Cryogenic sedimentation is effective to bring about the rapid separation of sub-micron particles from powder mixtures containing such particles without leading to significant agglomeration.

BACKGROUND OF THE INVENTION 
The present invention relates to the separation of finely divided powders 
into fractions with a specific size range and particularly to the 
separation of powders in the sub-micron size range. 
Historically the separation by size of such very fine powders has been done 
using a sedimentation technique that relies on the differential rate of 
sedimentation of particles as predicted by Stokes' law. In practice the 
larger the particle, the faster it is deposited therefore if chemically 
identical powders with a range of sizes are dispersed in a dispersion 
medium in which the particles are not soluble, such as water, the heavier 
particles settle out first, followed by the next heavier and so on until 
the finest are deposited. The separation is usually carried out in a 
column and the sediment at the bottom of the column after all particles 
have settled out will have the finer particles collected in the top layers 
and the heavier particles in the bottom layers. This is sometimes 
accomplished in a cascade of separator columns each separating out a 
fraction of the particle sizes to minimize the waiting time. This can be 
quite considerable since sub-micron particles dispersed in water can take 
weeks to settle out. Working with a cascade of columns allows the larger 
sizes to be removed relatively quickly so as to leave the finer particle 
sizes behind. 
In an attempt to speed up the process of settling out the finest particles, 
a settling agent of a lower density such as alcohol may be added to the 
dispersion medium and this is successful to some degree but the time taken 
to separate sub-micron particles is still very long. In addition the 
mixture then has to undergo dialysis to remove the settling agent before 
the powder is dried. 
A further problem arises when drying the sub-micron particles. If the 
separation from the dispersion medium involves heating this causes the 
particles to agglomerate. There has been some success in drying the 
powders using a freeze drying technique. This reduces the agglomeration 
but does nothing to shorten the separation process. 
Another separation technique involves elutriation in which a dispersion of 
the particles in, for example, water is cause to flow at a defined rate. 
This passes through a series of vessels of increasing diameter. The finer 
particles will travel further than the coarser such that a separation can 
take place. Once again however the problems of separation from the 
dispersion medium and drying discussed above are encountered. 
A process for separating very fine particles has now been developed that 
can be completed in a fraction of the time taken using prior art 
techniques. Moreover the process allows rapid controlled drying without 
agglomeration. In this way the process of the invention represents an 
inexpensive, convenient and effective means for producing fine, and 
particularly sub-micron, powders from powder mixtures. 
DESCRIPTION OF THE INVENTION 
The present invention provides a sedimentation process for the separation 
by particle size of fine powders in which the sedimentation medium is a 
liquid that is a gas above 0.degree. C. For this reason the process is 
referred to herein as "cryogenic sedimentation". 
It is found that cryogenic sedimentation occurs much more rapidly than 
separation using conventional separation media. In addition the removal of 
the medium after sedimentation has proceeded to the desired degree is very 
easily accomplished by simply raising the temperature above zero. 
The preferred medium depends on the powder to be separated but in general a 
liquified gas that is relatively inert and environmentally neutral is 
preferred. Liquifiable atmospheric gases are suitable including liquid 
nitrogen or oxygen and liquified rare gases such as argon, or neon could 
be substituted albeit at a higher cost. In addition other liquifiable 
gases such a lower hydrocarbons such as methane and mixtures of such gases 
and liquifiable halohydrocarbons and ammonia. Clearly however many of the 
potential options could only be safely or responsibly used in closed 
systems where the gases could not escape into the environment. In general 
therefore, for most applications, the preferred cryogenic separation 
medium is liquid nitrogen. 
The powders to be separated are not limited except by their stability under 
cryogenic conditions. The most usual powders to which the process might be 
applied are ceramic oxides such as alumina, magnesia, titania, zirconia 
and silica though this by no means a necessary limitation on the sort of 
powders to which the invention may be applied. 
In the operation of a preferred process of the invention a pressurized, 
insulated column is prepared and filled with liquid nitrogen. When 
equilibrium has been reached a powder having particles of a plurality of 
sizes is introduced at the top of the column and stirred to disperse the 
powder thoroughly in the medium. The particles are then allowed to 
sediment until the larger and undesired particles have sedimented out. 
Thereafter the liquid nitrogen remaining, which still has the finer 
particles dispersed therein, is separated and the nitrogen is removed from 
the pressurized enclosure so as to permit evaporation of the liquid 
nitrogen at a controlled rate. Generally this process should not be too 
rapid as some of the powder could be entrained in the evaporating liquid 
and be carried away. When all the nitrogen has been removed, the fine 
particles size fraction of the starting powder remains in an essentially 
unagglomerated and freely flowing form. 
The particle sizes to which the cryogenic separation process of the 
invention can be applied are not constrained by any of the necessary 
features of the process. Thus the range of particle sizes can be for 
example from 0.01 micrometer to 100 micrometer or more. However since 
larger particles generally sediment at a reasonably rapid rate in 
dispersion media such as water, the advantages of cryogenic separation in 
terms of speed of sedimentation are not so significant. Thus the cryogenic 
sedimentation process is most conveniently applied to separate powder 
particle sizes below about 5 micrometers and particularly below about 1 
micrometer such as for example from 0.1 to 1.0 micrometer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention is now described with reference ton the following example 
which is provide solely for the purpose of illustrating the invention and 
should not be understood to imply any necessary limitation of the scope of 
the invention. 
EXAMPLE 
A vertically oriented, cylindrical stainless steel pressure vessel was 
fitted with a superinsulating jacket and means to sense temperature and 
pressure. The bottom two thirds of the vessel was provide with an exterior 
coil wrap capable of circulating liquid at -20.degree. C. to +40.degree. 
C. The vessel was filled with liquid nitrogen while the circulating liquid 
in the coil was at -20.degree. C. The material to be separated by particle 
sizes was a synthetic diamond powder for which a particle size 
distribution curve had previously been generated. The diamond powder was 
mixed with a carrier of liquid nitrogen in an amount to give a 20-25% 
solids slurry and then introduced into the cylinder of liquid nitrogen 
which had been pressurized to about 200 atmospheres. The diamond particles 
were then allowed to settle to the bottom, with the largest particles 
falling fastest and the smallest slowest such that, at the bottom the 
particles are size segregated with the largest at the bottom and the 
smallest at the top. When the sedimentation is completed the temperature 
in the coil is allowed to rise and the vessel is opened to the atmosphere 
such that the nitrogen is evacuated leaving a cake of diamond powder. 
Because the powder has seen no heating, there is no agglomeration and the 
cake can be sliced tp remove volume proportions corresponding to the 
various size ranges previously identified in the powder. 
In this way a very accurate separation can be made of all the size ranges 
present in the mixture and the fractions are immediately usable because 
they are non-agglomerated. 
The separation process of the invention can be applied to any powder 
containing a variety of particle sizes providing a cryogenic solvent can 
be found that does not interact with the powder at the temperatures at 
which the cryogenic separation process is conducted.