FI68038B - FOERFARANDE FOER SAMTIDIG AOTERVINNING AV URAN SAELLSYNTA JORDMETALLER TORIUM OCH YTTRIUM FRAON EN SYRALOESNING - Google Patents

Description

ΓΜ .... ANNOUNCEMENT, q n v n JIT * ίΒϊ (11) UTLÄGGNINGSSKRIFT 6ÖU0Ö C (45) Pate: i 1: 1 r :-P! 3:? 1 ::: 3 (51) Kv.lk * / lnt.Cl.4 C 01 G 43/00, C 22 B 60/02 FINLAND —FINLAND (21) Patent application - Pitentansöknlng 802930 (22) Application date - Ansfikningsdag 1 7.09. 80 (FI) (23) Starting date - Giltighetsdag 17-09 * 80 (41) Become Public - Bllvlt offentlig 22.03 * 81

National Board of Patents and Registration. ^. Date of display and hearing. - 29.O3.85

Patent- och registrstyrelsen '' Ansökan utlagd och utUkriften publieerad (32) (33) (31) Requested privilege — Begärd prloritet 21.09.79 i5.O2.8O, 25.Ο6.8Ο, 25.Ο6.8Ο France-France (FR ) 7923517, 80033 ^ 9, 8014081, 8014082 (71) Rhone-Poulenc Industries, 22, Avenue Montaigne, F-75008 Paris,

France-France (FR) (72) Alain Leveque, Paris, Dominique Foraison, Thann,

Jean-Louis Sabot, Maisons-Laffitte, France-France (FR) (74) Oy Berggren Ab (54) Method for the simultaneous recovery of uranium, rare earths, thorium and yttrium contained in an acid solution -Förfarande för samtidig Stervinning av uran, sallsynta jordmetaller, torium och yttrium frln en syralösning

The present invention relates to a process by which uranium, rare earth metals, thorium and yttrium contained in an acid solution can be recovered simultaneously, and in particular to the simultaneous recovery of these metals from phosphoric acid obtained by a wet process by liquid-liquid extraction.

It has long been known, especially from studies by the United States Atonic Energy Commission (USAEC), to recover uranium and other metals from an aqueous solution of an acid containing them by liquid-liquid extraction and usually with the aid of cationic extractants, most often in an inert organic solvent. . In this way, uranium and other metals can be extracted more or less selectively from the aqueous phase to the organic phase, depending on the extractant selected, the starting phase and working conditions, after which the metal-enriched organic phase is treated by physicochemical methods (= precipitation, aqueous solution or other extraction). to recover precious metals.

68038

The initial aqueous solutions consist either of a solution of phosphoric acid obtained by the wet process, usually by dissolving phosphate minerals with sulfuric acid, or of a solution of nitric acid obtained by dissolving spent nuclear fuel from which uranium and possibly uranides are recovered.

It is also known that some minerals, such as apatites, contain quite significant amounts of yttrium, thorium, and rare earth metals in addition to uranium, which are economically viable to extract and recover together with uranium from the aqueous solutions used to dissolve these minerals.

Given the low concentrations of these species in the aqueous solutions used in mineral leaching and the high volumes required for industrial production, it is a prerequisite for the recovery of the above species in industry that they can be recovered simultaneously by a single extraction step and that the aqueous phase regenerates. That is, an effective extractant that is not selective for different species must be available. The metals are then recovered by conventional methods by precipitation or re-extraction and the different species are separated from solid or liquid concentrates, and these methods do not require large-scale equipment or installations.

In addition, it is known that acid solutions derived from the dissolution of minerals, especially in the case of phosphoric acid obtained by the wet process, contain considerable amounts of iron, up to 10 g / liter. It is therefore important that the extractant chosen for the method of aiding these metals has, in addition to a strong and non-selective extractability for uranium, thorium, rare earths and yttrium, a relatively poor extractability for iron, so that it is extracted only very little and interfere with subsequent recovery and separation operations.

Finally, the desired extractant must meet three additional conditions, namely: 3 68038 (a) it is combined with one of the selected inert solvents to form a homogeneous organic extraction phase with the highest possible concentration of extractant to increase the extraction capacity of the phase and suit the required hydrodynamic conditions; is stable on contact with strong acids such as phosphoric acid, sulfuric acid, hydrofluoric acid, etc., so that its extractability is maintained and it can be returned to the cycle for long periods of time, c) it has the weakest selectivity for the group of rare earth metals.

As for the extractants known and used in the art, they do not satisfy all the conditions required in the simultaneous aid process for uranium, thorium, yttrium and rare earths. Thus, for example, a synergistic extraction mixture of di- (2-ethylhexyl) phosphoric acid (HDEHP) and trioctylphosphine oxide (TOPO), as described in U.S. Patent No. 3,711,591, highly extracts uranium with an oxidation state of VI and rare heavier earth metals, but not suitable for extracting all rare earths. In addition, this extraction mixture does not extract uranium with an oxidation state of IV, which in some cases requires prior oxidation of the starting aqueous solution.

It is also known that the tetravalent uranium in an aqueous acid solution can be extracted by a mixture of octylphenylphosphoric acid (MOPPAj) and di- (octylphenyl) -phosphoric acid (DOPPA) dissolved in an aliphatic solvent (see U.S. Pat. However, the use of this mixture on an industrial scale presents some difficulties due to the fact that the extraction phase gradually loses its extraction capacity and the composition of this mixture has to be corrected from time to time.

There is also no guidance on how to extract and recover rare earth metals, yttrium and thorium with such a mixture.

4 68038

Thus, there is a need for a continuous process that allows the simultaneous extraction of uranium, thorium, utrium and rare earth metals so that the starting aqueous phase is virtually completely emptied by means of a stable organic phase.

The object of the present invention is to provide a continuous process for the simultaneous recovery of tetravalent and / or hexavalent uranium, rare earth metals, thorium and yttrium in an acid solution, and in particular for the simultaneous recovery of a liquid-liquid from an aqueous solution of phosphoric acid obtained by the wet process. by extraction with an organic extraction phase to give an organic phase containing the above-mentioned species and an aqueous acid solution which has been emptied.

Another object of the invention is to provide a process for recovering the above-mentioned species, which further comprises treating the organic phase containing the above-mentioned elements with a precipitating aqueous phase, whereby the above-mentioned species are recovered in the form of insoluble salts.

In its most general aspect, the process of the invention comprises treating an aqueous acid solution containing uranium, thorium, yttrium and rare earth metals with an organic homogeneous phase of a dialkylphenylphosphoric acid which is substantially free of a monoalkylphenylphosphoric acid derivative, a solvent selected from aliphatic or aromatic compounds, used alone or in admixture, and optionally a solvating derivative (modifying agent), which may be selected from a trialkylphosphine oxide, an alkylphosphinate, an alkylphenate and an alkylene, and an alkylene oxide phase containing uranium, thorium, yttrium and rare earths.

The invention further comprises a process for recovering metal species in the above-mentioned organic phase, characterized in that the enriched organic phase is treated with an aqueous phase of hydrofluoric acid in a strong acid solution selected from phosphoric acid or sulfuric acid to precipitate the metal species in the aqueous phase.

I Initial aqueous acid solution

The initial aqueous phase contains a strong mineral acid such as phosphoric, sulfuric or hydrochloric acid and the metal species sought as trace elements (uranium concentration is usually 0-500 ppm and rare earth, yttrium and thorium concentration is 100-3000 ppm). Typical examples of such an aqueous acid phase are the following: 1) Crude phosphoric acid obtained by the wet process by dissolving apatites with sulfuric acid and filtering gypsum. Generally, the crude solution is subjected to a pretreatment designed to clarify it and facilitate its use during subsequent extraction. The content of P 2 O 3 is usually 25 to 40% by weight.

2) An aqueous solution of an acid obtained by treating waste phosphor gypsum with a strong mineral acid, which contains metal species in an insoluble form and is derived from the dissolution of apatite minerals by means of sulfuric acid.

It has been found that when dissolving apatite minerals with sulfuric acid, the solubility of uranium is in most cases close to 100%, while for rare earths, thorium and yttrium, depending on the working conditions, very different solubility values are observed, ranging from some percentages to over 30%. Therefore, it has proven advantageous to recover the remaining species in insoluble form from phosphor gypsum obtained by dissolving apatite with sulfuric acid.

Depending on the origin of the rock species and the dissolution conditions, the content of rare earth metals, yttrium and thorium varies from a few mg to 20 g per kilogram of solid.

6 68038

In order to prepare an aqueous solution of said acid, the above-mentioned phosphor gypsum has to be treated with a strong acid. Various extraction experiments with strong acids show that a considerable part of said metal species in phosphor gypsum can be dissolved. Strong acids such as ^ SO 2, HCl, HNO 2 can be used; of these, t ^ SO 2 is preferred, which has good selectivity for the species in question, but leaves the gypsum insoluble, and does not alter the filtration properties of gypsum typical of apatite dissolution conditions. In addition, the process integrates well with the classical wet process for the preparation of phosphoric acid obtained by sulfuric acid leaching.

The concentration of strong acid used depends on its nature, contact time with phosphor gypsum, temperature and the liquid / solid ratio used. These conditions have been described in the literature, especially in A.M. In an article by Andrianov and Coll (Zhurnal Prikladnoi Khimii, Vol. 49, No. 3, pp. 636-638 C, 1976). For example, if sulfuric acid is used, the phosphor gypsum is usually treated with an aqueous solution having a concentration of 0.5 to 3 M. The contact time between the phosphor gypsum and the strong acid solution is not critical, but is usually 15 minutes to three hours for good dissolution of rare earths. When the reaction is complete, the residue of the emptied phosphor gypsum and the acid solution in which the species in question are enriched are separated, and this is then subjected to a liquid-liquid extraction step for their recovery. However, due to the economics of the method, it may be advantageous, in a preferred variant, to re-contact the obtained acid solution with fresh phosphor gypsum under the same conditions and this operation may be repeated several times in succession continuously or discontinuously. The emptied phosphor gypsum is then washed with water and the washing solution is combined with the above-mentioned acid solution. Various devices can be used for the contact and separation of phosphor gypsum to provide optimal economic conditions depending on the type of gypsum.

The resulting aqueous acid solution containing the disclosed metal species undergoes an extraction step, which will be described in the following. In this way, it is possible to industrially recover up to 90% of the rare earth metals, thorium and yttrium made soluble by washing gypsum or, depending on the minerals, usually 30-100% of the rare earth metals, yttrium and thorium present in the original phosphor gypsum.

This extraction and re-extraction method has the advantage that the acid used in the washing can be recovered and taken back into the cycle, or it can be re-used to extract metal species from gypsum, or it can be used to dissolve minerals.

3) Aqueous raffinate obtained from the extraction of phosphoric acid obtained by the wet process with solvents which are insoluble or sparingly soluble in water.

Purified phosphoric acid is in increasing commercial demand, in the human and animal food industry and as a feedstock for detergent compositions in polyphosphates. For this purpose, an increasing proportion of the crude phosphoric acid obtained by the wet process is purified by commonly known liquid-liquid extraction methods using organic water-insoluble or sparingly soluble solvents such as alkyl phosphates, e.g. tributyl phosphate, or aliphatic alcohols, e.g. . Such purification methods are described in French Patents Nos. 1,531,487, 1,553,095 and 2,259,787 in the form of non-limiting examples. According to these methods, the crude phosphoric acid solution obtained by the wet process resulting from the dissolution of phosphorus limestone species with a strong acid such as H 2 SO 4, after filtration, has to be extracted with the above-mentioned solvents to extract a substantial portion of phosphoric acid into the organic phase. After separation of the phases, the organic phase leaving the extraction coil, in which the P 2 O 5 content is enriched, and the emptied aqueous phase are recovered. (or Raffinate) with a low P2 ° 5 concentration and containing most of the cationic impurities present in the original aqueous solution. During said extraction, δ 68038 is also added, if necessary, to concentrated sulfuric acid to promote the extraction of phosphate species; as a result, there is a substantial proportion of added sulfuric acid in the aqueous raffinate. Therefore, a considerable, if not all, of the uranium, rare earth metals, yttrium and thorium are present in said raffinate, from which it is advantageous to recover them.

The aqueous raffinate used in the present invention, which is derived from a liquid-liquid extraction unit, contains e.g. phosphoric acid having a concentration by weight, expressed as P 2 O, generally less than 20%, predominantly about 10%, of sulfuric acid derived from an excess of added reactant added to the dissolution of phosphorus lime minerals and generally having a concentration of about 1-3% by weight, depending on the original excess used, other anions such as fluoride ions, etc., and cations such as iron, aluminum, magnesium, alkali, uranium cations, rare earth cations and yttrium and thorium cations, of which four the latter usually as trace elements. If the aid process has involved the addition of sulfuric acid, its concentration in the raffinate is naturally higher and can reach up to about 15% by weight, whereas, on the contrary, the weight content of P 2 O 5 can be as low as 2% (see therefore the above-mentioned patents).

The above aqueous raffinate is particularly well suited as a starting material in the extraction process of the present invention. As will be explained below, it is then possible, after re-extraction, to recover up to 90% of the uranium, rare earth metals, thorium and yttrium in the raffinate.

Organic extraction phase

The organic extraction phase used in the present invention contains di- (alkylphenyl) -phosphoric acid as an extractant. The alkylphenyl group of the above acid means a group in which the alkyl radical is an aliphatic straight-chain or branched radical containing 4 to 12 carbon atoms. The alkyl radical is preferably an octyl radical, particularly preferably it is the 1,1,3,3-tetramethylbutyl radical, preferably 68038 at position 4. The invention is not limited to the use of a single di- (alkylphenyl) -phosphoric acid, but also comprises the use of a mixture thereof. .

The organic extraction phase also contains an inert solvent, which may be selected from an aliphatic compound such as heptane, dodecane, hexane or kerosene type oil fractions, an aromatic compound such as benzene, toluene, ethylbenzene, xylene and Solvesso type distillates (EXXON). either alone or as a mixture.

The inert organic solvent is selected depending on the chemical nature of the aliphatic chain of the extractant. For example, in the case of the use of di- (octylphenyl) -phosphoric acid (DOPPA), which has a limited solubility in aliphatic hydrocarbons (e.g. a saturated solution in kerosene at 20 ° C contains less than 0.1 mol / liter), this solubility must be added to the organic phase to provide industrial extraction conditions using a mixture of aliphatic and aromatic hydrocarbons. However, in this case, the partition coefficients of the metal species shown decrease considerably as the weight ratio of aromatic hydrocarbon / aliphatic hydrocarbon in the solvent increases.

In general, the choice of the concentration of di- (alkylphenyl) -phosphoric acid in the organic phase and the choice of the weight ratio of aromatic solvent or aliphatic solvent is the result of a compromise which takes into account the solubility of said di- (alkylphenyl) -phosphoric acid in the system and the distribution of different metal species. between the aqueous phase.

In practice, the concentration of di- (alkylphenyl) -phosphoric acid is between 0.1 M and 1 M, preferably about 0.5 M, and the weight ratio of aromatic solvent / aliphatic solvent, in the case where it is necessary to add an aromatic solvent, is between 0.01 and 100 , mainly about 1.

In addition, in the present invention, it has been found that in order to improve the solubility of the extractant in the solvent mixture and to provide an advantageous extraction efficiency for the metal species sought, it is preferable to add a solvating agent (modifier) to the organic phase. . It is generally preferred to use trioctylphosphine oxide, dibutylbutyl phosphonate or tributyl phosphate. The content of the modifier in the organic phase is between 1 and 30% by weight, preferably about 5% by weight.

In addition, it has been found in the present invention that in some cases it is advantageous to add a modifying agent selected from an aliphatic or alkylaryl compound having hydroxyl groups and containing more than 4 carbon atoms in order to improve the typical hydrodynamic properties of the organic phase. For this purpose, an aliphatic alcohol is preferred, in particular one having 10 carbon atoms, or an alkylphenol such as nonylphenol. The concentration of the blowing agent in the organic phase is between 5-50%, mainly about 20%.

The organic extraction phase described above has a very good extraction effect with respect to uranium, thorium, yttrium and rare earth metals. In addition, it has been found that said extraction phase is suitable for aqueous acid solutions regardless of the reduction capacity of these aqueous solutions, i.e. regardless of the oxidation state of uranium / U (IV) or U (VI_) 7 · In particular, it is possible to treat solutions containing U ( IV) and U (VI). However, in order to make the separation from iron better, it may be advantageous to work in such a way that the initial aqueous phase is reduced so that the iron has two values and does not accompany the extraction, which also significantly increases the extraction coefficients of the searched elements.

The ratio of the flow rates of the aqueous phase to the organic phase is not decisively accurate during contact and is usually between 0.1 and 20, mainly between 1 and 5.

68038 11

The contact temperature is not precisely determined, it is sufficient for the extractants to be soluble in the solvents. It is usually carried out at a temperature in the range from 15 ° C to 60 ° C and mainly at about 40 ° C.

The contacting takes place in devices well known in the art, such as mixing-settling tanks installed in multi-level batteries, packed or pulsed columns, and the contact takes place either upstream or downstream. In this way, virtually complete depletion of the aqueous phase of uranium, repellents, yttrium and rare earths is achieved.

III. Re-extraction of the organic phase The process according to the invention also comprises the treatment of the above organic phase containing the above-mentioned species so that they are recovered in concentrated form.

Due to the strong extraction capacity of the organic phase, efficient re-extraction of the metal species is carried out by precipitation with hydrofluoric acid in a very acidic solution. Sulfuric acid, phosphoric acid, etc. are particularly suitable for this step. However, phosphoric acid is preferred because of its efficiency and the hydrodynamic properties of the solution HF-H-PO2. The concentration of ΗβΡΟ ^ ιη is most preferably between 30-60% of P 2 O 2, in particular between 45-55% of P 2 O 2 and the concentration of HF is between 3-10% by weight.

In this way, a mixture of salts of said species is precipitated, which is isolated from the aqueous phase and forms a yield. The separation of the species is carried out in the last step by known methods and is no longer part of the present invention.

The re-extraction takes place in known devices by means of a liquid-liquid contact at a temperature in the range from 20 ° to 60 ° C. After the organic phase and the aqueous phase containing the precipitate have been separated, the organic phase is returned to the cycle in the extraction unit. The precipitate is separated from the aqueous phase and refilled with HF and HgPO4 and also returned to the regeneration unit.

12 68038

In the first embodiment, working using a pre-reduced initial aqueous solution containing uranium having an oxidation state of four, the re-extraction zone consists of a single-level device. This embodiment is described in Figure 2 (Example 8).

In another embodiment, an initial aqueous solution in which the uranium has an oxidation state of six or six and four is used, and the re-extraction takes place in a region with only one level using an aqueous re-extraction solution containing hydrofluoric acid, phosphoric acid and a reducing agent for hexane uranium, such as for example iron (II) ions, precipitating the desired species in the form of salts. This embodiment is described in Figure 3 (Example 9).

In the third embodiment, an initial aqueous solution containing uranium with an oxidation value of two or six and four is used, and the re-extraction takes place in two steps. The first reuptake zone usually comprises one level and the organic starting solution is contacted with a first aqueous solution containing hydrofluoric acid and phosphoric acid, whereby yttrium, rare earth metals, thorium and part of the uranium precipitate in the form of salts.

The partially depleted organic phase then feeds a second re-extraction zone, usually with only one level, where it comes into contact with a second solution of hydrofluoric acid and phosphoric acid, which further contains a hexavalent uranium reducing agent such as iron (II) ions, leaving the organic phase leaving the first zone. the residues of uranium (VI) contained in the uranium (VI) are reduced, extracted and precipitated in the form of a salt of uranium (IV) very efficiently until depletion. This embodiment is illustrated in Figure 1 (Example 7).

IV. Recovery of HF and H 2 PO 4 from the organic phase In the above methods, when the species are re-extracted with an aqueous phase containing HF and H 2 PO 4 and possibly also a reducing agent, the organic 13 68Q38 phase is extracted in which no no HF or with very little and also very little I 2 PO 4, part of the phosphoric acid and hydrofluoric acid present in the aqueous solution used in the reconstitution. This is especially the case when the concentration parameters of the aqueous phase are adjusted to enhance the re-extraction of the species, i.e. the content of the aqueous phase I 2 PO 4 is between 30-85% by weight and the concentration of HF is more than 2%, i.e. 50% aqueous phosphoric acid calculated as P 2 O 2 and the HF concentration is 6%.

Extraction of this amount of acid, which is thus absent from re-extraction, means a very significant reduction in the concentration in the circulation of the aqueous phase used in the re-extraction, which means that provision must be made for acid re-loading, which in turn strains the economy of the process.

The invention therefore comprises, as an improvement, the recovery of hydrofluoric acid and phosphoric acid which go with the extraction into the organic phase in the extraction step. After the species have been re-extracted and the aqueous phase and the organic phase separated, the depleted organic phase of the metal species is subjected to an aqueous solution in the wash area before being re-introduced to the original aqueous acid solution to give an organic phase substantially empty of phosphoric acid and hydrofluoric acid. the aqueous solution leaving the acid and wash area is then combined with the aqueous solution used in the re-extraction.

The aqueous solution used to wash the depleted organic phase of metal species is water or a solution of phosphoric acid having a lower concentration than the phosphoric acid in the circulation of the aqueous solution used in the extraction.

The parameters in the above-mentioned contact, in particular the number of levels in the washing area, the ratio of the phase flow rates and the temperature, can vary within quite wide limits, but it is advantageous to select them so that the solution circulation in the re-extraction step is certainly economically balanced. These parameters are well known in the art. The washing device usually comprises at least one level and the flow rates 68038 organic phase / aqueous phase ratio are between 10 and 100, mainly about 45. The operating device usually consists of a mixing, settling tank, pulse or packed column and the phases flow upstream or downstream. The contact temperature is usually between 60 ° C and the ambient temperature.

After combining the above aqueous solution leaving the wash area with the aqueous re-extraction solution, the concentrations of phosphoric acid, hydrofluoric acid, and, if necessary, reducing agent in the combined solution stream are adjusted.

Since, in general, the concentrations of H 2 PO 4 and HF in the solution leaving the above wash zone are lower than in the aqueous solution of the re-extraction cycle, the acid concentration of the stream resulting from the combination of the two solutions is reduced. Therefore, a continuous concentration operation, usually evaporation by a known apparatus, must be performed in the combined stream to remove water and return the hydrofluoric acid back to the cycle. Thereafter, amounts of HF, H 2 PO 2 and, if necessary, a uranium reducing agent are continuously added to keep the various concentrations constant, to the feed to the regeneration zone, and to provide the best possible economic conditions.

This improvement in art adoption is better understood by the diagram of Figure 4, which shows an improvement over the diagram of Figure 3.

Along line 41, an aqueous acid solution, usually wet crude phosphoric acid, containing the desired metal species is added to the extraction zone 42, where it is contacted with the organic phase 50 in accordance with the present invention. In step 43, an aqueous solution is recovered, which is practically emptied and in step 44 the organic enriched phase. This then feeds a re-extraction zone 45, which generally contains only one level and where it comes into contact with an aqueous solution of hydrofluoric acid and phosphoric acid 46 containing a substance that reduces uranium VI, usually iron (II) ions. Once the phases are separated, the organic phase 47, from which uranium, thorium, rare earth metals, and yttrium have been virtually completely depleted, feeds the wash zone 48, where it is contacted with an aqueous solution 49, which is usually water or a dilute aqueous solution of phosphoric acid. From the area 48 leaves the organic phase 50, which has been virtually completely drained of acids and which is taken back into the cycle in the extraction area 42, and the washing solution 51.

In the above re-extraction zone 45, the searched species are re-extracted from the organic phase and precipitated in the form of salts from the aqueous solution of the re-extraction unit starting from zone 45 along line 52 and feeding the separation zone 53 to recover the filtration pack 54, which is washed in the washing zone 55. of water. At 57, a precipitate is recovered containing mainly the following salts: UF4, TrP3, YF3, ThF4 and CaF2, which form the yield. The mother liquor 58 remaining from the wash is preferably returned to the cycle in the extraction zone 42.

The mother water 59 leaving the separation zone 53 is connected to a stream 51 leaving the organic phase wash zone 48 and the combined stream 60 is concentrated in an evaporation zone 61 separating the water stream 62 and the aqueous solution 63 enriched in HF, which is combined with a concentrated water stream leaving the concentration zone 61 to form stream 64, the concentration and amount of H 2 O of the iron (II) ions and HF are checked, forming a stream 46 which feeds the re-extraction zone 45.

Thanks to this improvement, a considerable amount of phosphoric acid and hydrofluoric acid, which have been extracted into the organic phase during the extraction in region 45, can be recovered and can be recirculated to the extraction solution, thus achieving considerable savings in the reactants used. This makes it possible under normal industrial conditions to recover up to 100% of the extracted P 2 O 4 and up to 90% of the extracted HF, which would otherwise be lost.

This improvement in the recovery of hydrofluoric acid and phosphoric acid 16 68038 is, of course, applicable to other embodiments of the process according to the invention.

The following examples are provided to illustrate the matter and percentages are by weight unless otherwise indicated.

Example 1; This example determines the distribution coefficients of the species to be searched (ratios of the concentrations of the species in the organic phase and the aqueous phase).

Contact with one liter of crude wet-reduced non-reduced phosphoric acid with a p204 content of 30% and containing 135 mg / l of uranium (VI), 0.350 g / l of rare earths expressed as rare earth oxides (3 mg / l of Eu2O3) , 150 mg / l Y 2 O 2) '2 thorium (IV) and 7 g / l iron (III), per liter of organic phase containing 36% Solvesso (trade mark of aromatic hydrocarbons manufactured by EXXON), 36% kerosene, 23% DOPPA containing less than 1% MOPPA, 5% trioctylphosphine oxide, at ambient temperature. The partition coefficients are as follows: PTh4 +> 10 PU (VI) = 6 V3 10

Peu2 ° 3 l'2 PFe “° -4

Example 2:

The aqueous solution of Example 1 and the organic solution are contacted countercurrently in an extraction apparatus with six stirred-settling tanks at 20 ° C with a phase volume ratio of 1. After separation of the phases, more than 99% of uranium, 83% of rare earths (of which more than 95% of europium) are recovered from the organic phase. and yttrium), 99% thorium and 30% iron, which were originally in the aqueous phase.

Example 3:

Example 1 is repeated, this time using crude phosphoric acid 68038 previously reduced with metallic iron.

After contact of both phases at a volume ratio of 1/1, the partition coefficients are as follows: PTh4 +> 25 PU (IV)> 25 PY203> 25 PEu2 ° 3 '9'5 P „= 0.1

Fe

Example 4; Again, the aqueous and organic phases of Example 3 are used and contacted countercurrently in six theoretical levels in the batteries of the mixing-settling tanks with a volume ratio of aqueous phase / organic phase = 1 at 25 ° C.

After the phases are separated, more than 98% of the uranium (IV), rare earth metals, yttrium and thorium originally contained in the aqueous phase are recovered from the organic phase.

Example 5; This example demonstrates a non-selective recovery method for metal species from an enriched organic phase.

To this end, the organic extract containing the species obtained in Example 2 is again taken and contacted at 50 [deg.] C. in a stirred-settling tank with an aqueous solution containing 56% by weight of P2O2 and 5% of HF in an organic phase / volume ratio. aqueous phase = 2. Re-extract and precipitate from the aqueous phase 35% of uranium (VI) and more than 90% of the rare earth metals, yttrium and thorium present in the organic phase.

Example 6;

The experiment of Example 5 is repeated using the organic extract of Example 3 derived from the extraction in a six-level mixing-settling tank battery. In this case, more than 90% of the uranium and rare earths, thorium and yttrium in the organic phase are recovered as a precipitate from the aqueous phase.

Example 7:

Recovery from Crude Phosphoric Acid This example illustrates the continuous process shown in Figure 1 for recovering the above species.

Line 1 contains crude phosphoric acid obtained by the wet process, containing: Ρ20 ^ 30% by weight

Uranium (VI) 135 mg / l

Tr 2 O 3 + ThO 2 0.35 g / l of which Y 2 ° 3 0.15 g / l for extraction coil 2 (stirred-settling tanks) comprising 7 theoretical levels at 25 ° C upstream of the organic phase 17 containing a solvent consisting of 50% by weight kerosene and 50% by weight of Solvesso and contains 0.5 mol / liter of DOPPA and 5% by weight of TOPO, and the volume ratio of the phases is = 1. From 3 the emptied aqueous raffinate starts and from 4 the enriched organic phase. This feeds two single-level Regeneration Units 5 and 6 in succession. In the first Unit 5, the organic phase is contacted with an aqueous phase circulating in a closed circuit and containing 55% P 2 O 3 and 6% HF, the organic phase / aqueous phase of the volume ratio. at 1/2 at 50 ° C. After the phases are separated, the aqueous phase containing u (VI)

The precipitate of the salts of Tr, γ · η and Th is conveyed along line 7 to the filtration unit 8, where a solid part 9 is separated, which forms the output. To the filtrate leaving point 10, HF and P 2 O 3 are added to the above concentration and returned to the regeneration unit along line 11.

The partially emptied organic phase 12 then feeds the regeneration unit 6, where it comes into contact with a volume ratio of organic phase / aqueous phase 10/1 at a temperature of 50 ° C with an aqueous solution 13 having a P 2 O 4 content of 6%, 6% HF and 5 g / liter of iron ions (II).

68038 19

After the phases have been separated, the depleted organic phase 17 is returned to the cycle in the extraction coil 2 and the outgoing aqueous phase 14 containing the salt U (IV) is conveyed to a filtration unit 15 where the precipitate 16 is recovered. The distribution of uranium to the two concentrates is 35%. 9 with rare earths and 65% in precipitate 16.

Example 8: This example illustrates: with reference to Figure 2, an embodiment involving the reduction of crude phosphoric acid in advance by means of iron. Reduced crude phosphoric acid contains: P205 30%

Uranium (IV) 135 mg / l

Tr 2 O 3 + ThO 2 0.350 g / l of which Y90 · 3 0.150 g / l

2+ Δ J

Fe ^ 11 g / l This acid is introduced along line 21 into the battery system 22 of the mixing-settling tanks, comprising seven theoretical levels, at 25 ° C, upstream of the organic phase 27, which is identical to Example 1, the organic phase of the volume ratio / aqueous phase being = 1/5. At 23 the aqueous raffinate is removed and at 24 the organic enriched phase is removed. This phase feeds a regeneration unit 25 with only one level where it comes into contact with an aqueous solution 26 containing 51% P 2 O 5 and 6% HF at 50 ° C with a volume ratio of organic phase / aqueous phase = 5/1.

After the phases are separated, the depleted organic phase 27 re-feeds the extraction battery 22, while the aqueous phase 28 containing the precipitate is passed to a filtration unit 29 where a precipitate 30 containing the U (IV), Tr, Y and Th salts is recovered. , and which forms the output, and P 2 O 3 and HF are re-added to the filtrate 31 up to the above-mentioned concentrations and fed for the most part to the regeneration unit 25. The fraction 28 which is not returned to the cycle is removed.

Example 9: This example illustrates an implementation model using Figure 3.

20 68038 which includes the treatment of crude phosphoric acid containing uranium in the U (IV) and U (VI) forms, as well as reductive redistribution in one step. Crude phosphoric acid obtained by the wet process and containing: P205 29%

A total of 150 mg / l of uranium (of which U (IV) = 45 mg / l and U (VI) = 105 mg / l,

Tr 2 O 3 + ThO 2 0.42 g / l (of which Y 2 O 3 0.17 g / l)

A total of 9.1 g / l of Fe (of which Fe 2+ = 0.2 g / l) is introduced along line 21 into a battery 22 formed by seven mixing-settling tanks at a temperature of 40 ° C upstream of the organic phase 27, which is identical to Example 1, in volume ratio = 1. The enriched organic phase is removed from step 24 and the crude phosphoric acid emptied from step 23. The organic phase feeds the re-extraction unit 25 with only one level where it comes into contact with an aqueous solution 26 containing 48% P 2 O 5 and 6% HF and also reducing iron ions Fe 2+ with a concentration of 4 g / liter, with an organic phase / water phase of volume ratio = 10/1. Once the phases are separated, the depleted organic phase 27 contains less than 5 mg / liter of each of the elements uranium, thorium, rare earths, yttrium * it re-feeds the extraction battery 22. The aqueous phase 28 containing the precipitate suspended is passed to a separation unit 29 where the precipitated U (IV), Tr, Y, Th salts which form the output. The filtrate 31 is returned for the most part via cycle 26 to position 25 after 2+ after the HF, P 2 O 3 and Fe concentrations have been readjusted, and part of the solution 31 is withdrawn, 32.

During the performance of this method, it has been found that after 20 consecutive cycles of recovery, there has been no change in the titer of the solvent 27, in particular with respect to DOPPA, and no loss of extractability with respect to the elements sought.

Example 10: This example illustrates a process for crude, wet-obtained phosphoric acid supplemented with a batch of recovery of hydrofluoric acid and phosphoric acid which have gone to the organic phase. The method is shown in the attached Figure 4.

Crude phosphoric acid obtained by the wet process is introduced through point 41 and has the following composition: P205 30% U (VI) 100 g / m3

Tr 2 O 3 + ThO 2 400 g / m 3 (of which Y 2 O 2; 150 g / m 3)

A total of 9 kg / m3 of iron (mainly in the form of Fe (III)) in the battery 42 of the mixing-settling tanks with five levels at 40 ° C at a flow rate of 1 m3 / h. An organic phase is introduced countercurrently through point 50, as will be described below, containing:

Di- (octylphenyl) phosphoric acid 23%

Trioctylphosphine oxide 5%

Solvesso 150 (aromatic hydrocarbons, trade mark EXXON) 36%

Kerosene 36% P20 <- 3 kg / m3 HF 0.25 kg / m3

Iron 1.87 kg / m3 at a flow rate of 1 m / h.

An organic phase 44 is recovered from the battery outlet, which contains the searched metal species and additionally contains: HF 0.25 kg / m3 P205 3 kg / m3

Iron 1.95 kg / m3 and drained aqueous raffinate 43 containing:

Uranium 5 g / m

Tr203 50 g / m3 (mainly La203)

The aforementioned organic phase is contacted in stirred-settling tanks 45 with an aqueous phase 46 taken back into the cycle 68038, as will be described below, containing: P 2 O 5 52.7% HF 6.25%

O

Total iron 65 kg / mJ

of which iron (II): 225 g / m 2

Uranium 200 g / m3

Rare earth oxides 100 g / m3

Ratio of organic phase to aqueous phase flow rates = 5.

The aqueous phase leaving point 52, which contains precipitates of uranium (IV), rare earth metals, thorium and yttrium, contains 50% P 2 O 4, 6% HF and iron (III) ions and 65 kg / m3.

The above-mentioned aqueous suspension is filtered at 53, the filter tank 54 is washed at 55 with a stream of water at a speed of 0.01 m / h, and centrifuged to dryness. A residue is obtained which gives a yield of 5.3 kg / h and a flow of groundwater of 0.01 m / h containing: p2o5 10% HF 1.2%

Iron 13 kg / m3

The emptied organic phase 47 leaving the extractor 45 contains: P2Og 11 kg / m3 HF 1 kg / m3

Iron 2 kg / m3

It is led to a washing area 48 with a single level and washed countercurrently with a water flow 49 of 0.022 m3 / h and a phase ratio = 45.

An organic stream 50 is taken from the outlet of the washing area 48, which contains: P20j 3 kg / m3 HF 0.25 kg / m3

Iron 1.87 kg / m3 23 6 8 0 3 8 which is returned to the cycle in the extraction unit and water stream 51 containing: P205 26% HF 2.5%

Iron (III) 5 kg / min · ^

The above stream is combined with a stream 59 leaving the filtration zone 53 to form a stream 60 which is concentrated at 61, 62 by evaporation of 21 kg / h of water. To the resulting stream 64 is then added:

Iron 0.05 kg / h HF 0.45 kg / h H 2 O 0.19 kg / h forming a current 46 to be returned to the cycle re-extraction battery 45.

The following is a table comparing the acids required for re-extraction with respect to a process which does not use the process of the present invention: 3 Losses kg / m organic According to the invention Reference process via phase _ _ P 2 O ^ ^ 0.1 8 ä 10 HF £ .0 .25 0.75 ä 1

The following Examples 11-14 relate to the application of the process to an aqueous raffinate derived from the extraction of crude phosphoric acid with an insoluble solvent.

Example 11: A water raffinate derived from crude tributyl phosphate wet extracted by wet process from Tosphoric acid obtained by dissolving Togolese apatite with sulfuric acid is added, adding sulfuric acid to the extraction coil according to the instructions of the French patent No. 1,531,487 and working method. Its composition in weight is as follows: 68038 P205 3.5% H2SO4 12.2%

Fe 2 O 3 15.9 g / l Al 2 O 3 20.4 g / l

MgO 10.6 g / l

Na 2 O 16.8 g / l U (VI) 203 mg / l

Tr203 74 mg / l Y2O3 88 mg / l

ThO 2 approximately 0.5 mg / l

The above raffinate is extracted with an organic phase having the following composition in parts by weight:

Di- (octylphenyl) - 'phosphoric acid 23%

Trioctylphosphine oxide 5%

Kerosene 36%

Solvesso 150 (a mixture of aromatic hydrocarbons) is a trademark of VAM and EXXON 36% at 30 ° C, using a single-stage extractor. The following table shows the extraction yields of uranium (VI) and yttrium.

Phase ratio Uranium extract- Yttrium extract-_A / O_ Mist yield,% Mist yield,% 1 98 95 2 95 89 5 87 78

Example 12: The aqueous raffinate of Example 10 is used. The extraction is carried out as the solvent of Example 10 in a counter-current tank of mixing-settling tanks with four theoretical levels, maintained at 40 ° C, and the ratio of the flow rates of the organic phase to the aqueous phase is = 1.2. The battery discharges the depleted aqueous phase, which contains less than 10 mg / liter of uranium, rare earth metals, yttrium and thorium. The enriched organic phase contains: U (V) 165 mg / l

Tr20-3 57 mg / l Y2O3 J 73 mg / l

ThO 2 approximately 0.4 mg / 168038 The organic phase is then contacted with an aqueous solution containing P 2 O 3: 56%; HF: 6.5% and Fe: 1.5 g / liter in a stirred-settling tank at 40 ° C with flow rates of organic phase / aqueous phase = 5. A green precipitate forms which enters the aqueous phase and is then separated by centrifugation. The depleted organic phase contains less than 10 mg / liter of uranium, rare earth metals, yttrium and thorium. 0.35 g of dry uranium (IV), rare earth metals, yttrium and thorium fluoride per liter of solvent are obtained, which corresponds to a total recovery of 95% of uranium, rare earth metals, yttrium and thorium raffinate.

Example 13: An aqueous raffinate derived from Togolese apatite with sulfuric acid is dissolved in crude phosphoric acid obtained by the wet process and extracted with tributyl phosphate without adding excess sulfuric acid to the extraction coil.

Its composition is as follows: P205 15% H2SO4 4%

Fe 2 O 3 16 g / l Al 2 O 3 20 g / l

MgO 11 g / l

Na 2 O 17 g / l U (VI) 215 mg / l

Tr 2 O 2 75 mg / l Y 2 O 3 90 mg / l

The above raffinate is extracted with the organic phase of Example 10 so that the phases flow countercurrently in a stirred-tank tank with four theoretical levels maintained at 40 ° C and a flow rate ratio of organic phase / aqueous phase = 1. The evacuated aqueous phase leaves the extraction apparatus. Containing less than 20 mg / liter of uranium, rare metals, yttrium and thorium and an enriched organic phase containing: U = 210 mg / liter; Tr203 = 70 mg / liter and Y2C> 3 = 87 mg / liter, i.e. an average extraction rate of more than 90%. This organic phase is re-extracted as in Example 2 with a solution containing Ρ20 ^ ~ 56%; HF = 6.5% and Fe 2+ = 1.5 g / liter.

26 6 8 0 3 8

Example 14:

The aqueous raffinate of Example 13 is examined. It is pre-reduced on a scrap iron substrate by simultaneously reducing iron (III) to divalent iron (II) and uranium (VI) to tetravalent uranium (IV).

The extraction is then started so that the phase O / phase A ratio is = 1/5 in five planes using the solvent of Example 10. An organic phase is obtained containing U = 1.07 g / liter; Tr203 = 0.37 g / liter and Y2 ^ 3 = g / liter, which is re-extracted so that the phase ratio A / O = 1/5 using a solvent containing P2C> 5 = 50% and HF = 7%, whereby after centrifugation, a precipitate of the above species and an organic phase containing less than 10 mg / l of each element is obtained, corresponding to a recovery rate of more than 98% uranium, more than 96% yttrium and more than 95% rare earths.

The following Examples 15-19 relate to the application of the process of the invention to an aqueous acid solution derived from the treatment of waste phosphor gypsum.

Example 15: Waste phosphorus sparks from the dissolution of apatite by sulfuric acid followed by filtration containing 0.36% of rare earth oxides (Tr 2 O 3) and including yttrium and thorium oxide (ThC> 2) are used.

The phosphor gypsum is contacted by stirring for one hour at 25 ° C with a 2.3 M sulfuric acid solution in a weight / liquid = weight ratio of 4. After the gypsum has been separated by filtration and centrifugation, the resulting solution and a fresh fresh phosphor gypsum charge are re-contacted under the same conditions as above. This contacting operation is performed a total of five times with the same enriched solution containing H 2 SO 4 = 1.6 M; H 2 PO 2 = 0.8 M; Tr2Og + Th2 = 2.1 g / liter. The recovery of rare earths and thorium accounts for virtually 40% of the original phosphor gypsum.

27 68038

The above solution is contacted with an organic phase containing:

Di- (octylphenyl) -phosphoric acid 0.5 M / liter

Trioctylphosphine oxide 0.12 M / liter

Solvent: a mixture of an equal weight of kerosene and Solvesso 150 (trade mark of EXXON), a mixture of aromatic hydrocarbons, in a countercurrent stirred-tank tank with five theoretical levels and maintained at 25 ° C at flow rates : organic phase / aqueous phase = 1. The aqueous raffinate from the extractor outlet contains less than 100 mg / liter of rare earth, yttrium and thorium oxides, which corresponds to an extraction catch of more than 95% of rare earth, yttrium and thorium.

The enriched organic solution is then re-extracted in a single-stage stirred-precipitation tank using a phosphoric acid solution containing 52% P 2 O 3 and 6% hydrofluoric acid at flow rates of organic phase / aqueous phase = 5. A green precipitate containing rare earth metals, yttrium, is obtained. and thorium, and this precipitate is separated and dried to give 2.4 g of product per liter of treated organic phase. The organic phase leaving the re-extraction apparatus contains less than 10 mg / liter of rare earth metals, yttrium and thorium, which corresponds to a total recovery rate of more than 38% of the original phosphor gypsum.

Example 16: The waste phosphor gypsum of Example 15 is used. It is contacted with stirring for one hour at 50 ° C with a 2.7 M sulfuric acid solution, and then the procedures described in Example 1 are repeated. 90 liters of enriched solution containing H 2 SO 4 = 2 M are obtained; H 3 PO 4 = 0.8 M; Tr 2 O 3 + ThO 2 = 2.6 g / liter. Catches of rare earths, thorium and yttrium account for virtually 50% of the original phosphor gypsum.

28 68038

The present solution is contacted with the organic phase of Example 15 countercurrently in a stirred-tank tank with 8 theoretical levels maintained at 25 ° C with flow rates of organic phase / aqueous phase = 1. The aqueous raffinate from the extractor contains 550 mg / liter of rare earth metals. oxides, mainly oxides of lanthanum, cerium, praseodymium and neodymium, which account for about 80% of the extraction catch from the organic phase in the form of rare earth metals, yttrium and thorium. The rare earth metals, yttrium and thorium are then re-extracted under the conditions of Example 1.

Example 17; The waste phosphor gypsum of Example 15 is used. This is contacted with stirring for one hour at 50 ° C with a 1.8 M sulfuric acid solution, then the procedures described in Example 15 are repeated. 90 l of an enriched solution containing H2SO4 = 1 M are obtained; H 3 PO 4 = 0.8 M; Tr 2 O 3 + ThCl> 2 = 2.1 g / liter. The recovery yield of rare earths, yttrium and thorium corresponds to practically 40% of the original phosphor gypsum.

The above solution is contacted with the organic phase of Example 15, countercurrently, in a stirred-tank tank with 4 theoretical levels and maintained at 25 ° C with flow rates of organic phase / aqueous phase = 0.2. The aqueous raffinate leaving the extractor contains 50 mg / liter of rare earth oxides, especially lanthanum oxide, which corresponds to more than 97% of the extraction yield from the organic phase of rare earths, yttrium and thorium, which are re-extracted in the form of a fluoride precipitate with 6% HF and HF. ^ PO 4 containing 52% P2O5.

Example 18; Using the enriched solution of Example 17, the strength is: H 2 SO 4 = 1 M; H 3 PO 4 = 0.8 M; Tr 2 O 3 + ThCl 2 = 2.1 g / liter.

The above solution is contacted with an organic phase having a combination of: 68038 di- (octylphenyl) -phosphoric acid: 0.5 M dissolved in Solvesso 150, countercurrently in the battery of mixing basins with four theoretical levels and maintained at 25 ° C , in terms of flow rates the organic phase / aqueous phase is = 1. The aqueous raffinate leaving the extractor contains 55 mg / liter of rare earth oxides, which corresponds to 97% of the extraction yield from the organic phase of rare earths, yttrium and thorium.

Example 19: An enriched solution of Example 17 with a strength of: h 2 S 4 = 1 M H 3 PO 4 = 0.8 M is used; Tr 2 O 3 + ThO 2 = 2.1 g / liter.

Contact the above solution with an organic phase of the following composition: di- (octylphenyl) -phosphoric acid 0.2 M / liter nonylphenol 20% kerosene qsp 100% upstream in a stirred-tank tank with 4 theoretical levels maintained at 25 ° C at flow rates organic phase / aqueous phase Si = 1.25. The aqueous raffinate leaving the extractor contains 90 mg / liter of rare earth oxides, which corresponds to 95% of the extraction yield from the organic phase of rare earth metals, yttrium and thorium.

Claims (14)

30 68038 1. A continuous process for the simultaneous recovery of uranium, thorium, yttrium and rare earth metals from an acidic aqueous phase containing these substances, obtained by the wet process of phosphoric acid or obtained from waste phosphor gypsum containing said metals in insoluble form, by treatment with concentrated mineral acid or which is an aqueous raffinate obtained by extracting crude phosphoric acid prepared by the wet process with water-insoluble or sparingly soluble solvents, characterized in that said acidic aqueous phase is contacted with a homogeneous organic phase containing di- (alkylphenyl) -phosphoric acid, free of mono- (alkylphenyl) phosphoric acid derivatives, and an inert organic solvent which is an aliphatic or aromatic hydrocarbon, used alone or in admixture to give uranium after phase separation, thorium an aqueous phase virtually completely emptied of yttrium and rare earth metals and an organic phase containing said metals, which is then further treated to recover said metals. Process according to Claim 1, characterized in that the alkyl radical of the alkylphenyl group of the di- (alkylphenyl) -phosphoric acid is an aliphatic, straight-chain or branched radical containing four to twelve carbon atoms. Process according to Claim 1 or 2, characterized in that the alkyl radical is an octyl radical. Process according to Claim 3, characterized in that the octyl radical is a 1,1,3,3-tetramethylbutyl radical. 31 68038 Process according to Claim 4, characterized in that the di- (alkylphenyl) -phosphoric acid is di- [4- (1,1,3,3-tetramethylbutyl) -phenyl] -phosphoric acid. Process according to any one of the preceding claims, characterized in that the organic phase further contains a solvating compound (modifier) selected from trialkylphosphine oxide, alkylphosphinate, alkylphosphonate, alkylphosphate or alkylsulfoxide. Process according to any one of the preceding claims, characterized in that the organic phase also contains a modifier selected from a compound containing hydroxyl groups, an aliphatic or alkylaryl compound containing more than 4 carbon atoms. Process according to Claim 7, characterized in that the modification agent is an alkylphenol present in an amount of 5 to 50%, in particular about 20%. Process according to Claim 8, characterized in that the alkylphenol is nonylphenol. Process according to any one of the preceding claims, characterized in that the concentration of di- (alkylphenyl) phosphoric acid in the organic phase is from 0.1 M to 1 M, in particular about 0.5 M. Process according to any one of the preceding claims, characterized in that the organic phase, after extraction and separation of the precipitate, is re-extracted by contacting it with an aqueous phase containing hydrofluoric acid dissolved in concentrated mineral acid, followed by extraction of the aqueous phase precipitate. containing uranium, thorium, rare earth metals and yttrium in the form of salts which are separated and the depleted organic phase is returned to the extractor. Process according to Claim 11, characterized in that phosphoric acid or sulfuric acid is selected as the concentrated mineral acid. Process according to one of Claims 1 to 12, characterized in that the aqueous raffinate which forms the acidic aqueous raffinate starting material is obtained by extracting the phosphoric acid obtained by the wet process with an alkyl phosphate or an aliphatic alcohol which is practically insoluble in water. Process according to any one of Claims 11 to 13, characterized in that, after the metals have been re-extracted and the aqueous and organic phases have been separated, the organic phase from which the metals have been emptied is treated with water in a washing zone before being returned to the acid solution. or with a solution of phosphoric acid lower than the concentration of the aqueous solution obtained from the re-extraction when it enters the re-extraction zone to give an organic phase from which hydrofluoric acid and phosphoric acid are practically completely discharged and an enriched aqueous solution which can be recombined after the metals have been precipitated and before it is returned to the re-extraction zone. 33 68038