A method of obtaining a sintered material based onmodification of aluminum oxide

 

(57) Abstract:

The invention is: to provide a material using alumina trihydrate or nicobariensis alumina. Crush the original components of the grinding equipment grinding bodies of a-modification of aluminum oxide to obtain a suspension, and then it is injected 1-5 wt.% fine germ crystals a-Al2O3, dried and calcined at a temperature of 1100-1500oC. In a suspension can be introduced advanced modifiers and carried out the evacuation of the suspension, and the pH of the suspension < 5. As the source component, you can use the aluminum hydroxide formed during the production of aluminium by the Bayer process. 4 C.p. f-crystals.

The invention relates to a method for producing the sintered material, in particular material on the basis of aluminum oxide from aluminum hydroxide (gibbsite, or hydrargillite) or annealed at a low temperature of aluminum oxide.

Due to the high hardness of Al2O3(corundum) has for many decades been used as abrasive. The traditional way of obtaining suitable as abrasive corundum is aluminum oxide melting (glenozemsk prozivom it to the desired size of abrasive grain. However, because of the high melting point and hardness of the material and the melting and disintegration of the melt is very energy-intensive process, requiring equipment, the acquisition and content of which are very expensive. In addition, the properties of the thus obtained abrasives, in particular the viscosity, not all optimal assignments.

So been trying to get abrasives corundum-based by heating containing aluminum compounds below the melting point of corundum (about 2050oC). It turned out that not only is it important that the sintered material was dense and non-porous, but that he had a definite microstructure. In particular, the material should have a thin structure with a crystallite size of 1 μm in the absence of a single large crystals.

According to the prior art the above-mentioned problem is solved by the method of translation of Zola in the gel, on the basis of pure boehmite (oxihydroxide aluminum (AlOOH))(European patent 24 099), if necessary with the addition of germ crystals, preventing the formation of large crystallites by providing fast flowing process complete crystallization of the desired modification, so that the individual crystallites due to limited what the products are very high quality, but they are relatively expensive, because the source material was obtained by hydrolysis of aluminum alcoholate, which in turn can be quite expensive. However, the desired low content of alkali metals, particularly sodium, is difficult to achieve by other means. In fact, low sodium is particularly important to prevent the formation of b-Al2O3when heating material. This oxide has a particularly negative impact on the abrasive properties of the material as it is formed in the form of large crystals. Also tried to get abrasives comparable quality, based on less valuable boehmite. However, this has been accomplished only by adding significant amounts of additives promoting sintering (atomic ratio to aluminum is 1:35 1:2), and compliance with certain heating rate (patent Germany 32 19 607). However, when using these additives are formed extension phase, for example, described in the cited European patent application spinel whose presence because of the "sleek" abrasive effect is undesirable.

Object of the invention to develop a method of producing sintered abrasives on the basis of aluminum oxide coming from cheap raw materials and simple way is spent in paragraph 1 of the claims.

It has been unexpectedly found that by a suitable combination of stages of the method provides the possibility of obtaining from the usual technical aluminum hydroxide (Al(OH)3, gibbsite or hydrargillite) formed by the method Bayer, sintered a-Al2O3high density and hardness with crystallite size less than 1 micron, sometimes even less than 0.5 microns.

Although the process of obtaining a-Al2O3technical aluminum hydroxide is widely known he is one of the stages krupnopanelnogo aluminium production obtained in this way aluminum oxide has a high porosity and a low sintering, making it totally unsuitable for conventional methods of grinding. Sintering of such aluminum oxide begins only at such high temperatures at which there is a strong growth of crystals, because of what it gives, although dense, but coarse product with unsatisfactory mechanical properties, do not have any special advantages compared with conventional corundum. Similarly receive, for example, so-called tabular alumina, characterized by large plate crystals (size neskol processing, for example, polishing, in which the removal of material plays a relatively small role (patent DD 76 485). For the above-mentioned transfer method Zola in the conventional gel aluminum hydroxide is not suitable, because it is poorly dispersed and cannot be translated into a gel-like state (about the properties of aluminum hydroxide see, for example, Ullmann''s Encyclopedia of Industrial Chemistry, so A1, S. 557-594, published by VCH Verlagsgesellschaft mbH, Weinheim, 1985).

According to the proposed method, the original substance is first disagglomerated, i.e. milled in the wet state, to divide the process results in the production of alumina agglomerates into individual crystallites.

The process of desagglomeration preferably implemented attritor, vibratory mill or a ball mill with agitator, and use of grinding bodies composed entirely or almost entirely of aluminium oxide. Required for the milling process, the amount of fluid is chosen so as to form a suspension with a solids content of 10 to 40 wt.

As the liquid used is preferably water, but the latter can partially or completely replace miscible with water and volatile solvents, for example, nessesity to pH below 5, preferably up to about 2-4, resulting removes dissolved or absorbed carbon dioxide.

As an acid it is advisable to use an acid from the group comprising nitric, hydrochloric, citric, formic, acetic and oxalic acid, but preferably a salt. The required amount of acid depends on the properties of aluminum hydroxide, primarily from its specific surface. But the addition of an acid can be partially or fully waived, replacing it with evakuirovannym suspension for degassing.

To a suspension of aluminum hydroxide, it is advisable to add the germ crystals, in particular a modification of aluminum oxide. The embryos receive, for example, a simple way by grinding a-modification, such as sintered alumina, up to a grain size <1 μm.

The germ is advisable to add, mixing them well with the resulting suspension, in an amount of from 1 to 5 wt. considering the total number (in terms of Al2O3). Preferably the germ add at the beginning or during the process of desagglomeration aluminum hydroxide. In addition to the germ can be added excipients or additives such as defoamers, additives, contributing spec is SS="ptx2">

Thus obtained slurry is dried. The process of drying, it is advisable to exercise at a temperature below the boiling point of the suspension in order to prevent the formation of bubbles of the groove. Under normal pressure, for example, it is advisable to dry the slurry at a temperature of approximately 70oC. When, for example, the thickness of the layer of sediment is about 10 cm, the drying may be carried out at this temperature for about 2-3 days. When applying excessive pressure temperature drying can be increased by increasing the boiling point, reducing this time of drying the suspension. During drying as descending liquid content decreases volume (thickness) of the suspension without the occurrence of significant porosity. Thus it is easy to obtain a material with an open porosity of 0.05 ml/g and an average pore diameter of 10 nm (according to mercury porosimetry).

The precipitate, if necessary, ground to the desired grinding of grain size (taking into account the shrinkage on sintering) and is sintered. The sintering temperature is advantageous to choose within 1100-1500oC. the sintering Time depends on the temperature and is, for example, about 2 h at 1400oC.

Despite the strong compression volume is now sintered product, which does not require annealing.

Instead of aluminum hydroxide can be used and fired at low temperatures, aluminum oxide or mixtures thereof with aluminum hydroxide.

Annealed at low temperatures, oxides of aluminum contain a small amount of water, for example, about 8 wt. In the processing according to the invention due to the absorption of water they again turn to aluminum hydroxide, which can be proved by thermogravimetric analysis of the dried suspension. Due to the fact that annealed at low temperatures, oxides of aluminum on its part is obtained from aluminum hydroxide and their use does not give special advantages, as the starting material for the proposed method should be preferred aluminum hydroxide.

Obtained according to the invention the sintered material is thin crystallites, as well as high density and hardness. Its viscosity reaches 2.5 Nam-2and more. It is suitable for use not only as an abrasive, but as other materials that have these properties play an important role.

Example 1.

In attritor capacity 0.6 l in demineralized water with the addition of 1.5 wt. is luminia diameter of 1 mm grind and desagglomeration 100 grams of pure aluminum hydroxide (type MartinalROL-104, firms Martinswerk GmbH, D-W-5010 Bergheim). The particle size of the agglomerate to his grinding 100% is less than 10 μm, and after refining - 100% less than 1 μm. Embryos alpha-modification of aluminum oxide was obtained by grinding in attritor annealed at a high temperature of aluminum oxide to achieve a particle size less than 0.5 microns. The pH was brought to 2 by adding before milling about 20 ml of 37% hydrochloric acid. The suspension was dried at 70oC for 2 days (initial layer thickness of 5 cm). The average diameter of pores after drying was 9.5 nm (according to mercury porosimetry), and the open porosity was lower than 0.05 ml/g Data thermogravimetric differential thermal analysis showed that the bound water evaporates at a temperature below 250oAnd that at 1010oWith crystallization starts to alpha-Al2O3.

After sintering for 2 h at 1400oWith the receiving material with the crystallite size < 0.5 microns, a density of >3.8 g/ml (>95 theoretical density) and Vickers hardness (load 500 g) > 19 GPA.

Example 2.

Acted as in example 1 except that instead of the aluminum hydroxide used 70 g annealed at low Elo 0.2 wt. other metals of about 0.05 wt. the particle size 99% of particles < 1 μm, specific surface area 200 m2/g). Thermogravimetric analysis of the dried material showed a weight loss of 35% at 400oIn accordance with the composition Al(OH)3.

After a 2-hour sintering received the product with the same properties as the product of example 1.

Example 3.

Acted as in example 2 except that as the starting material used is annealed at a low temperature alumina type AH (firm Martinswerk) (loss on ignition of about 5 wt. the content of Na2O about 0.2 wt. other metals of about 0.06 wt. particle size: < 106 nm to about 25% < 45 μm, about 10% of the specific surface area of about 175 m2/g). The sodium content after sintering (1400oS, 1 h) was 0.11 wt. Using x-ray structural analysis in the sintered material was not detected b-Al2O3(the limit of sensitivity of about 1 wt.).

Example 4

(Comparative example)

Acted as in example 2 except that the grinding process is carried out not in attritor, and in a colloidal mill (type Ultra TurraxRcompany Janke und Kunkel) with mixing within the armed forces is the ETP of 400 nm and an open porosity of 0.06 ml/year After annealing at 1200o(5 h) received the material with an open porosity of 0.11 ml/g

Compared with this material, obtained according to example 2, after 5 hours of annealing at 1200oWith had an open porosity of only 0.03 ml/year

1. A method of obtaining a sintered material based on the modified alumina by cooking mist any grinding the source component, drying and heat treatment, characterized in that as the source component using the alumina trihydrate or nicobariensis alumina, grinding lead the grinding equipment grinding bodies of a-modification of aluminum oxide in suspension enter 1 a 5 wt. fine germ crystals a-Al2O3and the heat treatment is carried out at 1100 1500oC.

2. The method according to p. 1, wherein upon receipt of a suspension imposed modifying additives.

3. The method according to p. 1, characterized in that regulate the pH of the suspension, bringing its value to < 5 the addition of acid from the group of nitric, hydrochloric, acetic, citric, formic acid, oxalic acid.

4. The method according to PP.1 and 2, characterized in that the suspension vaccum.

5. The method according to PP.1 to 4, characterized in that use is

 

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2 cl, 4 tbl, 1 ex

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3 cl, 1 dwg, 1 tbl

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