WO2017221173A1

WO2017221173A1 - Process for treating a solid carbonaceous material containing aluminum, fluorides and sodium ions

Info

Publication number: WO2017221173A1
Application number: PCT/IB2017/053699
Authority: WO
Inventors: Massimo Maccagni; Edoardo GUERRINI
Original assignee: Engitec Technologies S.P.A.
Priority date: 2016-06-24
Filing date: 2017-06-21
Publication date: 2017-12-28
Also published as: ITUA20164638A1; RU2019100218A3; CN109689939B; RU2019100218A; EP3475470A1; EP3475470B1; CN109689939A; RU2742864C2

Abstract

The present invention relates to a process for treating a solid carbonaceous material containing aluminum, fluorides and Na⁺ ions, comprising the following steps: (a) leaching said solid carbonaceous material with at least one aqueous alkaline solution to form: (i) an extraction solution comprising said aluminum in hydrosoluble form, said fluorides and said Na⁺ ions, and (ii) at least one solid insoluble residue; (b) separating said insoluble solid residue from said extraction solution; (c) subjecting said extraction solution free from said solid insoluble residue to a membrane electrolysis process to form at least one precipitate comprising said aluminum and at least one aqueous solution of NaOH. The above process is particularly suitable for the treatment of spent potlinings (SPLs) deriving from electrolytic reduction cells of alumina for the recovery of aluminum, fluorides, and sodium ions contained therein.

Description

PROCESS FOR TREATING A SOLID CARBONACEOUS MATERIAL CONTAINING ALUMINUM, FLUORIDES AND SODIUM IONS

The present invention relates to a process for treating a solid carbonaceous material containing aluminum, fluorides and sodium ions. The process according to the present invention is particularly suitable for the treatment of spent potlinings deriving from production processes of primary aluminum.

Aluminum is generally produced by the reduction of aluminum-based minerals, mainly bauxite. The reduction reaction is carried out in a electrolytic reduction cell (pot) containing molten cryolite (NasAlFe) and other salts, such as aluminum fluoride (A1F₃) , alkaline metal fluorides (e.g. NaF, LiF) and alkaline-earth metal fluorides (e.g. CaF₂, MgF₂) . The walls and bottom of the electrolytic reduction cell are internally coated with a lining based on a carbonaceous material. When the cell is operating, the coating gradually deteriorates allowing various materials to penetrate into its interior. The deterioration of the lining continues with time until the functioning of the cell must be interrupted to replace the deteriorated lining {spent potlining) with a new lining. The spent potlining, also indicated hereafter as SPL, generally also comprises a part of the wall of the refractory material located outside the cell which is removed together with the internal lining.

Due to the operating conditions of the cell, SPLs contain significant quantities of aluminum, fluorides, alkaline metals - in particular sodium - and alkaline- earth metals, and other substances such as, for example, nitrides and cyanides. The presence of these substances, which are dangerous for the health of human beings and for the environment, and their relative ease of extraction by water, require that SPLs be suitably treated in order to reduce the content of dangerous substances to acceptable levels, before being definitely disposed of in landfills.

Various treatment processes of SPLs are known in the state of the art, aimed at recovering at least a part of the substances contained therein, in particular aluminum and fluorides, and possibly recycling them in the production process of primary aluminum.

Known treatment processes comprise, for example, combustion, roasting, or the reaction of SPLs with quicklime. Hydrometallurgical treatment processes of SPLs are also known, based on the extraction of the substances of interest from the carbonaceous material by means of acid or caustic solutions.

The treatment processes of the known art generally involve high energy consumptions and/or considerable costs linked to the high quantities of reagents used for treating the SPLs. Furthermore, in many cases, the processes of the known art only allow an effective recovery of some of the substances of interest, generating further residues containing contaminating substances which in turn require specific treatments before allowing them to be safely disposed of.

An example of hydrometallurgical treatment of SPLs aimed at recovering fluorides, is described in US 4,816,122.

In a first embodiment, the process described in US 4,816,122 provides the extraction of SPLs with a concentrated solution of caustic soda to form an extraction solution containing sodium fluoride (NaF) and sodium aluminate (NaAl(OH)₄) . Due to the lower solubility of fluorides in concentrated solutions of caustic soda, part of the fluorides, if not all, remain in the solid insoluble residue in the form of NaF. After separating the extraction solution, the NaF is extracted from the solid insoluble residue with water or with a diluted aqueous solution of NaF.

In a second embodiment, the process described in US

4,816,122 provides the extraction of SPLs with a solution of diluted caustic soda for dissolving a greater quantity of sodium fluoride and forming an insoluble solid residue. The caustic solution containing sodium fluoride is concentrated by evaporation to precipitate solid NaF which, after being separated from the concentrated solution, is re- dissolved in water.

Both of the above embodiments lead to the separation and recovery of solid NaF, which is then re- dissolved to form an aqueous solution to be subjected to electrodialysis for producing an aqueous solution of HF and an aqueous solution of NaOH. In a subsequent step, the aqueous solution of HF can be reacted with alumina (AI₂O3) to produce A1F₃, so as to obtain a reusable product in the bath of the electrolytic reduction cell to produce primary aluminum. The solution of NaOH produced by electrodialysis can be recycled to the purification process of the aluminum mineral (Bayer process) to produce alumina to be fed to the electrolytic reduction cell.

When the extraction is carried out with a concentrated caustic solution, most of the aluminum contained in the SPLs is solubilized in the form of sodium aluminate. Also in this case, the sodium aluminate can be recycled to the Bayer process, possibly after being dried by means of spray-drying. When, on the other hand, the extraction is effected with a diluted caustic solution, a significant part of the aluminate remains in the caustic solution which has been concentrated by evaporation; this fraction of aluminum is destined for disposal.

The process described in US 4,816,122 has various drawbacks. Even if it allows the decontamination of the SPLs from the fluorides and the recovery of the same as salts of A1F₃ in the electrolytic reduction cell of aluminum, this process, in fact, involves a relatively high number of operative steps, and is therefore somewhat complex to manage.

In addition, the use of a diluted caustic solution for leaching the SPLs, involving a solvent evaporation step, makes the SPL treatment process unattractive due to the high energy consumptions.

Furthermore, the aluminum originally present in the SPLs is not separated and recovered from the caustic extraction solution in which it is dissolved in the form of aluminate and it can therefore only be usefully recycled to the Bayer process.

A further drawback of this process lies in the fact that the separation of the aluminate from NaF is not sufficiently forced (part of the aluminate remains in the solutions of NaF and part of the NaF remains in the solutions containing the aluminate) for allowing the final production of reusable materials having a high degree of purity.

In consideration of the above state of the art, the need is thus felt for having a process for the treatment of SPLs and other carbonaceous materials containing aluminum, fluorides and sodium ions, which allows at least some of the drawbacks of the processes of the known art to be overcome.

The Applicant has therefore established the primary objective of providing a process for the treatment of carbonaceous materials containing aluminum, fluorides and sodium ions, in particular SPLs, in a simple and effective way, at least partially avoiding the drawbacks of the known art.

In particular, an objective of the present invention is to provide a process for the treatment of the above-mentioned carbonaceous materials, in particular SPLs, which allows the content of substances potentially dangerous for the health of human beings and the environment to be almost completely eliminated therefrom, or at least reduced.

A second objective of the present invention is to provide a process which allows substances to be recovered from the above carbonaceous materials, in particular SPLs, which can be re-used in the production process of primary aluminum, in particular in the electrolytic reduction cell of alumina, in an effective and economically convenient way.

A further objective of the present invention is to provide a process which, starting from the above- mentioned carbonaceous materials, in particular SPLs, allows the selective production of the most desired compounds containing aluminum, in relation, for example, to the specific requirements of the plant in which the production cycle of primary aluminum is carried out.

The Applicant has now found that the above and other objectives, which are better illustrated in the following description, can be reached by a treatment process in which SPLs or other carbonaceous material containing aluminum, fluorides and sodium ions, is subjected to leaching with a diluted or concentrated alkaline aqueous solution, to form an extraction solution containing the above-mentioned elements in a hydrosoluble form; the extraction solution is subsequently subjected to a membrane electrolysis process which allows the precipitation of one or more of aluminum compounds and, separately, the formation of an aqueous solution of NaOH.

Through the membrane electrolysis treatment, the precipitated aluminum compounds (for example cryolite, aluminum fluoride, aluminum hydroxide or a mixture of the same) can be easily recovered, also with a high degree of purity, and re-used in the production cycle of primary aluminum. In particular, these products can be re-used by feeding them, also directly, into a cell for the electrolytic reduction of alumina.

Advantageously, furthermore, by suitably modifying the composition of the extraction solution obtained in the leaching step, it is possible to easily control the chemical composition of the aluminum compounds formed during the membrane electrolysis. This makes the process of the invention particularly versatile and adaptable to the requirements of a specific production plant in which it is implemented. The membrane electrolysis treatment also generates an aqueous solution of NaOH which can be recovered and re-used for leaching additional SPL, with a consequent saving of reagents.

By suitably recirculating the electrolytic solutions used in the membrane cell, the process of the invention can be carried out in a continuous mode, in a simple and effective way, with a reduced consumption of chemical reagents and electric energy with respect to the processes of the known art.

The insoluble solid residue resulting from the leaching step can be safely disposed of in landfills. Depending on the content of impurities still present, this solid residue can also be used as auxiliary fuel in combustion processes or, if particularly pure, as material for constructing the lining of the electrolytic reduction cell of alumina.

The implementation of the process for the treatment of SPLs according to the present invention within an electrolytic production cycle of primary aluminum, therefore allows the overall productivity of the latter to be increased, as it allows a more effective exploitation of the materials fed to the reduction cell (in particular alumina, cryolite and sodium fluoride) and, at the same time, it simplifies the disposal of SPLs deriving from the same production cycle of primary aluminum.

According to a first aspect, the present invention relates to a process for treating a solid carbonaceous material containing aluminum, fluorides and Na⁺ ions which comprises the steps:

(a) leaching said solid carbonaceous material with at least one aqueous alkaline solution to form: (i) an extraction solution comprising said aluminum in a hydrosoluble form, said fluorides and said Na⁺ ions, and (ii) at least one solid insoluble residue;

(b) separating said solid insoluble residue from said extraction solution;

(c) subjecting said extraction solution free from said solid insoluble residue to a membrane electrolysis process to form at least one precipitate comprising said aluminum and at least one aqueous solution of NaOH.

For the purposes of the present description and relative claims, the expression "carbonaceous material containing aluminum, fluorides and sodium ions" is used for indicating various solid materials that contain aluminum, fluorides and sodium ions, including SPSs deriving from electrolytic reduction processes of alumina, carbon dust coming from aluminum melting processes and wastes of spent baths used in electrolytic reduction cells (aluminum dross) . For the sake of simplicity, in the following description, reference will be made to the treatment of SPLs, as an example of carbonaceous materials containing aluminum, fluorides and sodium ions that can be treated with the process of the present invention.

For the purposes of the present description and relative claims, the expression "membrane electrolysis" indicates an electrochemical process implemented in a electrolytic cell in which at least one ion-selective membrane is present, which separates at least two compartments; the ion-selective membrane can only be crossed by positively charged ionic species (cationic permselective membrane) or only by negatively charged ionic species (cationic permselective membrane) , that migrate from one compartment to the other of the cell under the influence of an electric field generated by a difference in electric potential applied to the electrodes of the cell; membrane electrolysis allows the separate recovery of at least one precipitate comprising aluminum and a solution of NaOH. For the purposes of the present invention, membrane electrolysis also comprises electrodialysis processes.

For the purposes of the present description and relative annexed claims, the verb "comprise" and all terms deriving therefrom, also includes the meaning of the verb "consist" and terms deriving therefrom.

The numerical limits and ranges expressed in the present description and enclosed claims also include the numerical value (s) mentioned. Furthermore, all the values and sub-ranges of a limit or numerical range should be considered as being specifically included as if they were explicitly mentioned.

According to the present invention, the leaching step is carried out with an alkaline aqueous solution (leaching solution), preferably an aqueous solution of NaOH, KOH or mixtures thereof. As the quantity of sodium ions present in SPLs is generally prevalent with respect to that of the other alkaline ions, the leaching solution is preferably an aqueous solution of NaOH.

The leaching solution preferably has a pH within the range of 10-14, more preferably within the range of 12-14.

The leaching can be effected with diluted or concentrated leaching solutions (e.g. aqueous solutions of NaOH or KOH) . As is known, with an increase in the concentration of the base in the leaching solution, the solubility of the fluoride ions decreases, whereas that of the aluminate ions increases. Consequently, if the leaching is aimed at mainly solubilizing in the extraction solution, the fluoride ions contained in the SPL, a diluted leaching solution is preferably used. If, on the other hand, the leaching is aimed at solubilizing the aluminum contained in the SPL by bringing it into the extraction solution in the form of aluminate ions, a concentrated leaching solution is preferably used.

In the case of aqueous solutions of hydroxides of alkaline metals (e.g. NaOH or KOH), the diluted leaching solution preferably has a concentration of hydroxide equal to or higher than 1% by weight and lower than 10% by weight, more preferably within the range of 5% - 8%, said percentages referring to the weight of the diluted leaching solution.

In the case of aqueous solutions of hydroxides of alkaline metals (e.g. NaOH or KOH), the concentrated leaching solution preferably has a concentration of hydroxide equal to or higher than 10% by weight and lower than 40% by weight, preferably within the range of 15% - 25%, said percentages referring to the weight of the concentrated leaching solution.

In a preferred embodiment, the leaching step comprises at least a first leaching operation carried out with a concentrated leaching solution and a subsequent washing operation of the resulting insoluble residue with water or with a very diluted leaching solution, preferably having a concentration of hydroxide equal to or lower than 3% by weight with respect to the weight of the solution. In this way, in fact, the extraction of both aluminum and fluorides from the SPL is maximized.

The extraction solutions deriving from leaching with diluted or concentrated leaching solutions together with the washing solutions, can be joined to form a single extraction solution to be subjected to the subsequent membrane electrolysis step.

The leaching can be effected with a leaching solution having room temperature (25°C) or higher than 25°C, more preferably within the range of 70°C - 95°C.

The duration of the leaching is generally selected in relation to the quantity of SPL to be treated, the temperature and concentration of the leaching solution. Preferably, the duration of the contact between SPL and solution ranges from 15 to 300 minutes.

Preferably, the ratio between the quantity of SPL and leaching solution is within the range of 20 - 400 g SPL/1 leaching solution, more preferably within the range of 70 - 150 g/1.

In order to decompose the cyanides possibly present in the extraction solution, the extraction solution can be treated with one or more oxidizing agents, before subjecting said extraction solution to electrolysis. The extraction solution, for example, can be subjected to treatment with ozone (O₃) to chemically decompose the possible cyanides dissolved. Alternatively, the SPL can be subjected to leaching after being calcined to thermally decompose the possible cyanides present.

As the aluminum is mainly present in SPLs in the form of alumina (AI₂O₃) , cryolite (Na₃AlF₆) and aluminium fluoride (AIF₃) , the main dissolution reactions that involve these compounds during leaching are the following (1) - (3) :

AI2O3 + 2 NaOH + 3 H₂0 → 2 NaAl(OH) (1)

Na₃AlF₆ + 3 NaOH → 6 NaF + Al (OH) ₃ (2) AIF3 + 3 NaOH → 3 NaF + Al (OH) ₃ (3) . At the end of the leaching step, the insoluble solid residue can be separated from the extraction solution by means of filtration and/or decanting, for example, or with other techniques known to skilled person in the art.

The insoluble solid residue can also be washed in order to reduce the quantity of fluorides remaining after the leaching with a diluted aqueous solution of NaF. The washing solution containing the fluorides extracted from the insoluble solid residue can then be joined with the extraction solution obtained from the leaching step or it can be treated according to known techniques for producing, for example, an aqueous solution of NaOH and A1F₃, as described for example in US 4,816,122.

According to the present invention, the extraction solution comprising the leaching products, namely at least fluoride ions, sodium ions and aluminum in the form of sodium aluminate, is subjected to electrolysis in a membrane cell to recover the electrolysis products separately, i.e. a precipitate containing aluminum and a solution containing NaOH.

The present invention exploits the electrolytic process for causing, through the H⁺ ions produced in the anodic reaction, the precipitation of an insoluble compound of aluminum in a first compartment of the membrane cell. The OH^~ ions produced in the cathodic reaction simultaneously form NaOH in a second compartment of the cell, together with the Na⁺ ions present in the extraction solution.

In a first preferred embodiment, the membrane electrolysis process comprises the following steps:

(i) providing at least one electrolytic cell comprising :

- at least one anode compartment comprising at least one anode immersed in an anolyte;

- at least one cathode compartment comprising at least one cathode immersed in a catholyte;

said anode compartment being separated from said cathode compartment by at least one permselective cationic membrane;

(ii) supplying said extraction solution to said anode compartment .

According to this embodiment, during the electrolytic process, due to the electrolysis of the water, H⁺ ions are formed in the anode compartment which, by locally lowering the pH of the anolyte, cause the formation of a solid precipitate containing aluminum. An aqueous dispersion of a substantially insoluble compound of aluminum is therefore formed in the anode compartment, which can be easily separated from the aqueous fraction.

During the electrolysis, moreover, due to the electric field present in the cell, the Na⁺ cations of the extraction solution present in the anode compartment migrate towards the cathode compartment through the permselective cationic membrane. In particular, the permselective cationic membrane allows the Na⁺ cations present in the anode compartment to migrate into the cathode compartment and prevents, instead, the anions present in the cathode compartment (OH-) from migrating into the anode compartment.

The electrolysis of the water in the cathode compartment, produces OtT ions which, together with the Na⁺ cations that have selectively migrated from the anode compartment to the cathode compartment, form NaOH. In the possible presence of other cations, in particular cations of alkaline and alkaline earth metals, the above OtT ions also form the corresponding hydroxides .

The catholyte supplied to the cathode compartment is preferably an aqueous solution of NaOH. In this way, at the end of the electrolysis, a solution of NaOH can be recovered, having a higher concentration with respect to that of the starting catholyte.

The solution of NaOH which is formed in the cathode compartment can be at least partially used as leaching solution for leaching further SPL.

The precipitate containing aluminum which is formed in the anode compartment can, for example, be an aluminum salt (e.g. NasAlFe) , an aluminum hydroxide (Al(OH)₃) or a mixture of the above compounds depending on the composition of the anolyte.

The composition of the precipitate containing aluminum which is formed in the anode compartment during the membrane electrolysis can be advantageously determined by suitably varying the composition of the extraction solution before the electrolysis.

By adding, for example, further fluoride ions to the extraction solution before the electrolysis, a precipitate substantially composed of Na₃AlF₆ can be obtained. Alternatively, by precipitating the fluoride ions initially present in the extraction solution and separating them from the same before the electrolysis, on the contrary, the formation of a precipitate substantially composed of Al(OH)₃, can be obtained, which, after drying, can be converted into AI₂O₃ (alumina) . The precipitation of the fluoride ions can be obtained, for example, by adding ions of at least one alkaline-earth metal, preferably Ca²⁺ ions, to the extraction solution.

The process according to the present invention can therefore be easily adapted to the requirements of the user, based, for example, on the need for obtaining cryolite or alumina to be fed to the bath of the electrolytic cell to produce primary aluminum.

According to a second preferred embodiment of the present invention, the recovery of a precipitate containing aluminum and the separate recovery of an aqueous solution of NaOH, can be advantageously obtained by effecting the membrane electrolysis in a cell comprising at least one permselective cationic membrane and at least one permselective anionic membrane.

According to this second preferred embodiment, the membrane electrolysis process comprises the following steps :

(i) providing at least one electrolytic cell comprising:

at least one anode compartment comprising at least one anode immersed in an anolyte; - at least one cathode compartment comprising at least one cathode immersed in a catholyte;

at least one supply compartment interposed between said anode compartment and said cathode compartment ;

said supply compartment being separated from said anode compartment by at least one permselective cationic membrane;

said supply compartment being separated from said cathode compartment by at least one permselective anionic membrane;

(ii) supplying said extraction solution to said feeding compartment.

During the electrolysis, due to the electric field present in the cell, the aluminate and fluoride ions present in the extraction solution migrate from the central supply compartment towards the positively charged anode, passing through the permselective anionic membrane. The Na⁺ cations, on the other hand, migrate in the opposite direction, towards the negatively charged cathode, passing through the permselective cationic membrane. As the H⁺ ions that are generated in the anode compartment cannot migrate towards the cathode compartment due to the permselective anionic membrane, they combine, in the anode compartment, with the aluminate and fluoride ions that have migrated from the supply compartment, causing the precipitation of aluminum as A1F₃, cryolite or as a mixture thereof.

Advantageously, in order to favour the formation of A1F₃, fluoride ions can be added in the anode compartment, for example by feeding an aqueous solution of HF during the electrolytic process.

As the Otr ions that are generated in the cathode compartment cannot migrate towards the anode compartment due to the permselective cationic membrane, they combine instead with the Na⁺ ions that have migrated from the supply compartment forming NaOH.

Also in the case of a cell having at least two membranes, the catholyte supplied to the cathode compartment is preferably an aqueous solution of NaOH.

The anolyte supplied to the anode compartment is preferably an acid electrolytic solution. The pH of the anolyte is preferably within the range of 0-5, more preferably within the range of 0-3. The anolyte, for example, can be selected from halogen acids, in particular HF, sulfuric acid, phosphoric acid and the like .

According to a third preferred embodiment of the present invention, the membrane electrolysis process can be carried out in an electrodialysis apparatus. The electrodialysis technique, using one or more cells comprising a cationic membrane and an anionic membrane, allows a precipitate containing aluminum to be obtained and, separately, a solution of NaOH, substantially exploiting the same separation principle described above for the cell comprising a cationic membrane and an anionic membrane.

Further examples of electrodialysis processes and the relative equipment that can be used for the purposes of the present invention are described in US 4, 107, 264.

The membrane electrolysis process according to the present invention can be carried out either batchwise or in continuous. Said process is preferably carried out in continuous, i.e. by continuously feeding the extraction solution to the membrane cell (for example, in the anode compartment or in the supply compartment) and continuously extracting from the same, an equal volume of treated solution containing precipitated aluminum.

The electrolytic solution used as catholyte is also preferably fed in continuous to the membrane cell and continuously extracted from the same in an equal volume, so as to become progressively enriched with NaOH or another alkaline metal hydroxide that is formed during the electrolytic process. In another preferred embodiment, the catholyte is recirculated in continuous to the cathode compartment and water is simultaneously fed to the same compartment in such a quantity as to keep the concentration of NaOH constant in the catholyte extracted from the same compartment.

In general, the additions of material in the compartments of the electrolytic cell and the withdrawals of material from the same can be effected through one or more ducts positioned directly in the compartments or along the recirculation circuits of the electrolytic solutions to the same compartments. The additions and withdrawals are preferably effected along the recirculation circuits, as this helps to keep the electrolysis conditions stable in the cell.

In a preferred embodiment of the invention, the extraction solution is supplied to the cell in a compartment in which an electrolytic solution having a desired pH is present, for example a pH value equal to or close to that at which the electrolytic precipitation of the aluminum can start (pH about 8- 10.5) . In this case, the extraction solution to be treated is preferably fed to the cell in such a quantity as to not substantially alter the pH of the electrolytic solution.

The electrolytic solution having a desired pH is preferably an aqueous solution comprising at least one salt composed of at least one cation of an alkaline or alkaline-earth metal and at least one anion having an oxidation potential higher than the oxidation potential of water under the conditions in which the membrane electrolysis step is carried out. The anion is preferably soluble in water within the pH range of the leaching solution. The anion is preferably selected from sulphate, nitrate, perchlorate, phosphate, borate and mixtures thereof, more preferably from sulphate, nitrate, perchlorate and mixtures thereof. The above cation is preferably Na⁺.

The addition of the above salt - hereinafter also indicated as "support electrolyte" - offers the advantage of allowing the membrane electrolysis to be exerted with continuity and reduced energy consumptions .

In the case of electrolysis, the formation of the aluminum precipitate leads to a progressive reduction in the conductivity of the anolyte with a consequent increase in the cell voltage and, therefore, energy consumptions of the process. The presence of the support electrolyte, on the other hand, ensures a sufficiently high conductivity of the anolyte, reducing the energy consumptions.

The support electrolyte can be added to the extraction solution or, more preferably, introduced into the recirculation line of the anolyte in the anode compartment. Alternatively, the electrolysis can be started using an aqueous solution comprising the support electrolyte as anolyte, to which the extraction solution to be treated is subsequently added.

The support electrolyte is preferably present in the anolyte in a quantity within the range of 0.1-3 moles/1 of monovalent cations.

The precipitate containing aluminum can be separated from the anolyte by means of conventional solid/liquid separation systems known in the art. The precipitate can be separated, for example, by decanting. The decanted precipitate can then be filtered, washed and dried.

The supernatant fraction that is recovered from the precipitate during the decanting or filtration can be advantageously partially recycled to the electrolytic treatment step, joining it with the alkaline extraction solution to be treated, so as to recirculate in the cell, the aluminum still present in the same in hydrosoluble form.

The process of the invention can be carried out in electrochemical cells of the type known in the state of the art. The density of current applied to the electrodes is preferably selected within the range of 100-5, 000 A/m².

Some preferred embodiments of the present invention are now described with reference to the following enclosed figures:

Figure 1, which schematically represents the process of the invention wherein the membrane electrolysis is effected in a single-membrane cell;

- Figure 2, which schematically represents the process of the invention wherein the membrane electrolysis is carried out in a two-membrane cell.

With reference to the scheme of Figure 1, an aliquot of SPL 18 is treated in a leaching unit L with an alkaline aqueous solution 19 under the conditions previously described, in order to obtain an extraction solution 21 comprising at least aluminum in hydrosoluble form, fluorides and sodium ions. The extraction solution 21 is fed, through lines 11 and 1, to the anode compartment 2 of an electrolytic cell 3. The cell 3 comprises a cathode compartment 4. The anode compartment 2 and the cathode compartment 4, respectively comprise an anode 5 and a cathode 6. The anode 5 and cathode 6 are separated in the respective compartments 2 and 4 by means of a permselective cationic membrane 7.

An electrolytic solution (catholyte) is fed in continuous to the cathode compartment 4, which, after being subjected to electrolysis, is extracted from the cathode compartment 4 and recirculated to the head of the same compartment by means of the recirculation line 8; said electrolytic solution is preferably a solution of NaOH.

By applying an adequate difference in electric potential to the electrodes 5 and 6, the water present in the cell is electrolyzed with the formation of H⁺ ions and gaseous O2 in the anode compartment 2, and OH^~ ions and gaseous ¾ in the cathode compartment 4. Due to the effect of the difference in electric potential applied, the alkaline ions present in the anode compartment 2 (mainly Na⁺) migrate towards the cathode compartment 4. The oxygen formed at the anode 5 is recovered through line 12. The hydrogen formed at the cathode 6 is also recovered through line 14.

In the anode compartment 2, the H⁺ ions produced cause a lowering of the pH with the consequent formation of a precipitate containing aluminum.

The reaction which takes place in the anode compartment when aluminum is present in the alkaline extraction solution mainly in the form of sodium aluminate, is the following (4) :

2 NaAl(OH) + 2 H⁺ → 2 Al(OH)₃| + 2 Na⁺ + 2 H₂0 (4)

The reaction (4) leads to the formation of a precipitate of aluminum hydroxide. In the presence of more or less high quantities of fluoride ions, the following reaction (5) can also take place in the anode compartment, which leads to the formation of a cryolite precipitate NaAl(OH) + 6 NaF + 4 H⁺ → 2 Na₃AlF₆| + 4 H₂0 + 4 Na⁺

(5)

The anolyte containing aluminum in the form of a precipitate is collected from the anode compartment 2 through line 9 and fed to a solid/liquid separation system 10. In the solid/liquid separation system 10, the precipitate is separated, by means of line 15, from the aqueous dispersion generated by the electrolysis. In the scheme of figure 1, for example, the precipitate comprising aluminum is fed to an electrolytic reduction cell C of alumina to produce primary aluminium.

The liquid fraction (supernatant) separated in the solid/liquid separation system 10 and containing aluminum remaining in hydrosoluble form, is fed, through line 20, to an accumulation tank A. At least a fraction of the solution present in the accumulation tank A is recirculated to the leaching unit L through line 17. A second fraction of the solution present in the accumulation tank A is recirculated to the head of the anode compartment 2 through the recirculation line 1.

Optionally, the chemical composition of the extraction solution fed to the electrolysis cell can be modified to favour the formation of a precipitate containing aluminum having a pre-determined composition. For example, Ca²⁺ ions can be added, in the leaching unit L, to the extraction solution in order to precipitate the fluorides in the extraction solution 21 in the form of CaF₂. The CaF₂ can then be separated in a liquid/solid separation system 25, by removing it from the extraction solution 21 through line 22. The extraction solution free from fluoride ions 23 leaving the separation system 25, is fed to the cell 3, joining it with the recirculation line 1.

In the cathode compartment 4, the OH^~ ions generated by the electrolysis combine with the Na⁺ cations that have migrated from the anode compartment, also conveying the hydration water with them, forming a solution of NaOH.

In a preferred embodiment, water is also fed to the cathode compartment 4, so as to allow the electrolytic process to take place in the presence of a catholyte having a constant concentration. The water can be introduced, for example, into the recirculation line 8 of the catholyte, through line 16.

Due to the hydration water carried by the Na⁺ cations and possibly the water added through line 16, there is an increase in volume of the catholyte in the cathode compartment. In order to compensate this increase in volume, a part of the catholyte containing NaOH in solution is extracted from the recycling line 8, through line 13, and fed to the accumulation tank A.

Before feeding the extraction solution to the cell, the electrolysis process is preferably started by applying a potential difference to the electrodes of the membrane cell and recirculating the respective electrolytic solutions (anolyte and catholyte) in the anode and cathode compartments until a pH value is reached in the anolyte, which is close to the starting value of the precipitation of aluminum. For this purpose, the desired pH value can also be reached by adding suitable acid or base compounds. The extraction solution is then fed to the anode compartment preferably with a volumetric flow-rate which is such as to not substantially alter the pH of the solution recirculated in the same compartment. In this way, the substantially instantaneous precipitation of the aluminum is obtained, which is then removed from the flow leaving the compartment, preventing it from accumulating inside the anode compartment where it could damage the ion-selective membrane.

In a particularly preferred embodiment, the extraction solution is fed to the anodic solution leaving the anode compartment, before the introduction of said anodic solution to the separation system 10. In this way, the formation of the aluminum precipitate occurs outside the cell, preventing possible damage to the ion-selective membrane. When, on the other hand, the precipitation is effected inside the anode compartment, the membrane can be protected, for example by positioning a diaphragm of polymeric material, e.g. polyester, close to the ion-selective membrane so as to mechanically protect the membrane from the possible abrasive action of the aluminum precipitate.

In a further preferred embodiment, the process according to the present invention can be carried out as schematically illustrated in figure 2.

With reference to Figure 2, the electrolytic cell 3 used for the membrane electrolysis of the extraction solution comprises: an anode compartment 6 in which an anode 7 immersed in an anolyte is present; a cathode compartment 4 in which a cathode 5 immersed in a catholyte is present. The anode compartment 6 and cathode compartment 4 are separated by a central supply compartment 2, which is separated from the anode compartment 6 by a permselective anionic membrane 9 and from the cathode compartment 4 by a permselective cationic membrane 8.

An aqueous alkaline solution (catholyte) is fed in continuous into the cathode compartment 4, which, after being subjected to electrolysis, is extracted from the cathode compartment 4 and recirculated to the head of the same compartment through the recirculation line 12; said electrolytic solution is preferably a solution of NaOH.

An acidic electrolytic aqueous solution is fed as anolyte to the anode compartment 6, for example a solution of HF. After being subjected to electrolysis, the anolyte is extracted from the anode compartment 6 and recirculated to the head of the same compartment 6 through the recirculation line 16.

The oxygen formed at the anode 7 is recovered through line 21. The hydrogen formed at the cathode 5 is recovered through line 22.

An aliquot of SPL 23 is treated in the leaching unit L with an alkaline aqueous solution 19 under the conditions previously described, so as to obtain an extraction solution 1 comprising at least aluminum in hydrosoluble form, fluorides and sodium ions.

The extraction solution 1 is fed to the supply compartment 2. The supply of extraction solution 1 is preferably effected with a flow-rate which is such that the electric conductivity in the cell during the electrolysis is kept substantially constant and consequently the cell voltage is also kept constant.

Due to the electric field generated by the difference in electric potential applied to the electrodes 5 and 7, the Na⁺ ions of the extraction solution circulating in the central supply compartment 2, migrate towards the cathode compartment 4 through the permselective cationic membrane 8 whereas the anions present in the extraction solution (OH-, Al(OH)4^~ and F^~) migrate towards the anode compartment 6 through the permselective anionic membrane 9.

In the anode compartment 6, the H⁺ ions produced by the anodic reaction cause a lowering of the pH with the consequent formation of a precipitate containing for example A1F₃. Diluted hydrofluoric acid can optionally be fed to the anode compartment 6, for favouring the precipitation of A1F₃, for example on the recirculation line of the anolyte 16 through line 15.

The reaction that takes place in the anode compartment, when the aluminum is present in the alkaline extraction solution mainly in the form of sodium aluminate, is the following (6) :

Al(OH)₄ ^~ + 2 H⁺ + 3 F^~ → A1F₃| + 4 H₂0 (6)

The precipitate containing aluminum is extracted from the anode compartment 6 through line 17 and fed to a solid/liquid separation section 20 (e.g. decanter) . The decanted precipitate leaves the separator through line 18. The precipitate containing aluminum can be filtered, washed and dried before being re-used. In Figure 2, for example, the precipitate of A1F₃ is fed to an electrolytic reduction cell C of alumina for the production of primary aluminum.

Advantageously, the liquid fraction (supernatant) which is separated in the separation system 20 can be partially recirculated through line 16 to the head of the electrolytic anode compartment, so as to recover, by precipitation, the aluminum still present in the same in hydrosoluble form.

In the cathode compartment 4, the OH^~ ions generated by the electrolysis combine with the Na⁺ cations that have migrated from the anode compartment, which also carry the hydration water, forming a solution of NaOH. In a preferred embodiment, water is also fed to the cathode compartment 4, so as to allow the electrolytic process to be carried out in the presence of a catholyte having a constant concentration of NaOH. The water can be introduced, for example, into the recirculation line 12 of the catholyte, through line 11.

Due to the hydration water carried by the cations and possibly the water added through line 11, there is an increase in the catholyte volume in the cathode compartment. In order to compensate this increase in volume, a part of the catholyte containing NaOH is extracted from the recirculation line 12, through line 13, and fed to the leaching unit L through line 24.

The addition of the extraction solution 1 in the recirculation line 10 of the central supply compartment 2 causes an increase in the overall volume of solution to be treated. In order to compensate this increase, a fraction of the extraction solution treated leaving the supply compartment 2 is fed to an accumulation tank A, through line 14. A second fraction of the above- mentioned treated extraction solution, on the other hand, is recirculated to the head of the supply compartment 2 through line 10 to recover the residual aluminum present in hydrosoluble form.

Some embodiment examples of the present invention are provided for purely illustrative purposes and should not be considered as limiting the protection scope defined by the enclosed claims.

EXAMPLES

The effectiveness of the treatment process of carbonaceous materials comprising aluminum, fluorides and sodium ions according to the present invention was evaluated by subjecting certain alkaline solutions containing aluminum in hydrosoluble form, to hydrolysis .

An electrolytic cell was used for the electrolysis, with electrodes having surfaces equal to 1 dm² and equipped with a permselective cationic membrane. A 20 A current (current density: 200 A/m²) was fed to the electrodes .

EXAMPLE 1 - Extraction solution containing

NaAl (OH)

In a first experiment an alkaline extraction solution was used (pH = 13.1) containing NaAl (OH) at a concentration of 81.2 g/1 and free NaOH at a concentration of 9.5 g/1. The extraction solution was fed to the anode compartment containing an electrolytic solution of Na₂S0₄ (80 g/1) . 3.04 1 of a NaOH solution having an initial concentration of 210.4 g/1 were fed to the cathode compartment .

Both the anolyte and catholyte were circulated in the respective anode and cathode compartments with a flow-rate equal to about 30 1/min and subjected to electrolysis until a whitish precipitate had started to form. Upon the appearance of the precipitate, the electrolysis was interrupted in order to characterize the catholyte. Upon the appearance of the precipitate, 3.05 1 of catholyte solution having a concentration of NaOH of 212.3 g/1, were present in the cell. The solution in the anode compartment reached a pH equal to about 9.8.

The electrolysis process was then restarted, supplying further extraction solution to the anode compartment and extracting from the same, the dispersion containing the precipitate which was fed to a decanter. The extraction solution was fed to the anode compartment joining it with the recirculated anolyte in such a proportion, with respect to the latter, as to not substantially modify the pH of the solution in the anode compartment.

The clarified solution at the outlet of the overflow of the decanter was sent to an accumulation tank from where it could be taken for being recirculated to the electrolytic cell.

The decanted fraction containing the precipitate was collected in a beaker and filtered, obtaining a cake containing aluminum. The cake was washed with water and dried at a temperature of 120°C for 12 hours.

The electrolytic process was carried out for 3 hours, feeding a total quantity of about 2.3 1 of a solution of NaAl(OH)₄.

At the end of the test it was found that:

- 3.10 1 of NaOH solution having a concentration of 236.0 g/1 (faradic yield 94.0%) were present in the cathode compartment;

the filtered, washed and dried (3 hours at 650°C) solid obtained in the anode compartment weighed 122.0 g and was substantially composed of AI₂O₃.

EXAMPLE 2 - Extraction solution containing Na₃AlF₆

A second experiment was carried out using, as anolyte, an alkaline extraction solution (pH = 12.9) in which 63.6 g/1 of Na₃AlF₆ were dissolved together with 21.3 g/1 of AI₂O₃. This solution was fed to the anode compartment containing a solution of Na₂SC>4 at a concentration of 80 g/1. 3.14 1 of a solution of NaOH having a concentration of 236.8 g/1 were fed to the cathode compartment.

The electrolysis process was carried out in the same cell and under the same operating conditions described for Example 1.

The anolyte and catholyte were circulated in the cell and electrolyzed until a precipitate had started to form in the anode compartment. The electrolysis was then interrupted in order to characterize the catholyte. Upon the appearance of the precipitate, 3.16 1 of catholyte, having a concentration of NaOH equal to 238.4 g/1, were present in the cathode compartment, whereas the solution in the anode compartment reached a pH of about 9.6.

The electrolysis process was then restarted, supplying further extraction solution to the anode compartment and extracting from the same, with the same flow-rate, the dispersion containing the precipitate, which was fed to a decanter.

The extraction solution was fed to the anode compartment joining it with the recirculated anolyte in such a proportion, with respect to the latter, as to not substantially modify the pH of the solution in the anode compartment .

The decanted fraction containing the precipitate was collected in a beaker and filtered, obtaining a cake containing aluminum. The cake was then washed with water and dried at a temperature of 120°C for 12 hours.

The electrolytic process was carried out for 5 hours and 30 minutes, feeding a total quantity of about 2.0 1 of alkaline extraction solution.

At the end of the test it was found that: 3.19 1 of NaOH solution having a concentration of 286.4 g/1 (faradic yield 96.0%) were present in the cathode compartment;

the filtered, washed and dried solid obtained in the anode compartment weighed 161.1 g and was substantially composed of a mixture of Na₃AlF₆ (88% by weight) and Al (OH) ₃ (12% by weight) .

EXAMPLE 3 - Extraction solution containing NaAl (OH) ₄ A third experiment was carried out using, as extraction solution, 2.50 1 of an alkaline solution (pH = 12.8) containing NaAl (OH) 4 at a concentration of 49.86 g/1, NaF at a concentration of 30.06 g/1 and free NaOH at a concentration of 35.46 g/1. The extraction solution was fed to the central compartment of a cell equipped with a cationic permselective membrane and an anionic permselective membrane as described above with reference to figure 2. 2.04 1 of NaOH solution at a concentration of 122.4 g/1 (catholyte) were fed to the cathode compartment, whereas 3.22 1 of a solution of HF at a concentration of 82.3 g/1 (anolyte) were fed to the anode compartment. The anolyte, catholyte and extraction solution were recirculated in the respective compartments of the cell for the whole duration of the test .

The electrolysis process was carried out for about

8 hours and 30 minutes with a current density of 2,000 A/m².

At the end of the electrolysis, the composition of the anolyte and catholyte was determined, obtaining the following results:

in the cathode compartment, 2.42 1 of a solution of NaOH having a concentration of 203.4 g/1 (faradic yield 94.3%) were present;

the filtered, washed and dried solid obtained in the anode compartment weighed 71.6 g and was substantially composed of A1F₃.

The final extraction solution circulating in the central compartment had a volume of 2.12 1 and the following composition: NaAL(OH)4 at a concentration of 20.53 g/l, NaF at a concentration of 7.07 g/l and free NaOH at a concentration of 1.50 g/l.

Claims

1. Process for treating a solid carbonaceous material containing aluminum, fluorides and Na⁺ ions that comprises the steps of:

(a) leaching said solid carbonaceous material with at least one aqueous alkaline solution to form: (i) an extraction solution comprising said aluminum in hydrosoluble form, said fluorides and said Na⁺ ions, and (ii) at least one solid insoluble residue;

(b) separating said solid insoluble residue from said extraction solution;

(c) subjecting said extraction solution free from said solid insoluble residue to a membrane electrolysis process to form at least one precipitate comprising said aluminum and at least one NaOH aqueous solution.

2. Process according to claim 1, wherein said precipitate comprising said aluminum is a compound selected from: Al(OH)₃, Na3AlF6, A1F₃ and mixtures thereof .

3. Process according to claim 1, wherein fluoride ions are added to said extraction solution to promote the formation of Na3AlF6 in said precipitate containing aluminum.

4. Process according to claim 1, wherein the fluoride ions present in said extraction solution are precipitated and separated from said extraction solution before said step of membrane electrolysis to promote the formation of Al(OH)₃ in said precipitate comprising aluminum.

5. Process according to the preceding claim, wherein said fluoride ions present in said extraction solution are precipitated by addition of ions of at least one alkaline earth metal to said extraction solution .

6. Process according to claim 1, wherein at least one part of said precipitate comprising said aluminum is fed to a cell for the electrolytic reduction of alumina to produce aluminum.

7. Process according to claim 1, wherein at least one part of said NaOH aqueous solution is used as an aqueous alkaline solution in said leaching step (a) .

8. Process according to one or more of claims 1-7, wherein said membrane electrolysis process comprises the steps of:

(i) providing at least one electrolytic cell comprising :

(ii) supplying said extraction solution to said anode compartment .

9. Process according to the preceding claim, wherein said extraction solution is added with at least one salt comprising at least one cation of an alkali metal or alkaline earth metal and at least one anion having an oxidation potential higher than the oxidation potential of water under the conditions in which said step of membrane electrolysis is carried out.

10. Process according to the preceding claim, wherein said anion is selected from: sulphate, nitrate, perchlorate, phosphate and borate.

11. Process according to claim 8, which comprises:

- taking said anolyte from said anode compartment after said step of membrane electrolysis;

- combining said extraction solution to said anolyte taken from said anode compartment so as to form said precipitate comprising said aluminum dispersed in an aqueous solution;

separating said precipitate comprising said aluminum from said aqueous solution;

- supplying said aqueous solution free from said precipitate comprising aluminum to said anode compartment .

12. Process according to one or more of claims 1-7, wherein said membrane electrolysis process comprises the steps of:

(i) providing at least one electrolytic cell comprising :

at least one anode compartment comprising at least one anode immersed in an anolyte,

- at least one cathode compartment comprising at least one cathode immersed in a catholyte,

at least one supply compartment interposed between said anode compartment and said cathode compartment;

(ii) supplying said extraction solution to said supply compartment.

13. Process according to claim 8 or 12, wherein said catholyte is an aqueous solution of NaOH.

14. Process according to claim 12, wherein said anolyte is an aqueous solution of hydrofluoric acid.

15. Process according to one or more of claims 1-7, wherein said membrane electrolysis process is carried out in an electrodialysis apparatus.