RU2801847C1 - Method for producing alumina, mainly from high-silica bauxite - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the production of metallurgical alumina during the processing of high-silicon bauxite with a silicon module μ_Si≤ 3 to obtain alumina of metallurgical grades. Obtaining alumina from high-silica bauxite includes preparing a reagent solution from a mixture of ammonium bisulphate and sulphuric acid, followed by leaching bauxite with it, filtering the resulting pulp, cooling to precipitate ammonium aluminium sulphate alum from the sulphate-ammonium solution, followed by their separation from the mother liquor. The alum is washed, dissolved in water, aluminium hydroxide is precipitated by raising the pH of the solution with gaseous ammonia, filtered and the resulting pulp is washed, after which the aluminium hydroxide is calcined in air. Leaching is carried out with a solution of a mixture of ammonium bisulphate and sulphuric acid with a volume ratio of solutions of 12.5:87.5-87.5:12.5 vol.% and a concentration of 30-40% and 25-40%, respectively. The precipitation of aluminium hydroxide is carried out for 60-120 minutes and the temperature of the solution is 80-95°C with the addition of a seed of gibbsite (Al(OH)₃) with an average particle size of 40-60 microns in the amount of 100-300 g/l.

EFFECT: method allows to intensify the leaching process and increase the size of alumina particles.

4 cl, 3 dwg, 2 tbl, 4 ex

Description

The invention relates to aluminum metallurgy and can be used in the production of metallurgical alumina, which is used to obtain metallic aluminum by electrolysis.

The traditional technology for obtaining alumina involves the processing of bauxite raw materials with a high silicon modulus μ _Si (the ratio of Al ₂ O ₃ /SiO ₂ in the raw material is above 7). These methods are based on the use of caustic alkali to bring aluminum into solution in high pressure reactors. In this case, waste red mud is formed, which is stored in the sludge fields, and can serve as a source of man-made pollution.

For the use of aluminum-containing raw materials with a low silicon modulus μ _Si < 7, acid methods are mainly proposed. Their advantage is the transfer of aluminum into solution without dissolving the main impurity: silicon oxide (SiO ₂ ) - silica, which remains in the solid residue.

A known method of obtaining alumina from oxidized polymetallic ore during the preliminary sintering of raw materials with Na ₂ CO ₃ and NaOH at T = 250-300°C and further leaching of the cake with hydrochloric acid (HCl). Leaching lead 4.7 M HCl at T = 60°C and a duration of 60 min [Patent RU No. 2373152, class. C01F 7/30, C01D 7/00, C01B 33/14, publ. November 20, 2009].

The disadvantage of this method is the need for preliminary firing of raw materials, which significantly increases the labor and energy costs of the entire technological scheme.

A known method of processing kaolin clays, [Patent RU No. 2572119, class. C22B 21/00, publ. December 27, 2015]. The main stages of this method: pre-calcination of clay with ammonium chloride, further hydrochloric acid leaching of the calcined product in an autoclave at T = 160°C, pulp filtration, precipitation of aluminum chloride hexahydrate AlCl ₃ ⋅ 6H ₂ O (HCA) from hydrochloric acid solution with gaseous hydrogen chloride at T = 70°C, calcination of HCA at T = 600°C to obtain crude alumina, alkaline processing of crude alumina in the Bayer cycle: dissolution in NaOH solution at T = 1 50°C for 2 hours, decomposition of the alkaline aluminate solution and calcination of aluminum hydroxide at T = 1100°C to obtain commercial alumina.

The main disadvantage of this method is the use of gaseous hydrogen chloride, which is extremely dangerous for human health, in the process of salting out HCA and the need to recrystallize crude alumina, which increases the number of processing stages and thereby significantly increases the cost of alumina.

Known hydrochloric acid method of processing clay Troshkovsky deposits [Patent RU No. 2625470, class. C01F 7/62, C01F 7/20, C22B 3/10, publ. 07/14/2017] The main difference is the use of electrochemical purification of aluminum-containing chloride solutions from iron at pH 1.0-3.0, cathode current density 0.001-0.150 A/cm, anodic current density 0.015-0.200 A/cm ² , and solution temperature 20 -97°C.

The disadvantage is the low degree of iron removal, which does not exceed 70-95%, which does not allow to obtain alumina of the required quality.

A known method for producing alumina from mudstone [Patent RU No. 2471010, cl. C22B 21/00, publ. December 27, 2012]. The method involves preliminary roasting of the raw material at T=600°C, leaching with 6M HCl at T=100-110°C for 7 hours, and purification of solutions from iron using Cyanex 272 complexing agent.

The disadvantages of this method are significant labor and energy costs at the stage of preliminary firing of raw materials, the duration of leaching and the high content of alkali metals (Na, K, Ca, Mg) in alumina up to 0.88 wt. %.

Known acid method for processing the ashes of coal-fired power plants [Patent RU No. 2510365, class. C01F 7/02, B09B 3/00, publ. 03/27/2014]. The method involves magnetic separation of ash, autoclave leaching with hydrochloric acid, purification of hydrochloric acid solution from iron using macroporous cationic resins (D001, 732, 742, 7020H, 7120H, JK008 or SPC-1), crystallization of aluminum chloride hexahydrate (ACH) and further calcination of ACH at T=950-1100°C.

The disadvantage of this method is the shape of the obtained alumina particles, since when calcining the HCA, the powder retains the form of a salt (long hexagonal crystals), which does not allow the use of this alumina in the electrolysis process due to low flow rates.

Closest to the proposed one is the bisulfate method for producing alumina, mainly from the clays of the Kaichakskoye deposit, the nepheline concentrate of the Kolskoye deposit and the waste of alumina production - red mud, including the preparation of a reagent solution from a mixture of ammonium bisulfate and sulfuric acid, followed by leaching of bauxite, filtering the resulting pulp, cooling to room temperature to precipitate ammonium sulfate alum from the ammonium sulfate solution with their subsequent separation from the mother liquor, washing the alum with subsequent dissolution in water, precipitation of aluminum hydroxide by increasing the pH of the solution using gaseous ammonia, filtering and washing the resulting pulp with further calcination of the hydroxide aluminum in air [Patent RU No. 2574247, class. C01F 7/26, publ. February 10, 2016].

The opening of the raw material was carried out by leaching the raw material with a mixture of 40-45% NH ₄ HSO ₄ and 1-3% H ₂ SO ₄ at T=85-175°C for 3-5 hours, cleaning the solution from impurities using a strongly basic anion exchanger AB-17x8, precipitation of A1(OH) ₃ with gaseous ammonia, with the release of sulfuric acid and solid ammonium sulfate, from which ammonium bisulfate and gaseous ammonia were obtained.

The main disadvantages of the method are the high duration of the leaching process and the small particle size of alumina (1-5 μm) obtained at the stage of deposition of Al(OH) ₃ with gaseous ammonia, which reduces the efficiency of the subsequent electrolysis process to obtain metallic aluminum.

The objective of this invention is to create a bisulfate method for processing high-silica bauxites with silicon modulus μ _Si ≤ 3 to obtain alumina of metallurgical grades.

The technical result achieved by using the invention is the intensification of the leaching process and an increase in the particle size of alumina.

The specified technical result is achieved by the fact that in the method for producing alumina, mainly from high-silica bauxite, including the preparation of a reagent solution from a mixture of ammonium bisulfate and sulfuric acid, followed by leaching of bauxite, filtering the resulting pulp, cooling to room temperature to precipitate alum of ammonium aluminum sulfate from ammonium sulfate solution, followed by their separation from the mother liquor, washing of alum with subsequent dissolution in water, precipitation of aluminum hydroxide with increasing pH of the solution using gaseous ammonia, filtering and washing the resulting pulp with further calcination of aluminum hydroxide in air, leaching is carried out with a solution of a mixture of ammonium bisulfate and sulfuric acid with a volume ratio of solutions of 12.5:87.5 - 87.5:12.5% vol. and a concentration of 30-40% and 25-40%, respectively, the precipitation of aluminum hydroxide is carried out for 60-120 min and the temperature of the solution is 80-95 C with the addition of a seed of gibbsite (Al(OH) ₃ ) with an average particle size of 40-60 μm in amount of 100-300 g/l.

It is advisable to carry out the leaching of bauxite at T=150-170°C, T:W=1:8-1:12 and for 30-90 minutes.

Figure 1 shows the appearance of the particles of alumina according to example 1.

Figure 2 - the appearance of the nanogibbsite particles.

Figure 3 - the appearance of the particles of alumina obtained without the use of a seed.

The intensification of the leaching process occurs due to the use of more concentrated sulfuric acid and an increase in its amount in the mixture, which, in addition to reducing the duration of the process, makes it possible to prevent coprecipitation of ammonioalunite (NH ₄ Al ₃ (SO ₄ ) ₂ (OH) ₆ ), which can occur at temperatures above 170 °C and high concentration of aluminum in the solution.

The use of seed gibbsite allows nanogibbsite to be deposited on the surface of the particles, resulting in a spherical alumina with an average particle size of 80-100 µm.

The bauxite of the Severoonezhskoye deposit (Arkhangelsk region) was chosen as a raw material, the chemical composition is presented in Table 1.

Example 1. Bauxite (see table 1), crushed to a particle size of 0.1 mm, leached with a mixture of 30% NH ₄ HSO ₄ + 40% H ₂ SO ₄ with a solution ratio of 12.5:87.5% vol. at T=150°C, T:W=1:12 and duration 90 min. The pulp after leaching of bauxite is filtered and the resulting solution is cooled to room temperature (25°C), while the crystals of ammonium alum precipitate. Alum crystals are filtered off from the acid solution, washed and dissolved in distilled water. The composition of the alum solution is as follows, mg/l: Al - 3460; Ca - 2.1; Cr - 1; Fe - 2.2; K - 3; Mg - 0.39; Mn - 0.02; Na - 6.3; P - 0.2; Si - 0.29; Ti - 0.05; V - 0.17; Zn - 0.05.

When Al(OH) ₃ is precipitated from a solution of alum with gaseous ammonia, the pH of the solution slowly increases from 2.4 to 7. The temperature of the solution is 90°C, the process of gibbsite precipitation is carried out for 120 minutes. Gibbsite with an average particle size of 40 μm is used as a seed aluminum hydroxide, and its concentration is 150 g/l. At the end of the experiment, the solution is filtered, and the precipitated gibbsite is washed with water and dried at T=105°C for 4 hours. The degree of transition of aluminum from solution to precipitate is 99%. After precipitation of gibbsite, the resulting powder is calcined in an air atmosphere in a muffle furnace at T=900°C with a holding time of 60 min.

The degree of extraction of aluminum in the leaching process is 89.6%, the average particle size of alumina was 82 microns.

The resulting alumina samples are analyzed for the content of impurities and compared with the requirements in accordance with GOST 30558-2017 “Metallurgical alumina. Specifications". The chemical composition of alumina obtained from bauxite by the bisulfate method is presented in table 2. Alumina samples have a spherical shape (figure 1).

Example 2. Bauxite crushed to a particle size of 0.1 mm, leached with a mixture of 40% NH ₄ HSO ₄ + 30% H ₂ SO ₄ with a solution ratio of 50:50% vol. at T=160°C, T:W=1:10 and duration 60 min. The pulp after leaching of bauxite is filtered and the resulting solution is cooled to room temperature (25°C), while the crystals of ammonium alum precipitate. Alum crystals are filtered off from the acid solution, washed and dissolved in distilled water.

When Al(OH) ₃ is precipitated from a solution of alum with gaseous ammonia, the pH of the solution slowly increases from 2.4 to 7. The temperature of the solution is 80°C, the process of gibbsite precipitation is carried out for 60 minutes. As the seed aluminum hydroxide, gibbsite with an average particle size of 50 μm is used, and its concentration is 200 g/l. At the end of the experiment, the solution is filtered, and the precipitated gibbsite is washed with water and dried at T=105°C for 4 hours. The degree of transition of aluminum from solution to precipitate is 99%. After precipitation of gibbsite, the resulting powder is calcined in an air atmosphere in a muffle furnace at T=900°C with a holding time of 60 min.

The degree of extraction of aluminum in the leaching process is 91.2%, the average particle size of alumina is 87 microns.

Example 3. Bauxite crushed to a particle size of 0.1 mm, leached with a mixture of 40% NH ₄ HSO ₄ + 25% H ₂ SO ₄ with a solution ratio of 87.5:12.5% vol. at T=170°C, T:W=1:8 and duration 30 min. The pulp after leaching of bauxite is filtered and the resulting solution is cooled to room temperature (25°C), while the crystals of ammonium alum precipitate. Alum crystals are filtered off from the acid solution, washed and dissolved in distilled water.

When Al(OH) ₃ is precipitated from a solution of alum with gaseous ammonia, the pH of the solution slowly increases from 2.4 to 7. The temperature of the solution is 80°C, the process of gibbsite precipitation is carried out for 90 minutes. As seed aluminum hydroxide, gibbsite with an average particle size of 60 μm is used, and its concentration is 300 g/l. At the end of the experiment, the solution is filtered, and the precipitated gibbsite is washed with water and dried at T=105°C for 4 hours. The degree of transition of aluminum from solution to precipitate is 99%. After precipitation of gibbsite, the resulting powder is calcined in an air atmosphere in a muffle furnace at T=900°C with a holding time of 60 min.

The degree of extraction of aluminum in the leaching process is 94.84% o, the average particle size of alumina is 95 microns.

Example 4. Bauxite crushed to a particle size of 0.1 mm, leached with a mixture of 40% NH ₄ HSO ₄ + 25% H ₂ SO ₄ with a solution ratio of 50:50% vol. at T=190°C, T:W=1:10 and duration 60 min. With these parameters, precipitation of water insoluble ammonioalunite occurs, which significantly reduces the degree of aluminum extraction. The pulp after leaching of bauxite is filtered and the resulting solution is cooled to room temperature (25°C), while the crystals of ammonium alum precipitate. Alum crystals are filtered off from the acid solution, washed and dissolved in distilled water.

When Al(OH) ₃ is precipitated from a solution of alum with gaseous ammonia, the pH of the solution slowly increases from 2.4 to 7. The temperature of the solution is 90°C, the process of gibbsite precipitation is carried out for 120 minutes. The process is carried out without seeding. At the end of the experiment, the solution is filtered, and the precipitated nanogibbsite is washed with isopropyl alcohol and dried at T=105°C for 4 h (figure 2). The degree of transition of aluminum from solution to precipitate is 99%. After precipitation of gibbsite, the resulting powder is calcined in an air atmosphere in a muffle furnace at T=900°C with a holding time of 60 min.

The degree of extraction of aluminum in the leaching process is 65%, the average particle size of alumina is 10 μm (figure 3).

Thus, the proposed method significantly reduces the duration of leaching from 3-5 hours to 60-90 minutes and allows you to increase the degree of extraction of aluminum to 94.84%. At the same time, in the process of precipitation of gibbsite from a solution of alum with gaseous ammonia, seed aluminum hydroxide is added, which makes it possible to obtain spherical particles with an average particle size of 80-100 μm. The resulting alumina complies with GOST 30558-2017 in terms of chemical composition and can be used to produce aluminum metal by electrolysis.

Claims (4)

1. A method for producing alumina, mainly from high-silica bauxite, which includes preparing a reagent solution from a mixture of ammonium bisulfate and sulfuric acid, followed by leaching of bauxite, filtering the resulting pulp, cooling to room temperature to precipitate ammonium aluminum sulfate alum from the ammonium sulfate solution, followed by by separating them from the mother liquor, washing the alum with subsequent dissolution in water, precipitating aluminum hydroxide with an increase in the pH of the solution using gaseous ammonia, filtering and washing the resulting pulp with further calcination of aluminum hydroxide in an air atmosphere, characterized in that the leaching is carried out with a solution of a mixture of ammonium bisulfate and sulfuric acid with a volume ratio of solutions of 12.5: 87.5-87.5: 12.5 vol.% and a concentration of 30-40% and 25-40%, respectively, the precipitation of aluminum hydroxide is carried out for 60-120 min and temperature a solution of 80-95°C with the addition of a seed of gibbsite (Al(OH) ₃ ) with an average particle size of 40-60 μm in the amount of 100-300 g/l. 2. The method according to p. 1, characterized in that the leaching of bauxite is carried out at T = 150-170°C. 3. The method according to p. 1, characterized in that the leaching of bauxite is carried out at a ratio of T:W = 1:8-1:12. 4. The method according to p. 1, characterized in that the leaching of bauxite is carried out for 30-90 minutes.