CA2071593A1 - Process for the production of sintered material based on .alpha.-aluminum oxide, especially for abrasives - Google Patents
Process for the production of sintered material based on .alpha.-aluminum oxide, especially for abrasivesInfo
- Publication number
- CA2071593A1 CA2071593A1 CA002071593A CA2071593A CA2071593A1 CA 2071593 A1 CA2071593 A1 CA 2071593A1 CA 002071593 A CA002071593 A CA 002071593A CA 2071593 A CA2071593 A CA 2071593A CA 2071593 A1 CA2071593 A1 CA 2071593A1
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- Prior art keywords
- aluminum oxide
- alpha
- process according
- suspension
- acid
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/44—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
- C01F7/441—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by calcination
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
- C04B35/10—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on aluminium oxide
- C04B35/111—Fine ceramics
- C04B35/1115—Minute sintered entities, e.g. sintered abrasive grains or shaped particles such as platelets
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K3/00—Materials not provided for elsewhere
- C09K3/14—Anti-slip materials; Abrasives
- C09K3/1409—Abrasive particles per se
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2002/00—Crystal-structural characteristics
- C01P2002/60—Compounds characterised by their crystallite size
Abstract
ABSTRACT OF THE DISCLOSURE
A process for manufacturing a sintered material usable as an abrasive is disclosed. The material is based on .alpha.-aluminum oxide, and is produced from Al(OH)3 or aluminum oxide calcined at a lower temperature. Aluminum hydroxide resulting from the Bayer process can be used as the Al(OH)3. The sintered material is distinguished by a crystallite size of typically 0.5 µm, high density and great hardness.
A process for manufacturing a sintered material usable as an abrasive is disclosed. The material is based on .alpha.-aluminum oxide, and is produced from Al(OH)3 or aluminum oxide calcined at a lower temperature. Aluminum hydroxide resulting from the Bayer process can be used as the Al(OH)3. The sintered material is distinguished by a crystallite size of typically 0.5 µm, high density and great hardness.
Description
2071~93 The present invention relates to a process for the production of sintered material, especially abrasive grain, based on aluminum oxide made from aluminum hydroxide (gibbsite or hydrargillite) or aluminum oxide calcined at a lower temperature.
~ -Aluminum oxide (corundum), because of its great hardness, has been used for many decades as an abrasive.
The standard process for the production of corundum suitable for grinding purposes comprises melting aluminum oxide (alumina) or raw materials containing aluminum oxide (bauxite) in an electric arc furnace and, after cooling, crushing and screening the solidified mass to the desired abrasive grain size. Both melting and size reduction use a lot of energy because of the high melting point and great hardness, and thus require units that are expensive to acquire and maintain. Moreover, the properties of the abrasive grains thus obtained, especially their toughness, are not optimal for many uses.
Therefore, efforts were undertaken at an early stage to obtain corundum abrasive grains by heating compounds containing aluminum helow the melting point of corundum (approximately 2050C). However, in this connection, it turned out that it was not only important that the material to be sintered be dense and nonporous, but that a decisive role is played above all by the microstructure of the sintered material. It is especially important that a uniformly fine texture with crystallite sizes of < 1 ~m result without the inclusion of some course crystals.
The known solutions achieve this aim by the so-called sol-gel process using highly pure boehmite (aluminum oxide-monohydrate, AlOOH) as the initial material (see, for example, European Published Patent Application No. 24099) and optionally adding crystallization seeds which prevent the formation of large crystallites, making possible a quick, complete crystallization in the desired 2071~93 modification, so that at a lower temperature there is neither time nor space for excessive growth of the individual crystallites (ref. European Published Patent Application No. 152768). The thus-obtained products actually are qualitatively of very high grade, but relatively expensive, since the initial material is produced by hydrolysis of aluminum alkoxides which are quite expensive.
The necessary low content of alkali metals, especially sodium, however, can hardly be achieved in any other way. A low sodium content is especially important to avoid the formation of B-aluminum oxide during heating.
The latter has an especially disadvantageous effect on the abrasive properties because it is formed as course crystals.
It has also been attempted to obtain abrasives of comparable quality starting from less high grade boehmite.
This, however, has only been attained by the addition of considerable amounts (atom ratio to aluminum 1:35 to 1:2) of sintering auxiliary agents and adhering to a specific rate of heating (ref. West German Patent No. 3,219,607).
However, by these additions, additional solid phases are formed, such as, for example, the spinels already described in the above-mentioned European Published Patent Application No. 24099, which are undesirable because they make the abra~ive grain "softer".
A main object of the invention is to provide a process for the production of a sintered aluminum oxide abrasive that starts from reasonably priced raw materials and yields, in a simple way and without special additions, a product with high grinding performance.
According to the present invention, there is provided a process for the production of sintered materials based on a-aluminum oxide, comprising the steps of subjecting aluminum hydroxide [Al(OH)3] or an aluminum oxide calcined at a lower temperature, either being a precursor 207~593 of ~-aluminum oxide, to a grinding and deagglomeration treatment; producing a suspension from the deagglomerated precursor of ~-aluminum oxide and optionally additives;
drying the suspension; and thereafter sintering the dried suspension.
Thus, the invention involves a process for the production of sintered materials based on ~-aluminum oxide.
A suspension of a precursor of ~-aluminum oxide and optionally usual additives, is prepared. Aluminum hydroxide [Al(OH)3] or an aluminum oxide calcined at a lower temperature is used as the precursor of Q-aluminum oxide and is subjected to a grinding and deagglomeration treatment for the formation of the suspension. The process continues by drying the suspension and sintering the dried material.
Preferably the grinding and deagglomeration treatment is performed with an attrition mill, a vibratory mill or a stirred ball mill. Preferably the attrition mill, vibratory mill or stirred ball mill is operated with grinding media consisting mainly of ~-aluminum oxide.
Pre~erably crystallization seeds of ~-aluminum oxide are added to the suspension to prevent grain growth during sintering. Advantageously, the suspension is adjusted to a pH of less than 5 by adding an acid, preferably nitric acid, hydrochloric acid, acetic acid, citric acid, formic acid or oxalic acid. Preferably the suspension is subjected to a vacuum treatment to remove dissolved and/or adsorbed gases. Preferably the sintering is performed at a temperature of 1100 to 1500C. Aluminum hydroxide resulting from the alumina production by the Bayer process is the preferred source of aluminum hydroxide.
The invention also includes sintered material based on ~-aluminum oxide and optionally additions of other oxides, carbides, nitrides, silicides or metals, obtained by the process of the invention.
The invention further includes the process of using the sintered material of the invention as abrasive grain.
It has been found that it is possible, by a suitable combination of process steps, to produce from ordinary industrial aluminum hydroxide (Al(OH)3, gibbsite, also called hydrargillite) as it is yielded in the Bayer process, sintered ~-aluminum oxide of great density and hardness with a crystallite size of less than 1 ~m, for example, even less than 0.5 ~m.
The production of ~-aluminum oxide from industrial aluminum hydroxide is generally known (it is a step in the industrial-scale production of aluminum), but the thus-obtained aluminum oxide usually has properties that are completely unsuitable for normal grinding purposes, namely, a high porosity and poor sintering properties. Sintering commences at temperatures so high, that it is connected with strong crystal growth and finally actually yields a dense, but -- because of the coarse texture -- mechanically unsatisfactory product, which does not offer any significant advantages relative to usual corundum. The so-called tabular alumina, for example, is produced in a similar way which is distinguished by its large (several hundred ~m) tabular crystals. Only for certain types of surface treatment, such as polishing, in which it is less important for the material removal, are such aluminum oxides suitable (ref. East German Patent Specification No. 76485).
The usual aluminum hydroxide is not suitable for use in the above-mentioned sol-gel process since it disperses poorly and cannot be gelled (for more information on the properties of aluminum hydroxides see, e.g., Ullmann's Encyclopedia of Industrial ChemistrY, Volume A1, VCH Verlagsgesellschaft mbH, Weinheim, (1985), pages 557 to 594)-207~93 According to the invention, the initial materialis first subjected to wet grinding or deagglomeration to divide the agglomerates present as a result of the production process, into individual crystallites. The deagglomeration is preferably performed in an attrition mill, a vibratory mill or a stirred ball mill, wherein grinding media are preferably used which consist entirely or predominantly or aluminum oxide. The amount of liquid is preferably selected so that a suspension results with a solids content of 10 to 40 percent by weight. Water is preferably used as the liquid, but it is also possible to replace the water partly with water-miscible and easily vaporizable solvents, such as lower alcohols or acetone.
The thus obtained suspension is advantageously adjusted by acid addition to a pH of less than 5, by which dissolved or adsorbed carbon dioxide is expelled.
Preferably the suspension is adjusted to a pH of about 2 to 4. The acid suitably is nitric acid, hydrochloric acid, citric acid, formic acid, acetic acid or oxalic acid, preferably hydrochloric acid. The necessary amount of acid in this case depends on the properties of the aluminum hydroxide, and above all, on its specific surface. An acid addition can be completely or partially dispensed with by instead using a vacuum treatment of the suspension for degassing.
Cry~tallization seeds, preferably composed of a-aluminum oxide, are suitably added to the aluminum hydroxide suspension These seeds can be obtained, for example, simply by grinding of a-aluminum oxide, for example, in the form of calcined alumina, to a particle size of < 1 ~m. Preferably the seeds are added in an amount of 1 to 5 percent by weight, relative to the total amount (calculated as Al2O3), and thoroughly mixed in.
Addition of the crystallization seeds at the beginning or during the deagglomeration is especially preferred. In addition to the crystallization seeds, optionally auxiliary 2071~3 or additional substances, such as foam separators, sintering auxiliary agents, grain growth inhibitors, etc., can be added. However, such auxiliary substances are not necessary for operation of the process according to the invention.
The suspension thus obtained is subsequently dried. The drying is suitably performed below the boiling point temperature to prevent the formation of vapour bubbles. At standard pressure, a drying temperature of about 70C is advantageous. If the suspension is present, for example, in a layer thickness of about 10 cm, drying can be performed at this temperature in 2 to 3 days. With the application of higher pressures the drying temperature can be increased corresponding to the higher boiling point and the drying time can thereby be shortened. The volume or the layer thickness corresponding to the decreasing liquid content is reduced during the drying operation without resulting in a significant porosity. An open porosity of ~ 0.05 ml/g and an average pore diameter of <
10 nm tdetermined according to the mercury-penetration method) can be achieved.
The thus-obtained dried cake is subsequently sintered, optionally after grinding to the grain size corresponding to the desired abrasive grain size (taking into consideration shrinkage during sintering). The sintering temperature is suitably at 1100 to 1500C. The sintering time depends on the temperature and i8, for example, about 2 hours at 1400C.
Despite the strong volume contraction (about 30 percent linear shrinkage), a dense sintered product is obtained with the process according to the invention by the conversion of aluminum hydroxide into ~-aluminum oxide without a separate calcination step being required.
Instead of aluminum hydroxide, an aluminum oxide calcined at a lower temperature, or mixtures of the latter and aluminum hydroxide, can also be used for the process 207~593 according to the invention. Aluminum oxides calcined at a lower temperature still contain some water, for example about 8 percent by weight, and are again converted into aluminum hydroxide by water absorption with the treatment according to the invention, as shown by thermogravimetric examination of the dried suspension. since the aluminum oxides calcined at a lower temperature are produced, on their part, from aluminum hydroxide and use thereof in the invention offers no special advantages, aluminum hydroxide is to be preferred as the initial material for the process according to the invention.
The sintered material produced according to the invention is distinguished by a very fine crystallite size, high sintered density and great hardness. Its toughness is in the area of 2.5 MPa-m~ or greater. It is suitable not only as an abrasive, but also for other uses in which these characteristics are important.
The following Examples illustrate the performance of the process according to the invention.
Example 1 In an attrition mill (0.6 liter), lOO g of pure aluminum hydroxide (Martinal~ OL-104), Martinswerk GmbH, D-W-5010 Bergheim) was ground for 2 hours in desalinated water with the addition of 1.5 percent by weight (relative to the aluminum hydroxide) of ~-aluminum oxide seeds with aluminum oxide-grinding balls (d = 1 mm) and deagglomerated. The particle or agglomerate size before grinding was 100 percent smaller than 10 ~m, after grinding it was 100 percent smaller than 1 ~m. The ~-aluminum oxide seeds were obtained by grinding aluminum oxide (calcined at high temperature) in the attrition mill to a particle size of ~ 0.5 ~m. The pH was adjusted to 2 by addition of about 20 ml of 37 percent hydrochloric acid before the grinding.
The suspension was dried for 2 days in an initial layer thickness of 5 cm at 70C. The average pore diameter after drying was 9.5 nm (mercury-porosimetry), and the open 2071~93 porosity was less than 0.05 ml/g. An examination of the dried material with thermogravimetry or differential thermal analysis showed that the bound water escapes below 350c and the crystallization to ~-Al2O3 set in at 1010C.
After 2 hours of sintering at 1400C, a material with a crystallite size of < 0.5 ~m, a density of > 3.8 g/ml (> 95 percent th.D.) and a hardness according to Vickers (500 g load) > 19 GPa, was obtained.
Example 2 The process was performed as described in Example 1, except that, instead of aluminum hydroxide, 70 g of an aluminum oxide (type HLS, Martinswerk), calcined at a lower temperature was used (loss on ignition about 5 percent by weight; Na20, about 0.2 percent by weight; other metals, 0.05 percent by weight; particle size, 99 percent < 1 ~m;
specific surface, 200 m2/g). A thermogravimetric examination of the dried material yielded a weight loss of 35 percent at 400C, corresponding to the composition Al(OH)3. After 2 hours of sintering at 1400C, a product with the same properties as in Example 1 was obtained.
Example 3 The process was performed as described in Example 2, however, with aluminum oxide of type AX (Martinswerk) calcined at a lower temperature (loss on ignition about 6 percent by weight; Na2O, about 0.2 g by weight; other metals, 0.06 percent by weight; grain size distribution, about 25 percent < 106 ~m, about 10 percent < 45 ~m;
specific surface, about 175 m2/g) as the initial material.
The sodium content after sintering (1400C, 1 hour) was 0.11 percent by weight. No ~-A12O3 was detectable in the sintered material by x-ray diffraction analysis (detection limit about 1 percent by weight).
2071~93 Example 4 ~Comparison Example) The process was performed as described in Example 2. However, instead of the attrition mill treatment, the aluminum oxide was mixed for only 30 minutes in a colloid mill (Ultra Turrax~, Janke and Kunkel). The cake obtained after three days of drying at 70OC had an average pore diameter of 400 nm and an open porosity of 0.06 ml/g. A
material with an open porosity of 0.11 ml/g was obtained by calcining at 1200C (5 hours).
The material according to Example 2, on the other hand, had an open porosity of only 0.03 ml/g after 5 hours at 1200C.
~ -Aluminum oxide (corundum), because of its great hardness, has been used for many decades as an abrasive.
The standard process for the production of corundum suitable for grinding purposes comprises melting aluminum oxide (alumina) or raw materials containing aluminum oxide (bauxite) in an electric arc furnace and, after cooling, crushing and screening the solidified mass to the desired abrasive grain size. Both melting and size reduction use a lot of energy because of the high melting point and great hardness, and thus require units that are expensive to acquire and maintain. Moreover, the properties of the abrasive grains thus obtained, especially their toughness, are not optimal for many uses.
Therefore, efforts were undertaken at an early stage to obtain corundum abrasive grains by heating compounds containing aluminum helow the melting point of corundum (approximately 2050C). However, in this connection, it turned out that it was not only important that the material to be sintered be dense and nonporous, but that a decisive role is played above all by the microstructure of the sintered material. It is especially important that a uniformly fine texture with crystallite sizes of < 1 ~m result without the inclusion of some course crystals.
The known solutions achieve this aim by the so-called sol-gel process using highly pure boehmite (aluminum oxide-monohydrate, AlOOH) as the initial material (see, for example, European Published Patent Application No. 24099) and optionally adding crystallization seeds which prevent the formation of large crystallites, making possible a quick, complete crystallization in the desired 2071~93 modification, so that at a lower temperature there is neither time nor space for excessive growth of the individual crystallites (ref. European Published Patent Application No. 152768). The thus-obtained products actually are qualitatively of very high grade, but relatively expensive, since the initial material is produced by hydrolysis of aluminum alkoxides which are quite expensive.
The necessary low content of alkali metals, especially sodium, however, can hardly be achieved in any other way. A low sodium content is especially important to avoid the formation of B-aluminum oxide during heating.
The latter has an especially disadvantageous effect on the abrasive properties because it is formed as course crystals.
It has also been attempted to obtain abrasives of comparable quality starting from less high grade boehmite.
This, however, has only been attained by the addition of considerable amounts (atom ratio to aluminum 1:35 to 1:2) of sintering auxiliary agents and adhering to a specific rate of heating (ref. West German Patent No. 3,219,607).
However, by these additions, additional solid phases are formed, such as, for example, the spinels already described in the above-mentioned European Published Patent Application No. 24099, which are undesirable because they make the abra~ive grain "softer".
A main object of the invention is to provide a process for the production of a sintered aluminum oxide abrasive that starts from reasonably priced raw materials and yields, in a simple way and without special additions, a product with high grinding performance.
According to the present invention, there is provided a process for the production of sintered materials based on a-aluminum oxide, comprising the steps of subjecting aluminum hydroxide [Al(OH)3] or an aluminum oxide calcined at a lower temperature, either being a precursor 207~593 of ~-aluminum oxide, to a grinding and deagglomeration treatment; producing a suspension from the deagglomerated precursor of ~-aluminum oxide and optionally additives;
drying the suspension; and thereafter sintering the dried suspension.
Thus, the invention involves a process for the production of sintered materials based on ~-aluminum oxide.
A suspension of a precursor of ~-aluminum oxide and optionally usual additives, is prepared. Aluminum hydroxide [Al(OH)3] or an aluminum oxide calcined at a lower temperature is used as the precursor of Q-aluminum oxide and is subjected to a grinding and deagglomeration treatment for the formation of the suspension. The process continues by drying the suspension and sintering the dried material.
Preferably the grinding and deagglomeration treatment is performed with an attrition mill, a vibratory mill or a stirred ball mill. Preferably the attrition mill, vibratory mill or stirred ball mill is operated with grinding media consisting mainly of ~-aluminum oxide.
Pre~erably crystallization seeds of ~-aluminum oxide are added to the suspension to prevent grain growth during sintering. Advantageously, the suspension is adjusted to a pH of less than 5 by adding an acid, preferably nitric acid, hydrochloric acid, acetic acid, citric acid, formic acid or oxalic acid. Preferably the suspension is subjected to a vacuum treatment to remove dissolved and/or adsorbed gases. Preferably the sintering is performed at a temperature of 1100 to 1500C. Aluminum hydroxide resulting from the alumina production by the Bayer process is the preferred source of aluminum hydroxide.
The invention also includes sintered material based on ~-aluminum oxide and optionally additions of other oxides, carbides, nitrides, silicides or metals, obtained by the process of the invention.
The invention further includes the process of using the sintered material of the invention as abrasive grain.
It has been found that it is possible, by a suitable combination of process steps, to produce from ordinary industrial aluminum hydroxide (Al(OH)3, gibbsite, also called hydrargillite) as it is yielded in the Bayer process, sintered ~-aluminum oxide of great density and hardness with a crystallite size of less than 1 ~m, for example, even less than 0.5 ~m.
The production of ~-aluminum oxide from industrial aluminum hydroxide is generally known (it is a step in the industrial-scale production of aluminum), but the thus-obtained aluminum oxide usually has properties that are completely unsuitable for normal grinding purposes, namely, a high porosity and poor sintering properties. Sintering commences at temperatures so high, that it is connected with strong crystal growth and finally actually yields a dense, but -- because of the coarse texture -- mechanically unsatisfactory product, which does not offer any significant advantages relative to usual corundum. The so-called tabular alumina, for example, is produced in a similar way which is distinguished by its large (several hundred ~m) tabular crystals. Only for certain types of surface treatment, such as polishing, in which it is less important for the material removal, are such aluminum oxides suitable (ref. East German Patent Specification No. 76485).
The usual aluminum hydroxide is not suitable for use in the above-mentioned sol-gel process since it disperses poorly and cannot be gelled (for more information on the properties of aluminum hydroxides see, e.g., Ullmann's Encyclopedia of Industrial ChemistrY, Volume A1, VCH Verlagsgesellschaft mbH, Weinheim, (1985), pages 557 to 594)-207~93 According to the invention, the initial materialis first subjected to wet grinding or deagglomeration to divide the agglomerates present as a result of the production process, into individual crystallites. The deagglomeration is preferably performed in an attrition mill, a vibratory mill or a stirred ball mill, wherein grinding media are preferably used which consist entirely or predominantly or aluminum oxide. The amount of liquid is preferably selected so that a suspension results with a solids content of 10 to 40 percent by weight. Water is preferably used as the liquid, but it is also possible to replace the water partly with water-miscible and easily vaporizable solvents, such as lower alcohols or acetone.
The thus obtained suspension is advantageously adjusted by acid addition to a pH of less than 5, by which dissolved or adsorbed carbon dioxide is expelled.
Preferably the suspension is adjusted to a pH of about 2 to 4. The acid suitably is nitric acid, hydrochloric acid, citric acid, formic acid, acetic acid or oxalic acid, preferably hydrochloric acid. The necessary amount of acid in this case depends on the properties of the aluminum hydroxide, and above all, on its specific surface. An acid addition can be completely or partially dispensed with by instead using a vacuum treatment of the suspension for degassing.
Cry~tallization seeds, preferably composed of a-aluminum oxide, are suitably added to the aluminum hydroxide suspension These seeds can be obtained, for example, simply by grinding of a-aluminum oxide, for example, in the form of calcined alumina, to a particle size of < 1 ~m. Preferably the seeds are added in an amount of 1 to 5 percent by weight, relative to the total amount (calculated as Al2O3), and thoroughly mixed in.
Addition of the crystallization seeds at the beginning or during the deagglomeration is especially preferred. In addition to the crystallization seeds, optionally auxiliary 2071~3 or additional substances, such as foam separators, sintering auxiliary agents, grain growth inhibitors, etc., can be added. However, such auxiliary substances are not necessary for operation of the process according to the invention.
The suspension thus obtained is subsequently dried. The drying is suitably performed below the boiling point temperature to prevent the formation of vapour bubbles. At standard pressure, a drying temperature of about 70C is advantageous. If the suspension is present, for example, in a layer thickness of about 10 cm, drying can be performed at this temperature in 2 to 3 days. With the application of higher pressures the drying temperature can be increased corresponding to the higher boiling point and the drying time can thereby be shortened. The volume or the layer thickness corresponding to the decreasing liquid content is reduced during the drying operation without resulting in a significant porosity. An open porosity of ~ 0.05 ml/g and an average pore diameter of <
10 nm tdetermined according to the mercury-penetration method) can be achieved.
The thus-obtained dried cake is subsequently sintered, optionally after grinding to the grain size corresponding to the desired abrasive grain size (taking into consideration shrinkage during sintering). The sintering temperature is suitably at 1100 to 1500C. The sintering time depends on the temperature and i8, for example, about 2 hours at 1400C.
Despite the strong volume contraction (about 30 percent linear shrinkage), a dense sintered product is obtained with the process according to the invention by the conversion of aluminum hydroxide into ~-aluminum oxide without a separate calcination step being required.
Instead of aluminum hydroxide, an aluminum oxide calcined at a lower temperature, or mixtures of the latter and aluminum hydroxide, can also be used for the process 207~593 according to the invention. Aluminum oxides calcined at a lower temperature still contain some water, for example about 8 percent by weight, and are again converted into aluminum hydroxide by water absorption with the treatment according to the invention, as shown by thermogravimetric examination of the dried suspension. since the aluminum oxides calcined at a lower temperature are produced, on their part, from aluminum hydroxide and use thereof in the invention offers no special advantages, aluminum hydroxide is to be preferred as the initial material for the process according to the invention.
The sintered material produced according to the invention is distinguished by a very fine crystallite size, high sintered density and great hardness. Its toughness is in the area of 2.5 MPa-m~ or greater. It is suitable not only as an abrasive, but also for other uses in which these characteristics are important.
The following Examples illustrate the performance of the process according to the invention.
Example 1 In an attrition mill (0.6 liter), lOO g of pure aluminum hydroxide (Martinal~ OL-104), Martinswerk GmbH, D-W-5010 Bergheim) was ground for 2 hours in desalinated water with the addition of 1.5 percent by weight (relative to the aluminum hydroxide) of ~-aluminum oxide seeds with aluminum oxide-grinding balls (d = 1 mm) and deagglomerated. The particle or agglomerate size before grinding was 100 percent smaller than 10 ~m, after grinding it was 100 percent smaller than 1 ~m. The ~-aluminum oxide seeds were obtained by grinding aluminum oxide (calcined at high temperature) in the attrition mill to a particle size of ~ 0.5 ~m. The pH was adjusted to 2 by addition of about 20 ml of 37 percent hydrochloric acid before the grinding.
The suspension was dried for 2 days in an initial layer thickness of 5 cm at 70C. The average pore diameter after drying was 9.5 nm (mercury-porosimetry), and the open 2071~93 porosity was less than 0.05 ml/g. An examination of the dried material with thermogravimetry or differential thermal analysis showed that the bound water escapes below 350c and the crystallization to ~-Al2O3 set in at 1010C.
After 2 hours of sintering at 1400C, a material with a crystallite size of < 0.5 ~m, a density of > 3.8 g/ml (> 95 percent th.D.) and a hardness according to Vickers (500 g load) > 19 GPa, was obtained.
Example 2 The process was performed as described in Example 1, except that, instead of aluminum hydroxide, 70 g of an aluminum oxide (type HLS, Martinswerk), calcined at a lower temperature was used (loss on ignition about 5 percent by weight; Na20, about 0.2 percent by weight; other metals, 0.05 percent by weight; particle size, 99 percent < 1 ~m;
specific surface, 200 m2/g). A thermogravimetric examination of the dried material yielded a weight loss of 35 percent at 400C, corresponding to the composition Al(OH)3. After 2 hours of sintering at 1400C, a product with the same properties as in Example 1 was obtained.
Example 3 The process was performed as described in Example 2, however, with aluminum oxide of type AX (Martinswerk) calcined at a lower temperature (loss on ignition about 6 percent by weight; Na2O, about 0.2 g by weight; other metals, 0.06 percent by weight; grain size distribution, about 25 percent < 106 ~m, about 10 percent < 45 ~m;
specific surface, about 175 m2/g) as the initial material.
The sodium content after sintering (1400C, 1 hour) was 0.11 percent by weight. No ~-A12O3 was detectable in the sintered material by x-ray diffraction analysis (detection limit about 1 percent by weight).
2071~93 Example 4 ~Comparison Example) The process was performed as described in Example 2. However, instead of the attrition mill treatment, the aluminum oxide was mixed for only 30 minutes in a colloid mill (Ultra Turrax~, Janke and Kunkel). The cake obtained after three days of drying at 70OC had an average pore diameter of 400 nm and an open porosity of 0.06 ml/g. A
material with an open porosity of 0.11 ml/g was obtained by calcining at 1200C (5 hours).
The material according to Example 2, on the other hand, had an open porosity of only 0.03 ml/g after 5 hours at 1200C.
Claims (13)
1. A process for the production of sintered materials based on .alpha.-aluminum oxide, comprising the steps of subjecting aluminum hydroxide [Al(OH)3] or an aluminum oxide calcined at a lower temperature, either being a precursor of .alpha.-aluminum oxide, to a grinding and deagglomeration treatment; producing a suspension from the deagglomerated precursor of .alpha.-aluminum oxide and optionally additives; drying the suspension; and thereafter sintering the dried suspension.
2. A process according to claim 1, wherein the grinding and deagglomeration treatment is performed with an attrition mill, a vibratory mill or a stirred ball mill.
3. A process according to claim 2, wherein the attrition mill, vibratory mill or stirred ball mill is operated with grinding media consisting mainly of .alpha.-aluminum oxide.
4. A process according to claim 3, wherein crystallization seeds of .alpha.-aluminum oxide are added to the suspension to prevent grain growth during sintering.
5. A process according to claim 4, wherein the suspension is adjusted to a pH of less than 5 by adding an acid.
6. A process according to claim 5, wherein the acid is selected from the group consisting of nitric acid, hydrochloric acid, acetic acid, citric acid, formic acid and oxalic acid.
7. A process according to any one of claims 1 to 6, wherein the suspension is subjected to a vacuum treatment to remove dissolved and/or adsorbed gases.
8. A process according to any one of claims 1 to 6, wherein the sintering is performed at a temperature of 1100° to 1500°C.
9. A process according to any one of claims 1 to 6, wherein aluminum hydroxide resulting from alumina production by the Bayer process is used as the aluminum hydroxide.
10. A sintered material based on .alpha.-aluminum oxide and optionally at least one member selected from the group consisting of carbides, nitride, silicides, other oxides and other metals, obtained by a process according to any one of claims 1, 2, 3, 4, 5 or 6.
11. A sintered .alpha.-aluminum oxide material according to claim 10, having a crystallite size of less than 1 µm.
12. A sintered .alpha.-aluminum oxide material according to claim 10, having a crystallite size of less than 0.5 µm.
13. A process comprising using the sintered material according to claim 10 as abrasive grain.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH184491 | 1991-06-21 | ||
| CH1844/91 | 1991-06-21 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2071593A1 true CA2071593A1 (en) | 1992-12-22 |
Family
ID=4219935
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002071593A Abandoned CA2071593A1 (en) | 1991-06-21 | 1992-06-18 | Process for the production of sintered material based on .alpha.-aluminum oxide, especially for abrasives |
Country Status (17)
| Country | Link |
|---|---|
| US (1) | US5236471A (en) |
| EP (1) | EP0524436B1 (en) |
| JP (1) | JPH05194026A (en) |
| CN (1) | CN1068092A (en) |
| AT (1) | ATE110353T1 (en) |
| BR (1) | BR9202236A (en) |
| CA (1) | CA2071593A1 (en) |
| CZ (1) | CZ188492A3 (en) |
| DE (1) | DE59200406D1 (en) |
| ES (1) | ES2060438T3 (en) |
| HU (1) | HUT61949A (en) |
| MX (1) | MX9202929A (en) |
| NO (1) | NO922439L (en) |
| PL (1) | PL294867A1 (en) |
| RU (1) | RU2076083C1 (en) |
| TR (1) | TR25916A (en) |
| ZA (1) | ZA924471B (en) |
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| US5645618A (en) * | 1993-11-12 | 1997-07-08 | Minnesota Mining And Manufacturing Company | Method for making an abrasive article |
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-
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- 1992-05-29 CN CN92104241A patent/CN1068092A/en active Pending
- 1992-06-11 RU SU925011807A patent/RU2076083C1/en active
- 1992-06-11 PL PL29486792A patent/PL294867A1/en unknown
- 1992-06-15 BR BR929202236A patent/BR9202236A/en not_active Application Discontinuation
- 1992-06-17 MX MX9202929A patent/MX9202929A/en unknown
- 1992-06-17 US US07/899,719 patent/US5236471A/en not_active Expired - Fee Related
- 1992-06-18 CA CA002071593A patent/CA2071593A1/en not_active Abandoned
- 1992-06-18 TR TR92/0563A patent/TR25916A/en unknown
- 1992-06-18 ZA ZA924471A patent/ZA924471B/en unknown
- 1992-06-19 AT AT92110426T patent/ATE110353T1/en not_active IP Right Cessation
- 1992-06-19 EP EP92110426A patent/EP0524436B1/en not_active Expired - Lifetime
- 1992-06-19 NO NO92922439A patent/NO922439L/en unknown
- 1992-06-19 HU HU9202063A patent/HUT61949A/en unknown
- 1992-06-19 DE DE59200406T patent/DE59200406D1/en not_active Expired - Fee Related
- 1992-06-19 ES ES92110426T patent/ES2060438T3/en not_active Expired - Lifetime
- 1992-06-19 CZ CS921884A patent/CZ188492A3/en unknown
- 1992-06-22 JP JP4161570A patent/JPH05194026A/en active Pending
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5593467A (en) * | 1993-11-12 | 1997-01-14 | Minnesota Mining And Manufacturing Company | Abrasive grain |
| US5645618A (en) * | 1993-11-12 | 1997-07-08 | Minnesota Mining And Manufacturing Company | Method for making an abrasive article |
| US5651801A (en) * | 1993-11-12 | 1997-07-29 | Minnesota Mining And Manufacturing Company | Method for making an abrasive article |
Also Published As
| Publication number | Publication date |
|---|---|
| RU2076083C1 (en) | 1997-03-27 |
| HUT61949A (en) | 1993-03-29 |
| DE59200406D1 (en) | 1994-09-29 |
| NO922439L (en) | 1992-12-22 |
| CZ188492A3 (en) | 1993-01-13 |
| PL294867A1 (en) | 1993-03-22 |
| ZA924471B (en) | 1993-03-31 |
| MX9202929A (en) | 1992-12-01 |
| NO922439D0 (en) | 1992-06-19 |
| EP0524436B1 (en) | 1994-08-24 |
| ATE110353T1 (en) | 1994-09-15 |
| JPH05194026A (en) | 1993-08-03 |
| HU9202063D0 (en) | 1992-10-28 |
| TR25916A (en) | 1993-11-01 |
| CN1068092A (en) | 1993-01-20 |
| EP0524436A1 (en) | 1993-01-27 |
| US5236471A (en) | 1993-08-17 |
| ES2060438T3 (en) | 1994-11-16 |
| BR9202236A (en) | 1993-02-02 |
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Legal Events
| Date | Code | Title | Description |
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| EEER | Examination request | ||
| FZDE | Discontinued |