CA2154133A1

CA2154133A1 - Process for the recovery of molybdenum from impure calcium molybdate resulting from the treatment of uraniferous ores

Info

Publication number: CA2154133A1
Application number: CA002154133A
Authority: CA
Inventors: Cecile Brassier; Pascal Baron
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1994-07-20
Filing date: 1995-07-18
Publication date: 1996-04-18
Also published as: AU698786B2; FI953493A0; FR2722701A1; AU2485495A; FI953493A; FR2722701B1

Abstract

The invention relates to a process for the recovery of the molybdenum present in impure calcium molybdate result-ing from the treatment of uraniferous ores.
According to this process (in 40) the calcium molyb-date is dissolved in an aqueous solution containing at least 2 moles/? of H2SO4 and then the molybdenum is ex-tracted (in 21) in an organic solvent comprising a dialkyl phosphoric acid such as di-(2-ethylhexyl)-phosphoric acid.
After extraction, it is possible to wash the solvent (in 23) using an aqueous sulphuric acid solution and then recover the molybdenum extracted in the reextraction solvent in an ammoniacal solution.
The choice of a dissolving solution having a H2SO4 concentration of at least 2 mole/? avoids the formation of difficultly extractable heteropolymolybdic components.

Description

~ 2I54133 PROCESS FOR THE RECOVERY OF MOLYBDENUM FROM IMPURE CALCIUM
MOLYBDATE RESULTING FROM THE TREATMENT OF URANIFEROUS ORES
.
DESCRIPTION
The present invention relates to a process for the recovery of molybdenum from impure calcium molybdate resulting from the treatment of uraniferous ores.
In the uranium industry and in particular in hydro-metallurgical processes, carbonated solutions are producedwhich contain uranium and various metals such as molyb-denum, which it is necessary to separate from the uranium.
This separation can be performed after acidification by precipitation with lime, which leads to the formation of an impure calcium molybdate precipitate, which more partic-ularly contains impurities such as Si, P and/or V liable to form complexes of the heteropolyanionic type with molybdenum.
In orfler to recover the molybdenum from this precipi-tate, it is necessary to dissolve the latter in a sulphuric solution and then extract the molyb~emlm present in said solution, particularly in the form of heteropolyanions and/or isopolyanions, by means of an appropriate organic solvent.
FR-A-2 691 373 describes a process for the recovery of molybdenum using these stages of dissolving and extracting by solvent. According to said document, the impure calcium molybdate is firstly dissolved in an aqueous sulphuric acid solution at approximately 0.25 mole/e, i.e. a pH-value close to 2 after dissolving and then the molyb~Pnl]m dissolved in the aqueous solution is extracted in an organic solvent comprising a first extractant constituted by a dialkyl phosphoric acid such as di-(2-ethylhexyl)-phosphoric acid and a second extractant chosen from tertiary amine salts, quaternary ~mmo~;um salts, tributyl phosphate and phosphine oxides.

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215~133 This process suffers from the disadvantage of requir-ing the use of an organic solvent incorporating two ex-tractant types in order to achieve satisfactory extraction yields.
- Thus, due to the presence of heteropolymolybdic components in the aqueous dissolving solution, firstly there is a transfer of molybdenum heteropolyanions into the organic phase by means of the second extractant in anion or acid form as a function of the nature of the second ex-tractant and then the proton form of the polynuclearanionic molybdenum complex in the organic phase into a di-alkyl phosphoric (MO2 2 + ) acid complex by means of the first extractant, whilst releasing the phosphoric acid, vanadic acid and/or orthosilicic acid, as well as the water in the aqueous phase.
US-A-3 751 555 and US-A-4 275 039 also illustrates the use of an organic solvent having two extractant types, such as di-(2-ethylhexyl)-phosphoric acid and tributyl phosphate in order to recover ~he molybdenum from molvbdite or scheelite.
The present invention specifically relates to a pro-cess for the recovery of molybdenum from impure calcium molybdate by dissolving and then extracting in an organic solvent, which avoids the formation of heteropolyanion com-plexes between the molybdenum and the impurities such as silicon, phosphorus and/or vanadium present in the starting calcium molybdate, whilst as a result simplifying the molyhA~nllm extraction procedure and at the same time obtaining very good extraction yields.
According to the invention, the process is character-ized in that it comprises the following steps:
a) dissolving the calcium molybdate in an aqueous sulphuric acid solution having a sulphuric acid concentra-tion of at least two mole/~, b) contacting the aqueous dissolving solution from step a) with an organic solvent incorporating a dialkyl phosphoric acid, and B 11985.3 MDT

c) separating from the aqueous solution the organic solvent which has extracted the molybdenum.
In this process, the use of an aqueous solution having at least 2 mole/~ of sulphuric acid, e.g. 2 to 4 mole/~ of sulphuric acid for dissolving the calcium molybdate avoids the formation of heteropolymolybdic components in the dissolving solution. Therefore it is no longer necessary to then use for the extraction of the molyb~nllm two extractants in order to ensure on the one hand the transfer of the heteropolymolybdic components into the solvent and on the other their transformation into the dialkyl phos-phoric (MoO22+) acid complex.
Thus, it has been found that the existence range in aqueous solution of heteropolymolybdic components was in the pH range 1 to 3, which corresponds to sulphuric acid concentrations of 0.2 to 2 mole/e and that the degradation of the components formed by the subsequent acidification of the solution was very slow.
In addition, by performing in accordance with the present invention the dissolving of the calcium molybdate with a sulphuric acid concentration above 2 mole/~, troublesome heteropolybdic components are not formed.
After dissolving the calcium molybdate in the aqueous sulphuric acid solution, said solution is contacted with an organic solvent incorporating a dialkyl phosphoric acid.
In general, the organic solvent is constituted by a dialkyl phosphoric acid solution in an appropriate organic diluent.
The dialkyl phosphoric acids which can be used are those in which the alkyl radicals are straight or branched radicals with 4 to 16 carbon atoms.
As an example of such an acid reference can be made to di-(2-ethylhexyl)-phosphoric acid, which is hereinafter designated by the abbreviation HDEHP.
The organic diluents used are e.g. aliphatic or aromatic hydrocarbons, e.g. hydrogenated tetrapropylene (TPH).

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The dialkyl phosphoric acid concentration of the - organic solvent is generally high, because the extractionrate increases with the extractant content. Preferably, - -the dialkyl phosphoric acid concentration is 15 to 40 vol.%.
According to the invention after contacting the or-ganic solvent with the dissolving solution, an organic solvent is obtained which contains the extracted molyb-denum.
0 Preferably said solvent is washed with an aqueous, dilute sulphuric acid solution, e.g. having a sulphuric acid concentration of 0.5 to 1 mole/e. The molyb~nllm can then be reextracted from the organic solvent by means of an aqueous ~mmo~acal solution.
The process according to the invention can be per-formed in conventional cocurrent or countercurrent ex-traction installations such as mixer-settlers, pulsed columns, etc.
It is possible to carry out contacting at ambient pressure and temperature, but it is also possible to use higher temperatures, particularly for improving the extraction kinetics.
It is also possible to operate continuously by bringing about a countercurrent circulation of the organic solvent and the aqueous solutions and recycling the aqueous washing solution of the organic solvent with the starting aqueous solution containing the molyb~Pn-lm to be extracted.
Other features and advantages of the invention can be better gathered from the following non-limitative descrip-tion with reference to the attached drawings, wherein show:Fig. 1 A diagram representing the heteropolymolybdic complex quantities present in a sulphuric medium as a function of the sulphuric acid concentration of the medium (in mole/~).
Fig. 2 A diayLallullatic representation of an exemplified embodiment of the process of the invention.

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,, 215~133 Fig. 3 A diagrammatic representation of another embodi-ment of the process of the invention.

Fig. 1 illustrates the behaviour of the following heteropolymolybdic complexes:

- phosphomolybdic complex (Mo:P atomic ratio 20:1) absorbing at 400 nm, - phosphov~n~Aomolybdic complex (Mo:P:V atomic ratio 11:1:1) absorbing at 490 nm, and - silicomolybdic complex (Mo:Si atomic ratio 12:1) absorbing at 430 nm, at a concentration of 0.1 mole/e of Mo(VI) in an aqueous sulphuric acid solution, as a function of the H2 S04 concen-tration (in mole/e).
The abundance of the complex in solution is repre-g sented by the optical density of the solution at the wave-length corresponding to the absorption peak of the con-sidered complex.
In Fig. 1, curve 1 relates to the phosphomolybdic complex, curve 2 to the phosphov~n~Aomolybdic complex and curve 3 to silicomolybdic complex.
Thus, it can be seen that beyond a sulphuric acid con-centration of 2 mole/e, there is no longer any complex in solution.
This is why, according to the invention, use is made for the purpose of dissolving the calcium molybdate of an aqueous sulphuric acid solution having a H2 S04 concentra-tion of at least 2 mole/e.
Thus, the dissolving solution incorporates virtually no heteropolymolybdic components. This result cannot be achieved rapidly by carrying out the dissolving with a dilute H2 S04 solution and then acidifying the dissolving solution, because it is found that these heteropolymolybdic components and in particular silicomolybdic components have an excessively slow degradation kinetics in concentrated acid medium. For example, a period of 15 days is necessary B 11985.3 MDT

,, ` 215~133 at ambient temperature in order to degrade silicomolybdic - components in a 3M sulphuric medium The following examples illustrate the results obtained - with the process of the invention EXAMPLE 1: DISSOLVING CALCI~M MOLYBDATE
In this example, lg of calcium molybdate having the composition given in the following Table 1 is dissolved in 20ml of an aqueous solution containing 3 mole/e of sul-phuric acid, accompanied by stirring, at ambient tempera-ture and the composition of the dissolving solution is checked by periodically taking samples and analyzing them by spectrophotometry and emission spectroscopy JCP for determining their Mo, P, V, Si and U contents After stirring for 45 minutes, the dissolving yield no longer undergoes any significant variation and a dissolving solution is obtained having the Mo, P, V, Si and U concen-trations given in Table 1 This Table includes for comparison purposes the results obtained when carrying out the dissolving under the same conditions, but using a 0 25 mole/e sulphuric acid solution (pH = 2) as in FR-A-2 691 373 CALCIUM DISSOLVING DISSOLVING
ELEMENTMOLYBDATESOLUTION SOLUTION
(wt ~)3 mole/e H2 S04 H2 S04 (pH=2) (in mole/e) (in mole/e) Mo 33 660 0 212 0 159 P 1 360 5 99-10- 3 1 . 10-10- 2 V 1 610 1 69-10- 2 1. 49-10- 2 Si 0 760 1 81-10- 3 5 80-10- 3 The results of Table 1 show that according to the - invention an increased molybdenum dissolving is obtained.

EXAMPLE 2: MOLYBDEN~M EXTRACTION
Contacting takes place, accompanied by stirring, of one volume of a dissolving solution obtained as in example 1 either with 2M H2 S04 or with 3M H2 S04, four days after it was obtained, with 1 volume of an organic solvent consist-ing of 30 vol.~ di-(2-ethylhexyl)-phosphoric acid and 70 vol.~ of hydrogenated tetrapropylene.
After stirring for 5 minutes at 23C, the two phases are allowed to settle and their respective molybdenum con-tents are determined. This operation is repeated twice contacting the aqueous solution separated from the organic solvent resulting from the preceding contact with 1 volume of new organic solvent.
The composition of the starting solution and the results obtained are given in Table 2. In this Table, the extraction yields correspond to the cumulative values ob-tained after 1, 2 or 3 contacts.
This Table gives for comparison purposes the results obtained when working in the same way with a dissolving solution having a H2 S04 concentration of 1 mole/e.

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,, 2154133 - DISSOLVING H2 S04 ( in 1 2 3 SOLUTION mole/~) Mo(VI) in 0.161 0.165 0.15 mole/~

After 1 contact Organic Mo(VI)0.07530.135 0.129 (in mole/ e ) Aqueous Mo(VI)0.08570.030 0.021 (in mole/~) Extraction yield (~) 46.7~ 81.8~ 86~

After 2 contacts Organic Mo(VI)0.03150.0164 0.0129 (in mole/~) Aqueous Mo(VI)0.05420.0136 0.0081 (in mole/~) Extraction yield (%) 66.3~ 91.7~ 94.6~

After 3 contacts Organic Mo(VI)0.02360.0028 0.0017 (in mole/~) Aqueous Mo(VI)0.03060.0108 0.0061 (in mole/~) Extraction yield (~) 80.9~ 93.4~ 95.9 The results of Table 2 show that better results are obtained with the dissolving solutions obtained according to the process of the invention.

EXAMPLE 3:
In this example the molybdenum is recovered from a dissolving solution obtained according to the invention, by using the installation operating in continuous manner and in countercurrent form, as is diay~auu--atically shown in Fig. 2.

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215~133 g This installation comprises an extraction stage 21 and - a washing stage 23 in which the organic solvent flows con-tinuously, being introduced by the pipe 25. In this ex-- traction stage, the organic solvent is countercurrent con-tacted with the dissolving solution of the calcium molyb-date stored in the tank 27. This solution is introduced by the pipe 29 in the extraction stage 21.
On leaving the extraction stage pipe 31 recovers an aqueous efflue~t containing P, Si, V and U and virtually no molybdenum and by the pipe 33 the organic solvent contain-ing molybdenum, as well as a few impurities.
At the washing stage 23, this organic solvent is brought into countercurrent contact with an aqueous washing solution, e.g. a 0.5 mole/e sulphuric acid solution, intro-duced by the pipe 35.
On leaving the washing stage pipe 37 recovers the molybdenum-containing organic solvent and pipe 39 an aqueous effluent containing the impurities deextracted from the solvent.
In this installation it is possible to use circulation flow rates such that the volume ratio of the organic phase to the aqueous phase is 2 in the extraction stage and in the washing stage.
This stage can be constituted by several individual stages each constituted by a mixer-settler. For example, the extraction stage can have 12 individual stages and the washing stage 4 individual stages.
In the following Table 3 are given the results obtained in this installation with a 3 mole/~ sulphuric acid dissolving solution. For comparison purposes this Table also gives the results obtained with a dissolving solution in a weakly acid medium (pH 2), then acidified in order to have a 3 mole/~ sulphuric acid concentration. In this Table, the extraction yields are calculated on the basis of concentrations measured on the different aqueous phases.

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TABLE 3 f `

Mo(VI) concentration Extraction Reflux E/S(.) (mole/l) yield ratio(*) ~o ~ DissolvingOn entry Extraction Washing Organic (%) (%) r 3 solution (in 29) effluent effluent solvent (3 mole/l) (in 31) (in 39) (in 37) dissolving solution (3 mole/l~ 0.212 6.1.10 2.2.10 0.10 99~7 0 05 1.056 Dissolving solution (pH2) acidified to 3 mole/l in H2S04 0.138 1.5.10 2.90.10 35.55.10 88.1 1.0 1.08 t~
~n Mo flow (mole/h) in the aqueou~ phase to be treated (*) Reflux ratio = ~-~
Mo flow (mole/h) in the dissolving solution entering the extraction operation C~

(.) E/S = Entering Mo flow ~departing Mo flow The results of Table 3 ~emonstrate that an extraction yield of 99.7~ is obtained with the process according to the invention, whereas it is well below this figure (88~) when the dissolving is carried out at pH 2, followed by an aci~dification to 3 mole/l of H2 S04 .

EXANPLE 4:
In this example, the dissolving and then the extrac-tion of the molybdenum take place from calcium molybdate, using the installation diagrammatically shown in Fig. 3.
Fig. 3 uses the same references as in Fig. 2 for designat-ing the components common to both installations.
In this installation, firstly the calcium molybdate is dissolved in the container 40 using 20 m~ of 3M H2 S04 per tonne of calcium molybdate. Dissolving is carried out, accompanied by stirring, for 1 hour and at 23C and then the solution is separated from solid materials by filtra-tion in the filter 42.
The separated solution containing approximately 0.2 mole/~ of molybdenum and impurities (V, U, Si, P) is supplied by the pipe 29 to the extraction stage 21, which consists of six individual stages with the washing solution leaving the washing stage 23.
The organic solvent (30~ HDEHP - 70~ THP) is intro-duced by the pipe 25 and circulates in countercurrent manner with the aqueous solution.
On leaving the extraction stage 21, it is introduced into the washing stage 23 consisting of four individual stages, where it is countercurrent contacted with a 1 mole/~ H2 S04 aqueous solution introduced by the pipe 35.
On leaving the washing stage 23, the organic solvent, which virtually contains no molybdenum, is treated with a view to recovering the molybdenum by deextraction in an o~;acal medium (pH 8.5 to 9) and then undergoes decon-tamination with respect to uranium in a carbonate medium.
It is then reacidified by H2 S04 and recycled to extraction stage 21. However, it is possible to avoid this B 11985.3 MDT

215~1~3 reacidification of the solvent, because the latter can be performed by the aqueous solution in the first extraction stage.
- The molybdenum can be separated from the ammoniacal solution by ammonium molybdate precipitation in the acid medium (pH _ 1).
Fig. 3 indicates the volumes of solutions introduced at this stage.
With this installation, molybdenum is recovered with a yield of 99.7~ using a single extractant.

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Claims

1. Process for the recovery of the molybdenum present in impure calcium molybdate obtained from the treatment of uraniferous ores, characterized in that it comprises the following successive steps:
a) dissolving the calcium molybdate in an aqueous sulphuric acid solution having a sulphuric acid concentra-tion of at least 2 mole/?, b) contacting the aqueous dissolving solution from stage a) with an organic solvent incorporating a dialkyl phosphoric acid and c) separating from the aqueous solution the organic solvent which has extracted the molybdenum.

2. Process according to claim 1, characterized in that the sulphuric acid concentrations of the aqueous solution used in step a) is 2 to 4 mole/?.

3. Process according to either of the claims 1 and 2, characterized in that the dialkyl phosphoric acid is di-(2-ethylhexyl)-phosphoric acid.

4. Process according to any one of the claims 1 to 3, characterized in that it involves a supplementary washing stage of the organic solvent which has extracted the molybdenum by an aqueous solution constituted by dilute sulphuric acid.

5. Process according to claim 4, characterized in that the sulphuric acid concentration of the washing solution is 0.5 to 1 mole/?.