Note: Descriptions are shown in the official language in which they were submitted.
04
Technical Field
This invention is in the field of mixing solids and
liquids.
Background of Prior Art
The present invention deals with the mixing of
pulverized solids and liquids and can be applied, for example,
to hydraulic binders preparation and specifically to the
continuous process preparation of a plaster powder and water
mixture.
It is a widespread practice, in order for the mixing
of pulverized solids and liquids, to use mixers having teeth
or blades and consisting of a cylindrical container having a
vertical shaft fitted with one or several sets of revolving
radial arms bearing blades or teeth. These blades or teeth
scrape the container's walls, mix the products and thus
operate a stirring. But such mixers do not provide a satis-
factory dispersion of pulverized solid into the liquid; hence
there is a lack of homogeneity in the fluidity of the dis-
charged mixture. On the other hand, there are turbine mixers
consisting of a container in which a disc, a propeller or a
turbine is revolving at very high speed. The solid and the
liquid reach the turbine which disperses them instantly.
Contrary to teeth mixers, turbine mixers achieve a high
shearing rate and an intensive turbulence in all points of the
system, so that product dispersion and homogenization are
satisfactory. But the study of such a mixer's performance,
through the introduction of some colored substance, or gener-
ally any tracer substance which can easily be detected,
reveals that a variation in the supply is reflected without
any change at the outlet after a very brief delay of the order
of one second. Thus, in a turbine mixer, the time during
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which the product remains in the mixer is very short, indeed,
so short that irregularities in supply are not suppressed by
the mixing operation and still exist unchanged at outlet.
When an even fluidity of final product is desired, which is
impossible to obtain through evenness of supply, turbine
mixers will not be satisfactory.
The present invention obviates the drawbacks of both
known systems; it makes it possible to provide a continuous
process mixing of pulverized solids and liquids with an even
fluidity.
In addition this invention overcomes the problem of
premature setting of a fluid evolutive product in the mixer.
A fluid evolutive product is a liquid in which a reaction
resulting in a physical or chemical transformation takes place
producing a solid phase or modifying the characteristics of a
solid phase initially carried by the liquid. A plaster powder
and water mixture is exemplary of such a product.
Brief Summary of the Invention
The invention offers a continuous mixing process of
pulverized solids and liquids comprising:
introducing in continuous process the liquid phase
in a mixing container,
introducing in continuous process the solid phase in
said container at such a rate that desired proportions of
solid and liquid are observed,
providing for the rotation of products contained in
the container in such a way as to establish a vortex,
regulating solid and liquid supply rates in such a
way that a given level of container filling is constantly
maintained,
emptying in continuous process the products mixture
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so that level of filling is maintained.
The process also advantageously uses the liquid
introduced in the container to wash down the water of the
container to prevent the formation of solids. Preferably the
mixed product is withdrawn along the lower walls of the
container to avoid the retention of solids in the container.
A sustained running is established after a starting
phase including the following steps:
introduction in a mixing container or tank of liquid
and solid in accordance to a weight ratio selected beforehand,
up to reaching a given filling level of said mixing container,
stirring in the container the introduced products
and sustaining of stirring for a selected period of time,
And then at the same time:
continuous process intake, in the container, of
liquid and solid at predetermined rates so that the mixture
weight ratio is observed, and
continuous process emptying of the mixing container
at such a rate that the filling level is maintained.
With given rates of introduction of solid and
liquid, the mixing container filling level determines the mean
time during which the products to be mixed remain in the
mixing container and said mean time of stay is at least equal
to 3 seconds and is preferably set between 15 and 30 seconds.
As a typical example, the process may be used for
continuous process mixing of plaster powder and water.
In addition, the invention offers a mechanism to
implement the mixing process. Such a mechanism comprises a
mixer characterized by the fact that it includes various
combinations of the following:
a container made from a hollow solid of revolution
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whose lower end is equipped with an outflow opening and
outflow rate regulating means, said container being fitted
with an inner wall constituting an intermediate bottom with
its side edges spaced away from the side wall of the container;
a turbine mounted within the container above the
intermediate bottom and revolving around a vertical axis
placed along container's axis; liquid supply means advan-
tageously including means for supplying liquid to the side
wall of the container and to the shaft of the turbine; and
solid supply means for supplying a solid to the container.
Brief Description of the Drawings
Figure 1 is a sketch of mixing mechanism,
Figure 2 is a schematic showing of the supply and
mixing mechanism;
Figure 3 is a horizontal section taken immediately
above the cone in the mechanism of Figure l;
Figure 4 is a horizontal section taken immediately
above the bottom of the ejection of the mechanism of Figure l;
Figure 5 is a schematic drawing of part of a mixer
having an impact crown ejector.
Detailed Description of the Invention
Referring to Figure 1, a mixer M for pulverized
products and liquids has a vertical cylindrical container or
tank 1 tapered in its lower section 2 to lead to a drainage
opening 2'. A turbine 3 is mounted on the side of tank 1 with
its impeller inside of container 1 on a vertical shaft 4
placed along the container's axis and driven by a motor 5.
There is a container intermediate bottom 6 constituted by the
top surface of a cone 7 designed to create an obstruction
inside lower tapered section 2 of container 1. This ob-
struction is a solid of revolution whose lower section is
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tapered. It is centered along the container's axis and its
dimensions are smaller than the inner dimensions of lower
tapered section 2 of the mixer, thus providing a ring shaped
opening between it and the lower section 2 of container 1.
Cone 7 is a cone with its point down inside of lower tapered
section 2 of the container which also is in the shape of a
cone, with the intermediate bottom 6 constituted by the flat
base of said inverted cone. Cone 7 is supported by bars 7A
which appear in Figure 3.
The lower section 2 of the mixer leads to an in-
verted cyclone shaped ejection device 8, i.e. constituted by
conical casing 9 mounted with its point up, with flat base 10
and a collector pipe 11 whose end is flush with base 10,
tangential to conical casing 9 and extending in the direction
of rotation of turbine 3. This collector pipe 11, then runs
vertically downward and is equipped with flow rate regulating
valve 12 which brings the mixture to a pipe 13 leading to
mixture utili~ation facilities (not shown). Valve 12 may be,
for example, a modulated pressure controlled elastic sleeve
valve of the type disclosed in our copending Canadian appli-
cation 321,010 entitled "Process and Mechanism for Evolutive
Pulp Flow Regulation" and filed concurrently with this
application. The top rim of container 1 is fitted with a
covered ring-shaped pouring spout 14, supplied with liquid
through a flexible hose 15. A pipe 16 connected to flexible
hose 15 and equipped with a regulating valve 17 directs liquid
at shaft 4 of turbine 3 to keep it clean.
By way of specific illustration a mixer with the following
specifications can deliver from 30 to 65 KG/MN of mixture:
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Speed of our turbine1275 revolutions/mn
Diameter of the turbine impeller blade 150 mm
Diameter of the mixer impeller blade 292 mm
Height of the mixer top above the485 mm
5 intermediate bottom
Angle of the base of the cone145 degrees
Diameter of the outlet of the mixer 35 mm
Distance between turbine blade and 15 mm
intermediate bottom
10 Angle of the bottom of the mixer, about........... l45 degrees.
Figure 2 shows a complete mixing installation.
Elements already described, such as mixer M with its container
1, its obstructing cone 7 inside of the container tapered
section 2, intermediate bottom 6 constituted by the top part
of cone 7, ring shaped opening between cone 7 and container
lower section 2, inverted cone shaped ejection device 8,
collector pipe 11 equipped with outcoming mixture flow rate
regulating valve 12, liquid supply through pouring spout 14
and pipe 16, and turbine 3 driven by motor 5 are there.
Pulverized solid supply system S and a liquid supply
system L for mixer M are shown in Figure 2. The solid supply
system S includes a hopper 18 mounted above a weight sensitive
conveying belt 19, balanced on a knife 20 when loaded with a
selected weight of product. Such a device is known as a
constant weight weight-sensitive conveyor. This weight-
sensitive conveyor 19 is combined with a trap 21 for regu-
lating the thickness of the layer of powder supplied by hopper
18. A vibrating metallic channel 22 equipped with an over-
lying screen is mounted below the discharge end of the
constant weight-sensitive conveyor 19. This channel is
inclined in relation to the horizontal direction at an angle
which depends on the pulverized product and which for plaster
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11'~1804
powder, will be preferably approximately 45 degrees. Channel
22 is mounted in such a way that its lower end hangs over
container 1 of mixer M and that the powder brought by it falls
into the center of container 1 on turbine 3. The solid supply
system S is known to the art so it need not be further
detailed.
In the liquid supply mechanism L of Figure 2, liquid
supply is effected from a constant level tank 23; a regulation
of liquid flow rate is insured by a valve 24, a flow-meter 25
making it possible to control the rate of flow into hose 15
with precision.
The mixing facility operates as follows. Plaster
powder (P) will be taken as example of the pulverized solid
and water (W) as example of the liquid.
Prior to the start, a ratio WpO is selected, Wo and
Po being respectively the mass flow rate of water (Wo) and the
mass flow rate of plaster powder (Po) which flow rates are
first set. The water flow rate is regulated by valve 24 to
selected Wo value. Then the plaster flow rate is regulated at
Po value: plaster powder contained in hopper 18 spreads on
constant weight weight-sensitive conveyor 19 set in equi-
librium on knife 20 for a selected weight of product in tank
1, and then flow rate Po is obtained by regulating the trans-
lation speed of the weight-sensitive conveyor 19. A length of
stay To of mixed plaster in mixer container 1 is selected.
Rotation of turbine 3 is started. Mixer container 1 is closed
by plugging pipe 13 or by closing valve 12. Liquid supply
mechanism L, adjusted to supply a flow rate Wo is opened for
the time To selected. Water is introduced through pouring
spout 14, and through pipe 16. By turning at high speed,
turbine 3 stirs up the water. At the end of time To, the
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water supply is shut off. Then the plaster powder supply
mechanism S, adjusted for a flow rate Po, is put in operation
for a period of time To. At the end of time To, the plaster
powder supply is shut off.
Turbine 3 is allowed to mix water and plaster powder
for a period of time of approximately To/2 starting with
plaster powder supply shutoff. Then, after this mixing time
To/2, at the same time, the water supply, still adjusted for a
flow rate To, is opened, the plaster supply still adjusted for
a flow rate Po is opened, mixture contained in the mixer
container is allowed to run out either by opening pipe 13 or
by opening valve 12, and by adjusting valve 12 so that the
amount of product in the mixer M remains constant and equal to
the amount present in the container at startup. Thus, a
permanent running condition is quickly reached. Water and
plaster powder supply is in continuous process with respective
flow rates Wo and Po, mixing is continuous, a constant amount
of mixture remains in the mixer container, the average time of
stay of the mixture in mixer is constant and equal to time To
selected at start, and running off of mixture is also a
continuous process with a (Wo + Po) flow rate.
Water introduced into ring-shaped pouring spout 14
is uniformly distributed all around it and overflows along the
inner wall of container 1. Water from pipe 16, controlled by
valve 17, sprays and cleans shaft 4 of turbine 3. Plaster
powder contained in hopper 18 spreads on constant weight-
sensitive conveyor 19 balanced on knife 20. The weight-
sensitive conveyor 19 being set for a flow rate Po, any tempo-
rary oversupply or undersupply of plaster powder results in an
unbalance which leads to a change of position of trap 21 for
regulation of the thickness of the plaster powder layer, a
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change which tends to reestablish balance.
At the end of weight-sensitive conveyor 19, plaster
powder falls on the screen that covers vibrating metallic
channel 22, breaking into blocks and plaster powder runs into
channel 22. The channel 22, through its vibrations, spreads
the plaster powder and then forces it through a spout formed
in its end to fall into turbine 3 revolving at high speed
inside of container 1 of mixer M. The sheet of water formed
on the wall of container 1 and the water sprayed on turbine
shaft 4 prevent any deposit of plaster and any unwanted
beginning of setting of plaster on container 1 wall and on
shaft 4.
High speed revolving turbine 3 moves the powder and
water inside container 1. The turbine speed is set so that a
single vertical axis vortex becomes established, i.e. a hollow
whirlpool covering the inside of the tank walls. Then the
outer surface of the mixture assumes a conical form, centered
on shaft 4 of turbine 3.
Vortex depth depends on geometric data of mixer M
container 1 and on the revolving speed of turbine 3 which is
adjusted so that bottom of the vortex touches turbine 3 and
eliminates any dead spots of mixture on bottom 6. This
optimal speed depends on mixture fluidity, which is a function
of ratio:
Wpo and of To.
With too low a speed there is an excessive covering
of the turbine blade by the mixture and an excessively flat
surface of the mixture on which blocks of solid powder may
remain because the powder is not dispersed.
On the other hand, too high a speed tends to hollow
out the vortex excessively so as to uncover all of turbine 3
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804
and to cause the mixture to climb too high along the container
1 wall, the latter falling back periodically on the turbine 3
and thus causing an irregular revolving motion.
Plaster powder supplied by vibrating channel 22
falls in the center of the vortex upon turbine 3 revolving at
high speed. It is instantly dispersed and project2d outwardly
into the preexisting mixture in container 1.
Rotation of the mixture insures homogenization, and
the slope of the liquid surface prevents stagnation of solid
products in agglomerates. The plaster/water mixture assumes
turbine 3 flow lines, i.e. mixing zone circulation lines
adjacent intermediate bottom 6. Thus, there is no deposit
forming on the intermediate bottom 6 because the mixture
sweeps across it. The plaster/water mixture flows out of the
tank in an even manner through the ring-shaped space between
obstructing cone 7 and mixer lower section 2 wall without
leaving any solid residue. The position of cone 7 in relation
to mixer lower tapering section 2 wall defines the dimensions
of this ring-shaped space and thus determines a limit on the
flowing-off of the contents of the container 1. The mixture
flows off through this space with a sufficient speed so as not
to induce setting of the mixture. When cone 7 is a cone and
when mixer tapered section 2 outer wall is itself conical,
plaster mixture speed as measured along cone 7 is preferably
at least 30 cm (11.81 in.) /second and generally about lm
(3.28 ft.) /second. Sections of mixture runoff piping located
downstream is selected so that this minimal speed can be
reached, thus preventing premature deposits and mass setting
of the mixture.
The mixture gathers toward opening 2' of mixer lower
section 2 with the mixture still revolving as it flows into
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11'~1804
inverted cyclone-shaped ejection device 8. The mixture hugs
the conical walls of the device 8 and flows down along these
walls as far as base 10 in a spiral descent. This way, no
uncontrolled vortex is likely to create a motionless zone
where mass setting could occur. Then the mixture in rotating
flow is received in collector pipe 11 and forms a full
cylindrical stream whose rate of flow can be accurately regu-
lated by flow regulating valve 12 located at the end of
collector pipe 11.
However, since supply flow rates Po and Wo are not
perfectly stable and may be subject to fluctuations which
would result in mixture fluidity fluctuations, valve 12 is
constantly adjusted to maintain a constant amount of mixture
in mixture container 1 and hence a constant time of stay of
the mixture in the mixer. This time of stay makes it possible
for the mixture to be homogenized and for the supply uneven-
ness to be suppressed.
The setting of valve 12 can be arrived at in several
ways. It may be a manually set one, but in the case of
plaster, taking into account pulverized plaster's rapid evo-
lution process as soon as it mixes with water, if a constant
fluidity mixture must be available, consistent with a very
precise time of stay in mixer, it preferably is automatically
set as by weighing the mixer, for example, as set forth in our
aforesaid copending application entitled "Process and
Mechanism for Evolutive Pulp Flow Regulation".
Valve 12 may, for example, be a direct channel valve
having a rigid housing, an elastic inner sleeve, and a fluid
intake between rigid housing and sleeve, said fluid being
capable of compressing the elastic sleeve to decrease the
valve flow rate. In order to prevent any plaster deposit or
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mass setting in such a valve it is advantageous to modulate
the control fluid pressure as disclosed in our aforesaid co-
pending application entitled "Process and Mechanism for
Evolutive Pulp Flow Regulation".
It is beneficial to control the valve l2 with a
pneumatic escape type regulating mechanism that causes a vari-
ation of valve l2 opening as a function of the weight of the
mixer and use oscillations induced by vibrations resulting
from mixture and turbine motion in the mixer container. Such
an escape pneumatic mechanism includes essentially a pneumatic
circuit and a force balance beam. The pneumatic circuit is
supplied with a constant compressed air flow; it includes two
branches, one of them leading to valve l2, the other one
having a nozzle for which the balance beam acts as a flapper
plate, thus providing a certain escape of air that varies with
the position of the beam. Thus, the balance beam is constant-
ly monitoring the mixer's weight. Its equilibrium is set for
a determined weight of the mixer and is disturbed when this
weight varies. It then causes an increase or a decrease of
pneumatic circuit fluid escape and consequently, causes a
decrease or an increase of the pressure of air directed toward
the valve, thus modifying the valve aperture and consequently
the flow rate from the mixer. In addition, the turbine
motions make the beam vibrate and faintly oscillate continu-
ously and these faint oscillations are picked up by thepneumatic circuit and create valve control fluid pressure
modulation to change the shape of the elastic sleeve and
vibrate it. Since the valve elastic sleeve is constantly
changing shape, no plaster deposit whatsoever can form there.
Such a pneumatic escape type regulating mechanism is disclosed
in our aforesaid patent application entitled "Process and
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Mechanism for Evolutive Pulp Flow Regulation".
The ejection device 8 can be any standard fluid
mechanic means which has the capacity of transforming any flux
whatsoever, and particularly a revolving flux, into a full
stream. Thus, a cylindrical impact crown 26 in Figure 5, made
from a cam with a bottom and a lateral discharge pipe can be
used.
Retention time To must always remain shorter than a
value Tp corresponding to the start of setting of the mixture.
Once supply flow rates Po and Wo and hence running off rate
(Po + Wo) are set, this mean retention time To is determined
by the mixing container filling level, and it is by maintain-
ing this filling level that mean retention time is kept
CGnStant. Mean retention time is at least three seconds and
preferably between 15 and 30 seconds in order for a satis-
factory homogenization of solid and liquid products to be
achieved.
So far a plaster powder and water mixture has been
described, but the process remains the same and mechanism
operates in the same manner if additives are added at one of
the various mixing steps, additive being understood as
reactive or inert, solid or liquid products, preferably finely
powdered for the solid ones. Thus, it is possible to intro-
duce solid additives with pulverized plaster, either with
addition made beforehand by the plaster powder manufacturer,
or by spreading the additive in hopper 18 or on weight-
sensitive conveyor belt 19. It is also possible to introduce
solid or liquid additives into the water, or else directly
into the mixer. Said additives can be chemical catalysts~ or
plaster reinforcing elements such as chopped or finely divided
fibers.
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Thus, it is necessary to take the words "plaster
powder" and "water" in a broader sense, and to use terms such
as solid phase or solid to designate plaster powder by itself
and mixtures of plaster with other solids, and to use terms
such as "liquid phase" or "liquid" to designate water by
itself as well as water containing solid or liquid additives.
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