AU2003100934A4 - Method for producing alumina - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: OTKRYTOE AKTSIONERNOE OBSCHESTVO "VSEROSSIISKY ALJUMINIEVO-MAGNIEVY INSTITUT" Invention Title: METHOD FOR PRODUCING ALUMINA The following statement is a full description of this invention, including the best method of performing it known to us: METHOD FOR PRODUCING ALUMINA FIELD OF THE INVENTION The present invention relates to non-ferrous metallurgy and is applicable in the production of alumina by the Bayer's method.

SBACKGROUND OF THE INVENTION In conventional methods for producing alumina from bauxite ores by the Bayer's method, decomposition mother liquors are regenerated by evaporation thereof (Light Metals 1992, The Minerals, Metals Material Society, 167-171, 1992).

A common problem with the conventional methods is that the heat-exchange surfaces are considerably overgrown with deposits in the process of evaporating the mother liquor. The deposits generally consist of aluminosilicates, sulphates, fluorides, organic acid salts (e:g.

sodium oxalates), etc.

In several patents, a number of additional steps were suggested to prevent the overgrowing of evaporator surfaces with the deposits.

By way of example, to prevent overgrowing of the heat-exchange surfaces with deposits and to raise the efficiency of evaporators, it was proposed to add an organic silicon liquid to the liquor with Na2Ok concentration of 20-200 g/l, and to add polyacrylamide to the liquor with Na2Ok concentration of 200-300 g/l (see Inventor's Certificate SU 526,137).

A disadvantage of the prior art method is that the liquors are polluted with organic compounds and silicon compounds, what impairing the recovery of alumina.

Most closely related from the point of technical subject-matter and the effect achieved to a method in accordance with the present invention is a method for producing alumina, including: grinding a starting material; heat treating a suspension in the presence of lime; extracting alumina to the liquid phase; separating an aluminate liquor from a residue; decomposing the aluminate liquor to obtain a mother liquor and aluminum hydroxide; evaporating the mother liquor and calcinating the aluminum hydroxide; directing the evaporated mother liquor to grinding Liner et al., Production of Alumina, 2-d edition, Metallurgy, 1978, pages 62,63, in Russia).

A disadvantages of the closest prior art method is that the heat-exchange surfaces of evaporators are intensively overgrown with the deposits when a considerable amount of impurities (soda, sulphates, fluorides, oxalates) is present in the liquors, and this effect reduces efficiency of the evaporators and increases heat consumption for the production of alumina.

SUMMARY OF THE INVENTION An object of the present invention is to increase efficiency of the evaporators and to reduce the heat consumption for evaporation of the liquors.

The object is achieved by a method for producing alumina, including the steps of: grinding a bauxite ore; leaching the bauxite ore in the presence of calcium-containing additives; separating an aluminate liquor from an insoluble residue; decomposing the aluminate liquor to produce aluminum hydroxide and a mother liquor, and evaporating the mother liquor, wherein the mother liquor is evaporated in two stages: to a concentration of caustic alkali (Na2Ok) of 180-220 g/1 in the first stage, and to a concentration of Na2Ok of 250- 300 g/l in the second stage, the liquor after the first evaporation stage being directed to the leaching, and the liquor after the second evaporation stage being mixed with the starting mother liquor.

Prior to said evaporating, 1-20 g/l of a calcium-containing additive in the form of oxide, hydroxide or calcium.carbonate is added to the mother liquor.

The calcium-containing additives are added in the second evaporation stage.

Said mixing of the evaporated liquor with the mother liquor is accomplished in the course of said evaporation.

Tests has shown that the evaporation of the mother liquor to a concentration of 180- 220 g/l, followed by dividing the liquor in two flows, the first of which being directed to the leaching of the bauxite ore, and the second one being evaporated to a concentration of 250- 300 g/l and being mixed with the mother liquor, raises efficiency of the Bayer's process due to increased heat transfer coefficient of the evaporators.

Reduction of the concentration in the first evaporation stage to less than 180 g/l is inexpedient because it reduces efficiency of the leaching, while at the concentration in the first stage increased to above 220 g/l, crystallization of carbonates and sodium sulfates starts, which necessitates the provision of a salt extracting assembly.

Substantially no gain in efficiency of the evaporator is attained at the concentration of Na2Ok in the second stage reduced to below 250 g/l, while the temperature depression noticeably increases at the concentration increased to above 300 g/l, what impairing the evaporation efficiency.

Addition of a calcium-containing additive in the amount of less than 1 g/l substantially does not affect the efficiency of deposit formation on the heat-exchange surfaces, while the concentration of the calcium-containing additive above 20 g/1 is inexpedient, since it does not practically affect the operational characteristics of the evaporator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Tests were conducted in an experimental-industrial apparatus (amounts of materials are given per 1 ton of A1 2 0 3 2.8 t (ton) of the bauxite ore with the A1 2 0 3 average content of 42.5 was mixed with 8.5 m 3 of a recycle liquor with the concentration of 210 g/l (gram per liter); lime was added to the resulting pulp on the basis of 72 kg CaO; after grinding in ball mills the pulp was heattreated in pressure vessels at the temperature of 230 0 C. After the heat treatment, the pulp was diluted to 130 g/l Na2Ok; a sludge was separated in a thickening and washing system; the liquor was subjected to decomposition in a continuously working battery of decomposers at the seed ratio of 3.8 during 52 hours. The mother liquor was separated from aluminum hydroxide and was directed to evaporation. The evaporation was accomplished in a five-effect evaporator to the concentration of 210 g/1 Na2Ok.

The selected concentration was dictated by the fact that no carbonates and sodium sulfates precipitated to solid phase at this concentration.

In a reference test, the partly evaporated iother liquor with a concentration of 210 g/l Na2Ok after the fourth effect was divided into two flows: the first flow was directed.to the leaching, and the second one was evaporated to the concentration of 280 l/g and was mixed with the starting mother liquor. In the evaporation up to 280 g/l Na2Ok, sulfates and sodium carbonates precipitates to from a solid phase, but they dissolves after mixing the evaporated liquor and the mother liquor.

Efficiency of the process was determined in terms of alumina yield and heat consumption per 1 t of alumina. The state of heat-exchange surfaces of the multi-effect evaporator was also inspected.

The characteristics were inspected during a month.

In the second test, 2.0 g/1 of a calcium-containing additive (CaO) was added to the mother liquor delivered to the first evaporation stage. This allows the thickness of deposit on the fourth and fifth evaporator effects to be decreased to about 0.5-0.8 mm.

In the third test, the calcium-containing additive was added in the second evaporation stage. Analysis of the deposits has shown that the content of sodium oxalate in the deposits on the heat-exchange surfaces was reduced, and the total thickness of deposits was decreased to 0.4-0.6 mm.

In the test carried out in accordance with the closest prior art method, wherein the entire flow of the mother liquor was evaporated to the concentration of 210 g/1 Na2Ok, the average alumina yield was 28.7 t/h, heat consumption was 3.6 Gcal/t, and the thickness of deposits on pipes of the fourth and fifth effects of the evaporator was 2 to 3 mm. The deposit contains 80 of sodium aluminosilicate. The multi-effect evaporator was washed with acid four times.

Mixing the mother and evaporated liquors in the evaporation process allows the heat exchange efficiency to be increased due to a heat of the evaporated liquors being fully used after the second evaporation stage.

In the reference test, the monthly mean efficiency of the multi-effect evaporator was 29.8 t/h, i.e. it was increased by 3.8 the heat consumption was 3.2 Gcal/t, i.e. it was reduced by 11.1 The evaporator effects operating at increased temperature (110-150 0

C)

were not cleaned in the inspected period, and the thickness of deposit on pipes of the fourth and fifth effects of the evaporator was 1.0-1.5 mm.

As compared to the closest prior art method, the process efficiency (yield of alumina) was increased by 3.8 the heat consumption for evaporation was decreased by 11.1 and expenses for cleaning the multi-effect evaporator were substantially eliminated, what increases the degree of evaporation equipment use from 78 to 92%.

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 recites ones.

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.

Claims (4)

1. A method for producing alumina, including the steps of: grinding a bauxite ore; leaching the bauxite ore in the presence of calcium-containing additives; separating an aluminate liquor from an insoluble residue; decomposing the aluminate liquor to produce aluminum hydroxide and a mother liquor, and evaporating the mother liquor, said method is characterized in that the mother liquor is evaporated in two stages: to a concentration of caustic alkali (Na2Ok) of 180-220 g/l in the first stage, and to a concentration of Na2Ok of 250- 300 g/1 in the second stage, the liquor after the first evaporation stage being. directed to the leaching, and the liquor after the second evaporation stage being mixed with the starting mother liquor.

2. The method of claim 1, characterized in that 1-20 g/l of a calcium-containing additive in the form of oxide, hydroxide or calcium carbonate is added to the mother liquor prior to said evaporating of the mother liquor.

3. The method of claim 1, characterized'in that said calcium-containing additives are added in the second evaporation stage.

4. The method of claim 1, characterized in that said mixing of the evaporated liquor and the mother liquor is accomplished in the course of said evaporation. Dated this 11th day of NOVEMBER 2003 OTKRYTOE AKTSIONERNOE OBSCHESTVO "VSEROSSIISKY ALJUMINIEVO-MAGNIEVY INSTITUT" By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia