AU714732B2

AU714732B2 - Process for treating a mixture comprising a rare-earth metal compound and radioactive elements of the radium, thorium and uranium type

Info

Publication number: AU714732B2
Application number: AU27547/97A
Authority: AU
Inventors: Thierry Delloye
Original assignee: Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1996-07-01
Filing date: 1997-06-26
Publication date: 2000-01-13
Anticipated expiration: 2017-06-26
Also published as: AU2754797A; FR2750414B1; FR2750414A1; ZA975669B

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

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Address of Service: Invention Title: Rhone-Poulenc Chimie Thierry DELLOYE SHELSTON WATERS 60 MARGARET STREET SYDNEY NSW 2000 "PROCESS FOR TREATING A MIXTURE COMPRISING A RARE-EARTH METAL COMPOUND AND RADIOACTIVE ELEMENTS OF THE RADIUM, THORIUM AND URANIUM TYPE" The following statement is a full description of this invention, including the best method of performing it known to us:- (File: 19876.00) -la- PROCESS FOR TREATING A MIXTURE COMPRISING A RARE-EARTH METAL COMPOUND AND RADIOACTIVE ELEMENTS OF THE RADIUM, THORIUM AND URANIUM TYPE RHONE-POULENC CHIMIE The present invention relates to a process for treating a mixture comprising a rareearth metal compound and radioactive elements of the radium, thorium and uranium type.

It is known that rare-earth metals are present in the natural state in ores which may contain larger or smaller amounts of radioelements such as radium, thorium or uranium.

Rare-earth metals are extracted from these ores by processes which use successive attacks by acidic or basic solutions in order to obtain the rare-earth metals in the form of "solutions. These solutions are liable to contain the radioelements of the starting 15 minerals. For obvious reasons of safety and respect for the environment, it is necessary to obtain rare-earth metal solutions which are virtually free of these radioelements and which can be used without any subsequent problems which can be used without any subsequent problems.

19876-00DOC -2- According to a first aspect, the invention provides a process for treating a mixture comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: the mixture is placed together with a source of an anion capable of forming an insoluble compound with an alkaline-earth metal; a source of alkaline-earth metal is added to the medium thus obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium; i 10' a base and optionally a source of alkaline-earth metalare added to the resulting medium, whereby a precipitate comprising thorium anduranium is obtained; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained.

According to a second aspect, the invention provides a process for treating a mixture comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: the mixture is placed together with a source of alkaline-earth metal; a source of an anion capable of forming an insoluble compound with an alkaline-earth metal is added to the medium thus obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium; 19876-OO.DOC I X-l^-xl- -3a base and optionally a source of alkaline-earth metal are added to the resulting medium, whereby a precipitate comprising thorium and uranium is obtained; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained.

According to a third aspect, the invention provides a process for treating a mixture in solid form comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following.

steps: the mixture is attacked with an acidic solution; a basi cbmpound is added to the resulting medium, whereby a precipitate comprising..

thorium and uranium is obtained; the precipitate is separated from the liquid medium; a source :of an anion capable of forming an insoluble compound with an alkaline-earth metal and optionally a source of alkaline-earth metal are added to the medium obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained.

According to a fourth aspect, the invention provides a process for treating a mixture in solid form comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: 19876-00.DOC

I

-4a basic compound is added to the mixture in liquid form, whereby a precipitate comprising thorium and uranium is obtained; the precipitate is separated from the liquid medium; a source of an anion capable of forming an insoluble compound with an alkaline-earth metal and optionally a source of alkaline-earth metal are added to the liquid medium obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained.

Unless the context clearly.requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising' and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Other characteristics, details and advantages of the invention will become even more fully apparent on reading the description which follows, as well as :a a a 19876-00.DOC

I

the various concrete, but in no way limiting, examples intended to illustrate it.

The process of the invention applies to the treatment of any mixture comprising at least one rareearth metal compound, radium and at least one element which may be thorium or uranium. Such a mixture may be obtained by attack, with a base such as an alkaline hydroxide or an alkaline carbonate, of an ore comprising at least one rare-earth metal and also containing the abovementioned radioelements. Such an attack gives a solid based on, for example, rare-earth metal hydroxides or carbonates, which also contains radioelements as impurities. Ores which may be mentioned as examples are monazite, xenotime and bastnaesite.

The mixture may also be a liquid solution of a rare-earth metal salt, such as a solution of rareearth metal nitrates or chlorides which comprises the :.abovementioned radioelements as impurities.

20 The process according to the first embodiment and according to the first variant will now be described more specifically.

This process includes a first step in which the mixture is placed together with a source of an anion capable of forming an insoluble compound with an alkaline-earth metal. This source may more particularly be sulphuric acid. When the mixture to be treated is in solid form, this placing-together may be carried out by 6 forming a suspension in acidic medium of the said mixture with the source of anion. As it is common, for their subsequent use, to prepare nitric or hydrochloric solutions of rare-earth metals, it is possible, in order to form such a suspension in acidic medium, to use as source of anion a solution which may also comprise nitric acid or hydrochloric acid, it being possible for this acid to be in a predominant amount relative to the anion capable of forming an insoluble compound and, more particularly, sulphuric acid, so as thus to obtain, after this attack, one or more rareearth metals in nitric or hydrochloric medium.

The abovementioned placing-together with the source of anion generally takes place under conditions such that the reaction medium has free H' ions, more particularly in a concentration of between 0.01 and 1 mol/l.

The temperature at which the placing-together .fl takes place is approximately between 20 and 100°C.

20 The second step of the process consists in adding a source of alkaline-earth metal to the medium "obtained after the placing-together with the source of anion. As alkaline-earth element, barium is generally used.

25 The source of alkaline-earth metal may more particularly be an alkaline-earth metal salt such as a .nitrate, a carbonate, a chloride or alternatively a hydroxide or an oxide.

The addition of the alkaline-earth metal has the effect of causing the precipitation of a compound of the abovementioned anion and of the alkaline-earth metal, for example barium sulphate in the case of the use of sulphuric acid and barium. The precipitate thus obtained also contains the element radium. It may also comprise other metal impurities such as titanium, iron, lead, zirconium or silica, depending on the nature of the ore or of the starting compound.

The pH, temperature and concentration conditions will be chosen so as to obtain as complete a precipitation as possible. Thus, it is preferable to place together the abovementioned mixture, the source of anion and the source of alkaline-earth metal under conditions such that the pH of the medium obtained has a value of not more than 4. More particularly, this pH value may be between 0.5 and 2.5. In addition, it may also be preferable for the abovementioned placing-together to be carried out under conditions 20 such that, in the medium obtained, there is an excess of anion capable of forming an insoluble compound with the alkaline-earth metal.

The term excess is understood to mean that the medium comprises an amount of the abovementioned 25 anion in solution.

In the specific case of sulphuric acid, the S content of free S0 4 2- may be between 10 and 30 g/l, for S rare-earth metal concentrations, expressed as oxide, of

I

8 between 50 and 400 g/l.

The precipitate is separated from the liquid medium by any known means.

In a subsequent step of the process, the liquid medium thus obtained is treated by adding a base and optionally a source of alkaline-earth metal thereto. The aim of adding the base is to adjust the pH of the medium to a value which is sufficiently high to cause the thorium and/or uranium to precipitate selectively with respect to the rare-earth metals. The base may more particularly be sodium hydroxide or aqueous ammonia. In order to improve the precipitation, it is possible to add a source of alkaline-earth metal, in particular in the form of a basic salt such as a carbonate or a hydroxide. The addition of a salt of basic nature makes it possible to decrease the amount of base. The amount of base and, optionally, of source of alkaline-earth metal added is such that the pH of oIo the medium obtained has a value of not more than *0 20 This value may more particularly be between 3.5 and .o e o The addition of the base and, optionally, of O the source of alkaline-earth metal results in the formation of a precipitate comprising thorium and/or te 25 uranium and, possibly, traces of residual radium. The nature of this precipitate may vary depending on the cote type of reactant used. It may thus be a precipitate based on a hydroxide or a carbonate of the radioelements thorium and uranium and also an insoluble compound of an alkaline-earth metal, such as barium sulphate.

The precipitate is separated out by any known means and a solution of rare earth metals freed of radium, thorium and uranium is thus obtained. The degree of removal of radium may reach at least 90 For the other elements, this degree may usually be at least 99 Depending on the elements, in particular for thorium and radium, this degree may be greater than 99.9 The embodiment which has just been described may form the subject of a second variant in which the first two steps mentioned above are reversed. Thus, in a first step, the mixture to be treated is placed together with the source of alkaline-earth metal. When the mixture is in solid form, this placing-together may be carried out by forming a suspension in acidic SI, medium, for example in nitric or hydrochloric medium, 20 of the said mixture with the source of alkaline-earth .metal. In a second stage, the abovementioned source of *a anion is added to the medium obtained above. Everything which has been described above for the first two steps of the first variant also applies for the corresponding 25 steps of the second variant. Lastly, the subsequent steps are identical to the corresponding ones of the first variant.

:first variant.

Lastly, according to a third variant, it is possible to carry out the first two steps simultaneously.

The second embodiment of the invention will now be described.

This embodiment differs from the first essentially in the order of precipitation of the various radioelements. Thus, the thorium and uranium are precipitated first, followed by the radium.

This second embodiment also includes two variants. The first applies to the case where the mixture is in solid form.

In this case, the first step consists in attacking the mixture with an acidic solution. This acidic solution will preferably be a nitric or hydrochloric solution for the reason given above.

In a second step, a basic compound is added to the solution thus obtained. The conditions under which this second step is carried out are similar to those already given for the first embodiment in the 20 description of the step of addition of the base. Thus, the base may more particularly be sodium hydroxide or Sa. aqueous ammonia, and it is possible also to add a source of alkaline-earth metal, in particular in the form of a basic salt such as a carbonate. Lastly, the 25 amount of base and, optionally, of alkaline-earth metal added is such that the pH of the medium obtained preferably has a value of not more than 5. This value may more particularly be between 3.5 and 4.5. A solid I I 11 comprising the thorium and/or uranium is thus precipitated and is of the same nature as the solid described above.

After separation, a source of an anion capable of forming an insoluble compound with an alkaline-earth metal and, optionally, a source of alkaline-earth metal are added to the medium obtained.

A source of alkaline-earth metal will be used in this step if one has not been used in the previous step during the addition of the basic compound.

As in the first embodiment, the source of anion capable of forming an insoluble compound with an alkaline-earth metal may more particularly be sulphuric acid. The alkaline-earth metal may be barium and the source of alkaline-earth metal may be an alkaline-earth metal salt such as a nitrate, a carbonate, a chloride, an oxide or a hydroxide.

Here also, the pH, temperature and concentration conditions will be chosen so as to obtain 20 as complete a precipitation as possible. Thus, the addition of the source of anion and optionally of the .e C source of alkaline-earth metal is preferably carried out so as to obtain a pH for the medium with a value of c,.

not more than 4.5. In addition, it is advantageous to have a medium having an excess of anion, in particular in the same content as that given above.

After this step, a precipitate based on the abovementioned anion and on the alkaline-earth metal, 12 for example barium sulphate, is obtained, which precipitate, as in the first embodiment, also contains the element radium.

After separation, a solution of rare-earth metals freed of radium, thorium and uranium and whose degrees of removal are the same as those given above is obtained.

The second variant of this second embodiment applies to the case where the mixture to be treated is in liquid form. In this case, the basic compound is added directly to the mixture. The rest of the process is identical to that which has been described for the first variant.

Examples will now be given.

In the following examples, the source of rare-earth metals consists of a concentrate of rareearth metal hydroxides, obtained from soda attack on monazite. The composition of this concentrate is as follows: 20 Moisture content: 30 Rare-earth metal hydroxides: 56.4 i.e.

48.5 expressed as rare-earth metal oxides or RO Thorium hydroxide: 5.6 i.e. a ThO 2

/RO

ratio 10.2 25 Uranyl hydroxide: 0.3 i.e. a U 3 0O/RO ratio 0.62 Other impurities: (ZrO 2 TiO,, Fe(OH),, SiO 2 CaO) 7.5 13 Ra 228: 365000 Bq/kg RO Ra 226: 52500 Bq/kg RO EXAMPLE 1 1000 g of crude hydroxide are transferred into a reactor. This hydroxide is placed in suspension in 1100 ml of water.

560 ml of 69 of nitric acid and 30 ml of 98 of sulphuric acid are added to this suspension.

This mixture is kept stirring for one hour at 80C. The concentration of free H* ions is 0.03 mol/l.

g of barium nitrate are then added to this suspension. The mixture is stirred for one hour.

The suspension is filtered. The solid residue is washed with 600 ml of water and the washing solution is incorporated into the filtered mother liquor.

6.4 g of sodium hydroxide are added to the resulting solution of rare-earth metal nitrate, so as to bring the pH of the suspension to about 4.

The solution is maintained at 80 0 C for one 20 hour.

The suspension obtained is filtered.

The analysis of the solution is as follows: RO: 170 g/1

S

UThO/RO: 20 ppm, precipitation yield 99.98 U304/RO: 500 ppm, precipitation yield 91.9 Ra 228/RO: 1000 Bq/kg, precipitation yield 99.7 Ra 226/RO: 140 Bq/kg, precipitation yield 99.7 14 EXAMPLE 2 1000 g of crude hydroxide are transferred into a reactor. This hydroxide is placed in suspension in 1100 ml of water.

This mixture is kept stirring for one hour at 80 0 C. The concentration of H' ions is 0.03 mol/l.

g of barium nitrate are then added to this suspension. The mixture is stirred for one hour. The suspension is filtered. The solid residue is washed with 600 ml of water and the washing solution is incorporated into the filtered mother liquor.

g of barium nitrate and 6.4 g of sodium hydroxide are added to the resulting solution of rareearth metal nitrate, so as to bring the pH of the suspension to about 4.

The solution is maintained at 80 0 C for one hour.

20 The suspension obtained is filtered.

The analysis of the solution is as follows: RO: 170 g/l ThO 2 /RO: 10 ppm, i.e. precipitation yield 99.99

S**

U30s/RO: 100 ppm, i.e. precipitation yield 98.4 25 Ra 228/RO: 100 Bq/kg, i.e. precipitation yield 99.97 Ra 226/RO: 14 Bq/kg, i.e. precipitation yield 99.97 EXAMPLE 3 1000 g of crude hydroxide are transferred into a reactor. This hydroxide is placed in suspension in 1100 ml of water.

565 ml of 69 of nitric acid and 30 ml of 98 of sulphuric acid are added to this suspension.

This mixture is kept stirring for one hour at 80 0 C. The pH of the solution is in the region of 0.8.

23 g of barium carbonate are then added to this suspension. The mixture is stirred for one hour.

42 g of barium carbonate and 0.7 g of sodium hydroxide are added to the resulting solution of rareearth metal nitrate, so as to bring the pH of the suspension to about 4.

The solution is maintained at 90 0 C for one hour.

o 20 The suspension obtained is filtered.

The analysis of the solution is as follows: RO: 170 g/l Tho,/RO: 5 ppm, i.e. precipitation yield greater than 99.99 25 U 3 0 8 /RO: 80 ppm, i.e. precipitation yield 98.7 Ra 228/RO: 50 Bq/kg, i.e. precipitation yield 99.9 6* o Ra 226/RO: 7 Bq/kg, i.e. precipitation yield 99.99

Claims

1. Process for treating a mixture comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: the mixture is placed together with a source of an anion capable of forming an insoluble compound with an alkaline-earth metal; a source of alkaline-earth metal is added to the medium thus obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; '10 the precipitate is separated from the liquid medium; 'a base and optionally a source of alkaline-earth metal are added to .the resulting medium, whereby a precipitate comprising thorium and uranium is obtained; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained.

2. Process for treating a mixture comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: the mixture is placed together with a source of alkaline-earth metal; a source of an anion capable of forming an insoluble compound with an alkaline-earth metal is added to the medium thus obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium;

19876-00.DOC -17- a base and optionally a source of alkaline-earth metal are added to the resulting medium, whereby a precipitate comprising thorium and uranium is obtained; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained.

3. Process for treating a mixture in solid form comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: the mixture is attacked with an acidic solution; a basic compound is added to the resulting medium, whereby a precipitate comprising thorium and uraniumis obtained; the precipitate is separated from the liquid medium; a source of an anion capable of forming an insoluble compound with an alkaline-earth metal and optionally a source of alkaline-earth metal are added to the medium obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium, whereby a solution freed of *:thorium, uranium and radium is obtained.

4. Process for treating a mixture in solid form comprising at least one rare-earth metal compound, radium and at least one element from the group thorium and uranium, said process comprising the following steps: a basic compound is added to the mixture in liquid form, whereby a precipitate comprising thorium and uranium is obtained; the precipitate is separated from the liquid medium; 19876-00.DOC -18- a source of an anion capable of forming an insoluble compound with an alkaline-earth metal and optionally a source of alkaline-earth metal are added to the liquid medium obtained, whereby a compound of the abovementioned anion and of the alkaline-earth metal, containing the element radium, is precipitated; the precipitate is separated from the liquid medium, whereby a solution freed of thorium, uranium and radium is obtained. Process according to one of the preceding claims, .wherein the source of an anion capable of forming an insoluble compound with an alkaline-earth metal is a sulfuric acid solution. 10 6. Process according to one of the preceding& 1 laims, wherein the alkaline-earth metal is barium. 0.* Process according to one of the preceding claims,, wherein the source of alkaline- earth metal is a carbonate or a nitrate. 8. Process according to any one of claims 1, 2 and 5 to 7, wherein the abovementioned mixture is placed together with the abovementioned source of anion :and the source of alkaline-earth metal is added under conditions such that the pH of the medium obtained has a value of not more than 4. 9. Process according to any one of the preceding claims, wherein the abovementioned source of anion is added under conditions such that there is an excess of anion in the medium obtained. Process according to any one of claims 1, 2 and 5 to 9, wherein the base and optionally the source of alkaline-earth metal are added under conditions such that the pH 19876-00DOC -19- of the medium obtained has a value of not more than 5, more particularly between and 11. Process according to any one of claims 3 to 7, wherein a basic compound is added to the medium resulting from attack of the abovementioned mixtures by an acidic solution, or to the mixture in liquid form, under conditions such that the pH of the medium obtained has a value of not more than 5, more particularly between 3.5 and 12. Process according to any one of claims 3 to 7, wherein a source of an anion capable of forming an insoluble compound with an alkaline-earth metal and optionally the source of alkaline-earth metal are added under conditions such that the pH of the 'fieditim obtained has a value of not more than '',L'Process according to any one of the preceding claims, wherein a mixture is treated which is obtained from attacking an ore with a base, this ore comprising at least one rare S* earth metal, radium and at least one element from the group thorium and uranium, or a mixture which is a liquid solution of a rare-earth metal salt comprising radium and at least one element from the group thorium and uranium. Process according to claim 13, wherein the ore is monazite, xenotime or bastnaesite. 15. A process according to any one of the preceding claims substantially as herein described with reference to any one of the examples. DATED this 2nd Day of August 1999 RHONE-POULENC CHIMIE Attorney: PAUL G. HARRISON Fellow Institute of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS 19876-00.DOC