RU2375308C1 - Method of alumina receiving from bauxite - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method of alumina receiving from bauxite by Baeyer process includes washing of aluminium hydrate by hot water from sodium oxalate, desoxalation of part or whole flow of industrial water by lime milk, separation of formed sediment from alkali solution and return of alkali solution into the process after evaporation. Desoxalation is implemented in two stages, herewith at the first stage treatment of industrial water is implemented by formed in the end of desoxalation process sediment in amount not less than 20 g/l of solid, and at the second stage - by lime milk, which is introduced up to finish of desoxalation process from the calculation of molecular ratio CaO_ak/Na₂O_ox=1÷1.1. Separation of formed sediment during the desoxalation process is implemented on chamber filter-press with return of filtrate for evaporation, of sediment - partly on the first stage of desoxalation, and excess is withdrawn from hydrochemical process.

EFFECT: minimisation of lime consumption, introduced into the process, losses of alumina and alkali with formed sediment, reduction of lime consumption in the form of lime milk 2-3 time and reduction of alumina losses in proportion to amount of introduced lime milk at redistribution of industrial water desoxalation.

1 dwg, 1 tbl, 2 ex, 2 cl

Description

The invention relates to the production of alumina from bauxite.

A known method of producing alumina from bauxite (Patent W0 00/75073) according to the Bayer process, in which the alkali-aluminate solution is purified from organic impurities, mainly sodium oxalate, is carried out by means of lime treatment of a part of the evaporated working solution with subsequent phase separation. The clarified solution is mixed with the main circulating stream, which is sent to the leaching of bauxite, and the precipitate is washed with industrial water (sub-sludge water, alkali-containing condensate) in order to decarbonize, and in some cases, re-deoxalization (lime may be added additionally) and re-separation of phases with the return of clarified promvu for washing red mud.

The disadvantages of the method include:

excessive complexity of the circuit due to the need for double phase separation during deoxalization of the circulating solution;

increased secondary loss of alumina in the interaction of lime with a concentrated working solution;

difficulties in separating finely divided sediment from a concentrated circulating solution at the first stage of deoxalization, etc.

A known method (Patent 4335082, USA) for producing alumina from bauxite according to the Bayer process, in which the alkali-aluminate solution is purified from organic impurities, mainly sodium oxalate, is carried out at the hydrate washing stage. The hydrated slurry is treated with lime in an amount excessive with respect to Na ₂ CO ₃ . The resulting solution was evaporated to a concentration of Na ₂ O corresponding to precipitation of calcium oxalate. The precipitate is separated from the solution by filtration or centrifuges and then removed from the system.

The disadvantages of this method include:

- poor filterability of finely divided calcium oxalate formed after evaporation of the solution;

- loss of a strong alkaline-aluminate solution with a precipitate that cannot be washed due to the partial presence of soluble organics;

- the need to build a special repository for the storage of hazardous chemical waste.

A known method (Patent 1390057, France) for producing alumina from bauxite by the Bayer process, in which the seed fine hydrate is washed with hot water from soluble sodium oxalate adsorbed on the surface of fine hydrate particles and a wet cake in the mother liquor. Part or all of the flow of the obtained promotions is treated with milk of lime for the purpose of deoxalization - precipitation of dissolved organics. The precipitate is separated from the solution by filtration and removed from the process, and the filtrate, a slightly alkaline solution, is sent for concentration together with the mother liquor and then returned to the process. In this method, the excess of lime introduced into the dressing will interact not only with sodium oxalate and sodium carbonate, but also with sodium aluminate, converting alumina into precipitate irretrievably lost for production.

The last of the considered methods, as the closest in essence to the claimed, adopted as a prototype.

The objective of the invention is to minimize the consumption of lime introduced into the process, as well as the loss of alumina and alkali with the resulting precipitate, which will reduce the consumption of lime in the form of milk of lime by 2-3 times and reduce the loss of alumina in proportion to the amount of lime milk added to the redistribution of deoxalization promv.

The technical result is achieved by the fact that in the method of producing alumina from bauxite according to the Bayer process, which includes washing aluminum hydroxide with hot water from sodium oxalate, deoxalizing part or all of the industrial wastewater stream with lime milk, separating the precipitate formed from the alkaline solution and returning the alkaline solution to the process after evaporation, desoxalization is carried out in two stages, while at the 1st stage, the treatment of promvods is carried out by the sediment formed at the end of the deoxalization process in an amount of at least 20 g / l of solid, and at the 2nd diium - with milk of lime, which is introduced before the end of the process of deoxalation from the calculation of the molecular ratio CaO _ac / Na ₂ O _ox = 1-1.1.

Separation of the precipitate formed in the process of deoxalization is carried out on chamber filter presses with the return of the filtrate to evaporation, the precipitate is partially in the 1st stage of deoxalization, and the excess is removed from the hydrochemical process.

A schematic diagram of the process of deoxalation is shown in the drawing. According to the scheme, part or all of the flow of promoters in a volume of L ₀ is supplied to the reactor-mixer of the 1st stage of deoxalization, where a part of the sludge S ₃ formed at the end of the process is directed. This precipitate is pre-repulped with hot water in a volume of V ₀ sufficient to dilute the stain to the optimum concentration. A mixture of slurry and sludge L ₁ , S ₂ after a certain exposure in the reactor-mixer at a temperature of 60-80 ° C is sent to the 2nd stage of deoxalization, where lime milk / lime is supplied in an amount of S ₀ . The process ends with the interaction of "fresh" lime and unreacted in the sludge with sodium oxalate and sodium aluminate according to the reactions:

or the formation of other calcium hydrocarbon aluminates.

The diluted suspension of L ₂ , S _{2 is} sent to clarification by settling or filtering. The clarified alkaline solution L _{3 is} evaporated and then used as a working solution for leaching bauxite, and the precipitate is divided into two parts: S _{3 is} wrapped in the 1st stage of deoxalization, S ₄ is removed from the hydrochemical process.

Confirmation of the effectiveness of the proposed method is shown below in the following examples.

Example 1 (Experiments 1-3). A sample taken from the industrial stream of the product from washing aluminum hydroxide obtained from bauxite by the Bayer process in the first three experiments was subjected to one-stage (for comparison) deoxalization. The mode and result of processing, as well as the sequence of operations are shown in the table.

In industrial conditions for lime treatment, as a rule, milk of lime is used, prepared by extinguishing lime with recycled water. Therefore, in our case, based on the material balance, the sample of L ₀ promoters was previously diluted with water in a certain volume of V ₀ to a concentration of Na ₂ O _about ≅30 g / l in the total alkali. Lime S ₀ was introduced in excess with respect to the content of sodium oxalate in terms of Na ₂ O _ox in the promo, in particular in the 1st experiment from the calculation of the molecular ratio (mo.o.) CaO _ac / Na ₂ O _ox ≅ 3.0 . The precipitate S ₁ formed in the process of deoxalation in a mixture with an alkaline solution L _{1 was} filtered off and sent for reuse completely in the first three experiments and then partially. The drawing shows that the part of the filtered precipitate S ₃ returned to deoxalization is repulped with water V ₀ , intended for dilution of the product, and introduced into the reactor in the form of a suspension, where deoxalization is carried out. Excess sediment S4 is removed from the process. In subsequent experiments, the dosage of lime was reduced to a molecular ratio of ~ 1, and the solid content in the processed promvod was brought to 20 g / l.

Table Technological indicators of decontamination of hydrated promises NGZ No. of experiments Processing Conditions (t = 80 ° C) Concentration of promvod, g / l Loss with sediment,% m.o. CaO _ac / Na ₂ O _ox τ _arr. hour With _tv . g / l Na ₂ O ₀ Al ₂ O ₃ Na _{2 Occ} ΔNa ₂ O _oks ΔAl ₂ O ₃ Exodus. end Exodus. end Single Stage Deoxalization one* 3.0 2.5 11.5 30.7 11.66 9.8 1.75 0.91 48 15.9 2 1.05 “ 17.0 31.6 13.09 12,4 1.48 1.37 7.4 5.27 3 1,02 “ 19.1 15,5 6.4 6.2 0.73 0.69 5.5 3,1 Two Stage Deoxalization four 1,07 0 + 2 ∑2.5 19.8 31,0 12.8 12.0 1.46 1.13 23.0 6.2 5 1,02 0 + 1.5 (∑2.5) 21,2 32.6 13.3 12.24 1,53 0.9 41.2 7.9 6 1.06 0 + 1.0 (∑2.5) 23.0 31,2 12.9 12.3 1.47 1,08 26.5 4.6 Note: * In op.1 - only lime was introduced (CaO _ak = 93.9%), in op.2-6 lime + sediment from previous experiments; PCA precipitate - CaO = 41.6%, Al ₂ O ₃ = 14.5%, SPP = 42.3, CO ₂ - 7.8%

X-ray phase composition: mainly Ca - 3CaO · Al ₂ O ₃ · CaCO ₃ · 11H ₂ O hydrocarbon aluminate and then in descending order 3CaO · Al ₂ O ₃ · 6H ₂ O Ca (OH) ₂ , CaCO ₃ , CaC ₂ O ₄ .

As can be seen from the above data, in the 1st experiment, as a result of a single-stage deoxalization at a dosage of lime, respectively, m.o. = 3, a relatively high degree of purification of the product from sodium oxalate or in terms of ΔNa ₂ O _ox = 48% was achieved. However, up to 16% of alumina precipitated. The main reason for such a large loss of Al ₂ O ₃ was excess lime, see equation 2. Therefore, in subsequent experiments, in order to reduce the loss of Al ₂ O ₃ during deoxalation, the dosage of lime was reduced to mo.o. = 1. Under these conditions, while the loss of ΔAl ₂ O _{3 was} reduced to 5-3%, the deoxalation indices sharply worsened: in experiment 2, the degree of purification of ΔNa ₂ O _ox was 7.4%, in experiment 3, 5.5% on a more diluted dressing.

According to the results of the phase composition of sediment S ₄ removed from the process, it can be seen that it contains a certain amount of unreacted lime. Consequently, the tried-and-true single-stage deoxalization was not effective enough.

Example 2 (experiments 4-6). In these experiments, washing L ₀ from washing aluminum hydroxide of the same concentration as in previous experiments was subjected to 2-stage deoxalization according to the scheme shown in the drawing with the introduction of the precipitate S ₃ formed at the end of the process at the 1st stage in an amount of not less than 20 g / l of solid, and at the 2nd stage of So milk of lime, which was introduced 60-90 minutes before the end of the process (total processing time 2.5 hours). The amount of lime was fed based on m.o. CaO _ac / Na _{2 Occ} = 1-1.1, the sludge to be wrapped was sought to be minimized in order to reduce the cost of clarification of the treated product line in the industrial scheme.

The highest rates of promvody degree of purification oxalate ΔNa _{aux 2} O = 41.2% were obtained in the experiment is 5 when lime is injected 90 minutes before the end of the process and largest losses ΔAl ₂ O ₃ = 4.6% in experiment 6 - when entering lime in 60 minutes This interval of lime input at the 2nd stage of deoxalation, equal to 60-90 min, can be considered optimal.

Thus, with fractional injection of reagent sludge reagents at the 1st stage with a flow rate of at least 20 g / l of solid and lime - at the 2nd stage, the degree of deoxalization of the sludge was achieved on average ~ 30%, and alumina losses decreased almost 3 times by Compared with industrial data for single-stage processing of industrial products and dosage of lime, based on the calculation of md = 3. The minimum consumption of sludge to be wrapped provided in experiments 4–6, along with injected lime, the necessary degree of industrial deoxalization. water. An increase in sludge flow rate would complicate its separation, which would lead to an increase in settling / filtering surface.

The resulting precipitate had good sedimentation-filtration properties. The moisture content of the filtered precipitate was in the range of 20-25%. Given these factors and the low solid content in the mixture, which is subject to filtration (L ₂ , S ₂ ), the chamber filter press should be considered the most suitable apparatus for separating the solution from the precipitate. Precipitation removed from such filters will be transportable.

Claims (2)

1. The method of producing alumina from bauxite according to the Bayer process, including washing aluminum hydroxide with hot water from sodium oxalate, deoxalizing part or all of the streaming water with lime milk, separating the precipitate formed from the alkaline solution and returning the alkaline solution to the process after evaporation, characterized in that it is desoxalizing carried out in two stages, at the same time at the first stage the treatment is carried out by the precipitate formed at the end of the deoxalization process in an amount of not less than 20 g / l of solid, and at the second stage by lime mole eye, which is introduced before the end of the process of deoxalation from the calculation of the molecular ratio CaO _ak / Na ₂ O _oks = 1 ÷ 1,1. 2. The method according to claim 1, characterized in that the precipitate formed during deoxalization is separated on chamber filter presses with the filtrate returned to evaporation, the precipitate is partially in the 1st stage of deoxalization, and the excess is removed from the hydrochemical process.

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