was shaped using a vibrocasting technique into the form of a hardened concrete in accordance with the invention, with dimensions equal to 23015080 mm.sup.3, which were suitable for the characterizations to be carried out.

[0283] Comparative Example 1 was manufactured in a manner identical to Examples 2 and 3, and 7 to 12, but no additive precursor was added.

[0284] Example 4 was a hardened concrete identical to the hardened concrete of Example 1, with the exception that one of its faces had been coated with additive precursor having the composition shown in Table 4 below:

TABLE-US-00004 TABLE 4 Composition of additive precursor, as a percentage by weight on the basis of the total weight of additive precursor Glass comprising iron 77.5% Silicon carbide 36-70 6.2% Silicon carbide 80-180 6.2% Silicon carbide 220 F. 6.2% Aluminum triphosphate 2.9% Phosphoric acid, H.sub.3PO.sub.4 .sup.1%

[0285] The glass powder comprising iron had a median size equal to 13 m, and the following chemical analysis: SiO.sub.2=56.1%, Fe.sub.2O.sub.3=9%, Al.sub.2O.sub.3=17.4% Na.sub.2O=2.4%, K.sub.2O=1.7%, CaO=7.9%, MgO=3.8%, TiO.sub.2=1.2%, others=0.5%. The silicon carbide powders had a purity of more than 98%. The aluminum triphosphate powder was a M13-01 powder from Budenheim.

[0286] The components of the additive precursor were mixed together, and 29% of water, on the basis of the total quantity of additive precursor, was added. The total mixing time was 10 minutes, in order to form a coating.

[0287] The test samples of Examples 1 and 4 were in the form of cylinders with a height equal to 50 mm and a diameter equal to 150 mm. For Example 4, the coating was applied was applied with a trowel to one of the two faces with a diameter equal to 150 mm and the total quantity of additive, on the basis of the weight of the coated sample, was equal to 4%.

[0288] The product of Example 5 was manufactured in accordance with steps A) and B) described above, starting from the following starting materials: [0289] chromium oxide Cr.sub.2O.sub.3 pigment with a purity of more than 95%, having a specific surface area equal to 4 m.sup.2/g and a median size of 0.7 m, [0290] titanium oxide, in the form of rutile, with a purity of more than 93% and having a median size of 1.5 m, [0291] high chromium 3 oxide content particles containing 98% Cr.sub.2O.sub.3 and having an open porosity of less than 3%, [0292] zirconia with a purity of more than 99% and having a median size equal to 3.5 m, [0293] a tungsten oxide, WO.sub.3, with a purity of more than 99% and having a median size equal to 35 m.

[0294] The weight contents of the various starting materials are summarized in Table 5 below:

TABLE-US-00005 TABLE 5 high chromium 3 oxide content particles, 2-4 mm 12.1 high chromium 3 oxide content particles, 0.5-2 mm 31.9 high chromium 3 oxide content particles, 0-0.5 mm 37.4 Chromium oxide pigment 10.1 Titanium oxide 0.4 Zirconia 8.1

[0295] The order in which the starting materials were introduced was as follows: a hydroxyethyl methyl cellulose, Tylose MH 4000 P2, marketed by Shin Etsu, and a calcium lignosulfonate, BRETAX C marketed by Brenntag, in a quantity equal to 0.2% and 0.5%, respectively, were added to 2.5% of water, the percentages being percentages of the total weight of the starting materials, including the additive. The particles with a high chromium 3 oxide content were then added and mixing was carried out for 10 minutes. The chromium oxide pigment, the zirconia, the titanium oxide and 1% of tungsten oxide WO.sub.3 were then added, and mixing was carried out for an additional 10 minutes in order to produce the feedstock. The quantity of tungsten oxide used was as a percentage by weight on the basis of the weight of particles with a high chromium 3 oxide content, the chromium oxide pigment, the zirconia and the titanium oxide.

[0296] In step B), the feedstock was shaped, using a compression technique under a pressure equal to 800 bar, into the form of an additive-containing product with dimensions equal to 23011435 mm.sup.3, which were suitable for the characterizations to be carried out.

[0297] Comparative Example 6 was produced in a manner identical to that of Example 5, without tungsten oxide.

[0298] The samples of Examples 5 and 6 to be tested were in the form of cylinders with a height equal to 50 mm and a diameter equal to 150 mm.

[0299] The product of Example 13 was manufactured in accordance with step A) and shaped in accordance with step B) described above.

[0300] In step A), the following starting materials were mixed: [0301] fused alumina-chromium oxide particles comprising a Cr.sub.2O.sub.3 content equal to 13% and an alumina content equal to 82%, [0302] electrofused alumina with a purity of more than 99%, [0303] magnesia with a purity of more than 96% and having a median size equal to 35 m, [0304] iron phosphate, E53-98 marketed by Budenheim.

[0305] The contents by weight of the different startinq materials are summarized in Table 6 below:

TABLE-US-00006 TABLE 6 Fused alumina-chromium oxide particles, 5-10 mm 20% Fused alumina-chromium oxide particles, 1-5 mm 20% Electrofused alumina, 0.5-1 mm 10% Electrofused alumina, 0-0.5 mm 22% Electrofused alumina, <0.15 mm 8% Electrofused alumina, <50 m having a median 5% size equal to 15 m Fused alumina-chromium oxide particles, <50 m 5% having a median size equal to 15 m Magnesia, <75 m 10%

[0306] Dextrin was then added in a quantity equal to 0.5% of the weight of said mixture of starting materials.

[0307] 0.5% of iron phosphate E53-98 marketed by Budenheim was then added, the quantity of iron phosphate being as a percentage on the basis of the fused alumina-chromium oxide particles, of electrofused alumina and of magnesia, in a manner such as to obtain a ready-to-use mixture.

[0308] 3% of water, as a percentage by weight on the basis of the weight of the ready-to-use mixture, was then added in a manner such as to produce the feedstock. The mixing time was 15 minutes.

[0309] In step B), the feedstock was shaped using a uniaxial compression technique at a pressure equal to 800 kg/cm.sup.2 in a manner such as to obtain an additive-containing product in accordance with the invention having dimensions equal to 23015080 mm.sup.3, which were suitable for the characterizations to be carried out.

[0310] Comparative Example 14 was produced in a manner identical to that of Example 13, but no additive was added.

[0311] Measurement Protocols

[0312] The bulk density and the open porosity of the products were measured by hydrostatic weighing.

[0313] The measurements of the bulk density and of the open porosity of a granulate were carried out in accordance with the following method: [0314] Drying at 110 C. for at least 12 hours, of 4 samples of 35 grams, each constituted by particles the size of which was in the range 2 to 5 mm. The dry weight of each of the samples was denoted Ps.sub.1, Ps.sub.2, Ps.sub.3 and Ps.sub.4. Note that Ps=Ps.sub.1+Ps.sub.2+Ps.sub.3+Ps.sub.4. [0315] Placing each sample in a flask. [0316] With the aid of a vacuum pump, generating a vacuum of at least 0.07 MPa in each of the flasks and maintaining this vacuum for 7 minutes. Next, introducing water into the flask in a manner such as to cover the particles with at least 2 cm of water, allowing the particles to be permanently covered with water when subsequently being placed under vacuum. [0317] Regenerating a vacuum of 0.08 MPa in each flask containing the particles and the water, and maintaining this vacuum for 7 minutes. Breaking the vacuum. [0318] Regenerating a vacuum of 0.08 MPa in each flask, and maintaining this vacuum for 7 minutes. Breaking the vacuum. [0319] Regenerating a vacuum of 0.08 MPa in each flask, and maintaining this vacuum for 7 minutes. Breaking the vacuum. [0320] Determining the immersed weight of each sample, Pi.sub.1, Pi.sub.2, Pi.sub.3 and Pi.sub.4. Note that Pi=Pi.sub.1+Pi.sub.2+Pi.sub.3+Pi.sub.4. [0321] Next, pouring the contents of the 4 flasks onto a sieve with a 2 mm square mesh in order to eliminate the water. Next, pouring the particles onto a dry cotton fabric in order to eliminate excess water and draining the particles until the moist sheen has disappeared from their surface. [0322] Determining the moist weight Ph of the assembly of particles.

[0323] The bulk density of the assembly of particles is equal to Ps/(PhPi).

[0324] The open porosity of the assembly of particles is equal to (PhPs)/(PhPi).

[0325] These measurements correspond to averaged measurements for the material constituting the particles, i.e. do not take the interstices between the different particles into account.

[0326] The median circularity of a set of particles of a granulate was evaluated using the following method:

[0327] A sample of particles with sizes in the range 0.5 to 2 mm was poured onto the glass plate of a Morphologi G3 instrument marketed by Malvern provided for this purpose. The selected magnification was 1. The analysis was commenced. In order to avoid counting any scratches on the glass plate and dust, the measurements corresponding to particles with a width of less than 0.4 mm were eliminated from the count by generating a filter (width<400). The number of particles counted after filtration was more than 250.

[0328] The instrument provided an evaluation of the distribution of the circularity, the particles being counted by number.

[0329] For the elements other than chromium 6, the chemical analysis of the products was carried out using Inductively Coupled Plasma or ICP for the elements the quantity of which did not exceed 0.5%. In order to determine the content of the other elements, a bead of the product to be analyzed was manufactured by melting the product, then the chemical analysis was carried out using X ray fluorescence.

[0330] The measurements of the chromium 6 contents were carried out by extraction by leaching in accordance with the NF standard EN12457-2, the quantity of Cr.sup.6+ then being measured by an analysis using liquid phase ion chromatography.

[0331] In order to measure the capability of a product of generating chromium 6, two tests were carried out as a function of the location of the additive: a test when the additive is located within the product (Examples 2, 3, 5 and 7 to 13), and a test when the additive is located at the surface of the product (Example 4). The same test was carried out on a cylinder of the same product containing no additive (Examples 1, 6 and 14).

[0332] The test when the additive was located within the product was as follows: samples of the products to be tested were placed in a curing oven. They were then heated to a temperature T in air, the time over which the temperature T was maintained being equal to 24 hours, the temperature T ramp-up rate being equal to 50 C./h and the temperature ramp-down rate being equal to 50 C./h. After the test, the chromium 6 content was determined.

[0333] The test when the additive was located on a surface of the product was as follows. The cylinder of Example 4 was disposed in a tube furnace with an internal diameter equal to 150 mm, in a manner such as to substantially block a portion of the tube, the coated face being orientated on the side for introducing an alkaline mist of a solution of 0.5 g/L of NaOH, injected into the furnace at a flow rate of 32 mg/h and per m.sup.3 available in the furnace, over 24 hours, said furnace being maintained at a temperature of 800 C. during the injection of the alkaline mist. The same test was carried out on a cylinder of the same product wherein the faces had not been coated with additive (Example 1). The varying degrees to which a yellow color was present on the large face of the product oriented on the side where the alkaline mist was introduced was related to the presence of chromate: the more intense the yellow color, the large the quantity of chromate.

[0334] Table 7 below summarizes the results obtained for Examples 1 to 3, and 5 to 14.

TABLE-US-00007 TABLE 7 Example 1.sup.(*.sup.) 2 3 5 6.sup.(*.sup.) 7 8 9 10 Chemical analysis of product comprising chromium oxide 3 pigment (exclusively of the additive) (%) Cr.sub.2O.sub.3 46.5 46.5 46.5 89.3 89.3 46.5 46.5 46.5 46.5 Al.sub.2O.sub.3 44.8 44.8 44.8 0.1 0.1 44.8 44.8 44.8 44.8 CaO 0.3 0.3 0.3 0.05 0.05 0.3 0.3 0.3 0.3 SiO.sub.2 2.1 2.1 2.1 0.05 0.05 2.1 2.1 2.1 2.1 MgO 0 0 0 0 0 0 0 0 0 Fe.sub.2O.sub.3 0.3 0.3 0.3 0.1 0.1 0.3 0.3 0.3 0.3 ZrO.sub.2 4.2 4.2 4.2 8 8 4.2 4.2 4.2 4.2 TiO.sub.2 1.3 1.3 1.3 2.1 2.1 1.3 1.3 1.3 1.3 Cr.sub.2O.sub.3 + Al.sub.2O.sub.3 + CaO + 99.5 99.5 99.5 99.7 99.7 99.5 99.5 99.5 99.5 ZrO.sub.2 + MgO + Fe.sub.2O.sub.3 + SiO.sub.2 + TiO.sub.2 Cr.sub.2O.sub.3 + Al.sub.2O.sub.3 + MgO 91.3 91.3 91.3 89.4 89.4 91.3 91.3 91.3 91.3 Other oxides 0.5 0.5 0.5 0.3 0.3 0.5 0.5 0.5 0.5 Other characteristics Bulk density (g/cm.sup.3) 3.55 3.55 3.55 4.31 4.32 3.53 3.57 3.56 3.55 Open porosity (%) 15.7 16.2 16 15.3 15 16.2 16.2 15.2 16 Chromium 6 content after test 690 390 210 90 280 250 430 230 500 at T = 400 C. (ppm) Reduction in the chromium 6 43.5% 69.6% 63.8% 37.7% 66.7% 27.5% content with respect to Example 1 Reduction in the chromium 6 67.9% content with respect to Example 6 Chromium 6 content after test 870 460 100 190 280 210 390 at T = 800 C. (ppm) Reduction in the chromium 6 47.1% 88.5% 78.2% 67.8% 75.9% 55.2% content with respect to Example 1 Chromium 6 content after test at T = 600 C. (ppm) Reduction in the chromium 6 content with respect to Example 14 Example 11 12 13 14(*) Chemical analysis of product comprising chromium oxide 3 pigment (exclusively of the additive) (%) Cr.sub.2O.sub.3 46.5 46.5 5.9 5.9 Al.sub.2O.sub.3 44.8 44.8 81.1 81.1 CaO 0.3 0.3 0.6 0.6 SiO.sub.2 2.1 2.1 0.3 0.3 MgO 0 0 9.9 9.9 Fe.sub.2O.sub.3 0.3 0.3 0.5 0.5 ZrO.sub.2 4.2 4.2 0 0 TiO.sub.2 1.3 1.3 0 0 Cr.sub.2O.sub.3 + Al.sub.2O.sub.3 + CaO + 99.5 99.5 98.3 98.3 ZrO.sub.2 + MgO + Fe.sub.2O.sub.3 + SiO.sub.2 + TiO.sub.2 Cr.sub.2O.sub.3 + Al.sub.2O.sub.3 + MgO 91.3 91.3 96.9 96.9 Other oxides 0.5 0.5 1.7 1.7 Other characteristics Bulk density (g/cm.sup.3) 3.51 3.58 3.00 3.00 Open porosity (%) 17.9 16.2 19 19.2 Chromium 6 content after test 490 320 at T = 400 C. (ppm) Reduction in the chromium 6 29% 53.6% content with respect to Example 1 Reduction in the chromium 6 content with respect to Example 6 Chromium 6 content after test 600 500 at T = 800 C. (ppm) Reduction in the chromium 6 31% 42.5% content with respect to Example 1 Chromium 6 content after test 150 300 at T = 600 C. (ppm) Reduction in the chromium 6 50% content with respect to Example 14 .sup.(*.sup.)comparative examples

[0335] A comparison of the product of Example 1, exclusively of the invention, with the additive-containing products of Examples 2 and 3 and 7 to 12, in accordance with the invention, shows that the additive-containing products of Examples 2 and 3 and 7 to 12 have a far weaker aptitude for generating a quantity of chromium 6 than the product of Example 1 after exposure for 24 hours to a temperature equal to 400 C. or 800 C.

[0336] A comparison of the products of Examples 1 to 3 and 7 to 12 demonstrates the effectiveness of the invention when the additive is distributed in the product in a substantially homogeneous manner.

[0337] Furthermore, after the test, the additive-containing product of Example 4 did not have a yellow coloration on the coated surface disposed inside the furnace, in contrast to the product of Example 1.

[0338] A comparison of the products of Examples 1 and 4 demonstrates the efficiency of the invention when the additive is disposed on a surface of the product.

[0339] A comparison of the additive-containing product of Example 5, in accordance with the invention, with the product of Example 6, not in accordance with the invention, shows that the additive-containing product of Example 5 has a far weaker capability of generating a quantity of chromium 6 than the product of Example 1 after exposure for 24 hours to a temperature equal to 400 C.

[0340] A comparison of the product of Example 14, not in accordance with the invention, with the additive-containing product of Example 13, in accordance with the invention, shows that the additive-containing product of Example 13 has a far weaker capability of generating a quantity of chromium 6 than the product of Example 1 after exposure for 24 hours to a temperature equal to 600 C.

[0341] As can clearly be seen here, the invention can be used to reduce the capability of a product, and in particular a concrete, for generating chromium 6 during its manufacture or its use, in particular at temperatures in the range 100 C. to 1200 C.

[0342] Clearly, the invention is not limited by the examples, which are provided solely for the purposes of illustration.