Microcrystalline abrasive agent and process for its preparation.

The invention relates to a sintered, microcrystalline abrasive material on the basis of aluminum oxide, and a method for its production.

It is known that various Al203-containing raw materials such as alumina or bauxite to be used for the preparation of sintered abrasive materials.

Thus, the DT-AS 14 71 331 describes an abrasive grain material, which is produced by compacting comminuted calcined bauxite nature at pressures up to 703 kp / cm², followed by sintering at temperatures between 1370 ° and 1570 ° C. Most of the crystals of this abrasive grain material has a size of 5-30 μ on.

The US Patent 4,086,067 describes a porous, sintered abrasive grain, wherein the alumina before sintering are plastic particles, cork particles, inter alia, is added to achieve a defined pore size distribution and a defined pore volume.

The US Patent 4,252,544 describes a sintered at a density of about 3.75 g / cc, and high hardness. This material consists of a mixture of molten or calcined alumina powder, a fine fraction <1 μ and a coarser fraction having a particle size of 3 - 10 μ mixed in the presence of water and binder and then kneaded and extruded. The sintering of the dried and cut pieces is performed at temperatures between 1550 ° and 1650 ° C.

The invention is now a non-molten, microcrystalline abrasive agent consisting of at least 98.5% α-Al₂O₃, having a density of at least 95% of theoretical density, wherein the size of the crystallites desα-Al₂O₃ is less than 4μ preferably less than 3μ. This abrasive material is characterized further still by high hardness and toughness and is excellent suitable for the high-pressure grinding.

Contents of the invention is furthermore a simple and cost effective method for producing such an abrasive material.

Surprisingly, it was found that the use of a highly dispersed α-alumina monohydrate as an additive to clay allows an abrasive material, which is characterized by very high density and toughness and its individual grains have a harder surface than the marketed similar abrasive materials.

The process for producing this abrasive material consists in the fact that a mixture of calcined, finely ground clay and 1-60 wt .-% of a highly dispersed α-aluminum oxide monohydrate, where appropriate, with additions of one or more elements from the group Mg, Si, Ti, Zr Cr, Ni, Ce, Fe, Mn, manufacturing, this mixture is combined with water and acid, homogenised, crushed the moldable material or pressed into regular shaped bodies, and subsequently at temperatures of between 1300 ° and 1700 ° C for a period of 5 minutes to 2 sinters hours.

When using calcined alumina and finely divided α-alumina having a particle size <2 μ, preferably <1 μ ensures that the final product has a crystallite size of less than 4 μ.

As a highly disperse α-aluminum oxide hydrates put on the market Pseudeboehmite can be used, which are known under the name Pural®, Disperal® and Versal®.

Furthermore, it was found that even small additions of one or more elements from the group Mg, Si, Ti, Zr, Cr, Ni, Ce, Fe and Mn can improve the performance of the abrasive material even further. These additives are of the order of max. 1 wt .-% calculated as oxide, based on the α-Al2O3 content. This can be geklären in that severely limited by these additives, the crystal growth during the sintering process and therefore a highly microcrystalline structure is achieved. Another advantage of the invention is that the compression of the moldable material no additional binders are required, which then could adversely affect the density due to increased pore formation during sintering.

If the starting materials (calcined alumina and α-alumina monohydrate) in sufficient fineness not available, so these materials are subjected to known grinding method of grinding with the aim that the grain size of the alumina-containing materials 2 μ, does not exceed vorzusgweise 1 μ. It is advantageous to wet grinding in a ball mill or ball mill with alumina balls as grinding media. In any case, however, the addition of acid is necessary when using the highly dispersed Al-Oxidmonohydrats, since thereby preventing agglomeration of the Al-Oxidmonohydrats or under mechanical effect a de-agglomeration of the finely divided material is effected.

After the wet milling, the milled acid clay-aluminum oxide hydrate suspension, optionally cast with additions of one or more elements from the group Mg, Si, Ti, Zr, Cr, Ni, Ce, Fe and Mn, from the milling apparatus and to the extent dewatered that a moldable material is produced, which is then crushed or pressed into regular geometric moldings.

In carrying out the process according to the invention are obtained due to the addition of the dispersible α-Al Oxidmonohydrats a moldable material that is compressible without addition of a further organic or inorganic binder and having also after sintering very high hardness and strength.

The Green grain formation is preferably carried out by pressing or extruding the moldable material through a die to form strands of uniform cross-section, which are then cut into pieces of certain length.

The green grain is then dried and sintered. The sintering is carried out in an oxidizing atmosphere at temperatures of 1300-1700 ° C, preferably 1350-1550 ° C. The sintering time varies depending on the temperature, the type of the sintering furnace and in particular of the composition of the green grain and is in the range of 5 minutes to 2 hours. As sintering both rotary kilns as well Pendelöfen and pusher type for making an abrasive material according to the invention are suitable. The abrasive grits obtained have a density of more than 95% of the theoretical density, and have, inter alia due to the low sintering temperature, a micro-crystalline structure, wherein the crystallite size of the α-Al203 crystallites does not exceed 4 μ.

With a share of more than 60 wt .-% of dispersible Al-oxide monohydrate, the material exhibits high shrinkage during drying and sintering, the grain yield and the efficiency of this material is characterized negatively affected.

Abrasive materials prepared by the process according to the invention were processed to grinding wheels and subjected to a comparative test series against stainless steel. It was the attrition ratio, di material removal / wheel wear determined (= 100% for Elektroschmelzkorund). It significantly higher values ​​were obtained for the abrasive material according to the invention compared with commercial abrasive materials in coarse grinding wheels. These values ​​are given in the table.

example 1

A mixture of 67.5 g wt .-% clay (grain size <1 μ), 6.75 wt .-% Disperal® (particle size <1 μ), 24.1 wt .-% water, 1.1 wt % HNO₃ (65%) and 0.27 wt .-% Mg (NO₃) ₂ were homogenized in a duplex kneader for about 1 hour and then extruded. The press pressure was 50-90 bar, which die diameter was 2 mm. The strands were then cut into approximately 4 mm in length, dried, and sintered for 1 hour at 1450 ° C. The density of the sintered material was 3.82 g / cm³. The determined on the scanning crystallite desα-Al2O3 was less than 3 μ.

This material was then processed to grinding wheels and determines the attrition ratio compared to Elektroschmelzkorund against stainless steel. The figure was 295%.

example 2

15 kg of an alumina hydrate, which was calcined at 1200 ° C (particle size <30 μ), was milled with 15 liters of water in a ball mill with Al-oxide spheres as grinding media 48 hours. The grain size of the alumina hydrate after grinding was <2 μ. Subsequently, the aqueous clay suspension was added 1.5 kg of the α-dispersible alumina hydrate (Disperal®; particle size 75% <45 μ) was added, and 300 g of 65% nitric acid and 103 g of magnesium nitrate hexahydrate and milled for an additional 2 hours. The suspension thus prepared was then applicable dehydrated until a doughy consistency was obtained. A laboratory extruder feed roller, the material was then forced through a die and cut by Kopfgranulieranlage to Grünkorn which have an average length from 2.5 to 4.4 mm and a diameter of 2.2 mm was obtained. The green particles were coated in ceramic cups and sintered in an electric resistance furnace in air atmosphere at 1500 ° C for 1.5 hours. The sintered material had an average diameter of 1.7 mm and an average length of about 3.3 mm. The density of the abrasive grain was 3.80 g / cc, Knoop hardness 2100 kg / mm², which crystallite size was <4 μ. In the grinding wheel, these Mat an attrition provided. Compared to fused alumina of 260%.

example 3

A suspension of calcined at 1200 ° C alumina hydrate (particle size <30 μ) in water in the weight ratio 1: 1. Was in an agitated ball mill, which was lined with polyurethane, ground 6 hours The fineness of the ground material was <1 μ. After addition of 50 wt .-% α-alumina monohydrate (based on the clay), 1.5 wt .-% nitric acid (65% strength) (based on the suspension) and 0.64 wt .-% of magnesium nitrate hexahydrate (based Al2O3 content), the mixture was subjected to a further Kugelmühlenmahlung for 1 h.. Subsequently, a green particles were as described in example 2 prepares, which was subjected to sintering at 1450 ° C for 1 hour. The sintered grain had a density of 3.88 g / cm³, the crystallite size was less than 2 μ. The removal rate of this abrasive grain in the grinding wheel was 310% based on Elektroschmelzkorund.

example 4

10 kg of calcined alumina hydrate was ground with 10 liters of water in a ball mill with Al-oxide spheres for 36 hours. The output fineness was <30 μ, the end fineness <2 μ. After addition of 1.0 kg Al-oxide monohydrate and dispergierbaremα-200 g nitric acid (65% strength), the suspension was milled for 3 more hours. The material was in a 5 - drained 10 cm thick layer on cups at 80 ° C and dried to form hard, brittle plates. These were crushed in a hammer mill to grains preclassified in a screening machine and the resulting fine fraction is recycled in the process. The green granules were then sintered 2 hours in an electric resistance furnace at 1500 ° C. The density of the sintered grain was 3.79 g / cc, which crystallite size was less than 4 μ. The removal rate of the material in the grinding wheel was 260% based on Elektroschmelzkorund.

Table: attrition ratio

Elektroschmelzkorund 100% Zirconia 209% Sintered n. DE OS 1471331126% Sintered by US Patent 4,252,544 252% inventive sintered according to Example 1 295% Example 2 260% according to Example 3 310% according to Example 4 260%