Method for extracting and separating rare earth elements from coal-based solid waste

Abstract

The invention discloses a method for extracting and separating rare earth elements from coal-based solid waste. Coal gangue and fly ash are used as raw materials. The extraction and separation method comprises the following steps: S1, roasting activation pretreatment of the coal-based solid waste; s2, carrying out sulfuric acid leaching on the coal-based solid waste subjected to activation pretreatment in the step S1, and carrying out solid-liquid separation to obtain a sulfuric acid leaching solution and solid residues; s3, an anti-solvent is slowly added into the sulfuric acid leaching solution separated in the step S2, and rare earth elements are selectively precipitated and recycled; and S4, evaporating the filtrate obtained by separation in the step S3, and recovering the anti-solvent for reutilization. According to the method provided by the invention, the technological process can be shortened, the technological steps are simplified, and the selective recovery of rare earth elements in the leachate is easier.

Description

Technical Field

[0001] The invention relates to the technical field of rare earth recovery, and in particular to a method for extracting and separating rare earth elements from coal-based solid waste. Background Art

[0002] Rare earth elements are a general term for scandium, yttrium and 15 lanthanide elements. Their special electronic structure gives them unique and excellent physical and chemical properties such as magnetism, electricity and light, which makes them widely used in modern high-tech products. However, due to the scarcity of rare earth resources and the influence of relevant policies of various countries, there is an urgent need to develop new rare earth resources and rare earth extraction and separation technologies.

[0003] At present, coal is the mainstay of my country's energy. During its mining and processing, the output of coal-based solid waste has increased dramatically, mainly including coal gangue and fly ash. However, due to technical and cost limitations, the disposal and utilization of these coal-based solid wastes face huge challenges. Coal gangue and fly ash are associated with or enriched with a large number of rare earth elements. Extracting and recovering rare earth elements from coal gangue and fly ash can provide a possibility to alleviate the development needs of high-tech industries in the future and broaden the existing production channels of rare earth elements. The existing hydrometallurgical combined with solvent extraction or chemical precipitation method is an effective method for extracting and separating rare earth elements from solid minerals, but it faces many problems, such as high acid concentration in the leachate, extremely high content of impurity elements (hundreds or thousands of times), difficulty in selective recovery of rare earth elements, and long and complex process flow. Summary of the invention

[0004] The purpose of the present invention is to provide a method for extracting and separating rare earth elements from coal-based solid waste, which can shorten the process flow, simplify the process steps, and make the selective recovery of rare earth elements in the leaching solution easier.

[0005] To achieve the above object, the present invention provides a method for recovering rare earth elements from coal-based solid waste, wherein the coal-based solid waste is subjected to roasting activation, sulfuric acid leaching, and anti-solvent precipitation in sequence to recover the rare earth elements. The specific process is as follows:

[0006] S1. Calcination and activation: The coal-based solid waste is placed in an air atmosphere for calcination and activation pretreatment; the calcination temperature is 500-900°C, and the calcination time is 30-240min;

[0007] S2, sulfuric acid leaching: the activated coal-based solid waste obtained in step S1 is mixed with a dilute sulfuric acid solution, and the mixture is continuously stirred at a certain temperature to carry out a leaching reaction, and a sulfuric acid leaching solution and a solid residue are obtained by solid-liquid separation; the solid-liquid ratio of the activated coal-based solid waste to the dilute sulfuric acid solution is 1: (5-20) g / mL, the concentration of the dilute sulfuric acid solution is 0.5-3 mol / L, the leaching time is 2-6 h, and the leaching temperature is 30-90° C.; the obtained sulfuric acid leaching solution contains rare earth ions and a large amount of impurity ions such as aluminum, iron, and magnesium, with a rare earth content of about 0.01-50 mg / L, an aluminum content of about 3-150 g / L, an iron content of about 0.3-2.5 g / L, and a titanium content of about 80-1200 mg / L;

[0008] S3, anti-solvent precipitation: slowly adding a certain volume of anti-solvent to the sulfuric acid leaching solution obtained in step S2, and continuously stirring under a certain temperature condition to form a mixed rare earth precipitate product, and then filtering and separating to obtain a mixed rare earth precipitate product; the volume ratio between the sulfuric acid leaching solution and the anti-solvent is 1: (0.6-2), the stirring temperature is 15-25° C., and the stirring time is 0.5-24 h;

[0009] S4, anti-solvent evaporation cycle: evaporate the solution filtered in step S3, recover the anti-solvent added in the precipitation process, and use the recovered anti-solvent in step S3 of the next batch recovery process to achieve the recycling of the anti-solvent; the evaporation temperature is 90-100°C.

[0010] Preferably, in step S2, the solid-liquid ratio of the activated coal-based solid waste to the dilute sulfuric acid solution is 1: (5-10) g / mL, the concentration of the dilute sulfuric acid solution is 1-2 mol / L, the leaching time is 3-4 h, and the leaching temperature is 60-90°C.

[0011] Preferably, the anti-solvent in step S3 is one or more of methanol, ethanol, n-propanol, and isopropanol.

[0012] Preferably, in step S3, the volume ratio between the sulfuric acid leaching solution and the anti-solvent is 1:(1-1.8), the stirring temperature is 20-25° C., and the stirring time is 0.5-2 h.

[0013] Preferably, the coal-based solid waste is coal gangue or fly ash.

[0014] Furthermore, when the coal-based solid waste in step S1 is coal gangue, it needs to be crushed and ground to less than 120 mesh before being subjected to roasting activation pretreatment; the roasting temperature is 500-600° C., and the roasting time is 60-120 min.

[0015] Furthermore, when the coal-based solid waste in step S1 is fly ash, the fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1: (0.5-3) and then subjected to roasting activation pretreatment.

[0016] Preferably, the mass ratio of fly ash to potassium sulfate is 1:(1-2); the roasting temperature is 600-800° C., and the roasting time is 120-180 min.

[0017] Preferably, in step S4, the evaporation temperature is 90-95°C.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The method for extracting and separating rare earth elements from coal-based solid waste provided by the present invention comprises four steps: roasting activation, sulfuric acid leaching, anti-solvent precipitation, and anti-solvent evaporation cycle. The sulfuric acid leaching solution does not need to be pre-enriched and impurity-removed, which simplifies the extraction and separation process and reduces costs. At the same time, the anti-solvent is recovered by evaporation operation and can be reused.

[0020] (2) The present invention adds a specific anti-solvent (methanol, ethanol, n-propanol or isopropanol) in the precipitation step, which can preferentially and selectively precipitate the rare earth elements in the coal-based solid waste sulfuric acid leaching solution without adjusting the solution pH environment, and has higher efficiency, with a precipitation recovery rate of up to 98.7%. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a process flow chart for extracting and separating rare earth elements from coal-based solid waste proposed by the present invention. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with specific examples. The following examples are only used to illustrate the content of the present invention and are not used to limit the protection scope of the present invention. Simple changes made to the present invention using the concept of the present invention are all within the scope of protection claimed by the present invention.

[0023] In the following examples, the gangue used was taken from the Harwusu Mine in the Jungar Mining Area (the rare earth element content was 423.5 mg / kg), the fly ash used was collected from the gangue power plant of Shenhua Junneng Group (the rare earth element content reached 1005.8 mg / kg), and the reagents used, unless otherwise specified, were all from conventional commercial stores.

[0024] Example 1

[0025] like Figure 1 As shown, a method for extracting and separating rare earth elements from coal gangue is provided, wherein the coal gangue is subjected to roasting activation, sulfuric acid leaching, and anti-solvent precipitation in sequence to recover the rare earth elements. The specific process is as follows:

[0026] S1. Calcination and activation: crush and grind the coal gangue to less than 120 mesh, and directly place it in air atmosphere for calcination and activation pretreatment. The calcination temperature is 600°C and the calcination time is 120min.

[0027] S2, sulfuric acid leaching: the activated coal gangue obtained above is mixed with 2 mol / L dilute sulfuric acid solution at a solid-liquid ratio of 1:10 g / mL, and the leaching is continued at 90° C. for 3 hours, and solid residue and sulfuric acid leaching solution containing rare earth elements are obtained by solid-liquid separation;

[0028] S3, anti-solvent precipitation: recovering rare earth elements from the sulfuric acid leachate by anti-solvent precipitation method, using ethanol as anti-solvent, slowly adding ethanol to the sulfuric acid leachate, stirring continuously for 3 hours at 25°C to form a mixed rare earth precipitation product, and then filtering and separating to obtain a mixed rare earth precipitation product, the volume ratio of sulfuric acid leachate to anti-solvent is 1:1;

[0029] S4, anti-solvent evaporation cycle: evaporate the solution obtained by filtration in step S3 to recover the ethanol added in the precipitation process, which can be used in step S3 in the next batch recovery process to achieve the recycling of the anti-solvent. The evaporation temperature is 90°C.

[0030] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the coal gangue) is 72.3%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 84.6%.

[0031] Example 2

[0032] Only step S2 of this embodiment is different from that of embodiment 1, and the other steps are consistent with those of embodiment 1.

[0033] Step S2 of this embodiment, sulfuric acid leaching: the activated coal gangue obtained above is mixed with 1 mol / L dilute sulfuric acid solution at a solid-liquid ratio of 1:15 g / mL, and the leaching is continued at 60° C. for 3 hours, and solid residue and sulfuric acid leaching solution containing rare earth elements are obtained by solid-liquid separation;

[0034] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the coal gangue) is 68.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 85.2%.

[0035] Example 3

[0036] Only step S3 of this embodiment is different from that of embodiment 1, and the other steps are consistent with those of embodiment 1.

[0037] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, ethanol is used as anti-solvent, ethanol is slowly added to the sulfuric acid leachate, and stirring is continued for 0.5h at a temperature of 25°C to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.5;

[0038] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the coal gangue) is 72.3%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 98.7%.

[0039] Example 4

[0040] Only step S3 of this embodiment is different from that of embodiment 1, and the other steps are consistent with those of embodiment 1.

[0041] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, ethanol is used as anti-solvent, ethanol is slowly added to the sulfuric acid leachate, and stirring is continued for 24 hours at a temperature of 25° C. to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.5;

[0042] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the coal gangue) is 72.3%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 92.4%.

[0043] Example 5

[0044] like Figure 1 As shown, a method for extracting and separating rare earth elements from fly ash is provided, wherein the fly ash is subjected to roasting activation, sulfuric acid leaching, and anti-solvent precipitation in sequence to recover the rare earth elements. The specific process is as follows:

[0045] S1, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:0.8, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0046] S2, sulfuric acid leaching: the activated fly ash obtained above was mixed with a 2 mol / L dilute sulfuric acid solution at a solid-liquid ratio of 1:10 g / mL, and the mixture was continuously leached at 90° C. for 4 h, and solid residue and sulfuric acid leaching solution containing rare earth elements were obtained by solid-liquid separation;

[0047] S3, anti-solvent precipitation: recover rare earth elements from the sulfuric acid leachate by anti-solvent precipitation method, using ethanol as anti-solvent, slowly adding ethanol to the sulfuric acid leachate, stirring continuously for 24 hours at 25°C to form a mixed rare earth precipitation product, then filtering and separating to obtain a mixed rare earth precipitation product, the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.5;

[0048] S4, anti-solvent evaporation cycle: evaporate the solution obtained by filtration in step S3 to recover the ethanol added in the precipitation process, which can be used in step S3 in the next batch recovery process to achieve the recycling of the anti-solvent. The evaporation temperature is 90°C.

[0049] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) is 71.2%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 90.5%.

[0050] Example 6

[0051] Only step S1 of this embodiment is different from that of embodiment 5, and the other steps are consistent with those of embodiment 5.

[0052] Step S1 of this embodiment, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:1.5, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0053] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) is 73.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 92.6%.

[0054] Example 7

[0055] Only steps S1 and S3 of this embodiment are different from those of embodiment 5, and the other steps are consistent with those of embodiment 5.

[0056] Step S1 of this embodiment, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:1.5, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0057] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, ethanol is used as anti-solvent, ethanol is slowly added to the sulfuric acid leachate, and stirring is continued for 6 hours at a temperature of 25° C. to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.5;

[0058] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) is 73.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 95.3%.

[0059] Example 8

[0060] Only steps S1 and S3 of this embodiment are different from those of embodiment 5, and the other steps are consistent with those of embodiment 5.

[0061] Step S1 of this embodiment, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:1.5, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0062] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, methanol is used as anti-solvent, methanol is slowly added to the sulfuric acid leachate, and stirring is continued for 3 hours at a temperature of 25° C. to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.2;

[0063] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) is 73.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 92.3%.

[0064] Example 9

[0065] Only steps S1 and S3 of this embodiment are different from those of embodiment 5, and the other steps are consistent with those of embodiment 5.

[0066] Step S1 of this embodiment, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:1.5, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0067] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, using n-propanol as anti-solvent, and n-propanol is slowly added to the sulfuric acid leachate, and stirred for 3 hours at 25° C. to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.2;

[0068] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) is 73.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 60.5%.

[0069] Example 10

[0070] Only steps S1 and S3 of this embodiment are different from those of embodiment 5, and the other steps are consistent with those of embodiment 5.

[0071] Step S1 of this embodiment, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:1.5, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0072] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, isopropanol is used as anti-solvent, isopropanol is slowly added to the sulfuric acid leachate, and stirring is continued for 3 hours at a temperature of 25° C. to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of sulfuric acid leachate to anti-solvent is 1:1.2;

[0073] The rare earth element leaching rate (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) obtained in this embodiment is 73.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 75.3%.

[0074] Embodiment 11

[0075] Only steps S1 and S3 of this embodiment are different from those of embodiment 5, and the other steps are consistent with those of embodiment 5.

[0076] Step S1 of this embodiment, calcination activation: fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1:1.5, and then placed in an air atmosphere for calcination activation pretreatment, the calcination temperature is 800°C, and the calcination time is 120min;

[0077] In step S3 of this embodiment, anti-solvent precipitation: the above sulfuric acid leachate is used to recover rare earth elements by anti-solvent precipitation method, and the ethanol recovered in step S4 of Example 5 is used as the anti-solvent, and the ethanol is slowly added to the sulfuric acid leachate, and stirred for 3 hours at a temperature of 25° C. to form a mixed rare earth precipitation product, which is then filtered and separated to obtain a mixed rare earth precipitation product, and the volume ratio of the sulfuric acid leachate to the anti-solvent is 1:1.2;

[0078] The rare earth element leaching rate obtained in this embodiment (the content of rare earth elements in the leaching solution obtained in step S2 / the content of rare earth elements in the fly ash) is 73.9%, and the precipitation recovery rate (the content of rare earth elements in the mixed rare earth precipitation product obtained in step S3 / the content of rare earth elements in the leaching solution obtained in step S2) is 95.6%.

Claims

1. A method for extracting and separating rare earth elements from coal-based solid waste, characterized in that: The coal-based solid waste is subjected to roasting activation, sulfuric acid leaching, and anti-solvent precipitation in sequence to recover rare earth elements. The specific process is as follows: S1. Calcination and activation: The coal-based solid waste is placed in an air atmosphere for calcination and activation pretreatment; the calcination temperature is 500-900°C, and the calcination time is 30-240min; S2, sulfuric acid leaching: the activated coal-based solid waste obtained in step S1 is mixed with a dilute sulfuric acid solution, and the mixture is continuously stirred under a certain temperature condition to carry out a leaching reaction, and a sulfuric acid leaching solution and a solid residue are obtained by solid-liquid separation; the solid-liquid ratio of the activated coal-based solid waste to the dilute sulfuric acid solution is 1: (5-20) g / mL, the concentration of the dilute sulfuric acid solution is 0.5-3 mol / L, the leaching time is 2-6 h, and the leaching temperature is 30-90° C.; S3, anti-solvent precipitation: slowly adding a certain volume of anti-solvent to the sulfuric acid leaching solution obtained in step S2, and continuously stirring under a certain temperature condition to form a mixed rare earth precipitate product, and then filtering and separating to obtain a mixed rare earth precipitate product; the volume ratio between the sulfuric acid leaching solution and the anti-solvent is 1: (0.6-2), the stirring temperature is 15-25° C., and the stirring time is 0.5-24 h; S4, anti-solvent evaporation cycle: evaporate the solution filtered in step S3, recover the anti-solvent added in the precipitation process, and use the recovered anti-solvent in step S3 of the next batch recovery process to achieve the recycling of the anti-solvent; the evaporation temperature is 90-100°C.

2. The method for extracting and separating rare earth elements from coal-based solid waste according to claim 1, characterized in that: In step S2, the solid-liquid ratio of the activated coal-based solid waste to the dilute sulfuric acid solution is 1: (5-10) g / mL, the concentration of the dilute sulfuric acid solution is 1-2 mol / L, the leaching time is 3-4 h, and the leaching temperature is 60-90°C.

3. A method for extracting and separating rare earth elements from coal-based solid waste according to claim 1 or 2, characterized in that: In step S3, the anti-solvent is one or more of methanol, ethanol, n-propanol, and isopropanol.

4. A method for extracting and separating rare earth elements from coal-based solid waste according to claim 1 or 2, characterized in that: In step S3, the volume ratio between the sulfuric acid leaching solution and the anti-solvent is 1:(1-1.8), the stirring temperature is 20-25° C., and the stirring time is 0.5-2 h.

5. A method for extracting and separating rare earth elements from coal-based solid waste according to claim 1 or 2, characterized in that: The coal-based solid waste is coal gangue or fly ash.

6. The method for extracting and separating rare earth elements from coal-based solid waste according to claim 5, characterized in that: When the coal-based solid waste in step S1 is coal gangue, it needs to be crushed and ground to less than 120 meshes before being subjected to roasting activation pretreatment; the roasting temperature is 500-600° C., and the roasting time is 60-120 min.

7. The method for extracting and separating rare earth elements from coal-based solid waste according to claim 5, characterized in that: When the coal-based solid waste in step S1 is fly ash, the fly ash and potassium sulfate are uniformly mixed in a mass ratio of 1: (0.5-3), and then subjected to roasting activation pretreatment.

8. The method for extracting and separating rare earth elements from coal-based solid waste according to claim 7, characterized in that: The mass ratio of fly ash to potassium sulfate is 1:(1-2); the roasting temperature is 600-800°C, and the roasting time is 120-180 minutes.

9. A method for extracting and separating rare earth elements from coal-based solid waste according to claim 1 or 2, characterized in that: In step S4, the evaporation temperature is 90-95°C.

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