SEPARATION OF MATTER BY FLOTATION

Abstract: Matter is floated to the surface of a liquid by bonding ions to the surface of the matter to give the matter a charge, and forming a froth with the aid of a frothing agent having groups of opposite charge to the ions so that the frothing agent bonds to the matter and is carried in the froth to the surface of the liquid. By removing the froth the matter can be separated from any inert matter present in the liquid. The oxidation state of the surface of the matter may be changed before bonding takes place with the ions to one which facilitates that bonding. The matter can be particulate or dissolved ions. For example, uranium dioxide particles are oxidised with hydrogen peroxide, sodium carbonate added to produce a negatively charged uranyl carbonate complex and a froth formed with the aid of cetyl trimethylammonium bromide. Cationic groups in the latter bond to the uranyl carbonate complex, causing the uranyl carbonate complex to be concentrated in the froth at the surface of the liquid. The froth is then skimmed off to remove the uranium dioxide particles.

Description: This invention relates to the flotation of matter at the surface of a liquid medium to assist its separation and is particularly, but not necessarily exclusively, concerned with the flotation of actinides in magnesium hydroxide based media. In the reprocessing of nuclear fuel elements it is necessary to first separate the spent fuel from its cladding. For fuel which had been irradiated in the so-called Magnox reactors, the cladding is made from Magnox, a magnesium alloy containing small quantities of aluminium, manganese and zirconium. Once the cladding has been removed it is transferred to concrete silos where it is stored under water. Small quantities of spent fuel matter, that is uranium dioxide and traces of other actinide oxides become associated with the cladding and are therefore transferred to the storage silos. During prolonged storage under water the cladding reacts with the water to give a magnesium hydroxide based sludge. The sludge also contains the particles of spent fuel that were associated with the cladding. Sludges containing actinides are also encountered in other industries, such as the uranium mining industry. It is desirable to remove the particles of spent fuel and/or actinides from such sludges. In the prior art EP-A-0004953 and GB 911,792 describe processes involving the selective separation of uranium from a liquid medium by a flotation method. However, the starting material for the processes in each case is an aqueous uranium solution in which uranium-containing complex anions are formed by bonding with the cations of an added surface active agent. Particulate uranium-containing materials such as uranium dioxide are notoriously difficult materials to separate by froth flotation but the present invention provides a method of carrying this out without having to cause the uranium to go into solution. Changing the oxidation state of an actinide-containing material is known per se, eg US-A-3,000,695 describes changing the oxidation state of plutonium groups; however changing the state of the surface of the particles of a particulate actinide-containing material for the purpose of separating the material in accordance with the present invention has not hitherto been suggested in the prior art. The aim of the present invention is to provide a method of separating matter by flotation. According to the present invention there is provided a method of causing matter to float at the surface of a liquid medium to assist its separation, the method comprising the steps of: (i) forming a mixture in a liquid medium of the matter to be separated and a frothing agent for the liquid medium, the matter to be separated and the frothing agent having oppositely charged groups; and (ii) causing the mixture to froth whereby the oppositely charged groups are attached to one another and are concentrated in froth at the surface of the liquid medium; wherein the matter to be separated comprises particulate matter whose particles have a surface whose oxidation state is changed before step (i) is carried out to facilitate the attachment thereto of ionic groups and which surfaces have attached thereto such ionic groups suitable for bonding with oppositely charged ionic groups of the frothing agent the said particulate matter comprising an actinide-containing material. Preferably the froth is removed from the said solution thereby effecting separation of the matter from any other inert material present in the medium which is not floated in the froth. Further frothing agent may then be added, a froth produced and the froth removed so that substantially all of the matter is removed from the medium. The ions associated with the matter to be separated may be anions, in which case the frothing agent of has one or more cationic groups. Alternatively, the ions associated with the matter to be separated may be cations, and the frothing agent may have one or more anionic groups. For example, the oxidation state may be changed by oxidising the matter using oxidants such as hydrogen peroxide, ozone, oxygen-enriched air or potassium permanganate. Alternatively, the oxidation state of the matter may be changed by reduction with, for example, hydroxylamine hydrogenchloride. The oxidation state may be changed only at the surface of the matter. The liquid medium may be alkaline, neutral, or mildly acidic (for example pH 3 to 6). For example, the liquid medium may be an alkaline medium based on substantially magnesium hydroxide. The matter to be separated may comprise particles of an actinide or an oxide of actinide. Typically the matter may comprise uranium or an oxide of uranium, such as uranium dioxide. It is preferable that the matter comprises particles of 150 µm in diameter or less. In the case where the ions associated with the matter to be separated are anions, the ions are typically carbonate but other ions such as sulphate, chloride, phosphate, thiocyanate, and anions of carboxylic acids such as citric acid and ethylenediaminetetra-acetic acid may be used. Examples of frothing agents having cationic groups for bonding with such anions are cetyl trimethyl ammonium bromide and cetyl pyridinium chloride. By changing the oxidation state of the surface of the actinide-containing matter, bonding of the ions to the matter is facilitated. The bonding of the ions to the matter gives the matter a charge which allows the matter further to bond to a frothing agent having groups of opposite charge. When a gas such as air is bubbled into the liquid medium the frothing agent produces a froth at the surface of the liquid medium. Since the actinide-containing matter bonds to the frothing agent, the matter is floated to the surface of the liquid medium. Removal of the froth allows the matter to be separated as particles from any species in the liquid which does or do not bond with the ions in step (i). An illustrative experiment will now be described by way of example, which will make clear the principles of the invention. Uranium dioxide powder (lg) is mixed with 10g of corroded Magnox (a sludge including magnesium hydroxide) and conditioned overnight in a solution of hydrogen peroxide (6% w/v). This has the effect of oxidising the surface of the uranium dioxide particles present in the powder to uranium trioxide. The solution is filtered and the solid collected, re-bulked in a solution containing sufficient sodium carbonate to give a pH of 8 to 9. After this stage the surface of the particles contain a negatively charged complex of UO₂(CO₃)₃⁴⁻. A solution comprising an excess of cetyl trimethylammonium bromide in alcohol (about 0.0001 M) is then added and air bubbled into the mixture to form a froth. The negatively changed particles of uranium dioxide stick to the positively charged end of the cetyl trimethylammonium bromide and are thus concentrated in the froth, which can be skimmed off the rest of the mixture, leaving behind cost of the magnesium hydroxide. Typically 90% of the uranium dioxide particles are removed in about 40% of the water together with 25% of the Magnesium hydroxide. When applied to the treatment of irradiated nuclear fuel, in particular the Magnox cladding thereof, oxides of actinides, such as uranium dioxide, plutonium dioxide and americium oxide present in the magnesium hydroxide based sludge, produced during prolonged storage of Magnox fuel cladding under water, are floated. The sludge is first conditioned with a solution of hydrogen peroxide (6% w/v) to oxidise the surface of the actinide particles and then treated with a complexing agent such a citrate, followed by cetyl trimethyl ammonium bromide. Air is bubbled into the mixture to form a froth and float the actinide particles. The froth is then skimmed off to effect separation of the actinide particles from the remainder of the sludge. The frothing process may be repeated by adding further cetyl trimethylammonium bromide and bubbling air into the mixture. In this way substantially all of actinide particles may be removed from the sludge. It is envisaged that the flotation process may be used to float uranium dioxide particles in the uranium mining industry. It should be appreciated that the floatation process is not limited to use within the nuclear industry.

Claims: A method of causing matter to float at the surface of a liquid medium to assist it separation, the method comprising the steps of: (i) forming a mixture in a liquid medium of the matter to be separated and a frothing agent for the liquid medium, the matter to be separated and the frothing agent having oppositely charged groups; and (ii) causing the mixture to froth whereby the oppositely charged groups are attached to one another and are concentrated in froth at the surface of the liquid medium wherein the matter to be separated comprises particulate matter whose particles have a surface whose oxidation state is changed before step (i) is carried out to facilitate the attachment thereto of ionic groups and which surfaces have attached thereto such ionic groups suitable for bonding with oppositely charged ionic groups of the frothing agent, the said particulate matter comprising an actinide-containing material. A method as claimed in claim 1 and in which the oxidation state is changed by oxidising the surface of the particles of matter. A method as claimed in claim 2 and in which the surface of the particles matter is oxidised using hydrogen peroxide. A method as claimed in any one of the preceding claims and in which the ions associated with the matter to be separated are anions, and in which the frothing agent has one or more cationic groups. A method as claimed in claim 4 and in which the anions are selected from carbonate, citrate, ethylene diaminetetra-acetate, cyanide, sulphate, chloride, phosphate and thiocyanate ions. A method as claimed in any of claims 4 or 5 and in which the frothing agent is selected from cetyl trimethyl ammonium bromide or cetyl pyridinium chloride. A method as claimed in any one of the preceding claims and in which the liquid medium is alkaline. A method as claimed in any one of claims 1 to 6 and in which the liquid medium is neutral. A method as claimed in claim 7 and in which the liquid medium includes magnesium hydroxide. A method as claimed in any one of the preceding claims and in which the particulate matter comprises particles of diameter 150 µm or less. A method as claimed in any one of the preceding claims and wherein the actinide is uranium.

Inventors: TAYLOR, JOHN MARTIN, WHITE, DAVID ANTHONY
Applicants: BRITISH NUCLEAR FUELS PLC
Application year: 1987
Main classification: B03D1
IPC codes: B03D1, B03B1, G21C19