"PROCESSES FOR NIOBIUM OXIDE EXTRUSION CASTING OR CONFORMATION AND PREPARATION OF A HYDROLIZED AND AMORPHOUS NIOBIUM OXIDE AND USE OF A NIOBIUM OXIDE IN THE EXTRUDED FORM"

TECHNICAL FIELD

The present invention relates to niobiuhi oxides casting or conformation, and more specifically, niobium oxides with distinct textural and mechanical properties, such as pores distribution, specific area and mechanical crushing strength, obtained from niobium precursors, with the help of several casting or conformation techniques, such as extrusion, pelletization and pressing.

ART-RELATED DESCRIPTION The preparation of pastes susceptible to casting or conformation operations is known in the art, such as extrusion, pelletization and pressing. It is also known that ceramic compounds with a certain configuration may have pores, as a result from changes of its crystalline structure, or elimination, during the thermal treatment, of compounds (additives, dispersers and/or pore-forming agents) added to the precursor paste.

A relevant feature of the ceramic paste is to be castable in the numerous required shapes, depending on their application, due to their appropriate rheological properties. Although the several casting or conformation methods may be performed in "casts" with batch system, the paste extrusion process is the most appropriate concerning material production on a large scale.

As described in North- American Patent US 4,814,300, ceramic compounds may contain hydrogel aluminum-silicate mixtures and refractory ceramic materials, such as carbides, nitrides, borides, silicates and oxides (such as alumina, chromia, zirconia, magnesia and titania). Particulate metals, surfactants and agents responsible for alterations in molecular interaction may also be part of the formulation. Refractory fibers may also be added to the composition, in order to ensure appropriate mechanical strength after casting.

North- American patent US 5,534,468 relates to the preparation of a ceramic paste, whose components, when mixed, may form a hydrogel, or hydrosol, or simply an oxides "mud". The hydrogel is a colloidal solution from a liquid in a solid, usually containing additives in order to ensure conformation, such as: refractory ceramics / fibers, surfactants, particulate metals, etc. The hydrosol is a colloidal solution containing additives (organic or inorganic acid, strong or weak bases), which favor the precipitation of electrically charged nanometric particles. The mud is the aqueous solution containing the paste- forming oxides, and also additives (refractory ceramic material, surfactant, gel, particulate metal and refractory fibers), and the oxides may, in certain cases, show a Gaussian distribution of particles size, in order to ensure better paste homogeneity. During the thermal treatment step for the precursor cast paste, gases may be eliminated, which fact leads to pores formation in the final product. Some treatments may be used, depending on the extruded application, in order to remove most of the additives, such as ionic exchange. Still according to this invention, there is a probable way to make the product in the extruded form, which process includes the following steps:

- Extrusion of moist ceramic compound or its precursor paste;

- Drying this extruded profile;

- Supporting the extruded profile during extrusion and drying to obtain conformation; - Calcination of dry extruded profile.

The supported moist profile prevents gravity action, avoiding breaking or deformation of the material obtained after casting or conformation. The extrusion shall be performed preferably in the vertical position, in order to obtain extruded pieces with no tortuosities. If it is otherwise performed, problems may be observed, such as deformation, breaking, curve formation and differences in the extruded transversal section.

The profile extrusion rates and heating rates shall be adjusted, in order to obtain a product with the properties desired for each application. The extrusion may be a selected paste, or paste mixed with one or more ceramic or precursor materials. According to North-American patent US 6,063,323, the following factors are taken into account, when ceramic materials are selected or mixed to be used in accordance with the process in the present invention:

1. Final product characteristics:

- Porosity;

- Specific area;

- Mechanical crushing strength; - Impact strength;

- Abrasion strength;

- Chemical strength;

- Thermal shock strength;

- Thermal and electrical conductivities;

- Expansion coefficient;

2. Extrusion attributes

- Ability to be effectively cast or conformed;

- Plasticity;

- Lubricity; - Reproducibility

3. Drying and/or calcination / syntherization attributes

- Drying behavior, temperature and time profile;

- Binding agent behavior through the several temperature regimens;

- Stability;

- Dielectric features and conductive features, i.e., dielectric constant and loss factor along the cycle;

When the extrusion is conducted in one or more steps, additional criteria or attributes for strengths in the intermediate steps shall be considered.

SUMMARY OF THE INVENTION

The present invention relates to ceramic composition of a niobium oxide, which, due to its preparation with appropriate rheological properties, is able to be cast or conformed in the desired configuration, with controlled and

stable porosity as a result from internal reactions and/or elements interactions or addition of agents and/or additives. Among the significant advantages of the composition, there is its components handling, which are added in varying amounts, allowing to obtain products with different features, mainly concerning texture. The niobium precursor compound is initially submitted to the action of a chemical agent, which makes it susceptible to a processing step involving direct casting by extrusion or with the help of any other conformation process.

According to this invention, there is a paste composition comprising a hydrated niobium oxide, or mixed with refractory ceramic material, such as refractory oxides, carbides, nitrides, borides, silicates, such as aluminum, silicon and the like. Particulate metals, surfactants, pore-forming and binding agents - as gels - may also be included as part of the paste composition, all of them added in a lower proportion when compared to the niobium oxide / precursor. Additionally, refractory fibers may also be added to the composition, in order to obtain higher mechanical strength for the cast product.

According to the present invention, the formulation components are classified within a distinct granulometric distribution, and thereafter, mixed so as to obtain a castable composition, usually by adding chemical agents to a niobium precursor / oxide mud. The chemical agent acts in the niobium compound structure, increasing its volume as a consequence of gases generation and/or molecular rearrangements. The addition of pore-generating agents, associated to thermodynamic properties effects, such as temperature and pressure, results in pores formation in the ceramic profile extruded.

DETAILED DESCRIPTION OF INVENTION

The present invention provides casting a niobium oxide ceramic composition, and thus, the final preparation of the extruded profile shape, size and use. The initial formulation composition and the additives addition also allow to obtain extruded profiles with the porosity desired, in keeping with each application requirements, such as support for catalysts, catalyst, filling for column of absorption and adsorption of chemical elements in liquid or gaseous phase, particulates filter and the like. In the present process, the composition and the process in the invention offer substantial advantage upon processing, as the common extrusion forms do not provide the pure niobium oxide with the properties required to obtain porosity, mechanical crushing strength and appropriate and controlled specific areas. In this casting or conformation process, a chemical agent is used with the purpose of promoting Van der Waals- type bindings, hydrogen bridges and/or covalent bindings with niobium oxide / precursor with an organic material. The chemical agent is later removed in the drying and calcinations steps.

The extrusion may involve a particular mixture, comprising an appropriate ceramic composition, such as ceramic grade niobium oxide, or optical grade niobium oxide, or crystal grade niobium oxide, or hydrated niobium oxide (niobia HY® ), or organic precursor, or niobium salt, with granulometry between 0.1 to 100 μm, preferably between 17 to 100 μm, most preferably between 18 to 75 μm. The particles size distribution will be selected, mainly, in keeping with the textural properties desired (volume and pores distribution), while the binder type and content will be established considering the susceptibility to conformation by extrusion (lubrication, green strength, etc.).

The extrusion occurs preferably from a viscous paste, containing 5 to 50% moisture in weight, preferably from 8 to 30% in weight. The green material may contain an organic binder or a plasticizer within the range from 1 to 20% in weight, typically 2 to 4% in weight, based on the dry mixture.

Specifically in this invention, a ceramic composition appropriate for use as catalytic support, or catalyst, or absorbing bed, or particulates filter, may include appropriate proportions of niobium oxide or its precursor compounds, such as niobium pentachloride, niobium alkoxide, niobium acid, niobium ammoniacal oxalate, potassium niobate, sodium niobate, niobium citrate, hydrosol niobium, niobium carbide and niobium nitride, of binders and/or plasticizers, such as methylcellulose, hydroxicellulose, nitrocellulose, starch - originating from cassava, corn or potato - polymeric amines based on ethylene and gum Arabic and of lubricant agents, such as the ethylene glycol. As preferential precursors to obtain extrusion-molded niobium oxide, niobia HY® and/or soluble salts may be used, such as niobium ammoniacal oxalate and potassium niobate.

Niobia HY® is an acid, amorphous solid, insoluble in water, containing about 80% Nb ₂ Os and 20% water. The niobium ammoniacal oxalate is a water-soluble salt, containing about 25% Nb ₂ Os, 46% oxalic acid, 14% ammonium and 15% water. Potassium niobate is a water-soluble salt, containing about 50% Nb ₂ O ₅ , 17% K ₂ O and 33% water.

The final product, cast as any profile, is obtained by means of a thermal treatment / calcination of the green extruded profile, which is formed by

means of casting a niobium oxide paste with rheological properties appropriate to extrusion. This paste with appropriate properties is a result from the peptization process of the hydrolyzed and amorphous niobium oxide / precursor, which is formed by the action of certain chemical agents on the niobium- containing compounds.

The processing conditions for the niobium precursor compounds depend on their physical-chemical characteristics, in order to obtain a hydrolyzed and amorphous niobium oxide. To process the niobia HY® , a first classification step is performed, concerning granulometric distribution. This granulometry may influence the superficial dispersion process of agglomerates composed of secondary and tertiary particles, when in contact with the chemical agent, and, thereby, reduce the mechanical strength of the green extruded profile, and later, of the final product. Thereto, a fraction of particles with dimensions between 0.1 and 100 μm, preferably, between 17 and 100 μm, most preferably between 18 and 75 μm, is the most appropriate granulometric distribution for the acid compound - niobia HY® . For niobium soluble salts processing, such as potassium niobate and niobium ammoniacal oxalate, a solution, with known concentration, is previously prepared, as required by each product features. These salts are, preferably, solubilized in water or in a water- alcohol mixture, alcohols such as methanol, ethanol, isopropanol or a water- polyoils mixture, such as ethylene glycol. In a precipitation process, whose processing conditions occur, preferably, with pH rate within the range from 3 to 10, most preferably within the range from 5 to 9 and at reaction temperature, preferably within the range from 2 to 30 ⁰ C, most preferably within the range from 5 to 25°C, a basic ammonium hydroxide, or sodium hydroxide, or

potassium hydroxide solution is added, preferably with concentration within the range from 1 to 50%, most preferably with concentration within the range from 1 to 25% is added to a niobium and ammonium oxalate precursor salt solution, with Nb concentration preferably within the range from 1 to 10%, most preferably within the range from 1 to 5%. The result of this reaction is the formation of colloidal niobium hydroxide, which is left to rest for a period not under 24 hours, in order to promote niobium precursor compound aging. In the next step, the material is isolated by filtration with the help of a vacuum pump and a funnel. The material retained in the filter is dried at 50 ⁰ C for a period not over 20 hours, ground and classified within the range 18 - 75 μm, resulting, therefore, in hydrolyzed and amorphous niobium oxide. Also with a precipitation process, whose processing conditions occur, preferably, with pH within the range from 1 to 10, most preferably within the range from 1 to 9, and at reaction temperature, preferably within the range from 2 to 30 ⁰ C, most preferably within the range from 3 to 25 ⁰ C, nitric acid, or hydrochloric acid, or sulfuric acid, preferably nitric acid with concentration within the range from 5 to 70%, most preferably within the range from 25 to 70% is added to a potassium niobate precursor salt solution with Nb concentration preferably within the range from 1 to 20%, most preferably within the range from 2 to 15%. The result of this reaction is the formation of colloidal niobium hydroxide, which is left to rest for a period not under 24 hours, in order to promote oxide aging. In the next step, the material is isolated by filtration with the help of a vacuum pump and a funnel. The material retained in the filter is dried at 5O ⁰ C for a period not over 24 hours, ground and classified within the range 18 - 75 μm, resulting, therefore, in hydrolyzed and amorphous niobium oxide. As the niobium precursors originating from niobium ammoniacal oxalate and potassium niobate generate

hydrated oxides under colloidal state, these may be submitted to a hydrothermal treatment or not.

The precursor compound niobai HY® , granulometrically classified, is submitted to a hydrothermal treatment. The hydrothermal technique as described by G. W. Morey, J. Am. Ceram. Soc, 36 (1953) 279, is based on minerals synthesis, using an autoclave-type reactor or acid or basic digester pump, being the material submitted to high temperatures (250 ⁰ C) and pressures (15 MPa). hi the present invention, the hydrothermal technique will be used to promote forced hydrolysis between the cation and the hydroxyls groups. In hydrothermal conditions, the cation hydrolysis is stronger than at room temperature, and this hydrolytic reaction may result in direct formation of polynucleated oxides. Such conditions make the niobium precursor compound susceptible to extrusion, when associated to a chelating, basic or acid chemical agent. The chelators are compounds which promote partial solubilization of an oxide or oxides mixture, minimizing the size of an oxides mass particles, generating a hydrolyzed ceramic precursor compound. Among the chelating chemical reagents able to promote niobium oxide polymerization, there are: a base, such as ammonium hydroxide and the organic compounds, such as ethylene glycol and organic acids, such as acetic, tartaric, citric and oxalic acids. Niobia HY® precursor hydrolysis or oxides mixture is made with a hydrothermal treatment, which is performed by adding, to a certain amount of the precursor, a certain volume of the concentration chelating chemical agent, preferably, within the range from 1 to 25%, most preferably within the range from 5 to 10%. Thereafter, the suspension is placed in an autoclave and heated at a temperature within the range from 80 to 25O ⁰ C, preferably within the range

from 85 to 200 ⁰ C, most preferably within the range from 90 to 17O ⁰ C, for a period of time not under 72 hours, preferably within the range from 1 to 72 hours, most preferably within the range from 3 to 18 hours. In this case, the chelating chemical agents that better promote the hydrolysis between the cation and the hydroxyls groups are oxalic acid and ammonium hydroxide. During the hydrothermal treatment, the precursor compound structure is modified, favoring the covalent bindings between the precursor and the chemical agent. F. Fairbrother and J.B. Taylor, J. Chem. Soc. (1956), 4946, reports that the niobium oxide is easily hydrolizable and stable in the presence of excessive chelating chemical agent. This is probably due to the acid anion or basic cation coordination with niobium. The reaction involves the oxygen ions and the oxalic acid -OH groups, which work as complex-forming acid, and concerning the ammonium hydroxide, ions NH ₄ ⁺ and OH ^" as complex-forming base. After autoclaving, the hydrolyzed solution is then filtered with the help of a vacuum pump and a syntherized porous plate funnel. The material retained in the filter is dried at room temperature for a period not over 24 hours, resulting, therefore, in hydrolyzed and semi-crystallyzed niobium compound.

The hydrolyzed and semi-crystallized compound, originating from niobia HY® precursors, or niobium ammoniacal oxalate, or potassium niobate, is then peptized / redispersed, with the help of a chemical agent, usually by adding an organic acid, such as acetic acid, or citric acid, or oxalic acid, preferably oxalic acid, with concentration preferably within the range from 5 to

50%, most preferably within the range from 7 to 20%. This peptization step is usually conducted by heating at a temperature not under 100 ⁰ C. The oxalic acid in the appropriate concentration is added, slowly, to the hydrolyzed and

amorphous niobium compound or oxides mixture, and is simultaneously homogenized by a sigma-type mixer. During this peptization process, the inorganic oxides polymeric chains are formed, generating a niobium compound paste with rheological properties appropriate to casting or conformation by extrusion, and providing the green extruded profile with the mechanical strength appropriate for its handling in the coming steps. The viscous gel formed is then extruded as pellets, depending on the extruder matrix, such as briquettes, tubes or cylinders. The extrusion step is usually performed with the help of an extrusion equipment - which may be vacuum-operated - consisting, basically, of a supplying inlet, a helicoidal thread, whose type varies according to the rheological features of the paste to be cast, arranged inside a cylinder, and an outlet plate, named matrix, which is usually equipped with several orifices with several diameters and shapes, providing the green extruded with the shape desired. The highly viscous paste containing the niobium precursor compound is supplied through the supplying inlet, and pumped by the helicoidal thread, and finally forced to cross the orifices plate. It may also be observed that any bubbles that may form in the extruded, may reduce mechanical crushing strength, hi order to avoid this problem, the extruding machine is required to have a relief point, i.e., it shall be vacuum-operated.

After hydrothermal treatment and extrusion steps, the green extruded profile is obtained with the mechanical strength required for handling. The green extruded profile undergoes a mild thermal treatment, which involves drying steps at room temperature for a period not under 12 hours. Thereafter, heating is performed, until a temperature between 25 and 170 ⁰ C is attained, at heating rate 0.25 and l°C/min., remaining on that temperature path for a period

not under 5 hours. At the final step of the process, during which the precursor compound transforms into cast niobium oxide, a calcination is performed on the green extruded profile, at a temperature depending on its specific application. Concerning its use in catalytic reactions and where the catalyst acidity is important, calcination is performed with the temperature between 200 and 470 ⁰ C, resulting in an semi-crystallized material. In catalytic support function, some applications require acidity absence, however, even in this case, the cast material shall have a high specific area, so as to obtain a uniform dispersion of the active phase on the support surface, mostly a metal or a mixture thereof, resulting in a catalyst with higher specific activity. In these cases, the calcination temperature shall be established between 470 and 850 ⁰ C, preferably between 470 and 700 ⁰ C, most preferably between 470 and 600 ⁰ C, resulting in a niobium oxide extruded with predominantly orthorhombic crystalline structure. During this step, the material micro, meso and macroporous structures are generated, resulting from chemical reactions involved in the transformation of the precursor in niobium oxide and of the thermal decomposition of compounds added during the several steps, such as organic, inorganic, plasticizing, dispersing, lubricant, inert chemical agents, pore-forming compounds, etc. For other applications, such as particulates filter and chemical elements absorbers, the calcinations temperatures may reach 1400 ⁰ C. The calcination step heating rate is under 20 ⁰ CnUn. ^"1 , preferably between 0.5 and 2O ⁰ CnUn ^'1 , most preferably between 0.5 and 10 ⁰ CmIn. ^"1 , precisely between 0.5 and 5 ⁰ CnUn ^"1 .

Examples of the present invention are shown below, illustrating the variety of compositions / formulations, processing techniques, chemical agents contents, etc. The product characterization involved establishing the mechanical

crushing strength, measured following the methodology described in ASTM D4179/82 - Test Method for Single Pellet Crush Strength of Formed Catalyst Shapes. The method is intended to establish the individual mechanical strength to compression of particles (spheres, cylinders, etc.) whose dimensions (length and diameter) do not vary more than 10% from the nominal value. The specific area has been measured with the help of N ₂ physisorption, using the BET method (Brunauer, Emmett and Teller). The micro and mesopores distribution has been obtained by applying BJH method for N ₂ desorption. The macropores distribution and volume have been established by mercury porosimetry. The classification concerning micro, meso and macropore has been established according to IUPAC (D. H. Everett, Pure Appl. Chem., 31 (1972) 579). The porous structure has been assessed with the help of Scanning Electron Microscopy (SEM) and the prevailing crystalline step has been established with the X-ray difractrometry technique and Transmission Electronic Microscopy (TEM).

EXAMPLE 1

About 0.075 moles of niobia HY® , with average agglomerates diameter - φ _m (values used for the several parameters are described on table 01) and 100 ml oxalic acid at a concentration - CAC are mixed and placed in an autoclave or 150-ml digester pump. The "mud" formed was heated up to the temperature - TAC, and kept on that path for a time - tAC. The solution is then cooled and the excessive oxalic acid is eliminated by filtration, with the help of a vacuum pump and a syntherized porous plate funnel. The material retained in the filter was dried at room temperature for a 24-hour period. Thereafter, the powder is transferred to a sigma mixer, to which the oxalic acid is added at 10%,

and homogenized for one hour. The paste susceptible to extrusion is supplied to a mono-thread or piston-type extruder, equipped with a matrix with a single outlet, with approximate diameter of 2.5 mm. After extrusion, the green extruded profile is cut in such a way to obtain cylinders, varying from 3 to 4 mm long, leaving them to rest at room temperature for a 24-hour period. After the cut, the green extruded profile thermal treatment is performed mildly, involving heating room temperature up to 12O ⁰ C, at a 0.7°C/min. rate, and remaining on this path for 10 minutes. Thereafter, the temperature is raised from 12O ⁰ C to a temperature - TCAL, at a 0.75°C/min rate, under 600 ml/min air flow, remaining at this temperature for 6 hours.

Table 1 shows the experimental conditions used in the assays performed, in which it has been searched to investigate the influence of the distribution of the niobium precursor compound particles size, of the hydrothermal treatment temperature, of the hydrothermal treatment time, of the oxalic acid content, of the green cast precursor calcination temperature.

Table 1: Experimental conditions used upon assays for production of niobium oxide precursor cast by extrusion, obtained from niobia HY ^® .

Average Temperature - Time - Concentration Temperature Assays diameter - TAC tAC - CAC -TCAL φ _m (μm) (°C) (h) (%) ( ⁰ C)

Assay 01 48.8 120 3 5 350

Assay 02 18.5 120 6 5 350

Assay 03 18.5 170 6 10 350

Assay 04 48.8 120 6 5 350

Assay 05 48.8 120 3 10 350

Assay 06 18.5 170 3 5 350

Assay 07 48.8 170 6 5 350

Assay 08 18.5 120 3 10 350

Assay 09 48.8 170 3 10 350

Assay 10 18.5 170 6 5 350

Assay 11 18.5 120 3 5 350

Assay 12 48.8 170 3 5 350

Assay 13 18.5 170 3 10 350

Assay 14 48.8 120 6 10 350

Assay 15 18.5 120 6 10 350

Assay 16 48.8 170 6 10 350

Assay 17 18.5 170 6 10 500

Assay 18 48.8 120 6 5 500

Assay 19 48.8 170 6 5 500

Assay 20 18.5 120 3 5 500

The series of experiments resulted, after calcination in the respective calcination temperatures, in niobium oxides with different textural and morphological properties. These differences were analyzed by means of specific area and individual crushing strength properties, summarized on table 2.

Table 2: Mechanical strength and specific area of calcined extruded profiles

Assays Mechanical strength (N/mm) Specific area (m ² /g)

Assay 01 18.2 82

Assay 02 20.7 66

Assay 03 12.8 151

Assay 04 18.9 98

Assay 05 12.3 28

Assay 06 9.5 138

Assay 07 8.0 133

Assay 08 18.0 30

Assay 09 9.5 14

Assay 10 15.2 132

Assay 11 30.7 65

Assay 12 9.7 119

Assay 13 23.8 88

Assay 14 8.3 19

Assay 15 14.8 21

Assay 16 0.0 0

Assay 17 15.6 90

Assay 18 16.1 49

Assay 19 8.1 73

Assay 20 3L0 41

Mechanical crushing strength decreases as increases the average diameter of niobia HY® precursor compound particles agglomerate and of hydrothermal treatment conditions (temperature, operation time and oxalic acid concentration). The specific area decreases as decreases the hydrothermal treatment temperature, as increases the average diameter of niobia HY® particles and the oxalic acid concentration used in the hydrothermal treatment.

The pores distribution has been assessed in extruded profiles calcined at 35O ⁰ C, which had larger specific area, higher mechanical crushing strength and intermediate values for specific superficial area and mechanical strength. Table 3 summarizes the results from these analyses.

Table 3: Pores distribution (%) and total pores volume

Total pores

Assays 100<d _p <500 A 500<d _p <2500 A d _p >2500 A volume (ml/g)

Assay 03 21% 22% 57% 0.122 Assay 11 19% 61% 20% 0.132 Assay 12 12% 27% 61% 0.123 d _p : pores diameter

EXAMPLE 2

Hydrolized and amorphous niobium oxide is formed by the reaction between ammonium niobium oxalate (ANO) precursor, previously sohibilized in deionized water, containing a known concentration of Nb in the solution - CR (the values used for the several parameters are described on table 04), and a basic chemical agent. An ammonium hydroxide solution at 25% is added to the

precursor salt solution, precipitating the niobium oxide in pH - ADpH. The system is shaken and kept at reaction temperature - RT for 20 minutes. Upon precipitate formation, the solution is kept at rest for 24 hours at room temperature. After this period, the solution containing the precipitate is filtered with the help of a vacuum pump and a Bϋchner funnel. The precipitate retained in the funnel is then dried at 5O ⁰ C for 15 hours. The material is ground and classified, and a fraction is selected, whose dimension is lower than 62 μm. Thereafter, the powder is transferred to a sigma mixer, to which oxalic acid at 10% is added, and it is homogenized for 1 hour. The paste susceptible to extrusion is supplied into a mono-thread or piston-type extruder, equipped with a matrix with a single outlet, with about 2.5 mm diameter. After extrusion, the green extruded profile is cut so as to obtain cylinders between 3 and 4 mm long, leaving them to rest at room temperature for a 24-hour period. After cutting, the thermal treatment for the green extruded profile cut is performed mildly, comprising heating at room temperature up to 130 ⁰ C, at a 0.7°C/min. rate, and remaining on this path for 10 minutes. Thereafter, the temperature is raised from 130 ⁰ C to 500 ⁰ C, at a 0.75°C/min. rate, under 600 ml/min air flow, remaining at this temperature for 6 hours.

Table 4 reports experimental conditions used in the assays performed, varying the precursor niobium salt concentration, reaction medium temperature and pH.

Table 4: Experimental conditions used in assays performed upon production of extrusion cast niobium oxide precursor obtained from niobium and ammonium oxalate.

_A Nb Reaction Reaction pH - ADpH concentration in Temperature - RT

the solution - (°c)

CR (%)

Assay 21 1.0 5 6.5

Assay 22 1.0 25 6.5

Assay 23 2.0 5 8.5

Assay 24 2.0 25 8.5

Assay 25 1.0 5 6.5

Assay 26 1.0 25 6.5

Assay 27 2.0 5 8.5

Assay 28 2.0 25 8.5

Assay 29 2.0 25 8.5

In this 9-assay series, the precipitate formed was not washed. The extruded profiles formed were then calcined at 500 ⁰ C temperature for 6 hours, and the niobium oxides physical properties, such as specific area and individual crushing strength, were analyzed and are summarized on table 5.

Table 5: Mechanical strength and specific area of calcined extruded profiles

Assays Mechanical Strength (N/rnm) Specific Area (m /g)

Assay 21 13.7 42

Assay 22 25.5 36

Assay 23 7.4 42

Assay 24 7.4 45

Assay 25 6.3 46

Assay 26 8.8 42

Assay 27 10.1 26

Assay 28 8.7 27

Assay 29 6.1 43

Some of the pores distributions of the extruded profiles calcined at 500 ⁰ C are shown on table 6.

Table 6: Pores distribution (%) and total pores volume

Assays 100<d _p <500 A 500<d _p <2500 A d _p >25Oθ A ^T ° ^tal _t $°* ^{GS J v _ P v _ volume (ml/g)}

Assay 21 63.5% 1.5% 35% 0.110 Assay 22 39% 6% 55% 0.105

d _p : pores diameter

In another series with 3 assays, the precipitate formed was washed. Table 7 shows the experimental conditions adopted in the precipitate formation process. The profiles obtained after extrusion were then calcined at 500 ⁰ C temperature for 6 hours, and their textural and mechanical properties were assessed by means of specific area and individual crushing strength properties, summarized on table 8.

Table 7: Experimental conditions used in assays performed upon production of extrusion cast niobium oxide precursor obtained from niobium and ammonium oxalate

Nb concentration Reaction temperature - Reaction pH - Assays in the solution - RT ADpH CR (%) CQ

Assay 30 1.0 25 6.5

Assay 31 2.0 25 6.5

Assay 32 ZO 25 8J>

Table 8: Mechanical strength and specific area of extruded profiles calcined

Assay 30 349 32 Assay 31 18.2 14 Assay 32 2jU 17

In the assays performed, the effects of the variants investigated on mechanical crushing strength and specific area were observed. Based on these assays, it is possible to state that the mechanical crushing strength increases when a precipitate washing step is introduced, with reaction temperature at about 25°C, and when Nb concentration and reaction medium pH decrease. The specific area decreases when the precipitate washing step is introduced, and when Nb concentration and reaction medium pH increase.

EXAMPLE 3

Hydrolized and amorphous niobium oxide is formed by the reaction between potassium niobate (KNb) precursor, previously solubilized in deionized water, containing a known concentration of Nb in the solution - CR (the values used for the several parameters are described on table 9), and an acid chemical agent. The nitric acid with concentration between 65 and 70% is added to the precursor salt solution, precipitating the niobium oxide in pH - ADpH. The system is shaken and kept at reaction temperature - RT for 20 minutes. After this period, the solution containing the precipitate is filtered with the help of a vacuum pump and a Bϋchner funnel. The precipitate retained in the funnel is washed with an ammonium chloride solution (0.5%), and then dried at 50 ⁰ C for 24 hours. The material is ground and classified, and a fraction is selected, whose dimension is lower than 62 μτn. Thereafter, the powder is transferred to a sigma mixer, to which oxalic acid at 10% is added, and it is homogenized for 1 hour. The paste susceptible to extrusion is supplied into a mono-thread or piston-type extruder, equipped with a single outlet, with approximate 2.5 mm diameter. After extrusion, the green extruded profile is cut so as to obtain cylinders between 3 and 4 mm long, leaving them to rest at room temperature for a 24- hour period. After cutting, the thermal treatment for the green extruded profile cut is performed mildly, comprising heating at room temperature up to 130 ⁰ C, at a 0.7°C/min. rate, and remaining on this path for 10 minutes. Thereafter, the temperature is raised from 130 ⁰ C to 500 ⁰ C, at a 0.75°C/min. rate, under 600 ml/min air flow, remaining at this temperature for 6 hours.

Table 9 reports experimental conditions used in the assays

performed, varying the precursor niobium salt concentration, temperature and reaction medium pH.

Assay 34 4.0 25 4.0

Assay 35 7.5 3 8.0

Assay 36 7.5 3 8.0

Assay 37 7.5 25 8.0

Assay 38 4.0 3 4.0

Assay 39 7.5 25 8.0

Assay 40 4.0 3 4.0

Assay 41 4.0 25 4.0

The 9-assay series produced calcined extruded profiles at 500 ⁰ C for 6 hours. Specific area and mechanical crushing strength properties were established and are shown on table 10.

Table 10: Mechanical strength and specific area of calcined extruded profiles

Assays Mechanical Strength (N/mm) Specific Area (m /g)

Assay 33 16.6 24

Assay 34 12.0 32

Assay 35 10.2 16

Assay 36 11.8 17

Assay 37 9.6 30

Assay 38 13.3 22

Assay 39 6.2 32

Assay 40 8.8 37

Assay 41 16.0 30

Some pore distributions of the extruded profiles, calcined at 500 ⁰ C

are shown on table 11.

Table 11: Pores distribution ( %) and total pores volume

Total pores

Assays 100<dp<500 A 500<dp <2500 A d _p >2500 A volume (ml/g)

Assay 33 43% 57% 0% 0.110 Assay 39 21% 79% 0% 0.105 d _p : pores diameter

In the assays performed, the effects of the variants investigated on mechanical crushing strength and specific area were observed. The mechanical crushing strength increases when the reaction temperature increases, Nb concentration decreases and reaction medium pH increases. The specific area, on its turn, decreases when the Nb concentration increases, pH 1 increases and reaction medium temperature decreases.