Separation of matter by floatation

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 oxidized 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.

BACKGROUND OF THE INVENTION 
This invention relates to the floatation of matter at the surface of a 
liquid medium to assist its separation and is particularly, but not 
necessarily exclusively, concerned with the floatation 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 has 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. 
The aim of the present invention is to provide a method of separating 
matter by floatation. 
FEATURES AND ASPECTS OF THE INVENTION 
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) changing the oxidation state of the matter; 
(ii) bonding ions to the surface at least of the matter; 
(iii) making a mixture comprising the product of step (ii), a liquid medium 
and a frothing agent for the liquid medium, the frothing agent having one 
or more groups of opposite charge to the ions of step (ii); and 
(iv) causing the mixture to froth whereby the frothing agent and the 
product of step (ii) form a bond and are together concentrated in the 
froth at the surface of the liquid medium. 
Preferably, the froth is removed 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 of step (ii) may be anions, in which case the frothing agent of 
step (iii) has one or more cationic groups. Alternatively, the ions of 
step (ii) may be cations, and the frothing agent may have one or more 
anionic groups. 
The oxidation state of the matter may be changed in step (i) to a state 
which facilitates the bonding of the ions to the matter in step (ii). 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 may be particulate or may be dissolved ions. For example, the 
matter may comprise particles of a metal or an oxide of a metal such, as 
an actinide or an oxide of an actinide. Typically the matter may comprise 
uranium or an oxide of uranium, such as uranium dioxide. The matter may 
also comprise soluble ions such as the uranyl ion. 
It is preferable that the matter comprises particles of 150 .mu.m in 
diameter or less. 
In the case where the ions in step (ii) 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 which may be used are cetyl trimethyl ammonium 
bromide and cetyl pyridinium chloride. 
By changing the oxidation state of the matter of the surface of the 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 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 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 from any species in the 
liquid which does or do not bond with the ions in step (ii). 
An illustrative experiment will now be described by way of example, which 
will make clear the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Uranium dioxide powder (1 g) is mixed with 10 g 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 UO.sub.2 (CO.sub.3).sub.3.sup.4-. 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 charged 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 most 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 maxnesium 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 floatation process may be used to float uranium 
dioxide particles in the uranium mixing industry. 
It should be appreciated that the floatation process is not limited to use 
within the nuclear industry.