DE19629690A1 - Sintered microcrystalline corundum grinding medium production - Google Patents

Abstract

A process for producing sintered, microcrystalline alpha -Al2O3 based bodies involves (a) grinding or disintegrating an alpha -Al2O3 powder of <= 3 microns mean particle size to form a slip of less than 1 (preferably <= 0.4) micron particle size; (b) granulating the slip using a fluidised bed spray granulator to form green bodies of \-40 (preferably \-50)% theoretical density and less than 6 (preferably <= 1) wt.% residual moisture content; and (c) carrying out shock sintering at 1300-1550 deg C. Preferably, the slip contains 0.5-10 wt.% (based on solids content) of one or more binders selected from methyl cellulose, dextrin, sugar, starch, alginates, glycols, polyvinyl pyrrolidone, lignin sulphonate, gum arabic, polyvinyl alcohol and polyvinyl acetate.

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, they are made of Gesin known material for more than 50 years.

In US-A 3 909 991 polycrystalline α-Al₂O₃ body are described, the Crystallite size is in the submicron range and its density is over 95% of theo retic density. The production takes place by hot pressing from one Mixture of carbon black and granulated α-Al₂O₃ bodies, which according to US-A 3,079,243 can be obtained by crushing cold-pressed α-Al₂O₃ moldings.

In recent times, similar sintered abrasives based on α-Al₂O₃ have been used knows that due to their microcrystalline structure advantages over the be known corundum abrasives. So in EP-B 0 152 768 Abrasives disclosed that about the sol-gel technique at sintering temperatures of approx. 1400 ° C. Crystallization nuclei are added as a sintering aid puts. Similar processes and substances are known from EP-A 0 024 099, DE-A 32 19 607, US-A 4 518 397, US-A 4 574 003, US-A 4 623 364, EP-A 0 168 606, EP-A 0 200 487, EP-A 0 228 856, EP-A 0 209 084 and EP-A 0 263 810 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 will. The relatively expensive raw materials that are involved in the hydrolysis of Aluminum-organic compounds are obtained, and the energy expenditure process engineering, the costs of sol-gel corundum are many times over of conventional corundum. Another disadvantage of these methods is that the colloidal solutions are usually more volatile with relatively large amounts inorganic acids are stabilized, which is procedural and environmental Brings problems.  

DE-C 36 04 848 describes a method of dispersing alumina-containing raw materials, silicic acid-containing compounds and other additives add (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. The product thus obtained has a crystallite size of less than 5 microns not yet the fine structure of a substance using the sol-gel method corresponding sintering additives is produced.

In addition, the method disclosed in DE-C 36 04 848 silicic acid compounds are added, which act as a sintering aid. During sintering, mullite is formed from this by reaction with the aluminum oxide. It is known that due to the presence of silicate phases the performance of an abrasive grain is reduced.

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 0 524 436 discloses a method whereby instead of the expensive boehmite other inexpensive precursors of aluminum oxide (e.g. hydrargillite) are used will. By grinding and subsequent deagglomeration Suspen Sions with a solids content between 10 and 40 wt .-% obtained, the analog can be further processed into the sol-gel process. Even with this ver the remaining water has to be removed with a lot of energy. With the exception of the expensive raw material, this is demonstrated in EP-A 0 524 436 Process described all procedural disadvantages of the sol-gel method on.

Compared to the sol-gel process, however, all ages known so far have Native process for the production of microcrystalline sintered corundum by inexpensive ten raw materials run out, the disadvantage that after sintering significantly coarser Structures exist as in the case of the pro obtained using the sol-gel process products. The reasons for this are obvious. The sol-gel processes start from particularly finely divided starting materials, as with the addition of sintering aids  can be densely sintered at very low temperatures due to driving. There the crystal growth is suppressed by. The alternative procedures require basically higher sintering temperatures, leading to a reinforced crystal lead growth. In addition, the one that takes place during sintering Phase transformations to α-Al₂O₃ a homogeneous and uncontrolled crystal growth initiated. The power of ge over the alternative method The sintered corundum obtained is therefore significantly lower than that of the sol-gel corundum.

EP-A 0 402 686 shows a process, microcrystalline corundum on the electrophoretic deposition from an α-Al₂O₃-containing organic Win suspension. It is thus possible to achieve a relatively dense green body hold, which - despite the dense and homogeneous packing - only with Temperatures of 1600 ° C can be densely sintered, so that it is here again increased crystal growth occurs. The structure of one over the electro The sintered corundum obtained from the phoretic deposition is significantly coarser than that of the sintered corundum produced using the sol-gel process. Is correspondingly lower also the power of the sintered corundum obtained by electrophoresis.

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 surprisingly been found that these requirements are met by a Process for the production of sintered, microcrystalline bodies based on α-Al₂O₃, characterized in that an α-Al₂O₃ powder with an average grain size of 3 µm to form a slurry with a particle size <1 µm is ground or deagglomerated, the slip with the help of a Fluid bed spray granulator to green body with a density of 40% of theoretical density and a residual moisture content of 6% by weight and then shock sintering at temperatures in the range from 1300 to 1550 ° C is subjected.

The first process step consists of wet grinding or a disagglo Meration of an α-Al₂O₃ powder that is already as finely divided as possible. Aim of Nassver Grinding or deagglomeration is an even finer, very homogeneous To get α-Al₂O₃ suspension with an extremely fine grain distribution curve. The Wet grinding and deagglomeration is advantageous in an aqueous medium  carried out. In the second step, the grinding or des agglomeration obtained aqueous suspension in a fluidized bed granulator spherical granules with a diameter between 1 and 10 mm granulated.

DE-A 35 07 376 describes a method and an apparatus for the manufacture development of granules with a narrow particle size distribution, in which the granulate to be granulated Product in liquid form sprayed into a fluidized bed and there on correspond germs are applied. The granule size is determined by the strength of the Visible gas flow of a zigzag classifier set. Similar procedures or Further developments of the so-called fluidized bed spray granulation are in DE-A 38 08 277 and DE-A 43 04 405.

In the third step, the granules are subjected to shock sintering.

The granules can be cut to the desired grain shape before or after sintering size and size. The preparation and classification for the finished Abrasive grains are made using the usual methods.

Surprisingly, starting from the process according to the invention of inexpensive raw materials, sintered corundum, which can be obtained in their Properties are comparable to the sol-gel corundum. Because the crystallite size in the sintered end product significantly from the average grain size and the Grain distribution in the starting material depends, it is appropriate, if possible use finely divided α-Al₂O₃ types, which then have the desired grain size be ground or deagglomerated.

The solids are preferably of an average particle size of less than 1 µm, particularly preferably less than 0.4 µm, ground down and / or deagglomerated to obtain the desired starting grain size or to split existing agglomerates into individual crystallites. The grinding or Deagglomeration is preferably wet in vibration mills, attritors or Ball mills performed. The grinding time depends on the initial grain size and the type of mill used. Although it should be obvious, one if possible Choosing finely divided starting material often speaks economic benefits against; because the super fine α-aluminum oxides are often so expensive, that one of the main advantages of the method according to the invention, namely  the use of an inexpensive raw material is lost. With fiction However, this method can advantageously also coarser α-Al₂O₃-Aus powder can be used.

Since the grinding power is inversely proportional to the primary crystal size in Abrasive grain behaves, it may be advantageous to use the starting material additional components, sintering aids and / or crystal growth inhibitors offset. Suitable additional components, sintering aids and crystal wax inhibitors are the oxides of the elements Mg, Co, Ni, Zn, Hf, Ti, Ce, Zr, Cu, Li, Sr, Ba, K, Nb, Si, B and / or rare earths.

The amount of liquid during grinding is preferably chosen so that the resulting suspension has a solids content of 15 to 80% by weight, preferably 30 up to 70% 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, crystal growth can be suppressed , it is often sufficient to continue grinding until the middle one Grain size in the suspension is less than 0.4 micron.

Suspensions whose solid particles have a bimodal grain can be advantageous have distribution curve, are used.

The best results can be achieved with slip, their bimodal Grain size distribution a maximum in the range between 0.1 and 0.3 µm and a have another maximum in the range between 0.2 and 1.0 µm. The Grain size distribution is determined using a laser dispersion method (Microtrac Type MIC 2, Micromeritics) in an aqueous solution and Na₄P₂O₇ as Dispersion aid measured.

The suspension can be steric or electrostatic by inorganic or organic compounds are stabilized. In the case of steric stabilization all known dispersing aids can be used. Suitable as such especially polyacrylic acids, polyglycolic acids, polymethacrylic acids, organi  bases such as triethylamine or carboxylic acids such as acetic acid or propion acid. The suspension preferably contains between 0.5 and 5% by weight organic stabilizers. In the case of electrostatic stabilization can advantageously volatile inorganic acids such as nitric acid or hydrochloric acid and ammonia can be used as the base.

The suspension is stabilized either during or after Grinding with the help of a disperser, which ensures a quick and even Distribution of the stabilizer is guaranteed. Preferred before, but also during or after grinding or stabilizing the suspension Sintering additives and binders can be added. Coming as sintering additives all known for Al₂O₃ sintering aids or their precursors in question.

The suspension according to the invention preferably contains 0.5 to 10% by weight of one or several binders from the group alginates, dextrin, glycols, gum arabic, Lignin sulfonate, methyl cellulose, polyvinyl acetate, polyvinyl alcohol, polyvinyl pyrro lidon, starch and sugar, based on the solids content of the suspension. As well The suspension can advantageously be 0.5 to 10% by weight of one or more binders from the group boehmite sol, silica sol and water glass.

The suspension is dried and the solid is compacted in one Step in a fluidized bed granulator. The granulation is preferably on Air is carried out and can be started in a fluidized bed apparatus that already contains starting granules. However, it is also possible to do the granulation in one empty apparatus to start, the fluidized bed granulation as spray drying begins and germs are generated in situ.

The suspension to be granulated is sprayed into the fluidized bed brings. The use of two-component nozzles is particularly advantageous. As Zer Dust gas can be any gas that is inert under the working conditions the. The amount of atomizing gas can vary within a wide range are and generally depends on the dimensions of the apparatus and the type and quantity of the product to be sprayed. The temperature of the atomization Bergas current or the air inlet temperature is also within a gr variable range. Generally one works at temperatures between 20 and 350 ° C. The sight gas temperatures are also in a larger range variable. Here too, work is preferably carried out in a range between 20 and 350 ° C.  The amount and speed of the sight gas depends on the Density and the desired grain size of the granulate.

The grain size is primarily determined by the gas flow and velocity of the classifying gas controlled. With the zigzag sifter described in DE-A 35 07 376 it is possible, a narrow grain band in the grain size range between 0.01 and 10 mm with a bandwidth of 1 mm. The finished granulate can be used directly - or after an intermediate calcining step at temperatures between 300 and 600 ° C - to be sintered.

Sintering takes place at temperatures between 1300 ° C and 1550 ° C. In order to the necessary sintering temperature is significantly lower than that otherwise required for sintering of conventional α-Al₂O₃ ceramic usual temperatures of about 1600 ° C but at the same time still much higher than that required for the sol-gel process temperature, which is preferably below 1300 ° C. All the more surprising it is that the process according to the invention succeeds in crystal growth suppress almost completely. In addition to the fineness and must narrow particle size distribution of the starting powder and the high density of the green body as a further requirement, the necessary sintering must be reached very quickly temperature can be guaranteed. That means that the green body in as quickly as possible should reach the hottest zone of the sintering furnace. Through this shock sintering the sintering process can be completed before crystal growth begins. At the same time, the good pre-compression accelerates the sintering process. The homogeneous grain distribution and the fineness of the starting material favor uniform dense sintering, whereby the primary crystals grow together larger crystallites can be avoided.

All furnace types or sintering processes are suitable for sintering allow sudden heating of the green body. Can be beneficial directly or indirectly heated rotary kilns, pendulum furnaces, push-through furnaces, fluidized beds sintering ovens or microwave sintering ovens are used. This is advantageous Shock sintering so that the green body in 60 seconds, preferably 30 Seconds, particularly preferably 10 seconds, on the required sintering temperature is brought. The holding time during sintering is 60 minutes, preferably 30 minutes, particularly preferably 15 minutes.  

The granules are crushed to the desired grain size from energeti reasons immediately after granulation. Depending on the application however, it may also be advantageous that after the calcining step or after the sine The crushing of the granules is carried out to make them particularly sharp Get cutting edges.

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.

Since the fineness of the crystallite structure is directly related to the performance of the Abrasive grain stands, can be an abrasive grain according to the inventive method with a higher stock removal rate. Another An advantage from an ecological point of view is that it works in an aqueous medium and environmental pollution from organic solvents is avoided.

Compared to the sol-gel process, there are also - in addition to the economic advantage through the use of inexpensive raw materials - not too underestimated environmental benefits. So in the sol-gel process Stabilization of the suspension of relatively large amounts of volatile acids used during drying and especially during calcining need to be evaporated. The acids are preferably Nitric or hydrochloric acid.

There are environmental impacts that, despite the complex technology, are not yet can be completely avoided. Another advantage over the Sol-gel method consists in the simplicity of the method according to the invention, whereby the production in the continuous production operation is facilitated, what ultimately brings economic benefits again.

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 are suitable these sintered bodies are excellent as abrasives, with the crystallite sizes here be between 0.1 and 3 µm.  

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 in the inventive method for the 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.

example

α-alumina with an average grain size (d₅₀) of 1.5 microns was in one Agitator ball mill (type PMC 25 TEX, Drais) to an average grain size (d50) of 320 nm ground in an aqueous medium. The resulting slurry with a solids content of 30 wt .-% and a d₉₀ of 630 nm was with a 10% aqueous suspension of a polyvinyl alcohol as a binder (Mowiol 8/88, Hoechst AG, Germany), so that the proportion of the binder was approx. 4% by weight, based on the solids content of the slurry.

The suspension was then in a fluidized bed spray granulator (AGT 400, from Glatt Germany) at an air inlet temperature of 320 ° C, one Layer temperature of 75 ° C and a spray rate of 2.4 kg / min processed. For Nucleation became a fine granule fraction with a medium granule size between 0.5 and 1 mm, which in a previous fluidized bed spray granule had been obtained via in situ nucleation.

The desired granules were separated off using a zigzag sifter. 60% by weight of the granules thus obtained had a diameter between 3 and 5 mm, about 30% by weight of the granules had a diameter between 1 and 3 mm and about 10% by weight of the granules had a diameter of <5 mm. The residual moisture of the granules was less than 1%.

The granules were crushed and then directly at 1480 ° C heated rotary tube sintered.

The product obtained has a Vickers hardness (HV 0.2) of 20.1 GPa and one average primary crystallite size of 0.38 µm. The density is 98.7% of the theoretical density.  

Grinding test (grinding belt)

Claims (15)

1. A process for the production of sintered, microcrystalline bodies based on α-Al₂O₃, characterized in that an α-Al₂O₃ powder with an average grain size of 3 µm is ground or deagglomerated as a starting material with a particle size of <1 µm, the slip granulated with the aid of a fluidized bed spray granulator to form green bodies with a density of 40% of the theoretical density and a residual moisture content of <6% by weight and then subjected to shock sintering at temperatures in the range from 1300 to 1550 ° C. 2. The method according to claim 1, characterized in that the density of Green body is preferably 50% of the theoretical density. 3. The method according to one or more of claims 1 and 2, characterized characterized in that the green body preferably has a residual moisture content of  1% by weight. 4. The method according to one or more of claims 1 to 3, characterized characterized in that the green bodies in the shock sintering in  60 seconds, preferably in 10 seconds, particularly preferably in  3 seconds, brought to the required sintering temperature. 5. The method according to one or more of claims 1 to 4, characterized characterized in that the slip further components, sintering additives and / or crystal growth inhibitors, preferably oxides from the group of the elements Mg, Zn, Ni, Co, Hf, Zr, Si, Ti, Cu, Sr, Ba, K, Nb, B and / or rare earths. 6. The method according to one or more of claims 1 to 5, characterized characterized in that the slip has an average particle size of 0.4 µm having. 7. The method according to one or more of claims 1 to 6, characterized characterized in that the particles in the slip have a bimodal distribution have curve.   8. The method according to one or more of claims 1 to 7, characterized characterized in that the bimodal distribution curve the first maximum in Range between 0.1 and 0.3 microns, preferably 0.1 and 0.2 microns, the second Maximum in the range between 0.2 and 1.0 µm, preferably 0.3 and 0.7 µm. 9. The method according to one or more of claims 1 to 8, characterized characterized in that 20 to 70 wt .-%, preferably 30 to 50 wt .-%, of Solids content of the slip a particle size between 0.1 and Have 0.3 μm and 80 to 30% by weight, preferably 70 to 50% by weight, the solids content of the slip a particle size between 0.3 and 1.0 µm. 10. The method according to one or more of claims 1 to 9, characterized characterized in that the slip has a solids content, calculated as α-Al₂O₃, from 5 to 80 wt .-%, preferably 15 to 50 wt .-%, and 0.5 to 5 wt .-% inorganic or organic stabilizers, based on the solids content, as dispersion aids. 11. The method according to one or more of claims 1 to 10, characterized characterized in that the suspension 0.5 to 10 wt .-% of one or more Binder from the group methyl cellulose, dextrin, sugar, starch, algi nate, glycols, polyvinylpyrrolidone, lignin sulfonate, gum arabic, poly vinyl alcohol and polyvinyl acetate, based on the solids content of the Contains suspension. 12. The method according to one or more of claims 1 to 11, characterized characterized in that the suspension 0.5 to 10 wt .-% of one or more Contains binders from the group of water glass, silica sol and boehmite sol. 13. The method according to one or more of claims 1 to 12, characterized characterized in that the spray granules have a residual moisture content of 6% by weight, preferably 1% by weight, based on the solids content. 14. The method according to one or more of claims 1 to 13, characterized characterized in that the granules before sintering at temperatures be calcined between 300 and 600 ° C.   15. The method according to one or more of claims 1 to 14, characterized characterized in that the granules before sintering to the desired Grain shape is crushed.