BE897802A - HIGH PURITY ALUMINA - Google Patents

Abstract

High purity, sintered, unmelted alumina material with closed, dense porosity and method for preparing such material by rapid heating to sintering temperature, electrophoretic deposition or gel alumina monohydrate, dried high purity, and use of this alumina material as an abrasive material.

Description

       

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  "High purity alumina" Priority of a patent application in the United States of America filed on September 23, 1982, under No. 421. 993, in the name of Alvin P. Gerk and Robert J.



  To help each other

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 "High purity alumina"
The present invention relates to a high purity, sintered, unmelted, dense alumina material with closed porosity. The present invention also relates to a process for producing such a material by rapid heating up to the sintering temperature of an electrophoretic deposit or gel of dried alumina monohydrate of high purity.



   Low porosity and high purity are desirable characteristics of abrasive materials requiring high hardness, high fracture toughness, and chemical and wear resistance. Typically, the denser materials have higher hardness, higher fracture toughness and are more resistant to mechanical and chemical attack than porous materials.



   Generally speaking, high purity alumina abrasives are produced by melting, forming, grinding, washing, drying and calibrating calcined "Bayer" alumina, so as to obtain a specific abrasive product. The Bayer process involves digestion of aluminous matter in an aqueous solution of a caustic base to form a solution of sodium aluminate, precipitation and / or separation of impurities, such as silica, iron and oxide titanium, the seed-

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 sodium aluminate solution with alumina trihydrate to precipitate the alumina in the solution as alumina trihydrate, and calcining the alumina trihydrate to form approximately 95% to 99% alumina weight, A1203.

   this alu-
 EMI3.1
 mine is then heated an electric oven above 2000 C, cooled at various rates so as to obtain specific shapes and dimensions of grains, crushed to release the grains, washed to separate the dust, dried, sieved mechanically to separate grain sizes and often impurities. An abrasive material formed from high purity alumina, which is melted generally has almost zero porosity and, consequently, a high hardness; characteristic microhardness values are of the order of 1680 to 2200 kg / mm2 K1 (Knoop scale, load of 100 g).



   However, Bayer abrasives made of high purity alumina are expensive because of their extensive processing required. In addition, in order to obtain high purity fused alumina abrasives, the feed material supplied to the Bayer circuit must already be high purity alumina, which further increases the
 EMI3.2
 production costs. Typical industrial compositions for standard "brown" molten alumina (% by weight) are as follows: 96, 70% of 19% of TiO, 49% of 21% of 13% of CaO, 0.11% of C 0.09% of 08% MgO and less than 0.01% of each of the constituents Na20 and Cr203.

   "White" industrial alumina, which is considered to be a high purity molten alumina abrasive material,

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 EMI4.1
 contains approximately (in% by weight): 99.61% of 16% of 0.06% 05% of J 0.03% of C, 0.02% of fie203'0.02% of CaO, 0.02% of MgO and less than 0.01% of each of the constituents Zoo 2 and Car203.



   High purity alumina abrasive material can be sintered rather than melted. In a characteristic alumina sintering process, precalibrated and preformed alumina raw materials are heated in ovens to a sintering temperature above 1200 C. The sintering of the particles makes it possible to form a finished abrasive material, rather than merging them and then performing a large number of subsequent processing steps as in a fusion process. These subsequent processing steps, such as grinding, forming, sizing, washing and drying, usually used in the manufacture of molten products, can thus be eliminated by producing a sintered product.

   However, to obtain the precalibrated and preformed raw materials for sintering, the alumina must be crushed initially and compressed or extruded to the desired shapes; and these initial processing steps raise the cost of producing a sintered alumina material above that of a molten alumina material. In addition, a high purity, sintered alumina abrasive material has a higher porosity than a molten alumina material and, therefore, a lower hardness.



   A high purity alumina abrasive material can also be obtained using a so-called sol-gel process. The sol-gel technology is illustrated by the

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 EMI5.1
 United States Patent No. 4,314,827 and by United States Patent Application No. 377,782, incorporated in connection with the present invention for reference.

   U.S. Patent No. 4,314,827 generally reports that a granular abrasive material based on aluminum oxide, synthetic, unmelted, free of alkali metal ions, can be obtained. and free of calcium ions by gelling a colloidal dispersion or an alumina monohydrate hydrosol and a modifier component, such as magnesium oxide, and then slowly heating the gel for several hours at a temperature of at least minus 1250 C. In example 13 of the patent of the United States of America No. 4,314,827, a reference alumina material containing approximately 100% of Al 2 O 3 is obtained by not adding any modifier component to the colloidal dispersion. or a-li alumina monohydrate hydrosol.

   This reference alumina material could not be used for severe abrasion applications, due to
 EMI5.2
 2 its very low Knoop hardness of 790 kg / mm2.



   United States Patent Application No. 377,782 generally describes a process for obtaining an abrasive material or grain from a liquid dispersion comprising aluminum oxide monohydrate containing more than 0.05% by weight of sodium plus calcium, and at least one sintering aid containing dissolved metal, such as magnesium oxide. The dispersion is prepared, gelled and dried. After drying, the solid is ground to form grains and is optionally calcined to separate the chemically bound water. The beans are then heated quickly,

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 less than 10 minutes, at a sintering temperature above about 1200 C.



   In U.S. Patent No. 4,314. 827 as well as in U.S. Patent Application No. 377,782, the resulting abrasive alumina material is doped with an oxide due to the addition of a sintering aid or a modifier component.



   In two other patents filed by the applicant, incorporated in the context of the present invention by way of reference, a method of obtaining alumina materials from a liquid dispersion is described by the use of electrophoresis. In the first of these patents, a liquid dispersion, containing aluminum oxide monohydrate and optionally a sintering aid containing dissolved metal, such as MgO, is formed and placed in an electrophoretic cell comprising an anode and at least one cathode, and in the second of these patents, the dispersion is placed in a cell containing at least one membrane arranged between the anode and the cathode. A direct current is applied between the anode and the cathode, which causes the deposition of a precipitate of concentrated aluminum oxide monohydrate on the cathode and / or the membrane.

   The deposit is then dried and the dried material is then heated to form the final product. If it is desired to obtain abrasive materials, one way of obtaining them, as described in these two patents, is then to add a sintering aid containing dissolved metal to the dispersion of alumina monohydrate, to deposit the material by electrophoresis. on the cathode and / or the membrane, to dry the deposit, to grind the deposit to form

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 grains, calcining the grains, if desired, to separate the chemically bound water, and then quickly heating the grains, in less than 10 minutes at a temperature
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 sintering strip above 1200 C.

   As with United States Patent No. 4,314,827 and United States Patent Application No. 377,782, the addition of a sintering aid to the dispersion of monohydrate alumina leads to the production of an oxide-doped abrasive alumina material.



   Therefore, the main object of the present invention is to form an alumina material, sintered, of high purity, undoped, of closed, dense porosity having a high hardness and a high resistance.



   Another object of the present invention is to provide an easy, efficient and economical process for producing high purity, closed porosity, dense alumina abrasive materials.



   The problems of the prior art are overcome by the finding that a sintered, unmelted, high purity, closed porosity, dense alumina material can be formed by rapidly heating to the sintering temperature, a monohydrate gel. dried alumina or a precipitate deposited by electrophoresis.



   In the process of the present invention, either a liquid dispersion comprising approximately 2 to approximately 60% by weight of aluminum oxide monohydrate is prepared and gelled, or particles originating from a liquid dispersion comprising approximately 0 are electrophoresed , 1 to about 40% by weight of aluminum oxide monohydrate on the cathode and / or the membrane of an electrophoretic cell.

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   The electrophoretic deposit or the gel is dried to separate the free water and then the dried material is ground to form grains. After grinding, the material can be calcined, before heating, at a temperature between about 250 ° C. and 800 ° C. The material is then rapidly heated or baked at a temperature
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 re being between 1200 ° C. and 1700 ° C. in less than 20 minutes, and preferably in less than 5 minutes, and maintained at the sintering temperature for at least 1 minute.



   It was found: 1) that there was an increase in the density of the high purity sintered alumina product of the present invention with an increase in the rate of heating or cooking to the sintering temperature; 2) that there was a decrease in the intrusion volume with an increase in the density for the high purity sintered alumina of the present invention; 3) that there was an increase in the hardness of the high purity alumina material of the present invention with an increase in density; 4) that there was an increase in the density of the high purity sintered alumina of the present invention with an increase in the sintering temperature; and 5) that the sintering time had only a slight effect on the density of the high purity sintered alumina of the present invention.



   A high purity alumina abrasive material obtained according to the present invention can be used in critical grinding processes, such as surface treatment of turbine blades, as abrasive grains, in refractory applications and in lost wax casting molds.

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   Other details and particularities of the invention will emerge from the description below, given by way of nonlimiting example and with reference to the appended drawings, in which:
Figure 1 is a graph illustrating the relationship between the density and the rate of cooking or heating of a high purity sintered alumina.



   Figure 2 is a graph illustrating the relationship between the intrusion volume and the density of a high purity sintered alumina.



   Figure 3 is a graph illustrating the relationship between hardness and density of a high purity sintered alumina.



   Figure 4 is a graph illustrating the relationship between the density and the sintering temperature of a high purity sintered alumina.



   Figure 5 is a graph illustrating the relationship between density and sintering time of high purity sintered alumina.



   The present invention relates to a sintered, unmelted alumina material, of high purity, of closed, dense porosity, as well as to a process for obtaining such a material by rapidly heating or baking to temperature. sintering a dried alumina monohydrate gel or a precipitate deposited by electrophoresis.



   The process of the present invention comprises either the preparation and gelling of a liquid dispersion of aluminum oxide monohydrate or the electrophoresis deposition of a precipitate of concentrated alumina monohydrate from a liquid dispersion of monohy -

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 alumina drate on a cathode and / or a membrane of an electrophoretic cell. The liquid dispersion is formed by any suitable means which may simply consist in mixing the aluminum oxide monohydrate with water. The liquid is almost always water but may be another liquid in which the aluminum oxide monohydrate will disperse.

   Any suitable mixing device can be used, including high and low shear mixers.
If the liquid dispersion of alumina monohydrate has to be gelled, the liquid dispersion can be a sol which is a colloidal dispersion. The amount of aluminum oxide monohydrate in the liquid dispersion may constitute from 2 to approximately 60, preferably from 10 to 40, and more preferably from approximately 15 to 35% by weight of the dispersion.



   If an alumina monohydrate precipitate is deposited by electrophoresis from a liquid dispersion of alumina monohydrate on a cathode and / or a membrane, then the liquid dispersion will contain approximately 0.1 to 40 and preferably 5 to 15 % by weight of alumina monohydrate.



   In the context of the present invention, the expression “aluminum oxide monohydrate” is understood to mean aluminum oxide hydrates corresponding to the stoichiometric formula of 1/2 (Al203. XH2o), in laqueDe x. is 0.5 to 3. Aluminum oxide monohydrate is also known as boehmite. Aluminum oxide monohydrate as used in the context of the present invention also includes, without limitation, pseudo boehmite.

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   A commercially available aluminum oxide monohydrate which is of interest in the context of the present invention is a by-product of the Ziegler process used for the manufacture of primary alcohol and is sold under the trade names " Dispural "(registered trademark) and" Catapal "(registered trademark). These products are characterized by exceptional purity, a high surface area and by a very small dimension of the crystallites. Aluminum oxide monohydrate can also be obtained by the hydrolysis of aluminum alcoholates, by precipitation from sodium aluminate solutions and by other methods.



   The liquid dispersion preferably comprises a dispersing aid, such as an acid. For example, about 0.02 to 0.25 and preferably 0.05 to about 0.15 mole of a volatile acid per mole of aluminum oxide monohydrate greatly facilitates dispersion. Examples of such acids are nitric, hydrochloric, acetic and formic acids. Nitric acid is preferred since it appears to provide the greatest stability in the colloidal dispersion of aluminum monohydrate.



   After preparing the dispersion, it can be gelled. If the solids content of the dispersion is very low, you may either have to vaporize the liquid to bring the dispersion into gel form or you may have to use another gelation method, i.e. the addition of 'a gelling agent. Gelation can simply consist in extracting water from the dispersion, for example by drying the dispersion.



   Or, after preparing the dispersion

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 liquid, it can be placed either in an electrophoretic cell comprising an anode and at least one cathode, or in an electrophoretic cell comprising an anode, a cathode and at least one membrane arranged between this anode and this cathode. The cathode includes an inert conductive material, such as platinum, platinum-coated titanium, or a corrosion-resistant, conductive material, such as stainless steel.



  The anode may either contain a similar conductive and inert material, or instead may include metallic aluminum, which will contribute to the deposition.



   If at least one membrane is positioned between the anode and the cathode of the electrophoretic cell, the essential conditions for the membrane are that colloidal particles cannot easily pass through it, and that ions, such as hydrogen ion and l 'nitrate ion, can cross the membrane. The membrane may include either an ion exchange membrane or a microporous membrane.



   A microporous membrane allows particles to pass through to a certain dimension. The microporous membranes of the invention can be made of cellulose; for example, membranes can be used
 EMI12.1
 dialysis / ultrafiltration Spectra / Por (registered trademark) n with a molecular weight range from 6000 to 8000.



   If an ion exchange membrane is used, no particle larger than an ion can diffuse through the membrane. Although the material of the ion exchange membrane used in the context of the present invention is not particularly limited, the membrane may comprise highly polymeric backbones

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 chemically resistant bridges to which a large number of anion and cation exchange groups, such as sulfonate groups, carboxylate groups, phenol groups, ammonium groups, etc., are attached as substituents.

   In addition, the membrane comprising particularly chemically resistant fluoropolymer resins proves to be particularly advantageous since the membrane comprising the fluoropolymer resin is very easily detachable from the electrophoretic deposit and that strong adhesion of the electrophoretic deposit to the fluoropolymer rarely occurs. Among the ion exchange membranes comprising these fluoropolymer resins, the preferred membrane comprises a copolymer of perfluorovinyl ether and tetrafluoroethylene, which has pendant type sulfonate groups (Nafion, a product of EI DuPont de Nemours g Co., Inc ..).



   Cathodes or membranes of different shapes can be used for various applications. For example, if a ceramic tube is the desired product, the cathode of the electrophoretic cell without membrane or the membrane of the electrophoretic cell with membrane will be tubular in shape. There is no limitation on the number of membranes that can be positioned between the anode and the cathode as long as there is a sufficient supply of energy.



   If the liquid dispersion containing the aluminum oxide monohydrate is placed in the anode, membrane and cathode chambers of an electrophoretic membrane cell, then after applying the electrical potential, particles from the dispersion

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 will settle on the cathode as well as on the membranes.



  In order to eliminate the deposit on the cathode, three methods can be followed. In the first method, at least one transverse hole is made in the cathode extending over the entire width of the cathode, preferably by drilling, and the cathode is then covered with a membrane.



  The purpose of these drilling holes in the cathode is to allow the release of hydrogen gas without passing through the deposit. In the second method which can be used to remove the deposit on the cathode, the electrophoretic bath is vigorously agitated in the vicinity of the cathode while the electrical potential is applied. One way to intensify agitation at the cathode is to use high pressure spray jets directed towards the cathode in the electrophoretic bath. In the third method which can be used, the liquid dispersion comprising the aluminum oxide monohydrate is placed in the anode and membrane chambers of the electrophoretic cell and a compatible conductive liquid is placed in the cathode chamber of the cell electrophoretic.

   The conductive liquid in the cathode chamber preferably comprises a dilute acid.



   After adding the liquid dispersion to an electrophoretic cell of the invention, an electrical potential is applied between the anode and the cathode, which causes the deposition of the dispersion particles on the cathode and / or the membrane (s). After allowing the current to flow for a sufficient period of time, the power supply is cut, disconnected and the electrophoretic cell is dismantled.



   If desired, electrophoretic cells of the invention can be connected in parallel. Of

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 even, a continuous electrophoretic cell can be used rather than a discontinuous electrophoretic cell. If a continuous electrophoretic cell is used rather than a discontinuous electrophoretic cell in membrane electrophoretic cells, a membrane is continuously fed to the electrophoretic cell while the electrical potential is applied.



  If a continuous electrophoretic cell is used rather than a discontinuous electrophoretic cell in electrophoretic cells without a membrane, the cathode, which preferably includes a strip of stainless steel or other non-corrosive material, is continuously fed to the cell electrophoretic while the electrical potential is applied.



   If the amount of aluminum oxide monohydrate in the dispersion to be electrophoretically deposited is between 30 and 40% by weight, it is not only difficult to keep the dispersion homogeneous, but a gel tends to form on the top of the precipitate deposited. On the other hand, if a very small amount is used, for example below about 2% by weight of aluminum oxide monohydrate in the dispersion, the efficiency of the electrophoretic cell is significantly reduced.



   After gelling or depositing the dispersion, it is dried by vaporizing the free water. Drying the dispersion can also be a way to gel the dispersion. Deposits on the cathode and / or membrane can be removed from the cathode or mem-
 EMI15.1
 brane either before or after drying. The terms "dried" or "drying", as used in connection with the

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 present invention, means that at least 90% of the free (unbound) water is removed to form a solid. Drying can be accomplished by any means known to those skilled in the art.

   When using heat, the drying temperature is usually about 800 to about 1100C. The drying time can vary between about 1 and about 72 hours depending on the amount of water or any other liquid present, the relative humidity and the drying temperature.



   After drying the solid, it is crushed or crushed by any suitable means, such as a hammer or ball mill to form particles or grains. Any suitable method can be used to break up the solid material and the term "grinding" is used to cover all of these methods.



   After grinding, the grains can be calcined at a temperature between about 2500C and about 8000C until essentially all of the water is separated and any acid residues are removed. The expression "essentially all", as used in this regard, means that all of the free water and more than 90% of the bound water are separated. The calcination period to essentially separate all of the water is usually between about 5 and about 20 minutes.



   After drying, grinding and calcination, the beans are quickly heated or cooked to a temperature
 EMI16.1
 rature above about 1200 C, and preferably above about 1400oc. Rapid heating is usually accomplished in less than 20 minutes, preferably

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 in less than 5 minutes, and more preferably in less than 1 minute. This rapid heating step allows the formation of a higher alumina abrasive grain with high density and hardness. Any suitable means or process can be used to quickly heat the beans, such as injecting the beans in a form other than in bulk, that is, separately, in an oven preheated to a room temperature. above 12000C and preferably at a temperature above 1400 C.



   It is possible to eliminate the calcination step and instead of rapidly heating the grains to a temperature above 1200 ° C. after drying. This process is particularly interesting for producing certain types of abrasives.



   After quickly heating the beans to
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 a temperature above 1200 ° C., they are continued to be heated to a sintering temperature lying between approximately 1200 ° C. and approximately 1700 ° C., and preferably between 1400 ° C. and 1600 ° C., for a period of sintering sufficient to obtain the density desired. Sufficient sintering time depends on the sintering temperature, and is usually between about 1 minute and about 30 minutes, and is preferably less than about 5 minutes, for example from about 1 to about 5 minutes.



   The invention includes bonded, coated and nonwoven abrasive articles comprising the grains of the invention. In general, the grains of the invention can be described as being sintered grains which preferably comprise more than approximately 99.80% by weight of Al 0.

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   The invention is illustrated in more detail by the following nonlimiting Examples.



   Examples l to 18
High purity alumina samples are prepared from an aqueous dispersion of 20% by weight of Dispural (registered trademark) and 0.06 mole of nitric acid (HNO per mole of AlOOH. After having gelled, dried and ground the material, half of the alumina is left unc calcined while the other half is calcined at 600 C. Samples of 10 g of calcined alumina and of non-calcined alumina are then heated to sintering temperatures 14000C and 1450C at different heating rates All samples are held at the sintering temperature for approximately 20 minutes.

   To obtain a heating rate or an evaluated time up to the temperature of 15 to 30 seconds, a small quantity of material is poured onto a ceramic substrate which is already at the sintering temperature. To obtain a heating speed of 2 to 5 minutes, a platinum crucible is introduced into the oven at the sintering temperature. To obtain a heating speed of 15 to 20 minutes, a platinum crucible is placed in an oven with a high heating speed programmed to reach the sintering temperature in 10 minutes. To obtain an hourly heating speed, a platinum crucible is placed in an oven with a high heating speed programmed to reach the sintering temperature in one hour.

   To obtain a heating speed of 5 hours, a platinum crucible is placed in an oven with a high heating speed programmed to reach the sintering temperature in 5 hours.

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 Mercury intrusion density and volume measurements using a Quantachrome Autoscan porosimeter and hardness measurements using Knoop ball hardness tests with a load of 100 g (K100) are then performed on these samples.

   The results are given in Table I.
 EMI19.1
 1

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TABLE I Variation of Knoop density, intrusion volume and Knoop hardness with the heating rate
 EMI20.1
 
 <tb>
 <tb> Tempé-Temps <SEP> eva-Calcined <SEP> No <SEP> calculated
 <tb> cross out <SEP> luke <SEP> up to <SEP> Densi-Vol. <SEP> int.
 <tb> of <SEP> fried- <SEP> the <SEP> tempera- <SEP> Example- <SEP> tee <SEP> Hardness <SEP> Exem-0 <SEP> Density <SEP> Vol ,. <SEP> int. <SEP> Hardness
 <tb> floor <SEP> ture <SEP> ple <SEP> (g / c <SEP> m) <SEP> (c <SEP> m / g) <SEP> K100 <SEP> ple <SEP> (g / c <SEP> m3) <SEP> (c <SEP> m3 / g) <SEP> K100
 <tb> 14000C <SEP> 15-30 <SEP> sec. <SEP> 1 <SEP> 3.42 <SEP> 0, <SEP> 0030-10 <SEP> 3.51 <SEP> 0, <SEP> 0030
 <tb> 2-5 <SEP> min. <SEP> 2 <SEP> 3.30 <SEP> 0.0045 <SEP> 1237 <SEP> 11 <SEP> 3.23 <SEP> 0, <SEP> 0090 <SEP> 1147
 <tb> 15-20 <SEP> min.

    <SEP> 3 <SEP> 3.16 <SEP> 0.0185 <SEP> 1027 <SEP> 12 <SEP> 3.11 <SEP> 0.0265 <SEP> 1052
 <tb> 1 <SEP> hr. <SEP> 4 <SEP> 3.12 <SEP> 0.0330 <SEP> 1043 <SEP> 13 <SEP> 3.03 <SEP> 0.0550 <SEP> 913
 <tb> 5 <SEP> hrs. <SEP> 5 <SEP> 2.93 <SEP> 0.0800 <SEP> 827 <SEP> 14 <SEP> 2.90 <SEP> 0.0865 <SEP> 738
 <tb> 1450 C <SEP> 2-5 <SEP> min. <SEP> 6 <SEP> 3.36 <SEP> N.D. <SEP> 1268 <SEP> 15 <SEP> 3.27 <SEP> N.D. <SEP> 1163
 <tb> 15-20 <SEP> min. <SEP> 7 <SEP> 3.17 <SEP> 0.009 <SEP> 1231 <SEP> 16 <SEP> 3.13 <SEP> 0.0245 <SEP> 1040
 <tb> 1 <SEP> hr. <SEP> 8 <SEP> 3.13 <SEP> 0.025 <SEP> 1019 <SEP> 17 <SEP> 3.07 <SEP> 0.0460 <SEP> 950
 <tb> 5 <SEP> hrs. <SEP> 9 <SEP> 3.00 <SEP> 0, <SEP> 074-18 <SEP> 2.98 <SEP> 0, <SEP> 0785
 <tb>
 There is no intrusion volume detected for Examples 6 and 15 (N.D .: not detected).

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   Examples 19 to 21
Samples of high purity alumina are prepared according to Examples 1 to 18. After having gelled, dried, ground, calcined and sintered the material, twelve Knoop hardness tests are carried out on each sample. The average of the results is given in Table II.



   TABLE II Variation of Knoop hardness with density
 EMI21.1
 
 <tb>
 <tb> Example <SEP> Density <SEP> (g / cm) <SEP> Hardness <SEP> (K-) <SEP>
 <tb> 19 <SEP> 3,400 <SEP> 1367
 <tb> 20 <SEP> 3,679 <SEP> 1734
 <tb> 21 <SEP> 3,673 <SEP> 1719
 <tb>
 
Examples 22 to 33
High purity alumina samples are prepared according to Examples 1 to 18. After having gelled, dried and ground the material, half of the material is left unc calcined while the remaining half is calcined at 600 C.

   Samples of 10 g of the calcined material and of the non-calcined material are then brought to the sintering temperature of 1400 C at a heating rate of 15 to 30 seconds, by pouring a small quantity of material onto a ceramic substrate. already found at 1400 C. Porosimetry measurements are then carried out on these samples. The results are given in Table III.

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   TABLE III Variation of the density and volume of intrusion with
 EMI22.1
 1 Calcined sintering time 1 Sintering
 EMI22.2
 
 <tb>
 <tb>, <SEP> Tempe- <SEP> Time <SEP> Exem-Density <SEP> Flight <SEP> int. <SEP>
 <tb> cross out <SEP> (min. <SEP>) <SEP> ple <SEP> (g / cm3) <SEP> (cm3 / g)
 <tb> (C)
 <tb> 1400 <SEP> 0.5 <SEP> 22 <SEP> 3.41 <SEP> 0.0055
 <tb> 2.0 <SEP> 23 <SEP> 3.38 <SEP> 0.0045
 <tb> 4.0 <SEP> 24 <SEP> 3.37 <SEP> 0.0040
 <tb> 8.0 <SEP> 25 <SEP> 3.38 <SEP> 0.0045
 <tb> 16.0 <SEP> 26 <SEP> 3.42 <SEP> 0.0030
 <tb> 30.0 <SEP> 27 <SEP> 3.46 <SEP> 0.0040
 <tb> No <SEP> calcined
 <tb> 0, <SEP> 5 <SEP> 28 <SEP> 3, <SEP> 37 <SEP> 0.0060
 <tb> 2.0 <SEP> 29 <SEP> 3.36 <SEP> 0.0035
 <tb> 4.0 <SEP> 30 <SEP> 3.42 <SEP> 0.0030
 <tb> 8.0 <SEP> 31 <SEP> 3.42 <SEP> 0.0015
 <tb> 16.0 <SEP> 32 <SEP> 3.51 <SEP> 0.0030
 <tb> 30.0 <SEP> 33 <SEP> 3.49 <SEP> 0,

  0035
 <tb>
 
Examples 34 to 37
Samples of high purity alumina are prepared according to Examples 1 to 18. After gelling, drying and crushing the material, the material is calcined at 600 C. Then samples of 10 g are brought to a series of temperatures sintering at a heating rate of approximately 1 to 5 minutes. We then perform on. these samples of density measurements. The results are given in Table IV.

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   TABLE IV Density variation with sintering temperature
 EMI23.1
 
 <tb>
 <tb> Example <SEP> Temperature <SEP> from <SEP> Density <SEP> (g / cm)
 <tb> sintering <SEP> (OC)
 <tb> 34 <SEP> 1400 <SEP> 3,460
 <tb> 35 <SEP> 1495 <SEP> 3,485
 <tb> 36 <SEP> 1545 <SEP> 3,510
 <tb> 37 <SEP> 1600 <SEP> 3,580
 <tb>
 
Examples 38 to 40
A sample of high purity alumina is prepared according to Examples 1 to 18. After having gelled, dried and ground the material, it is calcined at 600 ° C., it is rapidly heated in a rotary oven to a temperature of sintering of approximately 14000C and maintained at this temperature for approximately 10 minutes.

   After cooling, the material is baked in a gas oven to a sintering temperature of approximately 1500 C and maintained at this temperature for 1 hour. The sintered particles are then classified according to their size on an ordinary sieve to comply with ANSI 74 standards. 18-1977 for the size of 30 grit. These particles with a dimension of 30 grit are then brought in the form of a resin-bonded cutting wheel of 40.6 cm x 0.31 cm x 2.54 cm (Example 40).



   Control wheels also formed from particles of dimensions 24 and 30 grit (Examples 39 and 38 respectively) are also prepared in a similar manner, from standard brown molten alumina.



   The wheels are then tested on a 10 HP Tysaman machine using a dry type plunger (3 seconds) on 1020 carbon steel rods and

  <Desc / Clms Page number 24>

 
 EMI24.1
 made of 304 stainless steel. Twenty cuts 2 are made (approximately 129 per test.



   The results of the average grinding ratio (ratio-M = volume of metal removed / volume of wheel used) for successive grinding tests are given in Table V.



   TABLE V Grinding tests on cutting wheels
 EMI24.2
 
 <tb>
 <tb> Exem-Type <SEP> from a-Dimen-Acier <SEP> tried <SEP> Trial <SEP> Report-M
 <tb> pie <SEP> lumine <SEP> sion <SEP> in <SEP> n <SEP> medium
 <tb> grit
 <tb> 38 <SEP> brown <SEP> 30 <SEP> Carbon <SEP> 1020 <SEP> 1 <SEP> 4.82
 <tb> (witness)
 <tb> 39 <SEP> brown <SEP> 24 <SEP> Carbon <SEP> 1020 <SEP> 3 <SEP> 2.29
 <tb> (witness)
 <tb> Steel <SEP> inoxyda-4 <SEP> 12.67
 <tb> ble <SEP> to <SEP> 304
 <tb> 40 <SEP> Alumina <SEP> from <SEP> 30 <SEP> Carbon <SEP> 1020 <SEP> 1 <SEP> 9, <SEP> 12 <SEP>
 <tb> high <SEP> purity
 <tb> Steel <SEP> inoxy-7 <SEP> 20.75
 <tb> devil <SEP> 304
 <tb>
 
As can be seen from the average M-ratios in Table V, the high purity alumina abrasive material (Example 40)

   obtained according to the invention gives results of grinding wheel cb cut amsiuemt me better than standard brown molten alumina (Examples 38 and 39).



   Examples 41 to 48
High purity alumina sintered particles are prepared in a similar manner to Example 40.



   The sintered particles are classified according to their size on an ordinary sieve to satisfy the ANSI 74.18-1977 standard for the size of 50 grit.

  <Desc / Clms Page number 25>

 



   An abrasive material with a single coating is prepared by electrostatically applying particles of a size of 50 grit on a support of vulcanized fibers. The fiber chosen is a vulcanized fiber of 0.762 mm of abrasive quality, having a nominal weight of 30.4 kg per ream (480-9 x 11 sheets).



   A manufacturing adhesive mixture is prepared, comprising a liquid phenolic resin of the 1 stage commercial type with a formaldehyde / phenol ratio of approximately 1/1 and ground limestone with an average particle size of between 17 and 25 microns, using a mixing ratio by net weight of
 EMI25.1
 1/1. The mixture is then heated to 32 ° C. and applied with a roller on the fiber support.



  About 10.4 kg of adhesive are applied per ream.



   Using ordinary sandpaper making equipment, the 50 grit abrasive is electrostatically projected onto the fibers supporting the manufacturing mixture by applying approximately 28.1 kg per ream of grain.



   The support coated with abrasive adhesive is then heated to 79.50 ° C. for 1 hour and to 930 ° C. for 2 hours in the manufacturing ramp. After drying, a primer is then applied by roller coating methods, with an amount of approximately 10.4 kg / ream being applied. The primer mixture is made up of the same 1/1 phenolic resin charge ratio. Drying and hardening are then carried out by heating the coated material for 1 hour at 65 ° C., for 4 hours at 79 ° C. and for 16 hours at 1070 ° C.

  <Desc / Clms Page number 26>

 



   After the material has hardened, it is moistened in the usual manner to a moisture content of less than 8% by weight. The material is then flexed uniformly and cut into the matrix into three 17.8 cm discs. These discs are then tested on an ordinary pneumatic disc wheel using 4140 steel which has undergone a quenching and tempering operation (hardness 285-320 BHN) as a work piece and compared with a control disc produced and handled exactly. the same way except that the abrasive grains consisted of molten brown alumina.



  In these tests, the abrasive disc is placed on the disc wheel in the usual way and a work piece is placed so that it engages the disc on the 2.54 cm flat side at an angle of 10- 15. We pass the disc back then forward along the workpiece.



   The abrasive disc is rotated in this test at a nominal speed of 5500 revolutions per minute on a support pad of the hard rubber type with a diameter of 17.8 cm. A force of introduction of its own weight of 3.6 to 7.25 kg is exerted on the workpiece. The test is carried out for 30 seconds, after which the material removed from the bar is measured (weight before grinding minus weight after grinding) and it is recorded. This sequence is continued until the measured material removed is 5 g or less per grinding interval.



  The total material removed in this way for the test disc is compared to the total material removed for the control disc (cutting efficiency as a relative percentage).



  The average of the results of the grinding tests is given

  <Desc / Clms Page number 27>

 in Table VI. The percentages for the yield are taken in relation to 100% for the control.



   TABLE VI Tests of grinding wheels with resin discs
 EMI27.1
 
 <tb>
 <tb> 3.6 <SEP> kg <SEP> 7.2 <SEP> kg
 <tb> Type <SEP> from a-Exem-Poids <SEP> Rende-Exem-Poids <SEP> Yield
 <tb> lumine <SEP> ple <SEP> from <SEP> ment <SEP> in <SEP> ple <SEP> from <SEP> in <SEP>% <SEP> (g)
 <tb> cut <SEP>% <SEP> (g) <SEP> cut
 <tb> Brown <SEP> 41 <SEP> 66 <SEP> (100) <SEP> 45 <SEP> 166 <SEP> (100)
 <tb> (witness)
 <tb> Sol-gel <SEP> 42 <SEP> 50 <SEP> 76 <SEP> 46 <SEP> 142 <SEP> 86
 <tb> Sol-gel <SEP> 43 <SEP> 34 <SEP> 52 <SEP> 47 <SEP> 115 <SEP> 69
 <tb> Sol-gel <SEP> 44 <SEP> 32 <SEP> 48 <SEP> 48 <SEP> 83 <SEP> 50
 <tb>
 
The results of Table VI show cutting yields in relative% for the alumina abrasive resin discs produced according to the present invention, being between 48 and 86%, with an average of 64%,

   compared to the yield of a standard brown molten alumina abrasive resin disc.



   Examples 1 to 5 and Examples 10 to 14 are used to illustrate Figure 1, which shows an increase in density with an increase in the rate of heating or cooking to the sintering temperature for the alumina product. sintered, high purity of the present invention. Figure 1 gives values with non-calcined alumina, and calcined alumina at 600 C, which are sintered at 460 C for approximately 20 minutes.

   Densities greater than 3.4 g / cm3 are obtained with a rapid heating rate of approximately 20 seconds at a sintering temperature of 14000 C. Slow heating rates of 1 to 5 hours at a

  <Desc / Clms Page number 28>

 sintering temperature of 1400 C gives less dense materials with densities around 3.0 g /
 EMI28.1
 Examples 6 to 9 and Examples 15 to 18 also show that an increase in density is obtained as a function of an increased heating rate. This is how this relation is verified as well for calcined alumina as for non-calcined alumina, and at different sintering temperatures.



   Examples 1 to 18 are used to illustrate Figure 2, which demonstrates a decrease in the intrusion volume with an increase in density for the high purity sintered alumina of the present invention. Figure 2 gives values for non-calcined alumina, and calcined alumina at 600 C, which are sintered at 1400 C and 1450 C for approximately 20 minutes. As can be seen in FIG. 2, this relationship between the intrusion volume and the density is verified as well for calcined alumina as for non-calcined alumina at different sintering temperatures. Figure 2 shows samples with densities. lying between 3.3 and 3.5 g / cm3 having a practically zero intrusion volume.

   If we combine the results of Figures 1 and 2, we can say that a rapid heating rate of less than a few minutes gives a dense material with an almost zero intrusion volume.



   Examples 2 to 8, Examples 11 to 17 and Examples 19 to 21 are used to illustrate Figure 3, which shows an increase in Knoop hardness with an increase in density for the sintered alumina product. the present invention. That relation

  <Desc / Clms Page number 29>

 is equally true for calcined alumina as for non-calcined alumina, at different heating rates and at sintering temperatures.



  Figure 3 shows a hardness between 1300 and 1800 klos for the denser alumina materials (greater than 3.4 g / cm). These results can be compared to a hardness of around 2100 klos for standard molten brown alumina, which has a density of around 4.0 g / cm and zero porosity. If we combine the results of Figures 1,2 and 3, we can say that a rapid heating rate gives a dense material with an almost zero intrusion volume and high hardness.



   Examples 34 to 37 are given to illustrate Figure 4, which shows an increase in density with an increase in the sintering temperature for the high purity, sintered alumina of the present invention. This relationship is just as true for calcined alumina as for non-calcined alumina at different heating rates. Figure 4 shows alumina materials having densities lying in-
 EMI29.1
 3 be around 3, 4 and 3, 6 after having been quickly heated in 1-5 minutes and sintered at temperatures between 14000 and 1600 C.

   Using the results of Figures 1,2, 3 and 4, a rapid heating rate of only a few minutes combined with a sintering temperature above 1400 C, gives a dense material with a practically zero intrusion volume and a high hardness.



   Examples 22 to 33 illustrate Figure 5, which shows that the sintering time has little effect on the density of the high purity sintered alumina.

  <Desc / Clms Page number 30>

 the present invention. Figure 5 gives values for non-calcined alumina, and calcined alumina at 600 C, which are sintered at 1400 C, and rapidly heated in 15-30 seconds. This relationship between the sintering time and the density is verified both for calcined alumina and for non-calcined alumina at different heating rates and sintering temperatures.



  Figure 5 relates densities of alumina in a narrow range of about 3.35 to 3.5 g / cm3 for a wide range of sintering times from less than one minute to 30 minutes. From the results of Figures 1,4 and 5, it can be seen that the present invention achieves direct cost savings in fuel used for the furnace. The alumina materials simply need to be quickly heated in a short period of time to a relatively low sintering temperature, and held at that sintering temperature for only a few minutes, to obtain a dense alumina material with a practically zero intrusion volume and high hardness.



   It should be understood that the present invention is in no way limited to the above embodiments, and that many modifications can be made thereto without departing from the scope of this patent.


    

Claims (69)

 CLAIMS 1. A process for obtaining high purity sintered alumina grains, characterized in that it comprises the following stages: a) the preparation of a dispersion comprising approximately 2 to approximately 60% by weight of oxide monohydrate aluminum; b) gelation of this dispersion; c) drying the gelled dispersion; d) grinding the dried material to form grains; e) rapid heating of the grains to a temperature above about 1200 C; and f) continuing to heat the beans to a  EMI31.1  sintering temperature between about 1200 C and about 1700 C for a sufficient sintering time.  2. A process for obtaining high purity sintered alumina grains, characterized in that it comprises the following stages: a) the preparation of a dispersion comprising approximately 2 to approximately 60% by weight of oxide monohydrate d aluminum; b) gelation of this dispersion; c) drying the gelled dispersion; d) grinding the dried material to form grains; e) calcining the grains; f) rapid heating of the grains to a temperature above about 1200 C; and g) continuing to heat the grains to a sintering temperature between about 1200 C  <Desc / Clms Page number 32>  and around 17000C for a sufficient sintering time.  3. Method according to either of claims 1 and 2, characterized in that in step a) the dispersion contains about 10 to about 40% by weight of aluminum oxide monohydrate.  4. A process for producing high purity sintered alumina grains, characterized in that it comprises the following steps: a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of monohydrate d aluminum oxide; b) placing the dispersion in an electrophoretic cell comprising an anode and at least one cathode; c) applying an electrical potential between the anode and the aforementioned cathode to cause the deposition of particles from the dispersion on the cathode; d) drying of the deposit; e) grinding the dried deposit to produce grains; f) rapid heating of the grains to a temperature above about 1200 C;  and g) continuing to heat the particles to a sintering temperature between about 12000C and about 17000C for a sufficient sintering time.  5. Method for obtaining high purity sintered alumina grains, characterized in that it comprises the following steps: a) the formation of a liquid dispersion  <Desc / Clms Page number 33>  taking about 0.1 to about 40% by weight of aluminum oxide monohydrate; b) placing the dispersion in an electrophoretic cell comprising an anode and at least one cathode; c) applying an electrical potential between the anode and the aforementioned cathode to cause the deposition of particles from the dispersion on the cathode; d) drying of the deposit; e) grinding the dried deposit to produce grains; f) calcining the grains; g) rapid heating of the grains to a temperature above about 1200 C;  and h) continuing to heat the particles to a sintering temperature between about 12000C and about 17000C for a sufficient sintering time.  6. Method according to either of claims 4 and 5, characterized in that in step c), the cathode is supplied continuously in the electrophoretic cell while the electrical potential is applied.  7. Process for obtaining high purity sintered alumina grains, characterized in that it comprises the following stages: (a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of monohydrate aluminum oxide; (b) placing the dispersion in a cell  <Desc / Clms Page number 34>  Electrophoretic unit comprising an anode, a cathode and at least one membrane disposed between this anode and this cathode; (c) applying an electrical potential between the above anode and the cathode; (d) drying the resulting deposit; (e) grinding the deposited deposit to produce grains;  (f) rapid heating of the grains to a temperature above about 1200 C; and (g) continuing to heat the grains to a sintering temperature between about 1200 C and about 1700 C for a sufficient sintering time.  8. A method for producing high purity sintered alumina grains, characterized in that it comprises the following stages: (a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of monohydrate d aluminum oxide; (b) placing the dispersion in an electrophoretic cell comprising an anode, a cathode and at least one membrane disposed between this anode and this cathode; (c) applying an electrical potential between the above anode and the cathode; (d) drying the resulting deposit; (e) grinding the dried deposit to produce grains; (f) calcining the grains;  (g) rapid heating of the grains to a  <Desc / Clms Page number 35>  temperature above about 1200 C; and (h) continuing to heat the grains to a sintering temperature between about 1200 C and about 17000 C for a sufficient sintering time.  9. Method according to either of claims 7 and 8, characterized in that in step (b) the cathode is covered with a membrane.  10. Method according to claim 9, characterized in that the cathode comprises at least one transverse hole extending over the entire length thereof.  11. Method according to either of Claims 7 and 8, characterized in that the electrophoretic cell further comprises means for agitating this liquid dispersion in the area immediately adjacent to the cathode.  12. Method according to claim 11, characterized in that in step (c), the liquid dispersion is stirred while the electrical potential is applied.  13. A process for producing this high purity sintered alumina grains, characterized in that it comprises the following stages: (a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of monohydrate d aluminum oxide; (b) placing the dispersion in the anode and membrane chambers of an electrophoretic cell containing an anode, a cathode and at least one membrane disposed between this anode and this cathode; (c) placing a conductive liquid in  <Desc / Clms Page number 36>  the cathode chamber of the aforementioned electrophoretic cell; (d) applying an electrical potential between the anode and the aforementioned cathode; (e) drying the resulting deposit; (f) grinding the dried deposit to obtain grains;  (g) rapid heating of the grains to a temperature above about 1200 C; and (h) continuing to heat the grains to a sintering temperature between about 12000C and about 1700C for a sufficient sintering time.  14. Process for the production of high purity sintered alumina grains, characterized in that it comprises the following stages: (a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of monohydrate d aluminum oxide; (b) placing the dispersion in the anode and membrane chambers of an electrophoretic cell containing an anode, a cathode and at least one membrane disposed between the anode and the cathode; (c) placing a conductive liquid in the cathode chamber of the aforementioned electrophoretic cell; (d) applying an electrical potential between the above anode and the cathode; (e) drying the resulting deposit; (f) grinding the dried deposit to obtain grains;  <Desc / Clms Page number 37>  (g) calcining the grains;  (h) rapid heating of the grains to a temperature above about 1200 C; and (i) continuing to heat the grains to  EMI37.1  a sintering temperature between about 1200 C and about 1700 C for a sufficient sintering time.  15. Method according to either of claims 13 and 14, characterized in that in step (c) the conductive liquid comprises a dilute acid.  16. Method according to any one of claims 7,8, 13 and 14, characterized in that the membrane comprises an ion exchange membrane.  17. Method according to any one of claims 7,8, 13 and 14, characterized in that the membrane comprises a microporous membrane.  18. Method according to any one of claims 7,8, 13 and 14, characterized in that the membrane is continuously fed to the electrophoretic cell while the electrical potential is applied.  19. Method according to any one of claims 4, 5, 7,8, 13 and 14, characterized in that in step (a) the liquid dispersion comprises 5 to 15% by weight of oxide monohydrate aluminum.  20. A method according to any one of claims 4,5, 7,8, 13 and 14, characterized in that the cathode comprises inert metal.  21. A method according to any one of claims 4,5, 7,8, 13 and 14, characterized in that the anode comprises inert metal.  22. Method according to any one of claims 1, 2,4, 5,7, 8,13 and 14, characterized in that in step (a) the liquid dispersion comprises a  <Desc / Clms Page number 38>  aqueous dispersion.  23. Method according to any one of claims 1, 2,4, 5,7, 8,13 and 14, characterized in that in step (a), about 0.02 to about 0.25 mol are added of one volatile acid per mole of aluminum oxide monohydrate as a dispersion adjuvant.  24. The method of claim 23, characterized in that the acid is nitric acid.  25. A method according to any one of claims 2,5, 8 and 14, characterized in that the grains are calcined at a temperature of about 2500C to about 800 C.  26. Method according to any one of claims 1, 2,4, 5,7, 8,13 and 14, characterized in that rapid heating is carried out in less than 20 minutes.  27. The method of claim 26, characterized in that the rapid heating is carried out in less than 5 minutes.  28. The method of claim 27, characterized in that the rapid heating is carried out in less than 1 minute.  29. A method according to any one of claims 1,2, 4,5, 7,8, 13 and 14, characterized in that the sufficient sintering time is from about 1 to about 30 minutes.  30. The method of claim 29, characterized in that the sufficient sintering time is less than 5 minutes.  31. A method according to any of the  <Desc / Clms Page number 39>  vendications 1, 2,4, 5,7, 8,13 and 14, characterized in that the sintering temperature is between 1400 C and 1600 C.  32. Sintered alumina grains of high purity manufactured according to a process comprising the following stages: a) the preparation of a dispersion comprising approximately 2 to approximately 60% by weight of aluminum oxide monohydrate; b) gelation of this dispersion; c) drying the gelled dispersion to vaporize the free water; d) grinding the dried material to form grains; e) rapid heating of the grains to a temperature above about 1200 C; and f) continuing to heat the grains to a sintering temperature between about 1200 C and about 1700 C for a sufficient sintering time.  33. High purity sintered alumina grains produced according to a process comprising the following stages: a) the preparation of a dispersion comprising approximately 2 to approximately 60% by weight of aluminum oxide monohydrate; b) gelation of this dispersion; c) drying the gelled dispersion to vaporize the free water; d) grinding the dried material to form grains;  <Desc / Clms Page number 40>  e) calcining the grains; f) rapid heating of the grains to a temperature above about 1200 C; and g) continuing to heat the beans to a  EMI40.1  sintering temperature between about 1200 C and about 1700 C for a sufficient sintering time.  34. Grains according to either of claims 32 and 33, characterized in that in step a) the dispersion contains about 10 to about 40% by weight of aluminum oxide monohydrate.  35. High purity sintered alumina grains produced according to a process comprising the following stages: a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of aluminum oxide monohydrate; b) placing the dispersion in an electrophoretic cell comprising an anode and at least one cathode; c) applying an electrical potential between this anode and this cathode to cause the deposition of particles from the dispersion on the cathode; d) drying of the deposit; e) grinding the dried deposit to produce grains; f) rapid heating of the grains to a temperature above about 1200 C;  and g) continuing to heat the particles to a sintering temperature between about 1200 C and about 1700 C for a sufficient sintering time.  <Desc / Clms Page number 41>    36. Sintered alumina grains of high purity manufactured according to a process comprising the following stages: a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of aluminum oxide monohydrate; b) placing the dispersion in an electrophoretic cell comprising an anode and at least one cathode; c) applying an electrical potential between this anode and this cathode to cause the deposition of particles from the dispersion on the cathode; d) drying of the deposit; e) grinding the dried deposit to produce grains; f) calcining the grains; g) rapid heating of the grains to a temperature above about 1200 C;  and h) continuing to heat the particles to a sintering temperature between about 1200 C and about 17000 C for a sufficient sintering time.  37. Grains according to either of claims 35 and 36, characterized in that in step c), the cathode is continuously fed to the electrophoretic cell while the electrical potential is applied.  EMI41.1  than.  38. High purity sintered alumina grains produced according to a process comprising the following stages:  <Desc / Clms Page number 42>  (a) forming a liquid dispersion comprising from about 0.1 to about 40% by weight of aluminum oxide monohydrate; (b) placing the dispersion in an electrophoretic cell comprising an anode, a cathode and at least one membrane placed between this anode and this cathode; (c) applying an electrical potential between the above anode and the cathode; (d) drying the resulting deposit; (e) grinding the dried deposit to produce grains;  (f) rapid heating of the grains to a temperature above about 1200 C; and (g) continuing to heat the grains to a sintering temperature of between about 1200 C and about 17000 C for a sufficient sintering time.  39. High purity sintered alumina grains made by a process comprising the following steps: (a) forming a liquid dispersion comprising from about 0.1 to about 40% by weight of aluminum oxide monohydrate; (b) placing the dispersion in an electrophoretic cell comprising an anode, a cathode and at least one membrane disposed between this anode and this cathode; (c) applying an electrical potential between the above anode and the cathode; (d) drying the resulting deposit; (e) grinding the dried deposit to produce  <Desc / Clms Page number 43>  seeds ; (f) calcining the grains;  (g) rapid heating of the grains to a temperature above about 1200 C; and (h) continuing to heat the grains to a sintering temperature between about 12000C and about 17000C for a sufficient sintering time.  40. Grains according to either of claims 38 and 39, characterized in that in step (b) the cathode is covered with a membrane.  41. Grains according to claim 40, characterized in that the cathode has at least one transverse hole extending over the entire width thereof.  42. Grains according to either of claims 38 and 39, characterized in that the electrophoretic cell further comprises means for agitating  EMI43.1  ter liquid in the area immediately see the dispersed sine of the cathode.  43. Grains according to claim 42, characterized in that in step (c), the liquid dispersion is stirred while the electric potential is applied.  44. High purity sintered alumina grains made by a process comprising the following steps: (a) forming a liquid dispersion comprising from about 0.1 to about 40% by weight of aluminum oxide monohydrate; (b) placing the dispersion in the anode and membrane chambers of an electrophoretic cell comprising an anode, a cathode and at least one mem-  <Desc / Clms Page number 44>  brane disposed between this anode and this cathode; (c) placing a conductive liquid in the cathode chamber of this electrophoretic cell; (d) applying an electrical potential between the above anode and the cathode; (e) drying the resulting deposit; (f) grinding the dried deposit to produce grains;  (g) rapid heating of the grains to a temperature above about 1200 C; and (h) continuing to heat the grains to a sintering temperature between about 1200 C and about 17000 C for a sufficient sintering time.  45. Sintered alumina grains of high purity manufactured according to a process comprising the following stages: (a) the formation of a liquid dispersion comprising approximately 0.1 to approximately 40% by weight of aluminum oxide monohydrate; (b) placing the dispersion in the anode and membrane chambers of an electrophoretic cell comprising an anode, a cathode and at least one membrane disposed between this anode and this cathode; (c) placing a conductive liquid in the cathode chamber of this electrophoretic cell; (d) applying an electrical potential between the above anode and the cathode; (e) drying the resulting deposit; (f) grinding the dried deposit to produce  <Desc / Clms Page number 45>  seeds ; (g) calcining the grains;  (h) rapid heating of the grains to a temperature above about 1200 C; and (i) continuing to heat the grains to a sintering temperature between about 1200 C and about 17000 C for a sufficient sintering time.  46. Grains according to either of claims 44 and 45, characterized in that in step (c) the conductive liquid comprises a dilute acid.  47. Grains according to any one of claims 38, 39, 44 and 45, characterized in that the membrane comprises an ion exchange membrane.  48. Grains according to any one of claims 38, 39, 44 and 45, characterized in that the membrane comprises a microporous membrane.  49. Grains according to any one of claims 38, 39, 44 and 45, characterized in that the membrane is continuously fed to the electrophoretic cell while the electrical potential is applied.  50. Grains according to any one of claims 35,36, 38,39, 44 and 45, characterized in that in step (a) the liquid dispersion comprises 5 to 15% by weight of oxide monoxide hydrate aluminum.  51. Grains according to any one of claims 35,36, 38,39, 44 and 45, characterized in that the cathode comprises inert metal.  52. Grains according to any one of claims 35,36, 38,39, 44 and 45, characterized in that the anode comprises inert metal.  53. Grains according to any of the  <Desc / Clms Page number 46>  vendications 32,33, 35,36, 38,39, 44 and 45, characterized in that in step (a) the liquid dispersion comprises an aqueous dispersion.  54. Grains according to any one of claims 32.33, 35.36, 38.39, 44 and 45, characterized in that in step (a), about 0.02 to about 0.25 mol are added of one volatile acid per mole of aluminum oxide monohydrate as a dispersion adjuvant.  55. Grains according to claim 54, characterized in that the acid is nitric acid.  56. Grains according to any one of claims 33,36, 39 and 45, characterized in that they are calcined at a temperature of about 2500C to about 800 C.  57. Grains according to any one of claims 32.33, 35.36, 38.39, 44 and 45, characterized in that rapid heating is carried out in less than 20 minutes.  58. Grains according to claim 57, characterized in that rapid heating is carried out in less than 5 minutes.  59. Grains according to claim 58, characterized in that the rapid heating is carried out  EMI46.1  0 less than 1 minute.  60. Grains according to any one of claims 32,33, 35,36, 38,39, 44 and 45, characterized in that the sufficient sintering time is from about 1 to about 30 minutes.  61. Grains according to claim 60, characterized in that the sufficient sintering time is less than 5 minutes.  <Desc / Clms Page number 47>    62. Grains according to any one of claims 32.33, 35.36, 38.39, 44 and 45, character-  EMI47.1  ses in that the sintering temperature is between 1400 C and 1600 C.  63. High purity, sintered, dense alumina grains with closed porosity.  64. Grains according to claim 63, comprising at least 99.0% by weight of A1203.  65. A coated abrasive article comprising grains according to any one of claims 32, 33.35, 36.38, 39.44, 45 and 63.  66. A nonwoven abrasive article comprising grains according to any one of claims 32, 33.35, 36.38, 39.44, 45 and 63.  67. A nonwoven abrasive article comprising grains according to any one of claims 32, 33.35, 36.38, 39.44, 45 and 63.  68. A refractory article comprising grains according to any one of claims 32,33, 35, 36,38, 39,44, 45 and 63.  69. Sintered alumina grains of high purity, their preparation process and their use, as described above, in particular in the Examples given.