JPH04154924A - Method for recovering platinum group element from nuclear fuel reprocessing waste - Google Patents

Abstract

PURPOSE:To effectively separate and recover platinum group element from insoluble residue by applying selective oxidation and the whole oxidation successively under the specific condition to the insoluble residue produced in nuclear fuel reprocessing, and separating Mo, Ru and Pd successively. CONSTITUTION:The insoluble residue produced in the nuclear fuel reprocessing is selectively oxidized in gas stream containing 1-200ppm oxygen at 1000-1200 deg.C and the produced and vaporized Mo oxide is precipitated and recovered with a cooler. Further, the oxygen potential in the gas stream at 1000-1200 deg.C is raised to execute oxidizing evaporation, and Ru is recovered as Ru oxide oxidize-evaporated as the same way as Mo. Further, the insoluble residue after completing the recovery of Mo and Ru is treated at 1100-1200 deg.C under vacuum of <=10<-3> Torr, and Pd is evaporated, separated and recovered. By this method, platinum group element of Mo, Ru and Pd can efficiently be recovered from the insoluble residue.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for recovering platinum group elements from nuclear fuel reprocessing waste.

[Prior Art] In a nuclear fuel reprocessing plant, spent nuclear fuel generated from a nuclear reactor is chemically treated to separate nuclear fuel materials contained in the spent nuclear fuel from fission products produced by thermonuclear reactions. The former is reused as nuclear fuel in nuclear reactors. The latter contains radioactive isotopes with long half-lives, and their radioactivity is also very high. Therefore, it is necessary to isolate it from the biosphere for a long period of time, and there are plans to vitrify it and seal it in a corrosion-resistant container, store it for a certain period of time, and then dispose of it in deep geological formations. However, these fission products include metallic elements belonging to the platinum group and a group of elements having chemically similar properties. Examples of the elements include molybdenum, technetium, ruthenium, rhodium, and palladium. These element groups do not dissolve much in the acid dissolution step of reprocessing and exist in the form of fine particles, and are accumulated in the form of sludge at the bottom of high-level waste liquid storage tanks and the like after each reprocessing step.

Furthermore, in addition to these elements, water scales attached to the fuel cladding shear layer, the outer surface of the fuel cladding, and the like also enter the sludge component without being significantly dissolved in the acid dissolution process. They are collectively called insoluble residues.

This insoluble residue is difficult to incorporate into the borosilicate glass matrix as a homogeneous phase even during subsequent vitrification.
This has a significant negative impact on the radioactive material encapsulation properties of high-level vitrified materials.

[Problem to be solved by the invention] Platinum group elements (ruthenium, rhodium, and palladium) are hardly produced in Japan, and resources are scarce even in the world, and producing countries are unevenly distributed, making them extremely expensive metals. ing.

On the other hand, spent nuclear fuel contains a considerable amount of platinum group elements produced through nuclear fission, such as uranium and plutonium, and if all of this was recovered, it would account for more than several tens of percent of Japan's total consumption. It can be used as a quasi-domestic resource.

Currently, the existence form of platinum group metals contained in high-level waste (insoluble residue) generated in nuclear fuel reprocessing is different from that in natural ores, and alloys mainly composed of molybdenum, technetium, palladium, and rhodium are It is thought that it is forming. A method for separating, extracting, and recovering these elements has not yet been developed.

Therefore, an object of the present invention is to provide a method for separating and recovering platinum group elements from insoluble residues generated during nuclear fuel reprocessing.

[Means for solving the problem] That is, the present invention reduces the amount of insoluble residue generated during nuclear fuel reprocessing to 1
1000-12 in an air stream containing ~200 ppm oxygen
Step of separating molybdenum by selective oxidation at a temperature of 00°C; Next, 1000 to 1
A step of further oxidizing at a temperature of 200° C. to separate ruthenium; and a step of separating the insoluble residue from which molybdenum and ruthenium were separated in the above two steps in a vacuum of 10 torr or less.
The present invention relates to a method for recovering platinum group elements from nuclear fuel reprocessing waste, which comprises a step of evaporating and separating palladium by treatment at a temperature of 00 to 1200°C.

Further, the present invention includes a step of extracting insoluble residue generated in nuclear fuel reprocessing into metallic lead at a temperature of 1100 to 1200°C and forming a slag phase for use in the subsequent separation step;
The metallic lead-slag phase system obtained in the above step was heated to 1100
A process of blowing air into molten lead while heating it to a temperature of ~1200°C to form oxides of molybdenum and ruthenium; and a separation process of recovering palladium and rhodium from the lead by transferring these oxides to the slag phase. The present invention relates to a method for recovering platinum group elements from nuclear fuel reprocessing waste, which comprises a process.

[Function] The total amount of insoluble residue generated in the reprocessing of spent fuel as a raw material in the method of the present invention varies depending on the burnup of the fuel to be processed, but is approximately 2 to 6 kft per unit ton of fuel.
The composition breakdown is as follows.

Transuranium elements: 0.8 to 3.7% by weight Platinum group elements (including Mo) = 70 to 90% by weight Cladding tube sheared waste, etc. (Zr, Fe, Cr): 2.5 to 14% by weight, The composition ratio of each element of the platinum group elements (including Mo) is as follows.

Mo: IC1-55% by weight Ru: 30-50% by weight Rh: 1-20% by weight Pd: 5-10% by weight Tc: 2-20% by weight The method for separating and recovering platinum group elements according to the present invention includes (1) )l
those performed using the evaporation separation method and the vacuum evaporation separation method;
and (2) utilizing the force extraction method and the transition of oxides that are less compatible with metals into a slag phase.

Collection Method] First, an insoluble residue having the composition as described above is collected from a high-level waste liquid and inserted into a retort I. 100
0~1200℃ i blackness 1~2001'1ll11
The insoluble residue is heated and oxidized in an air stream containing a certain amount of oxygen. This treatment selectively oxidizes and evaporates mainly the molybdenum element from the insoluble residue.

Molybdenum oxide produced and vaporized by oxidation is precipitated and recovered in a cooler. Furthermore, at the same temperature, i.e. 1000
The ruthenium element is oxidized and evaporated in the same manner as molybdenum by increasing the oxygen potential in the air stream at ~1200°C (in the air or in an oxygen atmosphere). Collect the ruthenium oxide in a cooler.

In the insoluble residue after the above treatment, mainly palladium and rhodium elements remain without being oxidized. Rhodium is mainly present in this remaining insoluble residue.

The principles of the oxidation evaporation method and the vacuum evaporation method will be explained below. Note that FIG. 1 shows the relationship between oxygen partial pressure and temperature for oxides of insoluble residue constituent elements. In addition, the vapor pressure of volatile oxides is shown below.

Volatile oxides and other main L Flow ■ MOO310, 1mmHg (800℃) 760mmHg
(1155°C) Rub < 183mmHg (~100°C) As can be seen from Figure 1, for example, at a temperature of 1100°C, molybdenum element is selectively oxidized at an oxygen partial pressure corresponding to an atmosphere containing oxygen at a concentration of t ppm. Ru. Also,
The vapor pressure of the generated molybdenum oxide is close to 1 atm, and it is evaporated and separated from the insoluble residue. On the other hand, other constituent elements of the insoluble residue do not undergo oxidation at this oxygen partial pressure.
By increasing the oxygen partial pressure in the airflow atmosphere (air or pure oxygen atmosphere), the ruthenium element is again selectively oxidized, as is clear from FIG. Ruthenium oxide is recovered using a cooler, similar to molybdenum.

Through the above treatment, molybdenum and ruthenium are separated and recovered from the insoluble residue. Palladium and rhodium remain in the insoluble residue. Of these two elements, palladium has the highest vapor pressure at 1100°C, as shown in Figure 2.
in vacuum, e.g. 1100 ~ under reduced pressure of 10-3 torr or less
By heating at a temperature of 1200°C, it is possible to selectively evaporate and separate the palladium element, and ultimately the rhodium element remains alone.

Recovery Method 2 First, the insoluble residue having the composition as described above is collected from the high-level waste liquid, mixed with PB powder (platinum group element extractant) and slag forming agent, and then inserted into an electric furnace.

This mixture of insoluble residue and slag forming agent is melted at a temperature of 1100-1200°C to extract the insoluble residue into lead and form a slag. The reason why Pl) powder was used for extraction is that platinum group elements are selectively extracted by PB, and
The reason why borosilicate glass was used as the slag forming agent was because this glass has a relatively low melting point, is also used in high-level waste vitrification, and has excellent encapsulation properties. The composition of the slag forming agent used is within the following range.

Slag composition 5102: 45-50% by weight B2O2: 10-20% by weight AN203: 5-10% by weight Total of other trace oxides 2-10% by weight Also, the mixing ratio of insoluble residue, PB powder, and slag forming agent is PB The amount of powder needs to be such that the components transferred to the molten Pb do not reach a saturated state, and therefore, it is determined depending on the component composition of the insoluble residue used as the raw material. In addition, the mixing ratio of the insoluble residue and the slag forming agent is 1.2 by weight.
- About 3. The lead-slag system containing the produced insoluble residue is further heated to 1100-1200°C, and air is blown into the molten lead to selectively oxidize molybdenum and ruthenium to form oxides.

An example of a reaction formula for producing oxides of molybdenum and ruthenium is shown below.

Reaction formula for oxide production % Formula % These oxides produced transition to the slag phase. Mainly palladium and rhodium elements remain in the metal lead subjected to the above treatment without being oxidized.

The principle of the present invention will be described below. FIG. 3 shows the relationship between oxygen partial pressure and temperature for oxides of insoluble residue constituent elements.

As can be seen from FIG. 3, molybdenum and ruthenium elements are selectively oxidized in an air atmosphere at a temperature of, for example, 1100°C. At this time, transuranic elements (plutonium, etc.), cladding shear layers, etc. (Zr, Fe, Cr)
also has a strong tendency to become oxides, and the oxides produced similarly transition to the slag phase. Since MO oxide is highly volatile, a portion of it moves into the gas phase.

The same applies to Ru oxide. On the other hand, in this atmosphere, the other constituent elements of the insoluble residue, thumaladium and rhodium, do not undergo oxidation and therefore remain in the molten lead.

Through the above treatment, molybdenum and ruthenium are separated from the insoluble residue as oxides and transferred to a slag phase. Mainly palladium and rhodium remain in the insoluble residue after lead extraction. - [Example] Example 1 One embodiment of the recovery method 1 of the present invention will be described according to the flow sheet of FIG. 4.

Insoluble residue (1) recovered from nuclear fuel reprocessing process [here, the composition of Mo-Ru-Pd-Rb is 20
=60:10:10 (atomic %) using a simulated sample] is loaded into the rotating retort (2). Ar gas (3
) at a flow rate of 11/min, the temperature is raised to 1100°C to selectively oxidize the molybdenum element in the insoluble residue.

A rotating retort was used to ensure uniform oxidation. Further, Ar gas was used as the atmosphere because Ar gas usually contains about 1 ppI11 of oxygen and was suitable for selectively oxidizing molybdenum at a temperature of 1100°C. The molybdenum oxide produced by this treatment had a high vapor pressure, and was transported along with the air flow, cooled and condensed in the cooler (4), and recovered. After the separation of molybdenum was completed, the oxygen concentration of the atmosphere was increased (in this case, atmospheric atmosphere (3)) and heating was continued at 1100°C. Under these temperature and atmospheric conditions, the ruthenium element is selectively oxidized,
Since the ruthenium oxide produced is also highly volatile, it was recovered using a cooler like molybdenum.

The waste gas was exhausted through a scrubber (5).

Molybdenum and ruthenium were separated and recovered through these steps. 100% molybdenum by treatment in Ar gas atmosphere
At this time, about 10% of ruthenium is also separated by oxidation. In the air atmosphere, the remaining ruthenium element was 100% oxidized, evaporated, and separated.

Next, the insoluble residue from which molybdenum and ruthenium had been removed was loaded into a vacuum furnace (6) and heated at 1100° C. to evaporate and separate the palladium element, which was recovered in a cooler. Rhodium, a constituent element of the insoluble residue, remained in the vacuum furnace.

Example 2 An embodiment of recovery method 2 of the present invention will be described with reference to FIG.

Insoluble residue recovered from nuclear fuel reprocessing process (11) [
Here, the composition of Mo-Ru-Pd-Rh is 20:60.
:10:10 (atom %) was used], a raw material prepared by mixing lead powder and a slag forming agent in a weight ratio of 1:10:2 is loaded into an electric furnace (12). temperature 1100
℃ and melt the raw material to form a metallic lead phase (13) and a slag phase (1
4). By this operation, the insoluble residue was recovered in lead.

A slag phase (14) was formed on top of the lead phase (13).

By this operation, the insoluble residue was recovered in lead.

After completing this operation, air (1)) is slowly blown into the molten lead while continuing to heat it at 1100°C. Air must be blown slowly to reduce loss due to lead oxidation.

It was confirmed that this operation mainly oxidized molybdenum and ruthenium in lead, and that these oxides were incorporated into the glass phase. At this time, a small amount of lead was oxidized and transferred to the slag phase. Cold trap for waste gas (15)
and exhaust through the scrubber (16). Additionally, palladium and rhodium remained in the lead as metals.

[Effect of the invention] According to the present invention, from the high-level waste (insoluble residue) generated in nuclear fuel reprocessing plan 1, recovery method 1 can efficiently separate and remove platinum group elements such as ruthenium, rhodium, and palladium.
It can be recovered. In recovery method 2, platinum group elements can be separated from the insoluble residue by a combination of (ruthenium and rhodium) and (palladium and rhodium). Rhodium and palladium, which are relatively expensive, can be separated and recovered in their metallic state into a lead phase.

It can be processed with relatively simple equipment, and since the level is high, the actual processing is carried out in a concrete cell, but the use of chemicals is restricted in the cell, so the present invention does not use any other chemicals. It is also advantageous from that point of view.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the oxygen partial pressure and temperature of the oxide of the insoluble residue constituent elements, FIG. 2 is a graph showing the relationship between the vapor pressure and temperature of the insoluble residue constituent elements, and FIG. The figure is a flow sheet showing one embodiment of the method of the present invention, and the fourth
The figure is a diagram showing the relationship between the oxygen partial pressure of the oxide of the insoluble residue constituent element and temperature, and FIG. 5 is a diagram showing another embodiment of the method of the present invention. In the figure, 1... insoluble residue, 2...
Retort, 3...Ar gas, oxygen or atmosphere, 4.
...Cooler, 5...Scrubber, 6...Vacuum furnace, 7
...Separated platinum group elements (metal Ru, Pct), 8...
Platinum group oxide (Ru, Mo), 11... insoluble residue +
Lead powder + slag forming agent (raw material), 12... electric furnace, 1
3... Lead (including insoluble residue), 14... Slag phase (Mo and Ru oxides migrate), 15... Cold trap, 16... Scrubber, 17... Air. Patent applicant: Mitsubishi Atomic Crab Industry Co., Ltd. Procedural amendment (method) February 13, 1991

Claims (1)

[Claims] 1. Insoluble residue generated in nuclear fuel reprocessing from 1 to 200p
A process of selectively oxidizing molybdenum at a temperature of 1000 to 1200°C in an air flow containing pm of oxygen; a process of further oxidizing at a temperature of 1000 to 1200°C in an air or oxygen atmosphere to separate ruthenium. ; and the insoluble residue from which molybdenum and ruthenium were separated in both of the above steps in a vacuum of 10^-^3 torr or less at 1100 ~
A method for recovering platinum group elements from nuclear fuel reprocessing waste, comprising a step of evaporating and separating palladium by treatment at a temperature of 1200°C. 2. Insoluble residues generated during nuclear fuel reprocessing are mixed into metal lead.
A step of extracting at a temperature of 100 to 1200°C and forming a slag phase for use in the subsequent separation step;
A step in which air is blown into molten lead while heating it to a temperature of °C to form oxides of molybdenum and ruthenium; and a separation step in which palladium and rhodium are recovered from the lead by transferring these oxides to the slag phase. A method for recovering platinum group elements from nuclear fuel reprocessing waste, comprising: