DE4414570A1 - Mfr. of alpha-aluminium oxide sintered microcrystalline bodies - Google Patents

Abstract

A process for mfg. sintered microcrystalline bodies on the basis of alpha -Al2O3 is novel in that the solids from a suspension, contg. as solid particles alpha -Al2O3 and/or aluminium cpds. which form alpha -Al2O3 on sintering, are separated in a centrifugal field, then dried, sintered and comminuted.

Description

The present invention relates to a method for producing sintered microcrystalline body based on α-Al₂O₃ and their use.

A preferred application of α-Al₂O₃ sintered bodies is their use as Abrasives. In addition to molten corundum abrasives, these are also out sintered material has been known for more than 50 years.

In US-A-39 09 991 polycrystalline Al₂O₃ body are described, the Crystallite size is in the submicron range and their density is over 95% of theoretical density. The production takes place by hot pressing from one Mixture of carbon black and granulated Al₂O₃ bodies, which according to the US-A- 30 79 243 can be obtained by crushing cold-pressed Al₂O₃ moldings.

In recent times, similar sintered abrasives based on α-Al₂O₃ known, which have advantages over the microcrystalline structure have known corundum abrasives. So is in EP-B-01 52 768 Abrasives disclosed that about the sol-gel technique at sintering temperatures of approx. 1400 ° C. Crystallization nuclei are used as a sintering aid added. Similar processes and substances are disclosed in EP-A-00 24 099, DE-A- 32 19 607, US-A-45 18 397, US-A-45 74 003, US-A-46 23 364, EP-A-01 68 606, EP-A-02 00 487, EP-A-02 28 856, EP-A-02 09 084 and EP-A-02 63 810 forth.  

All of the latter methods have in common that they have a sol-gel method with finely dispersed aluminum oxide monohydrate of the boehmite type become. The relatively expensive raw materials that are involved in the hydrolysis of Aluminum-organic compounds are obtained, and the energy on it Manoeuvrable process engineering leaves the costs of sol-gel corundum at a lot times the conventional corundum. Another disadvantage of this ver driving is that the colloidal solutions mostly with relatively large amounts easily volatile inorganic acids are stabilized, what procedural and environmental brings technical problems.

DE-C-36 04 848 describes a method of dispersing raw materials containing alumina, compounds containing silicic acid and others Additives (e.g. compounds of the metals Co, Ni, Mg, Cr, Zr, Zn, Si or Ti) to grind a sinterable slurry, from which by gradual drying and sintering at temperatures up to 1700 ° C can, whose primary corundum crystallites have a diameter of less than 5 micron to have. Although it is possible to use inexpensive raw materials, one is still on complex grinding processes and high sintering temperatures instructed to obtain a microcrystalline abrasive. The product obtained in this way does not yet have a crystallite size of less than 5 microns the fine structure of a substance that can be processed using the sol-gel method Sintering additives is produced.

Numerous grinding tests on sintered corundums in recent years Years have shown that grinding performance is inversely proportional to size is the primary crystals. That is, the finer the structure, the higher the Rule grinding performance.

EP-A-05 24 436 describes a method whereby instead of the expensive boehmite other inexpensive precursors of aluminum oxide (e.g. hydrargillite) are used become. By grinding and subsequent deagglomeration Suspensions with a solids content between 10 and 40 wt .-% obtained that can be processed analogously to the sol-gel process. Also at this process must remove the remaining water with a lot of energy become. With the exception of the expensive raw material, this is shown in EP-A-05 24 436  described method all procedural disadvantages of the sol-gel method on.

EP-A-03 02 686 shows a process, microcrystalline corundum on the electrophoretic deposition from an α-Al₂O₃-containing organic Win suspension. It is thus possible to create a relatively dense green body obtained, which can be sintered at approx. 1600 ° C. Despite this - in comparison to the sintering temperatures that are common in the sol-gel process - high Sintering temperature occurs due to the dense and homogeneous packing of the Green body only during the sintering to a small crystal growth. Nevertheless, the structure is one about electrophoretic deposition obtained sintered corundum significantly coarser than that of the sol-gel Process produced sintered corundum.

The object of this invention is therefore to provide a method for producing a Show microcrystalline α-Al₂O₃ sintered body, which the described Does not have disadvantages of the prior art.

It has now been found that these requirements are met by Process in which α-Al₂O₃ and / or from a suspension as a solid Aluminum compounds, which form α-Al₂O₃ during sintering, contains the solid is deposited, sintered and crushed in the centrifugal field. This method is the subject of the present invention.

Surprisingly, it was found that by separation in the centrifugal field extremely dense green bodies can be obtained, which are in one Let the temperature range between 1250 and 1600 ° C compress. The Centrifugal separation therefore allows it to be depressurized in the extremely low Temperature range to sinter densely, which was previously only the sol-gel corundum was reserved. The solid (green body) obtained in the centrifugal field is built so dense and homogeneous that dense Al₂O₃ sintered body obtained can be recognized without crystal growth during sintering is. In addition to α-Al₂O₃, all precursors (e.g. hydrargillite, boehmite, γ-Al₂O₃, chi-Al₂O₃, bayerite, δ-Al₂O₃, amorphous alumina u. a.) are used.  

According to the inventive method, starting from inexpensive raw materials, sintered aluminum oxide can be obtained in their Properties are absolutely comparable with the sol-gel corundum. Since the Crystallite size in the sintered end product is significantly different from the average grain size and depends on the grain distribution in the starting material, it is advisable use as finely divided types as possible, which then have the desired grain size be ground or deagglomerated.

The solids for producing the suspension are preferred to an average Particle size of less than 1 μm, particularly preferably less than 0.4 μm, ground down and / or deagglomerated to the desired starting grain size to obtain or to split existing agglomerates into individual crystallites. The Grinding or deagglomeration is preferably wet in vibration mills, Attriters or ball mills performed. The grinding time depends on the Initial grain size and the type of mill used. Although it's in use of α-Al₂O₃ is obvious to choose a material as finely divided as possible, often speak against economic considerations; because the super fine α- Aluminum oxides are often so expensive that one of the main advantages of inventive method, namely the use of an inexpensive Raw material is lost. The examples below will therefore only called α-aluminum oxides, which make economic sense Production of the abrasive grain according to the invention can be used. The The same considerations apply to the corresponding preliminary stages.

The amount of liquid during grinding is preferably chosen so that the resulting suspension has a solids content of 15 to 50% by weight, preferably 10 up to 30% by weight. Water is preferably used as the solvent. Against other solvents such as. B. alcohols, acetone, which are also used ecological considerations speak above all.

Since a sintered body with an average primary crystal size is particularly preferred less than 0.4 micron is aimed for and if the sintering temperature is selected correctly In the case of the method according to the invention, the crystal growth largely can be suppressed, it is sufficient to continue grinding until the average grain size in the suspension is less than 0.4 micron.  

The suspension thus obtained is electrostatically or after grinding sterically stabilized.

In the case of steric stabilization, all known dispersing aids can be used be used. Particularly suitable dispersing aids are polyacrylic acids, Polyglycolic acids, polymethacrylic acids, organic bases such as triethylamine or Carboxylic acids such as acetic acid or propionic acid.

The suspension preferably contains 0.5 to 5% by weight of corresponding organic Stabilizers.

In the case of electrostatic stabilization, volatile can advantageously be used inorganic acids such as nitric or hydrochloric acid or ammonia as the base be used. The stability of the suspension only needs to be relatively short Time (until separation in the centrifugal field) can be guaranteed, so that one during the electrostatic stabilization of the suspension for the centrifugal divorce requires far less acid than comparatively with sol Gel procedure.

The suspension is stabilized either during grinding or after grinding with the aid of a dispersing device, whereby a rapid and even distribution of the stabilizer is guaranteed.

Are precursors of α-Al₂O₃ used, it is often necessary as a catalyst for the phase transition α-Al₂O₃ nuclei or similar nuclei to use. The nuclei are expedient with the corresponding Suspension added together with the dispersing aid.

The stabilized suspension becomes a sufficiently strong centrifugal force exposed so that the solids content in a dense layer that several Can become millimeters thick. The layer thickness can by suitable choice of the residence time in the centrifugal field can be varied.  

For the further process steps, it proves to be particularly advantageous if the solids in the centrifugal field for solids contents of greater than 80% by weight, preferably greater than 95 wt .-%, based on Al₂O₃, is concentrated.

A wide variety can be used as an aggregate for centrifugal separation Centrifuge types are used. So come separators, decanters, peelers centrifuges, inverting filter centrifuges or others in question. For those on a laboratory scale performed examples was expediently discontinuous with a Chamber separator worked. On a production scale, there are continuously working Prefer centrifuge types.

The centrifugate, from which the solids content depends on the centrifuge type, residence time and type of suspension up to 100% can be deposited in Cycle and used as a suspending agent before grinding become.

If the solids content is less than 95% by weight, the precipitate is separated Solid (green body) advantageously then subjected to drying. At the dryer has no special requirements. The drying itself preferably takes place at temperatures below 100 ° C. At a preferred solids content of greater than 95 wt .-% can be added Drying step can be dispensed with. In both cases the green body then preferably sintered at between 1250 ° C and 1600 ° C. As a sinter Furnaces can be used rotary kilns, push-through ovens or chamber ovens. The sintering temperature depends on the starting material used and is so chosen that a dense Al₂O₃ body is formed, but crystal growth still not taking place.

The method according to the invention has the advantage over electrophoresis that that with comparable grain sizes of the starting materials finer crystallite structure in sintered end product can be obtained. Another advantage is that there is no need to work in the organic medium.

There are also advantages over the sol-gel process. So when sol- Gel process the total water content (up to 80%) of the suspension after  the gelation is bound in the gel, with high energy expenditure during the Drying and calcining can be evaporated. It is to be attributed to that for Production of a sol-gel corundum compared to conventional grades, which are melted in the arc furnace, about 8 times the amount of energy is needed.

In the process according to the invention, however, the solid contains only a small amount Amounts of moisture that are either so low that they affect the sintering process do not disturb, or in a simple drying step after the centrifugal deposition can be removed.

In addition, the sol-gel process is used to stabilize the suspension uses relatively large amounts of volatile acids, which also during must be evaporated during drying and especially during calcining. The acids are preferably nitric or hydrochloric acid. Here environmental pollution occurs even today - despite complex technology - cannot be completely avoided yet.

After all, the inventive method does not require the use of the expensive ones Boehmite as starting substances.

The process according to the invention enables high-density to be produced sintered microcrystalline bodies with a high hardness, their crystallite size is adjustable between 0.1 and 10 µm.

Because of these properties, these sintered bodies are ideal as Abrasives.

This invention therefore also relates to the use of the invention produced sintered microcrystalline body as an abrasive and for the Manufacture of grinding and cutting tools.

Because the grinding properties differ significantly from the crystallite structure of each Depend on abrasive grain, it succeeds according to the inventive method, for  Various applications with one grit for the To provide optimal crystallite structure for each application.

In the following the invention will be explained by way of example without one Restriction can be seen.

Examples example 1

α-Alumina (A 16 SG ex Alcoa) was in a stirred ball mill type Grind PMC 25 TEX (Drais) to an average grain size of 320 nm. 5 kg of the resulting slurry with a solids content of approximately 18% by weight using a dispersant with 20 ml of dilute HNO₃. The so Stabilized suspension was with the aid of a chamber separator (type KAO5, Fa. Westfalia) centrifuged at 12,000 rpm and a throughput of 5 l / h. Here the solid was completely separated from the slurry. The drying of the Solids took place at 65 ° C. Sintering was carried out at 1475 ° C.

The product obtained has a Vickers hardness (HV₂₀₀) of 21.5 GPa and one average crystallite size of 0.4 micron.

Example 2

As in Example 1, a-alumina (A 16 SG) was used as the starting material puts. The suspension was stabilized with 3% polyacrylic acid. The other ver Driving steps were carried out analogously to Example 1. Was sintered 2 Hours at 1450 ° C.

The product thus obtained has a hardness (HV₂₀₀) of 19.8 GPa and an average 0.4 micron crystallite size.

Example 3

α-alumina (APA 0.2 X ex Condea) was so far in a ball mill deagglomerated that the solids content of the resulting slurry a medium Grain size of 300 nm. The slurry (1 kg) was diluted with 4 ml Nitric acid was added and then in the separator at 12,000 rpm centrifuged. Drying was carried out at 65 ° C. Sintering was carried out for 2 hours 1450 ° C.  

The product thus obtained has a hardness (HV₂₀₀) of 22.3 GPa and an average 0.3 micron crystallite size.

Example 4

500 g of aluminum oxide monohydrate (Disperal ex Condea) were in 2 l of water dispersed with the addition of acid (HNO₃). The colloidal solution became one 10% germ slurry (α-aluminum oxide with an average grain size of 400 nm) is added so that the proportion of germs, based on the end product, was about 3%. The mixture was in the chamber separator at 12,000 rpm centrifuged. Drying was carried out at 65 ° C. Sintering was carried out at 1300 ° C.

The product thus obtained has a hardness (HV₂₀₀) of 21.3 GPa and an average 0.3 micron crystallite size.

Example 5

Calcined alumina was in the PMC 25 TEX agitator ball mill (Drais) ground to an average grain size of 320 nm. The slurry was under simultaneous addition of crystallization nuclei (3% α-Al₂O₃ with a medium Grain size of 500 nm) stabilized with the help of nitric acid and at 12,000 Centrifuged rpm in the chamber separator. The green body was at 65 ° C dried and then sintered at 1480 ° C.

The product thus obtained has a hardness (HV₂₀₀) of 18.9 GPa and an average 0.8 micron crystallite size.  

The following are grinding tests with the grinding produced according to the invention grains compared to abrasive grains of the prior art.

Table 1 (grinding test 1 - fiber disc

Table 2 (sanding test 2 - sanding belt)

Table 3 (grinding test 3 - grinding wheel)

Claims (9)

1. A process for the production of sintered, microcrystalline bodies based on α-Al₂O₃, characterized in that the solid is deposited in a centrifugal field from a suspension which contains α-Al₂O₃ and / or aluminum compounds which form α-Al₂O₃ during sintering , then dried, sintered and crushed. 2. The method according to claim 1, characterized in that the solids to produce the suspension to an average particle size of <1 µm be ground down and / or deagglomerated. 3. The method according to claim 2, that the average particle size <0.4 microns is. 4. The method according to one or more of claims 1 to 3, characterized characterized in that the suspension has a solids content, calculated as Al₂O₃, from 5 to 50 wt .-%, preferably 10 to 30 wt .-%, has. 5. The method according to one or more of claims 1 to 4, characterized characterized in that the suspension 0.5 to 5 wt .-% organic Stabilizers, based on the Al₂O₃ content, contains. 6. The method according to one or more of claims 1 to 4, characterized characterized in that the suspension by adding acids or bases is electrostatically stabilized. 7. The method according to one or more of claims 1 to 6, characterized characterized in that the solids in the centrifugal field for solids contents of greater than 80% by weight, preferably greater than 95% by weight, based on Al₂O₃, is concentrated. 8. The method according to one or more of claims 1 to 7, characterized characterized in that the solid is dried and then at Temperatures in the range of 1250 ° C to 1600 ° C is sintered.   9. Use of the according to one or more of claims 1 to 8 produced sintered microcrystalline body as an abrasive and for the manufacture of grinding tools.