GB1590828A

GB1590828A - Separation processes

Info

Publication number: GB1590828A
Application number: GB4427976A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1977-10-21
Filing date: 1977-10-21
Publication date: 1981-06-10

Description

(54) IMPROVEMENTS IN OR RELATING TO SEPARATION PROCESSES (71) We, UNITED KINGDOM ATOMIC ENERGY AUTHORITY, London, a British Authority, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to separation processes and more particularly to separation processes for separating a substance from a liquid.

According to one aspect of the present invention there is provided a process for separating a substance from a liquid comprising the steps of treating the liquid with a chemical reagent (as hereinafter defined) so as to form a colloidal solution in which there are present colloidal particles carrying, or consisting of, substance to be separated, and separating the particles from the liquid by means of a semi-permeable membrane or a centrifuge.

The term "chemical reagent" (as used herein in relation to a chemical reagent used in treating the liquid) does not embrace organic surfactants.

It is to be understood that when the substance is carried by the colloidal particles it should be carried by the colloidal particles in a manner which allows the substance and the colloidal particles of the colloidal solution to be removed together, as one, from the liquid. It is also to be understood that "colloidal solution" means a solution which contains particles or agglomerates of particles of a size which do not inherently tend to settle readily to form a precipitate.

One example of a way in which the substance may be carried by the colloidal particles is by sorption thereby. Another example of a way in which the substance may be carried by the colloidal particles is by physical incorporation therein.

The substance to be removed from the liquid can be in true solution therein, or may be present in colloidal solution or in suspension. Furthermore some liquids which can be treated in accordance with the invention (e.g. effluents) may contain inherent, fine particulate material upon which such substance is carried e.g. by sorption.

The colloidal particles, carrying, or consisting of, substance to be separated, may be separated from the liquid by the use of a semi-permeable membrane by ultra-filtration or reverse osmosis. Examples of chemical reagents which may be used to produce the colloidal solution are iron compounds which can be reacted with alkaline reagents (e.g. NaOH) to give colloidal particles and other combinations of compounds capable of producing colloidal particles for example, copper sulphate/potassium ferrocyanide and calcium chloride/sodium phosphate. Also it is believed that in the separation of "heavy metals" (e.g. lead) from a liquid the liquid could be treated with a sulphide forming reagent in order to produce colloidal particles of metal sulphide (e.g. lead sulphide). Excess sulphide ion could be removed from the liquid, after separation of the colloidal particles by treatment with a material capable of combining with sulphide ions (e.g. iron).

The present invention is believed to offer advantages over known processes involving the production of a precipitate and subsequent separation of the precipitate in that only a comparatively small amount of chemical reagent is required to produce the colloidal solution whereas a substantial amount is required in known methods to cause the formation of a precipitate.

It has been found that the quantity of sludge produced in carrying out the present invention is less, by as much as a factor of 50, than with conventional chemical precipitation and separation methods. This is due to the very small chemical dosage required in carrying out the present invention since it is not necessary to obtain a precipitate which can be separated by gravity or air flotation.

Substances which may be removed from a liquid in accordance with the present invention include toxic materials such as mercury, cadmium, lead and other heavy metals and valuable materials such as copper. Thus the invention finds application in treatment of effluents to remove substances therefrom for environmental and/or economic reasons. The invention finds a particular application in the treatment of radioactive effluents to remove radioactive substances (for example strontium and caesium) therefrom prior to storage or discharge.

Thus, the present invention also provides a process for the separation of a radioactive substance from a liquid comprising the steps of treating the liquid with a chemical reagent (as hereinbefore defined) so as to form a colloidal solution in which there are present colloidal particles carrying, or consisting of, a radioactive substance and separating the colloidal particles carrying, or consisting of, a radioactive substance to be separated, from the liquid by use of a semi-permeable membrane or a centrifuge.

In one particular embodiment of the present invention radiostrontium, or radiocaesium, or both can be separated from an aqueous solution which also contains non-radioactive magnesium for example in the form of a solution of magnesium nitrate ions. In this embodiment the solution can be treated with a chemical reagent, or chemical reagents, to produce colloidal particles of either calcium oxalate or of strontium sulphate and/or barium sulphate carrying radiostrontium.

To produce colloidal particles carrying radiocaesium a chemical reagent, or chemical reagents, can be added to produce a colloidal ferrocyanide (e.g. a ferrocyanide of copper, of cobalt, of nickel, of iron, or of zinc).

Subsequently the solution can be subjected to ultrafiltration whereby the magnesium ions in solution pass through the membrane for subsequent discharge or treatment as low level radioactive waste whilst the colloidal particles carrying radiostrontium, or radiocaesium, or both do not pass through the membrane with the result that radioactive species are concentrated from the solution.

The immediately foregoing embodiment of the invention finds an application in the separation of radiostrontium and/or radiocaesium from large quantities of non-radioactive magnesium ions in solution.

In a second particular embodiment of the invention an essentially similar procedure to the immediately foregoing embodiment of the invention can be used to separate non-radioactive sodium ions from an aqueous solution which also contains radiostrontium or radiocaesium or both. The chemical reagent or reagents used to produce colloidal particles may be the same as those disclosed in connection with the immediately foregoing embodiment of the invention.

Alternatively reagents capable of forming calcium phosphate may be used to produce colloidal particles carrying radiostrontium.

In a third particular embodiment of the invention alpha activity can be removed from aqueous solution. In this embodiment a chemical reagent is, or reagents are, added to the solution to produce colloidal particles of ferric hydroxide upon which alpha activity is concentrated by absorption. The ferric hydroxide carrying the alpha activity is subsequently separated from the aqueous solution by ultrafiltration. One application of this embodiment of the invention is in the treatment of fuel reprocessing liquors which contain low concentrations of alpha radioactivity.

It is to be understood that the present invention can be used to separate a substance from a liquid where the substance is present in solution at very dilute concentration.

In addition, by appropriate choice of reagent or reagents, a substance may be selectively separated from solution leaving other substances in solution.

As hereinbefore disclosed the colloidal particles carrying, or consisting of substance to be separated may be separated from the liquid by use of a centrifuge as an alternative to using a semi-permeable membrane.

According to another aspect of the present there is provided an apparatus for separating a substance from a liquid comprising means for treating the liquid (with a chemical reagent as hereinbefore defined) so as to form a colloidal solution in which there are present colloidal particles carrying, or consisting of, substance to be separated, and means for separating colloidal particles from the liquid comprising a semi-permeable membrane or a centrifuge.

The means for treating the liquid may comprise means for introducing into the liquid a chemical reagent, or chemical reagents, capable of producing the colloidal solution.

In one embodiment apparatus in accordance with the present invention may comprise a tank in which the liquid and a chemical reagent, or chemical reagents, can be mixed to produce a colloidal solution, a semi-permeable membrane, means for contacting the colloidal solution with the semi-permeable membrane, means for withdrawing some of the liquid phase of the colloidal solution through the semi-permeable membrane, means for removing colloidal solution of increased concentration from contact with the semi-permeable membrane and a separating tank in which colloidal particles of the colloidal solution of increased concentration can separate from the liquid phase thereof to form a sludge.

According to a further aspect, the present invention provides a substance whenever separated from a liquid by a process in accordance with the invention.

The invention will now be illustrated and explained with reference to the following examples: Example I An aqueous radioactive effluent liquid was recirculated from a feed tank through a test cell, comprising a chamber divided into two portions by a flat cellulose acetate ultrafiltration membrane, and back to the feed tank. The cell was operated at a pressure of 380 kNm-2 (40 psig). A flow of effluent liquid was directed across the membrane surface and a high feed velocity of about 2m sec l was used in order to prevent solids accumulating on the membrane surface. Treated water was withdrawn from the opposite side of the membrane to that in contact with the effluent liquid.

The aqueous radioactive effluent liquid comprised a raw radioactive effluent spiked with Cs137 (added as the nitrate). The raw effluent was typical of the low level radioactive wastes arising from inactive and trace active operations associated with research in the field of atomic energy. It consisted mainly of water contaminated with very low levels of radioactivity and containing significant amounts of surfactants, inorganic ions, oil and dirt normally present in plant effluent. The spiked effluent (the exact composition of which was not important) was used in preference to spiked pure water in order to demonstrate that the process of the present invention is effective (see Examples 2 to 4) in treating liquids which contain the impurities commonly encountered in some applications.

No chemical reagents were added to the effluent liquid prior to subjecting it to circulation through the ultrafiltration test cell and on analysing the treated water it was found that only 8% of the beta radioactivity (mainly radioactive caesium) was removed from the feed solution.

Example 2 A sample of the spiked radioactive effluent liquid referred to in Example 1 was treated with chemical reagents to cause a colloidal solution to form and the colloidal solution was treated using the test cell and techniques of Example 1. The chemical reagents were 0.5 mg 1- l Cu2+ and 0.8 mg 1-1 Fe (CN)64- at pH 7. It was found that approximately 98% of the total beta radioactivity (mainly radioactive caesium) was removed from the effluent liquid. Comparing the results of Example 1 and Example 2 shows that where no reagents were used to form a colloidal solution the amount of radioactivity removed was very low whereas a very high percentage of the radioactive substance was removed when the effluent liquid was treated in accordance with the present invention.

The amount of copper sulphate and potassium ferrocyanide used above to effect the high removal observed in Example 2 was 1/50th of that which is necessary for conventional precipitation to obtain comparable radioactive caesium removal. It follows from this that the volume of radioactive concentrate obtained at the end of the process will also be reduced by a factor of 50.

Example 3 A sample of the raw radioactive effluent referred to in Example 1 was spiked with Cos137 and strontium 89 (added as the nitrate) and treated with chemical reagents prior to being recirculated in the apparatus used in Example 1. The chemical reagents on this occasion were 0.5 mg 1-l Cu2+, 0.8 mg 1-l Fe (CN)64- and 100 mg 1-l sodium alginate at pH7. The sodium alginate was added in order to facilitate the removal of the radioactive strontium.

It was found that after treating the effluent as in the foregoing examples about 90% of the total beta radioactivity and about 99% of the Cs'37 had been removed.

Example 4 The raw radioactive effluent referred to in Example 1 was spiked with aged fission products and treated in accordance with the procedure of Example 1 after the addition of the chemical reagents mentioned in Example 3.

It was found that 85%of the total beta activity had been removed and 98%of the Cos137.

Example 5 In this example a radioactive effluent having a higher radioactive materials content than described in the foregoing examples was treated in accordance with the procedure of Example 1 using the chemical reagents mentioned in Example 3 with the result that 98% of the total beta activity was removed. This is better than conventional chemical precipitation techniques. (The radioactive effluent used in this Example arose in a similar manner to the effluent used in the previous examples with the exception that it contained radioactive wastes from operations involving more radioactivity and consequently contained a higher level of radioactive contamination.

In the preceding examples of experiments conducted in accordance with the present invention it was generally noted that the percentage of radioactivity removed was constant and did not diminish as the concentration of material in the liquid feed to the ultrafiltration chamber increased. It will be appreciated that the effluent liquid becaome more and more concentrated in radioactive material because, in the experimental apparatus, the liquid was recycled from the ultra-filtration membrane to the feed tank, and back to the ultra-filtration chamber. It was observed that by using an ultra-filtration membrane the effectiveness of the separation process did not decrease as the liquid became more concentrated. If a reverse osmosis membrane is used instead of an ultrafiltration membrane it should be noted that the effectiveness of removal of substance can diminish as the concentration of materials in the feed liquid increases.

In the examples conducted in accordance with the present invention the recycling of liquid through the ultrafiltration chamber gave rise to the production of a sludge in the feed tank due to the increasing concentration of the colloidal solution. The filtration rates were equivalent to 2.0 - 1.6 m3 of filtrate per m2 of membrane per day.

The present invention will now be further described, by way of example, with reference to the single figure of the drawing accompanying the Provisional Specification which shows schematically apparatus for separating a substance from a liquid in accordance with the present invention.

Referring now to the drawing there is shown a feed tank 1 provided with a stirrer 2, an untreated feed liquid inlet 3 and a chemical reagent inlet 4. An outlet 5 is also provided for the passage of liquid from the feed tank 1 to an ultrafiltration chamber 6 via a feed pump 7.

The ultrafiltration chamber 6 is divided, by an ultrafiltration membrane 8, into two portions and is provided with a treated liquid outlet 9 and an outlet 10 which communicates with a separator tank 11 via a pressure regulating valve 12.

Separator tank 11 is provided with a cylindrical member 13 extending downwardly therein and a sludge outlet 14. A supernatant liquor overflow 15 is provided on the separator tank 11 for the passage of supernatant liquor either to the feed tank 1 via a pipe 16 or to join the treated liquid outlet 9 via pipe 17 (shown as a dotted line).

In operation, the liquid containing the substance to be separated is introduced into the feed tank 1 by means of the untreated feed solution inlet 3 and is mixed, by means of stirrer 2, with chemical reagents introduced through the chemical reagent inlet 4 so as to form a colloidal solution containing colloidal particles carrying, or consisting of, substance to be removed.

The colloidal solution is pumped from outlet 5 via pump 7 to the ultrafiltration chamber 6.

Some of the liquid phase of the colloidal solution passes through the ultrafiltration membrane 8 and is removed as treated liquid through treated liquid outlet 9. The colloidal particles which cannot pass through the ultrafiltration membrane 8 are retained on the feed liquid side thereof and are carried away by the flow of feed liquid over the membrane surface. The concentration of colloidal particles in the feed liquid is thereby increased.

This feed liquid containing an increased concentration of colloidal particles is then passed through outlet 10 to the separator tank 11 via the pressure regulating valve 12 which regulates the flow to the separator tank 11.

In the separator tank 11 the colloidal particles of the liquid from the ultrafiltration chamber 6 separate from the liquid phase under gravity to form a sludge which is collected by means of the sludge outlet 14. Supernatant liquid separated from the sludge passes via the supernatant liquor overflow 15, depending on the quality of the supernatant liquor, either to the feed tank 1 via the pipe 16 for further treatment or via the pipe 17 to the treated liquid outlet.

The sludge removed through outlet 14 contains the substance which it is desired to remove from the feed liquid.

WHAT WE CLAIM IS: 1. A process for separating a substance from a liquid comprising the steps of treating the liquid with a chemical reagent (as hereinbefore defined) so as to form a colloidal solution in which there are present colloidal particles carrying, or consiting of, substance to be separated, and separating the particles from the liquid by means of a semi-permeable membrane or a centrifuge.

2. A process as claimed in claim 1 where the semi-permeable membrane is an ultrafiltration membrane.

3. A process as claimed in claim 1 or claim 2 wherein the colloidal solution is produced by treating the liquid with an iron compound and an alkaline reagent to give colloidal particles.

4. A process as claimed in claim 1 or claim 2 where the colloidal solution is produced by treating the liquid with copper sulphate and potassium ferrocyanide, or with calcium chloride and sodium phosphate.

5. A process as claimed in claim 1 or claim 2 wherein the liquid is treated with a sulphide forming reagent in order to produce colloidal particles of metal sulphide.

6. A process as claimed in claim 5 wherein after separation of the colloidal particles from the liquid excess sulphide ion is removed by treatment with a material capable of combining with sulphide ions.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and did not diminish as the concentration of material in the liquid feed to the ultrafiltration chamber increased. It will be appreciated that the effluent liquid becaome more and more concentrated in radioactive material because, in the experimental apparatus, the liquid was recycled from the ultra-filtration membrane to the feed tank, and back to the ultra-filtration chamber. It was observed that by using an ultra-filtration membrane the effectiveness of the separation process did not decrease as the liquid became more concentrated. If a reverse osmosis membrane is used instead of an ultrafiltration membrane it should be noted that the effectiveness of removal of substance can diminish as the concentration of materials in the feed liquid increases. In the examples conducted in accordance with the present invention the recycling of liquid through the ultrafiltration chamber gave rise to the production of a sludge in the feed tank due to the increasing concentration of the colloidal solution. The filtration rates were equivalent to 2.0 - 1.6 m3 of filtrate per m2 of membrane per day. The present invention will now be further described, by way of example, with reference to the single figure of the drawing accompanying the Provisional Specification which shows schematically apparatus for separating a substance from a liquid in accordance with the present invention. Referring now to the drawing there is shown a feed tank 1 provided with a stirrer 2, an untreated feed liquid inlet 3 and a chemical reagent inlet 4. An outlet 5 is also provided for the passage of liquid from the feed tank 1 to an ultrafiltration chamber 6 via a feed pump 7. The ultrafiltration chamber 6 is divided, by an ultrafiltration membrane 8, into two portions and is provided with a treated liquid outlet 9 and an outlet 10 which communicates with a separator tank 11 via a pressure regulating valve 12. Separator tank 11 is provided with a cylindrical member 13 extending downwardly therein and a sludge outlet 14. A supernatant liquor overflow 15 is provided on the separator tank 11 for the passage of supernatant liquor either to the feed tank 1 via a pipe 16 or to join the treated liquid outlet 9 via pipe 17 (shown as a dotted line). In operation, the liquid containing the substance to be separated is introduced into the feed tank 1 by means of the untreated feed solution inlet 3 and is mixed, by means of stirrer 2, with chemical reagents introduced through the chemical reagent inlet 4 so as to form a colloidal solution containing colloidal particles carrying, or consisting of, substance to be removed. The colloidal solution is pumped from outlet 5 via pump 7 to the ultrafiltration chamber 6. Some of the liquid phase of the colloidal solution passes through the ultrafiltration membrane 8 and is removed as treated liquid through treated liquid outlet 9. The colloidal particles which cannot pass through the ultrafiltration membrane 8 are retained on the feed liquid side thereof and are carried away by the flow of feed liquid over the membrane surface. The concentration of colloidal particles in the feed liquid is thereby increased. This feed liquid containing an increased concentration of colloidal particles is then passed through outlet 10 to the separator tank 11 via the pressure regulating valve 12 which regulates the flow to the separator tank 11. In the separator tank 11 the colloidal particles of the liquid from the ultrafiltration chamber 6 separate from the liquid phase under gravity to form a sludge which is collected by means of the sludge outlet 14. Supernatant liquid separated from the sludge passes via the supernatant liquor overflow 15, depending on the quality of the supernatant liquor, either to the feed tank 1 via the pipe 16 for further treatment or via the pipe 17 to the treated liquid outlet. The sludge removed through outlet 14 contains the substance which it is desired to remove from the feed liquid. WHAT WE CLAIM IS:

1. A process for separating a substance from a liquid comprising the steps of treating the liquid with a chemical reagent (as hereinbefore defined) so as to form a colloidal solution in which there are present colloidal particles carrying, or consiting of, substance to be separated, and separating the particles from the liquid by means of a semi-permeable membrane or a centrifuge.

7. A process as claimed in any one of the preceding claims for the separation of radios

trontium and/or radiocaesium from a liquid comprising an aqueous solution containing radiostrontium and/or radiocaesium, which process includes treating the liquid with a chemical reagent, or chemical reagents, to produce a colloidal solution in which there are present colloidal particles carrying, or consisting of, radiostrontium or radiocaesium.

8. A process as claimed in claim 7 for the separation of radiostrontium from a liquid comprising an aqueous solution thereof which process includes treating the liquid with a chemical reagent, or chemical reagents, to produce colloidal particles of either calcium oxalate or of strontium sulphate and/or barium sulphate carrying radiostrontium.

9. A process as claimed in claim 7 wherein to produce colloidal particles carrying radiocaesium a liquid comprising an aqueous solution thereof is treated with a chemical reagent, or chemical reagents, to produce a colloidal ferrocyanide.

10. A process as claimed in claim 9 wherein the ferrocyanide is of copper, or of cobalt, or of nickel, or of iron, or of zinc.

11. A process as claimed in any one of claims 7 to 10 wherein the aqueous solution also contains non-radioactive magnesium.

12. A process as claimed in any one of claims 7 to 10 wherein the aqueous solution also contains non-radioactive sodium.

13. A process as claimed in any one of the preceding claims wherein reagents capable of forming calcium phosphate are used to produce colloidal particles carrying radiostrontium.

14. A process as claimed in any one of claims 13, 14 or 15 wherein subsequently to the production of the colloidal particles the colloidal solution is subjected to ultrafiltration whereby magnesium ions or sodium ions in solution pass throug the membrane, whilst the colloidal particles carrying radiostrontium, or radiocaesium, or both do not pass through the membrane with the result that radioactive species are concentrated from the solution.

15. A process as claimed in any one of claims 1 to 6 for the separation of alpha activity from a liquid carrying an aqueous solution containing the alpha activity, which process comprises treating the liquid with a chemical reagent, or chemical reagents, to produce colloidal particles of ferric hydroxide upon which alpha activity is concentrated by absorption, and separating the colloidal particles of ferric hydroxide carrying alpha activity from the liquid by ultrafiltration.

16. A process as claimed in any one of claims 1 to 6 wherein the substance separated is mercury, cadmium, lead or another heavy metal, or copper.

17. A process as claimed in claim 1 and for the separation of a radioactive substance from a liquid comprising the steps of treating the liquid with a chemical reagent (as hereinbefore defined) so as to form a colloidal solution in which there are present colloidal particles carrying, or consisting of, a radioactive substance and separating the colloidal particles carrying, or consisting of, a radioactive substance to be separated, from the liquid by use of a semi-permeable membrane or a centrifuge.

18. A process as claimed in any preceding claim wherein by appropriate choice of chemical reagent, or chemical reagents, a substance is selectively separated from solution leaving other substances in solution.

19. Apparatus for separating a substance from a liquid comprising means for treating the liquid with a chemical reagent (as herein before defined) so as to form a colloidal solution in which there are present colloidal particles, carrying, or consisting of, substance to be separated, and means for separating colloidal particles from the liquid comprising a semipermeable membrane or a centrifuge.

20. Apparatus as claimed in claim 19 wherein the means for treating the liquid with a chemical reagent comprises means for introducing into the liquid a chemical reagent, or chemical reagents, capable of producing the colloidal solution.

21. Apparatus as claimed in claim 19 or claim 20 wherein the means for separating colloidal particles from the liquid comprises an ultrafiltration membrane.

22. Apparatus as claimed in any one of claims 19 to 21 comprising a tank in which the liquid and a chemical reagent, or chemical reagents, can be mixed to produce a colloidal solution, a semi-permeable membrane, means for contacting the colloidal solution with the semi-permeable membrane, means for withdrawing some of the liquid phase of the colloidal solution through the semi-permeable membrane, means for removing colloidal solution of increased concentration from contact with the semi-permeable membrane and a separating tank in which colloidal particles of the colloidal solution of increased concentration can separate from the liquid phase thereof to form a sludge.

23. A process for separating a substance from a liquid substantially as hereinbefore described with reference to any one of the Examples 2 to 5.

24. A substance whenever separated from a liquid by a process as claimed in any one of claims 1 to 18 or claim 23.

25. Apparatus for separating a substance from a liquid substantially as hereinbefore described with reference to the single figure of the drawing accompanying the Provisional Specification.