Note: Descriptions are shown in the official language in which they were submitted.
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In the manufacture of particulate materials such
as thermoplastic polymers, e.g., high pressure polyethy-
lene, it is observed that certain lots will have a desired
property such as melt index which falls outside of product
specifications. To provide a maximum percentage of product
falling within product specifications, the manufacturer will
blend a product lot having an undesirably high melt index
with a product lot having an undesirably low melt index.
The resulting mixed lot will have a melt index within spe-
cifications. Such lots customarily are mixed in rotary
mixers and/or remelted and extruded. Such reprocessing
entails high labor costs and, in addition, high energy costs
when an extrusion step is employed.
In incorporating additives such as slip agents,
antiblock agents, and the like into such resins, it is custo-
mary to first prepare dry blends of the resin and addi-
tive(s) and then extrude the dry blend to form the homogeneous
mixture into pellets. Such processes are burdened with
high labor and energy costs.
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The present invention provides apparatus for
blending batches of particulate material that are composi-
tionally heterogeneous into batches of material that are
5 c:ompositionally uniform. The apparatus consists of two or
more cylindrical static mixers, each mixer being substan-
tially identical in volumetric capacity and construction.
Each mixer contains six or more vertically aligned parti-
tion walls that extend radially from the midoint of the
mixer to the wall to divide the mixer into at least six
storage compartments of substantially equal volumetric capa-
city. The vertical partition walls differ in height in a
regular descending order so that as material is charged to
and fills one compartment, it overflows the shorter wall to
fill the adjacent compartment. One embodiment of the inven-
tion includes means for feeding particulate material suc~
cessively through each of the static mixers. A second
embodiment of the invention includes means for simultaneously
discharging lots of particulate material from each of the
mixers at a substantially uniform rate into a common con-
veying means.
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In the accompanying drawings:
Fig. 1 is a perspective view of a single static
mixer with parts broken away.
Fig. 2 is a perspective view of two static mixers
in vertical alignment, including a conduit for feeding
material from the top mixer to the bottom mixer.
Fig. 3 is a top plain view of the static mixer
shown in Fig. 1.
Fig. 4 is a perspective view of a second embodi-
ment of a static mixer with parts broken away.
Fig. 5 shows a profile that the vertical parti-
tions included in Fig. 4 would present if they were rotated
counterciockwise.
Fig. 6 is a perspective view of the two static
mixers in horizontal alignment, including means for feeding
material from the first mixer to the second mixer.
Fig. 7 is an elevation showing the manner in which
several mixers are filled and subsequently emptied onto a
conveyor belt.
Fig. 8 is an enlarged view showing the manner in
which the particulate maerial from the several mixers are
ixed in being conveyed to a down stream packing station.
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As seen in ~iq. 1, the stati,c mixer has a prin-
cipal section 10 of cylindrical shape which terminates in
a bottom section 12 of conical shape to provide ready
gravity flow of particles from the discharge port 14 of
the mixer. A gate valve 15 is provided to discharge the
contents from the mixer. The top 16 of the mixer is pro-
vided with a product entry port 18, a vent port 20, and an
access port 22 to provide entry into the mixer to make
inspections and/or repairs. Suitable threaded covers 18a,
20a, and 22a are provided to seal ports 18, 20, and 22 when
the mixer is not in use. The covers 20a and 22a preferably
have a transparent section for visual inspection to deter-
mine the level of the contents in various sections of the
mixer. The interior of the mixer is provided with a series
of 6 vertical partition walls 31, 32, 33, 34, 35, and 36.
The vertical interior edge of each partition wall fits
tightly into channels provided in a centrally positioned
vrtical rod-like member 37. The partition walls extend
radially from member 37 and fit tightly against the inner
wall of the mixer to subdivide the mixer into a series of
6 pie-shaped compartments _, _, c, d, , and f. The cross
sectional area of each compartment is fixed by the angle
defined by its 2 partition walls; these angles being defined
as, respectively, C~C (31, 32) for compartment _, c~(32, 33)
for compartment b, and so forth. In the embodiment shown,
each of the ~Y~ angles is the same and is 60. The bot-
toms of each partition wall 31, 32, 33, 34, 35, and 36 are
cut to lie in a common plane (normal to the plane of gravity)
which is positioned as close as practical to the discharge
port 14. This construction prevents any significant upward
flow from any filled compartment into any unfilled com-
partment. In additon, this construction assures that the
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particles from each compartment will be discharged at a
substantially uniform volumetric rate when valve 15 is
opened.
In the embodiment illustrated in Fig. 1, the
shape of each of partition wall 31 through 36 is identical
except that each differs in height from the others. The
order of the heights is such that 31 ~ 32 ~ 33 > 34 ~ 35 >
36. The partition walls are actually higher than shown in
Fig. 1. They are shown in reduced height so that the dif-
ferences in height are seen more easily in the perspectiveview shown. The effective depth, and thus the volumetric
capacity, of each compartment a, _, c, d, _, and f is con-
trolled by the height of its shorter wall. As the angles
are the same for each compartment, the respective volu-
metric capacity of the compartments is >f. To provide a mixer in which each compartment has eaual
volumetric capacity, the partition walls should be arranged
so that o~(36, 31)~ o~(35, 36)> o~ (34, 35)~ o~ (33, 34)>
c~t32, 33)~ c~(31, 32). The precise differences in the
sizes of angles will be dependent upon the respective
heights of the partition walls and can be readily calculated
by those skilled in the art.
The apparatus of the invention shown in Fig. 2
consists of two static mixers in vertical alignment. The
mixers are arranged so that a conduit 40 feeds particles
discharged from the top mixer into the bottom mixer. The
two mixers are identical in size and construction.
The entry port 18 of each mixer is arran~ed so
that particulate matter charged to the mixer from a fill
tube 42 flows directly into compartment _. When compart-
ment _ is filled, additional material charged to the mixer
overflows partition wall 32 and falls into compartment b.
As the filling action is continued, particulate matter
successively and sequentially overflows partition walls 33,
34, 35, and 36 to fill compartments c, _, _, and f. The
respective heights of partition walls 31, 32, 33, 34, 35,
and 36 will be fixed to assure that the compartments of
the mixer are filled in this order. In particular, the
partition wall 31 will extend to the top of the mixer to
prevent any overflow of material from compartment a into
compartment f. The entry port 18 of the bottom mixer
also is positioned so that the particles discharged from
the top mixer through conduit 40 and fill tube 42 flow
directly into compartment _ and successively fill compart-
ments a, b, c, _, _, and f, as previously described.
The apparatus shown in Fig. 6 is similar to that
shown in Fig. 2, except that the two static mixers are
positioned in horizontal alignment rather than in vertical
alignment. The construction of the mixers and their opera-
tion are essentially similar to that shown in Fig. 2, except
for the means included to transfer particulate matter from
the first mixer to the second mixer. The product discharged
from the first mixer into conduit 40 flows into a transfer
line 44. Air admitted into line 44 through valve 46 blows
the discharged particulate matter from the first mixer
through line 44 into the second mixer. In lieu of employ-
ing a simple gate valve in the first mixer, it is desirable
to employ a rotary feeder valve to provide a positive dis-
charge of product when air pressure is applied to line 44.
Fig. 4 illustrates a modification of the mixer
of Fig. 1 in which like parts bear identifying numbers 100
units higher than the corresponding parts shown in Fig. 1.
Partition wall 131 has the same shape as corresponding
wall 31 shown in Fig. 1 and extends to the top of the mixer.
The top edge of each of the other partition walls 132, 133,
134, 135, and 136 is cut so that it either slopes from its
midsection (i.e., the section that fits in member 137) to
its wall section (i.e., the section that touches the inner
mixer wall) or from its wall section to its midsection.
g
The top edges of partition walls 133 and 135 slope from
their midsections to their wall sections. The top sur-
faces of partition walls 132, 135, and 136 slope in the
opposite direction, i.e., from their wall sections to
their midsections. Normal lines drawn from the point at
which partition walls 133 and 135 touch the inner mixer
wall to rod member 137 define acute angles s. Similarly,
normal lines drawn from the point at which the partition
walls 132, 134, and 136 touch rod member 137 to the inner
mixer wall define acute angles B. Typically, angles B are
approximately 15. If the partition walls were rotated
counterclockwise, the top surfaces of the partition walls
would show the cascading profile shown in Fig. 5. This
construction provides easier overflow of particles from one
compartment to the next compartment.
While the drawings illustrate mixers having 6
partition walls which divide the interior of the mixer
into 6 compartments of substantially equal volumetric capa-
city, it is feasible to provide 7 or more partition walls
to divide the mixer into 7 or more compartments. It has
been the inventor's experience that 6 compartment mixers
povide adequate mixing for most purposes when 2 mixers are
employed. Where mixing of extremely heterogeneous mate-
rials is required, somewhat improved results are obtained
when a third mixer is included in the apparatus of the
invention.
The specific heights of the partition walls in
the mixers will be somewhat dependent upon the flow charac-
teristics of the materials to be blended in the mixers.
The flow characteristics of particulate materials are pro-
portional to their angles of repose. For many materials,
such angles are known and reported in the literature.
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Where such angles are not known, they can be readily deter-
mined by known methods. The required differentials in
height between adjacent partition walls will be directly
proportional to the angles of repose of the materials to be
blended. The highest of the partition walls should extend
to the top of the mixer. This will prevent any flow of
charged material to the last of the concentrically arranged
storage compartments. Each of the remaining partition walls
should have its height reduced by the amount required to
provide ready overflow from one filled storage compartment
to the adjacent unfilled compartment.
The embodiment of Fig. 2 shows the two mixers
in direct vertical alignment. The arrangement has a minimum
space requirement, but requires that the transfer conduit 40
to be positioned at an angle from the field of gravity.
When adequate space is available, the lower mixer can be
offset somewhat from the first mixer to provide a direct drop
from the discharge port of the top mixer to the entry port
of the bottom mixer.
To provide optimum mixing efficiency in use, each
of the mixers should be completely emptied before the mate-
rial to be blended is charged thereto. The volume of each
batch of particulate material to be used should be, to the
extent practical, precisely equal to the volumetric storage
capacity of the several storage compartments.* It will be
recognized that some free space will remain above each of
the storage compartments. If the batch to be blended is
larger than the storage capacity of the several compartments,
*In subsequent discussions, the total capacity of the several
compartments will be referred to as the mixer's "effective
capacity."
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the excess particulate material will occupy a portion of
the designed free space provided in the mixer. As the con-
tent of each of the individual storage compartments is
discharged at the same rate, any excess material initially
occupying the designed free space will not be mixed with
any of the material stored in the storage compartments.
In situations where undersized batches of mate-
rial require blending, the batch size should be selected to
completely fill 2, 3, or more compartments of the mixers.
Such undersized batches, in a single pass through the
apparatus, will not be as well blended as full size batches.
If a more homogeneous blend is required, such undersized
batches should be passed through the apparatus two or more
times.
Where it is desired to prepare a blend from 2 or
more batches of particulate material, the precise order in
which the batches are charged to the top mixer ordinarily
is not critical. It is preferred, however, to charge each
batch to the mixer in sequence. By operating in this manner,
each batch will be concentrated in one or more compartments
in the top mixer. When the contents are discharged from the
top mixer, it will be well blended with the contents of the
other compartments. The same mixing action takes place in
the lower mixer(s) and it is readily seen that the final
` 25 lot of material discharged from the bottommost mlxer will be
homogeneously blended.
The apparatus is well suited to prepare blends of
an additive with particulate polymer at law cost. A typical
blend that can be prepared is polyethylene containing a
- 30 slip agent such as erucamide. In an initial step, the
erucamide will be dispersed in polyethylene pellets at a
concentration significantly higher than desired in the resin
to be delivered to the customer. For purposes of illus-
` tration only, the additive concentration will be six times
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the concentration desired in the final product. This con-
centrate will be prepared by any desired method as by
compounding in an estruder. A lot of this polymer con-
centrate having a volume equivalent to one-sixth of the
mixer's effective capacity will be charged to the top mixer.
When employing the apparatus illustrated in Figs. 1 and 2,
the concentrate will completely fill compartment a. A
second lot of the polymer containing no erucamide, having
a volume equivalent to five-sixths of the mixer's effective
capacity then will be charged to the top mixer. The
additive-free resin will completely fill compartments _,
c, _, e, and f. When valve 15 of the top mixer is opened,
the contents of each compartment a, _, c, d, e, and f are
discharged at essentially equivalent volumetric rates.
Thus the particles flowing through conduit 40 into the second
mixer will contain, on an average, equal volumes of ~ar-
ticles from each compartment of the top mixer.
As the particles from the top mixer flow into the
second mixer, each compartment will be filled with a mix-
ture containing on a volume basis, one part of the concen-
trate and five parts of the additive-free polymer. Any
deviations from the desired 1:5 ratio will be small. When
the final blend is discharged from the bottom mixer, the
uniformity of the blend will be further enhanced. The
final product will contain 1 particle of concentrate for
each 5 particles of additive-free polymer. When this prod-
uct is extruded by the ultimate user, the melt mixing in
the extruder will provide film having the erucamide suffi-
ciently well distributed throughout the film to be suit-
able for nearly all purposes.
In preparing a mixture of polymer products, a
manufacturer frequently is faced with the problem of pre-
paring a series of related polymers containing two additives,
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one of which is common to the entire family ~ products and
the second of which is specific for the individual polymer
products. A typical series of polymer products of this
type are film grade polymers containing a slip agent such
as erucamide and a specific colorant for each polymer prod-
uct. To prepare such a-series of polymer products, a first
polymer master batch containing erucamide will be prepared
as previously described. A master batch also will be pre-
pared for each polymer product and will contain the colorant
at a concentration substantially six times the concentration
desired in the finished product. The top mixer will be first
charged with one of the master batches and then the second
master batch. This charging order will fill compartment _
with one master batch and compartment b with the second
master batch. The mixer then is filled with uncompounded
or additive-free polymer. When the valve 15 of the top
mixer is open, the contents of each compartment a, , c,
d, _, and f are discharged at essentially equivalent volu-
metric rates. The particles flowing through conduit 40
into the second mixer will contain, on an average, one volume
part of the first master batch, one volume part of the second
master batch, and four volume parts of the uncompounded or
additive-free polymer.
As noted from the above descriptions, in preparing
batches of polymer particles having additives mixed there-
with, the first essential step is to prepare a polymer mas-
` ter batch of the desired additive(s) at a concentration such
that the master batch will be included in the final batch
in a volumetric proportion such that the master batch will
fill one or an even number of compartments of the top mixer.The master batch containing the additive should be charged
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to the first mixer in such a manner as to be contained
entirely within one or an even number of the compartments
of the mixer. This is done most conveniently by charging
the master batch to the mixer prior to charging the uncom-
pounded or additive-free polymer to the mixer.
Another application of the apparatus of the inven-
tion is to increase the homogeneity of a batch of polymer
particles which for any reason are more heterogeneous in
composition than desired. Again, in the manufacture of
polyethylene by high pressure autoclave process, it is some-
times noted that undesirably wide fluctuations of melt index
are present in the product exiting the reactor. By passing
such batches of polymer through the apparatus of the inven-
tion, the various segments of the initial batch become
blended so that the entire batch of polymer discharged from
the bottommost mixer is more homogeneous with respect to
melt index.
Fig. 7 shows five (5) mixers 1, 2, 3, 4, and 5
arranged in horizontal alignment. Each mixer is identical
in size and construction. A product delivery line 50 fitted
with valves 52 feeds particulate polymer to the mixers via
lines 54. As subsequently described, each mixer is filled
in~sequence. At the time of discharge, valves 15 of each
of mixers l, 2, 3, 4, and 5 are opened simultaneously 50 as
to discharge the particulate polymer onto an endless con-
veyor belt 60. Fig. 8 illustrates that the product from
the several mixers forms layers on the belt 60 with layer a
being the product discharged from-mixer 1, layer b being
the product discharged from mixer 2, and so forth. The par-
i~ 30 ticulate product is discharged from belt 60 to a packaaing
station not shown.
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In operation of the embodiment illustrated in
F:Lg. 7, each of mixers 1, 2, 3, 4, and 5 will be empty at
the beginning of the operational cycle. Particulate poly-
mer, such as pelleted polyethylene, is fed via line 50 to
the first valve 52 and through the 'irst line 54. This
material begins to fill compartment a of mixer 1. After
compartment a is filled, the polymer particles flowing into
mixer 1 overflow wall 32 and begin filling compartment b.
This action is continued until each of the compartments of
mixer 1 is filled. The flow of the particulate polymer to
mixer 1 will be measured and first valve 52 will be closed
at the appropriate time so that the volume of particulate
polymer charged will be, to the extent practical, pre-
cisely equal the volumetric storage capacity of the several
storage compartments of the mixer. It will be recognized
that some free space will remain above each of the storage
compartments of the mixer. This is desirable for reasons
discussed supra. After mixer 1 is filled, mixers 2, 3, 4,
and 5 will be filled in sequence in the same manner.
After all of the mixers have been filled, the con-
veyor belt 60 is started and the valves 15 of each of
mixers 1, 2, 3, 4, and 5 are opened simultaneously and
product from each mixer is discharged through lines 56.
The product being discharged from each mixer will contain an
equal volume fraction from each of storage compartments a,
_, c, _, _, and f. It is thus seen that the product being
discharged, by reason of containing an equal volume frac-
tion from each of the storage compartments, blends parti-
culate polymer produced over a significant time period and
tends to even out periodic variations in product properties
such as melt index.
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As illustrated in Fig. 8, the particulate polymer
tends to form layers a, b, c, d, and e on conveyor belt 60.
Each layer of course is homogeneous by reason of being a
blend formed by mixing material from the six storage com-
partments of each mixer. When the material is dischargedfrom the conveyor belt to the packaging station not shown,
the material in each of the layers a, b, c, d, and e becomes
intimately mixed. Thus the material that is packaged and
delivered to the user is made up of substantially equal
proportions of material from each of the 30 storage compart-
ments in the five mixers illustrated. Accordingly, the
periodic variations in polymer properties are evened out.
If desired, the apparatus of Fig. 7 can be modi-
fied by employing a pneumatic conveying system in lieu of
the conveyor belt shown. In this embodiment of the inven-
ton, lines 56 are connected to a large capacity pneumatic
conveying line. Gate valves 15 are replaced with rotary
feeders which function as discharge valves.
The apparatus and method of the invention are par-
ticularly well suited for the manufacture of large batches
of particular polymers. By way of example, by employing a
series of five bins, each of which contains an effective
volumetric capacity of about 6,500 ft3 (184 m3),it is pos-
sible to prepare single uniform batches of one million
pounds (472,000 kilos) of polyethylene. These figures are
based upon the consideration that the density of polyethy-
lene pellets is about 32 lbs/ft3 (513 kilos/m3). ~torage
bins of this capacity are easily manufactured. The bins
should be constructed to discharge product at a rate of at
lease about 650 ft3~hour (18.4 m3/hour~.
While the use of the appratus has been described
to prepare compositionally uniform blends of particulate
polymers, it obviously can be used to prepare uniform
blends of other types of particulate products.
