THICKENERS WITH VIBRATING INCLINED PLATES

EPAISSISSEURS AVEC PLAQUES INCLINEES VIBRANTES

English Abstract

A thickener with vibrating inclined plates placed inside is disclosed for
separating solids
from feed liquor of suspended solids, by gravity settling, to form a thickened
suspension
as thickener underflow effluent and recovering clarified liquid as thickener
overflow
effluent. The thickener is composed of a liquid-holding tank of rectangular or
circular
geometry, preferably with triangular or conical bottom section. Inside the
thickener,
vibrating inclined plates are placed, preferable in the compression zone of
the thickener.
The number and geometry of the plates, and their inclination angles are
adjusted
depending on the sliding characteristics of the sedimenting suspension. The
inclined
plates vibrate continuously or periodically at selected vibration directions,
amplitudes and
frequencies. Thickener overflow or the clarified fluid, which is the process
water for
most of industrial applications, is collected in a launder and continuously
discharged.
Thickener underflow is the thickened suspension, which is continuously
discharged from
the bottom of the thickener using screw conveyer. The present invention can be
used for
thickening of suspensions by means of batch or continuous operating modes.

Claims

Claims
Claim 1. A process apparatus for continuously separating suspended solids from
liquor
feed streams by gravity settling comprising:
a. a liquid-holding tank of rectangular or circular geometry, preferably
with a triangular or conical bottom section, for continuous thickening
of a body of liquid suspension;
b. triangular or conical bottom section of the said liquid-holding tank
according to Claim 1-a provides greater surface area in upwards
direction, by which the linear velocity of the release liquid in an
upwards direction in the thickener is reduced, which decreases the
interaction between the settling particles and the release liquid, and
consequently causes an increase in the rate of particle settling;
c. triangular or conical bottom section of the said liquid-holding tank
according to Claim 1-a improves the compaction of the thickened
suspension caused by natural gravity, which results in a better
compression of the thickened suspension, therefore high solid
concentration;
d. vibrating inclined plates placed inside the thickener, which could be in
varying number and geometry, mounted preferable within the
compression zone of the said liquid-holding tank according to
Claim 1-a;
e. vibrating inclined plates according to Claim 1-d, vibrating
continuously or periodically, at selected directions, amplitude and
frequency in the said liquid-holding tank according to Claim 1-a
provide disruptive energy to the solids suspension structure resulting
in accelerated reduction in excess fluid pressure, accelerated
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compression of the thickened suspension and faster release of the
liquid.
Claim 2. A process apparatus according to Claim 1, further including a
mechanism for
continuous discharge of thickened suspensions comprising:
a. a machine according to Claim 2 further consisting of a continuously
rotating screw conveyer mounted in the bottom section of the said
liquid-holding tank according to Claim 1-a, which continuously
discharges the thickened suspension; and,
b. a machine according to Claim 2-a the said continuously rotating screw
conveyer further increases the compression of the thickened
suspension during its discharge.
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Description

CA 02355575 2001-08-03
Descriution
Background of the Innovation
The present invention relates generally to the sedimentation process also
called
thickening, caused by natural gravity, which is used for the separation of
suspended
solids from solid-liquid suspensions.
Sedimentation apparatus, called thickeners, operating by gravity settling are
used to
separate suspended solids from streams of liquid, such as water and
wastewater. A
thickener is also called a clarifier, siince the recovered liquid would
contain a very minor
amount, i.e., generally less than 0.1 % by weight solids. The objective of the
use of a
thickener apparatus could be obtaining the thickened pulp, recovering the
liquid, or both.
In most industrial applications, the feed stream contains about 5 % to 30 % by
weight
solids, while the thickened suspensions contain about more than 30 % by weight
solids.
Particle size and particle density of suspended solids, suspension
temperature, fluid
viscosity, fluid density and thickener geometry are the paramount parameters
affecting
thickener performance. To accelerate liquid-solid separation, especially when
the size of
suspended particles is small, it is a common practice to employ chemical
additives, which
are called flocculating and/or coagulating reagents. 'These chemical additives
reduce the
solid-liquid interfacial tension in i;he suspension, by which the very fine
suspended
particles cannot be wetted strongly by the liquid and they form aggregates,
which are also
called flocs. These aggregates can settle rapidly since their sizes are
sufficiently large.
For large particles, gravity forces easily overcome viscous drag forces
exerted on the
particles by the upward motion of the release fluid and the downward motion of
the
suspended particles. In most cases, mixing of chemical additives in the
suspension is
accomplished outside the thickener, for example in a transportation pipeline,
in a
feedwell or in a mixing tank. In army case, sufficient retention time and
mixing intensity
are needed for the formation of the aggregates. Some chemical additives work
better
when the solid concentration of the; suspension is within a narrow
concentration range,
i.e., for some suspensions this range: is about 4 % to 8 % by weight solid
concentration.
If the solid concentration in the feed suspension is large, then dilution of
the feed

CA 02355575 2001-08-03
Thickeners with Vibrating Inclined Plates
suspension is needed, so that the chemical additive would work effectively to
form
aggregates.
Standpipe tests, I L or 2 L ( 1 L. is 0.001 cubic meter) capacity, are
generally used to
understand thickening mechanism for a given suspension. Thickening of
suspensions
takes place in four distinct zones, which are formed in the thickener:
~ Zone I, composed of clear recovered liquid.
~ Zone II or hindered settling zone, composed of pulp with uniform
concentration
with constant settling velocity.
~ Zone III, a thin transition zone between Zone II and Zone IV.
~ Zone IV or compression zone, composed of thickened pulp, in which solid
concentration gradient exists.
In standpipe tests, the settling characteristics of the pulp and operating
conditions such as
temperature and pulp solid concentration determine the thickness of each zone.
In
thickeners however, the thickness of each zone is a function of additional
variables such
as thickener geometry, thickener type, raking mechanism, etc.
It is well known that the settling .capacity of a thickener is related to the
cross sectional
area of the thickener and settling velocity of the particles in the pulp. The
settling
capacity of a thickener is definedf as the solid flux per unit cross sectional
area of the
thickener, i.e., kilogram solids fed into the thickener per square meter cross
sectional area
of the thickener, per hour. The minimum hindered settling velocity of Zone II
will
eventually control the settling cal>acity and therefore exerts a primary
influence on the
cross sectional area of the thickener.
Batch settling tests are generally suitable to measure hindered settling rates
and pulp
concentrations and are useful for predicting the required surface area of the
thickener.
The hindered settling rate can be determined by observing the motion of the
interface
between Zone I and Zone II as a function of time. The height of the thickener
and slow
raking of the pulp in Zone IV will affect the performance of the sediments
within the
compression zone. The best supporting data for the performance of Zone IV, the
compression zone, can be obtained by operating a continuous pilot unit, which
is usually
only economically feasible for large projects.
Summary of the Invention
A primary objective of the present invention is to provide an improved
sedimentation
apparatus for treating industrial suspension and tailings. The present
invention could be
used for the thickening of oil sands tailings, industrial tailings such as
pulp and paper,
food processing and malting wastewater, mineral slurnes such as coal and
mineral
processing tailings, municipal sewage, and others.
The present invention increases the settling capacity of the thickener. As a
result, with
the present invention, a thickener with a fixed cross sectional area would
have a high
settling capacity, therefore would handle the thickening of a high suspension
feed rate.
At the same time, the thickener still would provide good clarity in the
supernatant liquid,
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CA 02355575 2001-08-03
Thickeners with Vibrating Inclined Plates -
which is the thickener overflow effluent. It would also provide a high solid
concentration
in the compression zone and therefore a high solid concentration in the
thickener
underflow effluent.
The present invention uses vibrating inclined plates placed in the thickener,
triangular or
conical thickener bottom cross sectional geometry and rotating screw conveyer
mounted
in the bottom of the thickener. The present invention uses the combined
effects of
vibrating inclined plates, thickener bottom geometry and rotating screw
conveyer for the
most effective use of natural gravity to thicken the suspension. Using the
present
invention, high solid concentration in the compression zone is achieved
without
conventional raking mechanisms and the thickened suspension is easily
discharged from
the thickener.
The present invention is a sedimentation process apparatus with inclined
vibrating plates
placed in the thickener, preferably in the compression zone, Zone IV, of the
thickener.
Experimental tests performed using vibrating plates with inclining angles of
30, 45 and
60 degrees from the horizontal planes showed that small inclining angle from
the
horizontal plane for the vibrating plates performs betters. Small inclining
angles
however, would limit the downward sliding motion of the settled solids on the
vibrating
plates by the gravitational force. The most effective inclining angle for
vibrating plates
from the horizontal plane would not be universally constant. The optimal value
of the
inclining angle for the vibrating plates from the horizontal plane would be a
function of
the amplitude, frequency and direction of the vibration of the plates, as well
as the
physical characteristics of the thickened suspension. Therefore, the optimal
value of the
inclining angle from the horizontal plane would be determined experimentally.
The optimal inclining angle for the triangular or conical bottom section of
the thickener
would be experimentally determined using a laboratory scale thickener.
Preferably, the
inclining angle for the triangular or conical bottom section of the thickener
will be
parallel to the inclining angle of the vibrating plates.
As an example, a brief description of a rectangular thickener with a
triangular bottom
section using the art described in the present invention is depicted in Figure
1.
Other prior art considers thickening; apparatus of interest hereto includes
U.S. Pat. Nos.
4,787,978; 4,681,683; 4,544,487; 4,515,698; 4,263,137; 4,178,243; 4,151,084;
4,142,970;
4,055,494 and 3,975,266.

Representative Drawing