Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Mixer for cohesive powder materials
The present invention relates to a method and a mixer
for mixing of especially cohesive powder materials.
s In various industrial branches, many industrial
processes involve the mixing of different materials in one way
or another, be it raw materials or intermediate products. Such
industrial mixing processes can vary significantly in complexity,
from the quite simple process where desired homogeneity in the
to mixture is reached with simple means, through the mor difficult
processes which put greater demands on the equipment and the
process cycles.
Mixing processes can generally be divided into two
categories, namely mixing of liquids, gases, solids in liquids
is or a combination of such on one hand, and a mixture of parti
culate materials, that is powder in dry form or possibly with a
moistened content that is lower than the saturation limit. In
mixing of powders, there is no motion of particles without the
particles being affected, in contrary to fluids, where motion can
zo happen by diffusion or also as Hrownian movement. Powder affects
in a large degree the mixing process itself by the particles
physical properties as size, density, form, surface conditions,
and so on. Further, the individual particle in a mass of powder
at a size that is several orders of magnitude higher than the
Is particles in a fluid.
Many of the problems that arise in mixing processes of
various types, are due to differences in the above mentioned
properties which are not known, or that have not in sufficient
degree been considered in the selection of equipment or mixing
3o cycle. The problems that arise for this reason, result in for
example a series of failed attempts to use equipment that is
designed for mixing of liquids or the mixing of dry powder
materials. Poorly performed mixing processes can result in great
losses for the user, losses that can manifest themselves in the
35 form of unnecessary high consumption of raw materials, large
production of rejects, problems in the following equipment in the
production and dissatisfied costumers and possible loss of market
shares.
The primary function in the mixture of one or more
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cohesive powders in the same mixture requires a velocity gradient
that varies from zero and up to peripheral velocity, and at the
same time a shearing force larger than the slip force in the
micro plane.
s This slip force is the force that the individual
particle must have in order to escape the particle lump or heap
or agglomerate, that is a collection of single particles that
works as a collected particle, so that it works alone and
therefore also so that the entire surface, and not only a small
~o side surface becomes available for the mixture' s ultimate target,
which may be the colouring by pigments. This can be better
illustrated by considering on one hand that the pigment particles
consist of a large ball in a football field, which still will
look green. If the ball is divided into particles with for
15 example red colour and with the size of 2 um, the whole field
will look red.
I practice the situation is often that the pigment
particles work as a collection of particles, consisting of for
example ten particles of 2 um, and the colour yield will then
~o only make one half red, because the pigments are lying on the top
of each other and not beside each other.
We understand then, that for pigment processes the
trick is to convert collections of particles to single particles.
For example, in asphalt production the stone material must show
25 up in their smallest single particles to obtain the desired
reactivity with the emulsion. What is important here, is the
specific surface because the efficiency of the reactivity relates
to the area of the available surface that is exposed to reaction.
It is usual that the filler appears as collections of 1 mm in
3o dimension, it is evident that the end product will be totally
different than if the particles appear by themselves in the size
of about 10 um. Cohesive particles are particles that because of
some internal exchange of forces have tendency to appear as a
collection of single particles because the mass is not large
3s enough to dissolve the lump.
In for example medicines, the quantity of filling
materials with the present invention might be reduced because
less solution is required. The mass is dissolved into single
particles so that one can produce much smaller tablets and in
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fact make the medicines safer and easier to swallow.
Generally speaking, the mixture of powders is more
difficult than the mixture of liquids and gases, especially
mixture of dry powders. It is often merely impossible to reach
theoretically optimum conditions within the frame of the
available process techniques, that is use of time, production
volume and costs. Choice of proper equipment that could be used
for various assignments is essential for an economically
favourable result for the users of the mixing equipment. On the
~o other hand, there are many different types of equipment that at
first sight seem to be the same, on account of the many problems
around mixing processes.
Mixing of solids are carried out by applied diffusion,
applied convection or applied shearing force. Most processes and
equipment will work with a combination of these mechanisms, but
for one base mechanism.
The physical bulk properties of the powder materials
change in a significant degree, depending on the size of the
particles. If the mass has an average particle size of about 100
zo pm, the powder mass will usually behave as a free flowing mass
with a defined slide angle, without tendency to cohesiveness. The
mass will not acquire an inner strength after outside pressure,
nor will it withstand greater pressure without break down. In
order to obtain a successful mixture of such mass, it should be
zs diffusive or connective.
If the particle size on the other hand is smaller, that
is the main part of particles are substantially less than 100 pm,
the powder mass will change character from a free flowing mass
to a cohesive mass. It will then get an indefinite, but very
30 large slide angle, which could also be negative, and it will
acquire a relatively large strength after outside strain. Changes
in the physical properties also show up in the fact that the
powder get a significantly greater ability to form agglomerates,
that is there will be large lumps that each consists of many
ss small particles. The cause of this change in mass properties is
that the interparticular forces that contribute to the tying
together of the particles will be the dominating forces working
in the powder when the particle size is reduced. Interparticular
forces could be Van der Waal forces, electrostatic forces,
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chemical, magnetic or capillary forces. This means that the
effects of the force of gravity would be less compared to these
forces. As a result, the force of gravity will not by itself be
able to break down the powder to single particles, which is
necessary in order to obtain a good mixture at microlevels. The
powder or agglomerates must thus be exposed to shearing.forces
in the form of rubbing or high kinetic energy.
Use of high kinetic energy, as for example may be
obtained by fast rotating knives, is well known and used in many
to connections. It is an effective method, but may, however damage
the powder material if this is of such a nature that it is easily
crushed or in other ways is unable to withstand rough treatment.
Further, other problems may arise, for example pollution of the
powder in that the crushing tool itself wears down. If the powder
is is of an adhesive nature, the result could easily be that the
crushing tool soon be covered by powder and therefore stop
functioning satisfactorily.
The present invention supplies a possibility to supply
to the powder mass shearing forces without the above mentioned
zo draw backs. This is achieved by the method and mixing device
according to the invention as they are defined with features set
forth in the claims.
The present invention achieves a number of advantages
compared to the known art. Segregation properties are signi
zs ficantly improved on account of a fine distribution in the micro
plane. Environmental improvements are achieved by the fact that
the process can be trimmed very precisely so that smaller
quantities of harmful materials are being used.
Significant reduction in costs are achieved as a result
3o of small consumption of expensive component. In this connection
one could mention pigment processes, such as production of
powdered lacquers, in which the present invention can achieve
savings in the order of 15~ of the consumption of pigment, which
cost significantly more than filler materials. Other effects are
35 increased repetition reliability, simplified and at the same time
improved process control, better planning tool and more reliable
delivery.
The drawing shows schematically embodiments of the
invention, where
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Fig. 1 shows a cross section of a mixer with two rotors
in an embodiment with rubbing wanes,
Fig. 2 shows a corresponding partial section with
another embodiment of the rubbing wane,
s Fig. 3 shows, corresponding to Fig. 2, a third embodi-
ment of rubbing wanes, .
Fig. 4 shows a plane view of the mixer in Fig. 1, and
Fig. 5 shows an embodiment of a mixer where the rubbing
process takes place against the mixer's end wall.
With special rubbing wanes 1, 4, 5 provided between
regular wanes 3 which takes care of the transport and blending
in the mixer's housing 6, it is achieved that the powder is
broken down in several shearing planes, and that it therefore
takes place a crushing of agglomerates at the same time as the
mixing takes place at the micro plane. Rubbing wanes 1, 4, 5 rub,
by rotation by the rotor 2, materials against the bottom wall
against the housing 6, and thereby applies sufficient shearing
forces that the bonds in the agglomerates are broken . The rubbing
process can also be provided by the rubbing of the materials by
Zo rubbing wanes 7 against the end wall 8 of the housing 6, as shown
in Fig. 5.
Such a mixing process can be performed at low veloci-
ties, and therefore it is gentle to the powder mass. The method
and the mixer device can be combined with other mixing prin-
ciples, so that the three mechanisms for mixing can be utilised
at the same time, since the mixing will take place by means of
shearing in the micro plane while the entire mass of particles
is stirred and mixed by means of diffusion and convection.
A mixer's housing 6 for the embodiment of the method
3o according to the present invention can make use of rubbing wanes
1, 4, 5 mounted on two parallel rotor 2 with opposite rotation
direction, where the rubbing wanes press against a part of the
powder mass and rub this against the stationary housing 6. This
causes, because of the applied stress, that a number of breaks
3s will occur in the powder mass. In the break plane it will,
because of the shearing stresses, be a relative motion between
each side of the break plane, and because of this motion, it will
be a relative mass transport in the break zone itself. Agglo-
merates that happen to be in the break zone will therefore be
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broken down because of the shearing stresses, and the various
parts of agglomerates will immediately after the break down be
transported away from each other because of the relative motion
than happens between the two sides of the shearing plane. This
results in a mixing process on micro level that can be performed
at low velocity.
The method and mixing devices according to the present
invention can be utilized in all processes comprising mixing of
fine, cohesive powders where an active medium is to be evenly
to distributed in a mass of powder. Such tasks exist within several
branches of industry, for example, in production of medicines,
for mixing of pigments in raw materials for paint, chemical
reactants and catalyzers, flourmixtures, coloured chalk, coal or
binders, powders for toners and copying machines, etc.
The method and the mixing device may also advantage-
ously be used for mixing of small quantities of liquid in dry
powders or for mixing of products in the paste form.
The rubbing wanes can have the shape shown with the
rubbing wane 1 in Fig. 1, which consists of a pipe which is bent
zo in the form as shown in the Figure. Such bent rubbing wanes 1 in
the form of pipes are fastened to the rotors 2, distributed over
its length between the mixer's mixing wanes 3.
Fig . 2 shows another embodiment where the rubbing wanes
4 are bent so that the products are rubbed along rubbing wanes
zs 4 while they are thrown radially outwardly. Fig. 3 shows on the
other hand an embodiment of a rubbing wane 5 that has the form
of an elliptic disc, also shaped such that the products will rub
against the wall 6.
The width of the rubbing wanes 1, 4, 5, 7 against the
so wall 6, 8 are chosen, depending on the materials to be mixed, and
otherwise adapted to the special conditions. The rubbing wanes
may be produced in the form of plates or pipes.
The present invention has shown to provide specially
good results for gentle mixers with low peripheral velocities,
3s for example in the range from 0 to 3 m/s.
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