OA11960A - Process for purifying sodium halide aluminate containing sodium oxalate liquors for recovering residues - Google Patents

Abstract

A method for eliminating sodium oxalate from at least one fraction (L4) of decomposed Bayer aluminate liquor (L2) obtained after concentration (V) in a Bayer alumina production cycle, comprising the precipitation of said oxalate which is dissolved by means of a finely divided lime-based (C1) destabilizing agent followed by separation (F) of the solid residues from the deoxalation (S4) of the liquor (L6) which is recycled as an alkaline leaching liquor for bauxite whereby desolation residues (S4) are used to decarbonate and deoxalate industrial water.The method is particularly advantageous when said industrial water is raw water or water from a Bayer plant and said water is redirected towards the washing cycle for insoluble residues from the Bayer cycle once treatment has occurred.

Description

11960 - 1 -

PROCESS FOR PURIFYING SODIUM ALUMINATE LIQUEURS CONTAINING SODIUM OXALATE FOR ENHANCING LESRESIDUS

TECHNICAL FIELD The invention relates to a process for the purification of sodium aluminate liquors resulting from the alkaline attack of bauxites according to the Bayer process and containing sodium oxalate.

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STATE OF THE ART

EP 0 555 163 discloses a particularly efficient deoxalation process which advocates the use of a solid, finely divided and inexpensive material - lime - which serves as a primer for the precipitation of sodium oxalate in the liquor. concentrated decomposed aluminate. In fact, the sodium oxalate does not precipitate directly on the lime fines but on the solid products resulting from the reaction between the lime and the aluminate liquor belonging to the family of tricalcium aluminate hexahydrate: Al2O3.3CaO, 6H2O. For the rest of the presentation, we will group all of these reaction products under the name "AC3,6".

PROBLEM POSE 25

This process is used in many industrial plants and gives satisfaction for most bauxites to be treated. However, it is quite expensive in high capacity plants dealing with mixed or mixed bauxites, particularly of African or Indian origin, which are particularly rich in organic matter, these being degraded in the course of time. alkaline attack especially in the form of a large amount of sodium oxalate, which requires the implementation of frequent deoxalatation operations. This results in a high consumption of lime and a loss of soda and alumina entrained with deoxalation residues.

To reduce the consumption of lime and the loss of alumina and sodium hydroxide, the Applicant has sought, in WO 97/03924, to recycle the destabilizing agent, that is to say the AC3,6 resulting from the reaction of the lime with the aluminate liquor and which is in the form of finely divided particles: instead of discarding these particles coated with sodium oxalatepreecipitate, WO 97/03924 proposes washing them so as to redilute the sodium oxalate and then reintroducing these washed particles as deoxoxidation primer, the washing water enriched with sodium oxalate being further removed. But the dilution must be important and the industrial setting of the washing and filtration lines required by this operation is too expensive to be easily depreciated by the reduced lime consumption. There is indeed no reduction of sodium hydroxide and alumina, the impregnation of deoxalatation residues starting with the washing water.

The problem therefore remains to have a deoxalation process of aluminate liquor inexpensive, using the least destabilizing agentpossible and does not increase the losses of sodium hydroxide and alumina. 25

OBJECT OF THE INVENTION

The Applicant has succeeded in solving this problem by perfecting its method described in EP 0 555 163, that is to say by using the residues of deoxalation by lime to causticize, that is, to decarbonate and desoxalate industrial waters, for example the waters used for the chain of washing insoluble residues resulting from the alkaline attack of bauxite, called "inert residues".

The Applicant has in fact observed with surprise that, washed in a highly diluted medium by the water coming from industrial installations such as pond return water, raw water and sodium distilled water, said deoxalation residues - essentially AC3, 6 - allow to decaustify, that is to say to decarbonate and deoxalate these waters that can be redirected, directly after washing by dewatering deoxalatics, to industrial facilities, for example to the line of washing of inert residues. In order for the sodium oxalate to dissolve, the scrubbing of the deoxalatation residues must be carried out in a highly diluted sodium medium: the total Na2O content must be limited to 8 g / l and preferably to 3 g / l. l.

The solid products resulting from the jamming are evacuated. These are precipitates of carbonate and calcium oxalate. The CaC2C> 4, H2O compound has been identified by X-ray diffraction. They have a shape and size such that the liquid-solid separation can be easily performed by filtration, and even, using adjuvants such as anionic polyelectrolytes, by flocculation and decantation.

The Applicant has thus discovered that tricalcium aluminate hexahydrate is capable of precipitating calcium oxalate under conditions of concentration of the sludge basins, with limited sodium hydroxide concentrations: less than 10 g Na 2 O / l. The kinetics of calcium oxalate precipitation decreasing substantially when the sodium hydroxide concentration increases, it is preferable to limit this concentration to 8 g Na 2 O / l, the optimal effectiveness of the jamming being obtained with a concentration close to 3 This low concentration makes it necessary to use a lot of water or to obtain the lowest possible rate of impregnation of deoxalatation residues. Preferably, an impregnation rate of these residues of less than or equal to 50%, which can be easily achieved with a filter press, is aimed at. The kinetics of precipitation of calcium oxalate decreasing when the temperature increases, it will be sought to perform this scrambling in the temperature range 40 - 60 ° C, preferably in the vicinity of 40 ° C. During loathing, the released oxalate ion C2O42 'reacts with the compounds of the AC3,6 family to yield calcium oxalate CaC2CU, H2O precipitating, sodium aluminate and hydroxide. sodium. The lesoxalatation occurring during the blockage can be schematized thus: 3 CaO, Al2O3, 6H2O + 6 Na + + 3 C2O42 - »- 3 H2O <-> 3 CC1C2O4, H2O + 2 Na + 2 [Al (OH) 4] + 4 Na ++ 4 OH '

The reaction is, in fact, more complex because of the presence of non-fully transformed humic materials which have coprecipitated with the sodium oxalate and passed back into solution during the blockage. On the other hand some industrial waters are not pure. If they are carbonated - this is generally the case with basin return water from the Bayer chain facilities which are rich in sodium carbonate - the effect of the sludge is to precipitate calcium carbonate such that Subsequently, a decarbonated water is obtained which can be discharged to the industrial plants, all the more advantageously since these are sensitive to scaling of the pipes. The decarbonation occurring during the blockage can be schematized, for example: CaO, Al 2 O 3, 6H 2 O + &amp; Na + + 3CO32- ~ 3 CaCOs + 2 Na + + 2 [AI (OH, 4) - + 4 Na ++ 4 OH 'The carbonate ion CO32 · reacts with the compounds of the AC3,6 family to give carbonate of calcium CaCCb which precipitates, desodium aluminate and sodium hydroxide.Thus, the decarbonation performed bypatouching with residues deoxalatation by lime according to EP 0 555 163 allows to generate free soda from 10% carbonate of sodium sodium, existing product in carbonated waters or that can be added to raw water, because it is less expensive than soda.If the backwater basin is particularly carbonated, it may prove necessary to add lime in the mixture before or during the chaos 15

The backwaters of the basin can be oxalated themselves, this is often the case with the return water from the basin of the facilities of the Bayer chain - and this treatment makes it possible to deoxalate them before directing them, for example, towards the washing the residues of the alkaline attack. 20

In the preferred embodiment of the process according to the invention, the dewatering waters of the deoxalation residues are mixed with the incoming water in the washing chain of the insoluble residues of the alkaline attack of labauxite. In this case, the alumina, the soda, the humic substances, which were previously lost with the evacuation of deoxalatation residues, are no longer lost since these decarbonated and deoxalated waters are introduced into the Bayer cycle. In addition, soluble sodium hydroxide, generated as much by deoxalation as by decarbonation, is introduced through this way into the Bayer circuit. 30 1196 ο -9-

A L4 fraction is taken to undergo the deoxalation treatment and then, once treated, the L6 fraction is mixed with the main fraction L3 to form a liquor L7 alkaline attack (A) ore BX. The aluminate suspension SA from the alkaline etching (A) is then cooled and diluted (DI) with the washed ED water (LVBR) against the current of the insoluble residues br RESulting from the alkaline attack. The industrial waste water used for this washing line is raw water, distilled sodium water or, if the installation permits, pond return water. The supersaturated suspension SS is then poured into a decanter (DE) to separate these insoluble residues BR from the supersaturated aluminate liquor LO.

The fraction L4 removed for deoxalation in the concentrated decomposition liquor L 2 preferably represents 4 to 6% of the volume of said concentrated decomposed liquor L 2. The importance of the fraction to be collected is determined by the amount of sodium oxalate which must be removed at each cycle to avoid progressive poisoning of the sodium oxalate liquor and any risk of inadvertent precipitation of this oxalate on the trihydroxide grains. aluminum during decomposition. 20

After cooling (R) to a temperature between 40 and 60 ° C, the cooled L5 aliquer whose critical supersaturation threshold of the oxalated sodium content is reached or exceeded is brought into contact with finely divided C1 limevive in a first stirred reactor RI, to form a homogeneous suspen- sion SI of concentration.comprise preferably between 7 and 9 g of CaO per liter of liquor L5.

The suspension SI thus formed is transferred into a second agitated reactor R2 and a third reactor R3. The total time of contacting quicklime-aluminate liquor is between 3 and 5 hours. The S3 suspension leaving the 119 $ ο -7- by the evacuation of the inert residues is a little larger but the overall assessment is positive since the major part, not evacuated, alumina, soda and humic materials goes up against during the washing of the inert residues is poured into the suspension of aluminate resulting from the attack in order to dilute this suspension before decantation and decomposition.

The return of humic substances in the Bayer cycle is beneficial since it regulates the concentration of humic substances in the aluminate liquor. The Applicant has indeed found that the humic substances present in the aluminate liquor reduce the risk of premature precipitation of the oxalates during the decomposition of the liquor and that it is desirable to maintain as steady a concentration as possible of these humic substances in the liquid. aluminate liquor. In the case of the use of pond return water, the fouling of these waters with deoxalation residues by the lime according to the invention makes it possible to reduce significantly the consumption of lime used in the washing line for the decarbonation of the scrubber clear. Solid products resulting from fouling with deoxalatation residues are easily isolated from water redirected to industrial plants. Their evacuation has an advantage over the process described in EP 0 555 163 since residues consisting essentially of calcium carbonate and calcium oxalate, which are chemically inert with respect to the environment, are removed here. Π9 g q -8-

EMBODIMENTS OF THE INVENTION - EXAMPLES

The realization of the invention will be better understood from the description of the following examples. Example 1, illustrated in FIG. 1 and taken as a reference, is very close to example 3 of EP 0 555 163. Example 2, illustrated in FIG. 2, describes the enhancement provided according to the present invention to the deoxoxidation process of EP. No. 0 555 163, in which sodium distilled water from the industrial exploitation of a Bayer circuit is used. Example 3, also illustrated in FIG. 2, describes the improvement made in accordance with the present invention in the deoxalation process of EP 0 555 163, in which the backwater of strongly carbonated basins of an industrial plant operating a circuit is used for the fouling. Bayer. / 5

In both figures, the flows of water - washing or scrubbing - are represented by double lines.

20 EXAMPLES

Example 1 (reference of the prior art)

According to FIG. 1, the supersaturated aluminate liquor LO, optionally supplemented with an anionic polyelectrolyte 10 intended to raise, if necessary, critical oxalate supersaturation thresholds, is decomposed (P). Afterseparation of the precipitated aluminum trihydroxide, the aluminateresultante liquor, called the decomposed liquor, L1 is concentrated by evaporation (V), provided that its concentration of caustic soda is between 170 and 250 g Na2O / liter, preferably between 190 and 210 g Na2O / liter. Π960 -9-

A L4 fraction is taken to undergo the deoxalation treatment and then, once treated, the L6 fraction is mixed with the main fraction L3 to form a liquor L7 alkaline attack (A) ore BX. The suspension of aluminate SA from the alkaline attack (A) is then cooled and diluted (DI) with ED water after washing (LVBR, against the current of the insoluble residues BR resisting the alkaline attack. The supersaturated suspension SS is used for this washing line as raw water, sodium distilled water or, if the installation allows it, for return water to a decanter (DE) to separate these residues. insoluble BR of the supersaturated aluminate liquor LO.

The fraction L4 removed for deoxalation in the concentrated decomposition liquor L 2 preferably represents 4 to 6% of the volume of said concentrated decomposed liquor L 2. The importance of the fraction to be collected is determined by the amount of sodium oxalate which must be removed at each cycle to avoid progressive poisoning of the sodium oxalate liquor and any risk of inadvertent precipitation of this oxalate on the trihydroxide grains. aluminum during decomposition. 20

After cooling (R) to a temperature between 40 and 60 ° C, the cooled L5 aliquer whose critical supersaturation threshold of the oxalated sodium content is reached or exceeded is brought into contact with finely divided C1 limevive in a first stirred reactor RI, to form a homogeneous suspen- sion SI of concentration.comprise preferably between 7 and 9 g of CaO per liter of liquor L5.

The suspension S1 thus formed is transferred to a second agitated reactor R2 and a third reactor R3. The total time of contacting quicklime-aluminate liquor is between 3 and 5 hours. The suspension S3 leaving the third stirred reactor R3 is filtered. This filtration (F), carried out on press filter is very fast, of the order of 1.5 m3 / .hour of suspension per m2 of filtering surface. After filtering and spinning, the insoluble cake S4 whose Na2Olibre content is less than 3% is mixed with the inert residues for disposal.L6 liquor, depleted in sodium oxalate with a concentration between 0.15 and 0 , 25% of oxalic carbon reported in the caustic is mixed with the main fraction L3 of non-deoxalized liquor to form an alkaline liquor L7 recycled as a bauxite ore liquor.

40 m3 / h of industrial liquor L4 taken from the decompounded and concentrated L2 liquor circuit having a flow rate of 1000 m 3 / h are treated here. This liquor comes mainly from the 105 ° C alkaline attack of a tropical trihydrate bauxite. The liquor L4 removed had the following composition:

Na2O caustic: 205 g / l

Carbonated Na2O: 24 g / lAl2O3 = 120 g / l Sodium oxalate expressed as Cox (oxalic carbon): 0.88 g Cox / I (ieCox / Na2Octq = 0.43% and Cox / Na2Otot: 0.39%)

After cooling to 50.degree. C., the cooled liquor L5 is mixed with 320 kg / h of quicklime to form a homogeneous slurry of concentration 8 g CaO / liter and after 3 hours of contacting in a stirred reactor, the suspension having a volume of 40 m3 is filtered in less than one hour on a filter of 30 m2.

After wringing, wet insoluble cake S4 contains 3% of free sodium hydroxide expressed as Na2O and liquor L6 is returned to the Bayer circuit by mixing it with 96% of the unbound fraction of the concentrated decomposed liquor L3, representing flow rate of 960 m3 / h. The L6 liquor contains only 0.37 g Cox / I (Cox / Na2Octq = 0.18%). During this treatment, about 21 kg / h of oxalic carbon, corresponding to 117 kg / h of crystallized sodium oxalate, is eliminated.

After spinning, the impregnated wet S4 insoluble cake is discharged at a rate of 1680 kg / h. The precipitated oxalate causes the loss of 108 kg / h of sodium hydroxide expressed as Na2O. The impregnation contains soda linked to alumina (about 30 kg / h), carbonate soda (about 75 kg / h) and weldelibre (NaOH) whose loss, expressed in terms of Na2O, is the order of 55kg / h. The total loss of soda is of the order of 270 kg / h. The evacuation of deoxalatation residues therefore entails a loss of: -> 320 kg / h of quicklime - "270 kg / h of total soda -" 225 kg / h of alumina, whether insoluble or not she is dragged into impregnation. 20

This evacuation also leads to the loss of humic substances which have co-precipitated with sodium oxalate. According to the Bayer cycle, this loss can be between 10 and 20 kg / h of humic matter. 25 1196 Ο -12-

Example 2 (Figure 2) - Dewatering residue trapped with a mixture of raw water and sodium distilled water.

Figure 2 shows the previous deoxidation treatment perfected by upgrading the deoxidation residues. In this example, the jamming is performed only with raw water and distilled sodium.

The insoluble cake S4 obtained by filtration (F) is impregnated with its impregnation to a tank where it is washed (LVS4, by intermingling with distilled sodium distilled water EDS at a rate of 120 m 3 / h After a residence time of 2 hours, the mixture is then filtered.

The residue consists essentially of oxalate and calcium carbonate.

Its impregnation is very dilute soda, resulting in a loss of soda15 less than 1 kg / h. Its evacuation is done in non-aggressive conditions with respect to the environment.

The EF filtrate is mixed with the remainder of the washings. The water from the El washing line is a little more concentrated in soda and alumina because of the addition of the water resulting from the jamming. This slightly stronger concentration is responsible for losses of soluble soda in the wash chain which are slightly larger: around 186 kg / h whereas they were 104 kg / h when washing water did not follow the treatment according to the invention. 25

The balance is positive since: the 270 kg / h of total sodium hydroxide lost in the process of the prior art is partially recovered in the Bayer cycle, the overall loss being reduced to about 82 kg / h. The 225 kg / h of alumina lost in the original process is, for the most part, reintroduced into the Bayer cycle. The overall loss of alumina estramenée to 42 kg / h approximately. while in the process of the prior art, with the removal of the impregnation, 55 kg / h of free soda was lost, the 320 kg / h of lime intended for deoxalation generated 51 kg / h of free soda but the excess loss ensoude free at the level of the evacuation of insoluble residues here is 50 kg / h: the second use of 320 kg / h of lime has allowed to recover practically free soda lost in the art prior.

OJ

Example 3 - (Figure 2) Dewatering residue trapping with pond return water J5 In this example, the jamming is performed only with debris return water (ERB).

The latter have a total sodium hydroxide concentration of 1.5 g / l; they are strongly carbonated and are rich in oxalates. The introduction of such waters into the washing chain amounts to 84 kg / h of carbon-carbonated carbon and 0.6 kg / h of oxalate expressed as oxalic carbon (Cox). The total soluble Na2O value at the level of the evacuation of the insoluble residues is then 202 kg / h, a value to be compared with the 104 kg / h relative to the use of raw water and sodium distilled water of the preceding example. 25

The insoluble cake S4 obtained by filtration (F) is entrained with its impregnation to a tank where it is washed (LVS4) by intermingling with the waters around the basin at a rate of 120 m3 / h. After a residence time of 2 hours, the mixture is then filtered. It was not necessary in this case to add calcium C2 to complete the decarbonation. 119 6 0 - 14-

The residue consists essentially of oxalate and calcium carbonate.Its impregnation is very dilute soda, resulting in a loss of soda less than 1.5 kg / h. Its evacuation is done in non-aggressive conditions with respect to the environment.

The EF filtrate is mixed with the remainder of the washings. The water of the El washing chain is a little more concentrated in soda and alumina because of the supply of the water resulting from the jamming. This slightly higher concentration is responsible for losses of soda soluble in the washing chain slightly larger: of the order of 283 kg / h while they amounted to 202 kg / hen when the washing water did not follow the treatment according to the invention. The balance is positive since: the 270 kg / h of total sodium hydroxide lost in the process of the prior art is partially recovered in the Bayer cycle, the overall loss being reduced to about 81 kg / h. the 225 kg / h of alumina lost in the original process are reintroduced for the most part in the Bayer cycle. The loss was reduced to 45 kg / h. - while in the process of the prior art, was lost - with evacuationde the impregnation - 55 kg / h of free soda, the 320 kg / h of lime for deoxalation generated 179 kg / h of free soda but the excess loss of free void in the evacuation of insoluble residues is here 51 kg / h: the second use of 320 kg / h of lime thus caused, in addition to the recovery of the free soda lost in the prior art, a generation of 128 kg / h of free sodium hydroxide. 11960 '- 15-

BENEFITS OF THE PROCESS ACCORDING TO THE INVENTION • it makes it possible to reduce the losses of alumina and sodium hydroxide, entrained by the deoxalation process by adding lime to the concentrated compounded liquor of the prior art 5 · the return of humic substances in the Bayer cycle regulates the concentration of humic substances in the aluminate liquor, which has a beneficial effect vis-à-vis the risk of premature precipitation of oxalates during the decomposition. The method makes it possible to relieve causative treatment of the washing chain when recirculating highly carbonated waters, such as pond return waters.

Claims (11)

119 6 Ο - 16- CLAIMS 1. Process for the removal of sodium oxalate in an industrial plant employing the Bayer process for the production of alumina hydrate and using industrial water, said process comprising, in at least one fraction (L4) of the liquor of decomposed Bayer aluminate (L2) taken after concentration (V), precipitation of the dissolved oxalate by means of a finely divided lime-based destabilizing agent (Cl) and separation (F) of solid deoxalation residues (S4) of the liquor (L6) recycled as an alkaline liquor for attacking bauxite, characterized in that the deoxalation residues (S4) resulting from the separation are used to decarbonate and deoxalate said industrial waters. 2. Deoxalation process according to claim 1, wherein the treatment of said industrial water is a mess with the residues (S4) deoxalatation of the aluminate liquor (L4). 3. Deoxalation process according to claim 2, wherein the dewatering waters have a total sodium concentration of less than 10 g / l, preferably less than 3 g / l. 4. Deoxalation process according to any one of claims 1 to 3où impregnation rate of deoxalatation residues of the aluminate liquor is less than or equal to 50%. 25 5. Deoxalation process according to any one of claims 2 to 4 wherein said tamper is performed between 40 and 60 ° C, preferably around 40 ° C. 1196 0 -17- 6. deoxalation process according to any one of claims 1 to 5oWhich said industrial waters include raw water (EB), waters from the Bayer cycle such as sodium distilled water (EDS), and pond return water (ERB) . 5 7. Deoxalation process according to any one of claims 1 to 6 wherein said deoxalated and decarbonated industrial waters (EF) are directed to the washing chain insoluble residues of the Bayer chain (LVBR). 10 The deoxalation process according to any of claims 2 to 6, wherein a flocculant (fO) is added to the scrubbing water, for the purpose of improving the settling of the solid products resulting from the jamming. 9. Use of the deoxalation residues (S4) of a sodium aluminate liquor (L6) of a Bayer circuit for producing alumina, said residues resulting from the action of a destabilizing agent (C1) based on divided lime refinement which causes the precipitation of sodium oxalate, characterized in that it makes it possible to decarbonate and deoxalate the aqueous effluents having a total soda content of less than 10 gNa2Otot / liter 10. Use of the deoxalation residues (S4) of a sodium aluminate liquor (L6) of a Bayer alumina production circuit, said residues resulting from the action of a destabilizing agent (Cl) at divided lime-fining base which causes the precipitation of sodium oxalate, characterized in that it makes it possible to generate sodium hydroxide in a carbonated water, such as a pond return water or a raw water supplemented with sodium carbonate having a total sodium content of less than 10 g Na 2 Otot / liter 1196 11. A decarbonation and deoxalation process of aqueous effluents having a total soda content of less than 10 g Na2Otot / litcharacterized in that said effluents are used to wash the deoxalation residues (S4) of a liquor sodium aluminate (L6) of a Bayer alumina production circuit, said residues resulting from the action of a finely divided lime destabilizing agent (Cl) or reaction products of the lime with the sodium aluminate such as tricalcium aluminum hexahydrate. 10