AU760072B2 - Method for purifying sodium aluminate liquors containing sodium oxalate - Google Patents

Description

PROCESS FOR PURIFICATION OF SODIUM ALUMINATE LIQUORS CONTAINING SODIUM OXALATE IN ORDER TO INCREASE THE QUANTITY OF ALUMINA HYDRATE PRODUCED Technical domain The invention relates to a process for the purification of sodium aluminate liquors resulting from alkaline digestion of bauxite using the Bayer process and containing sodium oxalate.

State of the art The Bayer process can be used to produce alumina from bauxite ore, and particularly from alumina that will be transformed into aluminum by igneous electrolysis. According to this process, the bauxite ore is treated when hot by means of an aqueous solution of sodium hydroxide with an appropriate concentration, thus extracting the alumina to obtain a slurry containing the pregnant sodium aluminate liquor and insoluble residues. After separation of these residues, the pregnant sodium aluminate liquor, also called Bayer liquor, is precipitated by seeding with particles of recycled aluminum trihydroxide, until grains of aluminum trihydroxide are obtained with the required particle size and physicochemical properties. We will subsequently use the terms "aluminum trihydroxide", "aluminum trihydrate" and "hydrargillite" indifferently. The sodium aluminate liquor depleted in alumina is then recycled to the digestion step after having been concentrated in sodium hydroxide, or caustic soda, to restore the appropriate concentration to digest the ore.

With most bauxite ores used throughout the world, the content of organic compounds produced from the more or less complete degradation of organic materials contained in the ores, in the pregnant sodium aluminate liquor derived from digestion, gradually increases.

These organic compounds, degraded in the form of organic sodium salts and mainly in the form of sodium oxalate, are a hindrance. Oxalates accumulate and quickly reach their critical concentration threshold and precipitate in the form of fine needles over the aluminum hydroxide seed. These fine sodium oxalate needles then act as genuine seeds and cause an uncontrolled and undesirable increase in the number of fine aluminum hydrate particles formed during the crystallization of sodium aluminate.

Thus, precipitation of sodium oxalate affects the quality of the alumina trihydrate produced and in particular causes large variations in the particle size of the alumina produced and embrittlement of the grains that form major, or even unacceptable, disadvantages for the use of this alumina in the production of aluminum by hydrolysis.

Consequently, in industrial alumina production operations, it is necessary to control (or even better prevent) the precipitation of sodium oxalate during the aluminate liquor crystallization step.

Many processes have been proposed to limit the presence of sodium oxalate in solution in Bayer liquors. Thus, experiments have been carried out in processes designed to destroy or directly crystallize organic materials contained in the ore, for example by baking, but they are infrequently used due to their prohibitive cost.

Several processes recommend that the concentration of sodium oxalate during crystallization of the sodium aluminate liquor should be limited to a value less than its critical precipitation concentration, without unacceptably reducing the content of organic materials, these materials having a stabilizing effect on the aluminate liquor. To limit the oxalate concentration, the concentration of at least part of sodium aluminate liquor derived from the crystallization, but already supersaturated in sodium oxalate, is destabilized in order to precipitate and specifically separate sodium oxalate from the then desaturated liquor.

Thus, the process described in US patent 3899571 (EP-A-0013407) is designed to treat a Bayer liquor that is supersaturated with respect to the solubility of sodium oxalate at equilibrium by introducing a recycled sodium oxalate seed. After filtration, the purified liquor is returned into the Bayer cycle whereas a fraction of the solid phase of sodium oxalate is used for preparation of the seed crystals slurry, and the other fraction is eliminated from the cycle. Although this seeding process is efficient to provoke precipitation of sodium oxalate, it has disadvantages when applied industrially. Sodium oxalate crystals forming the seed quickly become inactive because their surface is poisoned by the organic materials present and this seed then has to be washed, which is very difficult in practice. If washing is insufficient, the activity of the seed drops and consequently the precipitation efficiency of sodium oxalate is also reduced. If washing is taken too far, partial extraction occurs and the particle size of the seed is refined making liquid solid separations very difficult, consequently reducing the purification efficiency.

Instead of using a sodium oxalate seed to destabilize the Bayer liquor supersaturated in sodium oxalate, US patent 4597952 (EP-A-0173630) recommends the use of calcium oxalate or barium oxalate seeds, which have an action that indirectly produces the same result. But in this process, it is impossible to avoid the formation of sodium oxalate precipitates finely dispersed in the Bayer liquor and therefore difficult to separate by settlement and/or filtration without the addition of additives. Furthermore, part of the precipitated sodium oxalate must be recycled to regenerate the calcium or barium oxalate seed after eliminating entrained organic materials.

In order to solve the two-fold problem of the separation of precipitated sodium oxalate fines in the sodium aluminate liquor and recycling of oxalate seeds for which regeneration is fairly difficult, the applicant has developed a process (FR-A-2 686 872 EP- A-0 555 163) that consists of destabilizing sodium oxalate in the sodium aluminate liquor from a finely divided lime based seed, therefore with a heterogeneous nature in the oxalate medium, rather than using an oxalate based seed. In the process described in FR-A- 2 736 908 (=EP 0 782 544), the applicant provokes destabilization of the oxalate in the depleted liquor concentrated by evaporation, starting from an insoluble and chemically inert seed such as alpha alumina or hexahydrated tricalcium aluminate. The advantage of this process is the possibility of regenerating and recycling the deoxalation seed under economically acceptable conditions if the bauxite is rich in organic materials, in other words if the content of oxalic carbon exceeds about 400 grams per tonne of alumina produced. When the content is lower, the depreciation of the installations necessary to cool the liquor that has just been concentrated in sodium hydroxide by evaporation, and the installations necessary to recycle and regenerate the seed, and finally the addition of a seed (alumina, hexahydrated tricalcium aluminate, etc.) in order to compensate for losses, increases the cost of this deoxalation process which becomes difficult to justify when the content of organic materials in the bauxite is low.

Statement of the problem Therefore, the objective is to find a less expensive process, useful specifically for processing bauxite with low contents of organic materials. The content of organic materials in the bauxite is characterized by the quantity of oxalate ions generated during its digestion, expressed as the mass of oxalic carbon generated per tonne of alumina produced. The field covered by this invention relates to the treatment of bauxite for which the content of oxalic carbon is less than 400 grams per tonne of alumina produced.

Purpose of the invention The process developed by the applicant is designed firstly to treat bauxite with low contents of organic materials under acceptable economic conditions, and also to increase the quantity of alumina produced. It consists of treating a fairly small proportion of the depleted unconcentrated aluminate liquor (typically 10 to by passing it through a circuit including a series of precipitator tanks at low temperature, the oxalate precipitating on part of the hydrate that is then retrieved after washing, with a quantity slightly greater than the quantity drawn off.

Accordingly, a first aspect of the present invention provides a process for the purification of sodium aluminate liquors resulting from the alkaline digestion of bauxite using the Bayer process and containing sodium oxalate, the said process comprising: a) drawing off an aliquot of the spent aluminate liquor before concentration; b) cooling of this aliquot, mixed in a seed tank with a destabilising agent that comprises alumina trihydrate; c) addition of alumina trihydrate originating from the crystallisation, into the slurry thus formed; d) crystallisation of the mix thus obtained which enters a series of precipitators called a mini-cooling series; e) filtration of a fraction of the slurry output from the last precipitator, the cake output from the filtration being reintroduced into the seed tank as the destabilising agent 20 in step the filtrate being mixed with the aliquot of the spent liquor that had not been drawn off to be concentrated by evaporation or addition of sodium hydroxide and to form 9• the green liquor; ••oo• f) filtration of the other fraction of the slurry output from the last precipitator with displacement of the cake impregnation with cold water, and then washing said cake with hot water; g) final filtration of the washed hydrate slurry.

A second aspect of the present invention provides sodium aluminate liquors when purified according to the process of the first aspect of the present invention above.

It is a process for the purification of sodium aluminate liquors resulting from the alkaline digestion of bauxite using the Bayer process and containing sodium oxalate, the said process comprising: S. drawing off an aliquot of the spent aluminate liquor before concentration; o* b) cooling of this aliquot, mixed in a seed tank with a destabilising agent that omprises alumina trihydrate; [R:\LIBAoso05508.doc:jjp -6ac) addition of alumina trihydrate originating from the decomposition, into the slurry thus formed; d) decomposition of the mix thus obtained which enters a series of precipitators called a mini-cooling series, the number of precipitators depending on the additional quantity of trihydrate that is to be retrieved; e) filtration of a fraction of the slurry output from the last precipitator, the cake output from the filtration being reintroduced into the seed tank as the destabilising agent in step the filtrate being mixed with the aliquot of the spent liquor that had not been drawn off to be @0 0000 0000S 0 0 sees .00.5 4*

S

14 [R:\LIBA]05508.doc:jp concentrated by evaporation or addition of sodium hydroxide and to form the green liquor; f) filtration of the other fraction of the slurry output from the last precipitator with displacement of the cake impregnation with cold water to reduce its soda content, and then washing with hot water. The effect of washing with hot water is to redissolve the oxalate.

g) final filtration of the washed hydrate slurry in order to recover a quantity of hydrate exceeding the quantity drawn off in c) at the end of the precipitation line. The wash water is removed, thus entraining the dissolved oxalate. It may also be partially directed to the precipitation head of the mini-cooling series to accelerate precipitation of the oxalate.

The deoxalation treatment is carried out on an aliquot of the spent aluminate liquor, preferably between 10 and 30% of this liquor, drawn off before reconcentration. Reconcentration is usually done by heating and evaporation, and it is advantageous to draw off before reheating, since less calories will have to be removed for cooling in step b).

At the end of crystallization, the temperature of the liquor is usually between 65 0 C and 50 0 C. It is cooled down to a temperature of between 400C and preferably 400C. It is possible to cool it to a lower temperature, but this would be more expensive and there is no benefit. Deoxalation is not more complete and more powerful cooling units would be necessary, requiring an investment that cannot be justified considering the expected gain in trihydrate production.

The cooled aliquot is mixed in a seed tank with a destabilizing agent that comprises alumina trihydrate originating particularly from the trihydrate cake, and precipitated oxalate originating from filtration of the overflow from the last precipitator in the mini-cooling series described in The slurry thus formed is mixed with alumina trihydrate drawn off at the end of the precipitation line, in other words with particle size characteristics corresponding to the target characteristics for the final product. In practice, it is better to take the slurry sample from the overflow from the last precipitator.

Since the rate of oxalate precipitation is significantly faster than the rate of trihydrate precipitation, trihydrate precipitates as soon as cooling starts by creating seeds or by being deposited on recycled hydrate and oxalate grains, the complete mix forming a number of fine particles that are "lost" in a very large number of much larger particles due to the addition (described in of part of the production trihydrate.

The mix then passes through a series of precipitators that we will call the crystallization "mini-cooling series" in which the aluminate liquor continues to crystallize. The number of tanks in this mini-cooling series depends on the additional quantity of trihydrate that is to be obtained. However, this quantity is limited since the liquor is already strongly depleted. However, the applicant has observed that under these particular crystallization conditions, the precipitated oxalate does not have any effect on the productivity in alumina; oxalate needles are deposited on hydrate grains, but this has no effect on the active surface area of the seed.

In one preferred embodiment of the invention, this slurry is separated into two parts at the output from the last precipitator tank, in order firstly to maintain a content of dry materials of between 400 and 800 grams per liter of slurry in the mini-cooling series, and secondly to evacuate the hydrargillite added and produced during crystallization of the minicooling series. Thus, for example the overflow and the underflow are taken off at two points, the slurry being regularly stirred in the last tank.

The overflow from the last precipitator is filtered. The filtrate is transferred to the spent aluminate liquor in the Bayer circuit, before concentration to form the green liquor. The cake is a mix of precipitated hydrate and oxalate particles that is added back at the beginning of the mini-cooling series as a crystallization seed.

The precipitation of oxalate in the form of fine needles causes filtration problems. These difficulties are characterized by the specific resistance of the filtration cake, expressed in m/kg. Thus, the specific resistance of the cake observed during the seed filtration in step e) is of the order of 1.2 1010 m/kg.

In order to solve this problem, the applicant had the idea of testing additives used in the crystallization step during the past, and which encourage a global increase in the grain size, due to their inhibiting role in the formation of seeds.

Surprisingly, it is found that most of these additives appear to affect the precipitation of oxalate and significantly reduce the specific resistance of the filtration cake.

These additives, used in the past during crystallization because they encourage the agglomeration of fines, comprise a surfactant and usually an oily solvent of the said surfactant. The surfactant is an organic compound chosen among oleic and stearic acids, lauryl sulfates, alkyl-aminobutyric acids, sulfonates and polymers with an alkyl chain containing at least ten atoms and comprising at least one of the following functions: carboxyl, ester (preferably ester sulfates), phenol, acrylate (preferably methacrylic acid and methacrylate stearyl copolymers), acrylamide and hydroxamate.

For example, the Crystal Growth Modifier (CGM) marketed by the NALCO CHEMICALS Company, a product designed to encourage the enlargement of trihydrate grains during crystallization and described in patent US 4 737 352, very much improves the filterability of the oxalate rich slurry at the seed filter when it is added at any point in the mini-cooling series. In the example described below, the specific resistance of the filtration cake was divided by 4 due to the addition of CGM. The improved filterability of the oxalate rich slurry can significantly reduce the size of installations that use the process according to the invention, and particularly the size of the seed filtration device.

Hydrate originating from the filtration of the overflow from the last precipitator is washed, preferably simply by displacing the impregnation in a belt filter using cold water, in order to reduce the soda content retained in the trihydrate. The displaced impregnation is returned to the Bayer circuit and added to the spent aluminate liquor before reconcentration.

The aqueous slurry obtained after cold washing is then washed with hot water in order to dissolve the oxalate precipitated on trihydrate grains. Preferably, the wash water is partially evacuated, entraining the dissolved oxalate with it, the other part being returned to the beginning of the mini-crystallization series in order to increase the oxalate concentration in the aluminate liquor. Once the trihydrate has been washed, it is recovered, for example by scraping on a drum filter and added to the production trihydrate since its size grading characteristics did not vary significantly. Thus, using the mini-cooling series, the quantity of trihydrate obtained thus exceeds the initially sampled quantity.

Embodiments of the invention Example The embodiment of the invention will be better understood from the description based on the general treatment diagram (figure 1) According to figure 1, the liquor LO supersaturated with sodium aluminate is derived from the digestion of a bauxite containing a low content of organic materials, the composition of which is defined by the following contents by weight: A1203 52% Fe203 12% SI02 Fire loss 20 to Miscellaneous remainder This bauxite generates 220 grams of oxalic carbon per tonne of alumina produced.

In order to control the risk of oxalate being precipitated during crystallization of the aluminate liquor, this aluminate liquor is characterized by a ratio of oxalic carbon to caustic soda, expressed as Cox (in g/l) Na 2 0 cstq (in Without a deoxalation device, this ratio can reach a critical value starting from which sodium oxalate can precipitate at the same time as the hydrate during crystallization. This threshold is between 0.2 and within the crystallization temperature range. Due to mini-cooling series according to the invention described below, this ratio is kept stable at a significantly lower value, for example less than 0.15% if the precipitation threshold is 0.2%.

After crystallization A of the pregnant aluminate liquor LO carried out in the presence of an alumina trihydrate seed, the resulting spent liquor L1, for which the ratio Rp of the A1203 sol (in g/l)/Na20 cstq (in g/l) concentrations is between 0.5 and 0.7, and the caustic soda concentration is preferably between 130 and 165 g Na20/l, is separated into two fractions: a principle fraction L3 and a minor fraction L4 that will be deoxalated using the process according to the invention.

The importance of the minor fraction L4 drawn off depends on the quantity of oxalate to be eliminated during each cycle in order to prevent the gradual enrichment of Bayer liquor in sodium oxalate and therefore any risk of unwanted precipitation of this oxalate on alumina trihydrate grains during the crystallization. In this case, the fraction L4 is equal to 20% of the total spent liquor L1.

The minor fraction L4 of the liquor is cooled in C to 40 0 C, such that the liquor is very close to its critical sodium oxalate concentration threshold. It is then sent to a first stirred reactor DI called the seed tank, in which it is put into contact with a recycled slurry S7 containing alumina trihydrate and precipitated sodium oxalate, at a content of 400 to 800 grams of dry material per liter of slurry. The slurry Si thus formed is transferred into another stirred reactor in which it is put into contact with a fraction S8 of the sampled slurry overflowing from the last precipitator in the Bayer circuit to maintain the content of dry materials.

The slurry thus obtained goes through a series of tanks called the crystallization mini-cooling series, which is designed to enable the crystallization of aluminate liquor to continue.

The slurry is separated into two parts S3 and S4 at the output from the last precipitator tank Dn.

The fraction S3 of the slurry is filtered on a disk filter F (seed filter). The cake S7 obtained is recycled in the seed tank. The filtrate L6 is mixed with the fraction L3 of the spent liquor L1 that was not drawn off.

The fraction S4 is placed in a belt filter and washed with cold water L'8 (G and I) such that the filtrate remains strongly concentrated in soda by displacement of impregnation, so that it can be added back to the major fraction L3 of the spent liquor L1.

The insoluble residue S'4, which has a lower concentration of soda following this cold water washing, is washed with hot water H, preferably by using a washing cylinder. The pH of the wash water is 6. It is produced by the external addition of pure water L8.

The wash water used in the wash cylinder must not be cooler than 40 0 C, to assure sufficiently complete and fast dissolution of oxalate and the coprecipitated organic materials.

The wash water L9 is partially evacuated L11, thus entraining the dissolved oxalate with it. The other part L13 is returned to the beginning of the minicrystallization series, in tank D2 and/or optionally in seed tank Di, in order to increase the concentration of oxalate in the aluminate liquor.

At the end of washing, the quantity of hydrate recovered S10 can exceed the quantity of hydrate S8 added in the mini-cooling series.

The applicant has observed that an addition S9 of Crystal Growth Modifier (CGM) which is also known for its inhibiting action on the formation on seeds during crystallization, has a significant effect on the improved filterability of the slurry enriched in sodium oxalate crystals. With the additive called CGM 7837 made by the NALCO CHEMICALS Company, the specific resistance of the cake obtained during seed filtration F increases from 1.2 1010 to 3 109 m/kg, in other words it is divided by a factor of 4.

With the filtration conditions thus improved, it is found that: the oxalic carbon/caustic soda ratio at the end of the mini-series stabilizes at about 0.1% by adding (S8) 50 kg of hydrate per m 3 of treated liquor (which is only one fifth of the total amount of aluminate liquor), about 62 kg of hydrate is recovered per m 3 of deoxalated liquor L'6 after washing (S10), the additional quantity of hydrate produced being equal to about 2.5% of the total production of alumina.

Optionally, a quantity S'8 of production hydrate may also be added directly into the seed tank D1, or into either of the other precipitator tanks.

Advantages of the process according to the invention As a complement to deoxalation, this process enables production of an increased quantity of trihydrate. In this case, the seed is not only recycled but it is "over-regenerated" in the form of a trihydrate similar to that derived from crystallization, and therefore perfectly usable.

Claims (9)

1. Process for the purification of sodium aluminate liquors resulting from the alkaline digestion of bauxite using the Bayer process and containing sodium oxalate, the said process comprising: a) drawing off an aliquot of the spent aluminate liquor before concentration; b) cooling of this aliquot, mixed in a seed tank with a destabilising agent that comprises alumina trihydrate; c) addition of alumina trihydrate originating from the crystallisation, into the slurry thus formed; d) crystallisation of the mix thus obtained which enters a series of precipitators called a mini-cooling series; e) filtration of a fraction of the slurry output from the last precipitator, the cake output from the filtration being reintroduced into the seed tank as the destabilising agent in step the filtrate being mixed with the aliquot of the spent liquor that had not been drawn off to be concentrated by evaporation or addition of sodium hydroxide and to form the green liquor; f) filtration of the other fraction of the slurry output from the last precipitator with displacement of the cake impregnation with cold water, and then washing said cake with hot water; S* 20 g) final filtration of the washed hydrate slurry.

2. Process for the purification of sodium aluminate liquors according to claim 1, characterised in that the aliquot sampled before concentration in step a) is 10 to 30% of the spent liquor.

3. Process for the purification of sodium aluminate liquors according to claim 1 or 2, characterised in that the crystallisation in step d) is done at a temperature between and 60'C and preferably about

4. Process for the purification of sodium aluminate liquors according to any one S"of claims 1 to 3, characterised in that part of the wash water in step g) is added back to the oo •"beginning of the crystallisation of the mini-cooling series. S 30

5. Process for the purification of sodium aluminate liquors according to any one of claims 1 to 4, characterised in that the content of dry material in the mini-cooling series is kept at between 400 and 800 g/litre of aluminate.

6. Process for the purification of sodium aluminate liquors according to any one claims 1 to 5, characterised in that the filtrate resulting from filtration step e) is [R:\LIBA]05508.doc:jjp A 1 -17- returned into the aliquot not drawn off from the spent liquor, by displacement of impregnation in a belt filter.

7. Process for the purification of sodium aluminate liquors according to any one of claims 1 to 6, characterised in that an additive comprising a surfactant, normally provided to encourage the agglomeration of fines during crystallisation, is added into the slurry of the mini-cooling series before or during the filtration in step e).

8. A process for the purification of sodium aluminate liquors resulting from the alkaline digestion of bauxite using the Bayer process and containing sodium oxalate, the said process comprising: a) drawing off an aliquot of the spent aluminate liquor before concentration; b) cooling of this aliquot, mixed in a seed tank with a destabilising agent that comprises alumina trihydrate; c) addition of alumina trihydrate originating from the crystallisation, into the slurry thus formed; d) crystallisation of the mix thus obtained which enters a series of precipitators called a mini-cooling series; e) filtration of a fraction of the slurry output from the last precipitator, the cake output from the filtration being reintroduced into the seed tank as the destabilising agent in step the filtrate being mixed with the aliquot of the spent liquor that had not been 20 drawn off to be concentrated by evaporation or addition of sodium hydroxide and to form the green liquor; f) filtration of the other fraction of the slurry output from the last precipitator with displacement of the cake impregnation with cold water, and then washing said cake *eo with hot water; g) final filtration of the washed hydrate slurry, substantially as hereinbefore described with reference to the accompanying drawing.

9. A process for the purification of sodium aluminate liquors resulting from the alkaline digestion of bauxite using the Bayer process and containing sodium oxalate, the said process comprising: S 30 a) drawing off an aliquot of the spent aluminate liquor before concentration; b) cooling of this aliquot, mixed in a seed tank with a destabilising agent that comprises alumina trihydrate; c) addition of alumina trihydrate originating from the crystallisation, into the lurry thus formed; [R:\LIBA105508.doc:jjp I Ia -18- d) crystallisation of the mix thus obtained which enters a series of precipitators called a mini-cooling series; e) filtration of a fraction of the slurry output from the last precipitator, the cake output from the filtration being reintroduced into the seed tank as the destabilising agent in step the filtrate being mixed with the aliquot of the spent liquor that had not been drawn off to be concentrated by evaporation or addition of sodium hydroxide and to form the green liquor; f) filtration of the other fraction of the slurry output from the last precipitator with displacement of the cake impregnation with cold water, and then washing said cake with hot water; g) final filtration of the washed hydrate slurry, substantially as hereinbefore described with reference to the Example. Sodium aluminate liquors when produced according to the process of any one of claims 1 to 9. Dated 28 February, 2003 Aluminium Pechiney Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON *s OOo* o **o [R:\LIBA]05508.doc:jjp