AU648215B2

AU648215B2 - Process for the removal of sodium oxalate from solutions of sodium aluminate from the Bayer cycle

Info

Publication number: AU648215B2
Application number: AU32197/93A
Authority: AU
Inventors: Benoit Cristol; Yves Michel Perret
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 1992-02-05
Filing date: 1993-02-03
Publication date: 1994-04-14
Anticipated expiration: 2013-02-03
Also published as: DE69300121D1; AU3219793A; DE69300121T2; FR2686872B1; HRP930104A2; EP0555163A1; ES2071533T3; FR2686872A1; GR3015945T3; EP0555163B1; HRP930104B1; OA09774A; BR9300494A; CN1076427A; CN1033379C

Description

S F Ref: 229375

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

648 oo oo Name and Address of Applicant: Aluminium Pechiney Immeuble Balzac 10 Place des Vosges, La 92400 Courbevoie

FRANCE

Defense r r r r r Actual Inventor(s): Address for Service: Invention Title: Benoit Cristol and Yves Michel Perret Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Hales, 2000, Australia Process for the Removal of Sodium Oxalate from Solutions of Sodium Aluminate from the Bayer Cycle The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845/5 PROCESS FOR THE EMdVAL OF SODIUM CVJ9CATE FRM4 SOLUICINS OF SODIUM ALHINATE FROM THE BAYER CYCLE TECNCAL FIELD The invention concerns a process for the removal of sodium oxalate contained in solutions of sodium aluminate resulting from the alkaline attack on bauxites using the Bayer process.

STATE OF THE ART The Bayer process which is widely described in the specialist literature constitutes the essential procedure for the production of alumina which is intended to be converted into aluminium by igneous electrolysis. In accordance with the process the bauxite ore is treated in the hot condition by means of an aqueous solution of sodium hydroxide at a suitable level of concentration, thus causing solubilisation of the alumina and the production of a supersaturated solution of sodium aluminate. After separation of the solid phase constituting the unattacked residue (red muds) of the ore, the super- .saturated solution of sodium aluminate, which is also referred to as the Bayer liquor, is decomposed by seeding with particles of :aluminium trihydroxide in order to cause precipitation in the form of aluminium trihydroxide of the aluminium in solution. The aluminadepleted sodium aluminate liquor is then recycled to the attack step after having been concentrated and re-charged with sodium hydroxide in order to restore the level of concentration which is suitable for the operation of carrying out the attack on the ore. However, at the sane time as the alumina of the bauxite is dissolved in the state of sodium aluminate, the supersaturated solution of sodium aluminate, which issues from the attack operation, becomes progressively charged with organic compounds produced by more or less complete decomposition of the humic materials contained in the ore. Those organic compounds which are degraded in the form of organic salts of sodium and primarily in the form of sodium oxalate are found to be highly troublesome. In particular the disadvantages associated with the accumulation of sodium oxalate in Bayer liquors, result from the fact that that oxalate, on reaching its critical supersaturation level, precipitates in the form of fine needles on the aluminium hydroxide seed. The fine sodium oxalate needles also act as true crystal nuclei for precipitation of the aluminium hydroxide, causing an increase in the number of fine particles which thus become too numerous to be effectively controlled during decomposition of the sodium aluminate.

Thus precipitation of the sodium oxalate affects the physicochemical qualities of the precipitated aluminium trihyrcKdd and in particular gives rise to major variations in the granulametry of the alumina produced as well as increased fragility of the grains, which 15 can represent major disadvantages in regard to the use of that alumina in the production of aluminium by electrolysis.

Hence, it is found necessary in industrial operations for the production of alumina to control or better avoid contamination of the aluminium trihydroxide seed during the step involving decomposition by precipitated sodium oxalate.

Various processes have been proposed for limiting the presence of sodium oxalate in solution in Bayer liquors.

The processes which aim to destroy or decompose directly the humic materials contained in the ore, for example by roasting, are little used on an industrial scale because of their prohibitive cost.

More widely known are processes which involve acting on the decomposition products of the supersaturated liquor by washing the aluminium trihydroxide which is intended for the production or a part of the aluminium trihydroxide which is recycled as a seed to the decomposition step. That procedure thus selectively renoves the dilute sodium oxalate in the washing waters by precipitation with lime and the formation of calcium oxalate. However such processes do not make -it possible to carry out the aluminium trihydroxide precipitation step under the best conditions from the point of view of productivity. Moreover, excessive contamination by the deposit of oxalate on the aluminium trihydroxide grains can cause fragilisation of those grains, which manifests itself at the stage of calcination upon thermal decomposition of the oxalate which is trapped in the crystalline edifice.

To eliminate those di. advantages, it becomes necessary to maintain the level of concentration of sodium oxalate in the sodium aluminate liquor, in the course of the decomposition operation, at a value which is lower than its critical supersaturation concentration; that has to be done without reducing the proportion of humic materials in solution, the stabilising effect of which on the liquor is now well known because it makes it possible to regulate the 15 critical supersaturation threshold of the dissolved sodium oxalate.

A number of processes have been proposed for limiting the amount of sodium oxalate in solution in Bayer liquors. Such procedures regularly involve taking a fraction or the whole of the decomposed liquor which however is already supersaturated in respect of sodium oxalate and effecting destabilisation of supersaturation for specific precipitation and separation of the sodiumi oxalate from the liquor which is then desaturated.

Thus the process described in US patent No 3 899 571 (EP-A-0 013 407) comprises treating a Bayer liquor which is supersaturated 25 with respect to solubility in the equilibrium state of sodium oxalate (such as liquors with a reduced content of sodium aluminate, which result from decomposition, with or without having been subjected to re-concentration), by the introduction of a recycled sodium oxalate seed to cause precipitation of the sodium oxalate in solution and arrive at the concentration of the equilibrium of solubility of anhydrous sodium oxalate. After a solid-liquid separation operation which is facilitated by the use of a filtration additive, the purified liquor is then re-introduced into the Bayer cycle while a fraction of the solid phase of sodium oxalate is used for preparation of the suspension of seed crystals, the other fraction being removed from the cycle.

Although that seeding process is found to be effective for causing precipitation of the sodium oxalate, it suffers from disadvantages when applied industrially. In fact the sodium oxalate crystals constituting the seed become fairly quickly inactive due to poisoning of their surface by the organic materials present and it is then expedient to effect washing of that seed, which is a highly delicate operation to carry out. In the case where the washing operation is found to be inadequate, there is found to be a fall in the activity of the seed and thus a reduction in the sodium oxalate precipitation yield. In the situation where the washing operation is 15 taken to excessive lengths, it is found that the granulometry of the seed is fined down, and that results in liquid-solid separation operations being highly difficult and thus involving falls in the purification yield.

Instead of providing for destabilisation of the Bayer liquor which is supersaturated with sodium oxalate by means of a sodium oxalate seed, US patent No 4 597 952 (EP-A-0 173 630) recamends the use of seeds of calcium oxalate or barium oxalate, the mode of action of which leads indirectly to the sane result. In fact, calcium or barium oxalate which are unstable in the strongly alkaline Bayer liquor liberate the oxalate ion to form sodium oxalate which contributes to increasing the level of concentration of sodium oxalate in solution beyond the critical supersaturation threshold, thus causing precipitation of the sodium oxalate. The amount of oxalate dissolved is thus restored to the limit of solubility of the oxalate under the conditions of the experiment, dependent in particular on the temperature of the liquor and the concentration of sodium hydroxide in the liquor.

This process also results in the production of sodium oxalate precipitates which are finely dispersed in the Bayer liquor and which are therefore difficult to separate by settlement and/or filtration without the aid of additives. Likewise the seed recycling operation is necessary and a part of the precipitated sodium oxalate therefore has to be recycled in order to regenerate the calcium or barium oxalate seed after removal of the entrained organic materials. That regeneration operation is carried out by at least partially putting the precipitated sodium oxalate back into aqueous solution and treating the suspension obtained by means of lime (CaO) or barium aluminate (Al 2 0 4 Ba). That treatment results in the production of a precipitate of calcium oxalate or barium oxalate, which is recyclced to the sodium oxalate precipitation stage.

15 SrATEMEN OF PROBLEM It is therefore in order to deal with the dual problem of separation of the precipitated fines of sodium oxalate in the Bayer liquor and recycling of the oxalate seeds, while maintaining the level of effectiveness and in particular selectivity of the processes for the precipitation of sodium oxalate by the addition of oxalates seed, that the applicants studied and developed the process according to the invention.

SUBJECT OF THE INVENTION 25 The invention is based on the discovery that, taking a heterogenous seed based on finely divided lime, it is possible to cause substantial precipitation of sod um oxalate in the Bayer liquor in the form of a precipitate which can be easily separated by filtration, without the addition of a filtration additive.

For selectively precipitating the sodium oxalate without entraining humic materials and thus not randomly modifying the critical supersaturation threshold of the sodium oxalate in the Bayer liquor as well as the filtrability properties of that liquor, it is expedient to operate under clearly defined operating conditions, in particular as regards the temperature at which the precipitation operation in the liquor is effected.

More precisely the invention concerns a process for the ranoval of sodium oxalate fran at least a fraction of the solution or liquor o f sodium aluninate which is taken off in the Bayer cycle for the production of alumina fron bauxite after the steps of decomposition and concentration of said liquor which is intended to be recycled as an alkaline liquor for attacking the bauxite ore, canprising precipitation of the dissolved sodium oxalate by means of an agent for destabilisation of the state of supersaturation of the 6.666:sodium oxalate, then separation by filtration of the sodium oxalate :which is precipitated in that way, characterised in that the agent for destabilisation of the state of supersaturation of the sodium oxalate, also acting as a filtration additive, which is brought into contact for more than one hour with the solution of sodium aluminate which is cooled at between 40 0 C and 60 0 C, is based on finely divided lime Magnesia is optionally added to that lime in a proportion which does not exceed 40%. by weight of the mixture which is formed in '.:that way.

It has in fact been surprisingly found that the lime which is scravetimes used as a filtration additive to facilitate the separation *of certain solid impurities frarn the Bayer liquor could very effectively replace the seeds of sodium, barium or calcium oxalates recarmended by the prior art, at between 40'C and 60'C. By virtue of the very strongly alkaline character of the Bayer liquor after deccomposition and concentration and up to 60*C, there cannot be any caunbination of the lime with the elements in solution in the liquor, in particular with the oxalate ion C 2 0 4 and the humic materials which remain in solution. It is to be noted that the lime can be brought into contact with the sodium aluminate solution in the form of quicklime CaO or in the form of milk of lime whose concentration of CaO is between 100 and 300) g/litre, but in no case is the equilibrium of solubility of the sodium oxalate altered. It must be admitted that the lime essentially acts as a seed for precipitation c. the oxalate in supersaturated solution due just to the mechnical effect of liquor/solid contact, which in the present case is considerable having regard to the large surface area developed by the finely divided lime in contact with the liquor. It is to be noted however that beyond 60 0 C that seeding effect quickly decreases and is no longer perceptible at In parallel in the temiperature range of 40 0 C 60 0 C, after a .00 sufficient contact time to permit precipitation of the small crystals of sodium oxalate in suspension with the particles of lime, it is :found that the solid phase whiich is obtained in that way can be easily filtered. In follows that, after drying of the insoluble cake to recover the maximum amount of impregnation liquor, the latter no longer contains materials which can be put to valuable use, and it V. can be completely dumped. This is unlike processes using an oxalate :seed which has to be recovered at least in part and regenerated by means of complex treatments.

It should also be recalled that the critical supersaturation threshold as from which the sodium oxalate precipitates spontaneously decreases with a de crease in temperature. At less than 400C, there can be a fear of untimely precipitations of very fine particles of sodium oxalate in the circuits outside the stations provided for that purpose, and those very fine particles are also found to be very difficult to filter off. If the increase in concentration of caustic soda in the liquor (concentration of free soda comnbined soda in A02 Naepesdi emo a2 0g/ 1) tends also to reduce the critical supersaturation threshold for sodium oxalate, it is influenced in particular by the presence of humic materials which are only slightly degraded in the liquor. That threshold rising in proportion to the charge of organic materials in the liquor, it can easily be concluded that the process according to the invention will enjoy increased effectiveness with a rising threshold and that therefore the amount of oxalate which can be precipitated is great for a given volume of liquor. That is the reason why the invention finds one of its best applications when treating tropical trihydrate bauxites. Such bauxites which are based on hydragillite and which are rich in humic materials undergo an alkaline attack at a temperature which is generally 4 1500 C so that the attack liquor is progressively enriched with sodium oxalate, as a consequence of .4 degradation of the humic materials in time. Accordingly the amount of soluble sodium oxalate can attain fram 0.3 to 0.6% by weight of oxalic carbon in relation to caustic soda (free soda soda in the :state of A10 Na) before being restored after precipitation in *2 accordance with the process of the invention to amounts of fran 0.15 to 0.25%. It will also be noted that, in the case of monohydrate bauxite containing little humic materials and which moreover are attacked at high temperature 200l 0 the critical supersaturation *threshold does not exceed 0.15 to 0.25% by weight of oxalic carbon relative to the caustic soda by -virtue of the presence in a very greatly reduced amount in the liquor of humic materials. That threshold can be artifically raised to 0.3 or 0.5% by the addition to the liquor, at any point in the Bayer cycle, of an anionic synthesis polyelectrolyte such as polyacrylanide, or polyacrylic acid, in accordance with the teaching of the present applicants' EP 0 173 630 (US No 4 597 952). The retarding action on the precipitation of sodium oxalate by that synthesis agent, which is similar to that of the only slightly degraded organic or humic materials naturally present in the Bayer liquor makes it possible to apply the process of the present invention to all bauxites with the sane effectiveness as for tropical trihydrate bauxites.

Generally speaking the treatment for the removal of oxalate frcm the Bayer liquor which is deccmrposed and then concentrated to reach a, concentration of caustic soda expressed in terms of caustic Na 20 of between 170 and 250 g/litre and a concentration of sodium oxalate expressed in terms of oxalic carbon relative to the caustic soda of bet-ween 0.3 and 0.6% is effected only on a fraction representing fran 3 to 20% of the total volume of the liquor after concentration and at a temperature of between 40 and 600 C, therefore requiring that fraction of the Bayer to be cooled. The quicklime in which about 2% by weight of the grains is smaller than 10 micrcmetres 4. a0 or the milk of lime in which about 20% of the grains is smaller than 10 micrcmetres, is introduced regularly into the cooled liquor which is maintained in an agitated state to form a highly hcmogenous 00 dispension of lime at a concentration of CaO of between 2 and 20 g per litre of liquor. For that purpose, after the introduction of lime, the suspension is still kept in an agitated state for more than 1 hour. The solid phase of the suspension which is then formed by a mixture of fine particles of lime and crystallised sodium oxalate is separated by settlement and filtration or direct filtration. It is to be noted that separation is relatively easy since it is possible :to collect at least 36 kg of dried cake per hour per m 2 of f ilItering s*oourface area with a proportion of free soda expressed as Na 02~3%o the dried moist cake. After separation the solid phase without useful material can be added to the red muds to be discarded while 000025 the liquor in which the reduced concentration of soluble sodium oxalate is less than 0.25% of oxalic carbon relative to the caustic soda is added to the main fraction of liquor from which oxalate has not been removed to form a strongly alkaline solution which is recycled as a bauxite attack liquor.

CAUZYI THE INVENTI@J IM1 kEFFBCT The way in which the invention is performed under the preferred operating conditions will be better appreciated from the description based on the general treatment diagram (Figure 1).

As shown in Figure 1, the Bayer liquor Lo, possibly with the addition prior to the decomposition step of an anionic polyelectrolyte lo, for exanple FLOERGER type AN934SH, in a proportion of 20 mg per litre of solution, to raise if necessary the critical supersaturation threshold of oxalate to the vicinity of of oxalic carbon with respect to the caustic soda is decomposed and after separation of the aluminium trihydroxide precipitated, the resulting liquor Ll is concentrated by evaporation in such a way that the concentration of caustic soda is between 170 and 250 g of Na 2 /litre and preferably between 190 and 210 g of A fraction IA which preferably represents from 4 to 6% of the volume of concentrated liquor L2 is taken off to be subjected to the 15 de-oxalatation treatment according to the invention. The magnitude of the fraction L4 which is off is determined by tiie amount of sodium oxalate which is to be removed in each cycle to avoid a progressive increase in the content of sodium oxalate in the Bayer liquor and any risk of untimely precipitation of that oxalate on the aluminium trihydroxide grains in the decaposition operation. It should be recalled that the increase in the content of oxalate in the Bayer liquor, which is more or less rapid depending on the nature and the origin of the bauxite, is caused by the oxalates present in the oxide ore going into solution in the alkaline attack operation, but also progressive degradation in the form of sodium oxalate of the organic materials which are already solubilised in the recycled Bayer liquor.

After cooling to a temperature of between 40°C and 60°C the cooled liquor L5 whose critical supersaturation threshold is attained or even exceeded, by virtue of the reduction in temperature, is brought into contact in a first agitated reactor R1 with finely divided quicklime So to form a homogenous suspension of a concentratiobn which is preferably between 7 and 9 g of CaO per litre of liquor The suspension Sl which is formed in that way is transferred into a second agitated reactor and then into a third agitated reactor R3. The total tine of quicklinie-liquor contacting which in the present case is effected in the three reactors in series is preferably between 3 and 5 hours. The suspension S3 which issues from the reactor R3 is filtered. That filtration operation which is carried out over a filter press is very quick, of the order of 1. m3/orosupninprm2of filtering surface area.

After filtration and drying the cake S4 of insoluble materials, whose content of free NapO is less than is mixed with the red muds to be discharged. The liquor L6 with a reduced sodium 0 C. oxalate content, at a level of concentration of between 0.15 and V, 15 0. 25% of oxalic carbon with respect to the causL.c soda, is mixed with the main fraction L3 of non-de-oxalated liquor to form an alkaline liquor L7 which is recycled as a bauxite ore attack liquor.

EXAMPLES OF USE Exariple 1 The treatment according to the process of the invention was 3 applied to 40 m /hour of industrial liquor L4 taken fran the ci rcuit of decomposed and concentrated liquor L2 whose flow rate m 3/hour and which essentially derives from the alkaline attack at a temnperature of 105'C on a tropical trihydrate bauxite. The liquor L4 taken off was of the following composition: Caustic Nap 2 O 200) g/l Carbonated Na 2 0 25 g/l A203 :120 g/l Sodium oxalate expressed as oxalic C :0.90 g/l Proportion of oxalic C/caustic Na 2 O: 0.45% After cooling to 400 C, the cooled liquor L5 is mixed with 270 kg of quicklime with the addition of 30 kg of magnesia in a first agitated reactor to form a homogenous suspension of a concentration of 8 g of CaO MgO/litre. After 3 hours of contact in the agitated reactor the suspension representing a volur-e of 40 m 3 was filtered in less than 1 hour over a filter of an area of 30 m. After drying the insoluble moist cake S4 titrated less than 3% of free soda expressed as Na 2 0 and the liquor L6 which was returned to the Bayer circuit to be mixed with the 960 m 3 of non-deoxalated liquor L3 which was recycled as the attack liquor contained only 0.37 g of oxalic carbon per litre, that is to say oxalic C/caustic Na 0 0.18%.

In that treatment (0.88 0.37) 4.10g, that is to say about S..20.4 kg of oxalic carbon corresponding to 113.8 kg of crystallised sodium oxalate was reoved in the course of the cycle.

*e 15 Exmle 2 The treatment carried out under the conditions of Example 1 was applied to 40 m3/hour of industrial liquor IA resulting from the attack at a temperature of 250'C of a monohydrate bauxite of S"Mediterranean origin. The liquor L4 whose critical supersaturation S. S threshold in respect of sodium oxalate has been raised to 0.68 g/l of oxalic carbon by the addition in a proportion of 20 mg/litre of .'"'.anionic polyelectrolyte Floerger AN 934 SH, was of the following composition: Caustic Na 2 195 g/l 25 Carbonated Na 2 0 22 g/ Al203 120 g/l Sodium oxalate expressed as oxalic C 0.58 g/l corresponds to a proportion of oxalic C/caustic Na20 of 0.30% substantially lower than the critical supersaturation threshold of 0.68 g/l corresponding to the proportion of 0.35%.

n.e .e e

C

I C

C@

Ct e* C C C After filtration in less than 1 hour and drying, the moist insoluble cake S4 titrated less than 3% of free soda expressed as Na 20 and the de-oxalated liquor L6 contained only 0. 19 g of oxalic carbon per litre, that is to say a proportion of oxalic C/caustic Na 2 O0 of 0.1%.

In that treatment (0.58 0.19) 4.10' g, that is to say about 15.6 kg of oxalic carbon corresponding to 87 kg of crystallised sodium oxalate, was removed in the course of the cycle.

lxn~e 3 The treatment according to the process of the invention was applied to 40 m /hour of industrial liquor L4 taken from the circuit of decomposed and concentrated liquor L2 whose flow rate is 100X0 m /hour and which essentially comes from the alkaline attack at a 15 teperature of 1050 C on a tropical trihydrate bauxite. The liquor 1A taken off was of the following composition: Caustic Nap 2 205 g/l Carbonated Na 2 O0 24 g/l Al203 :120 g/1 Sodium oxalate expressed as oxalic C :0.88 g/l Proportion of oxalic C/caustic Na 20 0.43% After cooling to 40 0 C, the cooled liquor L5 is mixed with 270 kg of quicklime with the addition of 30 kg of magnesia thereto in a first agitated reactor to form a homo~genous suspension of a 25 concentration of 8 g cf CaO MgO/litre. After 3 hours of contact in the agitated reactor the suspension representing a volume of 40 3 was filtered in less than 1 hour over a 30 m.

2 filter. After drying the moist insoluble cake S4 titrated less than 3% of free soda expressed as Na 0 and the liquor L6 returned to the Bayer circuit to be mixed with the 960 m 3of non-de-oxalated liquor L3 recycled as the attack liquor contained only 0.37 g of oxalic carbon per litre, that is to say oxalic C/caustic NapO 0.18%.

C

C C

CC..

C C .C C C C CC CC C C

CCC.

C

14 In that treatment (0.88 0.37) 4.l104 g, that is to say about 20.4 kg of oxalic carbon corresponding to 113.8 kg of crystallised sodium oxalate, was removed in the course of the cycle.

Example 4 This Exanple involved treating a second aliquot portion L4 of the liquor L2 of Example 3 under the same precipitation conditions as those of Example 3 except for the temperature of the liquor which was maintained at 80 0 C instead of 40 0 C. Independently of the less good permeability of the insoluble cake as specifically manifested by an increase in the filtration timfes at constant volume (Thia' for 40 m3 of liquor to be filtered), there was noted an insignificant reduction in the proportion of sodium oxalate expressed as oxalic C of 0.88 g/l *in the initial liquor to 0.86 g/l in the filtered liquor. On the *.15 other hand the proportion of organic carbon controlled at 7.2 g/l in the initial liquor is reduced to 5.3 g/l in the filtered liquor, confirming partial purification in respect of organic or humic materials which are slightly degraded, precipitated or insolublised, probably by the lime-magnesia mixture. It is therefore confirmed that, under those conditions, the addition of a lime-based seed has 5: no effect on the amount of oxalate in the liquor.

Example This Example involved treating a third aliquot por-tion L4 of the liquor L2 of Example 3 under the same precipitation conditions as those of Examaple 3, except for the temperature which was maintained at 60 0 C and the canposition of the 300 kg of stabilising agent which in the present case is formed by a mixture of 180 kg of CaO and 120 kg of MgO (60% /40% by weight). Good filtrability of the insoluble cake was noted, as in Example 3, as well as a significant reduction in the proportion from 0.88 g/l to 0.39 g/l corresponding to oxalic C/caustic Na 2 O 0.19% and the removal in the course of a treatment cycle on 40 m 3 of liquor of (0.88 0.39) 4.104 g, that is to say approximately 19.6 kg of oxalic carbon corresponding to 109.4 kg of crystallised sodium oxalate.

In parallel it is found that there is a slight reduction in the amount of organic carbon from 7.2 g/l in the initial liquor to 6.7 g in the filtered liquor after de-oxalatation, leading to the supposition that slightly degraded humic or organic materials are beginning to be insolublised.

ee 99 ft *o *oo

Claims

1. A process for the removal of sodium oxalate from at least a fraction of the solution or liquor of sodium aluminate which is taken off in the Bayer cycle for the production of alumina fron bauxite after the steps of decomposition and concentration of said liquor which is intended to be recycled as an alkaline liquor for attacking the bauxite ore, comprising precipitation of the dissolved sodium oxalate by means of an agent for destabilisation of the state of supersaturation of the sodium oxalate, then separation by filtration of the sodium oxalate which is precipitated in that way, characterised in that the agent for destabilisation of the state of •supersaturation of the sodium oxalate, also acting as a filtration additive, which is brought into contact for more than one hour with the solution of sodium aluminate which is cooled at between 40 0 C and is based on finely divided lime.

2. A process according to claim 1 characterised in that the 4 destabilisation agent is lime with the addition of magnesia in a proportion not exceeding 40% by weight of the mixture which is formed in that way.

3. A process acording to claim 1 characterised in that the destabilisation agent is quicklime in which about 2% by weight of the grains are smaller than 10 micrcmetres.

4. A process according to claim 1 characterised in that the destabilisation agent is a milk of lime in which the concentration of CaO is between 100 and 300 g/litre and in which about 20% by weight of the grains of lime in suspension are smaller than 10 micrometres. A process according to claim 1 characterised in that the solution or liquor of sodium alixninate which is taken off after decomnposition and concentration is of a concentration of caustic soda expressed as Na 2 of between 170 and 250 g/l and preferably between 190 and 210 g/l.

6. A process according to claim 1 or claim 5 characterised in that the solution or liquor of sodium aluninate which is taken of f after decanposition and concentration has a proportion of sodium oxalate expressed by the ratio by weight of oxalic carbon/caustic Na 20 of between 0.3% and 0.6% and preferably between 0.3% and *Ott 7. A process according to any one of claims 1, 5 and 6 characterised in that the fraction of concentrated liquor taken of f :represents from 3 to 20% and preferably fran 4 to 6% of the total volume of concentrated liquor.

8. A process according to any one of claims 1 to 7 characterised in that the ha-nogenous suspension of lime in the cooled liquor is of a a concentration of between 2 and 20 g and preferably between 7 and 9 g of CaO per litre. 9 1. A process according to any one of claim 1 to 8 characterised in that ater tim inoling contact ofte limteas ithu the oedlio forse fomn ah suspension afich itmataione in arng itdsaed while the liquid phase which is formed by the liquor with the reduced oxalate content is recycled mixed with the main fraction of non-de- oxalated liquor as the bauxite ore attack liquor.

11. A process according to claim 1 characterised in that an anionic polyelectrolyte Sis previously added to the solution or liquor of sodium aluminate in any step in the Bayer cycle and preferably prior to decomposition, if the critical supersaturation threshold in respect of sodium oxalate in the liquor which is taken off does not exceed 0.15 to 0.25% by weight of oxalic carbon with respect to the caustic soda.

12. A process according to claim 1 characterised in that the amount of anionic polyelectrolyte added is of the order of 20 mg per litre of liquor.

13. A process for the removal of sodium oxalate from at least a fraction of the solution or liquor of sodium aluminate which is taken off in the Bayer cycle for the production of alumina from bauxite after the steps of decomposition and concentration of said liquor which is intended to be recycled as an alkaline liquor for attacking the bauxite ore substantially as hereinbefore described with reference to any one of the Examples.

14. A process for the removal of sodium oxalate from at least a fraction of the solution or liquor of sodium aluminate which is taken off in the Bayer cycle for the 15 production of alumina from bauxite after the steps of decomposition and concentration of said liquor which is intended to be recycled as an alkaline liquor for attacking the bauxite ore substantially as hereinbefore described with reference to the accompanying drawings. SDated 3 February, 1993 Aluminium Pechiney Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON 0 a oe *o 0500 KEH/1664u ABSTRACT PROCESS FOR THE REMOVAL OF SODILM CDALATE FRCM SOLUTIONS OF SODIIM ALUMINATE FROM THE BAYER CYCLE Sodium oxalate is removed frcm at least a fraction of the solution or liquor of sodium aluminate which is taken off in the Bayer cycle for the production of alumina from bauxite, after the steps of deccmposition and concentration of said liquor which is intended to be recycled as an alkaline bauxite ore attack liquor, by precipitation of the dissolved sodium oxalate by means of an agent for destabilisation of the state of supersaturation of the sodium oxalate, then separation of the sodium oxalate which is thus precipitated. In accordance with the invention the agent for destabilisation of the state of supersaturation of the sodium oxalate which is brought into contact for more than 1 hour with the cooled sodium aluminate solution is lime which may optionally contain magnesia and which can be introduced into the sodium aluminate solution directly in the form of quicklime CaO or in the form of a milk of lime. Figure 1 Figure 1 C ••co