HU210196B - Process for producing coarse-grained awmina hydrate with<10% having a particle size < 45um - Google Patents

Abstract

The present invention provides an improved process for producing coarse-grained alumina hydrate containing less than 10 µm particles of less than 10% by weight of caustic Na2O at a concentration of 120 g / dm3, preferably 130-150 g / dm 3, with a two-stage mixing of Bayer sodium aluminate alkali. in which a product hydrate is prepared from the sodium aluminate solution by mixing in the first step by mixing the product hydrate, the second stage by mixing the vaccine, and then spreading the vaccine. The scope of the description is 8 pages (including 1 sheet) and partially or completely recycled into the process. and characterized in that, in the first step, the mixing is carried out at 470-600 g / dm3 coarse particle fractions of less than 45 μπι, containing 10 to 30% by weight of alumina hydrate vaccine for 6-12 hours at 7085 ° C, then the hydrate slurry in a manner known per se, preferably by filtration by separating the product hydrate, in which the proportion of particles with a particle size of less than 45 μιη is less than 10% and is fed to the remaining incomplete stairs where 450-

Description

950 g / dm ³ fine fractions of less than 45 μπι are mixed with 30-60 wt% alumina hydrate at a temperature of 45-55 ° C for 40-50 hours, and the resulting hydrate slurry is divided into two fractions in the first fraction. the fine fraction is recycled to the second stage.

The present invention relates to a process for the preparation of coarse alumina hydrate from Bayer's alumina liquors having a higher circular efficiency, at a concentration greater than 120 g / dm ^{3 of} caustic Na ₂ O, by at least two-stage mixing.

About 90% of the world's alumina production is produced by the Bayer process. In this process, aluminum-containing bauxites are dissolved in a solution containing sodium hydroxide, depending on the bauxite grades, either under atmospheric conditions or under pressure. The resulting metastable sodium aluminate solution is separated from the alkali-insoluble residue, red mud, by settling. The aluminum content of the supersaturated solution in the presence of a large amount of alumina hydrate in the form of Al ₂ O-3H ₂ O (alumina trihydrate, hereinafter referred to as "alumina hydrate") is a crystallize during mixing. At the end of the agitation, the alumina hydrate is separated from the mother liquor by filtration, part of which is returned to the agitation process as an inoculum and part of the alumina is calcined anhydrous A ₂ O ₃ after washing at a temperature of 1000-1200 ° C. The mother liquor, after evaporation, is recycled to the bauxite digestion operation.

When mixing, two basic requirements must be met on the one hand to crystallize as much alumina solution per unit volume of sodium aluminate solution (hereinafter referred to as "aluminum liquor") so as to recycle the bauxite to the lower alumina after evaporation , thereby increasing the efficiency of the alumina production cycle, on the one hand, and producing a physical alumina product that meets the requirements of aluminum metallurgy. Physical properties include granulometry of the product, dusting tendency, flow properties, specific surface area, etc. valid.

Until the 1970s, there were two basic technological variants of mixing. The so-called. In the US Bayer process, thymol hydrate was crystallized from a low concentration of caustic Na ² O, ca. 90 µg / dm ³ , at a relatively high temperature of 60-80 ° C. With this technology, coarse-grained, so-called. a sandy product is formed in which the weight ratio of particles smaller than 45 microns is less than 10% by weight of the total product. However, productivity in this case is rather low, ca. 40-50 kg solution of Al2O3 / m ³ . The other technological variant is the so called. European Bayer process, where a more concentrated solution of 130-160 d / dm ^{3 of} caustic Na ² O in the temperature range 45-60 ° C is much higher. 70-80 kg solution of Al ₂ O ₃ / m ^{3 with} a productivity of fine-grained so-called they produce flour-type alumina. Particles smaller than 45 microns are typical, with a weight ratio of about 40-60%.

In the 1970s, however, the environmental and occupational health standards of aluminum metallurgical engineering became stricter, introducing new types of tub systems and extending the use of dry gas cleaning and, as a result of dust reduction, the use of aluminum smelters. sandy alumina. The consequence of this was that each alumina factory sought to develop a new mixing technology that combines the benefits of Bayer technologies in the US and Europe, producing coarse-grained alumina hydrates with relatively high productivity.

Recently, a number of procedures have been known to modify this mixing operation in addition to this objective.

so e.g. No. 2,234,559. U.S. Pat. No. 4,123,195 discloses a solution in which the mixing process is divided into two distinct stages. The first, the so-called. In the agglomeration stage, at a temperature of 77-66 ° C, the fine alumina hydrate formed in the previous mixing cycle is converted into larger crystalline aggregates by agglomeration. The essence of agglomeration is to cement the crystals adhering by physical flocculation to freshly precipitated aluminum hydroxide from the supersaturated solution. The appropriate agglomeration condition is that the solution is saturated in g / dm ^{3 with} Al ² O ³ and the ratio of inoculant surfaces expressed in m ² to 1 dm ^{3 is} 7 to 25. After agglomeration for 6-8 hours, the hydrate slurry is cooled and a further 130-400 g / dm ³ coarse alum hydrate is added and stirred in the second stage for about 30-40 hours. It is classified as coarse seed hydrate and fine seed hydrate. The product of less than 45 micron grain fraction weight ratio of up to 15%, with productivity ₃ Al2O 70-80 kg / m ³ of solution.

The main disadvantage of this procedure is its limited applicability. The agglomeration can only be achieved in highly supersaturated solutions with good efficiency, which, in turn, can only be achieved by the so-called tropical origin. gibbsites (A1 ₂ O ₃ -3H ₂ O), available with bauxites. Due to the lower solubility of these bauxite grades, the required solution supersaturation cannot be achieved in the monohydrate (A1 ₂ O ₃ -H ₂ O), boehmite and diasporic ore processing plants.

HU 210 196 Β

No. 3,324,378. German Patent No. 4,198,123 discloses a different solution. According to this, the production of coarse alumina hydrate with high productivity in a single step is achieved by using an unusually large amount of seed crystals of 7001500 g / dm ³ . At the end of the mixing, an aliquot of the dense hydrate slurry is passed to a classifier and the coarse fraction obtained there forms the product. The remainder of the hydrate slurry is filtered and alumina hydrate is recycled as a inoculum, and the mother liquor, after evaporation, for digestion. This procedure makes the reproduction of extremely large quantities of vaccine quite expensive and, in addition, requires the installation and operation of a large filter park.

No. 4,511,542. Another patent describes a different process in which the fine-particle is agglomerated at a temperature of 68 ° C or higher for 1.5-6 hours at a very low temperature (1-3 m ² inoculum area / dm ³ solution) in the first stage of mixing. over time. In the second phase of mixing, the solid content of the suspension is increased from 30-40 g / dm ³ after the first phase to 250-700 g / dm ³ after the first phase by accumulating alumina hydrate. The temperature in the second phase tanks is maintained at 55-40 ° C. The hydrate slurry taken at the end of the mixing system is classified as product hydrate and fine inoculum hydrate, and after washing the fine inoculum is recycled to the first stage of the process. The weight ratio of the fraction less than 45 microns in the product is max. 5%. The available solution productivity is 80 kg Al ₂ O ₃ / m ³ solution.

The disadvantage of this process is both the cost of washing the inoculum hydrate as well as the fact that the second phase requires specialized tanks.

The object of the present invention was to provide a blending technology that allows the production of coarse alumina hydrate from any bauxite feedstock while further increasing the efficiency of the blending operation and thus of the Bayer process in a cost effective manner.

The present invention is based on the fundamental discovery made in studying the degradation of sodium aluminate solutions that, during the initial mixing phase, during the first 6-10 hours at a temperature above 70 ° C in the presence of a large amount of inoculum, nucleation is minimized and crystal growth is the dominant process. At this stage of the mixing, the structural organization of the solution, due to its rapid precipitation, only reaches the formation of hexameric aluminate ions, which, when deposited on the surface of the inoculum, results in significant crystal growth. The formation of new crystals at this stage is minimal, since the ordering of the aluminate complexes does not take place until colloid-sized associations are formed. Thus, there are different mechanisms involved in the two stages of mixing and their speed can be influenced by different means. It follows that the optimum of each step is different and therefore the technological ways of achieving them are different. Based on this fundamentally new discovery, we have come to the inventive idea of dividing the mixing operation into two steps such that in the first step it is fed back from the second step at a temperature favorable for the formation of coarse alumina hydrate, i.e. above about 70 ° C, preferably 70-85 ° C. with a relatively large amount of coarse-grained inoculum having a weight fraction of granule fractions less than 45 µm, producing a total product hydrate with crystal growth and non-agglomeration without striving for maximum efficiency at this stage, while maximizing the efficiency of the entire mixing process in the second stage, if, after cooling, the less saturated alumina liquor from the first stage is used to produce, in part, coarse-grained particles for use in the first stage and partly for use in the second stage, a more granular vaccine in which the weight fraction of the granule fractions less than 45 µm is 30-60%. Thus, according to the basic idea of the invention, in the first step, optimum conditions for coarse-grained product hydrate production are provided, whereas in the second step, optimum conditions for maximum mixing and thus maximum productivity are provided.

The present invention relates to a process for the production of coarse-grained alumina hydrate containing up to 10% by weight of alumina hydrate having a particle size of less than 45 µm, with a higher caustic Na2O concentration of 120 g / dm ³ , preferably 130-150 g / dm ³ caustic Na 2 O of the solution to produce a product hydrate in the first stage by mixing with the inoculum, then inoculating with the second stage inoculating the inoculum, and then partially classifying and recycle the inoculum, characterized in that the first stage is mixed at 470-600 g / dm ³ Fractions having a particle size of less than about 10-30% by weight are made with alumina hydrate containing 10-30% by weight for 6-12 hours at 7085 ° C, and the hydrate slurry is isolated in a manner known per se, preferably by filtration, less than 10 and the remaining sodium aluminate solution is cooled to 5055 ° C and introduced into the second stage, whereby 45950 g / dm ^{3 of} fine alumina hydrate vaccine containing 30-60% by weight of fractions having a particle size of less than 45 µm are added. After stirring at 55 ° C for 40-50 hours, the resulting hydrate slurry is divided into two fractions, the coarse fraction being recycled to the first stage and the fine fraction being recycled to the second stage.

It may be a characteristic of our process that some of the coarse fraction from the second stage is recycled as the vaccine in the first stage and the other part is directly utilized as the product hydrate.

Another advantageous feature of the invention may be that from the second stage some of the fine particles are recycled to the second stage as a vaccine and the remainder converted into coarse hydrate by agglomeration.

HU 210 196 Β

In another embodiment of the process, the first-stage inoculant is preferably 30-50% by weight of the product hydrate produced.

A further feature of our process may be that at least one, but preferably both, of the mixing steps consist of a series of inoculated tanks and add the total coarse fraction vaccine and the alumina slurry to the first tank of the first stage and feed the filtrate from the first stage. , both the filtrate of the product hydrate from the first stage, the fresh saturated aluminate alkali and the total fine fraction of the inoculum.

The process may also be carried out by taking the total amount of product hydrate from the bottom of the last stage of the first stage and the product to be graded from the last stage of the second stage.

Coarse-grained alumina hydrate, as used herein, includes the product hydrate having a fraction below 45 microns of not more than 10% by weight. However, the coarse-grained product used as the vaccine may be different and its fraction below 45 microns may exceed 10% by weight, e.g. It may be between 10 and 30% by weight. Alumina hydrate as the seed crystal to the unit volume of solution over extinguishing Relative mean weight ratio of A1 ₂ O ₃ content of A1 ₂ O ₃ content and supplied to the mixing solution.

The process according to the invention is fundamentally different from CA 1098 284/82. A two-stage mixing operation as described in Canadian Patent No. 5, the first step of which is to produce the product hydrate and the second step to produce a vaccine. This solution is based on the addition of the fresh, saturated aluminate solution to each unit of the first mixing line separately, i.e. in parallel, at the same time feeding the vaccine only in the first unit and at a lower seeding ratio of less than 1, with a saturated solution. It does not put weight on the coarse or fine particle size of the first stage vaccine. Although it also produces a vaccine in the second stage, it does not contain sufficient cooling prior to the stage to provide optimum mixing conditions, so that the overall mixing productivity is relatively low, with only 72 kg of Al ₂ O ₃ / m ³ solution.

According to the process of the invention, however, in a first step favorable to crystalline growth, the product hydrate is prepared after lowering the temperature of the alumina liquor from the digestion to 70-85 ° C, if necessary, depending on the temperature of the alumina to be digested. after about 6-12 hours, it is separated from the solution by filtration and after washing it is led to calcination. Preferably, the first mixing step comprises a plurality of series connected tanks, the first of which is fed with both the aluminum liquor to be mixed and the vaccine. Although this coarse-grained alumina hydrate is referred to as an inoculum, its granulometry justifies it being considered as a precursor. Product hydrate is removed from the last container in the queue. Cooling between the individual tanks is generally not necessary, or preferably not much.

The partially mixed alumina solution containing reduced Al ₂ O ₃ , i.e. the product alumina hydrate filtrate, and the unused alumina slurry are transferred from the first stage to a second mixing stage after significant cooling. This cooling should be maximized to increase agitation efficiency. This is limited by the temperature at which the particle size distribution is maintained, that is, that the resulting hydrate particles are of such a size that they do not have to be screened. It has been found that, if it is expedient to provide a cooling to 50-55 ° C, preferably by means of plate heat exchangers, the optimum temperature during the second stage mixing can be achieved. This cooling may be carried out in several stages. The alumina slurry and / or slurry fed to the second stage is stirred for about 40-50 hours in the presence of 2-4 seeding fine seed crystals having a weight fraction of less than 45 μπι, 30-60% by weight, for about 40-50 hours, preferably at 52-47 ° C, to ensure maximum mixing efficiency. Since the mixing conditions in this second stage are not optimal for the formation of coarse alumina hydrate particles, this second stage crystallizes an alumina hydrate consisting of a set of coarse and fine grains.

Therefore, the alumina hydrate formed in the second mixing step is subjected to classification. The coarse alumina hydrate obtained from the first removal of the classifier is filtered and recycled to the first mixing step as a precursor precursor. The fine-grained alumina hydrate leaving the classifier, i.e. the product having a particle size of less than 45 microns, is returned to the second mixing stage as an inoculum after filtration.

If either the amount of coarse grains produced or the fines is more than what we need as a vaccine, then this excess is not returned to the system, but this part of coarse alumina hydrate is treated as a product after washing and filtration. The excess fine particles, for example. agglomeration can be grown into coarse-grained sets in a third step and used accordingly.

The second mixing step may also consist of a series of connected units into which both the material from the first stage and the vaccine are fed into the first unit and the product passing through the screen is removed from the last unit. Incidentally, such mixing lines and alumina hydrate grades are known per se.

An important and novel element of the process according to the invention is therefore that the amount of feedstock precursor, which is also in the first stage as the inoculum, is about 30-50% by weight of the alumina hydrate produced and that the coarse-grained product hydrate from the first mixing step.

EN 210 196 Β to ensure that the product hydrate has the correct granulometry. In turn, the second mixing step is conducted with a fine alumina, high inoculum volume, and low temperature, such that a high alumina hydrate yield of about 80-90 kg is obtained. A solution of Al ₂ O ₃ / m ³ is achieved.

The process is explained in more detail using the technological scheme of Figure 1.

The saturated sodium aluminate solution obtained after settling the red slurry is cooled to 70-85 ° C on the plate heat exchangers (1). A sodium aluminate solution is used which has a caustic Na ₂ O concentration of 120 to 150 g / dm ^3, preferably between 130 to 150 g / dm ^3, the caustic Na2O / Al2O3 mole ratio in the solution is from 1.4 to 1.6, preferably 1, 45-1.55. To the solution is added a quantity of 40 to 60 kg of Al2O3 coarse 1030 wt% inoculum (prodrug) per m ^{3 in a} slurry tank under filter (2). The hydrate slurry thus obtained is introduced into the first container (3) of a first mixing step of a preferably multiple tank. The first step is the so-called. the amount of seed crystal in the solid retention tanks is greater than the seed 2 ratio. A solution with a solids content of less than 50 g / l and a hydrate suspension with a solids content of 500-1000 g / l at the bottom of the tanks are transferred to the next vessel at the upper overflows of the series connected tanks. Last (4) of the first container kikeverősor principle colorless bottom product is taken off in an amount of 80-90 kg Al2O ₃ / m ³ of solution which was filtered to termékszűrőn (5). The product alumina hydrate is subjected to washing followed by calcination. The filtrate in the tank (6) is combined with the overflow of the last mixing tank on the plate heat exchangers (7) cooled to 55-45 ° C and then in a slurry of filter (8) in an amount of fine 30 to 60% by weight. seed crystals containing a fraction below pm are added. The slurry thus obtained is fed into the first container (9) of a second mixing row, preferably of several series connected. From the last reservoir (10) of the second stage, the hydrate slurry is fed to the classifier (11). After sorting, the coarse precursor is filtered through the filter (2) and the fine product through the filter (8). After filtration, the mother liquor is recycled to the evaporation and after evaporation to the digestion, while the coarse precursor is recycled to the first stage inoculum and the fine product to the second stage inoculum.

The process of the present invention will be described in more detail with reference to the following example:

Example

A saturated sodium aluminate solution of 135 g / dm ³ kNa ² O and a caustic molar ratio (kNa 2 O / Al ² O ³ ) of red sludge settler was cooled to 76 ° C on a plate heat exchanger. The cooled aluminate slurry is fed to the filter (2) of Fig. 1 (516 m ³ / h), where 50 t of a 11 wt% alumina hydrate precursor is added continuously to the solution. The precursor feedstock contains 26% by weight of the alumina hydrate granule fraction of less than 45µm and 74% by weight of 45-120µm. The hydrate slurry thus obtained is fed to the first tank (3) of a first mixing step consisting of 4 1000 m ³ tanks. The slurry temperature is 75 ° C in the first mixing tank and 73 ° C in the fourth mixing tank. The average solids concentration in the first stage tanks corresponds to a seed ratio of 3 (530 g / dm ³ slurry). From the lower take-off of the last tank of the first mixing step, a hydrate slurry of 103 m ³ / h solids containing 670 g / dm ^{3 is} led to the product filter, while the partially mixed alumina slurry containing solids is discharged into the buffer tank 6 cooled to 53 ° C on a plate heat exchanger. The first stage of caustic mole ratio mixed liquor leaving the partially mixed amount of 2.2 at the first stage of alumina hydrate Al2O ₃ 47 kg / m ³ solution, taken from the total solid termékhidrátként 87 kg Al ₂ O ₃ / dm ³ solution. The fraction less than 45 µm in the product has a proportion of 7.5% by weight.

To the partially stirred alumina lye cooled to 53 ° C, a fine inoculum of 335 t / h containing 56% by weight of a fine particle of less than 45 µm and 44% by weight of a coarse fraction of 45 µm is continuously added to the slurry pan of the vaccine filter (8). The hydrate slurry is introduced into the first tank of the second mixing step and then to the subsequent tanks connected in series. The temperature of the hydrate slurry entering the second mixing stage is 52 ° C and the temperature of the hydrate slurry leaving the mixing line is 47 ° C. The caustic molar ratio of the alkaline phase is 3.62 and the amount of alumina hydrate mixed in the second stage corresponds to 40 kg of Al ₂ O ₃ / m ³ solution. The entire amount of hydrate slurry leaving the second mixing step is fed to a screening device (11), the cone product of which after filtration is fed as a precursor to the first product production phase and the overflow is also filtered after the filtration into the second phase. The mother liquor obtained as a filtrate is evaporated.

Of course, the present invention is not limited to the solution described in the example, but may be practiced in many other ways within the scope of the disclosure. .

Claims (6)

PATENT CLAIMS A process for the production of coarse-grained alumina hydrate, containing up to 10% by weight of particles smaller than 45 µm, of Bayer sodium having a caustic Na2O concentration of greater than 120 g / dm ³ , preferably of 130-150 g / dm ³ a two-stage mixing of the aluminate alkali, wherein the sodium aluminate solution is mixed with a vaccine to produce a product hydrate in the first stage, a second stage by mixing with a vaccine, then classifying the vaccine and partially or completely recycling the process to the first stage. HU 210 196 Β 470-600 g / dm ³ coarse fractions having a particle size of less than 45 μιπ are carried out with alumina hydrate containing 10-30% by weight for 6-12 hours at 70-85 ° C, and then the hydrate slurry is isolated by filtration. recovering the product hydrate having less than 10% by weight of the fraction fractions less than 45 μιπ and passing the remaining sodium aluminate solution to a second stage cooled to 50-55 ° C where 450-950 g / dm ^{3 is} fine at 45 µm smaller particle size fractions are mixed by adding alumina hydrate containing 30-60% by weight at 45-55 ° C for 40-50 hours, and the resulting hydrate slurry is divided into two fractions, the coarse fraction being recycled to the second stage. Process according to claim 1, characterized in that part of the coarse fraction from the second stage is recycled to the first stage and the other part is used directly as product hydrate. 3. The process of claim 1, wherein a portion of the fine particles from the second stage is recycled to the second stage as a vaccine and the remainder is converted to coarse-grained hydrate by agglomeration. 4. The process according to any one of claims 1 to 4, wherein the inoculum fed to the first stage is preferably 3050% by weight of the product hydrate produced. 5. A process according to any one of claims 1 to 4, wherein at least one, but preferably both, mixing steps consist of a series of inoculated tanks and add to the first tank of the first stage the total coarse fraction vaccine and to the first tank of the aluminous and second strain product hydrate filtrate, both fresh, saturated aluminate alkali, and total fine fraction inoculum. 6. A process according to any one of claims 1 to 6, wherein the total amount of product hydrate is taken from the bottom of the last stage of the first stage, and the product to be classified is extracted from the last stage of the second stage.