PROCESS FOR LEACHING RARE EARTH ELEMENTS

Abstract

Described herein is a process for stepwise leaching of all rare earth elements capable of forming peroxide and superoxide compounds, in particular cerium, lanthanum, neodymium, europium, from minerals containing these elements, namely from bastnaesites, orthites, chevkinites, and britholites.

Claims

1. A process for stepwise leaching of all rare earth elements capable of forming peroxide and superoxide compounds, in particular cerium, lanthanum, neodymium, europium, from minerals containing these elements, namely from bastnaesites, orthites, chevkinites, britholites, which have previously been comminuted to at least the particle size of 75 micrometers, comprising the following steps: an initial stage comprising the following steps: a first step of preparing the reaction mixture consisting of a leaching agent, a solvent and mineral raw materials containing rare earth elements; a second step of leaching REE by heating the obtained reaction mixture to a working temperature of 80-100 C. and maintaining it at the given temperature with simultaneous stirring and a pH between 1 and 3; and a third step of subsequently filtering the suspension thus obtained to form the liquid solution of REE compounds and a solid residue; one or more intermediate stages; a final stage comprising the following steps: a first step of treating the solution obtained as a result of the previous process steps with air until the divalent iron contained in the solution is completely transformed into trivalent iron; a second step of treating the solution obtained in the previous step with sodium hydroxide at room temperature until a pH between 9 and 12 is reached; a third step of filtering the solution obtained in the previous step and drying the sediment obtained thereby at 90-100 C. for about 4 hours; a fourth step of processing the sediment dried in the previous step with oxalic acid at room temperature, thereby forming a solution; a fifth step of final filtration of the solution obtained in the previous step, thereby forming a solid REE concentrate in pure form, wherein iron trichloride is used as leaching agent and water is used as solvent, and wherein the ratio solid:liquid in the reaction mixture is 1:5, the duration of holding the reaction mixture at the working temperature is at least about 1 hour and all intermediate stages of the process are repetitions of its initial stage, the total number of intermediate stages typically being 2 to 3, the solid residue formed as a result of each respective preceding process stage being used as a starting raw material in each respective subsequent intermediate stage, wherein a reduction-oxidation process takes place parallel to the hydrolysis of iron trichloride, which results in the formation of water-soluble peroxide and/or ozonide compounds of the rare earth elements and iron(II) chloride, so that both the iron and the REE compounds remain in the solution and do not enter in the sediment.

2. The process according to claim 1, wherein the minerals used are constituents of a stockpile material from residues of a previous leaching.

3. The process according to claim 1, wherein the consumption of leaching agent during carrying out the initial stage and each intermediate stage is constant and is from 3 to 20%, preferably 5%, relative to the initial mass of the mineral raw materials.

4. The process according to claim 1, wherein the amount of oxalic acid used in the fourth step of the final stage is equimolar relative to the trivalent iron contained in the dried sediment treated with the oxalic acid.

5. The process according to claim 2, wherein the consumption of leaching agent during carrying out the initial stage and each intermediate stage is constant and is from 3 to 20%, preferably 5%, relative to the initial mass of the mineral raw materials.

6. The process according to claim 2, wherein the amount of oxalic acid used in the fourth step of the final stage is equimolar relative to the trivalent iron contained in the dried sediment treated with the oxalic acid.

7. The process according to claim 3, wherein the amount of oxalic acid used in the fourth step of the final stage is equimolar relative to the trivalent iron contained in the dried sediment treated with the oxalic acid.

Description

[0063] The preferred embodiments described above and in the pending claims apply analogously to this process.

[0064] Example 1: The leaching process for rare earth elements was carried out as follows. Take 1 kg of crushed raw materials and put them in a container, pour 5 dm.sup.3 of tap water (solid:liquid=1:5) over them and add 200, 150, 100, 50 or 30 g FeCl.sub.3. The mixture was thoroughly mixed, then heated to 80-100 C. with simultaneous stirring and kept at this temperature for 1 hour, also with simultaneous stirring. The suspension was then filtered, dried and the solid residue weighed. The results of the mass loss experiments are shown in Table 1.

TABLE-US-00001 TABLE 1 Iron trichloride consumption, (in % to the initial mass of the raw materials) 20 15 10 5 3 Mass loss of raw materials, 5 7 11 15 7 (in % to their initial mass)

[0065] As can be seen from the table, the maximum mass loss is observed when the consumption of iron trichloride is 5% in relation to the initial mass of the raw materials studied. This can be explained by the fact that the degree of hydrolysis of iron trichloride increases with decreasing concentration and that the hydrochloric acid released during hydrolysis interacts intensively with the components of the mineral raw materials. It should be noted that under these conditions practically all of Fe.sup.+3 is converted to Fe.sup.+2 and the rare earth elements are converted to the water-soluble chloride, peroxide and ozonide compounds. This is shown by the fact that upon further processing of the resulting solution with oxalic acid, oxalates of the divalent iron precipitate, while the oxalate of the trivalent iron is a water-soluble compound. All ozonide compounds of rare earth elements are sublimable when heated, this effect was observed when the water soluble products were carefully evaporated and dried.

[0066] Already in the 19th century it was proven that all rare earth elements can form peroxide and superoxide compounds during their chemical transformations. Obviously, this could also affect the optimal leaching conditions. A further reduction in iron trichloride consumption could reduce these opportunities for leaching REE.

[0067] The process carried out with a 5% consumption of iron trichloride in relation to the initial mass of the raw materials investigated represents a preferred embodiment of the present process, which is present in patent claim 3 (dependent claim).

[0068] Example 2: Process as in example 1, except that only 50 g iron trichloride was added. Thereafter, the reaction mixture was thoroughly mixed, then heated to 80-100 C. with simultaneous stirring and kept at this temperature for 1 hour, also with simultaneous stirring, then filtered, dried, weighed and analysed. The same sequence of process steps was then repeated twice with the solid residue obtained after the previous repetition of this sequence. In total, all the above-mentioned process steps were carried out three times, with a corresponding decrease in the mass of the starting raw materials or the solid residue, respectively, after each repetition. The results of the analyses are shown in Table 2.

TABLE-US-00002 TABLE 2 number of the respective processing results of the analyses of the initial raw mass of solid residue degree of leaching of material sor solid after the respective REE after the respective residue processing, g processing, %. 1 850 50 2 750 78 3 700 90

[0069] With respect to the given data, the leaching of REE with iron trichloride is analogous to the extraction processes. Under the experimental conditions, the absolute value of fluorine in the original sample and after the treatments remained unchanged.

[0070] The solution obtained is then processed by treatment with air to convert the divalent iron into the trivalent state. Next, the solution is treated with NaOH up to a pH of 9 to 12, thereby precipitating rare earth elements and iron. The solution is filtered and the sediment is first dried for about 4 hours at 90-100 C. and then treated with an equimolar amount of oxalic acid in relation to the iron. The solution is filtered and a concentrate of rare earth elements is obtained in the sediment.

[0071] The following chemical processes take place in the application of the present process:

FeCl.sub.3+H.sub.2O=FeCl.sub.2OH+H.sup.+, the pH is 2.3 at room temperature, at T=80-100 C. -1.0.

Fe.sup.+3=Fe.sup.+2e

H.sup.++e=H

2H+O.sub.2=H.sub.2O.sub.2

Me(OX).sub.3+3H.sub.2O.sub.2+3H.sup.+=Me(OOH).sub.3+3H.sub.2O+3X

[0072] Thus, water-soluble ozonide compounds of rare earth elements are formed. During the treatment of the resulting solution with the oxygen contained in the air, oxidation from divalent iron to trivalent iron takes place:

Fe.sup.+2e=Fe.sup.+3

[0073] The oxalates of divalent iron are insoluble in water and those of trivalent iron are soluble in water.

[0074] When processed with sodium hydroxide, sodium salts of the ozonide compounds of rare earth elements and iron(III) hydroxide precipitate. During heating (drying) the ozonide compounds are decomposed and during treatment with oxalic acid only the iron(III) oxalate enters in the solution. According to standard methods, a 20-fold excess of oxalic acid is required for the extraction of rare earth elements and according to the proposed method an equimolar amount thereof relative to iron is sufficient.

[0075] Compared to known methods, the claimed invention has the following significant differences: [0076] The leaching of rare earth elements does not require the use of aggressive acids; [0077] The leaching of rare earth elements requires the use of iron(III) salts.

[0078] The following causal relationship exists between the distinguishing features of the leaching of rare earth elements with the iron trichloride and the technical problem to be solved [0079] The optimum concentration of iron trichloride in relation to the initial mass of the mineral raw materials is 5%. [0080] When rare earth elements are leached out with iron trichloride, the chemical equilibrium sets in. [0081] In order to obtain a high degree of leaching, a repeated treatment should be carried out. [0082] During leaching, chloride, peroxide and ozonide compounds of rare earth elements are formed. [0083] During leaching with iron trichloride, the trivalent iron is converted into a divalent compound. [0084] To obtain the REE concentrate, the solution is first treated with air, then with sodium hydroxide, the sediment is then separated, dried and treated with an equimolar amount of oxalic acid.

[0085] During the investigation of the given and related technical fields, no other technical solution was found to have the features that distinguish the present process from the prototype. Accordingly, the criteria of novelty and inventive step are fulfilled in this invention.