Note: Descriptions are shown in the official language in which they were submitted.
~L~374~7
"CENTRIFUGAL SEPARATION METHOD AND APPARATUS"
This invention relates to the separation out of
one of more components entrained in a liquid. The
invention has particular, although not of course
exclusive, application to the separation of heavier
and lighter particles where very low particle
concentration is being sought and where it is
therefore essential to collect substantially all of at
least one of ~he sets of particles.
An essential step in the analysis of stream
sediment samples for traces of kimberlitic indicator
minerals is the removal of quartz from a heavy mineral
concentrate. The traditional method of removing
quartz has been to immerse the sample in a liquid of
high specific gravity, e.g. tetrabromoethane (TBE), in
a separating funnel so that the quartz will float
while the heavy minerals sink. ~owever, this process
entails batches of typically 100 gms which must be
left to separate for at least 2 minutes before the
floats and/or sinks may be removed and a fresh sample
introduced. The process is thus tedious and time
consuming and~ in a typical laboratory installation,
~37~
- necessitates multiple samples being treated in funnels
simultaneously, usually with several operators. This
introduces a risk of cross contamination between
samples and confusion as to which concentrate belongs
to which sample. The process is also susceptible to
operator error, since insufficient settliny time may
result in the presence of heavy grains in the floats,
while an excessive s,ample may lead to light grains in
the sinks. Once separated from the ~uart~, the heavy
mineral concentrate is then usually microscopically
checked grain by grain to identify any grains of
indicator minerals that may be present.
The difficulty with mechanising the quartz removal
process is the extremely low incidence of kimberlitic
lS indicator mineral grains. It would be considered good
to find one indicator grain in 100 samples, which
represents a detection level of about one part in
2,000,000,000. Thus it is desirable that any
separation process permits, first, as thorough as
possible a separation of quartz from grains of heavy
mineral concentrate and, secondly, every single heavy
grain to be cleaned from the device and recovered for
identification. It is particularly important that
there be no carry over of even a single grain of
indicat~r mineral to the following sample as this
would most likely lead to not only failure to
recognize the sample which actually contained the
indicator but also result in very expensive resampling
and analyses from the geographical area corresponding
to the contaminated sample.
~374~37
- For these and other reasons, it has been found
that the quartz removal step cannot be satisfactorily
performed with any existing commercial centrifuge, nor
with elutriating columns, cyclones or dynawhirlpools.
5 Commercial bowl centrifuges generally have the floats
- outlet at the top of the bowl. In order to clean the
bowl between samples, the bowl, or a rubber liner,
must be removed and inverted. Moreover, ~ special
outer container is necessary to catch the liquid and
10 floats flung from the outlet rim.
It is therefore an o~ject of the invention to
provide an improved centrifugal separation process and
an associated centrifuge apparatus which permits
speedier, more efficient performance with due regard
15 to the requirement of a very low detection limit in
subsequent analysis.
The invention accordingly pxovides, in one aspect,
a method of separating out a component entrained in a
liquid, comprising:
continuously feeding the liquid through a
first opening into one end of a vessel and revolving
the liquid therein by rotating the vessel~ the vessel
being designed so that the liquid attains a dynamic
equilibrium at which an annular, centrifugally
25 induced, substantially static revolving volume of the
liguid of stable internal surface is confined by the
vessel about the axis of revolution while further
liquid flows on within said stable internal surface,
whereby the entrained component to be separated is
30 centrifugally directed into said annular volume of the
liquid while liquid substantially clarified of such
~L~3'7~
component flows on within said internal surface and
out a second opening in the other end of the vessel;
and
discontinuing said feeding of the liquid,
5slowing and stopping the rotating vessel to release
said annular volume of the liquid, which then, with
the entrained component, swirls towards and flows
through one of said openings.
By "substantially static volume" in the context of
lOthis specification is meant a volume that
substantially does not exchange liquid with the
on-flowing liquid which continues to pass through the
vessel.
The component may be particulate or a second
15 liquid heavier than the entraining liquid, e.g. oil in
water. Moreover, a particulate component may be
entrained with a lighter particulate component and the
method be primarily directed to separation of the two
sets of particles. Thus, in a particular aspect, the
20 invention provides a method of separating lighter and
heavier particles in a solids mixture, comprising:
entraining the mixture in a liquid of a
specific gravity intermediate the specific gravities
of the lighter and heavier particles;
feeding the liquid with entrained mixture into
one end of a vessel and revolving the liquid therein,
by rotating the vessel, the vessel being designed so
.that the liquid attains a dynamic equilibrium at which
an annular, centrifugally induced, substantially
30 static, revolving volume of the liquid of stable
internal surface is confined by the vessel about the
~2~ 7
- axis of revolution while further liquid flows on
within said stable internal surface, whereby the
heavier particles are centrifugally directed into said
annular volume of the liquid while the lighter
sparticles remain entrained in on-flowing liquid within
said internal surface which flows out a second opening
in the other end of the vessel; and discontinuing
feeding the mixture to the vessel, slowing and
stopping the rotating vessel to release said annular
10 vo.ume of the liquid, which then, with the heavier
particles entrained, swirls towards and flows through
one of said openings.
The method is advantageously applied where the
heavier particles are in very low concentration in the
15 mixture.
The method preferably comprises continuing to
feed the liquid, without entrained mixture, through
said first opening as the vessel slows and/or after it
has stopped, to flush out any residual heavier
20 particles remaining on the interior surface of the
- vessel. Moreover, liquid without entrained mixture
may be fed through said first opening for a period
before slowing said vessel, to flush out any residual
li~hter particles within or on said internal liquid
25 surface.
The axis of revolution is preferably vertical or
substantially inclined to the horizontal with the
first opening uppermost. The on-flowing liquid within
said internal surface will typically be a helically
flowing stream itself defining an internal surface.
Advantageously, the method of the invention further
~3~
- includes physically breaking the natural such surface
over a reglon sufficient to submerge heavier particles
thereon and thereby to encourage their movement to
said annular volume of the liquid.
In a further aspect the invention provides
centrifuge apparatus comprising:
a vessel having a pair of opposed openings;
bearing means to support the vessel ~or
rotation about an axis wbich passes through or near
said openings; and
an annular space within said vessel between
and laterally of said openings for retaining said
centrifugally induced, substantially static, revolving
volume of the liquid during performance of the method;
wherein, said vessel has a smooth continuous
interior surface, free of ne~ative slopes, edges or
bends for a liquid droplet swirling towards said one
opening, whereby sufficient swirling liquid is able to
clean the whole of said smooth interior surface of
heavier particles-
The vessel and bearing means are preferably somounted that said axis is vertical or substantially
inclined to the horizontal. Alternatively, if the
vessel rotates about a horizontal or near-horizontal
axis, it is preferable that it be mounted for
selective movement to a greater inclination, most
preferably vertical.
With the provision of the smooth continuous
interior surface as described, surface tension etfects
will ensure that further liquid will wash down t~e
inside surface of the vessel to remove any residual
~L~37~37
heavy particles, which will be observable, if the
vessel is transparent, by virtue of the identifying
bow wave.
Advantageously, the apparatus further includes a
5 separation member within the vessel between said
openings to facilitate the centrifugal action on the
liquid feed, and for physically breaking the natural
internal surface of the on-flowing liquid over a
region sulfficient to submerge heavy particles thereon
10 and thereby to encourage their movement to said
annular volume of the liquid.
It will be appreciated thatl as with other prior
classes of centrifuges, the apparatus may be employed
to classify particulate material, e.g. by using a bank
lS Of vessels in series with different liquid/speed
combinations.
The invention will be further described, by way of
example only, with reference to the accompanying
drawings, in which:
Figure 1 is a schematic diagram depicting both
centrifuge apparatus according to the invention and
peripheral equipment employed with the apparatus where
it is to be used for the separation of heavier and
lighter particles in a solids mixture.
Figure 2 is a side elevation/axial cross-section
of the said centrifuge apparatus; and
Figures 3A to 3D diagramatically indicate
successive stages in the centrifugal separation
process.
Refering firstly to Figure 1, the centrifuge
apparatus 10 includes a somewhat elongate, axially
symmetrical vessel 12 supported when in use with its
axis vertical in upper and lower bearings 14, 16.
Respective opposed openings 15, 17 at the bearings
respectively serve as an inlet opening 15 fed by a
5 funnel 18 and as a discharge opening 17 through which
contents of the vessel fall to an underlying
receptacle 39. Bearing 16 carries a pair of drive
pulleys 22 by which the vessel is rotated about its
vertical axis in the bearings 14, 16 by an electric
10 motor 24 via a drive belt 26. A separation member 13
is suspended within vessel 12, being fastened about
inlet opening 15 and rotatably supported in lower
bearing 16.
Peripheral equipment for performing the
15 centrifugal solids separation process includes, in
addition to motor 24 and belt drive 26, a vibratory
solids feeder 28 which discharges a solids mixture to
the funnel 18 at a controlled rate from a primary feed
bin 30, and a heavy liquid source including a head
20 tank 32 from which a liquid is supplied to the funnel
18 along tube 34 via filter 35 and valve 36~ As will
be explained, lighter particles are continuously
discharged from opening 17, being entrained in
on-flowing liquid. The contents of receptacle 38,
25 which has a sieve 39, are directed to a bin 40 where
the floats are collected and from where liquid is
recovered throu~h a 10-micron screen cloth 41 and
returned to head tank 32 by pump 44 from a sump 42.
Vessel 12 is preferably transparent and may be of
30 the general configuration of an upside down bottle,
1~3~d4137
with an aperture in its base, as shown, or may be more
like the shape o~ a conical separating funnel.
~ s will become apparent in due course, it is
necessary that, for a substantial segment of the axis
5 between openings 15, 17 the vessel is of greater
internal cross-section than the openings with respect
to the axis of the vessel: in the illustrated case~
much of the vessel between the openings meets this
requirement. The material of the vessel will
preferably be impervious to and non-reactant with the
chosen li~uid. Stainless steel can be used but has
the disadvantage of not being transparent (for the
purpose of detecting residual particles, as will
become apparent hereinafter)~ Glass may be used but
15 appropriate safety measures need to be taken in case
of accidental breakage. A typical heavy liquid is
tetrabromoethane ~TBE) for which a suitable vessel
material is clear polyethylene terephthalate (PET).
It will also be noted that the interior surface of
20 vessel 12 is smooth and continuous, being curved and
shaped so that no droplet or particle travelling (i.e,
swirling as will be appreciated hereinafter) from
opening 15 to opening 17 is confronted with a negative
- slope, edge or bend. The upper rim l5a (Figure 1) of
25 opening 15 is also curved, although this is not
readily apparent from all the drawings.
Opening 15 comprises a simple aperture through the
vessel wall. The rim of the aperture is retained
between a flange 46 on bearing 14 and an internal
30 retaining ring 48 by three equiangularly spaced
retaining screws 50. Screws 50 project through into
~3'74~7
separation member 13 to provide fastenings for th~
latter.
Bearing 14 has an internal low friction sleeve 52
which rotatably receives the depending outlet neck 54
5 of funnel 18. The main body of funnel 18 has an
underlying boss 56 with side flats 57 by which the
funnel is held against rotation in retaining arms ~not
shown) ~hat project from an upright stand or housing
such as motor housing 23 in Figure 1. Heavy liquid
10 supply tube 34 opens to the interior of funnel 18
substantially tangentially so that the liquid executes
a helically swirling ~low as it descends through the
funnel and outlet neck 54. A curtain of the liquid is
also introduced along sleeve 52 via capillary 55 for
15 the twin purposes of lubricating the bearing and
washing out particles which might otherwise cling
about the lower end of neck 54.
Separation member 13 comprises a broadly
bi-conical head 60 on an elongate co-axial stem 62.
Head 60 is an integral solid having a short
cylindrical portion 64 with peripheral sur~ace 65, a
shallow conical top 66, and an underlying truncated
conical portion 68 which is bored to receive stem 620
Three screw holes 69 receive screws 50 for suspending
the separation member below the retaining ring 48 so
as to maintain a finite annular spacing.
The lower end of stem 62 is thinned to vaned tip
70 which sits freely in the bore of bearing 16.
Bearing 16 carries an external flanged sleeve 72 of a
low friction material which rotatably seats a
depending neck 74 defining the discharge opening 17 of
~37~
- vessel 12. Firmly retained on the neck is a double
pulley 22 with a pair of grooves of different
diameters for selectively engaging belt 26 to effect
ro~ation of the vessel preferably in a rotation sense
sopposite to that with which liquid swirls in funnel 18
from tube 34. The pair of grooves permit, in
conjunction with a corresponding pair of pulleys Z7 on
the shaft of motor 24, a variation in rotational
speed.
Neck 74 also mounts a slinger 75 above the pulley.
This slinger has the dual function of (i) during
rotation of the centrifuge, throwing off any excess
liquid that has found its way onto and is dribbling
down the outside of vessel 12, befoxe the liquid gets
15 to the pulley drive system, and (ii) facilitating easy
manual rotation of the vessel during visual
inspection.
The lower end of bearing 16 is held in a tapered
sleeve 78 held against rotation by straps (not shown)
20attached to housing 23. Sleeve 78 has an internal
annular cavity 80 with peripheral capillaries 82 and
liquid supply tube 84 for applying a downwardly
flowing curtain of the clean liquid to prevent any
retention of grains in the region of the sleeve.
The operation of the apparatus in the centrifugal
separation of heavier and lighter particles will now
be described, with particular reference to Figures 3A
to 3D.
Vessel 12 is set into rotation about axis 11 and
30liquid, e.g. TBE, added gradually through tube 34,
funnel 18 and opening 15. Centrifugal forces spread
the revolving, helically descending liquid outwards
~3~
12
- until the liquid-air interface forms a truncated
paraboloid of revolution ~Figure 3A). As further
liquid is added, this interface gradually contracts
until maximum retainable liquid is present in vessel
12 ~Figure 3B). The liquid has now attained a dynamic
equilibrium at which an annular, centrifugally
induced, substantially static revolving volume 90 of
the liquid of stable internal surface is confined by
the vessel about axis 11. Any further liquid added
will flow freely helically downwardly within this
volume and pass through discharge opening 17, as shown
in Figure 3C~
The solids mixture is now added to the in-coming
liquid by being fed to funnel 18 by vibratory feeder
28. This mixing is assisted by the swirling action of
the liquid in funnel 18, especially if the rotational
sense of the swirling motion is opposite to the
direction of rotation of the vessel. The entrained
particles then ~low down the neck 54 of funnel 18 on
to the top 66 of separation member 13. A head of
liquid forms which forces a steady outward flow over
the conical surface of top 66 to the edge of the
separation member. This edge forms a separation point
at which the particles are subjected to an enhanced
centrifugal force in comparison to the force on the
particles in neck 54. The heavier particles are
directed into the annular static volume 90 of the
heavy liquid while the lighter particles are forced
inwards onto the cylindrical surface 65 of the
separation member 13. The current of excess liquid
washes these lighter particles downwards and also
releases any entrained heavier particles until the
- liquid air interface 92 under cone 68 is reached.
From here the lighter particles float down on the
surface of the out-flowing liquid until th~y pass out
through the discharge opening 17 to receptacle 38.
5 Separation member 13 is impor~ant in enhancing the
separation by physically breaking the natural internal
surface (air/liquid interface) of the on-flowing
liquid, to submerge stray heavier particles on the
interface and thereby to encourage their movement to
10 the static annular volume 90 of the liquid where they
are subject to higher g-forces.
When the whole original solids mixture sample has
passed through the vessel 12, clean liquid flow is
continued through funnel 18, receptacle 38 is removed,
15 a filter funnel is placed under discharge opening 17
and rotation of the vessel is stopped. The vortex of
static liquid in the vessel is thus released, swirls
towards and ~lows through opening 17, both as the
vessel slows to a stop and for some time thereafter,
20 washing most of the sink paticles into the filter
funnel. The few particles which may tend to remain in
the vessel are easily washed out with the continued
flow of the liquicl. Because of the smooth continuous
rounded design of the vessel including the rim 15a of
25 opening 15, surface tension effects ensure that
further liquid flows around the walls down the inside
surface of the vessel, and sufficient swirling liquid
is able to clean the whole of said smooth interior
surface of heavier particles. The separation member
30 13 is easily washed by directing liquid down the
centre of the feed funnel neck 34, or simply by
inserting a glass tube down the funnel necX
~;~3~07
14
- to displace liquid onto member 13. The transparency
of the vessel allows easy visual checking on whether
any particles remain in the vessel as such a particle
is rev~aled by the disturbance, e.g. bow wave, it
5 causes in the even flow of liquid over the
vessel surface.
This ease with which all particles may be cleaned
from vessel 12 and from separation member 13 renders
the illustrated centrifuge apparatus especially
10 applicable to analyses in which a very small detection
limit is involved. Vessel 12 and separation member 13
are cleaned by gravitional flow and any convenient
vessel may be employed to collect first the floats and
then the sinks, a situation in marked contrast to
lS various commercial bowl centrifuges which require
removal of the bowl or a rubber liner for cleaning and
which require a special outer container to catch the
liquid and floats which are flung from the outlet rim.
In contrast to elutriators, cyclones and
20 dynawhirlpools in which the size and shape of each
particle, as much as the specific gravity, determines
the result, it will be noted that the separation in
the inventive process is dependant primarily on the
specific gravity of the liquid used. The speed of
25 revolution of vessel 12 is not critical since it is an
open system, and the liquid-air interface will adjust
itself for whatever revolutions and liquid throughput
are in use. Finer particles do settle out lower down
in the vessel, so that the design of the vessel may be
varied to suit the particle size which is to be
separated. In general, the design of the vessel could
~37~
- be modified for other applications. Where visual
checking o~ the cleanliness of the apparatus is not
required, the vessel can be made ~rom a
non-transparent plastic or metal. Such a choice of
5 materials also facilitates easier or cheaper
fabrication of larger versions of the centrifuge
detailed here and allows much larger samples to be
treated. A higher rotational speed also allows finer
particle sizes to be treated.
An important'advantage of the process and
apparatus is that separation in a high standard
analysis may be partically mechanised without
compromising accuracy. The apparatus is
semi-continuous in that the vessel does require
15 emptying at regular intervals, but the arranyement is
nevertheless particularl~ suited to laboratory
situations involving smaller samples.
By way of example, the process and apparatus were
utilized for the quartz removal stage of the analys-s
20 of stream sediment samples for kimberlitic indicator
minerals. The heavy liquid chosen was
tetrabromoethane {TBE), specific gravity 2.96, which
is the traditional liquid for this separation. In the
conventional separation process, 100 gm samples of the
25 concentrate, which would typically weigh between 1 and
10 kgs, are introduced into separating funnels one at
a time and left for approximately 2 minutes.
Utilizing the inventive process, the vessel 12 was
rotated at about 630 rpm. The TBE flow rate was set
30 at 10.4 mls per second and the particle size ran~e of
the feed sample was 0.4 to 2.0mm. The vessel 12 had a
16
-- volume of 2 litres, resulting in a static TBE volume
of 1475 mls. The maximum throughput attainable was
600 gms per minute of dry mineral or up to 15 samples,
each of 5 kg, per 8-hour day. A recovery in excess of
99.9% was achieved with fine grains with a specific
gravity of 3.2. It was observed that the volume of
flowing TBE in vessel 12 was about 338 mls and the
maximum sinks retention in the vessel, approximately
1000 gm.
As earlier foreshadowed, the principles of the
invention may be utilised in the separation of a
mixture of substantially immiscible liquids. In such
case, the heavier liquid is centrifugally directed
into the revolving static volume of the lighter
liquid, whereby the on~flowing liquid is clarified or
purified. This process has particular application to
the recovery of objectionable immiscible liquid from
industrial waste water.
