US20160032419A1

US20160032419A1 - Method for selectively recovering the rare earths from an aqueous acid sulfate solution rich in aluminum and phosphates

Info

Publication number: US20160032419A1
Application number: US14/776,249
Authority: US
Inventors: Maxime VINCEC
Original assignee: Eramet SA
Current assignee: Eramet SA
Priority date: 2013-03-15
Filing date: 2014-03-13
Publication date: 2016-02-04
Also published as: BR112015023619A2; CN105229178A; FR3003270B1; CA2906251A1; WO2014140492A1; SE1551224A1; CN105229178B; FR3003270A1

Abstract

The present invention relates to a process for the selective recovery of the rare earth metals from an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, characterized in that it comprises the following successive stages: a) neutralization at a pH of between 3 and 4 of an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, the solution having a molar ratio Al/P>1 and a concentration of sulfates>100 g/l, by addition of a base, so as to precipitate the phosphate and the aluminum and the possible titanium, b) liquid/solid separation between the precipitate formed by the phosphate and the aluminum and the possible titanium and the aqueous sulfate solution, c) recovery of the aqueous sulfate solution, d) addition of phosphates to the aqueous sulfate solution obtained in stage c) such that the molar ratio of the solution obtained PO₄/REs>4, so as to precipitate the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates, e) liquid/solid separation between the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates and the aqueous sulfate solution, f) recovery of the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates.

Description

The present invention relates to a process for the selective recovery of heavy, medium and light rare earth metals from an acidic aqueous sulfate solution additionally comprising phosphates and aluminum and possibly titanium, iron(III) and iron(II).
Ores, such as pyrochlore ores, comprising numerous elements of interest, sometimes in low proportions. The rare earth metals are included among these elements. The rare earth metals can also be produced from monazite, bastnaesite and loparite ores. These rare earth metals have numerous advantageous applications in various fields. For example, lanthanum (La) is a component of catalysts employed in the refining of hydrocarbons, neodymium (Nd) is widely used in NdFeB magnets, europium (Eu) and terbium (Tb) are dopants for plasma screens and LCD screens. Yttrium (Y) is for its part used in YAG (Yttrium Aluminum Garnet) ceramics. It is thus advantageous to be able to extract them and to separate them from the other elements present. Rare earth metals can be divided chemically into three groups:

- Light rare earth metals: La, Ce, Pr, Nd
- Medium rare earth metals: Sm, Eu, Gd, Tb, Dy
- Heavy rare earth metals: Ho, Er, Tm, Yb, Lu+Y.

These elements, normally put into one and the same group, have a chemical behavior which is similar but rather different according to the reactions envisaged. In the context of the present invention, scandium is regarded as not being among the rare earth metals. This is because scandium (Sc), nevertheless often put into the family of the rare earth metals, has a different chemical behavior from the elements of the series of lanthanides (rare earth metals). The demand for medium and heavy rare earth metals is greater than for light rare earth metals whereas, in general, their content in ores is lower and whereas they are more difficult to recover. It is thus important to be able to find a process which makes it possible to recover them with a good yield.
During the leaching of pyrochlore ore, in particular the ore resulting from the Mabounié deposit, located in Gabon, the dissolution of the elements of value (Nb, rare earth metals (TR), Ta and U) is quantitative. The leachate obtained comprises not only light, medium and heavy rare earth metals but also iron, in particular ferric iron (FeIII), aluminum (Al), titanium and phosphates (P). This leaching is described in particular in the patent application WO 2012/093170. In point of fact, the presence of aluminum and to a lesser extent of ferric iron interferes with the recovery of the rare earth metals and in particular of the medium and heavy rare earth metals. As has been shown in the examples below, the conventional and known reactions for the recovery of the rare earth metals do not make it possible to recover them all, in particular to recover the heavy rare earth metals, or the whole of the medium rare earth metals:

- Precipitation of the double salts of rare earth metals: the heavy rare earth metals precipitate only slightly (yield less than 10%), the medium rare earth metals precipitate partially (moderate yield of 50%), and the light rare earth metals precipitate quantitatively (yield of 90%); this process thus does not make it possible to sufficiently recover heavy and medium rare earth metals.
- Solvent extraction/Ion-exchange resin: the presence of ferric iron and aluminum limits the extraction constants of the rare earth metals which, present in low amounts, cannot be correctly extracted.

The patent application US2009/0272230 describes a process for the recovery of rare earth metals from monazite and apatite ores. These ores contain a great deal of phosphates, aluminum and iron. The process provided comprises:

- a stage of leaching the ore with an acid in order to completely dissolve the apatite ore in the leaching liquor;
- a stage of precipitation of the rare earth metals from this liquor in the form of rare earth metal phosphates by addition of ammonia or calcium hydroxides;
- treatment of the residue by acid roasting and then by leaching with water in order to produce an aqueous leaching liquor rich in rare earth metals;
- the separation of the impurities, including thorium and iron from this liquor by introducing a neutralizing additive, such as magnesium oxide or magnesium carbonate;
- the precipitation of the rare earth metals from the post-neutralization liquor, in particular by using carbonate or double salt precipitation.

These ores essentially contain light rare earth metals. Thus, the problem of the recovery of the heavy rare earth metals is not tackled.
In addition, the amount of phosphates present in the residue from leaching the ores obtained is in excess with respect to the iron. It is thus recommended in this patent application to add iron to the residue in order to achieve the required stoichiometry (Fe/P=1).
Furthermore, the problem of the recovery of the rare earth metals in the presence of high contents of aluminum is not posed. This is because the solution containing the rare earth metals to be recovered does not contain high contents of aluminum. The presence of aluminum is thus not troublesome for said recovery.
Finally, due to the presence of the light rare earth metals which it is desired to recover, it is not possible to neutralize the liquor containing these rare earth metals with just any base. In particular, it is not possible to use a base containing calcium, which is nevertheless less expensive and easier to supply, as there would be precipitation of gypsum, which would entrain rare earth metals.
The inventors have noticed, surprisingly, that it is possible to recover, with a good yield, heavy rare earth metals despite the presence of ferric iron and in particular of aluminum in the starting solution. In order to do this, they have discovered that it is necessary to selectively precipitate the aluminum by using the phosphates already present with neutralization at a very precise pH, provided that the aluminum is in excess with respect to the phosphates. This stage makes it possible to purify (or deplete) the solution from (or in) aluminum and phosphates. It is subsequently sufficient to add phosphates to the solution obtained in order this time to precipitate the heavy rare earth metals.
The present invention thus relates to a process for the selective recovery of the rare earth metals from an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, characterized in that it comprises the following successive stages:

a) neutralization at a pH of between 3 and 4, advantageously at a pH of 3.5, of an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, the solution having a molar ratio Al/P>1, advantageously Al/P≧1.5, in particular Al/P≧2, and a concentration of sulfates>100 g/l, advantageously≧200 g/l, in particular of approximately 270 g/l, by addition of a base, so as to precipitate the phosphate and the aluminum and the possible titanium,
b) liquid/solid separation between the precipitate formed by the phosphate and the aluminum and the possible titanium and the aqueous sulfate solution,
c) recovery of the aqueous sulfate solution,
d) addition of phosphates to the aqueous sulfate solution obtained in stage c) such that the molar ratio of the solution obtained PO₄/REs is >4, advantageously PO₄/REs is ≦120, in particular PO₄/REs is ≦40, particularly PO₄/REs is ≦20, more particularly still PO₄/REs=5, so as to precipitate the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates,
e) liquid/solid separation between the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates and the aqueous sulfate solution,
f) recovery of the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates.

Within the meaning of the present invention, “rare earth metals (REs)” is understood to mean the rare earth metals chosen from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu+Y and their mixtures. In particular, scandium (Sc) is not included among the rare earth metals according to the present invention. Advantageously, the rare earth metals are classified into three groups:

- Light rare earth metals (LREs): La, Ce, Pr, Nd;
- Medium rare earth metals (MREs): Sm, Eu, Gd, Tb, Dy;
- Heavy rare earth metals (HREs): Ho, Er, Tm, Yb, Lu+Y.

Stage a) of the process according to the present invention makes it possible to purify (or deplete) the solution from (of) phosphates, aluminum and titanium (if titanium is present) in order to obtain a solution containing the heavy rare earth metals and the possible medium rare earth metals from which at least 90% by weight of aluminum, of the phosphates and the possible titanium have been removed, advantageously at least 95% by weight, with respect to the total weight present at the start in the solution. This is because, without being committed to theory, it appears that the majority of the phosphate, of the aluminum and of the possible titanium (at least 90% by weight, advantageously at least 95% by weight, with respect to the total weight present at the start in the solution) precipitate. In particular, as a result of its affinity for the aluminum, the phosphate appears to precipitate preferentially in the form of aluminum phosphate AlPO₄at a pH of between 3 and 4, advantageously of 3.5, preferentially to the rare earth metals phosphates. Once the phosphates have been removed, the remaining aluminum will quantitatively precipitate in the form of aluminum hydroxide, which will make possible the removal of the remaining aluminum. As the phosphates are deficient with respect to the aluminum, the heavy rare earth metals and the possible medium rare earth metals will not precipitate or will not precipitate very much in the form of phosphates (at most 40-50%). On the other hand, the precipitation of the aluminum in the form of phosphates is quantitative. Thus, the majority of the heavy rare earth metals and of the possible medium rare earth metals will remain in the sulfate solution (at least 50-60% by weight, with respect to the total weight of the initial acidic aqueous sulfate solution).
The base which can be used in stage a) of the process according to the present invention can be any base. It is advantageously chosen from NH₄OH, KOH, a basic sodium compound, such as, for example, NaOH or Na₂CO₃, a basic magnesium compound, such as, for example, MgO or MgCO₃, a basic calcium compound, such as, for example, CaCO₃, CaO and Ca(OH)₂, and their mixtures, more advantageously still chosen from MgCO₃, a basic calcium compound and their mixtures.
In a particularly advantageous embodiment of the process according to the present invention, the base of stage a) is a basic calcium compound advantageously chosen from CaCO₃, CaO, Ca(OH)₂and their mixtures; advantageously, it is CaCO₃. This type of base is particularly advantageous as it is relatively inexpensive. In addition, given that only the recovery of the heavy rare earth metals is being looked for, the use of such a base is possible since the precipitation in the form of gypsum entrains only predominantly the light rare earth metals, moderately the medium rare earth metals, and marginally the heavy rare earth metals.
Advantageously, the temperature of stage a) of the process according to the present invention is between 20 and 90° C.; in particular it is approximately 70° C.
Advantageously, the duration of stage a) is between 30 min and 6 h and it is advantageously 1 h.
Stage d) of the process according to the present invention is used to extract all the rare earth metals present in the solution by precipitation in the form of rare earth metal phosphates. Since the majority of the phosphates of the solution have already been removed during stage a) of the process according to the present invention (advantageously at least 90% by weight, advantageously at least 95% by weight, with respect to the total weight present at the start in the acidic aqueous sulfate solution), it is necessary to add it during stage d). The precipitation is then quantitative since there is virtually no more aluminum in the solution (advantageously, the solution contains less than 10% by weight of aluminum, with respect to the initial aqueous sulfate solution, advantageously less than 5% by weight).
Advantageously, the phosphate used in stage d) is chosen from Na₃PO₄, K₃PO₄, (NH₄)₃PO₄and their mixtures; in particular, it is Na₃PO₄.
Advantageously, the aqueous sulfate solution in stage d) has a pH of between 3 and 4, advantageously, it is 3.5.
Advantageously, the temperature of stage d) of the process according to the present invention is between 50° C. and the boiling point, which is advantageously 90° C.; in particular it is between 70° C. and the boiling point.
Advantageously, the duration of stage d) is between 30 minutes and 2 hours. Advantageously it is less than or equal to 1 hour.
In a specific embodiment of the present invention, the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, is the leachate obtained by acid attack on a pyrochlore ore in a sulfate medium, for example as described in the patent application WO 2012/093170. In particular, this solution contains:

- Al: at least 7 g/l, advantageously between 7 and 14 g/l;
- P: between 1 and 6 g/l, advantageously between 3 and 6 g/l, in particular between 4 and 6 g/l;
- Heavy and medium REs: at least 100 mg/l, advantageously between 200 and 300 mg/l;
- S: between 66 g/l and 100 g/l, advantageously between 70 and 80 g/l;
- Ti: ≦1 g/l; advantageously, if it is present, between 0.5 and 1 g/l.

The acidic aqueous sulfate solution can also contain iron (Fe), advantageously at least 50 g/l, advantageously between 50 and 70 g/l, in particular in the form of Fe(II).
In another specific embodiment of the present invention, the process according to the present invention comprises an additional stage g) of washing the precipitate obtained in stage f), advantageously by repulping with water, advantageously at ambient temperature.
The recovery of the heavy rare earth metals and of the possible medium rare earth metals from the precipitate obtained in stage f) or in stage g) can be carried out by methods well known to a person skilled in the art, such as, for example, purification by conversion into rare earth metal hydroxides.
Advantageously, the recovery yield of the heavy rare earth metals of the process according to the present invention is greater than 50%, advantageously greater than or equal to 60%.
In yet another embodiment of the process according to the present invention, the acidic aqueous sulfate solution comprising phosphates, aluminum, heavy rare earth metals and medium rare earth metals, and possibly iron(II) and titanium, additionally comprises light rare earth metals and an attempt is made to recover all the rare earth metals (medium, heavy and light). In point of fact, the light rare earth metals are partially entrained by the precipitation of gypsum during stage a), if the precipitation is carried out with a basic calcium compound. In addition, there are losses of light rare earth metals and of medium rare earth metals by precipitation in the form of phosphates during stage a), whatever the base used. Thus, if it is desired to recover the light rare earth metals and all of the medium rare earth metals, and thus to avoid the losses, the process according to the present invention comprises, before stage a), a prior stage A) of double salt precipitation of the light rare earth metals (advantageously at least 85% by weight, in particular 90% by weight, with respect to the total weight of the medium rare earth metals present in the initial acidic aqueous sulfate solution), so as to recover an acidic aqueous sulfate solution depleted (or purified) in (or from) light rare earth metals (advantageously, at most, there remains 15% by weight of light rare earth metals, in particular 10% by weight, with respect to the total weight of the light rare earth metals present in the initial acidic aqueous sulfate solution) and comprising phosphates, aluminum, heavy rare earth metals and medium rare earth metals, and possibly iron(II) and titanium. In particular, the stage of double salt precipitation precipitates not only the light rare earth metals but also a portion of the medium rare earth metals (approximately 50% by weight, with respect to the total weight of the medium rare earth metals present in the initial acidic aqueous sulfate solution) and advantageously a minority of heavy rare earth metals (at most 15% by weight, in particular 10% by weight, with respect to the total weight of the heavy rare earth metals present in the initial acidic aqueous sulfate solution). Thus, after this stage, there remains, in the acidic aqueous sulfate solution, at least 50% by weight of the medium rare earth metals, with respect to the total weight of the medium rare earth metals present in the initial acidic aqueous sulfate solution, and advantageously at least 85% by weight of the heavy rare earth metals, in particular 90% by weight, with respect to the total weight of the heavy rare earth metals present in the initial acidic aqueous sulfate solution.
The process for the double salt precipitation of light rare earth metals of stage A) is well known to a person skilled in the art. In particular, it concerns sodium, ammonium or potassium double salt precipitation, advantageously sodium double salt precipitation. In the case of the sodium double salt precipitation, stage A) advantageously takes place by addition of sodium sulfate, which results in the formation of an insoluble rare earth metal compound according the following reaction:
RE₂(SO₄)₃+Na₂SO₄+2H₂O→2NaRE(SO₄)₂.H₂O.
Advantageously, the addition of Na⁺ is carried out in excess with respect to the rare earth metals, so as to obtain a quantitative recovery of the light rare earth metals.
In order to recover these light rare earth metals, the precipitate is separated from the acidic aqueous sulfate solution depleted in light rare earth metals. It is advantageously washed, for example with water and a 5% Na₂SO₄solution.
Advantageously, the temperature of stage A) is between 50° C. and the boiling point, which is in particular 90° C. Advantageously, the duration of stage A) is between 30 minutes and 3 hours. In particular, it is 1 hour.
Advantageously, by virtue of this stage A), the recovery yield of the light rare earth metals is greater than 85%, advantageously greater than or equal to 90%, of the medium rare earth metals is greater than 50% and of the heavy rare earth metals is greater than 10%.
In an additional embodiment of the process according to the present invention, the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, titanium and iron(II), additionally comprises iron(III). In particular, the content of iron(III) is less than or equal to 20 g/l, advantageously between 5 and 20 g/l, very advantageously between 10 and 20 g/l. The presence of the iron(III) promotes the precipitation of phosphates in the form of ferric iron phosphates (FePO₄) during neutralization of the solution in stage a) of the process according to the present invention. As the amount of phosphates is deficient with respect to Al and Fe(III) (molar ratio (Al+Fe(III))/P>1 since molar ratio Al/P>1), the ferric iron also precipitates in other forms than that of phosphate, in particular by precipitation of ferric iron hydroxides, during stage a). However, such a precipitation has a tendency to entrain other elements, such as rare earth metals, in the precipitate. In addition, in order to precipitate all the ferric iron, it is necessary to add an additional amount of base in stage a), which results in the formation of additional gypsum when a base such as a basic calcium compound is used. It is thus advantageous, in order to improve the recovery yield of the heavy rare earth metals, of the medium rare earth metals and even of the light rare earth metals, when they are present, to add, to the process according to the present invention, a stage B), before stage a) and after the optional stage A), of reduction of the iron(III) to give iron(II), advantageously by addition of Fe(0) (for example in the form of iron powder), of SO₂or of another reducing agent. Advantageously, this stage makes it possible to obtain a content of ferric iron (Fe(III))<1 g/l in the acidic aqueous sulfate solution obtained comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, titanium and iron(II).
Advantageously, by virtue of stages A) and B), the recovery yield of the medium rare earth metals is >80%, advantageously greater than or equal to 85%.
In a final embodiment of the process according to the present invention, before stage a), the molar ratio Al/P of the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, iron(II), iron(III) and titanium, is <1. This means that the phosphates are in excess with respect to the aluminum. In this case, in order to be able to carry out the process according to the present invention, it is necessary to add a stage C), after the optional stages A) and B) and before stage a), of doping of the solution with aluminum, so as to obtain a molar ratio Al/P>1 which makes it possible to carry out stage a) of the process according to the present invention while minimizing the losses of heavy rare earth metals and of possible medium rare earth metals by precipitation in the form of phosphates.

A better understanding of the present invention will be obtained in the light of the description of the drawings and of the examples which follow.

FIG. 1 represents the precipitation yield (%) of the rare earth metals in the form of sodium and rare earth metal double sulfate salts as a function of the type of rare earth metal, obtained under the conditions of comparative example 1.

FIG. 2 represents the precipitation yield (%) of the aluminum or Gd (medium rare earth metal) phosphates as a function of the pH, obtained under the conditions of comparative example 2.

FIG. 3 represents the precipitation yield (%) of the light, medium and heavy rare earth metal (La, Gd and Y) phosphates as a function of the pH, obtained under the conditions of comparative example 2.

FIG. 4 represents the precipitation yield (%) of medium and heavy rare earth metals and of the aluminum in the form of phosphates, obtained under the conditions of example 1 during stage a) of the process according to the present invention.

FIG. 5 represents the precipitation yield (%) of the medium and heavy rare earth metals in the form of phosphates, obtained under the conditions of example 2 during stage d) of the process according to the present invention when the molar ratio PO₄/REs=P/REs=40 or PO₄/REs=120.

FIG. 6 represents the diagram of the process according to the present invention as used in example 3 (stages a), b), c), d), e) and f) according to the present invention).

FIG. 7 represents the recovery yield (%) of the light, medium and heavy rare earth metals, obtained under the conditions of example 3.

FIG. 8 represents the diagram of the process according to the present invention as used in example 4 (stages A), a), b), c), d), e) and f) according to the present invention) (DS=rare earth metal and sodium sulfate double salts, MRE/HRE=Medium Rare Earth Metal/Heavy Rare Earth Metal).

FIG. 9 represents the diagram of the process according to the present invention as used in example 5 (stages A), B), a), b), c), d), e) and f) according to the present invention) (DS=rare earth metal and sodium sulfate double salts, MRE/HRE=Medium Rare Earth Metal/Heavy Rare Earth Metal).

FIG. 10 represents the precipitation yield (%) of yttrium in the form of phosphates (that is to say, the losses of yttrium) and the residual concentration of aluminum (in g/l) in the solution as a function of the content of Fe(III) in g/l and of the amount of base Ca(OH)₂added during the neutralization stage a) according to the present invention under the following conditions: 70° C., 2 hours (example 5).

FIG. 11 represents the precipitation yield (%) of the rare earth metals Ce, Gd and Y and of Fe during stage d) of the process according to the present invention at a temperature of 100° C. for a period of time of 1 hour as a function of the molar ratio PO₄/REs (example 6).

FIG. 12 represents the recovery yield (%) of the rare earth metals at each stage, obtained by using the process according to the present invention under the conditions of the example 6.

COMPARATIVE EXAMPLE 1

Addition of Sodium in Order to Precipitate the Rare Earth Metals

This reaction is well known in a sulfate medium. Addition of sodium sulfate results in the formation of an insoluble rare earth metal compound according to the following reaction:
RE₂(SO₄)₃+Na₂SO₄+H₂O→2NaRE(SO₄)₂.H₂O.
The optimized parameters of this reaction which give the best yields are as follows:

- T=90° C.
- [Na₃₀]added=5 g/l
- Residence time: 1 h
- Washing of the solid: Water/5% Na₂SO₄

The solution (obtained by acid leaching in a sulfate medium of pyrochlore ore) on which this stage will be carried out exhibits the following composition:

- Fe: 50 to 70 g/l, such as Fe(III) at ˜10-20 g/l
- Al: 8 to 14 g/l
- P: 4 to 6 g/l
- Mn: 5 to 7 g/l
- REs: 1 to 3 g/l
- Th: 0.1 to 0.3 g/l
- SO₄: 250 to 300 g/l

Under these conditions, the recovery yields (%) of the light, medium and heavy rare earth metals are presented in FIG. 1.
Conclusion: the light rare earth metal double salts are insoluble, which makes it possible to recover 90% of them.
Disadvantage: the solubility of the rare earth metal double salts decreases with the increase in the atomic number of the element, resulting in a recovery of 50% of the medium rare earth metals and only 10% of the heavy rare earth metals for the Na contents under consideration.

COMPARATIVE EXAMPLE 2

Use of the Phosphates Present in Solution to Precipitate Rare Earth Metal Phosphates

The pyrochlore ore comprises a source of phosphates (originating in particular from the apatite): consequently, during the attack of sulfuric acid on the pyrochlore ore, all of these phosphates present are attacked and are reencountered in solution. Thus, a typical solution on which the process according to the present invention has to be carried out comprises ˜15 g/l of phosphates (PO₄ ³⁻) for ˜270 g/l of sulfates (SO₄ ²⁻).
One of the main rare earth metal ore sources is monazite, a rare earth metal phosphate (REPO₄). This ore is attacked by the sulfate route and the recovery of the rare earth metals takes place in a simple way:

- The solution obtained after attack essentially contains rare earth metals and thorium sulfates/phosphates (˜30 g/l of REs for ˜6 g/l of Th).
- Neutralization with an ammonia-type base makes it possible to increase the pH of the solution to 1.5-2, within a range where the rare earth metals phosphates are insoluble. A rare earth metal phosphate is then obtained with a good purity.
- In some cases, the thorium can be separated from the rare earth metals by a double neutralization: precipitation of ThPO₄before pH 1.5, and then precipitation of REPO₄at pH 2.

Following this idea, we carried out neutralization tests on the solution obtained by acid leaching in a sulfate medium of pyrochlore ore exhibiting the following composition:

- Fe: 50 to 70 g/l, such as Fe(III) at ˜10-20 g/l
- Al: 8 to 14 g/l
- P: 4 to 6 g/l
- Mn: 5 to 7 g/l
- REs: 1 to 3 g/l
- Th: 0.1 to 0.3 g/l
- SO₄: 250 to 300 g/l
  for the purpose of determining a pH range where the rare earth metal phosphates precipitate with a good selectivity with regard to the other metals present.

The operating conditions are as follows:

- Temperature: 70° C.,
- Residence time: 3 hours,
- Base added: NH₄OH 30%,
- Washing of the solid obtained with water.

The precipitation yields obtained for Gd from the solution for which the concentration is shown above as a function of the pH are represented in FIG. 2.
Given the concentration of salts, the precipitation pH curves are displaced toward the right: thus, the light rare earth metals begin to precipitate in the phosphate form at pH 2.5 and the heavy rare earth metals from pH 3. This process exhibits two disadvantages:

- The light rare earth metals precipitate with good yields but at the same time as all of the aluminum, which gives a very mediocre purity of the concentrate (molar ratio n(Al)/n(REs) ˜26);
- The medium and heavy rare earth metals precipitate only to 30%.

This low precipitation yield is not consistent with the low solubility product of the rare earth metals phosphates: this precipitation yield is understood better if the amount of phosphates present in solution during neutralization at a higher pH is monitored, as presented in FIG. 3.
Thus, from pH 3, all the phosphates present in solution are precipitated, in the form of aluminum, thorium and light rare earth metal phosphates. Consequently, phosphates no longer remain available at pH 3 to complete the precipitation of the medium and light rare earth metal phosphates. The precipitation yield of the MRE/HREPO₄compounds thus stagnates at 30% at higher pH.
Conclusion: Possibility of precipitating all of the light rare earth metals.
Disadvantages: the purity of the precipitate is mediocre as all the aluminum precipitates in the same pH range as the light rare earth metals. There are not enough phosphates available in solution to precipitate all the medium and heavy rare earth metals.

COMPARATIVE EXAMPLE 3

Doping of the Solution with Phosphates in Order to Promote the Precipitation of the Medium and Heavy Rare Earth Metals

The precipitation conditions of comparative example 2 are repeated, with furthermore the addition (approximately 3 g/l more) of phosphates (in the form of Na₃PO₄) in order to study the influence on the precipitation yield of the medium and heavy rare earth metals.
This addition of phosphates has no impact: the amount of P present is in such deficiency that, as the aluminum phosphate precipitates before the medium and heavy rare earth metals, the phosphate added is expelled in the form of aluminum phosphate.
Specifically, the molar ratios: [n(Al)/n(P)]_{solution comparative example 2}˜2 and in this instance [n(Al)/n(P)]_{solution comparative example 3}˜1.2.
Thus, even by virtually doubling the amount of phosphates, the latter serve above all to precipitate the aluminum in the phosphate form and there is no effect on the precipitation yield of the heavy rare earth metals.
Conclusion: the doping with phosphate has no effect on the precipitation yield of the rare earth metals.
Disadvantage: AlPO₄precipitates before the medium and heavy rare earth metal phosphates and the aluminum is in marked excess with respect to the phosphates; thus, the least addition of phosphate promotes the precipitation of the aluminum phosphate.

EXAMPLE 1

Implementation of Stage A) of the Process According to the Present Invention

We have seen that the amount of phosphates present in solution is too low (in particular with respect to the aluminum) to be able to recover MRE/HRE phosphates, even by doping the solution beforehand with phosphate. Thus, by operating the other way round, we can try to expel the phosphates and the aluminum in a first step in order to obtain a solution containing the medium and heavy rare earth metals which is purified from or depleted in Al and P.
Operating Conditions
The starting solutions used for the precipitation of the aluminum phosphate have approximately the composition shown in the following table 1:

TABLE 1

Fe	52	g/l
Al	7.7	g/l
P	4	g/l
Al/P (molar)	2.2
Mn	3	g/l
Ca	0.2	g/l
S	72.1	g/l
La	71	mg/l
Ce	130	mg/l
Pr	16.7	mg/l
Nd
70	mg/l
Sm	18	mg/l
Eu	7.5	mg/l
Gd	22	mg/l
Tb	5.1	mg/l
Dy	19	mg/l
Ho	4.3	mg/l
Er	11	mg/l
Tm	<0.5	mg/l
Yb	6.2	mg/l
Lu	<0.5	mg/l
Y	76	mg/l
Sc	16	mg/l
U	<0.5	mg/l
Th	97	mg/l

The conditions used are as follows:

- Addition of Ca(OH)₂in the form of 200 g/l limewater
- Neutralization of the solution at pH 3.5
- Residence time: 6 hours
- Temperature: 70° C.
- Washing of the solid obtained by repulping in water

Results
The precipitation yields of the moderate and heavy rare earth metals and of the aluminum in the form of phosphates are represented in FIG. 4. It is noticed that the precipitation of the aluminum is quantitative, whereas the precipitation of the medium and heavy rare earth metals is limited to 20-40% approximately. This stage makes it possible to obtain a solution resulting from the attack on the pyrochlore ore containing the moderate and heavy rare earth metals which is purified (or depleted) from (or in) aluminum and phosphates. It thus makes it possible to purify (or deplete) the solution from (or in) aluminum and phosphate while limiting the the coprecipitation of the rare earth metals. The presence in solution of phosphates resulting from the matrix of the ore has thus been used to precipitate the aluminum by neutralization in the form of aluminum phosphate, the remainder of the aluminum precipitating in the form of aluminum hydroxide, these precipitations being selective with regard to the rare earth metals.
This will make it possible to test several routes for the recovery of the medium and heavy rare earth metals which have failed to date due to the presence of aluminum.
This reaction is selective because the following conditions are combined:

- The phosphates are in deficiency with respect to the aluminum: the molar ratio Al/P is greater than 1;
- The medium has been concentrated in sulfates in order to promote the complexing of the rare earth metal sulfates. Specifically, the medium and heavy rare earth metal phosphates precipitate at a slightly higher pH and are thus lost to a lesser extent with AlPO₄.

Under these conditions, the precipitation of rare earth metals phosphates is limited. From an economic viewpoint, the addition of a base of lime (Ca(OH)₂) or limestone (CaCO₃) type makes it possible to obtain a satisfactorily profitable process

EXAMPLE 2

Implementation of Stage D) of the Process According to the Present Invention

The objective is to selectively and quantitatively precipitate the rare earth metals present in low concentration in an aqueous sulfate solution containing virtually no more aluminum or phosphates.
As the rare earth metal phosphates are very sparingly soluble, the addition of phosphates is carried out in order to promote their precipitation.
The composition of the solution for the precipitation tests on the rare earth metals is shown in table 2 below.

TABLE 2

La	36	mg/l
Ce	49	mg/l
Pr	8	mg/l
Nd	26	mg/l
Sm
10	mg/l
Eu	4.4	mg/l
Gd	14	mg/l
Tb	3.3	mg/l
Dy	13	mg/l
Ho	3	mg/l
Er	8	mg/l
Tm	<0.5	mg/l
Yb
4	mg/l
Lu	<0.5	mg/l
Y	53	mg/l
Fe	48.3	g/l
Al	0.16	g/l
P	15	mg/l
Mn	2.6	g/l
Ca	0.9	g/l
Sr	5.2	mg/l
Ti	4.5	mg/l
Zr	5.0	mg/l
S	49.7	g/l
U	<0.5	mg/l
Th	<4	mg/l
Sc	<1	mg/l

The precipitation of rare earth metal phosphates was carried out by addition of phosphates in the form of Na₃PO₄.
The conditions used for the precipitation are as follows:

- Temperature: 70° C.
- Residence time: 2 hours
- Addition of Na₃PO₄.10H₂O, such that:
  - Test 1: [Na⁺]_added=5 g/l, i.e. a molar ratio PO₄/REs=P/REs=40 mol/mol
  - Test 2: [Na⁺]_added=15 g/l, i.e. a molar ratio PO₄/REs=P/REs=120 mol/mol
- Washing of the solid by repulping in water at ambient temperature

Results
The precipitation yield of the rare earth metals are given in FIG. 5. The precipitation yields of the thulium and lutetium could not be calculated as these elements exhibited concentrations below the detection limits.
The precipitation of the light, medium and heavy rare earth metals is quantitative (approximately 90%, with the exception of praseodymium at 70%). The stoichiometry does not exert a significant effect on the precipitation yield, which allows it to be supposed that the amount of reactants to be added may be reduced in the future (such that the molar ratio P/REs=PO₄/REs is less than 40).

EXAMPLE 3

Implementation of Stages A), B), C), D), E) and F) of the Process According to the Present Invention

Operating Conditions
The initial solution for recovering the rare earth metals is shown in table 3 below.

TABLE 3

Fe	52	g/l
Al	7.7	g/l
P	4	g/l
Al/P (molar)	2.1
Mn	3	g/l
Ca	0.2	g/l
S	72.1	g/l
La	71	mg/l
Ce	130	mg/l
Pr	16.7	mg/l
Nd
70	mg/l
Sm	18	mg/l
Eu	7.5	mg/l
Gd	22	mg/l
Tb	5.1	mg/l
Dy	19	mg/l
Ho	4.3	mg/l
Er	11	mg/l
Tm	<0.5	mg/l
Yb	6.2	mg/l
Lu	<0.5	mg/l
Y	76	mg/l
Sc	16	mg/l
U	<0.5	mg/l
Th	97	mg/l

The conditions used for the precipitation of the aluminum phosphate (stage a) of the process) are as follows:

The conditions used for the precipitation of the rare earth metals phosphates (stage d) of the process) are as follows:

- Temperature: 70° C.
- Residence time: 2 hours
- Addition of Na₃PO₄.10H₂O, such that: [Na⁺]_added=5 g/l, i.e. a molar ratio PO₄/REs=P/REs=40
- Washing of the solid by repulping in water at ambient temperature.

The diagram of the process used is represented in FIG. 6.
Results
Subsequent to this sequence of stages, the recovery yield of the rare earth metals for the scheme of the process provided is represented in FIG. 7. The precipitation yields for the thulium and lutetium could not be calculated as these elements exhibited concentrations below the detection limits.
The process makes it possible to recover the rare earth metals in the phosphate form with very good yields from a solution initially containing large amounts of iron, aluminum and phosphorus.
The loss of rare earth metals during the neutralization can be reduced by optimization of the conditions for precipitation of AlPO₄. The recovery yields of the light rare earth metals vary between 50 and 60% and the medium and heavy rare earth metals are recovered with a yield of 65 to 75%.
The precipitation of the rare earth metals in the form of phosphates is thus:

- limited during the neutralization of AlPO₄(30 to 40% approximately of medium and heavy rare earth metals entrained);
- quantitative during the addition of sodium phosphate (precipitation yields of approximately 90%).

EXAMPLE 4

Precipitation of the Light Rare Earth Metals in the Form of Double Salts+Expelling the Phosphates by Precipitation of the Aluminum+Doping with Phosphates in Order to Recover the Unprecipitated Medium and Heavy Rare Earth Metals (Implementation of the Process According to the Present Invention: Stages A), a), b), c), d), e) and f))

It is not possible to recover the light rare earth metals in the phosphate form as AlPO₄precipitates at the same time during stage a). A precipitation of light rare earth metal double salts (according to comparative example 1) is thus carried out as first stage. Subsequently, we can expel the phosphates and the aluminum in a first step in order to obtain a solution containing the medium and heavy rare earth metals, purified from or depleted in Al and P. Doping at that moment with phosphates should make it possible to precipitate the medium and heavy rare earth metal phosphates.
We thus carry out the stages as shown in the scheme of the process of FIG. 8 starting from this solution and monitor the RE yield during each stage. The initial acidic aqueous sulfate solution has the following composition:

The operating conditions are the same as in example 3.
The stage by stage balance shows that:

- 90% of the LREs, 50% of the MREs and 10% of the HREs precipitate during the first stage of formation of the double salts;
- virtually all of the phosphorus (P) and aluminum (Al) precipitates by neutralization at pH 3.5, as well as 40-50% of the MREs/HREs;
- the remaining rare earth metals in solution precipitate with an excess of phosphate added (molar ratio n(PO₄)/n(REs)˜100 with a yield of 100%).

Conclusion: it is possible to recover the MREs/HREs after removal of the P and Al initially present, by doping the solution in phosphates.
Disadvantage: the RE losses during the stage of precipitation of Al and P are still high and the stoichiometric amount of PO₄with respect to the rare earth metals is high, which impacts the purity of the product.

EXAMPLE 5

Optimization of the Loss of the Rare Earth Metals During the Stage of Al/P Precipitation (Stage D)) by Decreasing the Concentration of Fe(III): (Implementation of the Process According to the Present Invention: Stages A), B), a), b), c), d), e) and f))

The presence of ferric iron (Fe(III)) originating from the upstream solution (acidic aqueous sulfate solution comprising the rare earth metals) results in two phenomena:

- The presence of Fe(III) promotes the precipitation of ferric iron phosphate (FePO₄) during the neutralization of the solution at pH 3.5 and the precipitation of gypsum and thus the entrainment of the rare earth metals in the gypsum;
- as the amount of phosphates is in marked deficit with respect to Al, Fe(III), Th and LREs, the ferric iron also precipitates in other forms than the phosphate form, in particular by precipitation of ferric iron hydroxides, which are known to soak up numerous elements in solution.

The idea is thus to neutralize the solution under the same conditions in order to precipitate the phosphorus and the aluminum but while studying the impact of the reduction of the solution: by addition of a reducing agent (Fe(0) or SO₂, for example), the Fe(III) is reduced to different concentrations before the neutralization reaction in order to study its effect. Use will thus be made of the scheme of the process illustrated in FIG. 9.
The precipitation yield (=loss yield) of the rare earth metals (yttrium Y in %) and the residual concentration of aluminum in the solution after precipitation during this neutralization stage as a function of the initial concentration of Fe(III) in the solution (0, 5 and 10 g/l) and of the amount of base added in g/l are combined in the graph illustrated in FIG. 10. The reaction conditions are as follows: temperature 70° C.; reaction time 2 hours; base used: Ca(OH)₂; composition of the initial aqueous sulfate solution: Fe: 50 to 70 g/l, such as Fe(III) at ˜10-20 g/l; Al: 8 to 14 g/l; P: 4 to 6 g/l; Mn: 5 to 7 g/l; REs: 1 to 3 g/l; Th: 0.1 to 0.3 g/l; SO₄: 250 to 300 g/l.
Conclusion: The lower the initial concentration of Fe(III), the lower the loss of MREs/HREs. It is thus highly probable that the presence of Fe(III) involves two disadvantages:

an additional amount of base has to be added in order to precipitate the ferric iron, which results in a greater precipitation of gypsum and thus more losses of medium and heavy rare earth metals;
the precipitation of Fe(OH)₃, which can soak up a portion of the rare earth metals.

It is therefore necessary to reduce virtually all of the ferric iron, in particular so that [Fe(III)]<1 g/l, in order to minimize this loss of rare earth metals and thus to increase the yield of the process according to the present invention.

EXAMPLE 6

Optimization of the Amount of Na₃PO₄Added During the Precipitation of the MREs/HREs (Stage d)) of the Process According to the Present Invention

The composition of the typical solution obtained after purification from Al and P is shown in table 4 below. The RE concentrations are capable of varying to +/−20%.

TABLE 4

Fe	40-60	g/l
Al	<200	mg/l
P	<50	mg/l
Mn	2-5	g/l
Ca	~1	g/l
Sr	~10	mg/l
Ti	<100	mg/l
Zr	<100	mg/l
S	40-60	g/l
U	<0.5	mg/l
Th	<4	mg/l
Sc	<1	mg/l
La	36	mg/l
Ce	49	mg/l
Pr	8	mg/l
Nd	26	mg/l
Sm
10	mg/l
Eu	4.4	mg/l
Gd	14	mg/l
Tb	3.3	mg/l
Dy	13	mg/l
Ho	3	mg/l
Er	8	mg/l
Tm	<0.5	mg/l
Yb
4	mg/l
Lu	<0.5	mg/l
Y	53	mg/l

The only element which from now on may present a problem is Fe(II): ferrous iron phosphate is more soluble than MRE/HRE phosphate and has to precipitate at a higher pH. However, the ratio between the two elements does not act in favor of the rare earth metals: n(Fe)=n(Fe(II)) and n(Fe)/n(REs)˜500.
By varying the following parameters: temperature, residence time, PO₄/REs stoichiometric amount (SA), the operating conditions can be optimized in order to decrease the amount of reactions to be added. The first precipitation test have been carried out at an SA of 100, which is totally unacceptable from an economic viewpoint.
The precipitation yield (%) of the rare earth metals (Ce, Gd and Y) and of iron in the aqueous solution as a function of the PO₄/REs SA is monitored, as illustrated in FIG. 11. The operating conditions of stage d) are as follows: temperature 100° C.; residence time: 1 hour, phosphate: Na₃PO₄.
The selectivity of the reaction is excellent: a low SA makes it possible to precipitate all of the rare earth metals with little ferrous iron. This is made possible by virtue of a high temperature (100° C.) and a deliberately short residence time (<1 h) which makes it possible to limit the reoxidation of Fe(II) to give Fe(III) over time and thus to limit the use of the PO₄groups which are present to precipitate an Fe(III) phosphate.
Thus, with an SA of less than 20, it is possible to precipitate all of the rare earth metals in the phosphate form, with only a 5% of yield for the iron. The tests have proved that an SA of 5 was the optimum.
Conclusion: under economically profitable conditions, it is possible to precipitate all of the rare earth metals remaining after the removal of the aluminum and phosphorus, with a relatively good purity (content of rare earth metal phosphate of ˜10% for an SA of 5). The presence of a large amount of ferrous iron is not thus a constraint.


Selectivity of the precipitation of
the rare earth metals	In solution before	In the concentrate
phosphates	precipitation	after precipitation

Fe/REs molar ratio	~500	~6-7

Overall conclusion: with the sequence of stages as illustrated in FIG. 9, the yields (%) obtained at each stage are represented in FIG. 12, i.e. 95% for the light rare earth metals, 85% for the medium rare earth metals and 75% for the heavy rare earth metals. The loss of rare earth metals during the stage of precipitation of Al and P (stage a) of the process according to the present invention) is thus deduced therefrom: 5% for the light rare earth metals, 15% for the medium rare earth metals and 25% for the heavy rare earth metals.

Claims

1. A process for the selective recovery of the rare earth metals from an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, wherein it comprises the following successive stages:

a) neutralization at a pH of between 3 and 4 of an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, the solution having a molar ratio Al/P>1 and a concentration of sulfates>100 g/l, by addition of a base, so as to precipitate the phosphate and the aluminum and the possible titanium,

b) liquid/solid separation between the precipitate formed by the phosphate and the aluminum and the possible titanium and the aqueous sulfate solution,

c) recovery of the aqueous sulfate solution,

d) addition of phosphates to the aqueous sulfate solution obtained in stage c) such that the molar ratio of the solution obtained PO₄/REs>4, so as to precipitate the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates,

e) liquid/solid separation between the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates and the aqueous sulfate solution,

f) recovery of the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates.

2. The process as claimed in claim 1, wherein, in stage a), the base is chosen from MgCO₃and a basic calcium compound.

3. The process as claimed in claim 2, wherein, in stage a), the base is a basic calcium compound.

4. The process as claimed in claims 1, wherein, in stage d), the phosphate is chosen from Na₃PO₄, K₃PO₄, (NH₄)₃PO₄and their mixtures.

5. The process as claimed in claim 1, wherein the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, is the leachate obtained by acid attack on a pyrochlore ore in a sulfate medium.

6. The process as claimed in claim 1, wherein the recovery yield of the heavy rare earth metals is greater than 50%.

7. The process as claimed in claims 1, wherein the acidic aqueous sulfate solution comprising phosphates, aluminum, heavy rare earth metals and medium rare earth metals, and possibly titanium and iron(II), additionally comprises light rare earth metals and in that the process comprises, before stage a), a prior stage A) of double salt precipitation of the light rare earth metals, so as to recover an acidic aqueous sulfate solution depleted in light rare earth metals and comprising phosphates, aluminum, heavy rare earth metals and medium rare earth metals, and possibly iron(II) and titanium.

8. The process as claimed in claims 1, wherein the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, iron(II) and titanium, additionally comprises iron(III) and in that the process comprises, before stage a) and after the optional stage A), a stage B) of reduction of the iron(III) to give iron(II).

9. The process as claimed in claims 1, wherein, before stage a), the molar ratio Al/P of the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, iron(II), titanium and iron(III), is <1 and in that the process comprises, before stage a) and after the optional stages A) and B), a stage C) of doping of the solution with aluminum, so as to obtain a molar ratio Al/P>1.

10. The process as claimed in claim 3, wherein the base is a basic calcium compound chosen from CaCO₃, CaO, Ca(OH)₂and their mixtures.

11. The process as claimed in claim 10, wherein the base is CaCO₃.

12. The process as claimed in claim 4, wherein in stage d), the phosphate is Na₃PO₄.

13. The process as claimed in claim 6, wherein the recovery yield of the heavy rare earth metals is greater than or equal to 60%.

14. The process as claimed in claim 8, wherein the stage B) of reduction of the iron(III) to give iron(II) is carried out by addition of Fe(0) or SO₂.