HRP930104A2 - Process of sodium oxalate elimination from sodium aluminate solutions through bayer cyclic process - Google Patents

Description

Technical feature

This invention relates to the process of eliminating sodium oxalate, which is contained in sodium aluminate solutions, and results from the decomposition of alkali in bauxites according to the Bayer process.

Technical regime

Bayer's process, widely described in professional literature, consists of the basic technique of producing alumina intended for transformation into aluminum by means of hot electrolysis. According to this procedure, bauxite ore is subjected to hot treatment with the help of an aqueous solution of sodium hydroxide, with an appropriate concentration, and this causes the melting of alumina and a solution supersaturated with sodium aluminate is obtained. After separation from the solid phase consisting of undecomposed ore precipitate (red precipitate), a solution supersaturated with sodium aluminate, also called Bayer's solution, is decomposed by sieving with particles of aluminum trihydroxide in order to promote separation (precipitation) in the form of aluminum trihydroxide in solution . The alumina-poor sodium aluminate solution is therefore recycled to the decomposition stage after it has been concentrated and filled with sodium hydroxide to achieve the appropriate concentration again during ore decomposition. But, at the same time as the bauxite alumina is converted into sodium aluminate, the solution supersaturated with sodium aluminate resulting from the decomposition becomes progressively turbid with organic compounds formed after the more or less complete decomposition of the moist substances contained in the ore. Those organic compounds that appear in the form of organic sodium salts and above all in the form of sodium oxalate prove to be very inconvenient. More precisely, these disadvantages are related to the accumulation of sodium oxalate in Bayer's solution, and result from the fact that said oxalate, which has reached a critical supersaturation level, appears in the form of fine needles on the surface of aluminum hydroxide. The fine needles of sodium oxalate act like real germs that affect the precipitation of aluminum hydroxide, provoking an increase in the number of fine particles that become so numerous that they can no longer be effectively controlled during the decomposition of sodium aluminate.

In this way, the precipitation of sodium oxalate affects the physio-chemical qualities of the aluminum trihydroxide that has been separated, and thus leads to large variations in the granulometry of the alumina produced in this way, as well as to the fragility of the resulting grains, which can represent the most unfavorable circumstance when using such alumina in the production of aluminum through electrolysis .

Therefore, it is necessary to control or even better avoid contamination with aluminum trihydroxide from the very beginning during the stage of decomposition by means of separated sodium oxalate in the industrial processes of alumina production.

There are various procedures for limiting the presence of sodium oxalate in Bayer solution solutions.

Processes that provide for the destruction or direct decomposition of wet substances contained in the ore, for example by roasting, are rarely used in industry due to their high cost.

There are more well-known procedures that consist in acting on the decomposition products of a supersaturated solution by washing the aluminum trihydroxide intended for production or a part of the aluminum trihydroxide recycled at the beginning of the decomposition stage. Thus, sodium oxalate dissolved in the washing liquids is selectively eliminated by means of hot separation, and calcium oxalate is formed. In any case, such procedures do not allow performing the aluminum trihydroxide separation step under the best productivity conditions. Namely, significant contamination of deposited oxalate on the grains of aluminum trihydroxide can cause brittleness of the grains, which manifests itself in the calcination phase during the thermal decomposition of the oxalate contained in the crystalline structure.

To avoid these disadvantages, it is necessary to maintain the concentration of sodium oxalate in the sodium aluminate solution during decomposition at a value lower than the critical supersaturation concentration; and this without reducing the amount of wet substances in the solution whose stabilizing effect on the solution is already well known by enabling the regulation of the critical threshold of sodium oxalate supersaturation in the solution.

Several procedures have been proposed to limit the amount of sodium oxalate in Bayer's solution. For this purpose, destabilization of the supersaturation is regularly induced on one part or on the entire decomposed solution, which is already supersaturated with sodium oxalate, in order to separate and separate the sodium oxalate from such a supersaturated solution.

Therefore, the process, which is described in patent USP 3899571 (EP-A-0013407), consists in treating a supersaturated Bayer solution until the sodium oxalate equilibrium is reached (such as solutions poor in sodium aluminate formed after decomposition, whether or not reconcentrated) by adding the recycled sodium oxalate compound to induce precipitation of the sodium oxalate in the solution and achieve the equilibrium solubility concentration of the dehydrated sodium oxalate.

After the solid-liquid separation, which was carried out with the help of filtration, the purified solution is subjected to the Bayer cycle process, and the sodium oxalate solid phase fraction is used to prepare a suspension of crystalline particles, while the other fraction is eliminated from the process.

Although the liquid addition process has proven to be effective in provoking the precipitation of sodium oxalate, it is still unsuitable for industrial use. Namely, the sodium oxalate crystals found in the substance suddenly become inactive due to the fact that their surface is polluted by the organic substances present, and therefore it is necessary to proceed with the washing of such particles, which is a very delicate procedure. In the case when the rinsing is not sufficient, there is a reduced activity of the substance, and thus a reduced effectiveness of sodium oxalate separation. In the case when the rinsing is too rapid, the granulometric fragmentation of the substance occurs, which makes the solid-liquid separation much more difficult, and hence the purification efficiency is reduced.

Instead of destabilizing the Bayer solution supersaturated with sodium oxalate by adding sodium oxalate, patent US 4597952 (EP-A-0173630) provides for the use of calcium oxalate or barium oxalate whose action indirectly leads to the same result. Namely, calcium or barium oxalate, unstable in a strong alkaline Bayer solution, releases the oxalate ion to form sodium oxalate, which contributes to increasing the concentration of sodium oxalate in the solution above the critical supersaturation threshold, thus promoting the precipitation of sodium oxalate. The amount of oxalate thus obtained reaches the solubility limit of oxalate under experimental conditions, which depends on the temperature of the solution and the concentration of sodium hydroxide in the solution.

With this process, sodium oxalate separates are obtained at the same time, which are finely dispersed in Bayer's solution, and therefore difficult to separate by decanting or filtering without auxiliary means. Therefore, recycling of the substance is required, so a part of the separated sodium oxalate must be recycled to regenerate the precipitate of calcium or barium oxalate after the elimination of the organic substances present. This regeneration is performed by placing at least partially separated sodium oxalate in a liquid solution, and treating the resulting suspension with lime (CaO) or barium aluminate (Al2O4Ba). This treatment leads to obtaining calcium oxalate or barium oxalate separates, which are recycled in the precipitation of sodium oxalate.

Problem setting

The method should solve a double problem: the separation of fine particles of sodium oxalate in Bayer's solution and the recycling of oxalate precipitates, with the aim of preserving the effectiveness and especially the selectivity in the sodium oxalate precipitation processes with the addition of oxalate substances, as outlined in the process of this invention.

The subject of this invention

This invention is based on the assumption that it is possible to induce considerable precipitation of sodium oxalate in Bayer's solution from a heterogeneous substance based on finely divided lime in the form of a single separate that is easily separated by filtration without having to use additional filtration. In order to selectively precipitate sodium oxalate without separating coarse substances and thus without modifying in a risky way the threshold of critical supersaturation of sodium oxalate in the Bayer solution, as well as the filterability properties of that solution, it is necessary to adhere to operating conditions that are firmly defined, especially with regard to the temperature at which this solution precipitation takes place.

More precisely, the invention relates to the process of eliminating sodium oxalate in at least one part of the solution or solution of sodium aluminate separated during the Bayer cycle of alumina production from bauxite through the stage of decomposition and concentration of said solution intended for recycling, such as the alkaline solution in crushed bauxite ore, which includes the precipitation of dissolved sodium oxalate with the addition of a factor destabilizing the state of supersaturation with sodium oxalate, then through separation by filtering the sodium oxalate thus precipitated, which procedure is characterized by the fact that the factor destabilizing the supersaturation with sodium oxalate is also a means of additional filtration. This factor is left in contact for over one hour with a solution of sodium aluminate which is cooled between 40°C and 60°C and which is based on finely divided lime. This lime is finally added to magnesium in a proportion that does not exceed 40% of the weight of the mixture thus obtained.

It can be surprisingly found that lime, which is sometimes used as a filtration agent to facilitate the separation of some solid impurities from the Bayer solution, can be very effectively replaced at a temperature between 40°C and 60°C with sodium barium and calcium oxalate substances as confirmed in earlier practice. Due to the strongly alkaline properties of the Bayer solution after decomposition and concentration up to 60°C, a mixture of lime with solution elements in the solution, especially not with the oxalate ion C2O4-2 and with coarse substances remaining in the solution, cannot be used. It should be emphasized that lime can come into contact with sodium aluminate solution in the form of quicklime CaO or in the form of milk of lime whose concentration of CaO ranges between 100 and 300 g/l, but the solubility balance of sodium oxalate has not changed in any way.

It should be noted that lime primarily acts as a substance that promotes sedimentation of oxalate in a supersaturated solution in the manner of ordinary mechanical liquid/solid contact, which in this case is significant considering the large surface that is created in contact between fine lime dust and the solution. However, it is important to note that above 60°C this deposition effect is suddenly lost, and above 70°C deposition no longer takes place.

Parallel to the temperature range of 40°C - 60°C, it can be stated that after a sufficiently long contact that enables the deposition of small particles of sodium oxalate crystals in a solution with lime particles, the solid substance thus obtained can be easily filtered. It follows that after drying the insoluble cake in order to extract the maximum of the impregnation solution, it no longer contains valuable substances and can be completely thrown away. This differs from processes in which the oxalate precipitate is used to collect at least one part and regenerate it through complex processes.

It should also be noted that the critical supersaturation threshold, after which sodium oxalate spontaneously sediments, decreases with decreasing temperature. Already from 40°C, one should be afraid of irregular precipitation of fine particles of sodium oxalate on parts that are not foreseen in the process, and these very fine particles are very difficult to filter. If the concentration of caustic soda in the solution (concentration of free soda plus combined soda in AlO2Na expressed in Na2O g/l) also tends to decrease the critical threshold of sodium oxalate supersaturation. This means that it is primarily affected by the presence of poorly decomposed solids in the solution. This threshold will be higher the more organic substances are in the solution, and therefore it can easily be concluded that the process according to this invention will be more efficient the higher this threshold is and the higher the amount of deposited oxalate in relation to the volume of the solution. This is the reason why this invention finds its best application in the processing of tropical bauxite trihydrate. These hydragylite-based bauxites are rich in solids and are subjected to an alkaline solution at an average temperature of ≤ 150°C in such a way that the solution is progressively enriched with sodium oxalate as a result of the gradual decomposition of solids. It follows from this that the content of soluble sodium oxalate can range between 0.3 to 0.6% of the weight of oxalic carbon in caustic soda (free soda plus soda in the AlO2Na group) before it reaches values of 0 .15 to 0.25 %. It should also be noted that in the case of bauxite monohydrate, which contains little solids and which in addition crushes at high temperature (≥200°C), the critical supersaturation threshold does not exceed 0.15 to 0.25% of the weight of the oxalic carbon present in the caustic soda, considering the fact that it is present in very small amounts in a solution of solids. This threshold can be artificially increased up to 0.3 or 0.5% by adding an anionic polyelectrolyte of synthesis such as polyacrylamide, polyacrylic acid to the solution at any time of the Bayer cycle, and in accordance with the elaboration from patent EP 0173630 (US 4597952), at the request of the customer. The process of retarding the precipitation of sodium oxalate using a synthesis factor, which is similar to that for solids or organics that are slightly decomposed and naturally present in the Bayer solution, allows the process of this invention to be applied to all bauxites with the same effectiveness as tropical bauxite trihydrate.

The general procedure for the purification of oxalate from Bayer solutions which is decomposed and then concentrated, in order to achieve a concentration of caustic soda expressed in Na20 in which the caustic soda is contained between 170 and 250 g/l and the concentration of sodium oxalate expressed in the carbon oxalate present in the caustic soda in the amount between 0.3 and 0.6%, it is performed only on one part (fraction), which represents 3 to 20% of the total amount of solution after concentration, and at a temperature ranging between 40°C and 60°C, which means that requires cooling that portion of the Bayer solution. Quicklime of which 2% by weight of grains is less than 10 micrometers or milk of lime of which 20% of average weight of grains is less than 10 micrometers are added regularly to the chilled solution which is continuously stirred to create a very homogeneous suspension of lime with a concentration of CaO between 2 and 20 g/l solution. For this purpose, after adding the lime suspension, it is still continuously stirred for over one hour. The solid phase of the resulting solution consists of a mixture of fine particles of lime and sodium oxalate, which is crystallized and separated by decantation and filtration or direct filtration. It should be noted that the separation is relatively easy considering that it is possible to collect at least 36 kg of dry cake per hour per m2 of filter area with a content of free soda expressed in Na20 up to 3% of the untreated dry cake. After separation, the solid phase without valuable substances can be added to the red mud or sludge for rejection, while the solution, whose concentration of soluble sodium oxalate is reduced and is lower than the 0.25% of carbon oxalate present in caustic soda, is added to the basic fraction of the unpurified mixture with oxalate to form a solution that is strongly alkaline, which is recycled as a solution by the decomposition of bauxite.

Practical application of the invention

The realization of the invention under favorable operating conditions will be easier to understand from the description based on the general scheme of the procedure (S. 1).

According to drawing 1, the Bayer solution Lo, to which an anionic polyelectrolyte 10 is optionally added before the decomposition stage, for example: FLOERGER (R) type AN 934SH in a ratio of 20 mg per liter of solution in order to raise the critical threshold if necessary supersaturation with oxalate to an average of 0.5% of carbon oxalate in relation to the caustic soda that has been decomposed and after the separation of the sedimented aluminum trihydroxide, the resulting solution L1 is concentrated by evaporation in such a way that the caustic soda concentration is contained between 170 and 250 g Na2O/l and preferably between 190 and 210 g Na2O/l.

Fraction L4, preferably present at 4 to 6% by volume of the concentrated solution L2, is separated to undergo the deoxalation treatment according to the present invention.

The significance of the separated fraction L4 is determined by the amount of sodium oxalate that must be eliminated from each cycle in order to avoid the progressive saturation of the Bayer solution with sodium oxalate and any risk of improper precipitation of this oxalate on the grains of aluminum trihydroxide during decomposition. It should be noted that the increase of oxalate in the Bayer solution, faster or slower depending on the nature and origin of the bauxite, is conditioned by the transfer of oxalates present in the solution to the bauxite ore during alkaline decomposition, but also by the progressive degradation in the form of sodium oxalate of organic substances already dissolved in recycled Bayer solution.

After cooling to a temperature ranging between 40°C and 60°C, the cooled solution L5 whose critical supersaturation threshold has been reached, even exceeded, with a decrease in temperature, is brought into contact in the first moving reactor R1 with finely divided quicklime SO in order to formed a homogeneous suspension whose concentration preferably ranges between 7 to 9 g of CaO per liter of solution L5.

The thus obtained suspension S1 is transferred to the secondary mobile reactor, then to the third mobile reactor R3. The total flow of time required to bring the quicklime into contact with the liquor in this case through three reactors in series is preferably between 3 and 5 hours. The suspension S3 leaving the reactor R3 is filtered. This filtration through the filter press is very fast and takes place at a rate of 1.5 m3/hour per m2 of filtering surface.

After filtering and drying the insoluble S4 cake whose free Na2O content is lower than 3%, it is mixed with red sludge for rejection. sodium oxalate-poor solution L6 with a concentration between 0.15 and 0.25% of carbon oxalate present in caustic soda is mixed with the main fraction of solution L3 which is not deoxalated to achieve alkaline solution L7 which is recycled as a solution from the decomposition of bauxite ore.

Application examples

Example 1

According to the process of this invention, 40 m3/hour of the separated industrial solution L4 was processed during the decomposition of the solution and concentration L2, the flow of which is 1000 m3/hour, which was basically obtained by the alkaline decomposition of tropical bauxite trihydrate at a temperature of 105°C. The obtained solution L4 had the following composition:

Na2O caustic: 200 g/l

Na2O carbonate: 25 g/l

Al2O3: 120 g/l

Sodium oxalate expressed in C oxalate: 0.90 g/l

Amount of oxalate C/caustic Na2O: 0.45 %.

After cooling to 40°C, the cooled solution L5 is mixed with 270 kg of quicklime to which 30 kg of magnesium is added in the first mobile reactor to create a homogeneous suspension with a concentration of 8 g Cao + MgO/l. Three hours after mixing in the reactor, the suspension, which has a volume of 40 m3, is filtered for at least one hour on a filter of 30 m2. After drying the insoluble cake S4 is treated with at least 3% of free soda expressed as Na2O, and solution L6 is re-introduced into the Bayer cycle to be mixed with 960 m3 of solution L3 which is not deoxalinized, and recycled as a decomposition solution containing only 0.37 g of carbon oxalate per liter, i.e. C ox/Na2O caust.=0.18 %.

During this process (0.88 - 0.37) 4,104 g, or approximately 20.4 kg of oxalate carbon corresponding to 113.8 kg of crystallized sodium oxalate, was eliminated during the cycle.

Example 2

According to the conditions from example 1, 40 m3/h of industrial solution L4 obtained by decomposition at 250°C of bauxite monohydrate of Mediterranean origin was processed. Solution L4, whose threshold of critical supersaturation with sodium oxalate was raised to 0.68 g/l of oxalate carbon by adding the anionic polyelectrolyte Floerger A N 934 SH, in a ratio of 20 mg/l, had the following composition:

Na2O caustic: 195 g/l

Na2O carbonate: 22 g/l

Al2O3: 120 g/l

Sodium oxalate expressed as C oxalate: 0.58 g/l corresponds to the amount of C ox./Na2O caustic. of 0.30%, which is significantly lower than the critical supersaturation threshold of 0.68 g/l, which corresponds to an amount of 0.35%.

After filtering for at least one hour and drying, the insoluble cake S4 is treated with 3% of free soda expressed as Na2O, and the deoxalinized solution L6 contains only 0.19 g of oxalic carbon per liter, i.e. the amount of C ox/Na2O caustic of 0, 1%.

During this procedure (0.58 - 0.19) 4,104 g or approximately 15.6 kg of oxalic carbon corresponding to the amount of 87 kg of sodium crystallized oxalate was eliminated during the cycle.

Example 3

According to the process of this invention, 40 m3/hour of industrial solution L4 obtained in the process of decomposition and concentration of L2, whose flow rate is 1,000 m3/h and which is essentially the result of the alkaline construction of tropical bauxite trihydrate at a temperature of 105°C, was processed. The obtained liqueur L4 has the following composition:

Na2O caustic 205 g/l

Na2O carbonate: 24 g/l

Al2O2: 120 g/l

Sodium oxalate expressed as C oxalate: 0.88 g/l

Amount of oxalate C/Na2O caustic: 0.43 %

After cooling to 40°C, the cooled solution L5 is mixed with 270 kg of quicklime to which 30 kg of magnesium is added in the first mobile reactor in order to form a homogeneous suspension with a concentration of 8 g CaO + MgO/l. After three hours of treatment in a moving reactor of a suspension with a volume of 40 m3, it is filtered for at least one hour on a filter with a surface area of 30 m2. After drying the insoluble cake S4, which contains less than 3% of free soda expressed as Na2O, the solution L6 is introduced into

Bayer cycle to mix with 960 m3 of solution L3 which is de-oxalinized and recycled as a digestate containing only 0.37 g of oxalic carbon per liter, i.e. C ox/Na2O caust. = 0.18%. During this procedure (0.88 - 0.37) 4,104 g, or about 20.4 kg of oxalic carbon, which corresponds to the weight of 113.8 kg of crystallized sodium oxalate, is eliminated during the cycle.

Example 4

A second aliquot of L4 solution L2 from Example 3 is subjected to the procedure under the same precipitation conditions as that of Example 3 with the exception that the temperature of the solution is maintained at 80°C instead of 40°C. It was determined, regardless of the less favorable permeability of the insoluble solid cake, that with an increase in the filtration time at a constant volume (1h10' for 40 m3 of filtration solution) there is a slight decrease in the amount of sodium oxalate expressed as C oxalate of 0.88 g/l in the initial solution to 0.86 g/l in the filtered solution. In turn, the content of organic carbon, which was 7.2 g/l in the initial solution, is 5.3 g/l in the filtered solution, confirming that coarse or slightly degraded organic substances have undergone partial purification, and that they are sedimented or probably insoluble after mixing with magnesia lime. This confirms that the addition of a lime-based mixture has no effect on the oxalate content in the solution.

Example 5

A third aliquot L4 of solution L2 from Example 3 is subjected to the process under the same precipitation conditions as those from Example 3 with the exception that the temperature is maintained at 60°C and that the mixture of 300 kg of stabilization factor consists in this case of a mixture containing 180 kg of CaO and 120 kg of MgO (60 % / 40 % by weight). A good filterability of the insoluble cake as in example 3 was determined, and a significant reduction in the amount from 0.88 g/l to 0.39 g/l, which corresponds to Coxal/Na2O caustic. = 0.19%, as well as that during the treatment cycle of 40 m3 solution, 4,104 g or about 19.6 kg of oxalate carbon was eliminated (0.88 - 0.39), which corresponds to the weight of 109.4 kg of crystallized sodium oxalate.

At the same time, a slight decrease in the content of organic carbon was determined from 7.2 g/l in the initial solution to 6.7 g/l in the filtered solution after deoxalination, which indicates the assumption that it started with the non-dissolution of slightly degraded solids or organic substances.

Claims (15)

1. A process for removing sodium oxalate from at least a portion of a solution or liquor of sodium aluminate taken from a Bayer cycle for the production of aluminum from bauxite after stages of decomposition and thickening of said liquor intended for recycling as alkaline liquor for etching bauxite ore, comprising precipitation of dissolved sodium oxalate by means of for destabilizing the state of supersaturation of sodium oxalate, then separating the thus precipitated sodium oxalate by filtration, characterized in that the agent for destabilizing the state of supersaturation of sodium oxalate, which also acts as a filter additive, and which is brought into contact with sodium aluminate cooled to a temperature between 40 °C and 60°C, during a period longer than one hour, on the basis of finely divided lime. 2. The method according to claim 1, characterized in that the destabilizing agent is lime with the addition of magnesium oxide in a proportion that does not exceed 40 wt. % of the mixture created in this way. 3. The method according to claim 1, characterized in that the destabilizing agent is quicklime. 4. The method according to claim 1, characterized in that the destabilizing agent is milk of lime in which the concentration of CaO is between 100 and 300 g/liter. 5. The process according to claim 1, characterized in that the sodium aluminate solution or liquor removed after decomposition and thickening has a concentration of sodium hydroxide expressed as Na2O between 170 and 250 g/l. 6. The process according to claim 5, characterized in that the sodium aluminate solution or liquor removed after decomposition and thickening has a concentration of sodium hydroxide expressed as Na2O between 190 and 210 g/l. %. 7. The method according to claim 1 or claim 6, characterized in that the sodium aluminate solution or liquor removed after decomposition and thickening has a sodium oxalate ratio expressed as a mass ratio of oxalate carbon and caustic Na2O between 0.3% and 0.6%. 8. The method according to any one of claims 1, 5, 6, 7, characterized in that the portion of concentrated lye removed represents 3 to 20% of the total volume of concentrated lye. 9. The method according to claim 8, characterized in that the portion of concentrated lye removed represents 4 to 6% of the total volume of concentrated lye. 10. The process according to any one of claims 1 to 9, characterized in that the homogeneous suspension of lime in the cooled liquor has a concentration between 2 and 20 g per liter. 11. The method according to claim 10, characterized in that the homogeneous suspension of lime in the cooled liquor has a concentration between 7 and 9 CaO g per liter. 12. The process according to any one of claims 1 to 11, characterized in that the time involving the contact of the lime with the cooled liquor for the formation of a suspension which is maintained in a restless state is preferably between 3 and 5 hours. 13. The method according to any one of claims 1 to 8, characterized in that after the CaO/lye contact time of at least one hour, the solid phase of the suspension is emptied after filtration and drying, while the liquid phase obtained from the lye with reduced oxalate content is recycled mixed with with the main fraction of non-des-oxalate liquor as liquor for etching bauxite ore. 14. The method according to claim 1, indicated by the fact that an anionic polyelectrolyte is first added to the sodium aluminate solution or liquor at any stage of the Bayer cycle, preferably before decomposition, if the critical supersaturation threshold with respect to the sodium oxalate in the liquor to be removed does not exceed 0.15 - 0.25 wt. % of oxalate carbon with respect to sodium hydroxide. 15. The method according to claim 14, characterized in that the amount of added anionic polyelectrolyte is of the order of 20 mg per liter of liquid.