AU2012379875B2 - Uranium solvent extraction method - Google Patents

Abstract

[Problem] To provide a uranium solvent extraction method capable of collecting uranium from a low concentration uranium exudate using a countercurrent extraction tower. [Solution] The uranium solvent extraction method comprises a process of extracting uranium in an exudate containing 50 - 800 ppm by weight of uranium into a solvent by bringing the exudate and the solvent into countercurrent contact in a countercurrent extraction tower in which the HTU based on raffinate is in the range of 1 - 5 m. The ratio (A/O ratio) of the supply flow for the exudate (A) with respect to the supply flow for the solvent (O) during same is 10 - 80.

Description

- 1 DESCRIPTION Title of Invention: SOLVENT EXTRACTION METHOD FOR URANIUM Technical Field [0001] The present invention relates to a technique of extracting uranium from a pregnant leaching solution of uranium ore. Background Art [0002] A uranium recovery process from uranium ore includes a step of obtaining a pregnant leaching solution (hereafter referred to as a PLS) containing uranium by leaching uranium ore into acid and then obtaining an extraction solution containing high-concentration uranium by extracting uranium from the PLS. [0003] A known example of a method for refining low concentration uranium in a PLS using sulfuric acid is an Eluex method including elution of uranium adsorbed onto ion exchange resin (IX (ion exchange) process) and solvent extraction (SX (solvent extraction) process) . By performing the IX process prior to the SX process, the scale of extractors with high equipment cost, such as a mixer settler and a vertical extraction column, is reduced (NPLs 1 and 2). [0004] 6847182_1 (GHMatters) P98389.AU DENISET - 2 For example, NPL 3 discloses that an Eluex method is economical when the uranium concentration is about 300 [wt ppm-U/L] (350 [wt ppm-U 3 0 8 /L] in terms of U 3 0 8 ) or less and a direct solvent extraction method (DSX (direct solvent exchange) method) not including the IX process is applied to a PLS containing uranium in a concentration of about 800 [wt ppm-U/L] (900 [wt ppm-U 3 0 8 /L] in terms of U 3 0 8 ) or more. [0005] With respect to this common knowledge, if the DSX method with a vertical extraction column could also be applied to a PLS having a low uranium concentration of 800 [wt ppm-U/L] or less, ion-exchange resin and sulfuric acid for elution would not be required and furthermore the equipment would be made simpler than a mixer settler or the like, which would considerably reduce the footprint and cost of the equipment. [0006] However, when a typical vertical extraction column is employed, the feed ratio (A/O = aqueous phase/organic phase) of the flow rate of the pregnant leaching solution (A) to the flow rate of the solvent (0) is increased. This makes it difficult to achieve stable operation of the vertical extraction column. As a result, the DSX method with a vertical extraction column has not been achieved. Citation List 6847182_1 (GHMatters) P98389.AU DENISET -3 Non Patent Literature [0007] NPL 1: Meritt, R., C.: "The Extractive Metallurgy of Uranium" (1971) NPL 2: International Atomic Energy Agency: "Significance of Mineralogy in the Development of Flowsheets for Processing Uranium Ores", Technical Reports Series No. 196, Vienna (1980) NPL 3: Tonder, V. D., Kotze, M.: ALTA 2007, Proceedings (2007) Summary of Invention [0008] In view of the foregoing, an embodiment of the present invention may provide a solvent extraction method for uranium in which uranium can be recovered from a pregnant leaching solution containing low-concentration uranium using a countercurrent extraction column. [0009] A first aspect of the invention provides a uranium extraction method comprising a step of bringing a pregnant leaching solution containing uranium in a concentration of 50 to 800 wt ppm into countercurrent contact with a solvent in a countercurrent extraction column whose raffinate-based HTU is in the range of 1 to 5 m to extract the uranium in the pregnant leaching solution into the solvent. 6847182_1 (GHMatters) P98389.AU DENISET -4 [0010] The uranium extraction method may have the following features: (a) the feed ratio (A/O ratio) of a flow rate of the pregnant leaching solution (A) to a flow rate of the solvent (0) is 10 to 80; (b) the pregnant leaching solution is obtained by performing leaching of uranium ore using sulfuric acid and the solvent contains a long-chain alkyl tertiary amine; and (c) the solvent contains a long-chain alkyl tertiary amine. [0011] According to an embodiment of the present invention, by employing a countercurrent extraction column whose raffinate-based HTU is in the range of 1 to 5 m, uranium can be recovered from a pregnant leaching solution having a uranium concentration of 50 to 800 wt ppm under the conditions that enable the operation of the countercurrent extraction column. [0011a] Another aspect of the invention provides uranium when extracted by a method according to the first aspect. Brief Description of Drawings [0012] Embodiment of the invention will now be described with 6847182_1 (GHMatters) P98389.AU DENISET - 5 reference to the accompanying non-limiting drawings, in which: [Fig. 1] Fig. 1 is a schematic process diagram illustrating a uranium extraction method according to an embodiment. [Fig. 2] Fig. 2 is an explanatory view illustrating an example of a countercurrent extraction column used in the process. [Fig. 3] Fig. 3 is an explanatory view showing extraction test results in Example 2. [Fig. 4] Fig. 4 is an explanatory view showing extraction test results in Example 4. [Fig. 5] Fig. 5 is an explanatory view showing extraction test results in Example 4a. Description of Embodiments [0013] [DSX process] First, an outline of a process for extracting uranium contained in a PLS by a DSX (direct solvent extraction) method will be described with reference to Fig. 1. Uranium ore is crushed and pulverized, and then mixed with an aqueous sulfuric acid solution and agitated in a leaching tank 2 so that the resulting slurry has a pH of 1 to 2. Thus, leaching of uranium is performed in the slurry. [0014] 6847182_1 (GHMatters) P98389.AU DENISET - 6 The slurry composed of uranium ore and an aqueous sulfuric acid solution is sent to a vacuum filter 3 from the leaching tank 2 and separated into the ore and a PLS by solid-liquid separation. As a result, for example, a PLS in which uranium is dissolved in the form of uranyl sulfate complex ions, U0 2

(SO)

4 0 3 4 , is obtained. In this DSX method, a PLS having, for example, a uranium concentration of 50 to 800 [wt ppm-U/L] and more suitably 200 to 500 [wt ppm-U/L] is processed. [0015] The PLS obtained by performing solid-liquid separation in the vacuum filter 3 is fed to the top of the uranium extraction column 1, which is a vertical countercurrent extraction column. As a result of countercurrent contact of the PLS with a solvent fed at the bottom in the uranium extraction column 1, uranium is extracted from the PLS to the solvent. [0016] An example of the solvent used in the extraction of uranium from the PLS is a solvent used in solvent extraction from an eluate in an IX process of a conventional method. As described above, in the case of a sulfuric acid-based PLS, a long-chain alkyl tertiary amine such as a mixed solution (Alamine (registered trademark)-336) of trioctylamine and tridecylamine is exemplified. 6847182_1 (GHMatters) P98389.AU DENISET -7 [0017] In the case of a tertiary amine solvent represented by general formula R 3 N, the following pretreatment proceeds in the uranium extraction column 1: an amine in the solvent that has contacted the sulfuric acid in the PLS is protonated through the reactions represented by formulae (1) and (2) below. 2R 3 N + H 2

SO

4 -> (R3NH)2SO 4 - (1)

(R

3 NH) 2

SO

4 + H 2

SO

4 -> 2 (R 3 NH) HSO 4 - - - (2) The protonated amine reacts with the uranyl sulfate complex ions through the reaction represented by formula (3) below. Thus, uranium is extracted from the PLS to the solvent. 4 (R 3 NH) HSO 4 + U0 2

(SO

4 ) 34 -> (R 3 NH) 4

UO

2

(SO

4 ) 3 + 4HSO4 (3) [0018] An extraction solution obtained by extracting uranium on the basis of the formula (3) is removed from the column top of the uranium extraction column 1 and subsequently subjected to re-extraction in a stripping process, a precipitation treatment, and a firing treatment to obtain

U

3 0 8 , which is referred to as yellowcake. A raffinate left after the extraction of uranium is reused for leaching of uranium. [0019] 6847182_1 (GHMatters) P98389.AU DENISET -8 [Extraction column] In order to recover uranium from a PLS having a uranium concentration of 50 to 800 [wt ppm-U/L] using the above described DSX process, a countercurrent extraction column with high extraction efficiency needs to be employed. An example of the countercurrent extraction column with high extraction efficiency is WINTRAY (registered trademark) patented by the subject applicant (e.g., Japanese Patent No. 2679630, U.S. Patent No. 5500116). [0020] As illustrated in a conceptual diagram of Fig. 2, WINTRAY has a basic structure in which a large number of trays 11 are arranged in a main body 10 of the column so as to be separated apart from each other. When the trays 11 are viewed from above, the trays 11 are arranged so that one end of a tray 11 overlaps another tray 11 adjacent to the tray 11 in a vertical direction. In each of the trays 11, a vertical wall 12 for collecting liquid (heavy liquid in the example shown in Fig. 2) of a dispersed phase on the tray 11 is disposed. An opening 13 for laterally discharging the dispersed phase is formed in the vertical wall 12. [0021] As a result, the dispersed phase fed to the upper side of the uranium extraction column 1 flows downward in the main body 10 of the column while repeatedly undergoing the 6847182_1 (GHMatters) P98389.AU DENISET - 9 coalescence that occurs when the dispersed phase is collected on the tray 11 and the dispersion that occurs when the dispersed phase on the tray 11 flows out through the opening 13. Since the dispersed phase is laterally (horizontally) discharged through the opening 13, the dispersed phase intersects a flow of a continuous phase (light liquid in the example shown in Fig. 2) that is fed to the lower side of the uranium extraction column 1 and rises in the uranium extraction column 1. Consequently, the dispersed phase is more effectively dispersed, and thereby high extraction efficiency can be achieved. [0022] Figs. 1 and 2 show the case where the PLS serving as a dispersed phase is heavy liquid and the extraction solution serving as a continuous phase is light liquid. [0023] [HTU of uranium extraction column] It has been confirmed that, even if such a countercurrent extraction column with high extraction efficiency is employed, higher performance of equipment is required when uranium is recovered from a PLS having a low uranium concentration of 50 to 800 [wt ppm-U/L]. Thus, the inventors of an embodiment of the present invention have found that, by setting HTU (height per transfer unit) to be an index and employing a uranium extraction column 1 whose 6847182_1 (GHMatters) P98389.AU DENISET - 10 HTU is within the predetermined range, uranium can be recovered from a low-concentration PLS by the DSX method under the constraint of operation of the uranium extraction column 1. [0024] When the extraction based on the reaction represented by the above formula (3) occurs and the uranium extraction column 1 can be treated as a differential column, for example, the axial mixing diffusion coefficients of the raffinate phase and the extraction solution phase are substantially negligible in the WINTRAY extraction column and therefore the raffinate-based HTU (height per transfer unit) of the uranium extraction column 1 is represented by formula (4) below. HTU = UR/Kpa ... (4) Furthermore, NTU (number of transfer unit) is represented by formula (5) below. [Math. 1] XF NTU= fdx(x-x*) --- (5) In the formulae (4) and (5), a represents a specific contact interfacial area [m 2 /m 3 ], Ka represents an overall volumetric mass transfer coefficient [1/s], U represents a superficial velocity [m/s], x represents a solute concentration in the raffinate phase [kg/M3], y represents a 6847182_1 (GHMatters) P98389.AU DENISET - 11 solute concentration in the extraction solution phase [kg/M 3 ], and x* represents an equilibrium solute concentration in the raffinate corresponding to the solute concentration y in the extraction solution [kg/M 3 ] . The subscripts F and R respectively indicate a raw material and a raffinate. Herein, the column height L [m] of the uranium extraction column 1 is represented by formula (6) below on the basis of the HTU and NTU. L = HTU-NTU ... (6) [0025] When the HTU is in the range of 1 to 5 [m] and more suitably 1 to 3 [m], uranium can be suitably recovered from a PLS having a uranium concentration of 50 to 800 [wt ppm U/L] by using the uranium extraction column 1. If the HTU is more than 5 [m], the cost of equipment increases. On the other hand, it is difficult to practically use a uranium extraction column 1 whose HTU is less than 1 [m] as commercial equipment. In the design stage, the HTU can be estimated from actual measurement data or the like in a pilot plant. [0026] It is clear from the above description that the specific structure of a countercurrent extraction column is not limited to the example of WINTRAY illustrated in Figs. 1 6847182_1 (GHMatters) P98389.AU DENISET - 12 and 2 as long as the HTU of the uranium extraction column 1 is in the range of 1 to 5 [m]. Therefore, if the HTU in the above range can be realized, a pulsed column having no possibility of clogging and an agitated extraction column can also be employed. [0027] [Operational conditions] In the uranium extraction column 1 whose HTU is in the range of 1 to 5 [m], the feed ratio (A/O = aqueous phase/organic phase) of the flow rate [kg/h] of the pregnant leaching solution (A) to the flow rate [kg/h] of the solvent (0) is in the range of 10 to 80 and preferably in the range of 15 to 25. If the A/O ratio is less than 10, the amount of the solvent required excessively increases, which increases the cost of equipment and the running cost. If the A/O ratio is more than 80, the uranium extraction does not effectively occur. [0028] The uranium extraction column 1 according to this embodiment can provide the following effects. By employing a countercurrent extraction column (uranium extraction column 1) whose raffinate-based HTU is in the range of 1 to 5 [m], uranium can be recovered from a pregnant leaching solution having a uranium concentration of 50 to 800 [wt ppm] under the conditions that enable the operation of the 6847182_1 (GHMatters) P98389.AU DENISET - 13 countercurrent extraction column. As a result, the DSX method can be applied to a PLS having a concentration range in which an IX process has been conventionally required, and thus there is no need of using ion-exchange resin and installing a mixer settler, which considerably reduces the footprint and cost of equipment. Examples [0029] (Experiment) Embodiments of the invention will now be described with reference to the accompanying non-limiting examples. A rectangular (width: 100 [mm], depth 40 [mm]) uranium extraction column 1 (height of extraction section: 4.9 [m], number of trays: 49) having a structure of WINTRAY illustrated in Fig. 2 was fabricated, and an extraction test was conducted. [0030] A. Experimental conditions [Preparation of pregnant leaching solution] A PLS with about 300 [wt ppm-U/L] (about 350 [wt ppm

U

3 0 8 /L]) was prepared under the following conditions. After 36.1 [tons] of uranium ore was crushed and passed through a screen with 3 [mm] mesh, a leaching treatment was performed in a leaching tank 2 equipped with an agitator and a steam jacket under the conditions shown in Table 1. The resulting 6847182_1 (GHMatters) P98389.AU DENISET - 14 slurry was filtered with a filter paper spread out on a two stage vacuum filter tray to obtain a PLS. Table 2 shows the composition of the obtained PLS. In actual operations, impurities accumulate in the equipment because a recycle line for a raffinate is provided in order to reduce the consumption of water and the loss of uranium. Therefore, a PLS was prepared by spiking the PLS having the composition shown in Table 2 with additional substances shown in Table 3 (adding the additional substances to the PLS) to simulate the influence of the accumulated impurities. [0031] [Table 1] Item Unit Condition Uranium content of uranium ore ppm-U 230-270 Solid content wt% 55-65 Temperature 0C 50 pH -_1.25 [0032] [Table 2] Component Unit Content

U

3 0 8 mg/L (wt ppm) 298-648 Cl mg/L 62-422 F mg/L 174-384 Na mg/L 168-333

NO

3 mg/L < 150

SO

4 g/L 10-29 Mg mg/L 687-1940 Al mg/L 777-2540 Si mg/L 671-1990 Ca mg/L 493-918 Ti mg/L 6-84 V mg/L 8-24 6847182_1 (GHMatters) P98389.AU DENISET - 15 Cr mg/L 12-26 Mn mg/L 132-714 Fe mg/L 1170-3790 Co mg/L < 2 Ni mg/L < 2-8 Cu mg/L < 2-10 Zn mg/L < 2-24 Mo mg/L < 2-5 Pb mg/L < 2-11 S mg/L 8390-16900 As mg/L < 2 6847182_1 (GHMatters) P98389.AU DENISET - 16 [0033] [Table 3] Added solution Target concentration Na 2 SiO 3 serving as Si source Si 1 g/L NaNO 3 serving as Na source NO 3 0.5 g/L NaCI serving as CI source Cl 0.5 g/L FeSO 4 serving as Fe source Fe 5 g/L Na 2

SO

4 serving as S source So 4 60 g/L [0034] [Solvent] A solvent for the extraction test was a mixture of Alamine-336 (hereafter abbreviated as Alamine) which is a tertiary amine and serves as an extractant, isodecanol serving as a modifier, and Shellsol (registered trademark) D70 serving as a diluent. Two concentrations, 7 [vol%] and 10 [vol%], were employed for Alamine. Table 4 shows the composition of each organic solvent used in the test. [Table 4] Alamine Isodecanol Shellsol D70 [vol%] [vol%] [vol%] Solvent 1 7 3.5 89.5 Solvent 2 10 5 85 [0035] [Extraction test] The test parameters are uranium concentration, A/O ratio, Alamine concentration, and total flux (sum of superficial velocities of PLS and solvent). Each extraction test was performed under a steady state, which reached over a volume three times the internal volume of the uranium 6847182_1 (GHMatters) P98389.AU DENISET - 17 extraction column 1 had been replaced with a solvent having a low flow rate. Each extraction test was performed at atmospheric temperature using a fresh solvent. As a result of a holdup test performed beforehand, the flooding flow velocity of the uranium extraction column 1 was 88 [m 3 /m 2 /h]. [0036] (Example 2) The extraction test was performed under the conditions of A/O ratio: 10/1, total flux: 70 [m 3 /m 2 /h], PLS: spiked, used solvent: solvent 1, and the interval of trays 11 in uranium extraction column 1: 100 [mm]. (Example 3) The extraction test was performed under the same conditions as in Example 2, except that the A/O ratio was changed to 15/1 and the used solvent was changed to a solvent 2. (Example 4) The extraction test was performed under the same conditions as in Example 3, except that the A/O ratio was changed to 20/1. (Example 4a) The extraction test was performed under the same conditions as in Example 4, except that a raffinate of Example 4 was used instead of the PLS. (Example 5) 6847182_1 (GHMatters) P98389.AU DENISET - 18 The extraction test was performed under the same conditions as in Example 3, except that the A/O ratio was changed to 20/1 and the total flux was changed to 26 [m 3 /m 2 /h]. (Example 6) The extraction test was performed under the same conditions as in Example 3, except that the A/O ratio was changed to 25/1 and a PLS without spikes was used. (Example 7) The extraction test was performed under the same conditions as in Example 3, except that the A/O ratio was changed to 20/1 and the interval of the trays 11 in the uranium extraction column 1 was changed to 150 [mm]. [0037] Table 5 shows the test conditions in Examples 2 to 7. 6847182_1 (GHMatters) P98389.AU DENISET - 19 [Table 5] PLS Solvent Uranium Flo Alamine Flo Total flux A/O Spik concentratio w concentratio w [m 3 /m 2 /hr rati s e n rate n rate ] o [wt ppm] [L/h] [vol%] [L/h] Exampl Yes 329 254 7 25.4 70 10 e2 Exampl Yes 322 263 10 17.5 70 15 e3 Exampl Yes 301 267 10 13.3 70 20 e4 Example Yes 40 267 10 13.3 70 20 Feed of e 4a raffinate Exampl Yes 296 99 10 4.9 26 20 e5 Exampl No 327 269 10 10.8 70 25 e6 Interval Exampl Yes 325 267 10 13.3 70 20 of trays: e 7 e 7_ 150 mm [0038] B. Experimental results (Example 2) The uranium extraction ratio was 94%, which was a high extraction ratio. The uranium concentration in the extract phase (solvent that flowed out from the column top of the uranium extraction column 1) was 3.09 [g/L], the uranium concentration in the raffinate phase was 19 [wt ppm-U/L], and the HTU was 1.3 [m]. Fig. 3(a) shows a change in the uranium concentration in the raffinate phase in a height direction from the column top of the uranium extraction column 1. Fig. 3 (b) shows a change in the uranium 6847182_1 (GHMatters) P98389.AU DENISET - 20 concentration in the extract phase in a height direction from the column bottom. A thick horizontal line shown in Fig. 3(b) indicates the maximum load of the solvent determined from an extraction isotherm (the same applies in Fig. 4 (b) and Fig. 5(b)). [0039] (Example 3) The uranium extraction ratio was 94%, which was a high extraction ratio. The uranium concentration in the extraction solution was 4.95 [g/L] and the uranium concentration in the raffinate was 19 [wt ppm-U/L]. The HTU was 1.5 [m], which was substantially the same as the HTU in Example 2. [0040] (Example 4) The uranium extraction ratio was 81%, which was a high extraction ratio despite high A/O ratio (= 20/1). The uranium concentration in the extraction solution was 5.52 [g/L], the uranium concentration in the raffinate phase was 56 [wt ppm-U/L], and the HTU was 2.3 [m]. Fig. 4(a) shows a change in the uranium concentration in the raffinate phase in a height direction from the column top of the uranium extraction column 1. Fig. 4(b) shows a change in the uranium concentration in the extract phase in a height direction from the column bottom. 6847182_1 (GHMatters) P98389.AU DENISET - 21 [0041] (Example 4a) Example 4a was conducted to confirm whether high extraction efficiency is achieved in a region where the uranium concentration in the PLS is 40 [wt ppm] or less. As a result, the extraction ratio was as high as 95% or more. Through the combination of the results of Example 4 and Example 4a, an extraction ratio of 99% or more was achieved (the uranium concentration in the PLS fed in Example 4 was 300 [wt ppm-U/L] whereas the uranium concentration in the raffinate in Example 4a was 2 [wt ppm-U/L] or less). According to Example 4a, extraction is effectively caused in a region where the uranium concentration in the PLS is low, and the uranium in the PLS is said to be substantially completely extracted. In Example 4a, the uranium concentration in the extract was 0.95 [g/L] and the HTU was 1.3 [m]. Fig. 5(a) shows a change in the uranium concentration in the PLS in a height direction from the column top of the uranium extraction column 1. Fig. 5(b) shows a change in the uranium concentration in the solvent in a height direction from the column bottom. [0042] (Example 5) Example 5 was conducted to confirm whether WINTRAY can exhibit its performance under various operational conditions. 6847182_1 (GHMatters) P98389.AU DENISET - 22 Example 5 was conducted at a total flux of 26 [m 3 /m 2 /h], which was 40% or less of the total flux in a normal operation and 30% of the flooding flow velocity. The extraction ratio was 77%, the uranium concentration in the extraction solution was 4.46 [g/L], the uranium concentration in the raffinate was 69 [wt ppm-U/L], and the HTU was 2.6 [m]. It was confirmed that WINTRAY had a wide operational range and substantially the same extraction efficiency as in a normal operational range was achieved. [0043] (Example 6) The extraction ratio was 95% and the HTU was 1.7 [m]. It was confirmed that, when the amount of impurities was small, the operation could be conducted at a high A/O ratio. The uranium concentration in the extraction solution was 9.67 [g/L] and the uranium concentration in the raffinate was 18 [wt ppm-U/L]. It was confirmed that, in consideration of the fact that an extraction ratio of 95% was achieved even at an A/O ratio which was higher than those of Examples 2 to 5 with spikes, impurities such as NO 3 , Cl, and SO 4 decreased the uranium concentration in the extraction solution (decreased the extraction efficiency). [0044] (Example 7) The extraction ratio was 74%, the uranium concentration 6847182_1 (GHMatters) P98389.AU DENISET - 23 in the extraction solution was 6.14 [g/L], the uranium concentration in the raffinate was 83 [wt ppm-U/L], and the HTU was 2.8 [m] . This test was conducted by changing the interval of trays from 100 [mm] to 150 [mm] in order to check a scale-up factor. [0045] Table 6 shows the summarized experimental results in Examples 2 to 7. [Table 6] Uranium Uranium Uranium concentration in concentration in concentration in PLS raffinate extraction HTU[i] solution [wt ppm-U/L] [wt ppm-U/L] [ tion [wt ppm-U/L] Example 2 329 19 3088 1.3 Example 3 322 19 4950 1.5 Example 4 301 56 5520 2.3 Example 4a 40 2> 948 1.3 Example 5 296 69 4458 2.6 Example 6 327 18 9665 1.7 Example 7 325 83 6144 2.8 In each of Examples, the SS (suspended particle) concentration in the PLS fed to the uranium extraction column 1 was as high as 80 to 170 [wt ppm], but did not affect the clarity of the extract phase. The extract obtained from an upper portion of the uranium extraction column 1 was transparent. In the extraction test, crud (scum) did not adhere to the tray, did not block the opening of the tray, and did not accumulate in the column. 6847182_1 (GHMatters) P98389.AU DENISET - 24 [0046] In WINTRAY, it is understood that a HTU 1 in the pilot plants of Examples 2 to 7 and a HTU 2 in an actual plant have a relationship represented by formula (7) below. (HTU 2) = (HTU 1) - (H2/H1)" - - - (7) where Hi represents the interval of the trays 11 in the pilot plant, H2 represents the interval of the trays 11 in the actual plant, and n represents a constant of an extraction system (0.52 in uranium extraction). The column height of the actual plant can be determined on the basis of the formula (7) . For example, the interval of the trays 11 in the actual plant is selected from the range of about 150 to 300 mm. [0047] The PLS contains suspended particles. When the PLS is brought into liquid-liquid contact with the solvent containing Alamine-336, crud and scum are generated in a liquid-liquid interface, which decreases the extraction efficiency (increases the HTU) or causes clogging in the extraction column. Therefore, a filtration treatment for adjusting the concentration of the suspended particles to 50 [wt ppm] or less is required as a pretreatment of extraction. However, according to an embodiment of the present invention, it was confirmed that extraction could be performed without problems even if the concentration of the suspended 6847182_1 (GHMatters) P98389.AU DENISET - 25 particles was 80 to 180 [wt ppm]. [0047a] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. [0047b] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. Reference Signs List [0048] 1 uranium extraction column 10 main body of column 11 tray 12 vertical wall 13 opening 2 leaching tank 3 vacuum filter 6847182_1 (GHMatters) P98389.AU DENISET