AU5226800A - Method for purifying sodium aluminate liquors containing sodium oxalate and enhancing residues - Google Patents

Description

PURIFICATION PROCESS FOR SODIUM ALUMINATE SOLUTIONS CONTAINING SODIUM OXALATE ENABLING RESIDUE RECYCLING FIELD OF THE INVENTION The invention relates to a purification process for sodium aluminate solutions resulting from the alkaline corrosion of bauxites according to the Bayer 5 process and containing sodium oxalate. STATE OF THE RELATED ART In EP 0 555 163, a particularly effective deoxalation process is known, specifying the use of a 10 solid, finely divided and inexpensive material - lime used as seed for the precipitation of sodium oxalate in concentrated decomposed aluminate solution. In fact, sodium oxalate does not precipitate directly on the fine lime particles but on the solid products of the 15 reaction between the lime and the aluminate solution belonging to the tricalcium aluminate hexahydrate family: A1203,3CaO,6H20. In the rest of this document, we will refer to all these reaction products together as "AC3,6". 20 PROBLEM POSED This process is used in numerous industrial facilities and gives full satisfaction for most bauxites to be processed. However, it proves to be 25 relatively expensive in high capacity facilities processing trihydrate or mixed bauxites, particularly of African or Indian origin, which are particularly rich in organic substances, said substances being broken down during the alkaline corrosion particularly 30 in the form of a large quantity of sodium oxalate, 2 which requires the implementation of frequent deoxalation operations. This is followed by a high consumption of lime and a loss of soda and alumina carried with the deoxalation residue. 5 To reduce the lime consumption and alumina and soda loss, the applicant attempted, in WO 97/03924, to recycle the destabilisation agent, i.e. the AC3,6 produced by the reaction of the lime with the aluminate solution, which comes in the form of finely divided 10 particles: instead of disposing of these particles coated with precipitated sodium oxalate, WO 97/03924 proposes to wash them so as to redilute the sodium oxalate and then reintroduce these washed particles to prime deoxalation, the washing water enriched with 15 sodium oxalate being evacuated elsewhere. However, the dilution must be significant and the industrial installation of the washing and filtration lines required by this operation are too costly to be easily paid off by the reduction in lime consumption alone. 20 Indeed, there is no reduction in the soda and alumina loss, with the impregnation of the deoxalation residues taken with the washing water. Therefore, the problem obtaining an inexpensive aluminate solution deoxalation process, using the least 25 destabilising agent possible and which does not increase soda and alumina losses remained unsolved. OBJECT OF THE INVENTION The applicant has succeeded in solving this 30 problem by perfecting its process described in EP 0 555 163, i.e. using the lime deoxalation residue to causticise, i.e. decarbonate and deoxalate the industrial water, e.g. the water used for the washing 3 line for the insoluble residue produced by the alkaline corrosion of the bauxite, referred to as "inert residue". The applicant was surprised to note that, washed in a highly dilute medium with water from industrial facilities such as water from red mud lake, current water and 5 alkaline condensate, said deoxalation residue - essentially AC3, 6 - makes it possible to causticise, i.e., decarbonate and deoxalate these waters which can thus be rerouted directly after the washing in mixing tank of the deoxalation residue, to the industrial facilities, e.g., to the inert residue washing line. To enable the dissolution of the sodium oxalate, the washing in mixing tank of the deoxalation residue must be carried out in a 10 highly dilute soda medium: the total Na 2 O content must be limited to 10 g/l and preferably to 3 g/l. The solid products resulting from the washing in mixing tank are evacuated. They consist of calcium carbonate and oxalate precipitates. The compound CaC 2 0 4 , H 2 0 was identified by X-ray diffraction. They have a shape and size such that liquid - solid is separation is easily performed by filtration, and even, using adjuvants such as anionic polyelectrolytes, by flocculation and decantation. Accordingly, a first aspect of the present invention provides a sodium oxalate elimination process for use in an industrial facility implementing the Bayer alumina hydrate production process and using industrial water, said process comprising, in at least 20 one fraction of the decomposed Bayer aluminate solution being removed after .. concentration, precipitation of the oxalate dissolved in the fraction by means of a finely divided lime-based destabilisation agent, followed by the separation of the solid deoxalation residue from the fraction, the resulting solution being recycled as a bauxite alkaline corrosion solution, said process characterised in that the solid deoxalation residue 25 from the separation is used to decarbonate and deoxalate said industrial water, said industrial water having a total caustic concentration of less than 10 g/l. A second aspect of the present invention provides use of the deoxalation residue of a sodium aluminate solution of a Bayer alumina production circuit, said residue resulting from the action of a finely divided lime-based destabilisation agent which induces the --. 30 precipitation of sodium oxalate, characterised in that it enables the decarbonation and deoxalation of water-based effluents with a total soda content of less than 10 g Na 2 O tot/litre. A third aspect of the present invention provides use of the deoxalation residue of a sodium aluminate solution of a Bayer alumina production circuit, said residue resulting 3a from the action of a finely divided lime-based destabilisation agent which induces the precipitation of sodium oxalate, characterised in that it enables the generation of sodium hydroxide in carbonated water, such as water from red mud lake or current water supplemented with sodium carbonate, with a total soda content of less than 10 g Na 2 O 5 tot/litre. A fourth aspect of the present invention provides use of decarbonation and deoxalation process for water-based effluents with a total soda content of less than 10 g Na 2 O tot/litre, characterised in that said effluents are used for the washing in mixing tank of the deoxalation residue of a sodium aluminate solution of a Bayer alumina production 10 circuit, said residue resulting from the action of a finely divided lime-based destabilisation agent or lime-sodium aluminate reaction products, such as tricalcium aluminate hexahydrate. Therefore, the applicant has discovered that tricalcium aluminate hexahydrate is capable of precipitating calcium oxalate under sludge tank concentration conditions, with 15 limited soda concentrations: less than 10 g Na 2 0/1. Since the precipitation kinetics of calcium oxalate decrease markedly when the soda concentration increases, it is preferable to limit this concentration to 8 g Na 2 0/1, 4 optimal washing in mixing tank efficiency being obtained with a concentration of approximately 3 g Na20/l. This low concentration makes it necessary to use large quantities of water or ensure that the lowest 5 possible deoxalation residue impregnation rate is obtained. Preferably, an impregnation rate of this residue less than or equal to 50%, which may be achieved easily with a filter press, is targeted. Since the precipitation kinetics of calcium oxalate decrease 10 when the temperature increases, this mixing should be carried out in the 40 - 60*C temperature range, preferably in the vicinity of 40*C. During the mixing, the oxalate ion C 2 0 4 2- released reacts with the AC3,6 family compounds to produce 15 calcium oxalate CaC 2 04,H 2 0 which precipitates from the sodium aluminate and sodium hydroxide. The deoxalation taking place during the washing in mixing tank may be represented as follows: 3 CaO, A1 2 0 3 , 6H 2 0 + 6 Na* + 3C 2 0 4 2- + 3 H 2 0 ++ 20 3 CaC 2 0 4 , H 2 0 + 2 Na' + 2[Al(OH) 4 ]~ + 4 Na+ + 4 OH~ The reaction is in fact more complex due to the presence of not entirely converted humic matter that co-precipitates ' with the sodium oxalate and is reintroduced into the solution during washing in mixing 25 tank. In addition, some industrial waters are not pure. If they are carbonated, which is generally the case with water from red mud lakes from Bayer line facilities which are rich in sodium carbonate, the 30 washing in mixing tank effect results in the precipitation of calcium carbonate such that a decarbonated water is subsequently obtained which may be rerouted to the industrial facilities, which is 5 particularly advantageous if they are liable to pipe scaling. The decarbonation taking place during the washing in mixing tank may be represented as follows: 3 CaO, A1 2 03, 6H20 + 6 Na* + 3C032- ++ 5 3 CaC03, + 2 Na* + 2[Al(OH)4] + 4 Na* + 4 OH~ The carbonate ion C032 reacts with the AC3,6 family compounds to produce calcium carbonate CaCO 3 which precipitates from the sodium aluminate and sodium hydroxide. In this way, the decarbonation produced by 10 the washing in mixing tank with the lime deoxalation residue according to EP 0 555 163 makes it possible to generate free soda from sodium carbonate, a product present in carbonated water or that may be added to current water, since it is less costly than soda. If 15 the water from red mud lake is particularly carbonated, it may be necessary to add some lime into the mixture before or during washing in mixing tank. The water from red mud lake may itself be oxalated. This is frequently the case with water from 20 red mud lake from Bayer line facilities and this treatment makes it possible to deoxalate said water before routing it, for example, to the alkaline corrosion residue washing line. In the preferred embodiment according to the 25 invention, the deoxalation residue washing water is mixed with the water entering the line to wash the insoluble residue from the bauxite alkaline corrosion. In this case, the alumina, soda and humic matter that were previously lost with the evacuation of the 30 deoxalation residue are no longer lost since this decarbonated and deoxalated water is reintroduced into the Bayer cycle. In addition, soluble soda, generated 6 both by the deoxalation and decarbonation, is introduced into the Bayer circuit in this way. In the inert residue washing line, the washing water flows in the opposite direction to the solid 5 alkaline corrosion residue, which passes through a series of vessels referred to as washers. In each of these vessels, the solid particles fall to the bottom by decantation and the underflow is pumped to supply the next washing vessel. In the opposite direction, the 10 washing water is gradually concentrated with alumina and soda, with the clear solution of each washer supplying the previous washer. At the end of this reverse-flow line, the clear solution from the first washer is used to dilute and cool the sodium aluminate 15 suspension in the Bayer circuit resulting from the alkaline corrosion of the bauxite, which favours the separation of the aluminate solution from the insoluble residue, e.g. by decantation, and initiates the decomposition of the aluminate solution, a complex 20 phenomenon with slow kinetics. In general, those skilled in the art perform a causticising operation on the clear solution from one of the washers (e.g. the second) to decarbonate the washing line water, which, since it is to be introduced 25 into the aluminate solution, must comprise the least carbonate ions possible to, among other things, improve Bayer solution productivity and prevent the scaling of the pipes forming the Bayer circuit. Due to the invention, use in the washing line of decarbonated 30 water following the washing in mixing tank with the deoxalation residue makes it possible, in certain cases, particularly when water from red mud lake is 7 used, to reduce the causticising performed on the clear solution from one of the washers of the washing line. Since the alumina, soda and humic matter are not evacuated with the deoxalation residue and since 5 soluble soda is generated by the washing in mixing tank, the total water entering the washing line, which is the mixture of this treated water and the rest of the washing water, is slightly more concentrated in alumina and soda. Due to this higher concentration at 10 the washing line inlet, the alumina and soda losses induced by the evacuation of the inert residue are slightly higher, but the overall result is positive since the majority, which is not evacuated, of the alumina, soda and humic matter returns in the opposite 15 direction of the inert residue washing and is then poured into the aluminate suspension produced by the corrosion so as to dilute this suspension before decantation and decomposition. The return of humic matter into the Bayer cycle is 20 beneficial since it regularises the humic matter concentration of the aluminate solution. The applicant has noted that the humic matter present in the aluminate solution reduces the risk of premature precipitation of the oxalates during the decomposition 25 of the solution and that it is advisable to maintain the most regular humic matter concentration possible in the aluminate solution. For the use of water from red mud lake, the washing in mixing tank of this water with the lime 30 deoxalation residue according to the invention makes it possible to reduce the lime consumption considerably in the washing line for the washer clear solution decarbonation washing line.

8 The solid products resulting from the washing in mixing tank with the deoxalation residue are easily isolated from the water rerouted to the industrial facilities. Their evacuation offers an advantage with 5 reference to the process described in EP 0 555 163 since, in this case, residue essentially composed of calcium carbonate and oxalate, products which are chemically inert with respect to the environment, are evacuated. 10 EMBODIMENTS OF THE INVENTION - EXAMPLES The embodiment of the invention is easier to understand through the description of the following examples. 15 Example 1, illustrated in figure 1 and taken as the reference, is very similar to example 3 of EP 0 555 163. Example 2, illustrated in figure 2, describes the improvements provided according to the present invention to the deoxalation process of EP 0 555 163, 20 wherein alcaline condensate from the industrial processing of a Bayer circuit is used for washing in mixing tank. Example 3, also illustrated in figure 2, describes the improvements provided according to the present invention to the decarbonation process of EP 0 25 555 163, wherein highly carbonated water from red mud lake from an industrial facility using a Bayer circuit is used for washing in mixing tank. In both figures, the washing or washing in mixing tank water flows are represented by double lines. 30 EXAMPLES Example 1 (prior art reference) 9 In figure 1, the supersaturated aluminate solution LO, possibly supplemented with an anionic polyeleCtrolYte 10 intended to raise the critical oxalate supersaturation threshold if required, is 5 decomposed P. After the separation of the precipitated aluminium trihydroxide, the resulting aluminate solution, referred to as the decomposed solution, Ll is concentrated by evaporation V such that its caustic soda concentration is between 170 and 250 Na20/litre, 10 preferably between 190 and 210 Na20/litre. A fraction L4 is removed to undergo the deoxalation treatment and then, once treated, the fraction L6 is mixed with the main fraction L3 to form an alkaline corrosion A solution L7 of the ore BX. The 15 aluminate solution SA produced by the alkaline corrosion A is then cooled and diluted DI with the water ED from the reverse-flow washing LVBR of the insoluble residue BR resulting from the alkaline corrosion. The industrial water EI used for this 20 washing line consists of current water, alcaline condensate or, if permitted by the facility, water from red mud lake. The supersaturated suspension SS is then introduced into a decanter DE to separate this insoluble residue BR from the supersaturated aluminate 25 solution LO. The fraction L4 removed for deoxalation in the concentrated decomposed solution L2 preferably represents 4 to 6% of the volume of said concentrated decomposed solution L2. The quantity of the fraction to 30 be removed is determined by the quantity of sodium oxalate to be eliminated at each cycle to prevent progressive poisoning of the solution with sodium oxalate and any risk of untimely precipitation of this 10 oxalate on aluminium trihydroxide particles during decomposition. After cooling R at a temperature between 40 and 60'C, the cooled solution L5 for which the critical 5 sodium oxalate content supersaturation threshold has been reached, or even exceeded, is placed in contact with finely divided quick lime C1 in a first stirred reaction vessel R1, to form a homogeneous suspension Si with a concentration preferably between 7 and 9 g of 10 CaO per litre of solution L5. The suspension S1 formed in this way is transferred to a second stirred reaction vessel R2 and then to a third R3. The total quick lime - aluminate solution contact time is between 3 and 5 hours. The 15 suspension S3 leaving the third stirred reaction vessel R3 is filtered. The filtration F, carried out on a filter press, is very quick, of the order of 1.5 m 3 /hour of suspension per m 2 of filtering surface area. After filtration and drying, the insoluble cake S4 20 with a free Na20 content of less than 3% is mixed with the inert residue to be dumped. The sodium oxalate depleted solution L6 with a concentration between 0.15 and 0.25% of oxalic carbon with reference to caustic soda is mixed with the main fraction L3 of non 25 deoxalated solution to form an alkaline solution L7 recycled as a bauxite ore corrosion solution. In this case, 40 m3/hour of industrial solution L4 removed from the decomposed and concentrated solution L2 circuit with a flow rate of 100 m3/hour is treated. 30 This solution is essentially produced by the alkaline corrosion at 105'c of a tropical trihydrate bauxite. The solution L4 removed had the following composition: Caustic Na20: 205 g/l 11 Carbonated Na20: 24 g/l A1203= 120 g/l Sodium oxalate expressed as Cox (oxalic carbon) 0.88 g Cox/l (or Cox/Na2Octq = 0.43% and Cox/na20tot: 5 0.39%). After cooling to 50 0 C, the cooled solution L5 is mixed with 320 kg/hour of quick lime to form a homogeneous suspension with a concentration of 8 g/CaO/litre and after 3 hours of contact in a stirred 10 reaction vessel, the suspension representing a volume of 40 m3 is filtered in less than one hour on a 30 m2 filter. After drying, the wet insoluble cake S4 has a free soda titre of 3% expressed as Na20 and the solution L6 15 is returned to the Bayer circuit by mixing it with the fraction not removed from the concentrated decomposed solution L3, representing a flow rate of 960 m3/hour. The solution 16 contains only 0.37 g of Cox /[(Cox/Na2Octq = 0.18%). 20 During this treatment, approximately 21 kg/hour of oxalic carbon, equivalent to 117 kg/hour of crystallised sodium oxalate, is eliminated. After drying, the impregnated wet insoluble cake S4 is evacuated at a rate of 1680 kg/hour. The 25 precipitated oxalate induces the loss of 108 kg/hour of soda expressed as Na20. The impregnation contains soda bound to the alumina (approximately 30 kg/hour), carbonated soda (approximately 75 kg/hour) and free soda (NaOH), the loss of which, expressed in terms of 30 Na20, is of the order of 55 kg/hour. The total soda loss is of the order 270 kg/hour. Therefore, the evacuation of the deoxalation residue induces the loss of: 12 - ~320 kg/hour of quick lime - x270 kg/hour of total soda - ~225 kg/hour of alumina, whether insoluble or carried in the impregnation. 5 This evacuation also induces the loss of the humic matter that co-precipitated with the sodium oxalate. Depending on the Bayer cycle, this loss may be between 10 and 20 kg/hour of humic matter. Example 2 (Figure 2) - Washing in mixing tank of 10 deoxalation residue with a mixture of current water and alcaline condensate Figure 2 shows the above deoxalation treatment improved by the recycling of the deoxalation residue. In this example, the washing in mixing tank is only 15 performed with current water and alcaline condensate. The insoluble cake S4 obtained by filtration F is carried with its impregnation to a vessel in which it is washed LVS4 in a mixing tank with the alcaline condensate EDS at a rate of 120 m 3 /hour. After a 20 contact time of 2 hours, the mixture is then filtered. The residue is essentially composed of sodium oxalate and carbonate. Its impregnation is highly dilute in soda, which induces a soda loss of less than 1 kg/hour. It is evacuated under conditions that are 25 not aggressive with respect to the environment. The filtrate EF is mixed with the rest of the washing water. The washing line water EI has a slightly higher concentration of soda and alumina due to the contribution of the water from the washing in mixing 30 tank. This slightly higher concentration is the cause of the slightly higher soluble soda losses in the washing line: of the order of 186 kg/hour compared to 13 approximately 104 kg/hour when the washing water did not undergo the treatment according to the invention. The result is positive since: - the 270 kg/hour of total soda lost in the 5 process according to the prior art are partly recovered in the Bayer cycle, with the overall loss reduced to approximately 82 kg/hour; - the 225 kg/hour of alumina lost in the process according to the prior art are mostly reintroduced into 10 the Bayer cycle. The overall alumina loss is reduced to approximately 42 kg/hour; - while in the process according to the prior art, with the evacuation of the impregnation, 55 kg/hour of free soda were lost, the 320 kg/hour of lime used for 15 deoxalation generated 51 kg/hour of free soda, but, in this case, the free soda loss excess in the insoluble residue evacuation is 50 kg/hour; in this way, the second use of 320 kg/hour of lime made it possible to recover practically all the free soda lost in the prior 20 art. Example 3 - (Figure 2) Washing in mixing tank of deoxalation residue with water from red mud lake In this example, the washing in mixing tank is only performed with water from red mud lake ERB. 25 Said water from red mud lake has a total soda concentration of 1.5 g/l; it is highly carbonated and rich in oxalates. The introduction of such water into the washing line represents a quantity of 84 kg/hour of carbonated soda and 0.6 kg/hour of oxalate expressed as 30 oxalic carbon Cox. In this case, the soluble total Na20 loss in the evacuation of the insoluble residue represents 202 kg/hour, compared to the value of 104 14 kg/hour for the use of current water and alcaline condensate in the above example. The insoluble cake S4 obtained by filtration F is carried with its impregnation to a vessel in which it 5 is washed LVS4 in a mixing tank with the water from red mud lake at a rate of 120 m 3 /hour. After a contact time of 2 hours, the mixture is then filtered. In this case, it is not necessary to add calcium C2 to complete the decarbonation. 10 The residue is essentially composed of calcium oxalate and carbonate. Its impregnation is highly dilute in soda, which induces a soda loss of less than 1.5 kg/hour. It is evacuated under conditions that are not aggressive with respect to the environment. 15 The filtrate EF is mixed with the rest of the washing water. The washing line water EI has a slightly higher concentration of soda and alumina due to the contribution of the water from the washing in mixing tank. This slightly higher concentration is the cause 20 of the slightly higher soluble soda losses in the washing line: of the order of 283 kg/hour compared to approximately 202 kg/hour when the washing water did not undergo the treatment according to the invention. The result is positive since: 25 - the 270 kg/hour of total soda lost in the process according to the prior art are partly recovered in the Bayer cycle, with the overall loss reduced to approximately 81 kg/hour; - the 225 kg/hour of alumina lost in the process 30 according to the prior art are mostly reintroduced into the Bayer cycle. The loss is reduced to 45 kg/hour; - while in the process according to the prior art, with the evacuation of the impregnation, 55 kg/hour of 15 free soda were lost, the 320 kg/hour of lime used for deoxalation generated 179 kg/hour of free soda, but, in this case, the free soda loss excess in the insoluble residue evacuation is 51 kg/hour; in this way, the 5 second use of 320 kg/hour of lime resulted, in addition to the recovery of the free soda lost in the prior art, in the generation of 128 kg/hour of free soda. ADVANTAGES OF THE PROCESS ACCORDING TO THE INVENTION 10 It makes it possible to reduce alumina and soda losses, carried by the deoxalation process, by adding lime to the concentrated decomposed solution according to the prior art. The return of humic matter into the Bayer cycle 15 regularises the humic matter concentration of the aluminate solution, which has a beneficial effect with respect to the risk of premature oxalate precipitation during decomposition. The process makes it possible to reduce the 20 washing line causticising treatment when highly carbonated water, such as water from red mud lake, is recycled.

Claims (11)

1. Sodium oxalate elimination process in an industrial facility implementing the Bayer alumina hydrate production process and using industrial water, said process comprising, in at least one fraction (L4) 5 of the decomposed Bayer aluminate solution (L2) removed after concentration (V), the precipitation of the oxalate dissolved by means of a finely divided lime based destabilisation agent (Cl) followed by the separation (F) of the solid deoxalation residue (S4) 10 from the solution (L6) recycled as a bauxite alkaline corrosion solution, characterised in that the deoxalation residue (S4) from the separation is used to decarbonate and deoxalate said industrial water.

2. Deoxalation process according to claim 1 15 wherein the treatment of said industrial water consists of washing in mixing tank the deoxalation residue (S4) of the aluminate solution (L4).

3. Deoxalation process according to claim 2 wherein the washing i~n mixing tank water has a total 20 soda concentration of less than 10 g/l, preferably less than 3 g/l.

4. Deoxalation process according to any of claims 1 to 3 wherein the impregnation rate of the deoxalation residue of the aluminate solution is less than or equal 25 to 50%.

5. Deoxalation process according to any of claims 2 to 4 wherein said washing in mixing tank is carried out between 40 and 60'C, preferably in the vicinity of 400C. 30

6. Deoxalation process according to any of claims 1 to 5 wherein said industrial water comprises current 17 water (EB), water from the Bayer cycle such as alcaline condensate (EDS), and water from red mud lake (ERB).

7. Deoxalation process according to any of claims 1 to 6 wherein said deoxalated and decarbonated 5 industrial water (EF) is routed to the insoluble residue washing line of the Bayer line (LVBR).

8. Deoxalation process according to any of claims 2 to 6 wherein a flocculent (fO) is added to the washing in mixing tank water, in order to improve the 10 decantation of the solid products resulting from the washing in mixing tank.

9. Use of the deoxalation residue (S4) of a sodium aluminate solution (L6) of a Bayer alumina production circuit, said residue resulting from the action of a 15 finely divided lime-based destabilisation agent (Cl) which induces the precipitation of sodium oxalate, characterised in that it enables the decarbonation and deoxalation of water-based effluents with a total soda content of less than 10 Na20tot/litre. 20

10. Use of the deoxalation residue (S4) of a sodium aluminate solution (L6) of a Bayer alumina production circuit, said residue resulting from the action of a finely divided lime-based destabilisation agent (Cl) which induces the precipitation of sodium 25 oxalate, characterised in that it enables the generation of sodium hydroxide in carbonated water, such as water from red mud lake or current water supplemented with sodium carbonate, with a total soda content of less than 10 Na20tot/litre. 30

11. Decarbonation and deoxalation process for water-based effluents with a total soda content of less than 10 Na20tot/litre, characterised in that said effluents are used for the washing in mixing tank of 18 the deoxalation residue (S4) of a sodium aluminate solution (L6) of a Bayer alumina production circuit, said residue resulting from the action of a finely divided lime-based destabilisation agent (Cl) or lime 5 sodium aluminate reaction products, such as tricalcium aluminate hexahydrate. 3 line for the insoluble residue produced by the alkaline corrosion of the bauxite, referred to as "inert residue". The applicant was surprised to note that, washed 5 in a highly dilute medium with water from industrial facilities such as water from red mud lake, current water and alcaline condensate, said deoxalation residue - essentially AC3,6 - makes it possible to causticise, i.e. decarbonate and deoxalate these waters which can 10 thus be rerouted directly after the washing in mixing tank of the deoxalation residue, to the industrial facilities, e.g. to the inert residue washing line. To enable the dissolution of the sodium oxalate, the washing in mixing tank of the deoxalation residue must 15 be carried out in a highly dilute soda medium: the total Na20 content must be limited to 8 g/l and preferably to 3 g/l. The solid products resulting from the washing in mixing tank are evacuated. They consist of calcium 20 carbonate and oxalate precipitates. The compound CaC 2 0 4 , H 2 0 was identified by X diffraction. They have a shape and size such that liquid - solid separation is easily performed by filtration, and even, using adjuvants such as anionic polyelectrolytes, by 25 flocculation and decantation. Therefore, the applicant has discovered that tricalcium aluminate hexahydrate is capable of precipitating calcium oxalate under sludge tank concentration conditions, with limited soda 30 concentrations: less than 10 g Na20/l. Since the precipitation kinetics of calcium oxalate decrease markedly when the soda concentration increases, it is preferable to limit this concentration to 8 g Na20/l,