JPH01134297A - Method for recovering platinum group element from waste of nuclear fuel reprocessing - Google Patents

Abstract

PURPOSE:To separate and extract platinum groups from the insoluble residues generated in nuclear fuel reprocessing by converting platinum group elements to fluorides. CONSTITUTION:The insoluble residues 1 are heated together with gaseous O2/N2 3 and thereafter, gaseous fluorine is fed thereto to effect fluorination reaction in a fluorinating and evaporating device 2. The formed and evaporated gas is sent to a cooling condenser 7. The fluorides of the solidified platinum group elements (ruthenium, rhodium, palladium, etc.) are separated from the carrier gas to obtain fine particles 9 of the fluorides in a cyclone capturing device 8. The fine particles 9 of the fluorides are charged into a fused salt electrolytic bath 10 which has the fused salt compsn. consisting of 66mol.% LiF and 34mol.% BeF and is heated to a high temp. Electrolytic refining is executed by inserting an anode electrode 11 and a cathode electrode 12 into the bath. Platinum group element-deposited lumps 13 deposited on the cathode electrode 12 are stripped and recovered. The platinum group elements are economically and efficiently recovered.

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.

[Conventional technology] In a nuclear fuel reprocessing plant, spent nuclear fuel generated from a nuclear reactor is dismantled, sheared, and dissolved in nitric acid, etc. In the subsequent process, nuclear fuel materials (uranium, plutonium, etc.) and fission products (so-called moe) are The slag) is separated, and the nuclear fuel material undergoes fuel processing and other processes and is reused as nuclear fuel in a nuclear reactor. On the other hand, since nuclear fission products have extremely high radioactivity levels over a long period of time, they need to be isolated from the biosphere. For this reason, after being treated by vitrification/sealing in a corrosion-resistant container, they must be placed in an appropriate facility. It will be stored and disposed of deep underground. However, these fission products include metals belonging to the platinum group elements and elements having properties similar to these metals, such as molybdenum, technetium, ruthenium, rhodium, and palladium. These elements do not dissolve much even in the acid dissolution process, and exist in the form of fine particles, which flow down through each reprocessing process and accumulate in a sludge state in high-level waste liquid tanks and the like. Furthermore, in addition to these elements, the shear layer of the fuel cladding tube, scales of water adhering to the outer surface of the fuel cladding tube, etc. are not significantly dissolved in the acid dissolution process and enter the sludge component. They are collectively called insoluble residues. This insoluble residue has a property that it is difficult to be incorporated into the glass phase as a homogeneous phase in the borosilicate glass matrix even during the subsequent vitrification treatment, and therefore has a significant adverse effect on the radioactive substance encapsulation properties of the high-level vitrified material.

[Problems to be solved by the invention] Platinum group elements (ruthenium, rhodium, palladium) are not only hardly produced in Japan, but also resources are scarce worldwide, and the countries where they are produced are unevenly distributed, making them extremely expensive. It is made of metal.

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.

High-level waste (insoluble residue) generated during nuclear fuel reprocessing
The existence form of the platinum group contained therein is completely different from that in natural ores that humankind has experienced up until now, and a method to separate and extract it has not yet been developed. .

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

[Means for Solving the Problems] That is, the present invention (a) fluorine only the platinum group elements in the insoluble residue by reacting the insoluble residue generated in the nuclear fuel reprocessing operation with fluorine gas at 400 to 600°C.・
Volatize and extract into the gas phase; (b) 1 fluorine gas containing the platinum group element fluoride obtained in the extraction step;
Cooling to a temperature of 0°C or lower to condense and finely particulate the fluoride of platinum group elements as a solid phase; (C) entrainment with fluorine gas;
Collecting floating fluoride particles of platinum group elements by a solid-gas separator; (d) collecting the unobtained fluoride particles of platinum group elements in a molten salt electrolytic bath with a composition of LiF30 to 67;
mol%, B e F 270 to 33 mol% in a molten salt electrolytic bath to dissolve the fluoride fine particles of platinum group elements, and then the electrolytic bath temperature was 400 to 600°C, and the electrolytic refining voltage was 1.
．． A nuclear fuel reprocessing operation characterized by performing electrolytic refining within a range of 0 to 10 volts to deposit platinum group element lumps on the cathode electrode, and (e) peeling off and recovering the platinum group element lumps from the cathode electrode. This invention relates to a method for recovering platinum group elements from generated insoluble residue.

[Function] The total amount of insoluble residue generated in the reprocessing of spent nuclear 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 kg per unit ton of fuel.
The composition breakdown is as follows.

Transuranic elements: 0.8 to 3.7% by weight, platinum group elements (including Mo), 0 to 95% by weight, cladding tube sheared waste, etc. = 2.5 to 14% by weight, and platinum group elements (including Mo).
The composition ratio of each element (including Mo) is as follows.

Mo: 10-55% by weight Ru: 30-50% by weight Rh: 1-20% by weight Pd: 5-10% by weight Tc: 2-20% by weight The platinum group metal recovery method according to the present invention uses a fluorination/volatilization device. ,
Uses cyclone collection equipment, molten salt electrolytic refining equipment, etc.

First, insoluble residue having the above-mentioned composition is collected from the high-level waste liquid and sent to a fluoridation/volatization device.

This fluorination/volatilization device reacts the platinum group elements contained in the insoluble residue with fluorine at high temperature to produce fluoride with a low boiling point, which is then extracted by volatilization and gasification. The operating temperature of the fluorination/volatization device is 400 to 600°C, and when reacted with fluorine gas at that temperature, the platinum group elements are converted to volatile fluorides. In addition, an example of the melting point and boiling point of a fluoride of a platinum group element is described below.

MuL similar 1-11 stop--boiling 1B5p- MoFa 17.5 35RuFs
54 227RI+ F s~6 70
Approximately 500 fluoride has a boiling point as indicated above, and at the operating temperature most of the fluoride is gasified.

On the other hand, transuranium elements other than platinum group elements and cladding shear layers in the insoluble residue components are fluorinated and oxidized under the above reaction conditions.
Since it does not volatilize, only all platinum group elements can be separated and extracted as fluorides in this fluorination/volatilization step.

The gaseous platinum group element fluoride obtained in the above-mentioned fluorination and volatilization step is sent to a cyclone collection device along with fluorine gas, which is a reaction gas. In the cyclone collection device and the cooling condenser attached thereto, the platinum group element fluoride sent in gaseous form is heated to a temperature below its melting point, e.g. 10°C.
It is rapidly cooled to a temperature below and condensed and solidified to form a fine particle-like solidified body of fluoride of a platinum group element, and this solidified body is separated from accompanying gas and collected in a cyclone collector. The entrained fluorine gas has a boiling point of 1188°C, so it accumulates in the gas phase, and the fine particle coagulation that is produced has a particle size of 0.01 to 1.0°C.
μ and a density of 2.0 to 5.0 y/am', it is collected in the cyclone collection device. Note that the gas cooling temperature in the cooling condenser is preferably selected to be 10° C. or lower because it needs to be sufficiently below the melting point of the fluoride of each platinum group element.

Next, the particulate platinum group element fluoride collected in the cyclone collection device as described above is charged into a fluoride molten salt electrolytic bath and dissolved in the fluoride molten salt. LiF and BeF2 were selected as the main components of the fluoride bath from the viewpoint of melting point, operability, corrosivity, etc. LiF and BeFz
The ratio is determined as follows using the phase equilibrium diagram of LiF-BeF in Table 1, under the conditions that the electrolytic refining operation temperature is as low as possible and the operation temperature is about 400 to 600 ° C. did.

LiF: 30-67 mol% BeF2nia 0-33 mol% Fluorides of platinum group elements dissolve very well in molten fluoride salts, and by dissolving them, the volatility of individual fluorides is significantly suppressed. . The operating temperature is determined from the liquidus temperature to the liquidus temperature + 100°C, which is determined by the bath composition (ratio of LiF and BeF), from the viewpoint of improving electrolysis efficiency and electrolysis rate, and suppressing corrosivity and volatility of the melt. 400-60
The temperature shall be 0°C.

After setting the operating temperature in this manner, electrodes are inserted into the electrolytic bath, and DC voltage is applied to start molten salt electrolytic refining. The minimum cell voltage required for electrolytic refining can be theoretically derived from the thermodynamic function in the reaction described below, but in reality it is 1.0 to 10, taking into account engineering factors such as electrolytic refining efficiency and electrode overvoltage. I chose the bolt.

M ”+ n −e−= M (cathode reaction) nF″″
-+n/2F2+ne-e- (anodic reaction) In the formula, M represents a platinum group element, and F represents a fluorine element.

In this case, the amount of platinum group element fluoride loaded into the electrolytic bath should be controlled to suppress its volatility as much as possible, to not significantly change the original physicochemical properties of the electrolytic bath, and to increase the electrolytic refining efficiency as much as possible. In view of this requirement, a range of 0.5 to 5% of the amount of electrolytic bath liquid is appropriate.

In addition, as a result of examining heat resistance, corrosion resistance, polarization characteristics, etc.
Hastelloy was selected for the anode electrode and electrolytic cell material.
Austenitic stainless steel was selected as the cathode electrode material.

After electrolytic refining is completed, the cathode is pulled up and cooled, and the platinum group elements deposited on the cathode are peeled off and collected. The platinum group elements can be obtained by thoroughly washing this to remove the attached molten salt. The obtained platinum group element group can be subjected to elemental separation and purification as required, and then provided for general demand.

[Example] Takumi Oimaru One embodiment of the present invention will be described according to the flow sheet shown in FIG.

Fluorinated and
It was loaded into the volatilizer (2). In order to carry out the fluorination reaction efficiently, the pellets were loaded in multiple stages. Fluoride/volatile bag! (2) First o2/N2 gas (3)
A heating bag while sending it? (5) to gradually heat the inside of the fluoridation/volatilization device (2) to 550°C, then switch the gas switching valve (6) to turn on the fluorine gas (4).
was sent to perform the fluorination reaction. The platinum group element fluoride thus generated and volatilized was discharged together with the fluorine gas and charged into the next cooling condenser ().

When the gas flowing into the cooling condenser () is cooled to below 10°C, the fluorides of platinum group elements solidify into fine particles and are entrained by a solid-gas separator such as a cyclone collector (8). Fluoride particles of platinum group elements were obtained by separating and collecting from the gas.

Next, the obtained fluoride fine particles (9) of a platinum group element were introduced into a molten salt electrolytic bath (10) adjusted to a predetermined temperature and dissolved therein. Molten salt composition is LiF: 66 mol%, BeFz:
The concentration was 34 mol%, and the bath temperature was 550°C.

Next, the anode electrode (11) and the cathode electrode (12) are inserted into this molten salt electrolytic bath, and electrolytic refining is performed by applying a voltage of 2.0 volts as a DC voltage. ) was precipitated.

When the precipitation of platinum group elements is completed, the cell voltage increases rapidly, and this point is considered the end point of the electrolytic refining operation.The electrode is pulled up, cooled, and the cathode ring is removed! Peel off and collect the platinum group element precipitated lump (13) precipitated in step (12).0 When the collected platinum group element precipitated lump (13) is sufficiently washed with water to remove the molten salt adhering to the surface, the platinum group element precipitated lump (13) is removed. A group of elements was obtained. The overall recovery rate through this operation was over 90%.

Note that the exhaust gas exiting the cyclone collection device (8) is treated by a fluorine gas scrubber (14), and is treated with fluorine gas (
15) and other fluoride impurities, the fluorine gas (14) is recovered and reused, and the other fluoride impurities are discharged as cleaning waste liquid (16).

[Effects of the invention] According to the present invention, platinum group elements (useful precious metals) such as ruthenium, rhodium, and palladium can be economically and efficiently recovered from high-level waste (insoluble residue) generated in nuclear fuel reprocessing plants. .

For example, assuming that the processing capacity of a nuclear fuel reprocessing plant is 800 tons in 2007, if platinum group elements are recovered by the method of the present invention from the insoluble residue generated there, rhodium: approximately 5
00 kg/year, Ruthenium: approx. 2400 kg/year,
Palladium: reaching approximately 1400 ky/year, the potential value of the method of the present invention is very large.

[Brief explanation of the drawing]

FIG. 1 is a phase equilibrium diagram of LiF-BeF2, and FIG. 2 is a flow sheet showing one embodiment of the method of the present invention. In the figure: 1... Insoluble residue 2... Fluorination/volatilization device 3
...N210□ gas 4...Fluorine gas 5...
Heating device 6... Gas switching valve 7... Cooling condenser 8... Cyclone collection device 9... Fluoride fine particles of platinum group element 10... Molten salt electrolytic bath 11... Anode electrode 1
2... Cathode electrode 13... Precipitated mass of platinum group elements 14... Fluorine gas scrubber 15... Fluorine gas 16... Washing waste liquid patent applicant Mitsubishi Atomic Crab Industry Co., Ltd. Temperature ('C) - m-to

Claims (1)

[Claims] (a) By reacting insoluble residue generated in nuclear fuel reprocessing operations with fluorine gas at 400 to 600°C, only the platinum group elements in the insoluble residue are fluorinated and volatilized and extracted into the gas phase. (b) Cooling the fluorine gas accompanied by the fluoride of the platinum group element obtained in the extraction step to a temperature of 10 ° C. or less to condense and finely particulate the fluoride of the platinum group element as a solid phase; (c) Collecting fine particles of fluoride of platinum group elements entrained and suspended in fluorine gas using a solid-gas separator; (d) The fine particles of fluoride of platinum group elements thus obtained are collected in a molten salt electrolytic bath. Composition is LiF30-67 mol%, BeF_270
The fluoride fine particles of the platinum group element were poured into a molten salt electrolytic bath with a concentration of ~33 mol%, and then the electrolytic bath temperature was raised to ~400 mol%.
Perform electrolytic refining at 600° C. and an electrolytic refining voltage within the range of 1.0 to 10 volts to deposit a platinum group element mass on the cathode electrode; and (e) peeling off and recovering the platinum group element mass from the cathode electrode. Features: A method for recovering platinum group elements from insoluble residues generated during nuclear fuel reprocessing operations.