CS220144B1 - A method for preparing alpha alumina modification from low temperature alumina modifications - Google Patents

Abstract

The aim of the invention was to prepare an alpha modification powder of alumina with predetermined physical properties. This was achieved by the process according to the invention in that the calcination of low-temperature modifications of alumina takes place in the presence of fluorides and/or organic fluorine derivatives.

Description

The invention relates to the preparation of a powdered high-temperature alpha modification of alumina, which is used for the production of alumina-based ceramics (hereinafter referred to as corundum ceramics).

To prepare the α-modification of alumina for the above purposes, low-temperature modifications of alumina are currently used, especially gamma-modifications of alumina, which are heated (hereinafter simply annealed) together with sintering additives, especially magnesium compounds, to a temperature higher than 150 °C, which is the temperature of polymorphic transformation to alpha-AkCb, for several hours.

The physical properties of the thus prepared alpha-modification of alumina, such as the size of the specific surface, polydispersity, grain size, cohesion or adhesion, are determined mainly by the time and temperature of annealing of low-temperature forms of alumina, while the regulation of these properties is a purely empirical matter.

The disadvantage of this process is that the preparation of alpha-modification of aluminum iodide with a predefined specific surface area or grain size is very difficult. Obtaining powdered alpha-AlO with an extremely low specific surface area is extremely energy and time-consuming or completely impossible under these conditions.

The above-mentioned shortcomings are solved by the method of preparing alpha-modification of alumina from low-temperature modifications of alumina by heating them to a temperature of 900 to 1400 °C according to the invention, in that the heating of low-temperature modifications of alumina takes place in the presence of inorganic fluorides or organic fluorine derivatives or inorganic fluorides and organic fluorine derivatives, in an amount containing 0.05 to 5% by weight of fluorine, based on AlO.

The advantage of the process according to the invention is lower energy and time requirements, better possibility of regulating the physical properties of the alpha-modification of alumina and the availability of powdered alpha-alumina; and _f exithematically low specific surface area, depending on the amount of fluorine present during annealing of low-temperature forms of alumina, since the amount and type of fluorine compounds at a given temperature and annealing time directly determine the physical properties of the resulting alpha alumina. The temperature of polymorphic transformation is reduced by up to 200 °C in the process according to the invention and the time required to achieve alpha modification of alumina is reduced by up to 75% compared to the previous preparation method.

Example 1

Low-temperature χ-AkCk with an additive of 0.25% by weight was annealed in the presence of 1% NH1HF2, based on AkOj, at different temperatures for different times. The achieved values of the specific surface area of alpha-AkOj, possibly with traces of unconverted γ-AhCk at lower times, are given in Table 1.

TABLE 1

Specific surface area of alpha-AhCk (m ² . g ¹ )

Temperature (°C)

0.25 0.5

1000 — _ 1200 40.0 30.8 1250 40.2 — 1320 31.3 —

Example 2

A mixture of low-temperature forms of ΑΙΌ3 containing 30 to 40% by weight of γ-AlgOj, 30 to 40% by weight of γj-AkOs and 20 to 40% by weight of p-AlzCb with the addition of oxides MgO, Y2O3 and AI2O3 in a total amount of 0.4% by weight was calcined in the presence of 1% by weight of AIF3, based on AI2O3, at a temperature of 1200 °C for various times. The average grain size of the resulting alpha- 45 13

-AI2O3 and d in micrometers is given in 55 14.5

240 16.5 table 2.

Time (h)

2 4' S'

38.6 16.0 — 2.3; 15.2 6.1 1.7 1.5· — 4.5 1.5 — 10.7 — 0.8

TABLE 2

Average grain size d <~»*»alpha AI2O3

50%

Time (min) d (^m)

50%

Example 3

Low-temperature 7--AI2O3 was heated at 1050 °C for one to two hours with the following compounds and their mixtures (weight percentage based on AI2O3):

a) 0.5% AIF3 + 0.5% NH1HF2

b) 3.0 °/o BaF2 + 0.5% CeHsF • c) 2% K2T1F6

d) 10% CFH2COOK

In all cases a) to d) alpha-AkOs was obtained, possibly with traces of y-AhOj, with a mean grain size d in the range of 7.7 to

50%

13.2 .um and a specific surface area ranging from 81 to 18 m ² . g ^_1 .

Claims (1)

A process for the preparation of an alpha-modification of alumina from low-temperature alumina modifications by heating them to a temperature of 900 ^° C to 1400 ^° C, characterized in that heating the low-temperature alumina modifications for <? The invention is run in the presence of inorganic fluorides and / or organic fluorine derivatives in an amount containing from 0.05 to 5% by weight of fluorine based on Al ₂ O ₃ .