WO2003065381A1

WO2003065381A1 - Process and apparatus for volume reduction of oil scale waste

Info

Publication number: WO2003065381A1
Application number: PCT/SE2002/000177
Authority: WO
Inventors: Maria Lindberg; David Bradbury; George Elder
Original assignee: Studsvik Radwaste Ab
Priority date: 2002-02-01
Filing date: 2002-02-01
Publication date: 2003-08-07

Abstract

A process for the volume reduction of radioactive oil scale, comprising: providing a de-oiled scale material, dissolving barium and strontium sulphates and associated radioactivity therefrom by means of a combination of cation and anion exchange resins, separating said resins from each other, subjecting the cation resin to a separation operation to separate radioactive and non-radioactive constituents from each other; and recovering separate radioactivate and non-radioactive fractions in solid form. An apparatus for such volume reduction.

Description

PROCESS AND APPARATUS FOR VOLUME REDUCTION OF OIL SCALE

WASTE

Technical Field

The present invention relates to the field of radioactive oil scale waste. More specifically, it relates to a process and an apparatus for the volume reduction of such oil scale so as to reduce handling and storage thereof, as well as to facilitate disposal of the radioactivity therein.

Background of the invention Radioactive oil scale arises in the oil extraction industry. It contains principally salts of alkaline earth metals, and it is found on pipework and equipment used in the oil extraction process. The radioactive content of the scale is of natural origin, consisting principally of radium isotopes (Ra-226 and Ra-228) but with a smaller quantity of thorium and possibly other isotopes. The scale is not highly radioactive, but sufficiently so to cause concern. Today it is generally treated by water jetting to remove it from the equipment, and is either stored or returned to the environment.

Facilities exist in the United Kingdom and elsewhere for disposal of low level radioactive waste. Conditioning and packaging followed by disposal in these facilities would probably be an appropriate disposal route for the radioactive content of oil scale. However, there is a considerable cost associated with such disposal, and (what may be of greater concern) the potential volume of scale to be disposed of might ultimately be too great for the waste disposal sites to handle conveniently. The sites were designed for the needs of the nuclear industry, which employs small facilities in comparison with oil extraction. It is the volume of the waste, not its radioactive content that is the main problem in this regard.

Dissolution of barium sulphate in oil scale has also been established as a means of treating it. There are many examples described directed towards "in-situ" dissolution of the oil scale itself, together with its associated radioactivity. Typically EDTA (ethylene diamine tetra-acetic acid) or DPTA (diethylene triamine penta-acetic acid) are used as the dissolving media (FJ Quattrini, US Patent 3,660,287, 1972, JB Olson and PK Nolan "The Chemical Dissolution of Barium Sulphate (Barite) " Corrosion '92, paper no 26. G.H. Hardy and Z.I. Khatib, SPE 36586, Treatment and Disposal Options for NORM Oil Field Waste, 1996) . It has been found that there is a significant enhancement of the dissolution rate when synergists are added, such as fluoride or oxalate anion (JM Paul, "An Improved Solvent for Sulphate Scales",

Corrosion '94) . Many other more exotic synergists, such as macrocycles, have also been described (e.g. F DeJong, DN Reinhoudt, GJ Torny-schutte and A Van Zon, Patent No. NO 146820, 1982) . Furthermore, scale inhibition programs have been proposed, e.g. by removing sulphate from seawater so as to prevent formation of scale or by adding scale inhibitors for the same purpose.

However, the prior art referred to does not disclose any process for the volume reduction of radioactive oil scale by means of which the major radioactivity is at the same time removed from the non-radioactive part of the scale . General description of the invention

From the above-mentioned, it can be gathered that there is a considerable potential benefit in processing the oil scale to reduce its volume before disposal .

This is accomplished by means of the present invention. In other words, one object of the invention is to reduce the volume of oil scale before disposal thereof . Another object is to remove and separate the radioactive constituents of the scale from the non- radioactive ones .

A further object is to accomplish dissolution of the oil scale and separation of undissolved solids in the absence of added chemicals.

Still another object is to accomplish completion of dissolution of the oil scale in the absence of dissolved species in the solution used therefor.

Another object is to be able to perform the separation of radioactive ionic species from non- radioactive ionic species in a cation exchange resin column.

Other objects of and advantages with the invention should be apparent to a person skilled in the art from the detailed description thereof below.

Thus, in accordance with the present invention it has been found that by using an ex-situ dissolution methodology a rather non-complicated as well as efficient volume reduction of an oil scale waste can be achieved, which also efficiently separates a radioactive fraction from a non-radioactive fraction from said scale.

The non-radioactive constituents could then be disposed of or recycled without radioactive material controls. Initially it was thought that sufficient volume reduction could be achieved just by burning or pyrolyzing the organic (oil) content of the scale, but tests on this have demonstrated that the resulting inorganic residue still occupies too great a volume.

The inorganic chemical make-up of this scale has been found to be principally barium/strontium sulphate, but there is a variability between samples, and at least one oilfield sample has been found to contain significant quantities of zinc blende. The co-precipitation of radium on barium sulphate is well known, so it is no surprise that barium/strontium sulphate in the scale concentrates environmental radioactive radium.

The dissolution of the barium/strontium sulphate constituent of the oil scale in accordance with the present invention is advantageous because

- if barium/strontium sulphate constitutes the majority of the sample, then any non-soluble residue can be combined with the radioactive waste for disposal, still leading to good overall volume reduction; or

- if barium/strontium sulphate does not constitute the majority of the sample, it is still likely that any radioactivity in the sample is co-precipitated on barium sulphate, in which case the residual solid after dissolution will be suitable for disposal as non- radioactive material .

As can be seen from the Background section above, dissolution of barium sulphate in oil scale is known per se as a means of treating scale. However, in these in- situ applications there are two conditions that are different in comparison with the present volume reduction process :

- In-situ dissolution must take place quickly by direct contact with the low surface area "asformed" scale. This places high demands on the kinetics of the dissolving system. By contrast there are various options for improving kinetics of dissolution of "ex-situ" scales, such as milling or grinding.

- The desired volume reduction process places extra demands on the dissolution chemistry because it must be compatible with the subsequent separation of non- radioactive constituents from the radioactivity. According to the invention, the dissolution operation is followed by separation steps ending in a separation of radium from barium, and it has been found that by the present invention sufficient radioactivity can be eliminated from the original scale in an integrated process, to allow the remaining materials to be disposed of as non-radioactive waste.

The separation of radioactivity from barium in the dissolved barium sulphate represents a challenge, because the principal radioactive constituent is radium, which has chemistry very similar to barium. Marie Curie originally achieved this separation (by repeated fractional crystallisation) . More modern methods of separation have since been reported in the literature. In particular, a method has been reported which separates radium from barium out of EDTA solutions using ion exchange methods (G. Gleason, "An Improved Ion Exchange Procedure for the Separation of Barium from Radium" , Proc Conf Anal Chem Energy Technol , Oak Ridge Tennessee, 1980) . In this paper it is suggested that, although EDTA solutions can be used to "load" the combined barium and radioactivity on to the ion exchange column, EDTA is not a suitable eluent for the ion exchange separation, and that DCyTA (diamino-cyclohexane tetra-acetic acid) is preferable .

These prior art methods of separating barium from radium have generally been developed for analytical purposes, and in particular have certain crucial differences from the objectives of the present invention.

In particular:

- Analytical methods require a completely "clean" separation, which, for laboratory convenience, must be achieved in a single column elution.

- Analytical methods do not have the same restrictions (due to cost) on the use of exotic materials as large- scale processes.

- Most analytical methods have not been developed to deal with the pattern of impurities present in oil scale.

Accordingly, the present invention seeks to overcome these problems by providing an integrated process for the dissolution of the radioactive constituents of the oil scale and the separation of the dissolved products into non-radioactive and radioactive fractions.

Detailed Description at the Invention

More specifically, according to a first aspect of the invention, a process is provided for the volume reduction of radioactive oil scale waste, which comprises : a) providing a de-oiled solid scale material; b) subjecting said de-oiled solid scale material to a dissolution operation by means of a combination of cation and anion exchange resins, cationic species from said scale being held on the cation exchange resin and anionic species from the same being held on the anion exchange resin; c) separating any undissolved solid from said mixture of cation and anion exchange resins; d) separating said cation and anion exchange resins, with their attendant respective ionic species, from each other; e) subjecting the cation exchange resin to an elution operation so as to separate radioactive cationic species from non- radioactive cationic species into separate radioactive and non-radioactive fractions, respectively; and optionally f) converting said radioactive and/or non- radioactive fraction(s) into solid form(s); the radioactive fraction from the process being volume reduced and in a form suitable for disposal. It is a purpose of the present invention that as many different types of oil scale feed as possible can be processed by a single oil scale processing plant. Generally, however, a scale based on barium sulphate is referred to. It is probable that the processing plant would have a central fixed location which serves many oil extraction facilities, rather than being a mobile plant which visits individual facilities. The latter option is not, however, precluded.

It is also a purpose of the present invention that wherever possible chemicals used in the process can be recycled and re-used to avoid the cost of re-purchasing and to minimise the generation of secondary waste. In the first stage of the process claimed, a de- oiled solid scale material is thus provided. Generally this means that radioactive oil scale is collected and brought to a processing plant . The means by which oil scale is collected are entirely conventional and could include mechanical removal from oilfield equipment or water jetting techniques to collect a slurry. Any method of recovery that does not significantly alter the chemical nature of the oil scale is compatible with the present invention. After receipt the oil scale is, if necessary, dried and treated to remove the oil content of the scale. The methods for removing the oil are conventional and include steam distillation, solvent extraction and heat treatment

(e.g. pyrolysis) . An example of the latter method of treatment is described in US Patent 5,909,654. Any oil that is recovered in this part of the process is unlikely to contain significant radioactivity and can be disposed of by conventional means (e.g. incineration) . The main reason for removing oil is the avoidance of oil contamination in later processing stages, particularly if ion exchange is being used.

The first stage of the process according to the present invention, however, also encompasses an in-situ dissolution approach to collect the oil scale. The solution obtained in such an approach could then, for instance, be fed directly to a pyrolysis unit to create a de-oiled solid scale material for treatment through the rest of the process.

In step a) of the process said solid scale material is preferably provided in particulate form. This can be accomplished by conventional grinding or milling, to enhance the kinetics of the subsequent dissolution. The necessity or otherwise of this step is dependent on the particle size of the input oil scale solid and the performance of the subsequent dissolution equipment, but preferably the average particle size should be reduced to a diameter of 50 μm or less. The de-oiled solid scale material is then subjected to a dissolution operation, which is advantageously performed in water. Thus, according to the present invention it is possible to dissolve the oil -scale without directly contacting the same with dissolved chemicals. In this way the liquid used for the physical separation of components after dissolution may be pure water. One great advantage therewith is that the liquid can easily be used in the subsequent separation steps and can be recycled as required without incurring losses of chemicals or requiring rinsing operations as would be required in prior art methods .

Preferably the dissolution operation is performed with a mixture of cation and anion exchange resins. Moreover, it is advantageously performed in a common vessel, the cation and anion exchange resins being first placed in said vessel together with the liquid used in the dissolution operation and the oil scale material being then introduced therein while agitating the resins.

The vessel referred to can optionally also serve as a device for separation of the solid components on completion of the dissolution.

The ion exchange resin can be any combination of cation exchange and anion exchange resins, generally hydrogen form cation exchange and hydroxide form anion exchange resin, respectively. Preferably "strong acid" (sulphonic acic functional group) and "strong base" (quaternary ammonium group) cation and anion resins, respectively, are used. Typical examples are Amberlite IR 120 and Amberlite IR 420. Cation and anion exchange resins having particles sizes in the range of 16-50 Mesh (0.29-1.18 mm) are particularly preferable, though this does not exclude the use of resins of other particle sizes, smaller or larger.

To aid subsequent separation of the resins the cation and anion resins should preferably either have the same particle size (particle size distribution) , or the cation resin be larger in particle size than the anion resin.

If the resins are not already in the correct ionic form they have to be converted by "regeneration" with for instance sulphuric acid and sodium hydroxide solutions in a manner consistent with commonly used industrial practice, or according to the resin manufacturer's recommendations .

Since the cation and anion exchange resins have a known equivalent capacity for absorbing barium and sulphate ions the amounts thereof to be used are easily calculated by a person skilled in the art. The amount of each resin used should be just sufficient to absorb the ionic contents of the charge of oil scale to be dissolved. It is generally assumed that the oil scale is entirely barium sulphate, but an allowance can be made if it is known that a significant proportion of the oil scale will be insoluble residue. The amount of liquid used should be just sufficient to allow proper mixing of the resin and oil scale solids. As was referred to above, it has been found that it is optimum practice to place the liquid and ion exchange resins in the mixing vessel first and then introduce the oil scale while the resins are agitated. Other practices may lead to the resin beads aggregating together, which will introduce difficulties of separation of the resins later on.

Generally, the dissolution proceeds over a period of a few hours, depending on the particle size of the oil scale. Analysis of the progress of dissolution may be difficult, because the barium sulphate is effectively dissolving directly on to the ion exchange resins. However, one possible technique is to measure the burden of suspended solids (below the particle size of the ion exchange resins) . Analysis of the pH can be made during dissolution. If said pH is acid or alkaline, the amounts of cation and anion resins have not been most properly balanced, and the dissolution may be incomplete.

Once the dissolution is complete, or when the desired dissolution has been reached, the mixture is allowed to settle. A preferable separation technique will then be separation (s) performed as density or particle size separation (s) .

This can easily be achieved by an upward liquid preferably water, the rate of which is adjusted to carry a lighter component upwards while a heavier one stays behind.

Generally this means that firstly a slow upwards flow of liquid is allowed through the mixture so as to suspend undissolved solids in the effluent liquid, while the cation and anion resins remain in the vessel. Such a flow rate can easily be determined for the particular apparatus in use, by a person skilled in the art. The liquid can be recirculated though a filter to collect the undissolved solids.

Next the flow rate is increased and the effluent liquid is diverted to an anion exchange vessel to collect the suspended anion exchange resin. Thus, generally an anion resin is less dense than a cation resin. Again the liquid can be recovered from this vessel and recirculated. The anion exchange resin may be regenerated, e.g. with sodium hydroxide, rinsed with water and returned for new use in the dissolution operation. Finally, the remaining cation exchange resin is recovered from the bottom of the vessel. The separation of step e) may then be performed directly on said cation exchange resin, after proper equilibration and adjustment operations as is known per se.

However, according to a preferable embodiment the elution operation in step e) comprises two stages, one stage being a pre-stage where the cation exchange resin from step d) is equilibrated with the solution to be used for the elution operation, and the other being the separation stage per se where the elution solution from said pre-stage is passed on to a new cation exchange resin so as to accomplish the separation into radioactive and non-radioactive fractions.

In other words, the cation exchange resin separated in step d) is advantageously used in a pre-column before a new, separate cation resin column, in which the real separation into radioactive and non-radioactive fractions is achieved.

Separation and recovery of the radioactive and non- radioactive constituents in step e) can be achieved by passage of an elution solution through the cation exchange column. Such elution can be performed in accordance with principles known per se . Prior art methods utilize chelating agents, such as EDTA, DTPA and DCyTA , and such chelating agents are preferably used in the process of the present invention. In one embodiment of the invention EDTA is used as the chelating agent because of its ready availability and its low cost.

Generally, the concentration of the chelating agent is in the range of 0.01-0.5 M, preferably 0.01-0.1 M.

The elution operation step e) is preferably performed within the pH-range of 5-9. The lower end of this range favours the adsorption of ions on the resin, while the higher end favours elution of ions from the resin. In other words, the lower end of the range is preferably used in the equilibrium stage and the higher end is preferably used in the separation stage referred to above .

From the above-mentioned it can be seen that the elution solution typically contains a chelating agent, such as EDTA, and will undergo a serious change of pH on encountering the resin, which may cause the precipitation of solid EDTA, thereby disrupting the rest of the process. The reason for the change in pH is the ionisation of barium EDTA complex, which occurs approximately as follows :

(Resin )₂-Ba^z"+ (Na^1r)₂(H₂EDTA)^/- — > (Na+)₂Ba ^T(EDTA)^ + 2(ResinΕT •+\)

The above description is not exact and is indicative only, but when the reaction above has occurred any additional di-sodium EDTA present will exchange sodium with the hydrogen form cation exchange resin, causing the pH to fall and free EDTA to precipitate as a white solid. To prevent this happening additional sodium hydroxide must be added as the cation resin is equilibrated with the elution solution before the separation starts.

To perform this operation the cation exchange resin is equilibrated in the pre-stage with typically about two bed volumes of the elution solution, and sodium hydroxide is added until the pH achieved is slightly below that required for the separation (for instance pH 5.8) . The required sodium hydroxide can, of course, be added to the elution solution before the mixing with the resin, provided the required amount is known. This supernatant solution contains barium and radium from the oil scale material and should therefore be passed through the pre- column and onto the separator column. More elution solution is then passed through the pre-column and separator column to complete the separation.

Once the cation species have been removed from the pre-column, the resin thereof should be rinsed with a minimum volume of water to remove any chelating agent from the resin, typically 2-3 volumes. The resin in the pre-column is now in the sodium form, and may then be regenerated with sulphuric acid and water rinsed, before preferably being recycled for further dissolution of oil scale .

As to the separation into radioactive and non- radioactive fractions, respectively, such separation is achieved as the constituents pass down the cation exchange column, under the influence of the elution solution. In this way the constituents will pass out of the column at different times. The output solution from the cation exchange column is diverted to collect the radioactive and non-radioactive fractions separately. The correct collection of fractions may be facilitated by equipment which continuously analyses the radioactivity and chemical composition of the output solution.

If perfect separation cannot be achieved on a single pass through the ion exchange column, the partial separation can be used to good effect by "cascading" ion exchange columns, i.e. feeding radium-depleted barium and barium-depleted radium to further ion exchange separation columns. It is also possible that the ion exchange conditions can be designed to concentrate the radium into a series of bands or "pulses" of radioactivity going down the ion exchange column, in-between bands of non- radioactive material .

For final effective separation of the fractions and disposal or use thereof, the radioactive fraction and/or the non-radioactive fraction is (are) optionally converted into solid form (s). Preferably this is accomplished by means of precipitation operation (s) , which can be performed in accordance with techniques known per se, e.g. by sulphate precipitation (s) with sulphate ions . The obtained radioactive and/or non- radioactive solids can then be used or disposed of in any suitable manner. Once radioactive and non-radioactive cationic constituents have been recovered, recovery of dissolving constituents (e.g. chelating agent) can be achieved, e.g. by the appropriate pH adjustment of the effluent solutions to cause precipitation. The filtered chelating agent can be recycled. Residual salt solutions can then be discarded as effluent, though if it is absolutely necessary to have a zero effluent operation, the salt solutions can be reconstituted into acid and alkali by appropriate electrical processes. An example of one such process is described in Helfferich "Ion Exchange", McGraw-Hill, 1962, p.400.

According to a second aspect of the invention a simple and efficient apparatus for performing a process as defined above is provided. Said apparatus comprises:

A) a dissolver vessel for a combination of cation and anion exchange resins ,-

B) means for separation of cation and anion exchange resins, respectively; C) a pre-column for equilibration of cation exchange resin separated in B) ;

D) a separator column for ion exchange separation of radium, and optionally other radioactive species, from barium and strontium, and optionally other radioactive constituent (s) ; and

E) recovery vessels for separated radioactive and non-radioactive fractions from said separator column, respectively.

As to preferable embodiments of such an apparatus and the operation thereof reference is primarily made to those preferable embodiments of the process which have been described above as well as to the description of that specific embodiment of the apparatus which is shown in Fig. 1 of the accompanying drawings.

However, some advantageous apparatus constructions can be summarized as follows: One preferable embodiment of the apparatus is an apparatus wherein the separator column is represented by several cascadingly arranged cation exchange columns.

Furthermore, the apparatus preferably also comprises a de-oiling unit, arranged before the dissolver vessel, for the removal of the oil contents of the starting radioactive oil scale waste.

To accomplish the conversion of the radioactive and/or non-radioactive fraction(s) into solid(s), the recovery vessels are preferably provided with pH- regulating means for the precipitation of at least the radioactive fraction into a solid radioactive waste.

The apparatus also preferably comprises a grinding unit, before said dissolver vessel, for reducing solid oil scale to a particulate material, preferably to a particle size of at most 50 μm.

Finally, an apparatus can be referred to, which comprises radioactivity analysis means in connection with the cation exchange separator column (s) to enable correct collection of fractions. Drawings

In the drawings the following is shown:

Fig. 1 shows a block diagram of one embodiment of the process according to the present invention;

Fig. 2 and 3 show oil scale dissolution results from Example 1 below, and

Fig. 4 shows barium/radium separation results from Example 2 below. More specifically, the block diagram of Fig. 1 shows one embodiment of the process, or apparatus, according to the present invention. The apparatus for performing the process comprises the following elements: a dissolver vessel 1; a cation exchange resin store 2 and an anion exchange resin store 3; a filter 4; a regeneration vessel

5 for anion exchange resin; a regeneration vessel 6 for cation resin; a separator column 7; an elution reservoir

8; a precipitation vessel 9; a holding column 10; a chelating agent recovery tank 11; a salt waste receipt tank 12; an evaporator 13; and a salt splitting device 14.

The process schematically shown in Fig. 1 can be described as follows. Water is charged in the dissolver vessel 1 and cation and anion exchange resins are added thereto from the cation resin store 2 and the anion resin store 3, respectively. De-oiled oil scale is passed into said vessel 1 and the mixture obtained therein is agitated so as to subject the de-oiled scale material to a dissolution operation. When dissolution is complete, undissolved solids are fluidised out of the vessel area and collected on the filter 4. The anion exchange resin is then fluidised out of the vessel into the regeneration vessel 5. The anion resin is regenerated with sodium hydroxide and rinsed with water in said vessel, and the resin is returned into the store 3 for new use in a following cycle.

The cation resin is fluidised into the regeneration vessel 6, or pre-column, where it is equilibrated with a pH-adjusted solution of EDTA. Barium, and other non- radioactive constituents, and radium, and other radioactive constituents, are then separated from each other on the separator column 7, by passing solution from the elution reservoir 8 through the resin in the regeneration vessel 6 and then through the separator column 7. The non-radioactive (barium) fraction is collected in the precipitation vessel 9, and the radioactive (radium) fraction is collected on the holding column 10. Sulphuric acid is added to the precipitation vessel 9, to precipitate and recover clean barium sulphate. The radium on the holding column is periodically regenerated and the radium recovered by precipitation with sulphuric acid.

The cation resin in the regeneration vessel 6 is washed with water, regenerated with sulphuric acid and then rinsed with water. The resin is then returned to the cation resin store 2. Waste solutions from the above operations may be added to the EDTA recovery tank 11 or the salt waste receipt tank 12. Sulphuric acid is then added to precipitate and recover EDTA in the tank 11. The solution is then passed to the tank 12. From the tank 12 solutions are passed to the evaporator 13 to separate them into concentrated salt solution and pure water, and the concentrated salt solution is optionally passed into the salt splitting device 1 . In this device a process of electrodialysis may be used to convert sodium sulphate into solutions of sulphuric acid and sodium hydroxide as in existing art. The order of the processes in the evaporator 13 and the salt splitting device 14 may optionally be reversed. Finally, water and sulphuric acid and sodium hydroxide solutions from the evaporator 13 and the salt splitting device 14 may be recycled to the process. Examples

The present invention will now be described more in detail by reference to the following working examples. Example 1 - Dissolution of oil scale with ion exchange resins

Samples of oil scale from three sources were separately heat treated to pyrolyse organics, and then each sample was ground to <50 μm particle size and homogenised using Retsch (S100) centrifugal ball mill with reversing mechanism. Portions (4 g) of the samples were subjected to dissolution by means of a combination of ion exchange resins using the procedure described below.

The ion exchange resins were prepared as follows . All ion exchange resins were first repeatedly washed with water and the supernatant discarded (to remove resin particle fines) . The cation resin was in the sodium form (Na⁺) (Amberlite 120H, 14-52 Mesh) and was regenerated with acid to attain the hydrogen form. For that purpose the resin was placed in an ion column. The volume occupied by the resin is known as bed volume (bv) . To regenerate the resin 10 bv of 1 M hydrochloric acid were passed through the column at a rate of 3 bv/h. The conversion was to the hydrogen form and the resin was then rinsed with demineralised water until the effluent from the rinsing operation had pH>5. The anion exchange resin (Amberlite IRA 420, 14-52 Mesh) in the chloride form was converted to the hydroxide form by passing through 5 bed volumes of sodium hydroxide (1 M) . After this the resin was rinsed with demineralised waster until the effluent from the rinsing operation had pH<9. Demineralised water (100 ml) was placed in a 400 ml beaker and the cation resin and anion resin (prepared as above, 20 and 37 ml respectively) were added. The combined mixture was stirred with a magnetic stirrer with just sufficient agitation to ensure that the resins were evenly suspended and mixed in the water, scale sample no.

1 being stirred with a conventional stirrer to investigate the effect of the stirring on the ion exchange resin. An oil scale portion, prepared as described above, was added and the combined mixture was agitated for a period of 16 hours. In some cases a solution of radium (0.2 ml 74kBq/ml diluted in water) was slowly added during the dissolution to act as a "spike" for the purpose of simplifying analysis in Example 2 below. The pH of the supernatant solution was measured. The supernatant was filtered and the solids added to the undissolved solids fraction described below.

The mixture was then transferred to a glass column of 3.7 cm diameter with an inlet connection at the bottom to introduce water and an outlet connection at the top. Demineralised water was passed into the bottom of the column and the outlet effluent passed through a filter system to allow any solid material suspended in the water to be removed. Water was passed at an upward velocity through the column of 0.3-0.4 cm/s for 20 minutes to collect the undissolved solids, then 1.3-3.3 cm/s for 30 minutes to collect the anion resin. The cation resin remained in the column at the end. The undissolved solids, anion and cation resin fractions were set aside. The wet settled volume of the collected cation and anion exchange resins was measured. Note that the resins are not expected to have exactly the same volume as before dissolution, since they have been converted to different ionic forms. In Figure 2 the volume of resin

(cation and anion ) before and after dissolution are shown, the black part representing the fractions by which the amount of resin decreased during dissolution. The undissolved solids were dried and weighed.

Because the undissolved solids also contained a small amount of the ion exchange resin fines, the dried solids were ashed in a platinum crucible and reweighed to determine the amount of non-combustible residue. The amount of this residue for the three samples is shown as a proportion of the initial weight of the 4 g sample portion in Fig. 3, together with an analysis of the original oil scale samples for comparison. It can be seen from this figure that the majority of the oil scale sample was dissolved by the procedure and that the proportions of undissolved residue correspond quite closely with the proportions of the oil scale samples which are not barium or strontium sulphate. It should also be noted that when dissolution took place with a conventional stirrer (scale 1) the dissolution was less.

Example 2 - Separation of barium and radium from the oil scale dissolution step The cation resin fraction from one of the samples in

Example 1 above was placed in a beaker with 2 bed volumes of EDTA (0.1 M) adjusted to pH 7 with sodium hydroxide. The resin was agitated in the solution and sodium hydroxide added to adjust the pH to 5.8. These will now be referred to as the "loading" solution and the

"loading" resin. A separator column was made from cation resin (75 ml) , prepared as above, and was equilibrated with EDTA solution (0.1 M) adjusted to pH 7 with sodium hydroxide. This was done by first passing 2 bed volumes of sodium chloride (2M) then 1 bed volume of water and then 4 bed volumes of the pH adjusted EDTA solution through the same. The effluent pH was checked and found to be 7.0. The loading resin was placed in a column in front of the separator column, and the loading solution was passed first through the loading resin and then through the separator column and the effluent collected in fractions, each being 100 ml. When all the loading solution had passed this was followed, using the same flowpath, by a solution of EDTA (0.1 M) adjusted to pH 7 with sodium hydroxide. The effluent from the separator column was again collected in successive fractions of 100 ml. After 10 fractions had been collected the pH of the feed solution was raised to 8.5 and three more fractions were collected. Each fraction was analysed for barium and radium and the results are shown in Fig. 4.

Claims

CLAIMS 1. A process for the volume reduction of radioactive oil scale waste, which comprises:

a) providing a de-oiled solid scale material; b) subjecting said de-oiled solid scale material to a dissolution operation by means of a combination of cation and anion exchange resins, cationic species from said scale being held on the cation exchange resin and anionic species from the same being held on the anion exchange resin; c) separating any undissolved solid from said mixture of cation and anion exchange resins; d) separating said cation and anion exchange resins, with their attendant respective ionic species, from each other; e) subjecting the cation exchange resin to an elution operation so as to separate radioactive cationic species from non-radioactive cationic species into separate radioactive and non- radioactive fractions, respectively; and optionally f) converting said radioactive and/or non- radioactive fraction(s) into solid form(s); the radioactive fraction from the process being volume reduced and in a form suitable for disposal .

2. A process according to claim 1, wherein, in step a) , oil is removed from collected oil scale by means of an operation selected from steam distillation, solvent extraction and heat treatment .

3. A process according to any one of the preceding claims, wherein, in step a) , said solid scale material is provided in particulate form, preferably by grinding, and especially with a particle size of at most 50 μm.

4. A process according to any one of the preceding claims, wherein, in step b) , said dissolution is performed in water, preferably without any added chemicals .

5. A process according to claim 4, wherein in step b) , said dissolution is performed in a common vessel, the cation and anion exchange resins being first placed in said vessel together with water and the oil scale material being then introduced therein while agitating the resins.

6. A process according any one of the preceding claims, wherein, in step b) , said cation and anion exchange resins are used in hydrogen form and hydroxide form, respectively, preferably in strong acid form and strong base form, respectively, especially sulfonic acid form and quaternary ammonium form, respectively.

7. A process according to any one of the preceding claims, wherein, in step b) said cation and anion exchange resins have a particle size in the range of 16- 50 Mesh (0.3-1.2 mm) .

8. A process according to any one the preceding claims, wherein, in step b) , the cation exchange resin is of the same particle size as, or larger in particle size than, the anion exchange resin.

9. A process according to any one of the preceding claims, wherein, the separation in step c) , and/or the separation in step d) is (are) performed as density or particle size separation (s) .

10. A process according to claim 9, wherein said separation (s) is (are) achieved by an upward flow of water, the rate of which is adjusted to carry a lighter component upwards while a heavier one stays behind.

11. A process according to any one of the preceding claims, wherein the elution operation in step e) comprises two stages, one stage being a pre-stage where the cation exchange resin from step d) is equilibrated with the solution to be used for the elution operation, and the other being the separation stage per se where the elution solution from said pre-stage is passed on to a new cation exchange resin so as to accomplish said separation into radioactive and non-radioactive fractions.

12. A process according to any one of the preceding claims, wherein the elution solution which is used in step e) contains a chelating agent.

13. A process according to claim 12, wherein said chelating agent is selected from EDTA, DPTA and DCyTA, preferably EDTA.

14. A process according to any one of claims 12 and 13, wherein the concentration of said chelating agent is in the range of 0.01-0.5 M, preferably 0.01-0.1 M.

15. A process according to any one of the preceding claims, wherein the elution operation of step e) is performed within a pH-range of 5-9.

16. A process according to any one of the preceeding claims, wherein said step e) is performed by cascading cation exchange columns so as to feed radium-depleted barium and barium-depleted radium to further separate cation exchange columns, respectively.

17. A process according to any one of the preceding claims, wherein the conversion of step f) is performed by means of a precipitation operation, preferably by means of sulphate precipitations (s) .

18. A process according to any one of the preceeding claims, wherein, after said radioactive and non- radioactive fractions have been separated, dissolving constituent (s) , such as chelating agent (s) , is (are) recovered.

19. A process according to claim 18, wherein said recovery is accomplished by precipitation.

20. An apparatus for the volume reduction of radioactive scale waste, which comprises:

A) a dissolver vessel for a combination of cation and anion exchange resins; B) means for separation of cation and anion exchange resins, respectively;

C) a pre-column for equilibration of cation exchange resin separated in B) ; D) a separator column for ion exchange separation of radium, and optionally other radioactive species, from barium and strontium, and optionally other radioactive constituent (s) ; and E) recovery vessels for separated radioactive and non-radioactive fractions from said separator column, respectively.

21. An apparatus according to claim 20, wherein said separator column is represented by several cascadingly arranged cation exchange columns.

22. An apparatus according to any one of claims 20-

21, which comprises a de-oiling unit, arranged before the dissolver vessel, for the removal of the oil content of the starting radioactive oil scale waste.

23. An apparatus according to any one of claims 20-

22, wherein said recovery vessels are provided with pH regulating means for precipitation of at least the radioactive fraction into a solid radioactive waste.

24. An apparatus according to any one of claims 20- 23, which comprises a grinding unit, before said dissolver vessel, for reducing solid oil scale to a particulate material, preferably to a particle size of at most 50 μm.

25. An apparatus according to any one of claims 20- 24, which comprises radioactivity analysis means in connection with said cation exchange column (s) .