High-speed centrifugal extractor having spiral liquid path

In a high-speed centrifugal extractor of the type wherein an inlet for a mixture of heavy liquid and light liquid is disposed at the bottom of a cylindrical rotor rotating at a high speed, a heavy liquid outlet and a light liquid outlet are disposed at the upper part of the rotor and a weir for facilitating selective extraction of the heavy liquid and a weir for facilitating selective extraction of the light liquid are disposed at the upper part of the rotor in order to guide the heavy liquid phase and the light liquid phase separated inside the rotor by centrifugal force to the heavy and light liquid outlets, respectively, an improvement resides in a spiral liquid path formed inside the cylindrical rotor and extends from the mixed liquid inlet to the selection weirs so that the mixture of the heavy and light liquids introduced from the mixed liquid inlet rises along the spiral liquid path while rotating therein.

BACKGROUND OF THE INVENTION 
The present invention relates to a high-speed centrifugal extractor capable 
of rapid liquid-liquid extraction by generating centrifugal force. 
The high-speed centrifugal extractor can be used particularly for 
separating uranium and plutonium contained in spent nuclear fuel of 
nuclear fission products when reprocessing the spent nuclear fuel by a 
solvent extraction method (such as a Purex process), but is not limited to 
this application and can be applied widely to liquid-liquid extraction of 
any mixture comprising a heavy liquid and a light liquid. 
A reprocessing method for spent nuclear fuel by employing the Purex process 
involves the steps of bringing a nitric acid solution (heavy liquid) 
containing uranium, plutonium and nuclear fission products into counter 
current flow contact with a tributyl phosphate (hereinafter called "TBP") 
diluted with hydrocarbon solution (light liquid) as an extraction solvent 
of uranium and plutonium in order to extract uranium and plutonium into 
the TBP from the nitric acid solution, then bringing once again this TBP 
into counter current flow contact with a new nitric acid solution in order 
to remove and wash the nuclear fission products that have been extracted 
in slight amounts into the TBP, and further bringing this washed TBP into 
counter current flow contact with a dilute nitric acid solution to 
accomplish a reverse extraction of uranium and plutonium in the TBP into 
the dilute nitric acid solution. 
Extractors such as a mixer settler, a pulse column, and the like, have been 
generally used for facilitating the extraction, washing and reverse 
extraction processes described above. In the mixer settler, however, a 
sufficient residence time must be secured because natural gravitational 
force is utilized to separate both liquids inside the extractor due to the 
difference of specific gravity of the liquids, and for this reason TBP as 
the extraction agent is likely to be damaged by radioactive rays. In the 
pulse column, on the other hand, it is known in the art that the 
dispersion state deteriorates due to wettability of the perforated plate 
fitted inside the column. These are the technical problems to be solved in 
order to obtain a high decontamination factor and a stable operating 
condition. In order to increase the processing capacity, a floor area must 
be increased in the mixer settler while the diameter and height of the 
column must be increased in the pulse column. This means that the overall 
size of the extractors must be increased. 
A high-speed centrifugal extractor has recently been developed as an 
extractor that solves the problems of the conventional extractors 
described above. The high-speed centrifugal extractor forcibly separates a 
mixture of the heavy liquid and light liquid into the heavy liquid and the 
light liquid by centrifugal force, and its typical construction is shown 
in FIG. 4 of the accompanying drawings. The high-speed centrifugal 
extractor fundamentally comprises a casing 1 and a cylindrical rotor 2 
that is rotated at a high speed by a rotary shaft 3 inside the casing. The 
heavy liquid (such as a nitric acid solution) and a light liquid (such as 
TBP as an extraction agent) are supplied into a mixing chamber 6 at a 
lower part of the casing 1 from respective supply pipes 4 and 5. After the 
liquids are sufficiently mixed inside the mixing chamber 6 by an impeller 
7 disposed at the lower end of the rotary shaft and rotating with it, the 
mixture is introduced into the rotor 2 through a center opening 9 of a 
rotor lower end plate 8. After the mixture is further stirred between the 
rotor end plate 8 and a baffle plate 10, the heavy liquid having a greater 
specific gravity is forced outwards while the light liquid having a 
smaller specific gravity remains inward of the heavy liquid due to the 
centrifugal force generated at the inner peripheral surface 2a of the 
rotor, and the liquids rise upwards on the rotor inner peripheral surface. 
Weirs 11 and 12 for facilitating selective extraction are disposed at the 
upper part inside the rotor 2 in order to separate the heavy liquid and 
the light liquid and to introduce them to respective outlets 13 and 14. 
The weir 11 for facilitating selective extraction of the heavy liquid has 
a heavy liquid draw port 11a that is open outwardly of the interface K 
between the outer heavy liquid phase and the inner light liquid phase, 
that is, on the side of the heavy liquid phase, and the heavy liquid 
passing through this draw port 11a flows over a plurality of weir plates 
11b, 11c, 11d and is introduced into the heavy liquid outlet 13, and is 
discharged therefrom to a heavy liquid discharge port 16 through a heavy 
liquid collection chamber 15 (as represented by the solid line arrow in 
the drawing). On the other hand, the light liquid selection weir 12 has a 
light liquid draw port 12a that is open inwardly of the interface K 
between the heavy liquid phase and the light liquid phase, that is, on the 
side of the light liquid phase, and the light liquid flowing through this 
draw port 12a is introduced into a light liquid outlet 14, and is 
discharged therefrom to a light liquid discharge port 18 through a light 
liquid collection chamber 17 (as represented by dotted line arrow in the 
drawing). 
Since the high-speed centrifugal extractor forcibly separates the heavy 
liquid and the light liquid by centrifugal force as described above, it 
provides the following advantages. 
(1) Since mixing is performed at a high-speed, the extraction efficiency is 
high. 
(2) Since the contact time is extremely short, damage of the extraction 
agent by radioactive rays is minimal. 
(3) Since the quantity of the liquid residing inside the extractor is 
small, the hold up quantity of nuclear substances and radioactive 
substances is small. 
(4) The size of the extractor required in order to secure the same 
processing capacity as that of the conventional pulse column or mixer 
settler is extremely small. 
(5) The time required before the equilibrium state is reached is short so 
that the time required from the start to the finish of processing is 
extremely short and also the quantity of resulting waste liquor is 
extremely small. 
In the centrifugal extractor of the type described above, sufficient 
centrifugal force will not act upon the mixed liquid introduced into the 
cylinder of the rotor 2 unless the mixed liquid rotates at a high speed 
with the revolution of the rotor. Therefore, in the conventional 
centrifugal extractor, a plurality of partitions 19 are disposed inside 
and in the longitudinal direction of the rotor and extend radially from 
the rotary shaft 3 to the rotor inner peripheral surface 2a, to divide the 
rotor cylinder into a plurality of chambers as shown in FIG. 5 and thus 
prevent the occurrence of slip between the liquid and the rotor inner 
peripheral surface 2a. (In FIG. 4, the partitions 19 are not shown.) 
However, a plurality of partitions 19 must be firmly fixed between the 
rotary shaft 3 and the rotor inner peripheral surface 2a by welding or the 
like. Accordingly, disassembly and inspection inside the rotor cannot be 
made easily. Since the rotor 2 rotates at a high speed, the partitions 19 
must be fitted in a well balanced condition and the production and 
assembly of the partitions 19 with an ordinary machining technique is 
rather difficult. 
If the mixture of the heavy liquid and the light liquid introduced into the 
rotor 2 contains any solid particles having a greater specific gravity 
than that of the heavy liquid, these solid particles will be deposited on 
the inner peripheral surface 2a of the rotor due to the centrifugal force 
inside the rotor. The solid particles thus deposited cannot be removed 
easily and eventually, the entire rotor must be replaced. 
The mixture of the heavy liquid and the light liquid formed inside the 
mixing chamber 6 at the lower part of the casing 1 is introduced into the 
rotor 2 by the pressure of the impeller 7 through the center opening 9 of 
the rotor lower end plate 8 and at this time, the mixed liquid cannot be 
directly fed into the upper part of the rotor by the action of the baffle 
plate 10. If the pressure generated by the impeller 7 is high, however, 
the mixed liquid is fed directly into the upper portion of the rotor 
without being separated, and the effective separation between the heavy 
liquid and the light liquid due to centrifugal force will not be made. 
SUMMARY OF THE INVENTION 
In order to solve these problems of the conventional high-speed centrifugal 
speed extractor described above, it is therefore an object of the present 
invention to provide a high-speed centrifugal extractor having a structure 
which eliminates the necessity of disposing the partitions for preventing 
slip between the rotor inner peripheral surface and the liquid when the 
rotor is rotating. 
Another object of the present invention is to provide a high-speed 
centrifugal extractor which has a simple structure and is easy to produce 
and assemble. 
Another object of the present invention is to provide a high-speed 
centrifugal extractor which prevents the deposition of solid particles on 
a rotor inner peripheral surface even if the mixed liquid introduced into 
the rotor contains any solid particles. 
A further object of the present invention is to provide a high-speed 
centrifugal extractor which prevents the direct feed of the mixture of the 
heavy liquid and the light liquid into the upper portion of a rotor. 
According to the present invention, there is provided a high-speed 
centrifugal extractor of the type wherein an inlet for a mixture of heavy 
liquid and light liquid is disposed at the bottom of a cylindrical rotor 
rotating at a high speed, a heavy liquid outlet and a light liquid outlet 
are disposed at the upper part of the rotor and a weir for facilitating 
selective extraction of the heavy liquid and a weir for facilitating 
selective extraction of the light liquid are disposed at the upper part of 
the rotor in order to guide the heavy liquid phase and the light liquid 
phase separated inside the rotor by a centrifugal force to the heavy and 
light liquid outlets, respectively, characterized in that a spiral liquid 
path is formed inside the cylindrical rotor and extends from the liquid 
inlet to the selection weirs so that the mixed mixture of the heavy and 
light liquids introduced from the mixed liquid inlet rises along the 
spiral liquid path while rotating therein. 
The spiral liquid path described above can be formed by a rotor rotary 
shaft having spiral projections threaded to a rotor cylindrical wall 
equipped with spiral grooves on the inner peripheral surface thereof. 
The apparatus of the present invention has a similar structure to the 
conventional centrifugal extractor in that the inlet of the mixed liquid 
of the heavy liquid and the light liquid is disposed at the bottom of the 
cylindrical rotor rotating at a high speed, the heavy liquid outlet and 
the light liquid outlet are disposed at the upper portions of the rotor 
and the weir for facilitating selective extraction of the heavy liquid and 
the weir for facilitating selective extraction of the light liquid are 
disposed at the upper portions inside the rotor in order to introduce the 
outer heavy liquid phase and the inner light liquid phase to the 
respective outlets. 
An important feature of the present invention lies in the internal 
structure of the cylindrical rotor. Namely, the partitions for preventing 
the slip between the rotor inner peripheral surface and the liquid 
employed in the conventional apparatus are not disposed inside nor do the 
same extend in the longitudinal direction of the rotor, but rather the 
spiral liquid path is formed inside the rotor and extends from the mixed 
liquid inlet at the bottom of the rotor to the upper selection weirs at 
the upper part of the rotor. 
The mixture of the heavy liquid and the light liquid that is introduced 
from the mixed liquid inlet into the rotor rotating at a high speed is 
introduced into the spiral liquid path and rises along the rotor inner 
peripheral surface while rotating inside this liquid path. During such a 
rotating and rising process, the heavy liquid of the mixture is forced 
outwardly while the light liquid remains inwardly of the heavy liquid due 
to centrifugal force and they then flow to the upper part of the rotor. 
The heavy liquid phase thus separated is introduced into the heavy liquid 
outlet through the heavy liquid draw port of the weir, for facilitating 
selective extraction of the heavy liquid, that is disposed at the upper 
part of the rotor, in the same way as in the conventional apparatus shown 
in FIG. 4, while the light liquid phase is introduced into the light 
liquid outlet through the light liquid draw port of the weir for 
facilitating selective extraction of the light liquid. 
As described above, the mixed liquid introduced into the rotor rises while 
rotating inside the spiral liquid phase 40. At this time, the rotational 
speed of the rotor is higher than the rotating moving speed of the liquid 
so that slip always occurs between the liquid and the rotor inner 
peripheral surface. Thus, even if any solid particles are contained in the 
mixed liquid, they cannot be easily deposited on the rotor inner 
peripheral surface. 
Since no portion of the vertical liquid path extending from the lower part 
to the upper part of the rotor is formed inside the rotor, the mixed 
liquid introduced from the mixed liquid inlet at the bottom of the rotor 
by the pressure of the impeller will not be forced directly into the upper 
part of the rotor. 
The above and other objects and novel features of the present invention 
will become more apparent from the following detailed description when 
taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described in further detail with 
reference to preferred embodiments thereof and to FIGS. 1 to 3. In these 
drawings, like reference numerals are used to identify like parts and 
elements of the conventional apparatus shown in FIG. 4. 
FIG. 1 is a schematic sectional view illustrating the overall construction 
of the high-speed centrifugal extractor in accordance with the present 
invention. The extractor fundamentally comprise a casing 1 and a 
cylindrical rotor 2 that is disposed inside and coaxially with the casing 
1 and can be rotated freely. A rotary shaft 30 rotated at a high speed by 
driving means (not shown) is disposed at the center of the rotor 2 so that 
the rotor 2 rotates with the rotary shaft 30. The rotary shaft 30 extends 
into a mixing chamber 6 at the lower part of the casing 1 through a center 
opening 9 of a rotor lower end plate 8, and an impeller 7 is fitted to the 
lower end of the rotary shaft 30. Weirs 11 and 12 for facilitating 
selective extraction of the heavy liquid and the light liquid, 
respectively, are disposed at the upper part inside the rotor 2. The heavy 
liquid weir 11 has a heavy liquid draw port 11a that is open to a heavy 
liquid phase H and communicates with a heavy liquid outlet 13 through a 
plurality of weirs 11b, 11c and 11d. On the other hand, the light liquid 
weir 12 has a light liquid draw port 12a open to a light liquid phase L, 
and communicates with a light liquid outlet 14. The bottom of the casing 1 
is connected to a heavy liquid supply pipe 4 and to a light liquid supply 
pipe 5, and a heavy liquid discharge port 16 and a light liquid discharge 
port 18 are disposed at the upper part of the casing 1 and communicate 
with the heavy liquid and light liquid outlets 13 and 14 of the rotor 
through annular heavy liquid and light liquid collection chambers 15 and 
17, respectively. 
The construction described above is substantially similar to that of the 
conventional apparatus shown in FIG. 4, but the present invention is 
different from the conventional apparatus in that the spiral liquid path 
is formed inside the rotor 2. 
In other words, the rotor principal portion in the present invention has a 
rotor cylindrical wall 20 as shown in FIG. 2 and a rotary shaft 30 having 
spiral projections 31 as shown in FIG. 3. The rotor cylindrical wall 20 
shown in FIG. 2 has spiral grooves 21 formed on its inner peripheral 
surface, while the rotary shaft 30 shown in FIG. 3 has the spiral 
projections 31 formed with the same pitch and extending in the same 
direction as the spiral grooves 21 on the inner peripheral surface of the 
cylindrical wall. When the spiral projections 31 are threadedly fitted 
into the spiral grooves 21, the rotary shaft 30 and the cylindrical wall 
20 comprise an integral structure. Thus, the principal portion of the 
rotor 2 shown in FIG. 1 can be assembled in a manner in which the spiral 
liquid path 40 having a square sectional shape is defined by the rotor 
inner peripheral surface 2a forming one of its side walls inside the rotor 
2. The rotor cylindrical wall 20 and rotary shaft 30 having such shapes 
can be produced easily by an ordinary screw-machining technique. 
Next, the operation of the high-speed centrifugal extractor having the 
construction described above will be explained. The heavy liquid (e.g. a 
nitric acid solution) and the light liquid (e.g. TBP as the extraction 
agent) are supplied through the respective supply pipes 4 and 5 into the 
mixing chamber 6 of the casing 1, then mixed sufficiently there by the 
impeller 7 and thereafter introduced into the rotor 2 from the center 
opening 9 of the rotor lower end plate 8. The mixed liquid thus introduced 
into the rotor is immediately fed into the spiral liquid path 40, and 
rises along the rotor inner peripheral surface 2a while rotating inside 
this path. During this rising process, the mixed liquid is separated into 
the outer heavy liquid phase H and the inner light liquid phase L. The 
heavy liquid phase H separated in this manner is discharged through the 
heavy liquid draw port 11a of the heavy liquid weir 11, overflows through 
a plurality of weir plates 11b, 11c and 11d, then flows into the heavy 
liquid collection chamber 15 of the casing through the heavy liquid outlet 
13 and is thereafter withdrawn from the heavy liquid discharge port 16. 
The light liquid phase L is discharged through the light liquid draw port 
12a of the light liquid weir 12, flows into the light liquid collection 
chamber 17 of the casing through the light liquid outlet 14 and is 
withdrawn from the light liquid discharge port 18. 
The construction of the high-speed centrifugal extractor shown in the 
drawings is merely for explaining a preferred embodiment of the present 
invention. Therefore, the present invention is not particularly limited 
thereto. For example, the shape of the casing 1, the mode of supply of the 
heavy and light liquids into the casing or rotor 2, the structure of the 
weirs 11, 12 for selecting the heavy and light liquids, and so forth, need 
not necessarily be the same as those of the embodiment shown in the 
drawings. And, the the characterizing feature of the present invention, 
wherein the spiral liquid path 40 is disposed inside the rotor, can be 
applied to any type of centrifugal extractor so long as it is of the type 
in which the mixed liquid comprising heavy and light liquids is supplied 
to the bottom of the cylindrical rotor that rotates at a high speed so as 
to be separated into heavy and light phases by centrifugal force. 
As can be understood from the description given above, the present 
invention is drawn to a structure wherein the spiral liquid path is 
disposed inside the rotor, and eliminates the necessity of disposing the 
slip prevention partitions in the longitudinal direction of the rotor as 
in the conventional apparatus. As a result, the mixture of the heavy and 
light liquids introduced into the rotor is gradually separated into a 
heavy liquid phase and a light liquid phase while rotating and rising 
along the spiral liquid path. 
Accordingly, since the liquid rises to the upper part of the rotor while 
slipping on the rotor inner peripheral surface, any solid particles 
contained in the liquid are not easily deposited on the rotor inner 
peripheral surface. Thus, the present invention eliminates the problem of 
the conventional apparatus in which the deposited solid particles cannot 
be removed and the entire rotor must be replaced. 
Since the vertical liquid path of the conventional apparatus that extends 
from the lower part to the upper part is not provided inside the rotor in 
the present invention, the mixture of the heavy and light liquids 
introduced from the rotor bottom is never forced directly to the upper 
part of the rotor which, otherwise, would result in ineffective separation 
of the liquid phases by the centrifugal force. 
Furthermore, the cylindrical rotor having the spiral liquid path formed 
therein can be assembled relatively easily by screwing the rotary shaft 
having the spiral projections into the rotor cylindrical wall formed with 
the spiral grooves on its inner peripheral surface. Therefore, the rotor 
of the present invention has a simpler construction and can be produced 
more easily than the rotor of the conventional apparatus, and disassembly 
and inspection of the rotor can be made more easily. 
While the preferred embodiment of the present invention has thus been 
described using specific terms, such description is for illustrative 
purpose only, and it is to be understood that changes and modifications 
may be made without departing from the spirit or scope of the following 
claims.