[0001] The present invention relates to a mixing machine in which a rotor in the form of
a circular rotary platform cooperates with a stator in the form of a stationary wall
which encircles the periphery of the rotor and extends above its rim. As the rotor
rotates, materials to be mixed are flung centrifugally outwards and upwards by the
rotor against the stator which deflects the materials inwards again onto the platform
so that the materials to be mixed are formed into a moving toroidal mass extending
around the platform with individual particles of the mixture travelling helically
around the toroid.
[0002] A mixing machine that operates on the above mentioned principle has been described
by Milik - see UK Patent Specification No.1365685 (assigned to Evans and Ryder). The
use of that machine has been described by the same inventor in relation to fibrous
materials (Patent Specification No.1365686), granular materials having a density below
70 lb per cubic foot which is less than the normal density of concrete (Patent Specification
No.1375943) and for the treatment of seed (Patent Specification No.1417694). The stator
part of the machine is a substantially vertically directed stationary wall that surrounds
and extends directly upwards from the rim of the rotor with an obtuse sharply defined
angle between the wall and the rim of the platform. But the mixer described by Milik
is difficult to use for the mixing of concrete in which water has to be mixed with
a nonhomogeneous mixture of particles ranging in size from very fine particles of
cement to particles of gravel and coarse aggregates that are retained by a 5 mm mesh
BS sieve. The blend of water or other liquid with such a complex mixture of particles
is liable to adhere to the inner surface of the stator at or adjacent to sharp angulations
to build up a static boundary layer with the result that complete and uniform mixing
is not achieved and the mixing operation may have to be prematurely discontinued.
Furthermore, sharp angulations in the circulation path of the particles being mixed
result in high rates of inter-particle impact and damage to the particles being mixed.
In concrete, the effect of inter-particle impacts during mixing is that the individual
particles become more rounded, and the cube strength developed by the resulting cured
concrete is less than would otherwise be obtained. A similar situation prevails for
other types of material to be mixed or blended. In the mixing of wood pulps during
processing to make paper it is necessary to avoid high material shear rates if product
damage and energy consumption are to be minimised and a similar situation prevails
in the mixing and blending of pigments.
[0003] The present invention is based on the realisation that more satisfactory results
are obtained by using a stator having a concave curved inner face to which the rim
of the rotor is tangential. With such an arrangement the particles to be mixed are
positively guided along an appropriate toroidal path and there is no sharp angulation
giving rise to a dead space for slow moving particles that could promote static boundary
layer formation. Uniform mixing can be achieved very rapidly without build up of an
unacceptable layer of incompletely mixed material at the stator wall and with a minimum
of particle damage and with minimum energy consumption.
[0004] Broadly stated the invention provides a mixing machine comprising:
a dished circular rotary platform that is smoothly curved towards an upwardly and
outwardly inclined rim, and
a stationary wall which closely surrounds and extends upwardly from the rim of the
rotary platform and defines on its inner face a concave part toroidal surface with
the rim of the rotary platform tangential to the said surface so that as the platform
rotates materials to be mixed are flung outwards and upwards by the platform against
the part toroidal surface which guides the materials along and confines them in a
toroidal circulation path so that the materials are formed into and mixed together
in a moving toroidal mass extending around the platform.
[0005] An embodiment of the invention will now be described by way of example only with
reference to the accompanying drawings in which:
Figure 1 is a diagram illustrating the principles of operation of the rotary mixer;
Figure 2 is a fragmentary view of the rotor and part of the stator showing a "breaker
bar" and a "drive-off" bar;
Figure 3 is a fragmentary view illustrating the variation of stator radius at substantially
constant rotor radius;
Figures 4 to 6 are respectively vertical sectional, front elevation, and left half
views of a practical embodiment of the rotary mixer;
Figure 7 is a view in horizontal section on the line A-A of Figure 4;
Figure 8 is a horizontal section of the right-hand half of the machine on the line
B-B of Figure 5; and
Figure 9 is a plan of the right-hand half of the machine.
[0006] In Figure 1 a mixing machine comprises a dished rotor a that is rotated by suitable
drive means in the direction of arrow b. Its top face is smoothly curved towards its
rim and although such smooth curvature is not essential and may be undesirable at
its centre, the intermediate and rim regions should be smoothly curved and devoid
of sharp angularities if damage to the material being mixed is to be avoided. It is
important that there should be sufficient friction between the rotor and the material
being mixed to drive the mix across the gap between the rotor a and the stator c.
The transfer of momentum may be improved by providing radially directed drive bars
d (figure 2) at equal angular intervals around the rim of the rotor and for example
there may be six such drive bars disposed at 60° intervals. The top face of the rotor
may also be coated with a plastics material such as a polyurethane. If the rotor is
covered with Linatex, however, which is a hard wearing rubber material having good
wet friction properties, there may be no need to use drive-off bars.
[0007] The stator c is formed with a concave inner face as shown that may be provided with
a plurality of "breaker bars" e also disposed at equal angular intervals about the
axis of rotation of the rotor a. In the absence of breaker bars the particles in the
material being mixed follow generally circular rather than toroidal paths, and there
is a tendency for material to adhere to the upper region of the curved part of the
stator. The function of the breaker bars which are vertically directed and made of
steel or other wear-resistant material is to prevent material sticking to the stator
and to encourage development of a toroidal circulation pattern. It will be noted that
the stator and rotor together define a smoothly curved surface with only a small gap
between them. Loss of fine material can be prevented by directing an upward current
of air f through the gap at a pressure of typically less than 172 kN/m
2 at a flow rate of 1.5 m3*/Min. This is in relation to a production size mixer of
overall diameter 1.0 metre. In such a mixer, as is apparent from Figure 3 the radius
of curvature of the part-toroidal inner face of the stator which is circular in profile
ranges typically from 0.127 metre to 0.203 metre, the size being selected to give
the desired mixing capacity.
[0008] In Figures 4 to 7 a motor 1 drives rotor 13 through gearbox 2 via a shaft 9 that
is covered by a domed cover 27. The rotor 13 cooperates with a dished stator 21 as
previously described. Also mounted beneath the rotor 13 is a compressor 3 that discharges
air through pipe 20 to the space beneath rotor 13. Air flows upwards into a chamber
30 defined between a false floor the rotor 13 and the walls of the housing of the
machine and thence upwards under appropriate pressure between the rotor 13 and the
stator 21. Portions of the upper part of the housing define a hinged discharge door
12 for mixed cement that may be opened and closed by means of a piston and cylinder
device 4, 6 to permit the contents of the mixer to be discharged when required.
[0009] It will be appreciated that modifications may be made to the embodiment described
above without departing- from the invention, the scope of which is defined in the
appended claims. For example, instead of using breaker bars on the inner face of the
stator, it is possible that an equivalent effect might be achieved by appropriately
arranging the air flow.
1. A mixing machine comprising a dished circular rotary platform (a) that is smoothly
curved towards an upwardly and outwardly inclined rim, and a stationary wall (c) which
closely surrounds and extends upwardly from the rim of the rotary platform characterised
in that there is defined on the inner face of the stationary wall a concave part toroidal
surface with the rim of the rotary platform (a) tangential to the said surface so
that as the platform rotates materials to be mixed are flung outwards and upwards
by the platform against the part-toroidal surface which guides the materials along
and confines them in a toroidal circulation path so that the materials are formed
into and mixed together in a moving toroidal mass extending around the platform.
2. A machine according to claim 1, characterised in that the rotary platform (a) is
surfaced with a high wet friction wear resistant material.
3. A machine according to claim 1 or 2, wherein vertically directed bars (e) are provided
on the inner face of the stationary wall (c) to promote toroidal circulation and prevent
build up of a static boundary layer.
4. A machine according to any preceding claim, wherein a compressor is arranged to
discharge gas (f) under pressure into a closed chamber defined by portions of a machine
housing beneath the rotary platform (a), whereby escape of gas upwards through the
annular gap between the rotary platform (a) and the wall (c) opposes the escape of
material being mixed therethrough.
5. A mixing machine according to any preceding claim wherein the rotary platform has
a raised zone coinciding with its axis of rotation to inhibit local build-up of material.