SK75397A3 - Process for the manufacture of abrasive grains based on alpha-al2o3 - Google Patents

Abstract

A process for preparing hydrosol by reducing thereof to gel, by drying, xerogel calcining, and sintering comprises the steps of controlled gelation to fix a desired microstructure by addition of a polyhydric organic acid solution, optionally containing alloying element, to afford a colloidal metastable solution of aluminium hydroxide in the presence of a mixture containing water and C1-4 alcohol (10 - 70 wt.%) with addition of a strong inorganic acid. Consequent combination of capillary drying and conventional heating upon alpha-Al2O3-containing ceramic membranes, or with a fixed alpha-Al2O3 layer thereupon, provides fast and effective drying along with the formation of a desired xerogel structure. Said xerogel is optionally subjected to an alloying element-containing alcohol impregnation. Obtained after baking are abrasive grains of a desired microstructure having density ranging between 92 and 99 percent of theoretical density.

Description

Technical field

The present invention relates to a process for the manufacture of grinding grains based on 01-Al2O3 from an alcoholic boehmite sol containing small amounts of oxonium salts of strong inorganic acids and organic alcohols by controlled gelatinization with a polybasic acid followed by drying the gel by capillary liquid transport from the gel in combination with convection or radiation , particularly suitable for drying gel layers of varying thickness and for obtaining dried gel grains of desired thickness with possible controlled nucleation and primary particle growth, depending on the crystallographic parameters of the lattice of the material serving as the porous membrane.

BACKGROUND OF THE INVENTION

The preparation of dense abrasive grains using the sol-gel method has long been known. One of the oldest references is U.S. Pat. U.S. Pat. No. 4,314,827, which discloses a process for preparing abrasive materials by gelatinizing an aluminum oxyhydroxide (boehmite) dispersion followed by drying and calcining using a modifying agent that is supplied in the form of a soluble magnesium nitrate salt. U.S. Pat. 4,181,532 and other successive patents, e.g. U. U.S. Patent No. 4,518,397; U.S. Pat. 4,574.003. These sources and subsequent others solve the technology of production of ceramic alumina for abrasive purposes. The result of most of these processes can be characterized by clusters of partially oriented α-Al 2 O 3 primary crystals larger than 6 gm. E.g. Kumagai and G.L. Messing in J.Am. Cer. Soc. 67, C 230 (1984), or Dynys, F.W. Halloren in Ultra-structure Processing of Ceramics, Glasses and Composites, ed. L. L. Hench and D. R. Ulrich (John Wiley, New York, 1984) Chap. 11, p. 142-181 and others.

The production of abrasive grains by sol-gel technology utilizes the chemical-ceramic technologies of gelling the alpha aluminum oxyhydroxide sol with one or more modifying components followed by dehydration and annealing. Modifying additives are mostly metal oxides of zirconium, hafnium or combinations thereof, or metal oxides forming with α-Al 2 O 3 spinels, e.g. nickel, magnesium.

The current sol-gel process used to prepare the abrasive grains can be described as follows:

1. Preparation and dispersion of boehmite (A10 (0H)) to form an aqueous dispersion (with or without the use of modifying additives)

2. Gelatinization of the dispersion (with or without the use of modifying agents) and concentration of the gel by reducing the water content

3. Drying the solid phase gel

4. Calcination to remove bound water and convert to a high temperature stable form of alumina

5. Sintering

This basic scheme may be supplemented by a series of steps that either complement or modify the above basic steps. E.g. impregnating a mixture of calcined aluminum oxide particles by modifying the additions of rare earth metal oxides to about 0.5%.

The prior art methods involve the preparation of an aqueous dispersion having a concentration of 2 to 60% aluminum oxyhydroxide (boehmite) by dispersing in water containing the peptide additive, or the peptide additive is gradually added. In the prior art, either monobasic acids or soluble salts of such acids, such as e.g. acetic, hydrochloric, nitric or formic acid. The most commonly used acid is nitric acid. Polyhydric acids are generally not used because they rapidly gel the dispersion and make it difficult to process by mixing with additives.

The gel microstructure appears to be one of the basic conditions conducive to the formation of the desired microstructure of the resulting abrasive grain. The formation of primary particles in the xerogel occurs diametrically differently in the presence of and without the inoculant. This difference is due to the nucleation process that must be applied to a gel not containing an inoculant. Conversely, it does not have to be applied in a vaccinated gel containing the seeds of the desired phase (X-Al 2 O 3.

The use of inoculants as nucleation centers results in a regular, dense grain structure compared to the inoculant-free process. In a macromerre, this operation results in a higher grain strength with a fine-grain structure, which is not only convenient but also necessary for efficient grinding. Suitable nucleation centers are e.g. fine-grained particles α-Α1 ₂ 03 · Other inoculants are substances that are crystallographically similar to alpha alumina as is the case with alpha iron oxide. The material thus produced has a density of 95% or more of the theoretical density.

When alpha alumina is used, the alumina particles are added to the dispersion in varying amounts (from 0.1% to 50%), the particle size of aAl2O3 being a very important parameter, ranging from 80 nm to 700 nm, but can be used even larger particles. The biggest role here is played by the largest particles.

Boehmit can be obtained from a variety of technologies. Among the known commercially available products, e.g. Disperal® · by Condea Chemie GmBH. These monohydrates are mostly relatively pure with a small amount of additives. If they are present, these are impurities that do not adversely affect the entire process. However, even the highest quality boehmites contain a certain proportion of unpeptifiable particles that can serve as undesirable nucleation centers (centers with inappropriate crystallographic parameters). Therefore, in order to obtain high quality soles, it is necessary to perform separation on ultracentrifuges.

A porous undesirable microstructure is likely to be formed by rapid nuclei growth through the alpha-A12O3 (nucleus) - --Α1Ο (defective phase) phase interface. Conversely, the primary particle formation process is strongly influenced by the addition of α-Α ^ Αβ fine crystals and the grafted gel is transformed into (X-Al 2 O 3 without homogeneous nucleation).

Thus, prior art xerogel processes require the elimination of undesirable nucleation centers from the sol preparation process, or the minimization of their negative impact and the use of crystallographic materials with very small particle sizes.

An important feature of all patents is that alloying additives are used which not only favor the sintering process, but also improve the final properties of the abrasive grains. These additives are usually salts of magnesium, zinc, cobalt, zirconium or hafnium, the effect of which is the formation of spinels or solid solutions. Rare earth metal ions (content ranging from 1 to 30%) are also used.

Existing methods of using alloying elements are based on the use of water-soluble salts containing a volatile anion, e.g. nitrates, formates, acetates and the like. Thus, anions decomposing at temperatures lower than the calcination temperature but leading to the formation of undesirable gaseous components. So far, in most cases, abrasive gels have been convective dried. The safe drying of the gels is dependent on the determination of the optimal parameters of the drying environment, i.e. in this case, in particular the temperatures and flow velocities of the drying medium, ensuring uniform drying in the volume of the gel formed in the form of a plate, a cylinder, etc. The condition of the safety of drying - i.e. Creation of the required conditions for the formation of primary particles in the xerogel - minimizing surface drying, preventing the formation of an impermeable surface layer, and achieving a low moisture gradient in the thickness of the dried gel layer. Achieving these conditions of harmless drying is associated with a very slow, time-consuming course of conventional drying. For example, tens of hours are not an exception for 10 to 20 mm gel layer thicknesses. The acceleration of the convective drying process of the gel layers leads to the preparation of dried grains with different properties of surface and internal grain layers with the possible presence of micro-cracks, which negatively influence the resulting properties of the abrasive grain.

The high time-consuming performance of conventional drying requires a large drying capacity in the continuous production of gel abrasives. In the production of abrasive grain by sol-gel technology, various methods are used to speed up the entire drying process to quickly remove excess water - extraction, vacuuming, and the like. However, the use of alloying elements in the form of electrolytes may leak this alloying element.

SUMMARY OF THE INVENTION

The essence of the process for producing grinding grains based on 01-Al2O3 with a density of 92-99% of the theoretical density, comprising the preparation of an aluminum oxyhydroxide sol, transformation into a gel, xerogel drying, xerogel grain calcination and sintering, according to the invention is the oxyhydroxide sol. Alumina is prepared in the environment of a mixture of alcohol and water with the addition of an inorganic acid containing 1 to 70% alcohol, the resulting sol is controlledly transformed into a gel, which is then dried on porous membranes with fixed nucleation centers, calcining and sintering with grains of dried gel containing active alloying elements.

The method according to the invention is based on the controlled transformation of an aluminum oxyhydroxide sol into a gel using the capillary liquid-phase drying of ceramic gels in combination with convection or radiation or microwave heating while allowing the growth of xerogel particles on a suitable porous membrane to achieve the desired microstructure. by thermal transformation to the desired abrasive grains α-Α ^ Οβ.

In essence, the preparation of a sol means deagglomeration of the original aggregates of boehmite particles into individual particles in an acidic medium. In this process, the boehmite particles adsorb hydrogen cations on their surface, thereby becoming positively charged and repelling each other, which in turn results in a certain time stability, which is then given by both the boehmite concentration and the pH at a given temperature.

Given the previously known proteolytic equilibrium theories, this stability is time-dependent, in this two-phase system, varies throughout the volume and thus does not ensure complete deagglomeration of the boehmite aggregates to the desired primary particles.

However, increasing the concentration of acids leads to a rapid gelatinization process to form a domain structure, and to the production of a large amount of environmentally undesirable waste gases throughout the technology. Decreasing the acid concentration does not ensure complete peptization. Reducing the concentration of boehmite in turn worsens the conditions for safe drying.

According to the invention, this drawback removes the use of organic alcohols as protophilic substances. The use of protophilic substances - organic alcohols - preferably ethyl alcohol, makes it possible to reduce the amount of inorganic acid. The use of protophilic substances makes it possible to increase the dissociation of the aqueous medium in which the dispersion takes place. In combination with a strong inorganic acid and an organic alcohol, the esterification process leads to the formation of oxonium salts which increase the solvation effect.

The colloidal gel is obtained by destabilizing the sol by changing the pH or by spontaneous gelatinization. Depending on the method used and thus the speed of the process, the microstructure of the original sol, the degree of perfection of the alloying elements, is fixed.

The use of polybasic acids, previously described as unsuitable, is used in the present invention to control the gelatinization process. The essence of the use of a polybasic organic acid, preferably citric acid, consists in the gradual destabilization of the sol as a function of the rate and concentration of the polybasic acid solution being added, which not only allows sufficient time for homogenizing the alloying additive, ie removing air from the solo. The effect of using an organic alcohol, preferably ethyl alcohol, is further manifested by the formation of an azeotropic mixture, a decrease in the evaporation temperature, thereby increasing the drying rate.

So far, in most cases, abrasive gels have been convective dried. The proper drying of the gels depends on a number of parameters which must ensure uniform drying in the volume of the gel formed into the shape of a plate, cylinder, etc. Achieving conditions of harmless drying is associated with a very slow, time-consuming, course of convection drying. The acceleration of the convective drying process of the gel layers leads to the preparation of dried grains with different properties of surface and internal grain layers with the possible presence of micro-cracks, which negatively influence the resulting properties of the abrasive grain. The high time-consuming performance of conventional drying requires a large drying capacity in the continuous production of gel abrasives. In the production of abrasive grain by sol-gel technology, various methods are used to speed up the entire drying process to quickly remove excess water - extraction, vacuuming, and the like. Thus, the use of alloying elements in the form of electrolytes may leak the alloying element.

According to the invention, on the other hand, a ceramic gel, e.g. in the form of a layer, it undergoes one or two-sided capillary suction of the liquid phase from the gel through a porous membrane made of a crystallographically suitable material, which allows to simultaneously influence the formation of primary particles in xerogel; followed by drying of the liquid phase by conventional or other type of in a controlled atmosphere having a relative humidity of 20 to 80%, a temperature of 30 to 200 ° C and a flow rate of 0 to 10 m / s.

The effect of the invention is that by capillary suction of the liquid phase and drying of the liquid from the surface of the porous membrane in a controlled atmosphere the drying of the gel surface is prevented, the liquid transport by the surface of the gel layer is reduced, more uniform distribution of the liquid in the gel layer is achieved resulting in uniform microstructure. thus increasing the quality of

process. The drying method according to the invention allows the drying of the ceramic gels at a higher drying rate than with the conventional type of drying. The process of preparing the gel of the invention with the application of alcohol in its preparation increases the drying advantages of the invention and further accelerates the transport of the liquid phase and shortens the time of safe drying of the gel.

In the known sources, the alloying elements are added to the resulting abrasive grain (X-Al2O3 in the process of preparing a sol or gel in the form of water-soluble salts, which may also be impregnated with xerogel calcines. It can also vary throughout the process due to the formation of concentration gradients during drying, as well as the method of introducing alloying ingredients by the saturation of calcined xerogel with subsequent sintering is characterized by an increased energy intensity due to the need to remove water from the solution.

According to the invention, on the other hand, the alloying element is introduced into the grain structure of 01-ΙΙ2Ο3

The principle is that the alloying element is introduced in the form of a reactive oxide in an acidic aqueous medium of an organic acid, preferably citric acid, in an unstable state, e.g. in caustic form, capable of participating in the chemical processes of gel formation. In this case, the individual boehmite particles are coated with complexes based on the organic acid used containing the respective alloying element, preferably magnesium, followed by drying, calcination and sintering.

Achieving a dense fine-grained microstructure is an essential condition for achieving the desired abrasive grain properties. To date, this requirement has been met by the use of inoculants, mostly based on 01-Al2O3, which requires the preparation of very fine, defined particles of a suitable nucleation substance. However, the preparation of these substances is very difficult and costly.

According to the invention, the ceramic gel is subjected to the aforementioned capillary suction of the liquid phase from the gel on a porous membrane, the surface of which simultaneously serves as a source of nucleation centers.

DETAILED DESCRIPTION OF THE INVENTION

The following examples illustrate the preparation of the grinding corundum grains according to the invention, but do not limit its scope.

Example 1

By dispersing boehmite (sold under the trade name Disperal, Condea Chemie, GmbH) at a temperature of 65 ° C, a suspension was prepared containing

28.6% boehmite. A 5.3% aqueous nitric acid solution was gradually added to the resulting suspension. This procedure yielded a sol, which was further dried according to Example no. 8. To gelatinize this sol, it was necessary to use the drying or plasticization mentioned below, i. removing excess water by stirring at 70 ° C.

The addition of an 11% citric acid solution to the same sol resulted in gelation during mixing.

Example 2

A dispersion containing 28.6% boehmite was prepared by dispersing boehmite at 65 ° C. A 2% aqueous nitric acid solution and ethyl alcohol were added successively to the resulting suspension. This procedure yielded a sol in which part of the water (10%) was replaced with ethyl alcohol, and 11% citric acid solution was gradually added thereto. Then the sol was poured onto a ceramic support and dried. The dried gel was crushed, sorted, calcined at 550-650 ° C for 1 hour and subsequently calcined at 1,380 ° C for 1 hour. The density of the grain thus obtained was 3.91 +/- 0.01 g / cm 2.

Examples 3 to 6

The procedure is the same as in the previous example, but the ethanol and citric acid contents were changed.

In the preparation of a sol containing 5% ethyl alcohol without the addition of citric acid solution, a final grain density of 3.55 g / cmL was achieved.

In the preparation of a sol with an ethanol content of 8% without the addition of a citric acid solution, a final grain density of 3.76 g / cm @ 2 was achieved.

In the preparation of a sol containing 10% ethyl alcohol, followed by gelatinization with a 19% citric acid solution, it was plasticized during mixing and reached a density of 3.80 g / cm @ 2.

In the preparation of a sol containing 10% ethyl alcohol followed by gelatinization with a 15% citric acid solution, gelation occurred after 3 min. After drying, calcining and calcination a density of 3.90 g / cm ^ was achieved.

Example 7

The procedure is identical to Example 1 except that the alloying element Mg was used in the form of caustic MgO, which was dissolved in 11% citric acid solution. After drying, calcining and calcination a density of 3.89 g / cm ³ was achieved.

Example 8

The gel prepared according to Example 1 was convectionally dried from a free surface on a non-porous substrate at an ambient temperature rise of 5 ° C / 2h from an initial temperature of 40 ° C to 105 ° C, drying the gel in 26h at a layer thickness of 1 cm. After a drying time of 12 h, an impermeable surface layer was formed with a temperature different from the undried gel layer.

The same gel was dried by a combined capillary convection method on a porous ceramic membrane with an ambient temperature of 70 ° C and a total drying time of 7 h, with drying over the entire volume of the gel at the same temperature.

Example 9

The gel prepared according to Example 2 was dried under the same mode as in Example 8 by a combined capillary convection method and the total drying time was 5 h while maintaining the properties of the gel layer throughout its drying.

Claims (4)

PATENT CLAIMS A method for producing abrasive grains based on (X-Al 2 O 3 with a density of 92-99% of theoretical density, comprising preparing an aluminum oxyhydroxide sol, transforming it into a gel, drying it to xerogel, calcining xerogel grains and sintering it, characterized in that the aluminum oxyhydroxide sol is prepared in an environment of a mixture of alcohol and water with the addition of an inorganic acid containing 1 to 70% alcohol, the resulting sol is controlledly transformed into a gel which is then dried on porous membranes with fixed nucleation centers, calcination and sintering carried out with dried gel containing active alloying elements. Method according to claim 1, characterized in that the controlled transformation of the sol into a gel is carried out by the addition of a polybasic organic acid. Method according to claim 1, characterized in that the alloying element is used in the form of an oxide reactive in an aqueous medium. Method according to claim 1, characterized in that the drying of the gel is carried out on membranes with an open porosity of 1 to 50%.