A vertical ribbon mixer
A vertical ribbon mixer is
manufactured by Mill Powder Tech Co., Ltd. Closely related to the
vertical ribbon mixer is the helicon mixer shown in figure1.the mixer is
similar to the helicon mixer it employs a rotating lift screw to create
convective mixing inside a conical mixing chamber. The mixer is
available in several configurations as shown in figure
figure2 the
mixer uses a single convective lift screw which also rotates around the
conical blending chamber.(a)internal structure of the Nauta
mixer.(b)three mixing actions which take place in the mixer:1.around the
lift screw;2.around the mixing chamber;3.convection currents.
Ribbon mixers
Here
we refer an active mixer as one in which there are either internal
moving parts to randomize ther positions of theingredients, or air jets
to create convection currents and turbulence in the powder container.
The first type of industrial mixer that we will consider is the ribbon
mixer.
In a ribbon mixer, a long complicated single paddle is mounted
axially and used to disperse the ingredients of the mixtur. The ribbon
is usually so constructed that the powder near the outside of the
container is moved in one direction, whereas in the middle it is moved
in the opposite diretion. Ribbon mixers are available from several
manufactuers and they all have their own type of ribbon. The reader
should note that many of the companies mentioned with respect to a
particluar type of mixer in the discussion often make other type of
mixer, as set out in their catalogs.
Ribbon mixers are widely used in
the food industry and pharmaceutical industry. This type of mixer can
have dead pockets, expecially near the end of the mixer close to the
axis of rotation. Loading and cleaning of a ribbon mixer
is usually done from the top of the trough. The discharge valve can be
located at either end or in the middle of the mixture.(The manufacturers
usually recommend emptying the system at the end of the mixer.) Van den
Bergh points out that attention must be paid to the design of the
mixers, Bearings and glands to avoid lubrication contamination. He also
states that ribbon mixers are not easy to clean and not recommended for
sticky materials. Because the mixing device turns over the whole charge
of powder, the power demands of this type of mixer are
relatively
high, thus van den Bergh gives an extimate of 12KW per 1000Kg of
charge.(Many of the figures on power demands and of available capacity
in industrial mixers are taken from an excellent review article by van
den Bergh. One does not normally fill this type of mixer to more than
50% of the available volume. Normally the capacity of this type of mixer
is restricted to an upper limit of 15.
Sometimes, to improve the
dispersion of ingredients in ribbon mixer, the ribbon blades are built
to be close to the cylindrical wall to give high shearing. Sometimes the
ribbons are also fitted with rubber wipers to ensure complete
intermingling of the ingredients by lifting any packed material near the
wall into the middle of the ribbon ensemble. It is difficult to predict
the scale-up performance if one attempts to increase mixing capacity by
going to a larger mixer of the same type.
In a ribbon mixer the
paddles move relatively slowly, as distinct from the movement in another
class of mixer which is variously referred to as a paddle or a
plowshare mixer.In the forberg mixer, the twin paddle system shown in
figure rotates very rapidly to generate a fluidized chaotic array of the
ingredients of the mixture. The forberg mixer consists of the two
horizontal drums that are joined on one side. The elements consist of
two shafts that retate anticlockwise along the axis of each of the two
drums. Scaling up the capacity of the Forberg mixer is expensive because
of its stainless steel construction.(This type of equipment has been
referred to as the tossed salad approach to mixing.)
One of the
advantages of the Forberg mixer is the very short mixing times; usually,
15 seconds are sufficient to achieve full mixing. There are virtually
no shear forces in the mixer, so it is not suitable for use with
cohesive materials. On the other hand, the way in which the powder is
dumped through a bottom door often prevents segregation in the initial
post-mix handling of the powder. If there are any fine components in the
mixer, the way that the mixture is dumped from the Forberg mixer can
give rise to fugitive emissions. Forberg mixers are available in sizes
ranging from 6 dm3 up to 5 m3. The operational power requirement can be
up to 50kW, but this is offset by the very short time of mixing. For
some purposes the Forberg mixer can be equipped with ancillary
transverse bars, as shown in figure. When the paddles are operating, the
ingredients impact on the lump breaking bar to deagglomerate any lumps
in the feed material. The flow distortion bar in this diagram is also
known as an intersifier, since it increases the efficiency of the mixing
system.The system can incorportaed a device for adding liquid to the
powder ingredients. The addition of liquid may be an integral part of
the ultimate mixture or it may be a technique for stabilizing the
mixture via agglomeration to prevent segregation when the system is
emptied and moved away from the mixer.
Another type of mixer in which
the rotation of the dispersing paddles is so intense that the
ingredients are suspended in the moving air to create a turbulent
mixture is the Littleford mixer shown in Figure. This mixture is known
in Europe by the term lodige mixer. Because the rotating paddles are
shaped like plowshares to enable the paddles to keep the wall clean as
the mixer operates, this type of mixer is sometimes referred to as a
plowshare mixer. Nommally the equipment can be filled with a range of
occupied volumes from 20% to 70% of the mixing drum. Van den Bergh
states that the entire material mass moves so that power consumption is
relatively high. The mixer is available in a range of sizes from 0.13 to
30M3. Mixing times are relatively short, and mixing is often complete
by the time the paddles are up to top speed, so that the limiting time
for the mixer is the time required for the paddles to reach top speed
and then to become stationary. To increase the rate of mixing with
difficult powder, and to disperse agglomeretes which can exist in the
ingredients, so-called intensifier choppers are mounted in the side of
the mixer and driven independently of the movement of the high-speed
paddles. If cohesive powders are mixed in this equipment, it may be
necessary to intimize the final mixture by passing it through a pinmill.
The basic system of the pinmill( or pegmill as it is sometimes
called)was described firstly. Equipment of this kind is also available
from the alpine Company, which is now owned by the Hosokawa Company.(See
trade literature on the pegmill from Hosokawa).
A vertical ribbon
mixer is manufactured by National Bulk Equipment Incorporated. Closely
related to the vertical ribbon mixer is the Helicone mixer shown in
Figure 7.2. The Nauta mixer is similar to the Helicone Mixer. It employs
a rotating lift screw to create comvective mixing inside a conical
mixing chamber. The Nauta mixer is available in several configurations
as shown in figure 7.3. Van den Berge has studied this type of mixer
extensively[20,21] and makes....
Ribbon mixer for industrial.
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A vertical ribbon mixer&Ribbon mixers
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