GB2345004A

GB2345004A - Treating off-gas containing ruthenium tetroxide

Info

Publication number: GB2345004A
Application number: GB9929620A
Authority: GB
Inventors: Andrew Derek Turner; Christopher Peter Jones; Jason Andrew Maude
Original assignee: AEA Technology PLC
Current assignee: Ricardo AEA Ltd
Priority date: 1998-12-22
Filing date: 1999-12-16
Publication date: 2000-06-28
Also published as: GB9828335D0; GB9929620D0

Abstract

Off-gas containing ruthenium tetroxide vapour is treated by scrubbing with a liquid containing a reducing agent then separating precipitated ruthenium dioxide from the scrubbing liquid. At least a proportion of the scrubbing liquid is treated to regenerate the reducing agent, and recycled to the scrubber. The scrubbing liquid may be sodium nitrite which reacts with the ruthenium tetroxide to form a precipitate of ruthenium dioxide and sodium nitrate. The nitrite may be regenerated by passing scrubbing liquid through a divided electrochemical cell. Off-gas is typically from radioactive material processing plant.

Description

Off-gas Treatment This invention relates to a process and an apparatus for treating off-gas, for example from a plant processing radioactive materials.

Where a plant is processing radioactive materials, for example vitrifying radioactive waste, it is conceivable that the off-gas might contain ruthenium tetroxide vapour. Since ruthenium is radioactive, it is desirable to remove this vapour before the off-gas is released. This can be achieved using a scrubber.

Ruthenium tetroxide is highly volatile, so that scrubbing using water requires very large volumes. If the scrubbing liquid contains a suitable reducing agent, however, this will react with the ruthenium tetroxide to form ruthenium dioxide, which is a non-volatile stable precipitate, and consequently improves the rate of removal. One is then faced with the problem of disposal of the spent scrubbing solution.

According to the present invention there is provided a process for treating off-gas to remove ruthenium tetroxide vapour, the process comprising scrubbing the off-gas with a liquid containing a reducing agent, separating precipitated ruthenium dioxide from the scrubbing liquid, treating at least a proportion of the scrubbing liquid to regenerate the reducing agent, and recycling the regenerated scrubbing liquid to the scrubber.

In a preferred process the scrubbing liquid is an aqueous solution containing an inorganic reducing agent.

The preferred reducing agent is a neutral nitrite salt, for example sodium nitrite. This reacts with the ruthenium tetroxide, forming sodium nitrate in solution.

The nitrite may be regenerated by passing at least a proportion of the scrubbing liquid through a divided electrochemical cell to reduce the nitrate back to nitrite. For example the flow rate through the divided cell might be less than 10 percent, for example 1 percent, of the flow used for scrubbing the off-gas.

Operation of this regeneration cell might be controlled in response to measurement of the concentration of the reducing agent in the scrubbing liquid.

The invention also provides an apparatus for treating off-gas to remove ruthenium tetroxide vapour, the apparatus comprising a scrubber to scrub the off-gas with a liquid containing a reducing agent, means to separate precipitated ruthenium dioxide from the scrubbing liquid, means to treat at least a proportion of the scrubbing liquid to regenerate the reducing agent, and means to recycle the regenerated scrubbing liquid to the scrubber.

The separation means may comprise a hydrocyclone, or a filter, or indeed a hydrocyclone to remove large particles followed by a filter to remove the smaller particles. Very small particles of ruthenium dioxide (preferably non-radioactive) may be added to the scrubbing liquid to act as nucleation centres, so that during operation larger particles of ruthenium dioxide are formed rather than very fine colloidal material. The scrubber may be a vortex scrubber for example as described in EP 0 331 343, as this contains no moving parts and has no packing material, and hence minimises the possibility of fouling by the precipitated ruthenium dioxide.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a sectional view through a vortex scrubber ; and Figure 2 represents diagrammatically the gas treatment apparatus of the present invention.

Referring to Figure 1 there is shown a sectional view of a gas/liquid contactor 10 suitable for use as a scrubber for treating an off-gas. This contactor is similar to that described in EP 0 331 343. It consists of a cylindrical vortex chamber 1 which has at least one tangentially oriented inlet port 2 for a gas stream (the stream of off-gas) and an axial outlet port 3 defined in the plate forming the top of the chamber 1 for the treated gas stream. A baffle plate 4 is provided above the outlet port 3 just above the top of the chamber 1, and the treated gas having flowed around the baffle plate 4 emerges through an outlet duct 5. Scrubbing liquid is supplied to the chamber 1 by a pump 6 via a Y-junction 7 to a pair of liquid jet outlet nozzles 8 which are opposed and coaxial with the chamber 1. The size and separation of the nozzles 7 are such that the liquid forms a wide spray 9 of droplets. The base of the chamber 1 is provided with perforations 11 near the outer edge so the liquid can drain into a sump 12 below the chamber 1. A drain pipe 13 enables any liquid droplets which reach the vicinity of the baffle plate 4 to drain back into the sump 12.

In use of the scrubber 10 the off-gas entering the inlet port 2 follows a spiral flow path towards the outlet port 3, its tangential velocity increasing as it approaches the port 3. The scrubber liquid droplets flow radially outwards from the nozzles 8 in counter current to the off-gas, so that thorough liquid/gas contact is ensured. The droplets are given a tangential component of velocity by the gas flow. Consequently they are accelerated towards the cylindrical outer wall of the chamber 1, disentrained, and run down into the sump 12.

The residence time of the gas in the contactor 10 is usually about 0.1 s. The volume of the chamber 1 is therefore selected in accordance with the expected offgas flow rate. The contactor 10 provides very vigorous mixing and contact of the scrubbing liquid and the offgas, so its volume (and the residence time) is about a tenth that of a conventional packed column scrubber, for the same gas throughput.

Referring now to Figure 2 an off-gas treatment plant 20 comprises a scrubber 10, an external sump 22, and a regeneration circuit 24. As explained in relation to figure 1, the off-gas is supplied to an inlet port 2, and the treated gas from which any ruthenium tetroxide has been removed emerges from an outlet duct 5. Scrubbing liquid, which in this example is an aqueous solution of sodium nitrite, is re-circulated by the pump 6 from the external sump 22 into which it drains from the sump 12 of the scrubber 10. If any ruthenium tetroxide is present in the off-gas, it will react with the sodium nitrite.

Some of the sodium nitrite will consequently be converted to sodium nitrate, and ruthenium dioxide particles will be carried in the scrubbing liquid. Some of the water will evaporate from the droplets as they pass through the scrubber 10, and therefore demineralised water is supplied to the external sump 22 to make up for this loss.

Some of the liquid from the external sump 22 is supplied by a pump 26 to a filter unit 28, the filtered liquid then passing through a divided electrochemical cell 30 before being returned to the sump 22. The flow through this regeneration circuit 24 might for example be 1 percent of the flow through the scrubber 10. The electrochemical cell 30 comprises an anode 31 and a cathode 32 separated by a cation-permeable membrane 33; the filtered scrubbing liquid is supplied to the vicinity of the cathode 32, while nitric acid is supplied to the vicinity of the anode 31. A potential difference is applied between the anode 31 and the cathode 32, so that any nitrate ions are reduced to nitrite ions.

Consequently the nitrite concentration in the scrubbing liquid is maintained.

The regeneration of nitrate to nitrite ions in this manner is facile, as this reaction occurs readily at the cathode 32 at a high current efficiency (as there are no significant competing reduction reactions). The standard electrode potential of this nitrate/nitrite reaction is about 0.4 V in neutral solution. The solution is preferably between pH 6 and 8.

In the above example the scrubbing liquid was described as a solution of sodium nitrite, but it will be appreciated that the solution may contain other salts which do not act as reducing agents. In particular the scrubbing liquid might be a 1 M solution cf sodium nitrate containing sodium nitrite at a concentration in the range 0.02 M up to 0. 1 M. If the volume of liquid being regenerated is say 1000 1/hr, the nitrite concentration in the liquid might increase by 0.002 M in its passage through the cell 30. The apparatus may include a redox probe (not shown) to monitor the concentration of the reducing agent (nitrite ions) in the scrubbing liquid, and the operation of the cell 30 in the regeneration circuit 24 might be controlled in response to signals from that probe to ensure that the scrubbing solution is always sufficiently reducing. If the off-gas contains ruthenium tetroxide only at rare intervals then the cell 30 might be operated only intermittently.

It will be appreciated that an off-gas treatment plant may differ from that described above while remaining within the scope of the present invention. For example it might use a different type of scrubber. There might be just a single sump in place of the internal sump 12 and external sump 22; and there might be a single pump in place of the pumps 6 and 26, most of the flow going to the scrubber and a small proportion being diverted to the regeneration circuit. The filter unit 28 might be replaced by a hydrocyclone, or alternatively the regeneration circuit might incorporate both a hydrocyclone and a filter in series. Furthermore a hydrocyclone might be provided in the pipe leading from the pump 6 to the Y-junction 7 to remove any large particles of ruthenium dioxide from the scrubbing liquid before it is supplied to the scrubber 10.

Claims

Claims 1. A process for treating off-gas to remove ruthenium tetroxide vapour, the process comprising scrubbing the off-gas with a liquid containing a reducing agent, separating precipitated ruthenium dioxide from the scrubbing liquid, treating at least a proportion of the scrubbing liquid to regenerate the reducing agent, and recycling the regenerated scrubbing liquid to the scrubber.
2. A process as claimed in claim 1 wherein the scrubbing liquid is an aqueous solution containing an inorganic reducing agent.
3. A process as claimed in claim 2 wherein the reducing agent is a neutral nitrite salt.
4. A process as claimed in any one of the preceding claims wherein the reducing agent is regenerated by passing at least a proportion of the scrubbing liquid through a divided electrochemical cell.
5. A process as claimed in any one of the preceding claims wherein the proportion of the scrubbing liquid which is regenerated is less than 5 percent of the flow through the scrubber.
6. A process as claimed in any one of the preceding claims in which small particles of ruthenium dioxide are included in the scrubbing liquid.
7. An apparatus for treating off-gas to remove ruthenium tetroxide vapour, the apparatus comprising a scrubber to scrub the off-gas with a liquid containing a reducing agent, means to separate precipitated ruthenium dioxide from the scrubbing liquid, means to treat at least a proportion of the scrubbing liquid to regenerate the reducing agent, and means to recycle the regenerated scrubbing liquid to the scrubber.
8. An apparatus as claimed in claim 7 in which the separation means comprises a hydrocyclone.
9. An apparatus as claimed in claim 7 or claim 8 in which the separation means comprises a filter.
10. An apparatus as claimed in any one of claims 7 to 9 in which the scrubber is a fluidic vortex scrubber.
11. A process for treating off-gas to remove ruthenium tetroxide vapour substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
12. An apparatus for treating off-gas to remove ruthenium tetroxide vapour substantially as hereinbefore described with reference to, and as shown in S accompanying drawings.