RU2613983C1 - Method of producing alumina from chromiferous bauxites - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of producing alumina from chromiferous bauxites comprises wet batch sintering, leaching of sintered material with flush water to produce aluminate solutions, whereof aluminum hydroxide is extracted. Bauxite is subjected to wet grinding on the concentrated reusable soda solution at a volume ratio of W:T=3:1 separate from the limestone. A pulp with a particle size of less than 0.05 mm is obtained. The pulp is subjected to magnetic separation to obtain magnetic and non-magnetic products. The magnetic product with chromium oxide (III) content from 25 to 30% is taken for processing to sodium chromate. The non-magnetic product with limestone fines and fresh soda is taken for the batch adjustment. Thereafter, the batch is sintered. The resulting sintered material is leached with flush water to produce aluminate solutions, whereof aluminium hydroxide is extracted. Aluminium hydroxide is filtered, washed and taken for calcination.

EFFECT: invention allows to increase the complexity of processing low-quality bauxite to produce alumina and sodium chromate, reduce the environmental pressure due to the extraction of the chromium compounds.

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Description

The invention relates to the production of alumina from bauxite.

A known method of producing alumina according to the Bayer method (Liner A.I. Production of alumina / A.I. Liner, Eremin N.I., Liner Yu.A., Pevzner I.Z. - M.: Metallurgy. - 1978. - S. 61-183), based on the interaction of bauxite ores with alkaline solutions and the subsequent decomposition of aluminate solutions with the release of aluminum hydroxide from them according to the reactions:

(уд. Вес 1,20-1,22), подобно растворам едкого натра, растворяет глинозем из боксита за 1,5 часа при перемешивании и под давлением в 3-4 ат. При этом хромсодержащие минералы, представленные трехвалентным хромом, переходят в красный шлам и, являясь водонерастворимыми формами, не оказывают вредного влияния на окружающую среду.The mother liquor from the decomposition of aluminate solutions by evaporation to 40-44 °

(beats. Weight 1.20-1.22), like sodium hydroxide solutions, dissolves alumina from bauxite in 1.5 hours with stirring and under a pressure of 3-4 at. At the same time, chromium-containing minerals represented by trivalent chromium pass into red mud and, being water-insoluble forms, do not have a harmful effect on the environment.

The disadvantage of this method is its suitability only for processing high-quality bauxite with a high silicon module (weight ratio of Al ₂ O ₃ to SiO ₂ ) of more than 7 ÷ 8.

A known method of producing alumina in a sequential version of the combined method Bayer-sintering (Liner A.I. Production of alumina / A.I. Liner, Eremin N.I., Liner Yu.A., Pevzner I.Z. - M: "Metallurgy" . - 1978. - S. 268-271), based on the fact that the autoclave slurry, rich in Al ₂ O ₃ and Na ₂ O, is sintered in a mixture with limestone and soda. The siliceous aluminate solution from sinter leaching is mixed with a diluted solution from an autoclave cooker for joint wringing, the red soda from the evaporation of the mother liquor is mixed with sludge before sintering.

The disadvantages of this method are the high capital costs per 1 ton of alumina, the composition of the red mud sometimes makes it difficult to sinter the mixture prepared from it, while bauxite with a high chromium content is sintered, chromium is oxidized during sintering and contaminated with water-soluble compounds of technological solutions, air and red mud.

A known method of processing high-silicon bauxite (Study of the material composition and enrichment of bauxite of the North Onega and Central Timan deposits. / Algebraistova N.K., Filenkova N.V., Markova S.A., Groo E.A., Kondratieva A.A. ., Sviridov LI, Shepelev II // Collection of reports of the II International Congress "Non-ferrous metals - 2010". - Krasnoyarsk. - September 2-4, 2010. - pp. 43-45), based on extraction of silicon dioxide and aluminum from them using microbiological leaching and subsequent selective separation of alumina from liquid th phase.

The disadvantages of this method are the length of the process and the need to use bulky equipment.

A known method of producing alumina by co-processing of bauxite with nepheline raw materials by sintering (Vinogradov S.A. Technology for the joint processing of nepheline and bauxite // Notes of the Mining Institute. - SPb .: SPGTI, 2007, T. 170, p. 153-155), based on the fact that the addition of 14% bauxite to nepheline raw materials can increase the alumina content in the cake to the current level (~ 15%), thereby allowing the extraction of alumina and alkalis to reach about 90%.

The disadvantage of this method is that the presence of the sintering stage of a two-component mixture will inevitably lead to the formation of water-soluble sodium chromates. In addition, the proportion involved in the processing of bauxite by this method is small.

A known method of producing alumina by the sintering method, (Liner A.I. Production of alumina / A.I. Liner., Eremin N.I., Liner Yu.A., Pevzner I.Z. - M.: Metallurgy. - 1978. - S. 184-263), adopted as a prototype, according to which bauxite is subjected to crushing and subsequent wet grinding together with limestone on one stripped off reverse soda solution. Soda is additionally dosed into the pulp and, after adjustment, the finished pulp is sintered. The cake is leached with water to obtain aluminate solutions from which aluminum hydroxide is recovered.

The disadvantage of this method is that during the processing of chromium-containing bauxite by sintering, chromium is oxidized during sintering and contaminated with water-soluble compounds of technological solutions, air and red mud. This is quite natural, since during chromite processing, oxidative firing is also carried out at 1100-1200 ° С with soda and dolomite. Compounds Cr (VI) have a local and general toxic effect, causing damage to the respiratory system, skin, mucous membranes, and the gastrointestinal tract.

The technical result of the invention is the partial removal of chromium-containing minerals from bauxite, followed by their possible conversion to sodium chromate.

The technical result is achieved by the fact that bauxite separately from limestone is subjected to wet grinding on a stripped off reverse soda solution with a volume ratio of W: T = 3: 1 to obtain pulp with particles smaller than 0.05 mm, then the pulp is subjected to magnetic separation to obtain magnetic and non-magnetic products, then a magnetic product with a chromium (III) oxide content of 25 to 30% is sent for processing to sodium chromate, and a non-magnetic product, together with crushed limestone and fresh soda, is sent for correction you, after which the mixture is sintered, the obtained cake is leached with washing water to obtain aluminate solutions, from which aluminum hydroxide is extracted, aluminum hydroxide is filtered, washed and sent to calcination.

The method is illustrated by the following figures:

FIG. 1 - a scheme for producing alumina from chromium-containing bauxite;

FIG. 2 - material composition of chromium-containing bauxites of the Iksinsky deposit;

FIG. 3 - distribution of chromium (III) oxide over bauxite minerals;

FIG. 4 - the results of magnetic separation of chromium-containing bauxite,

FIG. 5 - dependence of the extraction of chromium oxide (III) in a magnetic product on its output and particle size;

FIG. 6 - indicators of the criterion of the effectiveness of magnetic separation depending on the size of the fraction;

The method is as follows.

The starting chromium-containing bauxite is crushed in a jaw crusher. Next, crushed bauxite is fed together with a circulating soda solution from carbonization to a wet grinding ball mill with a volumetric ratio of liquid to solid (L: T) equal to 3: 1 in order to obtain a fraction of -0.05 mm. Next, the pulp is subjected to magnetic separation in a poly-gradient magnetic separator with a current strength in the coil of the magnetic separator from 6 to 12 A. A magnetic product containing chromium (III) and iron (III) oxides is processed to sodium chromate by a known technology. When the magnetic product yields 6.1-6.2%, the extraction of chromium (III) oxide in it is 25–32.0%, and the decrease in the content of chromium (III) oxide in the nonmagnetic product is 23–28%. A non-magnetic product, together with crushed limestone, fresh soda, white slurry from deoxidation of aluminate solutions and carbonate slurry obtained by causticizing soda solutions, is fed into large containers with air mixing, in which the charge is adjusted. The following molecular ratios must be maintained in the charge:

Na ₂ O: (Al ₂ O ₃ + Fe ₂ O ₃ ) = 1.00 ± 0.05 and CaO: SiO ₂ = 2.00 ± 0.05.

After adjustment, the pulp is pumped through feeding pools to rotary drum sintering furnaces with a diameter of 3 to 5 m and a length of 50 to 100 m. Since the bauxite charge contains a lot of free soda, which leads to pelletizing of the charge during drying, the bauxite-limestone pulp is fed into the tube rotary kiln sprayed through nozzles. During wet sintering, the sprayed droplets of the suspension are dried in a gas stream, the resulting dry granules fall on a layer of dry material and then move when the furnace is rotated to a zone of higher temperatures. The temperature of the formation of cakes 1150-1250 ° C. Spec is cooled in rotating drum refrigerators from 1000-1100 ° С to ~ 100 ° С. After cooling, the cakes are leached, and the exhaust gases from the sintering furnaces are cleaned of dust in a system of sequentially arranged dust chambers, cyclones and electrostatic precipitators. The sludge after leaching of bauxite cakes is washed with hot water, which is then used to leach cakes, after which the sludge is sent to the dump. To ensure the stability of the solutions, caustic alkali is introduced into the process as part of a circulating soda-alkaline solution. The aluminate solution is sent to desiliconization. Desiliconization is carried out in two stages. At the first stage, the solution is kept in autoclaves at 150-170 ° С for 1.5-2 hours, as a result of which silica precipitates in the form of sodium hydroaluminosilicate of the composition Na ₂ O⋅Al ₂ O ₃ ⋅l, 7SiO ₂ ⋅ 2H ₂ O, in the solution remains 0.4-0.5 g / l SiO ₂ . The thickened and filtered sludge from the first stage of desalination is sent to the head of the process for the preparation of the charge. The second stage of desalination is carried out in mixers with the addition of milk of lime at a temperature of 90-95 ° C for 1.5-2 hours. The milk of lime is dosed in an amount of 3-5 g / l CaO _act , which corresponds to the ratio by weight of CaO: SiO ₂ = 20 ÷ 40. As a result of the interactions formed, a poorly soluble calcium hydroaluminosilicate (hydrogarnet) of the composition 3CaO⋅Al ₂ O ₃ ⋅mSiO ₂ ⋅ (6-2m) H ₂ O, where m = 0.1 ÷ 0.4. After the second stage of desiliconization, the solution is separated from the sludge, filtered and sent for decomposition. The thickened and filtered sludge from the second stage of desalination is subjected to soda treatment to regenerate alumina and obtain an alkaline solution used to leach cake. Soda processing of sludge is carried out in two stages. About 50% of the carbonate sludge after the first treatment is used instead of lime in the second stage of desalination, the rest of the sludge after the second soda treatment is sent to baking and sintering. Soda-alkaline solution with a caustic module of 2.5-3.0, obtained by soda treatment, is used in the leaching of cakes as a source of caustic alkali. Sludge after the second stage of desiliconization can be used as a seed in the first and second stages of desiliconization, and then returned to the preparation of the charge. After deep desiliconization, the aluminate solution undergoes carbonization to a residual content of ~ 5 g / l Al ₂ O _{3 in it} . Carbonization is carried out in the presence of seed aluminum hydroxide, the amount of which is approximately equal to the content of aluminum hydroxide in the aluminate solution. After carbonization, the pulp is separated on hydrocyclones. The lower hydrocyclone product, containing mainly a large fraction of aluminum hydroxide, is filtered and washed. The washed and filtered aluminum hydroxide is directed to calcination (calcination) to obtain production alumina. The top drain is thickened, the thickened pulp containing a small fraction of aluminum hydroxide is used as a seed for carbonization. The mother liquors after carbonization are negotiable. They are subjected to control filtration and evaporation, after which they are used for magnetic separation of the initial bauxite. Part of the mother liquor is used for soda treatment of sludge after the second stage of desalination. A scheme for producing alumina from chromium-containing bauxite is shown in FIG. one.

The method is illustrated by the following examples.

Example 1

High-silicon chromium-containing boehmite-kaolinite bauxites of the Iksinsky deposit were processed (Fig. 2). Crystal-optical analysis of bauxite gave the following mineralogical composition in descending order: boehmite, kaolinite, gibbsite, zeolite, quartz, mica, calcite, feldspar, tourmaline, and organic residues. Among ore opaque minerals there are minerals of the chrome spinelide group and iron-containing minerals.

X-ray phase analysis established the following mineralogical composition: boehmite, gibbsite, kaolinite, hydromica, chlorite or montmorillonite, anatase, highly dispersed chromite. Minerals are listed in decreasing order of content. The distribution of chromium (III) oxide by minerals was determined using microprobe analysis (Fig. 3). 48% Cr ₂ O ₃ accounted for alumochromite. About 10% Cr ₂ O ₃ is isomorphically included in kaolinite, up to 25% is associated with boehmite and gibbsite, and about 15–17% are concentrated in iron minerals. In addition to aluminochromite, chrompicotite, as well as chromium-containing aluminogetite and magnetite, were found in the initial bauxite. Chromium spinelides in the bulk are non-magnetic or sometimes weakly magnetic. A significant degree of magnetism is observed in species enriched to a large extent with Fe ₂ O ₃ . Magnetite, which in our case contains Cr ₂ O ₃ , in turn, is highly magnetic.

The starting chromium-containing bauxite was crushed in a jaw crusher. Granulometric composition of crushed material:

Next, crushed bauxite together with a circulating soda solution was crushed in a ball mill to a particle size of -0.05 mm with a volume ratio L: T = 3: 1, the pulp was passed through a working cell of a polygradient magnetic separator (the current in the coils of the magnetic separator is 6 A). The magnetic product was washed with water and washed off after demagnetization. The pulp of the obtained products was filtered, dried, its yield was determined, and then subjected to chemical analysis.

The method made it possible to extract 25.41% of chromium (III) oxide into the magnetic product in an amount of 6.1% and, accordingly, reduce its content in a non-magnetic product by 23.33% (Fig. 4). Limestone and fresh soda are added to the non-magnetic product, and the mixture is processed into alumina by sintering. The magnetic product can be processed to the desired chromium-containing product Na ₂ CrO ₄ .

It was experimentally established that the efficiency of magnetic separation decreases with a particle size of the pulp of more than 0.05 mm (Fig. 5). Magnetic separation of the fraction with particles with a particle size of -2 mm does not allow satisfactory extraction of Cr ₂ O ₃ and Fe ₂ O ₃ into the magnetic purge even with an increase in the separator current, which indicates insufficient disclosure of minerals at this size. Increasing the efficiency of magnetic separation contributes to finer grinding.

The criterion for the effectiveness of magnetic separation is the ratio of the extraction of chromium oxide (III) into the magnetic product (ε) to the output of the magnetic product (γ), numerically equal to the slope of the approximating dependence on the abscissa axis line or the angle itself (Fig. 6).

Example 2

Chromium-containing bauxite, the chemical and mineralogical composition of which is shown in FIG. 2 and 3, they were preliminarily crushed to a particle size of less than 0.05 mm together with one stripped off reverse soda solution with a volume ratio L: T = 3: 1, the pulp was subjected to magnetic separation at a current strength of 9 A in the coils of a magnetic separator, and limestone was added to the non-magnetic product and fresh soda, and the mixture is processed into alumina by sintering. The magnetic product can be processed according to known technology to the target chromium-containing product Na ₂ CrO ₄ .

With this current strength, the method allows to extract 25.25% of chromium (III) oxide into the magnetic product and, accordingly, reduce its content in the non-magnetic product by 23.33%, the magnetic product yield is 6.16% (Fig. 4).

Example 3

Chromium-containing bauxite (Figs. 2 and 3), crushed on an evaporated reverse soda solution (volume ratio W: T = 3: 1) to a particle size of less than 0.05 mm, undergo magnetic separation (current strength in the coils of the magnetic separator 12 A). Limestone and fresh soda are added to the non-magnetic product, and the mixture is processed into alumina by sintering. The magnetic product can be processed according to known technology to the target chromium-containing product Na ₂ CrO ₄ .

When the current strength in the coils of the magnetic separator 12 A, the method allows you to extract 32.03% of chromium (III) oxide into the magnetic product in an amount of 6.2% and, accordingly, reduce its content in a non-magnetic product by 28.33% (Fig. . four).

The advantage of this method compared with the method adopted for the prototype is that the preliminary magnetic separation of the chromium-containing bauxite crushed on an evaporated soda solution reduces the environmental load on the environment by extracting part of the chromium compounds, as well as increasing the complexity of the processing of low-quality bauxite due to the processing of chromium compounds to the target product - sodium chromate.

Claims (1)

A method of producing alumina from chromium-containing bauxite, including wet sintering of the mixture, leaching the cake with washing water to obtain aluminate solutions, from which aluminum hydroxide is recovered, characterized in that bauxite is separately milled from limestone by wet grinding on an evaporated soda solution with a volume ratio W: T = 3: 1 to obtain pulp with particles smaller than 0.05 mm, then the pulp is subjected to magnetic separation to obtain magnetic and non-magnetic products, then a magnetic product containing 25–30% of it is sent for processing to sodium chromate for processing, and the non-magnetic product, together with crushed limestone and fresh soda, is sent to adjust the charge, after which the charge is sintered, the resulting cake is leached with wash water to obtain aluminate solutions, from which aluminum hydroxide is recovered, aluminum hydroxide is filtered, washed and sent for calcination.

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