RU2742864C2 - Method of treating solid carbonaceous material containing aluminum, fluorides and sodium ions - Google Patents

Abstract

FIELD: waste processing and disposal.SUBSTANCE: invention relates to a method of processing wastes of electrolytic production of primary aluminum in form of solid carbon-containing material containing aluminum, fluorides and Na+ions, in particular, for processing spent lining (PSL) used in production of primary aluminum. Method involves leaching solid carbonaceous material with at least one aqueous alkaline solution to form an extraction solution, containing aluminum in a water-soluble form, fluorides and Na+ions, and at least one solid insoluble residue. After that insoluble solid residue is separated from extraction solution. Method includes performing membrane electrolysis of extraction solution free from said solid insoluble residue to form at least one residue, containing said aluminum, and at least one aqueous solution of NaOH.EFFECT: method enables recycling PSL with removal of potentially hazardous substances and their reuse at production.14 cl, 3 ex, 2 dwg

Description

The present invention relates to a method for treating a solid carbonaceous material containing aluminum, fluorides and sodium ions. The process according to the present invention is particularly suitable for treating spent linings used in the primary aluminum production process.

Aluminum is usually obtained by reducing aluminum-based minerals, mainly bauxite. The reduction reaction is carried out in an electrolytic reduction cell (bath) containing molten cryolite (Na ₃ AlF ₆ ) and other salts such as aluminum fluoride (AlF ₃ ), alkali metal fluorides (eg NaF, LiF) and alkaline earth metal fluorides (eg, CaF ₂ , MgF ₂ ). The walls and bottom of the electrolytic reduction cell are internally lined with carbon based material. When the cell is in operation, the coating gradually breaks down, which allows various materials to penetrate inside. Over time, the lining continues to deteriorate, operation is interrupted to replace the damaged lining (waste lining) with a new lining. The spent lining, also referred to below as OTF, typically also includes a portion of the wall of the refractory material located outside the cell, which is removed along with the inner lining.

Under these operating conditions of the cell, the spent lining contains significant amounts of aluminum, fluorides, alkali metals, in particular sodium, and alkaline earth metals, and other substances such as, for example, nitrides and cyanides. The presence of these substances is hazardous to human health and the environment, and the relative ease with which they can be extracted with water requires that the spent liner be properly treated to reduce hazardous substances to acceptable levels before it can be safely disposed of in landfills.

Various methods are known in the art for the treatment of waste linings (OTF) aimed at recovering at least part of the substances contained therein, in particular aluminum and fluorides, and, possibly, for their reuse in the production of primary aluminum.

Known treatment methods include, for example, incineration, roasting, or interaction of OTF with quicklime. There are also known methods of hydrometallurgical treatment of OTF, based on the extraction of substances of interest from carbon-containing material using acidic or alkaline solutions.

These processing methods, known in the art, are usually associated with high energy costs and / or using large amounts of reagents required for treatment of OTP. In addition, in many cases, prior art methods can efficiently recover only some of the substances of interest, while generating other residues containing contaminants, which in turn require special treatment before they can be safely disposed of.

An example of a hydrometallurgical treatment of OTP for fluoride recovery is described in US Pat. No. 4,816,122.

In a first embodiment, the method described in US Pat. No. 4,816,122 involves extraction of OTP with a concentrated solution of caustic soda (sodium hydroxide) to form an extraction solution containing sodium fluoride (NaF) and sodium aluminate (NaAl (OH) ₄ ). Due to the lower solubility of fluorides in concentrated sodium hydroxide solutions, some, if not all, of the fluorides remain in the solid insoluble residue in the form of NaF. After separation of the extraction solution, NaF is extracted from the solid insoluble residue with water or a dilute aqueous NaF solution.

In a second embodiment, the method described in US Pat. No. 4,816,122 involves extracting OTP with a dilute caustic soda solution to dissolve more sodium fluoride and form an insoluble solid residue. The caustic solution containing sodium fluoride is concentrated by evaporation to precipitate solid NaF, which, after separation from the concentrated solution, is redissolved in water.

Both of the above embodiments result in the separation and recovery of solid NaF, which is then redissolved to form an aqueous solution subjected to electrodialysis to form an aqueous HF solution and an aqueous NaOH solution. In the next step, the aqueous HF solution can be reacted with alumina (Al ₂ O ₃ ) to form AlF ₃₎ thus obtaining a recyclable product in an electrolytic reduction cell to produce primary aluminum. The NaOH solution obtained by electrodialysis can be recycled to the aluminum mineral purification process (Bayer process) to produce alumina fed to the electrolytic reduction cell.

When the extraction is carried out with a concentrated caustic solution, most of the aluminum contained in the ATP dissolves in the form of sodium aluminate. Also in this case the sodium aluminate can be recycled to the Bayer process, possibly after drying by spray drying. When, on the other hand, extraction is carried out with a dilute caustic solution, a significant part of the aluminate remains in the caustic solution, which is concentrated by evaporation; this fraction of aluminum is intended for disposal.

The method described in US Pat. No. 4,816,122 has various disadvantages. Even if it allows ATP to be purified from fluorides and isolated in the form of AlF ₃ salts in an electrolytic cell for aluminum reduction, this method actually involves a relatively large number of technological stages and, therefore, is somewhat laborious.

In addition, when using a dilute caustic solution for leaching ATP, a stage of solvent evaporation is required, which makes the ATP processing method unattractive due to high energy costs.

Also, aluminum originally present in ATP cannot be separated and recovered from the caustic extraction solution, in which it is dissolved in the form of aluminate, and therefore can only be reused in the Bayer process.

Another disadvantage of this method is that the separation of aluminate from NaF is not good enough (part of the aluminate remains in NaF solutions, and part of NaF remains in solutions containing aluminate) to ensure the final production of reusable materials with a high degree of purity.

In view of the above prior art, therefore, there is a need for a method for treating OTP and other carbonaceous materials containing aluminum, fluorides and sodium ions that overcomes at least some of the disadvantages of prior art processes.

Thus, the applicant has established the main problem of creating a method for processing carbon-containing materials containing aluminum, fluorides and sodium ions, in particular OTP, in a simple and effective manner, at least partially eliminating the disadvantages of the prior art.

In particular, the object of the present invention is to provide a method for processing the aforementioned carbon-containing materials, in particular OTP, which makes it possible to almost completely remove from them substances potentially hazardous to human health and the environment, or at least reduce their amount.

In addition, the object of the present invention is to provide a method that allows you to extract from the above carbon-containing materials, in particular OTP, substances that can be reused in the production of primary aluminum, in particular, in an electrolytic reduction cell for aluminum oxide, in an efficient and economically convenient way ...

In addition, it is an object of the present invention to provide a process which, starting with the aforementioned carbon-containing materials, in particular OTP, makes it possible to selectively obtain the most desired aluminum-containing compounds, for example, in accordance with the specific requirements of the plant in which the primary aluminum production cycle is carried out.

Applicant has now found that the above and other objectives, which will be better illustrated in the following description, can be achieved by a treatment process in which ATP or other carbonaceous material containing aluminum, fluorides and sodium ions is leached with dilute or concentrated alkaline an aqueous solution to form an extraction solution containing the aforementioned elements in a water-soluble form; the extraction solution is then subjected to a membrane electrolysis process, which allows one or more aluminum compounds to be precipitated and an aqueous NaOH solution to be prepared separately.

With the help of membrane electrolysis treatment, precipitated aluminum compounds (for example, cryolite, aluminum fluoride, aluminum hydroxide or their mixtures) can be easily isolated, also with a high degree of purity, and reused in the production cycle of primary aluminum. In particular, these products can be reused by feeding them directly into the alumina electrolytic reduction cell.

Furthermore, preferably, by appropriately 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 membrane electrolysis. This makes the method according to the invention particularly versatile and adaptable to the requirements of the particular manufacturing plant in which it is implemented.

Membrane electrolysis also produces an aqueous solution of NaOH, which can be recovered and reused to leach additional OTP, thus saving reagents.

By appropriately recycling the electrolytic solutions used in the membrane cell, the process of the invention can be carried out continuously in a simple and efficient manner with reduced consumption of chemicals and electrical energy compared to prior art processes.

The insoluble solid residue from the leach step can be safely disposed of in landfills. Depending on the content of impurities that are still present, this solid residue can also be used as an auxiliary fuel in combustion processes or, if it is sufficiently pure, as a material for making a cell lining for the electrolytic reduction of alumina.

Thus, the implementation of the ATP processing method in accordance with the present invention within the framework of the electrolytic production cycle of primary aluminum makes it possible to increase the overall productivity of the latter, since this method allows more efficient use of the materials supplied to the reduction cell (in particular, aluminum oxide, cryolite and sodium fluoride) and, at the same time, this simplifies the utilization of OTPs formed in the same production cycle of primary aluminum.

According to a first aspect, the present invention relates to a method 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 containing said aluminum in water-soluble 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 of said solid insoluble residue, to a membrane electrolysis process to form at least one precipitate containing said aluminum and at least one aqueous NaOH solution.

For the purposes of the present description and the appended claims, the expression "carbon-containing material containing aluminum, fluorides and sodium ions" is used to denote various solid materials that contain aluminum, fluorides and sodium ions, including ATP obtained as a result of electrolytic reduction of aluminum oxide, carbon dust generated during aluminum smelting and waste from spent baths used in electrolytic reduction cells (aluminum slag). For simplicity, in the following description, reference will be made to the treatment of OTP as an example of carbonaceous materials containing aluminum, fluorides and sodium ions that can be processed using the method of the present invention.

For the purposes of the present description and the appended claims, the expression "membrane electrolysis" means an electrochemical process carried out in an electrolysis cell in which at least one ion-selective membrane is present which separates at least two compartments; the ion-selective membrane can be crossed only by positively charged ions (cationic selectively permeable membrane) or only negatively charged ions (anionic selectively permeable membrane), which migrate from one compartment of the cell to another under the influence of an electric field formed due to the difference in electric potential applied to the cell electrodes ; membrane electrolysis makes it possible to recover separately at least one aluminum-containing precipitate and a NaOH solution. For the purposes of the present invention, membrane electrolysis also includes electrodialysis processes.

For the purposes of the present description and the appended claims, the verb "include" and all terms derived therefrom also include the meaning of the verb "consist of" and its derived terms.

The numerical limits and ranges expressed in the present description and the appended claims also include the mentioned numerical values. In addition, all values and sub-ranges of a limit or numerical range are to be considered specifically included, as if they were explicitly mentioned.

According to the present invention, the leaching step is carried out with an alkaline aqueous solution (leach solution), preferably with an aqueous solution of NaOH, KOH or mixtures thereof. Since the amount of sodium ions present in the ATP usually prevails over the amount of other alkaline ions, the leach solution is preferably an aqueous solution of NaOH.

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

Leaching can be carried out using dilute or concentrated leach solutions (for example, 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 fluorine ions decreases, and the solubility of aluminate ions increases. Therefore, if the leaching is aimed mainly at dissolving the fluorine ions contained in the OTF in the extraction solution, then a diluted leaching solution is preferably used. If, on the other hand, the leaching is aimed at dissolving the aluminum contained in the OTP by introducing it into the extraction solution in the form of aluminate ions, then a concentrated leaching solution is preferably used.

In the case of aqueous solutions of alkali metal hydroxides (e.g. NaOH or KOH), the dilute leach solution preferably has a hydroxide concentration equal to or greater than 1 wt% and less than 10 wt%, more preferably the concentration is in the range of 5-8%, with the percentages indicated to the weight of the diluted leach solution.

In the case of aqueous solutions of alkali metal hydroxides (e.g. NaOH or KOH), the concentrated leach solution preferably has a hydroxide concentration equal to or greater than 10% by weight and less than 40% by weight, preferably in the range of 15-25%, with said percentages referring to the mass of the concentrated leaching solution.

In a preferred embodiment, the leaching step comprises at least a first leaching step carried out with a concentrated leach solution and a subsequent washing step of the resulting insoluble residue with water or a very dilute leach solution, preferably with a hydroxide concentration equal to or less than 3% by weight, based on the weight of the solution. ... Thus, the extraction of both aluminum and fluorides from ATP is actually maximized.

Extraction solutions obtained by leaching with dilute or concentrated leach solutions, together with the washing solutions, can be combined to form a single extraction solution, which will be subjected to a subsequent membrane electrolysis step.

The leaching can be carried out with a leach solution having room temperature (25 ° C) or higher than 25 ° C, more preferably in the range of 70 ° C to 95 ° C.

The duration of leaching is usually selected depending on the amount of treated AHP, temperature and concentration of the leaching solution. The contact time between OTP and the solution is preferably 15 to 300 minutes.

The ratio between the amount of OTP and the amount of leach solution is preferably in the range of 20 to 400 g OTP / L of the leach solution, more preferably in the range of 70 to 150 g / L.

To decompose 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. For example, the extraction solution can be treated with ozone (O ₃ ) to chemically decompose possible dissolved cyanides. Alternatively, OTP can be leached after calcination to thermally decompose possible cyanides present.

Since aluminum is mainly present in ATP in the form of aluminum oxide (Al ₂ O ₃ ), cryolite (Na ₃ AlF ₆ ) and aluminum fluoride (AlF ₃ ), the following reactions occur during the dissolution and leaching of these compounds (1) - (3) :

At the end of the leaching step, the insoluble solid residue can be separated from the extraction solution, for example, by filtration and / or decantation, or by other techniques known to the person skilled in the art.

The insoluble solid residue can also be washed to reduce the amount of fluoride remaining after leaching with dilute aqueous NaF solution. The wash solution containing fluorides extracted from the insoluble solid residue can then be combined with the extraction solution obtained in the leach step, or it can be processed according to known techniques to obtain, for example, an aqueous solution of NaOH and AlF ₃ , as described, for example, in US patent 4816122.

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

The present invention employs an electrolytic process in which an insoluble aluminum compound is deposited in the first compartment of the membrane cell ^{by means of H + ions generated during the anodic reaction.} At the same time, OH 'ions formed in the course of the cathodic reaction, together with the Na ⁺ ions present in the extraction solution, form NaOH in the second compartment of the cell.

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

(i) providing at least one electrolytic cell comprising:

- at least one anode compartment, including at least one anode immersed in anolyte;

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

moreover, the specified anode compartment is separated from the specified cathode compartment at least one selectively permeable cationic membrane;

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

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

In addition, during electrolysis, under the action of the electric field present in the cell, the Na ⁺ cations present in the extraction solution in the anode compartment migrate towards the cathode compartment through the selectively permeable cationic membrane. In particular, the selectively permeable cationic membrane allows Na \ cations present in the anode compartment to migrate into the cathode compartment and, conversely, prevents the migration of anions present in the cathode compartment (OH ") into the anode compartment.

During the electrolysis of water in the cathode compartment, OH ions are formed, which, together with Na ⁺ cations, which selectively migrated from the anode compartment to the cathode compartment, form NaOH. In the possible presence of other cations, in particular, cations of alkali and alkaline earth metals, the above OH ions also form the corresponding hydroxides.

The catholyte fed to the cathode compartment is preferably an aqueous NaOH solution. Thus, at the end of the electrolysis, a NaOH solution having a higher concentration than that of the original catholyte can be isolated.

The NaOH solution that forms in the cathode compartment can at least partially be used as a solution for leaching additional ATP.

The aluminum-containing precipitate that forms in the anode compartment can, for example, be an aluminum salt (eg Na3AlF ₆ ), aluminum hydroxide (Al (OH) ₃ ) or a mixture of the above compounds, depending on the composition of the anolyte.

The composition of the aluminum-containing sludge that forms in the anode compartment during membrane electrolysis can preferably be set by appropriately changing the composition of the extraction solution prior to electrolysis.

By adding, for example, additional fluorine ions to the extraction solution prior to electrolysis, a precipitate can be obtained essentially consisting of Na ₃ AlF ₆ . Alternatively, by precipitating the fluorine ions initially present in the extraction solution and separating them from it before electrolysis, conversely, a precipitate can be obtained essentially consisting of Al (OH) ₃ , which, after drying, can be converted into Al ₂ O ₃ (an oxide aluminum). The deposition of fluorine ions can be carried out, for example, by adding ions of at least one alkaline earth metal, preferably Ca ^{2+ ions} , to the extraction solution.

Thus, the method in accordance with the present invention can be easily adapted to the requirements of the user, based, for example, on the need to produce cryolite or alumina, which is fed to the bath of an electrolysis cell to produce primary aluminum.

According to a second preferred embodiment of the present invention, the recovery of the aluminum-containing sludge and the separate recovery of the aqueous NaOH solution can advantageously be carried out by membrane electrolysis in a cell comprising at least one selectively permeable cationic membrane and at least one selectively permeable 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, including at least one anode immersed in anolyte;

- at least one cathode compartment, including at least one cathode immersed in a catholyte;

- at least one feed compartment located between the anode compartment and the cathode compartment;

moreover, the specified feed compartment is separated from the specified anode compartment at least one selectively permeable cationic membrane;

said feed compartment is separated from said cathode compartment by at least one selectively permeable anionic membrane;

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

During electrolysis, due to the electric field present in the cell, the aluminate and fluoride ions present in the extraction solution migrate from the central feed compartment towards the positively charged anode, passing through the selectively permeable anionic membrane. Na ⁺ cations, on the other hand, migrate in the opposite direction, towards the negatively charged cathode, passing through a selectively permeable cationic membrane. Since Н ⁺ ions, which are formed in the anode compartment, cannot migrate towards the cathode compartment due to the selectively permeable anionic membrane, they combine in the anode compartment with aluminate and fluoride ions that have migrated from the feed compartment, resulting in the deposition of aluminum in the form AlF ₃ , cryolite or mixtures thereof.

Preferably, fluorine ions can be added to the anode compartment to promote the formation of AlF ₃ , for example, by feeding an aqueous HF solution during the electrolytic process.

Since OH ^- ions, which are formed in the cathode compartment, cannot migrate towards the anode compartment due to the selectively permeable cationic membrane, they combine with Na ⁺ ions that migrated from the feed compartment to form NaOH.

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

The NaOH solution that forms in the cathode compartment can at least partially be used as a solution for leaching additional ATP.

The anolyte supplied to the anode compartment is preferably an acidic electrolytic solution. The pH of the anolyte is preferably in the range of 0-5, more preferably in the range of 0-3. For example, the anolyte can be selected from hydrohalic 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 device. In electrodialysis using one or more cells comprising a cationic membrane and an anionic membrane, a precipitate containing aluminum and a separate NaOH solution can be produced, essentially using the same separation principle as described above for a cell comprising a cationic membrane and an anionic membrane.

Additional examples of electrodialysis processes and related equipment that can be used for the purposes of the present invention are described in US Pat. No. 4,107,264.

The membrane electrolysis process according to the present invention can be carried out either batchwise or continuously. This method is preferably carried out continuously, that is, by continuously feeding the extraction solution into the membrane cell (for example, into the anode compartment or into the feeding compartment) and continuously extracting from it an equal volume of the treated solution containing precipitated aluminum.

The electrolytic solution used as the catholyte is also preferably continuously fed into and continuously removed from the membrane cell in an equal volume, the solution being gradually enriched in NaOH or other alkali metal hydroxide that is formed during the electrolytic process. In another preferred embodiment, the catholyte is continuously recirculated to the cathode compartment, and at the same time water is fed into the same compartment in such an amount as to maintain a constant concentration of NaOH in the catholyte removed from the same compartment.

As a rule, the addition of material to and removal of material from the compartments of an electrolytic cell can be carried out through one or more channels located directly in the compartments or along the circuits of recirculation of electrolytic solutions to the same compartments. Addition and removal are preferably carried out along the recirculation loops as this helps to maintain stable electrolysis conditions in the cell.

In a preferred embodiment of the invention, the extraction solution is fed into the cell into a compartment in which an electrolytic solution is present having the desired pH, for example, a pH equal to or close to the value at which electrolytic aluminum deposition can begin (pH about 8-10.5) ... In this case, the extraction solution to be treated is preferably fed into the cell in such an amount that the pH of the electrolytic solution does not change significantly.

The electrolytic solution having the desired pH value is preferably an aqueous solution containing at least one salt consisting of at least one alkali or alkaline earth metal cation and at least one anion having an oxidation potential higher than the oxidation potential of water under conditions, at which the stage of membrane electrolysis is carried out. The anion is preferably water soluble in the pH range of the leach solution. The anion is preferably selected from sulfate, nitrate, perchlorate, phosphate, borate and mixtures thereof, more preferably from sulfate, nitrate, perchlorate, and mixtures thereof. The above cation is preferably Na ⁺ .

The addition of the aforementioned salt, hereinafter also referred to as "auxiliary electrolyte", has the advantage that the membrane electrolysis proceeds continuously and the energy consumption is reduced.

In the case of electrolysis, the formation of an aluminum deposit leads to a gradual decrease in the conductivity of the anolyte with a subsequent increase in the cell potential and, consequently, to an increase in energy consumption. The presence of the auxiliary electrolyte, on the other hand, ensures a sufficiently high conductivity of the anolyte, reducing energy consumption.

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

The auxiliary electrolyte is preferably present in the anolyte in an amount of 0.1-3 mol / l of monovalent cations.

The aluminum-containing precipitate can be separated from the anolyte using conventional solid / liquid separation systems known in the art. The precipitate can be separated, for example, by decantation. Then the decanted precipitate can be filtered, washed and dried.

The supernatant fraction, which remains after separation of the precipitate during decantation or filtration, can advantageously be partially recycled to the electrolytic treatment stage, where it is combined with the treated alkaline extraction solution and recycled to the cell, while aluminum is still present therein in a water-soluble form.

The method of the invention can be carried out in electrochemical cells of the type known in the art. The density of the current applied to the electrodes is preferably selected in the range of 100-5000 A / m ² .

Some preferred embodiments of the present invention are further described with reference to the following accompanying drawings:

FIG. 1 is a schematic illustration of a process according to the invention in which membrane electrolysis is carried out in a single membrane cell;

FIG. 2 schematically illustrates a process according to the invention in which membrane electrolysis is carried out in a double membrane cell.

In the diagram shown in FIG. 1, an aliquot of OTP 18 is treated in a leaching unit L with an alkaline aqueous solution 19 under the previously described conditions to obtain an extraction solution 21 containing at least aluminum in a water-soluble form, fluorides and sodium ions. The extraction solution 21 is fed through lines 11 and 1 to the anode compartment 2 of the electrolytic cell 3. Cell 3 includes a cathode compartment 4. Anode compartment 2 and cathode compartment 4 respectively include anode 5 and cathode 6. Anode 5 and cathode 6 are separated in the corresponding compartments 2 and 4 with a selectively permeable cationic membrane 7.

The electrolytic solution (catholyte) is continuously fed into the cathode compartment 4, which after electrolysis is removed from the cathode compartment 4 and recirculated to the beginning of the same compartment along the recirculation line 8; said electrolytic solution is preferably a NaOH solution.

When a sufficient electrical potential difference is created on electrodes 5 and 6, the water present in the cell is electrolyzed with the formation of H ⁺ ions and gaseous O ₂ in the anode compartment 2, as well as OH ^- ions and gaseous H ₂ in the cathode compartment 4. Under the action the applied difference in electric potential, alkali metal ions present in the anode compartment 2 (mainly Na ⁺ ) migrate towards the cathode compartment 4. Oxygen generated at the anode 5 is removed along line 12. The hydrogen generated at the cathode 6 is also removed through lines 14.

In the anode compartment 2, the formed H ⁺ ions cause a decrease in pH with the subsequent formation of a precipitate containing aluminum.

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

As a result of reaction (4), an aluminum hydroxide precipitate is formed. In the presence of relatively high amounts of fluoride ions in the anode compartment, the following reaction (5) can also occur, which leads to the formation of a cryolite deposit.

Anolyte containing aluminum in the form of a precipitate is collected from the anode compartment 2 through line 9 and fed to the solid / liquid separation system 10. In the solid / liquid separation system 10, the precipitate is separated through line 15 from the aqueous dispersion obtained by electrolysis.

For example, in the circuit shown in FIG. 1, an aluminum-containing precipitate is fed to an electrolytic cell C to reduce alumina to produce primary aluminum.

The liquid fraction (supernatant) separated in the solid / liquid separation system 10 and containing aluminum remaining in water-soluble form is fed via line 20 to storage tank A. At least part of the solution present in storage tank A is recirculated to the leach unit L along line 17. The second part of the solution present in the storage tank A is recirculated to the beginning of the anode compartment 2 through recirculation line 1.

The chemical composition of the extraction solution supplied to the electrolysis cell can be changed as necessary to facilitate the formation of a composition containing aluminum sludge. For example, Ca ^{2+ ions} can be added to the leach unit L to the extraction solution to precipitate fluorides from the extraction solution 21 in the form of CaF ₂ . The CaF ₂ can then be separated in the liquid / solid separation system 25 by removing it from the extraction solution 21 via line 22. The extraction solution free of fluorine ions 23 leaves the separation system 25 and is fed to chamber 3, which is connected to the recirculation line one.

In the cathode compartment 4, OH ^- ions formed during electrolysis combine with Na ⁺ cations, which migrated together with hydration water from the anode compartment, forming a NaOH solution.

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

Due to hydration water carried along with Na ⁺ cations and possibly due to water added through line 16, the volume of catholyte in the cathode compartment is increased. To compensate for this increase in volume, part of the catholyte containing dissolved NaOH is withdrawn from the recirculation line 8 through line 13 and fed to the storage tank A.

Prior to feeding the extraction solution into the cell, electrolysis is preferably started by applying a potential difference to the electrodes of the membrane cell and recirculating the appropriate electrolytic solutions (anolyte and catholyte) in the anode and cathode compartments until the anolyte pH is close to the initial aluminum deposition value. For this purpose, the desired pH can also be achieved by adding suitable acidic or basic compounds. The extraction solution is then fed into the anode compartment, preferably at such a volumetric flow rate that the pH of the solution recirculated to the same compartment is practically unchanged. Thus, almost instantaneous deposition of aluminum occurs, which is then removed from the stream exiting the compartment, while preventing its accumulation inside the anode compartment, where it could damage the ion-selective membrane.

In a particularly preferred embodiment, the extraction solution is fed into the anode solution leaving the anode compartment prior to the introduction of said anode solution into the separation system 10. Thus, the formation of an aluminum precipitate takes place outside the cell, thus preventing possible damage to the ion-selective membrane. When, on the other hand, deposition takes place inside the anode compartment, the membrane can be protected, for example, by installing a diaphragm made of a polymer material, such as polyester, in close proximity to the ion-selective membrane in order to mechanically protect the membrane from the possible abrasion of aluminum deposits.

In a further preferred embodiment, the method according to the present invention can be carried out as schematically shown in FIG. 2.

As shown in FIG. 2, an electrolytic cell 3 used for membrane electrolysis of an extraction solution includes: an anode compartment 6 in which an anode 7 is present immersed in anolyte; cathode compartment 4, in which there is a cathode 5 immersed in catholyte. The anode compartment 6 and the cathode compartment 4 are separated by a central feed compartment 2, which is separated from the anode compartment 6 by a selectively permeable anionic membrane 9, and from the cathode compartment 4 by a selectively permeable cationic membrane 8.

An aqueous alkaline solution (catholyte) is continuously fed into the cathode compartment 4, which, after electrolysis, is removed from the cathode compartment 4 and recirculated to the beginning of the same compartment along the recirculation line 12; said electrolytic solution is preferably a NaOH solution.

The acidic electrolytic aqueous solution is supplied in the form of anolyte to the anode compartment 6, for example, an HF solution. After electrolysis, the anolyte is removed from the anode compartment 6 and recirculated to the beginning of the same compartment 6 along the recirculation line 16.

Oxygen generated at the anode 7 is removed through line 21. Hydrogen generated at the cathode 5 is removed through line 22.

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

The extraction solution 1 is fed into the feed cell 2. The feed of the extraction solution 1 is preferably carried out at a flow rate such that the electrical conductivity in the cell is kept substantially constant during electrolysis and therefore the cell voltage is also kept constant.

Due to the electric field created by the difference in electric potentials applied to electrodes 5 and 7, the Na ⁺ ions of the extraction solution circulating in the central feed compartment 2 migrate to the cathode compartment 4 through the selectively permeable cationic membrane 8, while the anions present in the extraction solution ( OH ^- , Al (OH) ₄ ^- and F ^- ) migrate to the anode compartment 6 through the selectively permeable anionic membrane 9.

In the anode compartment 6, the H ⁺ ions formed as a result of the anodic reaction cause the pH to drop with the subsequent formation of a precipitate containing, for example, AlF ₃ . Diluted hydrofluoric acid may optionally be added to the anode compartment 6 to aid in the deposition of AlF ₃ , for example, into the anolyte recycle line 16 through line 15.

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

A precipitate containing aluminum is removed from the anode compartment 6 via line 17 and fed to a solid / liquid separation section 20 (eg decanter). The decanted sludge leaves the separator via line 18. The sludge containing aluminum can be filtered, washed and dried before reuse. FIG. 2, for example, an AlF ₃ precipitate is fed to an alumina reduction electrolytic cell C for the production of primary aluminum.

Preferably, the liquid fraction (supernatant) that is separated in the separation system 20 can be partially recirculated through line 16 to the beginning of the electrolytic anode compartment to recover aluminum by precipitation, still present therein in a water-soluble form.

In the cathode compartment 4, OH ^- ions formed during electrolysis combine with Na ⁺ cations, which migrated from the anode compartment and with which hydration water also migrated, forming a NaOH solution.

In a preferred embodiment, water is also fed to the cathode compartment 4 to allow the electrolytic process to be carried out in the presence of a catholyte having a constant NaOH concentration. 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, the volume of the catholyte increases in the cathode compartment. To compensate for this increase in volume, a portion of the catholyte containing NaOH is recovered from the recycle line 12 via line 13 and fed to the leach unit L via line 24.

The addition of the extraction solution 1 to the recirculation line 10 of the central feeding compartment 2 leads to an increase in the total volume of the processed solution. To compensate for this increase, a part of the treated extraction solution leaving the feed compartment 2 is fed to the storage tank A through line 14. On the other hand, a second part of the above treated extraction solution is recirculated to the beginning of the feed compartment 2 through line 10 to recover the residual aluminum present in water-soluble form.

Certain embodiments of the present invention are presented for illustrative purposes only and should not be construed as limiting the scope of protection defined by the appended claims.

EXAMPLES

The effectiveness of the method for treating carbonaceous materials containing aluminum, fluorides and sodium ions in accordance with the present invention was evaluated by electrolysis of certain alkaline solutions containing aluminum in a water-soluble form.

For electrolysis, an electrolytic cell was used with electrodes having a surface area of 1 dm ² and equipped with a membrane selectively permeable to cations. A current of 20 A was applied to the electrodes (current density: 200 A / m ² ).

EXAMPLE 1. Extraction solution containing NaAl (OH) ₄

In the first experiment, an alkaline extraction solution (pH = 13.1) was used containing NaAl (OH) ₄ with a concentration of 81.2 g / L and free NaOH with a concentration of 9.5 g / L. The extraction solution was fed into the anode compartment containing an electrolytic solution of Na ₂ SO ₄ (80 g / L). 3.04 L of NaOH solution with an initial concentration of 210.4 g / L was fed into the cathode compartment.

Both the anolyte and the catholyte were circulated in their respective anode and cathode compartments at a flow rate of about 30 L / min and were electrolyzed until a whitish precipitate began to form. When a precipitate appeared, the electrolysis was interrupted to study the characteristics of the catholyte. When a precipitate appeared in the cell, there was 3.05 L of a catholyte solution with a NaOH concentration of 212.3 g / L. The pH of the solution in the anode compartment reached about 9.8.

Then the electrolysis was resumed by feeding additional extraction solution into the anode compartment and extracting from it the dispersion containing the precipitate, which was fed to the decanter. The extraction solution was fed into the anode compartment, connecting it with the recirculating anolyte in such a proportion with respect to the latter so that the pH of the solution in the anode compartment did not change significantly.

The clarified solution at the outlet of the decanter was sent to a storage tank, from where it could be withdrawn for recirculation to the electrolytic cell.

The decanted fraction containing the precipitate was collected in a beaker and filtered to obtain a precipitate containing aluminum. The filter cake was washed with water and dried at 120 ° C for 12 hours.

Electrolysis was carried out for 3 hours, the total amount of NaAl (OH) ₄ solution supplied was about 2.3 L.

At the end of the experiment, it was found that:

- in the cathode compartment there was 3.10 l of NaOH solution having a concentration of 236.0 g / l (current efficiency 94.0%);

the filtered, washed and dried (3 hours at 650 ° C) solid obtained in the anode compartment weighed 122.0 g and consisted essentially of Al ₂ O ₃ .

EXAMPLE 2. Extraction solution containing Na ₃ AlF ₆

The second experiment was carried out using an alkaline extraction solution (pH = 12.9) as anolyte, in which 63.6 g / l Na ₃ AlF _{6 was} dissolved together with 21.3 g / l Al ₂ O ₃ . This solution was fed into the anode compartment containing a Na ₂ SO ₄ solution with a concentration of 80 g / L. 3.14 L of a NaOH solution with a concentration of 236.8 g / L was fed into the cathode compartment.

Electrolysis was performed in the same cell and under the same operating conditions as described in Example 1.

Anolyte and catholyte circulated in the cell and were subjected to electrolysis until a precipitate began to form in the anode compartment. Then the electrolysis was interrupted to study the characteristics of the catholyte. When a precipitate appeared in the cathode compartment, there were 3.16 liters of catholyte, the concentration of NaOH in which was 238.4 g / l, while the pH value of the solution in the anode compartment was about 9.6.

Then the electrolysis was resumed by feeding additional extraction solution into the anode compartment and withdrawing from it at the same flow rate the dispersion containing the precipitate, which was fed to the decanter.

The extraction solution was fed into the anode compartment, connecting it with the recirculating anolyte in such a proportion with respect to the latter so that the pH of the solution in the anode compartment did not change significantly.

The decanted fraction containing the precipitate was collected in a beaker and filtered to obtain a precipitate containing aluminum. Then the filter cake was washed with water and dried at 120 ° C for 12 hours.

The electrolysis was carried out for 5 hours and 30 minutes, the total amount of the alkaline extraction solution supplied was about 2.0 L.

At the end of the experiment, it was found that:

- in the cathode compartment there was 3.19 l of NaOH solution having a concentration of 286.4 g / l (current efficiency 96.0%);

the filtered, washed and dried solid obtained in the anode compartment weighed 161.1 g and consisted essentially of a mixture of Na ₃ AlF ₆ (88 wt%) and Al (OH) ₃ (12 wt%).

EXAMPLE 3. Extraction solution containing NaAl (OH) ₄

The third experiment was carried out using as an extraction solution 2.50 L of an alkaline solution (pH = 12.8) containing NaAl (OH) ₄ with a concentration of 49.86 g / L, NaF with a concentration of 30.06 g / L and free NaOH with a concentration of 35.46 g / l. The extraction solution was fed into the central compartment of a cell equipped with a cationic selective permeability membrane and an anionic selective permeable membrane as described above with reference to FIG. 2. 2.04 L of a 122.4 g / L NaOH solution (catholyte) was fed into the cathode compartment, while 3.22 L of an 82.3 g / L HF solution (anolyte) was fed into the anode compartment. The anolyte, catholyte and extraction solution were recirculated in the respective compartments of the cell throughout the test period.

The electrolysis was carried out for about 8 hours 30 minutes at a current density of 2000 A / m ² .

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

- in the cathode compartment there was 2.42 L of NaOH solution with a concentration of 203.4 g / L (current efficiency 94.3%);

the filtered, washed and dried solid obtained in the anode compartment weighed 71.6 g and consisted essentially of AlF ₃ .

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

Claims (33)

1. A method for processing waste from the electrolytic production of primary aluminum in the form of a solid carbon-containing material containing aluminum, fluorides and Na ⁺ ions, including the following stages: (a) leaching said solid carbonaceous material with at least one aqueous alkaline solution to form (i) an extraction solution containing aluminum in water-soluble form, fluorides and Na ⁺ ions, and (ii) at least one solid insoluble residue, (b) separating the solid insoluble residue from the extraction solution, (c) carrying out membrane electrolysis of an extraction solution free of said solid insoluble residue to form at least one aluminum-containing precipitate and at least one aqueous NaOH solution. 2. The method according to claim 1, wherein said aluminum-containing precipitate is a compound selected from: Al (OH) ₃ , Na ₃ AlF ₆ , AlF ₃ and mixtures thereof. 3. The method of claim 1, wherein fluorine ions are added to said extraction solution to facilitate the formation of Na ₃ AlF ₆ in said aluminum-containing precipitate. 4. The method of claim 1, wherein the fluorine ions present in said extraction solution are precipitated and separated from said extraction solution prior to said membrane electrolysis step to promote the formation of Al (OH) ₃ in said aluminum-containing precipitate. 5. The method according to claim 4, wherein the fluorine ions present in said extraction solution are precipitated by adding at least one alkaline earth metal ions to the extraction solution. 6. The method according to claim 1, wherein at least one part of said aqueous NaOH solution obtained in the membrane electrolysis step of the extraction solution (c) is used in the leaching step (a). 7. A method according to any one of claims. 1-6, wherein said membrane electrolysis step comprises the following: (i) providing at least one electrolytic cell comprising: - at least one anode compartment, including at least one anode immersed in anolyte, - at least one cathode compartment, including at least one cathode immersed in a catholyte, wherein said anode compartment is separated from said cathode compartment by at least one selectively permeable cationic membrane, (ii) feeding said extraction solution to said anode compartment. 8. A method according to claim 7, wherein said extraction solution is added with at least one salt containing at least one alkali metal or alkaline earth metal cation and at least one anion having an oxidation potential greater than that of water under conditions, at which the specified stage of membrane electrolysis is carried out. 9. The method of claim 8, wherein said anion is selected from sulfate, nitrate, perchlorate, phosphate, and borate. 10. The method according to claim 7, which includes: - removing the specified anolyte from the specified anode compartment after the membrane electrolysis stage, - combining the extraction solution with the anolyte extracted from the anode compartment, with the formation of a precipitate containing aluminum dispersed in an aqueous solution, - separation of the sediment containing aluminum from the aqueous solution, - supply of an aqueous solution free from the specified precipitate containing aluminum into the anode compartment. 11. The method according to any one of claims. 1-6, in which the membrane electrolysis step comprises the following: (i) providing at least one electrolytic cell comprising: - at least one anode compartment, including at least one anode immersed in anolyte, - at least one cathode compartment, including at least one cathode immersed in a catholyte, - at least one feed compartment located between said anode compartment and said cathode compartment, wherein said supply compartment is separated from said anode compartment by at least one selectively permeable cationic membrane, said feed compartment is separated from said cathode compartment by at least one selectively permeable anionic membrane, (ii) feeding said extraction solution to said feed compartment. 12. A method according to claim 7 or 11, wherein said catholyte is an aqueous NaOH solution. 13. The method of claim 11, wherein said anolyte is an aqueous solution of hydrofluoric acid. 14. The method according to any one of claims. 1-6, in which the membrane electrolysis step is carried out in an electrodialysis device.

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