WO2022081045A1

WO2022081045A1 - Method for producing alpha-aluminium oxide for growing single crystal sapphire

Info

Publication number: WO2022081045A1
Application number: PCT/RU2021/050318
Authority: WO
Inventors: Эдуард Напалионович МКРТУМЯН
Original assignee: Общество с ограниченной ответственностью "Империус Групп"; Эдуард Напалионович МКРТУМЯН
Priority date: 2020-10-14
Filing date: 2021-09-28
Publication date: 2022-04-21
Also published as: RU2742575C1

Abstract

The claimed method for producing alpha-aluminium oxide comprises oxidising a very high purity aluminium metal powder by atmospheric oxygen in the flame of a plasma generator at a temperature of 2040 to 3000°C to obtain particles of a fine gamma-aluminium oxide powder, concentrating the gamma-aluminium oxide on a pass-through electrostatic precipitator using a high-voltage DC power source with a current strength of 0.2 to 0.5 A and a voltage of 15 to 20 kV, and compressing the powder to a specific weight of 1 g/cm3. The gamma-aluminium oxide is then melted in an induction furnace at a temperature of 2050 to 3000°C in an electromagnetic field at a frequency of 700 to 800 kHz and a power of 120 to 150 kW for 1.5 to 2 hours, and the melt is then cooled at ambient temperature. The invention makes it possible to produce a compact and highly pure alpha-aluminium oxide suitable for producing single crystal sapphire.

Description

METHOD FOR OBTAINING ALPHA-ALUMINUM OXIDE FOR GROWING SINGLE-CRYSTAL SAPPHIRE

The invention relates to the field of chemistry, in particular to methods for producing alpha-alumina suitable for the manufacture of single-crystal sapphires.

Aluminum oxides used to obtain a single crystal of sapphire must have a strictly stoichiometric composition, since the presence of non-stoichiometric amounts of one of the chemical elements that form them (metal or oxygen), as well as the presence of oxides of the same name with a different oxidation state, significantly reduces their quality. Very high requirements are placed on the chemical purity of aluminum oxide (99.99%), and in order to optimize the crystal growth process, the density of the initial aluminum oxide is required comparable to the density of the future crystal (3.96 g/cm ³ ).

Aluminum alpha oxide is used to obtain single-crystal sapphires. To do this, alpha alumina is placed in a crucible, which is heated to a melting point in the range from 2040 °C to 2100 °C. Then, using a seed and temperature control, crystal growth is carried out from the alpha alumina melt. By slowly cooling the melt, over several days, the growth of the most perfect sapphire single crystal is achieved.

The production of high purity alumina, carried out by classical technologies for the production of alumina or by methods of processing mineral raw materials, is associated with the modernization of existing

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SUBSTITUTE SHEET (RULE 26) technological stages and the introduction of additional product purification operations. Methods that provide the production of aluminum oxide with an impurity content of up to 100 ppm require the use of special types of aluminum-containing raw materials. Modern technologies for obtaining high purity alumina are based on the oxidation of metallic aluminum. These technologies include the following stages: preparation of feedstock; oxidizing aluminum to produce aluminum hydroxide; aluminum hydroxide treatment; heat treatment with aluminum hydroxide to obtain Al2O3. The main difference between modern technologies for producing high-purity aluminum lies precisely in the stage of aluminum oxidation.

The prior art method for producing aluminum oxide suitable for the production of artificial corundum crystals, including anodic dissolution of aluminum with a purity of 99.950-99.999% in a chloride solution containing 5-150 g/l chloride ions at a temperature of 2095°C and a current density of 0.045-0 , 12 A / cm ² , separation of aluminum hydroxide, washing aluminum hydroxide with specially prepared water with a resistivity of 0.4-18.0 MΩ.cm and calcination to obtain aluminum oxide (RU 2466937, class C01F 7/42, C25B 1 / 00, 20.11.2012).

Also known is a method for producing high-purity alumina by electrolysis, including anodic dissolution of high-purity aluminum in an aqueous solution of ammonium chloride, separation of the hydroxyl precipitate, washing it with distilled water using three vertically arranged sieves and heat treatment to obtain alumina. Heat treatment is carried out in a multi-stage mode: aluminum hydroxide is dried at a temperature of 340-700°C and

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SUBSTITUTE SHEET (RULE 26) calcined to obtain aluminum oxide, which is then subjected to water treatment and additional drying in the temperature range of 100-300 ° C (RU 2538606, class C01F 7/42, C25B 1/00, 10.01.2015)

However, these methods are characterized by insufficient removal of sodium chloride during the processing of aluminum hydroxide, which does not allow to obtain aluminum oxide with a content of the main component of 99.995 wt.%; heterogeneity of the phase and granulometric composition of aluminum oxide, which is associated with an additional stage of water treatment of aluminum oxide and subsequent drying at low temperatures; the unsuitability of the obtained high-purity aluminum oxide for the growth of single-crystal corundum of the proper quality, since the total content of impurities of silicon (Si), potassium (K), sodium (Na) and iron (Fe) in aluminum oxide is at least 50 ppm.

A known method includes anodic dissolution of high-purity aluminum in an aqueous solution of ammonium nitrate, electrolyte refining by removing 50-100% of the first batch of aluminum hydroxide with preliminary settling in the electrolyte for 12-24 hours, separation of subsequent batches of aluminum hydroxide and electrolyte, washing subsequent batches of aluminum hydroxide with distilled water and their heat treatment, which is carried out by pre-drying for 12-24 hours at a temperature of 200-250 ° C and final calcination for 15-18 hours at a temperature of at least 100 ° C, while calcining every 3 h is the mixing of the product. The invention allows to obtain alpha-alumina with a content of the main component of 99.995-99.998 wt.% and co

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SUBSTITUTE SHEET (RULE 26) average dispersion 40-45 microns. (RU 2630212, class C25B 1/00, C01F 7/02, C01F 7/42, 09/06/2017).

The disadvantage of this method is its low productivity and the complexity of carrying out, due to the use of large amounts of water. All this leads to an increase in the cost of the final product.

There is also known a method for the synthesis of alpha alumina with a purity equal to 99.99% or more in the form of spherical particles with a size mainly equal to 850 microns or more, with a particle size distribution having a maximum at particle sizes of more than 850 microns, with a relative density of 50% or more from the theoretical density, which includes placing the gamma alumina (y) powder by means of supply on a silicon carbide plate and exposing said gamma alumina (y) powder to at least one CO ₂ laser beam. (RU 2568710, class C01F 7/02, SZOV 29/20, B01J 19/12, 11/20/2015).

The known method also has both low productivity and high cost of the resulting alumina.

The technical problem to be solved by the invention is the improvement of the method for producing alpha alumina.

The technical result of the invention is the production of compact and high-purity alumina in the form of alpha - oxide, suitable for the production of single-crystal sapphire.

The problem posed and the specified technical result are achieved by the fact that the method for obtaining alpha-alumina for subsequent growth of single-crystal sapphire, according to the invention, includes a preliminary stage of oxidation of highly pure

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SUBSTITUTE SHEET (RULE 26) aluminum metal powder with atmospheric oxygen in the flame of a plasma generator at a temperature of 2040 to 3000 ° C to obtain particles of a fine powder of gamma-aluminum oxide, followed by the stage of concentration of the resulting gamma-aluminum oxide powder on a through-line electrostatic precipitator using a high-voltage direct current power source with current strength from 0.2 to 0.5 A and voltage from 15 to 20 kV and the subsequent stage of compaction of the gamma-aluminum oxide powder to a specific gravity of 1 g/cm ³ . Next, gamma-aluminum oxide is melted in an induction furnace at a temperature of 2050 to 3000°C in an electromagnetic field at a frequency of 700 to 800 kHz and a power of 120 to 150 kW, for 1.5 to 2 hours. Why, a part of gamma alumina is preliminarily loaded into the furnace container, and then at least one ring of high-purity metallic aluminum is placed into it, which is necessary to initiate heating, followed by adding new portions of gamma alumina until the furnace container is completely filled with melt . Next, the melt is cooled at ambient temperature.

The air consumption during the combustion of aluminum powder in the flame of a plasma generator based on the stoichiometry of the chemical reaction of the interaction of aluminum with atmospheric oxygen is 350 liters per 108 g of aluminum powder or 67.2 liters of oxygen per 108 g of aluminum powder.

Oxidation in the plasma generator is carried out at a frequency of 2.45 GHz and a power of 3.5 kW

Compaction of gamma-alumina powder to a specific gravity of 1 g/cm ³ is carried out, as a rule, by tableting or granulating.

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SUBSTITUTE SHEET (RULE 26) The diameter of the high purity aluminum metal ring should be between 75% and 80% of the diameter of the induction furnace container.

At the initial stage of melting gamma-aluminum oxide in an induction furnace, it is advisable to maintain its power from 10 to 15 kW.

The invention is illustrated by the following graphics, where figure 1 is a trace element analysis of the combustion product of aluminum powder in air; figure 2 is a graph showing the trace element composition of the original high-purity aluminum powder; in fig. 3 - samples of the obtained single-crystal sapphires.

The method of obtaining alpha-alumina for the subsequent growth of single-crystal sapphire was carried out as follows.

A powder of highly pure metallic aluminum, having a size of up to 200 μm, was sprayed into a reactor, which was a water-cooled stainless steel cylinder. Spraying was carried out at atmospheric pressure and at room temperature in a mixture with air filtered from foreign particles. The air consumption for burning aluminum powder must correspond to the stoichiometry of the chemical reaction of the interaction of aluminum with atmospheric oxygen, that is, 67.2 liters of oxygen per 108 g of aluminum powder. In the case of burning aluminum powder in air, about 350 liters of air will be required per 108 g of aluminum powder. It can be sprayed with any powder pump. In the reactor, the mixture of air and aluminum metal powder ignites and burns to form finely dispersed gamma-aluminum oxide. The combustion of a mixture of suspended powder of aluminum metal and air filtered from dust occurs

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SUBSTITUTE SHEET (RULE 26) in the presence of a plasma generator flame. As a plasma generator, an experimental plasma torch with a frequency of 2.45 GHz and a power of 3.5 kW, manufactured by the Obninsk OOO SAPFIR, was used.

The flow of a mixture of finely dispersed gamma alumina with air enters the filter. The filter is a pass-through electrostatic precipitator using a high-voltage DC power supply with a voltage of 20 kV, in which, due to the field of static electricity with a voltage and current of up to 0.5 A, precipitation and concentration of particles of the resulting gamma-aluminum oxide occur on the cathode, followed by its collection in a bunker . The collection was carried out manually.

As a result, the gamma alumina powder obtained has a purity greater than or equal to 99.99% and a bulk density of less than 0.2 g/cm ³ .

For further melting, gamma-alumina powder was compacted by tableting or granulating in a press with polyethylene or fluoroplast walls to obtain tablets or granules having a specific gravity of 1 g/cm ³ . A press with walls made of polyethylene or PTFE is used in order to minimize the possibility of additional contamination with gamma alumina.

The resulting tableted or granulated gamma alumina was placed in an induction furnace container, up to 1 meter in diameter and up to 0.7 meters high. The induction furnace has a power consumption of 100 kW, the operating frequency of the electromagnetic oscillations of the inductor is 800 kHz/s.

For melting in an induction furnace of non-conductive materials such as aluminum oxide, it is necessary to preheat a certain amount of it. In this case, additional purification of aluminum oxide occurs due to the evaporation of foreign impurities at a temperature

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SUBSTITUTE SHEET (RULE 26) about 3000°C. In addition, the ingot obtained by fusion has a high specific gravity, which is necessary for optimal filling of the crucible during further growth of single-crystal leucosapphire.

Inside the inductor of an induction furnace, the material is heated and then melted in a high frequency electromagnetic field.

The container of the induction furnace was first loaded with half of the total amount of gamma alumina (about 150 kg). Then, at least one ring of high-purity aluminum metal was placed in it, which is necessary to initiate heating. In this case, the diameter of the ring should be about 80% of the diameter of the container. With such a cross section, the ring of metallic aluminum has sufficient electrical resistance so that it begins to heat up in the electric field of the inductor. The presence of the ring is necessary so that it continues to heat up all the time up to the melting of the adjacent areas of the poured gamma - aluminum oxide.

Then loaded the rest of the amount of aluminum oxide (150 kg), and the furnace was turned on at full capacity. And although the furnace power is 150 kW, at the initial stage, the power consumption is limited only by the resistance of the aluminum ring and is no more than 10-15 kW. After turning on the induction furnace, the heating of the aluminum ring begins and the gamma-alumina adjacent to the ring melts, and then the entire contents of the container. As the poured layer of gamma-aluminum oxide melted and settled, new portions were added. New portions of raw materials were added to the moment when their level no longer falls below the edges of the container filled with the melt.

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SUBSTITUTE SHEET (RULE 26) After the container of the induction furnace was fully loaded and the entire amount of gamma-alumina was melted, the furnace was turned off. When the melt solidifies, the dissolved oxygen of the air is released. Subsequently, bubble holes were observed in the chips of the ingot. The ingot was naturally cooled for 12 hours, after which the ingot cooled down enough to be removed from the container and subjected to further processing.

The entire process of melting the material in an amount of about 400 kg lasted about 2 hours, during which there is an additional purification of the resulting alpha-alumina from the presence of trace amounts of contaminants. The trace impurities of elements present in the obtained product (iron, nickel, chromium, silicon) have a very high vapor density at the temperature of compaction - melting in an induction furnace. Therefore, at a temperature of 2500-3000°C, their intensive evaporation occurs and, accordingly, further purification of the resulting compact alpha-alumina.

A special mechanism for tilting the container allows you to remove a monolithic ingot weighing 300 kg, having a relative density of 2.7 g/cm3 to ^3.0 g/cm3 ⁽ this is more than 70% of the theoretical density ^of 3.96 g/cm3), with the purity of the resulting product is 99.99%, which in its parameters is ideal for the production of single-crystal sapphire.

The resulting alpha-alumina ingot can be crushed in a press to pieces of the required size, convenient for subsequent melting to obtain single-crystal sapphire.

To evaluate the quantitative analysis of the chemical composition, an X-ray spectral fluorescence method was used.

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SUBSTITUTE SHEET (RULE 26) The results of the quantitative analysis of the chemical composition are presented in Table 1 and in FIG. 1 and 2.

Table 1

Based on the presented data, it can be concluded that the content of oxides in the BEL sample is three times higher than in the SER sample. Since no metals other than aluminum were found in significant amounts in the samples, all oxygen was converted to aluminum oxide. Due to the fact that the weight percentage of oxygen in the BEL sample is significantly higher than in the SER sample, the concentration of aluminum in the BEL sample became lower due to the dilution of aluminum metal with oxide, which is clearly visible in the spectra. (Figure 1 and Figure 2).

The microelement composition of both samples in terms of the Fe/Cr/Ni content ratio is identical to the common grade of stainless steel, which indicates possible contamination of the samples by stainless steel or its corrosion products. The detected impurities are initial (in raw materials), i.e. the sample is not contaminated (and not cleaned) during the oxidation process, but inherits impurities from the raw material. (Fig.1)

Melting aluminum oxide in air dissolves a significant amount of oxygen in excess of stoichiometric air, which is then released from the ingot during solidification.

This dissolved oxygen in the melt completely oxidizes those 7% of the metal that remains in the alpha alumina obtained by

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SUBSTITUTE SHEET (RULE 26) the declared method. As a result, it can be concluded that at the stage of compaction-melting, both the additional oxidation of the metal residues and the further purification of the resulting alpha-alumina occur.

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SUBSTITUTE SHEET (RULE 26)

Claims

Claim

1. A method for producing alpha-alumina for the subsequent cultivation of single-crystal sapphire, characterized in that the stage of oxidation of highly pure aluminum metal powder with air oxygen in a flame of a plasma generator at a temperature of 2040 to 3000 ° C is carried out in advance to obtain particles of a fine powder of gamma-aluminum oxide , with the subsequent stage of concentration of the obtained gamma-aluminum oxide powder on a through electrostatic precipitator using a high-voltage DC power source with a current strength of 0.2 to 0.5 A and a voltage of 15 to 20 kV, and the subsequent stage of compacting the gamma-aluminum oxide powder to specific gravity 1 g/cm ³ , after which gamma-aluminum oxide is melted in an induction furnace at a temperature of 2050 to 3000 ° C in an electromagnetic field at a frequency of 700 to 800 kHz and a power of 120 to 150 kW, for 1, 5 to 2 hours, for which a part of gamma alumina is preloaded into the furnace container, then placed at least one ring of high-purity metallic aluminum is added to it, which is necessary to initiate heating, followed by adding new portions of gamma-alumina until the furnace container is completely filled with melt, followed by cooling the melt at ambient temperature.

2. The method according to claim 1, characterized in that the air consumption during the combustion of aluminum powder in the flame of a plasma generator is 350 liters per 108 g of aluminum powder or 67.2 liters of oxygen per 108 g of aluminum powder.

3. Method according to and. 1, characterized in that the oxidation in the plasma generator is carried out at a frequency of 2.45 GHz and a power of 3.5 kW

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SUBSTITUTE SHEET (RULE 26)

4. The method according to p. 1, characterized in that the compaction of the gamma-alumina powder to a specific gravity of 1 g/cm ³ is carried out by tableting or granulating.

5. Method according to and. 1, characterized in that the diameter of the ring of highly pure metallic aluminum is from 75 to 80% of the diameter of the induction furnace container.

6. Method according to and. 1, characterized in that at the initial stage of melting gamma alumina in an induction furnace, its power is from 10 to 15 kW.

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SUBSTITUTE SHEET (RULE 26)