GB2046307A - Methods of and apparatus for separating particles from suspensions - Google Patents

Abstract

The apparatus for separating particles from a suspension comprises a cell (1) divided by a diaphragm (2) into an anode chamber (3) and a cathode chamber (4). The suspension is caused to flow from a supply vessel (5), by means of a pump (6), through the chamber (4), which contains a working electrode (7), and back to the vessel (5). The chamber (3) contains a counter- electrode (10) and is filled with nitric acid. An electrical charge opposite to the charge of the particles is applied to the suspension by means of the electrodes (7, 10). The particles coagulate and can be separated off. <IMAGE>

Description

SPECIFICATION Methods of and apparatus for separating particles from suspensions This invention relates to methods of and apparatus for separating particles from suspensions.

Embodiments of the invention can be used for separating suspended particles from highly acid solutions, as occur for example in the reworking of burnt-off fuel rods of nuclear power plants.

The chemical reworking of burnt-off nuclear reactor fuel elements essentially comprises recovering the unconsumed uranium and the plutonium that has been formed, in pure form. To this end, the fuel rods are crushed and dissolved in nitric acid. Solid particles are removed from the resulting solution after previously being decanted by way of a filter or by means of a centrifuge. While fuel rods with a low degree of burning-off provide clear solutions, higher melting or fire loss values have the result that the solutions are of a dark colour due to finely divided solid particles, for example because of the fission products zirconium, palladium, ruthenium and small amounts of uranium and plutonium. Electronmicroscopic photographs show a particle size distribution of less than 0.1 jilm up to relatively large agglomerates.As the suspensions have a very severe disturbing effect in the subsequent extraction process (Purex-process), effective clarification of the solution is absolutely essential.

It has been suggested that the solution should be clarified by filtration, centrifuging or by the addition of flocculating agents (Proceedings of the American Nuclear Society, Topical Meeting, 'Back End of the LWR Fuel Cycle', March 1978, Savannah, Georgia, page 111-23 ff). However, in performing the filtration operation it is found that there is a severe reduction in the filtration speed when there is a filter cake thickness of only a few #m. It is also observed that filtered or centrifuged solutions still contain small amounts of solid matter, presumably in colloidal form.

After repeated filtration at intervals of a few days, black deposits are still collected on the filter. Organic and inorganic flocculating agents such as KM nO4 with an excess of Mn(NO3)2 or Primafloc C-3 (Rohm and Haas Co., Philadelphia, Pennsylvania, U.S.A.) result in an increase in the amount of highly radioactive waste material which must be taken to a final storage location. In addition, expensive measuring and control processes are required for metering out the chemicals.

According to a first aspect of the invention there is provided a method of separating particles from a suspension, wherein an electrical charge which is opposite to the charge of the suspended particles is applied to the suspension, which is a highly acid suspension, and coagulated particles are separated off.

According to a second aspect of the invention there is provided apparatus for separating particles from a suspension, the apparatus comprising means including a working electrode and a counter-electrode for applying to the suspension an electrical charge which is opposite to the charge of the suspended particles, a cell which is divided by a diaphragm into an anode chamber and a cathode chamber, and means to cause the suspension to pass around the working electrode and acid to pass around the counter-electrode.

Embodiments of the present invention can overcome or at least alleviate at least some of the disadvantages of known methods and can effectively clarify solutions without the addition of chemicals.

According to a preferred manner of performing the invention, the electrical charge is supplied by means of electrodes. This is preferably carried out in a cell which comprises two electrode chambers which are separated from each other by a suitable diaphragm or an ion-exchange membrane. Processes for establishing the charge condition of the particles are known. The working electrode used is the cathode or the electrode, depending on whether there is a positive or a negative charge. When an electrical voltage is applied, the colloid-dispersed particles which generally carry a considerable surface charge are neutralised by the application of an opposite charge and are caused to flocculate out. After the particles have flocculated out, they can be separated in per se known manner as by decanting, filtration and centrifuging.

In a preferred embodiment of the invention, a dc voltage of periodically varying value or an a-c voltage at a frequency of from about 50 Hz to about 100 kHz is superimposed on the applied voltage.

The electro-coagulation action preferably occurs at the cathode, while a mineral acid, for example 1-8 n nitric acid, is preferably passed around the anode. After the suspended particles are coagulated, they can be separated off in a per se known manner. It is also possible for the cathode to be formed as a filter and for the particles to collect thereon.

In fact, the coagulated particles are retained by the cathode and can be obtained in the form of a concentrated sludge solution by backwashing of the electrolyte, possibly in conjunction with an ultrasonic generator.

Embodiments of the invention make it possible for suspensions with an amount of solid matter of about 100 ppm to be effectively clarified. A particular advantage of embodiments of the invention is that the particles flocculated out can be easily filtered and do not clog the filter even when they form a layer of substantial thickness. In addition, the solu tion is not observed to suffer from fresh clouding, even after the solution has been standing for a prolonged period. The very small particles which agglomerate after a filtration operation in the course of time and which result in fresh clouding of the solution are preferably effectively removed from the solution. An advantage of substance is also that the use of chemicals which are foreign to the process is unnecessary.Thus, the amount of radioactive waste can be reduced to a minimum, in particular in the case of suspensions from solutions of fuel rods. In addition, there is no need for expensive measuring and metering apparatuses which are prone to trouble.

The invention will now be further described, by way of illustrative and non-limiting example, in the following Examples and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of apparatus embodying the invention; and Figures 2 and 3 are respective graphs of transmission factor against coagulation time for suspensions processed by methods embodying the invention.

Fig. 1 shows an electro-coagulation cell 1 which is separated by a diaphragm 2 into an anode electrolyte chamber 3 containing an anode 10 and a cathode electrolyte chamber 4 containing a cathode 7. The diaphragm 2 is preferably of a porous structure or is semipermeable in order to prevent the anode and cathode flows from mixing. A suitable cathode material is one of the noble metals in particular platinum. However, it is also possible to use other materials which are resistant under the conditions obtaining, for example tantalum, titanium or graphite. If, as in the present embodiment, the cathode 7 is used as the working electrode, the anode material is of secondary importance. Besides nob metals, it is also possible to use platinum-plated titanium, tantalum or niobium.Such electrodes are almost as resistant as platinum and are already being used nowadays in large-scale technical processes.

In the embodiment shown in Fig. 1, a suspension flows from a supply vessel 5 by means of a pump 6 to the electro-coagulation cell 1. Here, the suspension flows into the electrolyte chamber 4, containing the working electrode 7, in an upward direction, flows through the cathode and flows back into the supply vessel 5. The flow speed can be measured by means of a rotameter 8 and adjusted by means of a valve 9.

The electrolyte chamber 3, containing the counter-electrode 10, is filled with 3n nitric acid. A voltage source is connected to current supply means. If the working electrode 7 is in the form of a porous filter, coagulated particles can be retained by the electrode and can be obtained in the form of concentrated sludge solutions by backwashing of the electrolyte.

The following examples illustrate methods embodying the invention.

Example 1 A mixture comprising a platinum suspension with a ruthenium dioxide suspension in 3n HNO3 which contains 1 n NaNO3 was produced. The darkly coloured suspension was left to stand for several weeks. It did not show any change in colour and no noticeable sedimentation.

100 ml of the solution was put into a beaker which contained a platinum plate with 5 cm2 of geometric surface area as the working electrode. The counter-electrode was a platinum wire which was separated from the suspension by way of a coarse glass frit as the diaphragm. A magnetic stirrer provided for good and thorough mixing.

The electro-coagulation operation was carried out with a current density of 20 mA/cm2.

The increase with time of the transmission factor of the solution was used as a measure- ment in respect of the speed of coagulation.

The result of this Example is shown in Fig.

2. The abscissae show the duration of the test in hours while the ordinates show the associated transmission factor as a percentage. Fig.

2 shows a slow but steady rise in the transmission factor. The progress of the electrocoagulation action can also be followed by the eye; the solution, which was orginally dark, becomes lighter and lighter. After a test time of about 20 hours, the solution is optically clear. In this way, particles of solid matter can be separated off from suspensions wherein the transmission factor does not change by filtration with a 0.2 ym filter. The coagulated particles are partly in a condition in which they are capable of being filtered, in the solution, and for the major part they are in the form of a black-blue loose layer on the electrode. This layer can be easily wiped off the electrode or can be re-dispersed in coarse flakes by ultra-sound.

Example 2 Platinum suspensions were processed in accordance with the process of Example 1. 100 ml of a dark-coloured suspension containing 0. 1% by weight of platinum was treated for half an hour in a beaker with a cathodic current of 100 mA. After this period of time, the solution was optically clear. Fig. 3 shows the results of a series of tests, the various symbols relating to different sets of tests. The abscissae show the period of the test in minutes while the ordinates show the associated transmission factor as a percentage. It will be seen that significantly shorter coagulation times can be achieved, under the same test conditions as in Example 1.

Claims (9)

1. A method of separating particles from a suspension, wherein an electrical charge which is opposite to the charge of the suspended particles is applied to the suspension, which is a highly acid suspension, and coagulated particles are separated off.

2. A method according to claim 1, wherein the charge is applied by means of electrodes.

3. A method according to claim 2, wherein the working electrode is a cathode.

4. A method according to any one of claims 1 to 3, wherein a direct current of periodically changing value or an alternating current is superimposed on the working current.

5. A method according to claim 1, substantially as set forth in Example 1 or Example 2.

6. Apparatus for separating particles from a suspension, the apparatus comprising means including a working electrode and a counter-electrode for applying to the suspension an electrical charge which is opposite to the charge of the suspended particles, a cell which is divided by a diaphragm into an anode chamber and a cathode chamber, and means to cause the suspension to pass around the working electrode and acid to pass around the counter-electrode.

7. Apparatus for carrying out a method according to any one of claims 1 to 4, the apparatus comprising a cell which is divided by a diaphragm into an anode chamber and a cathode chamber, and means to cause the suspension to pass around a working electrode and acid to pass around a counterelectrode.

8. Apparatus according to claim 6 or claim 7, wherein the working electrode is of a porous structure and comprises platinum, titanium, platinum-plated titanium, tantalum or graphite.

9. Apparatus for separating particles from a suspension, the apparatus being substantially as herein described with reference to the accompanying drawings.