Multistage centrifugal extractor

A centrifugal extractor comprising several superimposed stages (E1, E2, E3) has a fixed casing (10) within which rotate at high speed bowls (22) driven by a common vertical shaft (18). An immiscible light liquid and heavy liquid are emulsified and injected into a separating compartment (32) defined by each of the bowls (22) by ducts (54) forming ejectors and which are formed in the lower wall of the corresponding bowl. The light phase passes upwardly from one stage to the next by annular passages (46) surrounding the shaft (18), while the heavy phase redascends by gravity in return channels (44) formed in the fixed casing. This avoids turbulence in the separating compartments (32) and there is a reduction of the risks of clogging as a result of solid particles being present in the heavy phase.

BACKGROUND OF THE INVENTION 
The invention relates to a centrifugal extractor comprising several 
superimposed stages in which two normally immiscible liquids are brought 
together in order to ensure the treatment of a first of the liquids by a 
second one of the liquids. 
It should be noted that at least one of the two liquids brought together 
within the extractor can also contain suspended solid particles. 
Centrifugal extractors are apparatuses producing an emulsion of a liquid to 
be treated and a treatment liquid, which are normally immiscible, with a 
view to aiding the treatment and then ensuring the separation of the two 
liquids by centrifuging. The liquid to be treated can in particular be 
constituted by a heavy aqueous phase, whereas the treatment liquid is a 
light, organic phase. 
In practice, it is usually necessary to repeat these two operations a 
certain number of times in order to optimize the treatment. For this 
purpose, use is normally made of centrifugal extractors having several 
superimposed stages. In these centrifugal extractors the heavy aqueous 
phase to be treated circulates from top to bottom which is from the upper 
stage to the lower stage, whereas the light organic phase ensuring the 
treatment circulates from bottom to top which is from the lower stage to 
the upper stage of the apparatus. 
Among the existing centrifugal extractors with superimposed stages, 
reference is made to the centrifugal extractors of type LX marketed by 
ROBATEL SLPI. Within a fixed enclosure, these centrifugal extractors 
comprise a bowl which can be rotated at high speed around a vertical axis 
and which defines internally several superimposed separating compartments. 
The rotary bowl is in turn placed coaxjelly around a fixed, central, 
tubular barrel or shaft, which extends towards the inside each of the 
separating compartments. 
In the bottom of each of the separating compartments, the rotary bowl has a 
mixing chamber, which is separately penetrated by the heavy phase from the 
stage located immediately above, and the light phase from the stage 
located immediately below. A stirring disk, integral with the fixed 
central shaft or barrel, cooperates with the walls of the mixing chamber 
rotating at high speed in order to bring about an extremely fine emulsion 
of the light and heavy phases. Moreover, the assembly constituted by the 
mixing chamber, as well as the channels for introducing and discharging 
the two phases into and out of said chamber, together with the stirring 
disk, form a pump permitting the suction of the two phases from the 
adjacent stages. 
The two phases of the emulsion created in this way are then separated by 
centrifugal force in the separating compartment of the corresponding 
stage. The relatively light phase is admitted into the mixing chamber of 
the stage immediately above it by a passage located in the vicinity of the 
fixed, central shaft. The relatively heavy phase redascends towards the 
mixing chamber of the stage immediately below it by channels located on 
the outer periphery of the rotary bowl. 
Centrifugal extractors having several superimposed stages and constructed 
in this way are essentially satisfactory, but suffer from two significant 
disadvantages. 
The first of these disadvantages results from the circulation of the heavy 
phase from a given stage to the stage immediately below it by channels 
formed in the rotary bowl on the periphery thereof. In view of the fact 
that the bowl rotates at very high speeds and which can, for example, 
reach approximately 20 to 30 m/s, the solid particles generally present in 
the heavy phase tend to agglomerate under the effect of the centrifugal 
force, which leads to clogging actions necessitating the stopping of the 
apparatus. Moreover, the high density of the agglomerated solid materials, 
due to the centrifugal force, makes the unclogging operations long and 
difficult. 
A second disadvantage of centrifugal extractors with several superimposed 
stages of the type described hereinbefore is due to the air present within 
the apparatus. During the rotation of the bowl, the air is positioned 
between the fixed, central shaft and the liquid contained in the 
separating compartments. However, in view of the fact that the liquid is 
rotated at high speed by the rotary bowl, whereas the air layer in contact 
with the central shaft is virtually static, turbulence occurs and passes 
from the air into the liquid phases contained in the different separating 
compartments. This turbulence disturbs the separating efficiency obtained 
by centrifuging in the separating compartments, so that the efficiency of 
each stage is reduced. 
FR-A-2 093 055 also discloses a centrifugal extractor having several 
juxtaposed stages, in which each of the extractor stages comprises a fixed 
casing or housing within the interior of which a bowl internally defining 
a separating compartment is rotated about a vertical shaft. The heavy and 
light phases are brought into the bottom of the fixed casing, where they 
are mixed by paddles integral with the rotary bowl, before rising in an 
axial passage located in the bottom thereof. This rising action is brought 
about by a pumping effect obtained by the cooperation of fixed blades 
carried by an axial rod integral with the casing and rotary blades 
installed in the axial passage of the rotary bowl traversed by said rod. 
The heavy and light phases are consequently introduced into a separating 
compartment formed within the bowl, so as to be separated by centrifuging. 
Overflows provided at the upper end of the separating compartment make it 
possible to pass the heavy and light phases respectively towards the two 
adjacent stages using vertical channels formed in the fixed casing of the 
stage in question. 
This centrifugal extractor does not suffer from the disadvantages of that 
described hereinbefore. Thus, the path of the heavy phase between the 
adjacent stages takes place by gravity within the fixed casing, which 
considerably reduces clogging risks and facilitates unclogging when this 
proves necessary. Moreover, the air layer contained in the rotary bowl is 
in contact with the rotary shaft used for driving said bowl and with the 
liquid rotated in the latter. Consequently the turbulence caused in the 
liquid is reduced compared with the centrifugal extractor having several 
superimposed stages described hereinbefore. Therefore the efficiency of 
each individual stage is considerably improved. 
However, the centrifugal extractor described in FR-A-2 093 055 has several 
juxtaposed stages, which requires a separate motor for rotating each of 
the extractor stages. This increases the overall dimensions, the prime 
cost and the operating costs of the apparatus. Moreover, this contributes 
to an increase in breakdown risks. 
Moreover, the design of the centrifugal extractor described in FH-A-2 093 
055 is such that the transposition of the solution proposed in the case of 
an extractor having several juxtaposed stages to an extractor having 
several superimposed stages is virtually impossible to carry out. In 
particular, the pumping of the mixed liquid, enabling it to rise within 
the rotary bowl into the separating compartment is obtained by a 
cooperation of fixed and rotary blades formed respectively on an axial rod 
integral with the bottom of the fixed casing and in an axial passage 
formed in the bowl around said rod. It is therefore impossible to 
downwardly extend the vertical driving shaft of the rotary bowl in order 
to rotate the rotary bowls of other stages located below the first stage. 
The object of the present invention is to provide centrifugal extractor 
having several superimposed stages, whose original design makes it 
possible to take advantage of the advances provided by the centrifugal 
extractor with juxtaposed stages according to FR-A-2 093 055, while only 
requiring a single drive motor in the same way as is used in the 
centrifugal extractors having several superimposed stages of a 
conventional nature. 
SUMMARY OF THE INVENTION 
According to the present invention, the object is obtained by providing a 
centrifugal extractor comprising a fixed casing, a vertical shaft able to 
rotate within the casing a bowl internally defining a separating 
compartment, a single drive motor for rotating said vertical shaft, and 
means for mixing end injecting into the bottom of the separating 
compartment a relatively heavy liquid phase and a relatively light liquid 
phase. The bowl has at its upper end at least one central passage for the 
discharge of the relatively light phase and at least one peripheral 
passage for the discharge of the relatively heavy phase. The extractor 
comprises a plurality of superimposed stages, each having a bowl which can 
be rotated by the shaft. The central passage of each stage being an 
annular passage formed between the shaft and a tubular wall integral with 
the bowl of the stage and communicating with the mixing and injecting 
means of an upper stage, whereas the peripheral passage of each stage 
issues into a return channel for the relatively heavy phase formed in the 
fixed casing and communicating with the mixing and injecting means of a 
lower stage. 
Advantageously, the mixing and injecting means of the relatively heavy and 
relatively light phases in each separating compartment comprise ducts 
forming ejectors. The erectors are positioned along a horizontal plane in 
a lower wall of the bowl defining the compartment. The ducts are inclined 
with respect to a radial direction and located in the same horizontal 
plane. 
Preferably, a lower intake end of each of the ducts forming ejectors is 
rotated in front of guidance grooves for the relatively heavy phase. The 
guidance grooves are formed in the fixed vessel and radially and inwardly 
from the return channel for the relatively heavy phase. Connecting 
passages formed in the lower wall of the bowl connect an upper end of the 
annular passage to an outer discharge end of each of the ducts forming 
ejectors. 
In this arrangement, the inclined ducts forming ejectors carry out, in 
combination with the guidance grooves for the relatively heavy phase, the 
pumping of the two phases and their intimate mixing at the outlet of the 
inclined ducts. 
Preferably, the inclined ducts forming ejectors issue by their outer 
discharge ends into the guidance grooves for the relatively heavy and 
relatively light phase, formed radially in the lower wall of the bowl and 
whereof an outer end issues into the separating compartment. 
To ensure that the relatively heavy phase does not descend directly into 
the lower stage, the guidance grooves for said relatively heavy phase have 
a radially oriented portion, which extends upwardly until arriving in 
front of said inclined ducts forming ejectors by a vertically oriented 
portion. 
In order to facilitate the flow of the relatively heavy phase during its 
descent by gravity towards the lower stage, the return channel of the 
relatively heavy phase of each stage is angularly displaced by a given 
value relatively to the return channels of the adjacent stages. 
The fitting and dismantling of the centrifugal extractor can be facilitated 
by producing the bowls of the different stages in the form of separate 
parts mounted on a common vertical shaft. Furthermore, the fixed casing 
can be produced in the form of an elementary part for each stage with said 
elementary parts being assembled together in a tight manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is diagrammatically illustrated in FIG. 1, the centrifugal extractor 
according to the invention comprises a generally cylindrical, fixed outer 
casing or housing 10, centered on a vertical axis of the machine. This 
casing 10 has a certain number of superimposed, elementary parts 12, e.g. 
three in the embodiment shown and which are tightly assembled by not 
shown, appropriate means. 
The elementary parts 12 are all identical and are generally in the form of 
cylindrical vessels having a horizontal base 14 (FIG. 2) and a 
cylindrical, peripheral wall 16. Under the effect of the superimposing of 
the different elementary parts 12 during assembly, the base 14 of each of 
them forms a cover for the vessel located immediately below the same. The 
elementary part 12 located in the upper part of the apparatus is sealed by 
a cover 17. 
The elementary parts 12 of the fixed casing 10 define a plurality of 
superimposed stages (three in the embodiment illustrated in FIG. 1) and 
which are designated E1, E2 and E3 starting from the top in FIG. 1. 
The centrifugal extractor diagrammatically illustrated in FIG. 1 also 
comprises an assembly able to rotate at very high speed within the casing 
10 about the axis of the casing 10. This assembly comprises a vertical 
drive shaft 18, which traverses over its entire height the fixed casing 10 
and whose upper end is adapted for rotation at very high speed by a motor 
20. 
The shaft 18 carries the same number of bowls 22 as the extractor has 
stages, i.e. three in the embodiment shown. Each of these bowls 22 
constitutes a separate part placed within one of the elementary 
vessel-like parts 12 of the fixed casing 10. Consequently, each of the 
stages E1 to E3 of the centrifugal extractor has a bowl 22 which can be 
rotated at very high speed by the motor 20 by means of the vertical shaft 
18. 
The fitting of the bowls 22 to the vertical shaft 18 is brought about by 
means of sleeves 24 forming spacers and which are placed on the shaft 18. 
The internal peripheral edge of a lower wall 26, shaped like a disk (FIG. 
2), of each of the bowls 22 is gripped between two adjacent sleeves 24. 
As is more specifically illustrated by FIG. 2, each of the bowls 22 also 
has a cylindrical, outer wall 28 and a horizontal, upper wall 30, also 
shaped like a disk. The lower wall 26 of the bowl 22 of a given stage is 
positioned immediately above the base 14 of the elementary part 12 of said 
stage and the peripheral, outer wall 28 is placed immediately within the 
peripheral wall 16 of the elementary part 12 of said same stage. Finally, 
the upper wall 30 of the bowl 22 of a given stage is positioned 
immediately below the base 14 of the elementary part 12 of the stage 
immediately above it, or immediately below the cover 17 for the stage E1. 
Each of the centrifugal extractor bowls 22 internally defines, between its 
walls 26, 28 and 30, a separating compartment 32 in which immiscible, 
light and heavy phases of an emulsified liquid introduced into said 
compartment are separated by centrifuging. The introduction of the liquid 
takes place through the base of the separating compartment 32 using means 
34 for the mixing and injection of the light and heavy liquid phases into 
the base of the separating compartment 32. These means 34, which will be 
described in greater detail hereinafter, are partly formed in the lower 
wall 26 of the bowl 22 and partly in the base 14 of the elementary part 12 
of the corresponding stage. 
At the bottom of each of the centrifugal extractor stages, the means 34 
consequently make it possible to mix a relatively light liquid and a 
relatively heavy liquid, or more specifically bring about a finely divided 
emulsion of the two immiscible liquids, in order to ensure the treatment 
of the heavy phase by the light phase. The means 34 also make it possible 
to pump the thus formed emulsion in order to make it rise in the 
separating compartment 32 of the corresponding bowl 22. 
Under the effect of the very high speed rotation of the bowl 22, the heavy 
and light phases of the emulsion ejected into the separating compartment 
32 are automatically separated from one another under the effect of the 
centrifugal force. Thus, the heavy phase P1 is in the vicinity of the 
outer, peripheral wall 28 of the bowl 22, while the light phase P2 is in 
the vicinity of the sleeve 24 forming a spacer and from which it is 
separated by an air layer A. 
It is important to note that the air layer A is only in contact with the 
parts or products rotating at the same speed as the bowl 22. Thus, the air 
layer A is in contact with the different parts of the bowl 22, with the 
sleeve 24 forming a spacer and with the light phase P2 rotating with the 
bowl. Consequently, the air layer A also tends to rotate with the bowl 22, 
so that there is virtually no turbulence within the bowl and the 
separation of the two phases of the liquid under the effect of centrifugal 
force is obtained under optimum conditions. 
The upper wall 30 of each of the bowls 22 has in its thickness passages 35, 
which are oriented radially with respect to the vertical axis of the 
apparatus. The outer end of each of these passages 36 issues into an area 
of the separating compartment 32 adjacent to the cylindrical, outer wall 
28, so as to form a peripheral discharge passage for the heavy phase P1 
located in said area. 
The inner end of each of the passages 36 issues between the upper wall 30 
and the base 14 of the elementary part 12 of the stage located immediately 
above it about a tubular wall 48 projecting upwards from the inner edge of 
the upper wall 30 of the bowl 22. The heavy phase P1 passing out of the 
separating compartment 32 through the passages 36 then passes towards the 
outside between the upper wall 30 and the adjacent base 14, as is 
illustrated by the arrows in FIG. 2. 
When the heavy phase P1 strikes against the peripheral 16 of the elementary 
part 12 of the corresponding stage, the heavy phase P1 redascends by 
gravity between said wall 16 and a truncated cone-shaped deflector 38, 
which projects downwardly from the outer, peripheral edge of the upper 
wall 30 of the bowl 22. The base 14 of the elementary part 12 carries a 
ferrule 40, which projects upwardly, between the outer, cylindrical wall 
28 of the bowl 22 and the deflector 38, up to a level well above that of 
the lower edge of the deflector 38. The fall by gravity of the heavy phase 
P1 then continues in the substantially vertical passages 42 formed between 
the outer, peripheral wall 16 and the ferrule 40. 
These passages 42 extend downwardly by substantially vertical passages 44 
formed in the peripheral wall 16 of the elementary part 12 of the stage 
located immediately below the preceding stage. This leads to an automatic 
recycling of the heavy phase P1 of a given stage to the intake of the 
mixing and injecting means 34 of the stage immediately below it. 
With regard to the light phase P2 separated from the heavy phase P1 in each 
of the separating compartments 32, it passes out of the compartment by an 
annular, central discharge passage 46 between the sleeves 24 mounted on 
the shaft 18 and the tubular wall 48 projecting upwardly from the inner 
edge of the upper wall 30 of the bowl 22 of the stage in question. In 
reality, this passage 46 essentially contains the air layer A and a film 
of the light phase P2, which rises along the inner surface of the tubular 
well 48 under the effect of the centrifugal force. 
The light phase P2 from a given stage consequently rises up to the mixing 
and injecting means 34 of the stage immediately above it. 
In order to bring about the mixing and injecting effect of the two phases 
from two stages adjacent to a stage in question, the means 34 have a 
special structure, which will now be described in greater detail relative 
to FIGS. 2 end 3. 
When the heavy phase P1 has reached the lower end of each of the passages 
44 in the peripheral wall 16 of each of the elementary parts 12 of the 
fixed casing 10, it is passed radially towards the interior above the base 
14 of said elementary part using guidance grooves 50. These guidance 
grooves 50 are open on the side of the lower wall 26 of the bow; 22. The 
guidance grooves 50 have a first part 50a, which is oriented radially 
relative to the vertical axis of the apparatus. This first part 50a is 
extended upwardly by a second, vertically oriented part 50b, formed in the 
outer surface of a tubular wall 52 extending upwardly from the inner, 
peripheral edge of the lower wall 14 of the elementary part 12 about the 
tubular wall 48. This second, vertically oriented part 50b of the guidance 
grooves 50 is sealed et its upper end. 
To enable the lower wall 26 to pass around the tubular wall 52, the lower 
wall 26 of each of the bowls 22 has a central, planar, horizontal part by 
which said wall 26 is fixed to the shaft 18 above the upper edge of the 
tubular wall 52, a planer, horizontal, peripheral part adjacent to the 
base 14 and an intermediate, truncated cone-shaped part linking the upper, 
central part and the lower, peripheral part. It should be noted that the 
tubular wall 48 of the adjacent lower stage is connected by its upper edge 
to the central part of the lower wall 26. 
Ducts 54, forming ejectors, are provided in the lower wall 26 of the bowl 
22 at the junction of the planar, peripheral part and the truncated 
cone-shaped, intermediate part facing the vertically oriented parts 50b of 
the guidance grooves 50. These ducts 54 are curved inwardly and inclined 
with respect to a radial direction and are all located in the same 
horizontal plane. In combination with the guidance grooves 50, this 
feature makes it possible to ensure the injection or pumping of the heavy 
phase P1 from the guidance grooves 50 to the separating compartment 32 of 
the corresponding stage. 
The ducts 54 also have a section decreasing from their inner, intake end 
located in front of the vertically oriented parts 50b of the guidance 
grooves 50 and their outer, discharge end. This feature makes it possible 
for the inclined ducts 54 to fulfil an ejector function with respect to 
the light phase P2 from the stage immediately below the same of the 
apparatus. Thus, connecting passages 56 formed in the thickness of the 
central and intermediate parts of the lower well 26 of the bowl 23 link 
the annular, central passage 46 of the immediately lower stage to the 
outer discharge end of each of the ducts 54 forming ejectors. This brings 
about a pumping of the light phase P2 and its intimate mixing in the form 
of an emulsion with the heavy phase P1 admitted by the ducts 54. 
At the outlet from the ducts 54 and the connecting passages 56, the two 
liquids are mixed or emulsified and passed towards the separating 
compartment 32 by divergent, radial guidance grooves 58 formed in the 
thickness of the peripheral part of the lower well 28 of the corresponding 
bowl 22. These guidance grooves issue at their outer end into the 
separating compartment 32 in front of a horizontal deflector 60 carried by 
the outer, cylindrical wall 28 of the bowl 22. This deflector 60 
automatically brings the mixture to a central region of the compartment 
32. 
The light and heavy phases thus circulate in countercurrent within the 
centrifugal extractor between the different stages forming the latter, 
which has the effect of progressively bringing about the treatment of the 
heavy phase by the light phase, or vice versa, by means of mixing 
operations and then separating operations performed successively in each 
of the extractor stages. 
FIG. 1 shows at 62 an intake collector for the heavy phase P1, located in 
the upper part of the centrifugal extractor and communicating with the 
return channels 44 of the upper stage E1 by vertical ducts 64, into which 
the heavy phase P1 flows by gravity. 
The light phase P2 is introduced at the lower end of the apparatus by an 
annular collector 66. The latter issues into a lower annular passage 46, 
which is connected to the mixing and injecting means 34 equipping the 
lower stage E3 of the apparatus. Conversely, the heavy phase outflow takes 
place at the lower end or the apparatus by means of a collector 68, into 
which issue the ducts 70, each of which extends the return channels 44 of 
the lower stage E3. 
Finally, light phase discharge takes place in the upper part or the 
centrifugal extractor by means of a fixed collecting compartment 72, into 
which issues the annular passage 46 of the upper stage E1. 
The return channels 44 for returning the heavy phase from a given stage to 
the stage immediately below it are angularly displaced by a given value 
with respect to the return channels 44 of the immediately adjacent stages, 
so that channels can cross from one stage to the next without being 
interconnected. 
Apart from the absence of turbulence in the separating compartments 32, the 
centrifugal extractor according to the invention has the advantage of 
ensuring the circulation of the heavy phase between the stages by return 
channels 44 formed in the fixed casing 10. As a result of this 
arrangement, the risks of clogging of said channels by solid particles 
present in the heavy phase are very significantly reduced compared with 
apparatuses in which the circulation of the heavy phase between the 
different stages takes place in the rotary part of the extractor. 
Moreover, the cleaning of the apparatus is greatly simplified in the case 
of clogging. 
Moreover, manufacture and assembly of the centrifugal extractor are 
facilitated due to its modular design from this standpoint, the seal 
between the elementary parts 12 of the fixed casing 10 can be obtained 
either by placing O-rings between each of the elements, or by placing the 
complete casing in an outer sealing envelope. 
Obviously, the invention is not limited to the embodiment described in 
exemplified manner hereinbefore and in fact covers all variants thereof. 
In particular, the number of stages constituting the centrifugal extractor 
can differ from three without passing outside the scope of the invention.