Note: Descriptions are shown in the official language in which they were submitted.
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Reactor and a method of purifying a process solution
FIELD OF THE INVENTION
The invention relates to the equipment and method defined in the
independent claims 1 and 11 for purifying a process solution.
BACKGROUND OF THE INVENTION
Generally, agitation reactors are cylindrical and they have standard
diameters. Typically, they are provided with flow resistances, which are
attached to the walls of the reactor and the purpose of which is to eliminate
the central turbulence, which is considered harmful and which absorbs gas
from the surface. Solid-solution processes normally require mixing, wherein
both strong turbulences and sufficient circulation occur. One important
process is, e.g., the removal of cadmium by cementation. Cadmium is one of
the harmful substances in the electrolytic processing of zinc.
The feeding into the agitation reactor mostly takes place by feeding both the
solid matter and the solution into the reaction space from above. Generally,
in a continuous reactor, it is desirable that both the solid matter and the
solution escape approximately at the slurry density of the reaction space.
Thus, it is not desirable for the heaviest or coarsest particles to remain in
the
reactor. In that case, it is natural that the outlet of the slurry flow can
preferably be mounted on the reactor wall to mainly take place as an
overflow.
In the method according to US patent 3,954,452, the solution rises from a
fluidization part through a conical extension into a clarification part, from
where there is a discharge outlet of the solution on the wall of the
clarification
part. The process disclosed comprises the cementation of cadmium solution
and zinc powder. In this cementation reaction, cadmium powder is formed,
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which due to its porosity is lighter and, at the same time, also finer. One
object is to prevent the exit of solid particles, which are formed as reaction
products, out of the reactor along with the solution. A difficulty in this
case is
also the adherence of hook-like particles to each other, e.g., agglomeration.
Gradually, the agglomerates grow so large that the motion in the fluidized
bed weakens and, finally, stops completely. Therefore, a flocculation solution
that prevents the agglomeration of particles is fed into the fluidization
space.
As the prevention, in practice, is not quite perfect, a mixing member that
crushes the agglomerates is placed in the lower part and, correspondingly,
fairly small flow resistances that receive the impact forces and prevent
turbulences are placed on the walls. The solution flows as directly as
possible along the shortest route towards the exhaust unit, whereby the flow
field is rendered the form of a reducing curved cone. This, again, means that
the speed of the solution flow that carries possible particles increases and
the particles have no chance of detaching from the flow.
One problem with the equipment described above is that the bed material
that prevents the exit of solid matter should be quite coarse. However, with
the reactions advancing, the grain size of the solid matter in the bed
decreases, whereby the amount of solid matter drifting along with the
solution increases.
Present reactors face the disadvantage that the flow permeates through the
fluidized bed once in each reactor. This has a significant effect on the
purification of the solution and the number of circulation phases.
Additionally,
the present fluidized-bed reactors have no adjustment, but their properties
are determined according to the feed of the process flow and the particle
size. Naturally, this causes problems when these magnitudes vary according
to the status of the process. Furthermore, the reactors are of the batch type
and it is not easy to take into account the flexibility brought by the
capacities.
In the present fluidized-bed reactor system, the pressure losses are
controlled by the liquid bed accumulating on top of the exhaust units. As the
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potential energy should cover the pressure loss caused by the fluidized bed
of each reactor, the transfer of solution through the series of reactors then
requires a large liquid bed.
OBJECT OF THE INVENTION
The object of the present invention is to eliminate the disadvantages
occurring in the prior art described above. According to the invention, a
novel, more effective method and equipment for purifying solid matter from
the process solution by means of the fluidized bed are thus presented. By
means of the invention, the separation of solid matter is enhanced by
circulating the solution in the fluidized bed and the flexibility required by
the
process changes is increased by controlling the amount of solution to be
circulated in the fluidized bed.
SUMMARY OF THE INVENTION
The invention relates to a reactor for purifying solid matter from the process
solution in the fluidized bed, whereby the reactor comprises a means of
feeding and removing the process solution, the reactor being formed from at
least three parts, the lowermost of which is an essentially cylindrical
reaction
part for forming the fluidized bed; a conically upwards-widening calming part
is attached to the upper part of the reaction part and a cylindrical
clarification
part is connected to the upper part of this, its diameter being the same as
the
upper part of the calming part, whereby a mixing member is placed in the
reactor to circulate at least part of the process solution back to the
fluidized
bed and to control the amount of circulating solution in the fluidized bed. By
the solution according to the invention, the removal of solid matter is
enhanced by gaining a better purification result by circulating the solution
in
the fluidized bed. When the process conditions change, the solution
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according to the invention can respond to the changes without causing
breaks in the process.
According to an embodiment of the invention, the mixing member is placed in
the centre of the reactor to produce an axial flow in the solution in the
reactor. Consequently, the most advantageous flow conditions are reached
in the reactor. According to the invention, the mixing member comprises a
pipe element, the lower par of which extends below the fluidized bed.
Consequently, the flow can be directed to flow through the fluidized bed.
According to an application of the invention, the mixing member is a tunnel
propeller. According to an example of the invention, the lower part of the
reaction part of the reactor has a rounded shape. In that case, the flow that
is
fed from the pipe element of the mixing member into the lower part can most
preferably and evenly be directed to the fluidized bed.
According to an embodiment of the invention, the feeder pipe of the process
solution is placed above the mixing member, whereby the solution to be
purified can be guided directly to the pipe element through the mixing
member.
According to the invention, the upper part of the reactor comprises an
overflow tank for removing the clarified solution from the reactor. According
to the invention, the reactor comprises a means, such as a pump
arrangement, for transferring the solid matter out of the fluidized bed.
According to an embodiment of the invention, the amount of solution
circulating in the fluidized bed is larger than the amount of solution fed
into
the reactor, enhancing the purification of the solution that is fed. According
to
the example, the solid matter that is removed from the solution to be purified
is cadmium.
The invention also relates to the method of purifying solid matter from the
process solution in the fluidized bed in the reactor, into which the process
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solution is fed to form the fluidized bed in the essentially cylindrical
reaction
part that is the lowermost part in the reactor, from which bed the flow
further
moves to the calming part that widens conically upwards into the upper part
of the reactor part and, further, to the cylindrical clarification part that
is
5 connected to the upper part of the same, the diameter of the clarification
part
being the same as the upper part of the calming part, whereby at least part of
the solution that is fed into the reactor is circulated to the fluidized bed
more
than once, and that the amount of circulating solution is controlled in the
fluidized bed by means of the mixing member placed in the reactor.
According to an embodiment of the invention, the mixing member produces
an axial flow in the process solution in the reactor, extending the flow below
the fluidized bed. According to the invention, at least part of the solution
that
permeates the fluidized bed moves back to the pipe element that is
connected to the mixing member, from where the solution circulates back to
the fluidized bed.
According to an embodiment of the invention, solid matter is removed from
the fluidized bed at desired intervals without stopping the process and
emptying the reactor. The amount of solution flowing in the fluidized bed is
adjusted by the rotation speed of the mixing member. In that case, the
rotation speed of the mixing member is decreased, when the amount of
solution that is fed increases, whereas the rotation speed of the mixing
member is increased when the amount of solution that is fed decreases.
According to the invention, the energy needed to fluidize the particles in the
fluidized bed is produced by the mixing member.
According to the invention, the operation of the fluidized bed can be
adjusted, whereby any variations in the process flow and particle size do not
cause problems to the process. The changes caused by the variation in
capacity can preferably be implemented without having to stop the process.
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The density of the fluidized bed and the solid matter content of the overflow
can always be reactor-specifically optimized to suit the process status,
respectively. The dimensioning of a new reactor model can be made for a
wide feeding range and, in practice; the capacity of the system can be
controlled by the number of reactors. In the model according to the invention,
the mixing member that circulates the solution produces the energy needed
for the fluidization.
The essential features of the invention are disclosed in the appended claims.
LIST OF FIGURES
The equipment according to the invention is described in detail with
reference to the appended drawings, in which
Fig. 1 a shows a vertical section of the reactor according to the invention;
Fig. 1 b shows the reactor according to the invention as viewed in the
direction A.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 a shows the reactor 1 according to the invention, wherein a liquid
process solution 2 and solid matter are treated, so that the powdery solid
matter forms a fluidized bed 3 with the liquid and, at the same time, reacts
with the process solution 2 to be purified, which is fed into the reactor. In
the
fluidized bed 3, the flow fluidizes the solid matter that reacts with the
solution.
According to the example, the cementation reaction in question is to remove
cadmium from the zinc-bearing solution, where the aqueous solution, i.e.,
Cd-bearing solution of the substance to be cemented flows through the bed
of zinc powder. Now, a reaction takes place, according to which zinc
dissolves in the solution and cadmium is removed from the solution. In the
lower part of the reactor, i.e., the cylindrical reaction part 4, the
fluidized bed
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3 is formed. From the lower part, a conically upwards-widening calming part
and, further, a cylindrical clarification part 6 rise up for raising upwards
the
solution 9 that is mainly free of solid matter to be further removed to the
overflow tank 11 in the upper part 10 of the reactor and to be further
treated.
5 Generally, we talk about purifying the solution, whereby chemical
components, such as cadmium, are removed from the solution. At the next
process stage, cadmium can further be removed from the solution 16 that is
to be removed. The lower part 7 of the reaction part 4 of the reactor has a
rounded shape, which furthers the flow of solution 8 back to the fluidized bed
3.
According to the invention, the energy needed for the fluidization of the
fluidized bed is produced by a separate rotary mixing member 12, which is
placed in the reactor. The mixing member, i.e., a propeller, preferably a
tunnel propeller, is placed below the fluid level in the reactor and it is
protected by blades to prevent the absorption of air into the propeller.
Naturally, the mixing member 12 is attached, e.g., to the upper structures of
the reactor 1 and it is controlled by a control unit 13 outside the reactor.
The
mixing member can be controlled automatically according to the solid matter
content of the feeding flow or the overflow. In the reactor, the propeller
produces an axial flow in the process solution 2 that is fed along the feeder
pipe 15 above the same, enabling the circulation of the solution through the
fluidized bed 3 more than once. Circulation in the fluidized bed 3 further
enhances the separation of solid matter. The solution 2 that is fed into the
reactor moves to the pipe element 14 of the mixing member, such as a
circulating tube, and from there to below the fluidized bed, from where it
further flows through the bed 3, whereby the chemical component to be
purified reacts with the solid matter of the bed. When moving upwards from
the reaction part to the calming part in the reactor, the cross-sectional area
of
the reactor increases and when the flow velocity decreases, the particles
floating in the bed are separated from the solution. Thereafter, part of the
solution exits as an overflow and part moves back to the pipe element by
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means of the tunnel propeller to further flow through the fluidized bed. The
properties of the fluidized bed are controlled by the rotation speed of the
pumping mixing member, and conforming to the changes in capacity takes
place by adjusting the same. If the amount of solution fed into the reactor is
increased, the rotation speed of the pumping mixing member is
correspondingly decelerated; therefore, the fluidized bed remains stable.
Correspondingly, the procedure is reversed, when the amount to be fed
decreases. According to the invention, the solid matter used for purification
is
removed upstream from the fluidized bed by means of a piping and pump
arrangement 18, which is separate with respect to the flowing solution. The
removal of solid matter 17 is implemented by a suitable pump, such as an
airlift pump, into the upper part 10 of the reactor and from there to be
further
treated.
EXAMPLE
The invention is illustrated by means of the following example. According to
the example, a present well-known reactor for removing cadmium is
compared with the reactor according to the invention. Table 1 shows
measurement results in both cases mentioned above. 440 m3/h of process
solution to be purified were fed into the reactor, whereby the flow velocity
that floats the solid matter particles in the fluidized bed is 0.039 m/s.
According to the example, by using the reactor according to the invention,
the diameter of the fluidized bed can be increased to 3600 millimetres, the
diameter of the present reactor remaining at 2000 millimetres. The solution to
be purified is subjected to axial flow under the effect of the rotational
power
of the propeller placed in the reactor, whereby under the effect of the flow,
the solution is pushed into the pipe element of the propeller, i.e., the
circulating tube, at a velocity of 1.2 m/s. The diameter of the pipe element
of
the propeller, by which an advantageous stability of the fluidized bed is
achieved, is 550 millimetres in the solution according to the invention.
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The rate of flow required in conventional fluidization is always determined
according to the amount of solution to be fed, but according to the example
that applies the invention, the rate of flow can be increased to as much as
1023 cubic metres an hour. According to the invention, the solution can be
circulated according to changing conditions by adjusting the amount of
circulating flow in the fluidized bed by a separate tunnel propeller. The
amount of circulating flow needed for the fluidization is controlled by the
rotation speed of the tunnel propeller. According to the example, the control
range of the circulating flow is 1000-1500 m3/h, whereby the flow rate of the
solution fed into the reactor can be adjusted within a range of 0-900 m3/h.
According to the invention, the same reactor can thus conform to the
changes in the process conditions. According to the example, the volume of
the fluidized bed in the reactor can be increased to 15 m3. Furthermore, for
fluidizing the same flow rate, only one reactor is needed in the solution
according to the invention compared with a case, where conventionally a
series of many reactors is used.
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Table 1.
Reactor according
Magnitude to be measured Present reactor to the invention
Flow rate of the process solution
m3/h 440 440
Flow velocity required for fluidizing
the particles in the area of the
fluidized bed (m/s) 0.039 0.039
Diameter of the fluidized bed
m m 2000 3600
Diameter of the circulating tube
m m - 550
Velocity in the circulating tube
m/s - 1.2
Flow rate required for the
fluidization m3/h 440 1023
Control range of the circulating
flow m3/h 1000-1500
Variation allowed for the process _
solution m3/h 0-900
Volume of the fluidized bed m3 6 15
Number or reactors needed 2.3 1
It is obvious to those skilled in the art that with the technology improving,
the
5 basic idea of the invention can be implemented in various ways. Thus, the
invention and its embodiments are not limited to the examples described
above but they may vary within the claims.
