RU2843749C1 - Method of producing reactive alpha-aluminium oxide - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to chemical engineering, specifically to methods of producing reactive alpha-aluminium oxide (α-Al₂O₃). Method of producing a reactive α-Al₂O₃ includes dry grinding of alumina in a ball mill. Raw material used is metallurgical aluminium oxide with median particle size D₅₀ 9-22 mcm, containing crystalline phase γ-Al₂O₃ and θ-Al₂O_3. Into obtained charge additive is added, including chromium oxide (Cr₂O₃) in amount of 5 mol. % in terms of Cr³⁺ ion and ammonium fluoride (NH₄F) in amount of 0.5-1.5 mol. % in terms of F^- ion. Then one adds water till paste-like condition is obtained. Briquettes are prepared by laying into moulds and dried to constant mass at temperature 110±15 °C. Briquettes are subjected to calcination at temperature 1200-1450 °C for 2 hours. Further, the roasting products are crushed and then ground in a ball mill for 2-6 hours. Before grinding, 0.05 % polyethylene glycol is added to the roasting products.

EFFECT: low synthesis temperature of reactive alpha-aluminium oxide and high slag resistance of corundum ceramic made using finely dispersed powder of reactive alpha-aluminium oxide.

1 cl, 1 dwg, 2 tbl, 3 ex

Description

The invention relates to the field of chemical technology, namely to methods for producing reactive alpha-aluminum oxide (α-Al ₂ O ₃ ), which is used as a highly dispersed component in the production of biocompatible, structural and technical corundum ceramics, as a component of matrix systems in the technology of low-cement refractory casting masses, and also as a catalyst, adsorbent, and abrasive material.

Highly dispersed α-Al ₂ O ₃ powders are known in English technical terminology as reactive alumina. In reactive alumina, the material is dispersed to the size of primary crystals (0.5 - 2 μm). For large-tonnage production of finely and ultra-dispersed activated α-Al ₂ O ₃ , calcined alumina is currently used as the feedstock. Such reactive alumina is obtained by heat treatment (calcination) of Bayer aluminum hydroxide from 1300 to 2000 °C. The size of the resulting crystals of α-aluminum oxide is affected by both the temperature and the calcination rate. The higher the calcination temperature, the greater the size of the dense primary crystals and the content of Al ₂ O ₃ in the α-form. This in turn significantly affects the grindability of the material and the median particle size D ₅₀ of reactive alumina. Next, calcined alumina is subjected to dry or wet grinding to the size of primary crystals in vibration, jet and ball mills [Vorobyov N.D. Modeling the grinding process in ball mills // Mining Journal. 2004. No. 5. pp. 65-68].

It is known that during the synthesis of α-Al ₂ O ₃ from different precursors (Bayer aluminum hydroxide Al(OH) ₃ , low-temperature modifications of Al ₂ O ₃ ), mineralizing additives of different compositions are used, which significantly affect the processes of polymorphic transformations, morphology and particle size of the finished product.

The authors of the article Effect of AlF ₃ seed concentrations and calcination temperatures on the crystal growth of hexagonally shaped α-alumina powders [HS Kim, N. Park, TJ Lee, M. Kang//Ceram. Int. 2015. - Vol. 40. - P. 3813-3818] studied the effect of fluoride ions on the size of the formed α-Al ₂ O ₃ crystals. It was shown that the additive leads to a decrease in the temperature of formation of the α-phase, and also affects the size and morphology of the particles and ensures the intensification of the growth processes of primary α-Al ₂ O ₃ crystals and a low calcination temperature.

The effect of introducing a chromium-containing additive (Cr ₂ O ₃ ) on the microstructure and physicomechanical properties of corundum samples obtained by hot pressing at 1500°C is presented in the scientific article Effect of Cr ₂ O ₃ addition on microstructural evolution and mechanical properties of Al ₂ O ₃ [D. Riu, Y. Kong, H. Kim // J. Eur. Ceram. - 2000. - Vol. 10. - P. 1475-1481]. With the introduction of small amounts of Cr ₂ O ₃ (2 and 5 mol. %), the α-Al ₂ O ₃ grains became larger and were characterized by a bimodal distribution. It was noted that the fracture toughness, crack resistance, hardness and elastic modulus of such samples were significantly increased with the introduction of less than 5 mol. % Cr ₂ O ₃ . However, the mechanical strength of all the studied samples containing Cr ₂ O ₃ decreased. With an increase in the concentration of the introduced chromium oxide to 10 mol. %, no fundamental changes in the structural and morphological characteristics of the α-alumina grains were observed.

Known is the patent KR1020040077294, which presents the Method for preparing ultrafine aluminum oxide powder by introducing polymeric materials into a solution containing aluminum ions, or aluminum hydroxide, or a suspension of gamma-alumina. Nanosized powder of α-alumina (α-Al ₂ O ₃ ) is obtained using the following stages: adding polymeric materials or polymerized organic compounds to a solution of Al(NO ₃ ) ₃ , AlCl ₃ or Al ₂ (SO ₄ ) ₃ , or to freshly precipitated Al(OH) ₃ , or to a suspension of gamma-Al ₂ O ₃ . Low-molecular organic substances such as ethylene glycol and glycine are used as organic additives; then the solidified carbide resin containing aluminum ions is ground and the carbide powder is subsequently calcined at a temperature of 1000-1200°C to convert the amorphous aluminum oxide into the α-form. The remaining carbides are removed from the resulting product by heat treatment of the calcined powder at 600°C in air. The proposed method involves a large number of technological operations, the need to introduce various organic substances, and cannot be used to organize large-scale production.

The closest to the proposed technical solution, taken as a prototype, is patent RU 2791045 (published 01.03.2024), which describes a method for producing reactive alpha-aluminum oxide, including dry grinding of alumina in a ball mill using cylindrical grinding media with a volume of 6.3 ^cm3 each and a volumetric loading of the drum with grinding media of 35%. The feedstock used is metallurgical aluminum oxide with a median particle size D50 of 9-22 μm, containing crystalline phases of γ-Al ₂ O ₃ and θ-Al ₂ O ₃ , into which a complex mineralizer additive is introduced - ammonium dichromate in an amount of 0.005-0.5 mol.% in terms of chromium, then water is added to the resulting batch until a paste-like state is obtained, briquettes are prepared by placing them in molds and dried to a constant weight at a temperature of 110 ± 15 ° C, after which the briquettes are calcined at a temperature of 1350-1550 ° C for 3 hours, then the firing products are crushed, 0.05% polyethylene glycol is added to the calcined alumina in excess of the mass, and then ground in a ball mill using cylindrical grinding bodies with a volume of 6.3 cm ³ each and a volumetric loading of the drum with grinding bodies of 35% for 2-8 hours. As a result of dry grinding, powders of reactive alpha-aluminum oxide are obtained with an average size of primary crystals of 0.5-1 μm and a specific surface area of more than 27,000 ^cm2 /g.

The technical objective of the proposed technical solution is to expand the arsenal of means by developing a method for producing reactive alpha-aluminum oxide.

The technical result of the proposed technical solution is a decrease in the synthesis temperature of chromium-containing reactive alpha-aluminum oxide to 1200°C.

The second technical result is an increase in the slag resistance of corundum ceramics manufactured using highly dispersed powders of reactive alpha-aluminum oxide.

To achieve the set objectives, a method for obtaining reactive alpha-aluminum oxide is proposed, which is carried out in the following manner:

Commercially available metallurgical aluminum oxide with a median particle size of D ₅₀ ~ 9-22 μm, containing crystalline phases of γ-Al ₂ O ₃ and θ-Al ₂ O ₃ is used as the feedstock. To prepare the batch, metallurgical aluminum oxide is mixed with a complex mineralizing additive including chromium oxide (Cr ₂ O ₃ ) in an amount of 5 mol. % in terms of the Cr ³⁺ ion and ammonium fluoride (NH ₄ F) in an amount of 0.5-1.5 mol. % in terms of the F ^- ion. Water is added to the resulting batch until a paste-like state is obtained, then briquettes are prepared by placing them in molds and drying to a constant weight at a temperature of 110 ± 15 ° C. Then the briquettes are calcined at a temperature of 1200 - 1450 ° C for 2 hours. In the specified temperature range, the γ-Al ₂ O ₃ and θ-Al ₂ O ₃ are converted into α-Al ₂ O ₃ . The calcined products are crushed and then subjected to grinding in a ball mill for 2-6 hours using cylindrical grinding bodies with a volume of 6.3 cm ³ each, and a volumetric loading of the drum with grinding bodies of 35%, at a drum rotation speed of 100 rpm. The rotation speed of the ball mill drum depends on the technical characteristics of the equipment and does not affect the implementation of the proposed method for producing reactive alpha-aluminum oxide. The coefficient K, characterizing the ratio of the volume of the material being ground to the volume of voids between the grinding bodies, is taken to be equal to 1 in all cases. Before the start of grinding, 0.05% polyethylene glycol is added to the calcined alumina over the mass.

A distinctive feature of the proposed method for producing reactive alpha-aluminum oxide is the introduction of a complex mineralizing additive - chromium oxide (Cr ₂ O ₃ ) and ammonium fluoride (NH ₄ F) - into the initial batch, as well as a decrease in the synthesis temperature to 1200-1450°C. The synergistic effect of the introduced complex mineralizing additive containing a fluoride ion and a chromium (III) cation ensures the formation of a solid solution of Cr ³⁺ in the surface layer of α-Al ₂ O ₃ particles and optimal dispersion at temperatures of 1200-1350°C. The presence of such a surface chromium-containing solid solution significantly improves the grindability of the material after calcination, and also ensures an increase in the slag resistance of corundum ceramics made from highly dispersed powders of chromium-containing reactive alpha-aluminum oxide.

Examples of implementation of the invention.

Example 1

A complex mineralizing additive containing chromium oxide (Cr ₂ O ₃ ) in an amount of 5 mol. % based on the Cr ₃₊ ion and ammonium fluoride (NH _{4 F} ) in an amount of 0.5-1.5 mol. % based on the ^F - ion is added to metallurgical aluminum oxide with a median particle size of D _{50 ~} 9-22 μm, containing the crystalline phases of γ-Al ₂ O ³ . Water is added to the resulting batch until a paste-like state is obtained, then briquettes are prepared by placing them in molds and drying to a constant weight at a temperature of 110±15°C. The briquettes are then calcined at a temperature of 1200-1450°C for 2 hours. Next, the calcination products are crushed and then ground in a ball mill for 2-6 hours using cylindrical grinding bodies with a volume of 6.3 ^cm3 each, and a volumetric loading of the drum with grinding bodies of 35%, with a drum rotation speed of 100 rpm, while 0.05% polyethylene glycol is added to the calcination products before the start of grinding above the mass.

Fig. 1 shows the results of determining the phase composition of the calcination products of the original metallurgical alumina (control), metallurgical alumina with the preliminary introduction of a Cr ₂ O ₃ additive in an amount of 5 mol.% in terms of Cr ³⁺ , and metallurgical alumina with the preliminary introduction of a complex mineralizing additive including Cr ₂ O ₃ in an amount of 5 mol.% in terms of Cr ³⁺ and NH ₄ F in an amount of 0.5-1.5 mol.% in terms of the F ^- ion.

The data presented in Fig. 1 confirm that the obtained calcination products are alumina in the alpha modification. The content of the α-Al ₂ O ₃ phase reaches 100%. With the isomorphic substitution of Al ³⁺ cations in the crystal lattice of aluminum oxide by Cr ³⁺ cations, a shift in the maximum of the diffraction peaks occurs toward smaller 2θ angles compared to the control sample of α-aluminum oxide synthesized without the introduction of mineralizing additives. This confirms the formation of a solid solution in the Al ₂ O ₃ -Cr ₂ O ₃ system. It is also confirmed that the calcination temperature of 1200°C is sufficient for the formation of a solid solution during the synthesis of reactive alpha-aluminum oxide in the presence of a complex mineralizing additive including Cr ₂ O ₃ and NH ₄ F. The maximum temperature of the presence of fluoride ions in the system is 1300°C. Above this temperature, the decomposition of the fluorine-containing mineralizing additive and the complete removal of F ^- from the system occur. At a temperature of 1450°C, the processes of formation of the surface solid solution remain unchanged.

Example 2

In example 2, a complex mineralizing additive containing chromium oxide (Cr ₂ O ₃ ) in an amount of 5 mol % based on the Cr ₃₊ ion and ammonium fluoride (NH ₄ F) in an amount of 0.5-1.5 mol % based on the F _- ion is added to metallurgical aluminum oxide with a median particle size ^D50 of ~ 9-22 μm, containing the crystalline phases of _γ -Al ² O ₃ . Water is added to the resulting batch until a paste-like state is obtained, then briquettes are prepared by placing them in molds and drying to a constant weight at a temperature of 110±15°C. The briquettes are then calcined at a temperature of 1200°C for 2 hours. Next, the calcination products are crushed and then ground in a ball mill for 2-6 hours using cylindrical grinding bodies with a volume of 6.3 ^cm3 each, and a volumetric loading of the drum with grinding bodies of 35%, at a drum rotation speed of 100 rpm, while 0.05% polyethyleneglycol is added to the calcination products before grinding begins in excess of the mass. The effect of the composition of the introduced mineralizing additive on the granulometric composition of the resulting reactive alpha-aluminum oxide powders is given in Table 1.

Table 1

Amount of additive introduced, mol. % Median
particle size in powders after
6 h grinding, µm Average content of submicron fraction
(≥1 µm), % 0 (control) 1.68 34.0 1.0 mol.% F ^- 2.48 23.5 5 mol.% Cr ³⁺ +
0.5 mol.% F ^- 1.14 37.8 5 mol.% Cr ³⁺ +
1.0 mol.% F ^- 1.77 32.0 5 mol.% Cr ³⁺ +
1.5 mol.% F ^- 1.82 30.5

The data presented in the table confirm that without the introduction of a complex mineralizing additive it is impossible to obtain individual primary crystals of alpha-aluminum oxide by grinding in a ball mill; the control sample of reactive alpha-aluminum oxide is represented mainly by agglomerates of particles with a median size of 1.68 μm. When introducing only F as a mineralizing additive^-in the amount of 1.0 mol. % the median size of individual primary crystals of alpha-aluminum oxide is 2.48 μm along with a low content of submicron fraction - 23.5%. When introducing a complex mineralizing additive, assuming the presence of 5 mol % Cr³⁺and 1 mol.% F^-, the median crystallite size of alpha-aluminum oxide is 1.77 μm, the content of the submicron fraction is 32%. This is explained by the low-temperature energy-consuming process of formation of a solid solution of Cr³⁺in the Al lattice₂O₃(See example 1). Therefore, the presence of 1 mol % of fluoride ions does not lead to excessive growth of primary crystals, but ensures optimal dispersion of reactive alpha-aluminum oxide. Increasing the concentration of fluoride ions to 1.5 mol % in the complex mineralizing additive leads to active nucleation, i.e. the emergence of a large number of crystallization centers, which in turn prevents the growth of alpha-aluminum oxide crystals.

Example 3

The effect of the complex mineralizing additive introduced during the synthesis of reactive alpha-aluminum oxide on the slag resistance of the obtained corundum ceramics was estimated by the static method. For testing by the uniaxial pressing method, semi-finished products measuring 50×50 mm were made from reactive alpha-aluminum oxide powders synthesized without the use of a mineralizing additive (control), using an NH ₄ F additive at a concentration of 1 mol. % in terms of F ^- and a complex additive containing Cr ₂ O ₃ and NH ₄ F at concentrations corresponding to 5 mol. % Cr ³⁺ and 1 mol. % F ^- . The obtained pressed semi-finished products were fired at 1550 °C. Then, to study the slag resistance, a 3 g sample of metallurgical slag was poured into pre-formed depressions (20 mm in diameter and depth) on the surface of the sintered samples. After that, the prepared samples were kept for 2 hours at a temperature of 1450°C in a high-temperature furnace. At the end of the holding, the samples were cooled and sawn into two parts with a diamond tool along the axis of symmetry. Then, the degree of impregnation and the degree of corrosion were measured on the saw for each sample. Table 2 shows the average values for 5 samples of each composition.

Table 2

Sample Degree of impregnation, % Degree of corrosion, % Control 10 5 0 mol.% Cr ³⁺ +
1.0 mol.% F ^- 12 6 5 mol.% Cr ³⁺ +
1.0 mol.% F ^- 5 2

The data presented in Table 2 confirm that samples based on reactive alpha-aluminum oxide synthesized in the presence of a complex mineralizing additive including 5 mol.% Cr ³⁺ +1.0 mol.% F ^- have the highest slag resistance.

As a result of the examples given, the solution of the set technical problems was confirmed: a decrease in the synthesis temperature of chromium-containing reactive alpha-aluminum oxide to 1200°C and an increase in the slag resistance of corundum ceramics manufactured using highly dispersed powders of reactive alpha-aluminum oxide.

Claims (1)

A method for producing reactive alpha-aluminum oxide, comprising dry grinding of alumina in a ball mill using cylindrical grinding bodies with a volume of 6.3 cm ³ each, and a volumetric loading of the drum with grinding bodies of 35%, wherein metallurgical aluminum oxide with a median particle size D ₅₀ of 9–22 μm containing crystalline phases of γ-Al ₂ O ₃ and θ-Al ₂ O ₃ is used as the initial raw material, then an additive is introduced into the resulting batch, then water is added until a paste-like state is obtained, briquettes are prepared by placing them in molds and drying to a constant weight at a temperature of 110±15 °C, after which the briquettes are calcined, then the firing products are crushed and then ground in a ball mill at the specified process parameters, wherein 0.05% polyethylene glycol is added to the firing products in excess of the mass before the start of grinding, characterized in that a complex mineralizing additive is introduced into the resulting batch, including chromium oxide (Cr ₂ O ₃ ) in an amount of 5 mol. % in terms of the Cr ³⁺ ion and ammonium fluoride (NH ₄ F) in an amount of 0.5-1.5 mol. % in terms of the F ^- ion, and the briquettes are calcined at a temperature of 1200–1450 °C for 2 hours, while the roasting products are ground in a ball mill for 2–6 hours under the specified process parameters.