AU2003100932A4 - Method for producing alumina from bauxite - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant(s): OTKRYTOE AKTSIONERNOE OBSCHESTVO "VSEROSSIISKY ALJUMINIEVO-MAGNIEVY INSTITUT" Invention Title: METHOD FOR PRODUCING ALUMINA FROM BAUXITE The following statement is a full description of this invention, including the best method of performing it known to me/us: METHOD FOR PRODUCING ALUMINA FROM BAUXITE FIELD OF THE INVENTION The present invention relates to non-ferrous metallurgy, and more specifically to processing bauxite ores containing mainly alumina trihydrate and having a low content of silicon oxide.

BACKGROUND OF THE INVENTION A conventional method for processing bauxite ores contained of alumina trihydrate and having a low content of reactive silicon oxide (PCT/FR96/00450) includes treating the bauxite after grinding by leaching under the atmospheric pressure (according to the Bayer process), wherein before leaching the starting material is subjected to desiliconizing or predesiliconizing for no less than 30 minutes at a temperature of no greater than 108 0 C in presence of a part of the discharge liquor having the alumina/sodium oxide ratio in the range of 0.5-0.7 and containing alkaline in a concentration of 180-220 g/l, and subjecting the material to a secondary or subsequent desiliconizing for no less than 2 hours at a temperature no greater that 180 0 C with all the suspension formed at the leaching of the bauxite and having the alumina/sodium oxide ratio of 1.05-1.17. Before this step, the content of caustic soda in the suspension is brought to 140-180 g/1 by diluting the suspension.

Main disadvantages of the prior art method are: incomplete extraction of alumina from the bauxite during the leaching;.existence of hydrolytic losses of alumina from the aluminate liquor at the stage of thickening and washing the red slurry, and low degree of. alumina precipitation from the aluminate liquor at the decomposition redistribution, resulting in a great specific flow of liquors in the process cycle and high energy consumption in the production.

Most closely related in view of technical subject-matter and the effect achieved to a method according to the present invention is a method for producing alumina from bauxite, including mixing the bauxite with an alkaline/aluminate liquor; leaching the bauxite in pressure vessels; separating a slurry from the aluminate liquor; decomposing the aluminate liquor to produce aluminum hydroxide and a mother liquor; and evaporating the mother liquor Liner et al., Production of Alumina, 2-d edition, Metallurgy, 1978, pages 62,63, in Russia).

Main disadvantages of the closest prior art method are: incomplete extraction of alumina from the bauxite during the leaching; high hydrolytic losses of alumina from the aluminate liquor at the stage of thickening and washing the red slurry, and low degree of alumina precipitation from the aluminate liquor at the decomposition redistribution, resulting in a great specific flow of liquors in the process cycle and, hence, high energy consumption.

SUMMARY OF THE INVENTION An object of the present invention is to provide high extraction of the alumina product from the bauxite, to reduce the specific flow of liquors in the process cycle, and to decrease energy consumption in the production of alumina.

The object is achieved by a method for producing alumina from bauxite, including the steps of: mixing the bauxite with an alkaline/aluminate liquor; leaching the bauxite in pressure vessels; separating a slurry from the aluminate liquor; decomposing the aluminate liquor to produce aluminum hydroxide and a mother liquor; evaporating the mother liquor, wherein the slurry obtained after leaching the bauxite in the pressure vessels is mixed with the evaporated mother liquor having an alkaline concentration of 160-260 g/1 Na2Ok and a caustic modulus of 2.6-3.2 and is leached for 2-4 hours at a temperature of 98-110 0 C, the aluminate liquor is separated from the slurry and is directed to the pressure leaching of bauxite.

The leaching of bauxite in the pressure vessels is carried out at a temperature of 140- 250 0 C during 1-2 hours to produce the aluminate liquor having an alkaline concentration of 150-200 g/l Na2Ok, the caustic modulus in the final aluminate liquor being higher by 0.03- 0.10 units than its equilibrium level at the leaching conditions used.

At a temperature below 140 0 C in the pressure vessels, extraction of alumina from bauxite reduces due to reduction in solubility of hard forms of aluminum hydroxide (boehmite, diaspore, alumogoethite, etc). Increasing the temperature above 250 0 C is inexpedient and is generally defined by technical capabilities (pressure of heating yapor, design of the pressure vessels, etc).

Reduction in the Na2Ok concentration in the pressure leaching below 150 g/1 decreases the efficiency of recycling the liquors in the process cycle, while increase in the concentration above 200 g/1 aggravates the conditions of desiliconizing the aluminate liquors, what impairing the alumina quality.

Duration of the pressure leaching (1-2 hours) depends on the leaching temperature and the presence of alumina hard forms in the bauxite. At the 'leaching duration below 1 hour the alumina extraction is inadmissibly low, while the leaching step longer than 2 hours does not substantially affect the leaching efficiency, but increases the expenses for providing additional equipment.

Formation of the aluminate liquor having the caustic modulus exceeding the equilibrium one by less than 0.03 units is inexpedient in the pressure leaching as it requires an unjustified increase in the leaching duration, hence, in expenses for providing additional pressure vessels and increases heat energy consumption. If the caustic modulus is higher that the equilibrium one by 0.10, efficiency of recycling the liquors reduces and heat energy consumption increases.

In the atmospheric pressure leaching, it is inexpedient to reduce the Na2Ok concentration below 160 g/l, as it does not permit the extraction of residual aluminium hydroxide, while the increase in the Na2Ok concentration above 260 g/l does not substantially affect the leaching efficiency.

Increase in the caustic modulus above 3.2 in the liquor provided to the atmospheric pressure leaching is unreasonable, as it does not substantially affect the leaching efficiency, while the reduction in the liquor caustic modulus below 2.6 leads to increasing of heat energy consumption in the'general cycle.

Temperature of the atmospheric pressure leaching depends on the temperature depression of the liquors and must be below the boiling point of the liquors at the atmospheric pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a process flowsheet of the method in accordance with the invention.

Tests were conducted in experimental-industrial apparatus (amounts of materials are given per 1 ton of A1 2 0 3 Efficiency of the process was determined in terms of alumina yield and heat consumption per 1 t (ton) of alumina. The total hydrolytic losses in a system for thickening and washing the red slurry were also inspected.

The characteristics were inspected during a month.

In a reference test, 3.7 t of bauxite with the A1 2 0 3 average content of 48.5 was mixed with 7.1 m 3 of the recycle liquor having a concentration of 190 g/l; after grinding in ball mills the pulp was heat treated in pressure vessels at the temperature of 230 0 C during 2 hours. Liquid phase of the boiled pulp contained 190 g/1 Na2Ok and 220 g/1 A1 2 0 3 wherein a liquor caustic modulus was 1.42 (when the- equilibrium one was 1.30). The caustic modulus in context of the invention refers to a Na 2 0/A1 2 0 3 molecular ratio. After the heat treatment the pulp was diluted to 150 g/1 Na2Ok, the slurry was separated from the aluminate liquor, the aluminate liquor with cc, 1.45 was subjected to decomposition in a battery of continuously operating decomposers to produce aluminum hydroxide and a mother liquor, the mother liquor containing 155 g/1 Na2Ok and 91 g/1 A1 2 0 3 (c 2.8) was evaporated to the Na2Ok concentration of 210 g/1. The evaporated mother liquor was mixed with the slurry obtained in the pressure leaching, and directed to atmospheric pressure leaching conducted in mixers at the temperature of 105 0 C during 6 hours. The pulp was then diluted with industrial water obtained after washing the slurry to the concentration of 200 g/l, the aluminate liquor was separated from the slurry in a thickening unit, the slurry was washed with water and was sent to refuse disposal, while the aluminate liquor comprising 190 g/1 Na2Ok and 97.7 g/l A1 2 0 3 was sent to the pressure leaching. The end-to-end extraction of alumina from bauxite was 92.5 The hydrolytic losses were 0.5 Heat consumption per ton of the alumina produced was 2.3 Gcal/t.

In a test conducted in accordance with the closest prior art method, 3.7 t of bauxite with the A1 2 0 3 average content of 48.5 was mixed with 7.1 m 3 of the recycle liquor having the concentration of 190 g/l; after grinding in ball mills the pulp was heat treated at the first stage in mixers at the temperature of 105 0 C during 6 hours, and then at the second stage in the pressure vessels at the temperature of 230 0 C during 2 hours. Liquid phase of the boiled pulp contained 200 g/l Na2Ok and 222 g/1 A1 2 0 3 wherein the liquor caustic modulus (oxc) was 1.48 (when the equilibrium one was 1.30). After the heat treatment the pulp was diluted to 150 g/1 Na2Ok, the aluminate liquor was separated from slurry, the slurry was washed with water and was sent to refuse disposal, the aluminate liquor was subjected to decomposition in a battery of continuously operating decomposers to produce aluminum hydroxide and the- mother liquor, the mother liquor was evaporated to Na20k concentration of 210 g/l and was sentto the first stage of leaching.

The end-to-end extraction of alumina from bauxite was 87.5 Hydrolytic losses were Heat consumption per ton of the alumina produced was 2.6 Gcal/t.

Consequently, the use of a method in accordance with the present invention allows to increase the alumina extraction from bauxite by 5.7 to reduce the heat consumption by 11.5 and to decrease the hydrolytic losses by 90 as compared to the closest prior art method.

Although the above description was presented in the form of different preferred embodiments, it shall be understood that various modifications and changes can be done without departing from the matter of the present invention as defined in the claims.

In particular, the terms "comprise" and "include" in the description and the claims imply that in addition to the features listed after the aforementioned terms, the method steps in accordance with the invention can be defined by other additional features, or that the method in accordance with the invention can include other additional steps in addition to the Saforementioned.

4A It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

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Claims (2)

1. A method for producing alumina from bauxite, including the steps of: mixing the bauxite with an alkaline/aluminate liquor; leaching the bauxite in pressure vessels; separating a slurry from an aluminate liquor; decomposing the aluminate liquor to produce aluminum hydroxide and a mother liquor; evaporating the mother liquor, said method is characterized in that the slurry obtained after leaching the bauxite in the pressure vessels is mixed with the evaporated mother liquor having an alkaline concentration of 160-260 g/1 Na2Ok and the caustic modulus of 2.6-3.2 and is leached for 2-4 hours at a temperature of 98-110 C, the aluminate liquor is separated from the slurry and is sent to the pressure leaching of bauxite.

2. The method of claim 1, characterized in that the pressure leaching of bauxite is carried out at a temperature of 140-250°C during 1-2 hours to produce the aluminate liquor having an alkaline concentration of 150-200 g/1 Na2Ok, the caustic modulus in the final aluminate liquor being higher by 0.03-0.10 units than its equilibrium level at the leaching conditions used. Dated this 11th day of November 2003. OTKRYTOE AKSIONERNOE OBSCHESTVO "VSEROSSIISKY ALJUMINIEVO-MAGNIEVY INSTITUT" By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia