FR2750414A1 - Treatment of mixture comprising rare earth, radium , thorium and uranium - Google Patents

Abstract

A process for the treatment of a mixture comprising at least one rare earth compound, radium and at least one element from thorium and uranium, comprises: a) the mixture is contacted with an anion source capable of forming an insoluble compound with an alkaline earth; b) an alkaline earth source is added to precipitate a compound of the anion, containing radium; c) the precipitate is separated from the liquid medium; d) a base is added to the resulting liquid medium, and possibly an alkaline earth source, to produce a precipitate containing thorium and uranium; and e) the precipitate is filtered off and a purifies solution free from th radio-elements is obtained.

Description

PROCESS FOR TREATING A MIXTURE COMPRISING A
COMPOUND OF RARE EARTH AND ELEMENTS
RADIOACTIVES OF THE RADIUM TYPE. THORIUM AND URANIUM
RHONE-POULENC CHEMISTRY
The present invention relates to a process for treating a mixture comprising a rare earth compound and radioactive elements of the radium, thorium and uranium type.

 We know that rare earths occur naturally in minerals that can contain more or less large quantities of radioelements such as radium, thorium or uranium.

 The rare earths are extracted from these ores by processes which use successive attacks with acid or basic solutions to obtain the rare earths in the form of solutions. These solutions are likely to contain the radioelements of the starting minerals. For obvious reasons of safety and respect for the environment, it is necessary to obtain rare earth solutions which are practically free of these radioelements and which can be used without problem subsequently.

 The object of the invention is therefore the development of a process making it possible to obtain such solutions purified in radioelements.

For this purpose, and according to a first embodiment, the method of the invention for the treatment of a mixture comprising at least one rare earth compound, radium and at least one element of the thorium and uranium group is characterized in that that it includes the following stages:
- The mixture is brought into contact with a source of an anion capable of forming an insoluble compound with an alkaline earth;
- A source of alkaline earth is added to the medium thus obtained, whereby a compound of the anion and the no earth alkali, containing the element radium, is precipitated;
- The precipitate is separated from the liquid medium;
- A base and possibly a source of alkaline earth is added to the resulting medium, whereby a precipitate is obtained comprising thorium and uranium;
- The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained.

According to another variant of this first embodiment, the method of the invention is characterized in that it comprises the following steps:
- the mixture is brought into contact with a source of alkaline earth;
- Is added to the medium thus obtained a source of an anion capable of forming an insoluble compound with an alkaline earth, whereby a compound of the above anion and the alkaline earth, containing the element radium is precipitated ;
- The precipitate is separated from the liquid medium;
- A base and possibly a source of alkaline earth is added to the resulting medium, whereby a precipitate is obtained comprising thorium and uranium;
- The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained.

According to a second embodiment, the method of the invention for the treatment of a mixture in solid form comprising at least one rare earth compound, radium and at least one element of the thorium and uranium group is characterized in that it includes the following stages:
- the mixture is attacked with an acid solution;
- A basic compound is added to the resulting medium, whereby a precipitate is obtained comprising thorium and uranium;
- The precipitate is separated from the liquid medium;
a source of an anion capable of forming an insoluble compound with an alkaline earth and optionally a source of alkaline earth is added to the medium obtained, whereby a compound of the anion and of the alkaline earth is precipitated, containing the element radium;
- The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained.

According to another variant of this second embodiment, the method of the invention for the treatment of a mixture in liquid form comprising at least one rare earth compound, radium and at least one element of the thorium and uranium group, is characterized in that it comprises the following stages:
- A basic compound is added to the mixture in liquid form, whereby a precipitate is obtained comprising thorium and uranium;
- The precipitate is separated from the liquid medium;
- Adding to the liquid medium obtained a source of an anion capable of forming an insoluble compound with an alkaline earth and optionally a source of alkaline earth, whereby a compound of the aforementioned an ion and the alkali is precipitated - earthy, containing the element radium;
- The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained.

 Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as various concrete but nonlimiting examples intended to illustrate it.

 The method of the invention applies to the treatment of any mixture comprising at least one rare earth compound, radium and at least one element which may be thorium or uranium. Such a mixture can be obtained by attack with a base such as an alkali hydroxide or an alkali carbonate, of an ore comprising at least one rare earth and further containing the above-mentioned radioelements. Such an attack gives a solid based, for example, on hydroxides or rare earth carbonates, which additionally contains radioactive elements as impurities. By way of example of an ore, mention may be made of monazite, xenotime, bastnaesite.

 The mixture can also be a liquid solution of a rare earth salt such as a solution of nitrates or rare earth chlorides and which comprises as impurities the above-mentioned radioelements.

 The method according to the first embodiment and according to the first variant will now be described more particularly.

 This process comprises a first step in which the mixture is brought into contact with a source of an anion capable of forming an insoluble compound with an alkaline earth metal. This source can be more particularly sulfuric acid.

When the mixture to be treated is in solid form, this contacting can be done by forming a suspension in an acid medium of said mixture with the source of anion.

As is customary, for their subsequent use, to prepare nitric or hydrochloric solutions of rare earths, it is possible, to form such a suspension in an acid medium, to use as source of anion a solution which may further comprise nitric or hydrochloric acid, this acid being able to be in majority quantity compared to the anion likely to form an insoluble compound and, more particularly sulfuric acid, so as to thus obtain, at the end of this attack, a or rare earths in nitric or hydrochloric medium.

 The above-mentioned bringing into contact with the anion source is generally carried out 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 contact takes place is between approximately 20 and 100 "C.

 The second step of the process consists in adding a source of alkaline earth to the medium obtained following contact with the source of anion. As an alkaline earth element, barium is generally used.

 The source of alkaline earth can more particularly be an alkaline earth salt such as a nitrate, a carbonate, a chloride or even a hydroxide or an oxide.

The addition of alkaline earth has the effect of precipitating a compound of the aforementioned anion and the alkaline earth, for example barium sulfate in the case of
The use of sulfuric acid and barium. The precipitate thus obtained also contains the element radium. It can also include other metallic impurities such as titanium, iron, lead, zirconium or silica depending on the nature of the ore or the starting compound.

 The pH, temperature or concentration conditions will be chosen so as to obtain the most complete precipitation possible. Thus, it is preferable to bring the above-mentioned mixture into contact, 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 at most 4.

More particularly, this pH value can be between 0.5 and 2.5. In addition, it may also be preferable to carry out the abovementioned contact under conditions such that there is, in the medium obtained, an excess of anion capable of forming an insoluble compound with the alkaline earth metal.

 By excess is meant the fact that the medium comprises in solution a quantity of the abovementioned an ion.

 In the particular case of sulfuric acid, the content of free SO.sub.40 can be between 10 and 30 g / l, for concentrations of rare earths expressed in oxide of 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 possibly a source of alkaline earth. The purpose of adding the base is to adjust the pH of the medium to a value high enough to precipitate thorium and / or uranium selectively with respect to rare earths. The base can more particularly be soda or ammonia. It is possible, to improve precipitation, to add a source of alkaline earth, in particular in the form of a basic salt such as a carbonate or a hydroxide. The addition of a salt with a basic character makes it possible to reduce the amount of base. The added amount of base and, optionally, of alkaline-earth source is such that the pH of the medium obtained has a value of at most 5. This value can more particularly be between 3.5 and 4.5.

 The addition of the base and, optionally, of the alkaline-earth source leads to the formation of a precipitate comprising thorium and / or uranium and, optionally, traces of residual radium. The nature of this precipitate can vary depending on the type of reagent used. It could thus be a precipitate based on a hydroxide or a carbonate of radioactive thorium and uranium and also an insoluble compound of an alkaline earth such as barium sulfate.

 The precipitate is separated by any known means and a rare earth solution is thus obtained which is purified of radium, thorium and uranium. The uranium purification rate can reach at least 90%. For the other elements, this rate can usually be at least 99%. Depending on the elements, especially for thorium and radium, this rate may be higher than 99.9%.

 The embodiment which has just been described can be the subject of a second variant in which the first two steps mentioned above are reversed.

Thus, firstly, the mixture to be treated is brought into contact with the source of alkaline earth. When the mixture is in solid form, this bringing together can be done by forming a suspension in an acid medium, for example in a nitric or hydrochloric medium, of said mixture with the source of alkaline earth. In a second step, the abovementioned anion source is added to the medium obtained previously. All that has been described above for the first two steps of the first variant also applies to the corresponding steps of the second variant. Finally, the following steps are identical to those corresponding to the first variant.

 Finally, 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 mode differs from the previous one essentially by the order of precipitation of the different radioelements. Thus, we first precipitate thorium and uranium then 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 is to attack the mixture with an acid solution. This acid 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 description of the step of adding the base. Thus, the base can be more particularly soda or ammonia, and it is possible to additionally add a source of alkaline earth metals, in particular in the form of a basic salt such as a carbonate. Finally, the added amount of base and, optionally, of alkaline-earth source is such that the pH of the medium obtained preferably has a value of at most 5. This value can more particularly be between 3.5 and 4 , 5. A solid is thus precipitated comprising thorium and / or uranium which is of the same nature as that described above.

 After separation, a source of an anion capable of forming an insoluble compound with an alkaline earth and, optionally, a source of alkaline earth is added to the medium obtained. We will use in this step a source of alkaline earth if it was not 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 can be more particularly sulfuric acid. The alkaline earth can be barium and the source of alkaline earth an alkaline earth salt such as a nitrate, a carbonate, a chloride, an oxide or a hydroxide.

Here again, the pH, temperature or concentration conditions will be chosen so as to obtain the most complete precipitation possible. So,
The addition of the anion source and optionally the alkaline earth source is preferably done so as to obtain a pH of the medium of a value of at most 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.

 At the end of this stage, a precipitate is obtained based on the above-mentioned anion and on the alkaline earth metal, for example barium sulphate, which precipitate, as in the first embodiment, also contains the radium element.

 After separation, a rare earth solution is obtained which has been purified of radium, thorium and uranium and whose purification rates are the same as those given above.

 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 what has been described for the first variant.

 Examples will now be given.

In the following examples, the source of earth consists of a concentrate of rare earth hydroxides, obtained from a sodium attack of monazite. The composition of this concentrate is as follows: - Humidity: 30%
- Rare earth hydroxides: 56.4%, i.e. 48.5% expressed as rare earth oxides or OT
- Thorium hydroxide: 5.6%, i.e. a ThO2 / OT ratio = 10.2%
- Uranyl hydroxide: 0.3%, i.e. a U3Og / OT ratio = 0.62%
- Other impurities: (ZrO2, TiO2, Fe (OH) 3, SiO2, CaO) = 7.5%
- Ra 228: 365,000 Bq / kg OT
- Ra 226: 52 500 Bq / kg OT
EXAMPLE 1
1000 g of crude hydroxide are transferred to a reactor. This hydroxide is suspended in 1100 ml of water.

 560 ml of 69% nitric acid and 30 ml of 98% sulfuric acid are added to this suspension. This mixture is kept stirring for one hour at 80 ° C. The concentration of free H + ions is 0.03 mol / l.

 30 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 stock solution.

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

 The solution is kept for 1 hour at 80 ° C.

 The suspension obtained is filtered.

The analysis of the solution is as follows: -OT: 170g / l
- ThO2 / OT: 20 ppm, precipitation yield = 99.98%
- U3O8 / OT: 500 ppm, precipitation yield = 91.9% - Ra 228 / or: 1000 Bq / kg, precipitation yield = 99.7%
- Ra 226 / OT: 140 Bq / kg, precipitation yield = 99.7%
EXAMPLE 2
1000 g of crude hydroxide are transferred to a reactor. This hydroxide is suspended in 1100 ml of water.

 560 ml of 69% nitric acid and 30 ml of 98% sulfuric acid are added to this suspension. This mixture is stirred for one hour at 80 ° C. The concentration of H + ions is 0.03 mol / l.

 30 g of barium nitrate are then added to this suspension. The mixture is stirred for 1 hour. The suspension is filtered. The solid residue is washed with 600 ml of water and the washing solution is incorporated into the filtered stock solution.

 To the resulting rare earth nitrate solution, 55 g of barium nitrate and 6.4 g of sodium hydroxide are added so as to bring the pH of the suspension around 4.

 The solution is kept for 1 hour at 80 ° C.

 The suspension obtained is filtered.

The analysis of the solution is as follows: -OT / 170g / l
- ThO2 / OT: 10 ppm, i.e. a precipitation yield of 99.99%
- U3O8 / OT: 100 ppm, i.e. a precipitation yield of 98.4%
- Ra 228 / OT: 100 Bq / kg, i.e. a precipitation yield of 99.97%
- Ra 226 / OT: 14 Bq / kg, i.e. a precipitation yield of 99.97%
EXAMPLE 3
1000 g of crude hydroxide are transferred to a reactor. This hydroxide is suspended in 1100 ml of water.

 565 ml of 69% nitric acid and 30 ml of 98% sulfuric acid are added to this suspension. This mixture is kept stirring for one hour at 80 ° C. The pH of the solution is close to 0.8.

 23 g of barium carbonate 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 stock solution.

 To the resulting rare earth nitrate solution, 42 g of barium carbonate and 0.7 g of sodium hydroxide are added so as to bring the pH of the suspension around 4.

 The solution is kept for 1 hour at 90 C.

 The suspension obtained is filtered.

The analysis of the solution is as follows: -OT: 170g / l
- ThO2 / OT: 5 ppm, i.e. a precipitation yield greater than 99.99%
- U3O8 / OT: 80 ppm, i.e. a precipitation yield of 98.7%
- Ra 228 / OT: 50 Bq / kg, i.e. a precipitation yield of 99.9%
- Ra 226 / OT: 7 Bq / kg, i.e. a precipitation yield of 99.99%.

Claims (14)

1- Process for treating a mixture comprising at least one rare earth compound, radium and at least one element from the thorium and uranium group, characterized in that it comprises the following steps: - the mixture is brought into contact with a source of an anion capable of forming an insoluble compound with an alkaline earth; - A source of alkaline earth is added to the medium thus obtained, so as to precipitate a compound of the aforementioned anion and of alkaline earth, containing the element radium; - The precipitate is separated from the liquid medium; - Adding to the resulting medium a base and optionally a source of alkaline earth, whereby a precipitate is obtained comprising thorium and uranium; - The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained. 2- Process for treating a mixture comprising at least one rare earth compound, radium and at least one element from the thorium and uranium group, characterized in that it comprises the following stages: - the mixture is brought into contact with a source of alkaline earth. 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 above-mentioned anion and of the alkaline earth metal is precipitated, containing the element radium ; - The precipitate is separated from the liquid medium; - Adding to the resulting medium a base and optionally a source of alkaline earth, whereby a precipitate is obtained comprising thorium and uranium; - The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained. 3- Process for treating a mixture in solid form comprising at least one rare earth compound, radium and at least one element of the thorium and uranium group, characterized in that it comprises the following stages: - the mixture is attacked with an acid solution; - A basic compound is added to the resulting medium, whereby a precipitate is obtained comprising thorium and uranium; - The precipitate is separated from the liquid medium; - Adding to the medium obtained a source of an anion capable of forming an insoluble compound with an alkaline earth metal and optionally a source of alkaline earth metal, whereby a compound of the above-mentioned anion and the alkaline group - earthy, containing the element radium; - The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained.  4 Process for treating a mixture in liquid form comprising at least one rare earth compound, radium and at least one element from the thorium and uranium group, characterized in that it comprises the following stages: - the mixture is added under liquid form a basic compound, whereby a precipitate is obtained comprising thorium and uranium; - The precipitate is separated from the liquid medium; a source of an anion capable of forming an insoluble compound with an alkaline earth and optionally a source of alkaline earth is added to the liquid medium obtained, whereby a compound of the above-mentioned anion and of alkaline earth is precipitated, containing the element radium; - The precipitate is separated from the liquid medium, whereby a purified solution of thorium, uranium and radium is obtained. 5- Method according to one of the preceding claims, characterized in that the source of an anion capable of forming an insoluble compound with an alkaline earth is a solution of sulfuric acid. 6- Method according to one of the preceding claims, characterized in that the alkaline earth is barium. 7- Method according to one of the preceding claims, characterized in that the source of alkaline earth is a carbonate or a nitrate. 8- Method according to one of claims 1, 2 and 5 to 7, characterized in that the above mixture is brought into contact with the aforementioned source of anion and the source of alkaline earth is added under such conditions that the pH of the medium obtained has a value of at most 4. 9- Method according to one of the preceding claims, characterized in that the aforementioned anion source is added under conditions such that there is an excess of anion in the medium obtained. 10- Method according to one of claims 1, 2 and 5 to 9, characterized in that the base and optionally the source of alkaline earth is added under conditions such that the pH of the medium obtained has a value of at most 5, more particularly between 3.5 and 4.5. 11- Method according to one of claims 3 to 7, characterized in that one adds to the medium resulting from the attack of the aforementioned mixture with an acid solution, or to the mixture in liquid form a basic compound under conditions such as pH of the medium obtained has a value of at most 5, more particularly between 3.5 and 4.5. 12- Method according to one of claims 3 to 7, characterized in that one adds a source of an anion capable of forming an insoluble compound with an alkaline earth and optionally the source of alkaline earth under such conditions that the pH of the medium obtained has a value of at most 4.5. 13- Method according to one of the preceding claims, characterized in that one treats a mixture which comes from the attack by a base of an ore comprising at least a rare earth, radium and at least one element of the thorium group and uranium or a mixture which is a liquid solution of a rare earth salt comprising radium and at least one element from the group thorium and uranium. 14- Method according to claim 13, characterized in that the ore is monazite, xenotime or bastnaesite.