Note: Descriptions are shown in the official language in which they were submitted.
11f~57(~5
This invention relates to particle elassification,
and more particularly to a process for separating miea flakes
from other particulate material and for classifying the mica
flakes according to their aspect ratios.
Mica flakes have application in the field of rein-
forcement of plastics. Naturally oceurring miea (phlogopite and
muscovite) is eheap and readily available. Before use in
resin or plasties rei~foreeme~t, however, it must be separated
as mueh as possible from other, unwanted particulate material
(gangue~.
It has been reported that the strength and modulus
properties of miea flake-resin reinforeed composites depend
upon the aspect ratio (average flake diameter to thickness,
d/t) of the flakes. For example, Lusis, Woodhams and Xanthos,
"Polymer Engineering and Science", 13 No. 2, Mareh 1973,
reported that miea flakes with aspect ratios above 100 confer
a high degree of reinforcement to thermoplasties and thermosets.
The good strength, modulus values obtained using high aspect
ratio mica in thermosets or thermoplastics is offset, however,
by low impact strength. To obtain the best results, it is
desirable to use miea flakes having a narrow, pre-determined
range of aspeet ratios. There is therefore a need for a simple
and effieient method for separating mica Elakes into portions
of given aspect ratio range, as well as separating miea flakes
from gangue material.
3 1~5~(~5
Simple screening of mica flakes is not adequate for
classification according to aspect ratio. Screening only
separates mica flakes according to average diameter of the
flakes, and does not differentiate on the basis of thickness.
It is an object of the present invention to provide
a novel method and apparatus for separating mica flakes.
both from particulate gangue material, and into portions of
mica flakes of different aspect ratios.
It is a further object to provide such an appara-
tus and process which will cause rapid and efficient suchseparation, in a readily controlled manner, and on a large
or small scale.
The present invention is ~ased upon the principle
that particles can be separated from one another according to
their distinctive "terminal velocities", by dropping them into
a substantially uniform, low turbulence fluid flow stream, so
that the particles are carried downstream a distance which is
inversely proportional to the terminal velocity, a property
which is determined by the size, density and shape of the
2~ particles. Mica flakes have generally the same shape and
density as each other, and different from the shape and
density of particulate gangue materials with which they are
likely to be associated naturally. A given chunky particle
falling freely through a uniform gas flow has a unique tra-
jectory, determined by its terminal velocity. Flake-like
particles have trajectories which are additionally affecte
by the angle of the flake as it falls.
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11~5'~05
A mica flake can theoretically free-fall in a gas
stream in four different modes, namely edge-first, face-first
or at an intermediate angle or tumbling. The terminal velocity
Vt of a mica flake is proportional to the product of its dia-
meter (d) and its thickness ~t), i.e. dt o~ Vt. Thus, by
introducing a mass of mica flakes into a uniform gas stream,
e.g. in a wind tunnel, the mass can be divided into portions
of flakes of discrete d t ranges, and the flakes of a given
d t range can be collected in suitable receptacles placed at
different distances from the point of introduction. If the
mass of mica flakes has previously been screened so as to
contain flakes of a given diameter range only, the resultant
flake collectionsafter the air flow treatment each have given
aspect ratios. Alternatively, of course, the individual
collections can be screened subsequently, again to give mica
flakes classified according to aspect ratio.
Thus according to the present invention, there is
provided a process of separating mica flakes of a selected
aspect ratio range from mixtures of particulate materials in
which the selected mica flakes are contained, which comprises:
screening at least a portion of said mixture containing the
selected mica flakes; dropping at least a portion of the
mixture into a substantially uniform, low turbulence gas flow,
said gas flow having a velocity variation in the downstream
direction of not greater than 1~ ofaverage from point to point
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~l~S705
across a horizontal line extending transversely to the direction
of said gas flow; permitting the particles from said mixture
to fall and assume a falling trajectory under gravity in said
gas flow into discrete collection zones at varying horizontal
distances downstream of the location of introduction of the
particles into the gas flow; collecting separated portions of
the particles from the individual collection zones.
In the process of the inventio~ mica flake is
separated from gangue material by the gas flow process, since
the gangue particles have different densities and shapes from
the mica flakes, leading to different terminal velocities
and hence different trajectories in the uniform gas flow.
The mica flakes are classi~ied according to aspect ratio by
the combination of screening and gas flow separation, either
of which may be conducted first.
The process of the present invention can be con-
ducted on a laboratory scale, for analytical purposes, or
alternatively may be on a large industrial scale, e.g. for use
at a mine head or mineral processing facility. By varying
the size of the collection zones, namely their dimension in
the direction of flow of the gas current, different fineness
of classification can be arranged. Optimum gas flow rates
for a given apparatus and a given type of particle mixture
are readily determined by simple routine experiments, or by
prior calculation.
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In the accompanying drawings:
FIGURE 1 is a diagrammatic cross-section of an
apparatus for carrying out the process of the present
invention;
~ IGURE 2 is a perspective view, with parts cut away,
of an alternative form of apparatus;
FIGURE 3 iS a diagrammatic cross-sectional view of
a feed means for introducing flakes and particles under test,
into the apparatus of Figure 2.
l~ith respect to Figure l of the drawings, the
apparatus diagrammatically illustrated therein includes a
motor lO driving a centri~ugal-typeblower 12, communicating with
a plenum 13. A hori~ontal rectangular section air duct 14
for uniform, non-turbulent air flow therethrough, communicates
with the plenum 13, so that the ~lower 12 operates to cause
air flow through the duct 14 towards the plenum. At its up-
stream end, the duct 14 is provided with an entrance duct 15,
of larger cross-section than that of the duct 14. The entrance
duct 15 is open to atmosphere at its upstream end. Within
20 the entrance duct 15 is provided a honeycomb structure 16 and
a series of screens 17 of known type, for producing smooth,
uniform, non-turkulent air flow as the air flows therethrough.
The entrance duct 15 communicates with the main duct 14 via a
contracting section l~. The honeycomb structure 16, screens
25 17 and contracting section 18 serve to produce in the main
1125705
duct 14 substantially non-turbulent, uniform air flow.
The bottom wall of duct 14 is provided with a
continuous series of discrete collection zones in the form of
troughs la extending across the full width of the duct 14,
parallel to each other and transverse to the direction of air
flow in the duct 14. Various of the troughs 19 communicate
with collection hoppers 0, the number of troughs 19 communi-
cating with each hopper being selectable by the operator
depending upon the si7e of fraction to be collected in each.
A particulate substance introduction means is p_ovided at the
upstream end of the main duct 14, in the top wall thereof,
comprising a rotating air locked feeder 21 with a plurality of
cham~ers therein, being fed with particulate material including
mica flakes from storage bin 22, and feeding them into hopper
23. The hopper 23 contains therein a splash plate 24 inclined
downwardly and extending to near the side wall of hopper 23.
The edge of splash plate 24 has a saw-tooth configuration.
The splash plate is maintained in a state of vibration.
Material is fed through the gap beyond the edge of splash
plate 24 to a feed slot 25 extending across the full width of
the ~top wall of main duct 14. In this way, particulate material
including mica flakes is introduced and allowed to fall freely
in duct 14, across the full width thereof, evenly and with a
minimum of particle interference and interaction. Particles
with relatively high terminal velocities will assume trajec-
tories causing them to fall into collection troughs 19 close
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5'~5
to the upstream end of the duct 14. Those with lower terminal
velocities will be carried further downstream by the air flow,
into the downstream collection troughs. Screens of pre-
determined mesh size may be provided upstream of feeder 21, so
as to screen the material prior to entry into the duct 14 and
air flow separation therein. Alternatively such mesh screens
may be provided at the exits from collection hoppers 20, or
separately from the air flow apparatus, so that the material
is screened either before or after being subjected to the air
flow separation process. The final result of the combined screening
and air flow separation process is the separation of mica
flakes from particulate gangue, and classification of the
micra flakes according to aspect ratio. The breadth of aspect
ratio range in a classified section of the mica flakes depends
upon the downstream size of each of the collection troughs 19,
the number of adjacent troughs which deliver to an individual
collection hopper 20, and the selected mesh of the screens.
At its downstream end, the main duct 14 is provided
with hinged side wall portions 26, to throttle the air flow
2~ from the duct 14 into the plenum 13, and hence control the
rate of air flow in duct 14. An apparatus may comprise a
plurality of ducts 14, e.g. four in number, arranged parallel
to one another, and communicating with a single plenum 13 and
single blower 12 and motor lO. The flow ducts 14 may be of
the same or different cross-sectional siz~ and length. Air
~1257~)5
flows through the respective ducts can then be controlled by
adjustment of side wall ends 26 to create lesser and greater
degrees of throttling in the duct.
In the successful operation of the process according
to the invention, the air flow with the duct 14 should be in
a direction making a substantial angle with the vertical.
Clearly, in order to cause separation, the air flow must be
in a direction substantially different from the vertical fall
direction of the particles. It is preferred that the duct 14
be mounted in a substantially horizontal disposition and that
the air flow therein be substantially horizontal. However,
deviation from the horizontal can be tolerated for the air
flow, provided that it makes a substantial angle with the
vertical. As a practical matter, the direction of air flow
should be at an angle of at least 30~ to the vertical, hori-
zontal flow being the most convenient.
In order that the particles and flakes assume their
trajectories determined by their terminal velocities, on being
dropped into the air flow stream, it is important that the flow
be uniform in the horizontal direction transversely to the air
flow direction, and that turbulence in the air stream be at
a low level. If at any given downstream distance from the
point of introduction of the particles there are large
variations in the flow rate at different positions across a
horizontal line transverse to the air flow stream, the separa-
57()5
tion based upon terminal velocities of the flakes and particleswill be distorted. The flow rate, at any location across such
a horizontal line perpendicular to the direction of flow, at a
given downstream distance from the point of entry of the particl-
es, should be within 10~ and preferably within 1% of the average
flow rate at points along that line, to provide the necessary
uniformity of flow. It will of course be understood that
this degree of uniformity does not apply to the boundary
layers adjacent the four walls of the tunnel. As is well
known in aerodynamic systems, boundary layers exist adjacent
to the tunnel walls, to which the flow characteristics elsewhere
in the tunnel do not apply. Uniformity in the downstream
direction and in the vertical direction is of much less
importance.
In addition, tke flow within the duct 14 should be
of low turbulence so that the particles may assume the proper
trajectories. The turbulence of the air flow is the fluc-
tuation of the flow direction and velocity with time, at a
given point in the air flow. In the present case, the turbu-
lence at a point in the air flow is the "RMS turbulence",namely the root-mean-square (~S) of the velocity fluctuations
at the point, per unit of the mean velocity of flo~1 at that
point. In the process of the present invention, the R~S
turbulence should not exceed 10~, and should preferably not
exceed 5~. For best results, the R~IS turbulence should be
of the order of 1% or less. The turbulence at any point in
_ g _ .
l~'Z5705
the stream is readily tested by use of a hot-wire anemometer,
in the usual way.
In practice, air flows of the required uniformity
and low turbulence can be achieved in a variety of ways. Air
moving means such as fans and blowers capable of substantially
constant, steady operation at predetermined speeds should be
used to create the air flow. The uniformity and low turbulence
can be arranged by proper streamline design of the flow
chamber. It is preferred, however, to pass the air flow
through a plurality of metal screens, of various mesh sizes
prior to entering the chamber, in order to achieve these
desired flow characteristics. The use of screens for this
purpose is well known. It is preferred that the
screens be removable and replaceable, so that they can be re-
arranged, if necessary, to provide the required uniform, non-
turbulent flow, and so that they can be cleaned. Other pre-
ferred means fox use in the process of the invention are
honeycomb flow subdividers and contractor section in the flow
duct, as illustrated in Figure 1.
The air flow velocities which should be used in
the process of the invention are to some extent dependent
on the size and nature of the particles and flakes being
separated and their terminal velocities, and also upon the
air duct length. If the air speed is too great, the
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:~lZ57~5
particles and flakes with lower terminal velocities may be
blown beyond the collection means. If the air speed is too
low, insufficient separation will occur. The larger the
particles, in general, the larger the tunnel speed. In
general terms, the process of the invention operates best
when the air flow speed is between about six times the
terminal velocity of the lightest particle in the mass, and
about one~half the terminal velocity of the heaviest particle
in the mass. In practice air flow speeds in the tunnel of
from about 0.5 cm/sec to about 200 cm/sec are generally
adequate for mica flake classification. Preferred air flow
speeds are from about 3 cm/sec to about 150 cm/sec, these
speeds being easy to atta;n without large expenditure of
energy, and relatively easy to obtain in the necessary
uniformity and low turbulence.
In cases where it is desired to classify a mass of
particles and flakes having very wide extremes of particle
terminal velocities, it may be found necessary or advantageous
to subject the mass to two or more passes through the process
using different air flow speeds. Thus, at the first chosen
flow speed it may be found that classification into adequately
small ranges only occurs for flakes of low terminal velocities
i.e. the smaller particles, whereas the flakes and particles
of high terminal velocities are not effectively classified.
In such case, the process should be repeated using only the
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5705
mass of high terminal velocity flakes and partlcles separated
during the first pass through the process, but now using a
higher air flow velocity, to continue the classification.
In this way, with repeats of the process on parts of the
classified mixture, fine subdivision of the mass can be
achieved.
In order that the particles may assume the trajectories
determined by their terminal velocities, it is necessary to
ensure that the interference of one particle with another
during their introduction into and fall through the air flow
be minimized as far as practical. For this purpose, the rate
of feed of the flakes and particles into duct tunnel should
be controlled, and should ~e uniform. It is preferred that
the material be introduced through an entrance slot which
extends across the major part of the width of the tunnel,
such as a width generally corresponding to the width of the
uniform air flow in the tunnel, and stopping short of the non-
uniform boundary layers adjacent the tunnel walls. The
entrance slot should extend in a direction perpendicular to
the downstream flow direction, the slot being narrow in the
downstream direction. The rate at which the flakes and
particles can be introduced, without causing substantial inter-
ference with one another, is theoretically independent of the
slot width, in the downstream direction, and independent of
~5 the tunnel height, and can be up to at least about 100 g per
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minute per centimeter of slot length. There is no minimum
feed rate, except as determined for reasonable rates of process
performance. As the air flow velocity in the chamber is
increased, the maximum feed rate permissible without causing
excessive mutual interference between the flakes and particles
increases. As the flake diameter increases, this maximum
permissible feed rate increases. Specifically, a feed rate of
about 7 g/min/cm of slot length has been found to work ade-
quately for flakes in the 100 micron size range, at flow rates
of about 100 cm/sec. The geometry of the apparatus is also of
some significance, particularly the ratio of slot width, in
the downstream direction, to vertical height through which
the particles fall. This ratio is preferably from about
1 to about 1 .
Figures 2 and 3 illustrate an alternative, specific
form of apparatus according to the present invention. It will
be seen from Fig. 2 that the apparatus is disposed generally
horizontally, with collection means in the form of trays 27,
28 resting on the bottom wall of the duct 14 and extending
parallel to one another across the tunnel width. The trays
are merely placed on the bottom wall, and can be lifted out
and rearranged as required. They are placed adjacent to one
another so as to cover a section of the bottom wall and catch
all particles and flakes landing on that section. The front
side wall 34 of the apparatus is of transparent material and
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is hinged along its top edge 36 so that it can be raised to
provide access to the interior of duct 14 to remove and re-
arrange trays 27, 28. Suitable re easable fasteners 38 are
provided to hold the front wall 34 in its closed position to
form a substantially air-tight front wall for the duct 14 in
operation.
The screens 17 at the upstream end of the duct 14
~ e r~o~a I~
are ~e~ff~l~ mounted in the vertical plane, parallel to one
another, in a top holder 40 and a bottom holder 42 in the
apparatus. A removable access cover 44 is provided in the
top wall 46 of the apparatus, which access cover can be
removed to allow removal and rearrangement of screen 17 to
enhance or vary the air flow characteristics in duct 14. A
return duct ~r is included, disposed below main duct 14, so
that the apparatus forms essentially a closed circuit. Flow
smoothing screens 29 are semi-permanently mounted in the
return duct. An illumination means 48 namely a fluorescent
tube light is provided in the top part of duct 14, for obser-
~ation of the apparatus and process in operation. A trans-
2Q parent panel 50 permits the light to enter the duct 14.
The feeding means is mounted on the top wall 46of the apparatus, and comprises an elongated triangular
sectional vibratory hopper 52, the side walls of which
converge in the downward direction to define a narrow opening
slot 25 at the bottom, at the height of the top wall 46 of
11~57~5
the apparatus. The hopper 52 is mounted on the top wall 45
by means of brackets 54, 56 at its ends, the brackets
including thick rubber mounting pads 58, 60 so that the
hopper can vibrate end to end relative to the apparatus. A
vibrating electric motor 62 is mounted on the rubber pad 60
so as to vibrate the hopper 52 in this manner.
Above the hopper 52 is mounted a rotating, cut-away
feed tube 64 with an associated slow speed electric motor 66,
adpated to rotate the feed tube 64 slowly at constant rate above
the hopper 52. The feed tube 16 has a part-cylindrical side
wall and a bottom wall forming a chord of the circle of the
side wall, extending to and terminating at the centre of the
circle. When rotated at a constant speed, this feed tube has
the desirable property of feeding material at a constant rate.
~n operation, granular material to be classified is deposited
in feed tube 6~, which is set slowly rotating whilst beneath
it the hopper 52 is vibrating. As the feed tube slowly rotates,
it slowly and gradually deposits the granular material into the
hopper 52 at a constant rate. The upper part of the hopper
has double thickness walls, the inner layers 68 of which are
removable. A screen 70 extends across the hopper 52 along its
whole length, removably and replacably clamped into position
by the inner and outer hopper walls. The screen 70 vibrates
with the hopper 52 so that particles are vibrationally screened
on passing through the hopper. Another screen 72 cover~ the
bottom opening or slit 25 of the hopper 52, which serves to
give the particles another vibrational screening prior to
entering duct 14.
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ll'~S'705
The invention i5 further illustrated in the following
specific practical examples.
EXAM~LE 1
In this example, dry mica flakes 200-H were
classified according to the invention, using an apparatus
as illustrated in Figs. 2 and 3. The mica flakes were
screened through a 53 micron (270 mesh) screen, so as to
divide them into two types on the basis of diameter. Each
type was then classified by the air flow process, using an
air speed in the chamber of about 3 cm/sec. and the contents
of the individual trays observed by photomicrographs.
In each case, the photomicrographs showed that the
trays near the inlet collected thicker flakes, whereas the
thinner fla~es were collected in the downstream trays, so
that the mica flakes had effectively been classified according
to aspect ratio, by a process of screening and air flow
separation.
~XAMPLE 2
~ lica flakes with the commercial designation 60 - Z
(supplied by Suzorite ~ica) was processed in a laboratory
apparatus as generally described herein, with removable trays
placed on the ~ir duct floor.
The speed used in the duct was 150 cm./sec. and
the material was collected in 22 trays, some 5 cm. wide,
others 15 cm. wide, on the floor of the duct. P. sample of
wt. 250 gm. was fed in during 15 seconds.
The material collected was subsequently screened,
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using various combinations of screens. For example, material
from trays lO to 14 was passed through two screens of lO0 and
l~0 mesh. The material retained on the upper screen (28.6 gm.)
was found to be 99% pure mica, and that which passed through
the lower screen (12.3 gm.) was 99% gangue, (unwanted granules).
Thus the combination of screening and air classifying is
demonstrated to be capable of very effectively separating the
mica flakes from the gangue.
EXP~LE 3
Mica flakes designated 60 - H were classified in a
laboratory apparatus generally as described herein, and the
average diameter and average thickness of the material in
various trays was measured. It was found that diameter and
thickness both diminished with downstream distance from the
feed point, in such a way that the product (diameter x thick-
ness) varied inversely as the downstream distance. The contents
of two of the trays were then screened, and it was found that
substantial separation of "aspect ratio" (diameter , thickness)
was obtained. For example tray "r" was sieved so that the
material which was retained on a 60 mesh screen had aspect
ratio 91, whereas that which passed a 250 mesh screen had aspect
ratio of 24.
The process of the invention involving screening
and air flow classification, is thus very effective in
separating mica flakes from granular gangue, and classifying
~lZ5~7S~
the mica flakes according to aspect ratio, for best use in
reinforcement of plastics. The screening procedure may be
carried out before or after the air flow classification
procedure. It is in fact advantageous to screen the mixture
before subjecting it to the air flow classification, since
pre-screening enables a more suitable selection of air speed
in the duct for each screened fraction, producing more accurate
classification.
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