Note: Descriptions are shown in the official language in which they were submitted.
73~
The instant invention relates to a device, particularly
a rotor,for introduction of a gas in a finely divided form into
a liquid medium. The rotor accordin~ to the invention is
particularly suited for introduction of air in a finelv clivided
S form in a flotation device. Flotation is a well-known technique
bein~ used in order to separate one or more valuable constituents
from non-wanted constituents. A typical field of use is
enrichment of ores in which finel~ ~ivided air is introduced into
an aqueous suspension of finely divided ore and water. The
finely divided air bubbles will adhere to the metal values of
the ore and carry the same to the surface of the basin in which
the flotation is carried out. The metal values and the air
bubbles will usually form a foam on the surface which can be
removed by skimming.
In order to improve the selectivity and effectiveness oE the
flotation process certain adjuvants such as surface active
a~ents are added to the suspension, which bring about a be-tter
adhesion between the metal values and the air bubbles.
A ~eneral account o F the flotation process is inter alia
found in Kirk-Othmer: "Encyclopedia of Chemical Technology",
second edition, volume 9, the pages 3~0-39B.
The air can be introduced in the suspension of the finely
divided material in different ways, for instance, water
saturated with air under pressure can be introduced to the
suspension and,as a consequence of the pressure release,the
absorbed air will be liberated in the form of finely divided
bubbles.
Ilowever, the most common manner in which the air is
finely divided is by means of mechanical agitation, for instance
by means of a rotor wilich more or less "whips" added air into
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the suspension.
Such mechanical devices are inter alia described in
Norwec~ian patent No. 116.156, US-patents Nos. 2.609.0g7,
3.070.229, 3.3~7.~51, 3.393.~02, 3.393.803, 3.409.130, 3.437.203,
3.485.484 and 3.752.313, and DOS 26.59.230.
For flotation purposes it is desirable for the added air
to exhibit the ~reatest possible surface area with respect to the
added volume of air, that is, the air should be introduced in
the form of very finely divided and minute bubbles.
In US-~atent r~O . 2.609.097 a rotor is described into which
air is introduced centrally via a~supply pipe and is radially
distributed to the suspension via a number of conduits leadin~
to the periphery of the rotor, said conduits opening into a
~roove, one side of which is formed in such a manner that the
air, durinc3 the rotation of the rotor, is forced into the
adjacent suspension.
US-patent No. 3.485.434 discloses for instance a rotor in
which the air is centrally supplied and lecl out throucJh conduits
in the wings of the rotor and thus forced into the surroundincJ
suspension.
By means of the instant invention a rotor or an impeller
is provided for introduction of air to a liquid medium and in
which the rotor is so desic~ned that a more efficient distribution
of very finely divided air bubbles are obtained in the
suspension. The minute and finel~ divided air bubbles will
rise slowly throucJh the suspension ancl thereby increase the
possibility of contact beinc3 established between desired
particles to be flotated and one or rnore air bubbles, whereby
the effectiveness of the flotation is increased.
According to the invention this object is obtained in that
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centrally supplied air from a suitable source and at a suitable
pressure and further under the action of the rotation of the
impeller via conduits is ~uided tan~entially from a central
supply conduit. The air initially forced into the suspension
in the form of relatively lar~e air bubbles will further be
broken down to smaller bubbles due to vortex formation behind ~-
the rotor blade and by collision with the subsequent rotor blade.
The final distribution of the air bubbles thus formed occurs
when the suspension containin~ the air bubbles impin~es on
stationa~ry restrictions bein~ arranged in a circular manner
around and in a suitable distance from the impeller. Thus, the
air is first dispersed by the air bubbles formed bein~
disinte~rated to smaller bubbles by colliding with the rotor
win~s. Subsequently, the partially dispersed air is further
dispersed by collision a~ainst stator win~s arran~ed around the
impeller in a circular pattern.
'l'he object of the invention is attained in the preferred
embodiment of the invention described in the accompanyin~
description, and illustrated in the accompanyin~ drawin~s.
In fi~ure 1 a flotation device is shown partly in section,
employin~ the rotor or impeller in re.
Fi~ure 2 shows a side view of the rotor.
Fi~ure 3 is a side view, partially in section, of the
rotor.
Fi~ure 4 shows a cross-section alon~ the line A-A in
fi~ures 2 and 3; and
Fi~ure 5 shows how the injected air is broken down when
the rotor is in a workin~ mode.
In fi~ure 1 the essential elements of a flotation vessel
1 are indicated, comprisin~ side walls 2 and a bottom part 3.
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In the example shown two impellers 4 and 5 are driven by a common
driving device (not shown) in such a manner that both the
impellers rotate with the same rotational speed.
Further, air is supplied from the pressure tank 6 to the
impellers 4 and 5 via hollow drivin~ shafts 7 and a at a
pre-determined pressure varying in accordance with the consistenc~
of the suspension. ~`ixed to the lower part of the driving shafts
7 and 8 are impellers or rotors 9 the lower part 10 of which is
at a pre-determined distance from the bottom 3.
A number of stator wings 11 are secured to the bottom 3 in
a circular pattern around the rotor 9, with suitable clearence
between the inner edges 12 of the stator 11 and the impeller
wings-13 of the rotor 9.
A partition 14 partially divides the vessel 1 into two
compartments.
Figure 3 shows the hollow part 15 of the driving shaft 8,
through which compressed air is supplied to the impeller 9 from
the pressure vessel 6.
Figure 4 shows the air conduits 16 of the impeller 9.
Figure S shows the relative movement of the air with
respect to the rotating rotor. The air from the pressure tank 6
is supplied to the void 15 in the rotor 9 at a pressure being
adapted to the consistency of the suspension. For a relatively
dilute suspension a pressure of for instance 0.2 kp/cm2 will
be sufficient.
nuring production stops, for instance by failure of the
power supply, the suspended particles will descend to the bottom
3 and build a layer which can have a thickness sufficient to
cover the impeller. During start-up a considerable torque is
necessary in order to liberate the impeller from the settled
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particles, resulting in a very high power consumption during
start-up. According to the invention this problem can be
eliminated by constructing the impeller of rubber or a similar
elastic material permitting the wings 13 to vield during
S start-up and be forced a~ainst the boss of the rotor, whereby
the starting torque is considerabl~ reduced. In order to further
reduce the starting torque,the stiffness of the win~s can be
reduced by giving the wings a gradually decreasing thickness
such that the lower parts of the wings have a smaller thickness
than the upper part. In this manner the lower part of the wings
will be more flexible than the upper part. A reduced bending
stiffness of the lower part of the wings 13 can optionally be
obtained by providing the lower part of the wings with suitable
grooves or holes.
Tests with the device accordin~ to the invention in a
flotation plant have shown that when the wings are made from
an elastic material and provided with a declining thickness,
as indicated ~bove, the flotation plant can be re-started without
the manual aid normally being necessary when starting-up a
conventional plant with impellers of non-elastic material.
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