METHOD OF MANUFACTURING INORGANIC ION EXCHANGER FOR SELECTIVE EXTRACTION OF LITHIUM FROM LITHIUM-CONTAINING NATURAL AND TECHNOLOGICAL BRINES

Abstract

A method of manufacturing an inorganic ion exchanger for the selective extraction of lithium from lithium-containing natural and technological brines is performed by interacting at least one soluble niobium(V) compound with an acid that contains at least one iron(III) compound, thus forming an electrolyte that contains a hydrated niobium(V) oxide and a hydrated iron(III) oxide, which co-precipitate and form a precipitate of a mixed hydrated niobium(V) and iron(III) oxide. The precipitate is washed, an excess of the electrolyte is removed, and the product is granulated with subsequent conversion into a lithium form, which is calcined and is converted to an H-form of the inorganic ion exchanger by treating thereof with an acid solution. the addition of Fe.sup.3+ ions contained in the iron(III) compound to the sorbent composition allows obtaining inorganic ion-exchange sorbents with a specific structure, which provides high selectivity, especially for lithium ions.

Claims

1. A method of manufacturing an inorganic ion exchanger for selective extraction of lithium from lithium-containing natural and technological brines, the inorganic ion exchanger being represented by the following general formula:
H.sub.aNbO.sub.(2.5+0.5-a).Math.bLi.sub.2O.Math.cFe.sub.2O.sub.3.Math.dH.sub.2O; wherein: a is a number ranging from 0.5 to 2.0, b is a number ranging from 0.01 to 0.4, c is a number ranging from 0.05 to 0.11, and d is a number ranging from 0.1 to 2.0, wherein the method comprising: carrying out a process of coprecipitation by interacting at least one soluble niobium(V) compound with an acid solution of at least one iron(III) compound, thus forming a precipitate of a mixed niobium(V) and iron(III) hydrated oxide, which forms a suspension in a mother solution, the mother solution comprising a solution of salts resulting from the aforementioned interaction and constituting an electrolyte; washing the obtained mixed niobium(V) and iron(III) hydrated oxide by decanting for removing an excess of the electrolyte; granulating the mixed niobium(V) and iron(III) hydrated oxide by freezing thereof with subsequent defreezing, thus obtaining a granulated mixed niobium(V) and iron(III) hydrated oxide; converting the granulated mixed niobium(V) and iron(III) hydrated oxide into a lithium form by treating thereof with a lithium-containing compound selected from the group consisting of aqueous solutions of lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2CO.sub.3); calcining the lithium form of the granulated mixed niobium(V) and iron(III) hydrated oxide to obtain a mixed tripled lithium, niobium(V), and iron(III) oxide in a granulated form, which constitutes a lithium-form of an inorganic ion-exchanger; and converting the lithium-form of the inorganic ion-exchanger into an H-form of the inorganic ion-exchanger by treating thereof with an acid solution selected from the group consisting of a nitric acid (HNO.sub.3), a hydrochloric acid (HCl), a sulfuric acid (H.sub.2SO.sub.4) a perchloric acid (HClO.sub.4), and a trichloroacetic acid (CCl.sub.3COOH).

2. The method of claim 1, wherein the inorganic ion-exchanger comprises solid particles, which constitute a chemical non-stoichiometric compound in the form of an inorganic polymeric aqua-oxo-hydroxo complex.

3. The method of claim 1, wherein the polymeric aqua-oxo-hydroxo complex is a polymeric aqua-oxo-hydroxo complex of niobium and iron.

4. The method of claim 1, wherein the soluble niobium compounds are alkali metal orthoniobates selected from the group consisting of Li.sub.3NbO.sub.4, Na.sub.3NbO.sub.4, K.sub.3NbO.sub.4, Rb.sub.3NbO.sub.4, Cs.sub.3NbO.sub.4, and niobium halides selected from the group consisting of NbCl.sub.5, NbOCl.sub.3, NbBr.sub.5, and NbOBr.sub.3.

5. The method of claim 2, wherein the soluble niobium compounds are alkali metal orthoniobates selected from the group consisting of Li.sub.3NbO.sub.4, Na.sub.3NbO.sub.4, K.sub.3NbO.sub.4, Rb.sub.3NbO.sub.4, Cs.sub.3NbO.sub.4, and niobium halides selected from the group consisting of NbCl.sub.5, NbOCl.sub.3, NbBr.sub.5, and NbOBr.sub.3.

6. The method of claim 3, wherein the soluble niobium compounds are alkali metal orthoniobates selected from the group consisting of Li.sub.3NbO.sub.4, Na.sub.3NbO.sub.4, K.sub.3NbO.sub.4, Rb.sub.3NbO.sub.4, Cs.sub.3NbO.sub.4, and niobium halides selected from the group consisting of NbCl.sub.5, NbOC.sub.3, NbBr.sub.5, and NbOBr.sub.3.

7. The method of claim 1, wherein the soluble iron(III) compounds are represented by compounds selected from the group consisting of FeCl.sub.3, FeBr.sub.3, Fe(NO.sub.3).sub.3, Fe.sub.2(SO.sub.4).sub.3, Fe(CH.sub.3COO).sub.3.

8. The method of claim 2, wherein the soluble iron(III) compounds are represented by compounds selected from the group consisting of FeCl.sub.3, FeBr.sub.3, Fe(NO.sub.3).sub.3, Fe.sub.2(SO.sub.4).sub.3, Fe(CH.sub.3COO).sub.3.

9. The method of claim 3, wherein the soluble iron(III) compounds are represented by compounds selected from the group consisting of FeCl.sub.3, FeBr.sub.3, Fe(NO.sub.3).sub.3, Fe.sub.2(SO.sub.4).sub.3, Fe(CH.sub.3COO).sub.3.

10. The method of claim 4, wherein the soluble iron(III) compounds are represented by compounds selected from the group consisting of FeCl.sub.3, FeBr.sub.3, Fe(NO.sub.3).sub.3, Fe.sub.2(SO.sub.4).sub.3, Fe(CH.sub.3COO).sub.3.

11. The method of claim 1, wherein freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

12. The method of claim 2, wherein freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

13. The method of claim 3, wherein freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

14. The method of claim 4, wherein freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

15. The method of claim 5, wherein freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

16. The method of claim 2, wherein converting the granulated mixed hydrated niobium(V) and iron(III) oxide into a lithium form is carried out by treating thereof with a lithium-containing compound selected from the group consisting of aqueous solutions of lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2CO.sub.3).

17. The method of claim 5, wherein converting the granulated mixed hydrated niobium(V) and iron(II) oxide into a lithium form is carried out by treating thereof with a lithium-containing compound selected from the group consisting of aqueous solutions of lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2CO.sub.3).

18. The method of claim 6, wherein converting the granulated mixed hydrated niobium(V) and iron(III) oxide into a lithium form is carried out by treating thereof with a lithium-containing compound selected from the group consisting of aqueous solutions of lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2CO.sub.3).

19. The method of claim 1, wherein calcining is carried out at a temperature in the range of 360 C. to 460 C. and freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

20. The method of claim 18, wherein calcining is carried out at a temperature in the range of 360 C. to 460 C. and freezing is carried out for 24 to 48 hours at a temperature range of 4 C. to 10 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a graph illustrating a total exchange capacity of hydrated niobium pentoxide E.sub.Li with additives of Fe.sup.3+ ions (sorption of lithium from 0.1 N Li.sub.2CO.sub.3 solution; Fe/Nb is a molar ratio of the Fe.sup.3+ additive to the total amount of the Nb.sup.5+ ions in the sorbent composition).

[0026] FIG. 2 is a graph that illustrates the effect of the solution pH on the total exchange capacity (EU, meq of Li per 1 g of sorbent) of hydrated niobium pentoxide (HNP) obtained by titrating a HNP sample without addition (Curve 1) and with an addition of Fe.sup.3+ (Curve 2), the titration being carried out with 0.1N solution of UOH (Ionic Strength I=0.1; Fe/Nb=0.108 is a molar ratio of the Fe.sup.3+ additive to the total amount of the Nb.sup.5+ ions in the sorbent composition).

[0027] FIG. 3 is a graph that shows a 3D dependence of the selective capacity for lithium (E.sub.Li, mg of Li per 1 g of sorbentnumbers) on the heat treatment temperature of the Li-form of the sorbent; Fe/Nb is a molar ratio of the Fe.sup.3+ additive to the total amount of the Nb.sup.5+ ions in the sorbent composition.

[0028] FIG. 4 is a graph that shows an effect of the calcination temperature of materials on their sorption-selective properties during sorption from a model complex solution with a ratio Li/Na=1/10; Curve 1 shows E.sub.Li capacity for lithium (meq of Li per 1 g of sorbent); Curve 2 shows E.sub.Na capacity for sodium (meq of Li per 1 g of sorbent); and Curve 3 corresponds to Li/Na=E.sub.Li/E.sub.Namolar ratio of Lit and Na.sup.+ ions in the sorbent phase after completion of the sorption process; the samples had a ratio Fe(III):Nb(V) equal to 0.07.

[0029] FIG. 5 is a graph that illustrates dependences of lithium capacity (E.sub.Li; meq of Li per 1 g of sorbent), sodium capacity (E.sub.Na; meq of Na per 1 g of sorbent), and E.sub.Li/E.sub.Na ratio in the solid phase of the sorbent during sorption from a model complex solution with Li/Na=1/10, on the content of iron(III) ions in the sorbent; Fe(ill):Nb(V) is an ion ratio in the sorbent, where the curve 1 corresponds to E.sub.Li; the curve 2 corresponds to E.sub.Na; and the curve 3 corresponds to Li/Na=E.sub.Li/E.sub.Na; the samples were calcined at t=420 C.

[0030] FIG. 6 shows losses of sorbents for one cycle of operation in the mode of sorption-desorption for samples with the addition of Fe.sup.3+ ions at the ratio of Fe(III) to Nb(V) equal to 0.07; samples corresponding to the curve plotted by dots 1 were calcined 400 C., samples corresponding to the curve plotted by dots 2 were calcined at 540 C.

SUMMARY OF THE INVENTION

[0031] The invention relates to chemical technology and hydrometallurgy, particularly to the production of lithium-selective inorganic ion exchangers for lithium extraction from lithium-containing natural and technological brines. The invention may find use in extracting lithium from neutral and slightly alkaline solutions with a high content of sodium ions and ions of other alkali and alkaline earth metals. The method is based on the preparation and use of ion sieves. The invention makes it possible to improve the selectivity and exchange capacity for lithium on sorbents based on niobium oxide and improve the chemical stability of such sorbents in cyclic operations.

[0032] More specifically, the method of the invention is aimed at manufacturing an inorganic ion-exchanger in the form of solid particles, which constitute a chemical non-stoichiometric compound in the form of an inorganic polymeric aqua-oxo-hydroxo complex intended for selective extraction of lithium from lithium-containing natural and industrial brines, the inorganic ion-exchanger being represented by the following general formula:

H.sub.aNbO.sub.(2.5+0.5-a).Math.bLi.sub.2O.Math.cFe.sub.2O.sub.3.Math.dH.sub.2O;

[0033] wherein: [0034] a is a number ranging from 0.5 to 2.0, [0035] b is a number ranging from 0.01 to 0.4, [0036] c is a number ranging from 0.05 to 0.11, and [0037] d is a number ranging from 0.1 to 2.0.

[0038] The method consists of the following steps: [0039] carrying out a process of coprecipitation by interacting at least one soluble niobium(V) compound with an acid solution of at least one iron(III) compound, thus forming a precipitate of a mixed niobium(V) and iron(III) hydrated oxide, which forms a suspension in a mother solution, the mother solution comprising a solution of salts resulting from the aforementioned interaction and constituting an electrolyte; [0040] washing the obtained mixed niobium(V) and iron(III) hydrated oxide by decanting for removing an excess of the electrolyte; [0041] granulating the mixed niobium(V) and iron(III) hydrated oxide by freezing thereof with subsequent defreezing, thus obtaining a granulated mixed niobium(V) and iron(III) hydrated oxide; [0042] converting the granulated mixed niobium(V) and iron(III) hydrated oxide into a lithium form by treating thereof with a lithium-containing compound selected from the group consisting of aqueous solutions of lithium hydroxide (LiOH) and lithium carbonate (Li.sub.2CO.sub.3); [0043] calcining the lithium form of the granulated mixed niobium(V) and iron(III) hydrated oxide to obtain a mixed tripled lithium, niobium(V), and iron(III) oxide in a granulated form, which constitutes a lithium-form of an inorganic ion-exchanger; and converting the lithium-form of the inorganic ion-exchanger into an H-form of the inorganic ion-exchanger by treating thereof with an acid solution selected from the group consisting of a nitric acid (HNO.sub.3), a hydrochloric acid (HCl), a sulfuric acid (H.sub.2SO.sub.4) a perchloric acid (HClO.sub.4), and a trichloroacetic acid (CCl.sub.3COOH).

[0044] The aforementioned freezing is carried out for 24 to 48 hours at a temperature in a range of 4 C. to 10 C.

[0045] Calcining is performed preferably at a temperature in the range of 360 C. to 460 C.

[0046] In this invention, the use of niobium is based on the fact that, among other elements, niobium has one of the lowest neutrons capture cross sections, so it is, to a lesser degree, subject to the occurrence of induced radiation when exposed to neutron fluxes.

[0047] The term brines, as used in this patent specification, covers any natural or industrial solution that contains lithium. Ionic sieves mentioned in the specification are inorganic ion-exchange sorbents that exhibit the so-called ion-sieve effect resulting from separating ions that are contained in a solution according to the difference in their ionic radii. The sizes of crystallographic positions in the material's crystal structure correspond to ions of specific dimensions so that larger ions cannot enter the indicated positions. Thus, the ion-sieve effect ensures high selectivity in sieve-effect sorbents.

[0048] According to the present invention, a unique feature of the method is the addition of Fe.sup.3+ ions to the sorbent composition; this method allows obtaining inorganic ion-exchange sorbents with a specific structure, which provides high selectivity, especially for lithium ions.

[0049] It is also important to note that in the context of the present description, the term mixed niobium and iron hydrated oxides does not mean a mechanical mixture of hydrated niobium oxide with hydrated iron oxide but rather means a chemical compound of a non-stoichiometric composition.

[0050] Soluble niobium compounds for use in the method of the present invention are alkali metal orthoniobates selected from the group consisting of such compounds as Li.sub.3NbO.sub.4, Na.sub.3NbO.sub.4, K.sub.3NbO.sub.4, Rb.sub.3NbO.sub.4, Cs.sub.3NbO.sub.4, and niobium halides selected from the group consisting of such compounds as NbCl.sub.5, NbOCl.sub.3, NbBr.sub.5, and NbOBr.sub.3.

[0051] Soluble iron(III) compounds suitable for use in the method of the present invention may be represented by FeCl.sub.3, FeBr.sub.3, Fe(NO.sub.3).sub.3, Fe.sub.2(SO.sub.4).sub.3, and Fe(CH.sub.3COO).sub.3.

[0052] Examples of H-form sorbents suitable for use in the method of the invention are shown below in Table 1 (Li-forms are similar and therefore not included).

[0053] As disclosed in U.S. Pat. No. 10,434,497, a hydrated niobium pentoxide (HNP) can be obtained by using 0.1 N HCl on a solution of potassium orthoniobate K.sub.3NbO.sub.4 at pH=5.5. In the present invention, Fe.sup.3+ ion dopants were introduced into the hydrochloric acid solution by adding a calculated amount of salt FeCl.sub.3. When using niobium halides, Fe.sup.3+ ions are introduced into a solution of a corresponding niobium halide.

[0054] The co-precipitation method was used for introducing additives of Fe.sup.3+ ions into the phase of hydrated oxides. In this regard, the ion-exchange properties of the products of the co-precipitation of hydrated niobium pentoxide with Fe.sup.3+ ions were studied. The results of the study are shown in FIG. 1.

[0055] FIG. 1 shows the results of studying the properties of co-precipitation products of hydrated niobium pentoxide with the addition of Fe.sup.3+ ions. The dependence of hydrated niobium pentoxide properties as total exchange on the amount of the added Fe.sup.3+ was studied in a wide range of compositions (sorption of lithium from 0.1 N Li.sub.2CO.sub.3 solution; Fe/Nb is a molar ratio of the Fe.sup.3+ additive to the total amount of the Nb.sup.5+ ions in the sorbent composition.

[0056] As mentioned above, the obtained dependencies show that Fe.sup.3+ ions affect the ion-exchange properties of hydrated niobium pentoxide.

[0057] It was discovered that in the entire studied range of compositions (FIG. 1) the introduction of additives of Fe.sup.3+ ions into the hydrated niobium pentoxide structure led to a monotonic increase in the cation exchange capacity. A charge balance disorder can partially explain this effect since the charge of Fe.sup.3+ is less than that of Nb.sup.5+. It can also be assumed that such an effect results from structural changes caused in the Nb.sup.5+-containing hydrated niobium pentoxide by the introduction of Fe.sup.3+.

[0058] The added Fe.sup.3+ ions should affect the value of the cation-exchange capacity and the acidity of exchangeable OH-groups due to differences in the physical properties of Nb.sup.5+ ions and the added ions Fe.sup.3+. To clarify the extent of this effect, the inventors herein performed potentiometric titration of two hydrated niobium pentoxide samples by adding Fe.sup.3+ ions. Results of this test are shown in FIG. 2. The curves shown in this drawing illustrate the effect of pH of the sorbent solution on the total exchange capacity (E.sub.Li, mg-eqv of Li per 1 g of sorbent) of a hydrated niobium pentoxide obtained in the titration of a hydrated niobium pentoxide sample without addition and with the addition of Fe.sup.3+ in a ratio of Fe(III):Nb(V)=0.07, the titration being carried out with 0.1N solution of LiOH (ionic strength I=0.1).

[0059] At the same time, it was found that the potentiometric titration curves of the samples exhibit effects like those exhibited by analogous dependence for the initial HNP.

[0060] This dependence (Curve 1) was obtained by potentiometric titration of a sample of ion-exchange material with 0.1 N LiOH solution (I=0.1). The following products were used as samples: 1HNP without a dopant; 2HNP with the addition of Fe.sup.3+ in a ratio of Fe(III):Nb(V)=0.07 (FIG. 2).

[0061] Comparison of the obtained dependences reveal that the most acidic sorption OH-groups remained almost unchanged. Still, their number increased, but the weakly acidic groups were partially acidified (Table 1).

TABLE-US-00001 TABLE 1 Acidity of exchangeable OH-groups (pK.sub.a) in the initial HNP and in HNP with an additive of Fe.sup.3+ ions HNP with Addition of Fe.sup.3+ Initial cations in a a Fe(III):Nb(V) Type of Sorption Centers HNP ratio equal to 0.062 1 5.7 0.5 6.0 0.25 2 6.7 0.5 6.6 0.5 3 10.3 0.2 9.1 0.5 4 11.6 0.4

[0062] Based on the data presented for the cation exchanger with the addition of Fe.sup.3+ ions, one can make some assumptions about the nature of their entry into the solid phase of the material. With small additions of Fe.sup.3+ ions (Fe(III):Nb(V) equal to 0.07), the behavior of the presented dependence indicates to replacement of Nb.sup.5+ ions by Fe.sup.3+ ions a close to isomorphic. At large amounts, Fe.sup.3+ ions seem to interact with the ion-exchange centers of HNP.

[0063] Thus, it can be noted that HNP co-precipitation products doped with Fe.sup.3+ ions have good ion-exchange properties. Changing the pH of the solution makes it relatively easy to control the amount of alkali metal ions introduced into their composition, guided by potentiometric titration curves. The tests were conducted in compliance with the potentiometric titration tests. In this way, three-component oxides of various designs can be obtained.

[0064] The observed acidification of OH groups upon the introduction of Fe.sup.3+ ions into the HNP phase results from the higher electronegativity of iron compared to the electronegativity of niobium. [See: Mulliken P. S. A new electroaffinity scale, together with data on valence states and on valence ionization potentials and electron affinities.J. Chem. Phys., 1934, vol. 2, p. 782-793.].

[0065] Samples selected for the study were prepared by sorption of lithium from a 0.1 N solution of Li.sub.2CO.sub.3 on the H-form of an ion exchanger prepared with the addition of *Fe.sup.3+ ions. The lithium content in the solid phase was equal to the total exchange capacity of the related materials (FIG. 1). Optimal heat treatment, calcining, was carried out at a temperature of 43030 C. for 23 hours. All samples with different amounts of the additive were calcined simultaneously. Changing calcination duration, a selected time interval of 2 to 3 hours, had practically no effect on the ion-exchange properties of the obtained samples.

[0066] The performed X-ray phase analysis showed that calcining of the studied samples at a temperature exceeding the temperature of exothermic effects leads to the crystallization of the LiNbO.sub.3 phase. Moreover, the nature of the X-ray patterns does not depend on how the crystallization proceeded. The X-ray phase analysis of samples calcined at a temperature corresponding to the optimal conditions for the synthesis of cation exchangers selective to Li.sup.+ ions showed that all samples contain the LiNbO.sub.3 phase and some amount of the amorphous phase.

[0067] When studying the ion-exchange properties of the products of thermal treatment of HNP salt forms with the addition of various ions, the following characteristics of the aforementioned products were considered: their Li.sup.+ capacity (FIG. 3, 4, 5), Na.sup.+ capacity (FIG. 4, 5), the Li.sup.+/Na.sup.+ ratio in the exchanger phase (FIG. 4, 5), as well as chemical stability in sorption-desorption operation cycles (FIG. 6), during sorption from a solution of complex composition (Li/Na=1/10).

[0068] When studying the additive effect of Fe.sup.3+ ions on the properties of HNP, several samples were tested at different calcination temperatures, i.e., at 300 C. to 500 C. and separately at 540 C. The samples were tested at different temperatures to select an optimal range of calcination temperatures. The study was also aimed at comparing data obtained in the test of the method of the invention HNP (Li) with addition of Fe.sup.3+ with those disclosed in our previous patents and patent for pure HNP (Li).

[0069] Comparing the number of ions involved in the exchange in samples calcined at 400 C. with a total lithium content in the solid phase (FIG. 1), one can notice an almost complete absence of non-exchangeable lithium, up to the addition of Fe.sup.3+ in an amount exceeding the Fe/Nb>0.07 ratio. At the same time, no crystallization of the LiNbO.sub.3 phase is observed in these samples. This effect can be related to the difficulty in the crystallization process of the LiNbO.sub.3 phase at small amounts of addition of Fe.sup.3+ ions. A further decrease in the exchange capacity of the material is associated with the appearance of a well-crystallized lithium niobate phase.

[0070] To study the selectivity of the synthesized materials to Li.sup.+ ions, experiments Were carried out on the sorption of lithium from solutions of complex compositions. Experiments have shown (FIG. 3-5) that the samples calcined at 400 C. were less selective to Li.sup.+ ions up to the addition of Fe.sup.3+ in the amount of Fe/Nb<0.07, compared with the samples calcined at temperatures above 500 C., although they have a high capacity for target ions.

[0071] The addition of iron ions also affects the chemical stability of the obtained materials, which leads to a significant decrease in the losses of the sorbent during its multicycle use (FIG. 6). In addition, the losses of samples calcined at 540 C. turned out to be three to four times lower than in the samples calcined at 400 C.

[0072] The invention will be further illustrated by practical examples that should not be considered as limiting the scope of application of the present invention.

EXAMPLES

Example 1

[0073] At vigorous stirring, two liters of 0.05 M solution of K.sub.3NbO.sub.4 (pH=12.7) is combined with a predetermined amount of 0.05 M solution of FeCl.sub.3 in 1 M HCl. Precipitation is adjusted with HCl to maintain pH in the range of 5 to 6. The resulting precipitate is washed by successive decantation to a residual concentration of potassium ions equal to 0.08-0.09 g/l and frozen for 30 hours at a temperature ranging from 4 TC to 10 C. After thawing, the granulate is placed in an ion exchange column, and 4.5 liters of 0.1 M Li.sub.2CO.sub.3 solution are passed through the resulting precipitate. The precipitate is then unloaded from the column, dried in the air, heated to a temperature of 400 to-460 C. (at a temperature rise rate of 10 C. deg/min), and is maintained at this temperature for 3 hours. As a result, a sorbent is obtained in a Li-form, the main fraction of which is granules with a size of 0.2 to 0.8 mm.

[0074] The effects of operations conditions used for obtaining an ion exchanger on the sorption properties and purity of the resulting lithium salts are summarized in Table. 2. When testing the ion exchanger, solutions of the following compositions (g/l) were used: Li.sub.2SO.sub.4-5.5; NaCl, 56.0; NaOH, 3.0; pH 12.1. Lithium was desorbed from the sorbent with a 0.1 M HNO.sub.3 solution.

[0075] Examples of sorbents of various compositions obtained by the proposed method in the H-form are given below in Table 2 (Li-forms are similar and, therefore, not included).

TABLE-US-00002 TABLE 2 Effects of operations conditions obtained for an ion exchanger on sorption properties. Test results Operation conditions Selective Sorbent Nb(V):Fe(III) Thermal capacity Separation loss per ratio in solution treatment Sorbent for lithium, coefficient, operation during synthesis temperature C. composition * mg-eqv/g P.sub.Li, Na cycle, 1:0.03 380 H.sub.1.46NbO.sub.3.230.015Fe.sub.2O.sub.3 3.8 34.8 5.0 1:0.03 480 H.sub.0.86NbO.sub.2.930.015Fe.sub.2O.sub.3 2.3 100.8 2.8 1:0.14 380 H.sub.0.72NbO.sub.2.860.070Fe.sub.2O.sub.3 1.5 27.5 0.2 1:0.14 480 H.sub.0.27NbO.sub.2.640.070Fe.sub.2O.sub.3 0.8 64.2 0.2 1:0.05 460 H.sub.0.98NbO.sub.2.990.025Fe.sub.2O.sub.3 2.6 78.8 0.3 1:0.11 400 H.sub.0.40NbO.sub.2.700.055Fe.sub.2O.sub.3 2.4 110.0 0.3 1:0.11 460 H.sub.0.75NbO.sub.2.880.055Fe.sub.2O.sub.3 1.5 82.5 0.2 1:0.18 430 H.sub.0.93NbO.sub.2.970.040Fe.sub.2O.sub.3 3.2 97.2 0.5 1:0.01 300 H.sub.1.45NbO.sub.3.250.010Fe.sub.2O.sub.3 2.2 22.3 5.2 1:0.005 340 H.sub.1.45NbO.sub.3.250.005Fe.sub.2O.sub.3 2.7 31.5 4.8 1:0.15 500 H.sub.0.98NbO.sub.3.010.15Fe.sub.2O.sub.3 1.2 71.8 0.2 1:0.20 540 H.sub.0.71NbO.sub.2.840.20Fe.sub.2O.sub.3 1.0 62.7 0.1 * Composition of sorbent prepared for sorption of lithium, H-form indicates data missing or illegible when filed

[0076] The test data for sorbent samples of various compositions given in the above table and shown in FIG. 8 confirm that the Fe/Nb ratio optimal for the sorbent synthesis i& in the range from 0.03 to 0.10. An optimal synthesis temperature is in the range of 360 to 460 C.

[0077] The advantages of the sorbents obtained by the method of the invention under the optimal conditions over the prototypes obtained by conventional methods can be seen from the results of comparative tests shown in Table 3. The tests were carried out using solutions of the compositions mentioned above. The table shows average results for 5 cycles of sorbent operations.

TABLE-US-00003 TABLE 3 Comparison of properties obtained in the sorbents prepared by the method of the invention with those obtained by conventional methods. Sorption capacity, Separation Loss per mg/g coefficient, cycle, Sorbent Lithium Sodium P.sub.Li, Na macc. % Known 16.0 0.54 86.7 2.0 Proposed 27.5 0.37 100.8 0.3

[0078] As can be seen from Table 3, the sorbent obtained by the method of the invention demonstrates higher selectivity and capacity for lithium ions than the sorbent synthesized by the known method. This makes it possible to reduce the content of the NaNO.sub.3 impurity in the obtained lithium salt almost with a factor of 2. Also, the sorbent obtained by the method of the invention has a higher chemical resistance in the cycles of sorption and desorption, which makes it possible to increase the efficiency of its practical use.

[0079] The invention was described with reference to specific examples. It should be understood, however, that these examples should not be construed as limiting the scope of the practical application of the invention and that any changes and modifications are possible without departure from the scope of the attached patent claims. In particular, other practical examples may be derived from the graphs presented in the attached FIGS. 1 to 6.