RU2790705C1 - Method for producing ultrafine activated alpha alumina - Google Patents

Abstract

FIELD: chemical technology.

SUBSTANCE: method includes dry grinding of calcined alumina in a ball mill. To alumina before dry grinding, over the mass of raw materials, polyethylene glycol is added in an amount of 0.5% or polyethylene glycol simultaneously with polycarboxylate ether in an amount of 0.085-0.68%. As a feedstock, metallurgical alumina with a median particle size D₅₀ of 10-20 mcm is used, represented by a crystalline phase of γ-Al₂O₃, to which water is added until a pasty state is obtained, briquettes are prepared by laying in molds and dried to constant weight at a temperature of 110±15°C. The resulting briquettes are subjected to calcination at a temperature of 1250-1550°C for 1-3 hours, the baking products are crushed. Dry grinding is done in a ball mill for 0.5-8.5 hours.

EFFECT: obtaining particles with a primary crystal size from 0.2 to 1.1 mcm and a submicron fraction content from 65 to 87%, high fluidity of suspensions based on the obtained particles, as well as a high density of sintered samples at low baking temperatures.

1 cl, 1 dwg, 3 tbl, 4 ex

Description

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

Ultrafine powders α-Al ₂ O ₃ in English technical terminology is known as reactive alumina (Reactive alumina). In reactive alumina, the material is dispersed to the size of primary crystals (0.3 - 2 μm), while a significant proportion of particles have a submicron size, i.e. less than 1 micron (up to 90%). For mass production of finely and ultradispersed activated α-Al ₂ O ₃ , calcined alumina is currently used, as a rule. Such alumina is obtained by heat treatment (calcination) of Bayer aluminum hydroxide from 1300 to 2000°C [Gürel, S. Berrin. Reactive alumina production for the refractory industry / S. Berrin Gürel, Akin Altun Akin // Powder Technology. 2009. No. 196. P. 115-121]. The size of the formed crystals of α-alumina is affected by both the temperature and the rate of calcination. The higher the calcination temperature, the greater the size of 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 [Vorobiev N.D. Modeling of the grinding process in ball mills // Mining Journal. 2004. No. 5. S. 65 - 68].

Known patent RU 2625104 (publ. 07/11/2017), which describes a method for obtaining submicron powder of alpha alumina, including the following operations: processing of Bayer aluminum hydroxide in a mill with seed particles; drying; calcination and deaggregation of the resulting powder by grinding in an organic solvent. The obtained alumina powders consist of spherical alpha-alumina particles, weakly aggregated, with a narrow size distribution (0.1 - 0.3 μm). The scope of these powder products is the production of dense alumina ceramics. The disadvantage of this method is that a fairly large number of technological operations are used, which in turn makes it unsuitable for large-scale production of activated alumina.

Known patent US 3358937 (publ. 12/19/1967), which describes a method for producing activated alumina by dry grinding in a ball mill. Before grinding, an additive-intensifier is introduced into the aluminous raw material, which is a monohydric (ethanol, methanol, isopropanol, n-propanol, n-octanol) or polyhydric (ethylene glycol, triethylene glycol) alcohol, as well as compounds of the amine group (mono- and triethanolamine), or esters (n-butyl acetate). The grinding time is 8 hours. The introduction of such additives prevents the aggregation of particles when the level of dispersion is less than 2–3 µm and reduces the grinding time. The resulting products are used to manufacture high-density corundum ceramics. At the same time, the described method involves the introduction of a sufficiently high amount of additives-intensifiers - 0.5 wt.%, which leads to a large consumption of reagents in a large-tonnage production.

The influence of various technological parameters (number and size of grinding media, oscillation frequency) on the process of wet grinding of two types of alumina in a vibratory mill is presented in the scientific article Effect of selected parameters on grinding process of alumina in the rotary-vibration mill [MAWojcik, T. Gajda, J. Plewa, H. Altenburg, A. Lutterman, V. Figusch, M. Haviar // Fizykochemiczne Problemy Mineralurgii, 1997. - No. 31. - P. 115-124]. With the use of optimal technological parameters from the point of view of the authors of the work, it was possible to obtain powdered aluminum oxide with a D ₅₀ of 3 μm and a D ₉₀ of the order of 10 μm with a grinding time of 60 min. However, the wet grinding process has significant disadvantages, consisting in the need for an energy-intensive process of subsequent drying and the resulting aggregation of fine particles.

The influence of technological parameters on the granulometric characteristics of submicron aluminum oxide in the α-form is presented in a scientific article Study of the influence of technological parameters on the granulometric characteristics of submicron aluminum oxide in the α-form [Trubitsyn M.A., Volovicheva N.A., Furda L.V., Skrypnikov N.S. // Bulletin of BSTU im. V.G. Shukhova, 2021, No. 12, S. 84-97], where it is shown that in the case of using ball mills as grinding units for fine grinding of alumina raw materials, not only the choice of a grinding intensifier additive is relevant, but also the optimization of technological parameters of the grinding process . It is shown that dry grinding of α-Al ₂ O ₃ in a ball mill is most effective when using cylindrical grinding media, drum rotation speed equal to 100 rpm and volumetric loading of 35%, while grinding is carried out for 10 hours. The amount of additive-intensifier used (polyethylene glycol) with the specified parameters is 0.05%. In this case, the median particle size of the resulting product is about 3 μm, and the proportion of the submicron fraction (≤ 1.0 μm) is 20%.

The closest to the proposed technical solution, taken as a prototype, is patent RU 2770921 (publ. 04/25/2022), which describes a method for obtaining finely dispersed activated alpha alumina, including dry grinding of calcined alumina, in which the content of the phase α-Al ₂ O ₃ - not less than 98%, and the median particle size D ₅₀ - in the range from 40 to 60 microns, in a ball mill when using grinding bodies of a cylindrical shape and a volumetric loading of the drum with grinding bodies of 35%, while before grinding, over calcined alumina, polyethylene glycol is added to in the amount of 0.05% by weight of the raw material, simultaneously with the addition of polyethylene glycol, polycarboxylate ether is added in the amount of 0.085 - 0.68% by weight of the raw material. Grinding is carried out for 10 hours. In this case, the median particle size in the resulting product is 3.19-2.14 microns, and the content of the submicron fraction (less than 1 micron) reaches 25.7%.

The technical objective of the proposed technical solution is to expand the arsenal of tools by developing a method for obtaining ultrafine activated alpha-alumina.

The technical result of the proposed technical solution is to obtain ultrafine activated alpha alumina with a primary crystal size of 0.2 to 1.1 microns and a submicron fraction content of 65 to 87% by dry grinding; providing such rheological characteristics as high fluidity of suspensions based on ultrafine alpha-alumina at minimum operating humidity; providing high density of sintered samples of corundum ceramics at low firing temperatures.

The second technical result is a reduction in the grinding time, if it is required to obtain ultrafine activated alpha-alumina, in which the proportion of the submicron fraction (≤ 1.0 μm) is more than 50%.

To implement the tasks set, a method for obtaining ultrafine activated alpha-alumina is proposed, which is carried out in the following way:

Commercially available metallurgical alumina with a median particle size D ₅₀ ~ 10–20 µm, represented mainly by the γ-Al ₂ O ₃ crystalline phase, is used as a feedstock, to which water is added until a pasty state is obtained, then briquettes are prepared by laying in molds and dried to constant weight at a temperature of 110 ± 15°C. Next, the briquettes are subjected to calcination at a temperature of 1250 - 1550°C for 1 - 3 hours. In the specified temperature range, the transition of γ-Al ₂ O ₃ to α-Al ₂ O ₃ occurs. The roasting products are crushed, and then subjected to grinding in a ball mill for 0.5 - 8.5 hours using cylindrical grinding bodies with a volume of 6.3 cm ³ each, and a volume loading of the drum with grinding bodies of 35%, at a drum rotation speed equal to 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 ultrafine activated alpha-alumina. Coefficient K, which characterizes the ratio of the volume of the crushed material to the volume of voids between the grinding bodies, in all cases, is taken equal to 1. Before grinding, 0.05% polyethylene glycol is added to calcined alumina in excess of the mass, or polycarboxylate ether is added simultaneously with polyethylene glycol in the amount of 0.085 - 0.68% over the mass of raw materials.

A distinctive feature of the proposed method for obtaining ultrafine activated alpha-alumina is the use of commercially available metallurgical alumina, represented mainly by the γ-Al ₂ O ₃ crystalline phase, as a feedstock, which is subsequently subjected to calcination at a temperature of 1250 - 1550 ° C for 1 - 3 hours, followed by transition to α-Al ₂ O ₃ , as well as reducing the time of dry grinding of the resulting alpha-alumina to 0.5 - 8.5 hours, providing such rheological characteristics as high fluidity of suspensions based on ultrafine alpha-alumina at minimum operating humidity and ensuring high density of sintered samples of corundum ceramics at low firing temperatures.

Examples of implementation of the invention.

Example 1

To metallurgical alumina with a median particle size of D ₅₀ ~ 10 - 20 μm, represented mainly by the γ-Al ₂ O ₃ crystalline phase, water is added until a paste is obtained, briquettes are prepared by laying in molds, cast into molds and dried to constant weight at temperature of 110 ± 15°C, after which the briquettes are subjected to calcination at a temperature of 1250 - 1550°C for 1 - 3 hours. Next, the roasting products are crushed, and then grinding is carried out in a ball mill for 0.5 - 8.5 hours using cylindrical grinding bodies with a volume of 6.3 cm ³ each, and the volume loading of the drum with grinding bodies of 35%, at a drum rotation speed, equal to 100 rpm, while 0.05% polyethylene glycol is added to the calcined alumina before grinding, or polycarboxylate ether is added simultaneously with polyethylene glycol in an amount of 0.085 - 0.68% over the weight of the raw material.

In FIG. 1 shows the results of determining the phase composition of the calcination products of the original commercial alumina γ-Al ₂ O ₃ .

The data shown in Fig. Table 1 confirms that the calcination products obtained in a given temperature range of 1250 - 1550°C are alumina, which is predominantly in the alpha modification, the content of the α-Al ₂ O ₃ phase is at least 98%. The calcination temperature below 1250°C is insufficient, since there is no complete transition of γ-Al ₂ O ₃ to the alpha form, and the implementation of calcination at a temperature above 1550°C is not economically feasible.

Example 2

In example 2, to metallurgical alumina with a median particle size of D ₅₀ ~ 10 - 20 μm, represented mainly by the γ-Al ₂ O ₃ crystalline phase, water is added to obtain a paste-like state, briquettes are prepared by laying in molds, cast into molds and dried to constant weight at a temperature of 110 ± 15°C, after which the briquettes are subjected to calcination at a temperature of 1250 - 1550°C for 1 - 3 hours. Next, the roasting products are crushed, and then grinding is carried out in a ball mill for 0.5 - 8.5 hours using cylindrical grinding bodies with a volume of 6.3 cm ³ each, and the volume loading of the drum with grinding bodies of 35%, at a drum rotation speed, equal to 100 rpm. Coefficient K, which characterizes the ratio of the volume of the crushed material to the volume of voids between the grinding bodies, in all cases, is taken equal to 1. 0.05% polyethylene glycol is added to the calcined alumina before grinding starts in excess of the mass. The effect of calcination temperature on the dispersion characteristics of the obtained alpha alumina are shown in table 1.

Table 1

The data shown in Table 1 confirms that with an increase in the calcination temperature, an increase in the size of the primary crystals of α-Al ₂ O ₃ is observed. Thus, it is possible to purposefully obtain ultrafine α-Al ₂ O ₃ powders with the required primary crystal size, submicron fraction fraction, and specific surface area.

Example 3

Table 2 presents the results of the influence of the composition of grinding aids on the duration of grinding of α-Al ₂ O ₃ obtained at different calcination temperatures to a median particle size (D ₅₀ ) equal to 0.8 μm.

table 2

The results presented in Table 2 confirm that the preliminary introduction of polycarboxylate ether (PCE) simultaneously with polyethylene glycol makes it possible to reduce the grinding time of calcined alumina by 6-8 times, compared with the product obtained by grinding only in the presence of polyethylene glycol. The preferred minimum amount of polycarboxylate ester in the composition of the combined additive is 0.34%.

Example 4

The effect of calcination temperature on the rheotechnological properties of the products of 4-hour grinding of alpha-alumina (α-Al ₂ O ₃ ) was evaluated by the time of flow of 50 ml of aqueous suspensions on a VZ-1 viscometer with a funnel opening diameter of 5.4 mm.

The influence of the calcination temperature on the apparent density of the sintered specimens was evaluated as follows. An aqueous suspension (slip) was prepared by mixing water and powder in a drum on rollers with a ratio of balls : alumina = 1 : 1 for 24 h, and without balls (to remove air bubbles) for 3 h. The duration of the set of a raw product in a plaster mold was 10 min. Drying of raw blanks first took place in the mold itself for 12 hours, and then, after being removed from it, in an oven at 110 ± 15ºС for 6 hours. The products were fired in air at 1600ºС with an exposure of 2 hours. The results are presented in Table 3.

Table 3

Calcination of the original metallurgical alumina in the temperature range of 1250 to 1550ºС has a positive effect on the fluidity of aqueous suspensions and the apparent density of sintered samples obtained on the basis of crushed ultrafine calcination products (α-Al ₂ O ₃ ). In this case, not only a reduction in the time of suspension outflow by 1.6 - 9.8 times is observed, but also a decrease in the working humidity of the suspensions and the amount of polycarboxylate ether introduced, as well as a high density of sintered samples at low firing temperatures. The highest value of apparent density is observed in sintered specimens made using ultradisperse α-Al ₂ O ₃ powder obtained at a calcination temperature of 1450°C.

As a result of the above examples, the solution of the set technical problems was confirmed: obtaining ultrafine activated alpha-alumina by dry grinding with a primary crystal size of 0.2 to 1.1 μm and a submicron fraction content of 65 to 87%; providing such rheological characteristics as high fluidity of suspensions based on finely dispersed alpha-alumina at minimum operating humidity; ensuring high density of sintered specimens at low firing temperatures.

Claims (1)

A method for producing ultradisperse activated alpha-alumina, including dry grinding of calcined alumina in a ball mill using cylindrical grinding bodies with a volume of 6.3 cm ³ each, and a volume loading of the drum with grinding bodies of 35%; mass of raw materials add polyethylene glycol in an amount of 0.5% or polyethylene glycol simultaneously with polycarboxylate ether in an amount of 0.085-0.68%, characterized in that metallurgical alumina with a median particle size D ₅₀ of 10-20 μm is used as the feedstock, represented by a crystalline phase of γ-Al ₂ O ₃ , to which water is added to obtain a pasty state, briquettes are prepared by laying in molds and dried to constant weight at a temperature of 110±15°C, the resulting briquettes are subjected to calcination at a temperature of 1250-1550°C in for 1-3 hours, the roasting products are crushed, and then the mentioned dry grinding in a ball mill for 0.5-8.5 hours.

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